Mechanical Engineering

ADVANCED IC ENGINE NOTES
ME2041 ADVANCED I.C. ENGINES

LTPC
3003

OBJECTIVES:
 To update the knowledge in engine exhaust emission control and alternate fuels
 To enable the students to understand the recent developments in IC Engines
UNIT I SPARK IGNITION ENGINES 9
Air-fuel ratio requirements, Design of carburetor –fuel jet size and venture size, Stages
of combustion-normal and abnormal combustion, Factors affecting knock, Combustion
chambers, Introduction to thermodynamic analysis of SI Engine combustion process.
UNIT II COMPRESSION IGNITION ENGINES 9
Stages of combustion-normal and abnormal combustion – Factors affecting knock,
Direct and Indirect injection systems, Combustion chambers, Turbo charging,
Introduction to Thermodynamic Analysis of CI Engine Combustion process.
UNIT III ENGINE EXHAUST EMISSION CONTROL 9
Formation of NOX , HC/CO mechanism , Smoke and Particulate emissions, Green
House Effect , Methods of controlling emissions , Three way catalytic converter and
Particulate Trap, Emission (HC,CO, NO and NOX , ) measuring equipments, Smoke and
Particulate measurement, Indian Driving Cycles and emission norms
UNIT IV ALTERNATE FUELS 9
Alcohols , Vegetable oils and bio-diesel, Bio-gas, Natural Gas , Liquefied Petroleum
Gas ,Hydrogen , Properties , Suitability, Engine Modifications, Performance ,
Combustion and Emission Characteristics of SI and CI Engines using these alternate
fuels.
UNIT V RECENT TRENDS 9
Homogeneous Charge Compression Ignition Engine, Lean Burn Engine, Stratified
Charge Engine, Surface Ignition Engine, Four Valve and Overhead cam Engines,
Electronic Engine Management, Common Rail Direct Injection Diesel Engine, Gasoline
Direct Injection Engine, Data Acquisition System –pressure pick up, charge amplifier PC
for Combustion and Heat release analysis in Engines.
TOTAL: 45 PERIODS
TEXT BOOK:
1. Heinz Heisler, ‘Advanced Engine Technology,” SAE International Publications,
USA,1998
2. Ganesan V..” Internal Combustion Engines” , Third Edition, Tata Mcgraw-Hill ,2007
REFERENCES:
1. John B Heywood,” Internal Combustion Engine Fundamentals”, Tata McGraw-Hill
1988
2. Patterson D.J. and Henein N.A,“Emissions from combustion engines and their
control,” Ann Arbor Science publishers Inc, USA, 1978
3. Gupta H.N, “Fundamentals of Internal Combustion Engines” ,Prentice Hall of India,
2006
4. Ultrich Adler ,” Automotive Electric / Electronic Systems, Published by Robert Bosh
GmbH,1995

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UNIT

I

SPARK IGNITION ENGINES
Air-fuel Requirement in SI Engines
The spark-ignition automobile engines run on a mixture of gasoline and air. The amount of mixture the
engine can take in depends upon following major factors:
(i) Engine displacement.
(ii) Maximum revolution per minute (rpm) of engine.
(iii) Volumetric efficiency of engine.
There is a direct relationship between an engine’s air flow and it’s fuel requirement. This relationship is
called the air-fuel ratio.
Air-fuel Ratios
The air-fuel ratio is the proportions by weight of air and gasoline mixed by the carburetor as required for
combustion by the engine. This ratio is extremely important for an engine because there are limits to how
rich (with more fuel) or how lean (with less fuel) it can be, and still remain fully combustible for efficient
firing. The mixtures with which the engine can operate range from 8:1 to 18.5:1 i.e. from 8 kg of air/kg of
fuel to 18.5 kg of air/kg of fuel. Richer or leaner air-fuel ratio limit causes the engine to misfire, or simply
refuse
to
run
at
all.

Stoichiometric Air-Fuel Ratio
The ideal mixture or ratio at which all the fuels blend with all of the oxygen in the air and be completely
burned is called the stoichiometric ratio, a chemically perfect combination. In theory, an air fuel ratio of
about 14.7:1 i.e. 14.7 kg of air/kg of gasoline produce this ratio, but the exact ratio at which perfect
mixture and complete combustion take place depends on the molecular structure of gasoline, which can
vary somewhat.
Engine Air-fuel Ratios
An automobile SI engine, as indicated above, works with the air-fuel mixture ranging from 8:1 to 18.5:1.
But the ideal ratio would be one that provides both the maximum power and the best economy, while
producing the least emissions. But such a ratio does not exist because the fuel requirements of an engine
vary widely depending upon temperature, load, and speed conditions. The best fuel economy is obtained
with a 15:1 to 16:1 ratio, while maximum power output is achieved with a 12.5:1 to 13.5:1 ratio. A rich
mixture in the order of 11:1 is required for idle heavy load, and high-speed conditions. A lean mixture is
required for normal cruising and light load conditions. Figure 9.36 represents the characteristic curves
showing the effect of mixture ratio on efficiency and fuel consumption.

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Fig. 9.36. Effect of air-fuel ratio on efficiency and fuel consumption.

Practically for complete combustion, through mixing of the fuel in excess air (to a limited extent above
that of the ideal condition) is needed. Lean mixtures are used to obtain best economy through minimum
fuel consumption whereas rich mixtures used to suppress combustion knock and to obtain maximum
power from the engine. However, improper distribution of mixture to each cylinder and
imperfect/incomplete vaporization of fuel in air necessitates the use of rich mixture to obtain maximum
power output. A rich mixture is also required to overcome the effect of dilution of incoming mixture due to
entrapped exhaust gases in the cylinder and of air leakage because of the high vacuum in the manifold,
under idling or no-load condition. Maximum power is desired at full load while best economy is expected
at part throttle conditions. Thus required air fuel ratios result from maximum economy to maximum
power. The carburetor must be able to vary the air-fuel ratio quickly to provide the best possible mixture
for
the
engine’s
requirements
at
a
given
moment.
The best air-fuel ratio for one engine may not be the best ratio for another, even when the two engines are
of the same size and design. To accurately determine the best mixture, the engine should be run on a

3

dynamometer to measure speed, load and power requirements for all types of driving conditions.
With a slightly rich mixture, the combustion flame travels faster and conversely with a slightly weak
mixture, the flame travel becomes slower. If a very rich mixture is used then some “neat” petrol enters
cylinder, washes away lubricant from cylinder walls and gets past piston to contaminate engine oil. A very
sooty deposit occurs in the combustion chamber. On the other hand, if an engine runs on an excessively
weak mixture, then overheating particularly of such parts as valves, pistons and spark plugs occurs. This
causes
detonation
and
pre-ignition
together
or
separately.
The approximate proportions of air to petrol (by weight) suitable for the different operating conditions are
indicated below:
Starting
9: 1
Idling
12 : 1
Acceleration 12 : 1
Economy
16: 1
Full power
12 : 1
It makes no difference if an engine is carburetted or fuel injected, the engine still needs the same air-fuel
mixture ratios.
Carburetion
Introduction
Spark-ignition engines normally use volatile liquid fuels. Preparation of fuel-air mixture is done
outside the engine cylinder and formation of a homogeneous mixture is normally not completed in the inlet
manifold. Fuel droplets, which remain in suspension, continue to evaporate and mix with air even during
suction and compression processes. The process of mixture preparation is extremely important for sparkignition engines. The purpose of carburetion is to provide a combustible mixture of fuel and air in the
required quantity and quality for efficient operation of the engine under all conditions.
Definition of Carburetion
The process of formation of a combustible fuel-air mixture by mixing the proper amount of fuel
with air before admission to engine cylinder is called carburetion and the device which does this job is
called a carburetor.
Requirements of an automotive carburetor
The spark ignition engines fitted to automotive vehicles have to operate under variable speed and
load conditions. These engines present the most difficult and stringent requirements to the carburetors.
They are as follows:1. Ease of starting the engine, particularly under low ambient conditions.
2. Ability to give full power quickly after starting the engine.
3. Equally good and smooth engine operation at various loads.
4. Good and quick acceleration of the engine.
5. Developing sufficient power at high engine speeds.
6. Simple and compact in construction.
7. Good fuel economy.
8. Absence of racing of the engine under idling conditions.
9. Ensuring full torque at low speeds.
Factors Affecting Carburetion
Of the various factors, the process of carburetion is influenced by
i. The engine speed
ii. The vaporization characteristics of the fuel

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iii. The temperature of the incoming air and
iv. The design of the carburetor

Principle of Carburetion
Both air and gasoline are drawn through the carburetor and into the engine cylinders by the suction
created by the downward movement of the piston. This suction is due to an increase in the volume of the
cylinder and a consequent decrease in the gas pressure in this chamber. It is the difference in pressure
between the atmosphere and cylinder that causes the air to flow into the chamber. In the carburetor, air
passing into the combustion chamber picks up discharged from a tube. This tube has a fine orifice called
carburetor jet that is exposed to the air path. The rate at which fuel is discharged into the air depends on the
pressure difference or pressure head between the float chamber and the throat of the venturi and on the
area of the outlet of the tube. In order that the fuel drawn from the nozzle may be thoroughly atomized, the
suction effect must be strong and the nozzle outlet comparatively small. In order to produce a strong
suction, the pipe in the carburetor carrying air to the engine is made to have a restriction. At this restriction
called throat due to increase in velocity of flow, a suction effect is created. The restriction is made in the
form of a venturi to minimize throttling losses. The end of the fuel jet is located at the venturi or throat of
the carburetor. The geometry of venturi tube is as shown in Fig.16.6. It has a narrower path at the center so
that the flow area through which the air must pass is considerably reduced. As the same amount of air must
pass through every point in the tube, its velocity will be greatest at the narrowest point. The smaller the
area, the greater will be the velocity of the air, and thereby the suction is proportionately increased
As mentioned earlier, the opening of the fuel discharge jet is usually loped where the suction is maximum.
Normally, this is just below the narrowest section of the venturi tube. The spray of gasoline from the
nozzle and the air entering through the venturi tube are mixed together in this region and a combustible
mixture is formed which passes through the intake manifold into the cylinders. Most of the fuel gets
atomized and simultaneously a small part will be vaporized. Increased air velocity at the throat of the
venturi helps he rate of evaporation of fuel. The difficulty of obtaining a mixture of sufficiently high fuel
vapour-air ratio for efficient starting of the engine and for uniform fuel-air ratio indifferent cylinders (in
case of multi cylinder engine) cannot be fully met by the increased air velocity alone at the venturi throat.
The Simple Carburetor
Carburetors are highly complex. Let us first understand the working principle bf a simple or
elementary carburetor that provides an air fuel mixture for cruising or normal range at a single speed.
Later, other mechanisms to provide for the various special requirements like starting, idling, variable load
and speed operation and acceleration will be included. Figure 3. shows the details of a simple carburetor.

5

Figure: 3 The Simple Carburetor
The simple carburetor mainly consists of a float chamber, fuel discharge nozzle and a metering orifice, a
venturi, a throttle valve and a choke. The float and a needle valve system maintain a constant level of
gasoline in the float chamber. If the amount of fuel in the float chamber falls below the designed level, the
float goes down, thereby opening the fuel supply valve and admitting fuel. When the designed level has
been reached, the float closes the fuel supply valve thus stopping additional fuel flow from the supply
system. Float chamber is vented either to the atmosphere or to the” upstream side of the venturi.During
suction stroke air is drawn through the venturi.
As already described, venturi is a tube of decreasing cross-section with a minimum area at the
throat, Venturi tube is also known as the choke tube and is so shaped that it offers minimum resistance to
the air flow. As the air passes through the venturi the velocity increases reaching a maximum at the venturi
throat. Correspondingly, the pressure decreases reaching a minimum. From the float chamber, the fuel is
fed to a discharge jet, the tip of which is located in the throat of the venturi. Because of the differential
pressure between the float chamber and the throat of the venturi, known as carburetor depression, fuel is
discharged into the air stream. The fuel discharge is affected by the size of the discharge jet and it is
chosen to give the required air-fuel ratio. The pressure at the throat at the fully open throttle condition lies
between 4 to 5 cm of Hg, below atmospheric and seldom exceeds8 cm Hg below atmospheric. To avoid
overflow of fuel through the jet, the level of the liquid in the float chamber is maintained at a level slightly
below the tip of the discharge jet. This is called the tip of the nozzle. The difference in the height between
the top of the nozzle and the float chamber level is marked h in Fig.3.
The gasoline engine is quantity governed, which means that when power output is to be varied at a
particular speed, the amount of charge delivered to the cylinder is varied. This is achieved by means of a
throttle valve usually of the butterfly type that is situated after the venturi tube. As the throttle is closed

6

less air flows through the venturi tube and less is the quantity of air-fuel mixture delivered to the cylinder
and hence power output is reduced. As the” throttle is opened, more air flows through the choke tube
resulting in increased quantity of mixture being delivered to the engine. This increases the engine power
output. A simple carburetor of the type described above suffers from a fundamental drawback in that it
provides the required A/F ratio only at one throttle position. At the other throttle positions the mixture is
either leaner or richer depending on whether the throttle is opened less or more. As the throttle opening is
varied, the air flow varies and creates a certain pressure differential between the float chamber and the
venturi throat. The same pressure differential regulates the flow of fuel through the nozzle. Therefore, the
velocity of flow of air II and fuel vary in a similar manner. At the same time, the density I of air decrease
as the pressure at the venturi throat decrease with increasing air flow whereas that of the fuel remains
unchanged. This results in a simple carburetor producing a progressively rich mixture with increasing
throttle opening.
The Choke and the Throttle
When the vehicle is kept stationary for a long period during cool winter seasons, may be
overnight, starting becomes more difficult. As already explained, at low cranking speeds and intake
temperatures a very rich mixture is required to initiate combustion. Some times air-fuel ratio as rich as 9:1
is required. The main reason is that very large fraction of the fuel may remain as liquid suspended in air
even in the cylinder. For initiating combustion, fuel-vapour and air in the form of mixture at a ratio that
can sustain combustion is required. It may be noted that at very low temperature vapour fraction of the fuel
is also very small and this forms combustible mixture to initiate combustion. Hence, a very rich mixture
must be supplied. The most popular method of providing such mixture is by the use of choke valve. This is
simple butterfly valve located between the entrance to the carburetor and the venturi throat as shown in
Fig.3.
When the choke is partly closed, large pressure drop occurs at the venturi throat that would
normally result from the quantity of air passing through the venturi throat. The very large depression at the
throat inducts large amount of fuel from the main nozzle and provides a very rich mixture so that the ratio
of the evaporated fuel to air in the cylinder is within the combustible limits. Sometimes, the choke valves
are spring loaded to ensure that large carburetor depression and excessive choking does not persist after the
engine has started, and reached a desired speed. This choke can be made to operate automatically by
means of a thermostat so that the choke is closed when engine is cold and goes out of operation when
engine warms up after starting. The speed and the output of an engine is controlled by the use of the
throttle valve, which is located on the downstream side of the venturi.
The more the throttle is closed the greater is the obstruction to the flow of the mixture placed in the
passage and the less is the quantity of mixture delivered to .the cylinders. The decreased quantity of
mixture gives a less powerful impulse to the pistons and the output of the engine is reduced accordingly.
As the throttle is opened, the output of the engine increases. Opening the throttle usually increases the
speed of the engine. But this is not always the case as the load on the engine is also a factor. For example,
opening the throttle when the motor vehicle is starting to climb a hill may or may not increase the vehicle
speed, depending upon the steepness of the hill and the extent of throttle opening. In short, the throttle is
simply a means to regulate the output of the engine by varying the quantity of charge going into the
cylinder.
Stages of Combustion in SI Engine
In a spark-ignition engine a sufficiently homogeneous mixture of vaporized fuel, air and residual
gases is ignited by a single intense and high temperature spark between the spark plug electrodes (at the
moment of discharge the temperature of electrodes exceeds 10,000°C), leaving behind a thin thread of
flame. From this thin thread combustion spreads to the envelop of mixture immediately surrounding it at a

7

rate which depends primarily upon the temperature of the flame front itself and to a secondary degree,
upon both the temperature and the density of the surrounding envelope. In this manner there grows up,
gradually at first, a small hollow nucleus of flame, much in the manner of a soap bubble. If the contents of
the cylinder were at rest, this flame bubble would expand with steadily increasing speed until extended
throughout the whole mass. In the actual engine cylinder, however, the mixture is not at rest. It is, in fact,
in a highly turbulent condition the turbulence breaks the filament of flame into a ragged front, thus
presenting a far greater surface area from which heat is radiated; hence its advance is speeded up
enormously. The rate at which the flame front travels is dependent primarily on the degree of turbulence,
but its general direction of/movement, that of radiating outward from the ignition point, is not much
affected. According to Ricardo the combustion can be imagined as if developing in two stages, one the
growth and development of a semi propagating nucleus of flame called ignition lag or preparation phase,
and the other, the spread of the flame throughout the combustion chamber [see Fig. 9].

8

Figure: 9. Stages of combustion in SI engine
The former is a chemical process depending upon the nature of the fuel, upon temperature and pressure,
the proportion of the exhaust gas, and also upon the temperature coefficient of the fuel, that is, the
relationship between temperature and rate of acceleration of oxidation or burning. The second stage is a

9

mechanical one pure and simple. The two stages are not entirely distinct, since the nature and velocity of
combustion change gradually. The starting point of the second stage is where first measurable rise of
pressure can be seen on the indicator diagram, i.e., the point where the line of combustion departs from the
compression line. In Fig. 14.2(b), A shows the point of passage of spark - (say 28° before TDC), B the
point at which the first rise of pressure can be detected (say, 8°before TDC) and C the attainment of peak
pressure. Thus AB represents the first stage (about 20° crank angle rotation) and BC the second stage.
Although the point C makes the completion of the flame travel, it does not follow that at this point the
whole of the heat of the fuel has been liberated, for even after the passage of the flame, some further
chemical adjustments due to re-association, etc., and what is generally referred to as after burning, will to a
greater or less degree continue throughout the expansion stroke. The first stage AB, by analogy with diesel
engines is called ignition lag, which label is wrong in principle. In spark ignition there is practically no
ignition lag and a nucleus of combustion arises instantaneously near the spark plug electrodes. But during
the initial period flame front spreads very slowly and the fraction of burnt mixture is small so that an
increase of pressure cannot be detected on the indicator diagram. The increase of pressure maybe just one
per cent of maximum combustion pressure corresponding to burning of about 1.5per cent of the working
mixture, and the volume occupied by the combustion products may be about 5 per cent of the combustion
chamber space.
The stage II is the main stage of combustion. The end of second stage is taken as the moment at which
maximum pressure is reached in the indicator diagram (see Fig. 9). However, combustion does not
terminate at this point and after burning continues for a rather long time near the walls and behind the
turbulent flame front. The combustion rate in the stage III reduces, due to surface of the flame front
becoming smaller and reduction in turbulence. About 10 per cent or more of heat is evolved in the afterburning stage and hence the temperature of the gases continues to increase to point D in Fig.9. However,
the pressure reduces because the decrease in pressure due to expansion of gases and transfer of heat to
walls is more than the increase in pressure due to combustion.

Effect of Engine Variables on Flame Propagation
A study of the variables which affect the flame propagation velocity is important because the
flame velocity influences the rate of pressure rise in the cylinder, and has bearing or certain types of
abnormal combustion
.
There are several factors which affect the flame speed, the most important being fuel-air ratio and
turbulence.
1. Fuel-air ratio: The composition of the working mixture influences the rate of combustion and the
amount of heat evolved. With hydrocarbon fuels the maximum flame velocities occur when mixture
strength is 110% of stoichiometric (i.e., about 10% richer than stoichiometric). When the mixture is made
leaner or is enriched and still more, the velocity of flame diminishes. Lean mixtures release less thermal
energy resulting in lower flame temperature and flame speed. Very rich mixtures have incomplete
combustion (some carbon only burns to CO and not to CO2) that results in production of less thermal
energy and hence flame speed is again low.
2. Compression Ratio: A higher compression ratio increases the pressure and temperature of the working
mixture and decreases the concentration of residual gases. These favorable conditions reduce the ignition
lag of combustion and hence less ignition advance is needed. High pressures and temperatures of the
compressed mixture also speed up the second phase of combustion. Total ignition angle is reduced.

10

Maximum pressure and indicated mean effective pressure are increased.. Lastly, use of a higher
compression ratio increases the surface to volume ratio of the combustion chamber, thereby increasing the
part of the mixture which after-burns in the third phase. The increase in compression ratio results in
increase in temperature that increases the tendency of the engine to detonate.
3. Intake temperature and pressure: Increase in intake temperature and pressure increases the flame
speed.
4. Engine load: With increase in engine load the cycle pressures increase. Hence the flame speed
increases. In SI engines with decrease in load, throttling reduces power of an engine. Due to throttling the
initial and final compression pressures decrease and the dilution of the working mixture due to residual
gases increases. This makes the smooth development of self propagating nucleus of flame difficult and
unsteady and prolongs the ignition lag. The difficulty can be overcome to a certain extent by enriching the
mixture at low loads (0.8 to 0.9of stoichiometric) but still it is difficult to avoid after-burning during a
substantial part of expansion stroke. In fact, poor combustion at low loads and the necessity of mixture
enrichment are among the main disadvantages of spark ignition engines which cause wastage of fuel and
discharges of a large amount of products of incomplete combustion like carbon monoxide and other
poisonous substances.
5. Turbulence: Turbulence plays a very vital role in combustion phenomenon. The flame speed is very
low in non-turbulent mixtures. A turbulent motion of the mixture intensifies the processes of heat transfer
and mixing of the burned and unburned portions in the flame front (diffusion). These two factors cause the
velocity of turbulent flame to increase practically in proportion to the turbulence velocity. The turbulence
of the mixture is due to admission of fuel-air mixture through comparatively narrow sections of the intake
pipe, valves, etc. in the suction stroke. The turbulence can be increased at the end of the compression by
suitable design of combustion chamber that involves the geometry of cylinder head and piston crown. The
degree of turbulence increases directly with the piston speed. If there is no turbulence the time occupied by
each explosion would be so great as to make the high speed internal combustion engines impracticable.
Insufficient turbulence lowers the efficiency due to incomplete combustion of the fuel. However, excessive
turbulence is also undesirable.
6. Engine Speed: The higher the engine speed, the greater the turbulence inside the cylinder. For this
reason the flame speed increases almost linearly with engine speed. Thus if the engine speed is doubled the
time required, in milliseconds, for the flame to traverse the combustion space would be halved. Double the
original speed arid hence half the original time would give the same number of crank degrees for flame
propagation. The crank angle required for the flame propagation, which is the main phase of combustion,
will remain almost constant at all speeds. This is an important characteristic of spark ignition engines.
However, the increase in engine speed would lead to ignition advance due to the first phase of combustion.
This can be illustrated with a numerical example. Consider a petrol engine running at 1500rpm. Let us say
for the first stage of combustion the ignition lag, the time required in terms of crank angle, is 8° of crank
rotation, and for the second stage, the propagation of flame through the combustion space, 12oofcrank
rotation is required. Thus the total ignition period is 20°of crank rotation. Now if the engine speed is
doubled from 1500 to 3000 rpm, the time required for the second stage will again be 12° of crank rotation
(due to doubling of turbulence intensity time in milliseconds is halved and in terms of crank angle remains
constant), but for the first stage time in milliseconds is constant and hence in terms of crank angle it will be
doubled, i.e., it would be 16°.This would make the total ignition period of 16 + 12 = 28° crank rotation at
3000rpm compared to 8° + 12°= 20° at .1500 rpm. From this it follows that with increase in engine speed
ignition must be advanced. This is done in practice by automatic ignition advance mechanism.

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7. Engine size: Engines of similar design generally run at the same piston speed. This is achieved by
smaller engines having larger rpm and larger engines having smaller rpm. Due to the same piston speed,
the inlet velocity, the degree of turbulence, and flame speed are nearly same in similar engines regardless
of the size. However, in small engines the flame travel is small and in large engines large. Therefore, if the
engine size is doubled the time required (in milliseconds) for propagation of flame through combustion
space will also be doubled. But with lower rpm of larger engines the time for flame propagation in terms of
crank angle would be nearly same as in smaller engines. In other words the number of crank degrees
required for flame travel will be about the same irrespective of engine size, provided the engines are
similar.
Rate of Pressure Rise
The rate of pressure rise is a very important aspect of flame development from engine design and
operation point of view. It considerably influences the maximum cylinder pressure, the power produced
and the smooth running of the engine. The rate or pressure rise depends on the mass rate of combustion of
the mixture in the cylinder. Fig. 10 shows pressure-crank angle diagrams for three different combustion
rates. One is for a high, the second for the usual and the third for a low rate of combustion

12

Figure: 10. Relationship b/w pressure and crank angle for different rates of combustion
It is clear from the figure that with lower rates of combustion longer time is required for combustion that
necessitates the initiation of burning at an earlier point on the compression stroke. With higher rates of

13

burning the time required for combustion is smaller and the rate of pressure rise is higher. Also, the peak
pressure produced is close to TDC, which is desirable because it produces greater force acting through a
large portion of the power stroke. But peak pressure and hence peak temperature too close to TDC gives a
long time for rapid heat loss from the cylinder. The higher rate of pressure rise causes rough running of the
engine because of vibrations and jerks produced in crankshaft. If the rate of pressure rise is very high it
results in abnormal combustion called detonation. In practice the engine is so designed that approximately
one-half of the pressure rise takes place as the piston reaches TDC. This results in peak pressure and
temperature 10° to 15° after TDC. In this way very small portion of the expansion stroke is-lost and the
gain is smooth engine operation and saving an appreciable period of time during which loss of heat is
rapid. In the old engines with low compression ratios of 5 to 6 a rate of pressure rise of 2 bar per crank
degree used to be thought as optimum. Today with higher compression ratios of the order of 8 to 9, a rate
of pressure rise of 3 to 4 bar per crank degree may be employed if the engine mountings are sufficiently
stiff and efficient.
Gasoline Combustion
Vaporization of the hydrocarbons in gasoline and start of decomposition take place at temperatures
below 593 K, which exist in the combustion chamber before the initiation of ignition. The products of
combustion are mostly gases containing a large quantity of heat. The heat energy increases the gas
pressure in the combustion chamber to produce the force on the engine piston, required to operate the
engine. The liquid gasoline must be converted to a vapour to burn in an engine. In carburetted engines
vaporization of the gasoline must be done in one-third of a second at idle speeds and in one-thirtieth of a
second at normal operating speeds. In fuel injected engines this must occur much faster. The carburetor
during the process of mixing liquid fuel and air supports the vaporization process by breaking the liquid
gasoline into sudsy foam that rapidly mixes with the air. The molecules of fuel and the molecules of
oxygen in the air must combine in correct numbers. At sea level the air being dense a relatively small
quantity is required for a given amount of gasoline. The air becomes less dense at high altitudes and at
high atmospheric temperatures due to which the same volume of air contains a smaller number of oxygen
molecules causing the air-fuel mixture to become richer in fuel. This causes problem on some emission
controlled engines requiring leaner carburetor settings on automobiles used in the mountains than those
used at sea level. Since automobiles are frequently operated in both mountains and at sea level, carburetors
are provided with altitude compensation devices to prevent over-rich mixtures at high elevations.

the charge is trapped in the combustion chamber, the molecules of oxygen in the air come into close
contact with the hydrocarbon molecules of the gasoline. This causes rapid burning. A litre of gasoline if
completely burned produces nearly a litre of water as well as sulphur dioxide in an amount dependent on
the sulphur content in the gasoline. As the water is in a vapour form at normal operating temperatures it
leaves the cylinder as a part of exhaust gas. When the engine is first started in cold weather condensed
water vapour is visible in the exhaust. Condensed moisture with sulphur dioxide produces the acidic water,
which is corrosive. During low temperature operating conditions such as suburban driving when the engine
is cold, much of the moisture is condensed inside the engine. The combination of corrosion and wear under
these conditions is the major reason for excessive wear of the top ring area of the cylinder wall.
Normal Combustion
In a SI engine a homogeneous air-fuel mixture within the combustible range sustains the progress
of a definite flame front across the combustion chamber, and combustion takes place in any location where
fuel particle exists. In a CI engine, on the other hand, the air-fuel ratios in the various part of the chamber

14

very widely, so no definite flame front is evident, and hence combustion occurs in many locations within
the
chamber.
A spark plug ignites the charge in the combustion chamber near the end of the compression stroke. The
spark, produced across the spark plug electrodes at the correct time, must have sufficient energy to raise
the gas temperature between the electrodes at a point so that the charge burning becomes self-sustaining.
From this point, a flame front moves smoothly across the combustion. The flame front movement during
normal combustion is illustrated in Fig. 8.6. Burning of charge takes place during approximately fifty
degrees of crankshaft rotation due to which maximum force is exerted on the crankshaft. Actual
combustion is much more complex and the combustion gases pass through many phases during the
combustion process. For better understanding, the combustion is divided into two phases i.e. pre-flame
reactions,
and
combustion.

As the gases are compressed and the temperature rises, pre-flame chemical reactions take place in the
compressed charge thereby changing the character of the charge. These pre-flame reactions prepare the
charge
for
burning.
As ignition takes place, depending upon combustion chamber turbulence the flame front moves out in a
modified spherical fashion. The heat energy released behind the flame front increases combustion chamber
pressure and temperature. Due to higher combustion chamber pressure and temperature the pre-flame
reactions are increased in a portion of the charge, called the end gases, which remain ahead of the flame
front. Pre-flame reactions increase more rapidly at higher engine compression ratios. If pre-flame reactions
become
too
rapid,
abnormal
combustion
takes
place.

Abnormal Combustion
Abnormal combustion may be divided into two main types i.e. knock or detonation and surface
ignition. Each of these types causes loss of power and excessive temperature. Continued operation under
either type of abnormal combustion gives rise to physical damage of the engine.
Detonation.
Engine knock or detonation is the out come of rapid pre-flame reactions within the highly stressed
end gases. Due to the too rapid reactions spontaneous ignition of the end gases takes place as shown in Fig.
8.7. This causes very rapid combustion within the end gases, accompanied by high-frequency pressure
waves. These waves hit the combustion chamber walls; as a result vibration noise sets which is called
knock or detonation.
Detonation is affected by
(i) compression ratio,
(ii) the temperature and pressure at the end of compression, (Hi) the temperature of combustion chamber
wall,
(iv) engine speed,
(v) fuel mixture strength,
(vi) combustion chamber shape,
(vii) the type of fuel,
(viii) ignition timing,
(ix) position of spark plug, and
(x) position of exhaust valve.

15

Fig. 8.6. Flame front movement during normal combustion.

Fig. 8.7. Flame front movement during detonation.

The tendency of an engine to knock with a given fuel can be suppressed by lowering either
combustion pressure or temperature, or both ; or by reducing the time the end gases are subjected to high
pressures and temperatures. Also, using a fuel, which is less susceptible to rapid pre-flame reactions,
reduces the tendency to knock. Octane rating is a measure of the anti-knock properties of a fuel. A fuel,
which has high anti-knock characteristics, has a high octane rating.

16

Compression ratio has predominant effects on compression pressure. With the increase of compression
pressure the output power of an engine increases. This is due to the higher combustion pressures, which
are produced. High combustion pressures, however, increase the knock tendency. Fuels with high
antiknock properties are used in higher-compression ratio engines to run engine knock-free while
developing increased power. Lower compression ratios are used in low-emission engines so that they can
run knock-free on low-octane unleaded gasoline.
Combustion chamber design also affects knock tendency. If combustion chambers end gases are in a
squash or quench area, the engine has low knocking tendencies. This happens, as the
end gases are thin and close to a cool metal surface. Cooling the gases reduces and slows the end gas preflame reactions, thereby decreasing the engine knock tendency. This quenching of end gases is the main
reason for a rotating combustion chamber engine to run knock-free on low octane gasoline.
Combustion chamber turbulence, as illustrated in Fig. 8.8, also helps to reduce knocking tendency by
mixing cool and hot gases, thus preventing a concentration of static hot end gases where rapid pre-flame
reactions can take place.

Fig. 8.8. End gases cooled in the quench area.

17

Fig. 8.9. Flame front movement during pre-ignition.

The detonation can be reduced by
(a) decreasing the combustion pressure and temperature,
(b) reducing the time the end gases are subjected to high pressures and temperatures,
(c) the use of fuel with a high octane number,
(d) proper design of combustion chamber where end gases are in a squash or quench area, and
(e) increasing combustion chamber turbulence.
Surface Ignition.

18

Surface ignition or secondary ignition, an abnormal combustion, starts at any source of ignition
other than the spark plug. This is illustrated in Fig. 8.9. As surface ignition produces a secondary ignition
source, its effect is to complete the combustion process sooner than normal, thereby developing maximum
pressure at a wrong time in the engine cycle producing less power.
One potential source of secondary ignition is a hot spot, such as a spark plug electrode, a protruding
gasket, a sharp valve edge, etc. These items can become extremely hot during engine operation form ing a
second source of ignition. These sources rarely occur in modern engine designs provided the engines are
properly maintained. Another source of secondary ignition is combustion chamber deposits, which result
from the type of fuel and oil used in the engine as well as from the type of operation of the engine. A
deposit ignition source may be a hot loose deposit flake capable of igniting one charge before it is
exhausted
from the engine with the spent exhaust gases. This is called wild ping. Sometimes, the flake remains
attached to the combustion chamber wall. Under this situation, it ignites successive charges until the
deposit
is
consumed
or
the
engine
operating
conditions
are
changed.
When surface ignition occurs before firing of the spark plug, it is called pre-ignition. It may be audible or
inaudible. It may be a wild ping or it may be a continuous runaway surface ignition. If it occurs after the
ignition is turned off, it is called run-on or dieseling. Another phenomenon resulting from pre-ignition is
engine rumble. Rumble is a low-frequency vibration of the lower part of the engine that occurs when the
maximum pressure is reached earlier than normal in the cycle. Rumble has been almost eliminated from
modern
engines.
The knock-resistant fuels and antiknock additives generally tend to increase combustion chamber deposits
thereby increasing the tendency to cause surface ignition. Fuel manufacturers therefore, use additional
additives in the gasoline to reduce the deposit ignition tendency resulting from the antiknock additives
deposits. Abnormal combustion seldom occurs in modern mass-produced automotive engines provided the
recommended grade of fuel and motor oil is used and the engine is maintained and adjusted correctly.
Some problems may exist in engines that are used exclusively for low-speed, short-trip driving. Abnormal
combustion frequently occurs in engines modified for maximum performance and also some in emission
controlled engines.

Pre-ignition
Ignition of air fuel mixture by some hot spot which exists within the combustion chamber, before
the occurrence of spark is called pre-ignition.
In a spark ignition engine, the spark that jumps across the terminals of the spark plug initiates
combustion. Similarly if there is any other hot source in the combustion chamber it will heat up the air fuel
mixture surrounding it. Then preflame reaction will certainly be accelerated by this hot spot. The hot spot
may activate. The charge in its immediate vicinity and produce a flame. The flame may then propagate
from this point before the occurrence of spark. Pre-ignition combustion can be seen in fig.

As indicated under surface ignition, carbon deposit from fuel or oil, an over heated spark plug
center electrode or the edge of the gasket that protrudes into the combustion chamber can act as a hot spot
and cause pre-ignition. An overheated exhaust valve head or edge can cause preigniton. Using unsuitable
type spark plug (one that runs too hot or has a long reach) or igniton timing too far retarded or mixture too

19

weak or rich which gives too slow a burning rate may also cause preigniton. The minimum tendency to
preignite exists at fuel air ratios usually richer than the chemically correct. Tetra ethyl lead which is added
to a fuel to increase its antiknock characteristics also reduces the tendency to preignite.

The amount of charge that burns instantaneously due to preigniton depends upon the surface area
of the hot spot and the temperature of the hot spot. When a considerable amount of charge burns, steep
pressure rise and pressure pulsation may occur. A knock, metallic sound will be heard.

20

Different abnormal combustion that may take place in a SI engine
The definitions that follow the spirit of the CRC report 278, SAE special publication are as
follows:Knock – The noise associated with auto-ignition of a portion of the mixture ahead of a flame front
advancing at normal velocity (whether or not surface ignition is present).
Normal combustion - Combustion initiated by a timed spark, with the flame front moving in a uniform
manner at a normal velocity, without auto ignition.
Abnormal combustion – Combustion with surface ignition (phosphorous additives to the gasoline are used
for control of surface ignition and spark plug fouling).
Spark knock – Recurrent knock which can be controlled in intensity (or eliminated) by adjusting the spark
advance.
Surface ignition – Initiation of a flame front by a hot surface other than the spark.
Pre-ignition – Surface ignition occurring before the spark.
Post ignition – Surface ignition occurring after the spark.
Wild ping – Erratic pings or sharp cracks (probably as the result of early surface ignition from deposit
particles)
Rumble – A low pitched thud (probably caused by multiple, early, surface ignition raising the pressure
greatly with consequent deflection of mechanical parts).
Effects of combustion knock
The auto ignitions of the charge, steep pressure rise which sets up pressure wave, vibration of the
gas and increased heat transfer to the cylinder walls, piston and other engine components during knocking
combustion may result in the following:
1. Reduction in power output and efficiency.
2. Burning of piston crown due to increased temperature or due to blow by of very hot gases past the
piston rings from the piston top to the crankcase.
3. The impact of the high pressure wave that is set up might even fracture the piston crown.
4. Burning of cylinder head and valve head.
5. Gumming of piston rings in the piston grooves leading to ring sticking.
6. Loosening of valve seat inserts in the cylinder head.
7. Erosion of piston head may occur at the position of the end mixture. The eroded surface has the
appearance of being blasted and not melted.
Operating conditions causing detonation
The following are some of the operating conditions which may cause detonation in an engine.
1. Slow burning lean air fuel mixture supplied by faulty carburetor or fuel injector, fuel pump,
blocked fuel filter or fuel line, vacuum leak at higher engine speeds caused by bad positive
crankcase ventilation (PCV) valve or exhaust gas recirculation (EGR) valve.
2. Gasoline with low octane or anti clock rating. This is more common with unleaded gasoline.
3. Carbon deposits increasing compression ratio. This is the result of lubricating oil entering the
cylinders or poor detergent action of gasoline.

21

4. Engine operating at above normal temperature due to low coolant level or circulation, water jacket
blockage in the head.
5. Ignition timing very much advanced due to improper setting of initial ignition timing, inaccurate
distribution or advance curve etc.
6. Bad rings and / or valve seals allowing oil (low octane hydrocarbon) to be burned in the cylinders.
7. Air cleaner clogged, which allows too much hot exhaust gas to remain in the engine cylinder.
8. Excessive turbocharger boost pressure from a bad pressure limiting valve.

Ways and means of knock reduction
Investigations indicate that one or more of the following factors will decrease the possibility of knock
in the SI engine.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.

Decreasing the compression ratio or reducing the inlet pressure.
Decreasing inlet air temperature.
Decreasing coolant inlet temperature.
Decreasing temperature of the cylinder and combustion chamber walls or part opening of the
throttle (decreasing the load).
Retarding spark timing.
Decreasing the distance of flame travel in order to complete combustion within a shorter period.
Increasing the turbulence of the mixture and thus increasing the flame speed.
Increasing the engine speed, thus increasing the speed (movement) of the mixture and decreasing
the time available for preflame reactions.
Increasing octane rating of the fuel.
Supplying rich or lean mixtures.
Stratifying the mixtures so that the end gas is less reactive.
Increasing the humidity of the entering air.

Increase in variable

Major effect on
unburned reduce charge

Action to be taken
to Knocking

Compression ratio

Increases temperature
and pressure

reduce

Mass of charge
induced

Increase pressure

Reduce

Inlet temperature

Increases temperature

Reduce

Chamber wall temperature

Increases temperature

Reduce

Spark advance

Increases temperature & pressure

Retard

A / F ratio

Increases temperature & pressure

Make very rich

Turbulence

Decreases time factor

Increases

22

Engine speed

Decreases time factor

Increases

Distance of flame travel

Increases time factor

Reduce

Types of combustion chambers
Combustion chamber shape depends principally upon the valve arrangement, piston head and
combustion chamber contours. Different types of combustion chambers such as T head, L head, F head, L
head turbulent, valve in head, valve in head with inclined valves have been tried and used by different
engine manufacturers. These can be seen in figure.

The T head design stipulates the use of the lowest compression ratios to prevent knocking with a given
fuel. F head design is an improvement over the T head. In this the inlet valves are located in the cylinder
head and the exhaust valves are located in the cylinder or vice versa. This improves the volumetric
efficiency and also reduces the width of the combustion chambers. T head design stipulates two camshafts
one operating the inlet valves and the other operating the exhaust valves. F head and other designs can
have a single camshaft operating all the valves. However, F head design presents difficulties in the design
of the valve operating mechanism.
Overhead valve designs result in higher volumetric efficiency. These may have a single camshaft
located by the side of the cylinder operating the valves through tappets, push rods and rocker arms, or a
single camshaft located in the cylinder head and operating the valves through rocker arms or a single
camshaft located in the cylinder head and operating the valves directly.
In the turbulent combustion chamber, very small clearance is provided between the piston crown
and the cylinder head over a portion of the piston crown surface. This causes squish turbulence in the
mixture, better mixing of fuel and air and improves combustion. Further, this narrow space when made to
contain the end mixture, knocking is avoided because of better cooling. Even if knock occurs its severity
will be lesser. This feature was incorporated in the General Motors Research combustion chamber and this
permitted the use of a 12.5:1 compression ratio with 100 ON fuel. This principle was also incorporated in
the Ricardo turbulent combustion chamber.

23

QUESTION BANK OF UNIT I
Part A
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.

What is steady running
What is Transient operation
What is back firing
Define idling in an engine
What is the effect of inlet and exhaust pressure on mixture requirements
What are the factors that influence carburetion
What are the essential features of a carburetor
What is pre ignition
What are the effects of pre ignition
What are the effects of knock in SI engines
Name some types of combustion chambers in SI engines
Write a short notes on T- head combustion chamber
What are the various additives used to suppress knock in SI engines
Why rich mixture is required for idling
What is stoichiometric air fuel ratio
What are different air fuel mixtures on which an engine can operate
How can the location of the spark plug influence knocking tendency
What is delay period and what are the factors that affect the delay period?
Write any four factors that affect the process of combustion
What is a homogeneous and heterogeneous mixture?
What is meant by Carburetion?
What are the functions of a carburettor?
What is ignition lag
What is period of afterburning in SI engines
What are the variables that affect ignition lag
What is the effect of inlet temp and pressure on ignition lag
What is the effect of fuel - air ratio on flame propagation
What is the effect of compression ratio on flame propagation
Write short notes on effect of turbulence on flame propagation
What are the engine variables that effect Knock in SI engines.
How spark advance effects knock in SI engines
Write any four methods of controlling knock
What are the methods of detecting knock
What are the basic requirements of a good combustion chamber
Write short notes on Side Valve(I-Head) combustion chambers
Write short notes on Over head valve combustion chamber
What are the basic types of carburettor
What are the drawbacks of a simple carburettor

Part B
1.
Discuss the design criteria for a S.I engine combustion chamber
2.
Explain with figures various types of combustion chambers used in SI engines.
3.
Explain the effect of various engine variables on SI engine Knock
4.
Explain the phenomenon of knock in SI engines.what are the factors which influence the
knock.describe the methods used to supress it.
5.
Explain the fuel/air mixture requirements for an engine based on various speeds.

24

6.
With the help of neat sketch explain the working principle of a simple carburettor
7.
Explain the following related to SI engines:
i)Pre ignition
ii)Auto ignition
iii)Knock
8. What is Ignition Lag ?Discuss the effect of engine variables on ignition lag
9. Discuss the effect of the following engine variables on flame propagation :
a) Fuel - air ratio
b) Compression ratio
c) engine load
d) turbulence
e) engine speed
10. Discuss the ill effects of detonation
11. Explain the two theories of detonation
12. Explain the phenomenon of pre-ignition? How pre ignition leads to detonation

UNIT II
COMPRESSION IGNITION ENGINES
Four stages of combustion in a CI engine
Herry Ricardo has investigated the combustion in a compression ignition engine and divided the
same into the following four stages:
1. Ignition delay or delay period.
2. Uncontrolled combustion.
3. Controlled combustion.
4. After burning.

25

Fig: Pressure time diagram illustrating in a compression ignition engine.
1.
2.
3.
4.

Ignition delay
Uncontrolled combustion
Controlled combustion
After burning.

The details of these stages of combustion are given below:
Pressure Vs crank angle of a CI engine in a simplified from is shown in fig. The curved line
ABCG represents compression and expansion of the air charge in the engine cylinder when the engine is
being motored, without fuel injection. This curve is mirror symmetry with respect to TDC line. The curve
ABCDEFH shows the pressure trace of an actual engine.
Delay period
In an actual engine, fuel injection beings at the point B during the compression stroke. The
injected fuel does not ignite immediately. It takes some time to ignite. Ignition sets in at the point C.
During the crank travel B to C pressure in the combustion chamber does not rise above the compression
curve. The period corresponding to the crank angle B to C is called delay period or ignition delay (about
0.001 seconds).
During ignition delay, the following events take place. The injected spray enters the combustion
chamber and slowly (at about 55 m/min) bores hole in the air mass, while the fuel particles are stripped
away. Some of these particles are vapourized. Thus, the main body of the spray is surrounded by vapour
liquid particle air envelope. In small combustion chambers, the spray body may impinge on the walls.
Some of the impinged fuel may bounce off the surface, while the rest may glide on the walls.
Vapourization of fuel particles tends to lower the compression pressure and temperature slightly. At the
same time, the energy released in the pre-flame reactions tends to raise the pressure. Now in the outer

26

envelope of the spray, ignition nuclei are formed. Mostly, the nuclei are cool flame reactions, on the verge
of auto-ignition. By oxidation or cracking reactions, luminescent carbon particles are formed.
Uncontrolled combustion
At the end of the delay period i.e. at the point C, fuel starts burning. At this point, a good amount
of fuel would have already entered and got accumulated inside the combustion chamber. This fuel charge
is surrounded by hot air. The fuel is finely divided and evaporated. Majority of the fuel burns with an
explosion like effect. This instantaneous combustion is called uncontrolled combustion. This combustion
causes a rapid pressure rise.
During uncontrolled combustion the following take place. Flame appears at one or more locations
and spreads turbulently, with glowing luminosity. Flame of low luminosity marks regions of vapourized
fuel and air (premixed flame. Flames of higher luminosity marks regions of liquid droplets and air
(diffusion flame). The initial spreading of non luminous and luminous flame arises from auto-ignition and
flame propagation. This is the knock reaction with a high rate of energy release and correspondingly high
rate of pressure rise.
Combustion during crank travel C to D is called uncontrolled combustion. This is because no
control over this combustion is possible by the engine operator. Since this combustion is more or les
instantaneous, it is also called rapid combustion.
If more fuel is present in the cylinder at the end of delay period, and undergoes rapid combustion
when ignition sets in, the rate of pressure rise and the peak pressure attained will be greater. During this
combustion the piston is around TDC, and is almost stand still. Too rapid a pressure rise and severe
pressure impulse at this position of the piston will result in combustion noise called Diesel Knock.
The severity of the knock reactions is in proportion to the mass enflamed. The regions of
premixed flame are probably hotter (and older) than the regions where liquid droplets are present. As
such, the knock reaction may be propagated mainly in the low luminosity state of the flame.
The rate at which the uncontrolled combustion takes place will depend upon the following:
1. The quantity of fuel in the combustion chamber at the point C. This quantity depends upon the rate
at which fuel is injected during delay period and the duration of ignition delay.
2. The condition of fuel that has got accumulated in the combustion chamber at the point C.
The rate of combustion during the crank travel C to D and the resulting rate of pressure rise
determine the quietness and smoothness of operation of the engine.
Controlled combustion
During controlled combustion, following thing happen. The flame spreads rapidly (but less than
135 m/min), as a turbulent, heterogeneous or diffusion flame with a gradually decreasing rate of energy
release. Even in this stage, small auto-ignition regions may be present. The diffusion flame is
characterized by its high luminosity. Bright, white carbon flame with a peak temperature of 2500 o C is
noticed. In this stage, radiation plays a significant part in engine heat transfer.
During the period D to E, combustion is gradual. Further by controlling the rate of fuel injection,
complete control is possible over the rate of burning. Therefore, the rate o pressure rise is controllable.
Hence, this stage of combustion is called Gradual combustion or Controlled combustion.
The period corresponding to the crank travel D to E is called the period of controlled combustion.
The rate of burning during the period of controlled combustion depends on the following:

27

1.
2.
3.
4.

Rate of fuel injection during the period of controlled combustion.
The fineness of atomization of the injected fuel.
The uniformity of distribution of the injected fuel in the combustion chamber.
Amount and distribution of the oxygen left in the combustion space for reaction of the injected fuel.

At the point E, injection of fuel ends, the period of controlled combustion ends at this point. When
the load on the engine is greater, the period of controlled combustion is also greater.
During controlled combustion, the pressure in the cylinder may increase or remain constant or
decrease. Usually during this period, the combustion is more or less at constant pressure (on a PV
diagram) because the downward movement of the piston (i.e. increase in volume) compensates for the
effect of heat release and the consequent pressure rise.

After burning
At the last stage, i.e. between E and F the fuel that is left in the combustion space when the fuel
injection stops is burnt. This stage of combustion is called After burning (burning on the expansion
stroke). In the indicator diagram, after burning will not be visible. This is because the downward
movement of the piston causes the pressure to drop inspired of the heat that is released by the burning of
the last portion of the charge.
Increasing excess air, or air motion will shorten after burning i.e. reduce the quantity of fuel that
may undergo after burning).

THE PHENOMENON OF KNOCK IN CI ENGINES
In CI engines the injection process takes place over a definite interval of time. Consequently, as
the first few droplets to be injected are passing through the ignition delay period, additional droplets are
being injected into the chamber. If the ignition delay of the fuel being injected is short, the first few
droplets will commence the actual burning phase in a relatively short time after injection and a relatively
small amount of fuel will be accumulated in the chamber when actual burning commences.
Effect of Variables on the Delay Period
Increases in variable
Cetane number of fuel
Injection pressure

Effect on Delay Period
Reduces
Reduces

Reason
Reduces the self-ignition temperature
Reduces physical delay due to greater surface

Injection timing advance

Reduces

volume ration
Reduced pressures and temperatures when the

Compression ration

Reduces

injection begins
Increases air temperature and pressure and

Intake temperature
Jacket water temperature
Fuel temperature

Reduces
Reduces
Reduces

reduces auto-ignition temperature
Increases air temperature
Increases wall and hence air temperature
Increases chemical reaction due to better

pressure Reduces

vaporization
Increases density and also reduces auto-ignition

Intake
(Supercharging)
Speed

Increases in terms of

temperature
Reduces loss of heat

28

crank angle. Reduce in
Load (Fuel air ratio)
Engine size

Type of combustion chamber

terms of milliseconds
Decreases
Decrease in terms of
crank

angle.

effect

in

Increase the operating temperature
Larger engines operator normally at low speeds

Little

terms

of

milliseconds
Lower for engines with Due to compactness of the chamber.
pre-combustion

CHARACTERISTICS TENDING TO REDUCE DETONATION OR KNOCK
S.No
1.
2.
3.
4.
5.
6.
7.
8.

Characteristics
Ignition temperature of fuel
Ignition delay
Compression ratio
Inlet temperature
Inlet pressure
Combustion wall temperature
Speed, rpm
Cylinder size

SI Engines
High
Long
Low
Low
Low
Low
High
Small

CI Engines
Low
Short
High
High
High
High
Low
Large

Factors influencing diesel knock
The diesel combustion process which includes ignition delay, premixed burning due to delay
period and diffusion burning and injector needle lift and pressure variation with respect to crank angle can
be seen in fig. The premixed burning is responsible for diesel knock.
The following are the factors which influence ignition delay and thereby contribute to knock:
The different engine factors that control diesel knock can be seen in fig.

29

Fig: Diesel combustion and injector needle lift
Higher inlet air pressure, air temperature and compression ratio reduce knock. Supercharging
reduces knock. Increased humidity increases knock.
Combustion chamber design and associated air motion influence heat losses from the compressed
air. Tendency to knock will be lesser, with less heat losses. A combustion chamber with a minimum
surface to volume ratio and with lesser intensity of air motion is desirable.
Knocking tendency is lesser in engines where compressed air injects the fuel into the combustion
space. In the case of mechanical injection of fuel, finer the atomization of fuel, lesser is the tendency to
knock.
A fuel with a long preflame reactions (i.e. self ignition possible only at a higher temperature) will
result in the injection of a considerable amount of fuel before the initial part ignities. This in turn results in
a large amount or number of parts of the mixture to ignite at the same time and produce knock. Thus, a
good CI engine fuel should have a short ignition delay and low self ignition temperature, if knock is to be
avoided.

30

Fig: Factors influencing combustion knock in the CI engine.
Ignition delay of fuels is generally measured in terms of cetane number. Fuels of higher cetane
number have shorter ignition delay and thus will have a lesser tendency to knock.
The ignition delay of CI engine fuels may be decreased by the addition of small amounts of certain
compounds (called ignition accelerators or improves). These compounds are ethyl nitrate and amyl
thionitrate. These compounds affect the combustion process by speeding the molecular interactions.
Direct injection engines – These engines have a single, open combustion chamber into which the entire
quantity of fuel is injected directed directly. An open combustion chamber is one in which the combustion
space incorporates no restrictions that are sufficiently small to cause large differences in pressure between
different parts of the chamber during the combustion process.
Indirect injection engines – In these engines the combustion space is divided into two parts and the fuel is
injected into the auxiliary chamber which is connected to the main chamber via a nozzle or one or more
number of orifices. The main chamber is situated above the piston. The restrictions or throat are so small
to cause considerable pressure differences between them during the combustion process.
Combustion chambers for CI engines
Combustion chamber is the space wherein combustion of fuel with air takes place. In IC engines,
combustion chamber is the closed space formed by three engine components, namely, cylinder head, top

31

portion of cylinder and piston crown, when the piston is close to TDC, at the end of compression. This
space is more or less equal to the clearance volume in an engine.
Functions of the combustion chambers are as follows:
1. Efficient preparation of fuel air charge for combustion. This stipulates (i) an even distribution of the
injected fuel throughout the compressed air and (ii) a thorough mixing of the fuel with air to ensure
complete combustion, with minimum excess air supply.
2. Efficient and smooth combustion. This stipulates (i) a sufficiently high air temperature to cause
ignition of fuel, (ii) a small ignition lag or delay period, (iii) a moderate rate of pressure rise during
uncontrolled combustion stage, (iv) a controlled, even burning during controlled combustion stage, (v)
a minimum of after burning and (vi) minimum heat losses and energy losses to ensure high thermal
efficiency.

The open combustion chamber may be located either in the cylinder head or in the piston crown as shown
in fig.

Fig: Types of open combustion chambers, changing the shapes of the cavity in the piston crown
affects piston height and hence engine size.
It may also be partly in the cylinder head and partly in the piston crown. Presence of valves and
fuel injector in the cylinder head makes it difficult to locate the combustion chamber in the cylinder head.

32

Hence, it is usually located in the piston crown, either centrally or so. Locating the chamber in the piston
crown has an advantage i.e. reduced heat losses from the working fluid.
Open combustion chamber is invariably circular in plan. This helps organized rotational air
movement to prevail. The cross section is of different shapes. The chamber shape usually confirms to the
shapes of the fuel spray used.
The shape of some of the open combustion chambers used in automotive diesel engines can be
seen in fig. How the combustion chamber design affects weight and height of an engine can also be seen n
this figure.
The fig. shows the performance results of some direct injection engines having different shapes of
open combustion chambers. The torroidal shape seems to give better performance over the operating
range.

Fig: Performance of D.I. engines having different open combustion chambers
In the open combustion chamber, air mass is more or less quiescent in nature. As such
atomization (i.e. disintegration of the fuel jet into drops of different sizes), distribution of these drops and

33

mixture formation (i.e. mixing of fuel with air) are to be effected by the injection system. Hence, fuel
must be injected at high velocity. This means high injection pressure is required. Injector nozzle should
also contain more of holes of comparatively small diameter (d or = 0.15 to 0.25 mm)
In open combustion chamber even though there may be many sprays, still the air between the
sprays may be utilized fully, due to quiescent nature of the air charge. As such, in this chamber the
minimum possible excess air coefficient is amin > 1.5
The fuel injector is usually arranged along the chamber axis for effective distribution of fuel spray.
With a centrally located multihole injector nozzle, the design goal is to keep the amount of fuel which may
impinge on the piston bowl walls to be a minimum.
In some engines, injector and combustion chamber are located away from the cylinder axis. This
arrangement helps to increase the size of the valves, inlet manifold and exhaust manifold.
In the case of open combustion chambers the injection timing, rate of injection, injection pressure,
engine speed, size of each fuel orifice and viscosity and ignition quality of the fuel dictate the pressure rise
and completeness of combustion.
Pre-combustion chamber
In some CI engines, combustion space is divided into two parts, namely, pre-combustion chamber
and main combustion chamber. Pre-combustion chamber is always located in the cylinder head. Main
combustion chamber is enclosed between the piston and cylinder head. The two combustion chambers are
interconnected by one or more number of orifices.
Pre-combustion chamber is built in various shapes and relative sizes. Pre-combustion chamber
volume is about 30 to 40 percent of the total combustion space. The manner in which combustion of fuel
in this type of engine is taking place is discussed below:
During compression, part of the air in the cylinder enters the pre-combustion chamber. At the end
of compression, the whole of the fuel is injected into the pre-combustion chamber. The hot air ignitives
the fuel. Combustion starts in this chamber. Pressure rises in it. Rise of pressure in the pre-combustion
chamber forces out the products of combustion, partially burned and unburned fuel and remaining air into
the main combustion chamber.
These constituents flow out at high velocity into the main chamber. As such, these constituents
mix thoroughly with the air in the main chamber. The orifices connecting the two chambers are so sized,
and shaped and located to effect good mixing. Air motion thus created is called combustion induced swirl
or combustion turbulence.
In the pre-combustion chamber engine, ignition and combustion starts in the pre-combustion
chamber. But the combustion of the entire quantity of the injected fuel will not be completed in this
chamber itself. This is because only a smaller quantity of total air sucked in is present in this chamber.
About 20 percent of the fuel injected during each cycle, burns in the pre-combustion chamber and the
reminder burns in the main combustion chamber.

34

Merits and demerits of pre-combustion chamber- the merits and demerits of the ecombustion chamber
engines are as follows:
1. There is better mixing of air and fuel due to combustion induced swirl. Air movement is one of
turbulence in character. As such, lower fuel injection pressure (60 to 100 kscm) can be used. Lower
injection pressure eliminates dripping of fuel from the injector tip. Lower injection pressure
necessitates the use of fairly large injection orifices to deliver with carbon particles. Such injectors,
therefore, require less frequent maintenance. Because of larger orifices and lower injection pressures,
higher viscosity fuels can be used.
2. Brake mean effective pressures are much lower in these engines.
3. Only a fraction of the fuel is burnt in the pre-combustion chamber. Thus combustion process
proceeds at a slower rate. As such, rate of pressure rise and peak pressure (seldom exceeds 90 kscm)
will be lower. The engine will be very smooth running. The cycle becomes almost a constant
pressure cycle.
4. During compression , at any instant, pressure and temperature of air in the pre-combustion chamber
will lag behind in magnitude compared to those in the main combustion chamber. Throttling effect
of orifices is responsible for this. As such, at the start of fuel injection, pressure and temperature of
air in the pre-combustion chamber will be lesser. This factor increases ignition delay. Possibility of
knock of knock occurring is greater, especially during cold weather and while starting.

35

5. Heat losses through the orifices are greater during compression. Hence, cold starting is difficult. To
effect easy cold starting, electric heater or starting cartridges or higher compression ratios are used.
Using higher compression ratio (usually from 16 to 19) results in a relatively heavier engine.
6. Air flows from the main chamber into the pre-combustion chamber during compression. During
combustion and expansion, burning gases flow out from the pre-combustion chamber into the main
combustion chamber. These fluid flow through the orifices result in higher fluid friction, and energy
and heat losses. These aspects reduce power output by about 10 to 15 percent and also reduce
thermal efficiency. Specific fuel consumption is more by about 10 to 12percent compared to that of a
open combustion chamber.
7. Pre-combustion chamber imprisons the first combustion shock. This prevents high, knocking
pressure from being applied on the piston and through the connecting rod to the engine knock on the
ening ecomponents, inferior ignition quality fuels can be used.
8. Pre-combustion chambers are suitable and are being used in engines operating at relatively high
speed. This becomes possible because of reduced or elimination of the ill effect of knock.
9. Scavenging the pre-combustion chamber is difficult. This causes inefficient combustion.
10. Considerable amount of fuel that is injected burns after the same entering the main combustion
chamber. This combustion occurs relatively late in the expansion stroke. This aspect reduces
thermal efficiency.
11. Pre-combustion chamber utilizes the energy of initial combustion for creating air movement in the
main chamber. Greater will be the air movement if greater amount of fuel is burnt in the precombustion chamber. The amount of fuel that is burnt initially does not depend upon the speed. As
such this type of combustion chamber is very much suitable for engines meant for constant speed
operation.
Swirl combustion chamber
The swirl combustion chamber is also a divided type combustion chamber with certain differences
and modifications. Swirl chamber is usually located in the cylinder head. In one case, it is located in the
cylinder block itself, by the side of the engine cylinder.
Swirl chamber is spherical or cylindrical in shape. Volume of the swirl chamber is greater than
that of the precombusiton chamber. Volume of the chamber over the piston ranges from a minimum to
usually not more than half the total clearance volume. A much larger passage called transfer passage or
throat connects the swirl chamber with the chamber in the cylinder. This connection passage is tangential
to the swirl chamber. The figure shows the location of the swirl combustion chamber either in the cylinder
head or in the cylinder block and the air motion created in them.

36

Fig: Different arrangements of swirl combustion chambers

37

During compression, air from the cylinder is forced through the throat into the swirl chamber. A
tangential velocity of swirl is produced in the swirl chamber. This swirl is called compression swirl.
At the end of compression, fuel is injected into the swirl chamber. Vigorous swirl in the chamber
helps the injected fuel and air to mix well. Fuel injector is so located and fuel sprays are so aimed to
achieve this goal.
Ignition and combustion of fuel starts in the swirl chamber. Bulk of the injected fuel burns in the
swirl chamber itself. This becomes possible because of the presence of the major portion of air in it.
Combustion causes pressure rise. This pressure rise forces combustion products and air fuel mixture into
the engine cylinder. Piston is also pushed outward on the working stroke. Further mixing of unburned and
partially burnt fuel with air occurs and this results in efficient combustion. Hence, a swirl chamber engine
uses both compression induced swirl and combustion induced swirl.
‘M’ Combustion system
Dr Meurer of MAN, Germany has developed a simple but a peculiar diesel combustion chamber
based upon the following three rules:
1. The fuel must be allowed to oxidize slowly and gradually and must be heated only as vapour in the
mixed state.
2. The fuel quantity undergoing auto-ignition must be minimized
3. The mixture of fuel vapour and air must be done faster as combustion proceeds and the mixture must
never be richer than the stoichiometric ratio.
The M combustions chamber is located in the piston crown as shown in fig. It is open type and is
somewhat shallower. It has a recess at the top just below the injector nozzle. The nozzle directs the fuel
towards the combustion chamber walls, tangentially. The intake port is inclined. The intake valve has a
mask. These create an air swirl about the axis of the cylinder. The direction of the swirl is in the same
direction as that of the fuel jet.
The fuel particles injected at the first instance meet high resistance due to the dense hot air in the
chamber. Hence, these particles get well dispersed into the hot air. The succeeding particles due to lesser
air resistance get deposited on the combustion chamber walls, in the form of a thin film. At full load, the
thickness of the fuel film be about 0.150 mm. The fuel dispersed into the air mass is only about 5% of the
total fuel injected.
The bottom surface of the combustion chamber is cooled by the lubricating oil that is splashed
continuously from the crankcase. The combustion chamber wall temperature is maintained at about 330 oC.
The combustion of the 5% of the fuel which gets injected into air mass starts. It undergoes, usual
droplet combustion. But the combustion of the fuel sprayed on the cooled combustion chamber walls does
not follow immediately. The wall deposited fuel starts evaporating in the absence of the hot air and moves
towards the center. The swirling air removes the fuel vapours from the zone evaporation.

38

Fig: M Combustion chamber

The fuel vapours mix with air and after slow oxidation burns. The combustion of the air fuel
vapour mixture is initiated by the red hot carbon particles produced by combustion of air deposited fuel
(that act like spark produced in a spark ignition engine). As combustion proceeds, the chamber
temperature increases. This in turn increase the rate of vapourization and mixture formation. By this
controlled evaporation and slow combustion, the fuel has little or no chance to crack resulting in diesel
knock and smoky exhause. Hence, combustion is smooth and efficient in this system. A comparison of
the indicator diagrams of the conventional diesel engine and the M combustion chamber diesel can also be
seen in fig which will reveal this fact. Rate of pressure rise and peak pressure are lesser in the M
combustion chamber engine.

Merits and demerits of M combustion system: The advantages of the M combustion system are as
follows:
In the M combustion system, complete and effective burning of the fuel takes place. This controlled
burning eliminates diesel knock and free carbon particles in the exhaust.
1. About 5 – 10 % higher power output is realized.
2. Specific fuel consumption is lesser.

39

3. Smooth running of the engine even during idling becomes possible which is very rare in normal
diesel engines.
4. Much lower smoke density upto three fourths full load and almost identical with that of a
conventional diesel engine at full load. Smoke density is the ratio of carbon present in the exhaust to
the amount of carbon in the quantity of fuel injected.
5. Lesser contamination of insoluble in the lubricating oil. In the bohr test, in an ordinary diesel engine,
the insolubles were about 0.9% and in the M combustion engine, the insolubles where only about
0.25%.
6. M combustion system is more adaptable for multi fuel operation because of the elimination of diesel
knock.

Turbo charging
In turbo charging, the supercharger or blower is being driven by a gas turbine which uses the
energy in the exhaust gases. In this case, there is no mechanical linkage between the engine and the
supercharger. The major parts of a turbocharger are turbine wheel, turbine housing, turbo shaft,
compressor wheel, compressor housing and bearing housing.

40

During engine operation, hot exhaust gases blow out through the exhaust valve opening into the
exhaust manifold. The exhaust manifold and the connecting tubing route these gases into the turbine
housing. As the gases pass through the turbine housing, they strike on the fins or blades on the turbine
wheel. When the engine load is high enough, there is enough gas flow and this makes the turbine wheel to
spin rapidly. The turbine wheel is connected to the compressor wheel by the turboshaft. As such, the
compressor wheel rotates with the turbine. Compressor wheel rotation sucks air into the compressor
housing. Centrifugal force throws the air outward. This causes the air to flow out of the turbocharger and
into the engine cylinder under pressure.
In the case of turbocharging, there is a phenomena called turbolag. It refers to the short delay
period before the boost or manifold pressure increases. This is due to the time the turbocharger assembly
takes the exhaust gases to accelerate the turbine and compressor wheel to speed up.

41

If the supercharger is driven directly by the engine, part of the power developed by the engine will be used
in running the supercharger.

Fig: Comparative heat balance of naturally aspirated and supercharged diesel engines.
If is found that the gain in the power output of an engine due to supercharging will be many time
the power required to drive the supercharger. Of course, this is possible only with increased fuel supply to
the engine. It is to be noted that at full loads, the compression of the supercharger is not fully utilized.
This will result in greater loss. Therefore, the specific fuel consumption of a mechanically driven
supercharged engine will be more at part loads when compared to that of a naturally aspirated engine.
In the case of the exhaust gas turbine driven supercharger, the engine is not required to supply any
power to run the supercharger turbine. This type of supercharging is called turbo charging. The turbo
charging gives about 5% higher thermal efficiency at full load. This increase in efficiency results in
reduced fuel consumption compared to that of a naturally aspirated engine for the same power output.
Effects of turbocharging:
The following are the effects of supercharging engines. Some of the points refer to CI engines:
1. Higher power output
2. Mass of charge inducted is greater
3. Better atomization of fuel
4. Better mixing of fuel and air
5. Combustion is more complete and smoother
6. Can use inferior (poor ignition quality) fuels.
7. Scavenging of products is better
8. Improved torque over the whole speed range
9. Quicker acceleration (of vehicle) is possible
10. Reduction in diesel knock tendency and smoother operation
11. Increased detonation tendency in SI engines
12. Improved cold starting
13. Eliminates exhaust smoke

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14.
15.
16.
17.
18.
19.
20.
21.

Lowers specific fuel consumption, in turbocharging
Increased mechanical efficiency
Extent of supercharging is limited by durability, reliability and fuel economy
Increased thermal stresses
Increased turbulence may increase heat losses
Increased gas loading
Valve overlap period has to be increased to about 60 to 160 degrees of crank angle
Necessitates better cooling of pistons and valves.

QUESTION BANK OF UNIT II
Part A
1. What are the stages of combustion in CI engines.
2. What is Ignition delay period
3. What is period of rapid combustion
4. What is contrilled combustion in CI engines.
5. What is period of after burning.
6. What are the factors that affect delay period.
7. What is knock in CI engines.
8. State different types of combustion chambers in CI engines.
9.Write a short notes on Direct injection combustion chambers.
10. What is Pre - Combustion chamber.
11.What are homogeneous and heterogeneous mixtures.
12. What is turbo charging.
13. What are the advantages of turbo charging.
14. What are the dis-advantages of turbo charging.
15. What is ignition delay period in CI engines
16. What is Uncontrolled combustion in CI engines
17. What is controlled combustion in CI engines
18. What is period of afterburning in CI engines
19. What variables affect delay period
20. What is the affect of size of droplet on delay period
21. What is the affect of compression ratio on delay period
22. List various methods to control delay period in CI engine
23. What are the methods of generating air swirl in CI engine
24. What are the advantages of induction swirl
25. What are various cold starting aids in CI engine.
Part B
1. Explain the various stages of Combustion in CI engines.
2. Explain the phenomenon of Knock in CI engines.
3. Explain various types of Combustion chambers used in CI engines with figures.
4. What is meant by delay period. Explain about the types of delay period
5. What are the three methods of generating swirl in CI engine combustion chamber

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UNIT III
ENGINE EXHAUST EMISSION CONTROL
POLLUTION:
The mixing of unwanted and undesirable substances into our surroundings that cause undesirable
effects on both living and non living things is known as pollution.
AIR POLLUTION:
Air pollution is defined as the addition of unwanted and undesirable things to our atmosphere that
have harmful effect upon our planned life.
Major sources of Air pollution:
1. Automotive Engines
2. Electrical power generating stations
3. Industrial and domestic fuel consumption
4. Refuse burning of industrial processing, wastes etc.,
Sources of Pollutants from Gasoline Engine:
There are four possible sources of atmospheric pollution from a petrol engine powered vehicle.
They are
1. Fuel Tank
2. Carburettor
3. Crank case
4. Engine
The amount of pollutants contributed by the above mentioned sources are as follows.
a.. Fuel tank evaporative loss

5 to 10 % of HC

b. Carburettor evaporative loss 5 % of HC
c. Crank case blow by

20 to 35 % of HC

d. Tail Pipe exhaust

50 to 60 % of HC and
almost all Co and NOx

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Emittant as a Pollutant:
An emittant is said to be a pollutant when it has some harmful effect upon our surroundings.
The primary source of energy for our automotive vehicles is crude oil from underground which
typically contains varying amounts of sulphur. Much of the sulphur is removed during refining of
automotive fuels. Thus the final fuel is hydrocarbon with only a small amount of sulphur. If we neglect
sulphur and consider complete combustion, only water and carbon dioxide would appear in the exhaust.
Water is not generally considered undesirable and therefore it is not considered as a pollutant.
Likewise carbon dioxide is also not considered as pollutant in earlier days. But due to increase in global
warming due to CO2 which is a green house gas, now a days CO2 is also considered as unwanted one.

45

Then apart from this we get sulphur dioxide a pollutant which is a product of complete
combustion. Apart from this all the compounds currently considered as pollutants are the result of
imperfect or incomplete combustion.
Pollutants

Pollutant Effects

Unburned Hydro Carbons (UBHC)

Photochemical Smog

Nitric Oxide

Toxic , Photochemical Smog

Carbon monoxide

Toxic

Lead compounds

Toxic

Smoke combines with fog and forms a dense invisible layer in the atmosphere which is known as
Smog. The effect of Smog is that it reduces visibility.
Effect of Pollutants on Environment:
a. Unburned Hydro Carbons ( UBHC ):
The major sources of UBHC in an automobile are the engine exhaust, evaporative losses from fuel
system, blow by loss and scavenging in case of 2-stroke petrol engines.
Unburned or partially burned hydrocarbons in gaseous form combine with oxides of nitrogen in
the presence of sunlight to form photochemical smog.
UBHC + NOx

 Photochemical smog

The products of photochemical smog cause watering and burning of the eyes and affect the
respiratory system, especially when the respiratory system is marginal for other reasons.
Some of the high molecular weight aromatic hydrocarbons have been shown to be carcinogenic in
animals. Some of the unburned hydrocarbons also serve as particulate matter in atmosphere.
b. Carbon monoxide:
Carbon monoxide is formed during combustion in engine only when there is insufficient supply of
air. The main source is the engine exhaust.
The toxicity of carbon monoxide is well known. The hemoglobin the human blood which carries
oxygen to various parts of the body has great affinity towards carbon monoxide than for oxygen. When a
human is exposed to an atmosphere containing carbon monoxide, the oxygen carrying capacity of the
blood is reduced and results in the formation of carboxy hemoglobin. Due to this the human is subjected to
various ill effects and ultimately leads to death.
The toxic effects of carbon monoxide are dependent both on time and concentration as shown in
the diagram.

46

c. Oxides of Nitrogen ( NOx ) :
Oxides of nitrogen ( NO, NO 2 , N2O2 etc) are formed at higher combustion temperature present in
engines and the engine exhaust is the major source.
Like carbon monoxide, oxides of nitrogen also tend to settle on the hemoglobin in blood. Their
most undesirable effect is their tendency to join with moisture in the lungs to form dilute nitric acid.
Because the amounts formed are minute and dilute, their effect is very small but over a long period of time
cam be cumulatively undesirable, especially when the respiratory problems for other reasons are found.
Another effect is that, the oxides of nitrogen are also one of the essential component for the
formation of photochemical smog.
d. Sulphur dioxide:
Sulphur dioxide from automotive vehicle is very less when compared to that emitted by burning
coal. Sulphur dioxide combines with moisture in atmosphere and forms sulphuric acid at higher
temperatures. This comes to the earth as acid rain.
Much of the sulphur dioxide combines with other materials in the atmosphere and forms sulphates
which ultimately form particulate matter.
e. Particulates:

47

Particulate matter comes from hydrocarbons, lead additives and sulphur dioxide. If lead is used
with the fuel to control combustion almost 70% of the lead is airborne with the exhaust gasses. In that 30%
of the particulates rapidly settle to the ground while remaining remains in the atmosphere. Lead is well
known toxic compound

.Particulates when inhaled or taken along with food leads to respiratory problems and other infections.
Particulates when settle on the ground they spoil the nature of the object on which they are
settling. Lead, a particulate is a slow poison and ultimately leads to death.
CHEMISTRY OF SI ENGINE COMBUSTION:
In a Spark ignition engine a perfectly mixed air fuel mixture enters the engine during suction
stroke. The charge is compressed well and at the end of end of compression stroke, the charge is ignited by
means of spark from spark plug. The air fuel mixture is delivered to engine by means of carburettor.
The quantity and quality of charge entering the engine is controlled according to the engine speed
and load conditions.
GASOLINE ENGINE EMISSIONS
The emissions form gasoline powered automobiles are mainly

48

1. Unburned Hydro Carbons
2. Carbon monoxide
3. Oxides of nitrogen
4. Oxides of sulphur and
5. Particulates including smoke
Pollutant formation in Gasoline engine:
1. Hydrocarbons:
Hydrocarbon exhaust emission may arise from three sources as
a. Wall quenching
b. Incomplete combustion of charge
c. Exhaust scavenging in 2-stroke engines
In an automotive type 4-stroke cycle engine, wall quenching is the predominant source of exhaust
hydrocarbon under most operating conditions.
a. Wall quenching:
The quenching of flame near the combustion chamber walls is known as wall quenching. This is a
combustion phenomenon which arises when the flame tries to propagate in the vicinity of a wall. Normally
the effect of the wall is a slowing down or stopping of the reaction.
Because of the cooling, there is a cold zone next to the cooled combustion chamber walls. This
region is called the quench zone. Because of the low temperature, the fuel-air mixture fails to burn and
remains unburned.
Due to this, the exhaust gas shows a marked variation in HC emission. The first gas that exits is
from near the valve and is relatively cool. Due to this it is rich in HC. The next part of gas that comes is
from the hot combustion chamber and hence a low HC concentration. The last part of the gas that exits is
scrapped off the cool cylinder wall and is relatively cool. Therefore it is also rich in HC emission.
b. Incomplete combustion:
Under operating conditions, where mixtures are extremely rich or lean, or exhaust gas dilution is
excessive, incomplete flame propagation occurs during combustion and results in incomplete combustion
of the charge.
Normally, the carburettor supplies air fuel mixture in the combustible range. Thus incomplete
combustion usually results from high exhaust gas dilution arising from high vacuum operation such as idle
or deceleration.

49

However during transient operation, especially during warm up and deceleration it is possible that
some times too rich or too lean mixture enters the combustion chamber resulting in very high HC
emission.
Factors which promote incomplete flame propagation and misfire include:
a. Poor condition of the ignition system, including spark plug
b. Low charge temperature
c. Poor charge homogeneity
d. Too rich or lean mixture in the cylinder
e. Large exhaust residual quantity
f. Poor distribution of residuals with cylinder
Carburetion and mixture preparation, evaporation and mixing in the intake manifold, atomization
at the intake valve and swirl and turbulence in the combustion chamber are some factors which influence
gaseous mixture ration and degree of charge homogeneity including residual mixing.
The engine and intake system temperature resulting from prior operation of the engine affect
charge temperature and can also affect fuel distribution.

Valve overlap, engine speed, spark timing, compression ratio, intake and exhaust system back
pressure affect the amount and composition of exhaust residual. Fuel volatility of the fuel is also one of the
main reasons.

c. Scavenging:
In 2-stroke engine a third source of HC emission results from scavenging of the cylinder with fuel
air mixture. Due to scavenging part of the air fuel mixture blows through the cylinder directly into exhaust
port and escapes combustion process completely. HC emission from a 2-Stroke petrol engine is
comparatively higher than 4-Stroke petrol engine.

2. Carbon monoxide:
Carbon monoxide remains in the exhaust if the oxidation of CO to CO 2 is not complete. This is
because carbon monoxide is an intermediate product in the combustion process. Generally this is due to
lack of sufficient oxygen. The emission levels of CO from gasoline engine are highly dependent on A/F
ratio.
The amount of CO released reduces as the mixture is made leaner. The reason that the CO
concentration does not drop to zero when the mixture is chemically correct and leaner arises from a
combination of cycle to cycle and cylinder to cylinder mal distribution and slow CO reaction kinetics.

50

Better carburetion and fuel distribution are key to low CO emission in addition to operating the engine at
increased air-fuel ratio.
3. Oxides of Nitrogen:
Nitric oxide is formed within the combustion chamber at the peak combustion temperature and
persists during expansion and exhaust in non-equilibrium amount. Upon exposure to additional oxygen in
the atmosphere, nitrogen dioxide ( NO2) and other oxides may be formed.
It should be noted that although many oxides of nitrogen may be also formed in low
concentrations like, Nitrogen trioxide (N 2O3 ), Nitrogen pent oxide (N 2O5 ) etc., they are unstable
compounds and may decompose spontaneously at ambient condition to nitrogen dioxide.
A study of the equilibrium formation of the different nitrogen oxides showed that No is the only
compound having appreciable importance with respect to engine combustion. In engine terminology an
unknown mixture or nitrogen oxides usually NO and NO 2 is known as NOx. It is expected that higher
temperature and availability of oxygen would promote the formation of oxides of nitrogen.
Mechanism of NO formation:
The nitric oxide formation during the combustion process is the result of group of elementary
reaction involving the nitrogen and oxygen molecules. Different mechanism proposed are discussed below.
a. Simple reaction between N2 and O2
N2 + O2  2 NO
This mechanism proposed by Eyzat and Guibet predicts NO concentrations much lower that those
measured in I.C engines. According to this mechanism, the formation process is too slow for NO to reach
equilibrium at peak temperatures and pressures in the cylinders.

b. Zeldovich Chai Reaction mechanism:
O2  2 O

------------- ( 1)

O + N2  NO + N ------( 2 )
N + O2  NO + O ------( 3 )
The chain reactions are initiated by the equation ( 2 ) by the atomic oxygen, formed in equation ( 1
) from the dissociation of oxygen molecules at the high temperatures reached in the combustion process.
Oxygen atoms react with nitrogen molecules and produces NO and nitrogen atoms. In the equation ( 3 )
the nitrogen atoms react with oxygen molecule to form nitric oxide and atomic oxygen.
According to this mechanism nitrogen atoms do not start the chain reaction because their
equilibrium concentration during the combustion process is relatively low compared to that of atomic
oxygen. Experiments have shown that equilibrium concentrations of both oxygen atoms and nitric oxide
molecules increase with temperature and with leaning of mixtures. It has also been observed that NO

51

formed at the maximum cycle temperature does not decompose even during the expansion stroke when the
gas temperature decreases.
In general it can be expected that higher temperature would promote the formation of NO by
speeding the formation reactions. Ample O 2 supplies would also increase the formation of NO. The NO
levels would be low in fuel rich operations, i.e. A/F 15, since there is little O 2 left to react with N2 after the
hydrocarbons had reacted.
The maximum NO levels are formed with AFR about 10 percent above stoichiometric. More air
than this reduces the peak temperature, since excess air must be heated from energy released during
combustion and the NO concentration fall off even with additional oxygen.
Measurements taken on NO concentrations at the exhaust valve indicate that the concentration
rises to a peak and then fall as the combustion gases exhaust from the cylinder. This is consistent with the
idea that NO is formed in the bulk gases. The first gas exhausted is that near the exhaust valve followed by
the bulk gases. The last gases out should be those from near the cylinder wall and should exhibit lower
temperatures and lower NO concentration.
4. Particulate matter and Partial Oxidation Products:
Organic and inorganic compounds of higher molecular weights and lead compounds resulting
from the use of TEL are exhausted in the form of very small size particles of the order of 0.02 to 0.06
microns. About 75% of the lead burned in the engine is exhausted into the atmosphere in this form and rest
is deposited on engine parts.
Some traces of products of partial oxidation are also present in the exhaust gas of which
formaldehyde and acetaldehyde are important. Other constituents are phenolic acids, ketones, ethers etc.,
These are essentially products of incomplete combustion of the fuel.

52

53

Flame Quenching:
The phenomenon of flame quenching at the engine walls and the resulting unburned layer of
combustible mixture play a significant role in the overall problem of air pollution.
It has long been understood that a flame will not propagate through a narrow passage. It has been
found that the walls comprising the narrow passage quench the flame by acting as a sink for energy. The
minimum distance between two plates through which a flame will propagate is defined as the
quenching distance. The quenching distance is found to be a function of pressure, temperature and
reactant composition.
When a flame is quenched by a single wall as would be the case in the combustion chamber of a
S.I engine, the distance of the closest approach of the flame to the wall is smaller than the quenching
distance. This distance is called the dead space. In general, the dead space has been assumed to range from
0.33 to1.0 of the quenching distance.
Friedman and Johnson, Green and Agnes, Gottenbery and others have made significant work on
this area. The following points are drawn from their experiments.
1. Essentially the expression for quenching distance is of the form
1
qd = ------Pα Tβ
Where the values of α and β depends on the stoichiometry of the combustible mixture.
2. Lean mixtures have significantly large quenching distance than stoichiometry or rich mixture at
any given pressure.
3. There exist a direct linear relationship between the total exhausted hydrocarbon and surface to
volume ratio, a direct linear relationship between the representatives measured quench distance and the
quantity of unburned hydrocarbons in the combustion products.
4. The quenching distance of copper, mica, glass and platinum surfaces were the same and hence
they concluded that the quenching effect was independent of the surface material.
5. As the temperature of the wall increases, the flame can propagate closer to it. If high
temperature materials could be used to make the cylinder walls in an engine capable of withstanding 800
°C to 1200 °C temperature, the quench layer thickness can be reduced to bring down the concentration of
hydrocarbons.
Danial proposed that the unburned hydrocarbons that are exhausted during the cruise and
acceleration modes are due to the quenching of flames by the walls of the combustion chamber piston.
He measured the thickness of the dark zone between the flame and the combustion chamber wall
in a single cylinder engine that was fitted with a single quartz head. The dark zone or dead space was
measured by taking stroboscopic picture of successive cycle through the quartz cylinder head, and he
showed that the quantity of fuel trapped in the dead space was sufficient to account for the unburned

54

hydrocarbons emitted from the engine. He also reported that the thickness of the dark zone was a function
of temperature and pressure as referred by Friedman and Johnson.
Tabaczynski proposed that there are four separate quench regions in the cylinder of a S.I engine.
As shown in fig 3.1, these four quench layers may be expected to be exhausted from the cylinder at
different times during the exhaust stroke. Regions 1 and 2 shown in the figure are the head and side wall
quench layers respectively. Region 3 represents the piston face quench layer and region 4 corresponds to
the quench volume between the cylinder wall, piston crown and first compression ring.
It has been proposed that the head quench layer and part of the side wall quench layer nearest the
exhaust valve leave the cylinder when the exhaust valve opens. Due to the low flow velocities near the
piston face, the piston face quench layer will probably not leave the cylinder at any time during the stroke.
During the expansion stroke, the hydrocarbons from the crevice between the piston crown and the first
compression ring are laid along the cylinder wall.
As the piston begins its upward stroke, it has been shown that a vortex is formed which scraps up
the hydrocarbons along the wall and forces them to be exhausted near the end of the exhaust stroke.

55

56

EFFECT OF DESIGN AND OPERATING VARIABLES ON GASOLINE ENGINE EXHAUST
EMISSIONS
The exhaust emission of hydrocarbons, carbon monoxide and nitric oxide can be minimized by the
control of several inter related engine design and operating parameter. Fuel preparation, distribution and
composition are also factors. In this section the effects on emissions of factors which the engineer has
under his control when designing and tailoring his engine for minimum exhaust emissions are discussed.
The factors include:
•

Air fuel ratio

•

Load or power level

•

Speed

•

Spark timing

•

Exhaust back pressure

•

Valve overlap

•

Intake manifold pressure

•

Combustion chamber deposit build up

•

Surface temperature

•

Surface to volume ratio

•

Combustion chamber design

•

Displacement per cylinder

•

Compression ratio

•

Stroke to bore ratio

In the following discussions, the hydrocarbons and CO emissions are treated together; because
once they are formed they both can be reduced by chemical oxidation process in either the cylinder or the
exhaust system. On the other hand nitric oxide, once formed must be reduced by a chemical reduction
process.
In the first case for HC and CO reduction, excess O 2 is required where as in the second case for
NO reduction a deficiency of O2 is desirable.
EFFECTS ON UNBURNED HYDROCARBONS AND CARBON MONOXIDE
1. AIR – FUEL RATIO:
a. Hydrocarbon emission:

57

Hydrocarbon emissions are high at rich air fuel ratios and decrease as the mixture is leaned up to
about 17:1. When operation leaner than 17 or 18:1 is attempted, emissions increases because of incomplete
flame propagation and the engine begin to misfire.
The basic factor contributing to the shape of the curve for HC emissions are the effect of mixture
ratio on quench layer thickness and on fuel concentration within that quench layer, and the effect of
mixture ratio on the availability of excess oxygen in the exhaust to complete the combustion and on the
exhaust system temperature. When the temperature is over 650 °C and with oxygen available appreciable
exhaust after reaction does occur.
b. CO emission:
CO emissions are high at rich air fuel ratios and decreases as the mixture is leaned. On the richer
side, a change of only 1/3 air fuel ratio leads to a change of 1.0% in exhaust CO. The reason that the CO
concentration does not drop to zero when the mixture is chemically correct and leaner arises from a
combination of cycle to cycle and cylinder to cylinder mal distribution and slow CO kinetics.
2. POWER OUTPUT:
a. Hydrocarbon emission:
Hydrocarbon concentration does not change as load is increased while speed and mixture ratio are
held constant and spark is adjusted to MBT. This result is to be viewed as arising from effects of several
factors some of which tend to reduce HC while others tend to increase them, apparently counter balancing
one another.

58

A factor which increases the HC formation as load increases is the reduced time within the exhaust
system. The residence time of the exhaust gas in the very hot section of the exhaust system is very
important for increased exhaust after-reaction.

59

Factors tending to reduce HC concentration include decreased quench thickness and increased
exhaust temperature. Quench layer thickness decreases inversely as pressure increases and the mean
cylinder pressure increases linearly with increase in load. Increased temperature with increasing load tends
to increase exhaust after-reaction.
However, an almost linear increase in HC mass emissions is observed as load is increased. A light
car with low power is better than a large car on mass emission basis.
b. CO emission:
At a fixed air-fuel ratio there is no effect of power output on CO emission concentration. However,
as in the case of HC emissions, CO emission on mass basis will increase directly with increasing output,
giving advantage for a small light and efficient car.
3. ENGINE SPEED:
a. Hydrocarbon emission:
HC emission is considerably reduced at higher engine speeds. This is because with increase in
engine speed, the combustion process within the cylinder is increased by increasing turbulent mixing and
eddy diffusion. In addition, increased exhaust port turbulences at higher speeds promotes exhaust system
oxidation reactions through better mixing.

60

61

b. CO emission:
Speed has no effect on CO concentration. This is because oxidation of CO in the exhaust is
kinetically limited rather than mixing limited at normal exhaust temperatures.
4. SPARK TIMING:
a. HC emission:
HC emission has huge impact on spark timing. As the timing is retarded, the HC emissions are
reduced. This is because, the exhaust gas temperature increases which promotes CO and HC oxidation.
This advantage is gained by compromising the fuel economy.

62

b. CO emission:
Spark timing has very little effect on CO concentration. But at very high retarded timing, the CO
emission increases. This is due to lack of time, to complete oxidation of CO.
5. EXHAUST BACK PRESSURE:
a. HC emission:
Increasing exhaust back pressure increases the amount of residual exhaust gas left in the cylinder
at the end of the exhaust. If this increase in dilution does not affect the combustion process adversely, the

63

HC emissions would be marginally reduced. The reduction arises from leaving the tail end of the exhaust,
which is rich in HC, in the cylinder. This tail will be subsequently burned in the next cycle. If the back
pressure is increased more and more, HC emission would rise sharply because of the effect of excessive
dilution on combustion.
On the other hand, increased dilution at idle increases HC emission concentration. At idle, dilution
is already quirt high and combustion is marginal and the engine cannot tolerate much more exhaust
dilution.
6. VALVE OVERLAP:
a. HC emission:
Increasing valve overlap has an effect similar to increasing the back pressure. The charge is further
diluted with residual gases. A slight 2 overlap provided minimizes emission due to re burning of exhaust
tail gas which is rich in HC.
Combustion deteriorates with lean mixture as residual is increased. If the mixture ratio is richened
to provide stable idle and off-idle performance, then HC advantage will be lost and CO will be increased.
In general, minimum HC emissions are obtained with moderate or low back pressure with
minimum overlap.
b. CO emission:
There is no effect of overlap on CO concentration at a constant mixture ratio. However any
increase in richness of the mixture for smooth idle or off idle will increase the CO directly. This is due to
lack of insufficient supply of oxygen for complete oxidation of CO.

64

65

7. INTAKE MANIFOLD PRESSURE:
a. HC emission:
The intake manifold pressure variation reflects the variation in power output of an engine.
Between 22cm and 60cm of Hg manifold pressure, the A/F ratio is lean which minimizes HC and CO
emissions. Above 60cm of Hg, the engine power increases and the carburettor switch to rich mode. The
rich mixture increases HC and CO emissions. This holds good only in case of carbureted engine. At light
loads and low manifold pressure, additional HC emissions results from wall quenching accompanying rich
mixtures delivered from the carburetor and incomplete combustion at manifold pressures below 15cm of
Hg.
8. COMBUSTION CHAMBER DEPOSITS:
a. HC emission:
It is well known that in a normal engine the major source of combustion chamber deposit is TEL, a
fuel additive used to suppress combustion knock. The deposits act to increase the surface area of the
chamber because of their irregular porous nature. As a result, the mass of quenched HC increases. Deposits
act as a sponge to trap raw fuel which remains unburned and adds to exhaust. All these tend to increase the
HC emission.
Tests have indicated that removal of deposits, depending on the extent of deposit build up, would
reduce about 15% in HC emissions. Addition of fuel additives to reduce deposit build up may be helpful.
Ethylene dibromide is commonly added to motor fuel to reduce lead deposits from TEL. Any modification
to both fuels and lubricants can indirectly reduce HC emissions through deposit modification.
b. CO emission:
There is no effect of deposit build up on CO emission.
9. SURFACE TEMPERATURE:
a. HC emission:
Combustion chamber surface temperature affects the unburned HC emissions by changing the
thickness of combustion chamber quench layer and degree of after burning. Higher the combustion
chamber surface temperature, the lower are the HC emissions.
In addition to changing quench distance and after-reaction, changing engine temperature increases
fuel evaporation and distribution, and result in a faster reaction and hence reduced HC emission.
b. CO emission:
An increase in surface temperature of chamber increases the rate of oxidation of CO and hence
reduces CO emission. Further exhaust after reaction also increases resulting in decrease in CO emission.
10. SURFACE TO VOLUME RATIO:
a. HC emission:

66

Because hydrocarbon emissions arise primarily from quenching at the combustion chamber wall
surface, it is desirable to minimize the surface area of the chamber. The ratio of surface area to volume of
the combustion chamber (S / V) is useful for interpreting the effects of many designs and operating
variables on HC concentration. Lowering the S / V ratio reduces HC emission concentration.

67

68

b. CO emission:
CO concentration has no effect on surface to volume ratio.

69

11. COMBUSTION CHAMBER DESIGN:
One of the most important factors that the emission engineer has under his control is the
combustion chamber design. For a given clearance volume, reducing the surface area is an important way
of reducing HC emission. Designing a combustion chamber to create better turbulence will reduce both
HC and CO emission.
12. STROKE / BORE RATIO:
Another design factor is stroke to bore ratio. Engines with small bore and long stroke have lower S
/ V ratio. Engines with low surface to volume ratio provide a good emission reduction compared to the
engine with higher surface to volume ratio.
Displacement per cyl.
Bore
Stroke
Stroke/bore
s/v

41
4
3.25
.813
8

41
3.62
4
1.105
6.1

The engine with s/v 6.1 should provde good emission result.Unfortunately this requirement is
opposed to modern design practice of short stroke for reduced friction, increased power and economy.
Long stroke engines tend to be large, heavy and more expensive and they have poor fuel economy and
reduced peak power.
13. DISPLACEMENT PER CYLINDER:
For a given displacement, engines with larger cylinders have smaller surface to volume ratio. This
result suggests that for an engine of given displacement, hydrocarbon emissions can be reduced by
decreasing the number of cylinders and increasing the displacement per cylinder. On the other hand, for a
given number of cylinders, increasing the engine displacement reduces s/v ratio and reduces HC.
Displacement per cyl.
Bore
Stroke
Stroke/bore
s/v

41
4
3.25
.813
8

30.2
3.62
2.94
.813
9

14. COMPRESSION RATIO:
A decrease in compression ratio decreases surface to volume ratio. Decrease in compression ratio
increases the clearance volume greatly with little increase in surface area. Due to this decrease in surface to
volume ratio the HC emission is reduced.
A decrease in compression ratio decreases the HC emission on a second way also. With reduced
compression ratio, thermal efficiency is lowered and as a result exhaust gas temperature is increased. This
improves exhaust system after-recirculation and lowers the HC emission even more.

70

71

On the other hand, as engine efficiency is lowered, mass flow is increased for a given horse power
level which increases mass emissions.
On the other hand with large reduction in compression ratio, the temperature in chamber decreases
and it increases both HC and CO emission.
EFFECT OF NITRIC OXIDE:
The concentration of NO in the exhaust gases depends upon the difference between the rate of its
formation at the highest temperature in the cycle and the rate of its decomposition as the temperature
decreases during the expansion stroke. A study of the decomposition rate of NO indicates that the amount
decomposed is negligible because of the short time available during the expansion stroke.
1. EQUIVALENCE RATIO:
The equivalence ratio affects both the gas temperature and the available oxygen during
combustion. Theoretically an increase in the equivalence ratio form 1.0 to 1.1 results in an increase of
maximum cycle temperature by about 55C while oxygen concentration is reduced by 50%. At equivalence
ratio of 1.1, NO in the exhaust is very low. Maximum NO concentration occurs at an equivalence ratio of
0.8. The maximum cycle temperature with this lean mixture is lower than with a rich mixture but available
oxygen concentration is much higher.

72

73

With very rich mixtures, low peak combustion temperatures and low oxygen concentration lead to
low NO. For mixtures leaner than 15.5:1 there is enough oxygen but the temperature is very less and hence
lower the NO formation. Thus NO concentration is very low for very lean as well as very rich mixtures.
2. SPARK TIMING:
An advance in spark timing increases the maximum cycle temperature and therefore results in
increased NO concentration.

74

75

3. MANIFOLD PRESSURE:
An increase in manifold vacuum decreases load and temperature. As a result the ignition delay is
increased and the flame speed is reduced. Both these factors increase the time of combustion. This reduces
the maximum cycle temperature and thus reducing NO concentration in the exhaust.
4. ENGINE SPEED:
An increase in engine speed has little effect on ignition delay. Increase in engine speed results in
an increase in flame speed due to turbulence and reduces heat losses per cycle which tends to raise
compression and combustion temperature and pressure. If spark timing is held constant, a greater portion
of this combustion tends to occur during expansion where temperature and pressure are relatively low.

76

This is most pronounced for the slowest burning mixture ratio of 19:1. For richer mixtures which
burn faster, the effect of reduced heat losses at higher speeds predominates.

These are two opposing influences – an increase in the rate of NO formation due to reduced heat
losses opposed by a reduction in the rate of NO formation due to late burning. For rich mixtures where
combustion and NO formation are rapid, the former predominates. For lean mixtures where combustion
and NO formation are slow, the later effect predominates.
5. COOLANT TEMPERATURE AND DESPOSIT:
An increase in the coolant temperature results in a reduction of heat losses to the cylinder walls
and an increase in the maximum gas temperature. This results in an increase in NO concentration.
An increase in deposit thickness causes an increase in compression ratio, reduction in heat losses
to the coolant and an increase in NO concentration.

77

6. HUMIDITY:
The reduction in NO formation caused by an increase in mixture humidity is mainly due to the
drop in maximum flame temperature. Test on hydrogen-air, and ethylene-air mixture indicates that 1% of
water vapour reduces the flame temperature by 20C. This reduces the initial rate of NO production by
about 25%.

78

7. EXHAUST GAS RECIRCULATION:
Recycling of a portion of exhaust gas to inlet charge increases dilution. This reduces peak
combustion temperature, since the inert exhaust gas re circulated will act as a heat sink. This also reduces
the oxygen availability. About 15% recycle will reduce NOx emission by about 80%. The maximum
percentage which can be re circulated is limited by rough engine operation and loss of power.

79

80

8. SURFACE TO VOLUME RATIO:
Engine changes which decrease surface to volume ratio reduce heat loss to the coolant. As a result
NO concentration may increase.
EFFECT OF DESIGN AND OPERATING VARIABLES ON EXHAUST EMISSIONS
SL.NO.

VARIABLE

HC

CO

NO

1
2
3

INCREASED
Load
Speed
Spark retard

_
Decrease
Decrease

_
-

Increase
Increase/Decrease
Decrease

81

4
5
6

Exhaust back pressure
Decrease
Valve overlap
Decrease
Intake
manifold -

-

Decrease
Decrease
Increase

7

pressure
Combustion

chamber Increases

-

Increases

8
9

deposit
S/V ratio
Combustion

Increase
chamber Increase

-

-

10
11
12
13
14
15

area
Stroke to bore ratio
Displacement per cyl.
Compression ratio
Air Injection
Fuel injection
Coolant temperature

Decrease
Decrease
Decrease

Increase
Increase
Increase
Increase

Decrease
Decrease
Increase
Decrease
Decrease
Decrease

DIESEL ENGIEN EXHAUST EMISSIONS:
The pollutants from diesel engines can be categorized into two types:
1. Visible and 2. Invisible. The first one consists of smoke and metallic particulates. Smoke being so
conspicuous and odorous is objected to public and also reduces visibility and has smudging character but is
not harmful to health.
The second type consists of CO, un burnt hydrocarbons including poly nuclear aromatics, oxides
of N2, SO2 and partially oxidized organics (aldehydes, ketones etc.,)
Among these pollutants smoke, CO, UBHC and oxides of nitrogen are of most immediate concern.
FORMATION OF POLLUTANTS IN DIESEL ENGINES:
Unlike a gasoline engine, where fuel and air are premixed into a homogenous form before entering
the cylinder, in the diesel engine fuel is injected into the compressed air charge inside the cylinder. As the
mixing of air and fuel has to take place entirely in the combustion chamber, complete mixing is virtually
impossible and infinite variations in air-fuel mixture ratio takes place within the same cylinder. Also as the
load requirement is met through variation in the quantity of fuel injected, the overall air fuel ratio varies
within wide limits, about 20:1 to 60:1.
A normally rated and well maintained engine emits negligible amount of CO and unburnt
hydrocarbons, through considerable amount of oxides of nitrogen and smoke are emitted.
Carbon monoxide:
It is formed during combustion when there is insufficient oxygen to oxidize the fuel fully.
Compression ignition engines have long been known to produce low levels of CO because of excess
amount of air available for combustion. Theoretically it should not emit any CO as it always operated with
large amount of excess air. Nevertheless CO is present in small quantities ( 0.1 to 0.75%) in the exhausts.

82

This is possible because of the fact that fuel injected in later part of the injection does not find enough
oxygen due to local depletion in certain parts of the combustion chamber.
Unburnt Hydrocarbons:
The concentrations of hydrocarbons in diesel exhaust varies for a few parts per million to several
thousand parts per millions depending on engine speed and load. Hydrocarbons in engine exhaust are
composed of many individual hydrocarbons in the fuel supplied to the engine as well as number of
hydrocarbons partially unburnt produced during the combustion process. In addition some unburnt
hydrocarbons may be from lubricating oils. Tests on engine with single component fuels shows that these
engines contained hydrocarbons of higher and lower molecular weights, than original fuel as well as
molecules with different structures. Aromatic compounds have been observed in exhaust of engines
operated on pure paraffins. Poly nuclear aromatics found in exhaust are products of this synthesis.
During the normal operation the relatively cold walls “quench” the fuel air mixture and inhibit
combustion leaving a thick skin of unburnt air fuel mixture over the entire envelope of the combustion
chamber. The amount of unburnt fuel depends on the thickness of quench zone and the effective
combustion chamber area. The thickness of quench zone depends on many variables as combustion
temperature, pressure, mixture ratio, turbulence and residual gas dilution. Higher surface to volume ratio of
combustion chamber leads to greater fraction of unburnt hydrocarbon from the quench zone.
Partially oxidized hydrocarbons (aldehydes) have been associated with diesel exhaust. They
produce objectionable odor and are high when engine idles and under cold starting indicating poor
combustion.
OXIDES OF NITROGEN:
This is more significant. The formation of Nitric oxide, the major component of oxides of nitrogen
depends on number of operating conditions of diesel engine. The main factors that control this formation
are amounts of oxygen available and the peak temperature in the zones with sufficient oxygen and
residence times at temperatures above 2000K.
Both open and pre-combustion chamber produce small amount of oxides of nitrogen when air fuel
ratio is about 0.01 to maximum near air fuel ratio of about 0.035 ratios. Additional fuel tends to lower air
fuel ratio; the charge temperature also reduces which consequently reduces oxides of nitrogen.
Formation of oxides of nitrogen:
Since nitrogen is a high temperature species its formation is influenced by combustion temperature
and time available for combustion. Hence NO tends to increase with advanced injection timing. Also NO
produced increase with fuel supply. Notable exception is prechamber. In direct injection engines NO
reaches maximum value at stoichiometric air fuel ratio, as lean and rich mixtures tend to reduce
combustion temperatures. Increase in compression ratio leads to increase in combustion temperature and
hence higher NO formation.

83

Valve overlap has significant effect on NO formation. Higher valve overlap dilutes the incoming
air more and more leading to increasing in fuel/air ratio. This in turn reduces combustion temperatures and
hence lowers NO formation.
Earlier inlet valve opening before TDC leads to increased dilution of incoming air and hence lower
NO.
Extended inlet valve opening up to 20 has no effect on NO formation as it does not vary manifold
pressure.
Extended exhaust valve opening before bottom dead centre results in marginal increase in NO due
to better scavenging, conversely later exhaust valve opening leads to delayed scavenging and higher
dilution.
Exhaust valve closing determine effect of scavenging and pronounced effect on dilution and hence
Nitrogen formation.
DIESEL ENGINE SMOKE EMISSION:
Engine exhaust smoke is a visible indicator of the combustion process in the engine. Smoke is due
to incomplete combustion. Smoke in diesel engine can be divided into three categories: blue, white and
black.
Blue smoke:
It results from the burning of engine lubricating oil that reaches combustion chamber due to worn
piston rings, cylinder liners and valve guides.
White or cold smoke:
It is made up of droplets of unburnt or partially burnt fuel droplets and is usually associated with
the engine running at less than normal operating temperature after starting, long period of idling, operating
under very light load, operating with leaking injectors and water leakage in combustion chamber. This
smoke normally fades away as engine is warmed up and brought to normal stage.

Black or hot smoke:
It consists of unburnt carbon particles ( 0.5 – 1 microns in diameter) and other solid products of
combustion. This smoke appears after engine is warmed up and is accelerating or pulling under load.
Formation of smoke in Diesel engines:
The main cause of smoke formation is known to be inadequate mixing of fuel and air. Smoke is
formed when the local temperature is high enough to decompose fuel in a region where there is insufficient
oxygen to burn the carbon that is formed. The formation of over-rich fuel air mixtures either generally or
in localized regions will result in smoke. Large amounts of carbons will be formed during the early stage
of combustion. This carbon appears as smoke if there is insufficient air, if there is insufficient mixing or if

84

local temperatures fall below the carbon reaction temperatures (approximately 1000C) before the mixing
occurs.
Acceptable performance of diesel engine is critically influenced by exhaust some emissions.
Failure of engine to meet smoke legislation requirement prevents sale and particularly for military use,
possible visibility by smoke is useful to enemy force. Diesel emissions gives information on effectiveness
of combustion, general performance and condition of engine.
FACTORS AFFECTING SMOKE FORMATION:
The smoke intensity in the diesel exhaust is generally affected by many parameters. By controlling
them, smoke intensity may be reduced.
1. Injection timing:
Advancing the injection timing in diesel engines with all other parameters kept constant results in
longer delay periods, more fuel injected before ignition, higher temperatures in the cycle and earlier ending
of the combustion process. The residence time is therefore increased. All these factors have been fond to
reduce the smoke intensity in the exhaust. However earlier injection results in more combustion noise,
higher mechanical and thermal stresses, and high NO concentration.
In a recent study, khan reported that a very late injection reduces the smoke. The timing after
which this reduction occurs is that at which the minimum ignition delay occurs. He suggested that one of
the factor that contributes to the reduction in smoke at the retarded timing is the reduced rat of formation
due to decrease in the temperature of the diffusion flames as most of these flames occur during the
expansion stroke.
2. Rate of Injection:
Higher initial rates of injection have been found to be effective in reducing the exhaust smoke.
3. Injection nozzle:
The size of the nozzle holes and the ratio of the hole length to its diameter have an effect on smoke
concentration. A larger hole diameter results in less atomization and increased smoke. An increase in the
length/diameter ratio beyond a certain limit also results in increased smoke.
4. Maintenance:
The engine condition plays a very important role in deciding the smoke levels. The maintenance
affects the injection characteristics and the quantity of lubricating oil which passes across the piston rings
and thus a profound effect on smoke generation tendency of the engine. Good maintenance is a must for
lower smoke levels.

5. Fuel:
Higher cetane number fuels have a tendency to produce more smoke. It is believed to be due to
lower stability of these fuels. For a given cetane number less smoke is produced with more volatile fuels.

85

6. Load:
A rich fuel-air mixture results in higher smoke because the amount of oxygen available is less.
Hence any over loading of the engine will result in a very black smoke. The smoke level rises from no load
to full load. During the first part, the smoke level is more or less constant as there is always excess air
present. However in the higher load range there is an abrupt rise in smoke level due to less available
oxygen.

86

7. Engine type and speed:
Naturally aspirated engines have higher smoke levels at higher loads than turbo charged engines,
because the later have sufficient oxygen even at full loads. The smoke is worse at low as well as at high
speeds. This follows the volumetric efficiency curve of the engine in some measure as it drops at the
extremes of speed.
8. Fuel air ratio:
The smoke increases with richening the mixture. The increase in smoke occurs even with as much
as 25% excess air in cylinder, cleanly indicating that the diesel engine has a mixing problem.

87

CONTROL OF DIESEL ENGINE SMOKE:
Smoke can be reduced by some of the following methods:
1. Derating:
Derating is nothing but making the engine to run at lower loads. At lower loads more excess air is
present in the combustion chamber and hence the smoke developed is less as already discussed. However
this means a loss of output.
2. Proper maintenance of the engine:
Maintaining the engine properly, especially the injection system, will not only result in reducing
smoke but also keep the performance of the engine at its best.
3. Proper choice of combustion chamber design and operating conditions:
A proper choice of combustion chamber design results in better mixing of fuel and air in the
chamber and hence reduces the smoke level to a considerable level.
4. Use of smoke suppression additives:
Some barium compounds if used in fuel reduce the temperature of combustion, thus avoiding the
soot formation. Even if formed they break it into fine particles, thus appreciably reducing smoke.
However, the use of barium salts increases the deposit formation tendencies of engine and reduces the fuel
filter life.
5. Adopting fumigation technique:

88

This method consists of introducing a small amount of fuel into the intake manifold. This starts
pre-combustion reactions before and during the compression stroke resulting in reduced chemical delay,
because the intermediate products such as peroxides and aldehydes react more rapidly with oxygen than
original hydrocarbons. The shortening of delay period curbs thermal cracking which is responsible for soot
formation.

Fumigation rate of about 15% gives best smoke improvement. However this improvement varies
greatly with engine speed. At low engine speeds 50 to 80% smoke reduction is obtained. This decrease as
speed increases until a speed at which there is no effect of fumigation.
DIESEL ODOUR:
Ever since the first diesel engine was developed, the odor from its exhaust has been recognized as
undesirable. Determination of the cause of this odor has been difficult because of the complexity of the
heterogeneous combustion process and the lack of chemical instruments available. In practice the human
nose plays a significant role in odor measurement.
The members of the aldehydes family are supposed to be responsible for the pungent odors of
diesel exhaust. Though the amount of aldehydes is small being less than 30ppm, the concentration as low
as 1ppm are irritating the human eyes and nose.
Mechanism of odour production:

89

Some experimental results indicate that the products of partial oxidation are the main cause of
odor in diesel exhaust. This partial oxidation may be because of either very lean mixture or due to
quenching effect.

In diesel combustion there are most probably regions in which the fuel/oxygen/inert mixture is outside the
flammability limits. The fuel in these regions which are too lean to burn might only partially oxidize
resulting in odors. This is most likely to occur during idling and or part load operation of the engine. Also
the fact that chemical reactions take place during the second stage of diesel combustion suggest that if the
reactions are quenched during this period, partial oxidation products will result in odour in the diesel
exhaust.
Barns concluded in his research that diesel odor resulted from partial oxidation reaction in the fuel
lean regions which are almost inevitably formed in heterogeneous combustion. Graph shows the relative
odor producing capabilities of different air fuel regions. The data shown in graph were obtained using CFR
engine and varying air-fuel ratios and the inlet air temperatures.
Odor relevant compounds:
Until recently very little was known about the compound or compounds that contribute to the
odorous qualities of diesel exhaust. Rounds and Pearsall correlate odor with the aldehydes in diesel
exhaust gas. However Vogh says that aldehydes are not significant contributors to the overall odor
problem. Vogh also says that neither SO2 nor particulate contribute significantly to diesel odor emissions.
Research work at the Illinois Institute of Technology, Research Institute ( IITRI ) has contributed
significantly to the development of a better understanding of the chemical nature of the odor contributors

90

in diesel exhaust. Based on the IITRI work, various high molecular weight cyclic and aromatic
hydrocarbons including naphthalene, tetra ling and cyclo paraffins some with olefinic and or paraffinic
side changing were reported as major contributors to the burnt odor note of the exhaust.
Various non aromatic hydrocarbons with more than one double or triple bond were also reported
to contribute to the burnt odor note. Furan aldehydes, aromatic benzene and paraffinic aldehydes from
ethanol to n-octanol were found be important odor contributors and have individual odors that varied from
pleasant to pungent. Some heterocyclic sulfur compounds, thiophene and benzothiophene derivatives were
also reported to be odor contributors.
FACTORS AFFECTING ODOR PRODUCTION:
1. Fuel air ratio:
The fact that very lean mixtures result in odorous diesel exhaust has already been discussed.
2. Engine operation mode:
It has been found that the mode of operation of the engine significantly affects the exhaust odor.
Maximum odor occurs while accelerating from idle and minimum odor results when the engine is running
at medium sped and or at part loads.
Effect of engine operating mode on odor production ( 4-stroke normally aspirated medium speed
diesel engine)
Engine operation mode
Idle
Acceleration
Part load
Full load

Odor intensity ( Turk number)
3.6
4.1
3.0
3.5

3. Engine type:
The odor intensity does not vary with the engine type as can be seen from the table. The odor
intensity from all the engines is more or less the same.
Engine type
Odor intensity ( Turk number)
Two stroke
3.5
Four stroke normally aspirated 3.3
( medium speed)
Four stroke normally aspirated 3.5
( high speed)
Four stroke – Turbo charged

3.4

4. Fuel composition:
It is really surprising to find that the composition of the fuel has no effect on exhaust odor
intensity. The changes in fuel composition result in different second stage combustion time in diesel
combustion and it is expected that this will affect the degree of oxidation if quenching is taking place.
However the results contradict this expectation.
5. Odor suppressant additives:

91

It has been claimed from time to time, by different manufactures of odor suppressant additive
compounds that they reduce the odor. However small and rather insignificant effects upon the odor has
been found in comparison of exhausts from treated and untreated fuels. No predictable and reliable
correspondence between the additives and odor is found.
Odor Measurement Techniques:
Effective chemical or physical methods for the measurement of odor have not been developed, and
therefore the human nose plays a significant role in all odor studies. In practice odor is measured by a
specially selected, specially trained human panel.
When the nose is subjected to an odor, the physiological response to the odor can be classified by
either intensity or intensity and quality. The Turk kit contains a number of different standard odors that are
classified as a) burnt/smoky b) oily
c) pungent/acid and d) aldehydic/aromatic. It has been generally accepted as a standard for rating the
intensity and quality of an unknown odorous sample.
Odor-detection methods that have been developed todate may be placed in two general categories.
The first category includes methods that only rate the over all odor intensity, while the second group is
employed to classify odors by quality and intensity. The threshold dilution technique and natural dilution
technique that are described below fall into the first category.
The threshold dilution technique consists of presenting raw diesel exhaust synthetically diluted
with variable quantities of odor free air to a panel of “Sniffers” person who smells. A series of diluted
samples, both above and below the threshold dilution ratio, are presented to members of the panel and the
individual panel members are asked to determine whether or not any odor is detectable. The odor intensity
is assumed to be proportional to the dilution ratio at which the odor is just detectable to the panel.
The natural dilution technique was developed in order to determine whether diesel powered
vehicles could meet the motor vehicle exhaust odor on standards set by the state of California. During the
course of these tests, a panel is seated at varying distance from a vehicle. Both the vehicle and the panel
are located inside a large municipal hanger, in order to minimize the effects due to winds and the panelists
are asked to determine whether or not they can detect any odor from the vehicle. Their responses are
utilized to evaluate threshold response distances.
Variations of the direct method have been used to rate the quality and intensity of diesel odor and
hence thy fall into the second category of odor detection methods. When applying this method, the exhaust
from the diesel engine is usually diluted with odor free air at the engine exhaust pipe and the resulting
mixture of gases which consists of raw diesel exhaust mixed with odor free air in ratios ranging from 1 to
200 flows dynamically through a presentation system to the panelists. The panelists who have been
previously trained to evaluate both quality and intensity as determined by the Turk kit are asked to record
their response to test gases as a function of dilution ratio and experimental parameters.
CONTROL OF EMISSIONS FROM SI AND CI ENGINES
Design changes:

92

The effects of engine design and operating variables on exhaust emission were discussed in a
detailed manner already. Based on the discussions made already the engine design modifications
approaches to control the pollutants are discussed below.
1. NOx is decreased by
A. Decreasing the combustion chamber temperature
The combustion chamber temperature can be decreased by
1. Decreasing compression ratio
2. Retarding spark timing
3. Decreasing charge temperature
4. Decreasing engine speed
5. Decreasing inlet charge pressure
6. Exhaust gas recirculation
7. Increasing humidity
8. Water injection
9. Operating the engine with very lean or very rich air fuel ratio
10. Decreasing the coolant temperature
11. Decreasing the deposits
12. Increasing S/V ratio
B. By decreasing oxygen available in the flame front
The amount of oxygen available in the chamber can be controlled by
1. Rich mixture
2. Stratified charge engine
3. Divided combustion chamber
2. Hydrocarbon emission can be decreased by
1. Decreasing the compression ratio
2. Retarding the spark
3. Increasing charge temperature
4. Increasing coolant temperature
5. Insulating exhaust manifold
6. Increasing engine speed
7. Lean mixture
8. Adding oxygen in the exhaust
9. Decreasing S/V ratio
10. Increasing turbulence
11. Decreasing the deposits
12. Increasing exhaust manifold volume
13. Increasing exhaust back pressure

93

3. CO can be decreased by
1. Lean air fuel ratio
2. Adding oxygen in the exhaust
3. Increasing coolant temperature.
EXHAUST GAS RECIRCULATION:
In exhaust gas recirculation a portion of the exhaust gas is recirculated to the cylinder intake
charge. This reduces the peak combustion temperature, since the inert gas serves as a heat sink. This also
reduces the quantity of oxygen available for combustion.

The exhaust gas for recirculation is passed through the control valve for regulation of the rate and
inducted down to the intake p[ort, The recycle rate control valve is connected to the throttle shaft by means
of appropriated linkage and the amount of valve opening is regulated by throttle position. The link is
designed so that recycled exhaust is normally shut off during idle to prevent rough engine operation. This
is also shut off during full throttle, acceleration to prevent loss of power when maximum performance is
needed.
The NOx concentration will vary with the amount of recycling of gas at various air fuel ratios.
About 15% recycle will reduce NOx emission by about 80%. The maximum percentage which can be
circulated is limited by rough engine operation and loss of power.
The above figure shows a vacuum controlled EGR valve used to control the recycle rate. A special
passage connects the exhaust manifold with the intake manifold. This passage is opened or closed by a
vacuum controlled EGR valve. The upper part of the valve is sealed. It is connected by a vacuum line to a
vacuum port in the carburettor. When there is no vacuum the port, there is no vacuum applied to the

94

diaphragm in the EGR valve. Therefore, the spring holds the valve closed. No exhaust gas recirculates.
This is the situation during engine idling when little NOx is formed.
As the throttle valve opens it passes the vacuum port in the carburettor. This allows intake
manifold vacuum to operate the EGR valve. Then vacuum raises the diaphragm, which lifts the attached
valve off its seat. Now exhaust gas flows into the intake manifold. There the exhaust gas mixes with the air
fuel mixture and enter the engine cylinders.
At wide open throttle, there is little vacuum in the intake manifold. This produces a denser mixture
which burns cooler during the combustion process. Therefore at wide open throttle there is less need for
exhaust gas recirculation. Due to low vacuum, the EGR valve is nearly closed.

A thermal vacuum switch on many cars prevents exhaust gas recirculation until the engine
temperature reaches about 100 F 0r 37.8C. The thermal vacuum switch is also called a coolant temperature
override switch (CTO switch). It is mounted in a cooling system water jacket, so it senses coolant
temperature. If this temperature is below 100F, the switch remains closed. This prevents the vacuum from
reaching the EGR valve, so the exhaust gas does not recirculate. Cold engine performance immediately
after starting is improved. After the engine warms up it can tolerate exhaust gas recirculation. Then the
CTO valve opens. Now vacuum can get to the EGR valve, so that exhaust gas can recirculate.
EGR invariably results in drop in power, increased fuel consumption and rough combustion. In
addition excessive intake system deposit buildup and increased oil sludging occur.

95

Fumigation technique:
This method consists of introducing a small amount of fuel into the intake manifold. This starts
pre-combustion reactions before and during the compression stroke resulting in reduced chemical delay,
because the intermediate products such as peroxides and aldehydes react more rapidly with oxygen than
original hydrocarbons. The shortening of delay period curbs thermal cracking which is responsible for soot
formation.
Fumigation rate of about 15% gives best smoke improvement. However this improvement varies
greatly with engine speed. At low engine speeds 50 to 80% smoke reduction is obtained. This decrease as
speed increases until a speed at which there is no effect of fumigation.
CRANKCASE EMISSION AND CONTROL
During the compression and combustion strokes, highly corrosive blowby gases are forced past the
piston rings into the crankcase. The amount of blowby entering the crankcase generally increases with
engine speed. The amount of blowby also depends on other conditions including piston, ring and cylinder
wear. The actual amount of wear may be small, perhaps only a few thousands of an inch. But almost any
wear is enough to weaker the sealing effect of the rings and permit blowby to increase. Blowby gases

96

contain burned and unburned fuel, carbon and water vapour from the combustion chamber. When the
engine is cold, some of the water vapour of the blowby condenses on the cylinder walls and crankcase. It
forms into droplets and runs down into the oil pan. Gasoline vapour also condenses on cold engine parts
and drips down into the oil pan. This gasoline dilutes and thins the oil, reducing its lubricating ability.
The churning action of the rotating crank shaft can whip the water and engine oil into thick,
gummy substance called sludge. The acid compounds from the blowby can get into the sludge and cause
corrosion and faster wear of engine parts. Sludge can also clog oil passages and prevent normal engine
lubrication, thereby leading to early engine failure.
Blowby causes pressure in the crankcase. If this pressure is allowed to build up, engine oil is
forced past the oil seals and gaskets and out of the engine. To help to control the effect of blowby, there
must be a way to relieve the crankcase pressure caused by blow by gases.

CRANKCASE VENTILATION
To avoid the above said problems, the unburned and partly burned gasoline and the combustion
gases and water vapour must be cleared out of the crankcase by providing crankcase ventilation systems.
In early engines, the crankcase ventilation system was very simple. It provided crankcase
breathing by passing fresh oil through the crankcase. On almost all American made automobile engines
built prior to 1961, the fresh air entered through an air inlet at the top front of the engine. The fresh air is
mixed with the blowby fumes and other vapours in the crankcase. These vapours were routed out of the
crankcase through a large hollow tube called the road draft tube, which discharged under the car into the
atmosphere.
The fresh air inlet was usually the crankcase breather cap. On most engines it also served as the
cap for the crankcase oil filler tube. The cap was open, or vented with holes on both sides to let fresh air to
pass through. The cap was filled with oil soaked steel wool or similar material to serve as an air filter. The
filter prevented dust particle in the air from getting into the crankcase oil and causing engine wear.

97

ROAD DRAFT TUBE EMISSIONS:

98

The road draft tube system worked well to keep the crankcase free of fumes and pressure build up.
However it discharged all the crankcase pollutants into the atmosphere. This discharge through the road
draft tube represented about 20% of the total HC emissions from an automobile. Therefore controlling
blow by was the first step in eliminating atmospheric pollution from the automobile.
OPEN PCV SYSTEM:
An early system that partially controlled crankcase emission was installed on cars built for sale in
California beginning in 1961. The system was called open positive crankcase ventilation system.
In this system a tube is connected between a crank case vent and the intake manifold. While the
engine is running, intake manifold vacuum is used to pull vapour from the crankcase through the tube into
the intake manifold. Fresh ventilating air is drawn into the crankcase through an open oil filler cap. In the
intake manifold, the crankcase vapours are mixed with the incoming air-fuel mixture and sent to the
cylinders for burning.

99

For the engine to operate properly under all conditions of speed and load, a flow control valve is
required. Without a flow control valve, excessive ventilation air passes from the crankcase into the intake

100

manifold during idling and low speed. This upsets the engine air fuel ratio and results in poor idling with
frequent stalling.
The PCV valve is installed in a tube from the crankcase vent to the intake manifold. The PCV
valve is a variable orifice valve. A variable orifice is a hole that acts as a valve by changing the size to vary
the flow rate through it. This valve is also called a metering valve, a modulator valve and a regulator valve.
A typical PCV valve consists of a coil spring, a valve and a two piece outer body which is usually
crimped together. At idle or low speed, high intake manifold vacuum tends to pull the valve closed or into
its minimum flow condition. As the valve tries to close it compresses the valve spring. The smaller
opening now allows a much smaller volume of blow by gas to pass through. At high engine speeds, the
compressed spring overcomes the pull of the vacuum on the valve. The spring begins to force the valve
open towards the maximum flow condition. As the valve moves open, the flow capacity increases. This is
to handle the greater volume of blowby that results from an increase in engine load and speed.
CLOSED PCV:
The crankcase emission control system described above is not completely effective in controlling
crankcase emissions. In open type system, blowby in excess of the PCV valve flow rate escapes to the
atmosphere through the open oil filler cap. To overcome this problem, a closed positive crankcase
ventilation system was developed. All cars manufactured in California in 1963 and later used a closed type
of positive crankcase ventilation system.
The blowby gases are turned to the engine cylinder through the intake manifold and under
appropriate conditions, through the carburettor air cleaner. The PCV valve described earlier is generally
used as the flow control valve. A closed oil filler cap is used. Other possible outlets for blow by gases,
such as dipstick tube are sealed.

101

102

103

All cars are now being equipped with such closed PCV system wherever there are air pollution
regulations. These systems have completely eliminated the crankcase as a source of atmospheric
contamination and no additional control in future is required in this direction.
EVAPORATIVE EMISSIONS AND CONTROL
Hydrocarbon evaporative emissions from a vehicle arise from two sources as evaporation of fuel
in the carburettor float bowl ( 5-10 percent ) of fuel in the fuel tank ( about 5 percent ).
CARBURETTOR EVAPORATIVE LOSSES:
Carburettor hydrocarbon vapour losses arise from distillation of fuel from the float bowl.
Carburettor fuel temperature often reaches 55°C during warm weather engine operation and may rise up to
80°C during a hot soak. Hot soak is a condition when a running car is stopped and its engine turned off.
During the soak a significant fraction of the fuel will boil off and a large portion of the loss finds its way
into the atmosphere. There is a considerable rise in fuel system temperature following shut down after a
hard run.
The basic factors governing the mass of fuel distilled from carburettor during a hot soak period are
•

maximum fuel bowl temperature

•

amount of fuel in the bowl

•

amount of after-fill and

•

distillation curve of the fuel

Tests have indicated that a less volatile fuel would reduce the evaporative losses considerably. A
fuel with 20% distilled at 72 °C would give 22% less losses as compared to a fuel which distilled 25% at
72 °C .
The carburettor bowl volume has a significant effect on evaporative losses. Increase in the volume
of the bowl increases the losses linearly. If an insulated spacer is placed between the carburettor and the
inlet manifold, almost 50% reduction can be observed.
Filling of the carburettor (after-fill) to the original liquid level is similar to an increase in the bowl
volume and the distillation losses would increase by about 15%.
FUEL TANK EVAPORATIVE LOSSES:
Fuel tank losses occur by displacement of vapour during filling of petrol tank, or by vaporization
of fuel in the tank, forcing the vapour through a breather vent to the atmosphere. When the temperature is
low, the fuel tank breathes in air. When the temperature goes high it breathes out air, loaded with petrol
vapours. Fuel tank losses occur because the tank temperature is increased during the vehicle operation
which causes an increase in the vapour pressure and thermal expansion of tank vapour.
The mechanism of tank loss is as follows: When a partially filled fuel tank is open to atmosphere,
the partial pressure of vapour phase hydrocarbons and vapour pressure of the liquid phase are equal and
they are in equilibrium. If the temperature of the liquid is increased, say by engine operation, the vapour
pressure of the liquid will increase and it will vaporize in an attempt to restore equilibrium. As additional

104

liquid vaporize the total pressure of the tank increases and since the tank is open to atmosphere, the vapour
will flow out of the tank. This outflow to the vapour will increase if in addition to liquid temperature rise,
the vapour temperature is also increased.
The evaporation from the tank is affected by a large number of variables of which the ambient and
fuel tank temperature, the mode of vehicle operation, the amount of fuel in the tank and the capacity,
design and location of the fuel tank with reference to exhaust system and the flow pattern of the heated air
underneath the vehicle.
Less the tank fill, greater is the evaporative loss. The effect of the tank fill and the temperature are
shown in the table. This reflects the difference in the tank vapour space. Also when a car is parked in a hot
location, the evaporation of gasoline in the tank accelerates and so the evaporation loss is greater.
EFFECT OF FUEL TANK FILL ON EVAPORATIVE LOSS:
Tank fill

Ambient temperature Temp.

¼
½
¾
Full

C
19
16
18
22

rise

test in °C
7
4
2
3

during Loss

during

operations %
5.7
1.2
0.1
0.0

The operational modes substantially affect the evaporation loss. When the tank temperature rises,
the loss increases. The fuel composition also affects the tank losses. About 75% of the HC losses from the
tank are C4 and C5 hydrocarbons.
Design factors that affect the evaporative losses include the peak tank temperature, the area of the
liquid vapour surface, and the amount of agitation. It is obvious that nay design change which reduces the
peak tank temperature will reduce the tank loss. Such modifications include tank insulation, lower surface
to volume ratio of tank, better tank orientation or location for reduced heat pick up from solar radiation or
other heat sources such as the exhaust system.
The surface area for evaporation and tank agitation are factor which influence the speed with
which equilibrium is achieved. Baffles in the tank can reduce losses by maintaining concentration
gradients.
EVAPORATIVE EMISSION CONTROL DEVICES:
Evaporative emission control devices are designed to virtually eliminate the hydrocarbon vapours
emitted by the carburettor and fuel tank during both running and hot soak. During running, fuel tank
vapours are inducted and burned in the engine. Carburettor losses are vented to intake system. Vehicles
without evaporative controls are estimated to 10se 30 g/day of HC from fuel tank filling and breathing.
Another 40 g/day is lost by evaporation from the carburettor ( hot soak loss ) when the vehicle is parked
after being operated. ON this basis, evaporative losses are estimated to be 23% of total HC emissions.

105

The device as shown in the figure consists of an absorbent chamber, the pressure balance valve
and the purge control valve. The absorbent chamber which consists of a charcoal bed or foamed polyurethane holds the hydrocarbon vapour before they can escape to atmosphere. The carburettor bowl and
the fuel tank are directly connected to the absorbent chamber.

106

107

During hot soaks, vapours from the fuel tank are routed to a storage device. Carburettor vapours may be
vented to the storage system or retained internally in the carburettor or induction system volume. A
schematic diagram of this arrangement is shown in the figure.
Upon restart, filtered air is drawn through the stored vapours and the mixture is metered into the
intake system and burned in the engine. In this manner the storage device is purged (removed off the
retained vapour). The operations of the purge control valve are controlled by the exhaust back pressure.
The storage system consists of a canister containing activated charcoal located in the engine
compartment. Activated charcoal has an affinity for HC and on a recycle basis can store 30-35 grams of
fuel per 100 grams of charcoal without breakthrough. Typically 700-800 grams of charcoal are used in a
vehicle system.

One problem with any storage system is the possibility of liquid fuel entering the storage device. Ball
check valves or vapour liquid separators assure than only fuel vapours reach the storage device. In
addition, a dead volume in the tank allow for thermal expansions of a full fuel tank. About 10% of the tank
volume is partially walled off from the remainder of the tank. When the tank is filled, this volume remains
nearly empty. After a period of time, the fuel fills the additional volume thereby leaving room for
expansion in the rest of the tank. Otherwise expansion could force the liquid fuel into the charcoal canister
or the crankcase.

108

109

THERMAL REACTORS:
Thermal reactor is a chamber in the exhaust system designed to provide sufficient residence time
to allow appreciable homogeneous oxidation of HC and CO to occur. In order to improve CO conversion
efficiency, the exhaust temperature is increased by retarding spark timing. This however results in fuel
economy loss.

110

111

The air is supplied from an engine driven pump through a tube to a place very near to the exhaust
valve. To achieve a high degree of exhaust system oxidation of HC and CO, a high exhaust temperature
coupled with sufficient oxygen and residence time to complete the combustion is needed. Oxides of
nitrogen are not reduced. In fact, they may be increased if sufficiently high exhaust temperature results
from the combustion of CO and HC with the added air or if the injected air enters the cylinder during the
overlap period, thereby leaning the mixture in the cylinder.
Warren has derived the following equation for the concentration of hydrocarbons leaving the
exhaust system.
Co = Ci * exp Kr O2 P2V
K3 T2W
Where,

112

Co = Concentration of HC leaving the thermal reactor
Ci = Concentration of HC leaving cylinders and entering the thermal reactor
Kr = Specific reaction rate ft3/lbm – mole/sec
K3 = constant
O2 = oxygen concentration in exhaust gases. Volume percent
P = exhaust pressure ( Psi)
V = thermal reactor volume available for reaction Cu.ft
T = Absolute temperature C
W = Mass flow rate of air ( lb/sec)
Note the importance of pressure term. Increasing exhaust system back pressure promote after
reaction. However commercially, the possible back pressure increase is small.

The graph shows the effect of temperature on specific reaction rate Kr, calculated from the above
equation by warren from his experimental data. The nearness of his curve to a straight line suggests the
equation is a good approximation for the overall reactions occurring. Note that a decrease in exhaust
temperatures from 1100C to 1000F decrease the reaction rate by a factor of 10.

113

The graph shows the effect of temperature and reactor volume on exhaust hydrocarbon
concentration at an oxygen input concentration of 3%. Reactor volume may be viewed as the volume of
the exhaust system which is insulated and ant the high temperature needed for reaction.
Note that if the exhaust temperature were 1400F, only twice the convention system volume is
required for virtually complete elimination of the hydrocarbons. On the other hand, if the temperature were
only 1200F, eight times the volume would achieve only a 76% reduction. A pair of conventional exhaust
manifolds has about 0.09 ft3 of volume.

Increasing the exhaust system volume increases the residence time during which reactions can
occur. This is a benefit, providing the added surface area does not result in excessive cooling. Thus when
large volume exhaust manifolds are to be used, they must be well insulated.
Brownson and stebar have studied thermal reactor performance for a reactor coupled to a single
cylinder CFR engine. In their work an insulated exhaust mixing tank of 150 cubic inch was used for some
tests. They determined that the basic factors governing the combustion of CO and hydrocarbons in the
exhaust system are composition of the reacting mixture, temperature and pressure of the mixture, and
residence time of the mixture or time available for reaction.

114

The graph shows the hydrocarbon and CO emissions as a function of air-fuel ratio and injected air
flow rate. The emission concentration results were corrected for the added air. Injected air flow rate is
indicated as a percentage of the engine air volume flow rate. An insulated 150 cubic inch exhaust mixing
tank was used.

The minimum HC concentrations occurred at rich mixtures. When too much air was injected,
especially at lean mixtures, excessive cooling of the exhaust increased HC concentrations above those with
no air. Thus the normal oxidation process was apparently inhibited by this cooling. A small increase in CO
occurred slightly richer than stoichiometric. At stoichiometric mixtures and leaner, CO was very low. Best
results occurred for rich mixtures with air injection at 20-30% of inlet air flow. The air-fuel ratio for best
emission reduction was 13.5:1. Normally engine operation at such a rich mixture would reduce fuel
economy by 10%.
At each air-fuel ratio there exists one minimum air injection rate that provides maximum emission
reduction. Minimum air flow is desired in order to reduce pump power requirement, size and cost. Graph
shows the optimum air injection rate for both HC and CO emissions.

115

CATALYTIC CONVERTERS:
Catalytic converters provide another way to treat the exhaust gas. These devices located in the
exhaust system, convert the harmful pollutants into harmless gases.

116

In contrast to thermal reactors efficient catalytic oxidation catalysts can control CO and HC
emissions almost completely at temperature equivalent to normal exhaust gas temperatures. Thus the fuel
economy loss necessary to increase the exhaust temperature is avoided.
Inside the catalytic converter the exhaust gases pass over a large surface area coated with a
catalyst. A catalyst is a material that causes a chemical reaction without actually becoming a part of the
reaction process.
Catalytic reaction of NO can be represented as follows:
NO + CO  CO2 + ½ N2
NO + H2  H2O + ½ N2
10 NO + 4HC  2H2O + 4CO2 + 5 N2
HC / CO oxidation is represented by
CO + ½ O2  CO2
4HC + 5 O2  2H2O + 4CO2
The figure shows a single bed catalytic converter. The exhaust gas and air are passed through a
bed of platinum coated pellets or honeycomb core. HC and CO react with the oxygen in the air. Harmless
ware and carbon dioxide are formed. The catalyst platinum act on the exhaust gas in two ways, converting
HC and CO to carbon dioxide and water. So it is called a two way catalyst.

117

Figure show a dual bed catalytic converter. The exhaust gas first passes through the upper bed.
The upper bed contains a reducing catalyst ( example rhodium). NOx is reduced to nitrogen and oxygen in
the upper bed. Then secondary air is mixed with the exhaust gas. The mixture of exhaust gas and
secondary air flows to the lower bed. The lower bed contains an oxidizing catalyst ( example platinum).
HC and CO are oxidized to water vapour and carbon dioxide in the lower bed. Here the catalyst rhodium is
a one way catalyst since it acts o NOx only. Platinum is a two way catalyst since it acts on HC and CO.
A three way catalyst is a mixture of platinum and rhodium. It acts on all three of the regulated
pollutants ( HC, CO and NOx) but only when the air-fuel ratio is precisely controlled. If the engine is
operated with the ideal or stoichiometric air-fuel ratio of 14.7:1. The three way catalyst is very effective. It
strips oxygen away from the NOx to form harmless water, carbon dioxide and nitrogen. However the airfuel ratio must be precisely controlled, otherwise the three way catalyst does not work.

118

Figure shows a three way catalytic converter. The front section( in the direction of gas flow)
handles NOx and partly handles HC and CO. The partly treated exhaust gas is mixed with secondary air.
The mixture of partly treated exhaust gas and secondary air flows into the rear section of the chamber. The
two way catalyst present in the rear section takes care of HC and CO.

119

Generally catalysts are classified as:
1. Supported catalysts based on
a. Noble metals b. Transition metals
2. Unsupported metallic alloys
NO-Reduction Catalysts:
From the literature, it is seen that the following materials have been tried successfully as reduction
catalysts in the vehicle emission control

120

1. Copper oxide-chromia
2. Copper oxide – Vanadia
3. Iron oxide – Chromia
4. Nickel oxide pelleted on monolithic ceramic and metallic supports
5. Monel metal
6. Rare earth oxides
HC/CO oxidation catalysts:
1. Noble metal catalysts such as activated carbon, palladium or platinum
2. Transition metal oxide catalysts such as copper, cobalt, nickel and iron chromate as well as
vanadium or manganese promoted versions of these metals.
3. Copper chromite-alumina and platinum oxide –alumina catalysts were developed with sufficient
activity, stability and mechanical strength.
The catalysts chosen for vehicle emission control should satisfy the following:
1. High conversion efficiency under transient conditions
2. Effective for wide range of temperature ( for ambient to 1600 F)
3. Must withstand the poisoning action of additives in the gasoline that are emitted in the exhaust
4. Must be able to withstand thermal shock
5. Be attrition resistant to highly turbulent flows through the converter
6. Vehicle operation for 50,000 miles
7. Convert into harmless products
8. Cheap and readily available.
Converter Design:
Converter volume is fixed, based on the space velocity and exhaust flow rate
Space velocity = gas flow rate in cm^3 /hr / converter volume in cm^3
The reciprocal of this expression is the residence time. As the exhaust flow rate varies under
different modes of vehicle operation, an average gas flow rate of 0.85m^3/min and a space velocity of
15000/hr are normally selected for the preliminary design of the converter. This will give a converter
volume of 3,540 cm^3 in each stage.
Draw backs of the catalytic converter system:
1. Generally catalysts are active only at relatively high temperature. Emissions during warm-up
cannot be catalysed and this period has particularly heavy emissions.
2. Catalysts operate over a wide but not unlimited temperature range. A temperature control is
required to avoid burnout temperature at high speeds and loads.
3. Catalysts are poisoned by exhaust constituents in particular lead compounds. Hence conversion
efficiency decreases with use.
4. The catalytic bed offers considerable back pressure which increases with use.

121

5. Catalytic converters are expensive.
Importance of unleaded Petrol:
Vehicles equipped with catalytic converters must use only un-leaded gasoline. If the gasoline
contains lead, the lead will coat the catalyst and the converter will stop working.
OTHER EMISSIN CONTROL DEVICES:
1. Water injection:
In this a small amount of water is injected into the combustion chamber. Due to this the peak
combustion temperature is reduced and thus NOx emission is reduced.
Graph shows nitric oxide reduction as a function of water rate. The spark advance was kept
constant and the power loss was balanced by leaning the A/F ratio of the mixture. The specific fuel
consumption as clear from the graph, decreases a few percent at medium water injection ratio. So for no
attempts have been done to use water as a deice for controlling the NOx, perhaps because of complexity
varying the amount of injection rate in relation to engine requirements.
2. Direct air Injection:
In this compressed air is introduced into the combustion chamber in addition to air fuel charge
from the carburettor. This gives better combustion and hence reduced hydrocarbon and CO emission. This
will also give tremendous power boost with some saving in fuel. But extra equipment in the form of air
compressor and air valves will raise the cost very much. Also, exhaust gas recirculation will still be needed
to curb NOx emissions.
3. Ammonia Injection:
In this ammonia is injected into the exhaust gas. Ammonia reacts with NOx in exhaust and forms
nitrogen and water. Thus NOx emission is reduced.
As a fuel, ammonia does not hold much promise, but if used as an exhaust additive it can give
excellent control for NOx emission. Ammonia and nitric oxide interact to form nitrogen and water. Ford
motor company has been doing investigations with injecting Ammonia-water in the exhaust manifold,
downstream from the port.
For an effective utilization of Ammonia injection, the exhaust gas temperature has to be kept
within strict limits and the injecting device has to be put sufficiently down to bring the gas temperature to
165C. This also demands a very close tolerance in air-fuel ratio supplied by the carburettor. The present
carburettors are incapable of this and it might be necessary to adopt electronic injection system to keep it.
4. Electronic Injection:

122

It is possible to develop an electronic injection system with sensors for air temperature, manifold
pressure and speed which will precisely regulate the fuel supply giving only such air fuel ratio that will
give no hydrocarbon or CO emissions.
Since the injection can be affected in individual intake ports, the problem of fuel distribution
among various cylinders will automatically be avoided.
The emissions on deceleration can be completely removed by shutting off the fuel supply when the
throttle is closed. But this system will still not be able to control the HC emission. Combination of
electronic injection and ammonia as an exhaust additive has an attractive future.
MEASUREMENT TECHNIQUES EMISSION STANDARDS AND TEST PROCEDURE
VEHICLE EMISSION STANDARDS:
Federal exhaust emission test procedures for light duty vehicles under 6000 lb GVW covering the
period 1972 to 1975 assess hydrocarbon, carbon monoxide and nitric oxide emissions in terms of mass of
emission emitted over a 7.5 mile chassis dynamometer driving cycle. Results are expressed as grams of
pollutant emitted per mile.
There are two procedures in using the same test equipment which assess vehicle emissions. One,
which is termed as CVS-1 (constant volume sampling), employs a single bag to collect a representative
portion of the exhaust for subsequent analysis. This single bag system applied to testing of 1972, 1973 and
1974 vehicles. Based on this test, emission standards for vehicles have been set at
Hydrocarbons

3.4 g/mile ( 1972 to 1974 )

Carbon monoxide

3.9 g/mile ( 1972 to 1974 )

Oxide of nitrogen

3.0 g/mile ( 1973 to 1974 )

The second test procedure, termed CVS-3 uses three sampling bags and is designed to give a
reduced and more realistic weighing to cold start portion of the test. This three bag system applies to
testing of 1975 to 1976 vehicles. Exhaust emission standards based on this test are
Hydrocarbons

0.41 g/mile ( 1975 to 1976 )

Carbon monoxide

3.4 g/mile ( 1975 to 1976)

Oxide of nitrogen

3.0 g/mile ( 1975 )

One of the latest U.S standards ( 1982) for passenger cars and equivalents are
Hydrocarbons

0.41 g/mile

Carbon monoxide

3.4 g/mile

Oxide of nitrogen

1.5 g/mile

These are measured by following a prescribed test procedure.

Driving Cycle:

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The driving cycle for both CVS-1 and CVS-3 cycles is identical. It involves various accelerations,
decelerations and cruise modes of operation. The car is started after soaking for 12 hours in a 60-80 F
ambient. A trace of the driving cycle is shown in figure. Miles per hour versus time in seconds are plotted
on the scale. Top speed is 56.7 mph. Shown for comparison is the FTP or California test cycle. For many
advanced fast warm-up emission control systems, the end of the cold portion on the CVS test is the second
idle at 125 seconds. This occurs at 0.68 miles. In the CVS tests, emissions are measured during cranking,
start-up and for five seconds after ignition is turned off following the last deceleration. Consequently high
emissions from excessive cranking are included. Details of operation for manual transmission vehicles as
well as restart procedures and permissible test tolerance are included in the Federal Registers.

CVS-1 system:
The CVS-1 system, sometimes termed variable dilution sampling, is designed to measure the true
mass of emissions. The system is shown in figure. A large positive displacement pump draws a constant
volume flow of gas through the system. The exhaust of the vehicle is mixed with filtered room air and the
mixture is then drawn through the pump. Sufficient air is used to dilute the exhaust in order to avoid
vapour condensation, which could dissolve some pollutants and reduce measured values. Excessive
dilution on the other hand, results in very low concentration with attendant measurement problems. A
pump with capacity of 30-350 cfm provides sufficient dilution for most vehicles.
Before the exhaust-air mixture enters the pump, its temperature is controlled to within +or – 10F
by the heat exchanger. Thus constant density is maintained in the sampling system and pump. A fraction of
the diluted exhaust stream is drawn off by a pump P2 and ejected into an initially evacuated plastic bag.
Preferably, the bag should be opaque and manufactured of Teflon or Teldar. A single bag is used for the
entire test sample in the CVS-1 system.
Because of high dilution, ambient traces of HC, CO or NOx

can significantly increase

concentrations in the sample bag. A charcoal filter is employed for leveling ambient HC measurement. To
correct for ambient contamination a bag of dilution air is taken simultaneously with the filling of the
exhaust bag.
HC, CO and NOx measurements are made on a wet basis using FID, NDIR and chemiluminescent
detectors respectively. Instruments must be constructed to accurately measure the relatively low
concentrations of diluted exhaust.

124

Bags should be analyzed as quickly as possible preferably within ten minutes after the test because
reactions such as those between NO, NO2 and HC can occur within the bag quite quickly and change the
test results.
CVS-3 SYSTEM:
The CVS-3 system is identical to the CVS-1 system except that three exhaust sample bags are
used. The normal test is run from a cold start just like the CVS-1 test. After deceleration ends at 505
seconds, the diluted exhaust flow is switched from the transient bag to the stabilized bag and revolution
counter number 1 is switched off and number 2 is activated. The transient bag is analyzed immediately.
The rest of the test is completed in the normal fashion and the stabilized bag analyzed. However in the
CVS-3 test ten minutes after the test ends the cycle is begun and again run until the end of deceleration at
505 seconds. This second run is termed the hot start run. A fresh bag collects what is termed the hot
transient sample. It is assumed that the second half of the hot start run is the same as the second half of the
cold start run and is not repeated. In all, three exhaust sample bags are filled. An ambient air sample bag is
also filled simultaneously.
STANDARDS IN INDIA:
The Bureau of Indian Standards ( BIS ) is one of the pioneering organizations to initiate work on
air pollution control in India. At present only the standards for the emission of carbon monoxide are being
suggested by BIS given in IS:9057-1986. These are based on the size of the vehicle and to be measured
under idling conditions. The CO emission values are 5.5 percent for 2 or 3 wheeler vehicles with engine
displacement of 75cc or less, 4.5 percent for higher sizes and 3.5 percent for four wheeled vehicles.
IS: 8118-1976 Smoke Emission Levels for Diesel vehicles prescribes the smoke limit for diesel
engine as 75 Hatridge units or 5.2 Bosch units at full load and 60-70 percent rated speed or 65 Hatridge
units under free acceleration conditions.
EMISSION MEASURING INSTRUMENTS:
Terms of expressions:
In emission measurement, volume concentrations of the several components are characteristically
expressed in the following terms
1. Carbon dioxide and carbon monoxide are expressed as percent of the sample volume.
2. Nitric oxide and nitrogen dioxide are expressed as volume parts of NO or NO2 per million parts
of the sample ( ppm). The total of NO and NO2 is designated as NOx.
3. Hydrocarbon is expressed as i. Parts of hydrocarbon per million parts of the sample or ii. Parts
of carbon per million parts of the sample ( ppmc). The latter term is defined as the volume concentration of
hydrocarbon in the sample multiplied by the average number of carbon atoms per molecule of that
hydrocarbon. Thus 1ppm propane ( C3H8) is the equivalent of 3ppm C hydrocarbon. In the early days of

125

emission measurement hydrocarbon emissions were measured in terms of the carbon equivalent of hexane
or ppm hexane. Thus in early usage ppm values were often assumed to be ppm hexane even though not

126

designated as hexane, this usage is ambiguous and should be avoided.

127

128

Flame Ionisation Detector ( FID ):
The unburned hydrocarbons in the exhaust consist of about 200 different compounds, each with different
composition and different number of carbon and hydrogen atoms. It is impossible to detect each of these
hydrocarbons separately. The over all concentration of the unburned hydrocarbons may be found by
measuring the equivalent concentration of n-hexane ( C6H14). An accurate method of measuring the
unburned hydrocarbon emissions is to use the Flame Ionisation Detector ( FID ).
The working principle of FID is as follows: A hydrogen-air flame contains a negligible amount of
ions but if few hydrocarbon molecules are introduced into the flame a larger number of ions are produced.
The ion yield is proportional to the amount of hydrocarbon introduced into the flame.
The basic elements of a Flame Ionisation Detector are as shown in the figure, a burner and ion
collector assembly. In practice, a sample of gas is mixed with hydrogen in the burner assembly and the
mixture burned in a diffusion flame. Ions that are produced in the flame move to the negatively polarized
collector under the influence of an electrical potential applied between the collector plates. At the negative
collector, the ions receive, via a current network, electrons that are collected from the flame zone at the
positive collector. Thus a small current proportional to the amount of hydrocarbon entering the flame flows
between the collector plates. This small current is amplified using a high impedance direct current
amplifier, the output of which becomes an indication of hydrocarbon present.
The detector responds to carbon that is linked with hydrogen as in equation 1 and the response is
largely independent of the molecular configuration, i.e hydrocarbon species. Thus the detector is
essentially a carbon atom counter.
The output of the FID depends on the number of carbon atoms passing through the flame in a unit time.
Doubling the flow velocity would also double the output. Hexane
( C6H14) would give double the output of propane ( C3H8). Therefore FID output is usually referred to a
standard hydrocarbon usually as PPM of normal hexane.
Characteristics of the FID are improved with most burned designs if instead of using pure
hydrogen fuel, the hydrogen is mixed with inert gas to decrease flame temperature. This mixture of
hydrogen and inert gas is referred to as fuel gas or fuel.
The FID responds directly to the amount of hydrocarbon entering the flame. Therefore close
control of sample flow is required. In general, the sample flow rate is specified at the minimum amount
that will give the required sensitivity in any given instrument. Fuel and air flow rates also influence the
response characteristics of the detector. Response typically first rises and then fall with increased fuel rate,
as shown in the figure. Typical volume rates of instrument gases are sample 3-5 ml/min and fuel gas
mixture 75ml/min and air 200ml/min.

129

Presence of CO, CO2, NOx, water and nitrogen in the exhaust have no effect on the FID reading.
FID analyzer is rapid, continuous and accurate method of measuring HC in the exhaust gas
concentrations as low as 1ppb can be measured.
CHEMILUMINESCENT NOX ANALYZER
Chemiluminescence, a chemical reaction once dismissed as a laboratory curiosity, has become the most
widely used NOx emissions monitoring technique in stack emissions and ambient air-monitoring
instrumentation. More than 95% of the NOx CEMS used by the electric utility industry, and approximately
99% of the NOx analyzers used for EPA Reference Method 7E and 20 testing employ the
chemiluminescence measurement technology. The basic chemiluminescence chemistry was delineated by
Clough et al.
NO + O3 → NO2+ O2 (Eq. 1)
NO + O3 → NO2 + O2 (Eq. 2)
NO2 → NO2 + hv (~600 to 3,000nm) (Eq. 3)
NO2 + M → NO2 + M (Eq. 4)
hv = photons, M = any compound
Equations 1 and 3 describe the technique employed in commercial instrumentation to measure nitrogen
oxides. When NO reacts with O 3, some electronically-excited NO2 molecules are produced (Equation 1).
These molecules may give off energy in the form of light emission with intensity linearly proportional to
the concentration of NO (Equation 3). When the emitted radiation is monitored, it becomes a measure of
the concentration of the NO in the reacting sample. The light emission occurs between 600 and 3,000 nm,
with a peak at about 1,200 nm. Chemiluminescence NO x analyzers measure NO concentrations by using a
bandpass filter to select light in the region from about 600 to 900 nm.
The reactions described by equations 2 and 4 describe a potential limitation to the measurement technique.
Only a fraction of the NO reacts to form NO 2 and emits light. Fortunately, the percentage of NO in the
reactor that follows the pathway described by equations 1 and 3 is sufficient to ensure a proportionally
linear response in a properly designed instrument. By maintaining the O 3 concentration at a large excess to
the NO concentration and with a detection system designed for optimum light-collection efficiency,
analyzers have been developed with linear dynamic ranges over 10,000 ppm and with detection limits at
the parts per trillion level.

In order to measure NO 2 using the same basic mechanism outlined by Equations 1 through 4, NO 2 is first
converted to NO (i.e., NO2 to NO converter), after which the converted molecules react with ozone along
with the original NO molecules in the sample. This results in a signal that equals the sum of NO and NO 2.
By taking the difference between the converted and non-converted modes, a measure of the NO 2
concentration is obtained.
The major advantages of chemiluminescence method over other measurement methods for NO x monitoring
include:
• Increased sensitivity (detection limit)
• Improved specificity (accuracy)
• Rapid response time (control)
• Linearity over a wide dynamic range (precision)
• Continuous monitoring (control and reporting)
• Simplicity of design (maintenance)

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Basic NO-NO2-NOx Chemiluminescence Instrumentation
The simplified diagram of a NO-NO2-NOx chemiluminescence analyzer as previously described. To
measure NO concentrations, the sample mixes with ozone in a flow reactor. The ozone required for the
reaction is produced within the instrument from dry air or oxygen. The luminescence that results from the
reaction of NO with O3 is monitored through an optical bandpass filter by a high-sensitivity detector
positioned at the end of the reactor. The bandpass filter/detector combination responds to the light
emission in a narrow-wavelength band.
Figure 1 shows the spectral output of the chemiluminescence reaction (Trace 1) along with optical filter
(Trace 2) and typical detector (photomultiplier tube [PMT]) response (Trace 3) characteristics. As can be
seen, only a small portion of the spectrum is monitored. This aids in gaining specificity for the analyzer.
An electronic package takes the detector output signal and processes it to voltage, current level, or digital
signal.
The sample inlet generally has two flow modes. The first (NO mode) is a direct path of sample to the
reaction chamber. Ideally, only the NO in the sample reacts with the ozone to produce light emission
(chemiluminescence). In the second mode (NO x mode), the sample is routed through a converter that
transforms the NO2 to NO.

Figure 2 – Basic NO-NO2-NOx Chemiluminescence Instrumentation
DESCRIPTION OF A TYPICAL SAMPLE GAS FLOW SYSTEM
Figure 2 shows a flow diagram of a typical analyzer. The chemiluminescence method relies on the
measurement of the number of NO molecules entering the reaction chamber per time. In order to maintain
instrument stability, two gas flow systems are used to regulate a constant flow of ozone and sample gas to
the reaction chamber.
In Figure 3, a single capillary regulates the ozone flow. The function of the capillary is to restrict the flow
in the line such that a controllable backpressure is maintained. The pressure upstream of the capillary and
the capillary dimensions, and pump characteristics determine the flow. Other flow regulation schemes are
possible. Mass flow controller, critical orifices, and the regulation system describe above have been used.
All regulation systems are designed for the same purpose: to maintain a constant air or oxygen flow
through the ozonator. The prime requirement of the flow system is to maintain a constant flow of the
sample gas to the analyzer as the chemiluminescent reaction is extremely flow sensitive.
Figure 3 presents one standard industrial approach to fulfill these conditions. The total sample passes into
the analyzer through the inlet capillary restrictor. A fraction (about 5%) is bled off through a second
smaller restrictor and directed to a mode valve. The portion of the sample bypassing the mode valve (and
reaction chamber) exits the instrument through a bypass pump. The sample regulator functions to maintain
a constant pressure drop across the smaller capillary. This method of flow control maintains a reaction

131

chamber sample flow rate that is insensitive to the sample inlet pressure and total sample flow rate. This
type of bypass flow system has the additional advantage of bring the sample to the analyzer. It has the
disadvantage of using a fairly large sample flow, which can be a limitation in smog chamber or bag sample
studies. For typical process applications, however, there is no sample limitation.
Not all chemiluminescent analyzers use such a bypass-flow technique. Most ambient-air analyzers use
only one sample restrictor and no bypass flow. Nevertheless, sample flow remains one of the most critical
parameters to control. Mass flow controllers, critical orifices, and/or a regulator and capillary system using
a pressure sample have been used.
Figure 3 – Basic Chemiluminescence Analyzer Flow Diagram

DRIVING CYCLES IN UNIT-3
ENGINE EXHAUST EMISSION CONTROL
EMISSION (HC,CO,NO,NOX) MEASURING EQUIPMENTS
Vehicles generate potentially harmful and toxic emissions through their exhaust pipes,
especially when they are not properly maintained. Many states require that vehicles pass emissions testing
before they are deemed road-worthy. Special instrumentation is used to analyze emissions and detect gases
like nitrogen oxides (NOx), carbon dioxide (CO2), carbon monoxide (CO) and hydrocarbons(HC)
Gas Analyzers
Gas pipe exhaust is measured using special equipment such as the range of Dyne System five-gas
portable analyzers. These instruments test and measure the amount of engine exhaust for specific gas
components, including CO, CO2, NO, HC and oxygen. Equipment is first calibrated using ambient air

132

prior to each use to ensure accurate measurements. Analyzers feature a sample line to collect exhaust gases
and a two-stage filter system that is capable of detecting and capturing both large and small particles.
Systems have touch-screen interfaces that offer control over manual operations. Wall-mounted enclosures
are optional equipment to house instruments when not in use.
Dynamometer
A dynamometer is an electronic roller device used inside vehicle inspection bays to measure
tailpipe emissions. The test vehicle is driven onto the dynamometer rollers and then testing begins. This
instrument simulates the actual exhaust that the vehicle produces while in driving at low speed (15 mph)
and when accelerating, all while still sitting in the bay. It's used in conjunction with a five-gas analyzer for
instant data readout.
Mostly with the help of engine &chassis dyanamometers analysising of emissions are carried out.

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Chassis & engine dyanamometer used in automobile for measuring emission
Engine dynamometer
An engine dynamometer measures power and torque directly from the engine's crankshaft (or flywheel),
when the engine is removed from the vehicle. These dynos do not account for power losses in the
drivetrain, such as the gearbox, transmission or differential etc
Chassis dynamometer

134

Emissions development and homologation dynamometer test systems often integrate emissions
sampling, measurement, engine speed and load control, data acquisition, and safety monitoring into a
complete test cell system. These test systems usually include complex emissions sampling equipment
(such as constant volume samplers or raw exhaust gas sample preparation systems), and exhaust emissions
analyzers. These analyzers are much more sensitive and much faster than a typical portable exhaust gas
analyzer. Response times of well under one second are common and required by many transient test cycles
Scanning Tools
Many vehicles manufactured post-1980, and all models manufactured after 1996 have some form of
on-board computerized diagnostic systems (OBDs). These computers can monitor vehicular subsystems
like exhaust sensors and oxygen sensors. Scanning tools developed to exploit OBD technology can be
readily connected to these on-board computers to test emissions. Hand-held electronic scanning
instruments can provide real-time assessments when measuring vehicular emissions.
Gas Cap Testing
Gas cap testing checks for leaks from around vehicle gas cap seals. Invisible gasoline or diesel
fumes are volatile gases and seepage into the atmosphere may go otherwise undetected if inspections are
not made. According to the Illinois Environmental Protection Agency, about 40 percent of hydrocarbon
emissions in the air is due to evaporation of gasoline from leaky gas caps. The instrument used for testing
consists of a short, wide tube with a pressurized gauge attached. One end of the tube connects to the
uncovered gas cap on the vehicle, and the gas cap that was removed is screwed onto the other end.
Pressure is applied and the reading on the gauge reveals any leaks.
INDIAN DRIVING CYCLES & EMISSION NORMS
Table 1: Indian Emission Standards (4-Wheel Vehicles)
Standard

Reference

Date

Region

India 2000

Euro 1

2000

Nationwide

2001

NCR*, Mumbai, Kolkata, Chennai

2003.04

NCR*, 13 Cities†

2005.04

Nationwide

2005.04

NCR*, 13 Cities†

2010.04

Nationwide

2010.04

NCR*, 13 Cities†

Bharat Stage II

Euro 2

Bharat Stage III

Euro 3

Bharat Stage IV

Euro 4

* National Capital Region (Delhi)
† Mumbai, Kolkata, Chennai, Bengaluru, Hyderabad, Ahmedabad, Pune, Surat, Kanpur, Lucknow,
Sholapur, Jamshedpur and Agra
The above standards apply to all new 4-wheel vehicles sold and registered in the respective regions. In
addition, the National Auto Fuel Policy introduces certain emission requirements for interstate buses with
routes originating or terminating in Delhi or the other 10 cities.
For 2-and 3-wheelers, Bharat Stage II (Euro 2) will be applicable from April 1, 2005 and Stage III (Euro 3)
standards would come in force from April 1, 2010.
TRUCKS AND BUSES

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Exhaust gases from vehicles form a significant portion of air pollution which is harmful to human health
and the environment
Emission standards for new heavy-duty diesel engines—applicable to vehicles of GVW > 3,500 kg—are
listed in Table 2.
Table 2 Emission Standards for Diesel Truck and Bus Engines, g/kWh

Year

Reference

Test

CO

HC

NOx

PM

1992

-

ECE R49

17.3-32.6

2.7-3.7

-

-

1996

-

ECE R49

11.20

2.40

14.4

-

2000

Euro I

ECE R49

4.5

1.1

8.0

0.36*

2005†

Euro II

ECE R49

4.0

1.1

7.0

0.15

2010†

Euro III

ESC

2.1

0.66

5.0

0.10

ETC

5.45

0.78

5.0

0.16

2010‡

Euro IV

ESC

1.5

0.46

3.5

0.02

ETC

4.0

0.55

3.5

0.03

* 0.612 for engines below 85 kW
† earlier introduction in selected regions, see Table 1 ‡ only in selected regions, see Table 1
More details on Euro I-III regulations can be found in the EU heavy-duty engine standards page.
LIGHT DUTY DIESEL VEHICLES
Emission standards for light-duty diesel vehicles (GVW ≤ 3,500 kg) are summarized in Table 3. Ranges of
emission limits refer to different classes (by reference mass) of light commercial vehicles; compare the EU
light-duty vehicle emission standards page for details on the Euro 1 and later standards. The lowest limit in
each range applies to passenger cars (GVW ≤ 2,500 kg; up to 6 seats).
Table 3 Emission Standards for Light-Duty Diesel Vehicles, g/km

Year Reference CO

HC

HC+NOx NOx

PM

1992 -

17.3-32.6 2.7-3.7 -

-

-

1996 -

5.0-9.0

-

-

2000 Euro 1

2.72-6.90 -

-

2.0-4.0

0.97-1.70 0.14-0.25 -

136

2005† Euro 2

1.0-1.5

2010† Euro 3

0.64
0.80
0.95

2010‡ Euro 4

0.50
0.63
0.74

-

0.7-1.2

0.08-0.17 -

-

0.56
0.72
0.86

0.50
0.65
0.78

0.05
0.07
0.10

-

0.30
0.39
0.46

0.25
0.33
0.39

0.025
0.04
0.06

† earlier introduction in selected regions, see Table 1
‡ only in selected regions, see Table 1

The test cycle has been the ECE + EUDC for low power vehicles (with maximum speed limited to
90 km/h). Before 2000, emissions were measured over an Indian test cycle.
Engines for use in light-duty vehicles can be also emission tested using an engine dynamometer. The
respective emission standards are listed in Table 4.
Table 4 Emission Standards for Light-Duty Diesel Engines, g/kWh
Year

Reference

CO

HC

NOx

PM

1992

-

14.0

3.5

18.0

-

1996

-

11.20

2.40

14.4

-

2000

Euro I

4.5

1.1

8.0

0.36*

2005†

Euro II

4.0

1.1

7.0

0.15

* 0.612 for engines below 85 kW
† earlier introduction in selected regions, see Table 1
[edit]Light duty gasoline vehicles
[edit]4-wheel vehicles
Emissions standards for gasoline vehicles (GVW ≤ 3,500 kg) are summarized in Table 5. Ranges of
emission limits refer to different classes of light commercial vehicles (compare the EU light-duty vehicle
emission standards page). The lowest limit in each range applies to passenger cars (GVW ≤ 2,500 kg; up to
6 seats).
Table 5 Emission Standards for Gasoline Vehicles (GVW ≤ 3,500 kg), g/km
Year

Reference

CO

HC

HC+NOx

NOx

1991

-

14.3-27.1

2.0-2.9

-

1996

-

8.68-12.4

-

3.00-4.36

1998*

-

4.34-6.20

-

1.50-2.18

2000

Euro 1

2.72-6.90

-

0.97-1.70

2005†

Euro 2

2.2-5.0

-

0.5-0.7

2010†

Euro 3

2.3
4.17
5.22

0.20
0.25
0.29

-

0.15
0.18
0.21

2010‡

Euro 4

1.0

0.1

-

0.08

137

1.81
2.27

0.13
0.16

0.10
0.11

* for catalytic converter fitted vehicles
† earlier introduction in selected regions, see Table 1 ‡ only in selected regions, see Table 1
Gasoline vehicles must also meet an evaporative (SHED) limit of 2 g/test (effective 2000).
3- and 2-wheel vehicles
Emission standards for 3- and 2-wheel gasoline vehicles are listed in the following tables.
Table 6 Emission Standards for 3-Wheel Gasoline Vehicles, g/km
Year

CO

HC

HC+NOx

1991

12-30

8-12

-

1996

6.75

-

5.40

2000

4.00

-

2.00

2005 (BS II)

2.25

-

2.00

2010.04 (BS III)

1.25

-

1.25

Table 7 Emission Standards for 2-Wheel Gasoline Vehicles, g/km
Year

CO

HC

HC+NOx

1991

12-30

8-12

-

1996

5.50

-

3.60

2000

2.00

-

2.00

2005 (BS II)

1.5

-

1.5

2010.04 (BS III)

1.0

-

1.0

Table 8 Emission Standards for 2- And 3-Wheel Diesel Vehicles, g/km
Year

CO

HC+NOx

PM

2005.04

1.00

0.85

0.10

2010.04
0.50
0.50
Overview of the emission norms in India

0.05

1991 - Idle CO Limits for Gasoline Vehicles and Free Acceleration Smoke for Diesel Vehicles, Mass
Emission Norms for Gasoline Vehicles.
1992 - Mass Emission Norms for Diesel Vehicles.
1996 - Revision of Mass Emission Norms for Gasoline and Diesel Vehicles, mandatory fitment of
Catalytic Converter for Cars in Metros on Unleaded Gasoline.
1998 - Cold Start Norms Introduced.

138

2000 - India 2000 (Equivalent to Euro I) Norms, Modified IDC (Indian Driving Cycle), Bharat Stage II
Norms for Delhi.
2001 - Bharat Stage II (Equivalent to Euro II) Norms for All Metros, Emission Norms for CNG & LPG
Vehicles.
2003 - Bharat Stage II (Equivalent to Euro II) Norms for 13 major cities.
2005 - From 1 April Bharat Stage III (Equivalent to Euro III) Norms for 13 major cities.
2010 - Bharat Stage III Emission Norms for 4-wheelers for entire country whereas Bharat Stage - IV
(Equivalent to Euro IV) for 13 major cities. Bharat Stage IV also has norms on OBD (similar to Euro III
but diluted)
CO2 emission
India’s auto sector accounts for about 18 per cent of the total CO 2 emissions in the country. Relative
CO2 emissions from transport have risen rapidly in recent years, but like the EU, currently there are no
standards for CO2 emission limits for pollution from vehicles.
Obligatory labeling
There is also no provision to make the CO 2 emissions labeling mandatory on cars in the country. A
system exists in the EU to ensure that information relating to the fuel economy and CO 2 emissions of new
passenger cars offered for sale or lease in the Community is made available to consumers in order to
enable consumers to make an informed choice.

QUESTION BANK OF UNIT III
PART A
1. What are the harmful effects of various pollutants on human beings?
2. How unburnt hydrocarbons are formed in SI engines?
3. What is the effect of inlet air temperature on “NO” formation in SI engines?
4. How aldehyde emissions are formed in CI engines?
5. What is the effect of quantity of fuel injected on black smoke formation in SI engines?
6. What is a thermal reactor? What for it is used?
7. Name few catalysts used for oxidation and reduction of emissions in IC Engine exhaust.
8. What are the sources of pollution in I.C. Engine?
9. What are the factors contributing to NOx emission?
10. What is particulate emission?
11. What is aldehyde emission?
12. What is meant by fumigation?
13. What is gas chromatograph.
14. What are the major emissions that come out of engine exhaust.
15. What are the causes of HC emissions from SI engine.
16. What are the causes of incomplete combustion in an engine.
17. What is crankcase blow-by.
18. Write a short notes on HC emission in CI engine.

139

19. What are various Non-Exhaust emissions in CI engine.
20. What is positive crankcase ventilation.
21. What are the major emissions that come out of engine exhaust.
22. How oxides of Nitrogen are formed.
23. What are the causes of HC emissions from SI engine.
24. What are the causes of incomplete combustion in an engine.
25. What is crankcase blow-by.
26. Write a short notes on HC emission in CI engine.
27. What are various Non-Exhaust emissions in CI engine.
28. What is positive crankcase ventilation
29. What are the sources from which pollutants are emitted from SI engine.
30. How compression ratio affects emission control in SI engine.
31. What are the basic requirements of a catalytic converter.
32. What is a three way catalytic converter.
33. What are the various ways of controlling Oxides of nitrogen in exhaust emission
34. Write a short notes on Green house effect.
35. What is Diesel Smoke
36. What are the various types of smoke meters.
37. What is the effect of CO emission on human health.
38. What are the factors that affect the formation of NOx.
39. How are Hydro Carbons formed
Part B
1. Explain in detail about the various sources of IC engine responsible for emission and pollutant
formation.
2. Illustrate and explain the effects of combustion time and spark timing on “NO” formation in SI engines.
3. How the carbon monoxide is formed in SI engines? Explain the effect of O 2 concentration on “CO”
formation in SI engines.
4. Explain in detail about the combined effect of engine speed, cetane number and combustion time on
nitric oxide formation in CI engines.
5. Explain in detail about the following:
(i) Blake smoke
(ii) White smoke
(iii) Blue smoke
(iv) Particulates.
6. With the aid of a neat cross sectional sketch explain the construction, working and limitations of a 3 way
catalytic converter.
7. Explain in detail about the open and closed type PVC system. Also state that what for it is used.
8. With the help of a schematic diagram explain the working of CLA (Chemiluminescent analyzer). What
for it is used?
9. Explain the working of an instrument used for the measurement of CO and CO 2 from IC engines.
10. What are the operating variables and how they contribute on emission formation? How can it be
controlled?
11.Differentiate smoke and particulate emission. Explain the diffusion funnel method to measure
particulate emission in a diesel engine.
12. Explain different methods of smoke measurement.
13. (i)Explain PVC system in a diesel engine.

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(ii)What is the necessity of exhaust gas re-circulation? Explain in detail.
14. (i) What are thermal reactors? Explain their constructions.
(ii) Explain three way catalytic converter.

UNIT IV
ALTERNATE FUELS

141

142

143

144

145

146

147

148

149

150

151

152

Problems with alcohol blends :

Advantages of alcohol as Fuel :

153

Fumigation :

Surface ignition of ethanol :

154

155

156

Hydrogen
Advantages of Hydrogen as SI engine fuel

Disadvantages of Hydrogen as SI engine fuel

157

Combustion properties of hydrogen as fuel

158

Various ways of using Hydrogen as fuel for SI engine

159

160

161

Natural Gas

Methods of using Natural gas in engines

162

Conversion kits for use of natural gas in CI engine :

163

164

165

166

167

Liquefied Petroleum Gas

168

Bio Gas

Systems required for use of Bio Gas in engines

169

Use of Bio gas in diesel engine :

170

171

172

173

Bio – Diesel

Esterification

174

Transesterification

Advantages of bio diesel

175

Engine modifications:

176

Esterification process

177

Emission characteristics

178

QUESTION BANK OF UNIT IV
Part A
1. Why alcohols are considered as good SI engine fuels.

179

2. Mention important property changes that take place by the addition of alcohol to diesel.
3. Define Fumigation
4. What are the advantages of Hydrogen as fuel in SI engine
5. What are the demerits of using Hydrogen as fuel in SI engine
6. What are the methods of using Natural gas in CI engines.
7. What are the advantages of using LPG in engines.
8. What are the limitations of using LPG as fuel in IC engines.
9. Give the general composition of Bio gas.
10. What is Bio diesel.
11. What is meant by esterification.
12. What is meant by Transesterification.
13. What are the advantages of Bio diesel.
14. Name some alternative fuels for IC engine.
15. What is the reason for development of alternative fuels.
16. What are the advantages of using alternative fuels
17. What are the disadvantages of alternative fuels.
18. What is volatility
19. Why volatility is an important quality of SI engine fuel.
20. What are the advantages of alcohol as a fuel.
21. What are the advantages of using Bio-diesel as fuel for IC engines.
22. What are the disadvantages of using Bio-diesel as fuel for IC engines.
23. What are the properties of Biogas.
24. What are the advantages of using Biogas as fuel.
25. What are the disadvantages of using Biogas as fuel for IC engine.
26. What are primary fuels
27. What are secondary fuels.
28. What is the effect of fuel viscosity on diesel engine performance.

Part B
1. What are the methods of using alcohol fuels in diesel engine and explain any one method.
2. What are the methods of using Hydrogen in SI engine.
3. Discuss the emission and performance characteristics using hydrogen in SI engines.
4. Explain the conversion kit required for use of natural gas in CI engines.
5. Explain how Biogas is used in diesel engine. Discus the performance characteristics.
6. What is the engine modification required for using Biodiesel in existing diesel engine.
7. Discuss the suitability of Alcohols in diesl engines
8. With a schematic diagram explain the production of Methanol from coal
9. With a schematic diagram explain the production of Methanol from Municipal solid waste
10. Explain the engine performance of Ethanol over a petrol engine.

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UNIT V
RECENT TRENDS
HCCI engine
Homogeneous charge compression ignition (HCCI) is a form of internal combustion in which well-mixed
fuel and oxidizer (typically air) are compressed to the point of auto-ignition. As in other forms of
combustion, this exothermic reaction releases chemical energy into a sensible form that can be transformed
in an engine into work and heat.
Introduction
HCCI has characteristics of the two most popular forms of combustion used in SI engines: homogeneous
charge spark ignition (gasoline engines) and CI engines: stratified charge compression ignition (diesel
engines). As in homogeneous charge spark ignition, the fuel and oxidizer are mixed together. However,
rather than using an electric discharge to ignite a portion of the mixture, the density and temperature of the
mixture are raised by compression until the entire mixture reacts spontaneously. Stratified charge
compression ignition also relies on temperature and density increase resulting from compression, but
combustion occurs at the boundary of fuel-air mixing, caused by an injection event, to initiate combustion.
The defining characteristic of HCCI is that the ignition occurs at several places at a time which makes the
fuel/air mixture burn nearly simultaneously. There is no direct initiator of combustion. This makes the
process inherently challenging to control. However, with advances in microprocessors and a physical
understanding of the ignition process, HCCI can be controlled to achieve gasoline engine-like emissions
along with diesel engine-like efficiency. In fact, HCCI engines have been shown to achieve extremely low
levels of Nitrogen oxide emissions (NO x) without an aftertreatment catalytic converter. The unburned
hydrocarbon and carbon monoxide emissions are still high (due to lower peak temperatures), as in gasoline
engines, and must still be treated to meet automotive emission regulations.
Recent research has shown that the use of two fuels with different reactivities (such as gasoline and diesel)
can help solve some of the difficulties of controlling HCCI ignition and burn rates. RCCI or Reactivity
Controlled Compression Ignition has been demonstrated to provide highly efficient, low emissions
operation over wide load and speed ranges *.
HCCI engines have a long history, even though HCCI has not been as widely implemented as spark
ignition or diesel injection. It is essentially an Otto combustion cycle. In fact, HCCI was popular before
electronic spark ignition was used. One example is the hot-bulb engine which used a hot vaporization
chamber to help mix fuel with air. The extra heat combined with compression induced the conditions for
combustion to occur. Another example is the "diesel" model aircraft engine.

181

Operation
Methods
A mixture of fuel and air will ignite when the concentration and temperature of reactants is sufficiently
high. The concentration and/or temperature can be increased by several different ways:

182

•
•

High compression ratio
Pre-heating of induction gases

•

Forced induction

•

Retained or re-inducted exhaust gases

•

Once ignited, combustion occurs very quickly. When auto-ignition occurs too early or with too much
chemical energy, combustion is too fast and high in-cylinder pressures can destroy an engine. For this
reason, HCCI is typically operated at lean overall fuel mixtures.

Advantages

•
•

HCCI provides up to a 30-percent fuel savings, while meeting current emissions standards.
Since HCCI engines are fuel-lean, they can operate at a Diesel-like compression ratios (>15), thus
achieving higher efficiencies than conventional spark-ignited gasoline engines.

•

Homogeneous mixing of fuel and air leads to cleaner combustion and lower emissions. Actually,
because peak temperatures are significantly lower than in typical spark ignited engines, NOx levels
are almost negligible. Additionally, the premixed lean mixture does not produce soot.

•

HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels.

•

In regards to gasoline engines, the omission of throttle losses improves HCCI efficiency.

Disadvantages
•
•

High in-cylinder peak pressures may cause damage to the engine.
High heat release and pressure rise rates contribute to engine wear.

•

The autoignition event is difficult to control, unlike the ignition event in spark ignition (SI) and
diesel engines which are controlled by spark plugs and in-cylinder fuel injectors, respectively.

•

HCCI engines have a small power range, constrained at low loads by lean flammability limits and
high loads by in-cylinder pressure restrictions.

•

Carbon monoxide (CO) and hydrocarbon (HC) pre-catalyst emissions are higher than a typical
spark ignition engine, caused by incomplete oxidation (due to the rapid combustion event and low
in-cylinder temperatures) and trapped crevice gases, respectively.

Control

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Controlling HCCI is a major hurdle to more widespread commercialization. HCCI is more difficult to
control than other popular modern combustion engines, such as Spark Ignition (SI) and Diesel. In a typical
gasoline engine, a spark is used to ignite the pre-mixed fuel and air. In Diesel engines, combustion begins
when the fuel is injected into compressed air. In both cases, the timing of combustion is explicitly
controlled. In an HCCI engine, however, the homogeneous mixture of fuel and air is compressed and
combustion begins whenever the appropriate conditions are reached. This means that there is no welldefined combustion initiator that can be directly controlled. Engines can be designed so that the ignition
conditions occur at a desirable timing. To achieve dynamic operation in an HCCI engine, the control
system must change the conditions that induce combustion. Thus, the engine must control either the
compression ratio, inducted gas temperature, inducted gas pressure, fuel-air ratio, or quantity of retained or
re-inducted exhaust. Several control approaches are discussed below.

Variable compression ratio
There are several methods for modulating both the geometric and effective compression ratio. The
geometric compression ratio can be changed with a movable plunger at the top of the cylinder head. This is
the system used in "diesel" model aircraft engines. The effective compression ratio can be reduced from
the geometric ratio by closing the intake valve either very late or very early with some form of variable
valve actuation (i.e. variable valve timing permitting Miller cycle). Both of the approaches mentioned
above require some amounts of energy to achieve fast

responses. Additionally, implementation is expensive. Control of an HCCI engine using variable
compression ratio strategies has been shown effective. The effect of compression ratio on HCCI
combustion has also been studied extensively.

Variable induction temperature
In HCCI engines, the autoignition event is highly sensitive to temperature. Various methods have been
developed which use temperature to control combustion timing. The simplest method uses resistance
heaters to vary the inlet temperature, but this approach is slow (cannot change on a cycle-to-cycle basis).
Another technique is known as fast thermal management (FTM). It is accomplished by rapidly varying the
cycle to cycle intake charge temperature by rapidly mixing hot and cold air streams. It is also expensive to
implement and has limited bandwidth associated with actuator energy.

Variable exhaust gas percentage
Exhaust gas can be very hot if retained or re-inducted from the previous combustion cycle or cool if
recirculated through the intake as in conventional EGR systems. The exhaust has dual effects on HCCI
combustion. It dilutes the fresh charge, delaying ignition and reducing the chemical energy and engine
work. Hot combustion products conversely will increase the temperature of the gases in the cylinder and
advance ignition. Control of combustion timing HCCI engines using EGR has been shown experimentally.
Variable valve actuation

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Variable valve actuation (VVA) has been proven to extend the HCCI operating region by giving finer
control over the temperature-pressure-time history within the combustion chamber. VVA can achieve this
via two distinct methods:
•

•

Controlling the effective compression ratio: A variable duration VVA system on intake can control
the point at which the intake valve closes. If this is retarded past bottom dead center (BDC), then
the compression ratio will change, altering the in-cylinder pressure-time history prior to
combustion.
Controlling the amount of hot exhaust gas retained in the combustion chamber: A VVA system
can be used to control the amount of hot internal exhaust gas recirculation (EGR) within the
combustion chamber. This can be achieved with several methods, including valve re-opening and
changes in valve overlap. By balancing the percentage of cooled external EGR with the hot
internal EGR generated by a VVA system, it may be possible to control the in-cylinder
temperature.

While electro-hydraulic and camless VVA systems can be used to give a great deal of control over the
valve event, the componentry for such systems is currently complicated and expensive. Mechanical
variable lift and duration systems, however, although still being more complex than a standard valvetrain,
are far cheaper and less complicated. If the desired VVA characteristic is known, then it is relatively
simple to configure such systems to achieve the necessary control over the valve lift curve. Also see
variable valve timing.

Variable fuel ignition quality
Another means to extend the operating range is to control the onset of ignition and the heat release rate is
by manipulating fuel itself. This is usually carried out by adopting multiple fuels and blending them "on
the fly" for the same engine . Examples could be blending of commercial gasoline and diesel fuels ,
adopting natural gas or ethanol ". This can be achieved in a number of ways;

•

Blending fuels upstream of the engine: Two fuels are mixed in the liquid phase, one with low
resistance to ignition (such as diesel fuel) and a second with a greater resistance (gasoline), the
timing of ignition is controlled by varying the compositional ratio of these fuels. Fuel is then
delivered using either a port or direct injection event.

•

Having two fuel circuits: Fuel A can be injected in the intake duct (port injection) and Fuel B
using a direct injection (in-cylinder) event, the proportion of these fuels can be used to control
ignition, heat release rate as well as exhaust gas emissions.

Learn burn engines
Principle
A lean burn mode is a way to reduce throttling losses. An engine in a typical vehicle is sized for providing
the power desired for acceleration, but must operate well below that point in normal steady-speed
operation. Ordinarily, the power is cut by partially closing a throttle. However, the extra work done in

185

pumping air through the throttle reduces efficiency. If the fuel/air ratio is reduced, then lower power can be
achieved with the throttle closer to fully open, and the efficiency during normal driving (below the
maximum torque capability of the engine) can be higher.

The engines designed for lean burning can employ higher compression ratios and thus provide better
performance, efficient fuel use and low exhaust hydrocarbon emissions than those found in conventional
petrol engines. Ultra lean mixtures with very high air-fuel ratios can only be achieved by direct injection
engines.

The main drawback of lean burning is that a complex catalytic converter system is required to reduce NOx
emissions. Lean burn engines do not work well with modern 3-way catalytic converter—which require a
pollutant balance at the exhaust port so they can carry out oxidation and reduction reactions—so most
modern engines run at or near the stoichiometric point. Alternatively, ultra-lean ratios can reduce NOx
emissions.

Heavy-duty gas engines
Lean burn concepts are often used for the design of heavy-duty natural gas, biogas, and liquefied
petroleum gas (LPG) fuelled engines. These engines can either be full-time lean burn, where the engine
runs with a weak air-fuel mixture regardless of load and engine speed, or part-time lean burn (also known
as "lean mix" or "mixed lean"), where the engine runs lean only during low load and at high engine speeds,
reverting to a stoichiometric air-fuel mixture in other cases.

Heavy-duty lean burn gas engines admit as much as 75% more air than theoretically needed for complete
combustion into the combustion chambers. The extremely weak air-fuel mixtures lead to lower combustion
temperatures and therefore lower NOx formation. While lean-burn gas engines offer higher theoretical
thermal efficiencies, transient response and performance may be compromised in certain situations. Lean
burn gas engines are almost always turbocharged, resulting high power and torque figures not achieveable
with stoichiometric engines due to high combustion temperatures.

Heavy duty gas engines may employ pre-combustion chambers in the cylinder head. A lean gas and air
mixture is first highly compressed in the main chamber by the piston. A much richer, though much lesser
volume gas/air mixture is introduced to the pre-combustion chamber and ignited by spark plug. The flame
front spreads to the lean gas air mixture in the cylinder.

This two stage lean burn combustion produces low NOx and no particulate emissions. Thermal efficiency
is better as higher compression ratios are achieved.

Manufacturers of heavy-duty lean burn gas engines include GE Jenbacher, MAN Diesel & Turbo,
Wärtsilä, Mitsubishi Heavy Industries and Rolls-Royce plc.

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Honda lean burn systems
One of the newest lean-burn technologies available in automobiles currently in production uses very
precise control of fuel injection, a strong air-fuel swirl created in the combustion chamber, a new linear
air-fuel sensor (LAF type O2 sensor) and a lean-burn NOx catalyst to further reduce the resulting NOx
emissions that increase under "lean-burn" conditions and meet NOx emissions requirements.

This stratified-charge approach to lean-burn combustion means that the air-fuel ratio isn't equal throughout
the cylinder. Instead, precise control over fuel injection and intake flow dynamics allows a greater
concentration of fuel closer to the spark plug tip (richer), which is required for successful ignition and
flame spread for complete combustion. The remainder of the cylinders' intake charge is progressively
leaner with an overall average air:fuel ratio falling into the lean-burn category of up to 22:1.

The older Honda engines that used lean burn (not all did) accomplished this by having a parallel fuel and
intake system that fed a pre-chamber the "ideal" ratio for initial combustion. This burning mixture was
then opened to the main chamber where a much larger and leaner mix then ignited to provide sufficient
power. During the time this design was in production this system (CVCC, Compound Vortex Controlled
Combustion) primarily allowed lower emissions without the need for a catalytic converter. These were
carbureted engines and the relative "imprecise" nature of such limited the MPG abilities of the concept that
now under MPI (Multi-Port fuel Injection) allows for higher MPG too.

The newer Honda stratified charge (lean burn engines) operate on air-fuel ratios as high as 22:1. The
amount of fuel drawn into the engine is much lower than a typical gasoline engine, which operates at
14.7:1—the chemical stoichiometric ideal for complete combustion when averaging gasoline to the
petrochemical industries' accepted standard of C6H8.
This lean-burn ability by the necessity of the limits of physics, and the chemistry of combustion as it
applies to a current gasoline engine must be limited to light load and lower RPM conditions. A "top" speed
cut-off point is required since leaner gasoline fuel mixtures burn slower and for power to be produced
combustion must be "complete" by the time the exhaust valve open
Stratified charge engines
In a stratified charge engine, the fuel is injected into the cylinder just before ignition. This allows for
higher compression ratios without "knock," and leaner air/fuel mixtures than in conventional internal
combustion engines.

Conventionally, a four-stroke (petrol or gasoline) Otto cycle engine is fuelled by drawing a mixture of air
and fuel into the combustion chamber during the intake stroke. This produces a homogeneous charge: a
homogeneous mixture of air and fuel, which is ignited by a spark plug at a predetermined moment near the
top of the compression stroke.

187

In a homogeneous charge system, the air/fuel ratio is kept very close to stoichiometric. A stoichiometric
mixture contains the exact amount of air necessary for a complete combustion of the fuel. This gives stable
combustion, but places an upper limit on the engine's efficiency: any attempt to improve fuel economy by
running a lean mixture with a homogeneous charge results in unstable combustion; this impacts on power
and emissions, notably of nitrogen oxides or NOx.

If the Otto cycle is abandoned, however, and fuel is injected directly into the combustion-chamber during
the compression stroke, the petrol engine is liberated from a number of its limitations.

First, a higher mechanical compression ratio (or, with supercharged engines, maximum combustion
pressure) may be used for better thermodynamic efficiency. Since fuel is not present in the combustion
chamber until virtually the point at which combustion is required to begin, there is no risk of pre-ignition
or engine knock.

The engine may also run on a much leaner overall air/fuel ratio, using stratified charge.

188

Combustion can be problematic if a lean mixture is present at the spark-plug. However, fueling a petrol
engine directly allows more fuel to be directed towards the spark-plug than elsewhere in the combustionchamber. This results in a stratified charge: one in which the air/fuel ratio is not homogeneous throughout
the combustion-chamber, but varies in a controlled (and potentially quite complex) way across the volume
of the cylinder.

A relatively rich air/fuel mixture is directed to the spark-plug using multi-hole injectors. This mixture is
sparked, giving a strong, even and predictable flame-front. This in turn results in a high-quality
combustion of the much weaker mixture elsewhere in the cylinder.

Direct fuelling of petrol engines is rapidly becoming the norm, as it offers considerable advantages over
port-fuelling (in which the fuel injectors are placed in the intake ports, giving homogeneous charge), with
no real drawbacks. Powerful electronic management systems mean that there is not even a significant cost
penalty.

With the further impetus of tightening emissions legislation, the motor industry in Europe and north
America has now switched completely to direct fuelling for the new petrol engines it is introducing.

It is worth comparing contemporary directly-fuelled petrol engines with direct-injection diesels. Petrol
can burn faster than diesel fuel, allowing higher maximum engine speeds and thus greater maximum power
for sporting engines. Diesel fuel, on the other hand, has a higher energy density, and in combination with
higher combustion pressures can deliver very strong torque and high thermodynamic efficiency for more
'normal' road vehicles.
Four-valve engines

‡ A 4-Valve engine is designed for better performance than a regular 2-Valve engine
‡ More power: The 4-valve provides for a greater intake and exhaust area resulting in
more power More Mileage: 4-valve not only enhances the performance but also returns a very
good fuel economy
‡ More green: Comfortably
meets BSIII regulations
What is the 4-valve engine?
‡ An engine that has valves that let the air-fuel
mixture into the combustion chamber to be
burned and then draw out the exhaust gas
after the combustion. A conventional engine
has one intake valve to let in the air-fuel
mixture and one exhaust valve to let out the

189

exhaust gases. But the 4-valve engine has
two intake and two exhaust valves.
A typical 2-valve engine has just 1/3
combustion chamber head area covered by
the valves, but a 4-valve head increases that
to more than 50%, hence smoother and
quicker breathing.
‡ 4-valve design also benefits from a clean and effective combustion, because the spark plug can be
placed in the middle.
‡ 4 valves are better to be driven by twin-cam,
one for intake valves and one for exhaust
valves.
4-valve type and its shape is designed with the minimum
area necessary for the two intake and two exhaust valves.
‡ At the same time it is designed with a minimum intake and
exhaust valve angles to realize an optimum combustion
chamber shape.

M ERITS:‡ The first merit of 4-valve design is that it allows the spark
plug to be positioned in the center of the combustion
chamber to provide more efficient flame spread and
combustion. In other words, it enables highly efficient
combustion.
‡ Also, the 4-valve design enables greater overall valve area
than a 2-valve system for more efficient (per unit of area)
intake and exhaust function. This is the second major merit.
COMPARISION OF 4-VALVES OVER
2-VALVES
‡ A conventional engine has one intake valve to let in the
air-fuel mixture and one exhaust valve to let out the
exhaust gases. But the 4-valve engine has two intake
and two exhaust valves
‡ 4-valves is better than two because 4-valves give an engine steadier low-speed performance and a
better acceleration feeling
‡ That is why most race engines and high-performance
engines have four valves.
For example, Yamaha¶s YZR-M1 MotoGP race machine
has four valves.

190

The name of the game is velocity and turbulence/mixing of the intake charge
at differing engine speeds. At low engine speeds, one intake valve gives
increased velocity, hence better gas mixing and better cylinder filling. If you
open a second intake valve at low engine speed, the velocity drops
dramatically, leaving poor intake filling and a lean intake charge. The result
is engine knock and less torque. On the other hand, at high RPM, breathing
is the name of the game. The valves are open such a short length of time
you need the maximum available intake area. Therefore 2 intake valves
work better at high RPM. All of this is of course subject to the exhaust
system design. A proper extractor exhaust can make a significant difference
on a two valve system and a restrictive exhaust can nullify all the gains of a
4 valve system.

‡ When you have only 2 valves, the air/fuel mixture entering the cylinder can
be tangential to the circle of the cylinder, giving a high degree of swirl, better
air/fuel mixing and hence better performance at lower revs in an SI engine.
At higher revs, enough turbulence is available to create good mixing, and so
4 valves are better, as they allow greater airflow.

OHV engine design
OHV means OverHead Valve - an engine design where the camshaft is installed inside the engine block
and valves are operated through lifters, pushrods and rocker arms (an OHV engine also known as a
"Pushrod" engine). Although an OHV design is a bit outdated, it has been successfully used for decades.
An OHV engine is very simple, it has more compact size and proven to be durable.
On the downside, it's difficult to precisely control the valve timing at high rpm due to higher inertia caused
by larger amount of valve train components (lifter-pushrod-rocker arm). Also, it's very difficult to install
more than 2 valves per cylinder, or implement some of the latest technologies such as Variable Valve

191

Timing - something that could be easily done in a DOHC engine.

OHC or SOHC engine

4-cylinder 8-valve SOHC engine

OHC in general means OverHead Cam while SOHC means
Single OverHead Cam.
In a SOHC engine the camshaft is installed in the cylinder head
and valves are operated either by the rocker arms or directly
through the lifters (as in the picture).
The advantage is that valves are operated almost directly by the
camshaft, which makes it easy to achieve the perfect timing at
high rpm. It's also possible to install three or four valves per
cylinder
The disadvantage is that an OHC engine requires a timing belt
or chain with related components, which is more complex and
more expensive design.

DOHC or Twin Cam engine
DOHC means Double OverHead Cam, or sometimes it could be
called "Twin Cam". A DOHC setup is used in most of newer
cars. Since it's possible to install multiple valves per cylinder
and place intake valves on the opposite side from exhaust vales,
a DOHC engine can "breathe" better, meaning that it can
produce more horsepower with smaller engine volume.
Compare: The 3.5-liter V6 DOHC engine of 2003 Nissan
Pathfinder has 240 hp, similar to 245 hp of the 5.9-liter V8
OHV engine of 2003 Dodge Durango.
Pros: High efficiency, possible to install multiple valves per
cylinder and adopt variable timing.
Cons: More complex and more expensive design.
4-cylinder 16-valve DOHC engine

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Electronic Engine Management
electronic control systems
engines are subject to very high stresses during compression and ignition, and increasingly stringent
emission standards have made better control of the diesel combustion process necessary.
Electronic controlled diesel systems give very precise control of the fuel injection and combustion
process. Electronic controls have delivered other benefits besides a reduction in fuel consumption and
emissions, such as an increase in power and torque; improved engine responsiveness; a reduction in
engine noise and diesel knock; and improved and expanded diagnostic capabilities through the use of scan
tools.
electronic control systems monitor and control many variables, including:
•

•

Engine speed:
o to maintain a smooth functional idle,
o

and to limit the maximum safe engine speed, power, and torque;

o

and to keep the engine output to within safe limits.

Fuel injector operation:
o

•

Glow plugs and heater elements:
o

•

Control of pre-heating of the intake air to support quick cold starting and reduced cold run
emissions.

Exhaust emissions:
o

•

including the timing, rate and volume of fuel injected.

Analysis of exhaust gas to determine combustion efficiency and pollutants.

And the data bus:
o

An electronic communications network that allows exchange of data between computers necessary for efficient operation and fault diagnosis.

Other inputs monitored include:
•
•

crankshaft position,
throttle position,

•

brake and clutch operation,

•

battery voltage,

•

cruise control request,

•

air, oil fuel, exhaust and coolant temperatures,

193

•

and intake air, oil and fuel pressures.

The ECU is a micro-computer. It is constructed from printed circuitry, and contains a large
number of electrical components, including many semiconductor devices.

Its input devices receive data as electrical signals. They come from sensors and components at
various locations around the engine. Its processing unit compares incoming data with data
stored in a memory unit. The memory unit contains basic data about how the engine is to
operate. And an output device pulses the electrical circuit of the solenoid-type injection valves.
It is normally located in a safe place, behind a kick-panel in the foot-well, under the passenger
seat, or in the boot, and connected by a multi-plug, or plugs, to the vehicle’s wiring harness.

The core function of a basic ECU in an EFI system is to control the pulse width of the injector.
More sophisticated models also control other functions such as idle speed, ignition timing, and
the fuel pump. These wider systems are called engine management systems. The more precise
control they allow is very effective in reducing fuel consumption and exhaust emissions.

The ECU adjusts quickly to changing conditions by using what are called programmed
characteristic maps, stored in the memory unit. They are programmed into the ECU, just as data
is programmed into a computer. Characteristics means the engine’s operating conditions. And
they are called maps because they map all of the operating conditions for the engine.

They are constructed first from dynamometer tests, then fine-tuned, to optimise the operating
conditions and to comply with emission regulations. This data is stored electronically.

194

Ignition timing is crucial in this process. Between one spark and the next, the ECU uses data it
receives on engine load and speed to determine when the next ignition point will occur. It can
also correct the map value, using extra information such as engine coolant temperature, intake
air temperature, or throttle position. Putting all of this together, it arrives at the best ignition
point for that operating condition.

Common Rail Direct Injection or CRDI
CRDI is an intelligent way of controlling a diesel engine with use of modern computer systems. CRDI
helps to improve the power, performance and reduce harmful emissions from a diesel engine.
Conventional Diesel Engines (non-CRDI engines) are sluggish, noisy and poor in performance compared
to a CRDI engine.

CRDI or common rail direct injection system is also sometimes referred to by many similar or different
names. Some brands use name CRDe / DICOR / Turbojet / DDIS / TDI etc. All these systems work on
same principles with slight variations and enhancements here and there.

CRDI system uses common rail which is like one single rail or fuel channel which contains diesel
compresses at high pressure. This is a called a common rail because there is one single pump which
compresses the diesel and one single rail which contains that compressed fuel. In conventional diesel
engines, there will be as many pumps and fuel rails as there are cylinders.

As an example, for a conventional 4 cylinder diesel engine there will be 4 fuel-pumps, 4 fuel rails each
feeding to one cylinder. In CRDI, there will be one fuel rail for all 4 cylinders so that the fuel for all the
cylinders is pressurized at same pressure.

The fuel is injected into each engine cylinder at a particular time interval based on the position of moving
piston inside the cylinder. In a conventional non-CRDI system, this interval and the fuel quantity was
determined by mechanical components, but in a CRDI system this time interval and timing etc are all
controlled by a central computer or microprocessor based control system.

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To run a CRDI system, the microprocessor works with input from multiple sensors. Based on the input
from these sensors, the microprocessor can calculate the precise amount of the diesel and the timing when
the diesel should be injected inside the cylinder. Using these calculations, the CRDI control system
delivers the right amount of diesel at the right time to allow best possible output with least emissions and
least possible wastage of fuel.

The input sensors include throttle position sensor, crank position sensor, pressure sensor, lambda sensor
etc. The use of sensors and microprocessor to control the engine makes most efficient use of the fuel and
also improved the power, fuel-economy and performance of the engine by managing it in a much better
way.

One more major difference between a CRDI and conventional diesel engine is the way the fuel Injectors
are controlled. In case of a conventional Engine, the fuel injectors are controlled by mechanical
components to operate the fuel injectors. Use of these mechanical components adds additional noise as
there are many moving components in the injector mechanism of a conventional diesel engine. In case of a
CRDI engine, the fuel injectors are operated using solenoid valves which operate on electric current and do
not require complex and noisy mechanical arrangement to operate the fuel Injection into the cylinder. The
solenoid valves are operated by the central microprocessor of the CRDI control system based on the inputs
from the sensors used in the system.
Gasoline Direct Injection Engine :
Port fuel injected (PFI) engines are the most commonly used
spark ignition (SI) engine in current vehicles. In certain

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markets, a very small number of direct-injected spark ignition
(DISI) engines have been introduced. Both use gasoline fuel. In
PFI engines, fuel is injected into the intake port near the closed
intake valve, producing a well mixed fuel–air charge in the
combustion chamber. This is the most commonly used engine
type in current vehicles. These engines are typically operated
with a stoichiometric fuel–air ratio, which is the ratio that
permits complete conversion of the fuel and oxygen in the
intake charge to form CO2 and H2O. As a result of the premixed
combustion, it produces very low particulate emissions.
The levels of other emissions directly leaving the engine are
relatively high, and compliance with regulated emission
standards relies on the effectiveness of the three-way catalyst,
which reduces emissions by 95–99% as discussed in more detail
below.
In DISI engines, the fuel is injected directly into the
combustion chamber. At higher load, the fuel is injected
during the intake stroke to form a nearly homogeneous fuel–
air mixture at the time of ignition. At lower load, the injection
timing can be delayed until the compression stroke to produce
a ‘‘stratified’’ fuel mixture. This mixture is ideally uniform,
premixed, and stoichiometric near the center, and devoid of
fuel near the cylinder walls. This spatial localization translates
into a faster burn and allows the engine to be run more fuellean
overall than PFI engines, providing improved fuel
economy and better performance during transient acceleration/
deceleration. In practice, however, it is difficult to realize
this idealized mixing, and fuel-rich and lean regions result,
leading to reduced benefits. Additionally, because this engine
injects fuel droplets directly into the combustion chamber,
particulate emissions are increased substantially relative to PFI
engines. Like PFI engines, DISI engines rely on catalytic
devices to significantly reduce engine-out concentrations of
regulated emissions.

1. The engines use injectors that can spray fuel directly into the cylinder during the compression
stroke, along with an extremely high pressure fuel pump (2,000 PSI). Before GDI, it was far more
common to use port fuel injection, where the injector sprayed fuel at low pressure into the intake
manifold.
2. The direct injection process allows the fuel to evaporate in the cylinder and cool the air/fuel
mixture. That helps avoid premature ignition, so…
3. These engines can increase the compression ratio. The Mazda engine goes to a 14:1 ratio, which
has never been seen before in a production gasoline engine. The normal high is 12:1 or so, and that
would require premium fuel.
4. Many of the engines are using multiple injector sprays per stroke. One spray occurs as the air starts
flowing in on the intake stroke. The second occurs right before the spark plug fires. This creates a
stratified charge of fuel for a better burn pattern.

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the advantages of higher compression ratios ,The higher the compression ratio, the more closely
packed the molecules of fuel and air are when the mixture is ignited by the sparkplug, this causes a
more powerful explosion by making a more violent reaction which produces more power. Higher
compression makes the expansion ratio of the exploding hot gas greater which means that more energy
is impinged on the piston top, pushing it down harder, making more power. Increasing the
compression ratio improves the thermal efficiency of an engine and this is the primary reason why
higher compression increases power. Improving thermal efficiency improves fuel economy from
getting more power from the same amount of fuel and a reduction of combustion chamber surface area
to volume. This means less wasted combustion heat and more expansion being used to drive the piston
down.

UNIT V
Part A
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.

1.
2.
3.
4.

List the features of HCCI.
List the features of surface ignition engines.
Define charge stratification
What are the advantages of Four valve engine.
What is a Lambda sensor
List out various sensors used on an IC engine.
What is a Lean Burn engine.
What are the advantages of Gasoline direct engine over conventional gasoline engine
What is Surface Ignition.
What are the advantages of stratified charge engines.
What are the advantages of HCCI engine.
What are the disadvantages of HCCI engine.
What is the principle of Lean burn engines.
What is a Four valve engine.
What is a Over Head Valve engine.
What is an electronic control unit.

Part B
Explain the working of a Homogeneous charge combustion engine.
Draw the sketch of an engine and locate various sensors used on it.
Explain the working of Common rail direct injection diesel engine with a neat sketch.
Explain the working of Gasoline direct injection engine.

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5.
6.
7.
8.
9.

Explain the working of a Four – Valve engine and state the advantages of it.
Explain the working of a Lean Burn engine.
Explain the working of a Stratified Charge engine.
What are the methods of charge stratification
What is a stratified charge engine. What are the advantages and disadvantages of stratified charge
engine

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