Heat engines
g
• Heat engines are cyclic devices and that the working fluid of a
heat engine return to its initial state at the end of each cycle.
heat engine return to its initial state at the end of each cycle.
• Work is done by working fluid during one part of the cycle and
on the working fluid
h
ki fl id during another part. (Deference between
d i
h
(D f
b
these two equal to network delivered by the heat engine).
• To maximize efficiency: deliver most work and required least
work.
Internal combustion Engines: History,
History engine types and operation
of 2 & 4 stroke engines
• Maximum efficiency is given by ideal reversible cycle.
Dr. Primal Fernando
[email protected]
d@ d
lk
Ph: (081) 2393608
1
History of internal combustion (IC) engines
y
g
2
History of IC Engines
History of IC Engines
• Both
Both power generation and refrigeration are usually accomplished
power generation and refrigeration are usually accomplished
by systems that operate on a thermodynamic cycle: power cycles
and refrigeration cycles.
1860 Lenoir’s engine (a converted steam engine)
combusted natural gas in a double acting piston,
using electric ignition. Efficiency = 5%
i
l
i i ii
Effi i
5%
• Power producing devises: engines
• Refrigeration producing devices: refrigerators, air‐conditioners
and heat pumps.
• Steam engine ‐ 1700 (external combustion engines)
l
b
3
4
History ‐ continued
History
Classification of Engines
Classification of Engines
• 1876
1876 Nikolaus Otto patented the 4 cycle engine, it used gaseous
Nikolaus Otto patented the 4 cycle engine it used gaseous
fuel
• 1882 Gottlieb Daimler, an engineer for Daimler, left to work on
his own engine His 1889 twin cylinder V was the first engine to
his own engine. His 1889 twin cylinder V was the first engine to
be produced in quantities. Used liquid fuel and Venturi type
carburetor, engine was named “Mercedes” after the daughter of
his major distributor
his major distributor
• 1893 Rudolf Diesel built successful CI engine which was 26%
efficient (double the efficiency of any other engine of its time)
•
•
•
•
•
External vs Internal Combustion
Spark Ignition SI or Compression Ignition CI
Configuration
Valve Location
2 Stroke or 4 Stroke
2 Stroke or 4 Stroke
5
6
V Engine
g
Engine Configurations
Engine Configurations
In Line
(Automobile)
Horizontally
Opposed (Subaru)
Radial (Aircraft)
V
(Automobile)
Opposed Piston
(crankshafts geared
together)
7
8
Wankel Rotary Piston Engine
y
g
Rotary “Wankel”
Rotary
Wankel Engine
Engine
Ref. Internal combustion engines and air pollution, E. F. Obert
9
10
Basic considerations in the analysis of power cycles
y
p
y
• Cycles encountered in actual devices
y
are difficult to analyze because of the
presence of complicating effects such as
friction etc.
friction etc.
• Consider a cycle that resembles the
actual cycle closely but it made up
t l
l l l b t it
d
totally of internally reversible process
(ideal cycle)
Thermal efficiency, th
11
Wnet
Qin
or
wnet
qin
12
Net work of the cycle
Idealizations and simplifications
p
• Cycle does not involve any
fi i
friction: no pressure drop in the
d
i h
working fluid.
• Expansion and compression
process: quasi equilibrium.
• Pipes connecting various
components are well insulated.
• Neglecting changers in KE and
PE
13
14
Air‐standard assumption
p
Carnot cycle
y
• Gas power cycles (working fluid gas): spark ignition engines, diesel
engines, conventional gas turbines, etc.
• All these engines energy is provided by burning a fuel within the system
boundary.
• Working fluid (air) mainly contains nitrogen and hardly undergoing any
chemical reactions in the combustion chamber and can be closely
resembles to air at all times in the chamber.
– Assumptions: working fluid as air, behaves as ideal gas, internally
y
p
p
y
p
reversible cycle, combustion process replace by heat addition process
by a external source, exhaust process replace by heat rejection process
that re‐stores initial state of working fluid, specific heat values
determines at room temperatures (call cold‐air‐standard
assumptions)
assumptions).
• The Carnot cycle is the most efficient cycle
that can be executed between heat a source
that can be executed between heat a source
and a heat sink.
th,Carnot 1
TL
TH
15
16
Reciprocating Engines
Parts of an engine
g
Top Dead Center (TDC)
p
: Upper most position
pp
p
Bottom Dead Center (BDC) : Lower most position
Exhaust
valve
Intake
valve
Stroke : Length of piston travel
TDC
Stroke
Bore
BDC
Bore : Diameter of the cylinder
Clearance Volume (Vc) : V where piston is at TDC
Displacement Volume (Vd) :Swept Volume (V
Displacement Volume (V
) :Swept Volume (Vmax‐Vmin)
Compression Ratio (rv) = (Vmax/Vmin) = (VBDC/VTDC)
Mean Effective Pressure (MEP) :
Wnet = (MEP) x (Displacement Volume)
Reciprocating Engine is INTERNAL COMBUSTION ENGINE, and is Classified
into 2 types:
1.
Spark Ignition (SI): Gasoline Engine, Mixing air‐fuel outside cylinder,
ignites by a spark plug (Auto cycle)
2
2.
Compression Ignition (CI): Diesel engine fuel is injected into the
Compression Ignition (CI): Diesel engine, fuel is injected into the
cylinder, self ignited as a result of compression (Diesel cycle).
รศ.ดร.สมหมาย ปรี เปรม
17
Mean Effective Pressure, MEP Concept
18
Four Stroke Engine – spark ignition engine
Intake
Actual Processes
P
P
C
Compression
i
Power
Exhaust
Equivalent by MEP
Equivalent
Wnet
1. Intake Stroke piston moves from TDC to BDC,
drawing in fresh air-fuel mixture.
2. Compression Stroke piston moves from BDC to
TDC, compress air-fuel mixture.
3. Power Stroke piston at TDC, spark plug ignite
the air-fuel mixture. the combustion occur
very fast
f t that,
th t in
i theory,
th
the
th piston
i t still
till att
TDC. After that the piston is pushed to BDC.
4. Exhaust Stroke piston moves from BDC to TDC,
ppushes the combustion gases
g
out.
MEP
Wnet
vmin
TDC
vmax v
vmin
vmax
v
BDC
Wnet = (MEP) x (Displacement Volume)
= (MEP) x (Vmax-Vmin)
19
20
Actual and ideal cycle in spark ignition
engine
i
Two Stroke Engine
Compression
Intake &
Exhaust
Power
1. Compression Stroke piston moves from
BDC to TDC, compress air‐fuel mixture.
2. Power Stroke piston at TDC, spark plug
p
p
p g
ignite the air‐fuel mixture. After the
piston is pushed to BDC. Meanwhile,
about half way, combustion gases are
discharged and fresh air fuel mixture is
discharged and fresh air‐fuel mixture is
drawing in .
g
g
y
2‐stroke engines generally less efficient than 4‐stroke
engines since partial expulsion of unburned mixture
with exhaust gas. It has higher power/weight ratio.
21
Air Standard Otto Cycle (Nikolaus A. Otto 1876)
22
T
Energy balance –
gy
Otto cycle (I)
y
Ideal cycle of spark ignition engine, comprises of 44 Process:
Process 1-2 Isentropic Compression (piston moves from BDC to TDC)
Process 2-3 v = constant, heat added (piston stays still, represents combustion)
Process 3-4 Isentropic expansion (piston moves from TDC to BDC gives POWER)
Process 4-1 v = constant, heat rejection (piston stays still, represents EXHAUST and INTAKE stroke)
Neglecting changes in KE and PE
2
(qin qout ) ( win wout ) u (kJ
k / kg
k )
There are only 2-stroke of all 4-processes,
P
T
3
Heat transfer to/from the system is under
constant volume (no work)
qin u 3 u 2 c v (T3 T2 )
wout
2
2
win
v2=v3
TDC
1
1
v1=v4
v
s1=s2
q out u 4 u 1 cv (T4 T1 )
4
4
qout
s3=s4
4
qout
1
3
qin
3
qin
th ,Otto
s
BDC
w
q
net 1 out
qin
qin
Evaluate at room
tem: called cold air
standard assumption
standard assumption
T T1
1 4
1
T3 T2
s1=s2
P
s
s3=s4
3
wout
T
T1 4 1
T
1
T3
T2 1
T
2
2
4
win
v2=v3
1
v1=v4
v
What is the different of Otto cycle from Carnot cycle, the most efficient cycle
23
24
T
Energy balance –
gy
Otto cycle (II)
y
T
th ,Otto
T1 4 1
w
q
net 1 out 1 T4 T1 1 T1
qin
qin
T3 T2
T3
2
T2 1
T2
Processes 1‐2 and 3‐4 are isentropic and v
Processes
1‐2 and 3‐4 are isentropic and v2=v3
and v4=v1 (Pvk=constant)
T1 v 2
T2 v1
k 1
v
3
v4
k 1
qout
1
s1=s2
P
s
s3=s4
3
T4
T3
wout
V
V
v
r max 1 1
Vmin V2 v 2
th ,Otto 1
th ,Otto 1
4
2
Compression ratio
Compression ratio
Thermal efficiency of a Otto cycle (I)
y
y
()
3
qin
1
4
win
v2=v3
1
v
v1=v4
r k 1
1
r
k 1
• High compression ratios: temperature
of air/fuel mixture rises above auto
ignition temperature (premature
ignition)‐produces audible noise is
k=1.4
called engine knock.
• Improvement of thermal efficiency
was obtained utilizing higher
compression ratios (up to 12) gasoline
ble d ith tet aethyl lead (i
blend with tetraethyl lead (improving
o i
octane rating) but it has been
prohibited to use since the hazardous
Octane rating = measure of fuel
g
of lead to health
of lead to health.
quality (measure of engines knock
resistance)
25
Thermal efficiency of a Otto cycle (II)
y
y
( )
26
Compression Issues
p
Monatomic gas (He, Ar)
• Most
Most compression ratios are around 10:1,
compression ratios are around 10:1,
meaning that the gas let into the cylinder is
compressed to 1/10 times its original size.
air
CO2
k=1.2
• Efficiency is better with a higher
compression ratio but only to the limits
of the fuel quality.
ethane
Molecular weight of the
working fluid increases
• Problems can occur during a cycle if there is:
Problems can occur during a cycle if there is:
– Lack of Compression caused from gasses leaking past the
piston, a hole in the piston, or the intake or exhaust valves
i t
h l i th i t
th i t k
h
t l
are not sealing properly.
– Lack of Spark caused by malfunctioning spark plugs, dirty
spark plugs, mistimed firings, or bad connections between
plugs and the battery.
Thermal efficiency of actual spark ignition
efficiency of actual spark ignition
• Thermal
engine ~ 25‐30%
27
28
How Fuel is Handled
Chemical Energy of Gasoline
gy
• Structure of Gasoline
– Is mostly comprised of hydrocarbon molecules having
Is mostly comprised of hydrocarbon molecules having
from six to ten carbon atoms.
• The
The chemical energy of one gallon of gasoline is, on the average,
chemical energy of one gallon of gasoline is on the average
125,000 BTU per gallon (132×106 J per 3.8 L).
– Octane
Octane is a measure of the resistance to detonation. The
is a measure of the resistance to detonation The
octane number assigned to gasoline (87,89, 93, 100, 114,
120) represents the ratio of heptane, which easily
detonates, to isooctane, which does not want to detonate
detonates, to isooctane, which does not want to detonate
(better to say octane number above 100 as “performance
number”. It is calculated by different way. Often itʹs done
by pure extrapolation. ) . Eighty‐seven‐octane gasoline is
yp
p
)
g y
g
gasoline that contains 87‐percent octane and 13‐percent
heptane (or some other combination of fuels that has the
same performance of the 87/13 combination of
octane/heptane).
t
/h t
)
• Only about 25% of chemical energy in gasoline is converted to
mechanical energy.
• Basically out of a one dollar gallon of gasoline, 75 cents is
wasted.
29
30
Diesel cycle: The ideal cycle for compression
ignition (CI) engine (Rudolph Diesel 1890)
ignition (CI) engine (Rudolph Diesel 1890)
Cylinder
y
Configurations
g
• Similar to spark ignition engine differing mainly in the method
of initiating combustion.
fi ii i
b i
• In spark ignition (SI) engines (gasoline engines), air fuel mixture
p
g
g
g
g
compressed below auto ignition temperature of the air/fuel
mixture and combustion starts by firing spark plugs.
Straight Configuration
V Configuration
Flat
Configuration
• In combustion ignition (CI) engines (diesel engines) air
compressed above the auto ignition temperature of the air fuel
mixture and then fuel inject into the air.
Displacement refers to
the volume inside each
piston chamber.
chamber For
example: a 3.0 Liter
engine with 6 cylinders
will have 0.5 liters per
cylinder.
• SI engines has a carburetor and diesel engine has a fuel pump.
• The compression ratio of diesel engines typically higher (12 ‐24)
31
32
Energy balance – Diesel engine (I)
Diesel engine
g
(qin qout ) ( win wout ) u (kJ / kg )
• The fuel injection starts when the
p
piston reaches to TDC.
q in P2 (v 3 v 2 ) (u 3 u 2 ) h3 h2 c p (T3 T2 )
• Combustion process takes place
over longer interval.
over longer interval.
q out u 4 u 1 c v (T4 T1 )
• Because of this longer period the
heat addition process can be
heat addition process can be
approximated as constant
pressure heat addition process.
th , Diesel
Di l
wnet
q
1 out
qin
qin
(T T1 )
1 4
1
k (T3 T2 )
• Other parts are common for both
SI and CI engines.
T
T1 4 1
T
1
T3
kT2 1
T
2
33
Otto vs. Diesel
Energy balance – Diesel engine (II)
th , Diesel
q
w
(T T )
net 1 out 1 4 1 1
k (T3 T2 )
qin
qin
T
T1 4 1
T
1
T3
kT2 1
T
2
th ,Otto 1
1
r k 1
th, Diesel 1
1 rck 1
r k (rc 1)
k 1
th ,Otto th , Diesel (when both cycles operate on the same compressio n ratio)
V3 v3
Define new quantity; cutoff ratio rc
Define new quantity; cutoff ratio
V2 v 2
• Limiting value of rc=1; when efficiencies of both Otto and Diesel
cycles are identical.
Utilizing definition of isentropic ideal‐gas relations
g
p
g
th, Diesel
34
• Di
Diesel cycle operates much higher compression ratios, therefore
l
l
h hi h
i
i
h f
thermal efficiency of Diesel engines are usually higher than SI
engines (35 to 40%).
1 rk 1
1 k 1 c
r k (rc 1)
• Diesel engines burns fuels more completely than gasoline engines.
r is the compression ratio
Energy content of 1 gallon of diesel on average, 147,000 BTU per
gallon (155×10
ll (155 106 J per 3.8 L).
J
3 8 L)
35
36
Dual cycle
y
• More
More realistic way to model:
realistic way to model:
Combination of heat transfer
processes in gasoline and diesel
cycles.
l
• The relative amount of heat
transfer during each process can
be adjusted to approximate
actual cycle more closely.
37