Mechanical Engineering

MAJOR PROJECT ON APPLICATION OF TURBO-CHARGER IN PETROL ENGINE
A MAJOR PROJECT SUBMITTED IN THE PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE AWARD OF THE DEGREE OF

BACHELOR OF ENGINEERING IN MECHANICAL ENGINEERING
OF

MAHARSHI DAYANAND UNIVERSITY, ROHTAK

UNDER THE GUIDANCE OF Er. BHUBHANESH

BY VARUN JAIN (05/ME/228) JITENDER SAINI (05/ME/225) HARIPRASHAD (05/ME/234) PRASHANT SHARMA (05/ME/233) MUKESH KUMAR (05/ME/223) RAVINDER KAUSHIK (05/ME/238) TARUN (05/ME/237) RAHUL SHARMA (05/ME/229) AMIT SHRIVASTAV (05/ME/214)

DEPARTMENTAL OF MECHANICAL ENGINEERING B.P.R. COLLEGE OF ENGINEERING GOHANA (SONEPAT)

JUNE 2009
1

ACKNOWLEDGEMENT

To excel and develop in a field one has to have a sense of security and authority with the essence of responsibility. There are always people associated who help and guide for the successful achievement of the desired objective. Therefore these must be obliged too. While expressing our gratitude and indebtness to our elite guide Mr. VIPIN KUMAR. The words loose their worth for his valuable guidance, continuous encouragement and cooperation in every respect. His extreme inspiration and generous affection bring the work towards completion. Our special thanks to library department, B.P.R. college of Engg. For giving us valuable books and journals.

VARUN JAIN (05/ME/228) JITENDER SAINI (05/ME/225) HARIPRASHAD (05/ME/234) PRASHANT SHARMA (05/ME/233) MUKESH KUMAR (05/ME/223) RAVINDER KAUSHIK (05/ME/238) TARUN (05/ME/237) RAHUL SHARMA (05/ME/229) AMIT SHRIVASTAV (05/ME/214)

2

CERTIFICATE
It is to be certified that Varun Jain, Jitender Saini, Hariprashad, Prashant Sharma, Mukesh Kumar, Ravinder Kaushik, Tarun, Rahul Sharma and Amit Shrivastav, students of final year Mechanical Engineering has partially completed for 8th Sem. The project entitled “APPLICATION OF TURBOCHARGER IN PETROL ENGINE” under my guidance and direction as a requisite for the fulfillment of the degree of B.Engg. in Mechanical Engineering from Maharshi Dayanand University Rohtak.

Mr. Vipin Kumar (Lect.) Mechanical Engg. Dept. B.P.R. College of Engg.

Mr. Dinesh Panchal Head of Department Mechanical Engg. Dept. B.P.R. College of Engg.

3

CONTENTS

ACKNOWLEDGEMENT……………………………………………………………………….2 CERTIFICATE…………………………………………………………………………………..3

TITLE

PAGE No.

1. TURBOCHARGER- AN OVERVIEW………………………………………………………..5 1.1 1.2 1.3 1.4 1.5 1.6 1.7 INTRODUCTION……………………………………………………….……………..5 WORKING PRINCIPLE……………………………………………………………….7 FULL SECTIONAL VIEW OF TURBO-CHARGER…………..……………………..8 INSIDE A TURBO-CHARGER……………………….……………………………….9 DESIGN CONSIDERATION AND DETAILS………….…………………………….9 HOW TURBOCHARGER IS PLUMBED IN CAR…………………………………..54 NEED TO BE CONSIDER WHEN SELECTING TURBOCHARGER……………...55

2. APPLICATION OF TURBOCHARGER IN AUTOMOTIVE……………………………….56 3. ADVANTAGE OF TURBOCHARGER……………………………………….……………..57 4. DISADVANTAGE OF TURBOCHARGER…………………………………………………58 5. REFRENCES……………………………………………..……………………………………59

4

TURBOCHARGER- AN OVERVIEW

1.1

Introduction – The turbocharger or a just simply the turbo. It was invented by Swiss

engineer named Alfred Buchi in 1905 and was first used on the diesel engines of ships and locomotives from the 1920s. It was used on the engines of production airplanes from the 1930s and on truck engines from the late 1940s. A turbocharger, or turbo, is an air compressor used for forced-induction of an internal combustion engine. Like a supercharger, the purpose of a turbocharger is to increase the mass of air entering the engine to create more power. However, a turbocharger differs in that the compressor is powered by a turbine driven by the engine's own exhaust gases. OR As a forced induction system, a turbo is nothing more than an air pump that is driven by the exhaust gasses of a car engine. It consists of a compressor-wheel and a turbine-wheel that are connected by a common shaft. The compressor increases the density of the air that enters the intake manifold by forcing more air into the intake manifold than what the car would normally ingest. This higher intake air density contains more air molecules and produces more power when combined with the correct amount of fuel. This is similar to the way NOS allows more fuel to be burned by providing extra Oxygen. The major difference between NOS and a turbo is that the turbo provides a constant supply of extra Oxygen to the car engine while NOS only provides a limited supply. A turbocharger is an exhaust gas-driven turbine that compresses the intake air, increasing the horsepower and torque of an engine by increasing volumetric efficiency. This means that by compressing the air and increasing the density, you use a given volume of engine displacement more efficiently (volumetric efficiency). Denser air means more air atoms and more fuel atoms can be added into the engine. This makes more power. As a side note, you may have heard of nitrous or the brand name "NOS". Using nitrous oxide as an additive to gasoline engines dramatically increases the amount of oxygen in the engine's combustion cylinders, cools the air charge, and allows more fuel to be burned. More fuel + more air = more power. Top fuel dragsters use a fuel of 85% nitro methane and about 15% methane in engines only about the size of a ford mustang engine but with power measured in the thousands of horsepower (hp). Since the fuel is over 50% oxygen, it's concentration of energy possible from a 5

given space, its volumetric efficiency, is much greater than an engine burning only regular air which is only about 21% oxygen. Most modern turbocharged engines seem to be 4 cylinders and often have as much or more power than a 6 cylinder non turbo engine. You may be asking yourself why don't car manufacturers turbocharger all cars? Because it costs more money to design and build, larger engines usually have better low end power, and they can charge a premium for larger engines and that V8 sticker. And in many cases (like the Corvette) a big engine just works! Although you can add turbo charging to a non turbo car. So the ultimate goal of turbo charging is to increase air density to make more oxygen available to burn. The energy from this burning is what pushes the piston down, creating energy. This increase in air density, or boost, is normally expressed in pressure. In the US, the most commonly used unit of pressure is pounds/sq. in, or psi. Other common units of pressure are bar, or kPa. To help understand when the engine is under boost and under vacuum, consider these examples. If the engine is off, a vacuum/boost gauge would read 0. This means that the gauge is measuring a difference of 0 psi between the intake and ambient pressure. If the engine is running at idle, the gauge may show a negative reading, for example -7. This means that the intake is under vacuum and has a lower pressure than ambient air. If you press on the throttle pedal 100% while the engine is under load, the gauge will indicate a change from vacuum to boost, or positive pressure. This means that the turbo has pressurized the intake air more than ambient by whatever amount the gauge shows. Part of this is that most boost gauges get their reading from the intake manifold or piping. Also note that most diesel cars will not show any significant vacuum or negative reading in the intake manifold because there is no throttle plate to draw a vacuum against. Outside of North America, some cars may also show ambient atmospheric pressure instead of relative pressure. In other words, when at rest, the gauge will show about 14 psi or 1 bar instead of 0.

6

1.2 WORKING PRINCIPLE A turbocharger, often called a turbo, is a small radial fan pump driven by the energy of the exhaust flow of an engine. A turbocharger consists of a turbine and a compressor on a shared axle. The turbine inlet receives exhaust gases from the engine causing the turbine wheel to rotate. This rotation drives the compressor, compressing ambient air and delivering it to the air intake manifold of the engine at higher pressure, resulting in a greater mass of air entering each cylinder. In some instances, compressed air is routed through an intercooler before introduction to the intake manifold. The objective of a turbocharger is the same as a supercharger; to improve upon the size-to-output efficiency of an engine by solving one of its cardinal limitations. A naturally aspirated automobile engine uses only the downward stroke of a piston to create an area of low pressure in order to draw air into the cylinder through the intake valves. Because the pressure in the atmosphere is no more than 1 bar (approximately 14.7 psi), there ultimately will be a limit to the pressure difference across the intake valves and thus the amount of airflow entering the combustion chamber. This ability to fill the cylinder with air is its volumetric efficiency. Because the turbocharger increases the pressure at the point where air is entering the cylinder, a greater mass of air (oxygen) will be forced in as the inlet manifold pressure increases. The additional oxygen makes it possible to add more fuel, increasing the power and torque output of the engine. Because the pressure in the cylinder must not go too high to avoid detonation and physical damage, the intake pressure must be controlled by controlling the rotational speed of the turbocharger. The control function is performed by a waste gate, which routes some of the exhaust flow away from the exhaust turbine. This controls shaft speed and regulates air pressure in the intake manifold. The application of a compressor to increase pressure at the point of cylinder air intake is often referred to as forced induction. Centrifugal superchargers compress air in the same fashion as a turbocharger. However, the energy to spin the supercharger is taken from the rotating output energy of the engine's crankshaft as opposed to normally exhausted gas from the engine. Superchargers use output energy from an engine to achieve a net gain, which must be provided from some of the engine's total output. Turbochargers, on the other hand, convert some of the piston engine's exhaust into useful work. This energy would otherwise be wasted out the exhaust. This means that a turbocharger is a more efficient use of the heat energy obtained from the fuel than a supercharger.

7

WORKING PRINCIPLE

1.2

FULL SECTIONAL VIEW OF TURBO-CHARGER –

TURBO-CHARGER
8

1.3

INSIDE A TURBO-CHARGER - The turbocharger is bolted to the exhaust

manifold of the engine. The exhaust from the cylinders spins the turbine, which works like a gas turbine engine. The turbine is connected by a shaft to the compressor, which is located between the air filter and the intake manifold. The compressor pressurizes the air going into the pistons. The exhaust from the cylinders passes through the turbine blades, causing the turbine to spin. The more exhaust that goes through the blades, the faster they spin.

INSIDE TURBOCHARGER

On the other end of the shaft that the turbine is attached to, the compressor pumps air into the cylinders. The compressor is a type of centrifugal pump -- it draws air in at the center of its blades and flings it outward as it spins. 1.4

DESIGN CONSIDERATION AND DETAILS –

One of the main problems with turbochargers is that they do not provide an immediate power boost when you step on the gas. It takes a second for the turbine to get up to speed before boost is produced. This results in a feeling of lag when you step on the gas, and then the car lunges ahead when the turbo gets moving. One way to decrease turbo lag is to reduce the inertia of the rotating parts, mainly by reducing their weight. This allows the turbine and compressor to accelerate quickly, and start providing boost earlier. One sure way to reduce the inertia of the turbine and compressor is to make the turbocharger smaller. A small turbocharger will provide boost more quickly and at lower engine speeds, but may not be able to provide much boost at higher engine speeds when a really large volume of air is going into the engine. It is also in danger of spinning too quickly at higher engine speeds, when lots of exhaust is passing through the turbine. 9

Turbochargers provide boost to engines at high speeds
A large turbocharger can provide lots of boost at high engine speeds, but may have bad turbo lag because of how long it takes to accelerate its heavier turbine and compressor. Luckily, there are some tricks used to overcome these challenges. Most automotive turbochargers have a waste gate, which allows the use of a smaller turbocharger to reduce lag while preventing it from spinning too quickly at high engine speeds. The waste gate is a valve that allows the exhaust to bypass the turbine blades. The waste gate senses the boost pressure. If the pressure gets too high, it could be an indicator that the turbine is spinning too quickly, so the waste gate bypasses some of the exhaust around the turbine blades, allowing the blades to slow down. Some turbochargers use ball bearings instead of fluid bearings to support the turbine shaft. But these are not your regular ball bearings -- they are super-precise bearings made of advanced materials to handle the speeds and temperatures of the turbocharger. They allow the turbine shaft to spin with less friction than the fluid bearings used in most turbochargers. They also allow a slightly smaller, lighter shaft to be used. This helps the turbocharger accelerate more quickly, further reducing turbo lag. Ceramic turbine blades are lighter than the steel blades used in most turbochargers. Again, this allows the turbine to spin up to speed faster, which reduces turbo lag.

10

Turbo compressor blades
In order to handle speeds of up to 150,000 rpm, the turbine shaft has to be supported very carefully. Most bearings would explode at speeds like this, so most turbochargers use a fluid bearing. This type of bearing supports the shaft on a thin layer of oil that is constantly pumped around the shaft. This serves two purposes: It cools the shaft and some of the other turbocharger parts, and it allows the shaft to spin without much friction.

More Design Considerations
Some engines use two turbochargers of different sizes. The smaller one spins up to speed very quickly, reducing lag, while the bigger one takes over at higher engine speeds to provide more boost. When air is compressed, it heats up; and when air heats up, it expands. So some of the pressure increase from a turbocharger is the result of heating the air before it goes into the engine. In order to increase the power of the engine, the goal is to get more air molecules into the cylinder, not necessarily more air pressure. An intercooler or charge air cooler is an additional component that looks something like a radiator, except air passes through the inside as well as the outside of the intercooler. The intake air passes through sealed passageways inside the cooler, while cooler air from outside is blown across fins by the engine cooling fan. The intercooler further increases the power of the engine by cooling the pressurized air coming out of the compressor before it goes into the engine. This means that if the turbocharger is operating at a boost of 7 psi, the intercooled system will put in 7 psi of cooler air, which is denser and contains more air molecules than warmer air.

11

DESIGN DETAILS –
The turbocharger has four main components. The turbine (almost always a radial turbine) and impeller/compressor wheels are each contained within their own folded conical housing on opposite sides of the third component, the center housing/hub rotating assembly (CHRA).

Brass oil drain connection, braided oil supply line and water coolant line connections

The housings fitted around the compressor impeller and turbine collect and direct the gas flow through the wheels as they spin. The size and shape can dictate some performance characteristics of the overall turbocharger. Often the same basic turbocharger assembly will be available from the manufacturer with multiple housing choices for the turbine and sometimes the compressor cover as well. This allows the designer of the engine system to tailor the compromises between performance, response, and efficiency to application or preference.

Compressor impeller side with the cover removed

The turbine and impeller wheel sizes also dictate the amount of air or exhaust that can be flowed through the system, and the relative efficiency at which they operate. Generally, the larger the 12

turbine wheel and compressor wheel, the larger the flow capacity. Measurements and shapes can vary, as well as curvature and number of blades on the wheels.

Turbine side housing removed

The center hub rotating assembly houses the shaft which connects the compressor impeller and turbine. It also must contain a bearing system to suspend the shaft, allowing it to rotate at very high speed with minimal friction. For instance, in automotive applications the CHRA typically uses a thrust bearing or ball bearing lubricated by a constant supply of pressurized engine oil. The CHRA may also be considered "water cooled" by having an entry and exit point for engine coolant to be cycled. Water cooled models allow engine coolant to be used to keep the lubricating oil cooler, avoiding possible oil coking from the extreme heat found in the turbine. TURBO-CHARGER BASIC PARTS The turbocharger's basic parts are the compressor side, turbine side, and the center housing which connects the two sides. The turbine, or exhaust side, has a small pinwheel-like turbine that is spun by exhaust gasses. Built into the housing is an internal wastegate that lets excess exhaust gas and pressure out. If the turbo is an external wastegate, the wastegate is not built into the turbine housing and is somewhere else. Modern TDI turbos even have a one piece exhaust manifold and turbine housing. Earlier and many aftermarket TDI turbos use a separate exhaust manifold and turbo turbine housing

13

TURBO CHARGER The other side, the compressor or intake side, has a pinwheel-like impeller, powered by a straight shaft from the turbine wheel. Its job is to compress the intake air. The center housing, or center hub rotating assembly (CHRA), is the part that houses the shaft and bearings that the two wheels spin on, and normally contains oil and coolant to lubricate it all. Note that the turbo used in the VW TDI is oil cooled only.

14

The wheels and shaft can often reach speeds of 50,000 to 200,000 rpm which is why they require proper cooling and lubrication? Warning: do not reroute the CHRA oil or coolant lines without first considering any possible complications. A bent line could cause the CHRA to be starved of oil or coolant, damaging the turbo. After engine shutdown, the turbo cools off and causes some circulation in the turbo oil and coolant lines. Rerouting these lines improperly can starve the CHRA of this natural circulation, possibly causing long term damage.

15

TURBO

SELECION

AS

PART

OF

A

WHOLE

SYSTEM

AND

VOLUMETRIC EFFICIENCY
The most important characteristic of each turbo component is that they have to work well as a whole. If the exhaust housing was small, but it’s turbine was large, the airflow will get choked. If the exhaust housing is large but the turbine was small, the airflow will not be efficiently directed at the turbine. The exhaust and intake side also should be in harmony. The intake side compresses a certain amount of air into the motor and should expect an appropriately sized exhaust side to flow the exhaust gas back out. The whole system has to work in harmony to achieve efficient operation. Using a water funnel as an analogy, even if you put more and more water into it, there is a range of how much water can come out the bottom. An inappropriately sized and/or matched component will prevent the components from working in the area of good efficiency, performance, and value. Each mod should have a set of supporting modes working towards an overall goal. Before choosing components and modifying your car, have an estimate of about how much power you want, then design the modifications around making that overall goal. Meeting that power goal with the smallest turbo, the least turbo boost, and the most efficient intercooler, will all reduce engine stresses and maximize engine response. Also keep in mind that changing turbo components are only a part of increasing volumetric efficiency (VE). Adding camshafts, porting or tuning the intake manifold and cylinder heads, all change the volumetric efficiency and will further contribute to the efficiency of the engine. Note that modifying camshafts are more applicable to gasoline engines, but everything else listed can work on diesel engines too.

A/R RATIO
An important term to know when talking about turbo housings is the AR ratio. How does knowing this effect you in daily driving? Not at all, unless you plan on using anything other than the stock spec. turbo, but it is still a very useful aspect of turbo technology to know. The aspect ratio or AR is the ratio of the area of the cone to radius from the center hub. Basically, if you were to measure the cross section on any point on the turbo and divide by the distance from the center of that cross section to the center of the turbine wheel, you would get the AR ratio. Ideally, this 16

ratio should remain the same as you move in and out of the turbo housing because the housing gets smaller as you get closer to the center. This spiral shaped cone is called a volute. It begins right about the point in the housing where you can no longer see into it, after the inside diameter changes from the shape of the flange opening to the shape of the volute. Basically, it concentrates airflow at a point on the turbo wheels. Everything else being equal, increasing the AR will reduce spool up but increase top end performance by allowing more air to flow. Decreasing the AR will increase spool up but reduce top end performance. A larger AR will allow more air to flow through its passages.

A/R ratio is most useful when comparing flow capacities between like housings with similar exterior dimensions and different size volutes. In other words, turbo A with a .86 AR does not always flow more air than turbo B with a .64 AR. Turbo B could be a turbo 5 feet tall used in a power plant, and turbo A could be a 6 inch tall small motor turbo. When comparing AR ratio, the housings must be otherwise identical. Some VW turbos are called K03, K04, etc., this is only a general spec since there were many different K03 and K04 turbos, and most of them are not suitable for a TDI. In other words, don't buy a gasser VW turbo and bolt it onto your diesel because it won't work well.

INTERNAL VS EXTERNAL WASTEGATE
17

The exhaust housing may also house an internal wastegate. An internal wastegate is a hole cast into the exhaust housing and a flap door that opens to let excess gasses out instead of spinning the turbo and creating more boost on the intake side. If you look below, you can see pictures comparing an internal and external wastegate. If the shaft or wheels over speed, damage could result. A turbo wheel can spin from 0-100,000+ rpm. Metal turbine wheels are not as prone to damage as ceramic wheels, the VW TDI all use metal wheels. The wastegate door is is opened and closed by a spring loaded wastegate actuator. The wastegate actuator is basically a vacuum diaphragm which is normally closed from resistance from a spring. Once it receives boost pressure on one side of the vacuum diaphragm, it overcomes the spring pressure and pushes a lever that opens the wastegate. One method that chip tuners use to build power in turbo diesel cars is to reprogram the car's computer to hold the wastegate closed at higher than stock pressures to make more boost. In the below picture, you can see how the exhaust housing directs the exhaust gases onto the turbo wheel. The internal wastegate has a trap door that opens at a certain pressure; the external wastegate turbo requires a separate component, the external wastegate, placed upstream of the turbo to vent excess gasses. You can also see how the air flow out the external wastegate turbo matches the shape of the exhaust pipe, the internal wastegate has an empty spot where air turbulence can form. Some newer turbos have built in dividers and you can also make or buy an exhaust pipe with a divider to improve exhaust flow. This is in the 3rd picture and expanded on in the "split down pipe" section. An external wastegate is superior to an internal wastegate in terms of boost control and airflow. The piping exiting the exhaust housing can be made to match the size and shape of the exhaust turbine, creating a smooth transition from the turbo to the exhaust. This translates into more power everywhere in the rpm range. However, it often has a higher price since you have to pay for the wastegate separately from the turbo and takes up more space since it requires extra exhaust piping for the external wastegate. A good compromise between an internal wastegate and external wastegate is a split down pipe; see below for more details on down pipes. Some diesel turbos do not have a wastegate. Many VW turbos, specifically the VNT turbos, have a VNT actuator in the same spot as the wastegate actuator on conventional turbos. One of it's purposes is analogous to a wastegate because it redirects the turbo gases at the turbine wheel to control it's speed instead of dumping gasses out through the wastegate. 1996-1999 3rd generation (mk3) VW TDI turbos use a conventional turbo with an internal wastegate, all later generations used a VNT turbo. If the solenoid controlling pressure/vac to the wastegate malfunctions, the 18

default position is to open the wastegate. This is because if it were to fail in the closed position, the engine would create too much boost and incur serious damage.

TURBO EXHAUST FLOW

You want the least backpressure in the exhaust after the turbo for maximum performance, no exceptions. The problem is that you have to balance maximum performance with emissions and difficulty/cost of fabrication, etc.. Note that this does not apply to with non turbo or supercharged cars, where some exhaust backpressure is normal as a result of keeping exhaust velocity and the scavenging effect from individual cylinders high. Non turbo cars that keep their catalytic converters are not as significantly penalized by backpressure as turbo cars are. In most cases, non turbo exhausts want to restrict the piping diameter to some extent to keep exhaust gas velocity high and receive backpressure as a byproduct. With turbo exhausts, there is no scavenging effect 19

downstream of the turbo, so you want the least amount of backpressure after the turbo for the maximum performance and efficiency. A turbo exhaust should have the highest energy differential across the turbo (the exhaust gasses are also hot and have a lot of energy) to get the turbo spooled up, and the least backpressure and high velocity exhaust gases after the turbo. This is because a turbo gets its energy by a pressure ratio. Image a waterwheel: you want the pressure highest before the waterwheel and lowest after the waterwheel to give it the most energy. How much power is released by putting a straight pipe exhaust on a TDI? First, remember that total power is the area under a power curve, not just peak power. You may not gain much peak power with only an exhaust change but the total amount of power will increase. Also remember that an exhaust is a basic supporting mod for any future modifications such as a chip, larger turbos, fuel nozzles, etc. A TDI diesel is throttled by fuel and uses a relatively small turbo with computer controlled fueling. Peak power may not go up much but it will increase response and area under the power curve, and let any further mods reach their full potential. Below are some more details on individual components of exhaust systems. Because you want the least backpressure in a turbo car's exhaust, the ideal exhaust system would produce the least backpressure immediately after the turbo. Due to routing, emissions equipment, pipe diameter, exhaust gas temperatures/pressures, the perfect diameter changes from car to car, setup to setup. It's very difficult to know this without extensive testing, so as a rough rule of thumb, a consistent or increasing diameter exhaust as you head downstream towards the tailpipe is best in most cases. Mandrel bent exhausts are also always better. A mandrel bend is when piping is bent with a mandrel, or insert, to keep the inner diameter consistent at the bend. Crush bends reduce the diameter at the bend and reduce smooth exhaust flow. Most factory exhausts are non-mandrel bent crush style bends, so switching to a mandrel bent exhaust will increase power and efficiency of the turbos and engine with no other modifications. You also want to avoid very restrictive mufflers, sharp changes in piping diameter, and sharp bends. As a rough rule of thumb, each 90o bend in the piping has about the same resistance to airflow as 25 feet of straight piping! For an extreme level of modification, you could also switch to an equal length runner exhaust manifold for the turbo (the part that is between the turbo and cylinder head). Equal length runners make sure that the exhaust pulses are timed so that they take the same amount of time to hit the turbine and to keep cylinder reversion balanced across all cylinders. The stock VW exhaust manifold and most stock turbo exhaust manifolds are the log style manifold due to a number of factors. The log style is much cheaper and easier to make, are usually cast in one piece of iron so that they don't have weak welds that can crack with repeated heat cycling, expansion, and stress, 20

and also take up less room. However, the amount of custom fabrication is so high that you would be better off spending your money and time improving other areas in the turbo system first. This is because most turbo diesel passenger cars serve as daily drivers and are not yet at a level where a tubular manifold would be an economical power upgrade. Below is a picture of a tubular header.

WHY TO CHANGE OEM EXHAUST SYSTEM? You may be wondering how much gain in exhaust flow you will gain over your OEM exhaust. There will always be an increase in efficiency in switching to a quality aftermarket exhaust. Unless you make a measurement of backpressure, there is no way to quantitatively know how much. Even two identical cars may be slightly different due to manufacturing tolerances. Much like any other modification to your car, custom parts will cost a lot more than if it were a mass produced part by a parts supplier. All you can know for sure is that it will be an improvement over your OEM exhaust as long as the replacement parts are quality pieces. The OEM part has to conform to emissions and noise regulations that vary country to country, be easily produced and fabricated thousands of times, and may only be, as an example, 75% efficient. By replacing it with a part that is 95% efficient, you might end up spending $$$. As a result, work with your budget to reach your realistic power goals. If it's worth the money is ultimately up to you, some people would rather spend the money on something other than a car. 21

So why didn't your car maker just give you a 95% efficient exhaust? Remember that if all the parts on your car were just one level better, it would result in a car that is for example, $5000 more expensive. If they put all luxury car parts on an economy car, it wouldn't be an economy car would it? Car makers have to balance the quality of parts on a car to get the most perceived consumer desirability out of it. Since the interior and exterior are what buyers see and touch, some car makers prefer to spend the money there.

BACKPRESSURE IN EXHAUST HOUSING
One way to test how much back pressure you have is to take a reading. Tap the exhaust system before the turbo with any pressure gauge. An oil pressure gauge or low range air pressure gauge will both measure the backpressure in the exhaust. I suggest putting an air filter or fuel filter inline to dampen the exhaust pulses so you can get a steady measurement. Once you hit boost, note the peak hold value. Once the pressure has peaked, you have reached the engine's max VE. As a rule of thumb, you don’t want more than a 1:1.5 ratio of boost to backpressure. For example, if you are making 10psi of boost you don't want more than 15-18 psi of backpressure. If so, then the turbine side could benefit from more air flow and you’ll make more horsepower for every pound of boost you run. Keep in mind that the turbo wheels are not easily changed except by turbo rebuilding professionals, so for most users, the basic rule of thumb should be: Between the exhaust ports and the turbine housing, you want as much energy going through that turbo. This means metals that don't soak up the heat, heat reflecting coatings, short piping, and tubular headers. Keep in mind that if the turbine housing can't flow enough air, the effect of these improvements will be lessened. Also keep in mind that while an exhaust manifold made from stainless steel can be welded into to a better flowing manifold, it will get red hot if driven hard and will be more prone to cracking at the welds compared to a cast iron manifold. After the turbine housing, you want the greatest heat and pressure differential. This means a down pipe that is not coated and free flow exhausts. Test pipes or straight exhausts would be considered more or less free flow exhausts.

TEST PIPES VS. CATALYTIC COVERTERS AND BIODIESEL EFFECT
22

Tests pipes are basically pipes that replace the section of exhaust that contains the catalytic converters. It is for off-road use only and is illegal in every state! In fact, removing the catalytic converters and the O2 sensors will cause error codes to appear in many cars, especially obd2+ cars. OBD2+ gasoline cars often have an O2 sensor before and after the catalytic converter. Note that VW diesels did not use an O2 sensor in the exhaust except 2004-2006 cars that use pump Duse and later TDI. If removed and not worked around with a chip or resistor, it could set a check engine light and can cause a failure of any required emissions testing or inspections, preventing you from registering your car in some states. There are also fines for removing or tampering with factory emissions equipment on cars. If there are so many negatives to test pipes, then why do many people use them? Power and economy are both increased with test pipes, especially in turbo engines. In designing a turbo system, the engineers want to have the highest energy differential before and after the turbine wheel. This energy (exhaust gas velocity, heat, pressure) differential transfers energy to the turbo system. By removing the catalytic converters and that restriction in the exhaust system, you create greater a pressure differential for the turbo between the compressor and turbine side and let the turbo work "easier" and better. Keep in mind that this is for turbo cars only! Non-turbo or supercharged cars do not have turbos and the potential performance gains are not as great. Another factor is that while the catalytic converters act as a restriction in exhaust flow, they also add energy and velocity by burning off unburned hydrocarbons in an exothermic oxidation. This is still not enough to overcome their restriction in flow, but it's not like stuffing a potato in the exhaust pipe if that's what you were imagining. A catalytic converter is actually honeycombed or grid-like, to allow exhaust to flow through. All in all, especially for diesel applications, I would recommend leaving the catalytic converters in place. Leaving the catalytic converters in place will both clean the exhaust emissions, make the exhaust much cleaner and quieter, and is less expensive then making custom piping. The TDI is an excellent daily driver and I didn't want to tolerate the increased smoke, odor, and emissions for the trade off in increased peak power and throttle response. If you want an all out sports car, the TDI will not satisfy you and if it does, you never wanted a real sports car. In the end, it's up to you to determine if you want to remove it, but remember that in some states with emissions or inspection requirements, you may not pass without a muffler or catalytic converter. Another reason to bypass the exhaust filters is if you are using biodiesel.

23

Biodiesel, especially homebrew or contaminated biodiesel may cause the newest generation of diesel exhaust filters to become clogged with particulates. Some diesel filters, especially the Bluetec filter system sold in Mercedes, upcoming Audis and VW TDI, use a series of filters to catch diesel particulate emissions. At a set interval, the car's computer dramatically raises the exhaust gas temperatures to burn off the particulates and clean the filters. Home made biodiesel may put excessive byproducts and unreacted chemicals into the filters and cause them to become clogged. This is also a problem if you use the older non ultra low sulphur fuel, no longer available in the USA or Europe but still used in some parts of the world. There is still an ongoing debate since the Bluetec filter system is still so new and the urea injection systems are not widely tested with biodiesel. The filter is what gives petrol diesel such low emissions and the irony is that biodiesel is already a low emissions fuel. A resonator is welded on the left side to help quiet any droning resonating "booming" noise that many free flow exhausts will make at certain rpm. A louvered resonator causes turbulence and reduces exhaust flow but is quieter than a perforated hole resonator which has little effect on flow but is not as quiet. Remember loud = tickets and a catalytic converter is the best way to reduce emissions and keep the exhaust on the quiet side. control exhaust resonation. A common complaint with free flow straight pipe exhausts is exhaust resonation noise. In fact, many people have it but don't acknowledge it because they think it's just loud and actually like it. Resonation differs from loudness because it has a certain boominess, rattling, buzzing, or hollow vibration sounds at certain RPM. There are many possible causes, but some ways to get rid of it are installing a venturi along with a resonator at strategic positions along the exhaust, controlling flutter of the exhaust by smoothing out sharp corners in the exhaust or downpipe, or slowing the exhaust velocity by restoring the catalytic converter (contrary to performance increases but it will make the car better for daily driving). Pictured below is a venturi, this can also help

24

THE DOWNPIPE- SPLIT AND SINGLE PIPE A down pipe is the exhaust pipe immediately after the turbo. It could also be called an up-pipe, but due to the configuration of most engines, the exhaust is normally directed down after exiting the turbo. It normally is a single pipe that collects the exhaust from both the turbine output and wastegate output. From the above picture, you can see that there is also a lot of empty room for exhaust gases to become turbulent upon exiting the turbine in an internal wastegate housing. When the wastegate opens, the tumbling exhaust coming out of the wastegate collides with the spinning air exiting the turbine. This scenario is devastating to the goal of smooth airflow. This area of turbulence saps power because the air around the turbine isn't evacuated as smoothly as possible. Note that this is not a problem for a housing without an internal wastegate, such as 25

external wastegate and many diesel VNT turbos. Also note that some turbos have the initial section of downpipe as part of the exhaust housing. Another difference between your TDI downpipe and gasoline downpipe is that your TDI downpipe is just a pipe while gasoline car downpipes have a small catalytic converter immediately downstream of the turbo. The reason why is because the cast iron manifold and turbo absorb heat and can quadruple the time for the catalytic converter to heat up and start cleaning emissions. 90% of a car's emissions are during cold start and the small catalytic converter is needed to take care of these emissions. While removing it is illegal and will make your car's emissions much worse, removal will make a big difference in how the turbo spools up. A split downpipe is a downpipe with two separated pipes, one for the turbine exhaust, and one for the wastegate exhaust. It may have a machined separator for the empty space between the turbine outlet and wastegate or a section of pipe. By smoothing out the airflow, it enhances airflow all throughout the rpm range. The two split pipes then rejoin down the exhaust path. Here are some pictures of split downpipes. One has a split that is longer than the other. The point of diminishing returns is about 12"-18" for uninterrupted flow before rejoining the wastegate piping to the main exhaust flow. The second picture below also has detail of the machined wastegate separator at one end instead of using a section of pipe to separate the exhaust streams.

26

This last pictured downpipe is also slightly different in that it has an expansion chamber, a chamber where the diameter of the piping expands as you go downstream. A gradual expansion at the turbine outlet via a straight conical diffuser of 7-12° is ideal, depending on factors such as space within the engine bay, exhaust gas velocity, temperature, and volume. Too great or too abrupt of a transition, and you get flow separation and turbulence, reducing flow. Ideally, the best flow would be achieved by a "trumpet" shaped downpipe that exits into an area below the car, but this is obviously not legal or safe as it would be very loud without mufflers and the exhaust fumes would quickly injure or even kill you since the exhaust would surround and maybe leak into the cabin. You want the highest exhaust velocity after the turbine, and while bigger normally equals better, too large of an exhaust will cool the exhaust, reduce it's velocity, and create excess backpressure. A side effect of a more gradual expansion and wastegate pipe is that it sounds much smoother than a pipe which could cause resonation at certain rpm due to the fluttering of the exhaust.

27

Also note that these downpipes all have O2 sensor bungs welded in them because they are for gasoline cars, although exhaust flow theory is the same for diesel cars. Keep in mind that many diesel turbos, especially the newer diesel turbos do not have a wastegate, they use the variable nozzles (VNT) within the exhaust housing to control turbo speeds. Without a wastegate, an excellent downpipe would look the same as the below picture, except without the smaller pipe for the wastegate. Also note how the welds are ground down on the inside to smooth out the flow as the pipe diameter gradually increases.

TURBO LAG :The period between pushing on the throttle pedal and feeling the rush of acceleration is commonly referred to as lag. Lag is a symptom of the time it takes for the exhaust turbine wheel to overcome 28

its rotational inertia and for the intake impeller to create positive pressure in the intake. Just remember that although it changes the feeling of the power curve, a turbo car usually makes more power over every part of the power curve compared to an identical non turbo car. Lag can be reduced by lowering the rotational inertia of the turbine or by use of ball bearings. Manufacturers may use lighter parts such as ceramic turbo wheels to allow faster spool-up. Another way to reduce lag is to change the aspect ratio of the turbine by reducing the diameter and increasing the gas-flow path-length. Increasing the upper-deck air pressure and improving the wastegate response helps but there are cost increases and reliability disadvantages as well.

CHRA :The center housing rotating assembly (CHRA) is the center section that contains the bearings which hold the main shaft connecting the intake and exhaust wheels and the coolant and oil lines. Older turbos use bronze journal bearings, a machined bronze cylinder to hold the main shaft. Much like a crankshaft bearing, it is lubricated generously by oil from the engine and held in place by a thrust bearing. While pressurized by oil, the journal bearing is floating and spinning on a layer of oil. Some newer turbos use chromium/carbon steel ball bearings to hold the main shaft. The fastest turbos use ceramic ball bearings that are much more durable than steel ball bearings. Ceramic ball bearings can handle significantly higher safe operating rpm than comparable steel ball bearings. The advantages of ball bearings include better damping and control over shaft motion. In addition, the opposed angular contact bearing cartridge eliminates the need for a thrust bearing, a common source of damage and oil leaks. Ball bearings also spool faster and harder compared to an identical journal bearing at the same rpm. There is reduced drag on the turbo shaft which increases performance and can be felt. Ball bearings also require much less oil required to provide adequate lubrication than a journal bearing turbo. This lower oil volume also reduces the chance for seal leakage. But if they receive too much oil, the ball bearings will actually skid in their races, creating wear in one spot, quickly damaging the ball bearings. If you exchange your old journal bearing turbo for a new BB turbo and don't change the oil lines, be prepared for smoke due to excess oil leaking out the exhaust side. To the right is a picture of a ball bearing vs. journal

29

bearing oil feed restrictor. The journal bearing oil line is the larger diameter one - quite a significant difference!

The most problematic part

of a turbo is normally the

CHRA. The intake and exhaust housings are just nonmoving cast metal housings. They generally do not get damaged unless the exhaust side is overheated and cracks, breaks an inlet or outlet flange, or damage to the exhaust transmits force to the exhaust housing and cracks it. The turbine blades generally do not get break unless a foreign object falls into the turbo or air intake. But a worn or damaged CHRA can allow shaft play and damage the turbines. Below is a non VNT conventional turbo disassembled. Note the ball bearing instead of journal bearings and damage to the compressor wheel. A ball bearing is not rebuildable, the most reliable way to reuse your old turbo is to reuse old cast iron housings with a brand new CHRA and components.

TURBO RUNWAY IN A DIESEL ENGINE Another problem with the CHRA is that the oil can leak out from worn seals and cause a runaway engine. The turbo runaway is a variation of the diesel engine runaway. Older turbos use a 270o thrust bearing on the compressor side that holds the journal bearing in place. Some newer types use a 360o thrust bearing that holds the thrust bearing in place even better because they distribute the load across a wider area, see below for a picture. Some older VW TDI turbos use 270o bearings, some use a 360o. The VNT turbos use a 360o bearing. I wouldn't worry about the bearings used in the TDI since the difference in wear is marginal. With proper care and synthetic oil, the thrust bearing can last the life of the turbo. However, higher boost pressure and excessive thrust movement (caused by manufacturing issues and worn bearings) can cause excessive wear and play and can let oil leak out. Both compressor and turbine sides of the turbo can respectively leak oil out the intake or exhaust sides. If oil goes out the exhaust, it will cause black or "blue" smoke and soot and may shorten the life of the catalytic converter due to melting or clogging. If oil goes into the intake, it can cause a more serious problem for turbo diesels: the engine runaway. 30

In a gasoline turbo engine, oil in the fuel effectively reduces the octane of the fuel and can make the engine more likely to detonate. In a diesel engine, it can result in a runaway engine. Both conditions can result in damaged or destroyed engines. Since your diesel engine will run off engine oil, it actually increases the rpm by increasing the amount of fuel consumed (the engine oil). This is a diesel runaway. The line between a leaky turbo and an engine runaway is when the engine suddenly increases in rpm and draws the engine oil out of the turbo seals and feeds off that oil, raising the rpm, drawing even more oil out. The engine will run faster and faster until it overspeeds and breaks, or runs out of engine oil and seizes, both conditions resulting in total engine failure and a possible car crash. Once you reach a certain point, even taking your foot off of the accelerator pedal won't stop it since diesel engines don't have throttles! The engine will continue 31

to run faster and faster because it only needs air and fuel to run. Cutting off the diesel fuel won't 100% stop it because it is feeding off the engine oil. All mk4 ALH and later cars have anti-shudder valves or throttles that can shut off the air when you turn the ignition key to off. If so equipped, your first step to stop a runaway engine is to shut off the ignition and pull over as soon as practical. 1998-2003 engines use a vacuum operated valve, 2004-2006 engines use a more robust valve that may be better at stopping an engine runaway. Although the anti shudder valve can't stop the most severe runaway engines, I would still leave the valve in place. You can also put the car in the highest gear and step firmly on the brakes to slow you down and stall the engine. If you put the car in neutral or go to a lower gear, there will be less resistance on the engine and it will quickly over-speed and fail. A runaway engine can also be caused by a number of other problems such as excessive crankcase pressurization, older VW diesels had other conditions that could cause a runaway. Because this a turbo charging article focusing on modern TDI, a leaking intake turbo seal is among the most common reasons for a runaway engine on these modern engines. Also remember that an engine runaway from eating oil occurs in diesel engines only, gasoline engines can't run off oil. Always follow common safety practices! If you feel the engine runaway, don't risk getting rear ended on the freeway and personal injury to yourself and others, only pull over as soon as is safe and practical. It's not worth risking an accident to save the engine. Once the engine is stopped after a runaway, do not start it again. Have it towed to a diesel mechanic and explain that the engine had a diesel runaway. If you stopped it successfully, you should remove the piping around the intercooler. A little oil is normal but a lot could be a symptom of a runaway. You should do further diagnosis to make sure where the oil is coming from. If you let the engine runaway for a while and it stopped on it's own, it's likely that something was damaged. It either sucked enough oil that the engine seized from lack of lubrication or the engine internals were damaged from hydro lock. Further diagnosis is needed, don't try starting the engine again just to see if it starts.

32

OIL SUPPLY AND TURBO TIMERS The biggest area of concern in the turbo is the oil supply. Insufficient oil (especially journal bearing turbos) or excess oil (especially in ball bearing turbos) or dirty oil may wear out the bearings, causing wear and shaft play in the turbo. Because of the high turbo temperatures seen in turbocharged cars, the oil may also break down faster than a comparable non turbo car. Synthetic oil is recommended in turbo applications because it doesn't break down as quickly as conventional oil. Because the best engine oils for a diesel engine are synthetics, this is another reason to use synthetic in turbo and diesel applications if you are not already doing so. In addition, since the turbo can get hot when running, an engine idling period of 5-10 seconds once at a complete stop should be enough to let fresh oil circulate to the turbo bearings before engine shutdown. If driving very vigorously, a 1 minute idling period or a few minutes of sensible driving before shut down should be enough to let the turbo cool down and receive fresh oil. If the turbo is too hot and does not receive cooler oil upon shutdown, the oil could become burnt and "coking" may occur. This is more of an issue with non synthetic oils. Another issue is letting fresh coolant circulate to the CHRA. After engine shut down, the coolant heats and expands in the cartridge if the CHRA is too hot. This creates a natural circulation to drain away the heat and bring in fresh coolant. The reason it doesn't boil off is the same reason engine coolant doesn't boil off - the engine coolant is a sealed system. Some cars have auxiliary pumps that circulate coolant after engine shut down. There would be no benefit to this on a TDI since the turbos are oil cooled only and not water cooled, and because of the lower temperatures that you should see during engine shut down due to a diesel engine and because of good shut down practices. Even on gasoline water cooled turbos, if it didn't come from the factory with an auxiliary pump, I would not add one since the engineers didn't put one there and because there is some natural convection of coolant and oil. I do not recommend rerouting the oil or coolant lines in your turbo unless you are sure they are routed properly. If you improperly reroute the coolant or oil lines, this could disrupt the natural circulation after shut down. I also recommend never using radiator "stop leak" products because they can gum up and clog the turbo coolant lines. You should also not install any kind of inline oil prefilter upstream of the turbo oil supply line. Some newer Subaru gas turbo cars suffered destroyed turbos from oil starvation. These were traced to a change in how the factory routed the turbo oil supply - inline oil filters were added and became clogged, causing oil starvation. Use of VW approved engine oils in the TDI is also recommended to ensure proper lubrication to the turbo. The big shift for North American market cars was in 2004 with the introduction of the 33

pumpe duse engine and in 2009 with the common rail engine. These engines see very high pressures in the head and should use VW approved engine oil to keep your warranty intact. The common rail engine in the 2009 TDI uses VW spec 507.00 engine oil, you might be able to use other engine oils but to keep your warranty intact, stick to the VW spec, especially since this is a new engine and there isn't any aftermarket data out there yet. Some people install a turbo timer to keep the engine idling so they can walk away from their car during a cool down period. I do not recommend these products for a number of reasons. First, if you have a manual transmission, you should always put it in first or reverse gear when parking in addition to applying the parking brake, so the convenience of walking away with the car idling is not possible. Also, a turbo timer requires spending money on the timer, cutting wires and introducing an unnecessary failure point. Lastly, for diesel applications, coking is not as common of a problem due to the lower rpm and cooler exhaust gas temperatures, and you should be using synthetic oil anyways which is more resistant to coking. If you are truly concerned about turbo care, just make sure that you drive at medium rpms and low load when the engine is still warming up and just drive sensibly a few minutes before shutting the engine down. THE INTERCOOLER Another component essential to the turbo charging system is the intercooler. As the turbo compresses the air, it heats up - an intercooler lowers the air temps. The ideal gas law states that when all other variables are held constant, if pressure is increased in a system so wills temperature. The turbocharger also radiates some heat into the air because it's hot from all of the exhaust gasses passing through the exhaust side of the turbo. The hot under hood air also heats up all of the intake piping (turbo cars have more piping than non turbo cars). The mechanical agitation of the air by the turbo wheel also heats it up a little. Hotter than ambient air is one of the losses in efficiency associated with turbo charging because the air gets hotter than what an average nonturbo engine gets. This increases the likelihood of uncontrolled detonation and engine damage. An intercooler is basically heat sinks that takes away the heat of the intake charge and cool it as much as possible. Here is a picture of an intercooler in a Mk4 jetta TDI. The yellow outline marks the intercooler, the intercooler intake and outlet. The arrow marks the front of the car, where the cooling ambient air enters from.

34

You don't see intercoolers on non-turbo cars because the intake air is already at ambient temperature. An air intake directly connected to an intercooler or anywhere not after the turbo would actually decrease performance by restricting airflow. Below is a silly picture of an "intercooler", someone who put an intercooler on a non turbo car.

35

The goal of intercooling is to produce the least pressure drop and the most heat transfer to the metal and air or water, whatever the cooling medium is. Any well designed intercooler may have about .5-2.0 psi pressure drop due to pressure losses involved with the process of cooling the air. A good air-air intercooler can cool the air to within 20 degrees of ambient temperature if it has steady airflow to take away the heat. The advantage of a good air-water intercooler is more consistent intake air temperatures since the water (coolant) is not as quickly affected by rapid changes in ambient air temperatures and car speed. Some cars don't have the routing or space for a good air-air intercooler so they can also use an air-water intercooler. An air-air intercooler is preferred for diesels, more details below. An intercooler is acting more like a heat sink and less like a radiator when boosting. The intercooler gets the hottest after the turbo heats the turbo output air. After absorbing the heat, the intercooler releases the heat into the ambient air or coolant. In a gasoline engine, the engine is operating at vacuum or low boost most of the time. Low boost does not heat the turbo outlet air as much as hard boosting and as a result, doesn't transfer as much heat to the intercooler. In other words, a larger intercooler is not needed unless you need the extra heat sink capability! Most modified gasoline cars would benefit a little from a larger intercooler due to higher than stock boost levels. However, how much it's needed in only lightly modified cars is debatable due to variations between cars, ambient outside temperatures, intended use (street vs. track), desired safety margin and fuel octane, etc.. For example, a large front mount intercooler will cool better than a small intercooler but it may not fit, may be blocked by the bumper, cause overheating 36

problems due to blocking the radiator, etc.. Another issue is that like any other heat sink, after the intercooler absorbs heat, it releases it into ambient air AND the intake air. As long the car is in motion, most of the heat is carried away by ambient air.

Because of this, it's best to maximize the intercooler efficiency by leaving it unpainted and keeping the core unobstructed. The VW TDI naturally puts an oily mist on the inside of the intercooler but trying to keep the inside clean is like keeping the oil dipstick clean. Gasoline cars shouldn't have any oil inside the intercooler. Also check for leaves or dirt blocking the face of the intercooler. If you must paint it to help hide the intercooler, use 1-2 light sprays of radiator paint or even better, a heat shedding coating like Swaintech's "BBE heat emitting coating". Depending on ambient temperature, intercooler size, intake temps, etc., a heat shedding coating can lower intercooler temps by as much as 25o F. You can also spray coolant onto the outside of the intercooler, lowering the temperature of the intake air below ambient air temps. CO2 (compressed carbon dioxide gas), N2O (nitrous), and just regular water all work very well at increasing intercooler effectiveness but only work until your coolant runs out. If you are preparing to race, placing bags of ice on an air-air intercooler or chilling the coolant in a water-air intercooler works well too. Heat coatings won't lower the temps as much as using a coolant but are constantly working and don't run out. A diesel engine has a greater need for an effective heat sink than a comparable gasoline engine. In a diesel engine, turbos are normally smaller compared to a gasoline engine for a number of reasons, for example, the smaller rpm range. They also tend to use higher boost levels than a comparable gasoline engine. I think that even lightly modified VW TDI cars could benefit from more efficient intercooling for maximum peak power. With an air-water intercooler, the more stable temperature is harder to cool because once it's hot, it tends to stay hot longer than an air-air 37

intercooler. An air-air intercooler is also easier to fabricate with less chance for leaks. If there was a water leak into the intercooler core, it's possible that this could hydrolock the engine, so for these reasons I believe that air-air intercoolers are preferred for diesels. A air-water intercooler would be better on a mid engine car due to difficulty of intercooler packaging. Turbo pressure in most cars is regulated by how much pressure is seen at the intake manifold. Some also measure the air temp at or near the manifold. Regardless of intercooler efficiency, pressure at the intake manifold should remain about the same. For example, compare an engine that limits boost to 15 psi at the intake manifold. If you have two turbo setups, one with an efficient intercooler with only 1 psi pressure drop and the other with than an inefficient intercooler with 4 psi pressure drop, the turbo with the efficient intercooler only has to make 16 psi at the turbo whereas the inefficient setup has to make 19 psi at the turbo. The turbo making 19 psi is mechanically more stressed and is creating more heat than the turbo that has to make only 16 psi, assuming that they are both operating in an area of normal operation and efficiency. If the turbo is pushed beyond the normal area of efficiency for the turbo, it will create exponentially greater amounts of heat and pressure. Pressure does not equal density, you are still creating the same amount of pressure seen at the intake manifold that regulates the turbo, but the air is less dense and hotter, which creates less engine power and efficiency.

FLOW IMPROVEMENTS
Another way to increase the efficiency of your general setup is to improve the pre and post turbo and intercooler piping. This will reduce pumping losses. In the VW TDI, this can be difficult due to the turbo, intercooler, and battery locations. The best piping would be relatively smooth on the inside (mandrel bends), have a relatively straight path or gradual angles and transitions, and be as short as possible. The shortest, smoothest pipe routing on a transverse 4 cylinder engine would be from a turbo in the front, with a 180o loop to a front or side mounted intercooler and then a 180 o loop back to the intake manifold. This is not possible on the VW TDI due to the rear mounted turbo location but you can still improve the existing piping. When putting together an aftermarket setup, use piping that has mandrel bends with straight silicone couplers instead of using straight pipes with bent silicone couplers. Silicone couplers tend to collapse at tight spots and can bend, reducing the cross sectional area. Due to varying fitment, they also tend to have more gaps between the piping, disturbing airflow more than necessary. Shortening the intake piping, making the transitions between piping as smooth as possible, and and routing the piping as straight as possible will reduce the amount of required pressure to 38

produce a certain amount of power, increasing reliability and efficiency. A rough rule of thumb is that each 90o bend in pipe adds as much resistance to airflow as 25 ft of straight piping. Of course, this depends highly on diameter, smoothness of bend, etc., but generally speaking, short straight piping is best for flow in the intake. Some people think that larger piping or a larger intercooler increases lag. This is true because it takes longer to fill and pressurize the larger piping and intercooler. However, the difference is very small, especially considering the small, quick spooling turbos on the TDI. Everything else being equal, the difference in turbo response will not be greater than 1/10th of a second unless you are going from no intercooler to a huge intercooler. In addition, if the intercooler was the bottleneck in the system, the loss of throttle response is not even ANY factor because the gain of your other upgrades offsets any additional lag from pressurizing the greater volume. Exhaust backpressure, chip tuning, and turbo size is a far greater factor in throttle and turbo response than intake piping, so don't worry about piping too much. Again, intake piping makes a difference but on the TDI the priority is lower compared to a turbo, injectors, exhaust, and chip. The one thing to be wary of with the VW TDI is using high flow air filters. The mass air flow sensors (MAF or MAS) on the Mk4+ body seem to be sensitive to the additional dust and debris that a high flow air filter, especially aftermarket oiled cotton filters let into the intake tract. The stock air filter and housing was overbuilt and uses the same part as the 240 horsepower Golf R32, so there is little-no gain by switching to a high flow air filter anyways. Lastly, most cold air filters often don't use a cold air intake snorkel. This draws in hot underhood air and can actually reduce power.

TURBOCHARGING YOUR OWN CAR
All modern diesel passenger car and truck engines are turbocharged, but some readers may be wondering if you could turbocharge an older nonturbo diesel or nonturbo gasoline car. The short answer is yes! The long answer is that for most cars, it is such a large project, requiring such a large amount of custom fabrication, custom tuning, uncertain results, and lots of money, that the same amount of money could go towards buying another car that is already turbocharged and would not require such a large amount of effort and risk. In other words, if you have to ask if it's possible, the project is way over your head!

39

Some popular nonturbo cars have kits that have already been tried by many other people. In these cases, the risks are minimal because there are other people who can give you advice. But remember that it is often easier just to buy another car that is already turbocharged. The time that you spend on the project and then fixing all the problems that show up would be better spent working at a job so you can make more money and just buy the other car. For example, below is an advertisement by Porsche showing the upgraded parts between a 944 and 944 turbo. See all the extra parts that wouldn't be on your car if you just added a turbo and parts to make the turbo work?

With some turbo cars, they already sell higher end models with everything you want already on it, so it's not economical at all to spend money on increasing the performance of the base turbo car. For example, the Subaru WRX and Mitubishi lancer ralliart have less power, simpler suspension and all wheel drive systems, different interior and trim levels, etc., compared to the STI WRX and Evolution. You would spend 10x the money and time upgrading the base turbo car to the high end turbo car, it makes more sense to sell your car and just buy the higher end model. Ultimately, 40

it is your car, your money, and your responsibility, so FYI, here are some more cautions if you want to continue. The biggest problem is that a nonturbo car was not engineered for turbocharging and that people generally do not know the full consequences of turbocharging. For example, the transmission may only be designed to hold the amount of power from the nonturbo engine. If you were to increase the engine's power, the transmission could be more easily worn out and break. The clutch may not hold the amount of increased power, so you would have to replace the clutch and pressure plate with one that could withstand more power. But then, the clutch hydraulic system may not be able to handle the increased pressure required to actuate the clutch so you might have to change the components or rebuild them. The clutch pedal's metal may be designed for light pressure, and having high clutch pedal pressure could deform or wear out the clutch pedal levers and bushings. Some newer cars use plastic clutch pedals and they have cracked under very heavy and high pressure use. Some cars are susceptible to thrust bearing wear on the crankshaft from a stronger clutch pressure plate. The intake tract, including the various throttle gaskets and seals, piping, and vacuum lines may not be designed for positive pressure. Putting these components under boost can pop them off or cause small leaks that only show up under pressure and blow various seals. The engineers who built your car can't overbuild everything that they want to, otherwise your car would be as heavy as a tank and cost $100,000. So even if "x" is reliable at higher power levels, "y" breaks. Again, each car model is different. With modern traction control and stability controls, the car can also restrict power if it senses the car moving faster than it was designed to. As an extreme example, with the stock engine, even under the most favorable conditions, your car may accelerate to 0-60 in 7 seconds. If the car's computer sees your car accelerating to 0-60 in 3 seconds, it knows that something is wrong or assumes that the tires are spinning on ice, and reduces power, applies traction control, applies the brakes to regain control, etc. This is not a problem with VW or VW TDI cars, but this obstacle is starting to appear on some German cars. The compression ratio is also higher in nonturbo cars. This is true for both diesel and gasoline cars. Because of the higher compression ratio, it limits the amount of pressure and boost you can use. This pressure also creates the need for stronger pistons. The pistons in turbo cars also tend to have oil squirters that direct oil at the inside top of the piston which help carry away the additional heat of combustion.

41

Lastly, the typical nonturbo engines is not built as robustly as turbo engines. This includes the seals and gaskets, the moving metal parts of the engine, the bearings, and the engine block itself. If you're lucky, the engine's setup will result in cascading failures starting with easy to fix problems appearing first. If you're not lucky, the engine will be totally destroyed. For the same reason that you can't take a gasoline engine and turn it into a diesel engine (and expect it to last), most nonturbo engines are not designed to stand up to the stresses of turbocharging. For example, pictured below is a girdle or cage around the crankshaft bearings on a turbo car.

It depends on the car and the turbo kit, but if you want to turbocharge a nonturbo car and maintain the same reliability, your best bet is an engine rebuild with more robust components and a change of the compression ratio. You can add a turbo to your exiting engine but it will not be as durable and you will not get the same results without an engine rebuild with different components. Here is a picture of what can happen if you try to boost too much on an engine not originally engineered for turbocharging. Of course, this can also happen if you boost too much on a turbo engine, but turbo engines are normally engineered to be more resistant to abuse. If you think I am against turbocharging your own car, you are right. This section is written for the person who asks, "I saw a turbo kit on ebay that said it supports 500 horsepower and costs only $500". Even worse, "my ebay electric turbocharger is even better than your kit". Because most people run out of money or don't know how to do the job right, pictured right is what I think of when I see a DIY job. Ironically, the CRX is a car which a lot of people have successfully turbocharged with great results! There are many successful turbocharging jobs, but it requires either a lot of cash to pay someone else to do it, or a certain level of turbo and mechanical knowledge and experience. 42

A final (or first, depending on your view) consideration for DIY turbocharging is emissions and emissions testing. Catalytic converters need to heat up from the exhaust before they start to work well. Modern cars are so clean and catalytic converters so good that the majority of emissions are during cold engine starts. Adding a turbocharger between the engine and catalytic converter will result in much greater emissions during cold engine starts because a heavy cast iron lump absorbs heat energy instead of warming up the catalytic converter. It also takes away energy to spin the turbine wheel. Factory turbocharged cars are engineered from the factory to meet emissions and adding a turbo will result in significantly greater emissions during cold starts and the possible failure of emissions testing. A gasoline car with a DIY turbo that is warmed up, in good working order, and is tuned well, with catalytic converters, should pass emissions. If the car is cold and had to wait in line at the emissions testing facility, or is poorly tuned, it will probably fail. Without catalytic converters, there's no way it will pass emissions. Modern passenger car diesels are all turbocharged and many states don't have diesel emission testing, so this is more of a problem for gasoline cars

SEQUENTIAL TWIN TURBOS VS SYMMETRICAL TWIN TURBOS VS SINGLE TURBO
Some cars have twin turbos instead of single turbos and some cars that came from the factory with twin turbos are aftermarket converted to single turbos. The main configurations of twin turbos are parallel/symmetrical twin turbos, or asymmetrical sequential twin turbos. Parallel/symmetrical twin turbos are found mostly on V-configured engines such as the 300zx twin turbo or Audi S4 biturbo. They are most appropriate for V configured engines because each side of the V engine feeds one turbo and all the piping is kept equal. Both turbos should be equally sized to keep the engine balanced. Factory setups that use this configuration generally provide more low end power because the twin turbos will generally be smaller than one large turbo but a V engine can also produce more torque, so it really depends on the engine and design. Symmetrical twin turbos can also be found on the BMW 335i inline twin turbo gasoline engine but in a different alignment. Below is a cutaway picture of the 335i engine. Note that each turbo is fed from 3 cylinders only and lead into a shared outlet pipe before the intercooler (pointing to the right)

43

.

Inline engines can also be fitted with a another type of turbo configuration. A twin asymmetrical sequential configuration is used in the Supra or RX-7 twin turbo gasoline cars or the BMW 535d twin turbo diesel. Sequential twin turbos are most suitable for inline engines because the exhaust stream is coming out only one side and the piping is simple and short. If you tried to use sequential twin turbos in a V engine, the piping would have to be routed all the way around the engine, creating piping and space problems. Asymmetrical twin turbos use one smaller turbo for lower rpm and one larger turbo for higher rpm. The exhaust gasses are normally diverted to the smaller turbo until a certain air flow is achieved, then the exhaust gasses are diverted to the larger turbo to provide top end power. Sometimes the gasses go to both turbos at the same time. Mercedes Benz and Audi are working on asymmetrical twin turbo diesels that use one small and 44

one large turbo. Below are some diagrams of their systems.

45

46

A single turbo is most suited to inline engines instead of V engines mainly because of packaging and exhaust routing obstacles. Some turbocharged Saab gasoline cars use inefficient exhaust routing on a single turbo V engine that placed the turbo off to one side of the engine. They experimented with placing the turbo in the middle of the V engine on the top, but this actually melted the paint due to the red hot exhaust. Mercedes Benz's latest Bluetec turbodiesel engine do place the turbo near the top/rear of the engine, but they have a solution for heat control. I suspect it's also due to lower sustained temps in a diesel and heat shielding.

PROPERTIES AND APPLICATIONS Reliability
Turbochargers can be damaged by dirty or ineffective oil, and most manufacturers recommend more frequent oil changes for turbocharged engines. Many owners and some companies recommend using synthetic oils, which tend to flow more readily when cold and do not break down as quickly as conventional oils. Because the turbocharger will heat when running, many recommend letting the engine idle for one to three minutes before shutting off the engine if the turbocharger was used shortly before stopping (most manufacturers specify a 10-second period of idling before switching off to ensure the turbocharger is running at its idle speed to prevent damage to the bearings when the oil supply is cut off). This lets the turbo rotating assembly cool from the lower exhaust gas temperatures, and ensures that oil is supplied to the turbocharger while the turbine housing and exhaust manifold are still very hot; otherwise coking of the lubricating oil trapped in the unit may occur when the heat soaks into the bearings, causing rapid bearing wear and failure when the car is restarted. Even small particles of burnt oil will accumulate and lead to choking the oil supply and failure. This problem is less pronounced in diesel engines, due to the lower exhaust temperatures and generally slower engine speeds. A turbo timer can keep an engine running for a pre-specified period of time, to automatically provide this cool-down period. Oil coking is also eliminated by foil bearings. A more complex and problematic protective barrier against oil coking is the use of watercooled bearing cartridges. The water boils in the cartridge when the engine is shut off and forms a natural recirculation to drain away the heat. Nevertheless, it is not a good idea to shut the engine off while the turbo and manifold are still glowing.

47

In custom applications utilizing tubular headers rather than cast iron manifolds, the need for a cooldown period is reduced because the lighter headers store much less heat than heavy cast iron manifolds. Turbochargers can also suffer bearing damage and premature failure due to throttle blipping right before shutdown. This may cause the turbo to continue spinning after the engine has shutdown and oil pressure has dropped.

Turbo Lag
A pair of turbochargers mounted to an Inline 6 engine (2JZ-GTE from a MkIV Toyota Supra) in a dragster. The time required to bring the turbo up to a speed where it can function effectively is called turbo lag. This is noticed as a hesitation in throttle response when coming off idle. This is symptomatic of the time taken for the exhaust system driving the turbine to come to high pressure and for the turbine rotor to overcome its rotational inertia and reach the speed necessary to supply boost pressure. The directly-driven compressor in a supercharger does not suffer this problem. (Centrifugal superchargers do not build boost at low RPMs like a positive displacement supercharger will). Conversely on light loads or at low RPM a turbocharger supplies less boost and the engine acts like a naturally aspirated engine. Lag can be reduced by lowering the rotational inertia of the turbine, for example by using lighter parts to allow the spool-up to happen more quickly. Ceramic turbines are a big help in this direction. Unfortunately, their relative fragility limits the maximum boost they can supply. Another way to reduce lag is to change the aspect ratio of the turbine by reducing the diameter and increasing the gas-flow path-length. Increasing the upper-deck air pressure and improving the wastegate response helps but there are cost increases and reliability disadvantages that car manufacturers are not happy about. Lag is also reduced by using a foil bearing rather than a conventional oil bearing. This reduces friction and contributes to faster acceleration of the turbo's rotating assembly. Variable-nozzle turbochargers (discussed above) eliminate lag[citation needed]. Lag can be reduced with the use of multiple turbochargers. Another common method of equalizing turbo lag is to have the turbine wheel "clipped", or to reduce the surface area of the turbine wheel's rotating blades. By clipping a minute portion off the tip of each blade of the turbine wheel, less restriction is imposed upon the escaping exhaust gases. This imparts less impedance onto the flow of exhaust gases at low RPM, allowing the vehicle to retain more of its 48

low-end torque, but also pushes the effective boost RPM to a slightly higher level. The amount of turbine wheel clipping is highly application-specific. Turbine clipping is measured and specified in degrees. Lag is not to be confused with the boost threshold; however, many publications still make this basic mistake. The boost threshold of a turbo system describes the lower bound of the region within which the compressor will operate. Below a certain rate of flow at any given pressure multiplier, a given compressor will not produce boost. This has the effect of limiting boost at particular RPMs regardless of exhaust gas pressure. Newer turbocharger and engine developments have caused boost thresholds to steadily decline to where day-to-day use feels perfectly natural. Putting your foot down at 1200 engine RPM and having no boost until 2000 engine RPM is an example of boost threshold and not lag. If lag was experienced in this situation, the RPM would either not start to rise for a short period of time after the throttle was increased, or increase slowly for a few seconds and then suddenly build up at a greater rate as the turbo become effective. However, the term lag is used erroneously for boost threshold by many manufacturers themselves. Electrical boosting ("E-boosting") is a new technology under development; it uses a high speed electrical motor to drive the turbocharger to speed before exhaust gases are available, e.g. from a stop-light. The electric motor is about an inch long. Race cars often utilize an Anti-Lag System to completely eliminate lag at the cost of reduced turbocharger life. On modern diesel engines, this problem is virtually eliminated by utilizing a variable geometry turbocharger.

TWIN TURBOCHARGERS

Parallel
Some engines, such as V-type engines, utilize two identically-sized but smaller turbos, each fed by a separate set of exhaust streams from the engine. The two smaller turbos produce the same (or more) aggregate amount of boost as a larger single turbo, but since they are smaller they reach their optimal RPM, and thus optimal boost delivery, faster. Such an arrangement of turbos is 49

typically referred to as a parallel twin-turbo system. Examples of a parallel twin turbo automobile would be the Mitsubishi 3000GT VR-4 and the Nissan 300ZX.

Sequential
Some car makers combat lag by using two small turbos (such as Nissan, Toyota, Subaru, Maserati, Mazda, and Audi). A typical arrangement for this is to have one turbo active across the entire rev range of the engine and one coming on-line at higher RPM. Early designs would have one turbocharger active up to a certain RPM, after which both turbochargers are active. Below this RPM, both exhaust and air inlet of the secondary turbo are closed. Being individually smaller they do not suffer from excessive lag and having the second turbo operating at a higher RPM range allows it to get to full rotational speed before it is required. Such combinations are referred to as a sequential twin-turbo. Sequential twin-turbos are usually much more complicated than a single or parallel twin-turbo systems because they require what amounts to three sets of pipes-intake and wastegate pipes for the two turbochargers as well as valves to control the direction of the exhaust gases. An example of this is the current BMW E60 5-Series 535d. Another well-known example is the 1993-2002 Toyota Supra. Many new diesel engines use this technology to not only eliminate lag but also to reduce fuel consumption and reduce emissions. The Eunos Cosmo was the first production car with twin sequential turbochargers fitted as standard equipment from 1990-1995.

REMOTE TURBOCHARGERS
Turbochargers are sometimes mounted well away from the engine, in the tailpipe of the exhaust system. Such remote turbochargers require a smaller aspect ratio due to the slower, lower-volume, denser exhaust gas passing through them. For low-boost applications, an intercooler is not required; often the air charge will cool to near-ambient temperature en route to the engine. A remote turbo can run 300 to 600 degrees cooler than a close-coupled turbocharger, so oil cooking in the bearings is of much less concern. Remote turbo systems can incorporate multiple turbochargers in series or parallel.

Boost threshold
Turbochargers start producing boost only above a certain exhaust mass flow rate (depending on the size of the turbo) which is determined by the engine displacement, rpm, and throttle opening. 50

Without an appropriate exhaust gas flow, they logically cannot force air into the engine. The point at full throttle in which the mass flow in the exhaust is strong enough to force air into the engine is known as the boost threshold rpm. Engineers have, in some cases, been able to reduce the boost threshold rpm to idle speed to allow for instant response. Both Lag and Threshold characteristics can be acquired through the use of a compressor map and a mathematical equation.

AUTOMOTIVE APPLICATIONS
Turbo charging is very common on diesel engines in conventional automobiles, in trucks, locomotives, for marine and heavy machinery applications. In fact, for current automotive applications, non-turbocharged diesel engines are becoming increasingly rare. Diesels are particularly suitable for turbo charging for several reasons:
•

Naturally-aspirated diesels develop less power than gasoline engines of the same

displacement, and will weigh significantly more because diesel engines require heavier, stronger components. This gives such engines a poor power-to-weight ratio, which turbo charging can dramatically improve with only slight additional weight.
•

Diesel engines operate within a speed range, facilitating the use of a narrowly-optimized Diesel engines are not prone to the detonation that arises from high (or forced) cylinder Unlike gasoline (petrol) engines which experience higher fuel consumption when turbocharged,

turbocharger.
•

pressure and can damage gasoline engines.
•

turbo charging can reduce the fuel consumption of a diesel engine. The turbocharger's small size and low weight have production and marketing advantage to vehicle manufacturers. By providing naturally-aspirated and turbocharged versions of one engine, the manufacturer can offer two different power outputs with only a fraction of the development and production costs of designing and installing a different engine. The compact nature of a turbocharger mean that bodywork and engine compartment layout changes to accommodate the more powerful engine are not needed or minimal. Parts commonality between the two versions of the same engine reduces production and servicing costs.

51

Today, turbochargers are most commonly used on gasoline engines in high-performance automobiles and diesel engines in transportation and other industrial equipment. Small cars in particular benefit from this technology, as there is often little room to fit a large engine. Volvo and Saab have produced turbocharged cars for many years, the turbo Porsche 944's acceleration performance was very similar to that of the larger-engined non-turbo Porsche 928, and Chrysler Corporation built numerous turbocharged cars in the 1980s and 1990s.

AIRCRAFT
A more natural use of the turbocharger is with aircraft engines. As an aircraft climbs to higher altitudes the pressure of the surrounding air quickly falls off. At 5,486 m (18,000 ft) the air is at half the pressure of sea level, and the airframe only experiences half the aerodynamic drag. However, since the charge in the cylinders is being pushed in by this air pressure, it means that the engine will normally produce only half-power at full throttle at this altitude. Pilots would like to take advantage of the low drag at high altitudes in order to go faster, but a naturally aspirated engine will not produce enough power at the same altitude to do so.

ALTITUDE EFFECTS
A turbocharger remedies this problem by compressing the air back to sea-level pressures; or even much higher; in order to produce rated power at high altitude. Since the size of the turbocharger is chosen to produce a given amount of pressure at high altitude, the turbocharger is over-sized for low altitude. The speed of the turbocharger is controlled by a wastegate. Early systems used a fixed wastegate, resulting in a turbocharger that functioned much like a supercharger. Later systems utilized an adjustable wastegate, controlled either manually by the pilot or by an automatic hydraulic or electric system. When the aircraft is at low altitude the wastegate is usually fully open, venting all the exhaust gases overboard. As the aircraft climbs and the air density drops, the wastegate must continually close in small increments to maintain full power. The altitude at which the wastegate is full closed and the engine is still producing full rated power is known as the critical altitude.

TEMPERATURE CONDITIONS
One disadvantage of turbocharging is that compressing the air increases its temperature, which is true for any method of forced induction. This causes multiple problems. Increased temperatures can lead to detonation and excessive cylinder head temperatures. In addition, hotter air is less 52

dense, so fewer air molecules enter the cylinders on each intake stroke, resulting in an effective drop in volumetric efficiency which works against the efforts of the turbocharger to increase volumetric efficiency. Aircraft engines generally cope with this problem in one of several ways. The most common one is to add an intercooler or aftercooler somewhere in the air stream between the compressor outlet of the turbocharger and the engine intake manifold. Intercoolers and aftercoolers are types of heat exchangers which cause the compressed air to give up some of its heat energy to the ambient air. In the past, some aircraft featured anti-detonant injection for takeoff and climb phases of flight, which performs the function of cooling the fuel/air charge before it reaches the cylinders. In contrast, modern turbocharged aircraft usually forego any kind of temperature compensation, because the turbochargers are generally small and the manifold pressures created by the turbocharger are not very high. Thus the added weight, cost, and complexity of a charge cooling system are considered to be unnecessary penalties. In those cases the turbocharger is limited by the temperature at the compressor outlet, and the turbocharger and its controls are designed to prevent a large enough temperature rise to cause detonation. Even so, in many cases the engines are designed to run rich in order to use the evaporating fuel for charge cooling.

COMPARISON TO SUPERCHARGING
A supercharger inevitably requires some energy to be bled from the engine to drive the supercharger. On the single-stage single-speed supercharged Rolls Royce Merlin engine for instance, the supercharger uses up about 150 horsepower (110 kW). Yet the benefits outweigh the costs, for that 150 hp (110 kW), the engine is delivering 1,000 hp (750 kW) when it would otherwise deliver 750 hp (560 kW), a net gain of 250 hp (190 kW). This is where the principle disadvantage of a supercharger becomes apparent: The engine has to burn extra fuel to provide power to turn the supercharger. The increased charge density increases the engine's specific power and power to weight ratio, but also increases the engine's specific fuel consumption. This increases the cost of running the aircraft and reduces its overall range. On the other hand, a turbocharger is driven using the exhaust gases. The amount of power in the gas is proportional to the difference between the exhaust pressure and air pressure, and this difference increases with altitude, allowing a turbocharger to compensate for changing altitude without using up any extra power. 53

Another key disadvantage of supercharged engines is that they are controlled entirely by the pilot, introducing the possibility of human error which could damage the engine and endanger the aircraft. With a supercharged aircraft engine, the pilot must continually adjust the throttle to maintain the required manifold pressure during ascent or descent. The pilot must also take great care to avoid overboosting the engine and causing damage, especially during emergencies such as go-arounds. In contrast, modern turbocharger systems use an automatic wastegate which controls the manifold pressure within parameters preset by the manufacturer. For these systems, as long as the control system is working properly and the pilot's control commands are smooth and deliberate, a turbocharger will not overboost the engine and damage it. Yet the vast majority of World War II engines used superchargers, because they maintained three significant manufacturing advantages over turbochargers, which were larger, involved extra piping, and required exotic high-temperature materials in the turbine and pre-turbine section of the exhaust system. The size of the piping alone is a serious issue; American fighters Vought F4U and Republic P-47 used the same engine but the huge barrel-like fuselage of the latter was, in part, needed to hold the piping to and from the turbocharger in the rear of the plane. Turbocharged piston engines are also subject to many of the same operating restrictions as gas turbine engines. Pilots must make smooth, slow throttle adjustments to avoid overshooting their target manifold pressure. The fuel mixture must often be adjusted far on the rich side of the peak exhaust gas temperature to avoid overheating the turbine when running at high power settings. In systems using a manually-operated wastegate, the pilot must be careful not to exceed the turbocharger's maximum RPM. Turbocharged engines require a cooldown period after landing to prevent thermal shock from cracking the turbo or exhaust system. Turbocharged engines require frequent inspections of the turbocharger and exhaust systems for damage due to the increased heat, increasing maintenance costs. Today, most general aviation aircraft are naturally aspirated. The small number of modern aviation piston engines designed to run at high altitudes generally use a turbocharger or turbonormalizer system rather than a supercharger. The change in thinking is largely due to economics. Aviation gasoline was once plentiful and cheap, favoring the simple but fuel-hungry supercharger. As the cost of fuel has increased, the supercharger has fallen out of favor. Turbocharged aircraft often occupy a performance range in between that of normally-aspirated piston-powered aircraft and turbine-powered aircraft. The increased maintenance costs of a turbocharged engine are considered worthwhile for this purpose, as a turbocharged piston engine is still far cheaper than any turbine engine. 54

RELATIONSHIP TO GAS TURBINE ENGINES Prior to World War II, Sir Frank Whittle started his experiments on early turbojet engines. Due to a lack of sufficient materials as well as funding, initial progress was slow. However, turbochargers were used extensively in military aircraft during World War II to enable them to fly very fast at very high altitudes. The demands of the war led to constant advances in turbocharger technology, particularly in the area of materials. This area of study eventually crossed over in to the development of early gas turbine engines. Those early turbine engines were little more than a very large turbocharger with the compressor and turbine connected by a number of combustion chambers. The cross over between the two has been shown in an episode of the TV show Scrapheap Challenge where contestants were able to build a functioning Jet Engine using an exautomotive turbocharger as a compressor. Consider also, for example, that General Electric manufactured turbochargers for military aircraft and held several patents on their electric turbo controls during the war, then used that expertise to very quickly carve out a dominant share of the gas turbine market which they have held ever since.

55

1.5

HOW TURBOCHARGER IS PLUMBED IN CAR –

LOCATION OF TURBOCHARGER IN CAR –

56

1.6

NEED TO BE CONSIDER WHEN SELECTING TURBOCHARGER –

     

The capacity of your engine. The number of valves. At what RPM to you want the turbo to come in. The type of fuel you plan on using. The turbo boost you plan on running. The amount of horsepower you want.

57

APPLICATION OF TURBOCHARGER IN AUTOMOTIVE
Turbo charging is very common on diesel engines in conventional automobiles, in trucks, locomotives, for marine and heavy machinery applications. In fact, for current automotive applications, non-turbocharged diesel engines are becoming increasingly rare. Diesels are particularly suitable for turbo charging for several reasons:
•

Naturally-aspirated diesels develop less power than gasoline engines of the same displacement,

and will weigh significantly more because diesel engines require heavier, stronger components. This gives such engines a poor power-to-weight ratio, which turbo charging can dramatically improve with only slight additional weight.
•

Diesel engines operate within a speed range, facilitating the use of a narrowly-optimized Diesel engines are not prone to the detonation that arises from high (or forced) cylinder 58

turbocharger.
•

pressure and can damage gasoline engines.

•

Unlike gasoline (petrol) engines which experience higher fuel consumption when turbocharged,

turbo charging can reduce the fuel consumption of a diesel engine. The turbocharger's small size and low weight have production and marketing advantage to vehicle manufacturers. By providing naturally-aspirated and turbocharged versions of one engine, the manufacturer can offer two different power outputs with only a fraction of the development and production costs of designing and installing a different engine. The compact nature of a turbocharger means that bodywork and engine compartment layout changes to accommodate the more powerful engine are not needed or minimal. Parts commonality between the two versions of the same engine reduces production and servicing costs. Today, turbochargers are most commonly used on gasoline engines in high-performance automobiles and diesel engines in transportation and other industrial equipment. Small cars in particular benefit from this technology, as there is often little room to fit a large engine. Volvo and Saab have produced turbocharged cars for many years, the turbo Porsche 944's acceleration performance was very similar to that of the larger-engine non-turbo Porsche 928, and Chrysler Corporation built numerous turbocharged cars in the 1980s and 1990s.

ADVANTAGES OF TURBOCHARGER
 More specific power over naturally aspirated engine. This means a turbocharged engine Better thermal efficiency over both naturally aspirated and supercharged engine when

can achieve more power from same engine volume.  under full load (i.e. on boost). This is because the excess exhaust heat and pressure, which would normally be wasted, contributes some of the work required to compress the air.  Weight/Packaging. Smaller and lighter than alternative forced induction systems and may Fuel Economy. Although adding a turbocharger itself does not save fuel, it will allow a be more easily fitted in an engine bay.  vehicle to use a smaller engine while achieving power levels of a much larger engine, while attaining near normal fuel economy while off boost/cruising. This is because without boost, less fuel is used to create a proper air/fuel ratio.

59

DISADVANTAGES OF TURBO-CHARGER
 Lack of responsiveness if an incorrectly sized turbocharger is used. If a turbocharger that

is too large is used it reduces throttle response as it builds up boost slowly otherwise known as "lag". However, doing this may result in more peak power.  Boost threshold. A turbocharger starts producing boost only above a certain rpm due to a lack of exhaust gas volume to overcome inertia of rest of the turbo propeller. This results in a rapid and nonlinear rise in torque, and will reduce the usable power band of the engine. The sudden surge of power could overwhelm the tires and result in loss of grip, which could lead to understeer/oversteer, depending on the drivetrain and suspension setup of the vehicle. Lag can be disadvantageous in racing, if throttle is applied in a turn, power may unexpectedly increase when the turbo spools up, which can cause excessive wheelspin. 60



Cost. Turbocharger parts are costly to add to naturally aspirated engines. Heavily

modifying OEM turbocharger systems also require extensive upgrades that in most cases requires most (if not all) of the original components to be replaced.  Complexity. Further to cost, turbochargers require numerous additional systems if they are not to damage an engine. Even an engine under only light boost requires a system for properly routing (and sometimes cooling) the lubricating oil, turbo-specific exhaust manifold, application specific downpipe, boost regulation. In addition inter-cooled turbo engines require additional plumbing, while highly tuned turbocharged engines will require extensive upgrades to their lubrication, cooling, and breathing systems; while reinforcing internal engine and transmission parts.

REFRENCES

Books:A Text-Book OF Internal Combustion and Gas Turbine by V. GANESHAN

Websites:http://www.custom-car.us http://auto.howstuffworks.com 61

 