MAJOR PROJECTON

APPLICATION OF TURBO-CHARGER IN PETROL ENGINE

A MAJOR PROJECTSUBMITTED IN THE PARTIAL FULFILLMENT OF THE REQUIREMENT

FOR THE AWARD OF THE DEGREE OF

BACHELOR OF ENGINEERINGIN

MECHANICAL ENGINEERINGOF

MAHARSHI DAYANAND UNIVERSITY, ROHTAK

UNDER THE GUIDANCE OF BYEr. BHUBHANESH 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 ENGINEERINGB.P.R. COLLEGE OF ENGINEERING

GOHANA (SONEPAT)JUNE 2009

ACKNOWLEDGEMENT1

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)

CERTIFICATE2

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 TURBO-

CHARGER 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.) Mr. Dinesh Panchal

Mechanical Engg. Dept. Head of Department

B.P.R. College of Engg. Mechanical Engg. Dept.

B.P.R. College of Engg.

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CONTENTS

ACKNOWLEDGEMENT……………………………………………………………………….2

CERTIFICATE…………………………………………………………………………………..3

TITLE PAGE No.

1. TURBOCHARGER- AN OVERVIEW………………………………………………………..5

1.1 INTRODUCTION……………………………………………………….……………..5

1.2 WORKING PRINCIPLE……………………………………………………………….7

1.3 FULL SECTIONAL VIEW OF TURBO-CHARGER…………..……………………..8

1.4 INSIDE A TURBO-CHARGER……………………….……………………………….9

1.5 DESIGN CONSIDERATION AND DETAILS………….…………………………….9

1.6 HOW TURBOCHARGER IS PLUMBED IN CAR…………………………………..54

1.7 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

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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

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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.

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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

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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.

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.

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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.

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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

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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.

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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. 

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

15

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

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.  

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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

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 17

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

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. 

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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

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

19

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,

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.

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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.

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

21

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

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 22

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.

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

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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.    Pictured below is a venturi, this can also help

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).

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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

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

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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.

 

 

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

26

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.

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.

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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

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,

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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

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

29

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.

 

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 over-

speeds 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 30

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.

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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 32

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.

33

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. 

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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

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.

35

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

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

36

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 180o

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

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

37

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!

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? 

38

 

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,

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

39

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.

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.

40

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.

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

41

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)

42

.

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

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

43

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

44

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.

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.

45

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

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

46

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

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

47

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.

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.

48

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

turbocharger.

Diesel engines are not prone to the detonation that arises from high (or forced) cylinder

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 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.

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.

49

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

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

50

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.

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

51

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 turbo-

normalizer 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 turbo-

charged engine are considered worthwhile for this purpose, as a turbocharged piston engine is still

far cheaper than any turbine engine.

52

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 ex-

automotive 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.

53

1.5 HOW TURBOCHARGER IS PLUMBED IN CAR –

LOCATION OF TURBOCHARGER IN CAR –

54

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.

55

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

turbocharger.

Diesel engines are not prone to the detonation that arises from high (or forced) cylinder

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.

56

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

can achieve more power from same engine volume.

Better thermal efficiency over both naturally aspirated and supercharged engine when

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

be more easily fitted in an engine bay.

Fuel Economy. Although adding a turbocharger itself does not save fuel, it will allow a

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.

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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.

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.

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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

http://en.wikipedia.org

http://www.thehindubusinessline.com

http://www.team-bhp.com

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http://www. turbochargers .com

http://www.myturbodiesel.coM

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