Background

Turbocharger Theory

A turbocharger is basically an exhaust gas driven air compressor and can be

best understood if it is divided into its two basic parts, the exhaust gas driven turbine

and its housing, and the air compressor and its housing. The two parts resemble a set

of Siamese twins because each of them performs different functions but, because

they are joined together at the centre via a common shaft, which results in the

function of one impacting the function of the other.

If we take a perfectly set-up compressor section and mate it with an incorrect

turbine section, or vice versa, we end up with our Siamese twins trying to go in

different directions. The result is that our Siamese twins end up wasting all of their

energy fighting each other and go nowhere. When considering a turbocharger, most

people tend to look at the maximum CFM rating (rating according to capacity to

move air in cubic feet per minute) of the compressor and ignore everything else

under the assumption that the compressor and the exhaust turbine are perfectly

matched out of the box. This is acceptable in stock factory applications but, in all out

performance applications, it will not be ample because of the extremes of operation

in a performance application. The goal in a performance application is to get the

exhaust turbine up to speed as quickly as possible however; it must be mated to a

compressor wheel that will generate as much pressure as it can as soon as possible.

This is a contradiction because the exhaust turbine generates the drive power and the

compressor consumes that power. The larger the compressor and the higher the

pressure (boost) we want, the quicker the power from the exhaust turbine is used up.

Put in a larger exhaust turbine and it will take the engine longer to develop enough

hot expanding exhaust gas to spin it, slowing down the compressor and causing turbo

lag. Thus, a turbocharger should not be taken as a bolt on a piece of equipment but

rather as a system.

The turbine is powered by hot expanding exhaust gas, a lot of hot expanding

exhaust gas, the more and the hotter the expanding exhaust gas the better. Most

commonly, we might come across turbocharged engines with cherry red hot exhaust

systems and turbo housings. This outcome is often mistaken as outstanding horse

power numbers possessed by the engine. What the outcome really means is that the

engine was almost at its stall point for a prolonged period of time. A condition that

most turbocharged engines will never see. The real point here is that the exhaust

turbine will not generate enough power to turn the air compressor fast enough for it

to work properly unless the engine is feeding the exhaust turbine a lot of hot

expanding exhaust gas, a condition that can only be created when the engine is under

a load. There is where the selection of transmission gear ratios and the ring and

pinion ratio play a critical part. The fact that the engine must be under a load is the

reason why, no matter how highly-powered a turbocharged engine is, with no load on

it, we will not see the boost gauge move. This is also where the term 'turbo lag' came

from. Turbo lag is basically the amount of time it takes from the time we place a load

on the engine (stomp the accelerator pedal to the floor) until the moment the engine

develops enough hot expanding exhaust gas to spin the turbine fast enough for the

compressor to do its job.

Effectively, a turbocharged engine is a normally aspirated engine until the

turbine and compressor spin up. To minimize turbo lag, it is imperative that the

turbine and the compressor are properly matched to the engine as well as the engine

being properly matched to the transmission gears, the ring and pinion gears, and the

tires.

Turbocharging Principles

Figure 1: A Typical Turbocharger

Figure 2: Inside A Turbocharger

To better understand the techniques of turbocharging, it is useful to be familiar with

the internal combustion engine's principles of operation. Today, most cars and

commercial diesel engines are four-stroke piston engines controlled by intake and

exhaust valves. One operating cycle consists of four strokes during two complete

revolutions of the crankshaft.

i)

Induction

When the piston moves down, air (diesel engine or direct injection petrol engine) or a

fuel/air mixture (petrol engine) is drawn through the intake valve.

ii) Compression

The cylinder volume is compressed.

iii) Ignition

Figure 3: A Complete Cycle of Internal Combustion Engine

In the petrol engine, the fuel/air mixture is ignited by a spark plug, whereas in the

diesel engine fuel is injected under high pressure and the mixture ignites

spontaneously.

iv) Exhaust

The exhaust gas is expelled when the piston moves up.

Various Possibilities of Increasing the Engine's Power Output

i) Swept Volume Enlargement

Enlargement of the swept volume allows for an increase in power output, as more air

is available in a larger combustion chamber and thus more fuel can be burnt. This

enlargement can be achieved by increasing either the number of cylinders or the

volume of each individual cylinder. In general, this results in larger and heavier

engines. As far as fuel consumption and emissions are concerned, no significant

advantages can be expected.

ii) Increase in engine rpm

Another possibility for increasing the engine's power output is to increase its speed.

This is done by increasing the number of firing strokes per time unit. Because of

mechanical stability limits, however, this kind of output improvement is limited.

Furthermore, the increasing speed makes the frictional and pumping losses increase

exponentially and the engine efficiency drops.

iii) Turbocharging

In the previously-described procedures, the engine operates as a naturally-aspirated

engine (reciprocating internal combustion engine that depends solely on atmospheric

pressure to draw in combustion air). The combustion air is drawn directly into the

cylinder during the intake stroke. In turbocharged engines, the combustion air is

already pre-compressed before being supplied to the engine. The engine aspirates the

same volume of air, but due to the higher pressure, more air mass is supplied into the

combustion chamber. Some of the exhaust gas energy, which would normally be

wasted, is used to drive a turbine. Mounted on the same shaft as the turbine is a

compressor which draws in the combustion air, compresses it, and then supplies it to

the engine. There is no mechanical coupling to the engine. Consequently, more fuel

can be burnt, so that the engine's power output increases related to the same speed

and swept volume.

Working Principle of an Engine with Turbocharger

Engine power is proportional to the amount of air and fuel that can get into the

cylinders. All things being equal, larger engines flow more air and as such will

produce more power. If we require small engines to perform like large engines, or

simply make small engines produce more power, our ultimate objective is to draw

more air into the cylinder. By installing a turbocharger, the power and performance

of an engine can be dramatically increased.

Figure 4 shows how a turbocharger supplies more air into the engine followed by

description of overall operation of engine fitted with a turbocharger after Figure 5

and Figure 6:

1) Compressor Inlet

2) Compressor

Discharge

3) Charge air cooler

(CAC)

4) Intake Valve

5) Exhaust Valve

6) Turbine Inlet

7) Turbine Discharge

1 7

62

34

5

Figure 4:

Operation of Engine with

Turbocharger

•

The air filter (not

shown)

through

which

ambient air

passes

before entering the compressor. (1)

• The air is then compressed which raises the air’s density (mass / unit

volume). (2)

Figure 5: Automobile Engine Equipped with Turbocharger

Figure 6: Air Flow in a Turbocharger-Equipped Automobile Engine

• Many turbocharged engines have a charge air cooler/intercooler (3) that cools

the compressed air to further increase its density and to increase resistance to

detonation.

• After passing through the intake manifold (4), the air enters the engine’s

cylinders, which contain a fixed volume. Since the air is at elevated density,

each cylinder can draw in an increased mass flow rate of air. Higher air mass

flow rate allows a higher fuel flow rate (with similar air/fuel ratio).

Combusting more fuel results in more power being produced for a given size

or displacement.

• After the fuel is burned in the cylinder it is exhausted during the cylinder’s

exhaust stroke in to the exhaust manifold. (5)

• The high temperature gas then continues on to the turbine (6). The turbine

creates backpressure on the engine which means engine exhaust pressure is

higher than atmospheric pressure.

• A pressure and temperature drop occurs (expansion) across the turbine (7),

which harnesses the exhaust gas’ energy to provide the power necessary to

drive the compressor.

Components of Turbocharger

Figure 7: Components of Turbocharger (dissection view)

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).

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. 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 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.

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.

The layout of the turbocharger in a given application is critical to a properly

performing system. Intake and exhaust plumbing is often driven primarily by

packaging constraints. It is important to understand the need for a compressor bypass

valve (commonly referred to as a Blow-Off valve) on the intake tract and a wastegate

for the exhaust flow.

i) The Turbine Section

The turbine stage comprises of two components;

the turbine 'wheel' and the collector,

commonly referred to as ’housing’. The turbine wheel can be of radial or axial

design. Generally, in turbochargers used on high speed engines the turbines are of

radial design. On larger engines such as ship propulsion axial turbines are used. The

exhaust gas is guided into the turbine wheel by the housing. The energy in the

exhaust gas turns the turbine. Once the gas has passed through the blades of the

wheel it leaves the turbine housing via the exhaust outlet area. The speed of the

engine determines how fast the turbine wheel spins. If the engine is in idle mode, the

wheel will be spinning but at a minimal speed. As the accelerator pedal is stomped,

the wheel starts spinning faster. As more gas passes through the turbine housing, the

faster the turbine wheel rotates.

ii)

Compressor section

Compressors are the opposite of turbines. Again the compressor stage comprises of

two sections, the impeller or 'wheel' and the 'housing'. The compressor wheel is

connected to the turbine by a forged steel shaft. As the compressor wheel spins air

enters through an area known as the inducer and is compressed through the blades

leaving the exducer at a high velocity. The housing is designed to convert the high

velocity, low pressure air stream into a high pressure, low velocity air stream through

a process called diffusion. Air enters the compressor at a temperature equivalent to

atmosphere, however it leaves the compressor cover at a temperature up to 200

degrees Celsius. Because the density of the air decreases as it is heated up, even more

air can be forced into the engine if the air is cooled after the compressor. This is

called intercooling or aftercooling and is achieved either by cooling the charge air

with water or air through the use of intercooler.

Figure 8: Exhaust Flow in the Turbine Section (cross- section)

Figure 9: Ambient Air Flow in the Compressor Section (cross- section)

iii) Turbine Housing

Figure 10: Turbine Housing

Turbine housings are manufactured in various grades of

spheroidal graphite iron to deal with thermal fatigue and wheel burst containment. As

with the impeller, profile machining to suit turbine blade shape is carefully controlled

for optimum performance. The turbine housing inlet flange acts as the reference

point for fixing turbocharger position relative to its installation. It is normally the

load bearing interface.

iv) Turbine Wheel

Figure 11: Turbine Wheel

The Turbine Wheel is housed in the turbine casing and is connected to a shaft that in

turn rotates the compressor wheel.

v) Compressor Cover

Figure 10: Compressor Cover

Compressor housings are also made in cast aluminum. Various grades are used to

suit the application. Both gravity die and sand casting techniques are used. Profile

machining to match the developed compressor blade shape is important to achieve

performance consistency.

vi) Compressor Wheel

Figure 12: Compressor Wheel

Compressor impellers are produced using a variant of the aluminum investment

casting process. A rubber former is made to replicate the impeller around which a

casting mould is created. The rubber former can then be extracted from the mould

into which the metal is poured. Accurate blade sections and profiles are important in

achieving compressor performance. Back face profile machining optimizes impeller

stress conditions. Boring to tight tolerance and burnishing assist balancing and

fatigue resistance. The impeller is located on the shaft assembly using a threaded nut.

vii) Blow-Off Valve (BOV)

Figure 13: Blow-Off Valve (BOV)

The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent

the turbocharger’s compressor from going into surge. The BOV should be installed

between the compressor discharge and the throttle body, preferably downstream of

the charge air cooler (if equipped). When the throttle is closed rapidly, the airflow is

quickly reduced, causing flow instability and pressure fluctuations. These rapidly

cycling pressure fluctuations are the audible evidence of surge. Surge can eventually

lead to thrust bearing failure due to the high loads associated with it. Blow-Off

valves use a combination of manifold pressure signal and spring force to detect when

the throttle is closed. When the throttle is closed rapidly, the BOV vents boost in the

intake tract to atmosphere to relieve the pressure; helping to eliminate the

phenomenon of surge.

viii) Wastegate

Figure 14: Wastegate (External)

On the exhaust side, a wastegate provides us a

means to control the boost pressure of the

engine. Some commercial diesel applications do

not use a wastegate at all. This type of system is

called a free-floating turbocharger. However, the vast majority of petrol performance

applications require a wastegate. There are two configurations of wastegate, internal

or external. Both internal and external wastegate provide means to bypass exhaust

flow from the turbine wheel. Bypassing this energy (e.g. exhaust flow) reduces the

power driving the turbine wheel to match the power required for a given boost level.

Similar to the Blow-Off Valve (BOV), the wastegate uses boost pressure and spring

force to regulate the flow bypassing the turbine. There are two types of wastegate,

which is an external wastegate and internal wastegate (commonly known as

actuator). The external wastegate is usually installed at the extractor or exhaust

manifold just before the turbine while an internal wastegate or actuator is integrated

inside the turbo unit itself.

ix) Internal Wastegate (Actuator)

Figure 15: Internal Wastegate (Actuator)

Internal Wastegate is built into the turbine housing and consist of a “flapper” valve,

crank arm, rod end, and pneumatic actuator. It is important to connect this actuator

only to boost pressure. It is not designed to handle vacuum and as such should not be

referenced to an intake manifold.

x) Bearing Housing

Figure 16: Bearing Housing

A grey cast iron bearing housing provides locations for a fully

floating bearing system for the shaft, turbine and compressor

which can rotate at speeds up to 170,000 rev/min. Shell molding is used to provide

positional accuracy of critical features of the housing such as the shaft bearing and

seal locations. CNC machinery mills, turns, drills and taps housing faces and

connections. The bore is finish honed to meet stringent roundness, straightness and

surface finish specifications.

xi) Bearing System

Figure17: Bearing System

The turbocharger bearing system has to withstand high temperatures, hot shut down,

soot loading in the oil, contaminants, oil additives, dry starts. The manufacturing

process is designed to create geometric tolerances and surface finishes to suit very

high speed operation. Hardened steel thrust collars and oil slingers are manufactured

to strict tolerances using lapping. End thrust is absorbed in a bronze hydrodynamic

thrust bearing located at the compressor end of the shaft assembly. Careful sizing

provides adequate load bearing capacity without excessive losses. The bearing

system is lubricated by oil from the engine. The oil is fed under pressure into the

bearing housing, through to the journal bearings and thrust system. The oil also acts

as a coolant taking away heat generated by the turbine.

xii) Journal Bearings & Thrust Bearing/Ball Bearings

Figure 18: Journal Bearings

Figure 19: Ball Bearings

The journal bearing has long been the strength of the turbocharger (the conventional

bearing used in turbochargers). However, ball bearing is now an affordable

technology advancement that provides significant performance improvements to the

turbocharger. The ball bearing is initially termed as ‘cartridge ball bearing’. The

cartridge ball bearing is a single sleeve system that contains a set of angular contact

ball bearings on either end, whereas the traditional bearing system contains a set of

journal bearings and a thrust bearing

Figure 20: Thrust Bearing

xiii) Charge Air Cooler/Intercooler

Figure 21: Charge Air Cooler/Intercooler

An intercooler is a heat exchanger. That means there are two or more liquids or gases

that do not physically touch each other but a heat or energy transfer takes place

between them. At wide open throttle and full boost, the hot compressed air coming

from a turbocharger compressor is probably between 250 and 350 degree Fahrenheit

depending on the particular compressor, boost pressure, outside air temperature, etc.

When the hot air is cooled down, its volume reduces; thus more air molecules can be

supplied into the cylinders and reduce the engine's likelihood of knocking. Hot air

from the turbocharger compressor flows through tubes inside the intercooler. The air

transfers heat to the tubes. Outside air (or water) passes over the tubes and between

fins that are attached to the tubes. Heat is transferred from the hot tubes and fins to

the cool outside air.

Types of Turbocharger

There are generally five types of turbochargers which are commercially available

such as:

i. Water-Cooled Turbochargers

ii. Oil-Cooled Turbochargers

iii. Ball Bearing Turbochargers

iv. Journal Bearing Turbochargers

v. Twin-Entry Turbochargers

vi. Variable Turbine Geometry (VGT) Turbochargers

In this section, the types of turbocharger should not be confused with radial flow

turbine turbochargers and axial flow turbine turbochargers as we are concerned with

turbochargers/turbocharger test rigs for automotive application. Generally, in

turbochargers used on high speed engines/small-scaled engines and for automotive

applications, the turbines are of radial design. On larger-scaled engines such as ship

propulsion, axial turbines are used. On the other hand, there are also variable turbine

geometry (VGT) turbochargers which are generally used in diesel engines. It can

never be used on petrol engines because petrol engine exhaust gases are a lot hotter

than diesel engine exhaust gas, so generally the material used to make variable

turbine geometry turbochargers could not stand this heat

Turbocharger Test Rig

The turbocharger test rig is a system used for testing complete turbochargers

and turbocharger turbine and compressor components as used on petrol, diesel, or

natural gas engines.

The turbocharger test rig includes a combustion chamber in which a mixture

of fuel oil and compressed air is combusted to power the turbine of the turbocharger.

The compressed air is provided by a compressor. The inlet pipe of the turbine is of

single-flow design at the small test rig and of double-flow design at the big test rig.

The compressor’s in-coming air is pre-conditioned ambient air. Pressure and

temperature measuring points collect the stationary measurands. The turbine and

compressor mass-flow is calculated using venturi injectors. If necessary, unsteady

measuring points can be added.

To meet engine requirements for power output, efficiency, and emissions

reduction, turbochargers must be optimized for efficiency and matched to the

specific engine. Turbocharger test rig enables turbochargers to be tested without the

added complexity of an engine test cell setup. Turbocharger test rig is also able to

test turbochargers in ways not possible on an engine, including component and

durability tests.

In addition to testing the performance of new concepts and designs,

turbocharger test rig measures basic turbocharger performance, such as compressor

and turbine efficiencies and pressure ratios. Endurance tests are also possible. The

test rig takes ambient air and provides high-pressure, high-temperature air into the

turbine to simulate engine exhaust. Manufacturers often test turbochargers by

installing them on engines. Because the test method uses a rig instead of an actual

engine, users save the time and expense of engine installation. The rig may be

expanded to test two or more turbochargers simultaneously for additional savings.

A Turbocharger Test Rig offers the following features:

Turbochargers can be fully mapped, generating compressor and turbine

flow, pressure ratio, and efficiency maps.

A combustion chamber produces pressurized and heated air to the turbine

inlet

Throttles on the turbine inlet and compressor outlet control turbo speed and

pressure ratios.

A lubrication system supplies heated and pressurized oil to the turbocharger.

A PC-based data acquisition system records speeds, airflows,

temperatures, and pressures.

Turbocharger parameters are calculated in real time and recorded with

measured data.

Figure 22: Schematic of the Turbocharger Test Rig