23501289 Turbocharger Basics
Transcript of 23501289 Turbocharger Basics
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
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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