Forced Induction: Supercharger and Turbocharger
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Transcript of Forced Induction: Supercharger and Turbocharger
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Introduction
To increase the output efficiency of any engine, we have to burn more fuel and air to
make bigger explosion in every cycle. We have two options for this. One way to add
power is to build a bigger engine. But bigger engines, which weight more and cost moreto build and maintain, are not always better. Another way to add power is to make a
normal size engine more efficient.
We can accomplish this by forcing more air into the combustion chamber. More air
means more fuel can be added, and more fuel means a bigger explosion and greater
horsepower. This can be done by the help of supercharger and turbocharger.
Objective
a) To understand about forced induction elements; turbocharger and supercharger.
b) To learn how the turbocharger and supercharger work.
c) To know the need of turbocharging and supercharging.
d) To observe the construction and the components of turbocharger and
supercharger.
e) To detect the advantages and disadvantages of these two elements.
f) To apply the benefits of forced induction elements into automotive field.
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Turbocharger: Construction and Components
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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.
Wheel
The Turbine Wheel is housed in the turbine casing and is connected to a shaft that in
turn rotates the compressor wheel.
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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.
Compressor Wheel (Impellor)
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 metalis 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.
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Blow-Off (Bypass) Valves
The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the
turbos 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.
Wastegates
On the exhaust side, a wastegates provides us a means to control the boost pressure of
the engine. Some commercial diesel applications do not use a wastegates at all. This
type of system is called a free-floating turbocharger. However, the vast majority of
gasoline performance applications require a wastegates. There are two configurations of
Wastegates, internal or external. Both internal and external wastegates provide a 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
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boost level. Similar to the BOV, the wastegates uses boost pressure and spring force to
regulate the flow bypassing the turbine.
Internal wastegatesare 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.
External wastegates are added to the exhaust plumbing on the exhaust manifold or
header. The advantage of external wastegates is that the bypassed flow can be
reintroduced into the exhaust stream further downstream of the turbine. This tends to
improve the turbines performance. On racing applications, this wastegated exhaust flow
can be vented directly to atmosphere.
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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.
Bearing Systems
The bearing system has to withstand high temperatures, hot shut down, soot loading in
the oil, contaminants, oil additives, dry starts. Journal bearings are manufactured from
specially developed bronze or brass bearing alloys. The manufacturing process is
designed to create geometric tolerances and surface finishes to suit very high speed
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1 Compressor Inlet
2 Compressor Discharge
3 Charge air cooler (CAC)
4 Intake Valve
5 Exhaust Valve
6 Turbine Inlet
7 Turbine Discharge
The components that make up a typical turbocharger system are:
a) The air filter (not shown) through which ambient air passes before entering the
compressor (1).
b) The air is then compressed which raises the airs density (mass / unit volume)
(2).
c) Many turbocharged engines have a charge air cooler (aka intercooler) (3) that
cools the compressed air to further increase its density and to increase
resistance to detonation.
d) After passing through the intake manifold (4), the air enters the engines
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.
e) After the fuel is burned in the cylinder it is exhausted during the cylinders
exhaust stroke in to the exhaust manifold (5).
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f) 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.
g) 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.
Turbocharger: Control Systems
The drivability of passenger car turbo engines must meet the same high requirements as
naturally aspirated engines of the same power output. That means, full boost pressure
must be available at low engine speeds. This can only be achieved with a boost
pressure control system on the turbine side.
Control by turbine-side bypass (wastegate)
The turbine-side bypass is the simplest form of boost pressure control. The turbine size
is chosen such that torque characteristic requirements at low engine speeds can be met
and good vehicle drivability achieved. With this design, more exhaust gas than required
to produce the necessary boost pressure is supplied to the turbine shortly before the
maximum torque is reached. Therefore, once a specific boost pressure is achieved, part
of the exhaust gas flow is fed around the turbine via a bypass. The wastegate which
opens or closes the bypass is usually operated by a spring-loaded diaphragm in
response to the boost pressure.
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Boost Controller
Today, electronic boost pressure control systems are increasingly used in modern
passenger car diesel and petrol engines. When compared with purely pneumatic control,
which can only function as a full-load pressure limiter, a flexible boost pressure control
allows an optimal part-load boost pressure setting. This operates in accordance with
various parameters such as charge air temperature, degree of timing advance and fuel
quality. The operation of the flap corresponds to that of the previously described
actuator. The actuator diaphragm is subjected to a modulated control pressure instead
of full boost pressure. Boost pressure control of a turbocharged petrol engine by
proportional control pressure. This control pressure is lower than the boost pressure and
generated by a proportional valve. This ensures that the diaphragm is subjected to the
boost pressure and the pressure at the compressor inlet in varying proportions. The
proportional valve is controlled by the engine electronics. For diesel engines, a vacuum
regulated actuator is used for electronic boost pressure control.
Turbocharger: Turbo lag
All turbocharger applications can be roughly divided into 2 categories, those requiring
rapid throttle response and those that do not. While important to varying degrees, turbo
lag is most problematic when rapid changes in engine performance are required.
Turbo lag is the time required to change speed and function effectively in response to a
throttle change. For example, this is noticed as a hesitation in throttle response when
accelerating from idle as compared to a naturally aspirated engine. Throttle lag may be
noticeable under any driving condition, yet becomes a significant issue under
acceleration. This is symptomatic of the time needed for the exhaust system working inconcert with the turbine to generate enough extra power to accelerate rapidly. A
combination of inertia, friction, and compressor load are the primary contributors to turbo
lag. By eliminating the turbine, the directly driven compressor in a supercharger does not
suffer from this problem.
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Lag can be reduced in a number of ways:
a) Lower the rotational inertia of the turbocharger; for example by using lighter,
lower radius parts to allow the spool-up to happen more quickly. Ceramic turbines
are of benefit in this regard and or billet compressor wheel.b) Change the aspect ratio of the turbine.
c) Increase the upper-deck air pressure (compressor discharge) and improving the
wastegate response; this helps but there are cost increases and reliability
disadvantages.
d) Reduce bearing frictional losses; 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.
e) Variable-nozzle turbochargers (discussed below) greatly reduce lag.
f) Decreasing the volume of the upper-deck piping.
g) Using multiple turbos sequentially or in parallel.
Supercharger: Operation
Definition
Supercharger is an air compressor used for forced induction of an internal combustion
engine.The greater mass flow-rate provides more oxygen to support combustion than
would be available in a naturally aspirated engine, which allows more fuel to be burned
and more work to be done per cycle, increasing the power output of the engine. A
supercharger is typically powered mechanically by belt, gear, or chain-drive from the
engine's crankshaft.
Types
a) Centrifugal supercharger
b) Rotor (rootes) supercharger
c) Vane type supercharger
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Operation
Centrifugal Supercharger
The centrifugal supercharger has an impeller equipped with curved vanes. As the
engine drives the impeller, it draws air into its center and throws it off at its rim. The air
then is pushed along the inside of the circular housing. The diameter of the housing
gradually increases to the outlet where the air is pushed out.
Rotor (rootes) Supercharger
The rootes supercharger is of the positive displacement type and consists of two rotors
inside a housing. As the engine drives the rotor, air is trapped between them and the
housing. Air is then carried to the outlet where it is discharged. The rotors and the
housing in this type of supercharger must maintain tight clearances and therefore are
sensitive to dirt.
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Vane-type Supercharger
The vane-type supercharger has an integral steel rotor and shaft, one end
supported in the pump flange and the other end in the cover, and revolves in the body,
the bore of which is eccentric to the rotor. Two sliding vanes are placed 180 degrees
apart in slots in the rotor and are pressed against the body bore by springs in the slots.
When the shaft rotates, the vanes pick up a charge of air at the inlet port, and it is
carried around the body to the outlet where the air is discharged. Pressure is produced
by the wedging action of the air, as it is forced toward the outlet port by the vane. The
term superchargergenerally refers to a blower driven by a belt, chain, or
gears. Superchargers are used on large diesel and racing engines.
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The supercharger raises the air pressure in the engine intake manifold. Then, when the
intakes valves open, more air fuel mixture ( gasoline engine ) or air (diesel engine) can
flow into the cylinders. An intercooler is used between the supercharger outlet and the
engine to cool the air and to increase power ( cool charge of air carries more oxygen
needed for combustion ). A supercharger will constantly produce increased pressure at
low engine speed because it is mechanically linked to the engine crankshaft. This low
engine speed because it is mechanically linked to the engine crankshaft. This low
speed power and constant throttle response is desirable for passing and entering
interstate highways.
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Supercharger: Construction and Components
At 1 shown the inlet pipe for conducting the explosive mixture into the supercharger and
against the vanes 3 of the impeller 2. The impeller 2 is mounted on a shaft 4 and is
operated at high speed through suitable gearing, partly shown at 5 and 6, from the
crankshaft of the engine.
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The spacing of the impeller from the engine proper depends upon the space required for
the gearing 5, 6 and the inner wall 7 of the impeller casing extends at the right angles to
the impeller shaft for a distance equal to the diameter of the impeller. From the point 8
the inner wall 9 of the casing is slanted sharply toward the engine 10 and forms, with the
outer wall of the impeller casing, the slanting diffuser section 11 comprising the principal
movement of the supercharger.
Ordinarily, the diffuser section is radial to the impeller shaft but it will be seen that the
construction is substantially conical in shape, thus allowing the collector pipe 12, into
which the diffuser section merges, to be placed close to the engine 10 to which it is
secured at 13 forming with the engine a housing 17 for the gear train. The collector pipe
12 is partially separated from the diffuser section by a web 14 forming an extension of
the wall 9, this structure serving to reduce the necessary overall diameter of the
supercharger. The slanting diffuser section thus leaves the spaces at 16 radially of the
impeller and within the engine compartment free for mounting other accessories.
The foregoing construction which provides for terminating the impeller 2 at a point within
the diffuser casing 11, which is intermediate the fuel inlet pipe 1 and the collector pipe
12, so that the line of vertical axis of the impeller which is indicated at 15 crosses the
diffuser casing at a point between the extremity of the impeller 2 and the collector pipe
12, furnishes a very advantageous result.
Particles of liquid fuel reaching the impeller are thrown off the impeller vanes by
centrifugal force in the direction of the axial line 15, and thus cross the main stream of
fuel gas within the diffuser casing. The result is that instead of adhering to the inner
faces of the diffuser casing, particles of liquid fuel become thoroughly mixed with the
main fuel stream before entering the collector pipe 12.
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Conclusion
Both turbocharger and supercharger have their own advantages and disadvantages,
depend on what they are applied to. Both of them can be used to improve fuel efficiency,
or performance. But it is nearly impossible to get both of it. Both forced inductionsystems can be combined or doubled to create a greater engine either on gasoline or
diesel engine, even 2-stroke engine. History has shown what forced induction systems
contribute to. Through automotive any near related field, in commercial or performance
industry, may improve the existing forced induction systems. Maybe there will be a
system that can give both fuel efficiency and performance. No one knows.
Reference
http://en.wikipedia.org/wiki/Turbocharger
http://en.wikipedia.org/wiki/Supercharger
http://rusubaru.com/turbo-lag-subaru-wrx-sti/
http://forums.club4ag.com/zeroforum?id=26
http://www.allfordmustangs.com/forums/power-adders/1791-turbo-vs-supercharger.html
http://en.wikipedia.org/wiki/Turbochargerhttp://en.wikipedia.org/wiki/Turbochargerhttp://en.wikipedia.org/wiki/Superchargerhttp://en.wikipedia.org/wiki/Superchargerhttp://rusubaru.com/turbo-lag-subaru-wrx-sti/http://rusubaru.com/turbo-lag-subaru-wrx-sti/http://forums.club4ag.com/zeroforum?id=26http://forums.club4ag.com/zeroforum?id=26http://www.allfordmustangs.com/forums/power-adders/1791-turbo-vs-supercharger.htmlhttp://www.allfordmustangs.com/forums/power-adders/1791-turbo-vs-supercharger.htmlhttp://www.allfordmustangs.com/forums/power-adders/1791-turbo-vs-supercharger.htmlhttp://forums.club4ag.com/zeroforum?id=26http://rusubaru.com/turbo-lag-subaru-wrx-sti/http://en.wikipedia.org/wiki/Superchargerhttp://en.wikipedia.org/wiki/Turbocharger