Forced Induction: Supercharger and Turbocharger

8/3/2019 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

Forced Induction: Supercharger and Turbocharger

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