Chapter 6. Supercharging

SHROFF S. R. ROTARY INSTITUTE OF CHEMICAL TECHNOLOGY (SRICT)

DEPARTMENT OF MECHANICAL ENGINEERING.

Subject: Internal Combustion Engine

Chapter 6. Supercharging

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Chapter 6. Supercharging

6.1 Need for supercharging

6.2 Effect of supercharging

6.3 Types of Supercharger

6.4 Method of supercharging

6.5 Thermodynamic analysis of supercharging engine

6.6 Limitation of supercharging

6.7 Turbo charging methods

Outline

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The rate of fuel burned depends upon the air capacity of the

engine, that is the amount of air which the engine is capable

of drawing per unit time.

Also, air capacity is practically unaffected by the presence

of fuel.

Increasing the energy input requires the induction of more

charge per cycle.

Also at high engine speed, volumetric efficiency decreases

so the induction. Engine speed is also limited by thermal

stresses.


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At high altitude due to less density, available induction

charge is less. Also while climbing hill, racing and air craft

it is necessary to produce more power with less density.

“supercharging is used to increase the power output by

forcing the charge into the engine at pressure above

atmospheric”

It increase volumetric efficiency, performance, reduce

knocking, overheating or failure of some part.


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Output Power

It increase the power output of engine without increase its

weight and size.

Due to high inlet pressure, it compresses the residue gases

and facilitate more charger to fill.

It reduces gas exchange work. Because during suction work

is done by gas on the piston.

With more air, it facilitates more combustion.


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Fuel consumption

The brake specific fuel consumption for CI engine is somewhat

less than that for naturally aspirated engines due to better fuel

distribution, improved combustion and increased mechanical

efficiency.

Mechanical efficiency

An increase in the intake pressure increase the gas load, hence

large bearing area and heavier components are needed. Thus, the

friction losses are increased.

However, the increase in friction losses is less than the power

gained by supercharging. Therefore, the mechanical efficiency of

the engine is also increased by supercharging.


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Volumetric efficiency

Residual gases are compressed by inducted charge due to high

pressure in clearance volume.

The rate of increase of volume efficiency becomes progressively

less as the supercharging is increased, since the contraction of the

residuals becomes proportionately less.

Also increase in volumetric efficiency with the increase in the

intake pressure is higher at low C.R. ratio, since under these

conditions the volume occupied by the residuals is more and the

possibility of contraction is more.


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Lobe shape to rotor rotate in

opposite direction, fixed in

common casing

Both have contact with one

another and also with casing

Air trapped in the recesses (pocket) between the rotor and

housing is carried towards the delivery port without a

significant change in volume.

Roots blower

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As these recesses open to the

delivery line, since the suction

side is closed, the trapped air

is suddenly compressed by the

backflow from the higher

pressure delivery line.


Intermittent delivery produces pressure pulses.

Suitable for small pressure ratio 1.2

Three lobes rotor gives more uniform flow that two lobe

rotor.

Roots blower


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Positive displacement type,

It has a cylindrical rotor

mounted eccentrically with

respect to the fixed cylindrical

casing.

Deep slot are cut into the rotor to accommodate thin

rectangular vanes which are free radially.

Vane blower


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Vane blower

Two blades with rotor and

casing make closed pocket.

As rotor moves the volume of

pocket decrease from inlet side

to outlet side which compress

the air trapped in that pocket.

RPM 4000-5000 rpm.

Pressure 1.3 bar or above.


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It consist inlet pipe, impeller,

stationary diffuser, volute

casing and outlet pipe.

It primarily coupled with the

exhaust driven turbine in a

turbocharger.

20,000-30,000 rpm

Pressure ratio 2-3

Application Aircraft engines.

Centrifugal compressor


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Turbo charging

A compressor and turbine mounted on a single shaft is used

to increase inlet air density.

The exhaust gases from the engine having sufficient energy

drive the turbine. So, turbine drives the compressor.

Fig. (b) most commonly used arrangement, simple

Mechanical supercharging:

Blower or compressor driven by engine shown in Fig.(a)


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Turbo charging

Fig (c), Large marine engine

Fig (d) Very high boost pressure 4 to 7 bar


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Turbo charging

Fig (e) Second turbine used to increase the power of engine

Fig (f) Cooling increase the volumetric efficiency during

induction.

6.5 Thermodynamic analysis of supercharging

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The ideal dual combustion cycle of mechanically driven

supercharged engine and naturally aspirated engine are

shown in fig.


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Net Work given by supercharged engine.

Wsc = Engine work output + Gas Exchange work –

Supercharged work

It is be noted that positive gas exchange area may be

greater than negative supercharged area.

It is to be kept in mind that with increase in supercharging

pressure the negative work, will also increase and therefore

there will be a limit for supercharging.

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The ideal efficiency of supercharged engine will decrease

with increase in supercharging pressure.

However, it is to be understood that the not increase in

power output is due to increase in mass of charge.

For adiabatic compression the work done on the

supercharger

Considering efficiency of compressor,

,

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Increased charge density, increase burning and so temperature.

It creates cooling and knocking problems.

Also due to high peak pressure, high strength of cylinder

required.

It can be reduced by: lengthening ignition delay, lower C.R., too

lean or rich mixture, injection of water in inlet manifold, use of

high Octane fuel.

Considering above factors, the SI engines are rarely

supercharged, except when more output power is prime important

against efficiency and economy.

Supercharging of SI engine

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Supercharging of CI engine

It can be safely supercharged without any combustion

difficulties

Still at high pressure, violent pounding noise known as

diesel knock is produced.

So, it is essential to keep temperature of supercharge as low

as possible in order to get high volumetric and thermal

efficiency.

Supercharging of CI is limited by thermal and mechanical

loading, while SI is limited by knocking.

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Due to high thermal load, we have to use better coolant and

engine material

Large bearing and heavier engine component required to

withstand mechanical stresses.

SI engine is limited with supercharging due to the high

knocking tendency at high pressure and temperature.

More valve overlap may be used to overcome these problems.

Durability, reliability and fuel economy are the main

considerations that limit the degree of supercharging of an

engine.

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It is possible to use turbocharger alone in 4 stroke contrast

to 2 stroke to turbocharger for supply of air to the engine.

The main types of turbo charging methods namely (1)

Constant Pressure (2) Pulse (Buchi) operation (3) Pulse

converter.


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Exhaust into a common manifold at high pressure.

Exhaust gases expands to an approximately constant

pressure.

So, internal energy produce work in exhaust turbine

(Reaction Turbine)

Recovery of blow down is higher if the pressure ratio is high

Constant Pressure

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Advantages

At high pressure ratio, very efficient, low fuel consumption.

When the number of cylinder is divisible by 3 and turbine

pressure ratio is 3:1 or more, it operates at constant pressure

and temperature which gives high efficiency.

Exhaust piping is very simple for multi cylinder.

Engine speed is not limited by the pressure waves in the

exhaust pipes.

Constant Pressure

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Disadvantages

Large exhaust pipes, increases the size of engine.

Response of this system to load changes is poor.

Due to high pressure drop across the turbine, scavenging

is not proper.

Constant Pressure

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Considerable part of the blow down energy is converted

into exhaust pulses as soon as the exhaust valve opens.

These pulses are led through narrow exhaust pipes by the

shortest possible route to the turbine where this energy is

utilized.

A large proportion of energy is thus recovered.

Pulse Turbo charging

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Separate exhaust pipes are used so that exhaust process of

various cylinders do not interface with one another.

A common pipe is used for these cylinders whose exhaust

cycles do not overlap significantly in terms of time.

The turbine has separate inlets and nozzle segments for

each exhaust pipe.

Widely used for low pressure turbines where rapid

acceleration is needed.

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Advantages

Recovery of the exhaust blow down energy is quite

efficient.

Rapid acceleration of the turbocharger to a high speed,

almost no delay.

Less space required

Better scavenging

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Disadvantages

High throttling losses

Recovery of energy is poor when the pressure ratio is high

For multi cylinder, inlet and exhaust pipe arrangement

becomes complicated.

Poor turbine efficiency is obtained in case of one or two

cylinders.

If the waves take too long to travel to the turbine the

scavenging process is disturbed. Thus the length of the pipe or

engine speed is limited.

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Pulse converter allows the advantages of the pulse and the

constant pressure turbo charging to be utilized simultaneous,

while avoiding most of the drawbacks of both.

This is done by connecting the different branches of exhaust

manifolds together in a specially designed venturi junction

called pulse converter, before the turbine.

Pulse converter

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Pulse converter

Turbo charging turbine is a constant pressure machine and

for maximum efficiency requires steady flow conditions.

With pulse charging the turbine operates at relatively

lower efficiency due to partial admission operation.

Moreover, the low level of available exhaust energy

especially at part load required operation with pulse

charging for efficient utilization of this energy and good

scavenging. For this reason a combination of the two

system is needed for good efficiency of the turbine.

Chapter 6. Supercharging

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Transcript of Chapter 6. Supercharging