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8/12/2019 Combusion chambers2

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Internal Combustion Engines


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COMBUSTION CHAMBERS

COMBUSTION CHAMBERS- SI Engines

The design of combustion chamber has an important

influence upon the engine performance and its knock

properties. The design of combustion chamber involves

the shape of the combustion chamber, the location of the

sparking plug and the position of inlet and exhaust

valves. It has resulted in raising the compression ratio from 4: 1

before the First World War period to 8: 1 to 11:1


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HIGHER POWER OUTPUT REQUIRES THE FOLLOWING:

High compression ratio. The compression ratio is limited by the

phenomenon of detonation. Detonation depends on the design of

combustion chamber and fuel quality. Any change in design thatimproves the anti-knock characteristics of a combustion chamber

permits the use of a higher compression ratio which should result

in higher output and efficiency.

Small or no excess air.

Complete utilization of the air.

An optimum degree of turbulence.

High Volumetric Efficiency. This is achieved by having large

diameter valves with required clearance round the valve heads,proper valve timing and straight passage ways by streamlining the

combustion chamber so that the flow is with lesser pressure drop.

This means more charge per stroke and increase in the power

output. Large valves and straight passageways also increase thespeed at which the maximum power is obtained.


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HIGH THERMAL EFFICIENCY REQUIRES THE

FOLLOWING:

High compression ratio:

A small heat loss during combustion.

Good scavenging of the exhaust gases.

SMOOTH ENGINE OPERATION REQUIRES THE

FOLLOWING:

Moderate rate of pressure rise during combustion.

Absence of detonation which in turn means:

Compact combustion chamber, short distance of flame travel

from the sparking plug to the farthest point in the combustion

space.

Satisfactory cooling of the spark plug points (to avoid pre

ignition) and of exhaust valve head which is the hottest

region of the combustion chamber.


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Reduced exhaust pollutants

Exhaust pollutants can be reduced by

designing a combustion chamber that producesa faster burning rate of fuel. A faster burning

chamber with its shorter burning time which

reduces the oxides of nitrogen (NOX) in theexhaust gas without substantial increase in the

hydrocarbon emissions.


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DIFFERENT TYPES OF COMBUSTION

CHAMBERS

1. T-head combustion chamber.

2. L-head combustion chamber.

3. I-head (or overhead valve) combustionchamber.

4. F-head combustion chamber.


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T-head engine


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The intake valves are on one side of the engine

block and the exhaust valves on the other.

the cylinder and combustion chamberresembles a T - hence the name "T-head".

The design was very complex for its day,

requiring separate camshafts to operate theintake and exhaust valves. This made the

engine much more expensive


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L-head engine


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It is a modification of the T-head type of

combustion chamber. It provides the two valves

on the same side of the cylinder, and the valvesare operated by a single camshaft.

ADVTG:

Valve mechanism is simple and easy to lubricate.

Detachable head easy to remove for cleaning and

decarburizing without disturbing either the valve

gear or main pipe work. Valves of larger sizes can be provided.


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DADVTG:

Extremely prone to detonation due to large flame

length and slow combustion.

Extremely sensitive to ignition timing due to

slow combustion process


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Over head valve or I head combustion chamber

The disappearance of the side valve or L-head design

was inevitable at high compression ratio of 8 : 1because of the lack of space in the combustion chamber

to accommodate the valves.

Since 1950 overhead valve combustion

chambers are used. This type of combustion chamber

has both the inlet valve and the exhaust valve located in

the cylinder head. An overhead engine is superior to

side valve engine at high compression ratios.


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The overhead valve engine is superior to side

valve

- Lower pumping losses- Less distance for the flame to travel and

therefore greater freedom from knock

Removal of the hot exhaust valve from theblock to the head, thus confining heat failures

to the head.


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F- Head combustion chamber

In such a combustion chamber one valve is in head and

other in the block. This design

is a compromise between L-head and I-head combustionchambers. One of the most head engines (wedge type) is

the one used by the Rover Company for several years.

Another successful design of this type of chamber is that

used in Willeys jeeps


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Its advantages are :

o High volumetric efficiency

o Maximum compression ratio for fuel of givenoctane rating

o High thermal efficiency

o It can operate on leaner air-fuel ratios withoutmisfiring.

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