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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.