in which the cycle of operations is completed in two
revolutions (720º) of the
crank shaft or four strokes of the piston is known as the
four stroke engine. One stroke is completed when the piston
moves from Top dead centre to Bottom Dead Centre or when the
crank rotates through 180º.
In four stroke CI engine the combustion of fuel-air mixture
takes place with compression. The engine operates at a high
compression ratio of the order of 16 to 20. Due to high
compression ratio the mixtures reaches its ignition
temperature and the combustion takes place.
The major components of a four stroke compressed Ignition
It is a cylindrical vessel in which a piston makes up and
It is a cylindrical component making up and down movement in
It is the portion above the cylinder in which the combustion
of the Fuel-air mixture takes place.
The inlet valves allow the fresh fuel-air mixture to enter
the combustion chamber and the exhaust valve discharges the
products of combustion.
It is a shaft which converts the reciprocating motion of
piston into the rotary motion.
The connecting rod connects the Piston with the crankshaft.
The cam shaft controls the opening and closing of inlet and
It is located at the top of head to inject the fuel into the
A four stroke CI engine consists of the following four
or power stroke
1. Suction Stroke: This stroke starts when the piston is at the top dead centre. When it
moves downwards it will create suction and only air enters
the cylinder. The inlet valve is open at this time and
exhaust valve is closed. When the piston reaches at the
bottom dead centre the inlet valve closes and the suction
stroke ends. It all takes place in 180º
of the crankshaft rotation.
2. Compression stroke: In this stroke the piston starts moving upward. During this stroke
both the inlet and exhaust valves are closed. The air is
compressed by the upward movement of the piston. At the end
of the compression stroke the fuel is injected into the
combustion chamber. An injector is provided to inject the
fuel. At the end of compression stroke the temperature is
sufficient to ignite the fuel and the combustion of fuel-air
mixture takes place.
3. Expansion or Power Stroke: Due to the high pressure of the burnt gases the piston
moves towards bottom dead centre. Both the inlet and exhaust
valve remains closed during the stroke.
4. Exhaust stroke:
When the piston is at the bottom dead centre the exhaust valve opens.
As the pressure falls to atmospheric level. The piston moves
from Top dead centre to bottom dead centre and sweeps the
products of discharge out at nearly atmospheric pressure.
The exhaust valve closes at the end of exhaust stroke. The
gases are not fully exhausted. Some of the burnt gases
stills remains in the clearance volume.
remained gases mixed with the fresh fuel-air mixture
entering the chamber.