The engine 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.
If the combustion of the fuel-air mixture takes place with
the help of spark plug then it is known as four strokes
Spark Ignition Engine. The SI engine operates at a
compression ratio of 6 to 10.
components of a four stroke spark Ignition engine are.
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 cylinder head. It is used to initiate
the combustion process.
stroke engine SI consists of the following four strokes.
Suction or Intake stroke
Expansion or power stroke
This stroke starts when the piston is at the top dead
centre. When it moves downwards it will create suction and
draw the fuel air mixture into 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
fuel-air mixture is compressed by the upward movement of the
piston. At the end of the compression stroke the fuel-air
mixture is ignited by the spark plug. Burning of fuel-air
mixture takes place instantaneously when the piston is at
the top dead centre.
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.
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.
These remained gases mixed with the fresh fuel-air mixture
entering the chamber.