Manual of four stroke petrol engine

To study of
four stroke petrol
engines.

AIM: To study of four stroke petrol
engine.

APPARATUS: Model of  four stroke petrol
engine.

THEORY: The thermodynamic cycle is completed in four strokes of the piston or in two revolutions of the crankshaft.Thus , one power stroke is obtained in every two revolutions of the crankshaft.

1. WORKING PRINCIPLE OF FOUR STROKE PETROL ENGINES

There are four strokes which are as follows:

i) Suction stroke

ii) Compression stroke

iii) Expansion or working or power stroke

iv) Exhaust stroke

i) SUCTION STROKE: The suction stroke starts with the
piston at top dead centre position. During this stroke,
the piston moves downwards by means of crank shaft.
The inlet valve is opened and the exhaust valve is
closed. The partial vacuum created by the downward
movement of the piston sucks in the fresh charge
(mixture of air and petrol) from the carburetor through
the inlet value. The stroke is completed during the half
revolution (180 ) of the crank shaft, which means at
the end of the suction stroke, piston reaches the bottom
head centre position.

2.COMPRESSION STROKE: In this stroke both the inlet

and exhaust valves remain closed during the stroke.

The piston moves towards (T.D.C.) and compresses the

enclosed fuel-air mixture drawn. Just before the end of

this stroke the operating plug initiates a spark which

ignites the mixture and combustion takes place at

constant pressure.

3.POWER STROKE OR EXPANSION STROKE: In this stroke

both the valves remain closed during the start of this

stroke but when the piston just reaches the B.D.C. the

exhaust valve opens. When the mixture is ignited by the

spark plug the hot gases are produced which drive or

throw the piston from T.D.C. to B.D.C. and thus the

work is obtained in this stroke.

4.EXHAUST STROKE: This is the last stroke of the cycle.

Here the gases from which the work has been collected

become useless after the completion of the expansion

stroke and are made to escape through exhaust valve

to the atmosphere. This removal of gas is

accomplished during this stroke. The piston moves

from B.D.C. to T.D.C. and the exhaust gases are driven

out of the engine cylinder;