Intake Stroke (Suction Stroke) :- When we revolve the engine crankshaft through cranking handle or self starter, the inlet valve opens and the piston being tied up with crankshaft with the help of connecting rod travels toward Bottom Dead Centre (B.D.C). When the piston moves towards B.D.C. partial vacuum is created on top of piston in the combustion chamber. Due to this partial vacuum, air-fuel mixture is drawn into the combustion chamber through open inlet valve. The volume of air fuel mixture getting into the combustion chamber depends upon the speed of piston as well as on throttle opening. When the piston reaches B.D.C., the inlet valve closes, whereas in actual practice this valve remains open up to 50 degree after B.D.C.
Compression stroke :- After the piston has come down on suction stroke, it has to go up towards T.D.C. ( Top Dead Centre ) due to rotary motion of crankshaft. As soon as piston starts going up in compression stroke inlet valve which was kept open in induction stroke closes. Exhaust valve ( outlet valve ) is already closed. The mixture or charge starts compressing. We know it that if we press the charge its temperature and pressure rises. This rise in temperature and pressure depends upon the opening of throttle, the speed of engine and the compression ratio. In petrol engine the copression ratio is usually 6 to 12:1 and pressure reaches. This stroke finishes when piston reaches from B.D.C. to T.D.C.
Power stroke :- In this stroke both valves remain closed. Before the piston reaches T.D.C. in compression stroke, the charge is ignited with the help of spark plug. As soon as charge is ignited spontaneous burning of charge takes place. Due to burning of charge gases expand in the cylinder. Piston is the only moving part, as such these expanded gases put pressure on crown of piston making it to travel down with force. Since the piston is connected with crankshaft with the help of connecting rod the crankshaft revolves.In power stroke at full throttle the temperature of burning charge reaches up to 2000 degree.
Exhaust stroke :- In this stroke exhaust valve opens and piston starts moving up from B.D.C. to T.D.C. due to the momentum of flywheel. As the piston moves up, it pushes the burnt gases out through