The petrol engine is spark engine.
The working cycle has four strokes as
Intake/suction stroke: A fuel (petrol)
vapours and the mixture is drawn in the cylinder of the IC engine during the suction stock. Petrol (boiling point range 300C to 2000C) evaporates easily and forms an inflammable air-fuel mixture.
Compression stroke
The piston compresses the air-fuel mixture.
Power stroke
The compressed air-fuel mixture is ignited by an electric spark near the spark plug. The combustion produces hot gases, increasing the pressure and pushing the piston down.
Exhaust stroke
The piston moves up and expels the exhaust gas from the cylinder.
Compression stroke
The piston compresses the air to a pressure of about 40kg\cm2 this increased temperature to about 6000C.
Towards the end of the stroke fine droplets of diesel is injected into the compressed air. Diesel ignites spontaneously due to the high temperature of the air. This increase in pressure and temperature within the cylinder.
Power stroke
The combustion produces hot gases, increasing the pressure and pushing the piston down