working principle of four stroke petrol engine in Telugu

In four stroke petrol engine cycle is completed in two revaluations of crank shaft. These two revolutions constitute of four strokes
1.Suction stroke
2.Compression stroke
3.Expansion or power stroke
4. Exhaust stroke
1.Suction stroke: during this stroke, the inlet valve opened and the exhaust valve closed. The piston is moving downwards direction and create the partial vacuum in the cylinder ( i.e., pressure inside cylinder is less than atmospheric pressure). Due to pressure difference, the air fuel (petrol + air) mixture is sucked into the engine cylinder. The crank is revolute half revolution.
2.Compression stroke: during this stroke ,both valves are closed position, piston moving upwards and compress the charge (petrol + air) to required pressure and temperature. At the end of compression stroke ,the charge is ignited by spark given out by spark plug. The combustion takes place, due to combustion the pressure and temperature are increased further at nearly constant volume. The crank is totally complete one revolution .

3.Expansion or power stroke: during this stroke the both valves are closed and high pressure gas expands. The expansion of hot gases exerts a pressure on the piston. Due to rise in pressure ,the piston is pushed down a high pressure. At the end of the expansion stroke, exhaust valve is opens. The crank is revolute half revolution.
4. Exhaust stroke: In exhaust stroke ,exhaust valve is open and inlet valve is closed. Piston moves upwards (BDC to TDC) forces the spent gases through exhaust valve. At the end of this stroke exhaust valve is closes the inlet valve is opened. The fresh charge is sucked into engine cylinder. Totally the engine cycle is completed in two revolutions. Next cycle is repeated