Four stroke petrol engine 

It contains a cylinder, piston  inlet valve, exaust valve, spark plug etc.

It contains 4 processes which are as follows:-
1. Suction stroke
2.compression stroke
3. Expansion stroke
4. Exhaust stroke

complete diagram of four stroke petrol engine 

1. Suction stroke:- when piston moves TDC to BDC  inlet valve open and exaust valve remain closed air fuel mixture enters in cylinder and piston approach at BDC and suction stroke is complete .

2 compression stroke :- Now piston is at BDC . Piston moves BDC to TDC and inlet and exaust valve remain closed. When piston move towards TDC mixture of fuel and air compressed and compression stroke completed.

3. Expansion stroke:- Now piston is at TDC and spark plug spark in mixture of air and fuel and combustion of mixture takes place and creates pressure or power so piston moves towards BDC it is also known as power stroke .

4. Exhaust stroke :- when piston moves BDC to TDC exaust valve open and inlet valve remain closed so burned gases are move out of cylinder by piston moves upwards this is called exaust stroke.


Above all processes are very fast and 1 process complete another get started.