Petrol engine: Petrol engine works on the principle of Intake (fuel + air), Compression, Power and Exhaust. It means the fuel, and air will enter the combustion chamber, piston come upwards and compress the fuel. The compressed fuel comes in contact with spark plug and produces a spark that push piston down

Diesel Engine: In diesel engine the phenomenon is slightly different, there is no sparkplug in diesel engine. In here only air enters in the burning chamber, when piston is pushed up by motor, it will compress the air in the chamber, making it hot. Now valve opens and inject diesel, because of compressed air, the diesel blasts and push the piston down