Diesel and petrol engines: Diesel engine and petrol engines are both internal combustion engines. In a diesel engine vaporized diesel oil is mixed with the air which is ignited by the heat of light compression in the engine cylinder. The mixture, on the explosion, makes the piston move. The speed and power of the engine are controlled by the amount of fuel supplied. Diesel engines are used in heavy road vehicles and for industrial purposes because it is economical in operation. In a petrol engine, an explosive mixture of vaporized petrol and air is ignited by sparks in the engine cylinder.