This is an internal combustion engine. The vaporized fuel is mixed with air in carburettor. This mixture’s sucked by engine (Intake stroke - 1st stroke) through inlet valve which opens. The descending piston draws fresh petrol and air mixture into the cylinder. This mixture is compressed (Compression strokes 2nd stroke). During compression the mixture gets heated and ignition burns the fuel converting into a large volume of gas which pushes the piston. while the valves are closed the piston is pushed to the bottom of the cylinder by the gases of combustion (Power Stroke 3rd stroke) The exhaust valve opens due to momentum gained by wheels piston is pushed upwards and burnt gases are expelled (Exhaust stroke - 4th stroke).