The cycle of an internal combustion engine is called the four-stroke cycle, shown below. The piston rises and compresses the fuel air mixture before the spark ignites it, which increases the pressure and heat of the mixture near to its auto-ignition point. This will enable the mixture to burn at its fastest and consequently the fuels energy will be most efficiently harnessed. However if the compression ratio is increased too much then detonation could occur. The engine strokes can be summarised as follows:

· 1-2, Induction Stroke – The intake valves open, the piston moves down and the fuel / air mixture is injected into the combustion chamber.
· 2-3, Compression Stroke – During the compression stroke the valves are closed. The [piston moves up and compresses the mixture.
· 3-4, Working Stroke – The spark plug produces an electric spark that ignites the compressed mixture and forces the piston down.
· 4-1, Exhaust Stroke – During the exhaust stroke the exhaust valves are open. The piston rises and expels the exhaust gases from the cylinder.