Detonation

Also known as Knock.

A condition in which some of the unburned air-fuel mixture in the combustion chamber explodes spontaneously after the spark plug fires, as a result of the heat and pressure of air-fuel mixture that has already been ignited. This greatly increases the mechanical and thermal stresses on the engine and may be recognised by a 'pinking' sound. The use of higher octane fuel is usually recommended to reduce knocking.