Typically, when air and fuel burn in the combustion chamber, the burn is a gradual process, elapsing over a period of a few milliseconds as the flame front spreads from the spark plug which initiated the combustion. Under ideal conditions, the flame front expands uniformly, so that the combustion process exerts a continuous pressure on the piston during the piston’s down-stroke, resulting in a smooth strong twist on the crankshaft.
Detonation refers to an uncontrolled combustion, which elapses much more rapidly and which generates a shorter-lasting but more powerful pressure on the piston. Detonation can also be caused when the pressure of an expanding (spark plug initiated) flame front causes combustion to begin separately in other areas of the combustion chamber.
Pre-ignition is similar to detonation, but is caused when the air/fuel mixture ignites prior to the spark ignition.
The increased heat and pressure of detonation and pre-ignition are be very destructive, particularly to the piston heads and ring lands, making this a leading cause of engine failure in high-performance engines.
Factors contributing to detonation include: