Detonation
Detonation is an uncontrolled, explosive ignition of the fuel-air mixture within the cylinder’s combustion chamber. It causes excessive temperatures and pressures which, if not corrected, can quickly lead to failure of the piston, cylinder, or valves.
It's often caused by:
- Use of a lower fuel grade than that specified by the aircraft manufacturer
- Operation of the engine with extremely high manifold pressures in conjunction with low rpm
- Operation of the engine at high power settings with an excessively lean mixture
Preignition
Preignition occurs when the fuel-air mixture ignites prior to the engine’s normal ignition event.
Premature burning is usually caused by a residual hot spot in the combustion chamber, often created by:
- a small carbon deposit on a spark plug
- a cracked spark plug insulator
- other damage in the cylinder that causes a part to heat sufficiently to ignite the fuel-air mixture