An abnormal combustion event where the air-fuel mixture ignites too early in the compression stroke, usually caused by a hot spot in the cylinder like glowing carbon deposits or an overheated spark plug. This creates immense cylinder pressure pushing against the rising piston, leading to severe engine damage, melted pistons, or bent connecting rods.
Heard on the job
We pulled the head and found a hole melted right through piston number three from severe preignition.
Do not mix it up with
- Detonation: Preignition happens before the spark plug fires, while detonation happens after the spark plug fires when the remaining unburned mixture explodes spontaneously.