Detonation

Detonation (also known as engine knock, pinging, or spark knock) is an abnormal combustion phenomenon where the remaining unburned air-fuel mixture in the cylinder explodes spontaneously after the spark plug fires. In normal combustion, the spark plug creates a controlled flame front that burns smoothly across the chamber. During detonation, extreme cylinder heat and pressure cause the outlying fuel mixture to self-ignite and explode violently.

This uncontrolled explosion creates colliding flame fronts and pressure waves that slam into the cylinder walls and piston crown, producing a distinctive metallic pinging or knocking sound. Detonation is typically caused by low-octane fuel, advanced ignition timing, lean fuel mixtures, excessive combustion chamber deposits, or engine overheating.

Mild detonation is managed in modern cars by knock sensors, which detect the vibration and signal the ECU to retard ignition timing. However, severe or sustained detonation will cause rapid engine damage, including cracked pistons, blown head gaskets, and damaged spark plugs. Prevention involves using the correct fuel octane rating, maintaining fuel system health, and replacing failing knock sensors.