preignition

Preignition is the premature ignition of the air-fuel mixture in an internal combustion engine before the spark plug fires on the compression stroke. In a spark-ignition engine, the mixture is supposed to ignite at the moment of spark; preignition occurs when a hot spot or heat input ignites the fuel ahead of the spark. It is distinct from knocking (detonation), which is the spontaneous ignition of the remaining end gas after the spark has occurred.

Common causes include hot spots such as carbon deposits on the piston crown or combustion chamber, overheated or glowing spark plugs, and hot exhaust valves. High compression ratios, excessive combustion temperatures, advanced ignition timing, and elevated intake air temperatures from forced induction also raise the risk. Poor fuel quality (low octane) or improper air-fuel mixture can contribute to higher peak temperatures and preignition as well.

Preignition can produce very high peak pressures and temperatures, leading to piston and piston-ring damage, bearing damage, and valve damage. It often accompanies a loss of power, rough running, and unusual engine noise. Because it can occur abruptly, it may be difficult to distinguish from detonation without careful diagnostic checks.

Mitigation focuses on removing heat sources and avoiding conditions that promote premature ignition. Examples include cleaning carbon deposits, repairing cooling-system faults, using higher-octane fuel, selecting cooler spark plugs, and retarding the ignition timing or reducing boost to lower combustion temperatures. Regular maintenance to prevent hot spots is recommended to reduce preignition risk.