Engine knocking

Engine knocking is abnormal combustion in a spark-ignition cylinder that produces rapid pressure oscillations. It can damage pistons and other engine parts when severe or sustained.

End gas: The unburned fuel–air mixture ahead of the propagating flame in a spark-ignition engine cylinder. This trapped mixture can autoignite before the normal flame front reaches it.
Spark-ignition engine: An internal-combustion engine in which an electrical spark initiates combustion of a fuel–air mixture. Knock is defined by abnormal combustion in this engine type.
Knock sensor: A vibration sensor mounted on an engine to detect frequencies associated with combustion knock. Its signal lets the control system identify knock during operation.
Pre-ignition: Combustion that begins before the scheduled spark because a hot surface or other source ignites the mixture. Unlike knock, it starts before the spark and can trigger especially damaging pressure and heat.
Autoignition: Combustion initiated by heat and pressure without an external ignition source. Autoignition of the end gas creates the abrupt heat release that drives knock.
Four-stroke engine: An internal-combustion engine cycle with intake, compression, power, and exhaust strokes. Knock risk rises during the compression and power portions of its cycle.
Knock control: Engine strategies that detect knock and adjust operating conditions to suppress it. Control systems commonly retard ignition timing when knock is detected.
Surface ignition: Ignition of an engine charge by a hot surface inside the combustion chamber. It can cause pre-ignition or abnormal combustion, but is distinct from end-gas knock.
Detonation: A combustion wave coupled to a shock that propagates through a reactive mixture. Knock produces pressure waves, but it is not necessarily a self-sustaining detonation.
Air–fuel ratio: The mass ratio of air to fuel supplied for combustion. Mixture composition affects flame speed, temperature, and end-gas autoignition.

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