Abnormal combustion, often producing audible sound, caused by autoignition of the air/fuel mixture.
Knock produces extreme local temperatures in the cylinder. These high heat quantities place a severe strain on the piston, valves and cylinder head, and can lead to damage.
Tetraethyl Lead [Pb(C2H5)4] and Tetramethyl Lead [Pb(CH3)4] are effective additives that were used in the automobile fuel to reduce knocking. They have now been banned due to lead poisoning.
See also: Auto-Ignition Temperature, Internal Combustion Engine, Octane Number, Pre Ignition, Tetraethyl Lead, Tetramethyl Lead.
Subjects: Engines Noise & Vibration