knocking

In a spark-ignition petrol engine, a phenomenon that occurs when unburned fuel-air mixture explodes in the combustion chamber before being ignited by the spark. The resulting shock waves produce a metallic knocking sound. Loss of power occurs, which can be prevented by reducing the compression ratio, re-designing the geometry of the combustion chamber, or increasing the octane number of the petrol (formerly by the use of tetraethyl lead anti-knock additives, but now increasingly by MTBE – methyl tertiary butyl ether in unleaded petrol).