Tetraethyl lead is an ingredient that was once commonly added to gasoline (also called petrol) to improve the performance of engines. It is called an antiknock additive because it reduces the "knocking" or "pinging" sound that an engine makes. Knocking occurs when portions of the fuel in an engine cylinder ignite too soon, causing all of the fuel to burn too quickly. The same conditions that cause knocking also reduce an engine's power and can damage the cylinders.