knock, engine

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knock, engine (pre-ignition or pinking) Metallic noise produced in an internal combustion engine, caused by detonation of part of the compressed fuel-air mixture before a spark from the spark plug ignites the remainder. It lowers the efficiency of the engine but can be overcome by adding an anti-knocking agent to the fuel. The traditional additive is lead(IV) tetraethyl, but because this compound releases lead into the atmosphere and thereby contributes to pollution, its use declined with the introduction of unleaded petrol.

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