Knocking

Abnormal combustion in an engine in which the fuel-air mixture explodes before the spark ignites it, with a metallic knocking sound.

An engine becomes more efficient the more it compresses its fuel, but compress too much and the mixture ignites spontaneously all at once, damaging the engine. This was the barrier to higher efficiency. In the 1920s Thomas Midgley and T. A. Boyd at GM studied systematically how fuels and additives changed the tendency to knock^[1]; tetraethyl lead, found in 1921, suppressed it well.

Leaded petrol was used worldwide for decades until the health damage from lead released into the air became clear and countries banned it. Knock is now handled by high-octane fuel and electronic control of ignition timing.