Knock resistance

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The knock resistance is the property of the fuel used ( gasoline , gas ), in a gasoline engine uncontrolled by self-ignition to burn ( " knock "), but triggered by the spark, injection or compression .

In the case of a spark-ignition engine, in order to protect the plain bearings and all other components, a continuously moving flame front emanating from the spark plug or injection nozzle is desirable. When knocked, a large part of the mixture explodes almost simultaneously. Knocking often occurs in engines with high compression.

The knock resistance is expressed by the octane number for gasoline and by the methane number for gaseous fuels .

Engine knock

The knocking in the engine is an early ignition of the gasoline-air mixture. The compression and heat in the cylinders of the gasoline engine can lead to premature self-ignition of the gasoline-air mixture (= knocking). Unbranched hydrocarbons tend to ignite early, while branched and unsaturated hydrocarbons and aromatics have a relatively high knock resistance.

In the past, lead-containing, organometallic compounds such as tetraethyl lead were added to gasoline to increase the knock resistance (“leaded gasoline”). During combustion, the lead compound decomposed largely thermally, releasing lead dust in the exhaust gases. In contrast to the burnt, insoluble PbO 2 , the unburned tetraethyl lead is soluble, very bioactive and therefore harmful in small amounts even with short-term exposure. Nevertheless, this addition was only waived from 1985. In 2000 it was banned across the EU because it also affects the function of catalytic converters .

Older sports cars had corrosion problems on the valve seats due to the unleaded petrol. The lead coating had a protective effect. Even before lead was banned, corrosion-resistant valve steels had been developed. Lead additives are still available for older vehicles on the road.


  • Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. 25th edition, Friedr. Vieweg & Sohn Verlag, Wiesbaden 2003, ISBN 3528238763