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The self-ignition of a fuel is the spontaneous ignition of a fuel-air mixture without an ignition source , for example by compression in the engine. She's with gasoline engines , an undesirable process in diesel engines , it is the basis of operation. Compression ignition was described by the French railway engineer Alphonse Beau de Rochas in his famous patent on the four-stroke engine as early as 1862 .

The pneumatic lighter also works with self-ignition - here, for example, a piece of tinder sponge is made to glow.

In gasoline engines, compression ignition leads to what is known as knocking . The resistance of a petrol to knocking is described by the octane number . The ignitability of a diesel fuel is described by the cetane number .

Chemical processes

When the pressure in the cylinder rises, the rapid compression leads to temperature increases (ideal: adiabatic compression). In the first reactions between oxygen and hydrocarbons, oxygen is inserted into the hydrocarbon chains. In the process, peroxides and hydroperoxides are formed exothermically . These reactions are relatively slow. During normal operation in a gasoline engine, up to 10 percent of the total energy of the combustion can be released.

The CC and CH bonds can be cleaved by high temperatures and reactive radicals are formed . The peroxides can also be split into radicals. These then react very quickly exothermically with the oxygen. This leads to an explosive chain reaction at various points, which leads to pressure peaks that can be perceived as knocking .

If ignited by an ignition spark, radicals are presumably formed in the area of ​​the plasma . This creates a flame front emanating from the ignition source, which moves steadily through the combustion chamber. There are no critical pressure peaks.


  • Kurt-Jürgen Berger, Michael Braunheim, Eckhard Brennecke: Technology automotive engineering. 1st edition, Verlag Gehlen, Bad Homburg vor der Höhe, 2000, ISBN 3-441-92250-6
  • Peter Gerigk, Detlev Bruhn, Dietmar Danner: Automotive engineering. 3rd edition, Westermann Schulbuchverlag GmbH, Braunschweig, 2000, ISBN 3-14-221500-X
  • Hans Jörg Leyhausen: The master craftsman's examination in the automotive trade part 1. 12 edition, Vogel Buchverlag, Würzburg, 1991, ISBN 3-8023-0857-3

See also

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