Anti-knock agents

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An anti-knock agent is an additive in a fuel for gasoline engines .

General

An anti- knock agent reduces the tendency of a fuel-air mixture to knock in a gasoline engine. The opposite of the tendency to knock is (not lexically correct) known as knock resistance. This so-called knock resistance is given according to octane numbers , mostly according to the more easily measured RON (research octane number) and more rarely according to the more practice-relevant MOZ (engine octane number).

background

In a gasoline engine with spark plug ignition (unlike a diesel engine) it is desirable that the fuel-air mixture is only ignited by the arc on the spark plug. If the fuel-air mixture ignites by itself and therefore too early, the gasoline engine works poorly.

In this operating state, the efficiency is reduced and the relative thermal load and the relative mechanical load increased. The too early ignitions are acoustically noticeable and are known as knocking. If this unfavorable operating condition is intense (with a large throttle valve opening and high speed) and lasts for a long time (longer than about 100 seconds), the gasoline engine can overheat and be destroyed.

effect

An anti-knock agent increases the product of the pressure and the temperature necessary to ignite a fuel-air mixture.

history

The synthetic petrol Motalin , made from coal and refined with the anti- knock agent iron pentacarbonyl , has existed in Germany since 1928 . Its main disadvantage was that iron (III) oxide accumulated on the cylinders, which significantly increased the wear on them.

Tetraethyl lead (TEL) has been used as an anti-knock agent in the USA since 1921 and in Germany since 1936. Its great toxicity has been known since 1924. Nevertheless, it was unrivaled for decades because of its price-effectiveness ratio. It is still used today in aviation fuel for high-compression gasoline engines (such as the historically important "Av Gas 100" with the MOZ 100). A conceivable alternative is the combination of kerosene and (supercharged) diesel engines. The most important disadvantage (apart from the conversion price, if applicable) is the poorer mass-performance ratio of the diesel engines. However, this disadvantage is only relevant for small distances.

Because the lead produced when the tetraethyl lead is burned would destroy the vehicle catalytic converter, the tetraethyl lead in the unleaded motor fuels suitable for catalytic converters has been replaced by methyl tert-butyl ether ( MTBE ), ethyl tert-butyl ether ( ETBE ) and tert-amyl methyl ether (TAME) been.

Another class of substances for anti-knock agents are aromatics. A representative of this which is generally of little concern is toluene . The aromatic benzene used up to now, on the other hand, can cause malformations , is carcinogenic and otherwise toxic. That is why its share of motor gasoline in the European Economic Area has been limited by law to 1% vol. However, this proportion of benzene is no longer necessary.

In the past, low-quality petroleum distillates, refined with a relatively large amount of tetraethyl lead, were sold as fuel for gasoline engines. Later, the processes of chemical gasoline reforming were improved so much that the required octane numbers could be achieved with less (or even without) tetraethyl lead.

literature

  • Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Kraftfahrttechnisches Taschenbuch. Friedr. Vieweg & Sohn Verlag, Wiesbaden 2003, ISBN 3-528-23876-3 .
  • Jan Drummans: The car and its technology. Motorbuchverlag, Stuttgart 1992, ISBN 3-613-01288-X .
  • Peter Gerigk, Detlev Bruhn, Dietmar Danner: Automotive engineering. Westermann Schulbuchverlag GmbH, Braunschweig 1997, ISBN 3-14-221500-X .

See also

Web links