In a petrol engine, the combustion process is initiated by a spark whose heat starts a burning process in the fuel molecules in its immediate vicinity. This flame spreads outwards from the spark until it dies from a lack of fuel mixture, contact with the colder combustion chamber wall, or both.
The process, which takes only a millisecond or two, is meant to play out in an orderly manner. Whether it does so or whether it ends violently (detonation or ‘knocking’) depends partly on the fuel’s octane number as well as the fuel mixture, spark timing and compression ratio of the engine.
A high-octane number implies that the fuel will burn in an orderly manner; a low octane number is associated with a flame that may deteriorate into an uncontrolled explosion, sending shockwaves through the engine.
Fossil fuels require additives to raise the octane rating to levels above 90 to make them suitable for modern engines, but the biofuels mentioned above will need fewer, if any, additives.
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