Technique: The meaning of the octane number
The octane number of gasoline indicates its degree of knock resistance. The knock resistance of octane (with an octane number of exactly 100) is only a standard. Octane is a component of gasoline while the octane number is a number that indicates resistance to knocking. So much for the theory.
All production motocross motorcycles are designed to operate on 95 and 98 octane unleaded gasoline. You can simply fill up these two versions at your local gas station. Nowadays the Euro 95 is referred to as E10 and the Euro 98 as E5. The number after the E indicates what percentage of bio-ethanol it may contain. Standard engines therefore run on this petrol.
The octane number is a reference value that indicates the knock resistance of gasoline. The knocking is also called ping and is in fact premature spontaneous combustion of the gasoline. Knocking or pinging causes the engine to run irregularly and causes damage to the engine block. The pistons move irregularly and are pushed in the wrong direction. The strokes of the cylinder(s) in the block no longer run smoothly because the self-ignition causes several explosions in the block.
The octane number does not indicate the octane content of gasoline. Sometimes people incorrectly talk about the octane content, but that is not correct. A scale is used where the definition of knock resistance increases from 0 to 100. The knock resistance of n-heptane is 0 and that of iso-octane is 100. The octane number for gasoline is based on this. If gasoline has a high knock resistance, the octane number is high. In that case there are many branched hydrocarbons. The higher the octane number, the more anti-knock the gasoline is.
Preparing your gasoline (or using racing gasoline) so that you obtain a higher octane number does not provide extra power. Octane alone does not determine how much horsepower an engine produces. Used properly, however, higher octane can allow an engine to be tuned to produce more power. Then one speaks of engine tuning where adjustments are made to, among other things, the compression ratio.
Two-stroke engines are more sensitive to low octane than four-stroke engines. In addition, there are racing gas mixtures that add other elements and additives to produce more power – usually an increased oxygen content.
Scientists developed a detonation testing machine at the beginning of the last century. This device determines the octane number based on the RON number, the so-called Research Octane Number. In Europe, the anti-knock index number (read octane number) is based only on this RON number.
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