The octane rating of petrol is a measure of how smoothly it burns. Petrol which causes 'knocking' or 'pinking' in an engine has a low octane value and tends to combust too easily (low activation energy). One which burns smoothly without pinking has a high octane value.
The octane number is measured in a test engine. It is defined by comparison with the mixture of iso-octane and heptane which would have the same anti-knocking capacity as the fuel under test.
The percentage by volume of iso-octane (2,2,3 trimethyl pentane) is the octane number of the fuel.
For example, petrol with the same 'pinking' quality as a mixture of 80% iso-octane and 20% heptane would have an octane number of 80.
Octane rating can be regarded as a measure of a fuel's tendency to burn in a controlled way, rather than exploding in an uncontrolled way.
It is possible for a fuel to have an octane number greater than 100, because iso-octane is not the smoothest-burning substance available.
Some examples of low and high octane number fuels are shown below.
LOW OCTANE NUMBER
Straight chain alkanes, C4 to C12, e.g. pentane, hexane, heptane, octane.
HIGH OCTANE NUMBER
Highly branched alkanes, cycloalkanes, aromatics and alkenes, e.g. 2,2,3 trimethyl pentane, methylbenzene, 2-methyl pent-2-ene.
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