What is octane number?

The octane rating is a measure of the resistance of petrol and other fuels to autoignition in spark-ignition internal combustion engines.

The octane number of a fuel is measured in a test engine, and 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 in that mixture is the octane number of the fuel.

For example, petrol with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90.

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regards

Ramesh