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Octane rating is the measure of how resistant is gasoline and other fuels in the combustion engines to automatic ignition. It the measure of a fuel’s burning characteristics.

Octane rating can be determined by measuring the octane number of a fuel. It is measured as follows. The anti-knocking capacity of the fuel under test is determined first. A mixture of iso-octane and heptane with the same anti-knocking capacity and the percentage of iso-octane in the mixture gives the octane number of the fuel.

Fuels with higher octane ratings tend to have higher activation energies meaning that it takes a long time for auto-ignition to take place. When fuels with lower octane rating are used, the efficiency and the power output of the engine decreases. When fuels with higher octane rating are used, higher compression can be achieved; however the power output of an engine depends on the energy density of the fuel contained in it.

Octane rating is specified in octane numbers. There are different standards followed in different countries. However the most common method of octane rating is specifying the Research Octane Number (RON). This is determined by comparison of the performance of a fuel and the mixture of iso-octane and heptane. Though hydrogen has better anti-knocking capabilities and reduces the knock of a fuel by its high thermal conductivity, it is not used as a standard. Iso-octane is used as the standard for determining octane numbers. Other types of octane rating are Motor Octane Number and Anti-Knock Index (ANI). Motor Octane Number is a measure of the tendency of the fuel’s burning characteristics under loaded conditions while Anti-Knock Index is the average of Research Octane Number and Motor Octane Number. The Motor Octane Number of heptane is 0 and that of iso-octane is 100 and thus a good fuel can have a Research Octane Number as high as hundred. Motor Octane Number can be ten points lower than that of a Research Octane Number.

Octane number of a gas can also be put into simple words for better understanding. A gas octane is the value beyond which the compression of a fuel or a gas automatically ignites it. This is different for different gases and hence it is one of the important factors evaluating the performance of any gas. The properties of these gases are artificially changed with changes brought about in these values.