Octane number (octane rating) is a measure of the performance of petrol in the engine and the rating depends on the percentage composition of branched chain (iso-octane) to straight chain (n – pentane) hydrocarbons in the fuel.
Petrol consists of C7 (heptane), C8 (octane) and C9 (nonane) hydrocarbons. These hydrocarbons are present in either straight chain or branched chain isomers e.g.
It has been shown that straight chain hydrocarbons do not burn smoothly in combustion engines. They burn too rapidly and generate an explosion in the cylinder, thus, upsetting the normal up and down motion of the pistons, which results in abnormal pinking sound. This phenomenon is known as knocking in internal combustion engines.
The straight chain hydrocarbon n-heptane, (C7H16), is the worst offender and is assigned an octane number of zero while 2,2,4 – trimethyl pentane, Iso-octane, (C8H18), a branched chain hydrocarbon, is assigned an octane number of 100. Thus, the percentage of 2,2,4 trimethylpentane in a fuel is its octane number e.g. the petrol with octane number 80 would be equivalent to a fuel made up of 80% 2,2,4 trimethyl pentane and 20% n-heptane. Petrol which is graded as premium or super has an octane number of between 95 and 100.
These are substances that are manufactured from the by-products of petroleum. These include plastics, synthetic fibres (nylon and dacron), synthetic rubber, insecticides and detergents.
The manufacturer of these compounds has become an important industry.