Octane Number

Octane number of a gasoline is defined as the percentage of iso-octane present in a mixture of iso-octane and n-heptane which matches the fuel (gasoline) in knocking; higher the octane number of a gasoline better is its quality.

Ordinarily gasolines having an octane number of 74 are used in motor cars, scooters and motorcycles. These gasolines are known as regular gasolines. Gasolines having octane number lower than this value are called third grade gasolines, while gasolines having an octane number of 84 and above are known as premium gasolines.

Compounds having more than 100 and less than 0 octane numbers are also known. For example, triptane (2, 2, 3-trimethylbutane) has an octane number of 125, while n-nonane has an octane number of 45. Aviation gasoline (gasoline used as fuel in aeroplanes) has an octane number of 100 or more.

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Presence of following types of organic compounds increases the octane number of gasoline.

1. In case of straight chain hydrocarbons, octane number decreases with increase in the length of the chain.
2. Branching of chain increases the value of octane number.
3. Introduction of double or triple bond in an alkane increases the value of octane number.
4. Cyclic alkanes have relatively higher value of octane number than the corresponding n-alkanes.
5. The octane number of aromatic hydrocarbons are exceptionally high.

The Octane number of gasolines can be improved by following methods :

1. By cracking,
2. By alkylation,
3. By isomerisation,
4. By adding gasoline additives (e.g. TEL),
5. By adding BTX (benzene, toluene and xylene)