Octane number

Knocking tendency of petrol in a petrol engine is expressed by its octane number. The combustion characteristics of iso-octane i.e. 2, 2,4- Timothy pentane (C8H18) is very good. It has very little tendency to detonate when mixed with air and ignited at high temperature. Hence its octane number is taken as 100.
On the other, n-heptanes (C7H16) a straight chain hydrocarbon knocks readily when mixed with air ignited at high temperature. Hence its octane number is taken as zero.
The octane number of given petrol under test is defined as the percentage by volume of iso-octane in a mixture of iso-octane and n- heptanes which exactly matches to the knocking characteristics of the petrol under test.
Thus octane number of fuel given as 70 indicates that the knocking characteristics of the fuel matches with that of a mixture of 70% iso-octane and 30% n- heptanes
The octane number of hydrocarbon is as follows

Fuels with octane number more than 100are common. The octane number of such fuels is estimated by extrapolation it is found that octane number increase with branching and aromaticity of the hydrocarbon. The octane number can also be increased by using various additives like tetra lead (TEL).