Detonation means the sudden and violent knock experienced in a Petrol Engine,due to Anti-Ignition of a portion of the Fuel.The Octane Value of the Fuel will give us an idea of the tendency of the fuel to detonate.
This is mainly important in Spark Ignition Engines.To find it out,two Fuels,the iso-octane and the normal Leptane are selected for this purpose under modern practise.
The iso-Octane has little tendency towards detonation while Leptane,readily detonates.The actual fuel whose Octane Value is to be determined is first used to run standard engine and the conditions under which detonation occurs are noted.
The Detonation intensity is measured by "Knock Meter".Then several mixtures of iso-Octane and normal Heptane mixed up in different proportions are used in the same engine,under the same conditions,as before.
By making several trials,the mixture which gives the same intensity of Detonation is found out.
The percentage of iso-octane in this mixture gives the octane value of the Fuel.For example,with the mixture containing 75% of iso-Octane and 25% of normal Heptane,the Octane Value of the Fuel will be 75.
Higher the Octane Value,higher the Anti-knocking property of the Fuel.The Octane Value of the pure iso-Octane and normal Heptane are 100 and 0respectively.
The Octane rating indicates the quality of the petrol to resist detonation.