Gasoline or Petrol is the primary fuel used for powering engines that rely on spark ignition. This fuel is extracted from petroleum oil or crude oil mainly by distillation process. This Gasoline or petrol majorly consists of two major components viz. iso-octane and heptane. In a typical spark-ignition engine, if the fuel is not according to the right octane rating, the fuel entering the combustion chamber in the form of air-fuel mixture will not burn completely and these unburnt pockets of air fuel mixture remaining will self-ignite under excessive pressure and create an excess load on engine components. This phenomenon is known as knocking. Octane rating of petrol is in inverse proportion to the probability of knocking. Thus, higher the Octane rating, the chances of knocking are lower. Octane rating or octane number of petrol is the comparison between percentage by volume of iso-octane and heptane. When we say that the octane number of unleaded petrol in India is 91, we mean to say that if the fuel was a pure mixture of iso-octane and heptane, 91% of iso-octane and 9% of heptane is present in the fuel. The Octane rating preference of any engine also depends on its compression ratio. Higher compression engines require higher octane fuel to minimize knocking.
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In accordance with the Bharat Stage 3 vehicle emission norms, the minimum octane rating for petrol in India has increased from 88 to 91. Thus, prior to BS3 norms, petrol in India was classified into unleaded petrol at 88 octane rating and premium petrol at 91 octane rating. As on today, petrol in India is classified as follows:
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