Basics of Mechanical Engineering


What is meant by Octane Number of a fuel?

Rating of S.I (Gasoline or Petrol) Engine Fuels - Octane Number

The hydrocarbon fuels used in Spark ignition (S.I) engine have a tendency to cause engine knock when the engine operating conditions become severe. The knocking tendency of a fuel in S.I engines is generally expressed by its Octane Number.

The percentage, by volume of, iso-octane in a mixture of iso-octane and normal heptane, which exactly matches the knocking intensity of a given fuel, in a standard engine, under given standard operating conditions, is termed as the octane number rating of that fuel.

If a fuel has octane number as 75 it means that the fuel is having the same knocking tendency as a mixture of 75% iso-octane and 25% n-haptane by volume.

  • This octane number rating is an expression which indicates the ability of a fuel to resist the knock in a spark ignition engine.
  • Iso-octane is a very good anti knock fuel, therfore it assigned a rating of 100 octane number. on the other hand normal heptane has a very poor anti knock qualities, therefore it is given a rating of zero octane number.
  • It may be noted that, higher the octane number rating of a fuel, the greater will be its resistance to knock and higher will be the compression ratio.
  • Since the power output and specific fuel consumption are functions of compression ratio, therefore we may say that these are also the functions of octane number rating.
This facts indicates the extreme importance of the octane number rating in fuels for S.I Engines.
  • Octane Number of Natural gas is 130.
  • Octane number for Petrol, generally available is 80 to 100. 

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