Octane Number -Definition ,How to calculate Octane rating

What is Octane Number ?

Quality of petrol (gasoline) – octane number

The controlled combustion of fuel in the presence of air, gives an internal combustion engine its power. A low quality fuel does not burn smoothly and causes an occasional explosive sound, which is known as knocking. This greatly reduces the power of the engine.

The quality of a fuel is indicated in terms of its octane number. Different hydrocarbons have different knocking tendencies. A fuel that produces minimum knocking is considered as a good fuel.
An arbitrary scale of octane number has been set up with n-heptane and 2,2,4-trimethylpentane (iso-octane) as the reference compounds. All fuels are graded in between these two limiting values by comparing with a suitable mixture of the above two compounds. 2,2,4-trimethylpentane (generally called iso-octane) has excellent anti-knocking properties and has been arbitrarily assigned an octane number of 100, whereas n-heptane, which is very prone to knocking is assigned an octane number of zero (0). Therefore, the antiknock property of a fuel increases with the increase in its octane number.

octane number
octane number

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Definition Of Octane Number : 

Thus, the octane number of any fuel is defined ‘as the percentage of iso-octane in a mixture of iso-octane and n-heptane that has the same knocking as the fuel under examination’.
A fuel having an octane number of 80 behaves in a manner similar to a mixture having 80% of iso-butane, and 20% of n-heptane. Straight run gasolines may have octane values ranging from 20 to 73. Aviation fuel is rated as 100 octane.

How to measure Octane Number 

The octane number of a fuel is measured in a test engine against a mixture of iso-octane and heptane. If a gasoline sample has the same antiknock quality as that of a mixture containing 90% isooctane and 10% heptane, then the octane number for that sample is defined as 90. Some hydrocarbons have higher anti-knocking capacity than iso-octane. Hence, octane number definition is extended to allow octane numbers higher than 100.

Depending on the measurement techniques, there are following different types of octane numbers defined,

  • Research Octane Number (RON); is most commonly used octane number and it is determined by burning the fuel in a test engine under controlled conditions and variable compression ratios. Then the results are compared with mixtures of iso-octane and heptane.
  • Motor Octane Number (MON); testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed (900 RPM instead of 600 RPM used for RON), and variable ignition timing to further stress the fuel’s knock resistance. MON is a better measure of how a fuel will actually behave when under a higher load than normal.

Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON, however there is no direct link between RON and MON.

Some times an average of RON and MON known as Anti Knock Index (AKI) is specified in some locations. This is also known as Road Octane Number (RdON) or Pump Octane Number (PON).

Octane ratings of some compounds are:

  • n-Heptane -0
  • n-Pentane -62
  • tert-butyl alcohol -98
  • neo-octane Benzene -100
  • Ethanol -112
  • Methanol- 116
  • Toluene- 118
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