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What is Octane number?

Octane number is a scale which is used to determine the quality of the fuel in an internal engine. It has been found that straight chain hydrocarbons have higher tendency to knock than branched chain hydrocarbons. Two pure hydrocarbons have been selected as standards.

(1) n-hexane: A straight chain hydrocarbon, knocks very badly when used as fuel. Due to its poor knocking properties it has been arbitrarily assigned an octane number of Zero.
(2) Isooctane: A branched chain hydrocarbon, has the highest antiknocking properties. This has been given an octane number of 100.Isooctane is a commercial name for the hydrocarbon and is not in accordance with common name system.
All other fuels are then graded on the basis of their octen number ranging from 0 to 100. In order to do so, various mixtures of these two hydrocarbon are tried in a test engine till the mixture is formed which produces the same knocking as the fu being tested. The percentage  of Isooctane in this mixture is then taken as the Octane Number.

Thus Octane Number may be defined as the percentage of Isooctane in the mixture of isooctane and n-heptane. Which has the same anti-knocking properties such as the fuel under examination. For example, a fuel is assigned as octane number of 90 if it has same anti-knocking properties as a mixture of 90% I and 10% n-heptane.

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