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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