anti-knock property “octane” is one of the most important characteristics of gasoline. Knocking
occurs during the combustion of the air-fuel mixture, when the temperature and pressure of the
un-burnt portion ahead of the flame front has been raised to the point of spontaneous
When this point is reached,
combustion occurs instantly at the end charge, giving rise to high-pressure waves, which can damage
the walls of the combustion chamber that produces a “knocking”
This phenomenon is affected by the
chemical composition of the fuel and premature or spontaneous detonation is accompanied by loss off
engine power. Heat is wasted by having to flow through the piston rings, thereby causing the rings
to stick due to carbonisation of the oil.
In order to predict whether a
particular grade of fuel will give satisfactory performance in a particular engine, the anti knock
values have to be pre determined. This is done using standard ASTM-CFR laboratory test
Samples are rated against two
known octane standard reference fuels of simple composition and therefore reproducible, with
particularly good and particularly poor resistance to knock,
After much development work,
iso-octane (2, 2,4-trimethylpentane) and n-heptane were selected with arbitrary octane rating of
100 and 0 respectively.
The octane number of a sample fuel
is defined as the volumetric content of iso-octane in the reference fuel blend that matches the
identical knock behaviour under specified test conditions.
Gasoline’s are octane rated in the
under two different CFR engine conditions referred to as Research Octane Number (RON) and Motor
Octane Number (MON).
RON and MON
Research octane numbers are
generally higher than those obtained by the Motor Method and the difference between the two ratings
is known as the “sensitivity” of the fuel.
The “sensitivity” of low octane
fuels is usually small, but with high-octane fuels it varies greatly according to fuel composition.
For most commercial blends it is between 7 and 12 octane numbers in the 90 to 100 Research Octane
The actual octane performance of a
gasoline on the road is sometimes referred to as; “road index number”. This is referred to and
marketed in the United States and is calculated from both RON and MON results added and then
divided by two.
For quality reasons both RON and MON have to comply with min
and max octane specifications stipulated by its appropriate country’s