Octane rating
Octane rating measures the combustion quality of petrol or gasoline. Higher the octane number, lesser is the tendency to knock during combustion. To determine the octane number, two fuels iso-octane and normal heptane are mixed in a certain proportion by volume. If the mixture contains 80% iso-octane and 20% normal heptane by volume then the petrol has octane number of 80.
“The octane number can be defined as percentage by volume of iso-octane in a mixture of iso-octane and normal heptane”
The actual octane number for petrol is about 96 for premium and 90 for normal.