octane rating vs cetane rating

Octane rating

Definition-The octane rating of fuel is defined as percentage by volume of iso-octane in the mixture of iso-octane and normal heptane,which exactly matches the intensity of fuel in standard engine under a set of standard operating conditions.mainly applied in case of SI engine.

1. Octane rating is also called antiknocking rating because it signifies the ability to resist knocking when used in engine.

2.Higher octane number fuel permits higher compression ratio.The higher the octane number the better is the resistance of Gasoline to combustion prematurely, known as knocking.

3.It is the measure of resistance of gasoline against detonation or pre-ignition of fuel .

4.Higher the octane number, better the quality of petrol, and hence the vehicle gives higher efficiency.

5.Used in SI engine

                                    Cetane rating

Definition-The cetane rating of fuel is defined as percentage by volume of normal cetane in the mixture of normal cetane and α -methyl napthalene which has same ignition characteristics(ignition delay) as test fuel when combustion is carried out in standard engine under specified operating condition.Applied in case of CI engine



1. The Higher Cetane number means that the Diesel will ignite readily and therefore, perform better       in a diesel engine.
2.As cetane rating increases the time delay between injection and ignition decreases.
3.higher cetane number implies good quality diesel.
4.  Used in CI engine.

                                         thus we can say that study of octane reating and cetane rating is very important for the study of IC engine.In the design of ic engine these ratings are considered for smooth, noiseless,comfortsble ride,for longer life of component.