Pre-ignition , octane number and cetane number

Pre-ignition -: pre-ignition is caused by the spontaneous combustion of the mixture before the end of the compression stroke in spark ignition engine, i.e. , even before the spark occurs in the spark plug . It may occur generally due to the red hot carbon deposited , or the cylinder walls being too hot , or overheated spark plug , or the pressure wave due to detonation. It may also due to the faulty timings .

                 In S.I. engine the fuel air mixture is drawn into the chamber during the suction stroke and is compressed until the spark plug ignite it. When the engine is running on high loads , there might be a hot carbon deposite or overheated spark plug's tip that might lead to pre ignition. 

                    The low flash point temperature,  of the petrol also adds to this problem . This is completely eliminated by the intervention of fuel injection into petrol 

Octane number -: It is defined as the percentage by the volume of iso-octane in a mixture of iso-octane and n-heptane which show the same tendency to knock as the fuel in question under same conditions . Ignition quality of petrol is expressed by octane number. 

Cetane number-: Cetane number is defined as the percentage by the volume of cetane in a mixture of cetane and α-methyl-napthalane which has the same ignition delay as the fuel in question under same conditions. Ignition quality of diesel oil is expressed by cetane number.