What is meant by Cetane Number of a fuel?

Rating of S.I (Diesel) Engine Fuels - Cetane Number:

The knocking tendency is also found in compression ignition (C.I) engines with an effect similar to that of S.I Engine, but it is due to a different phenomenon. The knock is C.I engines is due to sudden ignition and abnormally rapid combustion of accumulated fuel in the combustion chamber. Such a situation occurs because of an ignition lag in the combustion of the fuel between the time of injection and actual burning. The property of ignition lag is generally measured in terms of Cetane Number.

Definition:

The percentage, by volume, of cetane in a mixture of cetane and alpha-methyl naphthalene that  produces the same ignitions lag as the fuel tested, in the same engine and under the same operating conditions is termed as the cetane number rating of that fuel.

Examples: 

If a fuel has cetane number 45, it means that the fuel is having the same ignition quality as a mixture of 45% cetane & 55% α-methyl naphthalene by volume.

>>  Rating of S.I (Gasoline or Petrol )Engine Fuels - Octane Number >>

• The cetane which is straight chain paraffin with good ignition quality is assigned a cetane number of 100 and alpha methyl-naphthalene which is a hydrocarbon with poor ignition quality, is assigned a zero cetane number.
• The knocking of C.I engines may be controlled by decreasing ignition lag. The shorter the ignition lag the lesser is the tendency to knock.
• The cetane number of Diesel oil, generally available is 40 to 55.