What is cetane number?
Knocking tendency of diesel in a diesel engine is expressed by its cetane number. The compression so it produces less knocking. Hence its cetane number is taken as 100.
On the other hand, ∝-methylnaphthalene, an aromatic hydrocarbon has very slow combustion rate in a diesel engine by compression so knocks readily. Hence its cetane number is taken as zero.
The cetane number of a given diesel under test is defined as the percentage by volume of cetane in a mixture of cetane and ∝-methylnaphthalene (2-methylnaphthalene) which exactly matches with the knocking characteristics of the diesel under test.
Thus cetane number of diesel given as 70 indicates that the knocking characteristic of the diesel matches with that of a mixture of 70% cetane and 30% 2-methylnaphthalene.
It is found that cetane number decrease as branching and atomicity of the hydrocarbon increase whereas it increases as the length of saturated hydrocarbons increase. The cetane number can also be increased by adding 2% dopes like ethyl nitrate acetone peroxide, ethyl nitrite etc.
