Increasing the fuel temperature, causes the fuel to ignite spontaneously, even it is not exposed to a flame. The minimum temperature that this occurs is called the autogenous ignition point (self-ignition temperature).
The fuel is injected into the combustion chamber and heated by the air of high temperature and pressure. Then, the fuel self-ignites and burns.
In a diesel engine, the fuel ignitability is improved because as the compression ratio increases, the temperature increases quickly.
Also, ignition performance is improved when using fuel with a high cetane number.
Cetane number The cetane number of the diesel fuel corresponds to the octane number of gasoline and represents ignitability of the fuel.
The higher the number is, the lower the ignition point and the better the fuel.
For diesel engine fuel, a cetane number of at least 40 - 45 is usually required.
Generally a cetane of 53 - 55 is used.
A high cetane number corresponds to the following effects.
