• Cetane Number:

    • Cetane number is a measure of the ease of autoignition of a diesel fuel, which is critical because this is the means of ignition used in diesel engines.

    • The cetane number measures the ability for auto ignition and is essentially the opposite of the octane number.

    • Cetane (n-hexadecane, \(\ce{C16H34}\)) itself performs very well in a diesel engine, meaning that there is negligible delay between the time of injection of the fuel into the cylinder and fuel combustion. Pure cetane is assigned a cetane number of 100. In contrast to this, \(\alpha\)-methylnaphthalene (\(\ce{CH3C10H7}\)), which performs very poorly (does not readily ignite) in a diesel engine, is assigned a cetane number of 0.

    • The higher the cetane number the lower the operating temperature at which a diesel engine may be successfully started.

    • Cetane number: 25–40 for slow speed engines; 40 for moderate speed, and 50–65 for high speed engines.

    • A petroleum crude which gives diesel oil of high cetane number, gives gasoline of low octane number and vice versa.

    • Cetane number increases in the order of \[\text{Aromatics} \rightarrow \text{Isoparaffins} \rightarrow \text{Naphthenes} \rightarrow \text{Olefins} \rightarrow \text{n-paraffins}\] Where as, octane number decreases in the same order.