Cetane Index Calculations

Cetane index is a measure of the ignition quality of a fuel in a diesel engine and is used as a specification for diesel fuels. Cetane index requirements are inverse with the temperature of the air being mixed with the fuel; i.e., the lower the air temperature, the higher the cetane index required for ignition to occur.

Ignition quality is also measured by Diesel Index, where the diesel index is defined as the API gravity times the aniline point divided by 100. While diesel index follows the same trend as cetane index, the numbers vary somewhat. Aniline point is determined experimentally and is the temperature at which an equal volume of aniline can be perfectly dissolved in a given volume of the oil being tested.

ASTM D976

The cetane index is calculated as a function of the API gravity and ASTM 50 percent distilled for a petroleum stock using ASTM Method D976. The index very closely approximates the cetane number which is determined by ASTM Method D613 using a special laboratory engine.