Cetane Index is used as an alternative of the Cetane number of diesel fuel. Cetane number measurement requires an engine that is a difficult and expensive test.
Therefore, CI is calculated from the density and distillation range of diesel fuel and does not include the cetane improver additives.
The most widely used methods to calculate the cetane index are, ASTM D-976 and ASTM D-4737. D976 uses the API gravity and the 50% distillation point, whereas D4737 uses gravity with the 10%, 50%, and 90% distillation points. The two methods are described as follows;
D 976 Cetane index = -420.34+0.016*API2+0.192*API*log(T50)+65.01*log(T50)–0.0001809 *T502
Where T50 is the D86 50% point indegrees °F
ASTM D-4737Cetane index=
45.2+0.0892*(T10-215)+[0.131+0.901* B]*[T50-260]+ [ 0 . 0 5 2 3 – ( 0 . 4 2 0 )*( B )*] [ T 9 0 -310]+[0.00049]*[(T10-215)2–(T90-310)2] + (107)*(B) + (60)*B2
Where: B = Exp[-3.5*(sp. gr. – 0.85)] –1 and the D86 temperatures are in °C