Diesel index

Diesel index is an empirical measure of the ignition quality of a distillate fuel, computed as the aniline point in degrees Fahrenheit times the API gravity, divided by 100. A high aniline point signals high paraffin content and good ignition quality, so a higher diesel index means a shorter ignition delay and smoother combustion. It correlates with, but is not identical to, cetane number, which is measured directly in a test engine. Diesel index applies to distillates; residual marine fuels are instead ranked by CCAI or the calculated ignition index, both derived from density and viscosity under ISO 8217.