Relation between Cetane Number and the Ignition Delay of Jet Fuels

Cetane number (CN) is a metric widely used to measure the performance characteristics of jet fuels. While it is a common parameter, the exact definition of cetane number is not easily correlated to typical combustion parameters that are measured through experiment such as gas phase ignition delay or atomization. In this study, ignition delay, a very common descriptor of fuel, will be related to cetane number to determine how well cetane number can be used as a predictive tool. Specifically, derived cetane number (DCN), which is a very similar but not an exact replacement for cetane number, is used in this study. A wide variety of fuels were selected for this study including standard multi-component jet fuels and non-standard sustainable jet fuels. These fuels cover a derived cetane number range from 16 to 55. The fuels used for this study were organized into the following sets: a blended fuel set, a set of fuels with constant properties such as average molecular formula, density and viscosity, and a set of fuels with the same derived cetane number. Ignition delay results were obtained using a combination of shock tube and rapid compression machine measurements. Results from the comparisons of experimental ignition delay data to DCN show that DCN does have correlation with ignition delay in the intermediate temperature or NTC region but does not show a useful correlation at low or high temperatures. Due to the variation shown in this study, the use of cetane number as a detailed descriptor of fuel may not be perfect, but with additional data and a larger variety of fuel types, a defined region may be created than provides a boundary for the possible ignition delay characteristics of a fuel with a measured CN or DCN.