A mathematical ignition delay model is developed which reflects not only engine load and speed operating conditions but also fuel quality through a cetane number correlation with activation energy. Thus, the model can be used to evaluate ignition delay, as part of a general engine thermodynamic code, for the complete speed and load range of the engine and for off-specification diesel fuels. Model constants are established, using a multivariable numerical optimization program, by direct correlation with experimental data from the 6V53T two stroke, turbocharged diesel engine. Further, the parameters which affect or control ignition delay are studied using the established model. Ignition delay was seen to have a direct correlation with fuel cetane number, fuel-air ratio and cylinder gas pressure, however, cylinder gas temperature was seen to control or dominate the actual delay period.