A Study of the Ignition-Delay Character of n-Heptane in the IQT™ Combustion Bomb Using CFD Modelling

The ASTM D6890 method predicts cetane numbers of diesel fuels based on the measured ignition delay in a combustion bomb (Ignition Quality Tester^™). The device is calibrated using n-heptane as the reference fuel. When n-heptane was tested in the IQT^™ device over a range of temperatures and pressures, the measured ignition delay did not correlate with the chemical autoignition delay normally associated with a stoichiometric, homogeneous mixture as predicted by detailed chemical kinetic models. This paper describes an investigation to study and reconcile this discrepancy, using (CFD) techniques to explore the physical conditions prevailing in the IQT^™ device. Significant insights regarding the flow patterns, thermal gradients and air/fuel ratio profiles were obtained from the analysis. The calculation yielded ignition-delay predictions (defined by the first appearance of a local autoignition) that correlated well with IQT^™ experimental data.