A study of the relationship between NOx and the ion current in a direct-injection diesel engine

This paper presents a modelling study of the ion current formed in a direct-injection diesel engine via chemical kinetics in order to investigate the strong correlation observed experimentally between tailpipe NOx and the ion current. An established n-heptane mechanism augmented with ionic and NOx sub-mechanisms is used to model the formation of ions and NOx in three combustion systems, including a phenomenological diesel engine model whose validation is also presented here. Sensitivity analyses are conducted on electron and NOx formation to determine species and reactions common to both. The O radical and, by extension, the mixture temperature are found to be important to both NOx and electron concentrations via mechanisms identified here.