Development of an Improved NOx Reaction Mechanism for Low Temperature Diesel Combustion Modeling

The development of a new Nitric Oxide (NO_x) reaction mechanism has been conducted by adding species, including hydrogen cyanide (HCN) and the CH radical to a reduced chemistry diesel combustion model. The additional chemical reactions were added to the ERC's reduced 12-step NO_x mechanism, which consists of N, NO, N₂O, and NO₂. The new NO_x mechanism was implemented into the KIVA/ERC-CHEMKIN code and was found to be able to predict the experimentally observed trend that the amount of engine-out NO_x decreases as engine load is increased, which is not reproduced by the current reduced NO_x mechanism. HCN and CH were found to be species that bridge C_xH_y products and N radicals via the reaction CH+N₂→HCN+N under high equivalence ratio conditions, and Zeldovich NO formation is suppressed by the formation of HCN, a species in the Fenimore NO formation pathway. The additional species and reactions were also found to influence the prediction of engine-out soot emissions. Predictions with the new NO_x mechanism and the ERC 2-step soot model showed a reduced amount of soot compared to the standard 12-step NO_x and 2-step soot models. Acetylene (C₂H₂), which is considered to be a precursor of soot in the 2-step soot model, serves as a CH radical precursor as well, and soot and CH radicals compete with C₂H₂ in fuel rich regions. It is concluded that the new NO_x reaction mechanism is able to predict NO_x emissions more accurately for fuel-rich, high-load diesel engine operating conditions.