Development of an Improved NOx Reaction Mechanism for Low Temperature Diesel Combustion Modeling 2008-01-2413
The development of a new Nitric Oxide (NOx) 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 NOx mechanism, which consists of N, NO, N2O, and NO2. The new NOx 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 NOx decreases as engine load is increased, which is not reproduced by the current reduced NOx mechanism. HCN and CH were found to be species that bridge CxHy 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 NOx mechanism and the ERC 2-step soot model showed a reduced amount of soot compared to the standard 12-step NOx 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 NOx reaction mechanism is able to predict NOx emissions more accurately for fuel-rich, high-load diesel engine operating conditions.