SAE International Development of an Improved NOx Reaction Mechanism for Low Temperature Diesel Combustion Modeling 2008-01-2413

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, N 2 O, and NO 2 . 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 x H y products and N radicals via the reaction CH+N 2 \u2192HCN+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 (C 2 H 2 ), 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 C 2 H 2 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.