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A skeletal mechanism for NOx emissions is incorporated into a cycle simulation code for direct-injection (DI) Diesel engines. The skeletal mechanism consists of seven chemical reactions associated with the extended Zeldovich and N2O mechanisms. In combining the skeletal mechanism with the cycle simulation code, both a two- and a one-zone combustion model are examined. In the former, NO forms in zone 1, which is characterized by the stoichiometric flame temperature, and decomposes in zone 2, which is represented by the overall bulk cylinder temperature. For the one-zone combustion model, it is postulated that both the NO formation and decomposition processes are characterized by the stoichiometric flame temperature. The main objective of this work is to examine the relative contribution of the Zeldovich and N2O mechanisms to the NO formation and decomposition processes occurring during Diesel combustion.

Inert injection is an often-used technique to reduce NOx emissions from engines. Here the effects of a new Mitsubishi water injection system for a direct injection (DI) Diesel engine on exhaust emissions are examined. Stoichiometric flame temperature correlations of thermal NOx emissions for conventional gas turbine combustors provide an activation energy to form NO of approximately 135 kcal/g-mol, the value for the Zeldovich mechanism with O/O2 equilibrium. Two theoretical limiting temperatures determined to bracket NOx emissions data for gas turbines are computed for the Diesel engine considered here. At low water to fuel ratios, the reductions of NOx for the DI Diesel engine are less than predicted for uniform distribution of an inert throughout the charge, but as the water to fuel ratio is increased the reductions are bounded successfully by the limiting temperatures.