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. Furthermore, comparisons are made between predicted and measured NOx values from four DI Diesel engines. For the two-zone modeling results, a correlation was found between predicted and measured NOx values for all engines examined, and the inclusion of the N2O mechanism slightly improves such a correlation. However, the bulk temperature is found to be too low to allow for the reactions associated with NO decomposition to proceed.
A correlation could also be obtained for measured NOx values when a one-zone model was used. However, such predictions require empirical selection of the amount of NO available to decompose and indicate that the N2O mechanism does not contribute to the net NO generation process in a one-zone stoichiometric model. Therefore, it is concluded that the two-zone model is more robust.