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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.