The objective of the work was to formulate and develop a combustion model for a direct injection diesel engine that would be capable of correctly predicting engine performance and exhaust emissions trends, be sensitive to changes in major combustion system variables, and yet be efficient enough to allow the model to be used in engine cycle simulations and formal mathematical optimization procedures. The model is multizone and is based on the air and fuel mixing process with a temperature and concentration dependent fuel burning rate. Nitric oxide kinetics are also included. The assumptions for the model are discussed in terms of the competing chemical and physical processes and the resulting implications for the level of complexity and detail required to meet the objectives of the work. The model predicts the spatially and temporally resolved temperatures and concentrations, as well as the bulk cylinder variables; including pressure, temperature, heat transfer, and nitric oxide mass emissions. The model predictions result in excellent qualitative and acceptable quantitative trends for the computed values of work, heat transfer, nitric oxide, and other variables in response to combustion system parameter changes. The resulting computational times and costs are very acceptable and meet the original objectives of the work.