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CFD simulations have been conducted to investigate the combustion in a direct-inject (DI) diesel engine using the Ricardo engine CFD program VECTIS and the Ricardo Two-Zone Flamelet (RTZF) combustion model. The simulation program covers full load and part load operating conditions, each including 6 to 7 cases forming an injection timing swing. CFD simulation results are compared against engine tests for the in-cylinder pressures and NOx emissions. The comparison shows that the RTZF combustion model performs well in all cases studied with no tuning of model coefficients necessary. The detailed time history of spray, fuel distribution and flame development obtained from the CFD simulation provides useful information towards gaining a better understanding of the features of combustion in DI diesel engines.