The combustion process of a heavy-duty DI-Diesel truck engine has been investigated using numerical simulation. The numerical modeling was based on an improved version of the KIVA-2 engine simulation code, employing a modified characteristic time-scale combustion model and a modified Kelvin-Helmholtz spray atomization model. The NO-formation process was modeled using the extended thermal Zeldovich mechanism. The simulation efforts included the effects of different injection characteristics such as varying the injection rate profile or number of injection holes and sizes. The physical sub-models used to improve the simulation of the mixture-formation and the combustion process were validated through comparison with single-cylinder engine experiments. Special attention was given to accurately model the in-cylinder flame propagation of the individual sprays and their effect on thermal NO-formation. All simulations were based on full load cases at medium speed. The simulation results showed good agreement with experiments for the cylinder pressure, the rate of heat-release and the final NOx levels. For the comparison of the injection rate shapes, the models were able to predict the correct trends of NOx formation, i.e. a lower NOx level for the gradually rising injection rate. The modeling efforts were also extended to study the effects of hydraulically similar 6- and 8-hole nozzles. The simulations revealed that, for the specific engine configuration, the 6- hole nozzle leads to more homogeneous combustion with lower overall NO-formation.