The main parameters affecting combustion of heavy-duty D.I. Diesel engines were studied by the utilization of a two-dimensional axisymmetric code taking into account air motion, spray formation and combustion.Model predictions are integrated with the analysis of the main combustion quantities coming from the recorded in-cylinder pressure, injection pressure and needle lift. The trend of the total air momentum and spray formation obtained by changing the swirl level, the combustion chamber shape, the in-cylinder air mass and the fuel jet distribution were compared with the trends of the smoke, fuel consumption, gaseous emissions and heat release rate. Experimental data and model predictions indicate that very different combustion chamber geometries can be adopted in medium and small bore engines. The ratio between the in-cylinder air mass and injected fuel momenta can be useful in defining the optimum air motion and spray formation of a given combustion system.