An investigation has been carried out to examine the influence of re-entrant combustion chamber geometry on mixture preparation, combustion process and engine performance in a high-speed direct injection (HSDI) four valves 2.0L Ford diesel engine by CFD modeling. The computed cylinder pressure, heat release rate and soot and NOx emissions were firstly compared with experimental data and good agreement between the predicted and experimental values was ensured the accuracy of the numerical predictions collected with the present work. Three ITs (Injection Timing) at 2.65° BTDC, 0.65° BTDC and 1.35° ATDC, all with 30 crank angle pilot separations were also considered to identify the optimum IT for achieving the minimum amount of pollutant emissions. In order to investigate the effect of combustion chamber, thirteen different types of combustion chamber configurations have been considered based on four categories including piston bowl depth, piston bowl width, and piston bottom surface and lip area. For all the studied cases, compression ratio, squish bowl volume and the amount of injected fuel were kept constant to avoid that the effects of changes in chamber geometry be masked by the effect of other engine parameters. The results confirmed that the combustion chamber geometry has significant effects on the combustion process. It showed that by changing the design of the piston crown, the amount of emission pollutants can be decreased while the other performance parameters of engine remain constant.