The effect of three injection rate modes on fuel spray-wall impingement and combustion process in a direct injection diesel engine is investigated in present work. A three-dimensional computational fluid dynamics model for flow field, spray, spray-wall interactions, combustion, and emission formation processes have been used to carry out the computations. The optimized omega combustion chamber geometry was used in the diesel engine model instead of baseline cylindrical geometry and the results were verified for this improved combustion chamber geometry. Results for different injection modes indicate that using the ramp injection rate curve, the spray-wall impinging is increased due to higher injection rate at the end of injection duration. Also the increased in-cylinder temperature, piston surface temperature and higher turbulence intensity leads to enhanced wall-film evaporation. Soot mass fraction also decreases due to improved air-fuel mixing and evaporation of wall-film by reduction of the fuel rich zones especially in the impingement regions. Results for different injection modes indicate that using the ramp injection rate shape slightly retards the combustion process and improves combustion characteristics while maintaining lower NOx and considerably lower soot emissions compared to the boot and rectangle injection modes. Also, in this injection mode, because of the high pressure injection, higher spray droplet velocities (higher Weber number) and increased wall spray height than the other modes, it could be said that the dominant impingement regime may be the splashing regime. The results of model for baseline diesel engine are compared with the corresponding experimental data and show good levels of agreement.