Post injections are a commonly used strategy to reduce soot and NOx emissions in DI diesel engines. This strategy has been widely explored and studied for several years, however, there are still some aspects of the behavior of the combustion process when it is used that are not completely well known. In this paper a numerical study is carried out in order to better understand the improvement on mixing/combustion phenomena using post injection compared with a single injection case. For this purpose, CFD simulations using commercial code Star CD were performed in realistic engine conditions: the combustion of a single injection case (Pilot + Main injection) was compared with that of post injection case (Pilot + Main + Post) evaluating different post injection timings and comparing the simulated results with previously developed experimental tests which reveal the mentioned improvement on combustion behavior. The results show that if the post injection is placed near enough the main injection, the end of combustion can take place even earlier compared to the case with a single main injection as in the experimental cases. These simulations have helped to understand why an acceleration of the combustion process takes place when a small and close post injection is used. As in many other cases, CFD has proved to be an interesting tool to understand the complex phenomena involved during combustion processes.