A comprehensive study was carried out in order to better understand combustion behavior in a direct injection diesel engine when using postinjections. More specifically, the aim of the study is twofold: (1) to better understand the mechanism of a postinjection to reduce soot and (2) to improve the understanding of the contribution of the postinjection combustion on the total soot emissions by looking at the effect of the postinjection timing variation and the postinjection mass variation on the soot emissions associated with the postinjection. The study is focused only on far postinjections, and the explored operating conditions include the use of EGR. The first objective was fulfilled analyzing some results from a previous work adding only a few complementary results. Concerning the second objective, the basic idea behind the analysis performed is the search of appropriate parameters physically linked to the processes under analysis. These parameters are found based on the state-of-the-art of diesel combustion. For the effect of the postinjection timing, the physical parameter found was the temperature of the unburned gases at the end of injection, T-ug_EoI. It was checked that a threshold level of T-ug_EoI (similar to 700 K for the cases explored here) exists below which soot is unable to be formed, independently of the postinjection size, and the amount of soot increases as the temperature increases beyond this threshold. For the effect of the postinjection size, the physical parameter that was found was DoI/ACT (the ratio between the actual duration of injection and the time necessary for mixing-the apparent combustion time). This parameter can quantify when the postinjection is able to produce soot (the threshold value is similar to 0.37 for the cases explored here), and the amount of soot produced increases as this parameter increases beyond this threshold value. A function containing these two parameters has been fitted to the experimental soot emissions associated with the postinjection obtained in many engine operating conditions, and the appropriate quality of the fit demonstrates that these two parameters explain the main behaviors of the soot emissions associated with a postinjection., (C) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.