Several studies have found that pilot injection enhances the mixture and combustion of the main injection fuel, which effectively improves performance and reduces emission. However, only a few investigations have focused on remediating the negative effect of pilot injection, which is that it restricts the advancement of the split injection. Therefore we study the influence of pilot-main injection matching on the combustion of diesel engines. The experiment was conducted using a single-cylinder direct injection diesel engine equipped with a high-pressure common rail system and an endoscopic visualization system. Through the endoscopic visualization method, the effect of pilot injection on mixing, combustion characteristics, and soot formation of the main injection fuel was investigated. The results show that the thermos atmosphere of a pilot injection with the correct parameters (a small pilot ratio and short dwell time) improves the mixture and combustion of the main injection fuel, which shortens the combustion duration. Compared with single injection as well as with split injections with longer dwell times or larger pilot injections, the pilot injection with a small pilot ratio and short dwell time obtained higher thermal efficiency. Also flame temperature peak and soot concentration decreased. For spilt injection with longer dwell time, the TA by pilot injection traversed far away from the main injection, thus not to promote evaporation, mixing, and ignition of the main injection fuel; for spilt injection with larger pilot injection, the TA by pilot injection will constraint the combustion of the main injection fuel, deteriorating mixing and combustion. (C) 2016 Elsevier Ltd. All rights reserved.