Engine-out emissions (NOx and soot) have led to serious air pollution problems, and consequently, increasingly stringent emission norms. In order to decrease the emissions and improve the combustion performance of diesel engines, the effect of split injections with swirl (swirl rate of 0 and 1) was experimentally researched, and the mechanism of the fuel/air mixture of split injections with swirl (swirl rate of 0.5-2.5) was numerically researched in this study. The experimental research was carried out on a modified 1132Z single cylinder diesel engine, equipped with an endoscope system. A two-color method was applied to record flame temperature distribution and KL factor. The experimental results indicate that the flame observed using split injections with swirl rotated obviously. Split injections with swirl had a positive influence on improving the fuel/air mixture, accelerating combustion progress and shortening combustion duration. The combustion duration decreased at swirl rate of 1, with a reduction in the range of 19.5-25.7% at various pilot quantities. In addition, brake-specific fuel consumption (BSFC) and soot emission were reduced. BSFC was lower at swirl rate of I than that at swirl rate of 0, with a reduction in the range of 1.1-2.01 g/(kW h)(-1). For KL factor at 12 CA ATDC, it was also observed that at 12 CA ATDC, the KL factor was lower at swirl rate of 1 than that at swirl rate of 0, with a reduction of 34.9%. Related numerical research on split injections with swirl was performed, and the results show that at a specific swirl, the main injection deviated from the pilot injection and entered the area between two sprays, enhancing the utilization of air in the chamber. In addition, the main combustion process accelerated due to the better thermal-atmosphere provided by the pilot injection, so that a better engine performance and lower soot concentration was achieved. (C) 2016 Elsevier Ltd. All rights reserved.