Abstract: Abstract: Direct injection (DI) diesel engines are used in the field of transportation and engineering machinery due to their excellent thermal efficiency and practical economic value. In order to reduce air pollution, diesel engines are continuously forced to improve the combustion processes, especially to reduce emitted particulate matter (PM) and nitrogen oxidants (NOx). Fuel-air mixing and fuel-air distribution play a vital role to reduce harmful emissions. Therefore, a homogeneous fuel/air mixture in the combustion chamber and an increase of the fuel/air mixture area can significantly improve the utilization of air and restrict the soot formation in diesel engines. Double swirl combustion system (DSCS) has excellent thermal efficiency in practical application, but its mass fraction of fuel in inner chamber has not been studied yet. It is hypothesized that mass fraction of the fuel in inner chamber is 10%. To verify the hypothesis, the best range of mass fraction in inner chamber and the effect of dynamic performance and the emission were researched in present paper. First, we analyzed the changes of mass fraction of fuel in inner chamber under different spray angles. Spray angle was conducted at 140?, 145?, 150?, 155? and 160?, and at this time, the injection advance angle was 12° and the speed was 2500 r/min. Meanwhile, the power characteristics and emission characteristics were analyzed with CFD (computational fluid dynamics) simulation. We researched the soot and NOx at different spray angles. We then analyzed the effect of injection advance angles on the mass fraction of fuel in inner chamber. Injection advance angle was conducted at 10? (corresponding to crank angle of 350?), 12?, 14?, 16?, 18? and 20?, and at this time, the spray angle was 145° and the speed was 2 500 r/min. Meanwhile, the mass fraction of fuel on TDC (top dead center) in inner chamber and mass fractions of NOx and soot were analyzed. We also researched the soot and NOx at different injection advance angles. The engine speeds (1 300, 1 600, 1 900, 2 200, 2 500 and 2 800 r/min) were selected to study its effect on mass fraction of fuel in inner chamber. The mass fraction of fuel on TDC in inner chamber and emission characteristics were also analyzed. At last we analyzed the combined influences of injection advance angle and speed. The result showed that the indicated power and NOx were the highest, and the soot formation was the lowest, when the mass fraction of the fuel in inner chamber was about 10%-20%. The reason could be explained as follows: the fuel/air mixing quality was improved when the mass fraction of the fuel in inner chamber was about 10%-20%. Thus, it can be concluded that about 10%-20% mass fraction of the fuel in inner chamber has a positive influence on improving the utilization of the air inside the cylinder. The air in DSCS is made full use of, thus making the cylinder have an increase in the fuel mixing and burning. With the increase of the fuel injection advance angle, the mass fraction of fuel in inner chamber increased. Soot emissions increased at first and then decreased with the increase of injection advance time, and NOx emissions decreased with injection advance time. With the increase of the speed, the mass fraction of fuel in inner chamber decreased, soot increased and NOx decreased. At last, we researched the comprehensive effect of injection advance angle and speed on the mass fraction of fuel in inner chamber. The result showed that when the speed increased, we could keep the fuel in inner chamber in about 10%-20% by increasing the fuel injection advance angle, which meant the optimal mass fraction of the fuel in inner chamber could be ensured when increasing speed and injection advance angle.