Decan Zhu’s research while affiliated with Beijing Institute of Technology and other places

There are few studies on the effect of turbulent jets on the combustion characteristics of passive pre-chamber (PC) gasoline engines, and their mechanism needs to be elucidated. Therefore, a 3-D simulation was performed using CONVERGE software. The results reveal that a modest expansion in the PC volume contributes to combustion due to the increased jet energy, but when it is too large, the above effect is weakened. In this study, at a smaller 0.8 ml volume, the combustion is still similar to traditional spark plug ignition. At middle 1.6 ml volume, the mixture concentration in the PC increases and the uniformity is better, resulting in the highest jet velocity and shortest jet duration, thus achieving the fastest flame propagation and the highest heat release rate (HRR). However, at a larger 1.8–2.0 ml volume, the mixture concentration near the spark-plug is sharply reduced, thus the reduced combustion rate in the PC leads to insufficient jet energy in the subsequent jets and resulting deterioration in combustion. An appropriate increase in nozzle number also improves combustion by introducing more ignition sources. But more nozzles not always improving the combustion. In the eight-nozzle case, three nozzles are close to the exhaust valve so that the intake is inhibited, resulting in a worsening of the ignition. The combustion under six-nozzle shows the highest HRR and shortest combustion duration. However, the combustion deteriorates at eight-nozzle, since the increase in the ignition point is not enough to compensate for the negative impact of the reduced jet intensity on combustion.

The effect of diesel engine continuous strengthening on combustion and emission remains to be clarified. This study tested combustion and emission processes at different intake pressures on a single-cylinder diesel engine. The results show that the combustion of diesel engine after continuous strengthening is concentrated in diffusion and post-combustion periods, the overall heat release slows down, and the power increment decreases gradually. When the intake pressure increases to 0.25 MPa, the premixed heat release peak almost disappears, while the value of diffusion combustion period increases significantly. Meanwhile, the gravity center of combustion moves backward and the cumulative heat release rate decreases. The power increases by 95% as the intake pressure increases from 0.15 MPa to 0.25 MPa, but only 44% as it further increases to 0.36 MPa. Meanwhile, the exhaust temperature increases after high supercharging. The above effects are more prominent at high excess air coefficient and low speed. Moreover, with the decrease of excess air coefficient, the proportion of heat release in the post-combustion stage to the whole combustion increases and the power increment is limited. However, the thermal efficiency decreases approximately linearly and the fuel consumption rate increases sharply. In addition, as the excess air coefficient decreases, the exhaust temperature increases, PM increases and NOx decreases. The above effects are more pronounced at higher boost ratio.

The entrainment coefficient reflects the air-fuel mixing quality, which is one of the foremost concerns for the development of cleaner and higher power-density internal combustion engines. Previous prediction models constructed by the change of axial mass flow rate have lower accuracy under high injection pressure conditions. During modeling in the work, a new construction method based on local entrainment velocity and local entrainment area is developed, and the influences of dilution effect and forces such as flow resistance, lateral pressure, etc. on local entrainment velocity are considered. With the modified model, its prediction accuracy can be effectively extended to high injection pressure and detailed information about entrainment can be provided for analysis. It is found that, with the increase of injection pressure, the entrainment coefficient rises in the whole flow field. When increasing to high injection pressure, the entrainment coefficient constantly decreases with distance in the far-field, which is consistent with the experiments, but not a constant value predicted by previous models. Besides, the decreased rate rises with the increase of injection pressure. Meanwhile, the increase of ambient pressure also makes the entrainment coefficient rise, but barely influences the decreased rate in the far-field. The large decrease of local entrainment velocity in the far-field caused by strong resistance can explain the decrease of entrainment coefficient with distance. Overall, the modified model can rapidly predict the spray mixing quality over a wider range of operating conditions and provide more detailed entrainment information for analysis.