Qianbo Sun’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 quality of spray atomization governs combustion, power output, fuel economy and emissions of internal combustion engines. Spray atomization is significantly affected by cavitation, and it is closely related to various factors, such as injection conditions, nozzle geometry and throttling effect. In this work, a 5-time up-scaled transparent nozzle was custom-made and utilized for the study of the dynamics and characteristics of the nozzle internal flow with cavitation under various injection pressures and throttling strengths (by varying needle lift). High-speed imaging technique was employed for the nozzle internal flow visualisation. To further investigate the 3D behaviour of cavitation, the simulation of nozzle internal flow was carried out with Fluent 18.0. It was found that cavitation was mainly induced by the throttling effect at the crevice between the needle and needle seat and the redirection of flow at the nozzle inlet. With the rise of the needle lift, initially the fraction of cavitation increased because of increasing hydraulic force, and then as the needle lift increased further, the fraction of cavitation decreased due to the reduced throttling effect. However, when the needle lift kept constant, the cavitation strengthened quickly with the injection pressure and then kept almost constant. Cavitation shedding was observed near the lower wall of the nozzle at the high injection pressure, but not at low injection pressure. In addition, at the low injection pressure, string cavitation was observed due to the throttling effect, originating from the nozzle and extending upstream to the needle.