Effect of injection timing on knock combustion and pollutant emission of heavy-duty diesel engines at low temperatures

The knock combustion and pollutant emission of heavy-duty diesel engines at low temperatures are still unclear, especially under different injection timings. Therefore, this study illustrates the above issues through CONVERGE simulation. The results show that with the start of injection (SOI) sweeps from - 7CA to 32CA, a large amount of liquid-phase fuel adheres to the wall, and the wet-wall ratio of fuel at SOI = - 32CA is as high as nearly 30%. The fuel film evaporates slowly, coupled with the effect of low temperature on chemical reactions, the high temperature ignition (HTI) is delayed seriously until the end of injection. The amount of premixed mixture formed during long ignition delay is significantly increased, but its uniformity is better and the concentration is more suitable for ignition. Once HTI is triggered, high-frequency and strong pressure oscillation occurs in the cylinder, and the maximum oscillation amplitude is as high as nearly 10 MPa, far exceeding the threshold of destructive knock combustion. Delayed fuel injection can effectively alleviate the above problems, such as the best when the SOI in this study is 17CA. In addition, HC emissions are positively correlated with the amount of fuel film, but the trend of CO quantity with injection timing shows the opposite result. NOx emission increases as the injection timing advances, while soot is the opposite, because the mixture concentration is leaner at the earlier SOI and the expanded high-temperature region leads to an accelerated oxidation rate of soot.

Automated summary

This study investigates the knock combustion and pollutant emission of heavy-duty diesel engines at low temperatures, focusing on different injection timings. The simulation results reveal that delayed fuel injection effectively alleviates issues such as fuel film adhesion and improves the uniformity and suitability of the premixed mixture. However, high-temperature ignition triggers strong pressure oscillation, exceeding the threshold for destructive knock combustion. Additionally, HC emissions are positively correlated with the amount of fuel film, while CO emissions show the opposite trend with injection timing. NOx emissions increase with advanced injection timing, while soot emissions decrease due to leaner mixture concentration and accelerated oxidation rate.