Effect of Injection Timing on Knock Combustion and Pollutant Emission of Heavy-Duty Diesel Engines at Low Temperatures

Abstract

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 problems through CONVERGE simulation. The results show that with the injection timing advancing from -7°CA to -32°CA, a large amount of liquid-phase fuel adheres to the wall surface, and the wet-wall ratio of fuel under -32°CA condition 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 range 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 10MPa, far exceeding the threshold of destructive knock combustion. Delayed fuel injection can effectively alleviate the above problems, such as the best when the injection timing in this study is -17°CA. 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 trend. NO x  emission increases as the injection timing advances, while soot is the opposite, because the mixture concentration is leaner at the earlier injection timing and the expanded high-temperature region leads to an accelerated oxidation rate of soot.

Keywords: Injection timing, Spray impingement, Knock combustion, Pollutant emission, Heavy-duty diesel engine, Low temperature