Effects of Split Injection on Combustion, Emissions, and Intermediate Species of Natural Gas High-Pressure Direct Injection Engine

Using natural gas (NG) as the primary fuel helps alleviate the fossil fuel crisis while reducing engine soot and nitrogen oxide (NO_X) emissions. In this paper, the influences of a novel split injection concept on an NG high pressure direct injection (HPDI) engine are examined. Four typical split injection strategies, namely split pre-injection of pilot diesel (PD) and NG, split post-injection of PD and NG, split pre-injection of NG, and split post-injection of PD, were developed to investigate the influences on combustion and emissions. Results revealed that split pre-injection of NG enhanced the atomization of PD, whereas the split post-injection of NG lowered the temperature in the core region of the PD spray, resulting in the deterioration of combustion. The effect of the split injection strategy on indicated thermal efficiency exceeded 7.5%. Split pre-injection was favorable to enhancing thermal efficiency, whereas split post-injection was not. Ignition delay, combustion duration, and premixed combustion time proportion were affected by injection strategies by 3.8%, 50%, and 19.7%, respectively. Split pre-injection increased CH₄ emission in the exhaust. Split post-injection, especially split post-injection of PD and NG, reduced the unburned CH₄ emission by approximately 30%. When the split post-injection ratio was less than 30%, the trade-off between NO_X and soot was interrupted. The distribution range of hydroxyl radicals was expanded by pre-injection, and NO_X was generated in the region where the NG jet hit the wall. This paper provides valuable insights into the optimization of HPDI injection parameters.