Effect of Split Injection and Intake Air Humidification on Combustion and Emission Characteristics of a Marine Diesel Engine in Partially Premixed Low-Temperature Combustion Mode

The objective of this study was to investigate combined effects of split injection strategies and intake air humidification on combustion and emissions of a partially premixed charge compression ignition (PCCI) marine diesel engine. In this research, a three-dimensional numerical model was established by a commercial code AVL-Fire to explore in-cylinder combustion process and pollutant formation factors in a four-stoke supercharged intercooled marine diesel engine under partial load at 1350 r/min. The novelty of this study is to combine different water-fuel ratios and fuel injection parameters (pilot injection timing and main injection timing) to find the optimized way to improve engine performance as well as NO_x-soot emissions, thus meeting the increasingly stringent emissions restriction. The results indicate that as the main injection timing advances (-14°CA to -20°CA aTDC), the in-cylinder peak pressure increases by about 10%, the main injection ignition delay (MI ignition delay) becomes longer, the CA50 is advanced near the top dead center (TDC), which is effective to improve the indicated thermal efficiency (ITE). Meanwhile, soot emissions are reduced by about 50% compared with the original engine at the -20°CA aTDC main injection timing. The early pilot injection timing can form relatively uniform temperature field and concentration field in the cylinder before the start of main injection (SOMI) timing, which is advantageous to fuel-air mixing. The high level of water-fuel ratio is utilized to reduce overall combustion temperatures and achieve low temperature combustion of the diesel engine. NO_x emissions significantly decrease by about 75% compared with the original engine when the water-fuel mass ratio is 2.0. All in all, the technical route to improve the NO_x-soot trade-off relationship is found through the coupling optimization of split injection strategies and intake air humidification. Meanwhile, the indicated specific fuel consumption (ISFC) is reduced and NO_x-ISFC trade-off relationship is improved.