Numerical Investigation on Effects of Oxygen-Enriched Air and Intake Air Humidification on Combustion and Emission Characteristics of Marine Diesel Engine

In order to meet the increasingly stringent emissions restriction, it is indispensable to improve the combustion and emissions technology of high-speed marine diesel engines. Oxygen-enriched combustion and intake air humidification are effective ways to control pollution from diesel engines and improve combustion of diesel engines. In this study, the combustion and emission characteristics of supercharged intercooled marine diesel engine with humidity ratio and intake oxygen concentration were investigated by using multi-dimensional CFD model. The combustion model was established by AVL Fire code. The combination strategy of intake air humidification and oxygen-enriched combustion were optimized under partial load at 1350 rpm. The simulation results indicate that under kept the constant value of the humidity ratio, the averaged in-cylinder temperature and pressure both increase with the rise of intake oxygen concentration, while the timing of the in-cylinder peak temperature and pressure are increased, the combustion duration got short and CA50 is delayed. Meanwhile, soot emissions are inhibited while the NO_x emissions increase remarkably. Intake air humidification can reduce NO_x emission significantly, whereas it has little effect on soot emissions and indicated power. With humidity ratio growing, both in-cylinder combustion pressure and temperature decrease as well as NO_x emissions. However, the ignition delay increased dramatically. The results reveal that the humidity ratios from 60% to 80% and the oxygen concentration from 22% to 23% can be matched to realize lower Soot-NO_x emissions than the original engine with loss of the indicated power no more than 3%.