Effect of Intake Air Humidification and EGR on Combustion and Emission Characteristics of Marine Diesel Engine at Advanced Injection Timing

The trade-off between NOx-soot and ISFC (Indicated Specific Fuel Consumption) brings new challenges for the development and application of innovative techniques that could reduce the NOx-soot emissions of diesel engine simultaneously with the lowest possible fuel penalty. In this study, the two coupling measures, advanced SOI (start of injection) respectively coupled with intake air humidification (IAH) and EGR, were used to achieve lower NOx-soot emissions than the original engine while minimizing the increase in ISFC. Numerical studies were conducted on a four-stroke supercharged intercooled marine diesel engine under 75% loads at 1350 r/min by using AVL Fire code. The advanced SOI varied from 14.5 degrees CA BTDC to 20 degrees CA BTDC; humidity ratio ranged from 0 to 100% in increments of 20%, and EGR rate ranged from 0 to 25% in increments of 5%. The technical route to achieve Tier III emission standards was also analyzed in this paper. The result indicates that lower in-cylinder pressure, temperature and NOx emissions, higher ISFC and soot emissions are observed when only IAH or EGR technology is applied, while opposite trends are found when only using advanced SOI. The proper combination of different SOI respectively with humidity ratio and EGR rate can improve the trade-off relationship between NOx and soot. Meanwhile, the increase in ISFC is improved by using advanced SOI under high EGR rate or humidity ratio. Compared with the advanced SOI coupled with EGR, advanced SOI coupled with IAH results in less loss of ISFC. Analysis results reveal that both above-mentioned coupling measures can achieve low NOx-soot emissions while ensuring that ISFC does not increase. As expected, there are nine combinations of advanced SOI coupled EGR that can achieve NO emissions to meet the Tier III standard, while advanced SOI coupled IAH has only one combination to meet this regulation.