Effects of ammonia energy fraction and diesel injection parameters on combustion stability and GHG emissions characteristics in a low-loaded ammonia/diesel dual-fuel engine

Ammonia, as a zero-carbon and widely sourced hydrogen carrier fuel, will play an important role in the decarbonization process of internal combustion engines, and the dual-fuel RCCI mode is a potential ammonia combustion technology route. In this study, ammonia/diesel dual-fuel (ADDF) combustion mode was implemented on a modified common-rail diesel engine, and the effects of ammonia energy fraction (AEF), diesel injection pressure (DIP), and diesel injection timing (DIT) on in-cylinder combustion, pollutant emissions and greenhouse effect of dual-fuel engine were investigated. The results indicated that the combustion process of ADDF was limited by the inert combustion and inhibitory effects of ammonia fuel. With the increase of AEF, the dual-fuel combustion process was delayed, the peak in-cylinder pressure decreased, and the ignition delay and combustion duration significantly increased, which was not conducive to the improvement of brake thermal efficiency (BTE). The introduction of ammonia fuel significantly reduced NOx, Soot and CO2 emissions, while unburned NH3 and N2O emissions sharply increased. At AEF = 60 %, CO2 emission decreased by 50.2 %, however, the in-cylinder combustion process deteriorated severely, with cyclic fluctuations reaching 5.06 %. Both increasing DIP and advancing DIT enhanced the chemical reactivity of in-cylinder mixture of ADDF engine, leading to earlier combustion heat release phase and shorter ignition delay, and reducing the variation of combustion cycle. Meanwhile, there was still a trade-off relationship between NOx and Soot emissions. When DIP increased from 100 MPa to 130 MPa, NH3 emission decreased by 20.8 %. Further increasing the DIP to 140 MPa would result in serious "wet wall" problems, leading to an increase in the incomplete combustion area and a decrease in BTE. When DIT was advanced from -4 degrees CA ATDC to -12 degrees CA ATDC, CO, HC and NH3 emissions decreased by 71.3 %, 52.7 % and 57.6 %, respectively, while N2O emission increased by 19.9 %, resulting in an increase in equivalent CO2 emissions. Under the optimized diesel injection strategy, the BTE of ADDF engine was 31.9 %, significantly higher than the original engine operating condition (BTE = 30.2 %).