The influences of hydrogen-enriched and exhaust gas recirculation strategies on the performance of ammonia-diesel dual-fuel engine

Under the dual-carbon strategy and global warming context, ammonia has been recognized as a highly promising carbon-free alternative fuel for the engines. Nevertheless, low combustion reactivity of ammonia necessitates the use of high-reactivity fuels to ignite the mixture in engines. A high ammonia energy share ratio (AESR) usually leads to excessive unburned ammonia (UA) emissions in the ammonia-diesel dual-fuel (ADDF) engine, which severely restricts its practical application. This study proposes hydrogen addition to enhance the ADDF engine performance, combined with exhaust gas recirculation (EGR) to regulate nitrogen oxide (NOx) emissions and combustion stability under high hydrogen energy share ratios (HESRs). Results indicate that hydrogen elevates in-cylinder OH radical concentration, accelerating chemical kinetic processes and increasing in-cylinder pressure and temperature. The maximum indicated thermal efficiency of the ADDF reaches 43.7% at 15% HESR. The minimum UA and N2O emissions are 0.51 ppm and 0.82 ppm, respectively. With hydrogen serving as an active combustion enhancer, the maximum indicated mean effective pressure (IMEP) of the ADDF engine reaches 1.145 MPa. However, an overly high HESR (above 35%) causes rapid pressure rise and deteriorated combustion stability. EGR effectively retards heat release by lowering intake oxygen concentration and increasing specific heat capacity of the mixture, mainly via suppressing OH radical. Although EGR slightly increases HC, CO and UA emissions, it reduces NOx emissions down to 6 g/kWh. Overall, hydrogen can effectively improve engine performance and emissions at high AESR, while EGR is critical to peak combustion pressure in the combustion chamber at high HESR.