The characteristics analysis and feasible improvement strategy evaluation for ammonia-diesel dual-fuel engine under heavy load and high ammonia ratio conditions

The decarbonization and reduction of dependence on fossil fuel for transportation industry relies on the rapid and large-scale adoption of carbon free fuels. This paper conducts systematical simulations for the ammonia- diesel dual-fuel engine with ammonia port injection and diesel direct injection with a validated numerical framework based on OpenFOAM and the Lib-ICE codes, focusing on the heavy load and high ammonia ratio conditions with significant carbon reduction potential. Facing the challenges of enlarged unburned NH3 emissions and low thermal efficiency under heavy load and high ammonia ratio conditions, the aim of this study is to explore the characteristics and feasible improvement strategies under this special and crucial operation condition. Wherein, the engine load and displacement of this study is 16 bar Indicated Mean Effective Pressure (IMEP) approximate 90 % of the maximum load and 1.327 L, respectively. Firstly, the reference cases with 70 %, 80 % and 90 % ammonia energy ratios (AER) are investigated. The energy balance and emissions analysis indicate that a significant portion of ammonia energy remains unutilized, leading to suboptimal greenhouse gas reduction effectiveness and low thermal efficiency. Given the limited diesel mass, optimizing diesel distribution for ignition and subsequent ammonia energy activation becomes crucial. Detailed analysis of unburned ammonia region indicates the potential of improvement through diesel injection strategy for AER70 and AER80 conditions, while the hydrogen introduction strategy is then proposed for AER90 condition. The comparison of advanced SOI and diesel split injection strategy under AER70 condition shows the superiority of split injection since comparing with the original case, it achieves 85.38 %, 99.88 % unburned NH3 reduction, 41.48 %, 81.70 % greenhouse gas reduction, 15.61 %, 13.45 % ITE increase with SOI1 =-60 degrees CA ATDC, SOI2 =-10,-15 degrees CA ATDC conditions, respectively. Under AER80 condition, due to proper diesel distribution and reactivity stratification, the case with SOI =-30 degrees CA ATDC obtains best improvement effectiveness, resulting in notable 16.41 % ITE increase, 95.81 % NH3 and 69.55 % N2O reduction. Under AER90 condition, hydrogen is introduced and homogenously premixed with ammonia through port injection. Hydrogen mainly makes a difference to later ammonia combustion, as evidenced by the increases in OH radical peak and second peak of heat release with the rise of HER. However, continuous hydrogen introduction fails to achieve significant improvement effectiveness. In conclusion, through proper combustion control strategies, the high ammonia ratio (70 %, 80 %) is promising for practical application with greenhouse gas reduction advantage and proper thermal efficiency under heavy load condition.