Insights into the combustion characteristics, emission formation sources, and optimization strategy of an ammonia-diesel dual-fuel engine under high ammonia ratio conditions

Ammonia is a promising carbon-free fuel for internal combustion engine to reduce greenhouse gas and meet more strict emissions regulations. Based on an ammonia-diesel dual-fuel engine with ammonia port injection, this study conducted detailed analysis for combustion and emission characteristics by three dimensional numerical simulations based on OpenFOAM and Lib-ICE codes, which are validated through experimental data with different ammonia energy ratios and diesel injection times. The effects of ammonia energy ratio (30%, 50%, 70%) for combustion and emission are analyzed in detail through energy distribution, heat release analysis, equivalence ratio vs temperature maps and key scalar distribution maps. Results show 70% ammonia energy ratio condition achieves fastest combustion rate in later premixed combustion stage between CA50-CA90 due to higher ammonia equivalence ratio and lowest N2O emissions due to the high-temperature flame front beneficial for thermal decomposition of N2O, indicating the huge application potential of high ammonia energy ratio in dual-fuel engines. Besides, equivalence ratio vs temperature maps indicate that stoichiometric or richer and high-temperature (1500 K–2500 K) combustion condition is beneficial for emissions reduction. Based on the 70% ammonia energy ratio condition, combustion acceleration in the primary combustion stage between CA10-CA50 through improvement of initial reaction activity and unburned NH3 reduction are the key points for enhancing combustion in initial stage and decreasing incomplete combustion loss. Adjustment of diesel injection time and hydrogen introduction are then applied as optimization strategies. Final comprehensive comparison indicates hydrogen introduction is more proper, which has similar combustion acceleration effectiveness in primary combustion stage, but higher reduction effectiveness for unburned NH3 with lower cost of NOx and PRR increase comparing with advanced SOI strategy. Only 10% hydrogen introduction achieves 30.43% combustion acceleration in primary combustion stage and 50.79% unburned NH3 reduction comparing with the original base case.