Exploring the GHG reduction potential of pilot diesel-ignited ammonia engines - Effects of diesel injection timing and ammonia energetic ratio

Ammonia-diesel dual-fuel engines have been demonstrated as a promising technology to reduce the greenhouse gas (GHG) emissions. However, understandings of the operational boundaries and combustion mechanism in the ammonia-diesel dual-fuel engines are far from adequate, and the published works are mostly done with relatively low ammonia substitution ratios. To bridge the gap between the technology status and the potential in engineering applications, a series of engine bench tests of the pilot diesel-ignited ammonia combustion with detailed analysis of the combustion performance and exhaust gas emissions are conducted in this study to explore the GHG reduction potential. As the first report, the effects of the diesel injection timing and ammonia energetic ratio on the performance and exhaust gas emissions of the dual-fuel engine at various loads and a fixed speed are evaluated in this paper. As the ammonia energetic ratio increases, the diesel injection timing for the stable engine operation becomes narrower. Increasing the ammonia energetic ratio decreases the indicated thermal efficiency. The ammonia-diesel dual-fuel engine can maintain the stable of COVIMEP below 3% at a wide ammonia energetic ratio range, the indicated thermal efficiency with the tested upmost 90% ammonia energetic ratio can reach about 34% if an optimized diesel injection timing is implemented. The unburned ammonia increases linearly with the ammonia energetic ratio while the unburned ammonia ratios of total input ammonia are similar at the different ammonia energetic ratios. In comparison to the conventional pure-diesel mode, while the CO2 emission decreases by 50% and 72%, the equivalent CO2 emission (i.e., CO2 + 265 N2O, labelled as CO2e) decreases by 24% and 55% at the 60% and 80% ammonia energetic ratios in the ammonia-diesel mode, respectively. In the ammonia-diesel dual-fuel mode, the specific CO2e, N2O and unburned ammonia decrease with the engine load increasing from 50% to 100%, while the highest indicated thermal efficiency is reached at the 75% load.