Combustion and Emission Characteristics of an Ammonia–Diesel Dual-Fuel Engine under High Ammonia Substitution Ratios

Abstract

To assess the potential for reducing carbon emissions, this study investigated the effects of fuel injection strategies, intake conditions, and engine speeds on combustion performance in ammonia–diesel dual-fuel engines. The results indicate that a high diesel injection pressure combined with advanced injection timing enhances the premixing of diesel and ammonia, shortens the ignition delay, and accelerates the combustion process, thereby improving the indicated thermal efficiency (ITE). Increasing the equivalence ratio reduces the compression pressure and temperature while decreasing the oxygen concentration around the diesel spray. This results in a longer ignition delay, a delayed combustion phase, and a combustion duration that initially shortens and then extends. Consequently, the ammonia combustion efficiency initially increases rapidly before gradually declining, while the ITE exhibits a similar trend, first increasing and then decreasing. At an ammonia energy fraction of 70%, the maximum ITE reaches 50.3%, representing an improvement of 6.7% compared with the pure diesel mode. At this point, the ammonia combustion efficiency is 94.6%, with NH₃ emissions of 14.5 g/kW·h, N₂O emissions of 0.17 g/kW·h, and NOx emissions of 2.9 times higher than the pure diesel mode. However, greenhouse gas (GHG) emissions are reduced by 67.5% compared with the pure diesel mode. Lower engine speeds of 1000 rpm result in lower greenhouse gas (GHG) emissions and ITE than 1500 rpm. Ammonia-fueled engines show promise in enhancing ITE and mitigating GHG emissions.