Numerical investigation and optimization of the ammonia/diesel dual fuel engine combustion under high ammonia substitution ratio

This study investigated the effects of initial temperature, equivalence ratio, and diesel injection timing on engine combustion and emission characteristics at high ammonia substitution ratios. Increased compression temperature and pressure significantly reduce ignition delay, enhance combustion speed and efficiency, and decrease N₂O and unburned NH₃ emissions. A strong correlation exists between the amount of N₂O produced and the mass of unburned NH₃ when ammonia combustion efficiency is high. The N₂O distribution is concentrated near the cylinder walls and the piston top surface, in areas with high concentrations of unburned NH₃. As the equivalence ratio increases from 0.6 to 0.75, flame propagation speed and indicated thermal efficiency (ITE) increase, while NOx, N₂O, and unburned NH₃ emissions decrease. The combustion performance and emissions were optimized by advancing the diesel injection timing and increasing the equivalence ratio to accelerate the combustion speed. This adjustment increases ITE to 47.6 % at an 80 % ammonia energy ratio. Post-optimization results show a reduction in unburned NH₃ emissions from 51.7 g/kW·h to 5.9 g/kW·h and a decrease in N₂O emissions from 0.930 g/kW·h to 0.370 g/kW·h, culminating in a 60.4 % reduction in greenhouse gas (GHG) emissions.