Combustion and emissions characteristics of ammonia/diesel dual-fuel engine with high premixed fractions in different injection modes

The performance of the ammonia/diesel dual-fuel (ADDF) engine still suffers from low indicated thermal efficiency (ITE) and misfiring with higher premixed fractions of ammonia. To improve the performance of the ADDF engine, this study compared two diesel injection modes, i.e., constant injection duration (CID) and constant injection rate (CIR) modes under high premixed fractions (f_NH3). This was achieved by conducting numerical simulations using the CONVERGE solver coupled with a newly developed skeletal mechanism of ammonia/n-heptane. In the simulation, the f_NH3 was increased from 0.4 to 0.9 at an interval of 0.1 with an engine speed of 910 rpm. Subsequently, the combustion and emissions characteristics of ADDF engines were analyzed. The results show that due to the ignition resistance of ammonia and the lower diesel injection pressure in the CID mode, the combustion deteriorates as f_NH3 increases. Moreover, CO, N₂O, and greenhouse gas (GHG) emissions increase with the increase of f_NH3 under CID mode, while NOx emissions remain unchanged. In contrast, the CIR mode could increase the injection pressure of diesel in the cylinder thereby overcoming the negative effects of the ignition resistance of ammonia in conditions with high f_NH3. Compared with the CID mode, the CIR mode can not only realize higher peak pressure, temperature, and heat release rate, but also improve the ITE with low ringing intensity, and reduce CO and GHG emissions. However, the NOx emissions of the engine in the CIR mode are higher due to the elevated combustion temperature.