A comparison between low- and high-pressure injection dual-fuel modes of diesel-pilot-ignition ammonia combustion engines

Abstract

Ammonia, as a carbon-free fuel, is drawing more and more attention due to its potential of decarbonization for marine engines. The dual-fuel mode that can compensate for the poor combustion properties of ammonia has exhibited some prospects for practical application, but more relevant researches are urgent to promote the carbon-free fuel, especially for the high-pressure injection dual-fuel (HPDF) mode. In this paper, the ammonia spray experiments under the diesel-like conditions are conducted using the high-speed photography to calibrate the spray sub-models. Then, the combustion and emission characteristics of the low- and high-pressure injection dual-fuel modes of diesel pilot-ignition ammonia combustion engines are numerically studied and compared. Particularly, the equivalent CO2 considering the global warming potential of CO2 and N2O are evaluated (i.e. CO2 + 300 × N2O). The results show that the maximum ammonia ratio about 80% by energy is recommended for the low-pressure injection dual-fuel (LPDF) mode and further increase the diesel replacement ratio will increase the probability of misfire, while the HPDF mode has the potential to achieve a 97% diesel replacement ratio. Compared with the pure diesel mode, the HPDF mode has similar indicated thermal efficiency, cooling and exhaust loss, while it can significantly reduce the equivalent CO2 with little increase in the NH3 emissions. Owing to the reduced heat transfer loss, the LPDF mode has the potential to achieve a higher indicated thermal efficiency than the pure-diesel mode, but its ability to reduce the equivalent CO2 would be relatively limited because the significant increase of N2O emissions if exhaust after treatments were not employed. The comparison between the LPDF mode and HPDF mode shows that the former case has the potential to achieve a higher indicated thermal efficiency, while the latter case can significantly reduce the engine-out NH3, NOx and greenhouse gas emissions.