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

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 CO 2 considering the global warming potential of CO 2 and N 2 O are evaluated (i.e. CO 2 + 300 × N 2 O). 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 CO 2 with little increase in the NH 3 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 CO 2 would be relatively limited because the significant increase of N 2 O 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 NH 3 , NOx and greenhouse gas emissions. • The low- and high-pressure injection dual-fuel modes of diesel pilot-ignition ammonia combustion engines are compared. • The maximum ammonia ratio about 80% by energy is recommended for LPDF mode. • The HPDF mode has the potential to achieve a 97% diesel replacement ratio. • The LPDF mode has the potential to achieve a higher indicated thermal efficiency than the pure-diesel mode. • The HPDF mode can significantly reduce the engine-out NH3, NOx and greenhouse gas emissions.