Under the guidance of carbon neutrality goals, ammonia is expected to become a
promising alternative fuel for internal combustion engines. Ammonia-diesel
dual-fuel combustion not only effectively reduces carbon emissions but also
addresses the issue of ammonia's slow combustion speed, ensuring good engine
performance. Ammonia-diesel engines with liquid ammonia direct injection have
the potential to further increase the ammonia energy ratio (AER) and reduce
unburned ammonia, greenhouse gas (GHG) emissions, as well as
NOx emissions. Based on a numerical model of
a liquid ammonia direct injection ammonia-diesel engine, this paper compares two
different injection system configurations: coaxial and non-coaxial liquid
ammonia direct injection, and investigates the effect of AER on combustion and
emission characteristics in the non-coaxial mode. The results show that,
compared to the non-coaxial mode, the coaxial mode achieves more even fuel
distribution and combustion distribution, higher indicated thermal efficiency
(ITE), and lower emissions of unburned ammonia and N2O. However,
NOx emissions increase significantly. In the
non-coaxial mode, as the AER increases from 50% to 90% or higher, the unevenness
of in-cylinder fuel distribution increases significantly. When AER reaches 95%
or higher, more ammonia diffuses near the cylinder walls, where it is difficult
to burn due to flame quenching, and the in-cylinder combustion temperature
decreases, leading to an increase in unburned ammonia and N2O
emissions. NOx emissions, on the other hand,
decrease as AER increases. Liquid ammonia direct injection reduces in-cylinder
temperature, helping to reduce heat loss, which becomes more pronounced as AER
increases. However, in the cases of AER = 95% and 98%, the deterioration of
in-cylinder combustion conditions leads to an increase in unburned ammonia and a
decrease in combustion efficiency, resulting in a significant drop in ITE. The
highest ITE occurs at AER = 90%, reaching 49.7%.