Research on the effects of ammonia substitution on the combustion and emission characteristics of a small displacement ammonia-diesel dual fuel engine

As an efficient hydrogen carrier, ammonia can be applied in internal combustion engines to reduce greenhouse gas emissions in the transportation sector. At present, small-displacement light-duty trucks account for a large proportion of urban transportation. However, limited research has been conducted on small-displacement ammonia–diesel dual-fuel (ADDF) engines. To promote the practical application of such engines, this study focuses on investigating the effects of ammonia on the performance of small-displacement diesel engines. This study employs a numerical simulation model to investigate the in-cylinder combustion and emission characteristics of a small displacement ammonia diesel dual-fuel under different ammonia energy ratios and start of diesel injection (SODI) conditions. Gaseous ammonia is supplied via the intake port, while diesel is directly injected into the combustion chamber. The simulation results indicate that under low ammonia energy ratio conditions, the increase in ammonia energy ratio raises the in-cylinder pressure and peak heat release rate, thereby improving the thermal efficiency of the engine. However, due to the decrease in the in-cylinder temperature during the late combustion process, the soot post-oxidation weakens, leading to an increase in soot emissions. Additionally, owing to the influence of the ammonia thermal nitrogen oxides reduction (de-NO_x) process, the increase in ammonia energy ratio from 0 % to 30 % reduces nitrogen oxides (NO_x) emissions by 31.7 %. Meanwhile, the nitrous oxide (N₂O) emissions increase. The advance of SODI results in excessively high in-cylinder combustion temperatures, significantly increasing NO_x emissions. Meanwhile, excessively advanced SODI increases engine negative work, consequently reducing thermal efficiency. At a SODI of −7.2 °CA, the engine achieves an indicated thermal efficiency of 45.47 %. Meanwhile, greenhouse gas emissions are measured at 483.3 g/kW·h, representing a 15.4 % reduction compared to pure diesel mode. The above results demonstrate the feasibility of using a low ammonia energy ratio in small-displacement engines. However, the excessively high NO_x emissions still require optimization of the injection strategy for further reduction.