Assessment of diesel split injection against pre-chamber combustion to enable efficient ammonia engine operation

Ammonia (NH 3 ) is a promising carbon-free fuel, but its low reactivity and high autoignition temperature pose challenges to efficient and clean combustion. By performing high-fidelity computational fluid dynamics simulations, this study examined two promising strategies in a light-duty engine: NH 3 -diesel dual-fuel compression ignition (DF-CI) and NH 3 ‑hydrogen active pre-chamber combustion (PCC). Key parameters were investigated to improve engine performance and enhance NH 3 utilisation. The results show that diesel split injection extends the feasible operating range compared to single injection by moderating the premixed combustion spike and improving heat release phasing. Further assessment of piston geometry, spray targeting, and nozzle number improved pilot distribution and charge consumption, with the best DF-CI case achieving an indicated thermal efficiency (ITE) of 47.4% at 70% ammonia energy fraction. For active PCC, improved performance was achieved by strengthening pre-chamber pressure build-up, delaying early jet-piston interaction, and improving jet alignment towards the bowl and squish regions. The best-performing PCC mode maintained a competitive ITE of 45.8% while relying almost entirely on carbon-free fuels, using only 0.75% hydrogen enrichment. Compared with the most favourable DF-CI case, active PCC reduced greenhouse gas and nitrogen oxides emissions by approximately 96% and 68%, respectively, although NH 3 slip increased due to the higher ammonia substitution. Overall, while DF-CI provides higher efficiency, active NH 3 ‑hydrogen PCC offers a more promising low-carbon pathway by decoupling ammonia combustion from hydrocarbon pilot fuel while maintaining high efficiency.