Study of performance and combustion in ammonia-fueled CI engines via diesel simultaneous injection

Abstract Ammonia is a promising carbon-free hydrogen carrier with wide industrial availability; however, its low flame speed, high latent heat, and narrow flammability characteristics create significant challenges for compression ignition engines. This study investigates ammonia/diesel dual-fuel operation at 50% and 80% load conditions, with diesel injection pressure varied from 200 to 340 bar and ammonia energy share (AES) ranging from 0 to 40%, to evaluate the sustainability of ammonia utilization in light-duty engines. Experimental results showed that increasing injection pressure enhanced cylinder pressure by nearly 10% at lower AES conditions and advanced peak pressure occurrence, partially compensating for ammonia’s slower combustion behaviour. Heat release rate (HRR), delayed by nearly 5–9°CA with increasing AES, improved considerably at higher injection pressures, with combustion shifting toward the controlled combustion phase. Ignition delay increased with AES, reaching nearly 4.5°CA at 40% AES and 200 bar, but reduced to 2.78°CA at 320 bar due to improved atomization and ignition support. Cycle-to-cycle variations quantified using CoV imep increased with ammonia substitution because of prolonged combustion duration, whereas optimized injection pressure reduced cyclic fluctuations by nearly 27%; however, beyond 300 bar at 40% AES, combustion stability improvement became marginal. Brake-specific fuel consumption increased by nearly 48% at high AES because of ammonia’s lower combustion reactivity, although higher load and injection pressure partially mitigated the increase. Increasing AES substantially reduced CO, HC, and CO 2 emissions due to lower diesel participation and weaker diffusion combustion, whereas NO x and unburnt NH 3 emissions increased because of intensified nitrogen oxidation pathways and incomplete ammonia conversion. The results suggest that optimized high-pressure injection strategies can considerably improve combustion stability and oxidation behaviour in ammonia-diesel dual-fuel engines, while AES near 30% offers a comparatively stable balance between carbon-emission reduction and combustion performance.