Experimental and simulation study of advanced injection strategies in an ammonia-diesel dual‑fuel engine for...

The present study focuses on evaluating split-injection strategies in an ammonia-diesel dual-fuel (ADDF) engine at 1800 rpm under medium load conditions, a load range of particular interest in stationary engine operation, with the broader goal of improving combustion stability and efficiency when operating near a 50% ammonia energy contribution. Medium-load ADDF operation, particularly in small compression ignition (CI) engines, remains sparsely explored, and the influence of diesel split injections on combustion phasing and efficiency is not well understood. In this study, a twin-cylinder common rail direct injection (CRDI) diesel engine was operated in ADDF mode at 1800 rpm and medium load, with ammonia energy share of approximately 50%. A systematic experimental investigation was conducted by varying pilot injection ratio (PIR) up to 50% and pilot injection timing from 46 to 66 crank angle degrees before top dead center (CAD BTDC), while maintaining a fixed main injection timing. The experiments were complemented with computational fluid dynamics (CFD) simulations using CONVERGE v3.0 to analyze in-cylinder combustion behavior. Results show that an optimized split-injection strategy (PIR of 20% and pilot timing of 60 CAD BTDC) significantly improves combustion efficiency (~ 95%) and enhances brake thermal efficiency under ADDF operation. Further increase in PIR leads to efficiency deterioration due to unfavorable combustion phasing. The pressure and heat release characteristics reveal a shift toward low-temperature combustion with reduced peak pressure compared to diesel-only operation, attributed to the higher specific heat capacity of ammonia. The optimized split strategy advances CA50, reduces combustion duration, and minimizes negative work, thereby improving overall combustion performance. OH-radicals from simulation corroborate a hierarchy of high-temperature reaction intensity: diesel-only > ADDF split > ADDF single. Stability remained acceptable with controlled COV of IMEP under 3%. This study provides an integrated experimental-CFD evaluations of split diesel injection for a medium-load ADDF engine, and identifies an optimized pilot-main injection strategy capable of enhancing ammonia utilization while mitigating efficiency penalties. These findings offer actionable guidance for ADDF operation in power generation sector and other medium-load CI applications.