Study on the pilot fuel injection strategy for a large-scale diesel/methanol dual-direct injection engine at high methanol substitution ratio

The diesel/methanol dual-direct injection (DMDDI) engine attracts wide attention because of its high methanol substitution ratio (MSR). This study establishes and validates a 3D CFD model for a 6400 kW two-stroke diesel/methanol dual-fuel (DMDF) engine. Under a fixed direct-injection conditions with an MSR of 90%, the effects of pilot fuel injection pressure (PFIP) and post-injection of pilot fuel (PFPI) strategy on combustion, performance, and emissions are investigated. Results indicate that excessively low or high PFIP weakens work output and fuel economy, whereas PFIP60 achieves the best power and economic performance. On the basis of PFIP60, PFPI reduces NOx, and delaying the post-injection timing (PIT) further suppresses NOx. At PIT14, the engine maintains higher performance while reducing NOx by about 12.39%, and it shows a favorable compromise for incomplete-combustion emissions. Furthermore, decreasing the post-injection mass ratio (PIMR) continues to reduce NOx; at PIMR10, the reduction reaches about 18.42%, and compared with the performance-optimal PIMR70, the differences in indicated power (IP) and indicated thermal efficiency (ITE) are only about 6 kW and 0.06%, respectively, although CO, HC, and CH2O emissions remain relatively higher. The findings clarify, under high MSR, the coupled effects of PFIP, PIT, and PIMR on combustion phasing, the evolution of high-temperature regions, and NOx emissions, and provide a basis for strategy design and engineering optimization of a large-scale dual-fuel engine.