Pilot-injection strategies for diesel/dissociated methanol gas dual-fuel engine: CFD analysis and RSM–NSGA-Ⅱ optimization

• A CFD, RSM and NSGA-II framework was established for dual-fuel engine optimization. • Hydrogen-rich DMG enables efficiency improvement with reduced soot emissions. • Visualizing the effects of injection parameters on performance and emissions. • The optimized strategy improves ITE by 1.23 % and reduces KI by 65.26 %. • Achieved stable hydrogen-rich combustion for clean renewable fuel utilization. Hydrogen-rich dissociated methanol gas (DMG), produced via exhaust-heat-assisted catalytic methanol dissociation, provides a promising route for on-board hydrogen utilization in diesel engines. However, hydrogen enrichment typically increases knock intensity (KI) and nitrogen oxide (NOx) formation, limiting its practical application. This study developed a computational fluid dynamic (CFD) based optimization framework to address these challenges by tailoring the pilot injection strategy. At the optimal blending ratio (φ = 30%), preliminary parametric analysis showed that an injection interval (II) of 20°CA and a pilot injection ratio (PIR) of 10% improved the indicated thermal efficiency (ITE) by 0.9%, accompanied by a 55.9% increase in NOx emissions and a 5.82% reduction in carbon dioxide (CO₂) emissions relative to conventional diesel operation. These trends guided the construction of a response surface methodology (RSM) model, which revealed that advancing the start of pilot injection (SOPI) enhances premixed combustion, while an appropriate PIR facilitates a more favorable combustion environment; the injection interval predominantly governed the interaction between the pilot and main combustion stages. Combining RSM with the non-dominated sorting genetic algorithm Ⅱ (NSGA-Ⅱ) yielded an optimized injection strategy that increased ITE by 1.23%, reduced soot emissions by 37.0%, and caused only a slight NOx rise of 15.8%. Importantly, KI decreased from 0.19 MPa to 0.066 MPa (−65.3%), ensuring stable operation under high hydrogen enrichment. Overall, the proposed DMG dual-fuel strategy, enabled by optimized pilot injection, demonstrates a practical pathway for achieving efficient, low-carbon, and knock-mitigated hydrogen utilization in diesel engines.