Numerical investigation on effects of water-blended methanol on combustion,performance,and NO x emissions of a two-stroke low-speed dual-fuel marine engine across the entire operating envelope

To address the issue of high NO_x emissions from two-stroke low-speed methanol/diesel dual-fuel marine engines, this study established a computational fluid dynamics simulation model to systematically investigate the effects of hydrous methanol on the engine’s combustion characteristics, power performance and emission behaviors across the entire operating envelope. A water-blending strategy that balances compliance with IMO Tier III emission limits and fuel economy was further proposed. The results demonstrate that as the water blending ratio (WBR) increase, both the peak in-cylinder pressure and the peak heat release rate decrease significantly under all investigated loads, and the heat-release process exhibits a distinctive three-stage profile. Meanwhile, water blending effectively suppresses the formation of high-temperature zones within the cylinder, resulting in a monotonic reduction in NO_x emissions across the full operating range. Notably, at the same WBR, NO_x reduction effect is more pronounced under low-load conditions. Water blending exerts a distinctly load-dependent effect on indicated mean effective pressure (IMEP), and a critical WBR threshold is observed under medium-to-high load conditions. When the WBR exceeds this threshold, in-cylinder heat losses intensify significantly, causing a sharp decline in IMEP. Trade-off analysis indicates that the recommended WBRs at 25%, 50%, 75%, and 100% loads are 55.8%, 49.7%, 44.0%, and 32.0%, respectively, corresponding to NO_x emission reductions of 78.4%–51.8% and increases in equivalent indicated specific fuel consumption of 3.3%–0.3%. This study provides a theoretical reference for optimizing the performance and emissions of two-stroke low-speed methanol/diesel dual-fuel marine engines.