Parametric study of injection strategy and two injector layouts for a large low-speed marine HPDI methanol engine

Methanol, as a promising green alternative fuel in marine engines, suffers from the challenge of balancing thermal efficiency and NOx emissions when applied to large low-speed marine high pressure direct injection (HPDI) engines. This study aims to investigate the influence of injector layout coupled with key injection parameters on in-cylinder combustion, energy distribution, and emission characteristics, and to identify the optimal configuration. Numerical simulation based on computational fluid dynamics was adopted. The results indicate that, compared to the original configuration, a uniform 120° arrangement of triple-injector layout (G3) improves fuel spray distribution within large-bore cylinders, increasing the by 1.63 %, although NOx emissions rise simultaneously; reducing the methanol injection pressure (MIP) from 600 to 300 bar achieves a good balance between ITE and emissions, whereas lowering the pressure to 200 bar significantly deteriorates CO and soot emissions as well as ITE; appropriately advancing the start of injection timing (SOI) can further improve ITE without exceeding the maximum cylinder pressure limit. As the methanol substitution ratio (MSR) rises from 88 % to 98 %, NOx emissions decrease monotonically while CO and soot emissions increase synchronously, and ITE presents an up-then-down tendency with a balanced overall performance achieved at an MSR of 93 %. The synergistic combination of the G3 layout, 300 bar MIP, and an SOI advanced by 1 °CA improves ITE by 1.4 % and reduces NOx emissions by 20.8 % relative to the original configuration, without imposing additional mechanical loads. This study demonstrates that the proposed injection strategy maintains favorable applicability with the MSR fluctuating by ±5 %.