Methanol wall-film dynamics in marine-engine port fuel injection: Effects of surface roughness and intake flow

The application of green methanol as a drop-in fuel for marine engine retrofits represents a promising pathway toward substantial greenhouse gas emission reductions. However, methanol’s high enthalpy of vaporization promotes wall film formation during port fuel injection, which can adversely affect mixture formation, combustion stability, and emissions. In this study, the influence of surface roughness and intake air mass flow on liquid film formation, transport, and evaporation is investigated using a down-scaled, optically accessible flow bench model of a marine engine intake port. Time-resolved measurements of wall film development were conducted using the refractive index matching (RIM) technique on roughened polymethyl methacrylate (PMMA) surfaces with different roughness levels. Methanol injection was compared to isooctane as a conventional reference fuel under identical operating conditions. The experiments reveal pronounced differences in film dynamics between the two fuels. Methanol exhibits faster film buildup and significantly longer persistence due to its higher latent heat of vaporization. The observed film evolution is governed by the combined effects of spray impingement, capillary-driven roughness invasion, aerodynamic shear forces, and evaporation. Increasing intake air mass flow accelerates film decay by enhancing both convective evaporation and shear-induced film removal. While higher roughness levels increase the liquid storage capacity and prolong film residence time, they also suppress large-scale liquid transport between roughness features. Additional RIM measurements in the combustion chamber show that higher intake mass flow rates promote liquid fuel transport into the chamber, despite increased evaporation rates. The results provide insight into methanol wall film behavior during port-fuel injection and highlight the critical role of surface roughness and intake flow in fuel transport, evaporation and mixture formation in methanol-fueled marine engines.