Numerical investigation of methanol-air mixture formation and combustion in a dual-fuel marine engine at low load

Port fuel injection (PFI) methanol-diesel dual-fuel is considered a promising retrofit solution for methanol adoption in marine engines. PFI enables fuel flexibility, improved thermal efficiency, and reduced greenhouse gas (GHG), soot, and NOx emissions. This work aims to provide insights for the optimization of PFI methanol-diesel marine engines, contributing to the decarbonization of the maritime sector. For this purpose, Computational Fluid Dynamics (CFD) simulations in CONVERGE-CFD are performed, incorporating spray modeling and combustion kinetics of both methanol and diesel to investigate high-pressure PFI methanol behaviours injected at different locations. The developed CFD model satisfactorily captured methanol-air mixture formation and methanol combustion under low-load dual-fuel operation measured at 3.1 bar Brake Mean Effective Pressure (BMEP). Two distinct phases of dual-fuel combustion are captured: compression ignition of n-heptane, as a surrogate for diesel, and fast flame propagation of premixed methanol, with results being sensitive to blending ratio (BR), start of injection (SOI), and injector location. At 3.1 bar BMEP, varying BR between 45%–65% yielded increased combustion efficiency (91.9%–94.8%) and gross Indicated Thermal Efficiency (ITE) (46.2%–47.3%), while BRs above 75% caused partial burn and efficiency drop. Advancing SOI improved mixture uniformity and flame propagation rate, however, it increased NOx and heat release rate of the second phase. Injecting more methanol from the short runner promoted homogeneity, raising combustion efficiency by up to 8%-points, thermal efficiency up to 4.3%-points, and NOx emissions by 50%. These results highlighted the model’s capability to simulate dual-fuel operation and advance the understanding towards efficient and low-emission methanol marine engines.