Parametric optimisation of methanol combustion in marine dual-fuel engines at low loads

• Stratified injection yields NOx emissions 4.5 g/kWh and higher efficiency to diesel. • Optimised multi-stage injection boosts efficiency in low-load marine engines. • Single injection requires increased CR and IVC temperature for complete combustion. • The three-stage injection strategy exhibits high NOx emissions 12 g/kWh. Methanol is considered an alternative fuel for the shipping decarbonisation, the properties of which, however, impact the marine dual-fuel engines ignition and combustion characteristics, especially at low load conditions. This study aims at parametrically optimising a marine dual-fuel engine operating with methanol high energy fraction at low loads to achieve knock-free combustion with the highest efficiency and lowest emissions. Computational Fluid Dynamics (CFD) modelling in the CONVERGE software is employed for the investigated large-bore marine four stroke engine considering four injection strategies including single, two stage and stratified injection. The Reynolds Averaged Navier Stokes (RANS) approach is employed to represent turbulence, the Lagrangian-Eulerian approach is used for the spray formation, and the SAGE detailed chemistry solver is used for modelling combustion. The CFD model was first developed and validated for the engine diesel mode. Subsequently, the validated model was expanded to accommodate the direct injection (DI) of both methanol and diesel fuels. Parametric runs are performed considering the compression ratio (CR) in the range 14–17 and the temperature range at inlet valve closing (T IVC ) 360–400 K. The results reveal that acceptable combustion efficiency and high thermal efficiency are achieved with CR and T IVC above 17 and 380 K respectively for single injection, above 16 and 380 K respectively for double injection, as well as above 14 and 360 K respectively for stratified injection. Stratified injection is proposed to improve engine performance and reduce NOx emissions. This study provides insights to achieve stable and efficient operation of methanol-fuelled marine engines at low loads, and as such it contributes to the maritime industry decarbonisation.