Numerical investigation of the effect of inhomogeneous hydrogen-air mixture on PFI marine dual-fuel engine performance and emissions

Hydrogen-diesel dual-fuel engines are regarded as a promising pathway toward low-carbon marine propulsion. However, when hydrogen is port-injected, mixture inhomogeneity inevitably arises, and its influence on combustion remains insufficiently understood. In this study, a CFD model was developed to investigate the hydrogen inhomogeneity effect on dual-fuel combustion. The simulation was conducted based on a large-bore marine four-stroke engine with a rated power of 4 MW at 750 rpm, assuming a 30% premixed hydrogen substitution ratio under full-load condition. The model was validated against the experimental data of the target engine’s diesel operating mode and literature data for a hydrogen-fuelled light-duty engine. Based on the benchmark of fully homogeneous distribution, four representative inhomogeneous premixed hydrogen distribution patterns were investigated. The results show that mixture inhomogeneity strongly affects hydrogen oxidation and advances the overall combustion phasing. The exterior hydrogen concentrated distribution pattern, represented by the Inverted Gaussian distribution, achieves the highest indicated thermal efficiency of 48.9%, but its intensive heat release leads to unstable engine operation. The centrally hydrogen concentrated distribution patterns, including Gaussian, Exponent Decay and Double Gaussian, exhibit distinct combustion behaviors depending on the stratification level. Among them, the Exponent Decay exhibits the least combustion completeness with the highest soot (0.197 g/kWh) and unburnt hydrogen (0.191 g/kWh) emissions. Overall, the homogenous assumption yields a 48.0% indicated thermal efficiency with acceptable emission levels, representing a trade-off between performance and emissions.