Hydrogen is a promising alternative fuel for internal combustion engines due to its high specific energy, fast flame speed, and carbon-free combustion. In dual-fuel operation, it offers a practical route to reducing greenhouse gas emissions while remaining compatible with existing engine hardware. This work evaluates how the hydrogen energy substitution ratio (HSR = 50, 70, and 90%) influences spray dynamics, combustion characteristics, and emissions in a heavy-duty compression ignition engine. Simulations are validated against experiments and use a URANS RNG k–ε framework with a hybrid combustion model: the Eddy Dissipation Concept (EDC) coupled with detailed kinetics (111 species, 768 reactions) for auto-ignition and diffusion burning of diesel, and a G-equation for propagation of a hydrogen-rich premixed flame. The results reveal clear spray–combustion linkages. At HSR 50, the higher Weber number induces stronger breakup, yielding a smaller Sauter mean diameter and higher number-averaged droplet velocity; at HSR 90, the spray is more stable and less atomized, with larger droplets and a shorter vapor penetration length. Increasing the HSR reduces unburned hydrocarbons (UHCs) by more than 50% from HSR 50 to HSR 90 while modestly altering combustion phasing (a later CA50 and a shorter burn duration due to faster hydrogen flame propagation). The validated model provides a practical tool for optimizing dual-fuel settings and HSR–EGR–SOI trade-offs to balance efficiency and emissions.

Hydrogen enrichment of compression ignition (CI) engines has emerged as a promising strategy to simultaneously enhance thermal efficiency and reduce carbon-based emissions. This study numerically investigates how hydrogen enrichment affects engine performance and emissions in methanol–diesel dual-fuel CI engines, a combustion mode gaining increasing attention for replacing fossil diesel with sustainable fuels, particularly in hard-to-abate sectors such as maritime transport. The simulations are based on the Unsteady Reynolds-Averaged Navier–Stokes (URANS) equations, incorporating the RNG k-ε turbulence model, the Eddy Dissipation Concept (EDC) for turbulence–chemistry interaction, and the G-equation for turbulent premixed flame propagation. The numerical model is validated against experimental data for in-cylinder pressure and heat release rate at 45% methanol substitution ratio (by energy). The results indicate that increasing the hydrogen enrichment ratio (HER, defined on an energy basis) from 5% to 20% raises the Sauter mean diameter (SMD) of the diesel fuel from 20.2 µm to 28.0 µm (+38%), driven by reduced aerodynamic breakup intensity associated with modified gas-phase properties under hydrogen enrichment. Furthermore, hydrogen’s elevated adiabatic flame temperature and superior mass diffusivity intensify combustion, raising peak in-cylinder pressure from 75.2 to 79.1 bar (+5.2%), amplifying the peak heat release rate from 129 to 211 J/°CA (+63.6%), and elevating maximum in-cylinder temperature from 1542 to 1735 K (+193 K). Under the investigated CFD operating conditions, these thermodynamic gains translate into an engine-level 6% improvement in indicated thermal efficiency and a 14% reduction in indicated specific fuel consumption (accounting for hydrogen, methanol, and diesel) at HER 20%. On the emissions front, CO₂ declines by 24% in direct proportion to the carbon-containing fuel mass displaced by hydrogen substitution, while NO_x increases approximately twofold from 0.10 g/kWh at HER 0 to 0.21 g/kWh at HER 20, driven by peak temperature elevation. These findings establish hydrogen-enriched methanol–diesel dual-fuel combustion as a viable pathway toward high-efficiency, low-carbon CI engine operation for heavy-duty transport applications.

Hydrogen is widely recognized as a promising zero-carbon fuel for internal combustion engines due to its zero carbon emissions, high laminar flame speed, and wide flammability limits. In dual-fuel compression ignition engines, substituting part of the diesel fuel with hydrogen can significantly reduce carbon-based emissions while maintaining or improving ther-mal efficiency. This study investigates the combined effects of hydrogen substitution ratio (HSR) and equivalence ratio (Φ) on combustion characteristics, engine performance, and emissions in a diesel-hydrogen dual-fuel engine. Three HSR levels (50, 70, and 90) are examined across equivalence ratios ranging from 0.5 to 0.8. A three-dimensional computational fluid dynamics (CFD) model is developed using ANSYS Forte. The model employs an Eulerian-Lagrangian framework, incorpo-rating the unsteady Reynolds-averaged Navier–Stokes (URANS) equations for flow resolution and the G-equation for turbu-lent flame propagation. Spray breakup dynamics are modeled using the Kelvin–Helmholtz/Rayleigh–Taylor approach, while turbulence is captured with the RNG k-ε model. Results indicate that increasing both the HSR and equivalence ratio reduces CO₂ emissions. At HSR 70, unburned hydrocarbon (UHC) emissions decrease by up to 82% as Φ increases from 0.5 to 0.8. Furthermore, the indicated specific fuel consumption (ISFC) decreases, particularly at high HSR, reflecting improved fuel economy. These findings highlight the potential of diesel-hydrogen dual-fuel operation for achieving cleaner and more efficient engine performance.