High hydrogen-fraction enrichment in a natural-gas spark-ignition engine: CFD analysis of combustion behavior and emissions

The integration of hydrogen into natural gas is considered a promising approach for developing low-carbon combustion systems. In this study, the combustion, performance, and emission characteristics of a hydrogen-enriched natural gas (HENG, NG/H 2 ) spark-ignition engine were numerically investigated using detailed-chemistry computational fluid dynamics simulations in Ansys Forte under a constant equivalence ratio condition (ϕ = 1.1). Simulations were performed on a Kubota WG1605-N-E3 engine for hydrogen blending ratios ranging from 0% to 60% by volume. The results show that hydrogen enrichment up to 50% H 2 accelerates flame propagation and shortens ignition delay, leading to improvements in both indicated mean effective pressure (IMEP) and thermal efficiency. The highest IMEP and thermal efficiency reached approximately 1.06 MPa and 20.5%, respectively. In contrast, CO and CO 2 emissions decreased continuously with increasing hydrogen fraction because of the lower carbon content of the fuel mixture and more complete oxidation characteristics. Although hydrogen addition enhanced flame speed, NO x emissions generally decreased, with a much stronger reduction observed at hydrogen fractions above 50% H 2 . This behavior was associated with lower volumetric energy density, thermal dilution effects, and increased water-vapor formation, which collectively reduced peak in-cylinder temperature and shortened the residence duration of high-temperature regions. At 60% H 2 , the combustion process shifted further into the expansion stroke, resulting in lower peak pressure and reduced thermodynamic effectiveness despite the faster chemical reactivity of hydrogen-rich mixtures. The results suggest that moderate hydrogen enrichment (≤50% H 2 ) provides the most favorable balance between combustion performance, thermal efficiency, and emissions under fixed operating conditions. At higher hydrogen fractions, additional combustion control strategies such as spark timing optimization, lean-burn operation, or exhaust gas recirculation may be required to maintain effective combustion phasing. Overall, the present study clarifies the combined influence of hydrogen-enhanced reactivity and thermodynamic dilution effects on combustion evolution and NO x formation behavior in HENG engines.