Modelling hydrogen enriched ammonia combustion through laminar flame speed and mass fraction burn analysis

Ammonia is a promising zero-carbon fuel for internal combustion engines; however, its low laminar flame speed makes lean combustion highly sensitive to turbulence and flame development. In this study, an entrainment-based two-zone combustion model is developed for hydrogen-enriched ammonia spark-ignition engines. The novelty of the approach lies in calibrating turbulence-related correction coefficients to simulate the combustion phasing using hydrogen-enriched ammonia laminar flame speed values. The results show that hydrogen enrichment accelerates both the early flame development and flame propagation phases due to enhanced chemical reactivity, leading to improved indicated mean effective pressure compared to neat ammonia. For a given excess air ratio, peak IMEP is achieved at 10% hydrogen addition by volume. The developed two-zone combustion model also captured the effects of exhaust gas recirculation through reduced flame speed and changes in thermodynamic properties. At 20% exhaust gas recirculation, nitric oxide emissions decreased by 48%, while the indicated specific fuel consumption increased by 10%.