Abstract Hydrogen is widely recognized as an effective combustion enhancer in internal combustion engines due to its high reactivity and wide flammability range. Although numerous studies have reported qualitative improvements in efficiency, combustion stability, and lean-burn capability through hydrogen addition, the quantitative identification of combustion stability limits and minimum hydrogen requirements under ultra-lean operating conditions remains insufficiently addressed. This study experimentally investigates a spark-ignition engine operating on a hydrogen–gasoline dual-fuel concept with port fuel injection of hydrogen and direct injection of gasoline. The engine was tested over a wide air excess ratio range from λ = 1.0 to 2.6 and hydrogen energy shares from 0% to 40%, with the objective of identifying threshold hydrogen concentrations required to maintain stable and efficient combustion. Combustion behavior was evaluated using brake thermal efficiency, in-cylinder pressure analysis, coefficient of variation of indicated mean effective pressure, and gaseous emission measurements.The results reveal the existence of a critical hydrogen energy share between 20% and 30%, beyond which combustion stability is preserved even under ultra-lean conditions. Hydrogen shares of 30–40% enable a substantial extension of the operable lean range while maintaining acceptable efficiency and pressure stability, whereas lower hydrogen fractions provide only marginal stabilization. These findings provide experimentally derived threshold values that support the practical design and control of hydrogen-assisted combustion systems.
Puškár et al. (Tue,) studied this question.
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