Hydrogen enrichment of gasoline in spark-ignition engines represents a promising pathway for improving combustion stability and extending lean-burn operating limits while reducing harmful emissions. However, the quantitative relationship between hydrogen energy share and the stability of ultra-lean combustion remains insufficiently understood, particularly in terms of identifying threshold values required to maintain stable engine operation. This study investigates the influence of hydrogen energy share on combustion stability in a hydrogen–gasoline spark-ignition engine operating under ultra-lean conditions. Experimental measurements were performed across a range of hydrogen energy shares and excess air ratios to evaluate combustion characteristics, pressure development, and cycle-to-cycle variability. Combustion stability was assessed using the coefficient of variation of indicated mean effective pressure (COVIMEP), while combustion performance and emission-related indicators were analyzed. The results reveal the existence of a critical hydrogen energy share threshold for stable ultra-lean combustion. When the hydrogen energy share exceeded approximately 30%, stable combustion was maintained even at excess air ratios approaching λ ≈ 2.6, while lower hydrogen shares resulted in increased cyclic variability and unstable combustion behavior. Hydrogen enrichment significantly improved flame propagation and reduced combustion instability, enabling a substantial extension of the lean operating limit. These findings demonstrate that hydrogen addition plays a key role in enhancing mixture reactivity and enabling stable ultra-lean combustion in hydrogen–gasoline spark-ignition engines. The identified hydrogen energy share threshold provides an important reference for the design and optimization of hydrogen-assisted combustion strategies in future low-emission internal combustion engines. • A distinct hydrogen reactivity threshold (∼30% energy share) is experimentally identified in ultra-lean SI combustion. • Lean stability limit extends abruptly from λ ≈ 1.6 to λ ≥ 2.0 beyond the threshold. • Combustion transitions from propagation-limited to propagation-sustained regime. • Post-threshold operation preserves brake thermal efficiency under ultra-lean conditions. • Ultra-lean hydrocarbon emissions are significantly suppressed without NOₓ penalty. • Stability mapping reveals non-linear hydrogen effects rather than incremental improvement.
Puškár et al. (Fri,) studied this question.