Impact of injection parameters on hydrogen combustion: Flame propagation and stability in ultra-lean mixtures

Hydrogen is a promising fuel for the decarbonization of internal combustion engines (ICEs) and is in line with global efforts towards climate neutrality. In this study, the combustion behavior of hydrogen in a single-cylinder spark engine was investigated under lean conditions (λ = 2.0–2.7) and at engine speeds of 2000 and 3000 rpm. Optical diagnostics and in-cylinder pressure analysis were used to evaluate the influence of lambda, modulated by injection duration (DOI), and spark timing (ST) on flame propagation and combustion stability. At 2000 rpm, an increase in DOI and thus a change in lambda from 2.7 to 2.0 led to a steady increase in flame front velocity, from 14 to 28 m/s at peak, and reduced the crank angle to reach the optical limit by about 10°. These results underline the critical role of DOI control in optimizing combustion under low turbulence conditions. In contrast, at 3000 rpm, where turbulence was significantly higher, DOI and thus lambda variations had a negligible effect on flame speed, suggesting that turbulence dominates combustion behavior at higher engine speeds. These results highlight the potential of parameter tuning to improve both the stability and efficiency of hydrogen-fueled engines. This study provides useful optical data under realistic engine conditions for ultra-lean hydrogen combustion and provides quantitative insights that support the optimization of hydrogen ICEs. The results help to bridge the gap between experimental research and simulation and contribute to the development of cleaner and more sustainable energy systems.