Experimental study on early flame dynamics in an optically accessible hydrogen-fueled spark ignition engine

Amid the urgent global shift toward a low-carbon economy, hydrogen has emerged as a promising carbon-free fuel for internal combustion engines (ICEs) due to its wide flammability limits and potential to reduce greenhouse gas and  emissions. However, lean hydrogen-air flames suffer from thermo-diffusive instabilities caused by their low Lewis numbers, which disrupt flame propagation and complicate combustion control. Compared to hydrocarbon fuels like methane, hydrogen’s higher diffusivity leads to significant differential diffusion effects, especially during early flame kernel development— a key factor in cycle-to-cycle variations. Despite previous numerical studies highlighting these challenges, experimental data under realistic engine operating conditions remain limited, creating a critical gap for validating combustion models.

The study employed high-speed planar sulfur dioxide laser-induced fluorescence (-PLIF) to visualize and analyze early flame dynamics in an optically accessible AVL single-cylinder spark-ignition engine. Researchers compared lean hydrogen () flames with stoichiometric methane () flames— a baseline fuel with near-unity Lewis number— under two operating conditions (OC A and B) differing in intake pressure and turbulence levels. Key flame characteristics were quantified, including curvature, -LIF intensity gradients, flame boundary tortuosity, and equivalent flame speed. The experimental setup ensured homogeneous fuel-air mixing, and image post-processing (including dewarping, filtering, and binarization) enabled high-resolution analysis of flame front evolution across four combustion phases (20%, 40%, 60%, and 80% mass fraction burned).

1. Flame Morphology Differences: Hydrogen flames exhibited more pronounced wrinkling, trough/cusp structures, and higher tortuosity than methane flames under low-turbulence conditions (OC B), driven by thermo-diffusive instabilities. Methane flames, in contrast, had smoother boundaries and lower sensitivity to such instabilities.
2. Curvature-Gradient Correlation: Hydrogen flames showed a stronger correlation between flame curvature and -PLIF intensity gradients (correlation coefficient = 0.52 for OC B) compared to methane flames (0.31), indicating enhanced reactivity at flame troughs and faster local propagation.
3. Turbulence Modulation: Higher turbulence (OC A) increased flame wrinkling and curvature for both fuels, but reduced the relative influence of thermo-diffusive instabilities on hydrogen flames. The tortuosity gap between hydrogen and methane flames narrowed under elevated turbulence.
4. Equivalent Flame Speed: Hydrogen flames consistently achieved higher equivalent flame speeds due to their high thermo-diffusivity. Both fuels accelerated under high turbulence, but hydrogen maintained a propagation advantage across all combustion phases.

This study provides unprecedented high-resolution experimental data on early hydrogen flame dynamics under engine-relevant conditions, filling a critical gap in previous research. The findings clarify the intricate interplay between thermo-diffusive instabilities and turbulence, offering valuable validation for next-generation combustion models. By elucidating how hydrogen’s unique thermo-physical properties shape flame propagation, the research supports the optimization of hydrogen-fueled ICEs— advancing their potential as efficient, low-emission power sources. Ultimately, these insights contribute to the broader transition toward carbon-neutral energy systems by addressing key technical barriers to hydrogen’s widespread adoption in transportation and industry.