The Characteristics of Hydrogen Flame Instability in a Highly Boosted Engine

Hydrogen is a promising alternative fuel for internal combustion engines due to its unique thermodynamics and combustion properties. Its carbon-free combustion and the fact that it can be derived from many abundant sources set it apart from other transport fuel alternatives. However, hydrogen-air flames are characterised by a high burning velocity but relatively lower sensitivity to the smallest-scale eddies. Consequently, the turbulent velocity fluctuation relative to the laminar flame speed, u′/ul, is significantly different from conventional gasoline-fuelled spark ignition engines. In this study, simultaneous pressure and Particle Image Velocimetry (PIV) measurements were employed to investigate the combustion characteristics of lean hydrogen and methane at engine-relevant conditions using the Leeds University Ported Optical (SI) Engine, version 2 (LUPOE). Turbulent flame speeds and burning velocities were derived at spark timings corresponding to pressures between 15 and 20 bar and two turbulence levels of ≈1.25 and ≈2.2 m/s RMS velocities at 750 and 1500 RPM, respectively. Results show that hydrogen offers broader flexibility for ultra-lean, high-efficiency engine operation with reduced knock risk and stronger tolerance to spark timing variation. The higher sensitivity of the flame to turbulence is consistent with its thinner flame fronts and stronger diffusivity. In contrast, methane requires richer mixtures and precise ignition phasing, constraining its lean-burn potential. Methane flames display more modest enhancement, reflecting thicker, more stable flames. Zimont constants were also derived from the Zimont model, showing practically reasonable values. Laminar burning velocities were also derived for stoichiometric hydrogen, methane, propane and lean hydrogen (a=1) at 5 bar, 439K, under mild turbulence less than 0.2 m/s. The values derived from the direct measurement and the density ratio method were very close to CHEMKIN-computed values. Flame wrinkling amplitude and sphericity were employed to characterize the flame wrinkling structure of flames in the wrinkled and corrugated flame regimes. The power spectral density (PSD) of the curvature for the laminar flames shows that the negative and positive curvatures were evenly distributed, while the PSD of wrinkles indicates that the flame wrinkling of all the conditions observed is close to the −5/3 law of the turbulent energy cascade. However, a clear conclusion on the relationship between flame cellularization and turbulent flow eddies in the high-wavelength region will require a higher resolution camera to capture the wave numbers of the smallest eddies.