<title>Laser-induced ignition characteristics of methane- and hydrogen-air mixtures at high pressures</title>

Abstract

A Nd:YAG laser was employed to ignite methane- and hydrogen-air mixtures to investigate relevant parameters of laser ignition. The lean side ignition limit of methane was found to be at air/fuel-equivalence ratios (λ) of 2.4 applying a laser pulse energy of 50 mJ. It has to be mentioned, however, that above λ = 2.2 only slowest combustions causing weak pressure rises could be observed. Successful ignitions of hydrogen-air mixtures were achieved up to λ = 8 but it was not possible to examine the lean side limit due to weakest pressure rises far below detection limits for λ >8. Despite much lower values of minimum ignition energy for reported hydrogen-air mixtures in the literature, the minimum laser pulse energies examined for ignition are of the same magnitude as for ignition of rich methane-air mixtures lying around 5 mJ. Minimum pulse energy needed for ignition was decreasing with increasing pressure for hydrogen-air mixtures showing the same trend as in case of methane. The ignition delay time for hydrogen at λ = 2.0 could be observed as ~7 ms being 40 times shorter compared to methane at the same air/fuel ratio. Unfavorable transmission losses of laser energy were observed for methane/air mixtures below λ = 2.1 demanding optimized focusing optics and temporal pulse shaping for future laser ignition systems.