Laser ignition of ultra-lean methane/hydrogen/air mixtures at high temperature and pressure

Laser-induced spark ignition of lean methane-hydrogen-air mixtures and multi-point laser-induced spark ignition of hydrogen-air mixtures were investigated using a Q-switched Nd:YAG laser at 1064 nm with a pulse duration of about 5 ns. Experiments were carried out under engine like conditions in a high pressure, constant volume chamber (up to 25 MPa peak pressure) at an initial temperature of 473 K and initial pressures up to 3 MPa. Both, ignition and combustion were characterized by measuring the transient pressure behavior and laser pulse energy. At the lean limit of methane-air mixtures hydrogen addition was used to improve combustion characteristics (flame speed, peak pressure). At an air/fuel equivalence ratio of λ = 1.9 a hydrogen addition of 15% (i.e. fuel gas: 15% H2 + 85% CH4) increased the peak pressure 12 times in comparison to methane-air mixtures and the combustion time was shortened. Beside hydrogen addition multi-point ignition is another way to accelerate the combustion which is important for engine applications. For the two point ignition experiments time until peak pressure was reduced to 50% in comparison to one point ignition. Three point ignition was carried out with a diffractive lens (f = 62 mm) which separated the laser beam into three focal points with a mutual distance of about 5 mm. No improved combustion characteristics were observed.