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

Abstract

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% H 2 + 85% CH 4) 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.