Application of laser ignition to hydrogen–air mixtures at high pressures

To optimise combustion in a wide field of applications, lasers represent attractive future alternative ignition sources, especially for internal combustion engines. Experiments were carried out in a high pressure, constant volume chamber (up to 25 MPa peak pressure and initial temperature of 473 K). Laser induced ignition of different hydrogen–air mixtures (air/fuel equivalence ratio λ=1.8–8) was investigated, using different filling pressures (p=0.5–4.2 MPa), different ignition energies (pulse energy PE=1–50 mJ), different chamber temperatures (T=393–473 K) and different focal length lenses (f=60, 120 mm). A Q-switched Nd:YAG laser at 1064 nm with a pulse duration of about 5 ns was used for ignition. An InGaAs photodetector (800–1800 nm) and a piezoelectric pressure transducer were used to characterise the combustion. Gas mixtures between λ=2.5 and 3.6 showed knocking combustions. With increasing initial pressures the minimum pulse energy was decreasing.