A compact high peak power, passively Q-switched, longitudinally diode-pumped laser was specially constructed for laser ignition. Nd:YAG was chosen as laser active medium and Cr 4+ :YAG as the saturable absorber medium. For pumping, a laser diode emitting at 808 nm with an output power of up to 300 W and pulse duration of up to 500 µs was employed. Experimental studies were carried out to find the optimum laser design parameters. These are the output coupler reflectivity, initial transmission of the saturable absorber, doping concentration of the Nd:YAG, oscillator length and the pump light distribution within the Nd:YAG crystal. Single pulses at 1064 nm with energies of 6 mJ and durations under 1.5 ns were achieved in a TEM00 mode.

Due to market demands aimed at increasing the efficiency and the power density of gas engines, existing ignition systems are rapidly approaching their limits. To avoid this, gas engine manufacturers are seeking new technologies. From the viewpoint of gas engine R&D engineers, ignition of the fuel/air mixture by means of a laser has great potential. Especially the thermodynamic requirements of a high compression ratio and a high power density are fulfilled well by laser ignition. Results of measurements on the test bench confirm the high expectations — with a BMEP of 1.8 MPa it was possible to verify NOx values of a non-optimized system of 30 ppm (70 mg/Nm3 @ 5 % O2) with very high combustion stability. Despite this, considerable developmental steps are still necessary to adapt the laser ignition concept fully to desired objectives (especially costs).

Methane-air mixtures at high fill pressures up to 30 bar and high temperatures up to 200°C were ignited in a high-pressure chamber with automated fill control by a 5 ns pulsed Nd:YAG laser at 1064 nm wavelength. Both, the minimum input laser pulse energy for ignition and the transmitted fraction of energy through the generated plasma were measured as a function of the air/fuel-equivalence ratio (λ). The lean-side ignition limit of methane-air mixtures was found to be λ=2.2. However, only λ<2.1 seems to be practically usable. As a comparison, the limit for conventional spark plug ignition of commercial natural gas engines is λ=1.8. Only with excessive efforts λ=2.0 can be spark ignited. The transmitted pulse shape through the laser-generated plasma was determined temporally as well as its dependence on input laser energy and properties of the specific gases interacting. For a first demonstration of the practical applicability of laser ignition, one cylinder of a 1 MW natural gas engine was ignited by a similar 5 ns pulsed Nd:YAG laser at 1064 nm. The engine worked successfully at λ=1.8 for a first test period of 100 hr without any interruption due to window fouling and other disturbances. Lowest values for NOx emission were achieved at λ=2.05 NOx=0.22 g/KWh. Three parameters obtained from accompanying spectroscopic measurements, namely, water absorbance, flame emission, and the gas inhomogeneity index have proven to be powerful tools to judge laser-induced ignition of methane-air mixtures. The following effects were determined by the absorption spectroscopic technique: formation of water in the vicinity of the laser spark (semi-quantitative); characterization of ignition (ignition delay, incomplete ignition, failed ignition); homogeneity of the gas phase in the vicinity of the ignition; and the progress of combustion.

Methane-air mixtures at high fill pressures up to 30 bar and high temperatures up to 200 °C were ignited in a high pressure chamber with automated fill control by a 5 ns pulsed Nd:YAG laser at 1064 nm wavelength. Both, the minimum input laser pulse energy for ignition and the transmitted fraction of energy through the generated plasma were measured as a function of the air/fuel-equivalence ratio (λ). The lean side ignition limit of methane-air mixtures was found to be λ = 2.4. However, only λ < 2.2 seems to be practically usable. As a comparison, the limit for conventional spark plug ignition of commercial natural gas engines is λ = 1.8. Only with excessive efforts λ = 2.0 can be spark-ignited. The transmitted pulse shape through the laser-generated plasma was determined temporally as well as its dependence on input laser energy and properties of the specific gases interacting. For a first demonstration of the practical applicability of laser ignition, one cylinder of a 1 MW natural gas engine was ignited by a similar 5 ns pulsed Nd:YAG laser at 1064 nm. The engine worked successfully at λ = 1.8 for a first test period of 100 hours without any interruption due to window fouling and other disturbances. Lowest values for NOx emission were achieved at λ = 2.05 (NOx = 0.22 g/KWh). Three parameters obtained from accompanying spectroscopic measurements, namely water absorbance, flame emission and the gas inhomogeneity index have proven to be a powerful tool to judge laser-induced ignition of methane-air mixtures. The following effects were determined by the absorption spectroscopic technique: formation of water in the vicinity of the laser spark (semi-quantitative); characterization of ignition (ignition delay, incomplete ignition, failed ignition); homogeneity of the gas phase in the vicinity of the ignition and the progress of combustion.

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