An optical and spectroscopic study of laser-induced sparks to determine available ignition energy

Abstract

This work conducts an optical and spectroscopic study of laser-induced sparks created in air. It is aimedat determining the minimum ignition energy associated with laser spark ignition. Gas breakdown was produced using a single-mode, Q-switched Nd:YAG laser. It produced a 0.6 cm diameter beam at a wavelength of 1064 nm with a 5.5 ns pulse duration. The beam deflection and the absolute two-line intensity ratio techniques were used for measuring the spark expansion and temperature. The results were used to determine the shock propagation energy, the radiation energy losses, and the energy of the hot gas that is left over after the shock has been propagated away. With spark energies ranging from 15 to 50 mJ, we found that the shock energy was from approximately 70% to 51%, the radiation energy losses were from 22% to 34%, and the energy of the remaining hot gas was about 7% to 8% of the spark total energy. As far as ignition is concerned, the shock propagation and the radiation energies are wasted. The energy of the hot gas is the energy source that causes ignition. Thus, within the limits of error of the present measurements and calculations, the ignition energy obtained by the laser spark ignition does not differ greatly from that obtained by the electric spark ignition as has been reported.