Abstract

The reduction of pollutant emissions and energy consumption represents a central objective in the improvement of internal combustion engines. This ambitious goal can be achieved by a clear optimization of combustion processes as well as ignition mechanisms. Enhanced compression ratios and leaner mixtures allow more efficient engine operation and lower emissions, respectively. Unfortunately, the established electrical spark plug reaches its physical borders and cannot fulfil such requirements. Laser ignition is one of the most promising alternative concepts where the electrical spark plug is replaced by a pulsed laser. The short pulses (~ ns) are focused into the combustion chamber, and thus a plasma is created igniting the gas mixture. The wavelength of the laser radiation affects the plasma formation and the ignition mechanism in a certain manner being topic of our investigations. Recent investigations on the development of a compact, competitive and reliable ignition laser were based on the fundamental wavelength of Nd:YAG (1064 nm) [1]. Since shorter wavelengths allow smaller focal spots the employment of the SH (second harmonic) wavelength could be advantageous with respect to a lower minimum pulse energy (MPE) required for plasma formation. Moreover, the mixture of two wavelengths (1064 & 532 nm) might be beneficial since the green fraction generates a plasma first and the infrared feeds it up due to better plasma absorption in the infrared regime. Adapting a KTP crystal for SHG (second harmonic generation), the emitted green (532 nm) and infrared (1064 nm) light is compared with respect to plasma characteristics and two-color laser ignition. The experimental setup including all relevant components is shown in Fig. 1.

© 2009 IEEE