Laser induced ignition of gasoline direct injection engines

A q-switched Nd:YAG laser as well as an excimer laser with an unstable resonator have been used for ignition of combustion processes. Following first experiments with a combustion bomb a gasoline direct injection engine has been modified for laser ignition by installation of a focusing element and a beam entrance window. It was possible with the q-switched Nd:YAG laser which delivers short pulses with a duration of less than 6 ns to ignite the engine for several 100 hours without problems. Compared to conventional spark ignition, laser ignition allows a more flexible choice of the ignition location inside the combustion chamber with the possibility to ignite even inside the fuel spray. Measurements of fuel consumption and emissions prove that laser ignition has important advantages compared to conventional spark ignition systems. Experiments with the direct injection engine have been carried out at the fundamental wavelength of the Nd:YAG laser as well as with a frequency doubled system. No differences in the minimal pulse energy needed for ignition could be found, since the minimal pulse energy for ignition is mainly determined by the ablation thresholds of combustion deposits at the surface of the window to the combustion chamber. Such combustion deposits reduce the transparency of the window where the laser beam enters the combustion chamber and a "self-cleaning" mechanism of the window by ablation is essential for successful operation. Experiments show that above a certain threshold intensity of the laser beam at the window even highly polluted surfaces could be cleaned with the first laser pulse which is important for operation in real-world engines. Theoretically calculated energy values for laser ignition are much lower since such mechanisms are usually not considered. Power and space requirements on possible future developments of laser ignition systems are discussed briefly. Several concepts for laser ignition, like diode-pumped solid state lasers (DPSS) with and without fiber coupling are presented and chances on realization are discussed.