Advances for laser ignition of internal combustion and rocket engines

Abstract

The scope of the PhD thesis presented here is the investigation of theoretical and practical aspects of laser-induced spark ignition and laser thermal ignition. Laser ignition systems are currently undergoing a rapidly development with growing intensity involving more and more research groups who mainly concentrate on the fi eld of car and large combustion engines. This research is primarily driven by the engagement to meet the increasingly strict emission limits and by the intention to use the limited energy reserves more efficiently. For internal combustion engines, laser plasma-induced ignition will allow to combine the goals for legally required reductions of pollutant emissions and higher engine efficiencies. Also for rocket engines laser ignition turns out to be very attractive. A highly reliable ignition system ike laser ignition would represent an option for introducing non-toxic propellants in order to replace highly toxic and carcinogenic hydrazine-based propellants commonly used in launch vehicle upper stages and satellites.<br />The most important results on laser ignition and laser plasma generation, accomplished by the author and, in some respects, enriched by cooperation with colleagues are presented in the following. The emphasis of this thesis is placed on the following issues:<br />- Two-color eects on laser plasma generation - Theoretical considerations about the focal volume concerning plasma generation - Plasma transmission experiments - Ignition experiments on laser-induced ignition - Ignition experiments on thermally-induced ignition - Feasibility study on laser ignition of rocket engines The purpose of the two-color laser plasma experiments is to investigate possible constructive interference effects of driving fields that are not monochromatic, but contain (second) harmonic radiation with respect to the goal of lowering the plasma genreation threshold. Such effects have been found in a number of related processes, such as laser ablation or high field laser physics.<br />Unfortunately, there is no standard de nition for the plasma threshold in the literature. Consequently, a clear de nition of the focal volume is missing. For this reason it was tried to find a theoretical formula for the volume. This formula is based on the assumption that the focal volume encloses the space where the threshold intensity is higher than Ith =I0/2 or, alternatively, Ith = I0/e2.<br />Laser energy transmission is one of the most important loss factors during plasma development by laser-induced optical breakdown and provides important information about the energy contained in the plasma.<br />Hence, a number of plasma experiments were carried out. In our experiments is was found that for decreasing focal volume the plasma threshold energy (MPE) and the energy transmission can be reduced respectively.<br />In order to investigate the possibility if laser-induced ignition can be made more efficient with respect to the laser pulse energy, several ignition experiments were performed. For these experiments a combustion chamber was employed at a fi lling pressure of 11 bar and a temperature of 110°C involving different focal sizes.<br />The thermal ignition experiments were carried out to demonstrate in principle the feasibility of thermal ignition via resonant absorption of IR radiation. By evaluating these results with respect to laser ignition of engines, it is conceivable to employ laser thermal ignition as an innovative ignition mechanism. As in HCCI (homogeneous charge compression ignition) engines and rocket engines, ignition occurs at specifi c elevated pressures and temperatures, it can be assumed that the ignition energies are in the range between 20 to 100 mJ.<br />Furthermore, different laser ignition system concepts were developed and evaluated regarding to their quali cation for rocket engine ignition. As a consequence of its highest rating in our study, resonant ignition should be considered an interesting alternative to laser spark ignition.