Development of a laser spark plug and comparative testing

Abstract

A prototype laser spark plug was developed to meet the requirements of an ignition system for lean burn gas engines, especially with respect to high pressure. The laser was designed as a longitudinally diode-pumped, passively Q-switched Nd:YAG - Cr4+:YAG system emitting radiation at 1064 nm. Experimental studies and simulations were carried out to find the optimum laser design parameters. Emphasis was set on the influence of pump beam profile and optimum fiber diameter on achievable pulse energies in the chosen case of fiber-coupling for the pump radiation. Several laser diodes emitting at 808 nm were tested for pumping with pump powers between 70 W and 600 W. Based on experimental results of discretely assembled setups, a monolithic laser/saturable absorber crystal arrangement was designed for the prototype spark plug. The monolithic system yielded pulses at room temperature with energies of up to 23 mJ and duration of about 1 ns at a pump power of 600 W. To gain knowledge about the temperature dependence of the laser and to simulate engine-like conditions, the laser was successfully operated at temperatures up to 200 °C. Additionally, comparative ignitibility studies between laser ignition, electrical spark ignition and Corona ignition were conducted. These experiments were performed with lean methane-air mixtures in a static constant-volume chamber. The results for static experiments clearly show an advantage of the Corona system over the Laser system. The Laser on the other hand outruns the Spark Plug system. However, the practicability of implementing the Corona system in a real dynamic engine while preserving the full scope of its advantages has to be regarded with skepticism. Especially the issues of influence of turbulence on the flammability and the high voltage need accompanied by insulation problems will have to be treated even further.<br />Finally, the capability of the hollow-core photonic bandgap concept for fiber transmission of ignition pulses was tested. It turned out, that the maximum transmittable energy together with satisfying modal properties is too low for laser ignition. The main problems are the low damage threshold of the fiber material and the problematic incoupling into the thin fiber core diameter.