Development of an Optical Spark Plug for Stationary Engines - A Theoretical Approach 2006-01-0194

Laser-ignition has been a vision as alternative to conventional spark-plug ignition for more than 40 years. Soon after the invention of the laser itself it was discovered that by focusing the light of an intense, pulsed laser beam, a plasma capable of ignition is generated in gases. Numerous experimental studies have been carried out ever since for demonstration. The first and foremost advantage of laser ignition is the virtually free choice of the location of ignition, the deposited energy and the timing.

However, no laser-based ignition system has come to commercial realization. A major reason for this is that only large, expensive laboratory devices have been used in the studies.

In order to develop a commercially viable, compact and purpose-oriented laser-ignition system, modeling was used to investigate the theory behind laser ignition. By understanding the interaction of light with matter and by calculating the minimum required energy for ignition, cost-effective and reliable laser ignition systems can be developed. A simulation tool was created which uses a numerical model based on Direct Numerical Simulation (DNS). It focuses on the first stage of the ignition process when the plasma is created by the laser beam. Especially it calculates the losses due to thermal radiation, which have a negative influence on the ignition probability. In the modeling, fluid mechanical aspects were combined with thermodynamic aspects in a novel approach.

Simulation work with different laser pulse lengths of 0.5 - 10 ns and laser pulse energies of 5 and 10 mJ in N₂ of 1 bar and 293K was carried out. Ignition and combustion were not modeled. The main simulation result is that the laser pulse length has much more influence on the plasma properties than the pulse energy. The losses due to heat radiation were much bigger and the temperatures were much higher with the plasmas generated by shorter laser pulse lengths.

The modeling results were compared to experiments, good agreement was found. As a key result, the asymmetric plasma and flame kernel growth could be understood.