Will laser ignition replace the venerable spark plug?

July 21, 2014 // By Christoph Hammerschmidt
Lowering fuel consumption and reduce the emission of pollutants - these aspects today are at the top of the design goals for new combustion engines. Not only for cars, but likewise for power plants and the like. So far, all concepts aiming at improving the combustion process have the side effect that the pressure in the combustion chamber is increasing significantly and the gas mixture has a much higher speed. This results in technical requirements to the ignition process that today's conventional spark plugs cannot meet any longer. Now, an alternative comes in sight: Laser ignition.

Actually, laser ignition has been discussed as the successor to the century-old spark plug for quite a while. However, a number of technological and economical issues prevented that laser ignition could become competitive. Now a research project at the University of Bayreuth (Germany) will dig deeper into the potential of laser ignition. Within the project, a group of researchers around professor Dieter Brüggemann, chair for Technical Thermodynamics and Transport Processes, will analyse the flow and ignition processes within the combustion chamber at the greatest possible precision.

The focus is on a specific type of 'laser spark plug' called passively Q-switched for their functional principles. These laser spark plugs are known as being very robust and thus withstand the high vibrations and temperatures in a combustion engine. What's more, this type is also relatively cost-effective. It transmits light pulses that succeed each other in extremely short intervals of 60 to 250 microseconds. If such an pulse chain is focused to a certain point in the combustion chamber, light-emitting plasma is generated that has a temperature of almost 100.000 degrees Celsius. Within several hundred nanoseconds, the plasma then cools down, emitting a pressure wave which propagates at supersonic speed within the combustion engine and ultimately causes the gas mixture around the plasma core to ignite.

Under certain conditions this principle enables motors to be driven with a much higher share of air in the combustion chamber - which can lead to better fuel efficiency or, alternatively, to significantly reduction of nitric oxides. According to project coordinator Sebastian Lorenz, a team from Argonne National Laboratory in the US recently has proved evidence that laser ignition bears the potential to reduce nitrogenous exhaust fumes by as much as 70%.

However, many theoretical benefits of this system cannot yet be applied to real-world power plants or engines. Therefore, the Bayreuth group has to acquire basic insights that directly could be used in an industrial environment.