Laser ignition

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The laser ignition provides a novel ignition system for internal combustion engines is, the principle of which is the beam of a pulsed laser by means of suitable lens optics within a combustible mixture in the combustion chamber to such focus that the resulting plasma , the fuel - air mixture ignites and replace such a conventional spark plug can.

Advantages of laser ignition

The reason for the search for alternative ignition sources , in particular for stationary gas engines in the MW output range, lies in the aim of meeting the increasingly strict emission guidelines and counteracting the increasing consumption of primary energy. From an engine point of view, this means an increase in the thermodynamic efficiency with a combustion of the fuel with as little nitrogen oxide as possible. The efficiency of an internal combustion engine increases with the compression ratio ε and the air ratio λ. Combustion with excess air (high λ) results in a lower flame temperature and thus also reduced NO x formation . Engines that follow this concept are called supercharged lean-burn engines.

An increase in the compression ratio also implies an increase in the ignition pressure and this in turn results in an increased breakdown voltage in conventional electrical ignition systems. This increased voltage , according to the Paschen-Back law , results in more pronounced electrode erosion, which drastically reduces the life of a candle. In addition, extremely lean fuel mixtures are extremely unwilling to ignite and therefore require geometrically optimal ignition conditions that cannot be guaranteed by electrical spark ignition. In this case, laser ignition as a new, innovative concept offers a number of promising advantages over conventional electrical ignition :

  • Ignition of extremely lean mixtures is possible → reduced flame temperature → reduction of NO x emissions
  • No electrical erosion effects on the spark plug → longer service life
  • Higher compression ratios possible → increase in efficiency → lower consumption
  • Any choice of focus → in the vicinity of the center of the combustion chamber possible to ensure optimal flame development.
  • No flame extinction effects (quenching) on ​​the electrodes of the spark plug → ignition of lean mixtures possible.

technical requirements

The minimum plasma energy, the analogue of the breakdown voltage, decreases with increasing pressure. Plasma formation during laser ignition is based on the phenomenon of non-resonant breakdown (i.e. the gas mixtures to be ignited do not absorb the laser radiation), for which optical intensities of about 10 11 W / cm 2 are necessary. In general, such an intensity can be achieved by focusing pulses with pulse energies of a few mJ and pulse durations of a few ns. However, in order to guarantee reliable ignition of the mixture, the pulse energy of the laser beam must be above the minimum ignition energy of the fuel-air mixture. Seen in detail, the minimum pulse energy depends strongly on the temperature and the air ratio and is in the range of 8 to 12 mJ.

Both conditions (plasma and ignition energy) must now be met for reliable combustion. In addition to the required output power of over 10 mJ with a duration of ~ 1 ns, the ignition laser must also be manufactured in a compact, robust and inexpensive design. Detailed aspects of laser ignition and its application can be found in.

Other potential areas of application for laser ignition

Laser ignition is also of great interest in aerospace for attitude control thrusters and liquid rocket engines , as current ignition systems are based on self-igniting toxic fuels such as hydrazine or nitrogen tetroxide or are heavier and more complex due to additional fuel supplies and valves. The trend towards “green propellants” (e.g. ethanol - oxygen ) implies an ignition system. Due to the electromagnetic interference , conventional electrical ones are only of limited interest. Furthermore, there are already studies on laser ignition of gas turbines .

Individual evidence

  1. ↑ R. Knystautas, JH Lee: Laser spark ignition of Chemically reactive gas. In: American Institute of Aeronautics and Astronautics Journal. Vol. 7 (2), 1969, pp. 312-317.
  2. ↑ JD Dale, PR Smy, RM Clements: Laser Ignited Internal Combustion Engines - An Experimental Study. SAE Technical Paper Series, Vol. 780329, (1978).
  3. ^ PD Ronney: Laser versus conventional ignition of flames. In: Optical Engineering. Vol. 33 (2), 1994, pp. 510-521.
  4. ↑ a b H. Kopecek, H. Maier, G. Reider, F. Winter, E. Wintner: Laser ignition of methane-air mixtures at high pressures. In: Experimental Thermal and Fluid Science. Vol. 27, 2003, pp. 499-503.
  5. ↑ a b G. Herdin: Gas Engines: Potentials and Future. Proceedings of DVV Colloquium, 2004.
  6. ^ A b G. Herdin, J. Klausner, E. Wintner, M. Weinrotter, J. Graf, K. Iskra: Laser Ignition: A New Concept to Use and Increase the Potentials of Gas Engines. Proceedings of ASME, Ottawa, Canada, Vol. ICEF 2005-1352, (2005), pp. 1-9.
  7. ↑ M. Weinrotter, H. Kopecek, E. Wintner: Laser ignition of engines. In: Laser Physics. Vol. 15 (7), 2005, pp. 947-953.
  8. ↑ J. Tauer, H. Kofler, E. Wintner: Laser-initiated ignition. 2009, doi: 10.1002 / lpor.200810070 .
  9. ↑ M. Lackner (Ed.): Lasers in Chemistry: Probing and Influencing Matter. Wiley-VCH, Weinheim 2008, ISBN 978-3-527-31997-8 , p. 1554.
  10. ↑ Michael Börner, Chiara Manfletti, Michael Oschwald: Laser Re-Ignition of a Cryogenic Multi-Injector Rocket Engine . July 1, 2015 ( researchgate.net [accessed September 6, 2016]).
  11. ^ Matt Thomas, John Bossard, Jim Early, Huu Trinh, Jay Dennis: Laser Ignition Technology for Bi-Propellant Rocket Engine Applications . January 1, 2001 ( nasa.gov [accessed September 7, 2016]).
  12. ↑ Chiara Manfletti, Michael Börner, Gerhard Kroupa and Sebastian Soller: You have to get into space first - future launch vehicles will ignite with lasers. German Aerospace Center (DLR), June 2016, accessed on September 6, 2016 .