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27 Jun 2007
A US team has for the first time used a nanosecond Nd:YAG hollow-core fiber laser as the ignition source in a commercial natural-gas engine, replacing conventional electrodes.
Laser ignition in stationary gas engines which are typically used for power generation and natural gas compression could play a vital role in achieving increased efficiency and reduced pollutant emissions.
Azer Yalin of Colorado State University and his group tested a variety of fiber-laser configurations to introduce a spark into an 18-liter 6-cylinder natural-gas engine. While laser ignition in laboratory environments has been widely demonstrated in liquids and solids, fiber-optic delivery into gas-filled combustion chambers has proved challenging owing to the high powers needed.
"The laser needs to deliver relatively high-energy pulses above about 10 mJ to ignite the lean fuel-air mixture, while maintaining a high-quality beam focused onto a very small area," Yalin explained to optics.org. "In theory, a conventional solid fiber would require a large diameter to supply the high energies, while the need for a high-quality beam without multiple spatial modes would suggest using a small diameter."
Yalin's group solved this conundrum by using a coated hollow-core fiber supplied by their collaborators at Tohoku University, Japan. The hollow fibers can carry intensities similar to those possible in solid fibers, but with the improved beam quality needed to form sparks (Applied Optics 46 19 4057).
The team's experiment used a 2 m length of hollow-core fiber to deliver a combustion spark into one cylinder of the six-cylinder turbocharged engine, a set-up chosen as a reasonable model for real-world multi-cylinder engines. Using the fundamental 1064 nm beam from a Q-switched Nd:YAG laser delivering 8 ns pulses at 10 Hz, the team achieved a spark efficiency of 98%, meaning that only 2% of laser shots failed to achieve a spark. Under operational engine pressures the efficiency should approach 100%, according to Yalin.
"We are working on improved fiber delivery, along with multiplexing to share the beam to multiple cylinders," he said. "The next step will be multi-cylinder operation of a megawatt-class gas engine, which should lay the groundwork for field prototypes and commercialization."
Using fiber lasers to deliver combustion sparks could enable the laser to be focused into any part of the engine cylinder to reach the optimal spark location. It also avoids the inherent tendency of electrodes to act as heat sinks, which limits their efficiency. Additional applications could become possible in gas-phase laser-induced breakdown spectroscopy (LIBS) once the fiber-delivered sparks have been optimized.
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