Lasers have been discussed as a promising alternative ignition source for efficient internal combustion engines since the mid 1970s. Laser ignition promises less pollution and greater fuel efficiency, but making small, powerful lasers has proven difficult. To ignite combustion, a laser must focus light to approximately 100 gigawatts per square centimeter with short pulses of more than 10 millijoules each.
At this year’s Conference on Lasers and Electro Optics (CLEO: 2011), to be held in Baltimore 1 - 6 May, researchers from Japan will describe a multibeam laser system small enough to screw into an engine’s cylinder head. The new laser system is made from ceramics, and could be produced inexpensively in large volumes, according to one of the presentation’s authors, Takunori Taira of Japan’s National Institutes of Natural Sciences.
|If engines ran leaner, they would produce lower NOx emissions. Spark plugs can ignite leaner fuel mixtures, but only by increasing spark energy. However, these high voltages erode spark plug electrodes faster, making such a solution not economical. By contrast, lasers have no electrodes and are not affected.|
|Conventional spark plugs sit on top of the cylinder and only ignite the air-fuel mixture close to them. The relatively cold metal of nearby electrodes and cylinder walls absorbs heat from the explosion, quenching the flame front just as it starts to expand.|
|Using a laser, the ignition plasma may be located anywhere within the combustion chamber. Optimal positioning of ignition apart from the cold cylinder wall allows the combustion flame front to expand rapidly and uniformly in the chamber and thus increases the efficiency. Without quenching, the flame front expands more symmetrically and up to three times faster than those produced by spark plugs.|
|Equally important, Taira says, lasers inject their energy within nanoseconds, compared with milliseconds for spark plugs. In addition, laser ignition has great potential for simultaneous, spatial multipoint ignition within a chamber. This shortens combustion time dramatically and improves the output and efficiency of engines effectively.|
|Further a laser can ignite leaner or high pressure mixtures that are difficult to be ignited by a conventional electric spark plug. A laser igniter is also expected to have a longer lifetime than a spark plug due to the absence of electrodes.|
In the past, lasers that could meet those requirements were limited to basic research because they were big, inefficient, and unstable.—Takunori Taira
Taira’s research team overcame this problem by making composite lasers from ceramic powders. The team heats the powders to fuse them into optically transparent solids and embeds metal ions in them to tune their properties. Ceramics are easier to tune optically than conventional crystals. They are also much stronger, more durable, and thermally conductive, so they can dissipate the heat from an engine without breaking down.
Taira’s team built its laser from two yttrium-aluminum-gallium (YAG) segments, one doped with neodymium, the other with chromium. They bonded the two sections together to form a powerful laser only 9 millimeters in diameter and 11 millimeters long.
The composite generates two laser beams that can ignite fuel in two separate locations at the same time. This would produce a flame wall that grows faster and more uniformly than one lit by a single laser.
The laser is not strong enough to light the leanest fuel mixtures with a single pulse. By using several 800-picosecond-long pulses, however, they can inject enough energy to ignite the mixture completely.
A commercial automotive engine will require 60 Hz (or pulse trains per second), Taira says. He has already tested the new dual-beam laser at 100 Hz. The team is also at work on a three-beam laser that will enable even faster and more uniform combustion.
The laser-ignition system, although promising, is not yet being installed into actual automobiles made in a factory. Taira’s team is, however, working with a large spark-plug company and with DENSO Corporation, a member of the Toyota Group.
This work is supported by the Japan Science and Technical Agency (JST).
Masaki Tsunekane, Takayuki Inohara, Kenji Kanehara and Takunori Taira, “Micro-Solid-State Laser for Ignition of Automobile Engines” in Advances in Solid-State Lasers: Development and Applications, Mikhail Grishin, ed. ISBN 978-953-7619-80-0, pp. 630, February 2010, INTECH, Croatia