Laser sparks hope of reduced auto emissions

Compiled by Photonics Spectra staff

Automakers are now one step closer to replacing spark plugs for internal combustion engines with laser igniters for cleaner, more efficient and more economical vehicles.

Lasers have been discussed as a promising alternative ignition source for efficient internal combustion engines because they promise less pollution and greater fuel efficiency. Until recently, it was difficult to make small, powerful lasers that could focus light to ~100 GW/cm² with short pulses of more than 10 mJ each, needed to ignite combustion.

For more than 150 years, spark plugs have powered internal combustion engines. Now, automakers are one step closer to replacing the plugs with laser igniters, which will enable cleaner, more efficient and more economical vehicles.

Scientists from Japan’s National Institutes of Natural Sciences overcame this problem by making composite lasers from ceramic powders. They heated the powders to fuse them into optically transparent solids, and embedded metal ions in them to tune their properties. Ceramics are easier to tune optically than conventional crystals and are much stronger, more durable and thermally conductive, enabling them to dissipate the heat from an engine without breaking down.

The research team built its laser from two yttrium aluminum gallium segments, one doped with neodymium, the other with chromium. The two segments were bonded together to form a powerful laser only 9 mm in diameter and 11 mm long. The composite generated two laser beams that could ignite fuel in two separate locations simultaneously, producing a flame wall that grows faster and more uniformly than one lit by a single laser.

Not strong enough to light the leanest fuel mixtures with a single pulse, the laser can, however, inject enough energy to ignite the mixture completely by using several 800-ps-long pulses. It was tested at 100 Hz – a commercial automotive engine will require only 60 Hz.

Although promising, the system is not yet being installed into automobiles. The team is working on a three-beam laser that will enable even faster and more uniform combustion. Supported by the Japan Science and Technical Agency, the work was presented at CLEO 2011.