Internal combustion engines are likely to remain in widespread use for some time yet, but it's possible that we may be bidding adieu to that most iconic of engine parts, the spark plug. Researchers from Japan's National Institutes of Natural Sciences (NINS) are creating laser igniters that could one day replace spark plugs in automobile engines. Not only would these lasers allow for better performance and fuel economy, but cars using them would also create less harmful emissions.
Located at the top of each engine cylinder, spark plugs send a high-voltage electrical spark across a gap between their two metal electrodes. That spark ignites the compressed air-fuel mixture in the cylinder, causing a controlled mini-explosion that pushes the piston down.
One byproduct of the process are toxic nitrogen oxides (NOx), which pollute the air causing smog and acid rain. Engines would produce less NOx if they burnt more air and less fuel, but they would require the plugs to produce higher-energy sparks in order to do so. While this is technically possible, the voltages involved would burn out the electrodes quite quickly. Laser igniters on the other hand, could ignite leaner mixtures without self-destructing because they don't have electrodes.
The NINS scientists also address another limitation of spark plugs – the fact that they only ignite the area of the air-fuel mixture closest to them (the top), with much of the heat of the explosion being absorbed by the metal cylinder walls before it can reach down to the piston. Lasers, by contrast, could focus their beams into the middle of the column, from which point the explosion would expand more symmetrically – and reportedly up to three times faster than one triggered by a spark plug.
Additionally, engine timing could be improved, as lasers can pulse within nanoseconds, while spark plugs require milliseconds.
In order to cause the desired combustion, a laser would have to be able to focus light to approximately 100 gigawatts per square centimeter with short pulses of more than 10 millijoules each. Previously, that sort of performance could only be achieved by large, inefficient, relatively unstable lasers. The Japanese researchers, however, have created a small, robust and efficient laser that can do the job. They did so by heating ceramic powders, fusing them into optically-transparent solids, then embedding them with metal ions in order to tune their properties.
Made from two bonded yttrium-aluminum-gallium segments, the laser igniter is just 9 millimeters wide and 11 millimeters long. It has two beams, which can produce a faster, more uniform explosion than one by igniting the air-fuel column in two locations at once – the team is even looking at producing a laser with three beams. While it cannot cause combustion with just one pulse, it can do so using several 800-picosecond-long pulses.
So far, the laser-ignition system hasn't been installed in an actual automobile. The scientists are reportedly in negotiations with a large spark plug manufacturer and with global auto components manufacturer DENSO Corporation.
In the meantime, drivers wishing an upgrade from their "old school" spark plugs might be interested in Pulse Plugs, which reportedly boost engine efficiency and performance by storing ignition energy, then discharging it in the form of intense plasma balls.
The NINS research will be presented next month at the Conference on Lasers and Electro Optics, in Baltimore.
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