Spark plugs have powered internal combustion engines for 150 years. Leave it to the Japanese to develop a way to replace spark plugs with lasers.
In the past, lasers that are strong enough to ignite an engine’s air-fuel mixtures were too big to fit under an automobile’s hood. At this year's Conference on Lasers and Electro Optics (CLEO), to be held in Baltimore May 1 - 6, researchers from Japan will display the first multibeam laser small enough to screw into an engine's cylinder head.
Of equal importance is the fact that the new laser system is made from ceramics, and could be produced cheaply in large volumes, according to one of the presentation's authors, Takunori Taira of Japan's National Institutes of Natural Sciences.
Yeah, Japanese scientists rule.
Taira says that conventional spark plugs are a barrier to improving fuel economy and decreasing emissions of nitrogen oxides (NOx), a key component of smog.
In case you didn’t know, spark plugs work by sending small, high-voltage electrical sparks across a gap between two metal electrodes. The spark burns the air-fuel mixture in the engine's cylinder—producing a controlled explosion that forces the piston down to the bottom of the cylinder, generating the horsepower required to move the vehicle.
Engines produce NOx as a result of combustion. If engines ran leaner – burnt more air and less fuel – they would make considerably smaller NOx emissions.
Spark plugs are able ignite leaner fuel mixtures, but only by increasing spark energy. Unfortunately, these high voltages wear away spark plug electrodes so fast, the solution is not cost-effective. By contrast, lasers, which ignite the air-fuel mixture with focused optical energy, have no electrodes and are not affected.
Lasers also increase efficiency. Traditional spark plugs sit on top of the cylinder and only explode the air-fuel mixture close to them. The fairly cold metal of nearby electrodes and cylinder walls sucks up heat from the explosion, satisfying the flame front just as it starts to swell.
Lasers can focus their beams right into the middle of the mixture. Without quenching, the flame front expands more evenly and up to three times faster than those produced by spark plugs.
Taira says that lasers inject their energy within nanoseconds, compared with milliseconds for spark plugs: “Timing – quick combustion – is very important. The more precise the timing, the more efficient the combustion and the better the fuel economy.”
Lasers offer less pollution and superior fuel efficiency, but making small, powerful lasers has, until now, been quite hard. To ignite combustion, a laser must focus light to around 100 gigawatts per square centimeter with short blasts of more than 10 millijoules each.
“In the past, lasers that could meet those requirements were limited to basic research because they were big, inefficient, and unstable,” Taira says. They couldn’t be located away from the engine either, because their powerful beams would destroy any optical fibers that brought light to the cylinders.
Taira’s research team fixed this problem by making complex lasers from ceramic powders. The team heats the powders to temper them into optically see-through solids and implants metal ions in them to tune their properties.
Ceramics are easier to tweak optically than regular crystals. They are also much stronger, more durable, and thermally conductive, so they can disperse the heat from an engine without malfunctioning.
Taira’s team assembled its laser from two yttrium-aluminum-gallium (YAG) segments, one doped with neodymium, the other with chromium. They joined the two sections together to form a powerful laser only 9 millimeters in diameter and 11 millimeters long (a bit less than half an inch).
The composite creates two laser beams that can ignite fuel in two separate places 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 powerful enough to light the thinnest fuel mixtures with a single pulse. By using several 800-picosecond-long pulses, however, they can insert 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, though highly promising, is not yet being put into actual automobiles made in a factory. Taira’s team is, nonetheless, 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). These guys are working on revolutionizing the automobile industry. Kudos to them.
Information provided by: The Optical Society.