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Spark plugs in petrol engines are set to be replaced by laser ignition systems, following development of new manufacturing techniques by Japanese boffins.

The new research is to be presented to the world at an optics conference in Baltimore next month by Takunori Taira of Japan's National Institutes of Natural Sciences and his colleagues. Taira and his team have been working with spark-plug firms and Toyota subsidiaries to address the problems facing ordinary petrol-engine ignition.

At the moment, as any automotive fule kno, the fuel-air mixture in a petrol engine's cylinder is ignited by a plug which generates a spark between its electrodes using high-voltage electricity. The spark plug is the limiting factor on how "lean" - how low in fuel - the mixture can be: in order to ignite a leaner mixture the spark must be hotter, and past a certain point this destroys the electrodes.

But designers would like to make leaner-running engines as this would improve fuel economy and cut down on emissions.

Lasers would potentially offer hotter ignition, and they have other advantages too. The timing of ignition would be more precise than with sparkplugs - on the very brief timescales over which cylinder mixtures change, the exact point at which a plug will spark is quite unpredictable.

“Timing – quick combustion – is very important," says Taira. "The more precise the timing, the more efficient the combustion and the better the fuel economy.”

Ignition by photon beam would also offer the option of focusing the hot spot in the centre of the cylinder rather than generating it at the top. This would allow the "flame front" of the exploding mixture to spread out in all directions and act more efficiently.

All in all, then, laser-ignition engines would be a boon. But the trouble thus far has been that the necessary laser machinery is large. It can't practically be deployed at the cylinder head, and there's no option to pipe the laser pulses in as they are of such frequency and energy as to destroy optical hardware.

“In the past, lasers that could meet those requirements were limited to basic research because they were big, inefficient, and unstable,” Taira says.

But not any more. The Japanese boffins have come up with a way of making small, high-power lasers out of ceramics. Their protoype unit is made from two yttrium-aluminum-gallium (YAG) segments, one doped with neodymium, the other with chromium. It is just 9mm in diameter and 11mm long, and by using bursts of pulses less than a nanosecond in duration it can ignite a volume of lean fuel-air mix at two points simultaneously - this develops power more quickly and efficiently than lighting at just one point.

The ceramic laser material is thought to be easily tough enough to stand up to conditions in a running car engine. The tech ought to permit much more efficient, lower-emission petrol cars in years to come - perhaps swinging the pendulum back towards petrol and away from diesel engines, which in recent times have become the fuel-sipping low-emissions choice.

As petrol is somewhat cheaper than diesel to buy, and petrol engines are somewhat cheaper to make, this could be very popular.

Taira and his colleagues will present full details of their work at the 2011 Conference on Lasers and Electro-Optics in Baltimore on 2 May. ®

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