Conventional gasoline engine makers may one day replace spark plugs with lasers, as Japanese researchers have designed and prototyped laser devices that are powerful enough to ignite the fuel and small enough to fit into the engine cylinder head (9 millimeters in diameter and 11 millimeters in length).
So far, lasers having that much power have been extremely large, and the possibility of embedding them under a car’s hood was out of the question. Trials that involved sending the laser through optical fibers failed, because the heat melted them. The new multibeam laser system, though, is made from ceramics, which are highly suited to high temperatures.
The advantages of a laser-ignited engine are the reduction of fuel consumption and the reduction of NOx emissions, because the fuel is burnt better and the air-fuel mixture is leaner (meaning that there is more oxygen available in the ignition chamber).
If it were to burn leaner on a spark plug-equipped engine, the plug’s electrodes, then the spark would have to be bigger, and so would the voltage on them. No problem with that in theory, but the reality is that the electrodes would get eroded very fast.
One other aspect and advantage of using the laser spark plug is that the firing time is reduced by several orders or magnitude, from milliseconds to nanoseconds, which translates into more precisely time-targeted explosions, associated with the a high spatial precision. With the laser plug, the explosion will be triggered right in the middle of the chamber, and not close to the top, where the nearby metal would dissipate much of the explosion’s efficiency.
Lasers this small had been hard to obtain in the past because of the power needed for such an application: 100 gigawatts per square centimeter with short pulses of more than 10 millijoules each. The pulse frequency needed for a regular automobile would be no more then 60 Hz, but Takunori Taira of Japan’s National Institutes of Natural Sciences says they tried it to 100 Hz successfully. By using several 800-picosecond-long pulses they can inject enough energy to ignite the mixture completely.
The invention will be presented at this year’s Conference on Lasers and Electro Optics in Baltimore from May 1 to 6. Toyota seems interested in the technology, but has not yet implemented the technology in any car engine.