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LASERS
It’s been around in one form or another for over a century, but is the trusted spark plug about to be replaced with all-new, cutting-edge laser technology? WORDS: KEITH WORDS: KEITH READ
ore than 100 years after Gottlob Honold, a powertrain engineer working for Robert Bosch, took the then-40-year-
and prototype systems is being revealed. Some of the latest key developments in laser-ignition systems have been highlighted in recent conferences on lasers and electro-
old concept of spark ignition for IC engines and invented the first commercially viable high-voltage spark plug, the automotive industry is poised to see a dramatic new alternative: lasers are set to breathe new life into gasoline engine design, offering increased efficiency levels, leaner combustion and substantially reduced emissions. However, similar benefits could be achieved even sooner with the interestingly named, Corona ignition system. In major automotive centers around the world, secret research and development into laser ignition – supported by leading OEMs and Tier 1 suppliers – has been quietly paving the way to practical production-possible
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optics in both the USA and Europe. One of the scientists heavily involved in the research is Dr Takunori Taira, associate professor with the National Institutes of Natural Sciences’ Institute for Molecular Science in Okazaki, Japan. Essential Ess entially, ly, his study is into micro m icro solid-state photonics based on Nd- or Ybdoped solid-state lasers and non-linear frequency conversion. In 2003, Taira was approached by Kenji Kanehara, whose career started with Denso and who is now with Nippon Soken. Kanehara had a vision of lasers being used to pulse intense heat into engine cylinders to ignite the fuel/air mixture. Only one year after the two engineers met, a program of collaboration
systems. As car the with announcement of thegasoline first production a laser-ignition engine moves ever closer, more and more information about the experimental advances
work started,Taira. but it“Laser-based has been a challenging task, admits ignition has been investigated for a long time – since just after the laser was invented,” he explains.
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// September 2011 // Engine Technology International.com
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TESTING CORONA Federal-Mogul Corporation has developed a replacement for the traditional spark plug, which is already in trials with customers. The system – known as Corona ignition – offer s improved fuel economy and could be in production in three years’ time according to Kristapher Mixell, senior manager of advanced t echnology. A number of major OEMs are currently undertaking trial runs with Corona, and their responses will dictate the start of production. Corona provides a much higher energy capability than spark plugs, creating streams of ions that ignite the fuel mixture throughout the combustion chamber. With more-complete combustion, leaner fuel mixtures containing more air – or recirculated exhaust gases – can be employed, thereby reducing fuel consumption and CO 2 to levels well below those achievable with spark ignition. Corona also allows engineers to develop combustion strategies such as stratified charge and dilute combustion for further reduced fuel consumption and CO 2 emissions. Corona works by using a high frequency field to produce repeatable, controlled ionization, distributed over a large area of the combustion chamber. FederalMogul has optimized the system to make it simpler t o implement in high-volume applications, avoiding many of the issues encountered by other industry researchers. It
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also used materials already proven in automotive applications, so it is robust and durable. Corona is designed to replace traditional coil and plug systems, with little or no impact on engine design or on assembly sequences. The beauty of the technology is that it suits alternative fuels and mixtures with high ethanol content, as it more easily accommodates the different characteristics. Federal-Mogul studied a number of alternative ignition systems before concluding, in consultation with some of its customers, that Corona was worth developing as ‘a game-changing ignition strategy’. The concept has b een around in patent form since the 1960s, and offers many possibilities, explains Mixell. “ We’d been studying it for some time. In 2007, we start ed studying it a bit more in depth and really started intensive R&D a couple of years ago.” He says a lot has been done to minimize the execution risks. “We’ve largely gone with proven materials and manufacturing processes as opposed to starting with the lab setting ideals and trying to productionize them.” Exact costings are unavailable. “But from a value perspective it will be competitive on a cost-benefit relationship,” says Mixell. Any cost premium would be recovered by reduced fuel consumption and the bonus of being environmentally greener through reduced emissions.
Above and below: Federal-Mogul’s innovative Corona ignition is being tested by several car manufacturers
LASERS
“The main challenge has been the downsizing of lasers to the size of a normal spark plug”
“This is because with laser ignition it is possible to realize ideal combustion. However, it has been limited to basic research because those early giant pulse lasers are bigger than our automobiles! The main challenge has been the downsizing of lasers to the size of a normal spark plug.”
Denso is one of the Tier 1 suppliers seriously investigating laser ignition
Benefits aplenty
Despite the vast size of the lasers used in early research, the work confirmed the potential of laser ignition. Subsequent investigations by Taira and his team concluded that the most suitable design for engine ignition would be an Nd:YAG laser, passively Q-switched by a Cr4+YAG saturable absorber. Although a sidepumping geometry could be used, an end-pumping system would ensure the compact dimensions of the laser that’s required for automotive application. The incentive for Taira and his fellow researchers to overcome the many challenges presented during their work has remained, but they keep going mainly because of the huge benefits that laser ignition offers gasoline engines. “Gasoline is the highest energy density – two orders higher than the Li-ion battery,” he explains. “However, its combustion efficiency is still not so high. If we see [gasoline fuel continuing into] the future, we have to improve the engine as a lean-burn, high-pressure system to save gasoline to reduce Thean spark plug, as we know and it, could neveremissions.” work in such engine, Taira maintains: “That is why everybody is investigating new ignition technology. The advantages of
laser ignition is that it offers multipoint ignition, multipulse ignition, high-energy (or short-time energy injection) ignition, flexible-position ignition, and it is free from metal electrodes [that erode].” All these, he says, will facilitate the lean mixtures necessary for increased efficiency and reduced emissions. “In addition, photonics ignition is free from EMI noise,” he adds. Unlike spark plugs, which ignite only the fuel mixture near the spark gap, lasers can focus two or three beams into engine cylinders at variable depths to achieve a more complete combustion. And, unlike spark plugs, there is no erosion of electrodes to adversely affect combustion. During his high-level research, Taira demonstrated that the cross-section area of a flame kernel generated by laser ignition is three times larger than that by a conventional spark plug, at 6ms after ignition in a stoichiometric mixture (air/fuel 15.2) of C3H8/air, even though ignition energy of the laser is one-third that of the spark plug. And 100% ignition was successfully demonstrated in a lean air/fuel mixture of 17.2 by laser ignition – a scenario where the spark plug failed. To overcome the problem of powerful laser beams destroying any optical fibers used to deliver light to the cylinder, Taira has been working on a system using ceramic powders pressed into containers roughly the same size and shape as a spark plug. Unlike the delicate crystals typically used in high-power lasers, the ceramics are robust and better able to withstand the heat within engine cylinder blocks. Forecasts regarding the likely timing of productionready laser-ignition systems vary. None of those involved in the research want to make predictions, as the decisions will inevitably be made by the large companies sponsoring R&D, and be based on the commercial viability of a replacement for spark plugs. However, Kanehara says his dream has always been “to drive a car fitted with laser ignition”. Hopefully the industry will not have to look too far into i nto the future for the 55-year-old to realize his dream. Leaner operation
Taira says that research has been challenging, but creating a laser ignition system will be well worth it
Meanwhile, extensive work at Argonne – the first science and engineering research laboratory in the USA – has shown that natural gas engines can be run much leaner by replacing spark plugs with a laser-ignition system. As well as having reduced fuel consumption (due to more efficient combustion), the lean-operation limit – the lowest possible fuel-to-air ratio for smooth engine operation – can be substantially extended, resulting in additional fuel savings. Also, with laser ignition the coefficient of variation of indicated mean effective pressure (COV IMEP) – the industry yardstick used to gauge ignition quality –
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remains low. And – so goes the formula – the lower the CVO IMEP, the more reliable the ignition. During development work, Argonne scientists noted a 70% reduction in NOx emissions for a given efficiency. Across the Atlantic from Argonne, Ford, through its UK engine R&D center and the University of Liverpool’s engineering faculty, has also achieved impressive emissions reductions with experimental laser-ignition systems on production Ford gasoline engines. Collaborative work in the university’s powertrain control laboratory and the laser engineering group has demonstrated nine key potential benefits of laser ignition: reduced fuel consumption through leaner operation; reduced CO2, uHC and NOx; a decrease in CO emissions; lower idle speeds; better cold-engine performance; and the absence of electrodes (no intrusion in i n cylinder). The other advantages of lasers according to Ford include quicker and more stable combustion, possibility of larger valves, the enabling of GDI by a control position of ignition focal point in the cylinders and possible multiple ignition points (time and space) sp ace) for improved combustion control. Dr Tom Shenton, one of the leaders of the research project, says a further advantage of laser ignition is that some of the laser can be reflected back from inside the cylinder to provide information for the car on the type of fuel being used, and the level of ignition, thus allowing the car to adjust the quantities of air and fuel automatically, automatical ly,
thereby optimizing performance. Such a setup raises the prospect of mixed-fuel cars that can run on a number of different biofuels while ensuring they still run efficiently. During tests, the university team built up a fourcylinder engine entirely by laser ignition, but delivering the laser beam via optical fiber initially proved more difficult than the team had hoped: “The fiber didn’t respond well to engine vibration, which increased the divergence of the output beam and reduced the beam mode quality,” explains Geoff Dearden, Shenton’s colleague from the university’s laser group. “Bending the fiber was also problematical: up to 20% of the beam energy was lost with small bend diameters, while tight bends caused the fiber to fail altogether after a period.” It was also discovered that the higher density of laser energy could cause immediate or long-term degradation, leading to loss of beam transmission – and therefore loss of ignition. “Careful design of laser parameters, fiber coupling, and choice of optical media is crucial to avoid this,” adds Dearden. Clearly there are still obstacles to be overcome before laser ignition becomes a viable system on production cars. However, the huge benefits it offers – coupled with the advances already made – should guarantee a sufficiently important incentive for continued R&D into a 21st century replacement for the spark plug, conceived and first seen in the closing years of the 19 th century.
“Careful design of laser paramete parameters, rs, fiber coupling, and choice of optical media is crucial” FIVE OTHER LASER BREAKTHROUGHS Laser bonding:
Laser welding:
Lightsaber:
Invented in 19 97 97,, this marking technique
Much different to laser cutting, laser welding
It may be fictional, but jus t im agi ne h ow m uch
makes of different types ofuse lasers to bond an additive marking substance to a wide range of substrates.
has carfor makers and helped suppliers many years to join together multiple pieces of metal through a beam that acts as a heat source, allowing for deep welds to be made.
easier automotive manufacturing would be if all engineers had this blade of plasma!
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Laser cutting:
LASIK:
This technology has revolutionized manufacturing industries worldwide since it came to the fore some half a century ago. By using a high-powered comput er, laser cutting works by directing the output of
Laser-assisted in situ keratomileusis is a groundbreaking type of surgical treatment of vi sio n pr obl ems , ju st th e ticket for those out there that are finding it difficult to tell the difference of various intricate
a high-power at the material tolaser be cut.
engine components!
September 2011 // Engine Technology International.com
Advanced electric motors, generators and drive systems
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