This looks like a spark plug. It looks that way by design. It is the igniter portion of the Federal-Mogul Advanced Corona Ignition System, which is being developed to improve fuel efficiency and reduce emissions in engines running advanced strategies.
The corona system on the left. Conventional spark plug on the right. If both are used in a cylinder to ignite the air-fuel mixture, which is more likely to reliably get it done?
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âLike the automobile itself, the internal-combustion engine had no single inventor. Two-cycle versions were patented by Stuart Perry, a New York inventor, in the United States in 1844 and 1846, and by Etienne Lenoir, a Belgian mechanic, in France in 1860. . . . Credit for being âfirstâ is generally given to Lenoir, because his engine was commercially successful. A major innovation of the Lenoir engine was the use of an electric spark plug powered by a battery and coil to ignite the fuel mixture of air and illuminating gas.ââThe Automobile Age, James J. Flink, The MIT Press
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Note well that last sentence. Lenoir is credited with the electric spark plug. That was 1860. And pretty much, the spark plug that he invented is like the spark plug that is still in use in automobiles today.
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Sure there have been changes. Changes in materials. Changes in configuration. Changes in positioning. Changes in how the plug is powered. And so on.
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But pretty much, the spark plug is the spark plug.
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But as Kristapher Mixell, advanced corona ignition system development, Powertrain Energy, Federal-Mogul (federalmogul.com), there are significant changes underway in the combustion strategies that OEMs are applying as they work to meet CAFE standards that call for 37.8 mpg for cars and 28.8 mpg for light trucks by 2016âand thatâs up from 33.3/25.4 mpg, respectively, for 2012.
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And the traditional coil-and-plug system may not be the correct means by which combustion can be ignited in these new approaches, such as stratified charge, lean burn, and using high levels of EGR. âWe can allow these strategies to work more effectively and in regions and loads they were never able to run before, and to be a salable strategy,â Mixell says for whatâs behind the development of the advanced corona ignition system (ACIS) that he and his colleagues are developing. Note the use of the word âsalableâ: this is something that is being developed for commercial deployment, not the stuff of white papers and fundamental research.
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As for the rationalization for why the ACIS is called for, he explains, âAs you add more exhaust gas and lean mixture out, the mixture is more difficult to ignite, and it burns really slow. Because this is a spark ignited system, you discover that the way weâve been igniting the fuel-air mixture for so many years works fantasticâI donât want to take anything away from spark plugsâwith todayâs strategies.â Note the âwith todayâs strategies.â Mixell continues, âWith a lambda 1 type mixture and certain areas of lean and EGR, it works quite well. But if you want to push these strategies further, you need something different.â
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Enter the idea for the ACIS.
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Actually, âthe idea for the ACISâ sounds a bit too theoretical. Although Mixell says that it isnât something that anyone can put into their production volume engines tomorrow, theyâve been working on this since 2007, and when we talk, in October, 2011, they are, he says, âon the cuspâ of moving from working with the R&D folks at OEMs to working with the advanced peopleâthe pre-production group.
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And while âthe idea for the ACISâ sounds along the lines of âoh, that would be a nice thing to have,â know that theyâve calculated that through the use of the high-energy, high-frequency electrical field that ionizes the air and fuel in the cylinder, theyâre able to generate rapid ignition and quick burning, which results not only in performance improvements (through more timely and thorough combustion, more work is performed) butâand hereâs where it goes from being nice to being a real game changerâas much as a 10% fuel efficiency improvement compared with standard spark ignition.
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(Yes, one of the brands under Federal-Mogul is Champion, as in âChampion Spark Plugs.â)
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Mixell makes a quick run through engine design in recent years. During the â60s and â70s, the location of the spark plug in the cylinder was generally off to the side. âNot ideal for combustion.â But then when four-valve cylinders came in, the spark plug was centered. Mixell says that the improvement in performance of those engines was as much as result of the proper placement of the spark plug as from the valve-train arrangement. With time there have been improvements ranging from improved fuel injectors to more powerful engine control units. âNow weâre at a point where fuel-conversion efficiency is quite good. Spark plugs do a good job; burn rates are quite acceptable for combustion as weâve been thinking about it.â
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But with the more advanced combustion strategies that are being developed, spark plugs donât do quite as well under these conditions.
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Mixell uses a great metaphor: Combustion is âone big probable crapshoot.â
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If you have the perfect amount of fuel and air and the right timing, there is a high probability that youâll have combustion. If you start removing fuel, the probability goes down. Add EGR and it goes down further. âYou donât want one roll of the die,â Mixell says. âYou want as many rolls as you can get at one seating at the table. Thatâs what corona allows you to do.â
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Rather than a spark being created in a gap, there are multiple streamers of ions from the nickel-steel tip of the ACIS. (In effect, it looks somewhat like a Tesla coilâand interestingly enough, Nikola Tesla held a patent for a spark plug.) This increases your odds of more thorough ignition, even when the probability is reduced. (Mixell says that it works well when the engine is at lambda 1, but the real objective is to provide benefit under the conditions that can provide better fuel economy). Whereas spark plugs initiate combustion through heat, the ACIS does it via a chemical reaction (the fuel and air are ionized, react with one another, and then as a result release heat).Â
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âWeâve taken the approach that we want this to be easily integrateable into an existing engine,â Mixell says. In other words, theyâve developed the ACIS so that there are two parts, the igniter which resembles a spark plug and an inductor that is basically like an ignition coil. Theyâre using materials that have been long proven in automotive service. So it is designed and engineered to be swapped out with traditional coil-and-plug systems without major re-architecting.Â
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In other words, this is far from something that has been bread-boarded. When we visit theyâre running the ACIS in one of the cylinders of a production 1.6-liter, turbocharged direct-injection engine to see how it performs compared to conventional spark plugs. And so far, very good.Â