BMW will use a new Federal-Mogul aluminum piston (right) for the triple-turbo, 93kW/liter engine (left) in its M550d xDrive sedan. The piston is design to satisfy the high heat and strength requirements of new diesel engines. (Source: Federal-Mogul)
The aluminum industry has been working on high-strength versions, including alloys such as aluminum-lithium, for some time now. For example, it's the major metal in aircraft, where it's mostly replaced steel (see my upcoming July feature article on aircraft materials). We've also covered a brake rotor prototype made of an aluminum composite: http://www.designnews.com/author.asp?section_id=1392&doc_id=239090 Aluminum rotors aren't new, but this one is managing to keep up with the increasing heat requirements of today's smaller, hotter engines, not a small feat. There are some under-hood applications of plastics, including carbon composites, such as this one I wrote about http://www.designnews.com/author.asp?section_id=1392&doc_id=243857 but AFAIK, not yet in car engines.
Ann, do you know what kind of aluminum Federal Mogul is using for this application? I assume it is an aluminum-silicon alloy. Do you know if it is hypereutectic, eutectic, or hypoeutectic? (This is metallurgy-speak for more than 12% silicon, about 12% silicon, or less than 12% silicon).
The DuraBowl re-melting process is interesting, and, like many good ideas, seems obvious in retrospect. Doing failure analysis of aluminum pistons, I've often observed that areas which melted (in service!) are much harder and have a much finer microstructure than the surrounding area. But I'm not sure that it would have occurred to me to exploit this as a manufacturing process.
Friction stir processing is another way to achieve improved properties in the piston bowl area. I'm not sure whether anyone is doing this commercially yet, but there is a lot of interest in it. At last year's Materials Science and Technology conference, Dr. Saumyadeep Jana gave a presentation on friction stir processing of cast aluminum alloys; during the Q&A session, it was clear that just about everyone in the audience (including myself!) had pistons on their mind. I strongly suspect we'll be seeing friction stir processed pistons in the next couple years. This would be a good topic for an article.
Dave, thanks for the feedback on the re-melting process. I agree, it doesn't sound intuitively obvious as a manufacturing process. Regarding the alloy, I'd like to know, too: no details on this product were given. On the company's website appears this general statement about their aluminum pistons. but whether it applies to the new diesel piston is not clear: "We offer aluminum diesel pistons made of Federal-Mogul's exclusive, high-strength B3 alloy (2006) for high-end light-vehicle diesel applications, giving a 10 percent higher fatigue resistance at 440º C to absorb extreme loads in performance engines."
Nice story, Ann. Do you know if these pistons use cast iron cylinder liners? Going way back to the old Chevrolet Vega (does anyone remember the Vega?), engineers have tried to use aluminum. When the Vega's engine had problems, engine builders started employing the cast iron liners. In the '90s, engineers got rid of the cast iron liners and started using hypereutectic aluminum alloys for the blocks and various coatings for the inside of the cylinder, but I don't know how that came out. Are they still using the liners?
BMW Motorcycles ran an all aluminum block with no cast iron liners with great success. I've seen them run over 100,000 miles with no loss of compression. I don't think the problem is purely aluminum but the overall design of the combustion process. The normal diesel "crack" that occurs at the time of ignition created enormous stresses in all the metal in the combustion chamber, not just the piston; spreading this explosion out over time would go a long way toward reducing stress. I'd guess that BMW has adopted more than just a triple turbocharger and an aluminum piston to make this engine meet their performance goals. Using aluminum pistons and reducing reciprocating mass is a significant step toward increased performance and durability.
@Chuck: I can't speak for the auto industry, but in the outboard engine industry, the status quo is a block made out of a lost-foam cast or die-cast alumium alloy (typically A356), a cast iron cylinder sleeve, and a hypereutectic aluminum alloy piston. However, Yamaha has started to use plasma-sprayed cylinder liners instead of cast iron sleeves. I believe the technology they use was developed by Ford and Flame-Spray Industries. Sulzer Metco has a different plasma-sprayed cylinder liner technology. One of the advantages of the plasma-sprayed liners is that they save weight compared to a cast iron sleeve. The properties of the coating be tailored to get additional benefits.
Thanks, Dave, for stirring my memory a bit. I do recall the outboard engine industry using aluminum alloy blocks. I looked back at an old article (from the '90s) and found that Mercury Marine was using (and maybe still uses) a hypereutectic aluminum-silicon block alloy known as Mercosil. Those engines went into production in '94. The silicon in the block was supposed to enhance wear resistance. Later, they used a nickel phosphorous/boron nitride piston coating with no cylinder liner, but I don't know how that turned out, or whether they still use it.
@Chuck: Yes, Mercury used their proprietary Mercosil low-copper hypereutectic alloy, along with a nickel-based piston coating, for some 15-25 HP engines in the late '90s. As far as I know, they never used it outside of that horsepower range. I'm not quite sure why it hasn't been more widely used, since it does seem to have offered a number of benefits.
Mercury's decision to keep most of their casting in-house and to develop it as a core competency has paid off; the Mercosil alloy is sold commercially, along with a series of die-casting alloys, called Mercalloy, which Mercury developed. Mercury casts these alloys for outside customers, and also licenses the alloys to foundries.
While Evinrude closed its die-casting division prior to the OMC bankruptcy, they have held on to their lost-foam casting facility; in fact, they recently won Best-In-Class in the AFS Casting of the Year competition. Like Mercury, they also do work for outside customers.
I believe the GM "LS" series V8 engines used in rear-drive cars since 1997-1998 is an all-aluminum block (no iron cylinder liners). It was first used in the 1997 Corvette. The LS engines are used in the current Corvette and Camaro high-performance cars.
Personally, I own a 1998 Chevrolet Camaro Z28 that is factory equipped with the LS1 5.7 liter V8 with aluminum block and heads (and 6-speed manual transmission). I have 135,000 miles on the car, and it still runs like new...brutal power and acceleration by normal car standards.
For most GM truck applications, the LS engine blocks are cast iron.
During the late 1970's, I had a 1973 Chevrolet Vega with the aluminum block. It drank about one quart of oil for every 100 miles, due to the aluminum cylinder wear.
Aluminum pistons, cast, hypereutectic and forged have been standard fare for piston technology for decades, some of them in very high stress applications.
They're talking about 93kW per liter. O.K. that's a 250 hp 3 liter diesel. Great. I was getting over 600 kw out of a turbosupercharged 7 liter V8 over two decades ago, and it had typical OEM forged aluminum pistons. They seemed to hold up just fine, even with over 20 inches of turbo boost...and I did all that design with a piece of paper and a slide rule and OEM components! :-)
Aluminum blocks? So? That's no big shakes either...and long before the Vega, too. They've have been around in various GM products since the early 1960's (Buick 215, Corvair flat six, etc.)
The much maligned Vega used aluminum block and cylinder with a spray coated cylinder wall which failed rather miserably in the Vega. But (like so many other technologies prototypd on the Vega, such as the HEI ignition system) very similar technologies eventually made their way into the GM product line, like the LS1.
Pardoning the pun, why are we suddenly so gassed about an aluminum pistoned turbo diesel?
@Ockham: Here is an article that describes in more detail why these pistons are a big deal. Was your 7L V8 turbodiesel for racing? If so, you probably didn't need to worry too much about thermal fatigue. Also, two decades ago, you probably didn't need to worry about emissions too much.
Aluminum pistons are obviously nothing new, but in order to reduce diesel emissions, it's desirable to get piston bowl temperatures as high as possible. Unfortunately, if you get them too high, the aluminum will melt. (Also, at temperatures near the melting point, the fatigue strength of the aluminum is greatly reduced).
Interestingly enough, when the aluminum melts and resolidifies, its mechanical properties are greatly improved. This is the idea behind Federal-Mogul's Durabowl process. I have no idea how they got the idea, but I wouldn't be surprised if it occurred to someone while looking at a failed piston.
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