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.
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