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)
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?
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."
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.
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.
I'm a little surprised that they found that aluminum is strong enough for this application. And very pleased, asuming it can work well over time. Is plastic next?
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