Federal-Mogul plans for the future with new diesel piston design

  • 16-Nov-2009 08:52 EST

Federal-Mogul's unique remelting process for the aluminum-silicon alloy used in DuraBowl, combined with a rapid cooling process, significantly changes the alloy's microstructure by reducing the size of hardening phases such as silicon particles and intermetallics. Shown is the beginning of the process using a tungsten gas electrode.

By most accounts, more and more of the powertrain engineers who have spent a good part of the past 10 years developing technologies that would reduce NOx and particulate matter emissions to essentially zero will next be confronted with a similar task but a different target: CO2.

One way to reduce CO2 emissions while keeping an eye on the ever present need to improve fuel economy is to develop “higher and higher power output from smaller and smaller engines,” says Federal-Mogul, which cites that typical performance outputs for automotive diesel engines rose from 50 kW/L 10 years ago to about 70 kW/L today.

“Rising specific outputs place higher mechanical and thermal loads on many of the components where Federal-Mogul has considerable expertise,” said Rainer Jueckstock, Federal-Mogul Senior Vice President of Powertrain Energy.

One such component is the piston. F-M is currently in high-volume production in Nürnberg, Germany, on an aluminum piston design, called DuraBowl, that it says can reliably withstand the mechanical and thermal loads produced by heavily boosted engines.

Temperatures in a piston’s combustion bowl can reach over 400°C (750°F) and pressures over 200 bar (2900 psi). Under such conditions, the rims of piston bowls often tend to have a high failure factor. After extensive analysis, F-M's engineers determined that both thermal and mechanical failures of the piston bowl can be traced to the presence of free primary silicon particles distributed throughout the aluminum matrix.

F-M explains that aluminum expands eight times as much as silicon, therefore stresses occur within the piston during temperature fluctuations. Also, repeated mechanical loads, each time the cylinder fires, could result in fatigue failure from the corners of the silicon particles. Silicon is a necessary constituent of the aluminum alloy, offering favorable properties such as low expansion and good castability, so it cannot be eliminated.

According to F-M, the only potential solution to this problem, until now, has been fiber-reinforced pistons.

“Fiber-reinforced pistons increase manufacturing complexity as the molten alloy has to infiltrate the fibers during casting,” said Frank Doernenburg, Federal-Mogul Director of Technology, Pistons and Pins. “Furthermore, there is not yet a reliable, nondestructive way to test the integrity of the finished part whereas, with our DuraBowl process, we can do an eddy current test to ensure the quality.”

F-M's solution was to premachine the cast piston and then remelt the alloy around the rim of the bowl, a procedure developed specifically for DuraBowl. F-M tells SOHE that the entire process—from idea conception to internal testing to customer validation—took in excess of five years.

“The strength and efficiency of our solution is that the process is physically simple,” said Doernenburg. “The sophistication is in the control of key parameters, which ensure consistent quality. The result is a technologically advanced, high-performing, and very cost-competitive product when compared to both fiber-reinforced and steel pistons.”

The remelted alloy cools a thousand times faster than it did when originally cast, which leads to much smaller silicon particles; only one-tenth of the previous size. Metallurgists refer to this as refinement of the microstructure; the result, says F-M, is a piston bowl rim whose first few millimeters provide significantly improved aluminum strength, further enabling efforts to downsize or turbo-boost engines for greater specific output.

Technological and cost benefits have been validated during extensive engine testing, both by F-M and its customers. 

“The remelting process certainly increases piston life and performance substantially while, at the same time, serving as a contributor to improved fuel efficiency and reduced CO2,” said Doernenburg. “A conservative estimate would be a fourfold improvement in the life of any cast piston which suffers from bowl rim failures.”

Besides reduced cost and process robustness, F-M tells SOHE that design flexibility rounds out the top three advantages of DuraBowl over fiber-reinforced pistons. The first application of the DuraBowl process is on a high-performance diesel engine recently launched for a “leading global vehicle manufacturer.”

F-M also says that the technology is adaptable to the off-highway industry and that it “already [has] one customer in the large displacement size class of engines.”

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