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Although the two photomicrographs seem to be showing two different materials, the diesel piston is all the same aluminum alloy. The area where there are smaller particles is the one that has undergone remelting then fast cooling, which results in the significantly smaller silicon particle size. This contributes to a longer piston life, as the cracking occurs due to thermal expansion and contraction during engine operation between the aluminum and silicon does not have the same deleterious effect when the particle size--and consequently crack size--is a tenth of the original.

Note the difference between the grain size as cast (above) and after remelting and cooling (below).

Better Diesel Pistons

Federal-Mogul has developed a process that improves the life for diesel pistons. Significantly improves their life. Here's how.

The implications of improving the fuel economy in diesel engines while, at the same time, reducing CO2 emissions, are rather significant, particularly as vehicle manufacturers are reducing the size of their diesels for both truck and car applications. According to Frank Doernenburg, director of Technology, Pistons and Pins, for Federal-Mogul Corp. (federalmogul.com), who works out of the company's facility in Nurnberg, Germany, where considerable research is conducted on diesel technology, about five years ago they began to work on addressing the issue of increasing engine pressures that are a consequence of the drive for efficiency.

Realize that while gasoline engines are spark-ignition, meaning that the fuel in the cylinders is ignited by spark plugs, diesel engines are compression-ignition, meaning that the pressure—and consequent heat—is such that the fuel explodes without the spark.

So, Doernenburg explains, whereas a typical gasoline engine may have an internal pressure of 100 bar and a turbocharged six-cylinder gasoline engine may have a pressure of 120 to 150 bar, "a diesel engine of five or six years ago had a 150- to 160-bar pressure. Now diesels have 180- to 190-bar pressure." The result of this is higher specific output. "Higher specific output means better fuel consumption," Doernenburg says, so companies are producing engines that produce 75 kW per liter of displacement (about 100 hp). One company Federal-Mogul is working with is developing a three-liter diesel that produces 300 kW (~402 hp).

This leads to a non-trivial problem. In Europe, in particular, the pistons for diesel truck engines are made of aluminum alloy. There's aluminum with the second most prevalent material being silicon, followed by other metals including copper and nickel. The alloying materials help increase the mechanical strength of the component. The pistons are cast, then machined.

Here's where the tricky part comes in. The temperature at the piston bowl rim within the cylinder of a modern diesel is on the order of 400 to 420°C. The first liquid phase of the aluminum alloy is 480°C. "So there is only 60° between the operating temperature and the melting temperature." Doernenburg says that there is no other part in an engine where the delta between the operating temperature and the melting temperature is so close.

But heat isn't the only issue. Of greater concern, in the long run, at least, is thermal cycling of the engine. During the combustion cycle, there is heating and cooling (comparatively speaking, of course) of the piston as the intake and exhaust valves are actuated. This thermal cycling gives rise to compression stresses in the aluminum. Complicating matters is the fact that the thermal expansion of aluminum is about eight times greater than that of the silicon particles within it. Consequently, through the repeated cycling of the engine, cracks are generated between the silicon particles and the aluminum structure. This can lead to component failure over time.

Which brings us back around to the work that Doernenburg and his colleagues undertook. There were a couple of approaches taken. One was to use an entirely different alloy, which helped address the problem, but proved to be less effective than the other process, the one that has subsequently been named "DuraBowl." As in, the new alloy provided an improvement in piston life on the order of 10 to 15% while this process, says Doernenburg, provides a 400% improvement. In testing, they've gone to a 700% improvement.

How is this phenomenal improvement being achieved? Via metallurgy. Not changing the alloying. Rather, changing the mircostructure of the silicon particles.

What they're doing is melting the piston bowl rim. They're using a tungsten inert gas (TIG) melting process to do this. Doernenburg says that they worked with a supplier that had welding equipment to develop the process parameters. The objective was to perform the melting to precise depths so that when solidified the melt would have a porosity-free, oxide-free microstructure. To assure this happens, they also developed an eddy current testing method, a nondestructive technique.

Doernenburg says that the melt depth on a standard piston from 80 to 85 mm in diameter is 4 mm.

They've developed an automated system to perform the operation. Cast and machined pistons are loaded into the system. The melting occurs. The top of the bowl is machined again (in effect, there is something of a weld bead created). The piston undergoes eddy current testing. It is unloaded. The time is predicated on the diameter of the piston to be processed. In one production application for a heavy-duty application, there are three layers of melting, so this takes additional time. But the process is scalable by adding equipment to meet production requirements.

So what happens? When the rim of the piston is melted, it cools very quickly, on the order of 1,000 times faster. As a result of this cooling, the size of the silicon particles is reduced to about 0.1 their original dimension. (The reason for the faster cooling is simple: when the piston is originally cast, the entire mass is molten. When you're melting just the top of the bowl, there is not only much less material to cool, but the thermal conductivity of the remainder of the aluminum helps draw away the heat.)

The thermal cycling of the piston in the high compression engines still occurs. The aluminum still undergoes the compression stresses. But the difference is that because the silicon particles are so significantly reduced in size, there are not the cracks created as their ordinarily are. Without that occurrence, significantly longer life is achieved.

According to Doernenburg, it is applicable to any aluminum alloy; process parameters would need to be adjusted depending on the metallurgy, of course. It can be used on almost any size piston, but because the electrode has a diameter of 6 mm, there could be a lower size limit, but not necessarily an upper limit (although again adjustment needs to be made for the time required to melt the necessary volume of aluminum).

Gasoline engine pistons? Not necessary, Doernenburg says. He cites a "highly loaded" turbocharged six-cylinder engine: "A maximum of 350°C on the bowl." Compared to the diesel, that's barely breaking a sweat.

 

What About A Laser?

Given that the DuraBowl process is predicated on melting an area on the piston, what about using a laser to do it rather than a TIG melting process? Frank Doernenburg, Federal-Mogul’s specialist on the process, says that they looked at it. In fact, he says, “We started with a laser but during the development we made a comparison with TIG and came to the conclusion that the TIG process is less expensive and the area that can be remelted is bigger.” He says that a competitive piston manufacturer is working with a university on a process that involves using a laser, but there are no production applications for it. The DuraBowl process has three production applications at present and a fourth to go on line later in 2010.