New piston thinking

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Why not go out on a limb? Isn’t that where the fruit is?” –Frank Scully

There’s a large slice of human philosophy predicated on the maxim that if the fundamentals are sound, don’t fiddle around with them – a line of thought given voice in a myriad of different sayings, from “don’t reinvent the wheel” to “if it ain’t broke, don’t fix it.”

However, there’s another school of thought that goes roughly like this: if you don’t constantly break down the basics and reinvent your business or products on a continual basis, you’ll find yourself on day left in the dust on the side of the road. Silicon chip maker Intel provides and excellent example of this: they’ve spent decades literally re-inventing the computer chip to keep making it smaller yet more powerful.

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That brings us to the lowly diesel engine piston – about as basic a component as you can get in the world of trucks these days. Yet pistons are now the focal point of a bevy of re-design efforts – some as part of an entire revamping of the diesel engine structure itself – as engineers seek ways to make the venerable diesel a more efficient and cleaner fuel-sipping workhorse.

Let’s start with Federal-Mogul Corp.’s new aluminum piston design, one the supplier touts can reliably withstand the mechanical and thermal loads produced by heavily boosted engines. This new piston, called “DuraBowl,” strengthens the crown of a piston by locally re-melting the alloy around the bowl, significantly improving the fatigue strength of the aluminum where it is most needed. The result is an extension of engine life to between four and seven times that achieved with a conventional cast piston, the company said.

[Neat idea – but couldn’t they have come up with a better name than ‘DuraBowl'? Sounds like a line of ceramic foodware for toddlers!]

Frank Doernenburg, Federal-Mogul’s director of technology, noted that combustion in a diesel engine takes place in a hollow bowl in the top of the piston, where temperatures can reach over 750 degrees Fahrenheit (400 degrees Celsius) and pressures over 200 Bar. Under these increasingly difficult combustion conditions, the risk of failure of the rim of the piston bowl increases significantly, he said.

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One reason for the thermal and mechanical failures of the piston bowl can be traced to the presence of free primary silicon particles distributed throughout the aluminum matrix, according to an analysis conducted by Federal-Mogul’s engineers. Silicon is a necessary constituent of the aluminum alloy used to make diesel engine pistons, offering favorable properties such as low expansion and good castability, so it cannot be eliminated.

However, the engineers found that aluminum expands eight times as much as silicon; therefore stresses are set up within the piston every time the temperature fluctuates. Furthermore, repeated mechanical loads, each time the cylinder fires, could result in fatigue failure from the corners of the silicon particles. The only potential solutions to this problem, until now, have been fiber-reinforced pistons, Doernenburg said.

"Fiber-reinforced pistons increase manufacturing complexity as the molten alloy has to infiltrate the fibers during casting," he added. “But there is not yet a reliable, non-destructive way to test the integrity of the finished part. However, with our DuraBowl process, we can do an Eddy Current test to ensure the quality."

Federal-Mogul's solution is to pre-machine the cast piston and then re-melt the alloy around the rim of the bowl. "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."

He added that the re-melted 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 – a technique known to increase the strength and durability of metal alloys.

Combined with a rapid cooling process, this re-melting technique significantly changes the alloy's microstructure by reducing the size of hardening phases such as silicon particles and inter-metallics, Doernenburg pointed out – and the result is a piston bowl rim whose first few millimeters provide significantly improved aluminum strength, further enabling engine manufacturer's efforts to downsize or turbo-boost engines for greater specific output.

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"The re-melting process certainly increases piston life and performance substantially, while at the same time, serving as a contributor to improve fuel efficiency and reduce carbon dioxide,” Doernenburg said. “A conservative estimate would be a fourfold improvement in the life of any cast piston which suffers from bowl rim failures."

Re-designed pistons also play a key role in a new thermodynamic engine process developed by the Scuderi Group -- a process that “fires” the pistons after top dead center. Originally conceived and designed by Carmelo Scuderi (1925-2002), the Scuderi engine is expected to produce up to 80% fewer toxins than a typical internal combustion engine while posting gains in efficiency – without all the aftertreatment systems.

I wrote about this unique design not too long ago. In a nutshell, the Scuderi split-cycle design divides the four strokes of a conventional combustion cycle over two paired cylinders: one intake/compression cylinder and one power/exhaust cylinder. By firing after top-dead center, it produces highly efficient, cleaner combustion with one cylinder and compressed air in the other, the company noted – and it uses re-designed pistons as part of the process to achieve these gains.

[Watch the video below for more details on the Scuderi engine design, including how the pistons and cylinders can be reshaped to deliver more efficient power.]

This all goes to show that when it comes to improving the efficiency and cleanliness of diesel engines, nothing is impossible if you are open to re-designing even the most basic of its components.

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