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That will be interesting if all goes to plan with the ceramics. Please keep us informed.
Trev

Since my ability to progress in the fabrication of this prototype is temporarily limited by the Covid 19 disruption I have decided to attempt to refine the design of the piston/displacer/regenerator.

I chose Approtehcies design for it's low tech simplicity and the steel wool regenerator piston is central to that.
It has many advantages and at least one predictable disadvantage.
Steel wool fibers are fragile and prone to disintegration.

My initial effort to remedy this is to use the coarsest steel wool available based on the theory that thicker fibers are less prone to breakage (due to microscopic cracks that develop over time) created to the inertial stress exerted on them by the pistons reciprocating motion. Realistically while this may reduce "shredding" it will not eliminate it.

Sintered steel wool and steel fleece appear to be an option. Unfortunately these add significant expense and are difficult for the average "hobbyist" to produce. While I am not rejecting this commercially available material I would prefer to develop another option if possible. Luckily researching sintered steel wool and fleece has (as is usually the case) provided some insight which might prove useful.

Even steel fibers bonded by sintering are subject to stress cracking of that bond. This has been minimized by providing a "skeleton" which supports the sintered steel wool material in the form of very light expanded metal. Imagine a "sandwich" of steel wool that consists of a steel wool interior and very light expanded steel mesh exterior. When bonded together the mesh supports the thin layer of sintered steel wool limiting the flex it must endure.

It is possible that by simply layering unbonded steel wool and light steel mesh a more durable steel wool regenerator might be created. A piston consisting of layers of steel wool alternating with light expanded metal (like a "dagwood sandwich") might prove much more durable than a simple spiral layering of steel wool alone produces. It might also be possible to create a spiral mesh support by simply "winding" a mesh support layer along with the spiral of steel wool which forms the regenerator portion of Appotechies piston design. I prefer this second method as it would appear to be simpler and aligns the "skeleton" with the forces created by the pistons reciprocating motion.

If simply layering steel wool with a mesh support were the best option sintering would not be required. Unfortunately this is not the case. The available evidence strongly indicates that bonding of the two materials is highly desirable.

In place of sintering I suggest that a high temp glue (such as sodium silicate) might suffice.

One might be able to dip a slightly oversize mesh and steel wool piston in "waterglass" and then place it in a heated jig consisting of an exterior cylinder and a centering device. This should slightly compress the outer steel wool layer providing a more durable piston wear surface while bonding the supporting mesh and steel wool into a single mass and thereby reducing or eliminating steel wool "shredding".

Thoughts?
Questions?

Dana

I dug up this old thread, I hope you're still working on it. It got awfully quiet after Covid, which makes me worried.

I followed along with your idea of the steel wool as displacer or on the displacer. But wouldn't it be more efficient to put it on the walls of the hot cylinder and move a displacer through it. That way the displacer is lighter and the steel wool doesn't get bounced around, reducing disintegration.

Just my two cents.