Diaphragm vs. Piston.....

[brian hughes]

....is there any clear consensus about the respective advantages and disadvantages? Diaphragms sound much easier to build than a low friction piston, but does the springiness introduced outweigh this?

[skyofcolorado]

Maybe add a rolling seal to that list as another type of power piston. Think miniature inner tube or squishy o-ring around a loose fitting piston that rolls along contacting the piston and cylinder wall as the piston moves. Not frictionless, but pretty good for the ease of construction.

Also, I don't see why a diaphragm needs to be springy? It just needs to seal within the range of travel and have a rigid component to connect with the flywheel. I've used material from a HDPE grocery bag as a diaphram with cardboard covering 80% of it to provide structure. Not at all springy, low friction, wide range of travel, easy construction.

[brian hughes]

Thanks for the reply- what I've got in mind is going to be fairly modular so that I can experiment. I want to see if I can incorporate a condom as the diaphragm :-)

[Tom Booth]

... but does the springiness introduced outweigh this?

Just my opinion, and don't know about any "clear consensus" at all anywhere on anything relating to Stirling engines. Overall, they are a complete mystery - to me anyway, but I have an inkling that the "springiness" may be an advantage rather than a disadvantage.

Take a bow and arrow for instance, or a slingshot or a trampoline or simply shooting a rubber band.

Whatever energy is used to stretch the elastic is stored as potential energy and returned. With a piston, the energy put into moving it in the first place, friction pushing it down the cylinder, and continually reversing direction is all energy permanently lost.

It may be better to have the elastic somewhat taut rather than loose in order to take full advantage of this springiness.

Probably the most impressive performance from any Stirling engines I've ever seen is TK Motors various tin can engines that use taut elastic diaphragms for both piston and displacer.

https://m.youtube.com/channel/UCkjcPbaP ... Uuw/videos

[skyofcolorado]

Whatever energy is used to stretch the elastic is stored as potential energy and returned. With a piston, the energy put into moving it in the first place, friction pushing it down the cylinder, and continually reversing direction is all energy permanently lost.

Good point, which is probably why many of those don't require a flywheel of any substance, if at all. My bag/cardboard diaphragm definitely needed a flywheel, but worked great otherwise.

TK's designs are great by the way! I just wish there were build videos to go along with it.

[Tom Booth]

.

TK's designs are great by the way! I just wish there were build videos to go along with it.

There are some, or at least one, sort of. Not exactly "build", but more like, take apart and show how it was built, but yeah, more would be helpful, though many are of similar construction, I believe.

[Bumpkin]

I’m a fan of diaphragms, particularly the truck air brake canister type, but to be fair to pistons; no they don’t take energy to reverse direction/reciprocate from a crankshaft — obviously the bearings will have higher friction with higher loads, but the mass gives back to the flywheel on deceleration what it takes on acceleration. And there is friction in the flexing of diaphragms. I forget the term —Hysterisis maybe? Anyway I like diaphragms.
Bumpkin

[spinningmagnets]

I stumbled across the term "Rolling Diaphragm", and suddenly I had lots of information about these. I've tried other terms for the thing I had a picture of in my head, but I previously had poor search results.

A

StirlingRllingDiaphragm1.png (41.52 KiB) Viewed 660 times

B

StirlingRollingDiaphragm2.png (134.85 KiB) Viewed 660 times

[spinningmagnets]

D

StirlingRollingDiaphragm3.png (106.3 KiB) Viewed 657 times

[Fool]

spinningmagnets, I can't thank you enough for that jewel. As you pointed out, not knowing that keyword kills any search for the device. I been looking for years for info on those. All I could do was reference Gabriel Hijacker air shocks, as they use them. I kept calling them "sock seals" as a rolling sock seal, to me, was a rolled up sock that rolled along in the cylinder, so the one that is just folded would just be a plain sock seal. LOL

Rolling diaphragm seal. Excellent. There is even a PDF on how to design them. I think they are used in potable water system pressure tanks.

The really cool part is they can be easily made out of any elastic tube, just like folding a sock back on itself, not necessarily a diaphragm. Attaching the inside to the piston, and the outside to the cylinder. Even a sock could be used if coated with RTV 100% silicone. By nature they must be in cooler parts of machines.

The only drawback I see is that they need a minimum pressure to prevent rubbing on each side. They also have a higher verses lower pressure orientation. They must bulge outward. The engine must be slightly pressurized.

Brian Hughes, yes they could be made out of a condom in low pressure applications. I'm not sure a condom would hold enough pressure for a rolling diaphragm seal except for very low delta T, just a degree F or two. They don't hold much more than a psi or so.

I agree with Tom. The springiness doesn't hurt. It returns most of the energy it is given. And I agree with Bumpkin. The thicker the diaphragm the more friction it has, hysteresis is a great word for that.

There is a compromise between thinness and ability to hold back the pressure and pressure changes, and fatigue life. The rolling seals probably have the best lifespans and travel distances. They probably have lower springiness and higher friction, until the diaphragms get very thick, then they may excell. Diameter is also an engineering compromise parameter in these piston/seals. Bigger diameter, thicker diaphragm.

[VincentG]

My 2 cents, in order for a Stirling engine to have and maintain a true "Stirling cycle" at the low rpms that most run at, there should be nearly no leakage at all. This almost exclusively rules out pistons and rings that aren't super high tech NASA stuff.

Sure it will run fine with leaks, but its altering the cycle, imo.

[spinningmagnets]

I recall wondering this for years, and then I read the patents for the Rider-Alpha Stirlings.

The Rider has an air-pump to make up internal air-mass due to leakage. It was suggested to maintain two atmospheres because the petroleum-based lube-oils would ignite if they got hot. When the oils would get hot they would smolder, but if enough oxygen was present, it could explode (or so I was told, who knows?)

Also, higher pressures seemed to create faster leakage, and "at the time" it appeared that two atmospheres was the practical limit with leather piston seals.

I mention this because we now have non-flammable lube, and also I have seen fully encapsulated engines running on helium [*except for the hot end of the displacer cylinder, of course].

On engines that had no leakage issues, I have seen many atmospheres of pressure inside the engine, to increase output, with no other changes.