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]

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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