Portable generator to Ringbom converstion

[Tom Booth]

Just an observation I made a few days ago. Not 100% definite but I think interesting, though probably many would think there is something wrong with .my observation skills.

My shop is in an unheated (this time of year anyway) basement under a large brick building and it still gets or stays quite cool down there.

I have several model Stirling engines of various types on the shelves. They normally all seem to require some time to get up to "operating temperature", but in trying to understand better why my Ringbom conversion is not working, I ran one model after the other, but they seemed to be taking an unusually long time to start up down there in the cool basement. Normally I'd bring them home to the (warmer) house to run.

Anyway, out of curiosity, and to test my theory about heat transfer in a Stirling engine, when an engine would not start I took the propane torch and waved the flame back and forth over the cold side as well as any transfer pipes and such. Quite surprisingly, engines that seemed to be stubbornly refusing to start, started right up quickly after this treatment; warming up the still cold parts of the engine, when given a little spin by hand, without even applying additional heat to the hot side, (although still warm or hot from the attempted startup)

This makes sense to me in terms of kinetic theory. The heated air molecules have to transfer the energy they have acquired to the piston. If the entire engine along the way is cold, the energy will be absorbed by the cold walls of the transfer pipe, cylinder walls etc. before it can carry the energy to the piston, or through a chain of collisions with other molecules.

I think normally, the transfer pipes will warm up from the inside in the ordinary process of heating the hot side and repeatedly cranking the engine over, the pipes and cylinder walls will eventually warm up from the hot air traveling back and forth.

This method of warming up the transfer tubes and the cold cylinder with a torch for a quick startup seemed to work consistently on one engine after the other.

Instead of heating with a torch or something, I was thinking that transfer tubes made of non-heat-absorbing/conducting material would result in quicker startup as not so much heat would be lost just warming the engine up to "operating temperature".

Anyway, the theory may be unconventional and contrary to "known science" or accepted ideas regarding how these engines are supposed to work, but I see what I see.

So on that basis I went and picked up some plastic plumbing pipe and elbows and such to use as transfer pipes in the Ringbom conversion effort, since the distance the hot air has to travel through these pipes is unusually long, I can't help but think there could be quite a lot of loss there.

In my compression tests, the pressure seemed OK, until it got to the canister. Most of the pipe is stainless steel, which is rather non conducting. But inside the canister was quite a lot of copper wire and copper coated scrub pads, so the apparent pressure drop might have been a combination of the dead air space AND a lot of cold steel and copper wool material.

[Bumpkin]

I’m still pursuing making my Ringbom engine a Beta, but some realities force consideration of a Gamma, wherein the connecting tube could be a length of automotive heater hose. I was mostly interested in simple connection and isolating engine vibration from the heater. I don’t know if it would survive temperatures you are dealing with, but it should handle as much heat as plastic plumbing fittings. Just a thought.

Bumpkin

[Tom Booth]

Thanks, I may try that. though either one would mostly just be for experimenting, to see if it actually makes any difference.

When and if this thing is ever actually operational I want to be able to use it in a campfire so probably anything plastic or rubber near the hot canister would be out, but should be alright just heating it with a torch.

I spent all afternoon and evening working on the displacer. The steel they make these little propane bottles out of is some really hard stuff, I kept trying to cut the top off on the metal lathe and the cutter blades just kept wearing off, though I used plenty of cutting oil. I finally just gave up and used a metal cutting disk in the drill press and that got chewed up and had to be replaced but I finally got through it.

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Then I had to cut the top off another one to use as a kind of clamp to hold the diaphragm.

At least now it fits inside the canister. I'm changing the design again though. I'm still not sure there will be enough room. The displacer will only have about, maybe an inch, or inch and 1/4 or 1 and 1/2 or something space to move.

I have a little larger (taller, same diameter) canister that might work identical to this one but with a lid on both ends, but the rubber gasket on the hot end would be a problem.

I got further along, drilled bolt holes, soldered nuts on the inside and made a new diaphragm or three, out of different materials. but I didn't take any other pictures.

The canister is heavy, so I'm going to try the truck tire inner tube again as that is the heaviest diaphragm material I have available at the moment. I just remembered I also have some rubber roofing material I could try.

Anyway, just getting home from the shop at 2:00 AM. Time to get some sleep.

I guess the bits for the mini-lathe are only good on soft metal.

I'm still trying to figure out a way to hang the displacer with such little space.