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David Kerzel, Pompano Beach, Florida david@FloridaAME.org
Stirling Cycle, Modified Ringbom configuration, for low differential temperature operation First posted 7/15/2003
One of the members, John A Hastings, sent a picture of a Ringbom engine. I wrote him to find out what it was and how it worked because it does not appear to have enough parts. He recommended books written by James Senft about Stirling and Ringbom engines.
The Ringbom is a derivative of the classic Stirling. It uses internal pressure to move the displacer rather than a second crank. The original patent, from 1907, had a extra hole to prevent pressure from building up inside the engine when heat was applied. Eliminating this hole makes it "modified" and more efficient.
5/31/2009 It is a good looking easy to start engine that runs well. It needs about 40°F to run and 45°F to run fast. As it runs the cold palte warms up over 15 minutes and it slows down.
As it runs there is a tick noise as the displacer hits the cold and hot plates. A true Stirling would have been silent. I suspect that would have been a better choice. Video running (Windows Media)
Like all Stirlings when it is cold it feels like it could never run. Give it a light flip and the compression stops the piston then it slowly drifts to the bottom of the stroke. Video (Windows Media)
Building the Engine
I had purchased Jerry Howell's full size low temperature Stirling hard to find parts and was going to build his engine. I became consumed with the simplicity of the Ringbom so I modified one of the plans in the Senft book "Miniature Ringbom Engines" to use Jerry's parts. Then I started to simplify the design to make it as simple as possible.
This should be a simple quick project. I will post in progress pictures. If it works the plans will be posted.
The engine will have glass cylinders and the cylinders will be the only support between the top and bottom sections. 7/4/2003
The heat exchange disks are made from 1/8 aluminum. They could be turned and the holes layout done later, a better edge finish would probably result.
I like the rotary table and just dialing in the numbers. I rough cut the disk on the band saw, held it to the wood disk with double sided tape, drilled the outer holes, and used #2 wood screws to hold it more securely when milling the edge.
It looked like a good idea, but it turned out to be a lot of work to make the flywheel from rectangular stock.
8/10/2003 All the main metal parts
are complete. Next will be fitting and getting the pistons and displacer to
Well for a simple project it has taken a long time. It appears back in 2003 I dropped one of the glass cylinders and it cracked. The engine went in a box and has been waiting. I got a new cylinder and it was in the box and ready to go. Looks like all that's needed is gluing the shaft to the displacer, sizing the displacer, getting the parts to move freely and it should be running, 6 years late.
I epoxied the displacer shaft in using a V block to insure it was perpendicular to the disk. I put the shaft in the lathe and used a sanding drum in a air powered tool to sand it round and to the correct diameter. I left the tape on from gluing to add more strength during this operation. I forgot how long it takes to get everything moving just right. The power piston falls trough the cylinder but if it rests on paper it takes about 15 seconds to slowly make the stroke. Just like in a sterling there can be no binding, everything needs to move very freely.
I re-read the books by James Senft, and understand how it works and how to get it working. I did not put a compression relief in the original design. This is a screw that allows air to be vented out of the cylinder to set the phase of displacer and power cylinder after a long down time. The book explains air leaks out and leaks back in but with the tight fits needed for a good low temperature engine it might take a while to get things back to operating. The vent allows the power cylinder to be easily moved with the displacer full down. After playing with he power cylinder I think I will need to add one
It is together and it runs. Like all Stirlings it is moderately hard to turn the thing when its cold. As it warms up it gets easier and easier to turn then suddenly it keeps running. When it is cold moving the piston down about 3/8 inch causes the displacer to pop up. When the vent is open, the piston just falls to the bottom. When the vent is closed it takes 20 seconds.
It takes more heat than I expected, probably 60F. I think the suction from the displace at top of bottom is slowing the operation. My displacer is solid and the book recommends adding regenerators which are heat transfer ports that allow more air movement .
I added the regenerator material, some blue air filter material. I think it ran better before. But it is dome now.
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