1/4 Scale Rider Ericsson

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hopperwhistle

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Almost finished but still not running quite right... now to build a base, and tinker with it for the next month...
 

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Very nice looking model, if I may ask what is not running for you the way you would like?

Bob
 
The model was partially built by someone else and had no prints when I acquired it. The prints I found later for it didnt quite seem to be the same as the prints the first owner was using, so I had to work around some slight differences to start with. It runs but takes a ton of heat, the displacer cylinder was made from copper which I have read on other posts doesnt work well. The displacer piston is rubbing on the cylinder wall very slightly and I think the clearance between the power piston and displacer piston is off a little. Do you know what the clearance at the top and bottom of the displacer cylinder should be?
 
Yes that displacer cylinder made of copper is an issue. A question is the bottom of the displacer cylinder copper? I have some ideas on how to minimize the copper issue. Remember friction is the bane of hot air engines, zero is the goal. The dragging of displacer piston is an issue and must be addressed. The displacer piston needs to be as light as possible. As for the clearance issues you ask about see attached PDF, go to the drawing and study it over. It has the information you want.

As for the heat issue, ie needs lots of heat you need to control that also. You should be able to put your finger on the base corner next to the cylinder. It should be warm to a little hot but not burning. After 5 hours of running my finger can stay on it. You need to keep that heat focused at the bottom of the displacer cylinder. How big of a burner do you have? Mine is about 3/4" diameter ceramic burner I made. If I had your engine I would wrap the displacer cylinder with about 1/8" to 1/4" insulating material ie asbestos like material. That would help keep the heat in the displacer cylinder. Next the gasket between the displacer cylinder and the base at least 1/8" insulating material.

I assume you have read the many fine articles on the web on this engine. Here is a link to a 98 page pdf file on my build, you might find some useful information.
https://www.dropbox.com/s/fzv8noejd...ot-Air Pumping Engine Build Complete.pdf?dl=0
Bob
 

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Thanks for the great information! After looking over everything I dont think the engine is far off. The displacer cylinder was made out of copper with a stainless steel bottom as shown in the print I have. I had already figured I might have to replace it but I like the idea of trying the insulation you suggested. I have not built a proper burner yet, for testing the engine I just used a propane torch pointed upwards towards the base of the cylinder, it took two torches and a very long time to heat up. The rubbing displacer piston I think can be corrected easily, the prints I was using showed one set screw securing the piston to the rod, another set of plans I recently found has two set screws 180 degrees apart, I think that will help keep it centered better. I think the clearances of the displacer piston are pretty close to being right
 
That displacer piston is the opposite, the copper should be on the bottom for max heat transfer, and stainless on the side to minimize heat transfer. Stainless is a poor conductor of heat. If you are daring. I would replace the bottom with copper, about .032. I used type M copper pipe, cut about 3" long, annealed several times and rolled flat with a large metal tube. How thick are the walls of the copper displace tube? If thick enough you could machine a heat dam near the top. A heat dam is a groove in the diameter. My wall thickness was .065, and I took it down to .030 for about .100 length. What this does is slow down the heat transfer. The thinner the material the slower the heat transfer. There are pictures of all this in the pdf, page 51.
 
The side walls of my displacer cylinder are also 1/16”. When I get time in the next few weeks I look forward to trying some of your suggestions. I’ll just try the insulation and the heat dam ideas for now, if that doesn't help enough I’ll make a new cylinder. So I am curious what was the original thinking when the engines were designed with the copper cylinder and stainless bottom? I have two different prints that both show that construction and an unfinished Denny hot air engine with the same copper cylinder.
 
I had never built a hot air engine, so I consulted some friends especially one who had built many of hot air engines as well as selling castings for them. Page 2 of my document has the changes I used and some of the whys. Here is a link to an article in Gas Engine magazine explaining some of what you are looking for.
https://www.gasenginemagazine.com/gas-engines/model-hot-air-pumping-engine/
Be carefully machining that copper it can grab. It is 1/16 ie .0625, I would not go deeper than .020 maybe .025 x .100 wide. Use your favorite lubrication for copper ( kerosene is one), no carbide use HSS cutter. Just my approach, limited copper work.
 
I made mine with a thin stainless cylinder and the bottom from copper to increase local conduction. I also made an all stainless cylinder for my Ky-Ko fan which works very well. Both engines will run on a simple spirit burner.
Fits are critical as mentioned previously and heat barriers too in the way of gaskets and local thinning of cylinder walls.
 
Thanks to both of you for the good tips. I have some thin wall stainless to use if I have to remake the part.
 

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