Stirlingengineforum.com

Good afternoon all.
Discovered sterling engines a while ago and have decided I want to build one for our wood-stove to circulate the air in the house.

Inspired by the stovetop fan that youtuber Turbinator built see link: Turbinator's 12" stovetop sterling

My dad's got a love of neat mechanical gizmo's and I have a career in industrial automation so it's a neat little project.

The Build
Started off with some scrap steel 4" pipe for the displacement chamber and it's being welded to a round 'heat sink' that will rest on the stove transferring energy into the displacement chamber.
On top is a bronze bearing sleeve with a delrin power piston and a series of brass crank-arms that I designed myself
The crank-shaft is held up by 2 identical aluminum pieces.

As I read more into the design process I think I may come across 2 problems
1: The displacement cylinder and piston cylinder ratios are unknown to me, I kinda eyeballed them from some other models I saw,
2: I'm uncertain of how I'm going to maintain a good seal in the piston sleeve and the displacer arm sleeve while reducing friction to a minimum

More to follow, this is just my initial post.

Worked on the build tonight, I've got to figure out how to post pictures on this forum for you all to see and help critique this thing.

Made the crank-shaft arms out of brass and finished off the top plate, while I was working on it I decided I'm entirely unsatisfied with using 1/4" dowels as the crankshaft rods, and (having sized it using TLAR) decided to go with 1/8" rod instead. This should help reduce friction and weight.

Question
Does the displacement chamber (and drive piston) have to be air-tight?

If your goal is to only circulate air from the oven you dont need a stirling engine
https://duckduckgo.com/?t=ffab&q=stove+ ... &ia=images

airpower wrote: ↑Wed Dec 08, 2021 12:24 am If your goal is to only circulate air from the oven you dont need a stirling engine

I love these things, and you're totally right. If my only reason for doing this was the practical application, I'd buy a peltier type fan like these.

The goal, though, is to specifically make a sterling engine fan as both a challenge and as a eye-piece in the livingroom.

Here's the CAD image for the design.

I figured out that it's an absolute bitch to make two parts with perfectly coaxial bearings so I'm switching to a different design

[image]https://ibb.co/f0mshS0[/image]

If anyone can help me embed images properly send me a message lol

anyway in this image you can see I've swapped over to a single pedestal and a bore-hole with the two bearings. this will make it easier to manufacture.
you can see I have a seal on the piston, we'll see if that introduces more drag than it's worth.

Figured out how to put images in lol

Anyway here's the old plan and the new plan.

Welcome to the forum. Woodstove Stirlings are always of interest to me. If the 4” pipe is very thick it will carry too much heat to the “cool” end and you won’t get the needed temp difference. It needs to be as thin and non-conductive as possible.Same for the displacer itself. Length helps but everything is a compromise. I think a good compromise volume ratio for the engine to run down to moderate stove temps is about 8/1 total air working volume (including dead space) to power cylinder displacement. The engine doesn’t need to be air-tight. The tighter the better, but friction is the killer for these low-power engines. Good luck and have fun. Bumpkin

Bumpkin wrote: ↑Wed Dec 08, 2021 11:46 am If the 4” pipe is very thick it will carry too much heat to the “cool” end and you won’t get the needed temp difference. It needs to be as thin and non-conductive as possible.Same for the displacer itself.

hmm, good call.
I was thinking of gasketing the aluminum top block to isolate it from the pipe but had not considered that the pipe itself would be heating up
I'm going to have a think on this and get back to you.

Thanks also for the displacement theory, numbers are hard.
Current displacement cylinder volume is at about 30 cubic inches with the displacer (quite arbitrarily) at 16.8 cubic inches... Power stroke uses about .5 cubic inches, which leaves us with a much greater total air vs power cylinder displacement of about 60:1

With the current design, I could bring that down quickly by increasing the size of the displacer and reducing it's stroke appropriately, what do you think?

Did that math wrong

30 - 16.8 = 13.2 cubic inches total working volume
13.2/.5 = 26.5/1 working air volume/power stroke

maybe I should make the displacer a little larger?

Half a cubic inch power cylinder seems small to get much power from an atmospheric pressure engine. If you could increase that to get the expansion ratio to fit the thermal ratio might be better. But either way should work to some degree if friction is low enough. Your displacer chamber dimensions seem more like an LTD (pancake) engine to me, which can certainly work for a stove-top engine but there are different issues. If the chamber wall is short enough it doesn’t have much thermal exchange relevance compared to the chamber ends, and a thermal break (or two) diminishes the loss from the hot to the cold (less hot) side. In that case you might want a moving regenerator more than a displacer, such as a lot of LTD engines use, only in your case of course the regenerator would need to be temperature safe well above 600 F. And some kind of heat shield might be useful to keep radiant heat from the upper fan-cooled portion. Good luck and have fun. Bumpkin

I've been thinking about what you've said, there are a few things that can be done to increase my odds of winning this build.

Power Cylinder Displacement:
Because of the location of the power piston, I am hesitant to try and grow its surface area, however that might just be the sunken cost bias I have because I've already made the top plate. When I get to work tomorrow I'll see if there are any larger bushings in the spare parts bin maybe I can find something a little larger, say 1.5" across or 1.25"... if I do I'll have to re-make the top plate but oh well that's the cost of iterative designs eh?

Design Core
You pointed out that this looks like a pancake engine (LTD) and you'd be spot on, I based this design off of pancake engines and the following youtube link... I don't know what his internals are but I suppose it's probably a similar design, what do you think?
TURBINATOR'S STOVETOP STERLING FAN

Regenerative Displacer
I'll probably be moving to a regenerative displacer per @Bumpkin's suggestion, not sure what material I'd use so if you have any suggestions I"m all ears!

Next post I make I'll have some pictures from the shop with progress made!

I can probably use some fireplace insulation sandwiched between some screening as a regenerator...

Got some pics from the shop for you all.
Top plate on top of the base, I've got some radiator fins I'm going to bolt down with some heat paste and you can see the beginnings of one of my crankarms. Here's the fan I've purchased for this Brass pushrods

Biscuit_Ovens wrote: ↑Sun Dec 12, 2021 9:38 am
...
Design Core
... I based this design off of pancake engines and the following youtube link... I don't know what his internals are but I suppose it's probably a similar design, what do you think?
TURBINATOR'S STOVETOP STERLING FAN

...

Not to throw a monkey wrench into the works, but watching the video, the lighting is rather poor, and the huge aluminum cooling fins stand out making the engine look short and wide, but at one angle I noticed the engine in the video is taller and skinnier than it looks at first.

The very dark, unadorned, lower half of the displacer chamber is barely visible against the dark background.

At least, my assumption would be that the displacer moves between the hot lower half and top cool half with cooling fins, separated from one another by a kind of wide flat "shield" to disperse rising hot air from the stove top away from the top cool half of the engine.

That would, I think, probably make it more a high temperature gamma type engine rather than an LTD pancake type design. If it matters, as far as your design considerations