Building a Woodstove Fan Stirling Engine

[newtechnician]

Howdy all,
First post here but I've been using the forum for a lot of research. The post title says a lot. I'm making an engine to drive a small fan on top of my friend's wood stove for a christmas gift. I'm borrowing heavily from the beer bottle stirling engine that you can find on youtube here:
https://www.youtube.com/watch?v=2hi2SB3IciI

However, since my skills with cutting glass have been falling short and I work on old vw engines and have some lying around, I thought it might be fun to use an old VW cylinder for the displacer cylinder. They look like this:


First question: Can I use this and expect it to keep my temperature differential adequate for running the engine on a wood stove? My buddy says it can get up to 300 on the surface of his stove. I'm hoping the cooling fins will help vent heat and I can always put some heat sink fins on top of the engine if not?

The cylinder's dimensions are 86mm diameter, 136mm height. I'm thinking I'll make about a 110mm displacer piston with about a 20 mm stroke.

I've found some good threads on the experimental best displacement ratio between displacer and power pistons. Sounds like it needs to be 1.5 or greater (Greater being for an LTD Stirling). I found these thread which go into the details:
http://www.stirlingengineforum.com/viewtopic.php?t=48
http://stirlingengineforum.com/viewtopic.php?t=855

The largest piping I could find at my local hardware store for a power piston was 3/4" so I'm using this, but when I plug my calculations in, the stroke is something insane like 30 cm on my power piston! Obviously I won't be doing that so I'm wondering:
Can I treat this system as an LTD stirling and still run the thing with a crazy ratio like 15 to 1 on the power piston?

I also don't much care how powerful the thing is so long as it turns. Both my friend and I would get a kick out of this if it worked at all, so it doesn't have to be the optimum design for a wood stove fan. More just for the geek appeal.

Thanks for your help folks! If I get it running I'll post up a video!

[Bumpkin]

Hi newtechnician. I think you would have a tough time getting a useful temperature ratio with the VW cylinder as the displacer chamber. Unless you could somehow insert a heat-break gasket, the heat will probably migrate along the cylinder faster than you could heat and cool the ends.

Displacement ratios depend on temperature ratios. As long as it can handle the heat, a low temperature engine will run at higher temperatures, it's just not as efficient as a more optimized ratio engine. On the other hand, a high temp-ratio engine won't run at lower temperature differences.

Your friend's stove top reaches 300 degrees - F. or C.? If it's Celsius and we assume the heat-sink air right above the stove is 40 C (it's probably higher) then the absolute (Kelvin) temperature difference is 313 to 573. That's the potential ratio for air expansion, but the potential for raising pressure is considerably less because internal compression heating decreases the useful difference. And of course it would be nice to have the fan run at more consistently available temperatures. Here's another forum thread that dealt with stovetop fan ratios; I think in the end he went with about an 8-1 displacement ratio. Sorry for rambling. Good luck and have fun.

http://stirlingengineforum.com/viewtopi ... fans#p6434

Bumpkin

[Ian S C]

I have two stove top fans that I'v built, both with 30 mm bore x 20 mm stroke. The first one a Ringbom motor is actually too powerful. The other one is just right, it's a GAMMA motor, they both drive 8" fans.
Ian S C

[Bumpkin]

Sorry for a bit off-topic newtechnician, but Ian, is your Ringbom motor self-starting? If so, I realize it might start in the wrong direction as a fan, but it would be handy to power an alternator.

Bumpkin

[derwood]

Hey, I have built several stirling engines using vw cylinders. They work great for the cool side of the displacer.

[newtechnician]

Hi Derwood, so do you mean you used a vw cylinder on the cool side, and used a heat break across the hot side as Bumpkin suggested? I was hoping to use the vw cylinder as the entirety of the displacer cylinder, just like the glass piece shown in the youtube video. I realize that glass is way less conductive so I might be in trouble once it heats up but maybe I could put a wooden ring between the top of the cylinder and the cool side plate on the top?

[derwood]

I used a 92mm vw cylinder with a tin can slid over the end of the cylinder. The end of the cylinder that fits into the vw block. I used red silicone and a large hose clamp to seal the can to the cylinder. I used a smaller can for the displacer. I have not been able to find a stock food can that fits around a stock vw cylinder. I used 1/4" steel plate for the head and all thread to fasten the head to the cylinder. No heat break was used and it worked fine. A heat break would be even better. I also made a dual displacer stirling and used a third vw cylinder for the power piston. the ratio works out well. Here is another engine I built using vw cylinders. https://youtu.be/wtW9fYVFXOY

[Ian S C]

No unfortunately the Ringbom is not self starting, although with a bit of work that might be changed. Yes it could power an alternator, and might give 5 Watts which would be good as a charger. Heres a pic of the GAMMA fan, I don't seem to have a pic of the Ringbom.
Ian S C
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[newtechnician]

OK So I got working on my engine and have made some good progress. I know I'm on the right track but I'm not to the point where the engine will keep spinning on its own. I do know I'm close though, because it definitely doesn't like spinning in reverse nearly as much as spinning forward. I've tested it with up to 4 tea light candles underneath it. It definitely gets hot on the bottom and stays pretty cool on top for at least long enough that it should get working for a bit.

I'm hoping y'all can help me figure out what's going on!

Here's what it looks like all together.

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Detail view of the crankshaft. Please don't laugh at my soldering job
The current arrangement is that the power piston moves 12mm in total and the displacer moves 9. I've switched it around as well to see if that made a difference but nothing big.
You'll see it's on ball bearings so it moves pretty freely. The crank is counterweighted but I haven't done much to perfect that balance.

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Here's the power piston out of its groove, it's sanded to 1500 grit and I think looks like a good finish. Same with the inside of the power cylinder. The fit between those two is that if I plug up the hole below the power cylinder, it takes the piston about a second to fall while the air slides past the pston. Seems like a good fit but maybe that's too loose?

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Here's the displacer piston with the cover removed. You can see the ring of RTV around the top too. When the whole thing is together the seal is quite good. I can't get much air to leak and what does is coming slowly past the displacer hole while I blow into the power cylinder.
I'm curious on your opinions for the displacer piston. Does it look too loose? When I drop it in, it drops smoothly but it is just a bit held up by the air it's displacing. I'd say it's probably closer to 1/8" (3mm) gap all round when it's centered. Another issue I'm running into is I'm not sure exactly how centered the piston is when the assembly is together. I did the best I could with that but still not sure.

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So my questions are:
Is the displacer piston too loose when it operates the way I describe?
How much does crank geometry play in and am I way off base with mine?
I definitely get some suction and some push from the power piston, but ideally should I see the piston really pulling and pushing on the crank or just pulling and kind of offering very little resistance on the up stroke?
Is the geometry totally wrong between my two pistons or can I make this arrangement work? It doesn't seem THAT far off the youtube video at the top of this thread.

I know you folks have talked about how the vw cylinder might not set up a great temperature gradient after a while but it should at least run for ten minutes until it gets hot on the top plate, right?

[Ian S C]

I think I would tidy up the crankshaft, I can't really see What's happening there, the displacer should lead the power piston by 90*. How well is the hot end of the displacer cylinder sealed, there must be no leaks, and while it may not leak when it's cold, there may be distortion when it's hot. The plate on the bottom can be quite heavy, say 1/4"/6 mm, the top one could be a similar thickness, or even greater, and aluminium would be good here. Can you pass the bolts through the holes in the cylinder (the original cylinder bolt holes).

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