LTD Stirling engine

[Powder keg]

This one uses a small piece of Chain to lift the displacer up and down. This might look kind of neat when running? I wonder how good it would work? It would be easy to make the pieces.

Wes

 

[mklotz]

So the top hat shaped aluminum piece fit in a hole in the foam? I imagine that there was a hole for the con rod to glue into? I'm thinking of gluing it and having a 2 piece con rod held together with some silicon tubing. If I can find some tubing small enough?

Maybe it would be better to have a more mechanical conection between the two conrod pieces that silicone tube?

Thanks for your input, Marv.

Wes

Not "into" but rather the brim of the tophat was glued directly to the displacer disk. There's no provision for attaching a con rod. A Ringbom Stirling has a free-floating displacer that is driven to and fro solely by the changing gas pressures inside the engine. In fact, it is that rather remarkable fact that distinguishes a Ringbom from more conventional designs.

Silicone tubing is fine as a connector for such a flea-powered engine but sort of visually tacky. A thin bit of brass tubing and a drop of Loctite might make it look more mechanical. (Loctite breaks down when heated if disassembly is required.)

 

[Powder keg]

Gotcha, Marv. I was thinking you had it inserted into the foam to make the connection between the con rod and the foam more durable. I was thinking a more conventional engine with a mechanical connection between the flywheel and the displacer.

I think the silicone looks tacky also. Are you talking about a really thin tubing that would flex back and forth as the piston went up and down? So it would be connected solid to the piston and the crank would have a bearing of some sort?

Thanks, Wes

 

[mklotz]

Wes,

Maybe I misunderstood you. An LTD Stirling is unlikely to have the power needed to flex a piece of tubing (if that was your intent). The connecting rod needs to have a very low friction connection. Exactly how you establish that is a design detail but it must be as low weight and friction as possible.

Stirling engines come closer than most engines to operating on a true Carnot cycle. That means that their efficiency is given by:

efficiency = 1 - Tc/Th

where Tc and Th are the cold and hot temperatures respectively (expressed in degrees Kelvin). Taking:

Tc = 32 F (ice) = 273 K
Th = 100f (sun warmed) = 311 K

we have:

efficiency = 1 - 273/311 ~= 12%

So we're talking about engines that, at best, can only convert about 10% of the energy they receive into useful work. (In fact, it's absolutely astounding that they can be made to run at all.) With that sort of dismal performance, anything you can do to reduce friction is very important. Never attempt to lubricate an LTD. The viscosity of any oil will overpower the engine. That's why the self-lubricating quality of graphite is so desirable.

 

[Powder keg]

Wes,

Maybe I misunderstood you. An LTD Stirling is unlikely to have the power needed to flex a piece of tubing (if that was your intent). The connecting rod needs to have a very low friction connection. Exactly how you establish that is a design detail but it must be as low weight and friction as possible.

I think We misunderstood each other LOL. I understand how little power these have. You were talking about using a thin brass tube to glue the two half's of the con rod together, right? (to make it look better) I think the engines that use the silicon tubing rely on that joint to flex because the rod is fixed solid into the piston's. I couldn't see how the brass would flex unless it was very thin.

The reason I want to use graphite in so many areas (main bearing, power piston, and displacer piston seal) is because you don't oil it. I want this engine to be maintainence free.

I can't wait to see what we come up with, Wes

 

[Powder keg]

I have some plans that makes use a heavy plastic storage container to make the displacer cylinder. I think I'll use this method because it will be cheep and easy. The plans even used the bottom of the container to put ice in, so the engine will run backwards. A ring is cut out of the middle to make the cylinder.

So far I've decided on Aluminum top and bottom plates. I'm going to use graphite for the displacer seal, power piston, and main bearing. I'll probably use a CD for the flywheel then in a later model, I'll use a flywheel machined on a CNC. (to make it look nicer) I'm thinking of making one piece that holds the power cylinder and the main bearing tower and maybe the displacer seal? It would bolt to the top plate. Then again they should bolt on seperatly?

I can't make up my mind on the details for the con rods? I want them to be durable and easy to build. I have seen them that are just strips of thin brass. They seem to work fine. Polished up they look nice also. I'm also thinking that with a engine this small they would last forever? Any opinions on this?

Later, Wes

 

[Powder keg]

If they can make a LTD almost completely out of paper, I shouldn't have any trouble, Right?

Wes

 

[rake60]

Now THAT is the coolest thing I've ever seen!

Rick

 

[Powder keg]

I haven't had time to scan my drawings yet, but I did get over to Walmart and found this.

It's a 58 oz. acrylic canister. I'm planing on cutting 2" off the bottom for ice storage. Then I'll cut a .750" long ring off on my bandsaw for the displacer cylinder. I'll then sand the end's smooth. How does this sound so far?

Wes

 

[Powder keg]

Here as a really rough sketch. I don't know how this will look?

The black parts will be graphite. I like the wire style con rods better. I think they will look better? The one for the power piston, will need a tab soldered onto the bottom. I think the bends in it will let you adjust the stroke some.

I'm planing on some stainless screws to connect the top and bottom half's to minimize heat transfer.

I can't decide on using a regular con rod on the power piston or a chain? I've seen several that used a chain and it looks neat while running.

Wes

 

[Bogstandard]

PK,
Why not use plastic rod for the spacers, something like delrin. That should stop most heat transfer. Or if you want to be a real cheapskate, pinch a couple of your grans plastic knitting needles.

John

 

[Powder keg]

Plastic would work fine also. Just some shafts with the end's threaded? That way I could use brass screws on the top to Bling it up:O) A shiny engine runs better, Right?

Wes

 

[Brass_Machine]

What sort of dimensions are you looking to do? Like the base... etc

Eric

 

[Powder keg]

I bought 4 of those containers. I was planing on cutting one to get the dims I need and to see how it will workout. I think there is enough jar to get 4 or so cylinders out of it? I'll start getting some of the other dims soon also. I want to have a good plan of attack before I start. The power piston is .625".

I'm probably going to use silicone to seal the cylinder. I'll make a bead round the rim and lay it down on some waxed paper to make the gasket. After the silicone has dried, I'll remove the paper, de-flash the silicone, and do the other side. Sound good?

Wes

 

[Powder keg]

It took about 30 min with my bandsaw and belt sander to whip out a ring for the displacer cylinder.

I also cleaned up the bottom piece to hold ice.

The inside of the cylinder is 4.600" and the outside is 4.800" and the legnth is a little over .750". I also found some aluminum that is .085" thick. I'll use this for my top and bottom of my displacer cylinder. I mainly did this to get dimensions and to see how it was going to be to work with. On the weekend of the build I'll re-build everything.

Wes

 

[Powder keg]

I found a [ame=http://www.youtube.com/watch?v=okYJV7giADI]Video[/ame] of a nicely made LTD. It uses wire Con Rods.

Some of these engines have longer strokes on the displacer. Is there an advantage to having a long stroke to a short stroke here?

Thanks, Wes

 

[Powder keg]

Thanks for the input birger. Do you have any pictures we can see?

Thanks, Wes

 

[mklotz]

On my LTD I simply used Vaseline to 'seal' the displacer chamber. Remember that these engines will never see any temperatures that your skin couldn't tolerate so sealing need not be a big deal.

 

[Powder keg]

Very nice job there. Thanks for the inspiration. What is that power piston made out of? It looks like some sort of plastic. That center bolt idea just saved me a bunch of work on my weekend engine.

Thanks again, Wes

 

[BobWarfield]

I'm probably over thinking things, but the center bolt seems like it would conduct the hottest part of the underside to the top, which is what you're trying to avoid in an LTD. As long as you're messing with that plastic anyway, I wonder if the center bolt could be threaded at both ends and the "nuts" made out of the plastic so it would stay insulated?

Nice looking LTD!

Best,

BW