LTD Stirling engine

[Powder keg]

I've been wanting a LTD Stirling so bad I can taste it. I want it to run next to my computer. The problem is I don't have any time. So What I've been thinking about doing is taking several plans I have and combining a few features, that I like, to make a new engine. This engine would be fairly simple, inexpensive to build, and a LTD. I have a few ideas that I want to try. I'd use proven piston sizes/strokes to ensure that it should run:O) Also I'd like it to be running in one weekend. From start to finish. We would call it the "Weekend engine" or something like that? It would then be posted in the plans section.

I was thinking this could be a group project. We could hash out the design and materials. Maybe someone would be kind enough to draw up a nice drawing of it? I've seen some neat 3D renderings(wink, wink) This would give us time to get any materials/tools we might need. The only exotic, harder to get, material I really want to use is Graphite. (I have a couple ideas I want to try) The rest would have to be off the shelf and stock easy to find items. After we get it all designed, The plans would be posted here for the whole world to enjoy.

The only other criteria, I can think of is no balloons for pistons. I want this to look expensive when its done.

What do you think? Is this doable? We could plan on a weekend and have a build log. Maybe a couple others would like to build one of their own the same weekend?

Later, Wes

 

[Brass_Machine]

Ok. I'm in.

This is a good idea. I have some other 'group' projects in the back of my head that I have been wanting to present and work on. This is a good way to find out if a group design and group build would work.

I also like the idea of "weekend Engines"

Not sure what skills I can lend to the project... I do have some CAD skills...


Eric

 

[BobWarfield]

We could easily build almost any engine in a weekend if we were to divide and conquer. How about a "pot luck" engine? An interested group designs it, each member is responsible for producing enough of one part for everyone, and then everyone mails each of the other guys the parts. Should be able to build a single part for an engine in a weekend, no? It would require some precision for it all to fit together at the end.

Just a thought...

Best,

BW

 

[wareagle]

That is a very interesting concept. I would say let's go for the Stirling, and see how it works out. No sure what help I could be with the design of the project, but I could do drawings of the final design for publishing if need be.

Another thought I have had goes along the lines of a group project. My thought is to have the members here all make one part for an engine and then send all of the parts to a designated assembly point (maybe a show). After it is assembled, then auction it off and donate the proceeds to a charity. If there is enough interest, maybe build up two engines for the purpose. We could most likely even fund the site this way.

 

[Powder keg]

Glad to see there is interest in my Idea. There are quite a few interested in Stirlings, but not that many actually building them? I thought a design that one person could build in a weekend might spark more builders to get off the puter and start making chips? Especially if they see that it can be done. Some of the designs are more complicated than I think they need to be. I have always used the KISS method when I'm designing and building something.

I'm going to start throwing ideas out here and younz (that was for you Rick:O) can join in. I'll draw up some sketches and we'll see where this goes.

Stirlings don't make much power. Usually, just enough to make them selfs run. For this reason they need to be friction free. I'm planing on using a brass cylinder and a graphite piston. I want to use graphite for the main bearing. I have never seen graphite used this way before. (might work?) I also want to use graphite for the seal for the displacer. I have machined graphite before and it is nice to work with. Dirty, but it machines really nice.

For the con rod bearings I want to use plastic with a groove machined in it to accept bent wire conn rod. These would be easy to machine and if reamed .001 over, they would be free running and light. This is where I was going with my "Eyes" topic.

I have some plans that use that foam with cardstock on each side. I think this would be one option? it looks easy to work with.

All Ideas are welcome, we all might find something usefull?

Later, Wes

 

[BobWarfield]

WarEagle, I like the charity idea too.

Anyway, I'm game to participate any way I can. I have drawing capability, a lathe, and a mill.

Sterlings are known to be finicky, so if we build a group project, we should do one that is proven. Otherwise, would love to see someone's "weekend" Stirling.

Best,

BW

 

[Powder keg]

I to like the charity engine idea. As well as the group effort idea. But I don't know if this will even work yet and I'd hate to build ten or more that don't work. I'm game for one built from a proven design. Steam might be a better option there, because these stirlings are known to be finicky.

I'll post some sketches tomorrow.

Wes

 

[Brass_Machine]

I to like the charity engine idea. As well as the group effort idea. But I don't know if this will even work yet and I'd hate to build ten or more that don't work. I'm game for one built from a proven design. Steam might be a better option there, because these stirlings are known to be finicky.

I'll post some sketches tomorrow.

Wes

Wes... (and everyone else)

I personally think that we should separate the "group build" charity engine from the "group design" LTD. I would like to do both.

The way I see it, we could do group designs using ideas from all of our members. Working out the design etc. I have a few design ideas for engines I would like to do group designs on.

Then on the other hand, we could do group builds for various engines and donate them for charity. If the group designed engine works into a group build, then so be it.

So what do you think? We have 2 separate ideas going? Group Design and Group Build?

Eric

 

[mklotz]

Building an LTD Stirling in a weekend is possible but something of a stretch.

If you're in a rush and not upset with the idea of using commercially available components, consider using a dashpot* for the cylinder-piston assembly. They come with a threaded fitting on the end so it's only a matter of drilling and tapping a hole, screwing it in and attaching the piston rod to the flywheel.

I used this approach in the Ringbom LTD Stirling I built

and it worked very nicely indeed. In this engine I used graphite for the displacer bearing and that worked well too. Graphite is self-lubricating and that's important for engines with such low specific power. Oil, even the lightest, can bring them to a halt.

Graphite rods are available as EDM electrodes from most of the usual suspects. Though fragile, it machines beautifully.

Although mechanically simpler, I don't recommend a Ringbom Stirling as your first attempt. They're very tricky to design and quirky to run in an LTD configuration. Use a standard design where the displacer is driven off the crank.

-------------------------

* A dashpot is a mechanical damping device which develops a force proportional to velocity (as opposed to a spring force which is proportional to displacement). It typically consists of a cylinder of quartz glass or clear hard plastic into which is closely fitted a graphite piston. A valve is fitted at the top of the cylinder. By adjusting the valve the resistance of the piston to motion can be changed. (With the valve fully open it functions as a simple piston in a cylinder.) Despite the advent of PLCs for machine control, dashpots are still used in some forms of machinery and are available commercially.

 

[Powder keg]

Hey Marv, I like that one:O) I know it's a stretch, to build one in a weekend, but I think it would be fun to try. Then if I suceed, I'll have my own LTD Stirling:O)

I have a few of those dashpots (mine aren't threaded so they have to be glued on) that I want to use eventually, but I want to keep the materials easer to get for this engine. I was planing on designing the engine around these, so they could be used in the future. That way people can see that they have different options.

I'm glad that the graphite bearings worked for you. I haven't seen this used before. I like the skinny bearing post that holds up your flywheel.

No I need to see if my scanner will work on my new puter?

Wes

 

[two dogs]

Marv,
I like the idea of using dashpots.
Where did you get those pictured on your engine?

Thanks
Mark

 

[Powder keg]

Does it matter much on these, what the top and bottom plates are made from? I've seen them run made with aluminum, brass, plastic, and I think stainless? I would imagine stainless would be the best? But they seen to work good on all these materials.

Wes

 

[Powder keg]

You can get them Here

Wes

 

[mklotz]

Does it matter much on these, what the top and bottom plates are made from? I've seen them run made with aluminum, brass, plastic, and I think stainless? I would imagine stainless would be the best? But they seen to work good on all these materials.

Wes

Aluminum is the most common material. I've never seen plastic used for the plates on an LTD and wouldn't recommend it. However, the displacer chamber is frequently made from plastic (PVC drainpipe in mine pictured above) because one wants it to be non-conductive so the temperature differential between the hot and cold end can be maintained.

Stainless would probably work. Brass would be pricey. For a first time engine, I'd stick with a proven material - aluminum in this case.

 

[Powder keg]

Here is the one with the plastic top plate Marv. It will run on solar power.

Solar Stirling

Later, Wes

 

[two dogs]

Thanks Wes. I'll check it out

Mark

 

[mklotz]

Here is the one with the plastic top plate Marv. It will run on solar power.

Solar Stirling

Later, Wes

Yes, but that one is heating the air via light absorbed by the displacer and re-emitted as infrared so, obviously, the upper plate must be transparent to allow the light to reach the displacer.

The more conventional design heats/cools the plates rather than the displacer piston so one wants a material with a reasonable coefficient of conduction.

 

[Powder keg]

I think I have some scrap aluminum around here somewhere:O)

What did you use for your displacer piston Marv? Nearly all of them use Styrofoam I've seen a couple that use that Styrofoam board with card stock on each side. I'm thinking the plain Styrofoam would be easiest? One had a piece of 1/2" wood dowel glued in the center to make the connection stronger.

Wes

 

[mklotz]

All my LTD Stirlings use styrofoam for the displacer. If you can't get sufficiently thin material, you can stick thicker stuff to a faceplate and 'machine' it down using a sanding disk in a Dremel held on the toolpost. The posterboard material faced with thin cardboard is unnecessarily heavy. I'd avoid it in an engine with such small specific power.

For the Ringbom, I made a 'tophat' shape with a wide brim of aluminum (drilled out to reduce weight) and glued that to the styrofoam disk. Be careful, many glues contain solvents that will attack styrofoam. Offhand, I don't remember what I used but it was probably Gorilla Glue or white woodworking glue. If you can't find a suitable glue, use double-sided carpet tape (everyone should have some of that in his toolbox).

Note that the displacer should not contact the chamber endplates at either end of its travel.

 

[Powder keg]

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