Possible LTD From Found Tube

[theropod2]

All,

I found a section of what I think is plexiglass. It is 7" long with an ID of 5 1/8" and an OD of 5 1/2". It has a few scratches but no cracks and it is cut very square now and with smooth ends.

Plexi Pipe

plexipipe01.jpg (250.68 KiB) Viewed 9973 times

I was thinking of building an LTD engine around this as the displacer chamber. I was honestly thinking of attempting to make an LTD version of the walking beam featured by our host. I want some opinions.

Should I cut the length down to make the stroke shorter? If so to what length and with what tooling?

I have a sheet of 1/8" fairly soft aluminum which is easily large enough to make both the heated end and the cool end. I also have a pile of 1" thick poly foam I could use for a displacer. I also have an Airpot® glass and graphite piston/cylinder. There is a small drawer full of bearings left over from salvaged VCR's.

I think this is far too nice of a find to just allow to lay around in a useless state. I'm open to suggestions or other ideas.

Well?

RS

[jimlarsen]

That is a great find for LTD applications! The displacer does not need to be very tall, since LTD engines of this type already have the hot side and cold side isolated from each other. Many of my LTD designs use 1/4" poster board, and they work very well.

The trick with the LTD design is to do everything you can to minimize dead space, eliminate friction, and maximize efficiency. Use a closed cell foam for the displacer if you can.

If you have a lathe, you can use that to cut your plexiglass tube.

You can find plans for a pancake style LTD engine in James Senft's book "Introduction to Low Temperature Differential Stirling Engines."

[Ian S C]

You could cut it in five 1 3/8" lenghts, if you mark it carefully, and work your way around carefully with a saw, then lay a large sheat of sand paper on a flat surface, and smooth the cut edge. with the 1" foam this would only give 1/4" stroke, it would be better if it was 3/4" thick. For the top and bottom, these could be attached direct to the perspex with small self tapping screws. Sounds as though you have enough bits in your junk box, so good building, could be worth making more than one of different design just to compair them. Ian S C

[theropod2]

You could cut it in five 1 3/8" lenghts, if you mark it carefully, and work your way around carefully with a saw, then lay a large sheat of sand paper on a flat surface, and smooth the cut edge. with the 1" foam this would only give 1/4" stroke, it would be better if it was 3/4" thick. For the top and bottom, these could be attached direct to the perspex with small self tapping screws. Sounds as though you have enough bits in your junk box, so good building, could be worth making more than one of different design just to compair them. Ian S C

What if I cut the pipe to 1 3/4" and use the 1" foam? That should leave me a 5/8+ stoke if I leave 1/16+- at the ends, or is that too much dead space? Would cutting the poly foam with a hot wire seal the edges good enough, or do I need a different material for the displacer?

I'm a little scared to try to cut it with my hand saw for wood. I'm afraid I'll get chatter and cut way off square. Perhaps I could use a hack saw to cut a little, make a turn and cut some more and repeat until through. I don't have a lathe, or know anyone that does. Sanding the cut end like that is a good idea, thanks.

I was thinking of using two blanks of plywood a little bigger than the OD, running a hunk of all thread through them and clamping the pipe with nuts while I spun the all thread with my drill. I could then vice up my saw and try to hold the drill as steady as I can. I'm a little afraid of doing that too.

I might be able to find some 3/4" foam if you think that would work better. I was wondering if I should visit a hobby shop and buy some balsa plank, or maybe the poster board jim mentioned.

What about the flywheel. Should I keep that as light as possible? Suggestions?

I was thinking of using the tube from a cheap portable radio antenna as the connecting rods and maybe a sleeve/gland made out of the glass beads they sell at sewing centers. Perhaps a fine knitting needle as the displacer rod.

Would JB Weld be good enough, or overkill, for attaching bits together? Perhaps the clear 5 minute epoxy would work. I don't have the tools to tap threads for the crank supports. Would short self tapping screws work OK as long as they don't protrude into the displacer chamber?

Thanks all for any help or advice. I've never tried an LTD before and as you can tell I'm a little nervous about screwing up this sweet pipe.

R

[Ian S C]

Cutting the foam with a hot wire worked Ok for me. First I had to cut the foam to the right thickness, so I rigged up a wire at the required hight above a board, hooked it up to my workshop low voltage DC supply and slid the foam across the board. I then put a hole in the middle, using a vertical wire, and the foam mounted in a pin , rotated it, all finished. It was a crude set up, I adjusted the heating by varying the length of the wire, I used wire from the heating element of an electric heater.
1/16" would be good for the end clearance, I think mine is about 3/32".
Cutting with a hacksaw, taking it slowly by hand is OK, but I don't think much of the idea of rotating it with the drill. Cut a groove right round, then cut through, wrap a bit of masking tape round the tube, and follow the edge of that, The tape will protect the perspex from scratching.
My motor is 6" dia, the top and bottom were frying pans, the discs are 7 5/8" dia, the top one was bigger, so I cut the extra metal off, and ended up with a bit 8 1/2" dia OD, 7 7/8" ID x 1/4" thick to make the rim of the flywheel. I made a hub from a bit of aluminium 3/4" dia with 6 holes around the edge. Ithen drilled 6 holes part way through from the insisde of the rim. I then cut up some bike spokes, and fitted them in the holes with some glue. The main thing is, you want a reasonably large dia with not too much weight.
Eventually I may get a photo up in my gallery, but don't hold your breath. Ian S C

[theropod2]

Cutting the foam with a hot wire worked Ok for me. First I had to cut the foam to the right thickness, so I rigged up a wire at the required hight above a board, hooked it up to my workshop low voltage DC supply and slid the foam across the board. I then put a hole in the middle, using a vertical wire, and the foam mounted in a pin , rotated it, all finished. It was a crude set up, I adjusted the heating by varying the length of the wire, I used wire from the heating element of an electric heater.

Since I live off grid using a 12V DC system I was thinking about a cigarette lighter coil. Take that wire and run it through a big rheostat like the dsahboard lights dimmer from an older car. I was thinking of setting the displacer rod with a dab of silicone sealer, but I'm open to suggestions there. I doubt my poly foam will accept most epoxies without melting. If I could find a slab of low density polyurethane would be best. I could thread the rod and sandwich it with small nuts, but those add weight.

That's one reason I've been thinking more about the balsa. I could build a framework around the displacer rod, being mostly hollow with radiating struts of very thin parts between the top and bottom, and wrap the sides like a banjo. Just about any good 2 part glue would work with no worries about a meltdown.

1/16" would be good for the end clearance, I think mine is about 3/32".
Cutting with a hacksaw, taking it slowly by hand is OK, but I don't think much of the idea of rotating it with the drill. Cut a groove right round, then cut through, wrap a bit of masking tape round the tube, and follow the edge of that, The tape will protect the perspex from scratching.

Should I get a new fine toothed blade for my hacksaw or do I need a little more coarse tooth count?

My motor is 6" dia, the top and bottom were frying pans, the discs are 7 5/8" dia, the top one was bigger, so I cut the extra metal off, and ended up with a bit 8 1/2" dia OD, 7 7/8" ID x 1/4" thick to make the rim of the flywheel. I made a hub from a bit of aluminum 3/4" dia with 6 holes around the edge. I then drilled 6 holes part way through from the inside of the rim. I then cut up some bike spokes, and fitted them in the holes with some glue. The main thing is, you want a reasonably large dia with not too much weight.
Eventually I may get a photo up in my gallery, but don't hold your breath. Ian S C

My top and bottom plate material is nice and soft. I can cut it with my jigsaw. I think I'm going with a square bottom and use some small self taping screws into pre-drilled holes in the plex through the aluminum. I think 10 will do. I can use all-thread to make the base stand off the running surface using holes out on the corners. Leaving room for a small heat source to slide under the engine. Do you think it best to apply a very thin bead of red silicone sealer at the bottom plate/plex contact point, and if so before or after setting those little screws? I was thinking after the screws so the displacer plex won't wiggle making a mess. I think a 1/8" bead would suffice.

The bottom of the engine with the displacer plex attached wouldn't change from that point on and I could make the cold end removable for fitting things that didn't work just so, which I expect to encounter. I was thinking of building a water jacket on the top section with extender tubes for the displacer/gland and power piston. That would leave room for 2 or 3 ice cubes to melt. I want that option to cool the hell out of one end especially during the first few run attempts if there might be issues.

I'll try to keep a picture record of things as I go, along with a series of posts here, to document this little project.

R

[Ian S C]

To run mine I either sit it on a small bowl of boiling water, or(and more often)I sit it on top of a pot while cooking my lunch. Boiling water is as hot as I have tried, any hotter endangers the foam displacer, I suppose one of those little tea candles would be OK, but if you don't need fire,why use it. Ian S C

[theropod2]

To run mine I either sit it on a small bowl of boiling water, or(and more often)I sit it on top of a pot while cooking my lunch. Boiling water is as hot as I have tried, any hotter endangers the foam displacer, I suppose one of those little tea candles would be OK, but if you don't need fire,why use it. Ian S C

I was thinking my elevated base section could sit in one place and I could insert or remove a heat source. I hope to be able to run the engine on hot water. If I have to use a candle I will know the engine isn't right.

I might also mount it on a heliostat to track the sun. I could use a mirror to deflect sunlight onto the base if it was elevated a little. I have one of those big magnifying glass things with a hoop florescent lamp. I could focus a lot of sunlight through that thing and onto the engine. I'd have to make sure not to fry the displacer/pex!

Does anyone think it would be possible to charge a modern smart phone if one were to build a tiny little axial flux 3 phase alternator into the engine flywheel? With a good set of low resistance rectifiers and a couple big capacitors I would think one could get up to the 5V USB standard. Wouldn't the phone self regulate the charge, as long as I used a LM7805 (1amp 5V solid state regulator) on the USB feed? It would be too cool to charge a phone with a bowl of boiling water or from the sun.

R

[Ian S C]

I have my doubts about generating any usable power with one of these motors, not to say that its not worth trying. If you heat the top side, and cool the bottom the motor goes in reverse, or in the sun shine you can turn the motor upside down on some sort of frame, I tried last summer but it didn't work (did'nt try very hard). I think you need direct sun to get the heat rather than a mirror that would absorb a large amount of the heat. You might be supprised by the amount of heat you get from the magnifying glass, I'v used a large fresnel lens on one of my high temp motors, and got a better performance than with LPG, but the focal point actually etched a line on the stainless steel displacer cylinder. The heat buils up in the local area because of the high heat resistance. The lens was designed to be placed in front of a TV set to enlarge the picture. Ian S C

[jimlarsen]

"Low Temperature Differential" means, among other things, low power. You can't get out any more power than you put in. The best example of a generator I have seen on a LTD motor was able to make a low voltage LED flash. If that is enough power to charge your phone, then the answer is yes. But it might take a week to get the phone fully charged.

[jimlarsen]

I just discovered a similar tube. The packaging containers from bulk CDs and DVDs has a similar plastic tube. It has a much thinner wall, but would work well for the sidewall of an LTD engine.

[theropod2]

I just discovered a similar tube. The packaging containers from bulk CDs and DVDs has a similar plastic tube. It has a much thinner wall, but would work well for the sidewall of an LTD engine.

My tube came out of the scrap bin at a local bakery. I think it was part of a flour delivery system, but I can't be sure. Yes, I do dumpster diving!

I think a very well made dual disc axial flux alternator might make a few watts of power. Heck 1 amp at 5V is still only 5 watts and while MY engine, if and when I get it built, might not make that much power surely getting 5 watts isn't impossible for an LTD.

R

[Ian S C]

You might get mili watts, its considered quite good to get 1 watt per cc from a high temp atmospheric stirling engine. The alternator/ generator at these small sizes will be 50% or less efficient, and the smaller you go the worse it gets, sorry theres not much you can do about it, modelers have had this problem for over a hundred years (there are also sites on the web about generators that put out more power than is put in). Ian S C

[Carnot]

To run mine I either sit it on a small bowl of boiling water, or(and more often)I sit it on top of a pot while cooking my lunch. Boiling water is as hot as I have tried, any hotter endangers the foam displacer, I suppose one of those little tea candles would be OK, but if you don't need fire,why use it. Ian S C

Tea lights can damage a polystyrene foam displacer - I have used a single tea light spaced about 100mm from the bottom of the displacer cylinder and it distorted the foam.
I even tried using a metal disk on the bottom of the displacer (actually a hard disk platter) but the foam still shrunk, increasing the dead space to the point where the engine would hardly run.
With a foam displacer, I would avoid using a flame.

[Ian S C]

Carnot, I suspected that would be the case with candles, I might get away with my motor, the base is 1/4" aluminium, that should conduct the heat away so that a hot spot does not occur, not that I'll try anyway. You could do similar damage with a magnifying glass, esspecially if the displacer plate was fairly thin.
Ian S C