Stirlingengineforum.com

Hey everyone, I'm new here and have learned a lot in the past week or so browsing the forum. I'm 18 years old and live in Oregon (usa). Just in this for the fun.
I was messing around with some coke cans trying to make a little engine, but I had some tolerance issues so i decided to make something a little better. I've looked into it, and am just wondering if anyone can see anything i might be doing wrong while i'm making it.

I got some plates laser cut so i could make a Stirling engine. (for free)
take a look :)





The plates are aluminum, and the cylinder is an appx. 6'' outer diameter, a few thousandths off. height of 1 1/8 inch (plexi-glass)
The displacer is a piece of dense foam insulation wrapped in aluminum foil, it's about 3/4 inch tall, and about 1/4 inch smaller diameter than the ID of the plexi cylinder.

Q: is the displacer diameter too small? should i do anything to it? I have more foam i can easily replace this with.

I did some math, and without including friction it should only require a temp difference of 27 degrees F or about 15 C
( assuming air expands approximately 1% for every 3 C you rise... i didn't feel like doing coefficients)
I'm gonna go get some tubing tomorrow so i can make a piston.
I figured a 1.5 inch copper tube (1inch id) and a 3/4 inch piston (just enough to fit in the other tube) to make a .5 inch stroke with an eighth inch space at either end.
I would like to make the piston sleeve out of aluminum so it matches, but would copper be better?
any suggestions here are welcome.

after that all i need is a flywheel and a crank case and i'll be in business, the extra holes are for stands to hold the crank shaft.

Note: the hole for the displacer rod still needs to be drilled, they missed that in cutting. what would the best way and size to do for that?


thanks for your time, Presentteck

Hi, I'll make a start, change the bolts that join the two plates, metal bolts conduct the heat from the bottom hot plate to the cold plate, you can make a direct swap by using nylon bolts the same size as the metal ones, or some plastic blocks, and self tapping screws, theres one or two more suggestions, but that will do for a start, best wishes Ian S C

Thanks Ian,

You make a good point, but in the end i'd like to keep the look of metal. also, I have nowhere near me where i have seen plastic bolts.
Do you think wrapping them in teflon tape where they make contact, or some kind of shielding material would work as well?
I know it wouldnt be as good, but i think it would help if i did it right.

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new thoughts:
I decided to make a bolt on plate that will have the piston & displacer rod. It would go on nicely with the holes i already have. The crankshaft stands will go on with the same bolts. (these are the 4 small holes you see on top)
I figure if i do this, i could TIG the tubes for the piston and displacer to that plate, and have no issues with damaging (or warping) the top plate.

As for a piston and linkage, i think i may just get some 1/8 inch carbon rods for the push rods, and a 1'' OD graphite rod from this site. http://www.beckergraphite.com/stock.html
less than $10... not bad
I may go with a copper piston for a while though since machining the graphite is a mess.

as for the crank shaft i have no idea, I have some fine roller bearings out of a HDD that i think would work great for that.
maybe an aluminum or steel rod for the shaft itself? (keeping with the color scheme.)

and for a flywheel i may eventually machine something, but for now i think i can use the motor and platters (or just the platters) from the HDD
I would take apart the motor and just load on platters till the platter holder was full, then attach it to the crank shaft... I need to think about this one

I don't know if you know a bit about electronic components, but if you do, you'll know the nylon washers used to isolate the likes of the bolts holding transistors to their heatsinks, a washer similar to these would be ideal.
here in New Zealand, We often feel isolated from the rest of the world, and have difficulies getting some materials, but I read of the dificulties people in larger countries have at getting quite simple things, it makes living here not so bad. I just have trouble finding retailers that have had to move because of the series of earthquakes our part of the country has suffered over the last 18 months. Ian S C

I'll look into those washers, my local electronics store just went out of business so i may have to go online...

I know all about those earthquakes, my teacher/friend (he's retired) was in NewZealand to see his son when a big one hit about 6 (i think) months ago, crazy stuff, good thing he wasnt on the gondala.

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Update, the bolt on piece will be bronze... brazing is easier and i found some material.

The big quake was 4th sept 2010, near where I live, not too much damage (most houses with chimnies lost them), things are a bit settled now, and I am filling my days replacing loose mortar in the brick work on the house (fairly minor), the second one was 22 feb 2011, thats the one that did most ofthe damage, and caused fatalities in Christchurch city about 40 km from here.
I would not recomend copper or aluminium for pistons or cylinders, they are two metals with high coefficients of friction. Aluminium can be used if it is either hard anodised, or (forgotten the name), coated with teflon (microns thick). Bite the bullet, and get into the graphite (keep a vaccum cleaner by you), unless you can get a suitable pression glass tube, brass, or stainless steel, or even mild steel, or cast iron with a polished bore would be my choise of materials. The system I use is cast iron piston, and cast iron cylinder, best, or steel cylinder good. The smallest (at the moment) motor in my gallery has a brass cylinder, with an aluminium piston with a thin skirt of carbon impregnated teflon over it. The bush for the displacer rod is made from the same teflon. Ian S C

Where might one be able to find 1'' ID brass tubing?
I look online and can find a little bigger and a little smaller...

The graphite material im looking at for a piston is 1'' OD, so I could go a little bigger for the pipe size but I dont know what kind of tolerances i need to keep a good seal, or how long it should be. I was planning 3/4 inch piston.

I have a 1 inch hole in the aluminum you see above, i dont think it would effect the piston or pressure if the tube above it were say: 1.12 or 1.17 ID?
The 1.12 is the closest i can come to an inch without going under... that would be 0.06 inches of space one either side of the piston and i assume that's too much.

any better pipe out there? I cant find any... PS: cant afford a full length of the stuff either, too expensive.

If aluminium is all you have, better use it. The clearance you need is a fraction of a thou, just enough for the piston to slide through under its own weight, and stop when you block an end, the exact measurement is unimportant. I'm not sure which thread its onb ut its not far down the page, one chap use the glass tube from a fluorescent light tube for a cylinder(just a thought).
The spacers between the plates could be made from wood, with short screws from top and bottom. Ian S C

well, looks like i have to make a slightly smaller piston, but im going with a piece of polished stainless tubing for a cylinder.
It'll have to turn the graphite down to the right size, might look into doing that elsewhere if possible...
on the plus side, looks like this will be a pretty slick project

I'll pressure fit that cylander into a thick sheet of aluminum, at least 0.100, i might go thicker if i can find any... I might have some 0.25 around.
the displacer rod will be press fit too, maybe some locktight.

the crankshaft will be brass stock with pressure fit aluminum blocks for the offset. and some nice bearings from an old HDD will let the thing spin I found a few nice rubber mounts that'll work great.

now then, what to do to hold the bearings in place, i'll let you know
maybe i could cut something out of that thick aluminum...

I had a question about tolerances,

1) With a graphite piston inside a polished stainless steel tube, how small is that fraction of a thousandth you need for clearance? 0.0005? less? I know less is better, but i don't know what i'll have for a lathe till i look for it. might go to the community college, i have some friends there.

2) say the tube i have my displacer in is 6 inches ID (really about 5.75. but lets make it easy)
How much space should i have between the displacer and the wall? right now i have about 1/8 inch either side... should i just make a new one?
it's just a foam disc covered in aluminum foil. i think the foam was R-14 insulation, something close.
the chamber is 1 1/8 inch tall, displacer is 3/4 tall

Should i use something else for a displacer, lighter foam? should i make something with a regenerator?
I have a bunch more circles i cut out of that foam i can make whatever size i like, they were cut to the tubes OD so i just have to be careful how i sand it.

Displacer: all you need is a plain foam disc, the aluminium just adds weight, the foam in my LTD motor was rather crumbly, so I painted it with silver spray paint used for Christmas decorations (its safe on foam). I cut the disc with a hot wire cutter, both the disc, then the thickness. Your clearance at the side of the displacer is about right.
The graphite piston: get it to a size where it will not quite fit, then polish it down. This is done with ordenary paper (not sand paper, but writing), graphite (imagine a pencil)polishes down this way very smoothly, the exact measurement is not so important as getting the right fit, as I said above somewere.
You could proberbly glue the power cylinder over the hole in the displacer cylinder, A bit of Araldite or similar, I'm even gluing some parts of my high temp engines these days. Ian S C

Thanks, i imagine it would polish nicely like that, so i can just cut it a hair oversize. It is a fine grain graphite though... wonder if that makes a difference. probably not.

The aluminum was mainly so it wouldn't look bad, i didn't have much in the way of paint that doesn't melt foam.

I will not do any gluing to the top of this, I have taken the easy way in far too many projects and it never looks as nice. I want this one to look perfect so I'm going at it a bit harder than usual. I need something that can impress... most of my stuff works, but looks like my gasifier >>http://www.youtube.com/watch?v=hXnYvl9EXS8
not that pretty :P

I need some other parts cut out anyway, so im gonna go for gold. good thing i learned CAD right :D

I'm affraid that some of my stuff is a bit scruffy, I build it as the bits come along, then I change my mind, then I add bits here and there, The worst one is the free piston motor, rearanging the springs, and the radiator for water cooling. Got rid of the spring on the power piston, now I use a magnet fixed to the base, opposing the magnet on the linear alternator, it works well, much better than mechanical springs. So don't worry too much unless its for show.
I'v never used graphite myself, but a fine dense material I would think would be the best you could get. Ian S C

well I'll be getting graphite from here soon: http://www.beckergraphite.com/stock.html
it's soo much cheaper than anywhere else, and shipping is just ups or usps, so you pick, i'd buy a few at the same time though, shipping shouldn't cost any more.

check this out though, linear actuators, with free samples. im getting one to see how it would work as a Stirling engine piston, it's graphite and responds to 0.5 psi or less... perfect seal, i might buy some in the future, but they're about $50... so i might just study the one i get and try to reproduce it.

and I would like to show this off, then i'll just make things work... my prototype was horable, take a look :)



everything was bad, a little off balance, piston made from wood, piston sleeve not round enough, leaks, paper mache and rubber cement... that was a joke
it kept me occupied in class for a while though

UPDATE!

got some new parts, need work though.

The first two will be the stands for my crank shaft, the laser cutter didn't like cutting metal that thick, so it got kinda mealy...
it can all be fixed with a little time, but I have to be careful not to make them uneven.



yes i know it's upside down, i tried flipping it about 7 times... wouldnt update


This one is my bearings, and the base for my flywheel. they are all out of HDD's and i found some little rubber expanders that fit the threads.
the long screw will be used to attach the flywheel base to the bearings. With this piece of a spindle for a base, i can make a variable weight flywheel using a combination of HDD platers and spacers. Just the purpose it was used for before!

hey if i get it up to 5500 rpm do you think i could read the discs XD