'cookie' engine diary

[tmk]

I'm building a stirling engine made out of a cookie tin, chicken wire, aluminum foil, and whatever else i find lying around the house. I'll update this thread as i make progress.

Since the body is a cookie tin (about 8" in diameter by 4" tall), i'm naming this one 'cookie'. Perhaps i'll rename it later if i can come up with something better.

Goals:
This will look like one of the LTD engines - wide and relatively short, except it will be much larger (8" diameter)

My hope is that i can get it working in the normal flywheel configuration, then start experimenting with ways of changing the mechanism such that the displacer can be made to 'dwell' longer at the resting states, and move only when the gas needs to be shifted (vs being in constant motion, as in a 'normal' stirling engine)

General design:
My initial attempts at construction have been relatively crude. I don't have fancy machine tools (pretty much just a hand drill and a dremel tool), so i'm trying to be creative.

I used a bolt with a hole drilled through it to act as the displacer rod sleeve (based on the tip on the main site here), and am using coat-hanger wire for all my connecting rods and such, since it's readily available.

For the displacer, i made a cylinder shape about 7" by 2" out of chicken wire, and covered it with aluminum foil.

My piston is epoxy formed in a PVC pipe (see my other post about how i made it), and the piston shaft is PVC pipe. Right now it's just attached with superglue because i was in a hurry.

I haven't got a flywheel design picked out yet, but it will probably be an old hard drive, since those generally have sweet bearings and are readily available.

Issues:
I didn't quite get the hole through the bolt straight, and i didn't quite get the displacer shaft hole in the center of the lid. We'll see how detrimental that turns out to be. The coat-hanger wire isn't quite straight, and it binds a bit as it slides in and out. I bought a bunch more bolts so hopefully i can re-do that part of it. I'm thinking i may just shave down the wire a little and hope the leakage isn't too bad.

The lid to the jar doesn't seal well, so i taped around it with masking tape. Surprisingly, this seems to work pretty well.

I also get a good seal between the bolt/displacer rod. I didn't expect that.

.. so does it work?
The current design needs a lot of tweaking (counterweights for the displacer, displacer rod isn't straight, etc, but at least in theory it's promising.

I did a test today after getting everything assembled, glued in and taped down. Here's how it went:

(keep in mind that right now there's just the power piston and the displacer rod, no flywheel, nothing connecting them together)

i put the engine on the stove and turned the burner (gas) on to low. i held the displacer at the 'high' position, and waited. I figure the air should expand and push the power piston up, and i should be able to see that happen.

Unfortunately, the thing just smoked as the labels and such burnt off. It was kind of disappointing. There was smoke coming out of the power cylinder as well, which seemingly indicates a loose-fit, so i got discouraged.

I thought maybe the piston was binding, so I tried a looser piston, but it didn't work either.. even more smoke came out the piston shaft. I thought i was sunk because the first piston i tried was already a pretty tight fit, and any tighter and i would have serious binding issues.

Then i tried simulating the actual stirling cycle, and moved the displacer down to the bottom, then pulled it up to the top.

Pop! The power piston shot out!! i was quite surprised. My piston shaft is about 1" tall, and the piston popped right out, and fell off onto the stove. i did it a few more times just for fun, and with the relatively high-heat of the burner, the engine can definitely move the piston.

That's all for tonight. My next step is to redo the displacer rod and shaft so those are nice and loose, and to decide what the best way to attach the piston/displacer so that the rotation of the flywheel translates to linear motion on the piston/displacer.

cheers
-tmk

[Administrator]

Take lots of pictures! You can't have too many!

Because it is made from "tin cans" you know I'm interested in seeing it run!

Sounds like me when I first built one from scratch. Spent more time "fixing and redoing it" that building it the first time! LOL That is when you really learn!

If you need it posted on a webpage (when done and running) let me know. I'd be happy to post anyones engines if they don't have the ability to.

Sounds promising!! Keep going and have fun!

Darryl
http://boydhouse.com/stirling/index.html

[tmk]

I'll put up a webpage a little later, i just wanted to share my progress.

I see you're in california... if you're in the bay area, the maker faire is this weekend.. There will be a guy who ran a company that built stirlings there

http://makerfaire.com/

http://makerfaire.com/pub/e/272 - stirling exhibit

-tmk

[Administrator]

That faire sounds like fun!

I'm am in the Bay Area. In Concord.

I'm going to try to get there! Looking forward to seeing your engine run!

Keep going!

Darryl
http://boydhouse.com/stirling/index.html

[tmk]

Here are a couple of pictures of what i have so far (click for a full-size version):



The pistons on top are made of epoxy. The piston cylinder is glued on with superglue. You can see some chicken-wire in the background.. same stuff i made my displacer out of.



This view has the thing taken apart so you can see the displacer.

I ran a few more tests:

There has to be a certain amount of air in the engine - if the piston pops out or stays at the top too long, the air inside leaks out, and if the piston is replaced, there's not enough air in the engine to push it back up.

I'll definitely need some sort of counterweight for the displacer.. it's not that easy to move.

I'm thinking of making a 'double' engine so the displacers can act as counterweights for each other.

cheers
-tmk

[tmk]

I had several hours today to fiddle and tweak my engine, and although it ran, in the end it was pretty much ruined.

I've been testing my prototypes out on the stove (it's nice and big so it fits over the burner like a pot), and it just wasn't happening. I'd added some weights to help it spin, and counterbalanced the 'walking beam', but although it had plenty of power, it didn't cycle.

I did some thinking and changed a few things.

First my 'CD' flywheel was no good, it would tilt during the power cycle and didn't really spin that well to begin with.

Second, my displacer was too heavy. The chicken wire + aluminum was too much, even though it seemed like it was hollow and made of almost nothing, it was actually pretty substantial :<

All my coat-hanger-wire links weren't working well, so i got some ring terminals and clamped them onto each end of each rod, and used some bolts i had to attach them at each point. That was a big help

Lastly, although it worked fairly well, my power piston wasn't tight enough, and didn't slide as smoothly as maybe it could have. (It was epoxy in PVC pipe for reference) When my engine was first starting up, some smoke would come out during the heating phase, and you could see all the loose points of the engine. A decent amount came out around the power piston.

I swapped out the CD flywheel for a Hard Drive platter and motor/bearing.. which was much much smoother (though i did break 2 drill bits trying to get the HD case off.. then i switched to the dremel and everything was fine)

For the new displacer, i found an 'O' of foam and wrapped it in aluminum foil. I threaded some thin steel wire through it to make an 'X' and attached the displacer rod to the center of the 'X' (just made a 'J' out of the end of the coat-hanger and clamped it real good)

I hot-glued a thumbtack (because i ran out of superglue) to the platter because i didn't think drilling through the platter was a good idea (anyone tried it?)

a few hours of work and assembly later, i was ready to test my creation.

I threw it on the stove, and noticed that during the heating phase, the whole lid would bow out, and during the cooling phase, it would bow in. I figured that was wasted power-piston energy so i started pressing down hard on the lid while it was running, that seemed to help a lot.

After some fine-tuning, it ran for 4 or 5 rotations, before the hot-glue melted, and the rods connecting the beam and the power piston to the flywheel stopped working. One of the key changes i made was a small weight opposite the power-piston-rod connection point to help it get around one of the 'humps' in the load cycle. A heavier flywheel would probably have had no problems and not needed the weight.

I reglued and tried again, but somewhere the displacer had come undone, and the foam was just sitting in there melting off, and filling the inside of the engine and the power cylinder with gooey stuff. This made the power piston bind up, and that was pretty much the end of the engine.

I salvaged what i could and scrapped the rest. At least it ran.

It was a fun learning experience though, and a relaxing fathers' day :)


If i build another one, here's the key things i will change:

* a flywheel with good rotational inertia
* better power piston
* heat-resistant/light displacer
* Body with good stiffness

Cheers,
-tmk

[speedless]

Hi
For a VERY lightweight displacer,check out "OASIS".
Its very fragile,hard to handle and glue but very light and porous.
Glue with hi-temp epoxy.
My pistons are made from "Chemical-metal"(2-comp.styrene for motor repair) ,then sprayed with a graphite coating,and polished with paper!
See my tread "hi there"
Jan

[tmk]

thanks for the tips, i'll check into those