Trying a Stirling

[dgjessing]

So I've gone back and forth on whether or not to do a "Work in Progress" thread on this, or to just build it, see if it works, then do a "Finished Project" post if it does. The first might jinx it or something... But what the heck:

;D

It's going to be similar to this - http://peswiki.com/index.php/Image:Stirling_engine_diagram.gif except I'm planning on moving the displacer with a yoke instead of a connecting rod. I thought it would be good to shorten the length of the tube to the power cylinder as much as possible, and the yoke arrangement helps with that.

Not much to show yet, but I made the displacer cylinder this evening. Started with a really nasty old piece of steel water pipe:

The white stuff on the end is teflon pipe tape,by the way. Teflon is an excellent thermal insulator so I'm thinking I'll make the cylinder + tape a press fit into the aluminum cold end.

Steel end cap brazed in & cleaned up, sitting on the not yet finished drawing:

The displacer cylinder has a 3/4" bore, the displacer will be 11/16" dia. The power cylinder will be 1/2" bore, and the stroke 3/4". The displacer will be aluminum, hollow, 1 1/2" long. The displacer rod and yoke will be brass. The finned cold end will be aluminum, 1 1/2" dia overall. The gland carrying the displacer push rod will be cast iron, and screwed into the cold end. I'm thinking the threads in the cold end will continue all the way through, on the theory that the "fins" formed by the threads will help with heat transfer. The power cylinder will be brass, it's piston aluminum

If any fatal flaws jump out at anyone, please let me know!

Waiting for some stock from Speedy Metals to arrive...

 

[dgjessing]

More or less completed the drawings today, and have roughed out the frame members:

My Speedy Metals order is allegedly out for delivery...

 

[dgjessing]

Boring the hole for the displacer assembly:

Finished frame on a zebrawood base:

Nothing from Speedy Metals yet... Apparently UPS does not subscribe to the "neither rain nor snow..." mantra

 

[dgjessing]

Added the main bearing, shaft, and flywheel:

I'm doing a lot of press fitting on this thing - the main bearing is pressed into the frame, and the flywheel is built up with a brass hub, aluminum disk, and steel rim. The rim is a piece of 3" schedule 40 pipe. Cold, the ID is 3.093", at 500 degrees 3.096". I made the disk 3.095" and it just dropped into place with the rim hot. Feels lie it's really "in there"

 

[Noitoen]

Keep up the good work

 

[NickG]

Nice work :bow:, it looks a similar design to the one I designed / made (I got the idea from one I found on the internet also).

You should be helped by the fact you have two plates square to each other, so long as you ensure everything else is made square. I designed mine so that it could be tweaked if I didn't quite get things inline which was good because it probably wouldn't have run otherwise! However, it makes it a bit fragile and susceptible to being knocked.

Nick
[ame=http://www.youtube.com/watch?v=lJ89IuZ3V_4]http://www.youtube.com/watch?v=lJ89IuZ3V_4[/ame]

 

[Longboy]

If any fatal flaws jump out at anyone, please let me know!

Naw, yer looking good. Scotch yoke somewhat unique. Hard to find other YouTube Stirlings with this feature. Dave.

 

[NickG]

Should be good. I screw threaded mine and used PTFE tape on it but my screw cutting wasn't very good so I had to wind quite a bit of tape around it. The way I'd designed mine I had to be able to remove it though.

Nick

 

[b.lindsey]

Looking good dj. The schotch yoke is unique and I am anxious to see how that works out from a friction standpoint. Looking forward to more on this one.

Bill

 

[NickG]

I was thinking about that myself Bill, if he gets it right, it should be lower friction than a plain bearing due to less contact area? I would think it'll work well with a loosish fit on it.

Nick

 

[dgjessing]

Thanks everyone!

No pictures today, but last night I did the crankshaft, and today I machined the cold end and the displacer shaft gland nut. Tomorrow I'll thread both of those & fit them together.

I was planning on making the gland nut out of a piece of cast iron I have laying around, but when I went to saw a piece off I experienced a problem - my band saw *seemed* to be happily cutting it at first, then it stopped making any progress. I checked things out and the blade was suddenly dull as a wet noodle... Put on a new blade but no help (and I stopped before ruining it). Perhaps it ran into a diamond or something?

Anyway, I made the gland nut out of aluminum instead. That ought to be good enough to see if the thing at least wants to try to run...

Friction (especially in the yoke) is my main worry. I'm expecting to have to run it in for some time with an electric motor to loosen things up.

We'll see! ;D

 

[Longboy]

Friction (especially in the yoke) is my main worry. I'm expecting to have to run it in for some time with an electric motor to loosen things up.

We'll see! ;D

Unlikely your yoke will give you any problems from friction. You can find bearings to ride in the slot on Ebay or hobby shop that has R/C cars. Sans a roller bearing, a Nylon screw through the slot to crank drives the yoke well too and may be a little queiter in running. The threads minumize contact in the slot and if rotating clockwise keeps the yoke up against the screw head for alignement. If you have to loosen up the engine by dry turning over with an electric motor, you are all done in a Stirling career! Got to be right (loose) from the start.

 

[NickG]

I'd agree with longboy on this - no running in required, get the clearances right in the first place

 

[dgjessing]

Threading the cold end:

Pressing it into the frame (frame fresh from the toaster oven, cold end right out of the freezer):

And the whole thing so far:

 

[dgjessing]

... and after that I made the hollow displacer and started on it's push rod. But I broke the end of the push rod off in my only 4/40 die and that kinda ground things to a halt for the evening :

 

[dgjessing]

OK, on to the power cylinder!

I'm optimistic - it tuns pretty freely with the yoke / displacer in play

 

[NickG]

Looking good, nice progress. What are your plans for connecting the power and displacer cylinders? Is your power cylinder a blind bore? That's the way I did mine and it works fine but in terms of boring it's probably easier to go right through. You just have the issue of putting a cover on it then though!

Nick

 

[dgjessing]

I'm running an 1/8" OD copper tube from the underside of the displacer cylinder to the base of the power cylinder. The power cylinder is bored all the way through so I can lap it better - I'll then solder a cap in it, and fix it to the base with a single screw down through the cap (with a dab of gasket sealer).

 

[NickG]

Should be ok. Will be interesting to see - not really sure what effect the bore of the pipe will have, mine is about 1/8" bore I think down the pillar that supports the displacer cyl then connected with aquarium pipe underneath. I didn't bother lapping mine, just reamed it and not even very well. It likes some thin oil on mine to help it seal.

Following the success with my graphite piston on my 'poppin' flame gulper, I want to change all of my pistons to graphite it works so well.

Nick

 

[dgjessing]

It *wants* to run - almost follows though, but not quite... I'm thinking the power piston is a little too tight of a fit and/or the flywheel isn't quite heavy enough.

I'll keep playing around with it.