# Trying a Stirling



## dgjessing (Feb 3, 2011)

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...


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## dgjessing (Feb 4, 2011)

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






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


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## dgjessing (Feb 5, 2011)

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


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## dgjessing (Feb 8, 2011)

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"


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## Noitoen (Feb 8, 2011)

Keep up the good work


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## NickG (Feb 8, 2011)

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]


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## Longboy (Feb 8, 2011)

dgjessing  said:
			
		

> 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.


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## NickG (Feb 9, 2011)

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


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## b.lindsey (Feb 9, 2011)

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


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## NickG (Feb 10, 2011)

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


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## dgjessing (Feb 10, 2011)

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


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## Longboy (Feb 10, 2011)

dgjessing  said:
			
		

> 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.


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## NickG (Feb 11, 2011)

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


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## dgjessing (Feb 11, 2011)

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:


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## dgjessing (Feb 11, 2011)

... 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 :


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## dgjessing (Feb 12, 2011)

OK, on to the power cylinder! 






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


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## NickG (Feb 13, 2011)

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


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## dgjessing (Feb 13, 2011)

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).


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## NickG (Feb 14, 2011)

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


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## dgjessing (Feb 14, 2011)

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.


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## Fingers (Feb 14, 2011)

Keep on fiddling around it sounds like it's just there nice job
Jamie


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## NickG (Feb 14, 2011)

Is the flywheel steel? If so, it's definitely heavy enough.

undo the tube from power piston but leave on at displacer end, take displacer assembly off (leave displacer and rod in) submerge it all in water and blow into the tube. There should be no bubbles. Maybe the odd 1 or 2 from around the rod but basically no leaks.

Friction wise, with the hot cap off the engine should spin over for about 20 revolutions from a flick of the flywheel.

Looks great.

Nick


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## dgjessing (Feb 14, 2011)

I've fooled around with a bit more and it's getting closer, but still no cigar. 

The flywheel has an aluminum disk with a steel rim. Don't have it in front of me but I'd guess the rim is about 3/8" wide and 1/4" thick (3" schedule 40 pipe). It's probably just fine. 

I think I'm doing pretty well air-tightness wise - with the rod disconnected from the crank, if I abruptly shove the piston to the bottom of the cylinder it pops back about half way. 

It's nowhere near that good friction-wise - it might spin 5 times, but nowhere near 20... The crank and flywheel alone may go 10 times :-[

I've ordered some little ball bearings to use in a new main bearing assembly.


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## b.lindsey (Feb 14, 2011)

Did it have ball bearings before or just sleeve bearings? These engines definitely need ball bearings to minimize friction as much as possible. Hang in there, it sounds like you are close.

Bill


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## dgjessing (Feb 14, 2011)

Plain brass bearing with a steel shaft currently. I've ordered a couple of 1/4 x 3/8 x 1/8 ball bearings and I'll make a hub that uses those with the existing crank. 

I just made a neat looking sheet metal hood to go over the hot end and (theoretically) hold the heat around it better. Looks nice anyway ;D

Seems like it's really close to running... It's to the point where it will pick up a little speed just after tdc or bdc but still not follow through. With any luck the better crank bearings are what it needs!


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## NickG (Feb 15, 2011)

If the leak test as I described (not power piston but the displacer assembly) is ok, it must be friction that is the enemy. It doesn't sound anywhere near free enough yet.

The crank and flywheel disconnected should spin over for many revolutions, for 20 seconds plus - it sounds extreme but you will find if you use the ball bearings it will do that.

The other thing you mentioned about the piston springing back is good in once sense, it shows there's a good seal. What is the bore and stroke of power and displacer pistons if you don't mind me asking? If it's springing back too much, it could mean the ratio of the two swept volumes is too small. This happened when I designed mine and I ended up making the displacer bore larger, I also made the displacer longer to minimise heat transfer from hot to cold end.

Sounds like you're nearly there. Have you tried a drop of light oil on piston and displacer rod?

Cheers

Nick


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## SignalFailure (Feb 15, 2011)

I made a simliar (though rather more, ahem, agricultural) version of this type of engine...no plans just winged it but it wouldn't run  I posted it on the stirling forum here http://stirlingengineforum.com/viewtopic.php?f=1&t=448 - sorry my picture has been removed but the follow up post might give you some clues to get yours going. Good luck, nice work deserves to run!

EDIT: ah, I found the picture...


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## dgjessing (Feb 15, 2011)

Thanks, Nick - the displacer bore is 3/4", the power cylinder 1/2", and the stroke for both 3/4". The displacer is 1 1/2" long and comes to within 1/8" of the ends of both the hot and cold ends of the cylinder (depending on which end it's pushed/pulled into). I don't know if you noticed or not but I cut threads all the way through the cold end on the theory that would help with heat transfer, and the hot end is insulated from the cold with several turns of teflon tape.


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## NickG (Feb 16, 2011)

Thanks for the info. How much clearance does the displacer have around it? Mine has about 1/32" so the displacer piston is 1/16" smaller diameter than the bore.

I think your displacer bore / piston is a little on the small side, even if you have 1/32" clearance all around like mine, it means your displacer swept volume is very similar to that of the power piston. It might be worth trying putting ice cubes on the cold end whilst attempting to run - having that ratio closer to 1:1 makes for an efficient engine but it means you need a higher temperature differential. If it runs with ice, you may need to make your displacer swept volume larger - the only way will be to bore your displacer cylinder out, new displacer and new hot cap. Note, the clearance at each end should be small too, same as the sides, about 1/32" is sufficient.

If you have no leakage on the displacer end, the only other problem would be friction. If it's trying now, the bearings may sort it so that would be the first thing to try. Does your power piston (with bottom off power cylinder) fall through the cylinder under its own weight? If so, it should be ok, if you put your thumb over bottom of cylinder and try to push piston in, there should be strong resistance from a sealing point of view - (sounds like you're ok on that front)

On mine it was leakage. The water jacket on mine was an after thought, I got it running on ice cubes initially, then water but now I've sorted the leaks I think it should run without water - can't remember whether I've tried. Could try tonight.

Hope this helps,

Nick


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## dgjessing (Feb 16, 2011)

Thanks Nick - I really appreciate it! 

If the ball bearings don't do the trick I'll make the displacer piston a bit larger in diameter and longer on both ends. 

I didn't get into the theory behind these things at all when designing it - I just looked at some other plans and guessed :

I _have_ tried putting ice on the cold end during my attempts to get it to run - my impression was that it didn't help much (but what do I know?). 

While waiting for the bearings to arrive I'm making an adjustable height pillar for the candle to sit on.


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## NickG (Feb 17, 2011)

No problem, you'll have it sorted soon.

I didn't consider these things enough at first either, hence modifcations down the line.

I think you may have just answered part of the problem though - again one that I encountered. I originally designed mine to be run from a tea light but I quickly realised / was advised that there is very little heat in a candle - not enough to run an engine like this. You need a meths burner with a fairly large flame that sort of wraps itself around the hot cap - look at mine again, note the blue flame from the meths- much hotter than a candle flame.

So I'd try, proper burner, bearings (I still think you'll need them judging by the friction test) then final resort the displacer bore. Increasing the diameter is the only way you can increase that swept volume on an engine in this configuration but it's the best way anyway as it's a squared relationship.

I think you're nearly there.

Nick


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## Chaffe (Feb 17, 2011)

Hi, i remember the joys of ironing out very similar problems with my first stirling, it has seperate cranks for displacer and power piston, and the crutial factor with it was timing, a few degrees either way and it wont run. But mine is prehaps exception to the rule as it also requires the whole thing (all of the displacer and some of the frame) to be hot before running fast, very bizzare but true all the same. Here is a picture, irt was the very first thing i machined so please excuse the charecterful (rough!) machine work, good luck with your engine, you seem very close James.


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## dgjessing (Feb 17, 2011)

It lives! (Kinda, sorta, in a manner of speaking...). Video later ;D


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## dgjessing (Feb 17, 2011)

[ame]http://www.youtube.com/watch?v=NG70omfIW74[/ame]


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## Chaffe (Feb 18, 2011)

Looking good, it runs very similar to my one when the timing is a little out, like it wants to go but just cant push it over the edge. Id say everything looks free enough and sealed very well. Good work! James


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## NickG (Feb 18, 2011)

Course it counts, it runs! :bow:

Is that with the new bearings or without? Just some fine tuning to do now, the timing can't be out, that's one thing you don't have to worry about with this layout!
I think this was the point when I discovered that there was a leak around the hot cap on mine and the pillar where the transfer port was drilled so I needed loads of ptfe tape to give a good enough seal.

If yours has no leaks and still requires the ice, it must either be friction, or the displacer is too small or a combination of the two.

Well done!

Nick

ps snap - can't remember if I've shown this before!

[ame=http://www.youtube.com/watch?v=xAgzQroi6PE]http://www.youtube.com/watch?v=xAgzQroi6PE[/ame]


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## dgjessing (Feb 18, 2011)

Ball bearings on the main shaft plus a new displacer piston - much better! ;D

[ame]http://www.youtube.com/watch?v=8TT2_B6LYFY[/ame]


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## NickG (Feb 19, 2011)

That's sorted it, very well done - brilliant feeling isn't it ;D :bow:


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## Chaffe (Feb 19, 2011)

well done! congratulations! runs very well


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## kcmillin (Feb 19, 2011)

Well done Indeed!! A fine runner!

Kel


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## dgjessing (Mar 7, 2011)

Well to follow up, I must say I'm kinda disappointed that I can't get it to run on a flame in a cup, but I've come to grips with that. Someday, I want to do another one that will run on a candle flame ;D

Anyway, this afternoon I made a new power piston for it, a couple of thousandths bigger, and that makes a huge difference - a whole lot less blow-by and no need for a big dollop of oil on top of the piston. With the new piston I can run it for 10 minutes or so before the entire thing gets really hot and it coasts to a stop. I think it could run forever but I don't have the patience to stand there blowing compressed air on the cold end's cooling fins (which extends the 10 minutes).

So what I've learned: 

 - "They" ain't kidding about keeping the friction down to a minimum; not a lot of power here, every little bit helps. 

- The scotch yoke thing is probably a mistake, unless it's done with great precision (which mine ain't). Too much friction, too much bangin' around. 

- This engine runs best when the "cold end" is a lot hotter than I expected would be good. If my HF non-contact thermometer is to be trusted, the best performance is achieved when the cold end is roughly 170 f and the hot end is about 400 f.


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## Chaffe (Mar 8, 2011)

another way to extract the most power i found is by adjusting the piston to displacer timing, prehaps a consideration when building another engine to make this a controllable peramiter, good work with your completed model though.


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## NickG (Mar 8, 2011)

Never played with the timing on a stirling but guess the easiest way would be to just have the two cylinders run off seperate cranks.

I set out to make mine run on a candle too but you need something with a much bigger displacer if you want that. Look at the LTD ones - larger ratio of swept volumes from displacer to power piston.

Nick


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## Chaffe (Mar 9, 2011)

NickG  said:
			
		

> Never played with the timing on a stirling


see my picture at the top of the post mate, timing is controlled by two crank discs, it wont run AT ALL if it is minutly out


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## NickG (Mar 10, 2011)

What is the timing set at though? 90 Deg apart? Presumably it will run at that as that is the whole point of a gamma type stirling like the one in this thread. The power and displacement cylinder centrelines are 90 deg apart running off a common crank giving you the 90 degree phase difference.

I can imagine 90 deg isn't the optimum for every engine but I would have thought most engines would work reasonably well at that angle.

Nick


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## Chaffe (Apr 13, 2011)

NickG  said:
			
		

> What is the timing set at though? 90 Deg apart? Presumably it will run at that as that is the whole point of a gamma type stirling like the one in this thread. The power and displacement cylinder centrelines are 90 deg apart running off a common crank giving you the 90 degree phase difference.
> 
> I can imagine 90 deg isn't the optimum for every engine but I would have thought most engines would work reasonably well at that angle.
> 
> Nick


You set the timing to the optimum setting by adjusting until it runs best. There is no way of saying what is the exact angle as the whole physics of the engine would vary the timing. Its similar to a four stroke engine, in theory the exaust valve and inlet valve should never be open at the smae time, but after experimentation it has been proven that overlap needs to occur for best performance.


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