Stirling Engine Question - Glass displacer cylinder

[Daryl_bee]

Hi

I asked a Stirling engine maker "Why don't you make all your engines have a glass displacer cylinder? Watching the displacer is a visual delight, helps to explain the engine, and glass is a good insulator so it should have good performance. Am I missing something?"

The answer I got back was "Glass is not used throughout simply because of thermodynamic optimisation and other pressure issues."

I'm not sure what that means. I thought the better the insulator (they use metal) between the hot and cold plates the better the efficiency would be. Anybody know what am I missing?

 

[minh-thanh]

Anybody know what am I missing?

Maybe....are you looking for high performance or observing the displacer movement ?
If you need high performance : metal
If you need to observe the displacer movement : glass
Using glass : you must make the engine according to commercial glass tube sizes., - unless you can make your own custom glass tube .
Using metal : you can make any size that you can design and make

I'm not sure what that means. I thought the better the insulator (they use metal) between the hot and cold plates the better the efficiency would be.

That is correct, it causes the temperature difference between the hot and cold parts to be higher, resulting in the air in the engine expanding when in the hot part and contracting when the air in the cold part is higher.

 

[Daryl_bee]

Maybe....are you looking for high performance or observing the displacer movement ?
If you need high performance : metal
If you need to observe the displacer movement : glass
Using glass : you must make the engine according to commercial glass tube sizes., - unless you can make your own custom glass tube .
Using metal : you can make any size that you can design and make


That is correct, it causes the temperature difference between the hot and cold parts to be higher, resulting in the air in the engine expanding when in the hot part and contracting when the air in the cold part is higher.

Well the engine I was looking at is a stove fan but it's that line in red (above) I don't understand. Metal conducts heat so brings the hot and cold plates closer together thermally and, with the lower temperature difference (compared to insulator separation) should give lower performance. Something like glass separating two silver plates should give better "thermodynamic optimization".

Is it that glass would limit input temperature? Pyrex (borosilicate glass) is good to 500C and stovetops are around 370C so there's a fair margin. It can take a temp differential of 165C so if gasketed top and bottom with another insulator against each plate should work fine. Could it be the air moving inside that would exceed it's max temp differential?

 

[Longboy]

Stirling doesn't care what materials you use in your model. You can distort Pyrex glass with heat from an alcohol burner but the engine should run long before with lower heat input. Don't let the 'Thermodynamic optimization" or "high performance" double talk shut your material choice down. For a stovetop Stirling, they are larger and the displacer/ power tubes would be metal per the duty they perform. Dynamic display models have the eye catching factor with glass cylinders but building a home version of available commercial stove fans.....use metal.

 

[minh-thanh]

Well the engine I was looking at is a stove fan but it's that line in red (above) I don't understand.

"high performance"... Maybe I should say: "more power"
With metal you can make the wall thickness of the hot cap as thin as possible, 0.6 or 0.5 or 0.... and with the displacer you can also make it as light as possible... that makes engine has more power
in my opinion, the thinner the wall, the faster it can absorb and lose heat - most homemade stirling engines are heated at the end of the hot cap
With glass , you can only buy commercial tubes , you can't decide the thickness yourself , ...
That's all I know

 

[Jasonb]

I would not make any of my hot air engines with glass cylinders. Simple reason is I like to make scale models and you don't find many full size ones with a test tube sticking out the side. These engines all did real work back in the day rather than sit on a table or stove just for display so efficiency was good enough for the job they did be that pumping water or driving small tools/items

 

[Steamchick]

Just an observation....
A stove top is usually black, - so the radiant heat expelled at 375deg.C is quite significant! Therefore I should design such an engine - if required for ornamental as well as operational benefit - as a large flat plate (Copper is good , especially if blackened on the side facing the stove top: thus Black-to-Black is maximum heating by radiation). But to have a glass tube, and show the displacer, I should use a parallel tube with seal onto the copper, and onto the body of the rest of the engine. This glass tube will in some way reduce the conducted heat from copper plate to main body, if you think that important. (I do not consider it significant, as the enclosed air is pumping so much heat from hot to cold zones that the glass will all be at some mid-temperature anyway. Probably a temperature gradient due to the expansion of the gas when compressed and hot at the stove end to expanded and cooler when it is passing down the anulus outside the displacer to the engine body and transfer port to power cylinder...?). The only worry I would have is the likelihood of the glass breaking when the vibration causes the whole thing to walk across the stove and fall on the floor...
Every job has to suit real life as well as aesthetics, function, form, cost, historical accuracy, etc., and all the other design factors.
Show us your design and logic behind the various decisions when to complete it please?
K2

 

[Steamchick]

Just an odd query re Stirling engines:
While I have "read a book and other stuff", I had a thought to own a Stirling engine, so I could play about a bit and get to appreciate its fundamental thermodynamic beauty.... (remove tongue form cheek?). So I bought a cheap device from China. Nice little toy, except the Pyrex glass tube has melted and deformed a bit while attempting to make it run. The plans had a displacer made from a "cylinder" of steel wire wool on a piece of piano wire (con-rod), but this is porous, so carried a lump of steel from Hot-to-cold-and-back, without any appreciable volumetric displacement of gas.
Anyhow, it didn't work.
So I have made a nylon displacer (because that's the material I had in that size). it still doesn't work.
It doesn't even attempt to do any work.
Check the timing: Set the displacer to LEAD the power piston by 90degrees. Correct to a "good eye"...
Friction? - Minimised, a flick of the flywheel and it spins for a lot of revolutions...
Suggestions of what may be wrong please?
Thanks,
K2

 

[minh-thanh]

Just an odd query re Stirling engines:
While I have "read a book and other stuff", I had a thought to own a Stirling engine, so I could play about a bit and get to appreciate its fundamental thermodynamic beauty.... (remove tongue form cheek?). So I bought a cheap device from China. Nice little toy, except the Pyrex glass tube has melted and deformed a bit while attempting to make it run. The plans had a displacer made from a "cylinder" of steel wire wool on a piece of piano wire (con-rod), but this is porous, so carried a lump of steel from Hot-to-cold-and-back, without any appreciable volumetric displacement of gas.
Anyhow, it didn't work.
So I have made a nylon displacer (because that's the material I had in that size). it still doesn't work.
It doesn't even attempt to do any work.
Check the timing: Set the displacer to LEAD the power piston by 90degrees. Correct to a "good eye"...
Friction? - Minimised, a flick of the flywheel and it spins for a lot of revolutions...
Suggestions of what may be wrong please?
Thanks,
K2

Hi K2 !
Looks like we talked about your engine problem in another thread
My experience with small stirling engines is limited to the engines I've done, so:
A stirling engine depends on the volume change of the air inside the engine when the air goes from the hot zone to the cold zone and vice versa and with engine rotation - several hundred rpm - that results The volume change of the air inside the engine is very small - extremely small - and almost instantaneous, so:
- Friction, not smoothness
- Airtight
The above 2 causes will kill it
Friction must be reduced, to the minimum that the engine can keep running
The engine must be airtight : at the joints, between the cylinder and the power piston, between the displacer's connecting rod and its bearing.
"Airtight" between the cylinder and the power piston, between the displacer's connecting rod and its bearing: that's really a problem, you have to make sure they're all smooth together BUT they have to be "airtight"
I put "airtight" in quotes because that tightness isn't completely airtight but it's good enough that small changes in the volume of the air inside the engine affect the power piston.
Displacer: is a part to move the air inside the engine from the hot zone to the cold area and vice versa., so make its weight as light as possible and suitable for the temperature of the engine.
Power piston: just a part that reflects the change in volume of the air inside the engine

 

[Nikhil Bhale]

The displacer cylinder has to do two jobs at same time.
1. To transfer heat to the air inside it and
2. To avoid heat travelling to other cooler parts of the engine.
These two jobs are totally contrary to each other. If you use glass tube which is insulator it does not do the first job efficiently but does the second job nicely. If you use a metal tube it does the first job nicely but fails in second job.
One thing that many do is to use a high heat conducting cap for the hot end, mostly copper and use a fairly bad conductor tubing of stainless steel for displacer cylinder. The lesser the wall thickness of tubing the lesser will be heat conduction along it. A teflon gasket between the displacer cylinder and the engine will further reduce the heat conduction to cooler parts.

Regards
Nikhil

 

[Steamchick]

Thanks Guys. Nothing new there for me, but good to confirm that all the things I have looked at are OK... On "Airtight", the con-rod for the displacer is a 1mm dia piano wire in a 1mm hole, so not perfectly "airtight" - but that's the design.... I wondered about greasing this instead of light oil, as that may help minimise air flow through the bearing? I can't detect any leakage though. I had used some WD40 to lubricate this, a bit too liberally and some got into the hot cylinder, when it made smoke.. This showed a tiny leak at the O-ring seal, but a 1/8 extra turn on the clamping screws stopped that, so I know "airtight" is what it is... I think I should use PTFE spray, not WD40... Any ideas?
K2

 

[minh-thanh]

I also thought about that before posting , Well really nothing new
It's nothing new about lubricating oil, I like to use sewing machine oil and graphite - take a pencil lead and crush it. , Or mix both

 

[Steamchick]

Aha! yes graphite! I have plenty from my candle flame on the outside of the glass tube... I leave grubby finger-prints EVERYWHERE from that! That's why I thought of PTFE spray (Dry lube sold in spray cans) as a cleaner equivalent.
K2

 

[minh-thanh]

Aha! yes graphite! I have plenty from my candle flame on the outside of the glass tube... I leave grubby finger-prints EVERYWHERE from that! That's why I thought of PTFE spray (Dry lube sold in spray cans) as a cleaner equivalent.
K2

It could also be a problem with your engine
Everything must be clean !!

 

[Steamchick]

It is! Until I put a candle beneath the cold glass, then Carbon is deposited.... Surely that cannot affect the performance? As a black surface it will increase the ability of the glass to absorb some radiant heat from the candle flame (yellow hot)? Or is it a symptom that the candle flame is too close to the glass? I am loath to apply a butane torch (cleaner, but more likely to melt the glass further?). The new displacer fills the hot-tube much more completely than the "designed" wire-wool lump... but thermally will not carry much heat into the cold zone, just pump the hot air much better. - Which I recall was the previous website suggestion? (Can't remember where that was...).
Thanks for applying your grey matter to this problem.

 

[Jasonb]

Wire wool should be OK, often the displacer is filled with it and has holes drilled in the top and bottom plates so air can pass through, my Robinson is made that way with stainless steel swarf, gigger versions have a couple of those stainless pot scrubbers in them. Look up "regenerator" which is what that type is called

Grease will add drag and dried out WD40 can make things sticky

Small spirit or gas flame is usually cleaner burning

 

[anday1]

Ask TED

 

[Daryl_bee]

Ask TED

You see this makes me wonder more why glass displacer cylinders aren't used on stove fans. Here (the movie above) heat (open flame) is being applied directly to the glass (insulator) which is less optimal than what I was asking - glass for cylinder walls only. Forgive my 10 minute CAD sketch

I'm just surprised, given the visual appeal of watching the displacer piston, it doesn't seem to be done anywhere. For the life of me I can't figure out why.

 

[anday1]

stainless steel as thin as possible is probably the best

 

[Jasonb]

Not easy to buy 100-150mm dia glass rings so probably why you don't see many stove engines using them

Clear cylinder walls of this sort of size are mostly found on coffee cup stirling engines where the lower temperatures means you can use readily available and easily machined clear acrylic tube