Jeroen Jonkman's Sterling 60

[Technical Ted]

I collected some aluminium cigar tubes for this... but have not made an engine... yet...

1. Does the end need to be hemispherical? - or is it just aesthetics? Does look good anyway.
2. Do you fill the displacer with wire wool or anything? A model I bought told me to do so, but didn't give any clues as to "how much" wire wool to use... Any suggestions?

Ta, K2

Not sure if the end needs to be hemi or not. My guess is that hemi is best because it better fits the end contour of the glass tube and will more completely move the air out of the glass tube, which is what the displacer needs to do...

Nothing inside the displacer; just hollow. Needs to be as light as possible so it doesn't create any more friction when running as possible. Also needs to be air tight.

I've made three of these engines now and, knock on wood, didn't have any problems making the displacer tubes and I made them quite thin. Finish the inside first and then take light cuts to finish the OD. I formed the inside radius with a round end end mill. I also made up a steel plug with a matching radius on the end to go inside for turning it around and forming the outside radius. Just take your time and take light cuts. Measure frequently so you know how thin you are getting. If you hurry, you might be sorry...

Ted

 

[almega]

These are only assumptions, but I think the purpose of the rounded end is to conform to the shape of the glass test tube to maximize the air displaced and to look cool. I have seen other Sterling engine plans where the displacer and cylinder were "square" in the bore and a metal cylinder was used rather than a glass tube.
I did not see anything about filling the displacer with anything in any of the plans I considered. To me, it would seem best to minimize the weight of the displacer but perhaps there is some thermodynamic reason for putting the wool inside the displacer. It could also just be for support in the case where the wall thickness is very thin. Perhaps someone here with more scientific knowledge about the Sterling cycle could give better information. I just know if I am reasonably diligent about following the plans and executing the plans well, the engine will (most of the time) run. That's my tuppence worth.

 

[almega]

I did similar to Technical Ted but for the outside rounded end, since I don't have a ball radius cutter thingy, I roughed it out close by pivoting the compound and taking several angled cuts. Then I VERY carefully filed it to shape. Perhaps not the best way to do it but I was successful twice as shown in my earlier photos. This is all a learning experience for me and I am always amazed at the outcome, even though sometimes I have to do it over a time or two to get it right.

 

[Technical Ted]

I did similar to Technical Ted but for the outside rounded end, since I don't have a ball radius cutter thingy, I roughed it out close by pivoting the compound and taking several angled cuts. Then I VERY carefully filed it to shape. Perhaps not the best way to do it but I was successful twice as shown in my earlier photos. This is all a learning experience for me and I am always amazed at the outcome, even though sometimes I have to do it over a time or two to get it right.

I hand ground a radius tool bit and formed the radius using that. I used sections of the cutting edge to rough it in and then, very lightly, used the whole cutting edge to finish it using very light cutting pressure. The inside steel radiused mandrel helps to support it. I used 360 brass which helped a lot since it machines very easily.

Any way you can form the radius is fine, just take it easy...

Ted

 

[Steamchick]

Thanks for the feedback. I hadn't thought too deeply about the action of the displacer - which (of course) displaces the heated air into the "cold" zone, then cooled air from the cold zone back to the hot zone. The alternate heating and cooling of the charge of air delevops the pressure oscillation that alternately drives and is pumped back out of the working cylinder. So thermodynamically, a low thermal inertia seems more useful than a high thermal inertia. Therefore the thinner the wall of the displacer, and the more completely it fills the space of the hot cylinder, the better, I reckon.
Thanks for helping me understand what is happening!
K2

 

[Technical Ted]

I just followed the original drawings for the clearance between the displacer and the tube. It should not be too tight by design.

Thermodynamics aside, my nemesis on all three of the engines I built was the CI bearing and the sliding fit of the displacer/rod. Being a horizontal design makes things harder because you are going to have more drag (friction) in the displacer rod and bearing than if it was a vertical design. The hole in the CI bearing MUST be perfectly concentric. Small drills love to wander off center when drilling and that was one of the biggest issues I had making mine. There are different ways to achieve a true bore and it doesn't matter which one you use, but the hole must be concentric.

If I were to make another one of these, I would also make the CI displacer rod bearing as long as possible. There is room to do so. Although this would add a bit more contact and friction, it would help with the cantilever effect when the displacer is fully extended into the diffuser tube. The longer this bearing is the less likely the shaft is to bind when the weight of the diffuser tube is hanging when fully extended.

YMMV,
Ted

 

[Ken I]

Can't really say - I've seen square ended displacers so yes it's probably more an aesthetic thing.

I don't see any point in adding anything to the displacer - all it does is (mostly) move the air about - null mass and thermal capacity would be optimum therefore unobtainium would probably do the job.

Starting with a cigar tube is a cool idea - but can you find a borosilicate tube to match ? Of course that doesn't need to be transparent either but that would detract from the aesthetics and nullify the need for a "pretty" displacer in any case.

Edit: when I hit save - Ted beat me to it.

Regards, Ken

 

[Technical Ted]

I am far from a stirling engine expert... but here's my take on it.

I agree with Ken that the whole concept of the glass tube and brass displacer are just to make things "pretty". It's neat being able to see that brass displacer move back and forth while it's running. The whole engine is "pretty"... nice to look at. Hey, it doesn't actually do anything! It's just to watch run and being pretty makes it nice!

If you could find a square ended glass tube then you would make a square ended displacer. Or, don't even use a glass tube. Make it out of aluminum, stainless steel, whatever as long as it will take the heat. You don't "need" to see through it. But, I think the end of the displacer needs to match the form of the end of the tube so the proper amount of air is being moved by the displacer. So, square with square, round with round. The glass see through tube and brass displacer just makes it pretty...

I'm currently making a Moriya stirling fan to run on my wood stove in my workshop. That design uses a square ended displacer and displacer tube, but still has fairly tight clearance at the end of the stoke to move the proper amount of air. That displacer hot end tube is made out of steel and the displacer is made out of stainless steel.

If there are any stirling engine experts out there please comment.

And, just to be clear, I wouldn't add anything to the displacer.... but I would to the displacer rod CI bearing as I said above. I would make it as long as possible.

YMMV,
Ted

 

[Steamchick]

Hi Ken 1: yes, that thought about a matching glass tube is pertinent. On the (bought) model I have, the tube has "softened and sagged" as I over-cooked it trying to get it to run. The timing had slipped in transit from China (rough seas, or a bumpy silk road?) so I have been trying to get it to run ever since... Do you (or anyone else?) have timing details between the displacer and power piston cranks? I assumed 90 degrees, but then again I am guessing! - and that doesn't work...
I also have a flame gulper I bought (again from China... - There is a lesson somewhere?) with "slipped" timing. that actually ran - straight out of the box - when I got a flame that was "right" - but after 3 to 5 seconds it stopped and I realised the screw securing the timing was loose. So the timing was "lost". Another one where I need to find out a typical timing, reset mine and try to optimise...
Both of these were bought to learn from... the lesson so far is "don't bother, Stupid!" I just like to understand an engine before I make one. (Am I crazy?) That way, I can appreciate all the odd foibles that need the best settings and are critical to successful running! But strange how the only 2 models I have ever bought both fail.... due to slipped timing... All the models I have made, or inherited, work fine...
K2

 

[Technical Ted]

I've read that the Piston should trail the Displacer by 90 degrees. And looking at the drawing for the Moriya it does show it timed that way.

Ted

 

[Steamchick]

Thanks Ted. I wasn't sure if there was supposed to be any lead or trail due to dynamics of hot and cold gases, and the relative pressures pumping gas through interconnecting passages? Perhaps I need to study a design in more detail and see what some numerical modelling analysis of the actual chamber sizes and passage sizes can tell me? As there are no valves, the modelling should be relatively easy to do on an Excel spreadsheet I think?
Back in the early 1980s, before powerful PCs were invented, I had to use a custom written software model on the Company finance mainframe computer to model up to 9 interconnected chambers, and a 7 mass-elastic model, to understand how to tune a piece of equipment. I could have 3 runs of the computer at up to 15mins run-time per model, overnight. Seriously crude empirical adjustments required to get the model to give "real" results. Each day I would manually graph the motions of parts, from the printout (all numbers), and tweak the 3 variants to get a "better" run the following night. Eventually, the "Doctor of Maths" inverted the program so I could put in the "final result" and the software he wrote could iterate to a set of dimensions that was within 10% of the real result. I then eliminated 5% with various "fudge factors" (constants developed from the difference between the model and reality) and fine tuned from this point to Quadruple the lifetime of the equipment, and increase performance by over 25%... We had to measure deflection of the equipment under various static loads (Hydraulic jack, air pressure on the piston and dial gauges all over) to determine the stiffness of sub-assemblies. The conclusion led to a 2-phase motion at one end to get the maximum performance at the other end of the equipment. (exchanging Stored energy for Kinetic energy and back again, with some parts doing 120g acceleration, when 7 mechanical spring connections were tuned to a model comprising up to 9 gas chambers and passages). The whole development took over 6 months for the project, but the result set a new standard for the competition to "chase".
So I wonder what can be done with a "simple" Sterling engine? (3 chambers and 3 interconnections?).
All I need are plans of a model (Thanks Ken 1 for the plans attached to this), some clues of temperatures of the hot and cold chambers, bore and stroke, etc...!
I see a small mental project to drive me bonkers!
Than

 

[almega]

Steamchick - Wow!!! Did you do all that with punch cards back in the day? That was how things were when I started out my life in the business world. Create and recreate your work ad infinitum day after day with numerous tweaks to your card deck to be run each night. It was a great day when we relegated the card reader to the trash.
But back to the sterling, you mention 3 chambers but I thought there were only two, the displacer cylinder and the power cylinder. Am I missing something?

 

[Steamchick]

Just spotted the obvious (I rarely manage to do that!). The displacer and power piston are both driven form a common crank, but the geometry sets the power piston 90degrees after the displacer:
I'll write it down, so I think I understand how it works, after which you can correct me where I miss the "obvious" points! (Or get it completely screwed-up wrongly).
The logic:

1. Crank at zero degrees. Displacer fully at Hot end, power piston at mid-stroke towards TDC. Gases cooling so pressure in power cylinder dropping, while gases pumped from power cylinder to cold cylinder.
2. Crank at 90 degrees. Displacer half-n-half, now filling cold end. Power piston at TDC, with gases heating in hot-end and pressure rising.
3. Crank at 180 degrees. Displacer in cold end, gases in Hot end and pressure rising. Power piston now half-way to BDC pushed by rising pressure.
4. Crank at 270 degrees: Displacer now half-way to hot end, so gases being pumped to cold end and pressure falling. Power piston has reached BDC. End of power stroke.
5. Back to 1...

I'll now attempt an excel model of the chambers (dimensions from the drawings) and see if I can draw the pressure cycle in the chambers.
Insanity beckons...
K2

 

[Steamchick]

Hi Almega,
As I shall be modelling the "gas chambers" and interconnections, there are the following:

1. Hot chamber - when displacer is at the cold end,
2. Cold Chamber - when the displacer is at the hot end,
3. Power cylinder - the piston bore x stroke plus ullage.

1 & 2 are interconnected by the annulus past the displacer, 2 & 3 are connected by a passage dia 2mm x 3.5 mm long (This has both volume and an inlet and exit, so could be considered a 4th chamber, but I shall look at it as infinitessimally short initially, as just a restricted connection. I'll add its volume half and half to the cold chamber ullage and the power cylinder ullage. But I'll check if that is close enough...
Incidentally, the Company Mainframe computer in 1984 didn't use punched cards... I left those at university in the early 1970s. Do you think I am a Dinosaur? I'm sure we had green screens in the computer department in the 1980s and the operators typed into those directly. Programs were stored on Magnetic tape.
Nostalgia fades with age... Neuralgia takes over?
K2

 

[Steamchick]

Just remember, we put a men on the moon and brought them back - safely - from pencil and paper calculations, done by girls called Computers! Because they computed the numbers... They called it Rocket Science back in the 1950s... (I was but a child). But it was only Engineering... developed by Sir Isaac Newton, based on maths by Keppler.
The 3 most powerful Computers on-board the Apollo missions were the 3 astronauts... with pencil and paper.
K2

 

[almega]

But it was only Engineering... developed by Sir Isaac Newton, based on maths by Keppler.

Which if taken back to the origins of algebra and geometry would be based on the science developed by the Egyptians.

Do you think I am a Dinosaur?

I would not insinuate that, nor was I, but as I recall there may have been a few of them running around yet when I started out earning my keep.
I would be interested in what you arrive at with the calculations re. the Sterling cycle.

 

[Steamchick]

Hi Almega, My intention is to publish what I achieve - if it is anything sensible and useful to others. But don't hold your breath!
Just spent 2 1/2 hours shredding the dross from a tree that blew down in a gale in November. Making swarf in a different sense, as it ends up as sawdust spread all over the garden. Nature then does the rest of de-fixing the CO2.... and releasing the minerals back into the earth for new growth in the spring.Satisfying when finished, but tedious at the time. My excuse is that it is much needed exercise - and it gets me in the garden in winter!
Too knackered for the machine shop or calculations now...
K2

 

[Steamchick]

Almega, No offence taken at anyone calling me "old". I take as recognition of my experience and the good life I have already lived - long may it continue! I Got used to being called "an old f@&t" when young graduate engineers reckoned I was older than their grandfathers... and a girl on the bus stood to let me sit in the Old persons' seat! When I declined I got some stick from other old people on the bus who told me I should "act my age" and respect the courtesy! All too rare when you need it apparently. 15 years on I am nearly old enough to be considered "Old", but my head is still 37 years old... - The "kids" are now 50+.
C'est les temps.
K2

 

[marvin hedberg]

aluminum cigar covers can be drawn to a smaller diameter by pushing them thru a sizing ring with a wood push stick with a rounded end. i did that to build a 1/10th size Erickson engine with a .800" bore. (back when i still had a dark beard

 

[almega]

Since this little engine won't be running for extended periods of time, I am wondering how much heat will actually be transferred to the o-rings that hold the glass tube in place. Do they need to be of a high temperature compound or will standard Buna-N at 70A hardness suffice? Another question is how much interference should the fit be between the o-rings and the glass tube? The glass tube I was able to find has an OD of 0.621" which is 15.77mm. I was looking at an o-ring of 3mm section with ID of 15.5mm. Will that be adequate?