Jeroen Jonkman's Sterling 60

[Steamchick]

I should guess that Viton is adequate, the o-rings should be at the temperature of the "Cold" end of the heat exchanger. Maybe less than 200deg.C? But you could use silicon - good for >300deg.C?
K2

 

[Technical Ted]

To tell you the truth, I don't remember what kind I used, so that leads me to believe they were not anything other than Viton. You need an air tight seal. There isn't much pressure so you just want to make sure that 1- it's air tight and 2- it holds the test tube in place.

Ted

 

[almega]

If I wanted to scale this up to make a larger version, would it scale directly or would I have to scale the relative volumes of the displacer and power cylinders? ie. if I wanted to make it double, could I just double all of the dimensions, which would result in the cylinder volumes being 8 times larger as opposed to keeping the proportions the same but making the dimensions so that the respective volumes are only doubled?

 

[Ken I]

K2 - Refer my diagram post #8
I used 2mm diameter Viton "O" rings in a gap of 1.95mm - not much interference at all - the amount of force required to keep the tube in place is negligible.
Using standard interference can make assembly somewhat difficult. Jonkman's original assembly instructions suggested a thin wax paper tube as an assembly "thimble" to get the tube past the "O" rings. Mine gave me no trouble.
Whilst the Viton is overkill - I would be worried about permanently bonding the tube in place with a melted "O" ring.

Regards, Ken

 

[creast]

When I built mine I used silicone O rings as these handle high temperatures easily and are softer so more compliant and lower squeeze force when assembling. Best to do a trial piece to ascertain fit .

 

[Steamchick]

Thanks Ken1:

Hi Almega,
I have been struggling to make a "model" on the spreadsheet, so I cheated and made a "best guess" as to what happens inside the engine. Of course, any experts can tell me I am wrong, because this is my best guess - and I am not an expert!
But I have decided:

1. that the annulus around the displacer is a connection between chambers of different temperatures and pressures. It separates the hot, high pressure zone from the colder "working" gas. As the hot zone fills with gas (displacer moving to cold end) the pressure rises in the large volume of gas displaced, as it rapidly heats up - as it transits the annulus. A delayed pressure curve is then seen in the cold chamber.
2. But the connection (to all intents, here) between the cold chamber and power cylinder is best made as short and large area as practical = same pressure seen by the piston as developed in the cold chamber.

So I have made a simple graph showing how the cold chamber pressure oscillates (assuming sinusoidal) - which will be similarly the pressure on the piston.
My GUESSES have deduced a pressure of just over 3 barg in the hot zone - if around 400deg.C.???
But the cold zone has residual gas and cooling parts so I GUESS the pressure only rises to just over 2 barg in that zone - connected to the piston. But because the engine is closed, and started at atmospheric pressure, the heating will raise the "background" pressure to something I guess could be as much as 1 barg?
It strikes me that the sealing of shafts through the body of the cylinder cooling block is as critical as the piston seal, which, on these plans, is a labyrinth seal, that should be effective.
To improve "power" efficiency, I guess that the colder the cold end and the hotter the hot end is better... but, as well as minimising friction, the minimum free space in both the hot-end and cold-end is desirable. e.g. the hemispherical end to the displacer, and minimal clearance to the glass from the displacer.
I'll not attach files as they are frankly, a mess, and unintelligable!
But the exercise was fun anyway.
Thanks for the game,
K2

 

[Steamchick]

Just a few further thoughts about the design, which may shed some light on the unknowns? (but probably not!):
Re: Post 38 and a few before. My thought experiments - to figure out what is important so I can fix my model - lead me to think that the displacer volume should be as high as practical inside the heated chamber. To that end, the ball-end of the displacer is necessary to fill the glass test-tube internal shape. The clearance between glass tube and thimble - I reckon - could easily be reduced to 0.5mm - as the drawings show an OD of the displacer thimble at 13.5mm and the ID of the glass tube as 14.5mm. If it worked for Jeroen it should be OK. Ken (I think it was your comment?) you mentioned 0.9mm clearance all around the displacer. I can see that this is nearly double the connection CSA between the hot zone and cold zone, but I think the effect is maybe only a loss of a small amount of performance (because your engine works!) and the more significant factor is the displaced volume. You are only "losing" around 7% of the "pumped" hot gas - so a similar effect on the of power of the beast. While it would be interesting to know if a 1mm larger displacer would make any difference, the assembly/disassembly of the displacer suggests you should not bother. But for new engines, making the Displacer OD only 0.5~1mm below the ID of the glass tube seems like a good idea to me.
On thermodynamics - I have been pondering the effect of a larger thermal mass of the displacer, such as the filling of mine with wire-wool - as suggested by the Chinese manufacturer. But logically, I cannot see any reason "why?"... The Stirling cycle is all about rapid pulsations of pressure developed by heating a large volume of gas quickly, moving the gas to a cold zone and rapidly cooling the gas. To have a "hot displacer" - radiating heat - and transiting the cold zone is only going to pump heat into the cold zone without affecting gas pressure. - or even raising the cold zone temperature and overall pressure. So the increase of thermal inertia in the displacer seems like a bad idea to me....?
Of course, experts who have made many models - seen at shows, usually running - often use such things a s polystyrene foam as displacers, where temperature permits - probably for the low mass, but I suspect also for the low thermal inertia of the displacer? An insulator (as opposed to the brass thimble?) would enable the "hot-end" of displacer to remain "hotter" and the "old=end" remain colder, so the thermodynamics are improved by less "wasted" heat transfer from Hot to Cold ends. The only path for the heat transfer should be by gas motion, to maximise the internal pressure fluctuation, which in turn maximises the work on the power piston. To this end, a shiny surface of displacer would also help, as the "hot" part moving into the cold zone is radiating heat, but this radiation can be reduced significantly when polished and shiny. I suspect that if I make a displacer from an aluminium tube, then polishing the aluminium - and possibly filling with polyurethane foam? - will give me the "best" displacer.
That's all my ideas used up now,
Thanks for teaching me about these engines! Theory is good, but practical results prove it.
K2

 

[almega]

Another question about the relative diameters of the displacer cylinder ID and the displacer OD to consider. If the difference between them is reduced to 0.5mm would not the resistance of forcing the air masses from hot to cold and cold to hot through such a restriction be so much as to stall the engine?

 

[Steamchick]

Almega: This is a consideration, but the annulus' CSA is still about 6.5sq.mm, for a volume of gas to be moved = 2771cu.mm. ... but you may be right as I estimated (CRUDELY!) the back-pressure on the displacer to be possibly as much as 6psi...? Really the calculations have to be dynamic, but my crude estimates are considering a "momentary static" condition.
But:
Having just re-checked drawing - for displacer 12.7mm. OD the glass tube can be 13.5 to 14.8mm. ID... so maybe the designer had considered the min clearance of 0.4 to max 1.05mm as OK?
I've missed something somewhere.
K2

 

[almega]

It would be an interesting experiment to do once (and if) I get my engine completed and operational. I have designed my displacer to be removable from the displacer shaft by unthreading it, so I could make different sized displacers to see what effects any changes will have in the function of the engine.

 

[Ken I]

K2, I agree with you on most points.

You do not want the displacer to convey heat to the cold side - only to displace the hot air - the clearance obviously is a double whammy in that a smaller clearance will mean more energy lost to compressing the hot side and squeezing it through the annulus - but then the pressure wave from the pressure cylinder also acts against it delivering some thrust while displacing the now cooler air beck into the hot zone.

There is almost certainly some horrible thermodynamic efficiency calculations - but from my experience at this scale the engine needs to be very free running and such thermodynamic musings are theoretical.

Regards, Ken

 

[Steamchick]

Thanks Ken. I agree with you, but without my attempting to understand the "theory" of how these models work, I personally can't figure out why my bought model kit has never run.... mostly because I don't understand the Chinese instructions. But this theoretical exercise and everyone's feedback has taught me a lot about what to look at (and corredct?) on my model.
Thanks for the feedback,
K2

 

[Bentwings]

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

I did very similar things with early analysis programs. The first fluid dynamics could take all week end to run then you had to sort out data on pages of printer paper. The dual processor and more memory helped but then compressible fluids came and it was back to setting up the event to run Friday as we were leaving the office . Make sure there was a full box of print paper, new ink cartridge push “ return” and go home for the weekend. Today I’ve forgotten how to even set up the events. I’d have to dig out some notes and books I think . I don’t miss that part. I did get pretty advanced info for the era and computers my current lap top has a rediculous amount of processing power the dumb games slow it down however. Getting rid of them as I can. I just have been away too long from the guts of the computer. My under the desk computer works pretty well. I don’t have a dedicated analysis program now just what ever I can squeeze out of the cad program. I need practice otherwise I have to dig through Roarks and use the computer calculator . Prehistoric times. Again LOL

BYRON

 

[minh-thanh]

Steamchick !

, I personally can't figure out why my bought model kit has never run....

If the engine you purchased doesn't run - and it is built according to a proven plan
Let's make it run smoothly and airtight. If necessary: disassemble everything and clean thoroughly
In my little experience with these little stirling engines: friction and airtightness determine if it will run or not.

The smoothness of the power piston with the cylinder, the shaft of the displacer with the bearing: It must be airtight and ensure they are smooth

 

[Steamchick]

Thanks Minh Thanh. You are right. That was basically all I had done - spending days checking to minimise friction. But I hadn't worried too much about air-tightness, so that needs reviewing. But I think I must have wrongly interpreted the use/design of the displacer. The wire wool packing was something that doesn't fit with running of other Stirling engines. I'll dig it out of storage and have a play when the garage comes above freezing (In a month?).
K2

 

[minh-thanh]

Hi K2 !

. But I think I must have wrongly interpreted the use/design of the displacer.
K2

Simply ....displacer only function is to move air from hot to cold and vice versa .

The wire wool packing was something that doesn't fit with running of other Stirling engines

Why not ?

 

[Steamchick]

HI again Minh Thanh,
" I think I must have wrongly interpreted the use/design of the displacer. " Before I joined this thread I thought the displacer must need the wire wool as some sort of heat exchanger: collecting heat when in the hot zone - then heating air when it moved to the cold zone. But my thought analysis suggested that it would also carry a lot of heat from hot zone to cold zone and waste it. If the displacer is "open" to pressure fluctuations, then it adds volume to the total gas chamber and therefore reduces the pressure difference between "hot-gas pressure" and "cold-gas pressure".
So my latest thinking suggests the sealed displacer means the gas heats when it moves to the hot zone - passing between the outside of the displacer and the hot glass, and back, and the gas is the medium that travels from hot zone to cold zone the change the pressure of a smaller volume of gas.
re: "The wire wool packing was something that doesn't fit with running of other Stirling engines " - I have seen a lot of Stirling engines using polystyrene foam or other light similar material (low thermal inertia, low conductivity = simple gas displacers, not "thermal" displacers?). Wire wool adds mass, thermal inertia, surplus extra volume for expansion, etc. which all seem negative to the Stirling cycle. - to me at least.
What ideas do you have?
K2

 

[minh-thanh]

Hi K2 !
I used polystyrene foam or other light similar material and Wire wool as a displacer
I think it's simple: All materials that are lightweight - the lighter the better, of course - and have the temperature fit for each type of engine is enough to make a displacer.
Can't claim displacer using wire wool with must be high efficiency as "standard" displacer because the volume of air it moves is less

 

[Steamchick]

The displacer design in the kit I bought said "make a tube from thin steel and fill it with wire wool". Which I did, and it doesn't work. So I'll make a lightweight insulation displacer and see if that is any better. I am fairly sure the displacer should displace the air from hot to cold and back, not just heat in the hot part and move to the cold part to cool down. And more thermal inertia means the process is slower, less pressure fluctuation and therefore less power....?
K2

 

[minh-thanh]

And more thermal inertia means the process is slower, less pressure fluctuation and therefore less power....?
K2

Sorry,,,I can't reply
when i do the engines i don't pay too much attention to the complicated calculations...and because i'm not a researcher or expert about the engine So I don't think it's necessary for me . And maybe I forgot most of the formulas too,