Stirlingengineforum.com

Hi all

I'm not sure exactly which ratio the displacer to piston should be, or if the piston is better to have long stroke or shorter stroke.

In anycase if I need to add dispacer volume, then I'll use 2 drums in parallel working together and perhaps union them together with a T both going into the head of the piston.

Anyhow FYI my current status


Still would like to know what would be the best total percent of heating and cooling area of the displacer.

Bottom and top only, or should a larger area of the displacer be heated and cooled ?

Please advise
Thanks

You may find the following formula useful:

Vmax/Vmin = 1 + ((Tmax-Tmin)/1100)

Where Vmax and Vmin indicate the maximum and minimum power piston volumes (ie. at top of stroke and at bottom of stroke).

fokker wrote:You may find the following formula useful:

Vmax/Vmin = 1 + ((Tmax-Tmin)/1100)

Where Vmax and Vmin indicate the maximum and minimum power piston volumes (ie. at top of stroke and at bottom of stroke).

Thanks,

Please clarify this a bit.

What does Tmax-Tmin refer to ? temperature ?
Perhaps I can toy with some equations for my design

I'm hoping this will clarify the subject of piston volume versus displacer volume requirements:and perhaps even an equation to determine the min. temperature differential for my design would also be nice

Please advise
Thanks

Hi, yes Tmax and Tmin refer to maximum and minimum temperature. This formula I provided is useful for knowing the stroke length (compression ratio) for your power piston.

There is also a formula I have called the Schmitt number that gives a good idea of power output based on several factors such as temperature, displacer volume, pressure, phase difference etc. The formula is a bit long to type out on here but you can probably google it.

Alternatively there are a couple of simpler formulae along the same lines; the Beale number and the West number - look them up in wikipedia, they may be of use.

Also, there is a formula (more of guideline really as are all these formulas) that gives an idea of displacer volume to piston volume based on several engine parameters. If you really want that I can try and dig it up from my pile...

fokker - thanks

I'll do that

Basically I guess I'm trying to get a simple idea of what my working design should look like on paper.

My displacer currently will be a 55 gallon drum, and piston size will be made from some pvc probably 4 inch piston for starters, and if that piston proves too large, then I will add displacer volume by using 2 drums, in series and tee them together into the piston chamber, they will go up and down simultaniously, instead of trying to figure out how to make a larger displacer etc.

But it would be nice to know these calculations to see if it will or should work on paper first.

Such as size of displacer piston versus the displacer drum itself, and versus the piston size, and of course min. operating temperature difference etc.

And if my design requires too much heat, then I will have to change the piston from plastic to some type of metal.

Also I'm considering a displacer, I've heard of the regeneration subject, and was considering a steel wool filled type of cage, not sure if this steel wool would be a good idea but I was thinking of trying to put a piece of wood in the center, to divide the cold side from the hot side of the displacer, and perhaps this would create a nice regeration type displacer, but I'm not really sure if this walking beam design is best used without a regeration type of displacer either.

Thanks for the info

As far as I understand it, a regenerator is a very good idea - the efficiency you will achieve without one will be abysmal. Not sure on the best type but you could either go for something like steel wool or mesh screens inside the displacer itself (ie so the air is forced through holes in the displacer body instead of around its edges). Or alternatively you can pipe the regenerator outside of the drum and force the air through it - but this would be more work. Getting the regenerator right is a matter of balancing factors such as having enough surface area without creating too much dead space, and having enough regenerative mesh without creating too great a pressure drop. Good luck with the engine.

fokker wrote:As far as I understand it, a regenerator is a very good idea - the efficiency you will achieve without one will be abysmal. Not sure on the best type but you could either go for something like steel wool or mesh screens inside the displacer itself (ie so the air is forced through holes in the displacer body instead of around its edges). Or alternatively you can pipe the regenerator outside of the drum and force the air through it - but this would be more work. Getting the regenerator right is a matter of balancing factors such as having enough surface area without creating too much dead space, and having enough regenerative mesh without creating too great a pressure drop. Good luck with the engine.

Hi thanks for the info,

After considering this topic some more I would like to get some input on the following subject:

If I my design can provide a heated bottom of the displacer piston itself, and a cooled top of the displacer.

I wonder if this could create an even better displacement.

Basically I'm considering that the lower plate of the displacer piston be a double wall disk in which hot water will circulate and remain the same constant temperature as the bottom of the displacer drum housing.

Also I would then do the same to the top plate of the displacer piston in which the top plate would also be double walled in which the inner very thin space between the double plate would be cooled and remain the same cooled temperature as the top of the displacer drum .

I'm not sure that could help things but I would assume that I would have to also make the top and bottom plates of the displacer piston, so that it had holes in it so that air could pass thru, into the regeration portion of the piston.

Basically in theory when the displacer piston goes to the bottom hot side, the top side would of course be cooled, and the top plate on the lower end of the cool side would also be cool causing the air to cool faster.

On the reverse cycle the opposite would be true, and the displacer piston would be at the top and the hot air in the bottom portion would also be heated by the bottom plate of the displacer piston which would now be toward top side of the hot air section. so that you would have heat on both the bottom and the top of that air section.

So basically the function would remain the same as a standard stirling, however when the air volume is at it's max on either side, the air would be either heated or cooled by both the top and bottom, but also the top and bottom of the displacer piston itself ?

I hope I'm making this clear.

Anyhow what do you think?
Will is Help? or hurt things ?

Any help with ideas are helpful. I've rounded up a few more parts and am now considering the bearing sleave and/or displacer piston materials.

Thanks

Not a bad idea at all but it will need to increase the power a fair amount to offset the extra energy required to run it. The weight and friction of plumbing fittings etc. I know your taking about a pretty large scale engine here and maybe it's less of a factor but it will add extra effort to your design. In my opinion, the displacer should be as light as possible to reduce the energy draw it causes the power side. If the chamber it's self can be heated and cooled in some new and efficient way that would be worth exploring further. I'm not putting your idea down by any means but it's going to be hard to build I think. Good luck, I look forward to seeing how this engine turns out.

Cartech wrote:Not a bad idea at all but it will need to increase the power a fair amount to offset the extra energy required to run it. The weight and friction of plumbing fittings etc. I know your taking about a pretty large scale engine here and maybe it's less of a factor but it will add extra effort to your design. In my opinion, the displacer should be as light as possible to reduce the energy draw it causes the power side. If the chamber it's self can be heated and cooled in some new and efficient way that would be worth exploring further. I'm not putting your idea down by any means but it's going to be hard to build I think. Good luck, I look forward to seeing how this engine turns out.

Hi, thanks for the reply.

I'm not expert. I really need to just get a working design in effect, then perhap tinker with it some more.

From what I've read I think the displacer should be light also from what I can tell.

I've got the plumbing and design figured out for this, if I decide to go with it and I don't believe it will add much weight due to the amount of liquid in the double walled displacer piston end pans.

I guess thats what I'll call these at this point is: displacer piston heating and cooling end cap elements.

I do think it could add too much additional weight unless I can figure out a way to only provide a very light double walled pan, perhaps a pizza pan or something very thin material that will allow a very small amount of liquid to pass thru it.

Perhaps 2 pizza pans with a small fitting on each side so that the liquid could pass into and exit the end cap.

The cooling side could use simple plastic hose which is very light, and the hot side could be more difficult if the heat is too hot then the plastic hoses will not work, so I'll have to consider this some more.

Anyhow does anyone think that this would really even make the displacer more efficient if too much weight is not added ?

I'm not exactly sure how it's suppose to work, but if the idea is to cool the cool side as quickly as possible, and the heat the hot side as quickly as possible, then it seems like both the top and bottom of the chamber should be heated and cooled, along with the top and bottom of the displacer piston itself.

I'll have to get my large standard walking beam built and see how it goes.

I've also considered toying with the idea of making the displacer drum a rotary type instead with the walking beam incorporated into it as well.

Anyhow I'm open for ideas about the displacer piston.

Regarding the wire mesh that people have talked about?

Is this like standard window screen that they are referring to ?

Or some other type of wire mesh ?

Please advise ?

Also is the wire meshed in sections or is the whole displacer piston shaped from this wire mesh etc. ???

Thanks

Agent86,

I'm no expert either and have just two working models under my belt. From what I have read, the mesh is usually steel wool and some times Scotch Brite pads but these are being used in models not large engines (to my limited knowledge) Maybe a suitable regenerator/displacer for you engine could be formed with chicken wire wrapped over a heavy guage wire form displacer and packed with steel wool? It would be fairly light and should regerate? One problem I see with the internally heated and cooled displacer would be keeping the hot side from heating the cooler side Perhaps a large and fairly thick insulating foam piston with the hot and cold coils on each end, maybe a copper line bent into a spiral coil for each one? Do you run the hot line out the bottom and the cold one out the top? If the fluid lines are run in a double walled pipe (the displacer connecting rod) I think heat transfer in this area would be a problem.

Cartech wrote:Agent86,

I'm no expert either and have just two working models under my belt. From what I have read, the mesh is usually steel wool and some times Scotch Brite pads but these are being used in models not large engines (to my limited knowledge) Maybe a suitable regenerator/displacer for you engine could be formed with chicken wire wrapped over a heavy guage wire form displacer and packed with steel wool? It would be fairly light and should regerate? One problem I see with the internally heated and cooled displacer would be keeping the hot side from heating the cooler side Perhaps a large and fairly thick insulating foam piston with the hot and cold coils on each end, maybe a copper line bent into a spiral coil for each one? Do you run the hot line out the bottom and the cold one out the top? If the fluid lines are run in a double walled pipe (the displacer connecting rod) I think heat transfer in this area would be a problem.

Hi,

Yes I was considering this chicken wire topic or perhaps garden fence to form the shape as well, and them perhaps some steal wool or something also.

As far as the plumbing goes I was considering just something similar to a refrigerator ice cube tubing or some sort of plastic line for the cool side of the displacer piston end cap.

I would run that in from the top, or the side of the upper portion. depending on how much clearence there was on the sides of the piston. Then also out the top or the side depending on how much room I have on the sides of the displacer piston.

Same for the hot side of the diplacer piston end cap.

I may not do this at first I think I may just get the large unit working first and just use the steel wool as you also suggested.

I'm wondering is the steel wool seem like a good idea, or what is most typical for this displacer piston.

Also I would guess that you fill the complete displacer piston with wool and then just insulate the center as to separate the hot from the cold ? At least thats what I'm considering.

Thanks for the replies

Also one more thing is that I'm not sure how much of the actual outer drum of the displacer should be heated or cooled ?

Is it best to just heat the top and bottom end cap of the drum or should the upper and lower half be completely heated and cooled ?

Your engine size is a bit over my head but I'll venture to say that the steel wool would be a good idea. I don't think you want it to separate the hot and cold side but the air should flow around and though it. The wool will catch and store some heat this way for use in next cycle. I don't think you want it too porous either. The examples of this type of displacer I've seen look like a movie film reel and the smaller inside circles are full of steel wool or scotch brite pads.

As for heating and cooling, I would heat the bottom plate and weld a jacket to the upper half (upper third?) and run water around it. You will have plenty of heat going up the side anyway and that isn't what you want much of. I'm guessing your using the 2/3rds of the drum occupied by the displacer right? I mean if the piston is too short then you have a considerable volume of air to heat and cool each cycle. This isn't going to be a LTD Stirling is it? Anyhow, much of your project is out of my relm of Stirling design and most of what I've offered is "best guess" so don't take my comments too seriously. It sounds like a fun project, let us know if it runs.

Cartech-
Thanks for the reply.

All ideas are good they help me collect my thought and even come up with other ideas.

Thats good to know about the air flow thru the displacer not being to free, and having some restriction. This is great news.

Yes to the 2/3rds.
I'm not sure what you mean by LTD I'm guessing low temp displacer ?

If so then yes I want a low temp displacer, however I'm not sure it should be a short type of displacer like of seen in the models, or should it be longer like the walking beam. I don't know if I've seen too many low temp walking beams but I'm going to try one anyhow. I can always use the parts in my next design if this proves to be a failure LOL.

Anyhow, Now that I think this sort of confirms my displacer materials and perhaps I'll just use a 10 spead bicycle rim with spokes for the top and bottom of the displacer, and then perhaps just wrapt the 2 rims with some garden fence, and fill it with steel wool or something similar that can be found at home depot.

I'm also considering the sort of angled displacer type that does not really go up and down but sort of pivots up and down like a swinging door, that way I don't have to worry too much about loosing air or what type of sleeve bearing to use. I can make a sleeve bearing and the shaft can come right out the side of the drum and have an arm on it for turning back and forth to make the swinging displacer door go up and down.
perhaps I'll use this concept anyhow and create a arm inside the unit to make the displacer go up and down also, instead of angled type swinging door more like a seasaw, but offset, and one half will be outside the drum, and the other half of the seasaw will be inside the drum.

Hmm this gives me some ideas, I'll post a drawing.

Why can't the displacer be 2 separate containers ?
Like for example 2 drums,
Heating one whole drum.
And cooling one whole drum

Then on one side of the round side of the drum you can tee them together, with a super insolated or steel wool filled pipe to connect them together.

Then on the power piston side, which would be the opposite side, you could just tee together the hot and cold drums, up to the piston, however just before the piston a 3 way pipe valve could be installed and could open the hot side for expansion of the piston, then switch to the cold side for contraction of the piston, and this valve could be controlled by the same walking beam concept and flywheel etc.

I may try that design too after I get one standard large walking beam working LOL

Anyhow this is going to be really fun. I can hardly sleep at night thinking of the cool stuff.

Anyhow thanks for the replies they have all been helpful.

I "think" LTD is low-temperature-difference. If you look at picture galleries, the LTD Stirlings dont make much power (can run on the heat of your hand or a cup of coffee!).

They need to emphasize heat exchange, so the displacer cylinder is a pancake.

Why can't the hot and cold side be two separate barrels? they can, but the additional volume will reduce the pressure.

Several Stirlings I have read about worked, but the builder realized too much heat was creeping from the hot end through to the cold end. The one I remember decided to machine the cylinder wall thinner, and it helped, especially at high speed.

I believe a useful configuration to consider might be the bottom 1/2s of two drums joined by a ceramic ring to isolate the two ends as much as possible.

I would also suggest making the flywheel spokes a fan that is ducted by a tube of some type to direct air to flow from the cold end tip towards the ceramic center. If you want to have water on the cold end tip, put a jacket just over the end, with cool air flowing from the end to the center inside a shroud.

To keep the water as cold as possible while cycling during demonstrations, make the water jacket a thermo-siphon loop, with the reservoir filled with salt-water and fresh-water ice chunks. For a constant duty configuration with no water, aluminum fins on the tip should be enough.

Hmmm, perhaps have the flywheel fan pull air across a large aluminum radiator to provide water-cooling?

The displacer piston should be as light as possible, unless you are certain you want a low RPM unit. In a very slow-RPM engine the piston can also be the regenerator, if you want.

Plan your displacer cylinder design around adding a regenerator onto its side, but you can get it running first and add the regen later.