h a l f b a k e r yWhy not imagine it in a way that works?
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Stirling hot air engines are based on using the expansion and contraction of a volume of air as it is alternately heated and cooled. The heating and cooling is normally achieved by moving the air from a hot chamber to a cold chamber and back.
My suggestion is that instead of moving the air to and fro
(with associated pumping losses), one could arrange a single chamber with a hot end (or side) and a cold end. The ends would have to have an insulating band between them. The air volume could them be alternately shielded from the hot and cold areas by moving a thin walled tube to and fro, displacing a minimal quantity of air.
Huge gains cannot be expected, but Stirling engines can already operate on low heat differentials (e.g. body heat) and any improvement means a smaller, lighter engine for the same power output.
Sterling engine
http://en.wikipedia...iki/Sterling_engine
at Wikipedia.org [BJS, Apr 06 2006, last modified Mar 08 2007]
Laminar Flow Engine possible explanation.
http://www.stirling...ng%20engines&topic=
[spacifique1, Mar 08 2007]
[link]
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Actually I think it is more efficient to expand and contract a volume of air as it is alternately heated and cooled, than to move mechanical parts. If I understand you correctly. (I could be wrong) |
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Interesting. Took me awhile to figure out what you meant... but now I get it, and I like it. |
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BJS; The first part of my description is a statement of prior art. (the way it's done up to now). The principal of expanding and contracting the air remains the same as conventional Stirling engines. The difference is that rather than moving a large displacer in one direction and pumping the air in the other, the bulk of the air remains static (ignoring intrinsic convection and turbulence) while a smaller mechanical part is moved to mask the hot and cold areas. The Beta Stirling engine also uses a displacer. |
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Is there any evidence that such a modification would have any benefit? You have a problem in that the Air on the Hot end will be hot and the Air on the cold end will be cold. Air is not the greatest conductor of heat energy and so I am concerned that by not moving the air you may not create enough heat transfer to make the engine go. Also based on what I can see there is no appreciable power draw from the displacer piston as a part of the system so why is this better. |
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jhomrighaus; The benefit is in the reduced pumping loss and reduced reciprocating mass. The air inside a Stirling engine is always moving as the expansion and contraction is used to drive the power piston. As far as I am aware, no Stirling engine draws any power from the displacer. I have no 'evidence' to show. This is, after all, the halfbakery and not a forum for highly funded research establishments. |
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You mean to say you are not properly funded? Why I never... |
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twizz, what i meant about power draw was not that the engine got power from the displacer but rather that it (the displacer) did not consume any noticable amount of power from the engine. The displacer isnt pumping the air like in the traditional sense(moving from one place to another using presure differental) It is only displacing the air. Its kind of like a big hollow Yo-Yo or ballon. The only impact on the system is that caused by turbulence and that is solved by properly shaping the displacer. Remember also that the displacer is not structural and does not transmit force so it can be constructed out of extremely light weight material with no loss in efficiency. |
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I think from what i've read that you need to move the air around in order to create enough heat differential to make the engine go. Try building one and see what happens. From what I can see your modification is to make the displacer an open ended cylinder instead of closed end. Should be easy to try but based on the theory I dont have enough information to know what would happen though. |
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ps i didnt mean to imply that you should have studied this in detail I was just wondering if you had any other information indicating the potenital benefits of this modification to the basic design. You have my vote for orginal thought but Im still not convinced that it will work as well as the exisiting design (+) |
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Twizz; I said "the first part of your idea" not "the first part of your statement". |
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jhomrighaus; Apologies for my mis-interpretation. The displacer draws power because it forces air to pass through a narrow annulus between itself and the chamber bore. AIr (or any fluid) only moves from one place to another because there is a pressure difference. It is the displacer that creates this pressure difference, especially as it moves air from hot to cold and the cooling air contracts, creating additional pressure difference. |
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I have to admit that I do not understand how heat is transferred to and from the air volume quickly enough for a Stirling engine to run at any speed. Any data I can find regarding thermal properties of air suggest that Stirling engines of the size I deal with (8mm power piston) should run at less than 100rpm!
There is clearly stuff going on with heat transfer that I don't understand, so experimentation will be needed... Unless anyone can explain? |
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The displacer-free Stirling exists and is well alive. Google LAMINA FLOW Striling or LAMINAR FLOW Stirling. I have an explanation as to how it might work. |
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Check out the link attached for details. |
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I'm currently building a page on PESWiki about LAMINAR FLOW Engines. I hope to complete it in a week or so, with full drawings. |
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they already have something that works better than that, it's a rankine cycle turbine, heat exchanger on both sides, pump the liquid to the hot side, expands goes through expander and compressor and back to cold side, driven by temperature differential |
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turbines do this very effectively, stirling and piston engines are very massive/bulky, turbines are small and compact, easily maintenanced and produce higher speeds allowing the mated generator to be smaller (and thus cheaper) as well |
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Moving the air from one chamber to another allows you to use a heat exchanger which improves efficiency dramatically. You idea would not allow for the air to be pre-heated or pre-cooled so would be much less efficient even if perfect in every other way. |
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I did however consider a piston where the cylindrical wall is hot on 1 side and cold on the other, with a rotating heat shield. |
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On one of the u tubes I saw someone who made the displacer allowing less air through, although still enough of it to fill the chamber. It stopped working. He then postulated that the the air must be blown towards and along the hot / cold source otherwise the whole thing doesn't work. The air was not getting a stream into the chambers. If this is correct, by just cutting off and insulating differently the hot or cold air in the middle, without displacing it wouldn't do the trick. |
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See my latest entry, [Liquid "Stirling" Hot Air Engine] where the hot or cold source are alternatively applied (via a radiator) to the same air chamber, which is a different but similar solution to what you want to achieve. |
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Manchester University had a bash at building a hot air engine while I was there. It was a wonderful device, conceived by a mind that clearly had no practical engineering experience... |
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Unfortunately due to the lack of experience, it didn't work. The flywheel was a 500kg steel disk, mounted in an enormous frame. It used car pistons and connecting rods in custom cylinders - but with normal car piston rings for keeping the gas in. |
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It's issues were mainly HUGE amounts of friction and lots of gas leaking around the piston rings - which happens if they're not moving fast enough to seat properly on top of the usual leak through the expansion gap. |
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Even if it didn't work, it was great to see the cold cylinder cooled with liquid nitrogen and the hot heated with nearly boiling vacuum pump oil. |
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They incorporated a regenerator – making it into a Stirling engine, but it still didn't work. Quite a faff. |
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Did anyone guess that this was the Physics department and not engineering...? |
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I've only seen 2 that do work. One in a steam punk exhibition in the Oxford History of Science museum, and the other I purchased for a present for my father. It's a wonderful little device powered by an alcohol burner. Once warmed up, it vibrates so much that it moves about on the desk, threatening to fall off the edge and spill the alcohol fuel all over his paperwork which is piled high from the floor. He greatly approves. |
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Any heat based engine has an upper limit to its efficiency set by the difference between the Thot and Tcold inputs.
Thats why the muscles cars of the 60's burned a rich fuel mix and had higher compression ratios--a hotter hot side makes it easier to extract horsepower.
When you dispense with the air pumping mechanism, you are getting rid of the only means to extract enough usable energy from a smaller temp differential to actually make an engine. |
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