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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 05 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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