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How about a Stirling Generator, where instead of pistons responsible for converting the changes in pressure into usable energy, having a liquid in place separating the main working gas with another separate sealed gas area. Changes in pressure would then force fluid in and out of the main working gas
In the link provided, I have animated a system design as such, utilizing a "Ross Yoke" Stirling system, but using wells of oil in place of piston/ring arrangement.
The flywheel itself could be mounted with rare earth permanent magnets and act as a rotor, with a fixed set of coils mounted next to it.
The entire system could be fully sealed, and the friction and leakages associated with a typical piston could be traded for the friction of the liquid.
As a result, higher efficiencies could be gained with the possible increases in gas pressure in the system.
Synthetic motor oil might be a useful liquid to use, as it already has anti-foaming agents incorporated into it, and is engineered to deal with much harsher and dirtier environments than the pressurized Helium that might be used as the gas.
Helium also has a very low solubility in petroleum products.
Wikipedia: Stirling engine
What I think the author is referring to (fixed spelling in title). Hm, does that illustration look like what I think it looks like? [jutta, Jun 08 2008]
Animated Concept, 2 liquid Valve
A possible implementation, animated, utilizing a modified Ross Yoke design. [Reynolds, Jun 14 2008]
Liquid piston stirling engine
Also known as "fluidyne" [8th of 7, Jun 14 2008]
Stirling engine links
Useful background [8th of 7, Jun 14 2008]
||I'm a little confused, are you are replacing the gas transfer piston, the power piston or both? Liquid friction would seem higher than piston sliding friction at high speed, but may switch at low speeds, but in any case I think you need more details.
||Initially I was thinking just the power piston, but later came up with the current design (animated version in link) that uses liquids (synthetic motor oil) for both.
||During initial run up, the floats would provide most of the lift to the linkages and displacers. At higher speeds, the drag forces between the plungers pictured and the cylinder walls would come into play.
||I see a link to the Fluidyne was provided. This is very interesting, I can see how the same concept I am describing can be used as a pump device by using a configuration of valves!
||This is very encouraging, as it validates the use of a fluid as a piston. The only remaining piece of the puzzle for me is to figure out if the oil drag + plungers will be a sufficient transfer mechanism for imposing the phase shift relationship between the hot a cold cylinders (see diagram).
||The differences between the Fluidyne and my design are primarily the lack of valves that control the flow, and the inclusion of a Ross Yoke and flywheel for the generation of power.