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Can a 2-cycle gas engine be converted to a Stirling Engine

12/22/2012 12:31 PM

I attempted to convert a 2-cycle RC plane engine to a Stirling engine. The "basis" was to supposedly use the intake port/piston as pre-compression piston, with no particular regnerator other than the heat left behind in the exhaust tube being sent back to the intake port without the carburetor. New aluminum head was installed (no leaks), with a small port on top leading to a gas tube of similar volume as the cylinder where the piston moves. This decidedly did not work with full heat from a propane torch on the head and on the gas tube. Does anyone out there know of anyone who has made this actually work? If not, then I will remove the inner sleeve, blank off the intake and exhaust ports, and then interconnect with another piston (cold side one) in an alpha configuration and continue.

This is for an at-home DIY project to make quiet running generators. I suppose I could also spend my time trying to perfect an exhaust silencer and go back to Plan A.

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#1

Re: Can a 2-cycle gas engine be converted to a Stirling Engine

12/22/2012 6:57 PM

Well, it might, but one wouldn't start from there. Stirling engines need two pistons that operate 90deg out-of-phase with each other and the forum isn't being told anything about the engine itself.

One might start the learning process with one of those cheap kits one finds on internet auction sites for a few quid, and go from there.

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#6
In reply to #1

Re: Can a 2-cycle gas engine be converted to a Stirling Engine

12/26/2012 1:32 PM

Yes, that is true. The engine (like many model-plane or Radio Controlled Cars) is a two-stroke engine. The intake where the carburetor normally mounts has a shaft mounted port that admits fuel-air under vacuum (in normal operating mode), and this is slightly compressed by the downward motion of the piston at the end of the power stroke, just as the exhaust port is opened by downward motion of the piston. At about 10-25 degrees (of rotation) later the piston exposes the intake port on the cylinder sleeve (opposite side from exhaust port).

What I was attempting to do (which I now believe to be completely impossible) was to loop a tube from the exhaust port back to the intake port (as a weak heat exhanger to dissipate heat left over after the power stroke, and return some heat at the intake), overcome the intake compression and the power piston upward stroke compression (with flywheel momentum), have enough time constant at the top of the stroke to pick up additional heat from the "furnace" (propane torch in first tests) sufficient to drive the power stroke, and re-charge the flywheel, etc. The hot temperature heat exchanger consists of a small diameter brass tube sealed at the distal end, and mounted to the head of the engine, similar to the volume of the piston in cylinder at top dead center. The object was to provide compressive heating of the gas within the tube to drive the power stroke. Having made numerous failed attempts with design #1, I will revert back to standard Alpha Stirling designs where there are indeed two cylinders, gas coupling, adjustable phase angle (at the flywheel/crank), hot source and cold source, then again.

Ultimate objective is to come up with a simple design that can be made even with fairly rudimentary tools (maybe even hand tools) - (not true in design #1), that runs, produces excess power to drive a small generator, and that is whisper quiet. (Quieter than commercially available Honda generator - makes this pretty stiff competition).

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#2

Re: Can a 2-cycle gas engine be converted to a Stirling Engine

12/22/2012 9:23 PM

I'm missing the point here, if one exists, do you have a propane torch that burns constantly and you want to convert lost energy?

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#7
In reply to #2

Re: Can a 2-cycle gas engine be converted to a Stirling Engine

12/26/2012 1:34 PM

See #6 Fredski - I just want to eventually make a whisper-quiet generator (hopefully that can be scaled up to 1.5 KWe or more), that can be solar, or alternative fuel (such as grass clippings, leaves, etc.) (never mind the smoke signals it will produce, I have a way of dealing with that).

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#3

Re: Can a 2-cycle gas engine be converted to a Stirling Engine

12/23/2012 4:10 PM

Here's basically how a Stirling engine works. The working gas is sealed up and is forced to move between a hot chamber and a cold chamber. The hot chamber stays hot and the cold chamber stays cold. The gas absorbs heat from the hot side and expands. When it does this, it moves some mechanism (call it a displacer) to move the gas to the cold chamber. where it contracts. As it contracts, it moves the displacer causing the gas to move back to the hot chamber, repeating the process. The beauty of this design is that only the gas (the working fluid) changes temperature.

The displace mechanism is the key and is implemented in a number of different ways. A google search will reveal a number of designs. One common way is to have two pistons 90 degrees out of phase in the cold and hot chambers. Connecting the chambers with a port moves the gas back and forth between the cold and hot side and also varies the volume allowing the expansion and contraction to do work. One interesting design I saw used two bicycle tire pumps with the handles connected to a bicycle wheel 90 degrees apart. The hoses are connected together confining the gas. One pump is heated and the other is cooled. The only problem I see is the seals withstanding the heat.

I bought a toy Stirling engine kit that is fun to play with and will run on an ice cube. It provides a good insight on the workings of this type of engine.

http://www.thinkgeek.com/product/9e84/?rkgid=275668648&cpg=ogpla&source=google_pla&gclid=CNXRyK-2sbQCFQqk4AodTiQAOA

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#4
In reply to #3

Re: Can a 2-cycle gas engine be converted to a Stirling Engine

12/23/2012 4:49 PM

Very well explained Rix.

There are some very cool models out there, here's one:

Sterling_engine_at_Amazon

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#10
In reply to #4

Re: Can a 2-cycle gas engine be converted to a Stirling Engine

12/29/2012 5:14 PM

This looks like the first one I bought that turned out to be a kit that required extensive machining to put together. I have an old Unimat but my machinist skills are very rudimentary, and it was way beyond my capability. It's still in the box!

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#8
In reply to #3

Re: Can a 2-cycle gas engine be converted to a Stirling Engine

12/26/2012 1:38 PM

I can easily see teflon replacing the bicycle tire pump seals for use up to near 500 C. To go higher, one would have to revert to metal piston with rings, I suspect, and some method of oiling.

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#5

Re: Can a 2-cycle gas engine be converted to a Stirling Engine

12/23/2012 6:22 PM

If you are looking for a direct heat to electricity conversion process then skip the mechanical conversion part all together and go with a solid state thermo electric generator system based on Peltier or Seebeck junction systems.

Cheap, mass produced, easy to work with, easily scaleable ,and durable.

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#9
In reply to #5

Re: Can a 2-cycle gas engine be converted to a Stirling Engine

12/26/2012 1:46 PM

Thanks for the kind answer, along with all you other gents, all with helpful suggestions. I did find early on in all my web searches one example (picture only) where one fellow had attempted to take a model airplane (2-stroke) engine and use it for the power piston of a Stirling (or was it the displacer...a bit hard to tell). He used a rectangular block to transfer gas from the one cylinder to the other. Bad part was there was no proof the thing actually ran. For some reason, this entered my mind as a challenge...along with watching too much "Bug-out Preppers" on NatGeo... wanting to bug-out by not bugging out at all, just living stealth mode when the time comes.

I think this is all just a bit humorous when between the wife and I, we have not even one .22 round to use for defense. But we do have some nasty walking staffs, seven cans of spam, dry beans, rice, etc. enough to make it from Good Friday to Easter.

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#11

Re: Can a 2-cycle gas engine be converted to a Stirling Engine

01/07/2013 4:27 PM

check tiny tech.com gj

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