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Rotary Stirling Concept

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Rotary Stirling Concept

Unread postAuthor: goose_man » Sun Feb 24, 2008 10:38 am

Hey, I was just looking at the "rand-cam" internal combustion engine. A discription of how it works can be found at http://www.regtech.com/Radmax_Technology/Rotary_Principle/.

Now I was thinking that this idea could be easily converted to be a true rotary stirling engine. Get rid of all the ports for fuel/air/exhaust and put a small hole in the center rotar between every two vanes (there are usaully 12). In that hole steel mesh could be used as a regenerator to increase efficiency (see: http://en.wikipedia.org/wiki/Stirling_engine#Stirling_cycle_thermodynamics under "The Stirling Cycle").

Applying a temperature differential to the top and bottom would create a stirling engine. Any criticisms, suggestions, etc? Thanks!

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Last edited by goose_man on Mon Feb 25, 2008 9:35 am, edited 1 time in total.
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Unread postAuthor: jrrdw » Sun Feb 24, 2008 11:31 am

Trying to solve the fuel crises are we? One question, how do you cool the prevesly heated section just to reheat it again? After many cycles, this would become difficult.
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Unread postAuthor: goose_man » Sun Feb 24, 2008 12:14 pm

In one type of simple stirling configuration, there are two pistons each in their own cylinder. The cylinders are connected by a tube/pipe. One of the cylinders is being heated, while the other is being cooled (with the simple addition of fins usually). In this configuration, all the heated "cylinders" are on one side of the engine (by which I mean the sinusidal plate is heated) and all the cooled (finned) "cylinders" are on the other. The hole connecting two regions is not in the vain, but in the central rotor. Each segment is the "cylinder" that connects to and compliments the one across from it, not next to it. So, in a configuration with 12 vanes, it would be like having 12 seperate cylinders.

Also, the regions between the sinusidal plates the oppose each other, would be 90 degrees apart (in their stroke) if they were connecting in a traditional piston fashion, just like a normal stirling engine.

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Last edited by goose_man on Mon Feb 25, 2008 9:35 am, edited 1 time in total.
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Unread postAuthor: Matheusilla » Sun Feb 24, 2008 3:31 pm

Wow, I think you've got your work cut out for you! I've made a few conventional stirlings and found them to be quite fussy to get to work. The biggest thing you're going to want to consider in your design is the amount of friction on all the moving parts because heat engines don't make to much power. Another thing is heat build-up. You'll want to seperate the hot and cold sides as much as possible. If you want to make a rotary engine that runs on heat differential may I suggest designing a rotary vacume engine or "flame eater" as well? I made one of those as well and it was a lot easier to get to work.

It's nice to see other people interested in this stuff :)
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Unread postAuthor: goose_man » Sun Feb 24, 2008 4:44 pm

Oh, I didn't mean that I could build this. Way out of my league. I was just wondering if people agreed that it could make a stirling engine. Seems like it would be a lot more efficient than other types.

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Last edited by goose_man on Mon Feb 25, 2008 9:35 am, edited 1 time in total.
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Unread postAuthor: clide » Sun Feb 24, 2008 8:02 pm

I don't know that it would work. You would be heating and cooling the gas in the cylinders as it rotated which would be increasing and decreasing the pressure. However since you you connect the top and the bottom parts the pressure wouldn't be able to rotate the engine.

To see what I mean imagine that you had the hole drilled through the center rotor and you pressurize one of the sections with compressed air. That compressed air won't be able to rotate the engine no matter what position it is in. It would be able to if you didn't have the hole, but once you have the hole the chamber becomes symmetric.

It might be able to work if you offset the phases of the sine wave patterns a bit, but then it becomes much more complicated because each side of the vanes needs to move independently of the other side.
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Unread postAuthor: goose_man » Mon Feb 25, 2008 7:35 am

I disagree. In one cylinder real stirlings (I'm refering to ones like this, though other types certainly exist: http://www.discoverthis.com/head-hand-stirling-engine.html) a "loose-fitting" piston is used so gas can get by. If you used a o-ringed piston, for example, the whole thing wouldn't work. The top and bottom of the cylinder in that example are seperated by the loose-fitting piston and their plates are at a different temperature.

Applying this concept to the rand-cam stirling idea, the "loose-fitting" piston would be the center rotor with a small hole in it. Too big of a hole would produce the faults your refering too, just like making the piston too "loose-fitting" in the example would do the same.

The second piston in that example (the one that is and needs to be o-ringed) that lies in a cylinder above the main cylinder would be unnecessary just how the internal combustion rand-cam doesn't need a piston.

Actually, since together, the two plates act as the piston in the rand-cam, a closer analogy would be heating the bottom of the outside of the cylinder. Heating the plate would be like heating the bottom of one of the cylinders in a stirling with two connected cylinders. That configuration also shows how a hole of the correct size is needed and will work, because the hole functions the same as the conecting tube in the two cylinder example.

Ignoring the fact that the rand-cam stirling is spinning (which we can do because the gas in any two opposing regions doesn't leave the pair) we can see that all we are doing is creating the same regions in the same proportions as the two cylinder example.

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Unread postAuthor: clide » Mon Feb 25, 2008 7:16 pm

The loose fitting piston is simply to move the gases back and forth from the hot and cold side to increase and decrease the pressure inside the engine, which as I said earlier this design would indeed do. However the real Stirling has a second smaller piston on the top which is where the actual power comes from. It does this by expanding the overall volume of the cylinder when the pressure is high and and decreasing the volume again when the pressure is low resulting in a net work production.

The volume of each of the cylinders in the rotary design would remain constant, so no work could be done.
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Unread postAuthor: goose_man » Tue Feb 26, 2008 6:42 pm

But won't the fact that the gases want to expand in that part of the cycle push the rotor around?

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Unread postAuthor: ammosmoke » Tue Feb 26, 2008 8:29 pm

It would be very difficult to get the timing right. You would have to be able to instantaneously turn each cylinder on and off. Say that it is a v6. (BC it is easier for me to illustrate.) If it looks like this..

{}--I--{}
{}--I--{}
{}--I--{}



{}=cylinder I=driveshaft ---=pushrod from the cylinders. If caused one whole side to go at once, it would be pushing the other ones not moving. It isn't very efficient that way. An engine goes in a series, so that it cranks as it is going down the line. But I am not sure how your idea of a stirling engine cylinders relate to the radmax engine, a type of rotary.
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Unread postAuthor: clide » Tue Feb 26, 2008 8:46 pm

Well if the holes are a decent size then the pressure is going to be even throughout the cylinder.
If you mean that they are so small that pressure wouldn't be able to equalize on each side as they are heated then I honestly can't say whether or not it would work, but I would still suspect it wouldn't. For one you would be trying to send the gas back through that hole after more heat has been transfered to it. In other words the cylinder has to expand and contract while it is on the same side of the engine, but you are trying to contract it right after the gas has its maximum amount of energy. A real Stirling contracts when the gas has a low temperature.

Another problem is that you still need to send gas from one cylinder to the other. When the hot side is trying to expand, the cold side would be contracting and that contracting would just increase the pressure on the cold side until the pressure was higher than the pressure on the hot side. So basically the pressure on each side would still end up being equal, if not because of flow through the hole then by the changing volume of each side.

The more I think about it the more I am thinking that it won't work. It would be a bit like trying to build that one cylinder Stirling that you linked to with only the large piston.

You might be able to make it work without the hole through the middle and alternating heat and cold in a checkerboard pattern around the outside of the cylinder, but that would be far from efficient.
Another way it could possibly work is having a small power cylinder for each cylinder, but once again that gets pretty complicated and probably wouldn't be any more efficient than a traditional one.
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Unread postAuthor: goose_man » Wed Feb 27, 2008 5:30 am

Hmmmm, maybe your right. I'm not really sure either way anymore.

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