Pressurizing a free piston stirling with a spring or rubber bands?

Discussion on Stirling or "hot air" engines (all types)
ciara8
Posts: 4
Joined: Sat May 13, 2017 11:02 am

Pressurizing a free piston stirling with a spring or rubber bands?

Postby ciara8 » Sun May 14, 2017 6:20 am

Hi, I'm looking at this type of simple stirling engine:

https://www.youtube.com/watch?v=1-XcDuyV2l4
https://www.youtube.com/watch?v=GrLH4AbmDuM
https://www.youtube.com/watch?v=Lyfo-N0HRWs
http://physicstoys.narod.ru/page/Freepi ... ston3.html

Where inertia/bouncing is what moves the displacer, which is also porous(but not necessarily) and serves as a regenerator...I ordered some glass syringes to make one, but they're quite hard to find and I'll wait a few weeks until they arrive from china.

In the meantime, I had the thought that by putting a tension spring/rubber bands between the tube and the piston, one can create pressurization of the engine, something that in the common type of stirlings requires a whole glass dome.

However, I then thought of two problems - when the engine is free, the mass of the moving cylinder creates both the Isentropic (reversible adiabatic) expansion of the gas and the Reversible isothermal compression that are in the stirling cycle.

When the engine is pressurized with powerful rubber bands to maybe triple the pressure, the intertia on expansion won't be enough to create the adiabatic expansion, that a flywheel or a free weight does. Would that cause a drop in efficiency (which makes the whole idea worthless) or even completely prevent the Carnot cycle from happening? I can't really solve that I've been thinking about it for an hour.

Also the rubber bands obey the hooke's law, so during elongation their force becomes stronger and stronger(not staying the same like a weight does). I can't really figure out how this fits in the whole idea and whether it's helping or hurting performance.

I saw that in stirling engines, doubling the pressure increases the work output by ~2-2.5 times for the same temperature difference(of course, increasing consumption as well, but in total, increasing efficiency). I see no reason why this https://www.youtube.com/watch?v=Lyfo-N0HRWs engine cannot be pressurized by increasing the weight of the top part a lot. But maybe I'm missing something.

Let's ignore mechanical issues, such as leaking for now.

derwood
Posts: 113
Joined: Sat Dec 11, 2010 10:15 pm

Re: Pressurizing a free piston stirling with a spring or rubber bands?

Postby derwood » Sun May 14, 2017 6:41 am

You could possibly pressurize the engine internally and use a spring to simulate external pressure.

ciara8
Posts: 4
Joined: Sat May 13, 2017 11:02 am

Re: Pressurizing a free piston stirling with a spring or rubber bands?

Postby ciara8 » Sun May 14, 2017 6:51 am

Yes, this is EXACTLY what I am thinking, guess that's a much better way to put it.. so is there any reason why this wouldn't work or it would create more inefficiencies than the increase in efficiency it produces?

I guess the equivalent for a flywheel engine would be adding a spring on the flywheel on the most compressed position and then pressurizing the whole thing so that the pressure force on the piston equals spring tension(and the engine is 100% hermetic). My thinking is the spring would take energy to expand, but then return the energy on contraction, so no work lost and we have a nicely pressurized stirling. But not sure if it would work in practice, I can't shake the feeling that I'm missing a very obvious reason why it wouldn't work at all.

derwood
Posts: 113
Joined: Sat Dec 11, 2010 10:15 pm

Re: Pressurizing a free piston stirling with a spring or rubber bands?

Postby derwood » Sun May 14, 2017 8:05 am

You would need to use a fairly long spring in order to keep a constant even tension. I considered this a couple of years ago but never tried it. I think it would work

Ian S C
Posts: 1954
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Pressurizing a free piston stirling with a spring or rubber bands?

Postby Ian S C » Mon May 15, 2017 4:29 am

I have an atmospheric free piston Stirling Engine, it is built as a parallel cylinder Gamma motor. The displacer is the same as in a Ringbom motor so is self acting. The power piston is magnetically sprung, ie., one magnet is attached to the bottom of the piston, and another magnet is attached to the base of the motor, like poles facing each other. I first started with a compression spring under the piston, then I had a pair of tension springs, but the magnets are by far the best I'v used, they are the magnets from a micro wave oven.
Ian S C

ciara8
Posts: 4
Joined: Sat May 13, 2017 11:02 am

Re: Pressurizing a free piston stirling with a spring or rubber bands?

Postby ciara8 » Mon May 15, 2017 11:47 am

Interesting, so you get a powerful compression just before the gas is about to be heated, but then it expands freely because magnet force will be almost nothing after the distance between the two magnets increases a bit. Maybe that's the secret to a really efficient stirling, or at least a high-torque one! Is it possible it be even better than constant pressurization?

Ian S C
Posts: 1954
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Pressurizing a free piston stirling with a spring or rubber bands?

Postby Ian S C » Wed May 17, 2017 2:02 am

Another method of springing, use in some commercial free piston engines is a gas spring, a small piston in a sealed cylinder.
At long last I'm able to post a photo of my machine, it's a high temperature motor using LPG as fuel. The linear alternator produces about 6volts at .5Watt.
Ian S C
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ciara8
Posts: 4
Joined: Sat May 13, 2017 11:02 am

Re: Pressurizing a free piston stirling with a spring or rubber bands?

Postby ciara8 » Wed May 17, 2017 12:02 pm

Nice, interesting. Since it's a high-temperature stirling, and I'm guessing you are putting in at least 100w of heat into it, shouldn't the linear alternator produce at least 10 watts of power? Is the low efficiency due to a sub-optimal linear generator? I'm guessing the generator is the two transformers and you're using a steel ring to short and open the magnetic flux or a steel bar in between two magnets in a repelling configuration?

Ian S C
Posts: 1954
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Pressurizing a free piston stirling with a spring or rubber bands?

Postby Ian S C » Thu May 18, 2017 5:40 am

The main problem I have with the alternator is the low frequency, 10Hz to 15Hz, I think a larger bore power piston, working on a shorter stroke would increase the frequency, I would like 50Hz +/3000strokes per min.
The two "transformers", are a pair of chokes out of a power supply with the Is removed and the Es still in place. To supply DC there is a bridge rectifier, and a 10,000uf electrolytic capacitor, this gives a smooth enough supply to run a radio.
Ian S C


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