Power Producing Engine # General Public Comments

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info74
Posts: 70
Joined: Thu Aug 02, 2001 7:41 pm
First Name: Brent
Last Name: Van Arsdell

Power Producing Engine # General Public Comments

Post by info74 »

I have been asked by the university where my father attended medical
school to build them a low cost, low tech, reliable Stirling engine.
Naturally I had to say yes.


After much discussion we have decided to post much of the information
about this design on our web site as the design develops. The initial
web site is located here.


Everyone who is interested in Stirling engines (whether you are a part
of this team project or not) is welcome to start their own free
Stirling engine web site using our server. To do that you simply
register as a user, then click on Homepage Maintenance to get your
site going. The page cited above was built very quickly using
Netscape Composer and the site editing tools built into our server.
You need to upload a file called index.html for your home page.


This thread is for use by everyone wants to offer helpful comments
about this Stirling engine project. Team members can also post
comments to the Team Comments thread.
jbondy
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Joined: Thu Sep 13, 2001 10:44 am
First Name: jon
Last Name: bondy

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Post by jbondy »

1) We have used graphite gaskets for our engines, of various thicknesses. Check out ChemStar Mechanical Packing, 863-425-5581 and ask about the GMH material. They can sell you raw material if you wish.

There will be LOTS of waste material if you cut 18" gaskets: don't throw it away! Other people can use it to make gaskets for smaller engines.

We actually had our gaskets cut by Flow Components, 863-425-4976. They are a fab shop local to ChemStar.

Note that this material will arrive with edges distorted by the cutting
operation. You will want to find a way to smooth the faces, either by
pressing the distortions down manually, or by running the gaskets across
some emery paper. Also note that you may need a LOT of pressure to seal
these gaskets. Get the specs and figure out how tightly the "sandwich" will have to be squeezed. If you use a series of identical gaskets, at least they should all seal at the same pressure...


2) Ray had some real doubts about your ability to find a rotating seal that will work effectively at high pressures. He said that most of the seals he's seen will press harder and harder on the shaft as the pressure increases, and this will increase friction and decrease power. I don't know if this will be a real problem or not. If it is, would you consider using magnetic couplings? These are used all of the time, these days, with aquarium pumps. The motor is totally sealed, and the pump rotor is driven by a pair of shaped ceramic magnets.
info74
Posts: 70
Joined: Thu Aug 02, 2001 7:41 pm
First Name: Brent
Last Name: Van Arsdell

Response to Power Producing Engine # General Public Comments

Post by info74 »

Graphite is a great gasket material. Unfortunately it is also a fairly good thermal conductor. A rough measure of a materials thermal conductivity is how good an electrical conductor it is, and while graphite is a long way from copper wire, it is a decent conductor.

I'm looking for a good gasket material, that is a good insulator and will handle the high temperatures.

I like the idea of magnetic coupling, but it tends to work well only in fast spinning engines. Power is torque x rpm and this engine will probably be a 750 rpm engine so the torque might get too high. The magnets you need aren't cheap in this size either. Having said that it probably could work.

Sunpower built a ringbom engine and a rice husk buring engine that had ordinary rotating shaft seals. I don't think sealing around a rotating shaft will be a big problem at 150 psi ~(10 bar) using air as the working fluid.
gerry
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Joined: Wed May 01, 2002 11:10 am
First Name: Dr Alfred Gerald
Last Name: Davison

Response to Power Producing Engine # General Public Comments

Post by gerry »

What kind of temperatures are you looking at ?

Regards
Gerry
info74
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First Name: Brent
Last Name: Van Arsdell

Temperatures

Post by info74 »

We are going to try to achieve typical Stirling engine temperatures. That means 900 K on the hot side and 300 K on the cold side. That will mean that we have to find a great way to insulate the hot and cold ends. Right now I am looking at castable ceramics.

For the ceramic idea to work we need to figure out how to keep the stresses in the ceramic low enough. My initial thoughts are to have soft metal and graphite gaskets both above and below the ceramic section. Ceramic fiber fabrics will probably not work at the pressures we need.
gerry
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Response to Power Producing Engine # General Public Comments

Post by gerry »

The ceramics idea is good - despite the problems with cracking under shock/vibration. Also, the soft metal gaskets you envisage are good enough. The weak spots in this construction will probably lie around the zone where the piston changes direction since at that time it is virtually standing still and the pressure is at its highest above the upper piston surface. Perhaps the answer to the cost/vibration criteria is a sandwhich construction composed of alternate ceramic and steel cross-sectional shapes with the part of the cylinder above the piston when at TDC being ceramic. I have not seen this construction before so I am not at all sure, but what is the softening point of Teflon ?

Regards
Gerry
info74
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Last Name: Van Arsdell

Teflon

Post by info74 »

I'm not sure at what temperature Teflon starts to get soft? Were you suggesting that it be used as a gasket?
raines___6941
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Joined: Fri Nov 01, 2002 8:13 pm
First Name: Tom
Last Name: Raines

Response to Power Producing Engine # General Public Comments

Post by raines___6941 »

[CERAMIC} Have you thought about using ceramic from Coors Yes Coors has in Coors Beer. Coors makes a ceramic that can be used for an interCombust' engine and I think thay have also made some tiles for NASA, for space craft.

Tom Raines
gerry
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Last Name: Davison

Response to Power Producing Engine # General Public Comments

Post by gerry »

The internal combusion engine operates at a lower flame temperature than the temperatures envisaged here, but the ceramic tiles thing I don't know about not having had anything to do with space construction. Where would one find a specification for these tiles ?

Gerry
info74
Posts: 70
Joined: Thu Aug 02, 2001 7:41 pm
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Last Name: Van Arsdell

High temperature "Caulk" ?

Post by info74 »

We have a reader named Joff Rolland who is chasing down a high temperature "caulk" or putty type material. It might be useful somewhere in a design like this.
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