pointers on pressurization - alpha engine.

[skunkworks]

I have started to experiment with stirling engines and have built a alpha stirling engine. The hot and cold are the same - 1" stroke 2.3" diameter. The engine overcomes its own friction and runs at around 450 - 500F. I am wondering how would I go about (other than building one) calculating how much lower that temp would be when the system is pressurized. At the moment it is just air - but would like to also try helium.

Thanks!
sam

[skunkworks]

one more thing..

about all I have for a regenerator right now is a coil of copper wire between the hot and cold. How much more would a decent sized regenerator help - even though it adds dead space?

thanks again
sam

[Ian S C]

I would suggest that stainless steel wire would be an improvement over the copper, but stainless foil would be even better. If wire you could perhaps try using stainless at the hot end, and copper at the cold end. One problem with copper is that it corrodes, and breaks down with heat, and the bits break away and can get into the power cylinder, damaging the bore.
You mention 450 to 500 F, is that 450 to 500 revs per miniute/ RPM, I would expect double that, although 500 to 700 I would think going by my own motors might be the speed under load. I find that the working speed is about half the free running speed.
Sorry can't help much with the temperature drop, other than to say that a motor that with air ran at red heat, when the gas is changed for helium, no longer runs at red, indicating better heat absorbsion(and letting you put in more heat). Ian S C

[Ferraccio]

The first question is very interesting because ia for me highly unexpected, at least with my (modest) understanding.

What does it mean that pressurization may reduce the temperature?
Pressurization increases (with the same temperature, both) output power.
The use of pressurized helium (instead of air) further increases the power, but to get this, it needs a different size of the compartments of the transit of gas (less) than air, and of course a pressurized body....

The use of copper for the regenerator is not considered a good solution, it is true that copper has the greatest capacity for heat exchange in very low-mass, with a high exchange surface area (all importants!!!), but it oxidizes (the oxide leads to bad) and collapses (if wires are very thin, (as they should be) at the end are in contact with each other, producing heat bridges between wires, and restricting the movement of gas around each single wire, this decreases greatly the surface of heat exchange with gas and then the feature.
Usually is usede "wool" or very very thin plate coils of stainless steel, (I will test the use of constantan wires, or very thin foil coils, is a copper nickel alloy, normally used for electrical heaters).
The use of stainless is for the maintenance of a rigid foam structure that holds the large exchange surface, while having a lower heat transfer.
A high exchange surface withy very small mass in the regenerator is essential.

Different speech is the size of the regenerator: here you asked the question a million dollars.
The problem of designing a regenerator (in all directions: height, width, net section of clearance, wire diameters, storage gas flow, turbulence, pressure drops at various speeds in a variable flow alternating sinusoidal or pseudo, increase in dead volume and so on., etc..) has been called one of the most complex problems for top-level math, and does not even agree with the theoretical maximum of pure mathematics.
The fact that easily solved with a good experimental aging.
Be careful not to miss, with the regenerator, to instrumentally optimize heating and cooling, which with the regeneration are the three pillars on which the operation, frictions and sealings are the collaterals to be optimized.

Compliment for your engine, size and temperature, seems to finally be addressed to a practical use.

[Ferraccio]

For Ian:
1) hello Ian from wich you have the match temperature versus rpm?
2) May be that Shunk has a solar system, ....and 700°F are not easy for him.
Thanks
Sergio

[Ian S C]

With the motors I have with stainless steel displacer cylinders, I run them at a dull to medium red heat, and motors with mild steel cylinders I try to keep them below red.
On my larger Beta motor (48 mm bore) I used a large fresnel lens, and found that the performance improved over the gas that the motor normally runs on, but the point were the light was focused became etched, as if a wire attached to a battery had been scraped across the cylinder, some indication of the heat in the 1 mm spot. (the lens is made for placing in front of a TV screen to enlarge the picture). Ian S C

[skunkworks]

thanks for all the replys.

To clarify - I was saying 450 to 500 degrees Fahrenheit is the point at which my engine overcomes it own friction and runs by itself.

I am hoping to lower that to 300ish F. (because of the design limitations of the engine (ptfe)) I would like the upper limit to be 500F.

(probably crazy)

I was wondering what effect pressurization would have on its starting temp.

thanks again
sam

[Ian S C]

Sam, I suspect that although more of the gas is heated, there is a possibility that the actual temperature may not need to be quite so high. With my ALPHA motor the hot cap on the hot piston is fairly long(could'nt find what length it should be), its about 60 mm long I think, its blue just over half its length, the bottom 20 mm is uncoloured, and it's attached to the top of the piston with loctite, this is an unpressurised engine. The hot cap/Heylandt Crown, and the hot cylinder are made of thin stainless steel, both pistons are made of cast iron. Ian S C

[skunkworks]

I have been goofing around some more. added a snifter. Unpressurized though the temp is just too high.

So - I did what any good experimenter does - I added water to the chamber. The thing started running around 200 degrees.

My next model will be pressurized. 100-200psi. I will see if that will lower the running temp using helium or hydrogen. maybe even some water.

(happy it ran at all :)

sam

[skunkworks]

btw - this is running a 16 inch cast iron pully as a flywheel at around 120rpm max. (with this goofy setup) With water/air mixture and a max hot side temp at 300 deg. Cooling the cold side heat sink with water externally.

[skunkworks]

correction - the flywheel is running about 360rpm.

sam

[jimlarsen]

I don't know about pressurization, but I can tell you that some of my engines show about a 10% improvement in the starting temperature when I change the air out with helium. My "heat of the hand" engine will run with a 10 degree differential using air, and will run on a 9 degree differential with helium.

[skunkworks]

Interesting. Well - I have started building a pressurizable model so I hope to have some info soon. :)

BTW - the actual engine runs around 120rpm at the moment. The flywheel on the engine was too small (as big as I could fit into what materials I was using to build it) - 70 tooth cast iron gear. So I have about a 25 tooth gear against that which also has the 16 inch cast iron pully on it. so the pully runs 3 times as fast as the engine is. (360rpm)

Thanks
sam