My stirling engine project

[jorgealdobr]

This is my latest attempt at designing a stirling engine.

I use the free software FreeCAD.

The engine is a square cylinder (heigth = diameter) with 220cc

Calculation was done using beale fomula.

I expect it to generate 80W of eletrical power (from 150W of mechanical power) when coupled with a generator.

This is a PDF print of the engine with all parts connected

Edit:
cant attach pdf files and the cad files .zip are too big :/

[Ian S C]

Hi, welcome to the forum, maybe you could start a gallery, and post some pictures, it sounds like an interesting motor. Ian S C

[Triangle.Stirling]

Hi jorgealdobr, welcome to the forum!

Maybe you can use a hosting service somewhere on the internet? It would be nice to see what sort of engine you have designed.
For photo's you can use upload-sites such as https://imageshack.com/ or other similar sites, it's free.

Looking forward to your photo's/ file's !

Krijn

[jorgealdobr]

Its an V-Alpha, with square cylinders and heylandt crowns on both pistons.

I calculated it using the beale formula :

0,15 = 412,5 / (0,5 * 25 * V)

0,15 = beale number
412,2 = power produced after efficiency correction (W_engine = W_burner * Nc).
0,5 = half mega pascal (or 5 ATM). the engine stopped pressure.
25 = frequency in hertz.

Where the Nc is the combined carnot limit + ineficiencies of heat transfer.

solving for V gives 220cc of volume.

I don't know if a 0,15 beale number is a realistic figure.

The pistons are supposed to be made out of aluminum with a graphite-brass ring around it, sealing and making contact with the aluminum block.

What i need to know is :

How to calculate maximum pressure allowed ?
How to calculate the strength needed on the piston rods ?
Is it OK to use heylandt crown on the cold side ? (Andy Ross models have the crown only on the hot side).

The cylinder coverings (the space around the crowns which doesn't touch the pistons) are made out of brass to increase thermal conductivity.

The cylinder is square : 220cc is the result of heigth * diameter (using the cylinder formula) where heigth = diameter.

Is it ok to have square expansion and compression spaces ? If not, whats the correct ratio ?

Phase difference between cylinders is 90º

The engine has a babington burner to burn liquid (non-volatile) fuels, mainly glycerin, due to price.

The idea is to have an open-source project of a small power generator

[Ian S C]

It is not necessary to have a Heylandt Crown on the cold side, its main use is to isolate the hot piston from the direct heat of the hot cap. The Crown should be made of stainless steel, thin as possible, to REDUCE heat conduction.
As far as the strength of con rods, my method is make it(usually aluminium), if it looks strong enough, weigh it, then take off as much metal as I dare, weighing it as I go (As I do when making pistons). I have not broken, or bent one yet, but I'm hoping, it will be an indication that there is more power than I thought. My biggest motor is not much over 10Watts out put. The important thing is the strength, with a low weight, but stiff enough not to flex under load.
You say aluminium for the cylinder, maybe not the best as far as friction goes, unless it is hard anodised, or as Andy Ross did with pistons, have them coated with Xylan, a Teflon resin paint that requires to be baked onto the metal. I would use cast iron for both the piston (no rings), and the cylinder, this could be a liner in an aluminium block. Aluminium generally is not a good bearing surface. Ian S C

[jorgealdobr]

The idea was that aluminum is cheap to work with (i can use a homebrew crucible o cast it, while steel is out of hope for home). Having the graphite-brass as a "ring" means that the bearing is aluminum vs graphite-brass, not aluminum vs aluminum.

Even if the aluminum is not all that hard, at least its harder than graphited brass.

and graphite-brass forms a kind of self-lubricating surface.

[Triangle.Stirling]

Aluminium in general is not the best choice for SE's.
Not only for the bearing surface, but is also has a relatively low melting point, what sort of burner are you going to use?
Also aluminium will expand a little bit with heat i believe? I don't know if aluminium pistons give problems?

correct me if i'm wrong!

Krijn

[jorgealdobr]

The part exposed to fire should be made out of brass, a specific alloy of brass that has 1200ºC melting point and has good thermal conductivity.

I dont know if the cylinder block will reach 600º to melt aluminum, but you are right, even if it doesnt melt, the heat is enough to modify its structure...

[Ian S C]

The hot cap should have low conduction lengthwise, hense the use of stainless steel, the heat only has to pass through the thickness of the metal, which is only a few thousandth of an inch. The stainless also, will not corrode like the brass will running at, or near red heat.
Any sort of steel, maybe a thin walled tube, pressed lightly into the aluminium of the cold end would be far better than aluminium.
I used aluminium for the displacer of one of my motors(ally aerosol can), it took three melt downs to persuade me to try something else, so I changed to thin walled stainless tube, with the hot end TIG welded on.
If nothing else, building these things is a good lesson in metallergy, thermodynamics, and just plain engineering. Ian S C

[jorgealdobr]

this is the machine, on dropbox.

V-Stirling mk2

[Ian S C]

sorry 10.59MB is too much for me to handle, either at home on dial up, or at the library on broadband. Ian S C

[Triangle.Stirling]

with wich programs can this be opened?

I tried Autodesk Inventor but that didn't work. Will try Solidworks in the near future.

Maybe you can try to save it in another format? I'd like to look at the model!

[jorgealdobr]

Its on the first post : FreeCAD

its a free cad

[Ian S C]

The photos in my gallery are loaded there direct from my hard drive, it gets you right to the rellivent photo with out having to go to another site. Ian S C

[jorgealdobr]

I am having problems with converting FreeCAD models to images, that why i dont have images of the engine.

The engine is yet purely hypothetical, i cant build until i get to the project class of the mechatronics course (and the teacher approves the project).