Beta Stirling Project

[Ian S C]

As it's really a research project, as long as you explain the effect of using aluminium, even a melt down could be see as a success. Ideally you would build two motors, one with aluminium, and one with stainless steel, and compare the two.
In my gallery there is a 2.5 cc GAMMA motor, it has a brass cylinder, with a piston in the form of an aluminium core, with a Carbon impregnated Teflon sleeve. The cylinder is just a bit of commercial brass tube, with no polishing of the bore. Other motors I'v used Teflon cup seals, and leather ones also, but I find that the best of all pistons is a well fitting cast iron one with labyrinth grooves in the skirt. If you'r like me, and make the piston skirt as thin as you can, don't say too many naughty words when the piston falls in half when you make the groove a little too deep(I think I'v done it twice, although there's only one laying around the work bench).
Ian S C

[Akya2120]

At what point are the piston an displacer closest together? When the displacer is at BDC and the piston is at half of it's stroke? I am just trying to get the connecting rod lengths and everything designed so that I can have the motor practically done with the design phase, save the operational tolerances.

[Aviator168]

IIRC. The bottom of displacer and top of the power piston is closest when the displacer is half way down and the power piston is at TDC. In other words, the power piston lags the displacer by 90 degrees.

http://www.animatedengines.com/stirling.html

[stirlingengine123]

for the displacer piston you can simply take the displacer cylinder making sure its open on both sides lay the tube vertical on some kind of surface and take aluminum foil fill the tube with it and then simply take some kind of solid metal cylinder or some kind of thing to compress the aluminum foil and you will hammer the cylinder which compresses the aluminum foil in the tube. after your done with this you simply take the compressed aluminum foil out sometimes you have to push the compressed aluminum out of the cylinder with a thick rod and after this you have a nice compressed aluminum displacer simply sand the whole displacer to make it smaller in diameter than the cylinder

[Ian S C]

For a regenerator type displacer, it can be made of stainless steel wool, either use a tube similar to the conventional displacer, but with holes at each end, or just wrap the steel wool around the displacer rod, I tried that on one engine, little difference to it's original displacer except that bits of steel wool fell off, and as it was a vertical engine, hot end on top, the bits fell on the piston, and jammed the motor. Another way is to use stainless steel foil. Aluminium foil would tend to oxidise fairly quickly at the hot end, and also it would conduct heat to the cold end.
Ian S C

[Akya2120]

I was planning to use some 400x400 stainless steel mesh to contain my choice in regenerator material in the displacer.

stainless mesh
http://www.amazon.com/Stainless-Steel-M ... B00DRMFWBU

regenerator material
http://www.artistsupplysource.com/produ ... fgodKjQAJQ

I figure the stainless mesh could be either tack welded in place or just cut to the right size to fit in the bottom and top of the tube.

[Ian S C]

The mesh should be ok just cut to fit, although Andy Ross found that stainless foil with dimples on it worked best. To do the dimples you use a tailors tracing wheel, its a small star shaped wheel, with a handle, and you just roll it in lines across the foil.
Ian S C

[Akya2120]

When the displacer is 90° from top dead center is it 50% below the top of it's stroke? I am just wondering what positions to account for my clearances. I don't want to end up building this thing and having my connecting rods too long then have the piston and displacer slam into one another. From the link Aviator188 posted it would appear that the piston and displacer are closest when the crank is 45° past the displacer at TDC.

[Akya2120]

I figured out the piston/displacer position using trig. Now the next questions.

-When making a piston for a Stirling I am curious of how critical I need to be with the thermal expansion of dissimilar metals.

-Should the cylinder and piston be the same metal?

-Do you normally lubricate the piston?

-How hot does the cold side piston get? Someone was talking about using teflon for a power piston so I can't imagine that the cold end gets too hot.

-If the piston is going to be machined from cast iron, is there anything other than having a puck with labyrinth grooves that I should design into it? Other than how I will mate it to the connecting rod of course.

Thanks for the help folks!

[Akya2120]

Oh, almost forgot!

-What is a typical tolerance between the cylinder wall and the displacer?

-What is a typical tolerance between the cylinder and piston?

Thanks!

[Akya2120]

Ian,

How hot does the cold end of your engine typically get? I ask because I am going to use a water cooled cold end and I have the option to but a single O-ring for 12 dollars or fifteen for the same price, the difference is one can handle 450°F and the other can only handle 250°F. My current design calls for two of them and I'd much rather have some to spare for 12$ than have none to spare for $24.

My current design as of now should be viewable here https://drive.google.com/file/d/0B69X9s ... sp=sharing . Sorry for the multi-color stuff going on :P.

I still have plenty of work to do on the design but for now the majority of the parts to be machined are there. The cylinder is 40mm in Dia, the power piston will sweep 40mm, and the displacer sweeps 60mm with a length of 120mm. I included the picture of the labyrinth piston up close to show how I decided to re-orient the labyrinth grooves, if it is even possible to cut them that way. It should lead to a better seal within the cylinder. More of the air trying to get by will be diverted down the grooves, much like the theory behind Nikola Tesla's passive one way valve.

I also considered making the piston Dia the size of the cylinder all the way down and increasing the length similar to some of the free piston engines I have seen. I feel like that would reduce the tendency for the piston to be pressed at an angle. I have lots of ideas for this project, but for now I am trying to keep it minimal since I have about a month to get this produced. I designed the hot cap and the center section so that I could make modifications later. I just need to get the basic engine running as I am doing this as a class project. It is also not a bad idea to start basic and then make adjustments, it will give me a good benchmark for what is better or worse. I am a little concerned with getting the displacer sealed sufficiently and the concept that this will be lubrication free. I am considering making a press ring from teflon to seal the displacer in the piston that will capture a small amount of grease to keep them sliding smoothly and well sealed. The reason you see no means of affixing the piston to a connecting rod is because I am going to produce a bracket which mounts to that tab at the end of the piston.

Thanks for your time!

[Bumpkin]

Hi Akya. Sorry to bring this up a week late, and you probably already accounted for it, but I just remembered something regarding the question of finding the point where the displacer and piston are closest. Make sure you account for the rod deflection angle: for instance, unless the rods are infinitely long the piston and displacer aren't nearly half-way up their strokes when the crank pin comes up to 90 degrees. In other words, in a typical crank-and-rod arrangement a piston always travels faster in the top half of its stroke.

Bumpkin

[Akya2120]

Yes I considered it the dynamic shortening of the connecting rod. The thing is I only needed the calculation for the OAL of the cylinder, when the piston is at BDC I don't have to consider that contraction. I was just having trouble knowing exactly how long I wanted the cylinder and where the piston would stop.

Has anyone collected temperature data on the different components of a Stirling engine? I am really curious about the cold end cylinder and piston temperatures.