Scaling Stirling Engine?

[john_k]

Good day all,

I have been considering for some time building a Stirling engine but the difficulty has arisen what to use as the displacer piston. The plan suggests an aluminium cigar tube (not an option) so have been looking for a substitute. I have now found a suitable part, however it is rather larger - Ø27mm. than the plan calls for - Ø20mm. I have read that some designs of model engine do not re-scale well (or not at all). Would this apply to the Stirling engine or is it feasible to increase the size to accommodate the larger component? Thanks for your comments and advice.

cheers John K.

 

[Longboy]

Go ahead and build your engine. Stirlings have been modeled with various size cylinders and the operating principle doesn't care.

 

[Cogsy]

Not sure about eBay in your part of the world but where I am eBay is full of cheap cigar tubes if you're having trouble sourcing them. From ~$5 for a stainless one LINK , $10ish for 5 aluminium LINK or $15ish for 10 aluminium LINK. The last 2 are even advertised as stirling displacers. I've never heard of them being used for displacers but it does seem like a good/simple way to go.

 

[Hopper]

Stirlings seem to scale OK. Especially a relatively small change like you mention. It's best to try to keep the ratio between the swept volume of the power cylinder to the swept volume of they dispacer cylinder about the same. So if you increase the bore of the displacer cylinder by a few per cent, just add about the same percentage to the bore of the power cylinder. But it's not critical.

 

[minh-thanh]

the plan calls for - Ø20mm.

cheers John K.

Why don't you make the displacer piston with a diameter 20 mm ?

 

[john_k]

Good day and thanks for the responses. Why I don't make the piston? From past experience of machining aluminium components with very thin sections this usually end in tears and the plan calls for the displacer to be as light as possible.

all the best John K.

 

[deverett]

Stainless steel maybe a better option than aluminium for your displacer. It is quite easy to machine a block of the said stainless down to less than 10 thou wall thickness.
See the description of my Robinson engine:
https://www.homemodelenginemachinist.com/threads/robinson-patent-hot-air-engine.30235/ reply #6

Dave
The Emerald Isle

 

[WOB]

Another possibility for a displacer piston is an aerosol can. If you look around in a drugstore ( Chemist?), you can find small size aerosol cans containing various female personal care products. There is one about 1" (maybe a little more) dia. aluminum drawn very thin ( like a soda can). It makes a very light piston.

WOB

 

[marvin hedberg]

another option that i have used for a displacer is a grill cleaning block. they are a closed cell, super lightweight pumice stone material. it can be drilled/machined easily and held to the rod with nut and washer and high temp adhesive.

 

[minh-thanh]

One more option for you!
it's always fine with my small engines (power piston 8mm)

 

[john_k]

Wow !

Thanks guys, you've answered the question and a couple I didn't ask. Trying to build up my competency and experience, so I'm going to take the easy way out for this one, and use what's at hand. The information here is going to be a great help when I move on to the next challenge. Thanks again . . .

cheers John K.

 

[Rocket Man]

If you double the diameter of a piston it increases cross sectional area 3 times this gives the engine 3 times more power. You only need to keep internal volume of the engine the same ratio as before, do the math before and after the piston diameter change. If you keep stroke of both pistons the same only thing you need to change is increase power piston same percentage as the displacer increased. Example, if displacer diameter doubles power piston diameter needs to double.

I make super light weight displacers with aluminum foil. Silver solder a very thin metal flat washer in the tip end of your displacer rod. Then silver solder another flat washer to the rod with the correct spacing between the 2 for the correct length of the piston. Then wrap the 2 flat washers with a sheet of kitchen aluminum foil, fold both ends over the ends of each flat washer. This works very well for any size piston even 3" and larger.

I like to design a sterling engines around a ready made over the country product like aluminum soft drink can or aluminum beer can. Use a quart size beer cans for large engines. Very light weight pistons make the engines run very fast 1500 rpm.





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[bluejets]

If you double the diameter of a piston it increases cross sectional area 3 times

That would be 4 times if I remember my maths correctly. Pi * radius squared..??