Square pistons/cylinders

Discussion on Stirling or "hot air" engines (all types)
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danalinscott
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Joined: Mon Nov 27, 2017 10:25 am

Square pistons/cylinders

Post by danalinscott » Wed Mar 27, 2019 2:44 pm

I am in the process of designing a 4 cylinder double acting radial stirling engine. To simplify fabrication I hope to use square structural steel tubing for the pistons and cylinders. The pistons will be sealed with teflon "overlays" on the four sides that contact the cylinder walls and the cylinder walls will be well polished where the PTFE and cylinder walls contact.

I know why square pistons are not practical in an IC engine but is there any reason that they would not work in a Stirling engine?

Ian S C
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Location: New Zealand

Re: Square pistons/cylinders

Post by Ian S C » Thu Mar 28, 2019 2:27 am

The cylinder of a Stirling Engine not having piston rings must be more accurate than an IC motor or steam engine, while it is possible to make square power units(you can't call a square a cylinder), there have been steam engines made that way. You don't really want steel on steel in rubbing contact. Are you going to get the piston coated with something like Xylon, this is Teflon that is baked on. You would be more likely to get a good fit with round tube that you could hone and lap to fit. The best metal for a piston is cast iron, and the cylinder, best CI, but steel quite good. For ready made cylinders the inside tube of an automotive shock absorber, get an old one at the car wreckers. Opening it is a bit messy, go careful and let the oil drain out.

Structural steel is rarely square in precision engineering terms.

Ian S C

danalinscott
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Joined: Mon Nov 27, 2017 10:25 am

Re: Square pistons/cylinders

Post by danalinscott » Thu Mar 28, 2019 1:18 pm

I agree that square steel tube is never actually "square" but machining it to true dimensions is pretty simple.
I have some experience with Stirling engines and prototype design and fabrication having owned a prototype fab shop for about a decade in my 30's.

Only the interior of the "cylinder" will have to be perfectly true and polished.
The sides of the piston will be fully sheathed and so the sheathing will be machined to true rather than the underlying piston.
There will be no steel to steel contact.

Teflon sheet can be machined to sheath the pistons lower section..with graphite sheathing the upper 1/3 for heat protection and sealing.

In the past 30 years I have seen a great deal of fabricated (welded) steel replace what had traditionally been cast iron components with very good results.

The engine contemplated is fairly large so salvage shock absorbers won't suffice.
8 double acting cylinders w 3.5" bores.
But I appreciate the suggestion.

What I am really asking is are there any reason that "square pistons" are not practical in a Stirling engine.

Thanks.

Ian S C
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Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Square pistons/cylinders

Post by Ian S C » Fri Mar 29, 2019 1:12 am

The main problem that I see is getting a good seal specially at the corners, even if left with a radius, round holes are always easiest( no corners). I think it sounds like an interesting design, a lot bigger than my biggest. Bore 2 1/4", power stroke 1 3/8".

Ian S C

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danalinscott
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Re: Square pistons/cylinders

Post by danalinscott » Fri Mar 29, 2019 5:56 am

I agree that trueing the radiused corners inside the "cylinders" is more difficult since they cannot simply be bored true.
Reciprocating rather than rotating tools must be used.

I am depending on the piston sheathing (graphite for the top third and Teflon for the bottom third) which can be easily machined true to provide an adequate seal. The graphite will be machined a few thousands larger than the bore and should "self lap" when mated with the cylinder prior to engine assembly. I plan to test a prototype piston/cylinder assembly for longevity by driving the assembly with the same reciprocating tool base used to polish the cylinder by monitoring pressure retention over a period of a few weeks using the initial prototype piston/cylinder assembly.

If the sealing capability of the piston sheathing degrades quickly the concept will have to be modified or scrapped,

Ian S C
Posts: 2198
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Square pistons/cylinders

Post by Ian S C » Sat Mar 30, 2019 1:35 am

There is always the possibility of using a block of graphite as the piston. Over all an interesting project. A similar motor I have read of was a rotary engine in which the cylinders and crankcase rotate(as in the rotary aircraft engines of WW1), I imagine your motor as horizontal with the crankshaft vertical.

Ian S C

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