Expansion on the hot end

Discussion on Stirling or "hot air" engines (all types)
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AnonAustria
Posts: 5
Joined: Tue Feb 06, 2018 10:53 pm

Expansion on the hot end

Post by AnonAustria » Sun Feb 11, 2018 4:36 pm

Hello, it's me again!

I am now almost finished with designing my Alpha Stirling engine and after looking at the design for a bit, I noticed that even a little bit of expansion of the piston or the cylinder would completeyl disable the engine. The two parts are made of aluminium/brass on my design and there's no rubber seal ect. in between. Is the expansion a problem and if it is, how do I fix it. Also, will the engine even work without some kind of seal?

Alfista
Posts: 91
Joined: Thu Feb 18, 2016 9:14 pm

Expansion is an issue

Post by Alfista » Mon Feb 12, 2018 7:34 am

AnonAustria,
expansion is definitely a problem. For this reason, a cast iron piston with a cast iron cylinder is a popular combination. Failing that, an undersized aluminum piston with rings of some kind may be in order. If I were building it with an aluminum piston, I would make cast iron rings. I must say that brass is not the best choice for the cylinder as it has a relatively high coefficient of friction. Here is a link to a helpful table :

https://www.engineeringtoolbox.com/fric ... d_778.html

In this table, they do not show aluminum against brass but I can tell you from experience that it is relatively high.

Some of the lowest figures are for oak, ptfe, cast iron, magnesium, glass, etc.

Good sealing and low friction are the defining characteristics of a successful engine.

I've never tried making a piston from oak but I think that it would be worth a try. For now, in my next one, I'll stick with cast iron ;-)



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

Re: Expansion on the hot end

Post by Ian S C » Tue Feb 13, 2018 4:16 am

AnonAustria, On my Ross Yoke ALPHA motor I have cast iron pistons in cast iron cylinders. When first built both cylinders and pistons were the same size, having the same clearance, but the engine seized on the first run, so it was stripped and the hot piston polished down about .0005" to allow for expansion.
If you did use Oak Alfista, it could cause bad corrosion in a steel or cast iron cylinder, bronze might be OK.
Aluminium, and most of it's alloys are a NO NO for any sliding contact specially the piston and cylinder of a Stirling Engine which must have minimum friction.
Glass with a graphite piston seems to be as good as you can get as far as minimum friction goes, but if you stick to a cast iron piston in either a steel or cast iron cylinder it will be the next best thing.
Ian S C

AnonAustria
Posts: 5
Joined: Tue Feb 06, 2018 10:53 pm

Re: Expansion on the hot end

Post by AnonAustria » Tue Feb 13, 2018 6:55 am

Thanks for the answers, but I don't have any cast iron available. I could make the piston out of steel, but I only have aluminium and brass in the diameter I need for the cylinder.

Alfista
Posts: 91
Joined: Thu Feb 18, 2016 9:14 pm

Re: Expansion on the hot end

Post by Alfista » Tue Feb 13, 2018 1:16 pm

Brass would be better than aluminum.

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