Question about Stirling Engine Design

Discussion on Stirling or "hot air" engines (all types)
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newbuilder12
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Joined: Tue Dec 01, 2015 3:20 pm

Question about Stirling Engine Design

Post by newbuilder12 » Tue Dec 01, 2015 3:31 pm

Hi,

I'm making a stirling engine for the first time and have a few questions. First off, I will be building one similar to the "dual syringe" designs such as this one:

However, I will instead use copper pipes for the cylinders and epoxy putty moulds for the pistons. Also for the tube that connects the two cyllinders, I will use a copper refrigerator "soft copper" tubing.

-Can both pistons displace the same amount of air? Obviously one will be the heated side and one won't. If not, is there a certain ratio for the amount of air displaced by the displacer piston (non-heated piston) that should be used to calculate the volume of this cylinder?

-Will the engine operate faster if I were to create a larger temperature difference in the non-heated cylinder? Such as some ice around the outside of that end of the cylinder? And if so, should the tube connecting the two cylinders be cooled also, or would this not help further?

Thanks so much for any help in advance.

I will post a picture or video when I finish building this if it turns out a success. May be a while since I will don't have some of the materials yet however. :cool:

Ian S C
Posts: 2221
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Question about Stirling Engine Design

Post by Ian S C » Wed Dec 02, 2015 2:24 am

newbuilder12, The engine you describe forms the basis o an ALPHA engine in which there are two power pistons, this would be difficult to get going as the piston on the hot side would get too hot for Epoxy.
I'll try and give you some ideas. (A) copper is not a good material, if you could get some brass, steel, or stainless steel with a polished bore that would make a good start for the power cylinder.(B) the displacer, the cylinder should at least be steel,(the preferred is stainless steel), the displacer it's self is a hollow steel, or stainless cylinder about 2 mm to 3 mm smaller in diameter than the bore of the displacer cylinder, It should be about 3 times its diameter long.
(C)Ratios, for a GAMMA, or BETA motor the displacer is of a larger volume, 1.5 : 1. Ian S C

Ian S C
Posts: 2221
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Question about Stirling Engine Design

Post by Ian S C » Wed Dec 02, 2015 2:38 am

cont. A simple way of designing a motor is to use the GAMMA type with the cylinders separate, and make it in a V form with the cylinders at 90*, that way you only need one crank pin for the two cylinders, and to get the ratio right you just increase the diameter of the displacer cylinder(the ratio only needs to be approximate).
Reasons for the choice of metals, the displacer cylinder; Copper XX, it conducts heat too fast, and the motor over heats at the cold end. The copper corrodes when heated, and it flakes off inside and out, and where the flame contacts it distorts. The displacer, same heating problems, but if you use a spirit burner you might just get away with aluminium if you find a suitable size container, if you are going to use a gas flame DON"T you will have a melt down(I'v done it 3 times on one of my engines, its got stainless now). Ian S C

Ian S C
Posts: 2221
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Question about Stirling Engine Design

Post by Ian S C » Wed Dec 02, 2015 2:44 am

cont. Important, you must have a minimum of friction, and leaks.
I don't know what sort of workshop you have, so this is a build I would suggest to some one with at least a lathe. If you go to the top of the index page you will find a thread by Boydhouse for a free plan for an engine that does not need a machine shop.
Happy building. Ian S C

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