More volume on the hot side?

[AnonAustria]

Hi,
I am drawing some plans for an alpha-type stirling engine I want to machine. In the process I noticed that many designs have some 'unused volume' on the hot side (usually in form of a glass piece that sticks out). Is this necessary/which function does it serve?

[Ian S C]

The hot side has the bit that "sticks out" so it can be heated, the hot piston has an extension on it similar to a displacer in other motors, this is called a Heylandt Crown, this fits in the hot cap with a gap same as a displacer, it does keep the direct heat off the top of the piston. Here's a photo of my Ross Yoke ALPHA motor.
Ian S C
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[AnonAustria]

Thank you for the answer. I have one more question: will the engine also run without the extension? I am limited in the time I can take to machine the thing, so I am trying to make it as simple as possible.

[Ian S C]

Not really, you have to have some thing to heat, and having a short hot end would get the heated area too close to the cold side, you just need a hot cap similar to a GAMMA, or BETA motor. I'm no sure if there is an ideal length for the Heylandt Crown, but some where about 1 to 1.5 times the diameter will work, I think I used 1.5 with a gap of 1.5 mm, about 1/16". I think the part on that motor that took the longest was the crankcase, I welded it up from scrap steel that I had to machine all over to get rid of the rust pits, and to get it a bit thinner. Here it is with it's little power hacksaw, it does work, takes about 1/2 hr to cut a bit of 10 mm rebar(steel reinforcing rod)
Ian S C
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[AnonAustria]

The hot side on my design is quite far away from the hot side, so heat creep shouldn't really be a problem. So if heat really is the only reason, my question is answered.

[cbstirling2]

The hot side on my design is quite far away from the hot side, so heat creep shouldn't really be a problem. So if heat really is the only reason, my question is answered.

The crown increases the surface area to conduct heat.
In my opinion, it's a good idea to have a crown on both pistons. I'd made both hot and cold identical since both jobs is to conduct heat. (Either to or fro...) This way you get efficiency on cylinder construction.

thanks

[Ian S C]

The cold piston should have as little bulk as possible to enhance the rate of cooling, the crown is only effective on the hot cylinder.
Ian S C

[cbstirling2]

The cold piston should have as little bulk as possible to enhance the rate of cooling, the crown is only effective on the hot cylinder.
Ian S C

Oh I see, the additional thermo mass outweighs the conduction from surface area?

thanks