ltd stirling

[ron parola]

OK, new here, have been in the OTHER external combustion world for years and have decided to see how the other half lives. To that end I'm thinking of making a larger version of a coffee cup motor I've got. Of course I've got a few questions, I was going to scale up my little motor to maybe a sauce pan size, maybe 9, 10 inches or so. The power piston sizes scale up and the displacer AREA would also scale but would the displacer stroke scale up or would it remain the same (assuming the thickness of the displacer is the same), since the temperature gradiant remains the same? The little (chinese) motor works well and doesn't bother with graphite anything (no real bearings anywhere for that matter) It seals the displacer rod with just brass and the piston is plastic in glass, has anyone tried grooves in the bushing and piston to act as seals?
and lastly has anyone measured the pressures in these little things or is it too low to really measure outside a lab? Thanks rp

[Alfista]

I would keep the ratio the same at about 1:60. Senft has quite a good book on the topic: "Low temperature differential Stirling engines" from Moriya Press. Have you considered an Airpot cylinder / piston ?

[Ian S C]

One of the chaps on the Model Engineer (UK) site has just built a LTD motor, and has a semiconductor pressure sensor on order, it will be interesting to see if it works, because even the pressure in a high temp atmospheric motor will barely get to double atmospheric pressure/ 30 psi.
Ian S C

[jimlarsen]

The key ratio to watch is not just the displacer stroke, but the volume of air displaced by each stroke. Compare that with the volume of the power piston stroke. Changing the length of the power piston stroke is usually an easy way to adjust the design.

[ron parola]

Thanks guys, and indeed I do have an airpot cylinder/piston on order rp