LTD piston/displacer/crankshaft questions

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LTD piston/displacer/crankshaft questions

Postby bnjwood » Fri Feb 15, 2008 7:38 pm

Hello,
I'm attempting to construct a Stirling engine that will be powered by
hot water or ice; something similar to the MM-5. I understand the
displacer and piston crank pins on the crankshaft are to be 90 degrees
apart, and the displacer pin should be constructed to allow for the
displacer to travel the full distance of the cylinder but not make
contact with the base or top of the cylinder. What I'm not sure about
is if the piston pin should ensure the piston travels the same
distance as the displacer, or something less or possibly more.

Also, regarding the piston, would it be better to use a
piston/cylinder assy. (as with the MM-7) or a diaphram assy (as with
the MM-5)?

Would appreciate any help in these matters.

Thanks much,
Bud Wood

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Response to LTD piston/displacer/crankshaft questions

Postby sgraehl » Sun Feb 17, 2008 11:20 pm

It doen't matter if they have the same stoke only that the displacer moves it's full travel (without touching of course) and that the power piston reaches the bottom end of it's stroke to keep the dead air space low. You can vary it to "tune" your engine so making it adjustable can be useful. I try to keep my power piston stoke and bore diamete dimensions close to equal but I did make one with a small bore that needed a longer stoke to run right.
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Response to LTD piston/displacer/crankshaft questions

Postby bnjwood » Sat Feb 23, 2008 6:26 pm

Thanks Steve for your response,
For an LTD, what would be a good piston diameter to use? I've been trying to use one made of 15mm dia. brass tubing; the cylinder is very slightly larger in dia., also made of brass, and provides for little leakage if any. I've got a feeling my piston/cylinder may be too large; true?

Thanks for your help,
Bud
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Response to LTD piston/displacer/crankshaft questions

Postby sgraehl » Sun Feb 24, 2008 12:34 am

Bud,

I have yet to find any exact formula (one that I understand well any way)for an LTD regarding the size difference between the displacer area and power piston. If I was building with a 15mm power piston I would try for about a 15mm stroke and I would use a 6 inch (about 150mm) displacer diameter with maybe a 10mm thick displacer in a 20mm thick chamber. As always, friction will be the main challenge and getting it to run on your hand may still not happen but is possible. Good luck with your engine.
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Response to LTD piston/displacer/crankshaft questions

Postby longboy4 » Tue Apr 01, 2008 11:10 pm

You can be square up on the strokes or a few mm. difference, such as a 22mm displacer stroke and an 18mm power piston stroke, these motors don't seem to care in this range differential. I have 3.5 in. dispacer with 15mm power cyl. and 2.5in. with 10 mm p.cyl. I never used diaphrams but only made three LTD's all piston type. 15 MM test tubes and 1/2in. copper pipe make great power cylinders. On the copper tubes, I use a 1/2in dia. nylon Dremel brush on the drill dipped in metal polish cream and make a few passes in the bore. I use a 3/4 stick of graphite custom cut to fit. If I suspect I may be undersized on the piston by low performance of the motor, I drag a q-tip dipped in rubbing alcohol across the top of the cylinder. The alcohol flows down the inside and around the piston increasing its seal and if the rpm increases I know I could use another .0005 fatter piston. The alcohol evaporates tracelessly after the test. I have an LTD that had a fat piston and was hell to get it to run but finally recognised that was the problem and took off that extra .0005in. and it came to life. Hard to tell sometimes the friction points by fingering the CD flywheel over. If you buy a section of copper water tube, make sure that you cut out your cylinder from that piece that is not embosed/stamped on the outside with size or other manufacture info as that does slightly deform the inside diameter. The glass test tubes adds that all important eye candy as you can see the full travel of the piston in its bore. Good luck in your build Bud!
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Response to LTD piston/displacer/crankshaft questions

Postby pwithu » Wed Jun 24, 2009 8:29 am

I had some issues myself while designing my Gamma type Stirling engine.
The piston and the displacer axis were perpendicular to each other but I connected the piston rod and the displacer piston rod to a circular crank mounted on a shaft at a phase angle of 90 degrees but i have no idea how to make a crank pin or a gudgeon pin and i also am not sure whether to connect the piston rod directly to the crank shaft or to use some other mechanism ( i heard about some ross mechanism i am not sure )
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