piston weight

Discussion on Stirling or "hot air" engines (all types)
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Joined: Fri Feb 02, 2007 8:28 am

piston weight

Post by jhead » Wed Feb 07, 2007 1:00 pm

I am building an engine similar to this one
and I found a piston and cylinder that seal perfectly out of some old air tool. The piston is 2cm across and about 2cm high and solid steel (pretty heavy). I can drill out the center and cut the weight by 1/2 if necessary. Is this necessary?

Also the piston is a little bigger than the one in the plans, should I bump up the size of the chamber and displacer or cut the stroke of the piston or leave it alone? if so how much?
Be ye fishers of men, you do the catchin and He'll do the cleanin.

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Post by SScandizzo » Thu Feb 08, 2007 11:28 am

Hi Joe,

Two very interesting questions. As far a weight goes, you can theoretically counter balance the mass of the piston with a mass on the flywheel. The final product should have a free spinning flywheel that shows no preference in terms of the piston's position. (Without a counter balance, the piston would most likely favor the bottom dead center position). If you don't drill out the piston, it means you will have to add more weight onto the flywheel. So keep that in mind.

In terms of volumes, the diameter of the power cylinder will have the most impact. (You can always adjust the stroke length but the diameter is pretty much a fixed value from the start of construction). If we are talking about a modest difference, I'd say go ahead and try what you have. However, if the diameter is say twice the suggested size, then a redesign will be necessary. Increasing the size of the displacer cylinder is fine provided you do not create deadspace. Make sure your displacer travels the entire height of the displacer cylinder and you should be fine. Keep in mind, however, that this may alter your heat requirements since it's a bigger volume.

Let us know how your project progresses!


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Post by partos » Thu Feb 08, 2007 6:34 pm

I've built a couple of engines following this general design. I found that nesting brass tubing, found in many hardware stores, works well for the power piston and cylinder.

There is a video of a two cylinder engine of this design that I built at

http://vids.myspace.com/index.cfm?fusea ... 1836069903
Richard Partos

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