Displacer cylinder length help

[jon james]

Hi all,

In determining the length of displacer and displacer cylinder in a high temp gamma engine, I have often seen that the hot end should be 2/3 the length of the cylinder and the cooling area 1/3 the length of cylinder. Also that the displacer length should be 2 times the diameter.

This seems somewhat arbitrary to me. It would seem to me that just as much, if not more area should be devoted to cooling as heating. Also, IMHO if the displacer is not used as a regenerator and is made of minimally heat conductive material, the displacer need only be as long as the stroke, in order to just displace an equal volume of air.

Does this make sense?

Your thoughts.

Thanks,

Jon

[Ian S C]

Jon, you'v got the figures a little wrong. The displacer should be about three times the diameter. One third is the hot end, and the cold end is the third at the other end, the center third is a trasitional area. There is I believe, a small amount of regeneration along theside of the displacer. The material that the displacer is made of should have a high resistance to heat conduction,ie., stainless steel (mild steel is OK), with the idea that the hot end shoul be hot, and the cold end a bit cooler. Apart from weight reasons, the metal in the displacer should be as thin a practical to resist the conduction of heat.
The three to one ratio is the one used by Robert Stirling in his engines, and is recomended by the likes of James G. Rizzo, and others. It does not apply to Low Temperature Differential LTD motors, or Robinson motors(these have a moving regenerator). The 3:1 ratio can be a wee bit longer, but I don't think there's too much advantage. Ian S C

[jon james]

Ian,

Thanks for the advise. I'll probably have more questions later.

Thanks again,

Jon