Novice displacer question Surface area vs volume [PIC]

[wirelessguy]

I have something of a rudimentary question. It's been some time since I looks at Stirling engine mechanics and thought I'dask the group rather than building things in trial/error.

What is important when it comes to displacers (other than the space gap between the cylinder wall)?




My photo here has three configurations.
Left is a solid (hollow) displacer
Middle is my reverse Popsicle. Heated side would be the bottom round end of the displacer
Right is an inversion of the middle where heated area would have more surface than the cool area on top.

... or is all of this wrong and some sort of spongy foam with tons of surface area more efficient?

Anyone here ever try these or know the answers off-hand?

Thanks
WG

[larsinist]

this is what i have learned from ian s c, the piston length should be 3 times as long as the diameter..
about those piston shapes, i only can say that i dont think they should be closed, cause then they can explode when they are getting warm.and you also need some space between the cylinder and the piston for the air to travel, i have read about a closed piston, and they drill a mini small hole in the top of them, so the air pressure can go out..

I am waiting for my new lathe/mill and then i also gonna build a macine with about the same piston as you use.

[Ian S C]

Don't drill a hole in the displacer, if you are worried about pressure, make the displacer rod out of tube, so that the inside is at atmospheric pressure, then all that can happen is the displacer could collapse, instead of exploding(that doesn't happen) because as the pressure increases in the displacer, it also increases in the displacer cylinder, the only time I'v had trouble is when I tried an aluminium displacer, when the temperature increased the aluminium collapsed.
Ian S C

[wirelessguy]

For those who are joining in, I'll repeat my original question and add one more.

Q#1: Displacer volume versus Surface area what's important?
It looks to me as if the displacer should be given volume to offset the power piston but that surface area could be used to good effect as well. In considering the LTD Gamma Stirling a finned displacer would allow for quicker cooling of air in the upper half of the large displacer cylinder.

Another crude Sketchup representation:


Here I just started to show some of the fins and didn't draw all of then in both directions. Nor did I pay any real attention to spacing.

Q#2: is there a good way to calculate the needed ratios for displacers and is that different for Gamma or Betas, or the same?

The LTD I have on my desk has roughly the following dimensions.
1. The length of the displacer chamber = 3 times is diameter.
2. The length of the heated chamber = 2/3 of the length of the displacer chamber (cylinder).
3. The length of the cooling chamber = 1/3 of the length of the displacer cylinder.
4. Swept volume of the displacer = 1 1/2 times the swept volume of the power piston.
5. Length of the displacer = 2/3 of the length of the displacer cylinder.
6. Stroke of the displacer = 1/3 of the length of the displacer cylinder.

Does anyone have any better suggestions or rules of thumb to work off of?