Designing a better Gamma chamber

[VincentG]

After much design and testing, it has become apparent that a flat pancake style chamber has many short comings. (Duh, I know)

-The flat plates are prone to flexing during peak pressure
-The flexing also causes sealing issues
-Unheated/cooled surface area is inherent to the design
-Heater/cooler surface area to volume is ok but not great
-Although seemingly simple, the optimal design and fabrication of this style chamber is too complex
-A valvular style displacer can be fashioned but it is far from ideal


When considering a new style of chamber, ease of construction and availability of ready-made materials is paramount. Other considerations are as follows.

-Chamber rigidity
-Maximizing heated and cooled surface area
-Utilizing the displacer itself as effective heating and cooling surface area
-Achieving a valvular type of displacer with no special fabrication or machining
-Minimizing dead volume, while maintaining the possibility of a hot reservoir space(thanks Matt Brown)
-Ease of external application of heating/cooling
-Using the displacer as a control valve to connect to cold side during compression and the hot side during expansion
-Possibility of adding a high efficiency regenerator

That's all for now. I'll update with some drawings and soon document the build of a 50cc chamber of this type. I have hopes of this 50cc chamber operating a 100-200cc power piston.

[matt brown]

When considering a new style of chamber, ease of construction and availability of ready-made materials is paramount.

Why not go 'green' (material, money and method). Besides, if you can't explain it to a 10 year old, it probably won't work.

Other considerations are as follows.

-Chamber rigidity
-Maximizing heated and cooled surface area
-Utilizing the displacer itself as effective heating and cooling surface area
-Achieving a valvular type of displacer with no special fabrication or machining
-Minimizing dead volume, while maintaining the possibility of a hot reservoir space(thanks Matt Brown)
-Ease of external application of heating/cooling
-Using the displacer as a control valve to connect to cold side during compression and the hot side during expansion
-Possibility of adding a high efficiency regenerator

that's quite a wish list, like Christmas in July

That's all for now. I'll update with some drawings and soon document the build of a 50cc chamber of this type. I have hopes of this 50cc chamber operating a 100-200cc power piston.

I think 100cc chamber is a better minimal (50cc is really small) but this also depends on chamber geometry.

[VincentG]

I'll bite Matt, how do I go green?

[matt brown]

I only meant using recycled green parts + saving green money = green method...

[VincentG]

Ha should have figured.

You're right about displacement. I'll aim for a 100cc chamber.

[VincentG]

This engine from 1904 is inspiring. A modern version of this with a valvular type displacer would be impressive and practical.
https://www.stirlingengineforum.com/viewtopic.php?t=732

[matt brown]

This engine from 1904 is inspiring. A modern version of this with a valvular type displacer would be impressive and practical.
https://www.stirlingengineforum.com/viewtopic.php?t=732

Xlnt link and the bugger I couldn't remember when someone asked about transfer port midway displacer a while back. The calcs in post are correct with engine akin 2.5w/cc @ 1000 rpm or 2500w/liter @ 1000 rpm if you could scale up original 4x and increase rpm 6x. Hmmm, Andy Ross had a hard time chasing 1w/cc, so pitching 2.5w/cc is a hard sell.

BTW, in that short thread, don't miss the last comment by Ian regarding torque. Also, in that thread, there's a link to this old Ted talk by Bill Gross

https://www.youtube.com/watch?v=TSMzKg6fwJ8

where Bill describes his Sunflower SE. He covers a lot of stuff rapid fire, but don't miss his "genetic algorythm" buzz that first appears ~4:25 and worth a few listens (I often use a similar scaled down approach).

Most guys that aim to make a hot air engine simply look at what's been done and say I can do that or I can do better. The flaw with this approach is that it's based on looks. A better approach is a first principle/s approach that doesn't focus on mechanical design, but on heat...only 4 common processes can supply heat externally and each has unique issues.

[VincentG]

"Genetic algorithm" translation- throw shit at the wall and see what sticks.

[VincentG]

After some digging it looks like the Bill Gross engine pumped out 250 watts of electrical power at peak. So maybe 300-350w shaft power.

[VincentG]

The first part so far is the displacer spacer ring, cast from JB Weld into a 3d printed form. The epoxy is poured into the form and set on a granite surface plate with a flat plate placed on top during the curing process. This method has produced extremely flat parts that make finish machining a breeze. Plastic wrap provides an easy release.

This ring will allow either end of the displacer to maintain as close to Tmax and Tmin as possible. The metal ends of the displacer will be held against the spacer with a small, centrally located stainless steel threaded rod. No welding or brazing will be necessary for the otherwise stainless steel displacer. The chamber itself should also not require any welding or special treatment, more to come on that later. These epoxy parts are extremely strong when used in compression.

Gamma v1.00 displacer casting 1.jpg (153.63 KiB) Viewed 10472 times

Gamma v1.00 displacer spacer 2.jpg (112.58 KiB) Viewed 10472 times

Gamma v1.00 displacer spacer raw casting.jpg (117.89 KiB) Viewed 10472 times

[matt brown]

I'm still trying to get my head around the values in this cold vs hot PP study (Wilcox thread)

http://www.stirlingengineforum.com/view ... 908#p23908

In this graphic, the hot PP model appears a nobrainer, but this is an illusion due to equal DP vs PP volumes. When hot PP model has dissimilar volumes,things get almost as squirrely as cold PP model.

I found a little equation to track molar ratio between volumes when cold PP, but inconclusive. However, staring at these values, something emerges...the pressure swing during compression (what might be considered the compression ratio) is independent of the thermal ratio (compare figs. 4 between A-B-C for each model) but that the pressure swing during expansion is dependent on the thermal ratio.

Ideally, the pressure swing during expansion should be as low as possible with Pmax constant (high MEP) and the pressure swing during compression should be as high as possible (low MEP). I'm gaming MEP and torque, but if thermal and volume values can coincide with high MEP during expansion, then torque 'location' doesn't matter.

[VincentG]

Matt, humor me and run a graphic of "hot pp essex" with 400cc pp and 200cc dp 300-600k. But adjust starting charge pressure according to planet Bob.

So starting charge pressure at 200cc @300k would be roughly 19.5psi.

And or run a 400cc pp and 400cc dp 300-900k cycle where in this case starting charge pressure is only 11psi at 400cc and 300k.

[matt brown]

planet Bob Essex.png (17.31 KiB) Viewed 10363 times

Hmmm, I was already gaming Essex DP/PP = 1, 2 and 1/2 (go figure LOL)

Note callout on pressure drop when DP moves (larger P drop when larger DP/PP ratio). Both graphics show "earth" values with same charge pressure such that B has 2x the gas mass. This allows better insight into how the volume and thermal ratios effect the pressure values. Meanwhile, planet Bob is simply P values 'transposed' thru each sequence.

[VincentG]

Note callout on pressure drop when DP moves (larger P drop when larger DP/PP ratio). Both graphics show "earth" values with same charge pressure such that B has 2x the gas mass. This allows better insight into how the volume and thermal ratios effect the pressure values. Meanwhile, planet Bob is simply P values 'transposed' thru each sequence.

Well hot dang that is a thing of beauty and I'd bet Bob would be grinning ear to ear when an engine like that fires up.


Back on earth I'd be happy with 50% of those numbers.

[VincentG]

The displacer itself and the chamber will be made from stainless steel nesting cups. They are cheap, sturdy and very well made. The conical shape not only eliminates the need for precision fitment but also allows the displacer to pick up and lose temperature as it contacts the chamber walls, all while bringing dead volume to near zero and providing a valvular action. No welding or brazing was needed to assemble the displacer.

First round of testing the displacer itself is complete. Here is the displacer with a mock assembly for testing. Not shown is a barrier inside the epoxy ring to stop convection from hot end to cold end. The final assembly will be sealed air tight with silicone but I didn't bother for initial testing. The whole thing is held in compression with 4-40 ss threaded rod(very small) to minimize heat transfer. Results are excellent. The next round of testing will be with higher temperatures. This unit should be easily capable of operating on an open flame, or the hot end can be directly inserted into a wood stove chimney.

For the thermal imaging I placed a thin strip of black tape along the displacer to accurately measure temperature.

Gamma v1.00 displacer 1.jpg (163.18 KiB) Viewed 10117 times

The displacer was cooled to below ambient with cold water and then the hot end was placed inside a vacuum cup with boiled water. After 5 minutes in still air, the cold end had risen to about 5F over ambient.

Gamma v1.00 initial displacer test cold end.jpg (373.39 KiB) Viewed 10117 times

The displacer was lifted out of the water to show the Tmax of the hot side, which was actually closer to 190F but this picture shows a high temperature of 168F.

Gamma v1.00 initial displacer test hot end.jpg (357.09 KiB) Viewed 10117 times