Cross section size.

[Aviator168]

Has anyone studied the effect of cross section size of hot-cold cylinder connecting tube to alpha stirling engine performance? Given the same length, the larger cross section reduces the energy required to push gas back and forth between the hot and cold cylinder; but it also increases dead volume which reduces power density. A smaller cross section area reduces dead volume; but requires much more energy to move the gas. Worse, the energy increase is exponential to the reduction of cross section area. Any good compromise?

[Ian S C]

I think my ALPHA motor would benefit from a bigger transfer tube, at the moment it's only 5 mm .
Ian S C

[Aviator168]

If the increase in cross section does not affect the heat exchangers, you should see rpm increase more than the increase in cross section area until the dead volume has an effect in performance. However, we can cheat by shortening the tube.

[Triangle.Stirling]

Interesting question. You would have to know some more parameters for setting up an equation. I think that is doable.

Would the influence of the dead air volume be big? If I made an engine, I wouldn't go big or small, more inbetween.

[Ian S C]

Within reason, dead space is not as important as transferring the air from hot to cold and back again efficiently. There was the results of an experiment posted in Model Engineer a good few years ago of a GAMMA motor run with a 6 foot rubber tube between the displacer cylinder, and the power cylinder, and very little reduction of power was noted, I don't know the bore of the tube, it was probably about 1/4", no more that 3/8".
Ian S C

[Aviator168]

I remember that Ian (I was here long time ago). As a matter fact, been thinking about that for some time after reading that report and could'nt figure it out until one day. Ah, it was a GAMA engine. Here is the catch. Try the same thing on an true (most of them we see are fake) alpha engine with a 6 ft tube separating the hot and the cold cylinder and see what happens.

[Ian S C]

It should work ok, I'm not sure how I could hook up my Ross Yoke ALPHA motor as the transfer tube is internal. It would need to be a V type GAMMA motor.
Ian S C

[Aviator168]

What I am saying is if you add dead volume between the hot and the cold exchanger, it will hurt performance. That's is how Andy Ross's speed control works.

[Andy]

There is a large number of variables concerning the transfer tube on a Alpha engine that will affect performance in many different ways.
On a Alpha engine the transfer tube is also the regenerator. If the tube is made of metal it will lose heat(energy) to the surrounding environment, thus losing torque. Using a rubber tube which is a insulator compared to metal, there is not so much heat loss which might cancel the effect of the extra dead volume.

The Gamma engine is not as effected by the dead volume within the transfer tube as heating and cooling takes place in the same cylinder, the displacer only shunts the working fluid(air) from hot to cold. The transfer tube only transfers this to the power piston.

The Alpha engine is very much affected by dead volume as it is dependent on moving as much air from one cylinder(hot) to the other cylinder(cold) in order to be really efficient.

Increasing the diameter will cause the engine to breath better and therefore there should be an increase in speed. All within reason though as there will be a point where the extra breathing volume will become dead volume.

Keep in mind though that the transfer tube is also your regenerator. The relation between diameter and volume is not 1:1. When you double the diameter of the transfer tube, the surface area(regeneration surface) will be 2 times more and the volume will be 4 times more. This will increase speed but not torque. For the same volume increase you can keep the small transfer tube and add 3 identical tubes, this way the surface area(regeneration surface) will be 4 times more and the volume will also be 4 times more. This will increase speed and torque.

[Aviator168]

Andy, your comments about the transfer tube only apply to text book alpha configuration. Most of the more practical alpha designs (including commerical ones) have the cooler on the skirt of the hot cylinder. The benefit is two parts. One, there is no transfer tube (i.e. dead space) between the heater <-> regenerator <-> cooler. Two, the seal ring of the hot piston can be in a much cooler region of the cylinder than otherwise would. This allows better sealing materials, which generally can't be in high temperature environments, to be used. So the dead volume of the transfer tube only affects the compression rate (which has a theoretical maximum value of 3) and in extension the effectiveness of the heater.

[Andy]

Thanks for the comments, can you maybe point me in the right direction concerning proper practical alpha designs. No point in me trying to go the stirling route if the basis of what i know is not correct.
Rather embarrassing.

[Aviator168]

Just look for one of "Andy Ross Stirling" video on youtube.