Stirlingengineforum.com

hi
i want to ask a couple of questions about gamma engines.
1) the clearance between displacer piston and cylinder at end of stroke should be as less as possible. right? this is to make sure the all enclosed gas move in hot and cold places. i read somewhere that people are using clearance down to 0.5mm. this will stretch my machining ability to the limits. what i am thinking is to make a threaded clevis that will be attached to displacer rod. by moving the clevis i would be able to adjust the clearance. will it work?
or i could somehow glue steel wool to both ends of displacer to reduce clearance.

2) is the end clearance between power piston and cylinder also important? i think if the clearance is bit more it will slightly increase the dead volume. i am thinking of inserting circular styrofoam "shims" inside the power cylinder to adjust the clearance. this way even if the piston bumps into them no harm will come. what do you think? will it work?
please let me know what you think about this.
thanks
rustybarrel

1- Just attempt to keep the gap as small as possible. Fact is, the displacer can make contact with either end with no ill effects, so long as it doesn't create friction/binding. I have used, and seen others use springs as the attachment between displacer shaft and con-rod, allowing the displacer to bottom out at both ends, which actually helps to produce a closer approximation of the "perfect stirling cycle." I would recommend against adding steel wool to the ends of the displacer, steel wool breaks down far too easily, and could end up fouling the mechanism. I've seen video of a gamma that used a ferrofluid power piston, and a steel wool displacer, and after two months of occasional running, the magnetic power piston was covered in steel wool fragments, causing it to begin to bind.

2- The power piston bumping against foam shims will cause you to loose power. It is much more critical to have smooth uninterrupted motion of the power piston than the displacer.

Keeping dead space to a minimum is important, but will only increase efficiency and performance a small amount compared to other details you should pay greater attention to. As an interesting experiment, try adding a fitting or two to allow a hose to be attached, increasing dead space. Then add/remove lengths of the hose to see how much various amounts of dead space truly effects your design.

I'd go for 1.5 - 2 mm end clearance, unless its a very small motor, then the .5 mm might be justified, I think thats what I used in my little one.
If your motor is a high temp one, forget about foam, it will be gone before it even gets warm enough to run. What would you glue steel wool to the hot end with? You don't want the displacer touching anywhere, it just causes resistance to movement, and makes a lot of noise. Ian S C