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I have a stirling hot air engine that I built about 15 years ago. It never would run. It tried to run but it did not have enough power to keep running. The engine was built from plans found in a stirling hot air engine book. It is a V2 engine with 1" crank and piston stroke. Power piston and displacer piston are both 1" diameter. The displacer piston is 2" long.

Last week I built a new displacer piston, double the diameter and double the length of the original displacer. The new 2" diameter displacer makes it run but it just barely has enough power to run.

This week I built a larger displacer it is now 4 1/4" diameter x 7" long. Now the engine runs fine. It appears to me that you can not have a displacer that is too large but you can have one that is too small. The problem with this engine is I built a common crank shaft with a 1" stroke. I made the rods 2" long to reduce piston side load. There is just too much crank case volume it is about 20 times more volume than a beta design engine. The large 4" displacer is probably 20 times larger volume than the original displacer. There is something to be learned here. The air volume inside the engine needs to be taken into consideration when selecting the displacer size. A law of physical says, pressure is proportational to volume. I built this engine with the idea of pressurizing the crankcase to 120 psi but I don't think I can pressurize it the displacer cylinder is not strong enough. I should have built the displacer cylinder from an old propane torch tank then it would be strong enough to hold 120 psi. Back to the drawing board.

Gary,
How about the power piston, what diameter did you use?

I think you are correct about ratio's and from what I have seen, the bigger the displacer, the lower the temp differential is. This being a Gama type discussion...I think the size, however, reaches a point that it will take too much power to move either it's mass (weight) or push the weight of the air (fluid) it's acting on. Somewhere in between is the perfect balance for both LTD type and regular, more powerfull versions. There seems to be a pretty large working ratio for a running model. I have no idea how this works out in a Alpha engine though.

Reread my post...I see you did state the size of the power piston. My bad!

That's interesting though about the relationship of the crankcase to the displacer. It does seem that compression ratio or whatever it's called in a Stirling would be lower with the larger displacer.

I've been thinking that if a sealed crosshead system is used in conjunction with the V-twin case you could get by with a smaller displacer and the sealed crossheads would trap and balance whatever pressure you want to run at. Sort of like a trapped piston in a blind hole.

I get carried away with ideas about adding a very small piston compressor to slowly "pump up" the cylinder pressures to a preset/adjustable relief valve limit.

How about two v-twins 180° out of phase. Pipe the crosshead of one to the crosshead of the 180° out companion cylinder. Would that be double acting?

Food for thought...or should I say; no sleep tonight.