Stirlingengineforum.com

Would appreciate some feedback on this idea. Dual displacers offset 180 degrees, both working on the same power piston, with all linkage and the flywheel sealed for zero air leakage. This mockup shows alot of dead-air-space around the flywheel and linkage, but most of that space will be filled in the final design. It also uses a diaphragm for the power piston, though an actual piston is probably a better idea.

How about a twin arrangement like a normal stirling but one end of the power piston working one displacer and the other end working the second displacer. A push-me-pull-you deal. Those AIr Pot actuators can be setup with an outlet on each end which could lead to the independent displacers. Timing might be more ticklish but the power piston acting in both directions might really get things hoppin'.

R

Thanks for your thoughts! I believe we are saying the same thing differently. As you can see in the animation, there is only one power piston (a diaphragm in this model), and it is being worked on by both displacers which have seperate air spaces. When one displacer is at the cold end, it's air-mass is being heated, so it's pushing the power piston. At the same time, the other displacer is at the hot end, so it's air-mass is being cooled, pulling on the piston. Instead of getting power during 1/4 of the cycle, you get it during 1/2 of the cycle, theoretically increasing performance by as much as 40-50% after extra mechanical losses are accounted for (with the simple see-saw displacer connection, there isn't much extra mechanical loss).

BTW- this design is for a high-power engine, so airpots will not be satisfactory, a true power piston (or diaphragm) will have to be used. I lean towards a diaphragm at this time due to ease of engineering and ease of maintenance. However, a piston would definitely be better in terms of performance (I'm pretty sure).

jerry, unless you plan to pressurise the motor to a high degree, air pots and graphite pistons will handlethe power, stirling engine pistons take the power in a graduated way, rather than the explosive way that an IC engine does. The air pots are used in door closers, and a door can put an extreem load on the mechanics of things. You say high power, how many watts/ hp. Ian S C

Thanks Ian, I didn't realize airpots were so robust.

I plan on building a test model with diplacer cylinders about the size of large soda cans. I'm sure an airpot will be fine for it.

If the small test model shows potential, then I will look into scaling up, including pressurization. I don't think I'm alone around here in my schemes to reach 3-5kw. But I'm also prudent enough to test before scaling up.

I've got to show my lack of engineering knowledge here, I don't know what to call the connection that will be required for the displacer swing arm. The rod will need to run between two rollers on the upper displacer connection point. I have no idea what to expect in terms of side-loading and friction. Will have to do some testing on a mock up.

I find this design really interesting.. hope to see your real-world prototype soon!

Thanks! I'm working on the bagel engine for the next week or two, not a lot of free time atm. Once it's running, I'll be attempting a prototype from either large soda cans or SS water bottles. Will post details once I get started.

I'm not sure this is what you're describing, but check out www.starspin.com/stirlings/ Look at Jim Dandy # 6. Bumpkin

Thanks for the link Bumpkin! That is similar in that it's a dual-acting system with a single power piston.

However, my design concept includes creating much less complex linkage in order to reduce friction, and enclosing all components within the closed system to prevent air leakage without the need for moving seals. Most dual-acting designs I've seen have to have 4 air tight seals, one for each displacer con-rod, and one for each side of the power piston. My design uses only two air seals between the two airspaces, the piston and the swing arm, and only the piston is a sliding seal, the swing arm seal is nothing more than a gromet.

Last nite I watched a video on the thermal migration in a stirling, and it leads me to believe I may be able to use UHMW for the roller bushings that the swing arm will move through at the ends of each displacer (the animation above doesn't show them, instead it shows the swing arm lengthening like a set of slide tubes). Anyone think I should stick with graphite for the roller bushings instead?

Nice job on the Cad. What are you using for the Cad software?

Thanks! I use Povray. Once you get a grip on CSG, it's easier than true CAD software (imho), and since it's based on ray tracing, you can get very realistic output (see my bagel design).

Has anyone seen or tried to build a dual piston stirling engine set up as a push me pull you kind of situation?

I have been working on a similar idea, but my first iteration will not be enclosed. I am making a twin cylinder arrangement with twin walking beam arms. One for the power and another for the displacer. This should be fine for a proof of concept, but I am ultimately leaning towards a boxer arrangement with the flywheel in the middle flanked by twin linked power pistons and then twin linked displacers. With cooling in the middle and heat at the ends. Your's looks easier to balance and only needs heat applied to one end. Mine lends itself to a single pressure vessel and possibly longer connecting rods for lower piston side loads.
I agree that a diaphragm is the wrong choice for a double acting piston. For piston with pressure on both sides, slow leakage would actually not be much of an issue and may be actually necessary to balance the two sides, so you could go looser on the piston fit for even lower friction. I am not a fan of elastic diaphragms in general, but a good rolling diaphragm is a good choice for single acting piston replacement with high differential pressure. That is what I am using.
My only real problem with your design is the working volume around the flywheel and the inequality versus the other cylinder. I know you said you'd minimize it, but you will have to do so in a way that matches the other cylinder's volume.