Created new blog just for Open Stirling Engines

Having my personal stuff mixed in with the Stirling engine project was awkward, so I created a new blog just for the engines. It is https://openstirlingengine.blogspot.com/ and I will start to post my prototype status reports there. Please make comments on that site, rather the this one. Also please visit the official Open Stirling Engine project site

Thanks

4 cylinder Alpha Stirling engine almost complete

After working all weekend to build another half of the engine. It is almost complete. Unfortunately the first 2 cylinders have lost pressure, so I will have to track that down before I begin pressure testing. The 2 new ones seem to hold air well Got to go to my real job now.

Diagam of sealed piston with pictures

Here is a diagram of the sealed piston that I am planning to use for the Stirling engine. I have exaggerated the gap to make drawing the bag easier. The kink in it on top of the piston is suppose to be the heat sealed end of the bag. A metal cap will be fitted over the cavity of the piston to hold it in place with a screw.


























The area will be pressured with a working gas, most likely methane, so the bag will be tightly pressed against the cylinder and piston, so in those areas it will not feel the pressure. Only where the bag makes the U turn, will it have to support the pressure of the gas in the cylinder. With a small gap, the force on membrane will be on the order of 1/10th the tensile strength of the material.

Here are some pictures of my tests on the sealed piston.

Piston at bottom of cylinder. The plastic bag is tucked into the top of the piston, but not completely. Then it goes down 1/3 of the length and goes back up to the top of the cylinder.
It is under ~10 psi pressure.


As the piston is pushed into the cylinder, the bag's U-turn moves further down the piston.

Piston at top of cylinder. Note: bag should not extend past the end of the piston.

Stirling Engine Project Status - gettng bags

I finally got a response from the KNF and they are processing the sample request.

I downloaded Sketchup from Google and begun making a 3D CAD model of the design. I should have model done this weekend. For now my priority is to start fabricating a second cylinder and piston assembly, so I will be ready when I get the bags for the seals. Also assemble the cam case. I am using acrylic for the heads for now, so I can see the top of piston. If the seals work, then I will switch to aluminum and tap them for 1/4" pressure hose connections.

I work on the machining in the evening because it makes noise and switch to assembly and software at night. I will try to take some pictures this time as I make the second set. The 3D modeling really helped, by showing where to put the head bolts, so they don't get in the way when assembling the cam case. There is no crank in this design, only a cam. This makes it possible to have up to four cylinders, two hot and two cold in a single radial plane. The shaft is straight through, The plan would be stack four packs to increase the total power of the engine. The current heads are just flat pieces that can be shared between adjacent layers of the stack.

The goal here is to keep the tops of the heads as flat as possible to keep the heat exchanger components simple to apply.

Initial testing will be on the minimum two cylinder configuration using low pressure air as the working gas. Once I up the pressure, we will need the opposing cylinders to balance the forces. The piston is > 3 sq in, so 100 psi is over 300 lb force on the cam. My main problem is providing support for the cam follower, so it doesn't jam or bend on the curved portions of its travels. I think I have that problem solved, just more stuff to build.

I am not have mentioned that assembling the seal is quite difficult. The bag material has to be stretched from under 2" in diameter over the outside of the cylinder which is 2 1/4 diameter. I will have to do some heat forming of the bag for production, so it all goes together much easier.