Sketchup model of 4 Cylinder section of alpha config

I finally got something I can show. This the Stirling engine design I would like to do as an open source project. It is modified alpha configuration that uses a cam instead of crank to drive the shaft. The horizontal cylinders are the hot ones and the vertical are the cold. There are still some pieces missing, in particular the push rods need a guide, otherwise they bend and bind in the hole through the case cover. There is also corner gussets in the case to hold the guides and to screw the end pieces to.

This is a 4 cylinder section. These sections are designed to stack along the shaft. The heads would just be extended to cover all the cylinders for each quadrants. The cam case pieces can just be extend too.

I am still learning Google Sketchup and had trouble making all the holes go through the head. I didn't finish added the head bolts and only installed one, it isn't even tightened. The head is suppose to be counter sank the the flat head bolts. The diagonal pipes are intended to be regenerators, that is why they are so wide. I am just going to use pressure hose for now. I still have to get the 3" wide 1/4" aluminum extrusion for the head. It can also be used for the case, since it is 3" deep, if you don't stack them. The case is intended to be sealed. A fly wheel will also be needed.

I am hoping to have the prototype to run with the cylinders pressurized to around 100 psi. The pistons are 2" diameter so the force pushing down on the push rod is over 300 lbs. That is one reason to have a sealed case. It could have back pressure to reduce the forces on the cam.

Now I can finish making the clear plastic prototype. It is made using mostly 3/8" thick sheet material. I am working on a spread sheet that would calculate the sized of the component based on other thicknesses.

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.

Looking for a few good bags

So far I have only found polyethylene bags in my area. I called every plastic bag supplier in my area.

Here is a request I sent to a Nylon supplier.

I am working on a prototype Stirling engine, that requires sealed pistons. I am looking for thin plastic film material with high temperature strength, fold crack resistance and low cost.

For my initial testing the working temperature will only be 210 degrees F and am building it out of acrylic plastic. This way it will be easy to keep an eye on the seals and to demonstrate the principles. Your Kenylon products seem like a good fit.

My plan is publish the plans for the engine Open Source using standard sized parts, so others could contribute and to speed the development. I will be making sealed piston/cylinder assemblies available as product, since based on my current experience, getting the plastic material with the correct properties and dimensions has not been easy. Each engine would have at least 4 cylinders, I am expecting that there will be a significant failure rate of the seals, so they may have to be replaced after a short period of time, until we can find the proper balance between thickness, pressure and folding life.

I almost have the prototype engine complete, except for the seals.

Right now I am using polyethylene bags, but they don't maintain their shape very well and begin leaking.
I need a stronger material.

The width is somewhat critical, so the bag fits snuggly around the piston which is 1.95" in diameter. A 3" bag is a little snug but should work. I am considering going to 1.90" diameter, because it is standard size for an aluminum tube, which I will switch to for production. In production I am expecting to have to reshape the bags, to different diameters in each section, to make assembly easier. Once the cylinder is under pressure the bag will stretch to fill the gap. So for now it just takes me more time get it fitted over the larger cylinder OD to make the seal.

I need at least 4 pcs of 3" wide nylon tube bags that have seamless sides and a heat sealed bottom. The length is 6" minimum. If I could get then with different thicknesses that would be great, but 4mil is a good start. getting 2 and 6 mil sample bags would be great.