Prototype status update - plugging the leaks

It has been a while since I have updated my progress.

LEAKS, call the plumbers!
I feel like president Nixon.
My main problem has been the leaks. I setup a pressure test gauge to check out each cylinder section. It seems the bags are getting damaged, but not where you would expect it. The leaks mainly occur near the top of the piston. The problem is that the lateral forces on the push rods is causing the piston to rub on sides of the cylinder bore.

I am hoping that adding the push rod cam follower guides is going to relieve much of this sidewards force. This should also reduce the friction at the bearing where the push rod enters the cam case.

Still looking for a few good bags
I bought a thousand polyethylene bags and am going through then. It may be a blessing that they are so soft, since I can identify potential ware problems early.

I will have to start sending out more requests for samples. These plastic people are not very professional. I had one guy say he was sending me some nylon bag samples and I haven't heard from him again. The company was KNF. I have also left messages with local suppliers but
I still haven't heard back from any of either.

Plans
I was trying to get the engine running without the cam follower guides, but now that I know it is going to cause more bag wear, I am going to take the whole thing apart and rework it.

To do list:

1. Make cam follower guides
2. Adjust each piston to make them more uniform in height and maybe diameter*
3. Adjust length of pushrods to make them more uniform
4. Drill hole in each push rod for follower pin and glue in place
5. Drill hole in shaft for cam set screw to stop the slippage
6. Make motor simple mount
7. mount heat exchanger water tubes to heads

* There are different diameter piston shells, originally I was shooting for 0.015" gap between cylinder bore and piston, the first two pistons have ~ 1.97" diameter.
Later when I realized that schedule 40 pipe has an inside diameter of 2.067, I have been shooting for a piston diameter of 1.930". Since I have a 2" ID clear acrylic cylinder, I have to turn down 2" OD tube to make the equivalent gap.
Since I don't have a lathe, I use a Dremel tool to "turn" the piston shells. I will post a picture of the setup. The results are quite rough, with gouges and variations of diameter.



Early technique of turning the piston shell using die grinder. The plastic tends to melt and create a clump of plastic. So I could only turn 1/3 before it gets in the way.

{Picture to be taken}

Newer approach, I use a piece of cylinder material and use it as guide. It is slightly less stable, but with the extra space between the holder and piston, I can turn continuously. It does bind every once and while, causing gouges about 4 mils deep. So if we were trying to make a perfectly smooth and piston, this is not the way to do it. The beauty of the bag seal system is that the piston and cylinder surfaces do not have to prefect.

Alpha Stirling engine protoype status

I was using my personal blog to post the status of this project, but now we have a site just for these Stirling engine projects. Also please go to the new project web page at http://sites.google.com/site/openstirlingengine for more detailed description of the project and a 3D Sketchup model. When the plans are finalized, I will also post a spreadsheet with the full parts list and some formulas for scaling the design for more cylinders.
The web site doesn't seem to support comments, so that is why I made this blog.

Please feel free to add comments and info on any other designs or engines the you know about. This site is not just for my design, but all open source designs and even to discuss commer

This one is an alpha configuration engine that is designed to be low cost and use mostly off the shelf materials. One goal of going open source is to create standard parts, which as the design is improved, replaced with better performing ones.

The pistons are 2" diameter in a 4" long cylinder. There is a small gap between the piston and the cylinder where a flexible seal is fitted. For now it is simply a plastic bag the fits over the piston, folds down and then back up to the top of the cylinder where it held in place by the head.

So far the only bags I can find are polyethylene, so the bags are not very tough and develop leaks, mainly where the bag exits the piston cap, which holds it to the top of the piston. The bags are just plain packaging grade, 3" wide by 6" long. It turns out that they are slightly wider so do not fit the piston as snugly as it should, so it gets creases. The bag is 4 mil thick and the gap between the piston and cylinder is 35 mils. This is because schedule 40 pipe has an ID of 2.069" and the piston can be exactly 2" diameter. Since I want clear cylinders, I am using 2" ID acrylic and have to "turn" the piston down from 2" to make the equivalent gap.

The push rods are just going through a 1/2 hole in the acrylic engine case and this is causing binding, because of the lateral forces from riding on the cam. I am going to add nylon sleeve bearing this weekend. Hopefully that reduce the friction enough so the current flywheel can provide enough inertia to keep the engine spinning.

Here is a picture of the 4 cylinder prototype without the flywheel, so you can see the cam and push rods.

Created new blog just for Stirling engine projects

I have been using my personal blog for posting the status of my Stirling engine project, but it was getting awkward. First I checked to see if stirlingengine was free, but it is being used for another project. So I used the same name as the project web site on Google sites, http://sites.google.com/site/openstirlingengine.

I am not sure how to move my existing posts and comments from my personal blog
use the preceding link to get to it so see my previous posts, until I start making more progress here.

Please visit the project web site if you want any of the design information, because that will be where I will be posting the project documents. So far all I really have is the SketchUp 3D model, which does not include the piston seal details. It was too curvy for me to figure out how to include it. All the other parts are there. There is a very rough sketch of the piston seal on my old blog, but I will post some pictures and more details soon.

Please leave any comments about the web site here, since Google project site doesn't seem to have that feature.