Thursday, August 7, 2008
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.