Tuesday, September 2, 2008

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.

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.

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