Stirlingengineforum.com

Some update from the prewious posting (see http://boydhouse.com/forums/viewtopic.p ... 8a0b7b1030 ).

Today I machined a new power piston out of polyacetal bar. The new piston Moves freely in the cylinder, but is so airtight that it feels like there's a spring under it when one presses it down. I purchased some gasket paper and cut appropriate shaped gaskets for the displacer cylinder. I also fixed some problems with the crank mechanism, and it seems to be alright now.

I then lit up the alcohol burner, and started to warm the hotcap. The engine started easily after half a minute of warming, and started to accelerate. It accelerated up to around 400 rpm, which is pretty much the max rpm I had anticipated. it could go faster if there was more heat available, but I quess the crank mechanism could't handle the increased loads at its present shape.

The engine then started to slow down for some reason. I sprayed some oil to the power cylinder, and the engine stopped. After a dissassebly I discovered that the oil actually created too much friction . Furthermore, there were some iron flings (probably from the hotcap insides) inside the power cylinder. I cleaned it up, grinded the hotcap slightly and assebled the engine. This time, with no oil, the engine ran very easily around 400 rpm. The slowing of rews was caused by the warming of the cold side (passive cooling). I really need to build a fan for this

I hooked up a 12 volt PC fan with a 9 volt battery to cool the cold side. The fan did it's job admirably regardless of the low power it had. The engine now ran for 15 minutes until it ran out of fuel. It could easily turn a fan to cool itself, now I only have to build one

I will also try to post in some pictures, but this forum doesn't accept direct postin. I will try to get some pics hosted somewhere...

Congratulations!! It is a great feeling to get something going after working hard on it!
As for posting the pictures, you might contact the owner of this website to post your pictures.
Thanks for sharing your info.

DonnieB

Some updates:

The engin now runs alright, but the alcohol burner came to and end of its way. There was some kind of leak issue about it's core, and lots of vaporised alcohol was allowed to escape the glass container. This eventually led to uncontrolled burning which had to be put down with water. The rapid temperature changes made the glass jar to crack so by this writing I have no heat source for my engine :(. What kind of heat sources do you use? candles, burners, etc...

My engines run on sterno cooking fuel...
I have tried candles but no go.

Donnie

polyacetal sounds like it'd make a great piston, but doesn't it have a really low melting point? like 120F?

Is yours a higher melting point version? Where did you get it?

-tmk

Hi!

Polyacetal is indeed NOT ideal piston material. Let it be self lubricating plastic with very low friction ratio, but its thermal expansion is the thing here that makes it of limited use. That is also why I was enquiring different piston materials here a while back. The only actual reason for me making pistons out of it was its easy machining. But yes, polyacetal (or teflon alloyed polyacetal) may work to some extent if the temperature is kept well under 100°C. It's the thermal expansion here which causes most trouble.

My power cylinder is made of brass and I have turned cooling fins to it to ensure it stays under 50°C (in some occasions it doesn't). The insides I drilled to rough size, turned in a lathe and finally polished by a special polishing tool I made. the cylinder has now very fine mirror surface, being ideal for a stirling engine. But if the power cylinder is heated too much the piston will expand more than the cylinder causing it to stuck inside stopping the engine. I still have no cooling fan attached, but I quess it may help to some extent.

I'm also mentally ready to make a new piston out of other suitable materials if necessary. Pure copper (softer than brass) might work, graphite would work but is close to impossible to get, brass piston with teflon rings is one, yet complicated, possibility.

Well, got some maintenance done for the Gamma engine.
I went to the machine shop, dismantled the engine, only to see that my displacer and hotcap had developed a rust problem (both been made of carbon steel, this wasn't a surprise). I had to re-polish the hotcap internals and the displacer, but i actually took the displacer design a bit further, and carefully turned around 0,2 millimeters off the walls in a lathe. It is now a bit lighter, and perhaps offers a better efficiency.

Earlier in the summer, the crank pin had failed (it was attached to the crank by machined threading). The threading simply failed under loads, and the pin was dropped clean off. I had to make a new one, and this time i did it in a lathe, and did very thorough work with the threading so it would be perfect... and it is almost. With more precise machining the crank mechanism is now less wobbly, and creates less friction.

I also modified the manifold between displacer and power cylinders by placing a spring into the hose, aiming to prevent the hose from folding shut. However, it is unclear if this actually helps or creates drag to airflow.

Anyway, I then re-assembled the engine, and started heating the hotcap with hot air gun (having maximum temperature setting at 500 deg C). To my surprise, the engine could run with the heat provided by the hot air gun! The temperature difference couldn't have been much more than 400 deg C although I had designed the engine for 800 deg C difference. So it is running very well. Or was actually... I had to clean up the power piston of metallic dust that had entered the engine, but after the cleanup, I couldn't get it running again. The friction imposed by the piston was too gread, and the reason was obvious. there was a long, deep scratch on the side of the piston. being machined to such tolerances, even the 0.05mm bump was enough to skyrocket friction. I turned around 0.05mm off the piston, but it didn't really seem to help...

So the next phase will be to build a new power piston for the engine. I have already purchased a piece of Polytetrafluoroethylene (PTFE) or "Teflon" which I plan to use as piston rings (building the piston frame itself from brass or aluminum). The sliding contact would be handled by the PTFE rings and lip seal (rendering the piston somewhat complicated to build). To simplify matter, I might try to build a solid PTFE piston first. Compared to polyacetal, PTFE boasts a friction coefficient of 0.04 over polyacetal's 0.2 so I might get much better results with PTFE. I dunno how easily it can be machined though, so it might require some trial and error before I can build a workable piston out of it.