Gallery, Projects and General > Project Logs
The Return of No. 83, a Hot Air Engine
vtsteam:
I returned the blank to the lathe and parted off the new graphite piston. I probably shouldn't have pushed that all the way, and sawed the last little bit off with a hacksaw blade because it broke a chunk off. Luckily that came out of the blank instead of the piston, so it was easily filed off.
Final step was to ream the journal to size and install the con rod.
vtsteam:
Lapped the piston with paper, reassembled the engine, and things are looking better! No 83 settled in at about 1300 RPM and hit a high of 1440 RPM. That's great as far as repairing the cylinder and fitting a new piston. But I'd actually hoped for better, since before the cylinder problem, I'd made some big changes to the displacer cylinder mounting, and added a copper end.
Well we're definitely back in business with a good baseline again. I'm going to think about the displacer cylinder change. I think definitely there is more cooling now, and it extends further along the cylinder with the heavy aluminum flange.
I think the problem may be that I've got too much cooling now compared to the amount of heat available, which is relatively low. I don't yet want to add more heat though I will in the future. I think my next change will be to make a displacer cylinder with a thinner flange thickness, and that will be made out of stainless or rather than aluminum. I'm not sure about the copper cap. Probably revert to stainless again just to compare with my earlier cylinder #2 which was that best performing one.
sorveltaja:
I built a Ringbom stirling engine in 2008, and it has kept good compression through the years.
Anyways, I can't remember the exact steps I used to hone the cylinder, but on the last stage, I used automotive polishing paste with - perhaps brass or aluminum rod (that's what I had at that time), against brass cylinder.
It was rather tedious process, as I did it manually. I didn't use split rod to lap - it was solid. It used to get stuck in the cylinder, when I went too far, while turning it. Good knock on the working end was enough to release it. Finally, it moved through the cylinder.
Then came the final stage - to fit graphite piston to the cylinder. I used the same procedure as with lapping. Solid graphite rod (part of it was then used as a piston) against lapped brass cylinder (as graphite wears itself relatively easily through the friction, when compared to metals).
That's just one possible way to minimize power piston/cylinder friction.
vtsteam:
Hey sorveltaja! :wave: :beer: Great to hear from you! How is your guitar? That was such an interesting project! :coffee:
Also interesting to hear about your lapping experiences. For me the dowel worked surprisingly well, I really hadn't expected it to do better than the aluminum lap.
I'm thinking that "aluminum" is actually a too general term for me to make judgements about-- aluminum varies a lot in hardness, and so maybe I chose a harder variety out of the scrap box. this time
Pure aluminum is dead soft, and I bet that makes a better lap. I think it's important that the lap get embedded with the abrasive, rather than it moving around between the two surfaces too much. The lead lap I used years ago worked very well, and I know the abrasive embedded deeply there. Same thing for the wood lap this time.
I have a pretty good honing rig for larger cylinders in a larger engine, which I'm hoping to start at some point in the future. It will still need lapping, too as a hot air engine, but it should have a uniform bore after honing, so only will need to be lapped for a good finish. That's my hope anyway.
vtsteam:
I ran the engine again tonight, and it is consistent at around 1300 RPM. I hear a rumbling sound and some slow beat volume changes at full speed. I think it might be the ball bearings.
It stands to reason that if the rest of the engine had rusted in a damp garage for 8 years that maybe the bearings might have some issues, too. Well, those are easily replaced, and I now have ABEC 9 bearings on hand. The ones in place were ABEC 7.
Another possible contributor is the main shaft. It's 5/16 (.3125") but the bearing takes an 8mm shaft, so there's a little play. I have some 8mm stainless steel now, so I'll make the switch when I change the bearings.
A third issue I noticed: I washed out the power cylinder before reassembling again tonight, and when giving the flywheel a spin, I saw a little moisture from not having dried it completely squeeze out from the edge of the cylinder where it seats in the base plate. This means a leak. Easily sealed with a little bearing compound -- Loctite 620, but good to know. It's probably always leaked, so that should help performance when fixed.
Fun stuff! :dremel:
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