Gallery, Projects and General > Project Logs

The Return of No. 83, a Hot Air Engine

<< < (2/21) > >>

vtsteam:
Well first evidence of clueless tampering in its past history. There are two allen screws (grub screws) holding the old displacer cylinder into the massive vertical aluminum bulkhead. They are situated 90 degrees apart. One comes in from the bottom of the bulkhead, and one comes in from the side. Well one of those screws was missing, and the other one seemed stuck (was it cross threaded?!)  :hammer:

Design note: I had originally hoped to mount the cylinder as a press fit into that bulkhead, but had bored it .002" oversize by mistake. I had wanted good heat transfer into the aluminum as a cooler hence the press fit. The remedy, since time was so short on this original birthday present project, was to immobilize the cylinder with the allen screws, after applying electronic heat sink compound to the bore to allow good heat transfer.)

Anyway, what to do about the stuck screw deep in the bulkhead? Well since the old cyinder was defunct anyway, I was able to pull it out, with some scratches -- but it was destined for the bin anyway. Luckily with the cylinder out, I was finally able to remove the stuck screw. Phew!  :med:

But when I tried to slide the new cylinder in place, It wouldn't go. What the heck? They were the same exact OD.

After taking a careful look at the bore, I notice what looked like the tip of the same grub screw I had removed, still in place. Another one? Well yes, mystery solved!  :smart:

I unscrewed that one, and could finally slide the new cylinder in place. Two screws had been put into the same hole. That explained the missing one from the other location 90 degrees away. Somebody had had the engine apart and then, uhhhhh... screwed it up. That would also have slightly changed the cylinder location from when originally tested. And possibly the angular alignment. It would also have allowed a little play, which is why two 90 degrees apart were originally intended. And jamming two on top of each other can make the top one hard to extract.

Okay, click, I'm starting to get a picture of events.....

vtsteam:
I went to town this morning and bought a can of Sterno. Not so easy to find any more. I guess not many people use chafing dishes. This was a wider can than what I had remembered using before, not really fitting all the way into the firebox. But with no door anyway, it was a moot point.

Time to try to get the engine going. I left it to heat for a minute or so and gave then gave flywheel a flip. It didn't spin on its own, maybe four grudging turns to stop no matter how hard I tried.

Encouragingly, flipping the flywheel backwards had much higher resistance and went to an immediate stop, which to me meant at least I had a hot air engine. Flipping backwards turns a Stirling into a heat pump, and that actually takes work, rather than producing it. The difference is a sign that there is potential.

I then tried a Mapp torch -- but being careful not to to melt the end cap silver braze (which is hard - 70% silver). The engine turned over when flipped and I'd say it "ran" but really haltingly. I had to keep waving the torch on it as it tried to stop. It's not the way I remember it, and I don't want to damage it by continuing. At least I could see it go on its own, so it is a hot air engine. Of sorts.

What's wrong?

I know it was a better runner than that. It's time to take the whole engine apart and measure everything. Is there wear? Is there corrosion? Something is definitely wrong.

vtsteam:
I've done some measurements of the cylinders and components, and calculated the swept volumes and free space. By my calculations, this engine ought to have a 1.5 to 1 compression ratio. I'm definitely not feeling that when spinning the flywheel. I hardly feel anything besides friction.

That's also an issue. This engine should be free spinning other than the compression resistance. It has twin ball bearing races on the crankshaft, and they are ABEC 7 rated skate bearings. The displacer is not contacting the walls of the displacer cylinder, and its polished pushrod is riding in a brass guide bushing that is still a close fit.

It's true that the crank linkages simply use 0.125" steel pins riding directly in holes in the aluminum connecting rods. But again the fit is still good, and their friction is tiny, and it's the same setup I had when it first ran 18 years ago. It's not enough to stop an engine.

Where I'm feeling a lot of friction is in the power piston and cylinder. Now this shouldn't be. That piston is graphite, and I remember lapping the brass cylinder it rides in with a lead lap and fine abrasive for probably an hour, trying to get a perfect fit. Why is it riding so badly now. And where is the compression?

The cylinder and piston both look good.... are they that far off now?

vtsteam:
Piston and cylinder apart.

 

The flywheel spins VERY freely for more than a minute by itself. The displacer slides easily in its bushing. But the graphite piston and cylinder do not.

(Later):

Somebody oiled the piston!  :wack: It's not meant to be oiled. That completely defeats the purpose of a graphite piston!

I was wondering what kind of oil they used. Maybe even kitchen vegetable oil, heaven forbid....  so I smelled the piston. It must have been "3-in-1" oil -- I remember that smell so well from when I was a kid and we were working on our bikes. Well I still have an old bottle of it. So distinctive.

I guess the friction problem was 18 year old oil sludge absorbed into the graphite of the piston. So I just washed the piston with dish detergent in hot water.  And the brass cylinder as well. And now they are VERY free running together. Plus, the compression is back.

I can locate the piston and the little aluminum connecting rod in the cylinder held vertical and they fall right through under their own weight -- which is tiny. But if I put my thumb over the end, the piston stays suspended indefinitely because of compression, until I remove my thumb.

That's a big relief. I thought I might have to make a new piston.

tom osselton:
I haven’t been doing much of anything lately besides watching marketplace for tools and such other than that I have to get off my butt and make some patterns for casting.

Navigation

[0] Message Index

[#] Next page

[*] Previous page