Gallery, Projects and General > Project Logs
The Return of No. 83, a Hot Air Engine
vtsteam:
I'm seeing some wear again in the displacer rod/bushing. I feel that the bushing is too short, compared with what I'd now like. Or at least with the side forces on it, considering the shortish connecting rod I now have for its throw.
However, increasing the bushing length isn't possible without major reconfiguration of the engine. I'd have to move the crankshaft, supports, bearings, and flywheel aft, and remake all of the rods. I also don't really want to increase the length of the engine for other reasons.
Instead, I'm considering a few more compact linkage systems to reduce the side forces on the displacer pushrod. Also making a Teflon displacer bushing. Among possible alternative linkages used on hot air engines are the Scotch yoke, the pivoted Scotch yoke, and the Ross linkage. I can probably add at least one of these without moving the crankshaft, and they might also allow a longer bushing and pushrod as well as reduced side forces.
The penalty is more friction and more moving mass. But I think it's a trade-off I want to make, because in future I'd like to try an axial regenerator in the displacer cylinder. That would require very accurate linear motion. Though presently working fine, I think there's too much play now in the bushing to fit anything in the small clearance space around the displacer without interference.
vtsteam:
Have thought a lot the past day about the Scotch yoke. I like it okay on its own, but what is giving me pause is the need for some kind of sliding support to keep it from flopping sideways.
It seems there are twenty animated examples of simplified Scotch yokes online, to every one photo of an actual physical working version. In practically all of the animations, the authors haven't bothered to restrain the yoke with a guide. So almost none are practical.
I think the guide is where the real friction is going to be. I've come up with a limited number of choices for guides:
1.) the crank pin, or pin and roller, bearing would need side rims to keep the yoke trapped between them.
2.) the yoke is grooved and a pin bearing rides in the groove
3.) the yoke is constrained between rubbing guides on either side, or a grooved guide at the bottom.
4.) the pushrod is flattened or square sectioned with a bushing to fit
5.) an additional pushrod above or below the active one is used as a second guide
6.) a pivot at the bottom of the yoke and a pivoting connection to the pushrod restrains the yoke (actually, this is technically a quick return linkage, not a Scotch yoke.) It is non linear in stroke speed. Hence the name.
I don't know. The #6 appeals to me because of the elimination sliding guides, but would the non-linear stroke be an advantage or disadvantage? And should it be favor the slower motion toward the hot or cold end of the displacer cylinder?
vtsteam:
I've finished making the test rig, and here's a photo of it, now all fastened down to a base. The electric furnace is trimmed down a little squarer and clamped. The engine is clamped, and the tachometer and scale are blocked up and screwed in place.
[ You are not allowed to view attachments ]
I did an initial test of No. 83 with displacer cylinder #4 and 100 watts input to the furnace and got a max RPM of 1300, and maximum power output of 0.575 watts at about 800 RPM.
Maximum free running RPM is probably higher, as the brake, even when loosened, weighs something. And that probably means the max power is also slightly under-represented. But it's good to have this initial set of figures anyway as a start on more accurate comparisons than I've done in the past.
vtsteam:
By comparison a new test today with cylinder #3 gave max RPM as 1254 while on the brake and maximum power as 0.489 Watts at 670 rpm. That makes cylinder #4 the winner.
But why? Not easily known, because there are two variables between them. #3 has a thick aluminum flange, and a copper end plate. Cylinder #4 has a thin steel flange and a stainless steel end plate.
As a guess though, the copper end plate has better heat transfer, so is probably not the problem for #3.
But the thicker aluminum flange likely does a better job of cooling the "cold" end of the displacer. I'm guessing that might be helpful with excessive heat from a furnace, but with only 100 watts going in to the furnace on the test rig. It may be doing too much cooling. Most small Stirling engine experiments use gas ring burners with far greater heat input.
The thick aluminum flange also effectively shortens the intermediate regenerative section of the displacer cylinder -- the no-man's land between hot end and cold end.
Well, just guesses.
vtsteam:
I've been working on the possibility of using a moveable regenerator instead of a displacer, along the lines of experiments done by David Urwick and Mick Collins in the late 70's. I made a metal wool mesh caged moving regenerator with the same overall proportions as No. 83's hollow aluminum displacer. Photo:
[ You are not allowed to view attachments ]
I only got to try it out on the Prony brake for about 15 minutes before the engine seized. The power cylinder was not the source of the seizure -- it was the displacer pushrod in the rod bushing that froze. I'm not entirely sure of the ultimate cause, but I found a foreign powdery substance, (possibly burnt off of the metal wool) deposited on the rod, and it had high enough friction to cant-lock it in the bushing. (Further proof that my bushing is too short, btw.)
While that was easily remedied, I also found tiny bits of wool in the power piston cylinder, and this was a more serious defect, so I called a halt to further running with this regenerator, as built. The photo above was taken after the run, and you can see some of the particles migrating out through the mesh.
While it was running, I did feel like the performance was easier running, but less powerful than the displacer versions. I can't say for sure, since I now wonder what the rod bushing friction was like during the trial.
Top speed was only 966 RPM and calculated power at the only speed tested was 0.134 watts at 935 rpm. That is less than half what the displacers were putting out at about the same speed, but again this isn't definitive since the rod friction had unknown effect at the time. Also, the displacer runs were measured at about a half hour in, when the engine was fully warmed, and showing best results. The regenerator run only got to 15 minutes before seizure.
Still, it's interesting to see the engine run without a displacer, using a moveable regenerator. I think the main difficulty with the latter is actually fabricating one. The requirement is that air be able to pass through it. End caps, of course tend to defeat that, unless perforated, and yet must strictly centralize a pushrod. Perforations aren't ideal, a web like structure would be better. The sides, if mesh, make chucking in a lathe impossible, so you would have to plan all structural machining before assembly.
Assembly also is problematic for a high temperature environment. Silver soldering if steel or stainless steel is used for mesh and end caps is difficult or impossible with finely divided materials, and most glues are tolerant of only lower temps. Press fits aren't possible with mesh.
These requirements would be easier to meet in a larger bore than the ~ 1" diameter of No. 83, so it was something of a victory figure out how to make this one work at all. Unfortunately the wool shedding problem with all the other difficulties leaves me less than enthused about going further in this direction on No. 83. But I'd certainly like to hear from others if they have.
One thing is for certain, they will run on a regenerator alone.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version