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Project Logs / Re: The Return of No. 83, a Hot Air Engine
« Last post by vtsteam on June 20, 2025, 05:45:27 PM »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:
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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.
[ 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.
