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The Return of No. 83, a Hot Air Engine

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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?

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