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| Kjetil:
This is a project I’ll devote some time to, and there will be a lot of waiting around for parts as I’m not going to do the machining myself. In other words, don’t expect the build to move rapidly. Plans started with a Cygnet Royal and some computations. Figured I needed a 2.5:1 scale in order to get 1HP at ~2000rpm and 2HP at ~3000rpm. I won’t need much more than 1HP, but some parameters are guesstimates, so I stay on the safe side. Also, it would be nice not having to push it towards its max all the time (I’ll team it with a 1HP boiler). I built a CAD model (SolidWorks, attached pic) after scaled drawings, played a bit around with it to learn the operating details like the orbital valve, and now I’m starting to build a second model I’ll use as a blueprint when machining the parts. Some changes are to be made thou, especially with the crank mechanism :scratch: crank arms offset from piston centre worries me, and with a master/slave arm system I just can’t seem to get enough room for the shaft dimensions and bushings I need without increasing stroke length :bang: I’ve had a look at the “fork & blade” system used on many V-engines, and a triple one is probably doable but I’m pretty sure a noob like me would mess it up. Best way I see is plain narrow crank arms and offset cylinders. The symmetry of the looks isn’t all that important to me, and I avoid over complicating things. Is there a big vibration penalty to this? We’re talking about 1cm offset between them :scratch: I’m also building a primary balance model for it :smart: in Matlab (see attached pic). I’ll treat piston and wrist pin as one unit, crank arm as one, then there is the offset crank shaft, and the orbital valve (out of phase with the crank). Plan is to have the model make vectors of locations of the centres of gravity over one full rotation, in order to assign mass to them and plot the total centre of gravity over the cycle, which I can hopefully use to calculate how to shave the counterweight :thumbup: Crank casing will be 1st part btw, I've located some 6" diameter 6082 alu round bar for it :ddb: But some more drawing will have to be done before I can cut :coffee: |
| Kjetil:
Had an hour this morning to play with Matlab, Black squares are CoG of the pistons, offset out along the line from the wrist pin by a variable. Yellow squares are CoG (axis) of the orbital valve and supporting pin Red squares are CoG (axis) of the offset crank shaft The rest of the coloured lines are the crank arms, and the square marker on them identifies the CoG of that crank arm. Inputs to the model will be material weights and CoGs as computed by SolidWorks, and there will also be tolerances, but it should get me as close as I can get. |
| Kjetil:
Back to the crank, by "fork and blade" setup I'm referring to something like the attached scribble, only built differently, probably aluminium with bronze bushings. I haven't seen such a triple setup anywhere, I don't know if that's for a good reason. I'm still a bit undecided on what to do here :scratch: any input on the subject would be highly appreciated :poke: fork & blade vs offset cylinders... |
| Kjetil:
I just had a revelation skimming through Elmer's radial engine :coffee: It uses way thinner shafting than the Cygnet, at the same cylinder volume, perhaps not advisable to run at the same max RPM, but it was reassuring reading. I did a sketch with my shafting dimension references somewhere inbetween the two, and was able to fit a bushed master rod large enough to accommodate bushed slave rods. Main shaft remains 20mm, crank shaft 1/2", and slave pins 5/16". Back to not eliminating any options at all, I'm more undecided than ever. This one would actually be my favorite ATM. |
| Kjetil:
Geometry of the primary balance model for the master/slave rods gets even more cryptic than before :smart: Red is the master rod, cyan and magenta is the slave rods. But now I can combine the entire crank into the same body mass as the master rod. Only other bodies are the two slave rods, the pistons/wristpins, and the orbital valve/pin. |
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