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Crank operated lever controls rotary valve and fries brain!
PaulR:
Someone please point me in the right direction with this, it's making my head hurt!
This render shows a version of the 1950 Popular Science magazine 'scraps and solder' rotary valve engine, the valve being operated by the side crank. The original plan has the inlet and exhaust ports (ie holes through the valve shaft) 115 degrees apart whereas the plan for this one shows 105 degrees. I'm trying to work out how things would change if I modified the dimensions further (eg cylinder/stroke length, valve housing size etc) but I can't seem to figure it out.
As I see it, the valve rod's crank pin is doing what an eccentric would but, because the pin connecting the valve rod to the lever isn't constrained horizontally, it seems to be introducing an order of complexity that my brain can't handle! I suppose the empirical way would be make some temporary parts and spot the points on the valve shaft through the cylinder or in/out pipes but I'd rather try to work it out.
crueby:
Interesting design!
It does work just like an eccentric would if the center of the eccentric was in the same place as the valve pin. It does look a whole lot different though! Its a common arrangement on locomotives, with the valve pin on the arm off the crank pin.
To play with offset lengths and positions, you could design it as an eccentric and then swap it for a pin and arm later, if that keeps the brain temperatures down! Some old hand cranked tools put a spiral arm out to the handle just for looks, but they behaved the same as if the arm was straight.
Charles Lamont:
The return crank acts, as you say, exactly as an eccentric would. At the valve end, the rocking of the lever means that end of the linkage has some up and down movement too, but not enough to have a great deal of effect on the behaviour of the valve. We would need to see more of the design of the rotary valve before commenting, and a whole lot of dimensions before advising on any calculations.
Jasonb:
Easiest thing is to draw it in cad, section and then you can watch what is happening as you make any changes.
I have done a couple of rocking valve engines but used a pocket in the valve rather than just holes but both were worked out in CAD.
As said the arrangement at the crankshaft end is no different to an eccentric.
Vertical movement at the rod end has little effect unless you make the lever very short
You need to watch any images taken from the likes of Grabcad, it is not unknown for errors to creap in
Jasonb:
Original on pages 196 & 197
https://books.google.co.uk/books?id=yywDAAAAMBAJ&printsec=frontcover&rview=1&source=gbs_ge_summary_r&redir_esc=y#v=onepage&q&f=false
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