Help! > Specific Engine Help
Eccentric operated rotary valve
PaulR:
Just thinking about a different rotary valve engine, having a valve crank operated by an eccentric on the crankshaft, ie the valve does a complete rotation in sync with the crankshaft. As far as I can see, the throw of the valve crank could just be some convenient measurement, there would always be a 1:1 rotational relationship. However, I'm concerned about binding.
I asked my AI friend about this and it says the important thing is the length of the valve rod, it needs to be such that the angularity error (deviation from pure linear motion) is less than about 10deg. If the throw of the valve crank is say 6mm, by trig the valve connecting rod would need to be at least ~35mm. Basically, in a horizontal configuration make the rod as long as possible to keep it as close to horizontal as possible at all times which sounds entirely reasonable!
Does that sound ok?
(The timing would be set by a set screw on the valve disk BTW)
Jasonb:
You will have difficulty getting a full rotation, the valve is more likely to rock back and forth. Basically as the rod movement from the eccentric gets to the top or bottom of it's movement there is nothing to say it will go over ctr and fully rotate the valve or more likely just move the valve lever back the way it came.
But if you can make do with partial rotation of the valve eg a "rocking valve" then that will work with an eccentric
PaulR:
--- Quote from: Jasonb on March 27, 2026, 09:36:15 am ---You will have difficulty getting a full rotation, the valve is more likely to rock back and forth. Basically as the rod movement from the eccentric gets to the top or bottom of it's movement there is nothing to say it will go over ctr and fully rotate the valve or more likely just move the valve lever back the way it came.
But if you can make do with partial rotation of the valve eg a "rocking valve" then that will work with an eccentric
--- End quote ---
I had a nagging feeling that might happen even though I don't know exactly why it should. I guess it'll be a complicated mix of small factors which are probably beyond my understanding :Lol:
I was trying to think of a way to get the valve at the far end of the cylinder and have it fully rotate...
For a rocking valve I'd probably go with the 'scraps and solder' type engine I built before as it's a lot easier than making an eccentric.
crueby:
For a fully rotating shaft, they would normally put a crank at either end like a bicycle foot pedal crank, so as one gets near top or bottom the other is in the middle. Large engines with offset valve shafts did id that way. Or they used an intermediate shaft with bevel gears, or a chain drive.
maury:
Try something like this.
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