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Joel Wickman
Aug 20, 2017

2 cyl, 1 valve... No Eccentric?

8 comments

If a guy were to build an engine with 180* timing of the cylinders on a double acting engine, would this work? I know the main reason of 2 cylinders is so that it is self starting and they are then timed at 90° to one another, and aranging them like this would defeat that, but is is possible. I was thinking as one cylinder was being forced to "expand" the other could cylinder could be hooked to the same supply from the valve in order to be forced to "contract".

 

Second, is there a specific reason that valves are operated on eccentrics from the crank shaft and not the traditional lobes like the piston connecting rods are attatched to? Would they work if there were conected to such lobes instead of the eccentrics?

Henry Artist
Aug 20, 2017

Poppet valves have been used on steam engines for a long time. James Watt used them on his beam engines in the 1770s. They were also used on some railway locomotives and steam waggons.

 

For a model engineer constructing such an engine can be quite a challenge hence the preference for engines with a slide valve or piston valve arrangement. Making a cam (eccentric) with a perfectly circular lobe is relatively easy on a lathe.

Gerald Gardiner
Aug 25, 2017

A twin cylinder engine with the cylinders timed at 180* will run perfectly well, it just will not be self starting. A number of full size engines are set up like that, but have methods of moving the engine off of centers if it stops on them.

Joel Wickman
Aug 25, 2017

Gerald, do you think if it was a double acting engine I could run both cylinders off of one valve, just have one cylinder hooked up backwards to the valving? I was thinking of using a decent sized spool valve. How big of a steam inlet and supply tubing would I need for a 1" bore x 1.75" stroke?

Gerald Gardiner
Aug 25, 2017

Hi Joel,

I can visualize it working, but I have never seen anyone set it up. It would be interesting to experiment with using some small air cylinders and control valve. 1" bore & 1.75" stroke is a bit above what I normally run, but with the setup you are describing your inlet and exhaust would be the same size, I would probably go with 1/4" ID tube.

pa-lewis
Aug 26, 2017

'Henry Artist' Well isn't the eccentric a cam lobe?

Joel Wickman
Aug 27, 2017

pa-lewis, what I am trying to ask is why we are using concentrics and not attatching the valve rods to the crank like we do the pistons. The valve already has a wrist joint, why would a larger throw hurt it?

KM6VV Marconett
Aug 28, 2017

You might want to look at the duplex pump. It uses the piston rod motion of one cylinder to run the valve of the other cylinder. Might give you some ideas. No eccentrics, just 'trip' latches (?) to work valves.

Joel Wickman
Aug 29, 2017

certainly food for thought.

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