On a compound locomotive, like a Mallet, are the piston valves the same diameter for both the high and low pressure cylinders? I am working on a scratchbuilt model, but my only plans are some basic section drawings. They don’t have that dimension.
No. The high pressure cylinders reduce the pressure, so when it reaches the low pressure ones it is at a lower pressure which requires a larger diameter cylinder. Do some google searching on such engines as N&W Y-6 and you will see the difference.
What about the valves that allow the steam into both the high and low pressure cylinders? Would they have to be different or could they be the same and allow fewer parts in inventory for maintenance?
The low pressure cylinders are significantly larger in area than the high pressure cylinders. The difference in piston area of the high and low pressure cylinders is proportional to the operating steam pressures expected at the inlet pipes. High pressure steam contains more energy than low pressure steam, so the high pressure piston requires less area to provide the same force to the drivers as is provided by the larger low pressure piston.
There is a partial answer on the NYC post today about the Elkhart yard, with a photo of a NYC class NU-1 mallet, used for switching of all things, and the high pressure cylinders have round valves and the low pressure cylinders have entirely different slide valves. I am not sure, but it once meant that the slide valves provided outside emission to the cylinders, whereas the newer valves provided inside emission. Good engineering must have been involved in synchronizing all of that stuff.
And, I finally found a pic of N&W Y-6a, and it is pretty obvious that the valves are not the same diameter for the high and low pressure cylinders, they are pretty much in scale with the cylinder diameters. Hope this helps you.
. Once again in my old age I forgot to attach the photo to the first post, and it won't allow in edit.
