Regarding SaltyChief's statement: "Take a look at the old three cylinder
marine engines. They had three different sized cylinders with the exhaust
from one cylinder feeding the next
cylinder. To use the exhaust from one set of cylinders to power another set
they have to be different sizes."
It took a while for the lightbulb to turn on in my brain, and then some more
time to figure out how to put it into words. The reason that each
successive cylinder must be larger is because the steam enters the cylinder
at a lower pressure than when it entered the previous cylinder. Because
volume is inversely proportional to pressure the steam's volume is larger.
If the lower-pressure cylinder is not big enough then it cannot hold all the
steam coming from the previous, higher-pressure cylinder. I believe this is
a fair explanantion of why a compound must have different sized cylinders.
A small complication is that the steam loses temperature also as it passes
through the high pressure cylinder and into the lower-pressure cylinder.
This will reduce the steam's volume compared to what it would be if the
steam temperature remained constant.
My conclusion from all this is that to get efficiency from a compound
requires a lot of calculation regarding steam temperature changes and
pressure drops through the cylinders and piping, and expected work output
from each set of cylinders. An inquiring mind could look up the published
specifications for a particular compound locomotive, do a few
straightforward calculations, and determine whether the loco manufacturer
was fibbing about the loco's hauling capability.
Steve
