Gary sez: "Sure thing, PV = nRT

n is the same for both the compressed air and dry steam at the same entry
pressure
and flow. R is different, but not a whole lot different as long as the
steam remains
hot enough to be non-condensing in the cylinder. T is very different for
the steam
and the compressed air. So the ratio of temperatures will give you an
approximate
ratio of relative performance after cut off."

Thanks, Gary! Totally agree; the ratio of temperatures is the key. The %
stroke, (timing of cutoff) it seems, would play greatly into this. Very hot
steam with an early cutoff would have more time to do its thing. The steam
"advantage" is enhanced by early cutoff.

*Now if we could only quantify all this - even in the most rudimentary (rule
of thumb) terms.*

Bob Swinney




"Gary Coffman" wrote in message
...
On Fri, 10 Sep 2004 18:59:49 -0500, "Robert Swinney"
wrote:
RichD sez:

"you really have answered your own question.
HEAT! A steam engine is a heat engine. This is the BIG difference.
Air has NO heat to give up.
That's it in a nut shell."

Yeah, but: Compressed air will follow the piston until the point of
cutoff.
From cutoff until the end of the stroke, the volume of air, trapped in
the
cylinder can do little more work as the piston moves away and increases
volume in the cylinder. For all practical purposes, the air is "dead" at
the point of cut off. Contrast this with live steam. Steam at boiler
pressure pushes the piston, much the same as air; but at the point of
cutoff
the steam and cylinder is still hot (it has lost some heat) and is still
expanding, doing more work against the piston. Performance after cutoff
is
one of the fundamental differences between compressed air and steam in a
steam engine. I would like to know if there is an easy "rule of thumb"
that
addresses this and other differences between the performance of
compressed
air and steam at the same input pressure.


Gary