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.

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.

Gary