Bob,

I don't agree with the statement that the air is dead. At cutoff the
pressure in the cylinder with air or steam is still 100PSI (actually
somewhat less but lets say 100PSI) now as the piston continues to move and
increases the volume the pressure will drop for both steam and air in
accordance with gas law pv=nrt. Steam doesn't any magical properties and in
fact if not superheated it will begin to condense and not perform as well as
air.

Joe


"Robert Swinney" wrote in message
...
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.