I think the difference in gammas tells the tale.

I'm about 4000 miles from my Machery's Handbook just now, but I think change
in pressure and volume will dictate outlet temp as fn of input temp so
actual inlet temp for a given adiabatic expansion is immaterial --
providing of course that steam does not condense. Outlet temp can be
above or below ambient.

"Gary Coffman" wrote in message
...
On Sat, 11 Sep 2004 10:41:47 +0000 (UTC), "Don Foreman"
wrote:
If ratio of temperatures governs relative performance, then hot
compressed
air would work better than cold compressed air?

Yes.

In any case, PV=nRT relates to an isothermal (constant temperature)
situation. Expansion after cutoff in a steam engine is usually regarded
as
adiabatic rather than isothermal expansion. In adiabatic expansion the
specific heat of the substance is relevant. Specific heat of steam may
be
quite different than that of air.

Indeed it is. But the important number with respect to the work done by
Carnot cycle engines is the ratio of specific heats of the particular
working
fluid. Gases have different specific heats depending on whether the
specific
heat is measured at constant volume or constant pressure. The ratio of
these
two values is called gamma. For air it is 1.4. For steam at 150 PSI it is
1.28.

T1 and T2 are still the dominant numbers (T1 is inlet temperature, T2 is
outlet
temperature, usually assumed to be ambient), but gamma does play a role in
the process. Gamma appears as an inverse exponent in the Carnot equations.
So the closer to 1 it is, the better. The ratio of gammas for steam and
air says
that steam should be a 9% better working fluid than air at the same
working
temperature.

Note that I'm assuming non-condensing operation. If the steam is allowed
to condense in the cylinder, then latent heats also have to be considered.

Gary