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