If ratio of temperatures governs relative performance, then hot compressed
air would work better than cold compressed air?

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.


"Gary Coffman" wrote in message
...


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