A conversation with a friend today, brought up the question of how well a
steam engine runs on compressed air: That answer is, "It will run on air
but not very well compared to performance on steam".

My question is this: Does anyone have a simple compressed air/steam "rule
of thumb" ?

Now, I know this gets into all sorts of complex thermodynamic calculations.
For example, the Brake HP of any engine is a direct function of pressure.
Pressure, however, in order to fit into conventional formulae must be given
in Mean Effective Pressure (MEP). Enter hairy thermo-math here. MEP would
be a sort of integral (mean) pressure in any heat engine. The type of
engine, amount of moisture in the steam, percentage of cut-off, insulation
of cylinder walls, size of passages including valve openings, on and on,
etc., etc., to nauseam, all enter into MEP. The old timers, at least those
mentioned in "Modern Locomotive Construction" circa 1892 (sold by Lindsay)
commonly used 90 psi as the MEP of a representative locomotive of the time.
So much for the math. Don't send me any formulae for calculating MEP - I've
got that. I'm looking for shortcuts, here, thank you.

What I'd like to see is a comparison of the HP output of a steam engine
running on a given amount of input (boiler) pressure compared to the HP
output of the same engine running on the same amount of input compressed air
pressure.

Analyze this from the standpoint of engine performance only, neglecting
boiler HP or compressor HP.

Ideas please.

Bob Swinney