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.







RichD wrote in message ...

On Fri, 10 Sep 2004 13:36:57 -0500, "Robert Swinney"
wrote:

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