Thanks to all respondents to my question re. a rule of thumb for performance
comparison of steam vs. air at the same input pressure in a steam engine.

After reading all the insightful replies, I am now more convinced than ever,
there is no such rule of thumb. Not even close. There are so many
variables that guesswork and gospel become intermixed. I broached the
question with the disclaimer it not be approached via the rigorous math of
thermodynamics. Seemingly, from most responses, there is no other way. Had
the question been given more thought it would have been obvious (to me)
there is no easy way to quantify such disparities as air and steam - in a
steam engine. The very idea borders on sacrilege!

In summary, the excellent responses boiled down to:

Air and steam at the same pressure input to a steam engine, with
temperatures of each as they come from respective generators, yield
different performance outputs. The amount of difference is not quantifiable
without much more general information and a trip through the "thermo-math"
jungle. The generalized answer was that "steam is better than air in a
steam engine". Most responses reinforced the notion that the point of
cutoff plays an important role in any such evaluation. Intuitively, at the
same cutoff point (original question criteria) steam performance is "better"
than air. It appears that an early cutoff for steam and a very late cutoff
for air would tend to place both entities in the best scenario. Those
scenarios, however, violate the basic premise of the question.

Again, thanks to all!

Bob Swinney



"Robert Swinney" wrote in message
...
Don sez: "If efficiency is not an issue, running a steam-type engine on
compressed
air is fine. But it is not a heat engine. A heat engine like a steam
engine is a device for converting chemical energy in a fuel into
mechanical energy via a thermal process."

Don, my disclaimer re. thermodynamics set all the issues of efficiency,
transport piping, etc., aside. My question was simply about comparing the
performance of a steam engine ran on steam to that same engine ran on
compressed air *at the same input* pressure.

Bob Swinney

"Don Stauffer" wrote in message
...
Let's put it this way, a compressed air engine cannot deliver more
energy than what is going into the compressor. In fact, because of
losses, you need to supply several times more energy to the compressor
than what you get out of engine/tank combination.

That said, compressed air is a way to store and transport energy.
Compressed air was used in mine locomotives because they did not need a
combustion process (safety issue). They didn't have much range (one
cannot store much energy in a compressed air tank), but they didn't need
much range. They could be recharged each time they came out of mine.


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

--
Don Stauffer in Minnesota

webpage- http://www.usfamily.net/web/stauffer