Don sez:
" Is it possible that some droplets of superheated water enter the cylinder
along with the steam? If so, that would cause a big difference."
Careful Don! You are about give away Robert's secret thermonucleur
reciprocating engine theory. Oh, alright! So it is a nucleur heated
cylinder with water injector. OK?
Bob Swinney
"Don Foreman" wrote in message
...
Neat experiment -- but perhaps not the same as a steam engine once the
cylinder is valved off from the boiler. She was compararing a charge
of
pressurized gas to a charge of superheated water.
Bob, I think steam does not behave the same as air in adiabatic
expansion.
Its behavior is treated in steam tables and/or indicator diagrams. It
might
be very interesting to instrument your steam engine with a cylinder head
pressure gage and displacement sensor to make indicator diagrams with air
and steam. It might be especially interesting to study how they did that
with mechanisms in the days long before electronic or even electric
instrumentation and perhaps try to replicate it in a model. Maybe a
linkage to the piston, another to a bourdon tube pressure gage and a
stylus
scribing the "indicator diagram" on a bit of metal smoked with soot.
"Glenn Ashmore" wrote in message
news:Vsu0d.112$iK2.21@lakeread08...
Back in Jr. High I had a wonderfule science teacher who had a great
experiment that she did every year to demonstrate the power of steam.
She
had a cylinder with a pipe near one end for compressed air and a
replaceable
cap on the other that he could mount an aluminum foil disk in to seal it
up.
There was also a spring loaded pin that would puncture the foil when you
pull a lanyard.
First she would fill the cylinder to 100 PSI with compressed air and pop
the
foil seal which resulted in a Whoosh!. Then she would drop in an ounce
of
water and heat the cylinder to about 340F to get 100 PSI of steam. When
she
broke the foil the entire school shook. I have been impressed by the
power
of steam ever since. :-)
Even from 2 blocks away in senior high we would heard that BOOM! and
break
out laughing knowing that Mrs. Harper was teaching thermodynamics again.
"Robert Swinney" wrote in message
...
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
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