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

In article , Robert Swinney says...

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.

To be fair I think you need to figure in some kind of flow rate
as well in this, ie 50 psi air at 10 cfm, vs 50 psi steam at
such and such a boiler water feed rate.

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

"Robert Swinney" wrote in message
...

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


I have measured the output of my 2 cylinder model engine on both steam and
air at the same pressure using a generator as a load. Measurements at 100
PSI and load adjusted via excitation to give the same RPM. Results are
steam 5 to 10 % greater than with air. I attribute this to the oil
viscosity change with temperature since the same oil and oil feed rate was
used for both tests. Cylinders were noticeably chilled when running on air.
Steam was superheated to 500 to 600 degree F area.

Joe Hanulec

Jim sez:

"To be fair I think you need to figure in some kind of flow rate as well in
this, ie 50 psi air at 10 cfm, vs 50 psi steam at such and such a boiler
water feed rate."

Each component is considered to enter the engine through the normal design
passages and at a volume of flow consistent with what the engine can take.
The boiler and the air compressor are adequate to "keep up" with the demand
of the engine. We want to know how the output of the engine compares with
the same amount of input pressure from steam as from air.

Bob Swinney

"jim rozen" wrote in message
...
In article , Robert Swinney says...

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.


Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================

Joe sez:

"I have measured the output of my 2 cylinder model engine on both steam and
air at the same pressure using a generator as a load. Measurements at 100
PSI and load adjusted via excitation to give the same RPM. Results are
steam 5 to 10 % greater than with air. I attribute this to the oil
viscosity change with temperature since the same oil and oil feed rate was
used for both tests. Cylinders were noticeably chilled when running on
air.
Steam was superheated to 500 to 600 degree F area."

Joe,
Good information. Do you know if the engine was running "valves wide open"
or if there was a cut off point such as 50% of stroke or etc.; and if the
same cutoff point was existent in both tests?

Bob Swinney





"Joe Hanulec" wrote in message
et...

"Robert Swinney" wrote in message
...

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


Joe Hanulec

To make things somewhat simplistic when superheated water turns to steam and
is allowed to escape it will expand something like 1600 times(this is why a dry
crown sheet is deadly) its volume as water. When you compress air say too 150
psi that is only 10 times the general atmophieric pressure. Compressed air
quickly looses its power when allowed to expand in the cylinder where as steam
keeps pushing as it expands. We are currently building a 12" guage steam
locomotive and have it running on compressed air at the present time. It will
run nicely down to about 25psi, but of course this is wheels up and no load.
Our hope is to have at least a running boiler pressure of 150psi if not alittle
higher.

tim

Bob,
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.
RichD

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

Bob,

The cutoff was the same for both air and steam, my recollection is 75 %.

Joe


" Joe,
Good information. Do you know if the engine was running "valves wide
open"
or if there was a cut off point such as 50% of stroke or etc.; and if the
same cutoff point was existent in both tests?

Bob Swinney

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

"Robert Swinney"

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.

One of the delights of early/modern steam technology was the indicator--done up
with literature and hardwood case--that produced a stylus's readout of engine
performance. Perhaps you might use one (or a modern substitute) to compare the
steam/air results on a given engine.
Frank Morrison.

Bob,

I don't agree with the statement that the air is dead. At cutoff the
pressure in the cylinder with air or steam is still 100PSI (actually
somewhat less but lets say 100PSI) now as the piston continues to move and
increases the volume the pressure will drop for both steam and air in
accordance with gas law pv=nrt. Steam doesn't any magical properties and in
fact if not superheated it will begin to condense and not perform as well as
air.

Joe


"Robert Swinney" wrote in message
...
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.

Thanks Joe,
I'm guessing that would account for the slight amount of difference for
steam.
Bob Swinney
"Joe Hanulec" wrote in message
et...
Bob,

The cutoff was the same for both air and steam, my recollection is 75 %.

Joe


" Joe,
Good information. Do you know if the engine was running "valves wide
open"
or if there was a cut off point such as 50% of stroke or etc.; and if
the
same cutoff point was existent in both tests?

Bob Swinney

Joe sez:

"I don't agree with the statement that the air is dead."

My statement was that the air is dead at cutoff for all practical purposes.
Air pressure will fall off rapidly as the piston retreats and increases cyl.
volume. Steam in a hot cyl. is still expanding and will continue to do work
against the piston. This would, I believe, account for the small difference
(steam over air) you reported at late cutoff. I wish there was some easy
way to estimate these effects.

Bob Swinney

"Joe Hanulec" wrote in message
et...
Bob,

At cutoff the
pressure in the cylinder with air or steam is still 100PSI (actually
somewhat less but lets say 100PSI) now as the piston continues to move and
increases the volume the pressure will drop for both steam and air in
accordance with gas law pv=nrt. Steam doesn't any magical properties and
in
fact if not superheated it will begin to condense and not perform as well
as
air.

Joe


"Robert Swinney" wrote in message
...
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.

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

Gee, Why can't teachers keep kids attention anymore?
Science was FUN! Look on EBAY at the old science kits and
all the damage, er a fun that you could do with one!

I'm sure Mrs. Harper would be serving time for a explosive device
detonation nowadays.

Bart

Glenn Ashmore wrote:

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











--
Bart D. Hull

Tempe, Arizona

Check http://www.inficad.com/~bdhull/engine.html
for my Subaru Engine Conversion
Check http://www.inficad.com/~bdhull/fuselage.html
for Tango II I'm building.

Remove -nospam to reply via email.

On Fri, 10 Sep 2004 18:59:49 -0500, "Robert Swinney" wrote:
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.

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

On 10 Sep 2004 23:08:59 GMT, (TSJABS) wrote:
To make things somewhat simplistic when superheated water turns to steam and
is allowed to escape it will expand something like 1600 times(this is why a dry
crown sheet is deadly) its volume as water.

That's true, but that happens in the boiler where the *liquid* water volume
turns to a *gas* (steam) volume. Once that happens, the flow of gas (steam
or compressed air) into the engine is determined by the pressure (same for
both), the valve aperture (same for both), and the valve opening duration
(same for both). So the fact that the volume of water expands 1600 times
when it changes from liquid to vapor is irrelevant for comparing engine
performance.

When you compress air say too 150
psi that is only 10 times the general atmophieric pressure.

The same is true for an equal volume of non-condensing steam at 150 PSI.
150 PSI is 150 PSI.

Compressed air
quickly looses its power when allowed to expand in the cylinder where as steam
keeps pushing as it expands.

Both continue to push equally until pressure falls to atmospheric.

Now if both start out at the same temperature, then both will reach
atmospheric pressure after the same amount of expansion. But we
know that the steam is at a temperature of at least 681 R while the
compressed air is at tank temperature, which for a big enough air tank
is close enough to room temperature to use that number, ie 469 R.
And we know that PV=nRT.

So, if the engine expansion ratio is large enough to allow both gases
to expand to atmospheric pressure, the advantage for steam is crudely
the ratio of the working gas temperatures. 681/469 = 1.45

Of course if the expansion ratio is less than 10 to 1 for an engine working
with a 150 PSI input pressure, there won't be any observable advantage
for steam over compressed air, since the expansion ratio will only be enough
to expand the compressed air to atmospheric, and not enough more to
take advantage of the higher temperature of the steam.

Gary

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.

Is it possible that some droplets of superheated water enter the cylinder
along with the steam? If so, that would cause a big difference.





"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

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

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.

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.

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

Mrs. Harper was unique. When she was teaching basic machines she had an old
Crosley engine that she would assemble and crank up in front of the class.
When she taught centrifugal motion you had to stay awake or you might get
hit by a flying ball. And what she could do with a neon sign transformer
was awe inspiring to a 9th grader. :-)

No way she could do that in a class room these days.

"Bart D. Hull" wrote in message
...
Gee, Why can't teachers keep kids attention anymore?
Science was FUN! Look on EBAY at the old science kits and
all the damage, er a fun that you could do with one!

I'm sure Mrs. Harper would be serving time for a explosive device
detonation nowadays.

Bart

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

Gary sez:
" Both continue to push equally until pressure falls to atmospheric."

They do not continue to push equally. The heat energy contained in steam
equates to more "push" if both steam and air are at the same pressure at
cutoff. Air pressure degrades rapidly after cutoff. Steam pressure after
cutoff falls more slowly because of contained heat. Perhaps my original
question would have been more relevant if I had said, "Oh, BTW, the steam is
superheated as it enters the cylinder".

Bob Swinney



"Gary Coffman" wrote in message
...
On 10 Sep 2004 23:08:59 GMT, (TSJABS) wrote:
To make things somewhat simplistic when superheated water turns to
steam and
is allowed to escape it will expand something like 1600 times(this is why
a dry
crown sheet is deadly) its volume as water.

That's true, but that happens in the boiler where the *liquid* water
volume
turns to a *gas* (steam) volume. Once that happens, the flow of gas (steam
or compressed air) into the engine is determined by the pressure (same for
both), the valve aperture (same for both), and the valve opening duration
(same for both). So the fact that the volume of water expands 1600 times
when it changes from liquid to vapor is irrelevant for comparing engine
performance.

When you compress air say too 150
psi that is only 10 times the general atmophieric pressure.

The same is true for an equal volume of non-condensing steam at 150 PSI.
150 PSI is 150 PSI.

Compressed air
quickly looses its power when allowed to expand in the cylinder where as
steam
keeps pushing as it expands.


Now if both start out at the same temperature, then both will reach
atmospheric pressure after the same amount of expansion. But we
know that the steam is at a temperature of at least 681 R while the
compressed air is at tank temperature, which for a big enough air tank
is close enough to room temperature to use that number, ie 469 R.
And we know that PV=nRT.

So, if the engine expansion ratio is large enough to allow both gases
to expand to atmospheric pressure, the advantage for steam is crudely
the ratio of the working gas temperatures. 681/469 = 1.45

Of course if the expansion ratio is less than 10 to 1 for an engine
working
with a 150 PSI input pressure, there won't be any observable advantage
for steam over compressed air, since the expansion ratio will only be
enough
to expand the compressed air to atmospheric, and not enough more to
take advantage of the higher temperature of the steam.

Gary

Gary sez: "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."

Thanks, Gary! Totally agree; the ratio of temperatures is the key. The %
stroke, (timing of cutoff) it seems, would play greatly into this. Very hot
steam with an early cutoff would have more time to do its thing. The steam
"advantage" is enhanced by early cutoff.

*Now if we could only quantify all this - even in the most rudimentary (rule
of thumb) terms.*

Bob Swinney




"Gary Coffman" wrote in message
...
On Fri, 10 Sep 2004 18:59:49 -0500, "Robert Swinney"
wrote:
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.


Gary

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

On Sat, 11 Sep 2004 10:41:47 +0000 (UTC), "Don Foreman" wrote:
If ratio of temperatures governs relative performance, then hot compressed
air would work better than cold compressed air?

Yes.

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.

Indeed it is. But the important number with respect to the work done by
Carnot cycle engines is the ratio of specific heats of the particular working
fluid. Gases have different specific heats depending on whether the specific
heat is measured at constant volume or constant pressure. The ratio of these
two values is called gamma. For air it is 1.4. For steam at 150 PSI it is 1.28.

T1 and T2 are still the dominant numbers (T1 is inlet temperature, T2 is outlet
temperature, usually assumed to be ambient), but gamma does play a role in
the process. Gamma appears as an inverse exponent in the Carnot equations.
So the closer to 1 it is, the better. The ratio of gammas for steam and air says
that steam should be a 9% better working fluid than air at the same working
temperature.

Note that I'm assuming non-condensing operation. If the steam is allowed
to condense in the cylinder, then latent heats also have to be considered.

Gary

I think the difference in gammas tells the tale.

I'm about 4000 miles from my Machery's Handbook just now, but I think change
in pressure and volume will dictate outlet temp as fn of input temp so
actual inlet temp for a given adiabatic expansion is immaterial --
providing of course that steam does not condense. Outlet temp can be
above or below ambient.

"Gary Coffman" wrote in message
...
On Sat, 11 Sep 2004 10:41:47 +0000 (UTC), "Don Foreman"
wrote:
If ratio of temperatures governs relative performance, then hot
compressed
air would work better than cold compressed air?

Yes.

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.

Indeed it is. But the important number with respect to the work done by
Carnot cycle engines is the ratio of specific heats of the particular
working
fluid. Gases have different specific heats depending on whether the
specific
heat is measured at constant volume or constant pressure. The ratio of
these
two values is called gamma. For air it is 1.4. For steam at 150 PSI it is
1.28.

T1 and T2 are still the dominant numbers (T1 is inlet temperature, T2 is
outlet
temperature, usually assumed to be ambient), but gamma does play a role in
the process. Gamma appears as an inverse exponent in the Carnot equations.
So the closer to 1 it is, the better. The ratio of gammas for steam and
air says
that steam should be a 9% better working fluid than air at the same
working
temperature.

Note that I'm assuming non-condensing operation. If the steam is allowed
to condense in the cylinder, then latent heats also have to be considered.

Gary

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

On Sat, 11 Sep 2004 23:08:43 +0000 (UTC), "Don Foreman" wrote:
I think the difference in gammas tells the tale.

I'm about 4000 miles from my Machery's Handbook just now, but I think change
in pressure and volume will dictate outlet temp as fn of input temp so
actual inlet temp for a given adiabatic expansion is immaterial --
providing of course that steam does not condense. Outlet temp can be
above or below ambient.

Outlet temp can't be below ambient unless external work is applied to
the piston to expand the gas below ambient pressure. Otherwise, you'd
be violating the Carnot limit.

Gary

Sure it can, if pressure is higher than ambient at ambient temp as in most
compressed air systems. Air tools get cold while in use, right? They
typically use vanes rather than pistons, but expansion is still going on.


"Gary Coffman" wrote in message
...


Outlet temp can't be below ambient unless external work is applied to
the piston to expand the gas below ambient pressure. Otherwise, you'd
be violating the Carnot limit.

Gary

On Mon, 13 Sep 2004 10:08:50 +0000 (UTC), "Don Foreman" wrote:
Sure it can, if pressure is higher than ambient at ambient temp as in most
compressed air systems. Air tools get cold while in use, right? They
typically use vanes rather than pistons, but expansion is still going on.

A compressed air system has external mechanical input to create higher
than ambient pressure at ambient temperature. A heat engine, such as
a steam engine, doesn't.

Gary

"Gary Coffman" wrote in message
.. .


Outlet temp can't be below ambient unless external work is applied to
the piston to expand the gas below ambient pressure. Otherwise, you'd
be violating the Carnot limit.

Gary

Right. Bob's original question had to do with an expansion engine
running on air vs running on steam, both coming from an external source at
given pressure without regard to how it got there. The Carnot limit,
dealing with efficiency, is irrelevant here.

"Gary Coffman" wrote in message
...
On Mon, 13 Sep 2004 10:08:50 +0000 (UTC), "Don Foreman"
wrote:
Sure it can, if pressure is higher than ambient at ambient temp as in
most
compressed air systems. Air tools get cold while in use, right? They
typically use vanes rather than pistons, but expansion is still going on.

A compressed air system has external mechanical input to create higher
than ambient pressure at ambient temperature. A heat engine, such as
a steam engine, doesn't.

Gary

"Gary Coffman" wrote in message
.. .


Outlet temp can't be below ambient unless external work is applied to
the piston to expand the gas below ambient pressure. Otherwise, you'd
be violating the Carnot limit.

Gary