Let's talk steam

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

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 Sw>

Joe sez:

"I have measured the output of my 2 cylinder model engine on both steam and

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

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,

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

Joe

"> Joe,

RichD sez:

"you really have answered your own question.

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.

calculations.

"Robert Swinney" snipped-for-privacy@comcast.net

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

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

At cutoff the

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.

calculations.

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:

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.

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

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

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

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.

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.

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, runn>