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