Fw: Fw: Holden Commodore, Aerodynamics. AND NOW: WATER INJECTION...

[Danny Barrett]

I see your point about the ideal gas law. Yes, it is only an approximation
to a real gas, but the "real gas law" is a bit harder to use if you can't
find the various extra constants, so the ideal gas law is better than
nothing. However, as you say, a steam table would be by the best idea, as
these are made up from actual experimental results, and reflect the real
characteristics of steam.

Danny Barrett


>I follow your reasoning, but I don't follow your use of the ideal gas
>law.
>
>You are saying (correct me if I am wrong) that the pressure at TDC is
>high enough to keep the water in the liquid phase because the boiling
>point is so high at that pressure.
>
>As the piston drops and the pressure tries to fall, more water will flash
>into steam, keeping the pressure higher than it would be without the
>water.  This will increase the power output.  The trick is to inject the
>proper amount of water.  You would want to run out of water on the way
>down so that the pressure can drop before the exhaust valve opens.
>
>To get an idea if this will work, you'll have to look at a steam table. 
>That table lists the boiling point of water at various pressures.  You
>can also get the mass density of the steam at various temperatures and
>pressures.  You would normally use the ideal gas law (PV=nRT) to
>calculate this, but steam isn't an ideal gas until you get well above the
>boiling temperature at that pressure.
>
>Even nitrogen and oxygen aren't ideal gases at room temperature and
>pressure.  They're really close, though.  We did an experiment in
>chemistry class where we did some measurements and compared them to the
>ideal gas law.  They were off by a few tenths of a percent.  We then used
>Van Der Wal's non-ideal gas formula to calculate the expeted pressure and
>volume and found it to match the ideal gas law to within about six or
>seven significent figures.  The difference was trivial compared to our
>experimental error.
>
>That isn't the case with steam.  The differences are significent.
>
>I'm sure that someone on the list can tell us what the typical and
>maximum temperatures and pressures are at TDC.  Also, maybe someone can
>point us to a steam table somewhere on the web.
>
>Hmmm... I just thought of something:
>If injecting water results in a lower exhaust temperature, more heat will
>be used to power the engine and less will be wasted out the exhaust.  If
>the water is vaporizing as the piston drops (as explained above), the
>exhaust temperature will be lower because the water is absorbing the
>heat.
>
>
>Ray Drouillard, BSEE