WATER INJECTION...(was Holden...etc.)

[Danny Barrett]

G'Day again...
I've just finished with the rest of my email, so I can give a better answer
to the one I just ave a few minutes ago... This email is partially directed
to Ray, and partially directed to others. Note that although I may seem to
have quite a few answers, I have MANY more questions than answers, and I am
by no means an expert on this topic...

>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.

Yes, this is what I am saying. However, since I used the ideal gas law, I
could be wrong (as I stated in the original email). However, what I am
trying to say is that the partial pressure of the steam should be at its
maximum somewhere after TDC, so direct injection into the combustion chamber
should not be required.

>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.

Yes, more water will flash into steam (around 1400 times as much volume
taken up as water), giving more power due to its expansion - in reality, if
the water was steam before it was compressed back into water, it only uts
back what it had put into it, BUT it will also ABSORB extra heat, and hence
expand further - this is where the power increase occurs.

>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.

Absolutely correct. However, the ideal gas law will at least give a "feel"
for what is going on. Of course, it is no substitute for a good set of steam
properties tables...

>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.

Yes... That is the whole idea of it (amongst other things for other
reasons). Someone who I have had the pleasure of knowing for about a year
has had water injection systems (home brew) on his car (normally aspirated
6cyl, early '70's plymouth), and it gave him better fuel economy, and better
power (although I can't remember what he got just now). As for my experience
with it, I had a home brew system, but it wasn't set up very well - it would
either use too much, or not enough - it was very hard to strike the "happy
medium." However, on such a day that I got it right, my fuel economy in my
1980 GM Holden VC Commodore (Australia), I got 33 MPG (Imperial Gallons),
instead of my usual 26 MPG to 28 MPG (Imperial Gallons again). As you can
see, there was a considerable improvement. Of course, a lot of it would have
been due to nice driving conditions, but I have had similar conditions, and
had been driving with a similar "style," and all I would get is a maximum of
about 28 MPG. So, where did the difference come from? Probably the water
injection.

To get maximum power from a forced flow engine (Turbo, Super charge), I am
told you need about 30% to 50% of the fuel volume as water. This is mainly
as an anti-knock agent to "intercool" the air, and keep the combustion
cooler. However, I am inclined to believe that the maximum benefit for a
naturally aspirated engine would be derrived from injecting steam, as it has
already been heated through its latent heat of evaporation (a major absorber
of energy for water injection) and into the gaseous state (although it will
condense on the compression stroke, but will re-vaporise when the
compression is taken away, no matter what heat is or isn't transfered to
it). However, this would be no-good for turbo engines, as they need the
latent heat of vaporisation for its anti-knock properties.

For fuel efficiency, I am told that not much is needed (only about 3 litres
of water per 100 litres of gasoline), however, I am always interested in
other peoples opinions, as the broader experience of many people gives a
larger knowledge base than the "not so broad" experiences of a smaller
number of people. Anyway, for fuel economy, from what I can tell, you need
only a small amount (as stated above). It is best to have this "trace"
amount being injected all of the time, once the engine has "warmed up."
However, if you are going to use the water as an anti-knock agent, then it
is best injected in larger quantitied at high load/boost. Look through the
archive on water injection (as I did in the last few days). There is a lot
of stuff there, and a lot can be learned (from both the for and against
sides). For fuel economy, it might be an idea to boil the water first with
exhaust heat, and then inject the steamy air into the intake charge. What
are other peoples ideas on this? Also, there is the point that just because
it seems to work for one engine, it doesn't mean it will work for them all.

Danny Barrett.