water injection and then some (long!)

[Chris Conlon]

Barry wrote:

>One point of contention however regarding the source of power from water
>injection.  I do not dispute what you state.  However, my understanding was
>that the inlet cooler charge allowed more fuel to enter the chamber which
>both keeps power high and reduces the possibility of detonation versus the
>situation where the inlet charge is hot and less able to hold atomized fuel.

Well, the cooler charge temp (and thus greater O2 mass flow) does
*definitely* help things. If you have high compressor outlet temps and
no other intercooling, you might get a power benefit even if all the
water did vaporize before getting into the cylinder. (Lots of vapor
would be formed, but lots of heat would be absorbed in the process.)
But consider the case in which still more water is injected, such
that it's near boiling before entering the cylinder. It will still
have absorbed a large amount of heat, though less than if it had all
evaporated, but produced very little steam. There's some trade-off
point and I honestly can't tell you how to calculate it beforehand in
the real world.  (Actually I suspect the peak power point involves
even more water, as Greg has suggested. The tremendous volume
expansion as a little liquid water turns into a lot of steam really
does provide a simple, direct way to recover a lot of the otherwise
wasted heat energy. Though I don't doubt you would run into various
other problems at high water:fuel ratios.)

As for the reduction of detonation, you definitely want the water
to be liquid droplets, not vapor. The high heat of vaporization plays
a large role in the anti-detonation effect. (Although it *may* be
true that charge air with an equally high amount of pre-evaporated
water would have almost as much anti-detonation effect, blah blah
blah, I personally don't feel this is the case, blah blah. And you'd
still get more power injecting water as liquid anyway.)


>In other words, I have always been led to believe that there are several
>things going on with water injection that ultimately benefit combustion.

There are definitely several things going on, and they all work better
if the water stays as liquid until entering the cylinder. The almost-
exception is the intercooling effect, which gives you more calories
absorbed per gram of water if the water vaporizes. But this
"efficency" is misleading, since *more* water will give you fewer
calories absorbed per gram, but *lower* intake temps and less charge
displaced by steam. And then give you the water-to-steam BMEP benefit
to boot!

I'll back off of my coarser droplets suggestion, and agree with
Bruce, that if you need a lot of intercooling a finer mist is
better. But do keep the water rate high enough to keep it liquid.
The reason that I suggested coarser droplets was in thinking of
a situation where you already have intercooling, and where
you're injecting well upstream of the port, thus at least some
of the water will have time to sit against hot metal in the
(closed) intake port. In this case larger droplets may make
somewhat less steam. Sucking heat from the ports/head, and
making steam, is a big lose because the head can source a lot
more heat energy than the compressor. You're never gonna cool
the head down much (barring insulating layers), and even if you
did it wouldn't be good. A larger droplet has more mass per
surface area, and so heats up more slowly than a smaller droplet.

In short: The intercooling effect is real and a benefit; there
are several different things going on here, and keeping the
water as liquid makes them all work better.


One mistake I made due to the late hour:

> Now if water/methanol formed a more stable azeotrope, in some ratio,
> that might give a higher heat of vaporization than either one alone.
> I think it's the other way round, though. Does anyone know offhand,
> if you mix a bunch of water and methanol, do they get hotter or
> colder? If they get colder, the heat of vap should be higher.

I got this backwards of course. If the mix gets *hotter* (releases
heat), the heat of vap will be higher. (Since you have to "pay back"
that heat to separate the molecules again.)


Ok, back to packing up...

   Chris C.