water injection and then some (long!)

[Greg Hermann]

>Hi everyone,
>
>The following is all sliced together from various posts. I'm mainly
>replying to Barry, but have quoted many other people too in here. It's
>long and long winded... consider yourselves warned. :)

Hi Chris--thanks for the effort! Here I thought that only code heads did
lots of caffeine and worked real late!!
>
>
>
 I am wondering what range should be considered usable.
>> I read one study that worked with 5-15% based on mass.

I suspect that above 15% (by mass) is where you start to displace
significant amounts of air with steam. Also prolly where you get into a
true "wet flow" situation in  the intake manifold. It should get
SPECTACULARLY complicated with wet flow involving two immiscible liquids!!
All sorts of interesting hiccoughs under transitional conditions if you are
putting the water in out by the air cleaner or so!!
>

>As for the boost control: Have a small dedicated air pump and a solenoid
>valve to actuate the wastegate. Keep the wastegate FULLY CLOSED at ALL
>TIMES except when the turbo nears overspeed. (Yes you need a turbine RPM
>measuring setup.) For boost control, get a fly-by-wire throttle, put it
>*before* the turbo, and have the throttle follow the driver's foot except
>when boost is too high. (Thus putting boost control where it should be
>IMHO, but without burdening the driver.)

I have become enamored of the variable vane turbine turbos, but this sounds
like a neat idea. A readaptation of the mechanical  control Aerocharger has
developed for the turbine vanes would work for controlling the second
throttle pretty well, I think. But one of the really nice thing about the
variable vanes is that all the back pressure goes away at light loads!

Cooling the intake charge is great, but not at the expense of
>*displacing* large amounts of that intake charge with inert steam. Ideally
>you want the water to enter the cylinder just under the boiling point,
>but still as liquid. This is because the main power increase due to water
>injection (assuming constant boost) is not due to cooler charge but due
>to the large increase in cylinder pressure as a small volume of water
>flashes into a large volume of steam.

Don't be forgetting about the increase in net output due to absorbtion of
heat (and consequent reduction in work done) during the compression stroke.
>
 *And* while displacing only a small %
>of the intake charge, as long as the water stays as liquid. This is why
>Greg is so insistent that the water be injected near the valve, and timed
>with the valve opening. Water evaporating outside the cylinder = big lose.
>Think of it as accidental EGR.

Thanks--you made that clearer than I did!!
>
>
>> If the droplets are small enough however, won't this effectively be the same
>> deal?  Clearly we don't want puddling, and we need "vapor" (small airborn
>> droplets) that can take heat from the air charge to allow more fuel to be
>> dumped in (but not so much that the water displaces
(OXYGEN)
required
(TO BURN)
fuel).
>
>
>
>> >A last thought on evaporation.  The evaporation rate of water is very low
>> >below its boiling point 100c stp.
>>
>> Suggest reading a steam table. It all depends on temperature and PRESSURE.
>
>Water and/or methanol will produce significant vapor pressure well below
>boiling. Temperature is a measurement of *average* molecular energy, the
>peak of a bell curve. This is why a swamp cooler can cool air below
>ambient, why wind chill happens, and so forth.
>
>> Simply atomizing water will not cause it to change from a liquid to a
>> vapor - it takes 100c STP to accomplish that.
>
>Nope. And atomizing it will greatly speed the process. Finely divided
>materials dissolve and react more quickly. Consider how gasoline mist
>burns compared to a pool of liquid gasoline.

You need the right t/p conditions to evaporate anything. You also need
available heat from the liquid's surroundings to provide the latent heat
needed for evaporation. THEN, you need to transfer some of the available
heat into the liquid being evaporated. The larger the surface to volume
ratio of the liquid, the faster this heat can be transferred (and the
faster the liquid will evaporate). The smaller the droplets, the larger the
surface to volume ratio, so the quicker the liquid will evaporate. The
cooling effect is due to the taking of available heat out of the
surroundings.
>
>...
>
>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.
>Otherwise the addition of methanol is mainly as antifreeze.

Guessing here, but I bet on them forming the more stable azeotrope--it
takes an awfully big (tall) tower to completely dry out alky, and alky and
H2O have a really high affinity for each other--which is part of why
methanol is such a good fuel tank dryer outer.
>
>
Regards, Greg
>