water injection

[Chris Conlon]

Hi,

Zack wrote:

> I think I disagree on the point of whether water injected at the 
> intake will stay in the liquid phase during compression.
> ...
> So, if we start with a charge of 
> 80 degree F intake air, and compress it by a factor of 8.5:1, the 
> temperature of the air at the end of the compression phase will be 
> more than 800 degree F at the end of the compression stroke.  
>
> At 80 degrees, the vapor pressure of water is about 0.003 atm.  My 
> steam tables only go up to 705 degrees (since that's the critical 
> point), at which point the vapor pressure is a whopping 218 atm.
>
> 	So... I think there's no question that if you inject water in the 
> intake, all of that water will have turned to steam before the 
> completion of the compression stroke.  In the process, that 

I think this is close, but not quite there. The water will absorb
a large amount of the heat energy due to compressing the charge,
and thus the temperature rise will be less than calculated. Possibly
*way* less depending on how much water. (OTOH the pressure rise will
be more, due to water vapor, etc etc.)  But as long as all the water
is liquid before the intake valves close, and vapor after combustion,
I don't think it makes a difference. (Guess I ought not nitpick.)

It seems like we're looking at 2 different things. First the use of
water to (inter)cool the intake charge, the goal being to get more
air mass into the cylinder. The coolant water should stay liquid,
as much as possible, because if it turns to steam (and expands
tremendously), it'll just displace air from the cylinder. (As someone
said, like adding an inert gas to the charge).

Once inside the cylinder, you want the water to still be liquid,
but as close to boiling as possible. This way it'll evaporate and
expand, giving you the desired pressure rise, but take up a minimum
of your combustion heat to do so.

How to do this? Research project :)

(As an aside, I wondered the other day about heat loss through the
cylinder walls, and the loss of energy due to that. I have heard of
ceramic coating pistons, is there anything similar that can be done
to the head and cylinder walls to keep heat in the chamber and thus contributing energy to the crank? Or would this push cylinder wall
temps so high that detonation would be unstoppable?)

   Chris C.