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

[Zack]

Danny,


I think I disagree on the point of whether water injected at the 
intake will stay in the liquid phase during compression.  I think the 
source of our disagreement would be, as you say, the fact that you're 
using the ideal gas law to calculate pressure.  You can only do this 
if the compression is isothermal.  In reality, the compression phase 
occurs quickly enough that it would be more correct to use an 
isentropic model.  In that case, the temperature of the gas is given 
by:

T  = T0 * ( V / V0 ) ^ (gamma - 1)

Where gamma for air is about 1.4.  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 
vaporization will absorb a large quantity of heat which would have 
otherwise gone to raising the temperature of the air, resulting in a 
cooler air charge at TDC.  
	If you were to vaporize the water prior to injecting it into the 
intake, you would lose the cooling benefits of injecting the water as 
a liquid.  In that case, injecting water vapor would be just like 
injecting any other inert gas.

Zack

> 
> >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
> 
> Yes, more water will flash into steam (around 1400 times as much volume
> taken up as water), giving more power due to its expansion -