WATER INJECTION

[Danny Barrett]

Well Gents, it seems we have a difinative answer. Thankyou Zack. So, now
that it has been shown that the steam WILL stay vaporised (which was the
opposite of my prediction), can anyone come up with anything that will show
whether the steam's maximum partial pressure will be before or after TDC?
Also, what if we were to inject the water at say 90 deg C (194 deg F). It
has not yet boiled, but it has gone nearly all the way to boiling, so the
energy completely lost due to latent heat of evaporation will be lowered as
much as possible? This way, we will not be injecting an "inert gas" but a
liquid that is just about ready to flash to a vapor. Please bear in mind
that the water would be heated by the exhaust manifold.

Danny Barrett.

>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 -
>
>