Injecting prior to turbo comp' impellers

[hotrod]

I usually shut off my WI when temps get below about 55 deg F. I simply don't need it for knock supression at those temps with my current set up, and it makes the mixture too cold -- leads to compressor surge problems on my setup in cold temps.



The oxygen in the water is not available for combustion so you can ignore it. The methanol does reduce the oxygen demand slightly as it consumes less oxygen as it burns than an equivalent weight of gasoline.

The colder intake air mass flow does increase the amount of oxygen available to burn simply due to its increased density, but the percent oxygen in the intake air is essentially unchanged from normal atmospheric 21% O2. You can ignore the very small dilution effect of the extra water vapor.


For example if you have dry air (0% RH) at sea level pressure, 59.9 deg F and 29.9 in/hg air pressure and call that 100% air density, if you bring it up to 100% humidity, the density is now 99.35%.

The effect on engine power from humidity change is quite small. If you take an engine with intake air temp of 102 deg F, 0% RH, and absolute air pressure of 29.9 in/hg and call that 100% engine power. If you changed only the relative humidity of the intake air the engine should make 92% of its rated power. The dyno correcton factor for that humidy change would be 1.087.

Okay you lost 8.7% power, but if you now look at the temperature drop effect of Water injection on power output, and drop the intake air temp to 62 deg F. you get your power back. This would be due to the cooling effect of the water evaporation in the intake tract, you now get an engine power output of 102.2%. So you have a net gain in power of 2.2% in spite of the dilution effects of the water thanks to its intake cooling.

Obviously in the real world you'd never start at 0% humidity, but on the same token most of us get extra evaporative cooling from the methanol in the WI mix. The example does show that even in a worst case condition you get more back than you loose.

This is totally ignoring the advantages you get on a boosted engine with increased detonation threshold and being able to crank up the boost.

If you want to play with some numbers and read a fascinating web page on atmospheric pressure effects on engine power check out:

http://wahiduddin.net/calc/calc_hp.htm

Follow the links he has a TON of information on his web site.
In fact we probably should invite him to participate in this discussion.
I'm going to send him an email and see if he'd like to join in on this thread.



Larry