Ok, I'm out on a limb here, but I did some thermodynamic calculations on water absorbtion of heat during compression at the turbo impeller. The calculation is in 4 parts and relates to my WRX STI 2.5 L turbo car.
Part 1 Assumptions and definitions
At 15-16 psi the engine is consuming ~250-275 g air/sec
Temperature of compression at 60% turbo efficiency, produces ~100 degrees C change at 7000 RPM
Heat of evaportation 540 calories/ gram of water
1 calorie = 1 degree C/ gram/sec at one atmosphere
1 calorie = 4.184 joules
Specific heat of nitrogen gas 0.25 calories/degree C (air is mostly nitrogen)
Part 2 Heat change under compression and amount of water to restore starting temperature
For 1 degree change in water temp we get 1 calorie*grams/sec.
For 100 C change (increase) of air (nitrogen) of 1 gram = 25 calories to heat 1g of air 1 degree C
540 calories/gram of water evaporated, heat of evaporation
Too cool 250 g air / sec heated 100 C costs 250*25 or 6250 calories / sec
6250 calories/ 540 calories/g water is 11.6 g water / sec = 11.6 ml water/ sec or 695 ml of water / min
Thus, 695 ml/min of water brings the compressor to isothermal compression with 100 C change in temp
during compression.
Part 3 Engine's consumption of air at 7000 RPM in a 2.5L engine
For my 2.5 L engine at one atmosphere and 7000 RPM, that's 3500 cylinder fillings per min,
or 2.5L * 3500, or 8750 L / min * 1000 ml/L =
8.8 million ml of air / min
At 15 psi of boost the amount of air is double or 17.5 million ml / min
695 ml/min cools 17.5 million ml/min air 100C or
0.004% (by volume) water to air ratio = isothermal compression with 100C temperature change
Part 4 Aquamist pre-turbo injection effects with my set up
100% stated "efficiency" of a turbo compressor is not isothermal, but adiabatic compression.
With Aquamist and a 0.5 mm jet I can inject about 125 ml / min, that is about 18% of 695 ml / min.
The turbo is operating at ~60% adiabatic efficiency at 7000 RPM, the 695 ml/sec water to brings it
to isothermal compression which corresponds to about to 118% efficiency at a pressure ratio of 2,
a difference of 58%.
18% (my 125 ml/min) of 58% is 10.5%
60% efficiency of turbo + 10.5% cooling from water = 70.5% efficiency of compression adiabatic.
250 ml / min water injection gives twice that or 81% compressor efficiency.
So with just 125 ml/min WI, compressor efficiency is near 70% at redline, quite a feat!
It's like having a 20% larger turbo.
I don't know the actual change in temperature during compression at 7000 RPM on my '04 Subaru STI with
the stock turbo. 100C is a reasonble guess, but it could be as high as 150C if the compressor efficiency
is lower than the predicted 60%. Thus, these numbers are only a rough estimate. While isothermal
compression would be nice to achieve, I can be happy with just extending the efficiency of the turbo
beyond it's normal bounds, and make up for the adiabatic heat by injecting more water after
the intercooler. Under this scenario maximum total water injection is 15% of fuel mass, very reasonable.
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