Water injection

[Schwanger]

34. We shall see the effectiveness of an intercooler.
    The efficiency of a intercooler relies on the following factors:

1. Frontal area.
2. Pressure drop across the cores (external and internal).
3. Road speed.
4. Air temperature differentials (external and internal).
   To simplify the calculations, given that there are so many variations in size and efficiency. We start with an intercooler with 50% efficiency. We will calculate how much heat is being removed for a given mass airflow based on a boost pressure ratio of 2 at 6000 rpm, using the same 2-litre engine. The result will be used to compare against a water injection system. Ambient air temperature is 25°C and the compressor exit temperature is 124°C. At 50% intercooler efficiency (reasonable assumption as both the engine and intercooler is operating close to their peak flow limits), the temperature drop across the inter-cooler should be:
   (124°C - 25°C)/2 = 49.5°C. The final air temperature entering the engine is 74.5°C.

35. We shall see the intercooling properties of water injection.
    Water temperature is 25°C and the compressor exit temperature is 124°C and Water flow rate of 250cc per minute. In the region we are working in the partial pressure of water vapour in the mixture is very low. We can assume that the enthalpy is a function of temperature alone within reasonable accuracy (e.g. for 250g /min of water and 8.64Kg/min of air, the partial pressure of water in about 0.06 bar). Consequently the final temperature is an inverse linear function of water proportional as nearest makes no difference. The calculation and results:
    At 200cc/min: 66.09°C
    At 250cc/min: 50.88°C
    At 300cc/min: 36.27°C
    So there we are, we have proved our point, water is just as effective as an intercooler. We decided to calculate a few more water flow rates so that a chart can be plotted, useful for visual prediction. Even at a modest flow rate of 200cc per minute, water is a better cooler than a 50% efficient intercooler by a long way. Notice at 350cc/minute of water, the final temperature dropped below the ambient of 25°C !!! there just isn't a 110% efficiency intercooler around yet ...

all of this was found at http://www.turbodynamics.co.uk/water.htm