Quickly. Normal compression is adiabatic, which to a first order means that as the gas is compressed, it gets hot. Now this heat is one of the biggest problems with forced induction, for 2 reasons. Firstly you have to get rid of the heat, and secondly you need to take power out the turbine shaft to perform the heating.( heat is work and work is heat).
Now with the right level of water injection, the heat is removed before it builds up, pushing the compression closer to isothermal (not all the way, but closer). In round terms this is about 30% more efficient (less exhaust gas required for the same boost, or more boost at the same exhaust flow).
problem 1 is that water on its own, whilst having a very high latent heat of vapourisation, doesn't have that good a saturation partial pressure. You can only put so much in before the air is at 100%RH. Above this the water will not cool the air until you compress it in the engine. However, if you add a second fluid, say methanol, Dalton proved with his law of partial pressures that the methanol doesn't know that the air is saturated with water vapour and goes on to vapourise as well. Add a 3rd, such as acetone (all available chemicals) and you get a 3rd tranch of cooling.
The net result is that you may be able to cool the air to slightly below ambient with the right mix. So your compressor becomes more efficient, you can throw away the intercooler, increasing your flow, and in some cases significantly improve the flow from turbo to inlet.
If you leave the intercooler in, you are generally warming the air back up again, so you have to take the plunge and remove it to gain the best benefit.
Its good, very very good.
Bill
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