Turbos are rated in terms of efficiency of compression, but they should really say efficiency compared to adiabatic compression. At best a turbo in a narrow RPM range is 80% efficient with respect to adiabatic compression. So, there is 20% of the heat that is not merely the result of compression, but comes from somewhere else--friction, turbulance, heating of the turbo materials. As the turbo moves toward stall speed (the speed of sound) the efficiency drops off exponentially. As mach 1 is reached, not just heat but castistrophic sound waves disrupt the whole stability of the spinning wheel.
All the talk above in this thread about isothermal compression, is compression without the generation of any heat. Isothermal is a theoretical idea that cannot be achieved. However, if we can cool the air with water droplets as it is being compressed we may achieve a quasi-isothermal compression. At the outer limits of the impeller speed (near mach 1) the efficiency can drop to 50% or less of the adiabatic value.
For the sake of arguement let's say we could "recover" all the energy lost to heat at these near maximum speed. We could realize a 50% increase in compression with no heat loss. Effectively, our turbo would be 50% larger. That is a major accomplishment for the injection of a little water.
Practically, we will never cool compression this much, but even a 10% or 20% gain would make a huge difference in the amount of air mass available for combustion. For a 300hp engine, that's 30-60 hp gain. Many folks would kill for a gain like that.
Ideally, I'd like to experiment with the turbo off of the car. Compare the compression with and without water injection pre-turbo and see if compression is increased (i.e. air flow) and if post turbo temperature is reduced. Also in this ideal set up, we would want not liquid water (droplets) to survive traversing the turbo (not that that "overspray" as it is called is a bad thing, we are just not measuring the change in compression/heat only).
In the real world the best we can do is measure post turbo pressure and boost with and without injection of water. My hope is that I'll see more compression (higher boost) and lower temp (adiabatic and "friction losses" absorbed my water evaporation).
We are basically talking about running the turbo outside of it's efficiency range, using water to compensate for adiabatic and turbo losses. Like, the fear of water shut down that may destroy the engine, similarly the turbo may spin out of control if water were suddenly removed.
Engine management to prevent catastrophic damage is essential, since the computer can react much faster than we can. All attempts to operate at the edge of tuning involve some form of risk. Sensors, redundant pumps, auto shut off fuel or boost blowoff, would all lead to a safer system. This is one of the reasons I was so excited about the now indefinitely postponed EcuTek-Aquamist offering. For now, I'll be using the Element Hydra to design a system as best I can (one I'm sure will not be as good as a professional product).
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