[hotrod]

If the centrifugal forces don't mess up the mist, then that's one less headache.

Very little of the mist survives the path through the compressor in liquid form. In the discharge you have much cooler air at very high relative humidity but almost no water droplets.

Literally 10's of thousands of military aircraft engines in the period 1939 - through the early 1950's used before the compressor injection with no problems at all. It is also widely used in the tractor pulls, and on diesel trucks, CART race cars I understand, used a pre-compressor injection of some of their fuel for the same purpose, a handful of production cars and trucks have also used the system.

All you have to do is avoid large water droplets or a solid stream of water from impacting the impeller.

So such a setup wouldn't be boost-driven like std water injection, would it?
To realise the gains of near iso-thermal compression we'd want to inject water from zero boost, yes?

Depends on how you are defining boost driven --- some systems use a pressurized water reservoir so that boost pressure provides the motive force for injection. These systems are best for trucks pulling heavy loads up mountain passes where they stay on boost for extended periods of time. A presurize reservoir system will not react quick enough in my opinion for a high performance application.

Other systems use a pump supplied pressurized system with a valve of some sort that opens to allow spray at a certain boost pressure.

You generally do not want a continous on system (except for an rv or truck as mentioned above). On my system I have a pressure switch which activates at about 8 psi to turn on the spray. It is a simple on/off WI system. It will spray anytime boost is over the 8 psi threshold I have currently set.

As far as computing max water flow, your correct to establish a limiting case you would figure at max air flow which would typically be at max power rpm and a true engine VE of about .85-.88 for most engines.

However for detonation control your most critical flow rate is in the midrange max torque rpm range, say 3000-5500 rpm. That is where the engine has max VE and by definition you have max torque because you have max cylinder pressures. This is the RPM range where a sudden loss of WI can be fatal to the engine.

If you look at a manifold pressure log of a typical turbocharged engine when the turbo begins to make serious boost you go from near normal atmospheric pressure to a significant boost pressure >6 psi for example in just a fraction of a second ( ie only a 1000 rpm change in engine speed at WOT).

On my WRX on a 3rd gear WOT blast, I go from -1.2 psig manifold pressure at cruise, to 13.5 psig manifold pressure in 0.922 seconds when I just stab the throttle wide open, on the shift to 4th I go from 10.4 psig to -8.4 psig manifold pressure in 0.234 seconds as I close the throttle for the shift, and then jump to +5.9 psig .235 seconds later as I go back to WOT and another 235 milliseconds later (the time resolution of this log), I am backup to a manifold pressure of 11.9 psig. (this is all at 5800 ft altitude).

Total time under load in Low gear is about 1.5 seconds, 2rd gear was 3.828 seconds, 3rd gear 2.359 seconds, and 4th gear only lasts a couple of seconds before I'm over 100 mph.

So in a highway acceleration type situation, you would seldom see more than 7 - 8 seconds of flow, with brief interruptions of about .5 second every 2 - 3 seconds, so your actual duty cycle for the spray activation will be something like 80% or so. In my case I have an accumulator in line with the solenoid so the system pressure changes very little over this short interval.

If you are looking for detonation supression you need it most in the midrange rpms near the engines torque peak. That is when the engine is most likely to detonate under high engine load. You want the WI to kick in just below the rpm range and boost pressure that you can first experience detonation under heavy load. If you turn it on early you cool the exhaust gasses just as the turbo needs hot EGT's to help it spool up. So in that case you delay the WI spray as long as you safely can.

If your looking for maximum efficiency of the compressor, you want the spray to come on at a boost pressure when the compressor is just moving out of its maximum effeciency island on the compressor map. What you are doing is artificially stretching the compressor map to the right so the compressor has a wider island of efficiency.

In my case I will eventually be using a compound system with some spray pre-turbo for the purpose of maximising the compressors efficiency, and if I need it, a secondary jet spraying pre-throttle body to control any detonation that is not supressed by the pre-turbo injection.

Larry