intake charge cooling
There are some differences between diesels and gasoline engines so that needs to be considered. As far as VE increase is concerned, it is clear that charge air cooling due to evaporation helps inprove VE all else being equal.
In one of the NACA studies they tried injecting the WI mix both well before the intake valves and immediately before the intake valve. The postion that was "pre-vaporization tank" was several feet from the intake valve and the other injection point was only inches from the intake valve.
Study E5E18 page 5.
"For vaporization-tank injection, the mixture temperature decreased as more internal coolant was injected until the fuel-air mixture became saturated (fig 12) and then the mixture temperature remained relatively constant. In a corresponding manner, the power at peak-power spark advance increased because of the increased charge weight inducted into the cylinder until the internal coolant-fuel ratio for saturation was reached ( fig 13), at which point the power leveled off. After complete saturation of the incoming mixture with the internal coolants, any additional cooling of the mixture must occur after the intake valve closes, which makes it impossible to increase engine power through an effect on air flow. From this point, the power obtainable at peak-power spark advance dropped slightly as the internal coolant-fuel ratio was increased because some heat of vaporization was extracted from the air during the compression stroke, resulting in a decrease in cycle efficiency. When the internal coolant was injected at the manifold elbow, the power increase at peak-power spark advance was relatively small (fig 13) because there was insufficient time for charge cooling before the intake valve closed."
As you can see on a gasoline engine the intake charge cooling due to evaporation of the WI mixture can have an effect on maximum cylinder filling and hence power up to the point you reach 100% humidity (saturation) of the intake charge, AND if there is suffecient time for the evaporation to take place. Spraying beyond, that will increase the cooling effect on the engine due to reduction in combustion chamber temperatures, but at a slight cost in power. You might be able to recover that loss in power by upping the boost more to levels that couldn't be reached at lower injection rates, so you need to consider the fact that you are not only changing cylinder filling but also how much boost, and spark advance you can get away with. This interaction of multiple variables means you have a very complex system to optimize.
Now in your case, your dealing with a diesel, which has too much air to begin with, so the power limit is mostly determined by the amount of fuel available to burn and maximum EGT numbers, and how much cylinder pressure you can tolerate before you start blowing head gaskets or lifting the heads.
If you went with a WI mix that was pure water or low in alcohol content you would maximize cooling at the cost of some reduction in the power gain possible. If you upped the alcohol content you would be increasing the fuel load, and power.
If you increased the alcohol content injected by overspraying a lower alcohol mix, you should be able to get both a power increase and a strong cooling effect on the combustion temps. Only some experimentation would tell you the ideal balance of the variables. For testing purposes you may want to experiment with a 2 nozzle system where you inject water alcohol from one and straight water from the other. That would allow you to sort out the ideal total injection weight of water and alcohol for your goals.
My guess is that on diesels you will need to spend more time sorting out how much extra fuel you can accept (ie alcohol ) to reach your power goals without blowing head gaskets and then control cooling by increasing or decreasing the total WI spray mix and increasing or decreasing the alcohol fraction so you don't exceed your max additional fuel level.
As far as engine cooling due to WI, this paragraph is worth noting.
NACA report 756 ( page 71 , Conclusions item 3.)
Water injection had a marked cooling effect on the engine head and cylinder. The exhaust-valve guide was the only point on the head at which the temperature showed a tendency to increase with indicated mean effective pressure. The temperature was less, however, than that obtained with a straight fuel permitting equivalent power.
Larry
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