Water injection will reduce the temperature of the combustion chamber. If you take an engine already tuned for the temperature of minimum emissions and inject water, then obviously you will upset that equilibrium.
If you took an engine properly tuned and just add boost (by boost, I mean increasing the pressure of the air at the intake manifold, which allows more oxygen into the combustion chamber), then you will increase engine knocking and raise the temperature of the engine, thus increasing NOx formation. If you then add water injection, that can balance things out again.
Obviously all of the above is an extreme simplification, and in addition, you have to deal with the case where there is no water to inject. Previously this would have been a real concern, but with modern computer controlled engines, it is possible to reconfigure the engine while it is running for some combination of reduced boost, less aggressive timings (including late intake valve timing, if your engine is setup for that; this effectively reduces the compression ratio of the engine), and richer fuel-air mixture.
Water injection will reduce the temperature of the combustion chamber. If you take an engine already tuned for the temperature of minimum emissions and inject water, then obviously you will upset that equilibrium.
If you took an engine properly tuned and just add boost (by boost, I mean increasing the pressure of the air at the intake manifold, which allows more oxygen into the combustion chamber), then you will increase engine knocking and raise the temperature of the engine, thus increasing NOx formation. If you then add water injection, that can balance things out again.
Obviously all of the above is an extreme simplification, and in addition, you have to deal with the case where there is no water to inject. Previously this would have been a real concern, but with modern computer controlled engines, it is possible to reconfigure the engine while it is running for some combination of reduced boost, less aggressive timings (including late intake valve timing, if your engine is setup for that; this effectively reduces the compression ratio of the engine), and richer fuel-air mixture.