Water injection on diesel is effective on controlling excessive exhaust gas temperature. In most case, it is not needed untill extra fuel is added to boost power.
On the older type of diesel engine with mechanical injection, upping the boost pressure will not yield more power because the fuel flow remains the same. But on the modern diesel enigne, all sensors are intergrated so when boost is increased, flow enrichment is automatic. In those application, water injection is more useful to keep the engine cool and knock free.
It is not as easy to be specific what signal should the water injection follow. The majority of systems avaible on the market follow boost pressure without RPM reference. I don't know if this type of method can be improved by referencing the IDC and common-rail pressure.
The direct-injetcion Gasoline engine is almost the same as the common-rail direct injection diesel system.
In answering your question, the biggest usage for water injection on diesel engine is keeping the incylinder combustion temperature stable. Any excess heat (destructive) is absorbed water through evaporation, keeping the cylinder pressure high (more torque) without the penalty of the high temperatures. It is almost liken to a having a secondary engine powered by steam.
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Richard L
aquamist technical support
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