What are your planned results?
Simply spraying water randomly into the exhaust is a no-starter.
The only answer that makes sense is an alternator driven pump.
And IF you're going the roundabout route of spraying the exhaust instead of the intake, you want to spray it before the EGR valve to maximize cleaning (clean the exhaust, clean soot out of the EGR... big issue for diesels driven in traffic or primarily at low rpms) and to increase the benefits of EGR cooling (Mazda specifically uses cooled EGR for its SkyActiv motors).
Also, the obsession with hydrogen. Dude, you need to set your practical goals, decide what you want to do first and what is most cost effecitve, and do it. Having both water injection and hydrogen injection means twice the complication and twice the added hardware.
Keep it simple. Do one thing. Test. Evaluate results. Evaluate cost-effectiveness. Then do another.
I specifically cited Mazda because this is the kind of research you should be doing. Research what the OEMs are doing. Research what the racing teams are doing. They do those things because they work. Sometimes, they won't use them specifically because of the expense (meth injection) or because of the added complication (cooled EGR... but it's now becoming more common).
Try to note what OEMs DO NOT research. And try to figure out why...
