Water injection systems were mostly used by racers to allow higher compression ratios on the available octane rated gasoline. The water cools the combustion chamber, preventing precombustion (knocking, pinging). It was drawn into the intake manifold, often controlled by a vacuum switch(es, sometimes it was a two stage system.) Water can be slowly poured into the intake to steam clean the carbon buildup from the combustion chamber. We used to do it to clean up engines that aren't driven hard enough, or long enough to completely burn the fuel charge. Too much carbon buildup will retain heat and glow, preigniting the fuel, causing it to burn uncontrollably. The flame front should travel smoothly across the combustion chamber. If another flame front occurs at the other side of the chamber, the two will collide, creating the noise we know as pinging or knocking. Nitrous oxide is also used to allow a very large amount of fuel to be burned. An adapter plate would be installed under the carburetor with two nozzles. one for the gasoline, one for the nitrous oxide. In gasoline or diesel engines, most of the heat is transmitted to the cylinder head, as opposed to the cylinder wall. A diesel engine doesn't really need all the heat. It will run while it is still warming up. Injecting water after the fuel is burned sounds like a great idea to me. As the water flashes to steam and expands, it will have to push the piston down, because that is the only part of the combustion chamber that will move. It should also keep the combustion chamber clean.
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