Electrolysis of water is not a very energy efficient process. So any device that splits water molecules on the fly from power taken from your engine isn't going to give you back any more energy.

That said there my be a use for stored hydrogen as a power booster on a diesel.

It was researched for use in airships

go here
http://ntrs.nasa.gov/search.jsp

search for
NACA-TR-535

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white 79 300SD 200K'ish miles "Farfegnugen" (RIP - cracked crank)
desert storm primer 63 T-bird "The Undead" (long term hibernation)

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