I've always wondered whether a 'catalytic cracker'
could me made to separate the oxygen and hydrogen atoms from molecules of water, thus providing a perfect energy source for a modified combustion engine. No emissions, not even an exhaust pipe or air intake, and electrical hybrid power generated under braking to boost energy efficiency -but how would the governments tax it - by GPS beacon to record distance covered?
That said, if millions of cars were running that way, just imagining it was possible, then wouldn't we ultimately be depleting the oxygen from the atmosphere (although we'd not be drawing it direct from the atmosphere as is presently the case with cars which run on oil derived fuel)
I'm not a scientist, nor an economist, but I can imagine such a concept would bankrupt the middle-east...