Water engine

So i've heard this myth or conspiracy theory my whole life,
some guy invented an engine that runs on water via separation of hydrogen from oxygen and combustion with said molecules. then the gas companies had him killed to ensure that people still bought gas
i just wanna know had anyone else heard this, and are there any science majors who could validate or dispel this? not the murder part im sure the rich could pull that off, i mean the engine.

ROFL i've heard the same, but i dunno if thats possible i sounds kinda futuristic

Actually I'd like to know the validity of this too. we should call mythbusters cause I've heard this about a million times. it seems very possible but I'm no chemist or physicist.