Ask HN: Running a car on water. Plausible or just more bad internet science?

I've got a friend who's pretty smart and a good programmer, who keeps going on about building a car to run on just water. He's convinced it's possible and that other people have done it but that there's a big conspiracy by big oil to keep it covered up.

I've sat through all the videos he's shown me, and I've read countless websites and other documents he's supplied, but so far I haven't seen anything like what I'd consider proof.

I try and keep an open mind, and I'd love for this to be possible, but all I keep seeing is bad science, appallingly bad science and stuff bordering on outright fraud. The basic premise behind most of these claims is a means of converting water to hydrogen and oxygen, that requires less energy that what's obtainable from burning the hydrogen.

So here's my question: Does anyone here know of any evidence that supports this? Do you know anyone who's played with this, or of anyone, anywhere, who has set up a simple closed system to prove it?