the most cockamamie one I had pitched to me lately was a car running on "brown's gas" - stoichiometric hydrogen and oxygen from electrolysis - so far so good. But then he wanted to use the alternator to do the electrolysis as you drove, replenishing the gas - so all you added was water.
He couldn't understand that even if your engine was 100% efficient, you'd still have to add energy equal to the work done in moving the car around.