I've thought about that stuff as well. especially when I'm choking down fumes stuck in rush hour. Unfortunately, There is no MACHINE that can generate more energy than it uses.
On a very practical note - It's sad that we can't get cars like the Volkswagen Lupo over here (America). Instead the Europeans get to benefit from the small VW diesel which gets over 80mpg. And that's old technology - not even hybrid.
Some other favorites: