There's a lot of heavy duty R&D going on these days to reinvent the engine or replace the engines we're using altogether. The free piston engine, though invented in the 1920s, is getting more attention because it is less expensive than fuel cells for generating electricity while potentially just as efficient.
The free piston engine has a straight rod that moves back and forth with a piston and 2 stroke combustion chamber on either end. The central portion of the rod has magnets attached which move past stationary coils to generate electricity. This creates an opposed 2 stroke, every stroke is a power stroke generating electricity. Used in place of a fuel cell, you still power the wheels with an electric motor. The engine could approach 50% efficiency which is about twice as efficient as standard internal combustion engines.
The potential high efficiency of free-piston engines gives them an advantage over conventional generators, and their ability to use a variety of fuels is an advantage over hydrogen fuel cells. What's more, free-piston engines don't require expensive materials such as the platinum catalysts needed in fuel cells, so they could be cheaper too.
Besides being a cool idea and having high efficiency, since it is a combustion engine, it gets around the silent electric vehicle problem which some say is a problem for pedestrians. Neat!