Sterling engines have been around for almost 200 years, although their use has been somewhat limited. They are usually reserved for high power applications that don't require much speed, but after some reengineering, Dean Kamen managed to install a sterling engine into a Th!nk City Car.
The sterling engine works on the theory of thermal expansion. As the air trapped inside is heated, its volume will increase, hence pressure will increase, and force a piston upward. Once the pressure reduces, the piston will return to the bottom of the "stroke" and the process will repeat.
The characteristics of the sterling engine make it very efficient when speaking in terms of piston engines. If you take the high level of efficiency and match it up with a wide range of fuel sources, you have a very flexible source of energy that can be adapted to almost any terrain and environment.
Details are still rather scarce as this is still very early in the development phase, but so far it looks promising. The system is currently being tested as the generator for the electric drive system on the Th!nk.