Worth keeping an eye on.

The approved patent, for a Stirling Hybrid Electric Automobile, has seemingly doubled the efficiency that NASA achieved by changing the design. The inventor, Mr. McDowell, has introduced a heat exchanger that converts the hot exhaust gas directly into electrical current, which is fed back into the hybrid electrical system. The design is much like that of the Toyota Prius, except that there is a Stirling engine and not an IC engine.

via Articles: The Search for the 100-MPG Car.