Presented at the recent SAE World Congress in Detroit, the idea from Toyota Central R&D Labs Inc. and involves what is called a Free Piston Engine Linear Generator (FPEG). Think of it as a sort of one-cylinder, two-stroke mini-engine that can work either as a generator (thank to magnets and a linear coil) or to directly drive a vehicle. The current prototype is a 10-kW unit that Toyota say would provide enough power to get a B- or C-segment electric vehicle up to highway speeds (75 miles per hour) when paired up to offer 20 kW. Pairing the FPEGs is also important to minimize vibrations. One system tested by Toyota had a 42 percent thermal efficiency, but the engineers are working to improve the overall efficiency even further.
You can watch an animated video of the piston in action here (click on "Outline") and see the SAE papers here and here. More technical details are available at Green Car Congress.