Green Car Congress: GM and U Mich receive patent on plug-in series hybrid/extended range electric vehicle powertrain using multiple free piston linear alternator engines)
Wow - what a radical design! There's no mechanical connection to the piston - it generates electricity directly by moving magnets past coils. And spark plugs at each end. Cool!
More info on the concept: Wikipedia - Free-piston engine Certainly a different series-hybrid-only approach, as opposed to the traditional ICE powerplant in the Volt and Karma.
It's an advantage of PHEV that as long as the motor is powerful enough engineers are free to use a generator instead of an engine and therefore they have greater flexibility. Potentially, anyway. They do need time and money. ;-)