http://gm-volt.com/2010/09/27/gm-patent-application-may-be-for-the-chevrolet-volts-transmission/ This is confirmed to be the patent for the drive train. Planetary gear but it has three clutches. The traction motor can be directly coupled to the wheels with ICE and generator disengaged completely. Generator is coupled over 70mph to allow 101mph in EV mode probably by lowering the effective ratio for the traction motor to the wheels. ICE can run the generator decoupled from the planetary gear which pumps power into the traction motor as needed to help run the car when the battery is low but apparently sometimes the ICE/Generator is also coupled directly to the wheels to aid in propulsion. It is confirmed that going up steep hills with the battery discharged yields a 40mph top speed.