the drivetrain switches to "series hybrid" when the energy level remaining in the battery pack is low or when the need arises for additional acceleration. in this mode the two electric motors are powered by the battery pack and the gasoline-powered generator. the vehicle can also switch to parallel hybrid mode. in this drive mode, the plug-in hybrid uses its full complement of available resources: the gasoline engine, the two electric motors and gasoline-powered engine generator. the "parallel hybrid" mode is the most commonly utilized when driving in a long-haul/high-speed steady-state cruise manner, such as on the open road or freeway. this is the most efficient drivetrain mode under these types of driving circumstances.