 The general layout of the initial production platform is considered by some to be a plug-in series hybrid design (since mechanical power initially drives the generator, which in turn charges the battery pack), despite its transmission design that makes it, by definition, a parallel hybrid.Power is drained from the batteries to run the electrical motors which move the vehicle.The internal combustion engine can run at a constant speed for both optimal efficiency and mechanical simplicity (i.e., there is no need for variable cam phasing).