US Patent 7240748 Hybrid vehicle

In a hybrid vehicle in which front wheels are driven by an engine and a front motor, and rear wheels are driven by a main rear motor and a sub rear motor, both main rear motor and the sub rear motor are driven when a large driving force is required as when the vehicle is driven at low speed to haul a trailer or the like or to run on a sandy ground, whereas when the required driving force is small, only the main rear motor is driven.