A power device for motor vehicles installed on a motor vehicle with an internal combustion engine is composed of a first variable displacement hydraulic activator (107), a second variable displacement hydraulic activator (108), a planetary gear mechanism (103) and a mechanical transmission (104). The power device further includes a hydraulic control device (109) and an electronic control device (112) which are used to control the first and second variable displacement hydraulic activators. The control device may carry out the following controls, that is, during the period when the engine outputs power, part of mechanical energy is converted into electric energy via the motor to be stored in the chargeable cell when the power requirement from the motor vehicle increases.