The IC engine (1) drives the main gearbox (4) via a planetary drive (2) coupled to the motor/generator (5). The processor control monitors the torque requirements of the motor/generator, to charge the battery and to drive the accessories and controls the engine torque for minimum fuel use ie. to deliver only the required amount of torque to the generator. At set times the power train is supplemented by torque from the electric motor/generator. The planetary drive has one input element (21) linked to the output of the IC engine and one output element (22) linked to the input of the main gearbox. The other element (22) is the reaction element linked to the motor/generator.
