Method of controlling a hybrid propulsion system of a vehicle

A method of controlling a hybrid propulsion system of a vehicle, wherein the hybrid propulsion system includes an in-series arrangement having an internal combustion engine, a first clutch, an electric motor, a second clutch and a gearbox having a first input shaft and an output shaft connected to drive wheels of the vehicle, said first clutch being arranged between the internal combustion engine and the electric motor, and said second clutch being arranged between the electric motor and said first input shaft of the gearbox, wherein said hybrid propulsion system is in an initial operating state where the internal combustion engine is turned off, the electric motor is active and said first clutch is in an open condition, whereby connection between the internal combustion engine and the electric motor is interrupted, the method comprising:

• said gearbox including a second input shaft and said hybrid propulsion system including a third clutch arranged between the electric motor and the second input shaft,
• a first operating mode of starting the internal combustion engine by the electric motor excluding actuation of a gear shift of the gearbox until start of the internal combustion engine is completed, wherein said first clutch is progressively closed, so that the internal combustion engine is driven in rotation by the electric motor, while at least one of the second and third clutches is set in a slip condition, so that the electric motor provides a torque higher than any torque transmitted to at least one of said corresponding first and second input shafts,
• a second operating mode of actuating the gear shift excluding start of the internal combustion engine until actuation of the gear shift is completed, wherein each gear shift is actuated by opening one of the second and third clutches and closing the other of the second and third clutches, and wherein said gearbox actuates a variation of a transmission ratio between said input shaft and said output shaft,
• a third operating mode which actuates in parallel the start of the internal combustion engine and the gear shift,
• passing from the first operating mode to the third operating mode even if the start of the internal combustion engine has not been yet completed, or passing from the second operating mode to the third operating mode even if the actuation of the gear shift has not been yet completed, so that the transition from one operating mode to another can be freely actuated at any time, depending upon the operating conditions of the hybrid propulsion system.