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 an 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 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:

• a first operating mode of starting the internal combustion engine by the electric motor, wherein said first clutch is progressively closed, so that the internal combustion engine is driven in rotation by the electric motor, while the second clutch is set in a slip condition, so that the electric motor provides a torque higher than torque transmitted to said input shaft,
• a second operating mode of actuating a gear shift, wherein said gearbox actuates a variation of a transmission ratio between said input shaft and said output shaft,
• wherein said first operating mode includes starting the internal combustion engine excluding actuation of the gearbox until start of the internal combustion engine is completed,
• wherein said second operating mode includes actuating the gear shift excluding start of the internal combustion engine until actuation of the gear shift is completed,
• 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.