Method and system for starting an internal combustion engine of a hybrid vehicle

A system for starting an internal combustion engine of a hybrid vehicle when the hybrid vehicle is powered by an electric machine, where the system comprises an internal combustion engine, an electric machine, and a transmission unit, where the transmission unit comprises a gearbox, where the gear box comprises a torque converter provided with a lock-up clutch, a second clutch and a transmission, where the internal combustion engine is adapted to rotate a drive shaft providing torque to at least one driving wheel of the hybrid vehicle via the transmission unit, where the first clutch connects the internal combustion engine to the input shaft of the gearbox, where the input shaft is connected to the torque converter, where the torque converter is connected to the second clutch connecting the torque converter to the at least one driving wheel through the transmission, where the input shaft further is connected to the electric machine, wherein the lock-up clutch is adapted to be engaged after the second clutch has been disengaged to a predetermined torque level such that there is a slip in the second clutch, that the internal combustion engine is adapted to be brought to a first rotational speed by the angular momentum stored in the torque converter by engaging the first clutch, and that the first clutch is adapted to be disengaged until the internal combustion engine has started and reached a second rotational speed.