Patent ID: 8733481

Claim:
A method of operating a hybrid powertrain having a battery, an engine configured to operate at an engine speed, a first electric machine configured to operate at a first machine torque and first machine speed, and a second electric machine configured to operate at a second machine torque and second machine speed, the method comprising: commanding an engine start, wherein the engine speed is approximately zero; commanding a spooling phase, including: accelerating the first electric machine with the first machine torque, such that the first electric machine begins rotating and the first machine speed changes from zero to non-zero, maintaining the engine speed at approximately zero, storing mechanical energy of the rotating first electric machine; accelerating the second electric machine with the second machine torque, such that the second electric machine begins rotating and the second machine speed changes from zero to non-zero, and storing mechanical energy of the rotating second electric machine; commanding a transfer phase, including: commanding an increase in the magnitude of the first machine torque, decelerating the first electric machine, such that the first machine speed moves toward zero, commanding an increase in the magnitude of the second machine torque, decelerating the second electric machine, such that the second machine speed moves toward zero, and transferring the stored mechanical energy of the first electric machine to the engine and transferring the stored mechanical energy of the second electric machine to the engine to increase the engine speed to greater than zero, wherein increasing the engine speed starts the engine; and balancing a power output of the hybrid powertrain during the transfer phase by one of charging the battery and discharging the battery.