Method for starting an engine of a hybrid powertrain

A method of starting an internal combustion engine in 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 first torque transmitting mechanism configured to operate at a first slip 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 speed to a non-zero speed,
• maintaining the engine speed at approximately zero,
• accelerating the first slip speed of the first torque transmitting mechanism, such that the first slip speed moves from zero speed to a non-zero speed, and
• storing mechanical energy of the rotating first electric machine;
• and
• 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,
• decelerating the first torque transmitting mechanism, such that the first slip speed moves toward zero speed, and
• transferring the stored mechanical energy of the first electric machine to the engine to increase the engine speed to greater than zero, wherein increasing the engine speed starts the engine.

A method of starting an internal combustion engine in a hybrid powertrain having a battery, an engine configured to operate at an engine speed, and a first electric machine configured to operate at a first machine speed, the method comprising:

• commanding an engine start, wherein the engine speed is approximately zero;
• rotating and the first electric machine such that the first machine speed changes from zero to non-zero and builds mechanical energy;
• maintaining the engine speed at approximately zero;
• and
• stopping rotation of the first electric machine, such that the first machine speed moves toward zero by transferring the mechanical energy of the first electric machine to the engine to increase the engine speed to greater than zero, wherein increasing the engine speed starts the engine.