Hybrid powertrain with geared starter motor and belt alternator starter and method of restarting an engine

1. A method of restarting an engine of a vehicle comprising:

determining via a controller vehicle operating conditions requiring a restart of the engine following cutoff of engine fuel during an automatic shutdown subsequent to a key start of the engine;

determining an operating parameter of at least one of the engine, a first energy storage device, and a second energy storage device; wherein the first energy storage device is operable in a first range of operating voltage having a first minimum voltage and a first maximum voltage; wherein the second energy storage device is operable in a second range of operating voltage having a second minimum voltage and a second maximum voltage; wherein the second maximum voltage is greater than or equal to the first minimum voltage and less than the first maximum voltage;

placing a switching device in an off-state to operatively disconnect both a starter motor and a motor/generator from the second energy storage device if the operating parameter is different than a predetermined reference parameter by at least a predetermined amount;

energizing a solenoid to thereby operatively connect the starter motor with the engine through a gear train; and

after said energizing the solenoid, powering the starter motor and operating the motor/generator as a motor to restart the engine.

2. The method of claim 1 , wherein the controller is operable to cause the motor/generator to be powered as a motor during the entire restart.

3. The method of claim 1 , wherein the controller is operable to cause the motor/generator to be powered during the restart only after the starter motor has caused the engine to reach a predetermined speed or after a predetermined amount of time has passed since the starter motor was powered.

4. The method of claim 1 , further comprising:

determining vehicle operating conditions indicative of a vehicle deceleration;

placing the switching device in the off-state if the switching device is in the on-state; and

operating the motor/generator as a generator to provide power to the first energy storage device and thereby slow an output member in a regenerative braking mode.

5. The method of claim 1 , further comprising:

determining vehicle operating conditions indicative of a need for vehicle acceleration after engine fuel cutoff; and

operating the motor/generator as a motor to accelerate the vehicle and extend engine fuel cutoff.

6. The method of claim 1 , wherein powering the motor/generator includes modulating torque provided by the motor/generator at a compression pulse frequency of the engine.

7. The method of claim 1 , further comprising:

determining vehicle operating conditions indicative of initiation of the key-start of the engine; and

closing the switching device and powering the starter motor as a motor when the key-start is initiated.

8. The method of claim 1 , further comprising:

determining vehicle operating conditions indicative of vehicle deceleration;

operating the motor/generator as a generator to provide power to the first energy storage device and to the second energy storage device to thereby slow an output member of the powertrain in a regenerative braking mode;

placing the switching device in the off-state when the second energy storage device reaches a maximum voltage level; and

reducing or ceasing operation of the motor/generator as a generator when the first energy storage device reaches a maximum voltage or maximum state of charge.

9. The method of claim 1 , further comprising:

determining vehicle operating conditions require acceleration of the vehicle while fuel is shut off to the engine; and

controlling the motor/generator to function as a motor to provide torque at a crankshaft of the engine.