Method for providing a controlled neutral in lieu of a conventional neutral in an electro-mechanical transmission

1. A method for controlling a powertrain system comprising an internal combustion engine, a first electric machine and an electro-mechanical transmission operative to transmit torque to a driveline, the method comprising:

in the presence of a controlled neutral operation of the electro-mechanical transmission being selected:

monitoring vehicle speed; and

only when the monitored vehicle speed is indicative of a low-speed zone, restricting a transition from a current engine operating state.

2. The method of claim 1 , wherein restricting the transition from the current engine operating state comprises:

limiting motor torque output from the first electric machine to zero only when the current engine operating state comprises an engine ON state.

3. The method of claim 2 , wherein the engine is operated at a desired engine idle speed.

4. The method of claim 1 , wherein restricting the transition from the current engine operating state comprises:

utilizing motor torque output from the first electric machine to maintain engine speed at zero when the current engine operating state comprises an engine OFF state.

5. The method of claim 1 , wherein the vehicle speed becomes indicative of the low-speed zone when the monitored vehicle speed comprises a magnitude that is less than a first speed threshold.

6. The method of claim 5 , wherein the vehicle speed is no longer indicative of the low-speed zone when the monitored vehicle speed comprises a magnitude that is greater than a second speed threshold.

7. The method of claim 1 , further comprising:

limiting motor torque output from a second electric machine to zero when the monitored vehicle speed is indicative of the low-speed zone.

8. The method of claim 1 , further comprising:

when the monitored vehicle speed is indicative of the low-speed zone:

monitoring a state of charge of an energy storage device;

allowing the state of charge of the energy storage device to deplete until reaching a minimum state of charge threshold; and

performing a key-off event when the state of charge of the energy storage device depletes below the minimum state of charge threshold.

9. The method of claim 1 further comprising:

forbidding engine starting and stopping events when the monitored vehicle speed is indicative of the low-speed zone.

10. The method of claim 1 further comprising:

forbidding charging of an energy storage device when the monitored vehicle speed is indicative of the low-speed zone.

11. The method of claim 1 further comprising:

forbidding an ability for the operator to request fluctuations in engine speed when the monitored vehicle speed is indicative of the low-speed zone.

12. The method of claim 1 further comprising:

coordinating torque commands between the engine and the first electric machine to establish a net zero output torque condition when the monitored vehicle speed is not indicative of the low-speed zone.

13. An apparatus for controlling a powertrain system including an electro-mechanical transmission of a vehicle in the presence of a controlled neutral operation comprising:

an internal combustion engine;

first and second electric machines;

an energy storage device;

an electro-mechanical transmission mechanically-operatively coupled to the internal combustion engine and the first and second electric machines adapted to transmit mechanical power to an output member that reacts with a driveline, the electro-mechanical transmission comprising:

first and second planetary gear sets each including a first element, a second element, and a third element,

a first clutch having one element coupled to the first element of the second planetary gear set and the other element coupled to a case ground, and

a second clutch having one element coupled to the first element of the first planetary gear set and the other element coupled to the first element of the second planetary gear set; and

a control module

monitoring vehicle speed, and

only when the monitored vehicle speed is indicative of a low-speed zone, restricting a transition from a current engine operating state.

14. The apparatus of claim 13 , wherein restricting the transition from the current engine operating state comprises limiting motor torque output from the first electric machine to zero when the current engine operating state includes an engine ON state.

15. The apparatus of claim 13 , wherein monitored vehicle speed becomes indicative of the low-speed zone when the vehicle speed is greater than a minimum entrance threshold and less than a maximum entrance threshold.

16. The apparatus of claim 15 , wherein monitored vehicle speed remains indicative of the low-speed zone when the vehicle speed is greater than a minimum exit threshold and less than a maximum exit threshold.

17. The apparatus of claim 13 , wherein the second clutch is always deactivated to decouple the first element of the second planetary gear set from the first electric machine when the monitored vehicle speed is indicative of the low-speed zone.

18. The apparatus of claim 13 , wherein the first clutch is one of:

activated to ground the first element of the second planetary gear set when the monitored vehicle speed is indicative of the low-speed zone, the first clutch providing a reactive torque to assist in establishing zero output torque from the transmission to the driveline; and

deactivated to un-ground the first element of the second planetary gear set when the monitored vehicle speed is indicative of the low-speed zone.

19. The apparatus of claim 13 , wherein the second electric machine provides a motor torque for maintaining a substantially zero slip speed across the first clutch.

20. The apparatus of claim 13 , further comprising the control module:

monitoring a state of charge of the energy storage device;

when the monitored vehicle speed is indicative of the low-speed zone, allowing the state of charge of the energy storage device to deplete until reaching a minimum state of charge threshold; and

performing a key-off event when the state of charge of the energy storage device depletes below the minimum state of charge threshold.