Speed controlling an electric machine of a hybrid electric vehicle

1. A hybrid powertrain comprising:

an engine having a crankshaft;

an electric motor including a rotor selectively coupled to the crankshaft via a disconnect clutch;

a transmission including a torque converter having an impeller fixed to the rotor and a turbine; and

a controller configured to, in response to the engine starting, generate a torque command for the motor that defines a magnitude that is based on a difference between a target impeller speed, that is based on a measured speed of the turbine, and a measured impeller speed.

2. The hybrid powertrain of claim 1 , wherein the torque converter further includes a bypass clutch configured to selectively lock the impeller and the turbine relative to each other.

3. The hybrid powertrain of claim 2 , wherein the controller is further configured to generate the torque command in response to the bypass clutch being open or slipping.

4. The hybrid powertrain of claim 1 , wherein the torque command further includes a feedforward component, and a feedback component that is based on the difference between the target impeller speed and the measured impeller speed.

5. The hybrid powertrain of claim 1 , wherein the magnitude increases in response to the difference between the target impeller speed and the measured impeller speed increasing.

6. The hybrid powertrain of claim 5 , wherein the magnitude decreases in response to the difference between the commanded impeller speed and the measured impeller speed decreasing.

7. The hybrid powertrain of claim 1 , wherein the controller is further configured to command the starting of the engine.

8. The hybrid powertrain of claim 1 further comprising a speed sensor disposed within the electric machine and configured to output a speed signal indicating the measured impeller speed.

9. A vehicle comprising:

an engine including a crankshaft;

a transmission including a torque converter having an impeller, and a turbine fixed to a turbine shaft that is driveably connected to driven wheels of the vehicle, wherein the torque converter further includes a bypass clutch configured to selectively lock the impeller and turbine relative to each other;

an electric machine including a rotor selectively coupled to the crankshaft via a disconnect clutch and fixed to the impeller;

a speed sensor disposed within the transmission and configured to output a speed signal indicating a measured impeller speed; and

at least one controller configured to, in response to a change in torque split between the engine and the electric machine, and the bypass clutch being open or slipping, generate a torque command for the electric machine that includes a feedforward component, and a feedback component that is based on an error between a target impeller speed and the measured impeller speed, wherein the target impeller speed is based on a measured speed of the turbine.

10. The vehicle of claim 9 , wherein the change in torque split includes starting of the engine.

11. The vehicle of claim 10 , wherein the controller is further programmed to command closing of the disconnect clutch in response to a request to start the engine.

12. The vehicle of claim 9 , wherein the feedforward component is based on a capacity of the disconnect clutch.

13. The vehicle of claim 9 , wherein the feedforward component is further based on a pedal position of an accelerator pedal of the vehicle.

14. The vehicle of claim 9 , wherein a magnitude of the feedback component increases in response to the error increasing.

15. A method of controlling an electric machine of a hybrid powertrain that includes an engine, a transmission, and a torque converter having a turbine, an impeller fixed to the electric machine, and a bypass clutch, the method comprising:

generating a command to start the engine; and

in response to the command to start the engine and the bypass clutch being open or slipping, generating a speed-control torque command for the electric machine that defines a magnitude that is based on a difference between a target impeller speed, that is based on a measured speed of the turbine, and a measured impeller speed.

16. The method of claim 15 further comprising, in response to completion of the engine starting, generating a torque-control torque command for the electric machine that defines a magnitude that is based on driver-demanded torque.

17. The method of claim 15 further comprising, in response to the bypass clutch being closed, generating a torque-control torque command for the electric machine that defines a magnitude that is based on driver-demanded torque.