Hybrid vehicle braking system and method

1. A vehicle comprising:

a powertrain having an electric machine;

a torque converter in connection with the powertrain configured to decouple the electric machine from a vehicle axle in response to vehicle speed reducing to less than a second threshold;

friction brakes configured to resist rotation of a wheel; and

at least one controller programmed to (i) issue a command for the electric machine to apply a regenerative torque to decelerate the vehicle in response to driver braking demand, and (ii) cause both a decay in the regenerative torque and a countervailing increase in the resistance of the friction brakes when vehicle speed is reduced to less than a first threshold such that braking demand is substantially satisfied by only the friction brakes when the vehicle speed is less than the second threshold.

2. The vehicle of claim 1 wherein the regenerative torque is limited by at least one of a braking stability limit, a powertrain regenerative torque limit, and a blend-out limit, and the blend-out limit varies from a current regenerative torque value to approximately zero when the vehicle speed approaches the second threshold.

3. The vehicle of claim 1 wherein a difference in speed between the first and second thresholds varies based on a rate of vehicle deceleration.

4. The vehicle of claim 1 wherein a difference between the first and second thresholds is a constant.

5. The vehicle of claim 1 wherein the first and second thresholds vary based upon a transmission gear ratio.

6. The vehicle of claim 1 wherein a rate of the decay of the regenerative torque and a rate of the increase in resistance of the friction brakes each vary based on vehicle deceleration.

7. A method of controlling deceleration of a vehicle having an electric machine comprising:

applying a regenerative torque from the electric machine to a powertrain torque output to satisfy a driver braking demand;

decoupling the powertrain output torque from a vehicle axle when vehicle speed is reduced to less than a first predetermined threshold;

decaying the regenerative torque such that a magnitude of the regenerative torque is approximately zero when the powertrain output torque is decoupled; and

increasing a friction brake resistance at wheels of the vehicle such that driver braking demand is satisfied by only the friction brakes when the regenerative torque is approximately zero.

8. The method of claim 7 wherein the decaying of the regenerative torque occurs at a rate that is countervailing to a rate of increase of the friction brake resistance.

9. The method of claim 7 wherein decaying the regenerative torque is initiated in response to vehicle speed decreasing to less than a second buffer threshold that is greater than the first predetermined threshold.

10. The method of claim 9 further comprising modifying a difference between the second buffer threshold and the first predetermined threshold based on at least one of a transmission gear ratio and a deceleration rate.

11. A vehicle comprising:

an electric machine configured to apply regenerative torque during a regenerative braking mode;

a torque converter to decouple the electric machine from vehicle wheels;

friction brakes to resist vehicle wheel rotation; and

a controller programmed to transition out of regenerative braking mode prior to decoupling the electric machine, wherein regenerative torque and friction brake resistance collectively satisfy braking demand during the transition and, regenerative torque is approximately zero after the transition.

12. The vehicle of claim 11 wherein the torque converter includes an internal bypass clutch.

13. The vehicle of claim 11 wherein the decoupling of the electric machine from the wheels occurs in response to vehicle speed decreasing to less than a first predetermined threshold, and the controller initiates the transition out of regenerative braking mode in response to vehicle speed decreasing to less than a buffer threshold speed that is greater than the first predetermined threshold.

14. The vehicle of claim 13 wherein a difference between the buffer threshold speed and the first predetermined threshold varies based on at least one of a transmission gear ratio and a deceleration rate.

15. The vehicle of claim 13 wherein a difference between the buffer threshold speed and the first predetermined threshold is a scalar value.

16. The vehicle of claim 11 wherein the controller is further programmed to issue a command to increase the friction brake resistance at a rate that is countervailing to a decay in regenerative torque during the transition out of regenerative braking mode.