Brake control for stop/start vehicle

1. A vehicle comprising:

an engine adapted to shutdown and restart during a drive cycle;

a first brake system adapted to generate a first brake torque;

a second brake system adapted to generate a second brake torque; and

a controller programmed to:

shutdown the engine in response to brake pressure exceeding a pressure threshold;

restart the engine in response to an accelerator pedal position exceeding a position threshold independent of brake pressure;

decrease the first brake torque and decrease the second brake torque in response to an engine restart; and

increase the first brake torque to a torque threshold in response to the brake pressure decreasing below the pressure threshold, wherein the torque threshold and the pressure threshold correspond to brake conditions at which the engine was shutdown.

2. The vehicle of claim 1 wherein the first brake system comprises an electric brake pump coupled to a hydraulic brake system and the second brake system comprises an electric park brake coupled to at least one brake caliper, and wherein the controller is further programmed to restart the engine in response to the electric park brake being applied and the accelerator pedal position exceeding the position threshold.

3. The vehicle of claim 1 wherein the controller is further programmed to:

increase the first brake torque to a torque threshold in response to an engine shutdown; and

release the first brake system and increase the second brake torque to the torque threshold in response to a battery voltage decreasing below a voltage threshold, wherein the torque threshold corresponds to brake torque at which the engine was shutdown.

4. The vehicle of claim 3 wherein the controller is further programmed to maintain a total brake torque above the torque threshold while increasing the second brake torque and decreasing the first brake torque in response to the battery voltage decreasing below the voltage threshold, wherein the total brake torque is equal to a sum of the first brake torque and the second brake torque.

5. The vehicle of claim 3 wherein the controller is further programmed to restart the engine in response to the battery voltage decreasing below a second voltage threshold, wherein the second voltage threshold is less than the voltage threshold.

6. A vehicle system comprising:

a first brake system adapted to generate a first brake torque;

a second brake system adapted to generate a second brake torque; and

a controller programmed to:

increase the first brake torque to a torque threshold in response to engine shutdown,

decrease the first brake torque and increase the second brake torque in response to a battery voltage decreasing below a voltage threshold, and

release the first and second brake systems responsive to an engine restart.

7. The vehicles system of claim 6 wherein the controller is further programmed to maintain a total brake torque above the torque threshold while increasing the second brake torque and decreasing the first brake torque in response to the battery voltage decreasing below the voltage threshold, wherein the total brake torque is equal to a sum of the first brake torque and the second brake torque.

8. The vehicle system of claim 6 wherein the controller is further programmed to restart the engine in response to the battery voltage decreasing below a second voltage threshold, wherein the second voltage threshold is less than the voltage threshold.

9. The vehicle system of claim 6 wherein the controller is further programmed to decrease the first brake torque and increase the second brake torque in response to a change in brake pressure exceeding a pressure loss threshold.

10. The vehicle system of claim 6 wherein the first brake system comprises an electric brake pump coupled to a hydraulic brake system and the second brake system comprises an electric park brake coupled to at least one brake caliper.

11. A method for controlling brake systems, comprising:

shutting down an engine responsive to brake pressure exceeding a pressure threshold;

controlling a first brake system to increase a first brake torque above a torque threshold in response to engine shutdown; and

controlling the first brake system to decrease the first brake torque and controlling a second brake system to increase a second brake torque in response to a battery voltage decreasing below a voltage threshold.

12. The method of claim 11 further comprising maintaining a total brake torque above the torque threshold while increasing the second brake torque and decreasing the first brake torque in response to the battery voltage decreasing below the voltage threshold, wherein the total brake torque is equal to a sum of the first brake torque and the second brake torque.

13. The method of claim 11 further comprising restarting the engine and decreasing the second brake torque in response to the battery voltage decreasing below a second voltage threshold, wherein the second voltage threshold is less than the voltage threshold.

14. The method of claim 11 further comprising decreasing the first brake torque and increasing the second brake torque to the torque threshold in response to a change in brake pressure exceeding a pressure loss threshold.

15. The method of claim 11 further comprising restarting the engine in response to an accelerator pedal position exceeding a position threshold and independent of brake pressure.

16. The method of claim 11 further comprising decreasing the first brake torque and the second brake torque in response to an engine restart.