System and method for vehicle hill hold release
A method and system for releasing a vehicle from a hill hold mode is described. In one example, torque output of an electric machine is adjusted to reduce a possibility of the vehicle moving in an unintended direction prior to or during release of the vehicle from hill hold mode.
1 . A method for releasing a vehicle from a hill hold mode, comprising:
via a controller, disengaging a vehicle motion restraining device;
in response to detection of a slip of the vehicle motion restraining device, estimating an amount of torque output via an electric machine based on a current pressure of the vehicle motion restraining device and a friction coefficient of the vehicle motion restraining device; and
adjusting the electric machine to produce the estimated amount of torque output.
2 . The method of claim 1 , where the vehicle motion restraining device is a friction brake.
3 . The method of claim 1 , where the vehicle motion restraining device is a transmission clutch.
4 . The method of claim 1 , further comprising locking a transmission output shaft to engage the vehicle in the hill hold mode prior to disengaging the hill hold mode.
5 . The method of claim 4 , where the transmission output shaft is locked via applying one or more transmission clutches.
6 . The method of claim 1 , where the pressure is lowered in response to an increase in depression of a driver demand pedal.
7 . The method of claim 1 , where the pressure is lowered in response to an adjustment of a shifter.
8 . The method of claim 1 , where the amount of torque is a constant.
9 . The method of claim 1 , where the amount of torque is a function of road grade.
10 . A vehicle hill hold system, comprising:
a transmission of a vehicle including one or more brakes that are operable to couple an output shaft of the transmission to a case of the transmission;
an electric machine configured to provide propulsive effort to the vehicle; and
a controller including executable instructions that cause the controller to:
disengage one or more brakes that operable to couple the output shaft of the transmission to the case of the transmission;
in response to detection of a slip of the vehicle motion restraining device, estimate an amount of torque output via an electric machine based on a current pressure of the one or more brakes and a friction coefficient of the one or more brakes; and
adjust the electric machine to produce the estimated amount of torque output.
11 . The vehicle hill hold system of claim 10 , further comprising additional executable instructions to adjust the torque output of the electric machine in response to a request to release a hill hold mode from being engaged.
12 . The vehicle hill hold system of claim 11 , where the hill hold mode includes applying the one or more brakes to couple the output shaft of the transmission to the case of the transmission.
13 . The vehicle hill hold system of claim 10 , where the torque output of the electric machine is adjusted based on a friction coefficient of a brake included in the one or more brakes and a pressure of fluid supplied to the one or more brakes.
14 . The vehicle hill hold system of claim 10 , further comprising additional instructions that cause the controller to reduce a pressure applied to the one or more brakes in response to a request to release a hill hold mode from being engaged.
15 . The vehicle hill hold system of claim 10 , where the torque output of the electric machine is adjusted to a torque that moves the vehicle in a forward direction when the one or more brakes are released and a driver demand pedal is not applied.
16 . A method for releasing a vehicle from a hill hold mode, comprising:
via a controller, disengaging a transmission clutch to unlock an output shaft of a transmission from a transmission case in response to a request to exit the hill hold mode;
in response to detection of a slip of the vehicle motion restraining device, estimating an amount of torque output via an electric machine based on a current pressure of the transmission clutch and a friction coefficient of the transmission clutch; and
adjusting the electric machine to produce the estimated amount of torque output.
17 . The method of claim 16 , where adjusting the amount of torque output includes adjusting the torque output to a constant value.
18 . The method of claim 17 , where the constant value is based on output of an inclinometer.
19 . The method of claim 16 , further comprising operating the vehicle in a speed control mode in response to slip of the transmission clutch.
20 . The method of claim 16 , where the transmission clutch is one of two transmission clutches capable of unlocking the output shaft of the transmission.