Transmission braking for vehicle speed control

1. A transmission braking system for a vehicle, the system comprising:

an automatic transmission having a plurality of clutches and a plurality of forward gears, the automatic transmission being adapted to transfer drive torque from a torque generating system of the vehicle to a driveline of the vehicle; and

a controller configured to:

determine a set of available clutches of the plurality of clutches that are not being utilized for engagement of a particular forward gear of the plurality of forward gears,

determine at least one of a modeled temperature, a measured temperature, and a modeled energy of each clutch of the set of available clutches to obtain a set of at least one of clutch temperatures and energies, and

based on at least the set of available clutches and the set of at least one of clutch temperatures and energies, selectively operate the transmission in a transmission braking mode that includes at least partially applying at least one clutch of the set of available clutches to mitigate or prevent powerflow through the transmission and thereby decrease or maintain a speed of the vehicle and/or reduce acceleration of the vehicle.

2. The system of claim 1 , wherein the particular forward gear is a currently active forward gear of the plurality of forward gears of the transmission,

wherein the transmission braking mode includes at least partially applying the at least one clutch of the set of available clutches while the transmission remains in the currently active forward gear to thereby absorb frictional energy in the transmission, and

wherein at least partially applying the at least one clutch of the set of available clutches while the transmission remains in the currently active forward gear provides at least a partial lockup state of the transmission whereby powerflow therethrough is mitigated or prevented.

3. The system of claim 2 , wherein the set of available clutches comprises at least a first available clutch and at least one second available clutch, and wherein the controller is configured to periodically and intermittently at least partially apply the first and the at least one second available clutch.

4. The system of claim 2 , wherein the controller is further configured to:

based on at least the set of available clutches and the set of at least one of clutch temperatures and energies, determine a need to exit the transmission braking mode;

in response to determining the need to exit the transmission braking mode:

determine a different set of available clutches that would be available in a different forward gear of the transmission,

determine at least one of a modeled temperature, a measured temperature, and a modeled energy of each clutch of the different set of available clutches to obtain a different set of at least one of clutch temperatures and energies, and

based on at least the different set of available clutches and the different set of at least one of clutch temperatures and energies, determine whether the transmission braking mode could be extended by shifting the transmission to the different forward gear; and

when the transmission braking mode can be extended by shifting the transmission to the different forward gear:

command the transmission to shift to the different forward gear, and

based on at least the different set of available clutches and the different set of at least one of clutch temperatures and energies, at least temporarily continue operating the transmission in the transmission braking mode including at least partially applying at least one clutch of the different set of available clutches to thereby absorb frictional energy and decrease or maintain the speed of the vehicle and/or reduce acceleration of the vehicle.

5. The system of claim 1 , wherein the transmission braking mode further comprises the controller at least partially disengaging at least one clutch that was being utilized for engagement of the particular forward gear of the transmission to provide at least a partial lockup state of the transmission whereby the powerflow therethrough is mitigated or prevented.

6. The system of claim 5 , wherein engagement of a different set of the plurality of clutches is required to establish powerflow in each of the plurality of forward gears, and wherein engagement of a unique set of the plurality of clutches not utilized to generate powerflow in any of the plurality of forward gears generates the at least partial lockup state of the transmission.

7. The system of claim 6 , wherein the transmission comprises eight forward gears and five clutches.

8. The system of claim 1 , wherein the controller is further configured to:

detect a park request provided by a driver of the vehicle or generated by an auto-park feature of the vehicle;

when the park request is detected and the vehicle speed is above a park speed threshold, operate the transmission in the transmission braking mode to decrease the vehicle speed to zero or within a threshold from zero; and

upon the vehicle speed decreasing to zero or within the threshold from zero, engage a park pawl system to stop the vehicle.

9. The system of claim 1 , wherein the controller is further configured to:

determine whether a cruise control mode of the vehicle is active; and

when the cruise control mode of the vehicle is active and the vehicle speed is above a cruise control speed setpoint, operate the transmission in the transmission braking mode to decrease the vehicle speed to the cruise control speed setpoint or within a threshold from the cruise control speed setpoint.

10. The system of claim 1 , wherein the controller is further configured to:

determine whether an exhaust braking speed hold mode of the vehicle is active; and

when the exhaust braking speed hold mode of the vehicle is active, operate the transmission in the transmission braking mode to decrease the vehicle speed to a speed hold setpoint or within a threshold from the speed hold setpoint.

11. The system of claim 1 , wherein the controller is further configured to:

determine whether an engine braking speed hold mode of the vehicle is active; and

when the engine braking speed hold mode of the vehicle is active, operate the transmission in the transmission braking mode to decrease the vehicle speed to a speed hold setpoint or within a threshold from the speed hold setpoint.

12. The system of claim 1 , wherein the controller is configured to operate the transmission in the transmission braking mode when the vehicle is traveling downhill and a downshift of the transmission cannot be performed.

13. The system of claim 1 , wherein the controller is configured to operate the transmission in the transmission braking mode in response to a manual activation request provided by a driver of the vehicle.