Vehicle transmissions having a park function

Abstract

In response to driver selection of PARK, a vehicle transmission is commanded to momentarily select and hold NEUTRAL before PARK, while a vehicle wheel brake is applied. Such an arrangement avoids trapping torque in the driveline, and thus can obviate pawl ‘thump’ when PARK is next disengaged.

Claims

1. A method of engaging a park pawl of a transmission of a vehicle, the method comprising: commanding engagement of the park pawl with an output shaft of the transmission; braking wheels of the vehicle against movement in response to said commanding engagement of the park pawl with the output shaft of the transmission; engaging a NEUTRAL condition of the transmission in response to said commanding engagement of the park pawl with the output shaft of the transmission; and engaging the park pawl with the output shaft of the transmission after a pre-determined delay, wherein the delay is from said engaging the NEUTRAL condition of the transmission or from said commanding engagement of the park pawl with the output shaft of the transmission, wherein braking the wheels of the vehicle wheel is during the delay and the transmission is in the NEUTRAL condition during the delay.

2. A method according to claim 1, wherein a duration of said pre-determined delay is selected based on a vehicle condition.

3. A method according to claim 1, wherein a vehicle wheel brake is applied automatically to achieve said braking wheels of the vehicle against movement.

4. A method as claimed in claim 1, wherein the transmission is maintained in the NEUTRAL condition for at least the duration of the pre-determined delay.

5. A method as claimed in claim 1, wherein said braking of the wheels is maintained at least until said park pawl is engaged with the output shaft of the transmission.

6. A control system for an automatic transmission of a vehicle having a park pawl engageable with an output shaft of the transmission, said control system comprising: a selector for commanding a condition of the transmission; a transmission actuator responsive to the selector to engage the commanded condition; a selector detector to detect selection of a PARK condition as the commanded condition; a brake controller to command application of a vehicle wheel brake; and an actuator controller to command the transmission actuator, wherein, upon selection of the PARK condition as the commanded condition: the control system gives a command for engagement of the park pawl with the output shaft of the transmission; in response to selection of the PARK condition as the commanded condition, the brake controller commands application of the vehicle wheel brake to brake the wheels of the vehicle against movement; in response to selection of the PARK condition as the commanded condition, the actuator controller controls the transmission actuator to cause engagement of a NEUTRAL condition of the transmission; and the control system causes engagement of the park pawl with the output shaft of the transmission after a pre-determined delay, wherein the delay is from said engagement of the NEUTRAL condition or from said command for engagement of the park pawl with the output shaft of the transmission, wherein the application of the vehicle wheel brake is during the delay and the transmission is in the NEUTRAL condition during the delay.

7. A control system according to claim 6, wherein said brake controller is configured to command application of the vehicle wheel brake when said selector is detected to have a PARK position corresponding to the PARK condition.

8. A control system according to claim 6, wherein said transmission actuator is configured to engage and hold said NEUTRAL condition for a period of 1 second or less.

9. A control system according to claim 8, wherein said period is less than 0.5 seconds.

10. A control system according to claim 9, wherein said period is less than 0.35 seconds.

11. A control system according to claim 8, wherein a duration of said period is selected based on a vehicle condition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically illustrates a vehicle incorporating an embodiment of the present invention.

(2) FIG. 2 schematically illustrates a control system designed according to an embodiment of this invention.

DETAILED DESCRIPTION

(3) With reference to FIG. 1 a vehicle 11 has an engine 12 and a front wheel drive automatic transmission 13. The selector input shaft of the transmission 13 is coupled to an electric actuator 14 mounted on the transmission casing, and is controlled via an electric cable 15 extending to a rotary selector 16 mounted on a vehicle console 17 in the vehicle interior.

(4) An illustrative example control system 300 is for the vehicle automatic transmission having a park pawl 310 that is engageable with an output shaft 312 of the transmission. The control system 300 includes the selector 16 for commanding a condition of the transmission and the transmission actuator 14 responsive to the selector 16 to engage the commanded condition. The control system comprises a selector detector 318 to detect selection of a park condition, a brake controller 320 to command application of a vehicle wheel brake 322, and an actuator controller 324 to command the transmission actuator 14 to momentarily engage and hold a neutral condition of the transmission prior to engagement of the park pawl 310.

(5) As illustrated the vehicle 11 is on a slope, and accordingly it will be appreciated that torque is applied to the transmission park pawl 310 if the vehicle wheel brake 322, which may be a parking brake, is not applied or is ineffective. Release of the pawl 310, even if the vehicle brake 322 is in the meantime applied, is thus under torque loading, and may be rapid, which results in a typically audible noise as the pawl 310 thumps against an abutment in the released condition. As noted earlier, torque may also be trapped due to early engagement of the park pawl 310 and wind-up of the transmission; this leads to the same pawl thump release phenomenon.

(6) In the invention selection of PARK via the rotary selector 16 does not immediately cause the park pawl 310 to engage. The brake controller 320 is commanded to engage an electronic park brake (e.g., the brake 322), and the actuator 14 is commanded to engage or hold the transmission 13 in NEUTRAL for a short period, typically less than 500 milliseconds before allowing the park pawl 310 to engage. This delay, while the vehicle park brake is engaged, allows any residual torque in the transmission to dissipate. The vehicle park brake 322 may be associated with a driven or non-driven axle.

(7) If the selector 16 is in DRIVE, the actuator 14 is commanded to pause in neutral while the selector is manually moved from DRIVE to PARK. If the selector 16 is in REVERSE, the actuator 14 is commanded to shift into NEUTRAL and pause, while the selector 16 is manually moved from REVERSE to PARK. The final movement of the actuator 14 from NEUTRAL to PARK passes through the REVERSE position, but is sufficiently rapid to avoid engagement of the REVERSE speed ratio—such rapid motion is conventional.

(8) It will be appreciated that in selecting PARK from REVERSE, the actuator 14 is commanded to first move away from the PARK condition towards NEUTRAL. This motion is momentary, and should be imperceptible to the vehicle driver.

(9) Thus the invention can avoid trapping residual torque in the transmission driveline, and so long as the vehicle park brake 322 is effective and can eliminate park pawl thump as PARK is disengaged.

(10) This invention is described with reference to an automatic transmission 13 having a conventional PRNDL selector 16. However it will be understood that the principle of the invention is applicable to any transmission having a park lock function and a neutral condition. The invention is also applicable to rear wheel drive and all wheel drive vehicles, and to vehicles having an automated manual transmission with a park function.

(11) Typically functionality of the invention is provided in an electronic control unit of the vehicle responsive to the selection of the transmission condition, and commanding operation of an appropriate actuator. Such a control unit is conventional, and may include logic elements to ensure appropriate command of the actuator upon selection of the park function.