Abstract:
A method for controlling a vehicle transmission during a park release event includes engaging a park brake to hold vehicle wheels against rotation, holding against rotation a first component and a second component of an epicyclic gear unit connected to the wheels, releasing the park brake, selecting a transmission drive range, releasing said first component, and operating the transmission in the selected drive range.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an automatic transmission, and more particularly to its control during a transition from Park to a forward drive range or a reverse drive range. 
     2. Description of the Prior Art 
     When rolling movement a vehicle is produced by placing a range selector of an automatic transmission in the Park range, the vehicle wheels are held against rotation by securing them through a drive connection to the transmission case. It is conventional practice to engage automatically a friction control element of the transmission that must be engaged in both first gear and reverse gear, when Park range is engaged. 
     When a transmission in this state is shifted out of Park on a grade, a large objectionable boom can occur when the stored energy of the loaded drive line is suddenly released. This abrupt release of energy can cause the driveline to oscillate for a brief but undesired period. 
     A need exists in the industry for a reliable technique that prevents unintended rolling of a vehicle using a park brake, and avoids a harsh, abrupt release of energy stored in the vehicle driveline when the park brake is released. 
     SUMMARY OF THE INVENTION 
     A method for controlling a vehicle transmission during a park release event includes engaging a park brake to hold vehicle wheels against rotation, holding against rotation a first component and a second component of an epicyclic gear unit connected to the wheels, releasing the park brake, selecting a transmission drive range, releasing said first component, and operating the transmission in the selected drive range. 
     The output shaft tie up will prevent the vehicle from rolling backwards on a hill. By applying this strategy when the vehicle is in Park, the stored energy from the vehicle&#39;s drive line with holding torque applied can be released in a controlled fashion by gradually releasing the tie-up rather releasing it abruptly. This gradual release of braking torque mitigates the boom that often occurs. 
     By using this strategy in Park, static engagements occur more smoothly because all torque disturbances are held internally to the transmission, and do not affect the driveline. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram showing gearing of a transmission to which the control can be applied; 
         FIG. 2  is chart showing the operating gear and the corresponding state of friction control elements of the transmission of  FIG. 1 ; and 
         FIG. 3  is cross sectional view showing a park pawl engaged with a parking gear of the transmission of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, there is illustrated in  FIG. 1  a transmission  10  having a hydrokinetic torque converter  12  driveably connected to an engine crankshaft  14 . The torque converter includes three bladed wheels: an impeller  16 , turbine  18 , and stator  20 , which together define a toroidal fluid flow circuit within the casing of the torque converter. The stator  20  is supported on an overrunning brake  22 , which anchors the stator to the shaft to prevent rotation of the stator in a direction opposite to the direction of rotation of the impeller and to permit free-wheeling rotation in the opposite direction. Turbine  18  is connected to a transmission input  24 . A bypass or lock-up clutch  26 , located between crankshaft  14  and input  24 , direct mechanically connects the engine shaft  14  and input  24  when clutch  26  is engaged. When clutch  26  is disengaged, engine shaft  14  and input  24  are hydrodynamically connected through operation of the torque converter  12 . 
     The range in which the transmission operates is selected by moving a range selector  28  manually among selected positions, PRNDL, representing park, reverse drive, neutral, forward drive and low gear, respectively. 
     A speed reduction epicyclic gear unit  30  includes a sun gear  34  secured to the transmission case  62 , ring gear  36  connected to input  24 , carrier  38  and planetary pinions  40 , rotatably supported on carrier  38  and in continuous meshing engagement with sun gear  34  and ring gear  36 . 
     A Ravigneaux gearset  38 , an epicyclic gear unit, includes sun gears  40 ,  41 ; a ring gear  42 , connected to the output  44  and the driven vehicle wheels  43 ,  45 ; a carrier  46 ; a first set of planet pinions  48 , rotatably supported on carrier  44  and in continuous meshing engagement with sun gear  40 ; and a second set of planet pinions  50 , rotatably supported on carrier  44  and in continuous meshing engagement with sun gear  41  and pinions  48 . 
     Forward clutch C1234, a hydraulically-actuated friction control clutch  52 , releasably connects carrier  24  and sun gear  40 . Direct clutch C35R, a hydraulically-actuated friction clutch  54 , releasably connects carrier  38  and sun gear  41 . Clutch C456, a hydraulically-actuated friction clutch  56 , releasably connects ring gear  36  and carrier  46 . Intermediate brake C26, a hydraulically-actuated friction brake  58 , releasably holds sun gear  41  against rotation on the transmission case  62 . Low/reverse brake CBLR, a hydraulically-actuated friction brake  60 , releasably holds carrier  46  against rotation on the transmission case  62 . An overrunning clutch  66 , arranged in parallel with brake  60 , provides a one-way drive connection between the casing  62  and carrier  46  one rotary direction and holding carrier  46  against rotation in the opposite direction. 
       FIG. 2  is a chart showing the engaged and disengaged status of the clutches  52 ,  54 ,  56  and brakes  58 ,  60 ,  66  corresponding to each forward speed ratios and reverse ratio produced by transmission  10 . In  FIG. 2 , “X” indicates engagement of the friction clutches and brakes and transmission of torque by the overrunning clutch  66 . A blank in  FIG. 2  indicates that no torque is being transmitted by the clutch or brake. 
       FIG. 3  shows details of a park brake assembly, which includes a park gear  70  formed integrally with ring gear  42  and output  44 ; a park pawl  72 , which pivots through an opening  74  in the servo piston  76  that actuates brake  60  and into engagement with the teeth  80  of park gear  70 ; and a park support  78 , secured to the transmission case  62  and supporting the pawl  72 , which pivots into and out of engagement with the park gear  70 . 
     Preferably the control elements of transmission  10 , i.e., clutches  52 ,  54 ,  56  and brakes  58 ,  60 , are hydraulically-actuated friction devices comprising interleaved plates  82  and discs  84 , which are brought into mutual frictional contact by a servo piston, such as piston  76 , when pressure in a servo cylinder, such as cylinder  86 , is high. When pressure in the servo cylinder is relatively low, frictional contact between the plates  82  and discs  84  is released due to the force of a return spring  88 , thereby disengaging the control element. 
     In operation, when the range selector is moved away from the Park range, pawl  72  pivots out of engagement with the teeth  80  formed on the outer periphery of park gear  70 . 
     When the range selector is moved into the Park range, pawl  72  pivots into engagement with the teeth  80  on the park gear  70 , thereby fixing the output  44  to case  62  and holding the output against rotation. When range selector is moved into the Park range, the low/reverse CLRB brake  60  and intermediate C26 clutch  58  are applied automatically. Engagement of brake  58  holds sun gear  41  against rotation on case  62 , and engagement of CBLR brake  60  holds carrier  46  against rotation on case  62 . While rotation of sun gear  41  and carrier  46  is prevented, each of the other components of the Ravigneaux gearset  38  capable of rotation, i.e., ring gear  42 , sun gear  40 , are held against rotation and the entire gear unit is locked. Because the C1234 brake  52  is released, the turbine  18  spins freely, only being resisted by open pack clutch drag, instead of the full stall of the torque converter. 
     When the range selector is shifted out of the Park range to the forward drive range, the low/reverse brake  60  remains engaged, the intermediate clutch  58  is disengaged and the forward clutch  52  is engaged, thereby enabling the transmission  10  to operate in the first forward gear, manual M 1 , as  FIG. 2  shows. When the range selector is shifted out of the Park range to the reverse drive range, the low/reverse brake  60  remains engaged, the intermediate clutch  58  is disengaged and the direct clutch  54  is engaged, thereby enabling the transmission  10  to produce reverse drive, as  FIG. 2  shows. 
     Although the control has been described with reference to a Ravigneaux gearset  38 , the epicyclic gear unit that is driveably connected to the output  44  and the wheels  43 ,  45  may be a simple or compound planetary gear unit, and may include a Simpson gear unit and variations thereof. 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.