Abstract:
A method for controlling a transmission gear change to a desired gear includes disengaging an offgoing transmission control element, changing engine speed to a synchronous speed of the desired gear, decreasing engine output torque, and engaging an oncoming transmission control element.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to the control of gear shifts in an automatic transmission for a motor vehicle, particularly to the control of upshifts between nonconsecutive gears. 
         [0003]    2. Description of the Prior Art 
         [0004]    Failure of a single point solenoid, hydraulic system valve or clutch can result in an automatic transmission being disabled, requiring the transmission to perform upshifts that skip gears, such as an upshift from first gear to third gear (1-3), an upshift from third gear to fifth gear (3-5), an upshift from second gear to sixth gear (2-6), etc. 
         [0005]    These shifts are much higher energy than upshifts between consecutive gears and can result in additional transmission damage due to excessive energy on an on-coming clutch, whose engagement is required to produce the target gear of the upshift. To avoid these problems when a failure has occurred, upshifts should be limited to lower vehicle speeds or full backouts, but this causes delayed upshifts until the vehicle operator backs out. A backout refers to the vehicle operator releasing the accelerator pedal. 
         [0006]    Conventionally in performing the commanded upshift with a failure present, the oncoming clutch causes the speed ratio change, but inertia torque can still be excessive leading to damage of previously undamaged clutches. 
         [0007]    Conventionally the transmission is locked in a single gear when a failure or fault occurs that would affect upshifting the transmission. Although this procedure has low risk of causing further transmission damage and is simple to execute its disadvantages include a need to maintain low vehicle speed particularly while operating in first or second gear, and compromised vehicle launch performance issues particularly while operating in a higher gear. 
         [0008]    A need exists in the industry for a method for controlling a transmission after a failure that affects upshifts such that all gears of the transmission are functional and available, thereby allowing the vehicle operator to have the vehicle serviced when it is convenient. 
       SUMMARY OF THE INVENTION 
       [0009]    A method for controlling a transmission gear change to a desired gear includes disengaging an offgoing transmission control element, changing engine speed to a synchronous speed of the desired gear, decreasing engine output torque, and engaging an oncoming transmission control element. 
         [0010]    The failure mode upshift energy management strategy converts high energy gear shifts into shifts in which virtually no torque is transmitted by the oncoming control element. The torque transmitting capacity of the offgoing clutch (sometimes referred to a torque capacity) is ramped off while the energy management control calculates a target engine speed based on the desired gear and current transmission output speed. Then the control uses engine speed limiting to pull engine speed down to the synchronous speed of the target gear. As engine speed approaches the target speed, the oncoming clutch is engaged. In this way virtually no energy is transmitted through the oncoming clutch because that clutch does not reduce engine speed to the synchronous speed. 
         [0011]    The oncoming clutch does not overcome either combustion torque or inertia torque to synchronize the speed ratio of the desired gear. 
         [0012]    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 
         [0013]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0014]      FIG. 1  is a schematic diagram of an automatic transmission; 
           [0015]      FIG. 2  is chart showing for each gear the applied and released states of the friction control elements of the transmission of  FIG. 1 ; and 
           [0016]      FIG. 3  is a graph show the variation of various parameters during an upshift of the transmission of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    Referring now to the drawings, there is illustrated in  FIG. 1  the kinematic arrangement of an automatic transmission. A torque converter  10  includes an impeller wheel  12  connected to the crankshaft  14  of an internal combustion engine, a bladed turbine wheel  16 , and a bladed stator wheel  18 . The impeller, stator and turbine wheels define a toroidal fluid flow circuit, whereby the impeller  12  is hydrokinetically connected to the turbine  16 . The stator  18  is supported rotatably on a stationary stator shaft, and an overrunning brake  20  anchors the stator to the shaft to prevent rotation of the stator in a direction opposite the direction of rotation of the impeller, although free-wheeling motion in the opposite direction is permitted. 
         [0018]    The torque converter  10  includes a lockup clutch  22  located within the torque converter impeller housing  23 . When clutch  22  is engaged, the turbine  16  and impeller  12  are mechanically connected to a transmission input shaft  24 ; when clutch  24  is disengaged, the turbine  16  and impeller  12  are hydrokinetically connected and mechanically disconnected. Fluid contained in the torque converter is supplied from the output of an oil pump assembly  25  and is returned to an oil sump, to which an inlet of the pump is connected hydraulically. 
         [0019]    Transmission  10  is enclosed in a transmission housing  27 , which is fixed against rotation to the vehicle structure. The input  24  is driven by the engine through torque converter  10 . An output  26  is driveably connected to the vehicle&#39;s wheels, preferably through a differential mechanism and a set of transfer gears (not shown). 
         [0020]    The transmission  10  includes two epicyclic gearsets  28 ,  30 . The first gearset  28  includes a first sun gear  32 , first ring gear  34 , first carrier  36 , and a first set of planet pinions  38 , supported for rotation on carrier  36  and meshing with first sun gear  32  and first ring gear  34 . Sun gear  32  is continuously fixed against rotation, preferably by a connection to the housing  27  or a front support, secured to the housing. Ring gear  34  is secured to input  24 . 
         [0021]    The second gearset  30  includes a second sun gear  40 ; third sun gear  41 ; third ring gear  42 ; second carrier  44 ; a set of short planet pinions  46 , supported for rotation on second carrier  44 ; a set of long planet pinions  48 , supported for rotation on second carrier  44  and meshing with third sun gear  41 , third ring gear  42  and short planet pinions  46 . The output  26  is secured to the third ring gear  42 . 
         [0022]    A forward clutch  50  (C 1234 ) selectively opens and closes a drive connection between carrier  36  and the third sun gear  41 . A direct clutch  52  (C 35 R) selectively opens and closes a drive connection between carrier  36  and the second sun gear  40 . An intermediate brake  56  (CB 26 ) alternately releases and holds second sun gear  40  and the output of clutch  52  against rotation. A low/reverse brake  58  (CBLR) alternately releases and holds carrier  44  against rotation. An overdrive clutch  60  (C 456 ) selectively opens and closes a drive connection between carrier  44  and ring gear  34 , which is secured to input  24 . 
         [0023]    A clutch alternately connects and disconnects two rotating components. A brake alternately holds a rotating component against rotation and releases that rotating component so that it can rotate freely. Transmission  10  includes three clutches  50 ,  52 ,  60  and two brakes  56 ,  58 , which are sometimes referred to as friction elements or control elements. 
         [0024]    Clutches  50 ,  52 ,  60  and brakes  56 ,  58  include plates, which are connected by a spline to a first member, and friction discs, which are connected by a spline to a second member, the plates and discs being interleaved. When hydraulic pressure is applied to a servo that actuates a control element, its plates and discs are forced together into mutual frictional contact, thereby increasing the torque transmitting capacity of the control element and driveably connecting the first and second members. When hydraulic pressure is vented from the servo, the control element transmits no torque, allowing the first and second members to rotate independently. 
         [0025]    Although clutches  50 ,  52 ,  60  and brakes  56 ,  58  have been illustrated and described as hydraulically actuated multi-plate clutches and brakes, the invention may be practiced with alternate types of releasable connections including but not limited to dog clutches and brakes, controllable one way clutches and brakes, magnetically actuated clutches and brakes, or electrically actuated clutches and brakes. 
         [0026]    As the table of  FIG. 2  shows, third gear is produced by engaging clutches  50 ,  52 . Fifth gear is produced by engaging clutches  52  and  60 . When producing an upshift from third gear to fifth gear, clutch  52  remains engaged, forward clutch  50  (C 1234 ) is the off going element, and overdrive clutch  60  (C 456 ) is the oncoming element. 
         [0027]    As  FIG. 3  shows when a 3-5 upshift is commanded by an electronic controller at time  70 , the actuating servo pressure  72  of the offgoing control element C 1234  decreases along a relatively steep ramp  74  to a substantially zero pressure  75  during the entire upshift. But servo pressure  76  of the oncoming control element C 456  remains low and vented during a delay period. Engine speed  68  is substantially constant before the upshift command occurs. 
         [0028]    After the upshift is commanded, the controller determines the synchronous speed  78  of the engine, i.e., the engine speed at which the speed of the transmission output  26  in the target gear (fifth gear) would be equal to the output speed in the current gear (third gear). The controller then controls engine speed  80  to decrease steadily toward the synchronous speed  78  while the oncoming control element C 456  remains disengaged. The controller also decreases engine combustion torque  82  to substantially zero torque  84 , while engine speed is decreasing along ramp  80 . 
         [0029]    Preferably when engine speed decreases to about 200-300 rpm greater than synchronous speed  78 , pressure  76  in the oncoming control element C 456  increases at  82  to a stroking pressure, which displaces the hydraulic piston of the servo that actuated oncoming control element C 456 , thereby removing clearances in the servo. 
         [0030]    Servo pressure may decrease at  86  after the clutch C 456  is stroked and increase thereafter to a start pressure  88 . Then torque capacity of the oncoming control element C 456  increases along a ramp  90  causing clutch C 456  to become fully engaged with actuating pressure at line pressure  92 . 
         [0031]    After the 3-5 upshift is completed, the controller increases engine combustion torque at  94 , and increases engine limit speed at  96 . Engine speed after the upshift  98  is lower than before the upshift  68 . 
         [0032]    Because oncoming clutch C 456  becomes engaged while engine speed is at synchronous speed, the clutch does not transmit inertia torque, which is normally required to synchronize engine speed with output speed in the target gear. Because oncoming clutch C 456  is engaged while engine combustion torque  84  is substantially zero, the clutch does not transmit engine combustion torque. Therefore, the upshift is produced with a low magnitude of torque being transmitted by the oncoming clutch C 456  until after the upshift is completed. 
         [0033]    Although the 3-5 upshift has been described with reference to the offgoing and oncoming control elements being clutches, the control is applicable to gear shifts in which brakes are the participating control elements, or both clutches and brakes participate in the gear shift. The control is applicable to both upshifts and to downshifts. 
         [0034]    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.