Abstract:
A method of down-shifting a transmission avoids output shaft oscillations by briefly increasing the torque capacity of an off-going element at the end of the inertia phase. The torque capacity of the off-going element is then reduced to zero when the measured transmission speed ratio begins to decrease. The method is suitable for downshifts that involve multiple off-going elements and multiple on-coming elements such as a shift from tenth gear to sixth gear in a ten speed transmission.

Description:
TECHNICAL FIELD 
     This disclosure relates to the field of automatic transmissions for motor vehicles. More particularly, the disclosure pertains to a method of changing among speed ratios. 
     BACKGROUND 
     Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. Transmission speed ratio is the ratio of input shaft speed to output shaft speed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. 
     When driving conditions change, an automatic transmission changes from one speed ratio to another speed ratio. For example, when a vehicle is cruising using a low speed ratio and a driver demands an increase in wheel torque, the transmission must downshift into a higher speed ratio. For sudden changes in driver demanded wheel torque, the transmission may skip over one or more available gear ratios in a single shift event. Many automatic transmissions have multiple shift elements, such as clutches or brakes, and select particular speed ratios by engaging particular subsets of the shift elements. To perform a shift from one speed ratio to another, one or more previously engaged shift elements are released and one or more previously disengaged shift elements are engaged. Passenger comfort is maximized if the transition is accomplished smoothly. Performance is maximized if the transition is accomplished quickly. These considerations are often in conflict. 
     SUMMARY OF THE DISCLOSURE 
     A method of shifting a transmission includes maintaining a first offgoing element in a fully engaged condition while operating in a first transmission speed ratio, reducing the torque capacity of the first offgoing element during an inertia phase, and then increasing the torque capacity of the first offgoing element at the end of the inertia phase to prevent output shaft oscillation. A first oncoming clutch is engaged at the end of the inertia phase. The torque capacity of the first offgoing clutch is then decreased to zero in response to the measured transmission speed ratio decreasing. The method may be utilized as part of double transition shift that includes releasing a second offgoing clutch and engaging a second oncoming clutch during the inertia phase. The elements may be either brakes or clutches. 
     In another embodiment, a transmission includes at least first and second elements and a controller programmed to downshift from a first speed ratio in which the first clutch is engaged to and the second clutch is disengaged to a second speed ratio in which the first clutch is disengaged and the second clutch is engaged by increasing the torque capacity of the first clutch at the end of the inertia phase. The transmission may also include a third clutch which is engaged in the first speed ratio and disengaged in the second speed ratio and a fourth clutch that is disengaged in the first speed ratio and engaged in the second speed ratio. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary transmission gearing arrangement. 
         FIG. 2  is a graph illustrating speed relationships during execution of a downshift. 
         FIG. 3  is a graph illustrating torque relationships during execution of a downshift. 
         FIG. 4  is a flowchart illustrating a method of shifting. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
     An example transmission is schematically illustrated in  FIG. 1 . The transmission utilizes four simple planetary gear sets  20 ,  30 ,  40 , and  50 . Sun gear  26  is fixed to sun gear  36 , carrier  22  is fixed to ring gear  58 , ring gear  38  is fixed to sun gear  46 , ring gear  48  is fixed to sun gear  56 , input shaft  60  is fixed to carrier  32 , and output shaft  62  is fixed to carrier  52 . Ring gear  28  is selectively held against rotation by brake  66  and sun gears  26  and  36  are selectively held against rotation by brake  68 . Input shaft  60  is selectively coupled to ring gear  48  and sun gear  56  by clutch  70 . Intermediate shaft  64  is selectively coupled to carrier  42  by clutch  72 , selectively coupled to carrier  22  and ring gear  58  by clutch  74 , and selectively coupled to ring gear  38  and sun gear  46  by clutch  76 . 
     As shown in Table 1, engaging the clutches and brakes in combinations of four establishes ten forward speed ratios and one reverse speed ratio between input shaft  60  and output shaft  62 . An X indicates that the corresponding shift element is engaged to establish the speed ratio. 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 66 
                 68 
                 70 
                 72 
                 74 
                 76 
                 Ratio 
                 Step 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Rev 
                 X 
                 X 
                   
                 X 
                 X 
                   
                 −4.79 
                 102% 
               
               
                   
                 1 st   
                 X 
                 X 
                 X 
                 X 
                   
                   
                 4.70 
               
               
                   
                 2 nd   
                 X 
                 X 
                   
                 X 
                   
                 X 
                 2.99 
                 1.57 
               
               
                   
                 3 rd   
                 X 
                   
                 X 
                 X 
                   
                 X 
                 2.18 
                 1.37 
               
               
                   
                 4 th   
                 X 
                   
                   
                 X 
                 X 
                 X 
                 1.80 
                 1.21 
               
               
                   
                 5 th   
                 X 
                   
                 X 
                   
                 X 
                 X 
                 1.54 
                 1.17 
               
               
                   
                 6 th   
                 X 
                   
                 X 
                 X 
                 X 
                   
                 1.29 
                 1.19 
               
               
                   
                 7 th   
                   
                   
                 X 
                 X 
                 X 
                 X 
                 1.00 
                 1.29 
               
               
                   
                 8 th   
                   
                 X 
                 X 
                 X 
                 X 
                   
                 0.85 
                 1.17 
               
               
                   
                 9 th   
                   
                 X 
                 X 
                   
                 X 
                 X 
                 0.69 
                 1.24 
               
               
                   
                 10 th   
                   
                 X 
                   
                 X 
                 X 
                 X 
                 0.64 
                 1.08 
               
               
                   
                   
               
             
          
         
       
     
     All single step and two step shifts are performed by gradually engaging one shift element, called an oncoming element (ONC) while gradually releasing a different shift element, called the offgoing element (OFG). During each of these shifts, three shift elements, called holding elements, are maintained fully engaged while one shift element is maintained fully disengaged. In other transmission arrangements, the number of holding elements may be different. 
     During a downshift, the engine speed must increase to match the new speed ratio. The output torque may decrease while some of the power is diverted to increasing engine speed rather than being transmitted to the output. Also, since shift elements are slipping during a shift, some of the power is converted to heat, exacerbating the output torque deficiency. 
     Sometimes, it is desirable to downshift by more than two ratio steps. For example, if the vehicle driver presses the accelerator pedal to pass another vehicle while cruising on the highway in top gear, the shift scheduling algorithm may demand a multiple step downshift. For some multiple step downshifts, two shift elements must be releases and two shift elements must be engaged. For example, to shift from  10 th gear in the example transmission to 6th gear in the example transmission, brake  68  (OFG 1 ) and clutch  76  (OFG 2 ) must be released and clutch  70  (ONC 1 ) and brake  66  (ONC 2 ) must be engaged. While it is possible to complete such a shift in two stages, by shifting temporarily into  8 th gear for example, completing the shift in that manner would require more time and result in more output torque disturbance than making the shift in a single process. Fluctuating output torque tends to be annoying to the driver as it translates directly into fluctuating vehicle acceleration. 
       FIGS. 2 and 3  illustrate speed and torque relationships for a shift from 10th gear to 6th gear in the transmission of  FIG. 1 . Line  60  in  FIG. 2  shows the input speed as a function of time assuming that output speed is substantially constant. The remaining lines depict the relative speeds across various clutches and brakes. The scale is not necessarily identical among lines. Line  62  in  FIG. 3  shows the output torque as a function of time assuming that input torque is substantially constant. The remaining lines depict the torque transmitted by various clutches and brakes. Again, the scale is not necessarily identical among these lines.  FIG. 4  is a flow diagram illustrating a method of controlling shift elements to effectuate a shift such as the shift illustrated in  FIGS. 2 and 3 . 
     The downshift is initiated at  122  in  FIG. 4  by gradually reducing the commanded torque capacity of brake  68  (OFG 1 ) as shown between  84  and  86  in  FIG. 3 . When the torque capacity becomes less than the capacity required to maintain 10th gear, brake  68  will begin to slip and input speed will begin to rise marking the beginning of the inertia phase. As shown by line  62  in  FIG. 3 , the output torque drops during this phase as power is diverted to increasing engine speed. The torque capacity of brake  68  (OFG 1 ) determines how much the output torque drops and how quickly the engine speed increases as shown at  124  in  FIG. 4 . If the torque capacity of brake  68  is close to zero, then very little of the engine power will be transmitted to the output but the engine speed will increase rapidly. On the other hand, if the torque capacity of brake  68  is maintained close to the level that brake  68  would transmit in 10th gear, then most of the engine power will be transmitted to the output shaft and engine speed will increase slowly. 
     As shown in  FIG. 2 , as the input shaft increases in speed, the speed difference across clutch  70  (ONC 1 ) and brake  66  (ONC 2 ) decrease. During this period, the pressure supplied to clutch  70  and brake  66  may be increased in order to prepare for later engagement, but not enough to exert substantial torque. When the speed difference across clutch  70  (ONC 1 ) reaches zero at  88  in  FIG. 2  and at  126  in  FIG. 4 , the torque capacity of clutch  76  (OFG 2 ) is rapidly ramped to zero and the torque capacity of clutch  70  (ONC 1 ) is rapidly increased as shown at  128  in  FIG. 4 . The torque capacity of brake  66  (OFG 1 ) continues to control the rate of change of the input speed as shown at  130  in  FIG. 4 . 
     When the speed difference across brake  66  (ONC 2 ) reaches zero at  90  in  FIG. 2  and at  132  in  FIG. 4 , the inertia phase ends and the torque phase begins. The torque capacity of brake  66  (ONC 2 ) is rapidly increased between  92  and  94  in  FIG. 3  and at  134  in  FIG. 4 . Engaging brake  66  before point  92  would cause a further reduction in output torque. Because brake  66  does not immediately reach sufficient torque capacity to stop ring gear  28 , the direction of rotation of brake  66  may briefly change as shown at  96  in  FIG. 2 . As shown at  98  in  FIG. 2 , the input speed may also temporarily exceed the speed associated with the final gear ratio. As brake  66  pulls the input speed back down to it final value, excess energy in various transmission components can result in windup in various shafts such as the vehicle driveshaft. Unless action is taken to dissipate this energy, the output shaft will oscillate as shown by the dotted lines at  100 . 
     Instead of immediately releasing brake  68  (OFG 1 ), the torque capacity of brake  68  is increased at a calibratable rate between  92  and  94  in  FIG. 3  and at  136  in  FIG. 4 . When the input speed begins to decrease at  98  in  FIG. 2  and at  138  in  FIG. 4 , the torque capacity of brake  68  is decreased at a calibratable rate as shown between  102  and  104  in  FIG. 3  and at  140  in  FIG. 4 . Point  102  may also be determined by a change in direction of the relative speed across either offgoing shift element  68  or  76  or of the second oncoming shift element  66 . Between points  92  and  104 , brake  68  absorbs energy, dampening any oscillation and resulting in the smooth torque transition illustrated at  106  in  FIG. 3 . 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.