Abstract:
A transmission includes four planetary gearsets and five torque transmitting devices operative to provide eight forward speed ratios. At least one of the torque transmitting devices is engaged during two different speed ratios. The torque transmitting device includes a piston partially defining two separate and independently pressurizable chambers. Accordingly, the transmission enables both chambers to be pressurized for speed ratios in which a high torque capacity is required of the torque transmitting device, and only one of the chambers to be pressurized for speed ratios in which a low torque capacity is required of the torque transmitting device.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/942,690, filed Jun. 8, 2007, and which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This invention relates to transmissions having planetary gear arrangements, torque transmitting devices for selectively coupling members of the planetary gear arrangements to other members, and pistons partially defining separate pressure chambers for actuating the torque transmitting devices. 
     BACKGROUND OF THE INVENTION 
     Certain multi-speed transmissions are characterized by differences in the amount of torque a clutch is subjected to in various gear states. The torque capacity of a clutch is, at least in part, determined by the axial force exerted on the clutch plates and discs by a hydraulically actuated piston. The force of the piston is determined by the pressure of the hydraulic fluid and the surface area of the piston exposed to the hydraulic fluid. Thus, the surface area of a clutch piston exposed to hydraulic pressure must be sized to produce the amount of axial force necessary for the highest torque condition of the clutch. 
     A clutch with a large torque capacity has a large gain between hydraulic pressure exerted on the piston and clutch torque. Small errors in pressure control may result in large clutch torque errors. In speed ratios with lower mechanical gains, the resulting clutch torque errors can be a significant percentage of the total torque required to complete a shift. 
     SUMMARY OF THE INVENTION 
     A transmission includes an input member, an output member, a stationary member, and first, second, third, and fourth planetary gearsets. Each of the gearsets has respective first, second, and third members. The first member of the first planetary gearset is continuously operatively connected to the first component of the second planetary gearset for unitary rotation. The second member of the second planetary gearset is continuously operatively connected to the second member of the third planetary gearset for unitary rotation. The third member of the first planetary gearset is continuously operatively connected to the third member of the fourth planetary gearset for unitary rotation. The first member of the third planetary gearset is continuously operatively connected to the first member of the fourth planetary gearset for unitary rotation. 
     The second member of the first planetary gearset is continuously operatively connected to the input member for unitary rotation. The second member of the third planetary gearset is continuously operatively connected to the output member for unitary rotation. 
     The transmission also includes first, second, third, fourth, and fifth torque transmitting devices that are operative to selectively connect members of the planetary gearsets with the input member, the stationary member, or with other members of the planetary gearsets to provide at least eight forward speed ratios. At least one of the torque transmitting devices includes a piston that partially defines two separate pressure chambers and that is selectively moveable in response to hydraulic pressure in the pressure chambers. 
     The transmission provided herein improves upon the prior art because, where a relatively high amount of torque capacity is required by one of the torque transmitting devices, both chambers of the piston may be pressurized to provide the required torque capacity. Where a relatively low amount of torque capacity is required by the torque transmitting device, only one of the chambers may be pressurized, thereby reducing the amount of torque disturbance resulting from pressure errors. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a lever diagram of a transmission in accordance with the invention; 
         FIG. 2  shows a stick diagram corresponding with the lever diagram of  FIG. 1 ; 
         FIG. 3  is a table indicating the torque capacity required of the torque transmitting devices of the transmission of  FIGS. 1 and 2  during different speed ratios; 
         FIG. 4  is a schematic side view of a portion of the transmission of  FIGS. 1 and 2 , including one of the torque transmitting devices; and 
         FIG. 4   a  is a schematic depiction of a pump and a portion of a hydraulic circuit of the transmission. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a lever diagram of a transmission  10  in accordance with the invention. The mechanisms will be described with specific reference to the stick diagram of  FIG. 2 , wherein like reference numerals refer to like components from  FIG. 1 . Referring to  FIGS. 1 and 2 , the transmission  10  includes a first planetary gearset  14 , a second planetary gearset  18 , a third planetary gearset  22 , and a fourth planetary gearset  26 . Each of the planetary gearsets  14 ,  18 ,  22 ,  26  includes respective first, second, and third members. 
     More specifically, planetary gearset  14  includes a sun gear member  30 , a ring gear member  34 , and a planet carrier  38 . Planet gears  40  are rotatably mounted with respect to the planet carrier  38  and are meshingly engaged with the ring gear member  34  and the sun gear member  30 . Planetary gearset  18  includes a sun gear member  42 , a ring gear member  46 , and a planet carrier  50 . Planet gears  52  are rotatably mounted with respect to the planet carrier  50  and are meshingly engaged with the ring gear member  46  and the sun gear member  42 . Planetary gearset  22  includes a sun gear member  54 , a ring gear member  58 , and a planet carrier  62 . Planet gears  64  are rotatably mounted with respect to the planet carrier  62  and are meshingly engaged with the ring gear member  58  and the sun gear member  54 . Planetary gearset  26  includes a sun gear member  66 , a ring gear member  70 , and a planet carrier  74 . Planet gears  76  are rotatably mounted with respect to the planet carrier  74  and are meshingly engaged with the ring gear member  70  and the sun gear member  66 . 
     It should be noted that, where used in the claims, first, second, and third members of planetary gearsets do not necessarily refer to a member of a particular type; thus, for example, a first member may be any one of a ring gear member, sun gear member, or planet carrier assembly member. Similarly, as used in the claims, the respective first, second, or third members of two or more gearsets may or may not be the same type of member. 
     The transmission  10  also includes an input member  80  that is continuously operatively connected to the planet carrier  38  for unitary rotation therewith. An output member  84  is continuously operatively connected to the planet carrier  62  for unitary rotation therewith. Sun gear member  30  and sun gear member  66  are continuously operatively connected to one another for unitary rotation by interconnecting member  88 . Planet carrier  50  and planet carrier  62  are continuously operatively connected to one another for unitary rotation by interconnecting member  92 . Sun gear member  42  and ring gear member  34  are continuously operatively connected to one another for unitary rotation by interconnecting member  96 . Planet carrier  74  and ring gear member  58  are continuously operatively connected to one another for unitary rotation by interconnecting member  100 . 
     The transmission  10  also includes a plurality of torque transmitting devices that are selectively engageable to couple members of the planetary gearsets with the input member, the stationary member, or with other members of the planetary gearsets. Torque transmitting device  104  is a clutch that is configured to selectively couple the input member  80  and planet carrier  38  to ring gear member  54  for unitary rotation. Torque transmitting device  108  is a clutch that is configured to selectively couple interconnecting member  96 , sun gear member  42 , and ring gear member  34  to sun gear member  54  for unitary rotation. Torque transmitting device  112  is a clutch configured to selectively couple ring gear member  46  to sun gear member  54  for unitary rotation. Torque transmitting device  116  is a brake that is configured to selectively couple interconnecting member  88 , sun gear member  30 , and sun gear member  66  to a stationary member, such as transmission housing  118 . Torque transmitting device  120  is a brake configured to selectively couple ring gear member  70  to the housing  118 . 
     Referring to  FIG. 3 , a shift logic sequence for torque transmitting devices  104 ,  108 ,  112 ,  116 ,  120  is depicted that provides eight forward speed ratios and one reverse speed ratio between the input member  80  and the output member  84 . Referring to  FIGS. 1-3 , a reverse speed ratio is achieved when torque transmitting devices  116 ,  120 , and  112  are engaged and torque transmitting devices  108  and  104  are disengaged. 
     A first forward speed ratio is achieved when torque transmitting devices  116 ,  120 , and  104  are engaged and torque transmitting devices  108  and  112  are disengaged. A second forward speed ratio is achieved when torque transmitting devices  116 ,  120 , and  108  are engaged and torque transmitting devices  104  and  112  are disengaged. A third forward speed ratio is achieved when torque transmitting devices  120 ,  108 , and  104  are engaged and torque transmitting devices  116  and  112  are disengaged. A fourth forward speed ratio is achieved when torque transmitting devices  120 ,  108 , and  112  are engaged and torque transmitting devices  116  and  104  are disengaged. A fifth forward speed ratio is achieved when torque transmitting devices  120 ,  104 , and  112  are engaged and torque transmitting devices  116  and  108  are disengaged. A sixth forward speed ratio is achieved when torque transmitting devices  108 ,  104 , and  112  are engaged and torque transmitting devices  116  and  120  are disengaged. A seventh forward speed ratio is achieved when torque transmitting devices  116 ,  104 , and  112  are engaged and torque transmitting devices  120  and  108  are disengaged. An eighth forward speed ratio is achieved when torque transmitting devices  116 ,  108 , and  112  are engaged and torque transmitting devices  120  and  104  are disengaged. 
     Referring to  FIG. 4 , wherein like reference numbers refer to like components from  FIGS. 1 and 2 , torque transmitting device  120  is schematically depicted. Torque transmitting device  120  includes a plurality of evenly-spaced pressure plates  124  operatively connected to the housing  118 . Torque transmitting device  120  also includes a plurality of discs  128  that are spaced apart from one another and operatively connected to the ring gear member  70 . At least part of each plate  124  is disposed between two discs  128 , as understood by those skilled in the art. 
     The torque transmitting device  120  also includes a piston  132  that is biased by a return spring, i.e., belleville spring  134 , into a disengaged position as shown in  FIG. 4 . When the piston  132  is in the disengaged position, it does not act on the plates  124  and discs  128 ; thus the discs  128  and the ring gear member  70  are free to rotate with respect to the plates  124  and the housing  118 . The piston  132  is selectively moveable axially into engagement with the plates  124  and discs  128 , thereby compressing the plates  124  and discs  128  together such that the plates  124  and discs  128 , and therefore the ring gear member  70  and the housing  118 , cannot rotate with respect to each other. 
     The surface of the piston  132  is characterized by a first area  136  and a second area  140 . The first area  136  cooperates with transmission structure  144  to define a first piston chamber  150 , and the second area  140  cooperates with the transmission structure  144  to define a second piston chamber  154 . The first and second chambers  150 ,  154  are in selective fluid communication with a source of pressurized fluid, such as the pump (shown at  160  in  FIG. 4   a ). 
     More specifically, and with reference to  FIGS. 4 and 4   a , valve  164  is selectively moveable between open and closed positions. When valve  164  is in its open position, the first chamber  150  is in fluid communication with the pump  160  via conduit  168 . When valve  164  is in its closed position, valve  164  prevents fluid communication between the pump  160  and the first chamber  150 . Valve  172  is selectively moveable between open and closed positions. When valve  172  is in its open position, the second chamber  154  is in fluid communication with the pump via conduit  176 . When valve  172  is in its closed position, valve  172  prevents fluid communication between the second chamber  154  and the pump  160 . Valves  164 ,  172  include actuators, such as solenoids, to cause the valves  164 ,  172  to move between their respective open and closed positions. The valves  164 ,  172  are operatively connected to a controller  180  to be controlled thereby. 
     The transmission structure  144  defines three axially oriented surfaces  184 ,  188 ,  192 . The piston  132  includes three axially oriented surfaces  196 ,  200 ,  204 . Surfaces  184  and  196  are positioned to remain in contact with one another during axial translation of the piston  132 ; surfaces  188  and  200  are positioned to remain in contact with one another during axial translation of the piston  132 ; and surfaces  192  and  204  are positioned to remain in contact with one another during axial translation of the piston  132 . 
     Chamber  150  is sealed by the contact between surfaces  184  and  196 , and by contact between surfaces  188  and  200 . Chamber  154  is sealed by contact between surfaces  188  and  200  and by contact between surfaces  192  and  204 . Seals (not shown) may be employed between surfaces  184 ,  188 ,  192 ,  196 ,  200 ,  204  to further seal the first and second chambers  150 ,  154 . The first and second chambers  150 ,  154  are not in fluid communication with one another; that is, fluid or fluid pressure from conduit  168  affects the first chamber  150  but not the second chamber  154 . Similarly, fluid or fluid pressure from conduit  176  affects the second chamber  154  but not the first chamber  150 . Accordingly, the first and second chambers  150 ,  154  may be separately and independently pressurized by the controller  180 . 
     More specifically, by opening valve  164  and maintaining valve  172  in its closed position, the controller  180  causes the first chamber  150  to be pressurized while the second chamber  154  remains unpressurized. By opening valve  172  and maintaining valve  164  in its closed position, the controller  180  causes the first chamber  150  to remain unpressurized while the second chamber  154  is pressurized. The controller causes concurrent pressurization of both chambers  150 ,  154  by causing both valves  164 ,  172  to be concurrently open. 
     The first area  136  is arranged such that pressure in chamber  150  acts on the first area  136  to overcome the bias of the belleville spring  134  and move the piston  132  axially into engagement with the plates  124  and discs  128 . Similarly, the second area  140  is arranged such that pressure in chamber  154  acts on the second area  140  to overcome the bias of the belleville spring  134  and move the piston  132  axially into engagement with the plates  124  and discs  128 . 
     Pressurizing only one of the chambers  150 ,  154  results in less piston force, and therefore less torque capacity of the torque transmitting device  120 , than pressurizing both of the chambers  150 ,  154 . A given amount of pressure error with only one of the chambers  150 ,  154  pressurized results in a smaller amount of torque disturbance than the same amount of pressure error subjected to both chambers  150 ,  154 . 
     Torque transmitting devices  116  and  112  are similar to torque transmitting device  120  in having respective pistons with separate chambers that may be separately and independently pressurized by controller  180 . As shown in  FIG. 3 , relatively high torque capacity is required from torque transmitting device  116  for the reverse and first speed ratios, and relatively low torque capacity is required from torque transmitting device  116  for the second, seventh, and eighth speed ratios. Relatively high torque capacity is required from torque transmitting device  120  for the reverse, first, second and third speed ratios, and relatively low torque capacity is required from torque transmitting device  120  for the fourth and fifth speed ratios. Relatively high torque capacity is required from torque transmitting device  112  for the reverse speed ratio, and relatively low torque capacity is required from torque transmitting device  112  for the fourth, fifth, sixth, seventh, and eighth speed ratios. 
     Where a relatively high torque capacity is required from any of torque transmitting devices  116 ,  120 ,  112 , the controller  180  is programmed to pressurize both piston chambers of the torque transmitting device. Where a relatively low torque capacity is required from any of torque transmitting devices  116 ,  120 ,  112 , the controller  80  is programmed to pressurize only one of the piston chambers of the torque transmitting device. 
     More specifically, both of the chambers of the piston of torque transmitting device  116  are pressurized in the reverse and first speed ratios, and only one of the chambers of the piston of torque transmitting device  116  is pressurized while shifting into the second, seventh, and eighth speed ratios. Both of the chambers  150 ,  154  of the piston  132  of torque transmitting device  120  are pressurized in the reverse, first, second, and third speed ratios, and only one of the chambers  150 ,  154  of the piston  132  of torque transmitting device  120  is pressurized while shifting into the fourth and fifth speed ratios. Both of the chambers of the piston of torque transmitting device  112  are pressurized in the reverse speed ratio, and only one of the chambers of the piston of torque transmitting device  112  is pressurized while shifting into the fourth, fifth, sixth, seventh, and eighth speed ratios. It should be noted that, where only one chamber is pressurized during shifting, it may be desirable to pressurize both areas after the shift in order to reduce overall line pressure (pump pressure) for improved fuel economy. 
     Thus, for speed ratios where a large amount of clutch torque capacity is required to finish a shift, both piston chambers or areas will be pressurized. While shifting into speed ratios where smaller amounts of clutch torque capacity is required, only one of the chambers or areas will be pressurized. The result is that any pressure error induced torque disturbance is small compared to the torque required to complete the shift. In addition, using the smaller area to stroke the clutch reduces the oil volume needed to start the shift. The result is reduced shift delay. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.