Abstract:
A dual clutch transaxle provides eight forward gear ratios and two reverse gear ratios. The gears are arranged in seven gear planes. Four coupler assemblies are arranged to selectively sequentially engage each adjacent pair of the eight forward gear ratios to permit shifting among the eight forward gear ratios with continuous transmission of torque through the transaxle.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to the field of automatic transmissions for motor vehicles. More particularly, the disclosure pertains to an arrangement of gears, clutches, couplers, and the interconnections among them in a power transmission. 
       BACKGROUND 
       [0002]    Dual clutch transmissions are a type of transmission employing two input clutches which connect a pair of input shafts to the prime mover, typically an internal combustion engine. One clutch is used for odd numbered gears and the other clutch is used for even numbered gears. Couplers establish power flow paths between the input shafts and the transmission output. While the vehicle is driving in an odd numbered gear, the couplers for the even numbered gears may be manipulated to select the next higher or lower gear ratio, and vice versa. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]      FIG. 1  is a schematic diagram of a dual clutch transmission. 
           [0004]      FIG. 2  is a cross-sectional view of a first gear plane of the transmission of  FIG. 1 . 
           [0005]      FIG. 3  is a cross-sectional view of a second gear plane of the transmission of  FIG. 1 . 
           [0006]      FIG. 4  is a cross-sectional view of a third gear plane of the transmission of  FIG. 1 . 
           [0007]      FIG. 5  is a cross-sectional view of a fourth gear plane of the transmission of  FIG. 1 . 
           [0008]      FIG. 6  is a cross-sectional view of a fifth gear plane of the transmission of  FIG. 1 . 
           [0009]      FIG. 7  is a cross-sectional view of a sixth gear plane of the transmission of  FIG. 1 . 
           [0010]      FIG. 8  is a cross-sectional view of a seventh gear plane of the transmission of  FIG. 1 . 
           [0011]      FIG. 9  is a schematic diagram of one of the coupler assemblies of the transmission of  FIG. 1 . 
           [0012]      FIG. 10  is a schematic diagram of another dual clutch transmission. 
           [0013]      FIG. 11  is a schematic diagram of one of the coupler assemblies of the transmission of  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    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. 
         [0015]    Automobile manufacturers are under increasing pressure to improve the fuel efficiency of vehicles. One means of improving fuel efficiency is utilizing transmissions with more speed ratios, thereby operating the engine closer to its most efficient speed at various vehicle speeds. Generally, increasing the number of available speed ratios requires making the transmission physically longer. However, automobile manufacturers are also under pressure to reduce the width of the engine compartment. For transverse mounted powertrains, this severely constrains the ability to use a longer transmission. 
         [0016]    A transmission that is designed to be mounted transversely in the vehicle is called a transaxle. The input axis of a transaxle is typically aligned with the axis of the engine crankshaft. The output axis is offset such that it is close to the axis of one set of wheels. A transaxle can include a differential assembly mounted on the output axis, which allows the wheels to rotate at slightly different speeds relative to one another to accommodate the greater distance that the outside wheel must travel during a turn. 
         [0017]    Two components are fixed to one another when they are constrained to rotate as a unit in all operating conditions. Components can be fixed by a spline connection, welding, press fitting, machining from a common solid, or other means. In contrast, two components are selectively coupled by a coupler when they are constrained to rotate as a unit whenever the coupler is fully engaged and they are free to rotate at distinct speeds in at least some other operating condition. Selective coupling can be accomplished by means of a friction clutch, a dog clutch, a synchronizer, or other means. 
         [0018]    Dual clutch transaxles typically utilize parallel axis gears, sometimes called layshaft gearing, to transfer power between the input shafts and the output axis. A dual clutch transaxle typically includes one or more layshafts which are located on axes offset from both the input shaft axis and the output axis. A small gear, called a pinion, is usually fixed to each layshaft and meshes with a large gear, called the output gear, which is fixed to the differential assembly. In common arrangements, each gear ratio corresponds to a pair of gears, one on the input axis and the other on the layshaft axis. Typically, one of the gears is fixed to a shaft while the other is free to rotate about a shaft until a coupler is engaged, fixing it to the shaft. Couplers are often combined in coupler assemblies that include a sliding sleeve which is positioned by a fork. When the sleeve is pushed in one direction, one gear is coupled to a shaft. When the sleeve is pushed in the opposite direction, a different gear is coupled to the shaft. When the sleeve is positioned in the middle, both gears are free to rotate at different speeds relative to the shaft and to each other. 
         [0019]    A factor in determining the length of a transaxle can be the number of separate planes of gears. Each additional plane of gears increases the length of the transmission by the width of the gears and any required clearance. A transaxle can have one plane of gears for the pinions and output gear plus one plane of gears for every gear ratio, including reverse. Thus, a transmission with eight forward gear ratios and one reverse gear ratio would have ten planes of gears if built according to the conventional arrangement. One technique for minimizing the number of planes is utilizing a single gear on the input axis meshing with gears on two different layshafts. This achieves two gear ratios with a single plane of gears. A disadvantage of this technique, however, can be that it restricts the freedom to select gear sizes in order to establish the desired progression of gear ratios. 
         [0020]    A high speed ratio between the input shaft and the output can maximize vehicle performance and reduce the amount of energy that the input clutch must absorb during a vehicle launch. In the typical arrangement described above, the maximum gear ratio achievable is restricted by the minimum physical size of a gear on the input axis. Similarly, the minimum gear ratio achievable is restricted by the minimum physical size of a gear on the layshaft axis. A low speed ratio in top gear is desirable to allow the engine to operate near its most efficient speed while cruising on the highway. 
         [0021]    An example transmission is schematically illustrated in  FIG. 1 . Transmission input  20  is driveably connected to an engine, preferably via some form of torsional damper. Solid input shaft  22  is selectively coupled to the transmission input  20  by even clutch  24 . Hollow input shaft  26  is positioned co-axially with solid input shaft  22  and is selectively coupled to the transmission input  20  by odd clutch  28 . Even clutch  24  and odd clutch  28  are preferably friction clutches capable of transmitting torque between elements that are rotating at different speeds. Either wet or dry friction clutches are applicable. 
         [0022]    Output gear  30  is driveably connected to the vehicle wheels, preferably via a differential assembly that permits some difference in wheel speed. Two layshafts  32  and  34  are supported on axes parallel to transmission input  20 . Pinion gear  36  is fixed to layshaft  32  and in continuous meshing engagement with output gear  30 . Similarly, pinion gear  38  is fixed to layshaft  34  and in continuous meshing engagement with output gear  30 .  FIG. 2  shows a cross section through the pinion gears and illustrates the relationships among the various axes of rotation. 
         [0023]    Gears  40  and  42  are fixed to hollow input shaft  26 . Hollow intermediate shaft  44  is supported for rotation on layshaft  32 . Gear  46  is fixed to intermediate shaft  44  and is in continuous meshing engagement with gear  42 . Gears  48  and  50  are supported for rotation about layshaft  34  and are in continuous meshing engagement with gears  40  and  42  respectively. These gears are illustrated in cross section in  FIGS. 3 and 4 . Gears  52 ,  54 , and  56  are fixed to solid input shaft  22 . Gear  58  is supported for rotation about layshaft  34  and is in continuous meshing engagement with gear  52  as shown in  FIG. 5 . Gear  60  is supported for rotation about layshaft  32  and is in continuous meshing engagement with gear  54  as shown in  FIG. 6 . Hollow intermediate shaft  62  is supported for rotation on layshaft  32 . Gear  64  is fixed to intermediate shaft  62  and is in continuous meshing engagement with gear  56  as shown in  FIG. 8 . Finally, gear  66  is fixed to intermediate shaft  62 . Gear  68  is supported for rotation about layshaft  34  and is in continuous meshing engagement with gear  66  as shown in  FIG. 7 . 
         [0024]    Coupler assembly  70  can be a synchronizer assembly as typically used in manual transmissions. Coupler  70  includes a hub fixed to layshaft  34 . The hub supports a sliding sleeve which can be moved axially by an external fork. When the sleeve is moved toward the transmission input, teeth on the hub are forced into engagement with teeth on gear  48 , forcing gear  48  to rotate with layshaft  34 . When the sleeve is moved in the opposite direction, teeth on the hub are forced into engagement with teeth on gear  50 , forcing gear  50  to rotate with layshaft  34 . When the sleeve is in a central position, layshaft  34 , gear  48 , and gear  50  are free to rotate at distinct (different) speeds. Similarly, coupler assembly  72  selectively couples gears  58  and  68  to layshaft  34  and coupler  74  couples gear  60  and intermediate shaft  62  to layshaft  32 . 
         [0025]    Coupler assembly  76  selectively couples gear  46  to either layshaft  32  or gear  60 . The structure of coupler assembly  76  is illustrated in  FIG. 9 . Hub  78  is fixed to intermediate shaft  44  and supports sleeve  80 . A portion of sleeve  80  extends through a set of holes  82  in gear  46 . These holes are also shown in  FIG. 4 . When the sleeve is moved toward the transmission input, teeth  84  fixed to intermediate shaft  44  are forced into engagement with teeth  86  fixed to layshaft  32 , forcing gear  46  to rotate with layshaft  32 . When the sleeve is moved in the opposite direction, teeth  88  fixed to intermediate shaft  44  are forced into engagement with teeth  90  fixed to gear  60 , forcing gears  46  and  60  to rotate as a unit. When the sleeve is in a central position, layshaft  32 , gear  46 , and gear  60  are free to rotate at distinct speeds. 
         [0026]    Another example transmission is illustrated in  FIGS. 10 and 11 . Coupler assembly  76 ′ selectively couples gear  46  to either layshaft  32  or gear  60 . Intermediate shaft  92  is supported for rotation about layshaft  32  and fixed to gear  60 . Intermediate shaft  44 ′ is supported for rotation about intermediate shaft  60 ′ and fixed to gear  46 . Hub  78 ′ is fixed to intermediate shaft  44  and supports sleeve  80 ′. When the sleeve is moved toward the transmission input, teeth  84 ′ fixed to intermediate shaft  44 ′ are forced into engagement with teeth  86 ′ fixed to layshaft  32 , forcing gear  46  to rotate with layshaft  32 . When the sleeve is moved in the opposite direction, teeth  88 ′ fixed to intermediate shaft  44 ′ are forced into engagement with teeth  90 ′ fixed to intermediate shaft  92 , forcing gears  46  and  60  to rotate as a unit. When the sleeve is in a central position, layshaft  32 , gear  46 , and gear  60  are free to rotate at distinct speeds. 
         [0027]    A suggested number of gear teeth and approximate pitch radius for each gear are listed in Table 1. 
         [0000]                                                  TABLE 1                           Radius       #   Used In   Teeth   (mm)                                30   all   83   113.23       36   1 st , 2 nd , 5 th , 6 th     17   23.19       38   R1, R2, 3 rd , 4 th , 7 th , 8 th     21   28.65       40   3 rd     19   22.91       42   R1, 1 st , 5 th , 7 th , 8 th     69   47.15       46   R1, 1 st , 5 th , 8 th     67   45.78       48   3 rd     49   59.09       50   7 th , 8 th     51   34.85       52   4 th     23   30.42       54   R1, 1 st , 6 th     38   52.71       56   R1, R2, 1 st , 2 nd     18   21.17       58   4 th     39   51.58       60   R1, 1 st , 6 th , 8 th     29   40.22       64   R1, R2, 1 st , 2 nd     61   71.76       66   R1, R2   42   50.97       68   R1, R2   47   57.03                    
The corresponding gear ratios and step sizes are listed in Table 2.
 
         [0000]                                                                      TABLE 2                       Clutch   Couplers   Gears   Ratio   Step                                    R1   28   72R, 76R   42-46-60-54-56-64-66-68-36-   −19.07   91%                   30       R2   24   72R   56-64-66-68-36-30   −15.01   1.27       1 st     28   74R, 76R   42-46-60-54-56-64-36-30   21.05       2 nd     24   74R   56-64-36-30   16.55   1.27       3 rd     28   70L   40-48-38-30   10.19   1.62       4 th     24   72L   52-58-38-30   6.70   1.52       5 th     28   76L   42-46-36-30   4.74   1.41       6 th     24   74L   54-60-36-30   3.73   1.27       7 th     28   70R   42-50-38-30   2.92   1.28       8 th     24   70R, 76R   54-60-46-42-50-38-30   2.30   1.27                    
Table 2 also indicates which clutch is applied and which couplers must be engaged to select each gear ratio. A number of gears are used in a variety of gear ratios. In addition to establishing the power paths for fifth and sixth gear, gears  42 ,  46 ,  60 , and  54  also establish a power path between the first and second input shafts whenever gear  46  is coupled to gear  60  by coupler assembly  76 . This power path between the input shafts is used in conjunction with the second gear power path to establish the first gear power path. As a result, the achievable first gear ratio is higher than would be achievable with a conventional arrangement. Also, this power path between the input shafts is used in conjunction with the seventh gear power path to establish an eighth gear power path with a lower gear ratio than would be achievable with a conventional arrangement. Consequently, first gear and eighth gear do not require any additional gears beyond those that are present for second, fifth, sixth, and seventh gears, reducing the number of gear planes relative to a conventional arrangement.
 
         [0028]    The transmission is prepared for forward vehicle movement by coupling gear  64  to layshaft  32  via coupler assembly  74  and coupling gear  46  to gear  60  via coupler assembly  76 . In this configuration, the speed of solid input shaft  22  is related to the speed of the output by the second gear ratio. In addition, the speed of the hollow input shaft is related to the speed of the solid input shaft by the ratio of the fifth gear and sixth gear ratios. Consequently, the first gear ratio is the product of the second gear ratio and the ratio of the fifth and sixth gear ratios. 
         [0029]    From this state, the vehicle can be launched in first gear by gradually engaging clutch  28  or the vehicle can be launched in second gear by gradually engaging clutch  24 . Launching in first gear will result in better acceleration. However, the very high gear ratio can result in a very early shift from first gear to second gear which drivers could find annoying. On the other hand, launching in second gear will result in reduced acceleration and will increase the amount of heat that must be absorbed by the clutch and eventually dissipated to the environment. Using clutches  28  and  24  in conjunction addresses these issues. Initially, clutch  28  is used for performance reasons. Gradually, the torque capacity of clutch  24  is increased and the torque capacity of clutch  28  is decreased such that the torque capacity of clutch  28  is zero by the time the hollow input shaft  26  reaches the input speed. From that point, the launch is completed in second gear using clutch  24 . This control strategy has the additional benefit of dividing the heat between the two clutches. 
         [0030]    As the vehicle continues in second gear with clutch  24  engaged, coupler assembly  76  is disengaged and gear  48  is coupled to layshaft  34  via coupler assembly  70 . The shift from second to third is accomplished by gradually releasing clutch  24  while gradually engaging clutch  28 . Similarly, while the transmission is in third gear, coupler assembly  74  is disengaged and gear  58  is coupled to layshaft  34  via coupler assembly  72 . The shift from third to fourth is accomplished by gradually releasing clutch  28  while gradually engaging clutch  24 . This pattern continues up through seventh gear with each odd numbered gear selected while driving in an even numbered gear and each even numbered gear selected while driving in an odd numbered gear. Fifth gear is selected by coupling gear  46  to layshaft  32  via coupler assembly  76 . Sixth gear is selected by coupling gear  60  to layshaft  32  via coupler assembly  74 . Similarly, seventh gear is selected by coupling gear  50  to layshaft  34  via coupler assembly  70 . Finally, to engage eighth gear, gear  46  is coupled to gear  60  via coupler assembly  76  while seventh gear remains engaged. Consequently, the eighth gear ratio is equal to the seventh gear ratio divided by the ratio of the fifth and sixth gear ratios. 
         [0031]    Downshifts are accomplished similarly. In general, any even numbered gear ratio can be selected while driving in any odd numbered gear ratio except first gear. Once an even numbered gear ratio is selected, a shift to that ratio is accomplished by gradually releasing clutch  28  while gradually engaging clutch  24 . Similarly, any odd numbered gear ratio can be selected while driving in any even numbered gear ratio except eighth gear. Once an odd numbered gear ratio is selected, a shift to that ratio is accomplished by gradually releasing clutch  24  while gradually engaging clutch  28 . 
         [0032]    The transmission is prepared for reverse vehicle movement by coupling gear  68  to layshaft  34  via coupler assembly  72  and coupling gear  46  to gear  60  via coupler assembly  76 . As with forward launch, the vehicle can be launched in reverse by gradually engaging clutch  24 , clutch  28 , or some combination of the two. 
         [0033]    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.