Abstract:
Four planetary gear sets, three rotating clutches, and two brakes can be arranged to produce eight forward speed ratios and a reverse speed ratio. The first and fourth gear sets can be substantially coplanar with each other, and the second and third gear sets can be substantially coplanar with each other. The lowest forward speed ratio could be suitable for use as a special purpose low range ratio, while the remaining seven forward ratios can provide adequate span for normal driving.

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 planetary gear sets, clutches, brakes, and the interconnections among them in a power transmission. 
       BACKGROUND 
       [0002]    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 increasing the number of gear sets and clutches and consequently 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. 
         [0003]    Four wheel drive vehicles based on longitudinal powertrains utilize a transfer case behind the transmission. Many transfer cases include a driver selected low range mode which provides additional speed reduction. This low range mode is useful when additional torque is needed or when improved vehicle speed control is needed. Vehicles with transverse powertrains do not use a transfer case and therefore have not offered drivers a low range mode. 
       SUMMARY 
       [0004]    Certain transmission systems disclosed herein comprise arrangements of planetary gear sets and couplers interconnected to produce up to eight forward speed ratios and at least one reverse speed ratio. Some embodiments are particularly suited to applications that require reduced axial length because two planetary gear sets can be located in a single axial plane. In some embodiments, the sun gears of the outer planetary gear sets can be directly fixed to the ring gears of the inner planetary gear sets. The overall diameter of two gear sets located in a single plane might be excessive if the tooth number ratio of the outer gear set is too large. However, in some disclosed embodiments, a small tooth number ratio can be selected for the outer gear sets while retaining favorable spacing among the speed ratios. The eight forward speed ratios can be widely spaced. As a result, the  1 st forward ratio may be used in a special purpose low range mode similar to the low range mode achieved through a transfer case. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a schematic illustration of a first embodiment of the gearing arrangement. 
           [0006]      FIG. 2  is a table showing suggested ratios of tooth numbers for the embodiment of  FIG. 1 . 
           [0007]      FIG. 3  is a table showing the clutch application chart and resulting ratios when the embodiment of  FIG. 1  has the ratios of tooth numbers shown in  FIG. 2 . 
           [0008]      FIG. 4  is a schematic illustration of a second embodiment of the gearing arrangement. 
           [0009]      FIG. 5  is a table showing suggested ratios of tooth numbers for the embodiment of  FIG. 4 . 
           [0010]      FIG. 6  is a table showing the clutch application chart and resulting ratios when the embodiment of  FIG. 4  has the ratios of tooth numbers shown in  FIG. 5 . 
           [0011]      FIG. 7  is a lever diagram for the gearing arrangement. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may 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. 
         [0013]    Two components are fixed to one another when they are constrained to have substantially identical rotational speeds in all operating conditions. In contrast, two components are releasably coupled by a control element when they are constrained to rotate at substantially the same speed whenever the control element is fully engaged and they have distinct speeds in at least some other operating condition. A component that is releasably coupled to a non-rotating component, such as a transmission case, is said to be releasably held against rotation and the control element may be called a brake. An element has a positive speed when it rotates in one direction and a negative speed when it rotates in the opposite direction. 
         [0014]      FIG. 1  illustrates a first embodiment of a transmission according to the present invention. Input  10  is driven by the vehicle engine, preferably via a hydrodynamic torque converter with a lockup clutch (not shown). Output  12  drives the vehicle wheels via output gear  14  and a differential assembly (not shown). The transmission case  16  is mounted to vehicle structure and does not rotate with respect to the vehicle. 
         [0015]    A first simple planetary gear set includes sun gear  18 , ring gear  20 , and carrier  22 . A set of planet gears  24  are supported on carrier  22  and mesh with both sun gear  18  and ring gear  20 . Similarly, a second simple planetary gear set includes sun gear  26 , ring gear  28 , carrier  30 , and planet gears  32 . Sun gear  26  is fixed to ring gear  20  and carrier  30  is fixed to sun gear  18 . The third simple planetary gear set, including sun gear  34 , ring gear  36 , carrier  38 , and planet gears  40 , is coplanar with and radially outside the second gear set. Ring gear  28  and sun gear  34  are combined into common member  42 . Ring gear  36  is fixed to the input  10 . Finally, the fourth simple planetary gear set, including sun gear  44 , ring gear  46 , carrier  48 , and planet gears  50 , is coplanar with and radially outside the first gear set. Sun gear  44  is fixed to carrier  22  while carrier  48  is fixed to output  12 . The ratio of the number of teeth on the ring gear to the number of teeth on the sun gear is called the beta ratio. Suggested beta ratios for these gear sets are shown in  FIG. 2 . 
         [0016]    Brakes  52  and  54  and clutches  56 ,  58 , and  60  are, in some examples, hydraulically actuated multi-plate friction clutches. When a control system supplies pressurized fluid to such a clutch, a piston forces the plates together such that the releasably coupled elements are forced to rotate at the same speed. When the pressure is released, a spring forces the plates apart such that the releasably coupled elements can rotate at different speeds with minimal drag torque transmitted. Brake  52  releasably couples the combination of ring gear  20  and sun gear  26  to transmission case  16 . Similarly, brake  54  releasably couples ring gear  46  to transmission case  16 . Clutch  56  releasably couples carrier  48  to the output  12 . Clutch  58  releasably couples carrier  38  to the combination of sun gear  18  and carrier  30 . Finally, clutch  60  releasably couples the combination of sun gear  18  and carrier  30  to the input  10 . 
         [0017]      FIG. 3  shows which clutches are engaged to select each of the available eight forward and one reverse speed ratios. Shifts between adjacent ratios are accomplished by the coordinated release of one clutch and engagement of another clutch, holding two clutches in the engaged state.  FIG. 3  also shows the resulting speed ratio when the tooth counts of the gearsets have the ratios shown in  FIG. 2 . In most driving conditions, the  1 st forward ratio may be skipped and the  2 nd forward ratio used to start from a stationary condition. In situations that require more wheel torque or more precise speed control, the driver may select a low range operating mode. In this mode, the vehicle would utilize all eight forward ratios. 
         [0018]    The transmission can optionally include one way clutch  62  which passively holds the combination of ring gear  20  and sun gear  26  against rotation in one direction while permitting rotation in the opposite direction. If one way clutch  62  is present, then it is not necessary to engage brake  52  in the  1 st and  2 nd forward ratios. This simplifies the shift from the  2 nd to the  3 rd ratio because the control system does not need to actively control the release of a clutch. As clutch  58  is engaged, one way clutch  62  automatically over-runs. 
         [0019]      FIG. 4  illustrates a second embodiment of a transmission according to the present invention. Components that are common to both the first and second embodiment are labeled with the same reference number on  FIG. 1  and  FIG. 4 . 
         [0020]    A double pinion planetary gear set includes sun gear  64 , ring gear  66 , and carrier  68 . A set of inner planet gears  70  are supported on carrier  68  and mesh with sun gear  64 . A set of outer planet gears  72  are supported on carrier  68 . Each outer planet gear meshes with one of the inner planet gears and with ring gear  66 . A simple planetary gear set includes sun gear  26 , ring gear  28 , carrier  30 , and planet gears  32 . Sun gear  26  is fixed to carrier  68  and carrier  30  is fixed to sun gear  64 . Another simple planetary gear set, including sun gear  34 , ring gear  36 , carrier  38 , and planet gears  40 , is coplanar with and radially outside the second gear set. Ring gear  28  and sun gear  34  are combined into common member  42 . Ring gear  36  is fixed to the input  10 . Finally, another simple planetary gear set, including sun gear  44 , ring gear  46 , carrier  48 , and planet gears  50 , is coplanar with and radially outside the double pinion planetary gear set. Ring gear  66  and sun gear  44  are combined into common member  74 . Carrier  48  is fixed to output  12 . Suggested beta ratios for these gear sets are shown in  FIG. 5 . 
         [0021]    Brake  52  links the combination of carrier  68  and sun gear  26  to transmission case  16 . Similarly, brake  54  links ring gear  46  to transmission case  16 . Clutch  56  links carrier  48  to the output  12 . Clutch  58  links carrier  38  to the combination of sun gear  64  and carrier  30 . Finally, clutch  60  links the combination of sun gear  64  and carrier  30  to the input  10 .  FIG. 6  shows which clutches are engaged to select each of the available ratios and the resulting speed ratio when the tooth counts of the gear sets have the ratios shown in  FIG. 5 . 
         [0022]      FIG. 7  illustrates a lever diagram. Each lever corresponds to three or more elements that rotate about a common axis and are constrained to rotate with speeds that are linearly related. In other words, the speed of any element of the lever in any operating condition is the weighted average of the speeds of any two other elements of the lever in the same operating condition. Components which are constrained to have the same speed as each other in all operating conditions, as measured relative to the common axis, are considered a single element. For example, a carrier and a corresponding set of planet gears form a single element. 
         [0023]    Every lever has two elements designated as the outer elements. In any operating condition, one outer element will have the slowest speed and the other outer element will have the fastest speed. These roles can be reversed in other operating conditions. The remaining elements, designated inner elements, have intermediate speeds in each operating condition. In a lever diagram, outer elements correspond to the endpoints of a lever, while the inner elements correspond to points along the lever. 
         [0024]    A simple planetary gear set corresponds to a three node lever with the sun gear as one outer element, the ring gear as the other outer element, and the carrier as an inner element. A double pinion planetary gear set, on the other hands, corresponds to a three node lever with the sun gear and carrier as outer elements and the ring gear as the inner element. Other devices can be represented as levers in a lever diagram. Lever  76  represents the third gear set in  FIG. 1  and in  FIG. 4 . Sun gear  34  and ring gear  36  are outer elements and carrier  38  is an inner element. Similarly, lever  78  represents the fourth gear set in  FIG. 1  and in  FIG. 4 . Sun gear  44  and ring gear  46  are outer elements and carrier  48  is an inner element. 
         [0025]    A four node lever can be created from two three node levers by fixing two of the elements of the first three node lever to two corresponding elements of the second three node lever. For example, four node lever  80  represents the first two gear sets of  FIG. 1 . Outer element  82  corresponds to ring gear  28 . Inner element  84  corresponds to the combination of sun gear  18  and carrier  30 . Inner element  86  corresponds to carrier  22 . Finally, outer element  88  corresponds to the combination of ring gear  20  and sun gear  26 . Four node lever also represents two gear sets of  FIG. 4 . Outer element  82  corresponds to ring gear  28 . Inner element  84  corresponds to the combination of sun gear  26  and carrier  30 . Inner element  86  corresponds to ring gear  66 . Finally, outer element  88  corresponds to the combination of carrier  68  and sun gear  26 . 
         [0026]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.