Abstract:
A transmission gearing arrangement includes four simple planetary gear sets, a range selection clutch, and five other clutches including one brake. The transmission is operated in high range by engaging the range selection clutch and in low range by disengaging the range selection clutch. In high range, the transmission produces eight forward speed ratios and one reverse speed ratio by selective engagement of various combinations of three of the five other clutches. In low range, the transmission produces five forward speed ratios by selective engagement of various combinations of two of the other five clutches. The transmission can shift from the fourth low range ratio to the third high range ratio while the vehicle is in motion.

Description:
TECHNICAL FIELD 
     This disclosure relates to the field of automatic transmissions for motor vehicles. More particularly, the disclosure pertains to an arrangement of gears, clutches, and the interconnections among them in a power transmission. 
     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. 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. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns. 
     Some vehicles are equipped with a two speed secondary transmission such that a driver can select a high range and a low range. The high range may be selected for on-road transportation while the low range may be used to provide higher speed ratios for off-road use. In some situations, such as transitioning from on-road to off-road or from off-road to on-road conditions, it is desirable to shift between high and low range while the vehicle is moving, preferably without interrupting the flow of power to the vehicle wheels. 
     SUMMARY OF THE DISCLOSURE 
     A transmission gearing arrangement includes a range selection clutch and five other clutches. The transmission is operated in high range by engaging the range selection clutch and in low range by disengaging the range selection clutch. The transmission can shift from the fourth low range ratio to the third high range ratio while the vehicle is in motion. 
     In one embodiment, a transmission includes input and output shaft and first, second, third, fourth, and sixth rotating elements. A first gearing arrangement fixedly constrains the speed of the input shaft to be between that of the first and second elements. The first gearing arrangement may be, for example, a simple planetary gear set with the sun gear as the first element, the ring gear as the second element, and the carrier fixedly coupled to the input shaft. A second gearing arrangement fixedly constrains the speed of the output shaft to be between that of the third and fourth elements. The second gearing arrangement may be, for example, a simple planetary gear set with the sun gear as the third element, the ring gear as the fourth element, and the carrier fixedly coupled to the output shaft. A third gearing arrangement fixedly constrains the speed of the fourth element to be between zero and the speed of the sixth element. The third gearing arrangement may be, for example, a simple planetary gear set with the sun gear as the sixth element, the ring gear fixedly held against rotation, and the carrier fixedly coupled to the fourth element. A fourth gearing arrangement selectively constrains the speed of the output shaft to be between that of the second and third elements. The fourth gearing arrangement may be, for example, a simple planetary gear set with the sun gear selectively coupled to second element, the ring gear fixedly coupled to the third element, and the carrier fixedly coupled to the output shaft. A range clutch selectively couples the sixth element to the input shaft. The third element is selectively coupled to the second element by a second clutch and to the input shaft by a third clutch. The first element is selectively coupled to the sixth element by a fourth clutch and held against rotation by a brake. 
     In another embodiment, a transmission includes input and output shaft and first, second, third, fourth, fifth, and sixth rotating elements. A first gearing arrangement fixedly constrains the speed of the input shaft to be between that of the first and second elements. The first gearing arrangement may be, for example, a simple planetary gear set with the sun gear as the first element, the ring gear as the second element, and the carrier fixedly coupled to the input shaft. A second gearing arrangement fixedly constrains the speed of the third element, the output shaft, the fourth element, and the fifth element to be linearly related. The second gearing arrangement may be, for example, two simple planetary gear sets with the first sun gear as the fifth element, the two carriers fixedly coupled to one another and to the output shaft, the second ring gear as the fourth element, and first ring gear fixedly coupled to the second sun gear as the third element. A third gearing arrangement fixedly constrains the speed of the fourth element to be between zero and the speed of the sixth element. The third gearing arrangement may be, for example, a simple planetary gear set with the sun gear as the sixth element, the ring gear fixedly held against rotation, and the carrier fixedly coupled to the fourth element. A range clutch selectively couples the sixth element to the input shaft. The third element is selectively coupled to the second element by a second clutch and to the input shaft by a third clutch. The first element is selectively coupled to the sixth element by a fourth clutch and held against rotation by a brake. A fifth clutch selectively couples the second element to the fifth element. 
     In another embodiment, a vehicle having a transmission with a range selection clutch and five other clutches is operated in high range with the range selection clutch disengaged and operated in a low range with the range selection clutch engaged in response to a driver selection. In the high range, the transmission is operated in up to eight distinct forward speed ratios and one reverse speed ratio by selective engagement of the five other clutches in combinations of three. In the low range, the transmission is operated in up to five distinct forward speed ratios by selective engagement of the five other clutches in combinations of two. A transition from low range to high range can be accomplished by sequentially engaging a third of the other five clutches and then disengaging the range clutch. A transition from high range to low range can be accomplished by sequentially engaging the range clutch and then disengaging one of the other five clutches. These transitions can be accomplished while the vehicle is moving in response to a change in range selection by a vehicle driver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a transmission gearing arrangement; 
         FIG. 2  is a lever diagram corresponding to the gearing arrangement of  FIG. 1 ; 
         FIG. 3  is a clutch application chart indicating the state of each clutch in the gearing arrangement of  FIG. 1  and the resulting speed ratios when operated in a high range; 
         FIG. 4  is a clutch application chart indicating the state of each clutch in the gearing arrangement of  FIG. 1  and the resulting speed ratios when operated in a low range; 
         FIG. 5  is a flow diagram illustrating the steps required to transition between low range and high range; and 
         FIG. 6  is a clutch application chart indicating the state of each clutch in the gearing arrangement of  FIG. 1  and the resulting speed ratio when operated in a combined low and high range. 
     
    
    
     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 may 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 may 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. 
     A gearing arrangement is a collection of rotating elements and clutches configured to impose specified speed relationships among elements. Some speed relationships, called fixed speed relationships, are imposed regardless of the state of any clutches. A gearing arrangement imposing only fixed relationships is called a fixed gearing arrangement. Other speed relationships are imposed only when particular clutches are fully engaged. A gearing arrangement that selectively imposes speed relationships is called a shiftable gearing arrangement. A discrete ratio transmission has a shiftable gearing arrangement that selectively imposes a variety of speed ratios between an input shaft and an output shaft. 
     A group of elements are fixedly coupled to one another if they are constrained to rotate as a unit in all operating conditions. Elements may be fixedly coupled by spline connections, welding, press fitting, machining from a common solid, or other means. Slight variations in rotational displacement between fixedly coupled elements can occur such as displacement due to lash or shaft compliance. In contrast, two elements are selectively coupled by a clutch when the clutch constrains them to rotate as a unit whenever the clutch is fully engaged and they are free to rotate at distinct speeds in at least some other operating condition. Clutches include actively controlled devices such as hydraulically or electrically actuated clutches and passive devices such as one way clutches. A clutch that holds an element against rotation by selectively connecting the element to the housing may be called a brake. 
     Shifts among speed ratios may be performed without interrupting the flow of power from the input shaft to the output shaft by carefully coordinating the engagement of one clutch with the disengagement of another clutch. During the transition, one of both of these clutches must transmit torque between elements moving at different speeds. In such a condition, heat is absorbed and dissipated by the clutch. The amount of energy absorbed is larger when the ratio of the two speed ratios involved, called the step size, is higher. Sometimes, providing a clutch with enough energy absorption capability dictates the sizing of the clutch and increases the amount of parasitic drag the clutch causes when disengaged. Also, shifts with very large step sizes are difficult to perform without generating large torque disturbances at the output shaft which may be uncomfortable for vehicle occupants. 
     Typical secondary transmissions have step sizes exceeding 2:1 . In order to minimize the parasitic drag associated with the secondary transmission, the secondary transmission is often configured such that shifting between the low range and high range is only possible when the vehicle is stationary. If range shifts are allowed while the vehicle is moving, it may be necessary to interrupt the flow of power by putting the primary transmission in neutral while shifting the secondary transmission. 
     An example transmission is schematically illustrated in  FIG. 1 . The transmission utilizes four simple planetary gear sets  20 ,  30 ,  40 , and  50 . A simple planetary gear set is a type of fixed gearing arrangement. A planet carrier  22  rotates about a central axis and supports a set of planet gears  24  such that the planet gears rotate with respect to the planet carrier. External gear teeth on the planet gears mesh with external gear teeth on a sun gear  26  and with internal gear teeth on a ring gear  28 . The sun gear and ring gear are supported to rotate about the same axis as the carrier. A simple planetary gear set imposes the fixed speed relationship that the speed of the carrier is between the speed of the sun gear and the speed of the ring gear. (This relationship is defined to include the condition in which all three rotate at the same speed.) More specifically, the speed of the carrier is a weighted average of the speed of the sun gear and the speed of the ring gear with weighting factors determined by the number of teeth on each gear. Similar speed relationships are imposed by other known types of fixed gearing arrangements. For example, a double pinion planetary gear set constrains the speed of the ring gear to be a weighted average between the speed of the sun gear and the speed of the carrier. A suggested ratio of gear teeth for each planetary gear set in  FIG. 1  is listed in Table 1. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Ring 28/Sun 26 
                 2.12 
               
               
                   
                 Ring 38/Sun 36 
                 2.14 
               
               
                   
                 Ring 48/Sun 46 
                 1.73 
               
               
                   
                 Ring 58/Sun 56 
                 3.56 
               
               
                   
                   
               
             
          
         
       
     
     Input shaft  10  is fixedly coupled to carrier  32 . Output shaft  12  is fixedly coupled to carrier  42  and carrier  52 . Ring gear  28  is fixedly held against rotation by transmission case  14 . Carrier  22  is fixedly coupled to ring gear  58 . Ring gear  48  is fixedly coupled to sun gear  56 . Sun gear  36  is selectively coupled to sun gear  26  by clutch  62  and selectively held against rotation by brake  64 . Input shaft  10  and carrier  32  are selectively coupled to sun gear  26  by range clutch  70  and selectively coupled to ring gear  48  and sun gear  56  by clutch  66 . Ring gear  38  is selectively coupled to sun gear  46  by clutch  60  and selectively coupled to ring gear  48  and sun gear  56  by clutch  68 . 
       FIG. 2  describes the transmission of  FIG. 1  in the form of a lever diagram. Gear elements which rotate about a common axis and have speeds with a fixed linear relationship are shown along a lever according to their relative speeds. The two elements that have the most extreme speeds are shown at the endpoints of the lever. The remaining elements are shown at intermediate points. The 1st through 6th rotating elements each correspond to one or more planetary gear elements. Gear sets  20  and  30  correspond directly to three node levers with the sun gear at one endpoint, the ring gear at the opposite endpoint, and the carrier at an intermediate point. Specifically, the 1st element corresponds to sun gear  36 , the 2nd element corresponds to ring gear  38 , the 4th element corresponds to carrier  22 , and the 6th element corresponds to sun gear  26 . Four node lever  72  corresponds to gear sets  40  and  50 , with the 3rd element corresponding to ring gear  48  and sun gear  56 , the 4th element corresponding to ring gear  58 , and the 5th element corresponding to sun gear  46 . Any four element fixed gearing arrangement that imposes the designated speed relationships with appropriate weighting factors may be substituted for gear sets  40  and  50  of  FIG. 1  without impacting the transmission speed ratios. Any combination of two planetary gear sets with two elements of each fixedly connected to two elements of the other forms a four element fixed gearing arrangement. Some fixed gearing arrangements will be preferable to others in terms of packaging, efficiency, and planet gear speeds. 
     The clutches and brakes may be hydraulically actuated multi-plate clutches or other types of clutches that are actively engaged and disengaged by a controller. As discussed below, it is possible to operate the transmission such that the controller does not need to accurately modulate the torque capacity of range clutch  70  and range clutch  70  does not absorb appreciable energy during shifts. Consequently, the design of range clutch  70  can be optimized for very low parasitic drag. Also, brake  64  can be a combination of a controllable friction clutch and a passive one way clutch. Such a combination may be engaged either actively by the controller or as a result of the one way clutch resisting rotation in a reverse direction. 
     As shown in  FIGS. 3-5 , engaging the clutches in combinations of three establishes a variety of forward and reverse speed ratios between input shaft  10  and output shaft  12 . An X indicates that the clutch is engaged to establish the speed ratio. When the gear sets of  FIG. 1  have tooth numbers as indicated in Table 1, the speed ratios have the values indicated in  FIGS. 3-5 . 
       FIG. 3  indicates which clutches are applied during high range operation. Range clutch  70  is disengaged in all high range gear states. All shifts between adjacent speed ratios can be accomplished by the coordinated release of one clutch and the engagement of one other clutch. Similarly, all two-step shifts and many of the three step shifts can be accomplished by the coordinated release of one clutch and engagement of one other clutch. If the transmission is equipped with a launch device such as a torque converter or launch clutch, then the transmission is prepared for forward driving by engaging clutches  62  and  66  and brake  64 . Alternatively, brake  64  can be utilized as a launch clutch. For reverse driving, clutch  60  is engaged instead of clutch  66 .  FIG. 4  indicates which clutches are applied during low range forward operation. Range clutch  70  is engaged in all low range gear states. 
       FIG. 5  illustrates a process for transitioning between low range and high range in response to a driver changing the selected range. Operation in high range is shown at  80 . Two sequences of clutch transitions effect the change from high range to low range. The controller selects between these two sequences at  82  based on vehicle speed. If the vehicle is stopped or nearly stopped, the controller disengages all currently engaged clutches except clutch  64  at  84 , placing the transmission in neutral. Then, clutch  70  is engaged at  86 . The only energy absorbed by clutch  70  during this engagement is the energy associated with the inertia of transmission components, which is small because all transmission components are rotating slowly if at all. Because the transmission is still in neutral even after clutch  70  is engaged, very little torque disturbance is transmitted to the output shaft. At  88 , clutch  60  is engaged. At that point, the transmission in is Low 1, and operation in low mode commences at  90 . If the vehicle is moving, a different sequence of transitions is selected at  82 . At  92 , the transmission is shifted to 3rd gear if it was in some other gear. If this shift is not allowed due to excessive vehicle speed, the controller waits until vehicle speed decreases and then shifts into 3rd gear. At  94 , clutch  70  is engaged. In 3rd gear, sun gear  26  and the input shaft, the two elements coupled by clutch  70 , rotate at the same speed. Consequently, engaging clutch  70  does not change the speed of any elements or cause any output torque disturbance. Then, clutch  62  is disengaged at  96 , placing the transmission in Low 4, and operation in low mode commences at  90 . The transmission continues to transmit power from the input shaft to the output shaft throughout the transition from high mode to low mode by this later sequence. 
     Similarly, two sequences of clutch transitions effect the change from low mode to high mode. The controller selects between these two sequences at  98  based on vehicle speed. If the vehicle is stopped or nearly stopped, the controller disengages all currently engaged clutches except clutch  64  at  100 , placing the transmission in neutral. Then, clutches  60  and  62  are engaged at  102  placing the transmission in 3rd gear. Operation in high mode commences at  80 . If the vehicle is moving, a different sequence of transitions is selected at  98 . At  104 , the transmission is shifted to Low 4 if it was in some other gear. At  106 , clutch  62  is engaged. Then, clutch  70  is disengaged at  108 , placing the transmission in 3rd gear, and operation in high mode commences at  80 . The transmission continues to transmit power from the input shaft to the output shaft throughout the transition from high mode to low mode by this later sequence. 
       FIG. 6  is a clutch application chart for a combined low and high range mode of operation. In this mode of operation, the transmission is automatically shifted between low range and high range based on vehicle speed or other factors like accelerator pedal position. At low speed, the transmission operates in low range, utilizing Low 1 through Low 4 speed ratios. If a further upshift is indicated, the transmission first shifts into 3rd gear by engaging clutch  62  and then disengaging clutch  70 . At higher speeds, the transmission operates in high range, utilizing 3rd through 8th gears. If a further downshift is indicated while operating in 3rd gear, the transmission first shifts into low range by engaging clutch  70  and then releasing clutch  62 . 
     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 may be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments may 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 may 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 may be desirable for particular applications.