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.
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. 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. A launch device such as a torque converter or launch clutch is often employed between the engine and the transmission input shaft such that the engine can idle while the vehicle and input shaft are stationary. Also, a differential assembly may connect the transmission output shaft to the vehicle wheels, providing an additional fixed speed ratio and permitting the left and right wheel to rotate at slightly different speeds as the vehicle turns. The ratio of transmission input shaft speed to transmission output shaft speed is called the transmission ratio. When the vehicle is at low speed, the transmission is usually operated at a high ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low ratio permits an engine speed associated with quiet, fuel efficient cruising.
Some vehicles are equipped with a transfer case which directs the power to both front wheels and rear wheels. Some transfer cases provide multiple transfer case ratios between the transmission output shaft and the differential 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. When a two speed transfer case is present, the overall ratio is the product of the transmission ratio and the transfer case ratio. 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.
A transmission arrangement may include both positive and negative transmission ratios suitable for use in a low range mode of operation.
In one embodiment, a transmission includes input and output shafts and first, second, third, fourth, fifth, and sixth rotating elements. A first gearing arrangement fixedly constrains the speed of the first element to be between the speeds of the input shaft and the second element. The first gearing arrangement may be, for example, a simple planetary gear set having a sun gear fixedly coupled to the input shaft, a carrier as the first element, and a ring gear as the second element. A second gearing arrangement fixedly constrains the speed of the third element to be between the speeds of the first element and the fourth element. The second gearing arrangement may be, for example, a simple planetary gear set having a sun gear as the fourth element, a carrier as the third element, and a ring gear as the first element. A third gearing arrangement fixedly constrains the speeds of the output shaft, third element, fifth element, and sixth element. The third gearing arrangement may be, for example, two simple planetary gear sets wherein the first sun gear is the third element, the second sun gear is the fifth element, the combination of the first carrier and second ring gear is the sixth element, and the first ring gear and second carrier are fixedly coupled to the output shaft. The third gearing arrangement may be, as another example, two simple planetary gear sets wherein the second ring gear is the third element, the combination of the first sun gear and the second sun gear is the fifth element, the combination of the first ring gear and the second carrier is the sixth element, and the first carrier is fixedly coupled to the output shaft. The transmission may include a number of clutches including first, second, and third brakes that selectively hold the fifth element, the fourth element, and the second element, respectively, against rotation. First and second clutches may selectively couple the second element and the sixth element, respectively, to the input shaft. A third clutch may selectively couple the fourth element to the sixth element and a fourth clutch may selectively couple the second element to the third element. When all seven clutches are present, the third brake may be utilized as a range clutch which is engaged to launch a vehicle in low range but is disengaged to launch the vehicle in high range. The range clutch may be re-engaged in either range to establish overdrive ratios.
In another embodiment, a transmission includes a transmission case, input and output shafts, and first, second, third, fourth, fifth, sixth, seventh, and eighth rotating elements. A first gearing arrangement fixedly constrains the speed of the first element to be between the speeds of the input shaft and the second element. A second gearing arrangement fixedly constrains the speed of the third element to be between the speeds of the first element and the fourth element. A third gearing arrangement fixedly constrains the speed of the sixth element to be between the speeds of the output shaft and the third element. A fourth gearing arrangement fixedly constrains the speed of the seventh element to be between the speeds of the fifth element and the eighth element. The fifth element is fixedly or selectively coupled to the transmission case, the seventh element is fixedly or selectively coupled to the output shaft, and the eighth element is fixedly or selectively coupled to the sixth element. In an exemplary embodiment, a first brake selectively couples the fifth element to the transmission case while the seventh element is fixedly coupled to the output shaft and the eighth element is fixedly coupled to the sixth element. Second and third brakes may selectively couple the fourth element and the second element, respectively, to the transmission case. First and second clutches may selectively couple the second element and the sixth element, respectively, to the input shaft. A third clutch may selectively couple the fourth element to the sixth element and a fourth clutch may selectively couple the second element to the third element. When all seven clutches are present, the third brake may be utilized as a range clutch which is engaged to launch a vehicle in low range but is disengaged to launch the vehicle in high range. The range clutch may be re-engaged in either range to establish overdrive ratios.
In another embodiment, a vehicle is launched in reverse using a first negative transmission ratio when high range is selected and is launched in reverse using a second negative transmission ratio when low range is selected. When launched in the second negative ratio, the vehicle may optionally shift to the first negative transmission ratio. When in the first negative transmission ratio, the vehicle may shift to a third negative transmission ratio. The vehicle may also be launched using a first positive transmission ratio when high range is selected and launched in a second positive transmission ratio when low range is selected followed by a shift to the first positive transmission ratio. The shift from the second positive transmission ratio to the first positive transmission ratio may include release a range clutch. The range clutch may be re-engaged to establish a third positive transmission ratio which may be an overdrive ratio.
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. Other speed relationships, called selective speed relationships, are imposed only when particular clutches are fully engaged. A discrete ratio transmission selectively imposes a variety of speed ratios, called transmission ratios, between an input shaft and an output shaft.
Certain rotating elements can be coupled to one another, and that coupling can be fixedly coupled or selectively coupled. 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 spline 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 or 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 two speed transfer cases have step sizes exceeding 2:1. In order to minimize the parasitic drag associated with the transfer case, the transfer case 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 transmission in neutral while shifting the transfer case.
An example transmission is schematically illustrated in
Input shaft 10 is fixedly coupled to sun gear 26. Output shaft 12 is fixedly coupled to ring gear 48 and carrier 52. Carrier 22 is fixedly coupled to ring gear 38, carrier 32 is fixedly coupled to sun gear 46, and carrier 42 is fixedly coupled to ring gear 58. Input shaft 10 is selectively coupled to ring gear 28 by clutch 62 and selectively coupled to carrier 42 by clutch 68. Note that engaging clutch 62 has the effect of forcing all of the components of planetary gear set 20 to rotate as a unit. This effect can alternatively be achieved by a clutch that selectively couples carrier 22 to either sun gear 26 or ring gear 28. Ring gear 28 is selectively coupled to the combination of carrier 32 and sun gear 46 by clutch 60 and selectively held against rotation by brake 66. Sun gear 36 is selectively coupled to the combination of carrier 42 and ring gear 58 by clutch 72 and selectively held against rotation by brake 64. Finally, sun gear 56 is selectively held against rotation by brake 70.
Another example transmission is illustrated in
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. Brake 66 may be a combination of a controllable clutch and a passive one way clutch each selectively coupling ring gear 28 to transmission case 14. Such a combination may be engaged either actively by the controller or as a result of the one way clutch resisting rotation of ring gear 28 in a reverse direction. Such a combination may be desirable to improve shift quality and to reduce parasitic losses because the actively controlled portion is not called upon to transmit as much torque.
As shown in
When the driver selects drive (forward) and low range, the transmission is prepared for vehicle launch by engaging brakes 64, 66, and 70. If brake 66 is implemented as a combination of a passive one way clutch and an actively controlled clutch, then the transmission would transmit power from the engine to the wheels without active engagement. However, if transmission of power in the opposite direction is desired, such as for engine braking, then brake 66 must be actively controlled. A shift to 1st may be accomplished by gradually disengaging brake 66 while gradually engaging clutch 60. If the brake 66 includes a one way clutch, the one way clutch will disengage passively as clutch 60 is engaged, eliminating the need to actively control the disengagement. Additional shifts into the remaining forward ratios may be accomplished as described above for high range. As shown in
When the driver selects reverse and high range, the transmission is prepared for reverse launch by engaging clutches 60 and 72 and brake 64. Optionally, a shift to a higher reverse ratio may be accomplished by gradually disengaging clutch 60 while gradually engaging clutch 62. When the driver selects reverse and low range, the transmission is prepared for reverse launch by engaging clutch 72 and brakes 64 and 66. Optionally, a shift into the high range reverse ratio may be accomplished as described above for the low to 1st shift.
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.