Patent Publication Number: US-7717819-B2

Title: 8-speed transmission

Description:
TECHNICAL FIELD 
     The present invention relates to a power transmission in a front wheel drive configuration having three planetary gear sets, three external gear sets and a final drive gear set that are controlled by five torque-transmitting devices to provide eight forward speed ratios and one reverse speed ratio. 
     BACKGROUND OF THE INVENTION 
     Passenger vehicles include a powertrain that is comprised of an engine, multi-speed transmission, and a differential or final drive. The multi-speed transmission increases the overall operating range of the vehicle by permitting the engine to operate through its torque range a number of times. The number of forward speed ratios that are available in the transmission determines the number of times the engine torque range is repeated. Early automatic transmissions had two speed ranges. This severely limited the overall speed range of the vehicle and therefore required a relatively large engine that could produce a wide speed and torque range. This resulted in the engine operating at a specific fuel consumption point during cruising, other than the most efficient point. Therefore, manually-shifted (countershaft transmissions) were the most popular. 
     With the advent of three- and four-speed automatic transmissions, the automatic shifting (planetary gear) transmission increased in popularity with the motoring public. These transmissions improved the operating performance and fuel economy of the vehicle. The increased number of speed ratios reduces the step size between ratios and therefore improves the shift quality of the transmission by making the ratio interchanges substantially imperceptible to the operator under normal vehicle acceleration. 
     Six-speed transmissions offer several advantages over four- and five-speed transmissions, including improved vehicle acceleration and improved fuel economy. While many trucks employ power transmissions having six or more forward speed ratios, passenger cars are still manufactured with three- and four-speed automatic transmissions and relatively few five- or six-speed devices due to the size and complexity of these transmissions. 
     Seven-, eight- and nine-speed transmissions provide further improvements in acceleration and fuel economy over six-speed transmissions. However, like the six-speed transmissions discussed above, the development of seven-, eight- and nine-speed transmissions has been precluded because of complexity, size and cost. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved transmission in a front wheel drive configuration having three planetary gear sets, three external gear sets and a final drive gear set controlled to provide eight forward speed ratios and one reverse speed ratio. 
     The transmission family of the present invention has three planetary gear sets positioned along first and second axes, wherein each planetary gear set includes a first, second and third member, which members may comprise a sun gear, a ring gear, or a planet carrier assembly member, in any order. 
     In referring to the first, second and third gear sets in this description and in the claims, these sets may be counted “first” to “third” in any order in the drawing i.e., left to right, right to left, etc.). Additionally, the first, second or third members of each gear set may be counted “first” to “third” in any order in the drawing (i.e., top to bottom, bottom to top, etc.) for each gear set. 
     Each carrier member can be either a single-pinion carrier member (simple) or a double-pinion carrier member (compound). Embodiments with long pinions are also possible. 
     An interconnecting member continuously connects the third member of the second planetary gear set with the first member of the third planetary gear set. 
     Three external gear sets are positioned for transferring torque between the first and second axes, and each including first and second intermeshed external gears. A final drive external gear set is connected to an output member. 
     The input member is continuously connected with the second member of the first planetary gear set. The output member is continuously connected with the second external gear (driven gear) of the final drive external gear set. 
     A first torque-transmitting device, such as a brake, selectively connects the first member of the first planetary gear set with a stationary member (transmission housing/casing). 
     A second torque-transmitting device, such as a clutch, selectively connects the first external gear of the third external gear set with the input member. 
     A third torque-transmitting device, such as a clutch, selectively connects the second member of the second planetary gear set with the third member of the third planetary gear set. 
     A fourth torque-transmitting device, such as a clutch, selectively connects the first external gear of the second external gear set with the first external gear of the third external gear set. 
     A fifth torque-transmitting device, such as a clutch, selectively connects the second external gear of the first external gear set with the third member of the third planetary gear set. 
     The five torque-transmitting devices are selectively engageable in combinations of three to yield eight forward speed ratios and one reverse speed ratio. 
     A variety of speed ratios and ratio spreads can be realized by suitably selecting the tooth ratios of the planetary and external gear sets. 
     The above features 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   a  is a schematic representation of a powertrain including a planetary transmission in accordance with the present invention; 
         FIG. 1   b  is a truth table and chart depicting some of the operating characteristics of the powertrain shown in  FIG. 1   a ; and 
         FIG. 1   c  is a schematic representation of the powertrain of  FIG. 1   a  depicted in lever diagram form. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, there is shown in  FIG. 1   a  a powertrain  10  having a conventional engine and torque converter  12 , a planetary transmission  14 , and a conventional final drive mechanism  16 . The engine  12  may be powered using various types of fuel to improve the efficiency and fuel economy of a particular application. Such fuels may include, for example, gasoline; diesel; ethanol; dimethyl ether; etc. 
     The front wheel drive planetary transmission  14  includes an input member  17  continuously connected with the engine  12 , a planetary gear arrangement  18 , and an output member  19  continuously connected with the final drive mechanism  16 . The planetary gear arrangement  18  includes three planetary gear sets  20 ,  30 ,  40 , three external gear sets,  50 ,  54 ,  58  and a final drive gear set  62 . The planetary and external gear sets  20 ,  30 ,  40 ,  50 ,  54  and  58  are positioned along first and second axes  91 ,  93 . 
     The planetary gear set  20  includes a sun gear member  22 , a ring gear member  24 , and a planet carrier assembly member  26 . The planet carrier assembly member  26  includes a plurality of pinion gears  27  rotatably mounted on a carrier member  29  and disposed in meshing relationship with both the ring gear member  24  and the sun gear member  22 . 
     The planetary gear set  30  includes a sun gear member  32 , a ring gear member  34 , and a planet carrier assembly member  36 . The planet carrier assembly member  36  includes a plurality of pinion gears  37  rotatably mounted on a carrier member  39  and disposed in meshing relationship with both the sun gear member  32  and the ring gear member  34 . 
     The planetary gear set  40  includes a sun gear member  42 , a ring gear member  44 , and a planet carrier assembly member  46 . The planet carrier assembly member  46  includes a plurality of pinion gears  47  mounted on a carrier member  49  and are disposed in meshing relationship with both the ring gear member  44  and the sun gear member  42 . 
     The external gear set  50  includes first and second intermeshed external gears  51  and  52 . The external gear set  54  includes first and second intermeshed external gears  55  and  56 . The external gear set  58  includes first and second intermeshed external gears  59  and  60 . The external gear sets  50 ,  54 ,  58  are positioned to transfer torque between the first and second axes  91 ,  93 . 
     The final drive external gear set  62  includes the external drive gear  63  and the external driven gear  64 . 
     The planetary gear arrangement also includes five torque-transmitting devices  80 ,  82 ,  83 ,  84  and  85 . The torque-transmitting device  80  is a stationary-type torque-transmitting device, commonly termed brake or reaction clutch. The torque-transmitting devices  82 ,  83 ,  84  and  85  are rotating-type torque-transmitting devices, commonly termed clutches. 
     The input member  17  is continuously connected with the planet carrier assembly member  26  of the planetary gear set  20 . The output member  19  is continuously connected with the driven external gear  64  of the final drive gear set  62 . 
     An interconnecting member  70  continuously connects the ring gear member  34  of the planetary gear set  30  with the sun gear member  42  of the planetary gear set  40 . 
     The sun gear member  22  of the planetary gear set  20  is continuously connected with the first external gear  51  of the first external gear set  50 . The ring gear member  24  of the planetary gear set  20  is continuously connected with the first external gear  55  of the second external gear set  54 . The sun gear member  32  of the planetary gear set  30  is continuously connected with the second external gear  56  of the second external gear set  54 . The sun gear member  42  of the planetary gear set  40  is continuously connected with the second external gear  60  of the third external gear set  58 . The planet carrier assembly member  46  of the planetary gear set  40  is continuously connected with the external drive gear  63  of the final gear set  62 . 
     A first torque-transmitting device, such as brake  80 , selectively connects the sun gear member  22  of the planetary gear set  20  with the transmission housing  90 . A second torque-transmitting device, such as clutch  82 , selectively connects the first external gear  59  of the third external gear set  58  with the input member  17 . A third torque-transmitting device, such as clutch  83 , selectively connects the planet carrier assembly member  36  of the planetary gear set  30  with the ring gear member  44  of the planetary gear set  40 . A fourth torque-transmitting device, such as clutch  84 , selectively connects the first external gear  55  of the second external gear set  54  with the first external gear  59  of the third external gear set  58 . A fifth torque-transmitting device, such as clutch  85 , selectively connects the second external gear  52  of the first external gear set  50  with the ring gear member  44  of the planetary gear set  40 . 
     As shown in  FIG. 1   b , and in particular the truth table disclosed therein, the torque-transmitting devices are selectively engaged in combinations of three to provide eight forward speed ratios and one reverse speed ratio, all with single transition sequential shifts and a double overdrive ratio. 
     As set forth above, the engagement schedule for the torque-transmitting devices is shown in the truth table of  FIG. 1   b . The chart of  FIG. 1   b  describes the ratio steps that are attained in the above described transmission. For example, the step ratio between the first and second forward speed ratios is 1.50, while the step ratio between the reverse speed ratio and first forward ratio is −0.31. 
     Referring to  FIG. 1   c , the embodiment of powertrain  10  depicted in  FIG. 1   a  is illustrated in a lever diagram format. A lever diagram is a schematic representation of the components of a mechanical device such as an automatic transmission. Each individual lever represents a planetary gearset, wherein the three basic mechanical components of the planetary gear are each represented by a node. Therefore, a single lever contains three nodes: one for the sun gear member, one for the planet gear carrier member, and one for the ring gear member. The relative length between the nodes of each lever can be used to represent the ring-to-sun ratio of each respective gearset. These lever ratios, in turn, are used to vary the gear ratios of the transmission in order to achieve appropriate ratios and ratio progression. Mechanical couplings or interconnections between the nodes of the various planetary gear sets are illustrated by thin, horizontal lines and torque transmitting devices such as clutches and brakes are presented as interleaved fingers. If the device is a brake, one set of the fingers is grounded. Vertical dashed lines with a grounded pivot point represent external gear sets. Further explanation of the format, purpose and use of lever diagrams can be found in SAE Paper 810102, authored by Benford, Howard and Leising, Maurice, “The Lever Analogy: A New Tool in Transmission Analysis”, 1981, which is hereby fully incorporated by reference. 
     The powertrain  10  includes an input member  17  continuously connected with the engine  12 , an output member  19  continuously connected with the final drive mechanism  16 , a first planetary gear set  20 A having three nodes: a first node  22 A, a second node  26 A and a third node  24 A; a second planetary gear set  30 A having three nodes: a first node  32 A, a second node  36 A and a third node  34 A; and a third planetary gear set  40 A having three nodes: a first node  42 A, a second node  46 A and a third node  44 A. The powertrain  10  also includes a first external gear set  50 A having two external gears: a first external gear  51 A and a second external gear  52 A; a second external gear set  54 A having two external gears: a first external gear  55 A and a second external gear  56 A; a third external gear set  58 A having two external gears: a first external gear  59 A and a second external gear  60 A; and a final drive gear set  62 A with two external gears: a external drive gear  63 A and a external driven gear  64 A. 
     The input member  17  is continuously connected with the node  26 A. The output member  19  is continuously connected with the external gear  64 A. 
     The node  34 A is continuously connected with the node  42 A via: interconnecting member  70 . 
     A first torque-transmitting device, such as brake  80 , selectively connects the node  22 A with the transmission housing  90 . A second torque-transmitting device, such as clutch  82 , selectively connects the external gear  59 A with the input member  17 . A third torque-transmitting device, such as clutch  83 , selectively connects the node  36 A with the node  44 A. A fourth torque-transmitting device, such as clutch  84 , selectively connects the external gear  55 A with the external gear  59 A. A fifth torque-transmitting device, such as clutch  85 , selectively connects the external gear  52 A with the node  44 A. 
     To establish ratios, three torque-transmitting devices are engaged for each gear state. The engaged torque-transmitting devices are represented by an “X” in each respective row of  FIG. 1   b . For example, to establish reverse gear, the brake  80  and clutches  83 ,  85  are engaged. The brake  80  engages the node  22 A with the transmission housing  90 . The clutch  83  engages the node  36 A with the node  44 A. The clutch  85  engages the external gear  52 A with the node  44 A. Likewise, the eight forward ratios are achieved through different combinations of clutch engagement as per  FIG. 1   b.    
     The powertrain  10  may share components with a hybrid vehicle, and such a combination may be operable in a “charge-depleting mode”. For purposes of the present invention, a “charge-depleting mode” is a mode wherein the vehicle is powered primarily by an electric motor/generator such that a battery is depleted or nearly depleted when the vehicle reaches its destination. In other words, during the charge-depleting mode, the engine  12  is only operated to the extent necessary to ensure that the battery is not depleted before the destination is reached. A conventional hybrid vehicle operates in a “charge-sustaining mode”, wherein if the battery charge level drops below a predetermined level (e.g., 25%) the engine is automatically run to recharge the battery. Therefore, by operating in a charge-depleting mode, the hybrid vehicle can conserve some or all of the fuel that would otherwise be expended to maintain the 25% batter charge level in a conventional hybrid vehicle. It should be appreciated that a hybrid vehicle powertrain is preferably only operated in the charge-depleting mode if the battery can be recharged after the destination is reached by plugging it into an energy source. 
     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.