Abstract:
A vehicle transmission includes a differential gearset having five coaxial gear elements and two output members. Two of the gear elements are controllable via torque-transmitting devices such as friction brakes or electric motors to establish a plurality of speed ratios between the input shaft and the two output members. The two output members are selectively operatively connectable to an output shaft via a gear arrangement that allows for a plurality of speed ratios between the output members and the output shaft. If the torque-transmitting devices are electric motors or hydraulic pumps, then a continuously variable speed ratio between the input shaft and the output shaft is achievable. The plurality of speed ratios between the two output members and the output shaft enable a plurality of compound split operating modes, facilitating smaller and less powerful motors or pumps compared to the prior art.

Description:
TECHNICAL FIELD 
     This invention relates to vehicle transmissions having a differential gear set and two output members operatively interconnecting the gear set and an output shaft. 
     BACKGROUND OF THE INVENTION 
     A vehicle transmission can deliver mechanical power from an engine to the remainder of a drive system, typically fixed gearing, axles, and wheels. A transmission allows some freedom in engine operation, usually through alternate selection of five or six different drive ratios, a neutral selection that allows the engine to operate accessories with the vehicle stationary, and clutches or torque converters that allow smooth transitions between driving ratios to start the vehicle from rest and accelerate to the desired highway speed with the engine turning. Transmission gear selection typically allows power from the engine to be delivered to the rest of the drive system with a ratio of torque multiplication and speed reduction, with a ratio of torque reduction and speed multiplication known as overdrive, or with a reverse ratio. 
     An electric generator can transform mechanical power from the engine into electrical power, and an electric motor can transform that electric power back into mechanical power at different torques and speeds for the remainder of the vehicle drive system. This arrangement allows a continuous variation in the ratio of torque and speed between the engine and the remainder of the drive system, within the limits of the electric machinery. An electric storage battery used as a source of power for propulsion may be added to this arrangement, forming a series hybrid electric drive system. 
     The series hybrid system allows the engine to operate relatively independently of the torque, speed, and power to propel a vehicle, so as to be controlled for improved emissions and efficiency. This system also allows the electric machine attached to the engine to function as a motor to start the engine and allows the electric machine attached to the remainder of the drive train to act as a generator, recovering energy into the battery by regenerative braking. However, a series electric drive requires that the electrical machinery be sufficiently sized to transform all engine power from mechanical to electrical form and from electrical to mechanical form, and useful power is lost in this double conversion. 
     A power split transmission can use what is commonly understood to be a “differential gearing” to achieve a continuously variable torque and speed ratio between input and output without sending all power through the variable elements. An electrically variable transmission can use differential gearing to send a fraction of its transmitted power through a pair of electric motor/generators and the remainder of its power through another, parallel path that is all mechanical and direct, of fixed ratio, or alternatively selectable. One form of differential gearing may constitute a planetary gear subset. In fact, planetary gearing is usually the preferred embodiment employed in differentially geared inventions, with the advantage of compactness and different torque and speed ratios among all members of the planetary gearing subset. However, it is possible to construct this invention without planetary gears, as by using bevel differential gears or other differential gears. 
     A hybrid electrically variable transmission system for a vehicle also includes an electric storage battery, which allows the mechanical output power to vary from the mechanical input power, engine starting with the transmission system, and regenerative vehicle braking. 
     An electrically variable transmission in a vehicle can simply transmit mechanical power. To do so, the electric power produced by one motor/generator balances the electrical losses and the electric power consumed by the other motor/generator. A hybrid electrically variable transmission system in a vehicle includes an electrical storage battery, so the electric power generated by one motor/generator can be greater than or less than the electric power consumed by the other. Electric power from the battery can sometimes allow both motor/generators to act as motors, especially to assist the engine with vehicle acceleration. Both motors can sometimes act as generators to recharge the battery, especially in regenerative vehicle braking. 
     One of the most successful substitutes for the series hybrid transmission is the variable, two-mode, input-compound-split, parallel-hybrid electric transmission. Such a transmission utilizes an input means to receive power from the vehicle engine and a power output member to deliver power to drive the vehicle. First and second motor/generators are connected to energy storage devices, such as batteries, so that the energy storage devices can accept power from, and supply power to, the first and second motor/generators. A control unit regulates power flow among the energy storage devices and the motor/generators as well as between the first and second motor/generators. 
     Operation in a first or second mode may be selectively achieved by using clutches in the nature of torque-transmitting devices. In one mode, the input-split mode, the output speed of the transmission is proportional to the speed of one motor/generator, and in the second mode, the compound-split mode, the output speed of the transmission increases along with the speed of the other motor/generator. 
     In some embodiments of the variable, two-mode, input-compound-split, parallel-hybrid electric transmission a planetary gear set is selectively employed for torque multiplication. In addition, some embodiments may utilize three torque-transmitting devices—two to select the operational mode desired of the transmission and the third selectively to disconnect the transmission from the engine. In other embodiments, all three torque transfers may be utilized to select the desired operational mode of the transmission. 
     As those skilled in the art will appreciate, a transmission system using a power split arrangement may receive power from two sources. However, the prior art does not include any practical gear schemes with more than three compound split operating modes. 
     SUMMARY OF THE INVENTION 
     A vehicle transmission is provided. The transmission includes a differential gearset having first, second, third, fourth, and fifth gear elements operatively interconnected with one another. An input shaft is operatively connected to the first gear element. A first selectively engageable torque-transmitting device is mounted to a stationary member and is operatively connected to the second gear element. A second selectively engageable torque-transmitting device is mounted to the stationary member and is operatively connected to the third gear element. A first output member is operatively connected to the fourth gear element, and a second output member is operatively connected to the fifth gear element. A third torque-transmitting device is selectively engageable to operatively connect the first output member to an output shaft. Similarly, a fourth torque-transmitting device is selectively engageable to operatively connect the second output member to the output shaft. The gearset is configured such that the rotational speeds of two of the gear elements may be independently established and determine the rotational speeds of the other three gear elements. 
     The transmission of the invention may operate with fixed ratios, or may be employed with motor/generators as the first and second torque-transmitting devices to provide a large number of compound power split ratio ranges as a continuously variable transmission. The power required from the motor/generators in continuously variable operation is kept to a small fraction of power through the transmission while the ratio spread can be wide. The overall capacity or “corner power” of the motor/generators can also be kept as low as practical differential gearing will allow. Since hydraulic motors or electric motors are relatively expensive and inefficient as compared with gearing, limiting their size will help make the transmission relatively inexpensive and efficient. 
     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  is a schematic illustration of a vehicle transmission including a differential gearset according to the invention; and 
         FIG. 2  is a graphical representation of the speeds of the gear elements of the gearset of  FIG. 1  in an exemplary operation of the vehicle transmission. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a vehicular transmission  10  is schematically depicted. The transmission  10  includes a compound differential, Ravineaux gear set  14 . The gear set  14  includes a first ring gear member  18 , a second ring gear member  22 , a planet carrier assembly member  26  including a first set of pinion gears  30  and a second set of pinion gears  34  rotatably mounted thereto, a first sun gear member  38 , and a second sun gear member  42 . The first sun gear member  38  is meshingly engaged with the first set of pinion gears  30 . The second sun gear member  42  is meshingly engaged with the second set of pinion gears  34 . The first ring gear member  18  is meshingly engaged with the first set of pinion gears  30 , and the second ring gear member  22  is meshingly engaged with the second set of pinion gears  34 . The first set of pinion gears  30  is meshingly engaged with the second set of pinion gears  34 . 
     The differential gearset  14  has five members  18 ,  22 ,  26 ,  38 ,  42  on a common axis A, and is configured so that the speeds of any two of the gear members are capable of being established independently of one another, and the speeds of the other three gear members are dependent on the speeds established for the two gear members. Thus, for example, the rotational speeds of the planetary carrier  26  and the first sun gear  38  may be established independently, and the rotational speeds of the second sun gear  42 , the first ring gear  18 , and the second ring gear  22  are determined by the speeds of the planetary carrier  26  and the first sun gear  38 . Similarly, the rotational speeds of the planetary carrier  26  and the second sun gear  42  may be established independently, and the rotational speeds of the first sun gear  38 , the first ring gear  18 , and the second ring gear  22  are determined by the speeds of the planetary carrier  26  and the second sun gear  42 . 
     An input shaft  46  is operatively connected to the planet carrier assembly member  26 . A first torque-transmitting device  50  operatively interconnects the first sun gear member  38  and a stationary member such as the transmission housing  54 . A second torque-transmitting device  58  operatively interconnects the second sun gear member  42  and the transmission housing  54 . The first and second torque-transmitting devices  50 ,  58  may be friction brakes, electric motor/generators, hydraulic motor/pumps, etc., within the scope of the claimed invention. In a preferred embodiment, the torque-transmitting devices  50 ,  58  are electric motors each having a stator  62  rigidly mounted to the housing  54  and a rotor  66  rigidly affixed to one of the sun gears  38 ,  42  for rotation therewith. The rotor  66  of the first torque-transmitting device  50  is affixed to sun gear  38  via a sleeve  70  around input shaft  46 . The rotor  66  of the second torque-transmitting device  58  is affixed to sun gear  42  via sleeve  74  around input shaft  46 . 
     The transmission  10  includes two output members, namely, a first countershaft  78  and a second countershaft  80 , that are operatively connected to the gearset  14  and that define two power paths by which power may flow to an output shaft  76 . The first countershaft  78  is operatively connected to the first ring gear  18  so as to be driven thereby. More specifically, the first countershaft is rigidly connected to gear  82 , which is meshingly engaged with gear member  84 . Gear member  84  is rigidly connected to the first ring gear member  18  for rotation therewith. Similarly, the second countershaft  80  is operatively connected to the second ring gear  22  to be driven thereby. More specifically, the second countershaft  80  is rigidly connected to gear  90 , which is meshingly engaged with gear member  92 . Gear member  92  is rigidly connected to the second ring gear member  22  for rotation therewith. 
     Two gear members  100 ,  104  are connected to the output shaft  76  for rotation therewith. Countershaft  78  has rotatably supported thereon a gear member  108  that is meshingly engaged with gear member  100 . Countershaft  78  also has rotatably supported thereon a gear member  112  that is meshingly engaged with gear member  104 . A clutch, such as a dog clutch or synchronizer assembly  116 , is connected to countershaft  78  and is configured to selectively operatively connect the countershaft  78  to the output shaft  76  via gear  108  or gear  112 . More specifically, synchronizer assembly  116  is configured to selectively establish a drive connection between gear  108  and the countershaft  78 . Synchronizer assembly  116  is also configured to selectively establish a drive connection between gear  112  and the countershaft  78 . Synchronizer assembly  116  is also characterized by a neutral position such that neither gear  108  nor gear  112  is drivingly connected to countershaft  78 . 
     Similarly, countershaft  80  has rotatably supported thereon a gear member  120  that is meshingly engaged with gear member  100 . Countershaft  80  also has rotatably supported thereon a gear member  124  that is meshingly engaged with gear member  104 . A clutch, such as synchronizer assembly  128 , is connected to countershaft  80  and is configured to selectively operatively connect the countershaft  80  to the output shaft  76  via gear  120  or gear  124 . More specifically, synchronizer assembly  128  is configured to selectively establish a drive connection between gear  120  and the countershaft  80 . Synchronizer assembly  128  is also configured to selectively establish a drive connection between gear  124  and the countershaft  80 . Synchronizer assembly  128  is also characterized by a neutral position such that neither gear  120  nor gear  124  is drivingly connected to countershaft  80 . 
     When gear member  108  is drivingly connected to countershaft  78 , a first speed ratio is established between countershaft  78  and the output shaft  76 . When gear member  112  is drivingly connected to countershaft  78 , a second speed ratio is established between countershaft  78  and the output shaft  76 . Similarly, when gear member  120  is drivingly connected to countershaft  80 , a first speed ratio is established between countershaft  80  and the output shaft  76 . When gear member  124  is drivingly connected to countershaft  80 , a second speed ratio is established between countershaft  80  and the output shaft  76 . It should be noted that the first countershaft  78  and the second countershaft  80  may be operatively connected to the output shaft  76  simultaneously so that both countershafts transmit power to the output shaft. 
     Thus, the transmission  10  of  FIG. 1  is characterized by two fixed gear ratios, such as the first and fifth speed ratios, where power is transmitted to the output shaft by only one of the countershafts. The transmission  10  is also characterized by three fixed gear ratios where both countershafts are operatively connected to the output shaft, such as the second, third, and fourth speed ratios. Input shaft  46  is also selectively engageable directly with output shaft  76  via a direct drive clutch  130  to bypass the countershafts for an additional fixed gear ratio, such as the sixth speed ratio. 
     The torque-transmitting devices  50 ,  58  selectively control the rotational speed of the sun gears  38 ,  42 . Different speed ratios between the input shaft  46 , the first countershaft  78 , and the second countershaft  80  are established through selective engagement of the torque-transmitting devices  50 ,  58  and corresponding control of sun gear speed. For example, when torque-transmitting device  50  is engaged to prevent sun gear member  38  from rotating, ring gear member  18  and countershaft  78  rotate slower than ring gear member  22  and countershaft  80 , respectively, with a constant input shaft speed. When torque-transmitting device  58  prevents sun gear member  42  from rotating, ring gear member  22  and countershaft  80  rotate slower than ring gear member  18  and countershaft  78 , respectively, with a constant input shaft speed. Thus, the selective application of torque-transmitting devices  50 ,  58  establishes a particular set of speed ratios among the elements of the gearset  14  and, correspondingly, a set of speed ratios between countershaft  78  and countershaft  80 . 
     If the torque-transmitting devices  50 ,  58  are friction brakes, or stationary clutches, shifting of the transmission  10  can be accomplished by disconnecting one of the countershaft gears, disengaging torque-transmitting device  58  while engaging torque-transmitting device  50 , and connecting the free countershaft to a gear that is synchronized at the new operating ratio. To shift to the next highest gear, the gear on countershaft  78  is uncoupled from countershaft  78 . Torque-transmitting device  50  is engaged while torque-transmitting device  58  is disengaged. The speed of countershaft  78  falls, so the speed of countershaft  78  is less than the speed of countershaft  80 . For a fixed output shaft speed, the input shaft speed also decreases. With torque-transmitting device  50  fully engaged, another gear on countershaft  78  with less speed ratio between itself and the output shaft, a “higher gear,” can have the necessary ratio to be coupled to countershaft  78 , completing an upshift. 
     For example, the gears on countershafts  78 ,  80  in use before the shift could have been those with the highest and the second highest ratios with the output shaft. Normally, those would be thought of as “first” and “second” in a dual countershaft or dual layshaft transmission. Instead, the gearset  14  is running countershaft  78  faster and countershaft  80  slower so that both gear members can work together to form the actual “second” gear through the transmission. During the shift, countershaft  80  carries the load through the transmission and stays constant in speed, while countershaft  78  changes speed and gear from what would normally be “first” to “third.” After the shift, the gearset  14  is running countershaft  78  slower than countershaft  80  and the input speed is lower, forming the “third” gear through the transmission. 
     While countershafts  78 ,  80  are employed as output members in a preferred embodiment, those skilled in the art will recognize a variety of different output member configurations that may be employed within the scope of the invention to form dual power paths from the gearset  14  to an output shaft  76 . For example, members of a second planetary gearset may be operatively connected to the first and second ring gears and selectively engageable via clutches to an output shaft. Moreover, those skilled in the art will recognize that it may be desirable to add additional gears to the countershafts and the output shaft to increase the number of speed ratios available between the countershafts and the output shaft. 
     In a preferred embodiment, the transmission  10  also includes an energy storage device such as battery  132  connected via conductive wires  134  to the motors of torque-transmitting devices  50 ,  58  to receive power therefrom and to supply power thereto. A controller  136  is operatively connected to the battery and the motors to regulate the flow of power therebetween. Thus, a hybrid transmission is formed. If torque-transmitting devices  50 ,  58  are hydraulic pumps or electric generators, then they can apply torque indefinitely even if their shafts are rotating. Thus, the shifts described above can be transformed into compound power split operating ranges. That is, the braking torque and speed applied by one of the torque-transmitting devices to spin the gearset element corresponding to one of the countershafts slower than the input can be transformed into power used by the other torque-transmitting device to spin the element corresponding to the other countershaft faster than the input. 
     When the torque-transmitting devices  50 ,  58  are motors or include motors, the transmission  10  can be operated as a CVT, by absorbing power with one motor and using it in the other motor. The transmission  10  of  FIG. 1  is capable of four continuously variable ranges or modes. Each CVT range or mode is covered as the speed of one torque-transmitting device is decreasing and the speed of the other is increasing. Thus, each CVT mode corresponds to part of the shift of the fixed speed ratio transmission as described above. Transitions between successive modes or ranges occur as the countershaft gearing is selectively engaged and disengaged. 
     For example, motor  58  might be holding sun gear  42  at zero speed, and countershaft  80  might be carrying the load through the transmission by means of countershaft gear  120  and output gear  100 . The speed ratio through the transmission might then be at the “first gear” speed. The other motor  50  would be turning rapidly, and could be generating electrical power to supply the stationary motor  58 . To change the transmission speed ratio smoothly and continuously from the “first gear” speed to the “second gear” speed, the speed of the stationary motor  58  would increase smoothly and continuously and the speed of the other motor  50  would decrease smoothly and continuously to zero speed. 
       FIG. 2  depicts an exemplary operation of the transmission  10  as a CVT. Rotational speeds of the members of the gearset  14  are depicted with respect to output shaft speed. Thus, reading the graph from left to right represents acceleration in vehicle speed. Referring to  FIGS. 1 and 2 , the speed of the input shaft  46 , and correspondingly the speed of planetary carrier  26 , represented by line  144  in  FIG. 2 , is held constant. In a first range or mode  146  of CVT operation, i.e., prior to a predetermined output shaft speed  148 , the controller  136  causes the speed of the first motor  50  and, correspondingly, the speed of the first sun gear  38 , represented by line  152  in  FIG. 2 , to start at a high value and decrease with increasing output shaft speed. Simultaneously, the speed of the second motor  58  and, correspondingly, the speed of the second sun gear  42 , represented by line  156 , starts at a low value and increases with increasing output shaft speed. The speed of the first ring gear  18 , represented by line  160 , and, correspondingly, the speed of the first countershaft  78 , rises proportionally with the output shaft speed, while the speed of the second ring gear  22 , represented by line  164  in  FIG. 2 , and, correspondingly, the speed of the second countershaft  80 , decreases proportionally with the output shaft speed. Synchronizer  116  is engaged to operatively connect gear  108  to the first countershaft  76 , so that the transmission output shaft  76  is connected to the first ring gear  18  via the first countershaft  78 . 
     At output shaft speed  148 , the transmission is shifted from the first CVT range or mode  146  to a second CVT range or mode  168  by engaging synchronizer  128  to operatively connect gear  120  to the second countershaft  80  and releasing synchronizer  116  to disconnect gear  108  from the first countershaft  78 . This change disconnects the first ring gear  18  from the output shaft  76  and connects the second ring gear  22  to the output shaft  76 . Thereafter, the speed of the second motor  58  and, correspondingly, the speed of the second sun gear  42 , decrease with increasing output shaft speed, while the speed of the first motor  50  and the first sun gear  38  rises. Concurrently, the speed of the first ring  18  gear decreases with increasing output shaft speed, and the speed of the second ring gear  22  and, correspondingly, the speed of the second countershaft  80 , rises proportionally with the output shaft speed. 
     A third range or mode of CVT operation may thereafter be established by disengaging synchronizer  128  to disconnect gear  120  from the second countershaft  80 , and by engaging synchronizer  116  to connect gear  112  to the first countershaft. The motor speeds would then behave as in the first range or mode of CVT operation, with the speeds of the first motor  50  and first sun gear  38  descending and the speeds of the second motor and the second sun gear ascending. In like manner, a fourth range or mode of CVT operation may be established subsequent to the third range or mode by releasing gear  112  from the first countershaft  78 , operatively connecting gear  124  to the second countershaft  80 , and causing the speed of the first motor and first sun gear  38  to ascend with output shaft speed, and causing the speed of the second motor and the second sun gear  42  to descend with increasing output shaft speed. 
     Thus, the same transmission gearing, compound planetary gearing, and dual countershaft gearing can be useful in both stepped ratio and continuously variable transmissions, and, in fact, a transmission can be constructed to operate effectively in both ways. If the torque-transmitting devices  50 ,  58  include motors and friction brakes, then the transmission can be operated practically as a stepped ratio transmission, CVT, or combination of the two. 
     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.