Abstract:
A transmission includes a first drive secured to an input, producing a fixed drive ratio; a second drive secured to an input, producing a variable drive ratio; a planetary gearset including a member connected to the second drive, and a second member; a brake releasably holding the second member rotatably fixed; a first clutch releasably connecting the first member and an output; and a second clutch releasably connecting the second member and the first drive.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a continually variable transmission (CVT) that is able to produce forward and reverse drive and to operate in a torque split mode. 
     2. Description of the Prior Art 
     CVT transmissions produce variable speed ratio required for a motor vehicle by changing the radial position at which a drive belt engages an input pulley and an output pulley. But CVT transmissions may have low mechanical efficiency and a limited range of drive ratio. 
     A need exist for a control technique or kinematic arrangement that extends the overdrive range of a CVT transmission, provides the reverse drive function required for a torque converter vehicle launch using a CVT, yet requires a minimum number of friction control elements, such as clutches and brakes. 
     SUMMARY OF THE INVENTION 
     A transmission includes a first drive secured to an input, producing a fixed drive ratio; a second drive secured to an input, producing a variable drive ratio; a planetary gearset including a member connected to the second drive, and a second member; a brake releasably holding the second member rotatably fixed; a first clutch releasably connecting the first member and an output; and a second clutch releasably connecting the second member and the first drive. 
     The transmission is a two mode CVT having a variator mode and a split torque mode. When operating in the torque split overdrive range, ratio range is extended and overdrive efficiency is improved, due to a portion of the torque being delivered directly. 
     The mixing planetary gear set also provides the reversing function required in a CVT transmission that relies on a torque converter to participate in the vehicle launch condition. 
     A chain drive having a fixed drive ratio provides direct torque input to the mixing planetary gearset. 
     Torque amplification of a torque converter, located between the power source and the transmission&#39;s input shaft, aids in launching the vehicle from a stop. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of the kinematic arrangement of an automatic transmission for a motor vehicle. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The transmission assembly  10  of  FIG. 1  includes a torque converter  12 ; an input shaft  14 , a fixed ratio chain drive  16 ; a variable ratio drive  18 ; a planetary gearset  20 ; a transfer gearset  22 ; a final drive  24 ; and an inter-wheel differential  26  for transmitting rotating power to right-hand and left-hand axle shafts  28 ,  30 . 
     The crankshaft  32  of an internal combustion engine, or the shaft of another power source, is connected to the torque converter&#39;s impeller  34 , which hydrokinetically drives its turbine  36 . Fluid flow exiting the blades of turbine  36  in the torque converter is directed through a stator  38  onto the blades of the impeller  34 . Stator is held fixed by a casing  40  through a stator shaft  42 . 
     The input sprocket  44  of the fixed ratio chain drive  16  is secured to input shaft  14 , and the output sprocket  46  is secured to a sleeve shaft  48 . A drive chain  50  engages sprockets  44 ,  46 . 
     An input pulley  52  of the variable ratio drive  18  is secured to input shaft  14 , and the output pulley  54  is secured to an intermediate shaft  56 , which passes through sleeve shaft  48 . A drive belt  58  engages pulleys  52 ,  54 , whose diameter of contact with belt  58  is variable. 
     The planetary gearset  20  includes a sun gear  60  secured to shaft  56 , a ring gear  62 , a carrier  64 ; and planet pinions  66  supported on the carrier and meshing with the sung gear and ring gear. 
     The transfer gearset  22  includes a pinion  68  secured to an output shaft  70  and a gear  72  meshing with the pinion  68  and secured to a shaft  74 . 
     The final drive  24  includes a pinion  76  secured to a shaft  74  and a gear  78  meshing with the pinion  78  and secured to the ring gear of differential  26 . 
     A torque split clutch  80  alternately opens and closes a drive connection between output sprocket  46  and carrier  64  when the assembly  10  is operating in a power split mode, i.e., when both the fixed ratio chain drive  16  transmits power to carrier  64  and the variable ratio belt drive  18  transmits power to sun gear  60 . 
     A reverse brake  82  is engaged to produce reverse drive by holding carrier  64  fixed against rotation. In reverse drive, ring gear  62  and output shaft  70  are underdriven in a reverse direction compared to the speed and direction of sun gear  60 , which is continually driveably connected to output pulley  54 . 
     When the assembly  10  is operating in CVT mode, i.e., when torque split clutch  80  is open, thereby disconnecting chain drive  16  from the planetary gearset  20 , clutch  84  is engaged, thereby producing a direct drive connecting between output pulley  54  through clutch  84  and output shaft  70  to the transfer gearset  22 . 
     In one example, the chain drive  16  has a 0.5:1 drive ratio; the variable drive  18  drive ratio range is 2.3-0.43:1; the transfer gearset  22  has a 2:1 drive ratio; the final drive  24  has a 3.5:1 drive ratio; and beta, i.e., the ratio of the ring gear pitch diameter to the sun gear pitch diameter, is 2.0. 
     Reverse drive is produced with power transmitted from the engine shaft  32 , through the torque converter  12 , variable drive  18  and planetary gearset  20  with brake  82  engaged. Clutches  80  and  84  are disengaged in reverse drive. 
     The assembly  10  operates in a torque split mode with power transmitted from the engine shaft  32  through the torque converter  12 , chain drive  16 , variable drive  18  and planetary gearset  20  with clutch  80  engaged. Clutch  84  and brake  82  are disengaged in the torque split operating mode. In this instance planetary gearset  20  performs as a torque split device mixing torque transmitted from chain drive  16  through clutch  80  to carrier  64 , and torque transmitted from variable drive  18  to sun gear  60 . The split torque mode provides extended overdrive range. In the split torque mode, the drive ratio can be reduced to 2.17, provided the variable drive  18  operates at its 2.3:1 ratio and the drive ratios of the other components are as stated in the example. 
     The assembly  10  operates in a torque split mode with power transmitted from the engine shaft  32  through the torque converter  12 , chain drive  16 , variable drive  18  and planetary gearset  20  with clutch  80  engaged. Clutch  84  and brake  82  are disengaged in the torque split operating mode. In this instance planetary gearset  20  performs as a torque split device mixing torque transmitted from chain drive  16  through clutch  80  to carrier  64 , and torque transmitted from variable drive  18  to sun gear  60 , ring gear  62  being the output of gearset  20 . The split torque mode provides extended overdrive range. In the split torque mode, the drive ratio can be 2.17 increased, provided the variable drive  18  operates at its 0.43:1 ratio and the drive ratios of the other components are as stated in the example. 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.