Abstract:
An improved electrically variable transmission, especially useful in a hybrid electric vehicle, includes a variable input power split and independent shifting of the gear ratios of the mechanical and electrical power paths through the transmission. A first electric machine varies the ratio of the mechanical power path, and a second electric machine defining the electrical power path is mechanically connected to the output at two or more ratios and electrically connected to the first electric machine and optionally to an electric storage device. The mechanical connection of the second electric machine can be reconfigured without disturbing the output whenever the power flow through the electrical power path is zero. The second electric machine and electric storage device can sustain the output when the mechanical power path is reconfigured. Reconfiguration of the mechanical power path and the mechanical connection of the second electric machine are achieved with automated manual transmission gearing, and independent reconfiguration of the mechanical power path and the mechanical connection multiplies the number of overall operating modes for improved efficiency at a low cost.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to an electrically variable transmission for a motor vehicle powertrain, and more particularly to an electrically variable transmission having variable ratio input split differential gearing and independently shifted power paths through the transmission.  
         BACKGROUND OF THE INVENTION  
         [0002]    An electrically variable transmission (EVT) utilizes one or more electric machines (which may be operated as motors or generators) and a differential gearing arrangement to provide a continuously variable ratio drive between input and output. An EVT is particularly useful in hybrid electric vehicle powertrains including an engine that is directly coupled to the transmission input and also including an electrical storage battery used to supply power for propulsion and to recover energy from braking. An EVT is also particularly useful with an engine that is designed for constant speed operation.  
           [0003]    Such a powertrain is shown and described in the U.S. Pat. No. 5,931,757 to Schmidt, issued on Aug. 3, 1999, and assigned to the assignee of the present invention. In Schmidt, the electric machines are conveniently arranged on a common axis with a compound planetary differential gearset, and operation over a wide output speed range without requiring undesirably high electric machine speeds is achieved by using multiple friction clutches to reconfigure or shift the operating mode of the gearset at a mid-range output speed. Increasing the number of operating modes will improve the powertrain efficiency and minimize the capacity, cost and weight of the electric machines. However, the number of operating modes in prior art EVT configurations have been minimized due to considerations of cost, gearing complexity, size, weight and so on.  
           [0004]    To minimize clutch energy and output torque disturbances during shifting, the clutches in the Schmitt configuration are operated synchronously—that is, with substantially zero relative speed between the two sides of each clutching mechanism. Another way to minimize clutch energy is to interrupt the power flow through the transmission, as has been very well known in manual automotive transmissions, and in the automated manual transmission (AMT). To avoid interrupting the power flow, an AMT can be configured with multiple power paths, where one power path remains active while the other shifts; however, such configurations typically require duplication of many parts.  
           [0005]    Accordingly, what is desired is an efficient EVT configuration with multiple power paths and multiple operating modes, and that is smaller and lower in cost and complexity than presently known EVT configurations.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention is directed to an improved EVT configuration including a mechanical power path from input to output through input split differential gearing, a first electric machine that regulates the ratio between input and output, a partly electrical power path to the output, and a second electric machine that functions as an electric power balancer. Both power paths are independently reconfigurable to provide multiple operating modes to reduce the torque and speed requirements of the second electric machine, and to prevent power interruption during shifting. For example, during reconfiguration of the mechanical power path, an external electrical source such as a storage battery can supply torque to the output through the second electric machine.  
           [0007]    Preferably, reconfiguration of the electrical power path is performed when the torque produced by the second electric machine is substantially zero. If electrical power is neither drawn from nor supplied to the EVT, the second machine will produce zero torque when the speed of the first electric machine is substantially zero. If electrical power is drawn from or supplied by a storage battery or other external source, the second electric machine will produce zero torque when the power required by the first electric machine is equal to the power that can be supplied by the external source.  
           [0008]    Preferably, reconfiguration of the power paths is achieved with clutches having sliding splines or other compact means typically found in the manual transmission and in the AMT. In the illustrated embodiment, a system of overlapping sliding splines is used to select all of the configurations of differential gearing available from a single planetary gearset.  
           [0009]    In a preferred implementation, the EVT of the present invention includes first and second independently reconfigurable multiple-mode gearsets serially coupled by a connecting shaft, and features automated manual transmission gearing for reduced cost, size and complexity. The first gearset differentially couples an input and the first electric machine to the connecting shaft, and the second gearset couples the connecting shaft to an output. The second electric machine is coupled to the second gearset with multiple-mode AMT gearing to provide power balancing, and independently reconfiguring the gearsets and the coupling of the second machine multiplies the number of overall transmission operating modes to provide improved efficiency without significantly increasing the cost, size and complexity of the transmission, and without sacrificing the advantages of known EVT configurations.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIGS. 1A, 1B,  1 C,  1 D,  1 E and  1 F are schematic representations of an EVT embodying the concepts of the present invention, and illustrating various possible EVT operating modes.  
         [0011]    [0011]FIG. 2 is a diagram illustrating the EVT of this invention as applied to a hybrid electric vehicle powertrain.  
         [0012]    [0012]FIG. 3, Graphs A-D, depict a representative operation of the EVT of FIG. 1. Graphs A, B and C depict various operating modes of the EVT relative to the output speed, and Graph D depicts various EVT shaft speeds relative to the output speed. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]    Referring to FIG. 1, the reference numeral  10  generally designates an embodiment of a motor vehicle electrically variable ratio transmission (EVT) according to this invention. The EVT  10  has an input shaft  12  that is preferably driven by an internal combustion engine, either directly or via an input clutch. The engine may take a variety of different forms and, in a typical hybrid powertrain, drives input shaft  12  at a constant speed during forward vehicle motion subsequent to a vehicle launch phase. An output shaft  14  of EVT  10  may be coupled to vehicle drive wheels through a conventional differential gearset (not shown).  
         [0014]    The EVT  10  includes first and second planetary gearsets  16 ,  18  serially coupled by a connecting shaft  20 , and two electric machines  22 ,  24 , with machines  22 ,  24  being coaxially aligned with the gearsets  16 ,  18  as shown. The machines  22 ,  24  are operable in either motoring or generating modes, and preferably are configured as induction machines, although other configurations are possible. The machine  22  includes a wound stator  22   a  and a rotor  22   b,  and machine  24  includes a wound stator  24   a  and a rotor  24   b.  The rotor  22   a  is mounted for rotation on sleeve shafts  26   a,    26   b,  and the rotor  24   a  is mounted for rotation on sleeve shafts  28   a,    28   b.    
         [0015]    In customary fashion, each planetary gearset  16 ,  18  includes an outer (ring) gear circumscribing an inner (sun) gear, and a plurality of planet gears rotatably mounted on a carrier such that each planet gears meshingly engage both the outer gear and the inner gear. Thus, the gearset  16  includes a ring gear  30 , a sun gear  32 , and a set of planet gears  34  mounted on a carrier  36 ; the gearset  18  includes a ring gear  38 , a sun gear  40 , and a set of planet gears  42  mounted on a carrier  44 .  
         [0016]    The sun, carrier and ring gears  32 ,  36 ,  30  of gearset  16  are reconfigurably coupled to input shaft  12 , rotor  22   b,  and connecting shaft  20  by an automated manual transmission (AMT) gearing arrangement in which the gearset  16  is axially shiftable with respect to axially fixed splines  46 ,  48 ,  50 ,  52  respectively coupled to input shaft  12 , sleeve shafts  26   a,    26   b,  and connecting shaft  20  to establish a desired configuration. FIGS.  1 A- 1 F depict six different configurations, including four forward configurations, and two reverse configurations, each of which provides an input split between connecting shaft  20  and rotor  22   b.  FIG. 1A depicts a first configuration of gearset  16  in which input shaft  12  is coupled to sun gear  32 , connecting shaft  20  is coupled to carrier  36 , and rotor  22   b  is coupled to ring gear  30  via sleeve shaft  26   b,  establishing a maximum forward speed reduction of connecting shaft  20  with respect to input shaft  12 . FIG. 1B depicts a second configuration in which input shaft  12  is coupled to ring gear  30 , connecting shaft  20  is coupled to carrier  36 , and rotor  22   b  is coupled to sun gear  32  via sleeve shaft  26   b,  establishing a minimum forward speed reduction of connecting shaft  20  with respect to input shaft  12 . FIG. 1C depicts a third configuration in which input shaft  12  is coupled to carrier  36 , connecting shaft  20  is coupled to ring gear  30 , and rotor  22   b  is coupled to sun gear  32  via sleeve shaft  26   a,  establishing a minimum forward overdrive of connecting shaft  20  with respect to input shaft  12 . FIG. 1D depicts a fourth configuration in which input shaft  12  is coupled to carrier  36 , connecting shaft  20  is coupled to sun gear  32 , and rotor  22   b  is coupled to ring gear  30  via sleeve shaft  26   a,  establishing a maximum forward overdrive of connecting shaft  20  with respect to input shaft  12 . FIG. 1E depicts a fifth configuration in which input shaft  12  is coupled to sun gear  32 , connecting shaft  20  is coupled to ring gear  30 , and rotor  22   b  is coupled to carrier  36  via sleeve shaft  26   a,  establishing a minimum reverse speed reduction of connecting shaft  20  with respect to input shaft  12 . Finally, FIG. 1F depicts a sixth configuration in which input shaft  12  is coupled to ring gear  30 , connecting shaft  20  is coupled to sun gear  32 , and rotor  22   b  is coupled to carrier  36  via sleeve shaft  26   b,  establishing a reverse overdrive of connecting shaft  20  with respect to input shaft  12 . In each configuration, of course, operating machine  22  to vary the speed of rotor  22   b  electrically controls the actual ratio between input shaft  12  and connecting shaft  20 , and the “mechanical point” of the configuration is defined as the ratio in effect when the speed of rotor  22   b  is zero.  
         [0017]    The second planetary gearset  18  reconfigurably couples the connecting shaft  20  with output shaft  14 . Output shaft  14  is coupled to carrier  44 , and the sun gear  40  is grounded, as shown. The carrier and ring gear  44 ,  38  are reconfigurably coupled to connecting shaft  20  by an automated manual transmission (AMT) gearing arrangement in which the gearset  18  is axially shiftable with respect to axially fixed splines  54  coupled to connecting shaft  20  to establish a desired configuration. FIGS. 1A, 1C,  1 D and  1 E depict a first configuration in which connecting shaft  20  is coupled to carrier  44  (which is also connected to output shaft  14 ), establishing a forward direct or 1:1 drive between connecting shaft  20  and output shaft  14 . FIGS. 1B and 1F depict a second configuration in which connecting shaft  20  is coupled to ring rear  38 , establishing a fixed speed reduction of output shaft  14  with respect to connecting shaft  20 .  
         [0018]    The rotor  24   b  of machine  24  is reconfigurably coupled to carrier and ring gear  44 ,  38  of gearset  18  by an AMT gearing arrangement in which a spline  56  coupled to the inner periphery of rotor  24   b  is shifted axially with respect to gearset  18  to establish the desired configuration. FIGS. 1A, 1C,  1 D and  1 E depict a configuration in which rotor  24   b  is coupled to ring gear  38 , allowing machine  24  to drive output shaft  14  at a reduced speed with respect to the speed of rotor  24   b.  FIGS. 1B and 1F depicts a configuration in which rotor  24   b  is coupled to directly carrier  36  and output shaft  14 .  
         [0019]    The AMT gearing arrangements coupling connecting shaft  20  to input shaft  12  and output shaft  14  are independently reconfigured to multiply the number of mechanical points between input shaft  12  and output shaft  14 . In the illustrated embodiment where the gearset  16  can be configured to establish four different forward input-split ratios and two different input-split reverse ratios, and gearset  18  can be configured to establish two different forward ratios, the EVT  10  is capable of providing twelve different speed ratios: eight forward and four reverse. The coupling between machine  24  and gearset  18  is capable of independent configuration as well, but is preferably reconfigured during zero torque operation of machine  24 . In a preferred implementation, the machine  24  is operated primarily in a motoring mode to assist the engine and to supply torque to output shaft  14  during reconfiguration (shifting) of the gearsets  16  and  18 , and machine  22  is operated alternately in a generating mode to develop electrical power for satisfying the energy requirements of machine  24  and in a motoring mode supplied by machine  24 . If no electrical power source is available, the zero torque points of machine  24  occur at the mechanical points of gearset  16  when the machine  22  is unable to generate power for energizing the machine  24 . On the other hand, if an external source of electrical energy is available for supplying power to machine  24 , the zero torque points need not coincide with the mechanical points of gearset  16 , and instead occur when the power required by machine  22  is equal to the power that can be supplied by the external source.  
         [0020]    The block diagram of FIG. 2 depicts the EVT  10  in the context of a hybrid electric vehicle powertrain. The gearset  16  is identified as variable ratio differential gearing, with the machine  22  controlling the ratio, and the gearset  18  is identified as variable (shiftable) ratio gearing. As explained above, an engine  60  (input) is coupled to the wheels  64  (output) through gearsets  16  and  18 , and machine  24  is also coupled to the wheels  64  through gearset  18 . Alternatively, of course, the machine  24  may be coupled to the wheels  64  (either the same wheels or different wheels) through another gearset packaged integral or remote from gearset  18 . Finally, both machines  22  and  24  are electrically coupled to an electric energy storage device  62 , which is capable of both supplying electrical power to machines  22 ,  24  and absorbing electrical power generated by machines  22 ,  24 .  
         [0021]    Graphs A-D of FIG. 3 depict full power forward operation of EVT  10 , with no external source of electrical power, in a six-speed implementation utilizing three forward configurations of the gearset  16  and the two configurations of the gearset  18 , and selective configuration of the machine  24 . The three forward configurations of the gearset  16  include the minimum speed reduction arrangement (MIN-RED) depicted in FIG. 1B, the minimum overdrive arrangement (MIN-OD) depicted in FIG. 1C and the maximum overdrive arrangement (MAX-OD) depicted in FIG. 1D, and the selected configuration is depicted in Graph A as a function of output shaft speed. Graph B likewise depicts the selected configuration—reduction (RED) or 1:1—of gearset  18 , and Graph C depicts the selected configuration—reduction (RED) or 1:1—of machine  24 , both as a function of output shaft speed. Graph D depicts the input shaft speed (IN), and the corresponding speeds of rotor  22   b  (M 1 ), rotor  24   b  (M 2 ), and connecting shaft  22  (CS).  
         [0022]    At low forward output speeds, the gearset  16  is configured in the MIN-RED mode, the gearset  18  is configured in the RED mode, and the rotor  24   b  is coupled to ring gear  38  (RED mode), as respectively indicated in Graphs A, B and C. At zero output speed, the connecting shaft  20  is held stationary, and an engine drives input shaft  12  at an idle speed Ni 1 ; in this state, input shaft  12  drives rotor  22   b  in reverse at an idle speed of −Ni 2 . To launch the vehicle, the engine  60  progressively increases the input speed IN to a steady-state running speed Nss; the machine  22  (M 1 ) is operated in a generator mode, and the developed power is used to drive machine  24  (M 2 ) in the motoring mode to assist the engine  60 . Once the input speed IN reaches Nss, the speed of machine  22  (M 1 ) is reduced to zero and then increased in a positive direction as shown to further increase the speed of connecting shaft  20  (CS). At output speed OS 1 , the gearset  18  is reconfigured in the direct or 1:1 mode, which requires the connecting shaft  20  to decrease to the output speed, which in turn, drives the rotor  22   b  of machine  22  in a negative direction. From this point, the speed of machine  22  (M 1 ) is reduced to zero and then increased in a positive direction as shown to further increase the speed of connecting shaft  20  (CS). At output speed OS 2 , the gearset  16  is reconfigured to the MIN-OD mode, and the gearset  18  is reconfigured to the RED mode; both reconfigurations require an increase in the speed of connecting shaft  20 , and the speed of machine  22  is not affected. At such point, the speed of machine  22  (M 1 ) is reduced to zero and then increased in a negative direction as shown to further increase the speed of connecting shaft  20  (CS). At output speed OS 3 , the gearset  18  is reconfigured in the direct or 1:1 mode, which requires the connecting shaft  20  to decrease to the output speed, which in turn, drives the rotor  22   b  of machine  22  in a positive direction as shown. The speed of machine  22  (M 1 ) is then reduced to zero and increased in a positive direction to further increase the speed of connecting shaft  20  (CS). At output speed OS 4  when the speed of machine  22  is zero (i.e., at the mechanical point of gearset  16 ) the machine  24  is reconfigured to the direct or 1:1 mode in which rotor  24   b  is coupled directly to the carrier  44  and output shaft  14 ; this requires the speed of rotor  24   b  to decrease to the output speed as shown. At output speed OS 5 , the gearset  16  is reconfigured to the MAX-OD mode, and the gearset  18  is reconfigured to the RED mode; this requires the speed of connecting shaft to increase to the speed of ring gear  38 , which drives rotor  22   b  in a positive direction as shown. At such point, the speed of machine  22  (M 1 ) is reduced to zero and then increased in a negative direction as shown to further increase the speed of connecting shaft  20  (CS). Finally, at output speed OS 6 , the gearset  18  is reconfigured in the direct or 1:1 mode, which requires the connecting shaft  20  to decrease to the output speed, which in turn, drives the rotor  22   b  of machine  22  in a positive direction. Thereafter, the speed of machine  22  (M 1 ) is reduced to zero and then increased in a negative direction as shown to further increase the speed of connecting shaft  20  (CS).  
         [0023]    In the above illustration, the machine  22  is operated alternately in the generating mode, and the generated electrical power is used to operate machine  24  in the motoring mode, and in the motoring mode with electrical power from machine  24  in the generating mode. Accordingly, no power is supplied to machine  24  at the mechanical points of the gearset  16  (i.e., at the zero speed points of machine  22 ), and the coupling of machine  24  to gearset  18  is reconfigured at such a point to minimize shift energy.  
         [0024]    In summary, the present invention provides an improved input-split EVT configuration including first and second independently reconfigurable multiple-mode gearsets  16 ,  18  serially coupled by connector shaft  20 , and featuring automated manual transmission (AMT) gearing for reduced cost, size and complexity. The capability of independently reconfiguring the gearsets  16 ,  18  and machine  24  with AMT gearing multiplies the number of overall transmission operating modes to provide improved efficiency without significantly increasing the cost, size and complexity of the EVT, and without sacrificing the advantages of known EVT configurations, such as elimination of an input clutch.  
         [0025]    While the present invention has been described in reference to the illustrated embodiment, it is fully expected that various modifications in addition to those mentioned above will occur to those skilled in the art. For example, while the operation of EVT  10  has been illustrated in the context of vehicle acceleration, it will be apparent that the EVT  10  (and engine  60 ) may also be used to assist vehicle braking, with the machines  22 ,  24  being operated as generators to apply reverse torque to the wheels  64 , and supplying electrical power to energy storage device  62  in the process. Also, common bevel differential gears or straight differential gearing may be used in place of the planetary gearset  16 , and different planetary configurations are also possible. Thus, it will be understood that electrically variable transmissions incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.