Abstract:
An electromechanical transmission is provided having only a single motor/generator, at least one differential gear set and two torque-transmitting mechanisms. A first of the torque-transmitting mechanisms is selectively engageable to establish a first continuously variable operating mode and a second of the torque-transmitting mechanisms selectively engageable to establish a second continuously variable operating mode and a synchronous shift between the two operating modes is achievable. A method of control is provided in which vehicle operating characteristics are analyzed to identify a target operating state in terms of operating mode and ratio based on the available motor torque and motor power and on maximizing energy efficiency. The transmission is controlled to approach this operating mode as closely as possible and the diverge away from the optimum torque operating with zero net battery use as the battery charge of the battery connected with the motor/generator accumulates or depletes.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to electrically variable transmissions having a single electric motor/generator and a method of control therefore.  
       BACKGROUND OF THE INVENTION  
       [0002]     Internal combustion engines, particularly those of the reciprocating piston type, currently propel most vehicles. Such engines are relatively efficient, compact, lightweight, and inexpensive mechanisms by which to convert highly concentrated energy in the form of fuel into useful mechanical power. A novel transmission system, which can be used with internal combustion engines and which can reduce fuel consumption and emissions, may be of great benefit to the public.  
         [0003]     The wide variation in the demands that vehicles typically place on internal combustion engines increases fuel consumption and emissions beyond the ideal case for such engines. Typically, a vehicle is propelled by such an engine, which is started from a cold state by a small electric motor and relatively small electric storage batteries, then quickly placed under the loads from propulsion and accessory equipment. Such an engine is also operated through a wide range of speeds and a wide range of loads and typically at an average of approximately a fifth of its maximum power output.  
         [0004]     A vehicle transmission typically delivers mechanical power from an engine to the remainder of a drive system, such as fixed final drive gearing, axles and wheels. A typical mechanical 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 a torque converter for smooth transitions between driving ratios and to start the vehicle from rest 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.  
         [0005]     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 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.  
         [0006]     The series hybrid system allows the engine to operate with some independence from the torque, speed and power required to propel a vehicle, so the engine may be controlled for improved emissions and efficiency. This system allows the electric machine attached to the engine to act as a motor to start the engine. This system also allows the electric machine attached to the remainder of the drive train to act as a generator, recovering energy from slowing the vehicle into the battery by regenerative braking. A series electric drive suffers from the weight and cost of sufficient electric machinery to transform all of the engine power from mechanical to electrical in the generator and from electrical to mechanical in the drive motor, and from the useful energy lost in these conversions.  
         [0007]     A power-split transmission can use what is commonly understood to be “differential gearing” to achieve a continuously variable torque and speed ratio between input and output. An electrically variable transmission can use differential gearing to send a fraction of its transmitted power through a pair of electric motor/generators. The remainder of its power flows through another, parallel path that is all mechanical and direct, of fixed ratio, or alternatively selectable.  
         [0008]     One form of differential gearing, as is well known to those skilled in this art, may constitute a planetary gear set. Planetary gearing is usually the preferred embodiment employed in differentially geared inventions, with the advantages of compactness and different torque and speed ratios among all members of the planetary gear set. However, it is possible to construct this invention without planetary gears, as by using bevel gears or other gears in an arrangement where the rotational speed of at least one element of a gear set is always a weighted average of speeds of two other elements.  
         [0009]     A hybrid electric vehicle transmission system also includes one or more electric energy storage devices. The typical device is a chemical electric storage battery, but capacitive or mechanical devices, such as an electrically driven flywheel, may also be included. Electric energy storage allows the mechanical output power from the transmission system to the vehicle to vary from the mechanical input power from the engine to the transmission system. The battery or other device also allows for engine starting with the transmission system and for regenerative vehicle braking.  
         [0010]     An electrically variable transmission in a vehicle can simply transmit mechanical power from an engine input to a final drive output. 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. By using the above-referenced electrical storage battery, 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.  
       SUMMARY OF THE INVENTION  
       [0011]     A reduced cost, compact hybrid electromechanical transmission having only a single motor/generator is provided. Benefits of an electromechanical transmission, such as emissions reductions and fuel economy improvement, may be realized while cost reduction is possible due to the elimination of a second motor/generator. Additionally, the hybrid electromechanical transmission herein accomplishes synchronous shifting between continuously variable operating modes. (As used herein, a “mode” is a particular operating state, whether encompassing a continuous range of speed ratios or only a fixed speed ratio, achieved by engagement of a particular torque-transmitting mechanism or torque-transmitting mechanisms.) In other words, the transmission offers synchronous shifting between variable ranges, thus optimizing shift feel and passenger comfort. Synchronous shifting means that torque-transmitting mechanisms used in an off-going speed ratio are released as torque-transmitting mechanisms used in an on-coming speed ratio are engaged, while the relative speeds of the two operative parts of each of the torque-transmitting mechanisms are very low (essentially zero). Because only a single motor/generator is employed, its ability to provide power (when acting as a motor) and to receive power (when acting as a generator) is limited by the energy storage capacity of a battery from which the single motor/generator receives power for powering the transmission or to which the motor/generator delivers power that is received from the transmission. Accordingly, a method described below permits control of the transmission at optimal energy efficiency in light of the limitations of the battery.  
         [0012]     Specifically, an electromechanical transmission within the scope of the invention includes an input member to receive power from an engine and output member for delivering power from the transmission. The transmission also includes a single motor/generator. A “single motor/generator” means that the transmission is characterized by an absence of any other motor/generators that affect power flow between the input member and the output member. The transmission also includes at least one differential gear set having a plurality of members including a first, a second and a third member. The differential gear set may a simple planetary gear set, a compound planetary gear set or multiple planetary gear sets may be employed. The input member and the motor/generator are each operatively connectable with different members of the differential gear set, either continuously or selectively via torque-transmitting mechanisms.  
         [0013]     At least two continuously variable operating modes are achieved by the transmission. A first torque-transmitting mechanism is selectively engageable to establish a first continuously variable operating mode that has a first preferred range of speed ratios. A second torque-transmitting mechanism is selectively engageable to establish a continuously variable operating mode that is characterized by a second preferred range of speed ratios. Preferably, the first torque-transmitting mechanism connects the single motor/generator with one of the members of the differential gear set and engagement of the second torque-transmitting mechanism connects the single motor/generator with another member of the differential gear set. The speed of the output member is a combination of the speed of the input member and the speed of the motor/generator. As is well understood by those skilled in the art, in a continuously variable operating mode, power is provided by the engine and also flows from or to the motor/generator. Having more than one continuously variable operating mode reduces the amount of motor/generator power necessary to control the speed ratio through the transmission from input member to output member.  
         [0014]     Synchronous shifting between the continuously variable operating modes is provided, that is engagement of one of the first and second torque-transmitting mechanisms and disengagement of the other of the first and second torque-transmitting mechanisms to shift between the respective continuously variable operating modes occurs at essentially the same time and when the relative speeds of the two components of each of the first and second torque-transmitting mechanisms is essentially zero. This synchronous shifting operation contrasts with most prior art transmissions, wherein the entire shift event includes substantial relative speeds across at least one torque transmitting mechanism. Preferably, a number of fixed speed ratios are also achievable by engaging other torque-transmitting mechanisms, as will be further described below. Some of these fixed speed ratios reverse the direction between of the output member relative to the input member, and the continuously variable operating modes may be used to reverse the direction of the output member.  
         [0015]     The differential gear set and the interconnections of the input member, the output member, the single motor/generator and the various torque-transmitting mechanisms are best described by a lever analogy. Specifically, within the scope of the invention, the differential gear set is represented by a first lever of a lever diagram having at least three nodes, that is a first, a second and a third node. When only one simple planetary gear set is employed, the lever has only these three nodes. In this instance, the first torque-transmitting mechanism is a clutch and is selectively engageable to connect the input member with the first node. The second torque-transmitting mechanism is also a clutch and is selectively engageable to connect the input member with the second node. The motor/generator is operatively connected with the third node. A third clutch is selectively engageable to operatively connect the output member with the second node. A fourth clutch is selectively engageable to operatively connect the output member with the first node. A first brake is selectively engageable to ground the third node to a stationary member such as the transmission housing. Engagement of the first and third clutches establishes the first continuously variable operating mode whereas engagement of the second and fourth clutches establishes the second continuously variable operating mode. Furthermore, the additional clutches and the brake allow fixed operating modes. For instance, engagement of the first and third clutches as well as the brake establishes a first fixed forward speed ratio. Engagement of either the first or the second clutch as well as both of the third and fourth clutches established a second fixed forward speed ratio. Alternatively, the second fixed forward speed ratio may be established by engagement of both of the first and second clutches and only one of the third and fourth clutches. Engagement of the second and fourth clutches as well as the brake establishes a third fixed forward speed ratio.  
         [0016]     Alternatively, instead of a simple planetary gear set, the transmission may employ a compounded planetary gear set such as a Simpson gear set, represented by a lever diagram with a four node lever having a first, a second, a third and a fourth node. In this instance, the input member is continuously connected with the first node and the output member is continuously connected with the second node. Engagement of the first torque-transmitting mechanism connects the motor/generator with the third node whereas engagement of the second torque-transmitting mechanism connects the motor/generator with the fourth node.  
         [0017]     In one embodiment having a four node lever, the first and second torque-transmitting mechanisms are first and second clutches, respectively. A first brake is selectively engageable to ground the third node to the stationary member and a second brake is selectively engageable to ground the fourth node to the stationary member. Engagement of the first brake establishes a first fixed forward speed ratio whereas engagement of the first clutch and the second clutch establishes a second fixed forward speed ratio. Engagement of the second brake establishes a third fixed forward speed ratio.  
         [0018]     In an alternative embodiment, the differential gearing of the transmission may be characterized by a five node lever including the four nodes described above and an additional fifth node. In one embodiment of a transmission within the scope of the invention, the first and second torque-transmitting mechanisms are first and second clutches, respectively, the first brake is selectively engageable to operatively connect the fifth node with the stationary member and the second brake is selectively engageable to operatively connect the third node with the stationary member. Additionally, a third brake is selectively engageable to operatively connect the fourth node with the stationary member. Four fixed forward speed ratios are achievable. Specifically, engagement of the first brake will establish a first fixed forward speed ratio. Engagement of the second brake establishes a second fixed forward speed ratio. Engagement of both of the first and second clutches establishes a third fixed forward speed ratio. Engagement of the third brake establishes a fourth fixed forward speed ratio. An electric-only mode (in which the transmission is powered only by the motor/generator) may be achieved by adding a fourth brake that is selectively engageable to connect the input member with the stationary housing.  
         [0019]     A method of operating the electromechanical transmission described above includes determining values of pre-selected vehicle operating characteristics at a first time at which the vehicle is characterized by a first operating state. The vehicle operating conditions may be vehicle speed, vehicle load, operator commands (such as accelerator input), motor speed and motor torque. The first operating state is one of the operating modes achievable by the transmission, such as a continuously variable operating mode characterized by a range of speed ratios or a fixed ratio mode. After the values are determined, they are analyzed according to an algorithm or look-up table stored in an electronic control unit to thereby identify a target operating state which includes identifying one of the continuously variable operating modes and a specific speed ratio at which the vehicle will be characterized by optimal energy efficiency given the vehicle operating characteristics determined. For instance, at a constant vehicle speed, vehicle load and operator command, the method determines the most efficient mode and speed ratio given the motor speed and motor torque available. Next, if the operating mode of the first operating state (i.e., the current operating state) is different than the operating mode of the target operating state, then the method includes engaging all of the torque-transmitting mechanisms that establish the identified operating mode which are not engaged in the first operating state and disengaging all of the torque-transmitting mechanisms that establish the first operating state that are not engaged in the identified operating mode to thereby establish the operating mode of the target operating state. Once the operating mode of the target operating state is established, the method includes controlling power flow between the battery and the motor/generator to target the speed ratio identified in the target operating state, which is either just approached or is in fact achieved, depending on the battery charge level available. The controlling step causes the battery to gradually reach either its maximum or minimum power level (depending on whether power is being supplied by or received by the battery) until net power flow from or to the battery is zero, at which point the transmission will be characterized by a fixed speed ratio established by the engaged torque-transmitting mechanisms of the identified operating mode. Thus, the method first seeks the most efficient operating state and then gradually moves away from the most efficient operating state towards the fixed speed ratio as the battery charge accumulates or depletes.  
         [0020]     The above features and advantages 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  
       [0021]      FIG. 1  is a schematic representation of one embodiment of a hybrid electromechanical transmission within the scope of the invention, represented in lever diagram form by a three node lever;  
         [0022]      FIG. 2  is a schematic representation of another embodiment of a hybrid electromechanical transmission within the scope of the invention, represented in lever diagram form by a four node lever;  
         [0023]      FIG. 3  is a schematic representation of another embodiment of a three node hybrid electromechanical transmission within the scope of the invention including a brake to achieve a fixed reverse speed ratio;  
         [0024]      FIG. 4  is a schematic representation of another embodiment of a hybrid electromechanical transmission representable by a four node lever diagram within the scope of the invention, including a brake to achieve a fixed reverse speed ratio;  
         [0025]      FIG. 5  is a schematic representation of another embodiment of a hybrid electromechanical transmission within the scope of the invention, represented in lever diagram form by a five node lever;  
         [0026]      FIG. 6  is a schematic representation of another embodiment of a hybrid electromechanical transmission within the scope of the invention, represented by a five node lever diagram including two additional clutches to achieve a fixed reverse speed ratio;  
         [0027]      FIG. 7A  is a schematic representation in stick diagram form of the transmission illustrated by the four node lever in  FIG. 2 , having an output countershaft arrangement;  
         [0028]      FIG. 7B  is a truth table illustrating engaged torque-transmitting mechanisms to achieve various operating modes in the transmission of  FIG. 7A ;  
         [0029]      FIG. 8A  is a schematic representation in stick diagram form of another four node hybrid electromechanical transmission as represented by the lever diagram in  FIG. 2 , having a coaxial layout;  
         [0030]      FIG. 8B  is a truth table illustrating engaged torque-transmitting mechanisms to achieve various operating modes in the transmission of  FIG. 8A ;  
         [0031]      FIG. 9A  is a schematic representation in stick diagram form of a hybrid electromechanical transmission as represented by the five node lever diagram of  FIG. 5 , having a coaxial layout;  
         [0032]      FIG. 9B  is truth table illustrating engaged torque-transmitting mechanisms to achieve various operating modes in the transmission of  FIG. 9A ; and  
         [0033]      FIG. 10  is a flow diagram illustrating a method of controlling a hybrid electromechanical transmission. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     Referring to the drawings, wherein like reference numbers refer to like components,  FIG. 1  shows a powertrain  10  including an engine  12  connected to one embodiment of an electromechanical transmission designated generally by the numeral  14 . The transmission  14  is designed to receive at least a portion of its driving power from the engine  12 . The engine  12  has an output shaft that is connectable to an input member  17  of the transmission  14 . The input member  17  is selectively connectable to a gear set member of transmission  14  represented by a first node A of a lever  20  via a first clutch C 1 . The engine  12  is operatively connected to node A of the lever  20  when C 1  is engaged. The lever  20  represents a simple planetary gear set, and may be referred to as such. The lever  20  includes the first node A as well as a second and third node B and C, respectively. The nodes A, B and C represent a first, second and third member of the planetary gear set  20 , preferably a ring gear member, a carrier member and a sun gear member. Alternatively, the input member  17  is selectively connectable to the node B via a second torque-transmitting mechanism or clutch C 2  to operatively connect the engine  12  to node B. A third torque-transmitting mechanism or clutch C 3  selectively connects an output member  19  to the second node B. The output member  19  is operatively connected with a final drive mechanism for powering wheels of the vehicle (not shown), as will be well understood by those skilled in the art. Alternatively, the output member  19  may be selectively connected to the node A by a fourth torque-transmitting mechanism or clutch C 4 . A single motor/generator  18  is operatively connected to the third node C for providing power to the lever or receiving power therefrom. A brake B  1  is selectively engageable to connect node C with the transmission housing  60 .  
         [0035]     Two continuously variable operating modes and three fixed ratio operating modes may be achieved by the transmission  14 . Specifically, a first continuously variable operating mode is achieved by engaging clutches C 1  and C 3 . A second continuously various operating mode is achieved by engaging clutches C 2  and C 4 . A shift between the first continuously variable operating mode and the second continuously operating mode may be accomplished by releasing C 1  and C 3  while engaging C 2  and C 4 . This shift is illustrated by the arrows in  FIG. 1 . This shift may be synchronous, that is it may be accomplished when the speeds of the members represented by nodes A and B are the same and therefore when the relative speeds across all of the clutches C 1 , C 2 , C 3  and C 4  are essentially zero immediately before, during, and immediately after the shift.  
         [0036]     To establish the first fixed forward ratio, the brake B 1  is engaged during the first continuously variable operating mode (i.e., while C 1  and C 3  are engaged). To establish the second fixed forward speed ratio, either C 1  or C 2  and both of C 3  and C 4  are engaged or both C 1  and C 2  and only one of C 3  and C 4  is engaged. Engagement of all of the clutches C 1 , C 2 , C 3  and C 4  also results in operation at the second fixed forward speed ratio. To establish the third fixed forward speed ratio, the brake B 1  is engaged during the second continuously variable operating mode (i.e., while the clutches C 2  and C 4  are engaged).  
         [0037]     Referring to  FIG. 2 , a powertrain  100  has a compound planetary transmission  114  that is represented by the lever  120  which is a compound planetary gear set having four nodes: a first node A, a second node B, a third node C and a fourth node D. The engine  12  is continuously connected with the second node B via input member  17  and the output member  19  is continuously connected with the third node C. A single motor/generator  118  is selectively connectable to the third node C via the first torque-transmitting mechanism C 1  or, alternatively, to the fourth node D via the clutch C 2 . A brake B  1  selectively connects the third node C with the transmission housing  160  and a brake B 2  selectively connects the fourth node D with the transmission housing  160 . Like the three node lever  20  in the transmission  14  of  FIG. 1 , the transmission  114  with the four node lever  120  achieves two continuously variable operating modes and three fixed ratio operating modes. The transmission  114  requires only two clutches and two brakes to achieve these five operating modes. The first continuously variable operating mode is achieved with the engagement of clutch C 1  to connect the motor/generator  118  to node C. The second continuously variable operating mode is achieved with the engagement of clutch C 2  to connect the motor/generator  118  to node D. A shift between the first continuously variable operating mode and the second continuously variable operating mode may be accomplished synchronously by releasing clutch C 1  while engaging clutch C 2  when the speeds of nodes C and D are the same. A first fixed forward speed ratio is achieved with the engagement of the brake B 1 . A second fixed forward speed ratio is achieved with the engagement of both clutch C 1  and clutch C 2 . A third fixed forward speed ratio is achieved with the engagement of the brake B 2 . The motor/generator  118  may be used in any of the operating modes by operatively connecting it to the lever  120  by means of engaging clutch C 1  or C 2 , for assisting the engine in driving the vehicle or generating electricity for the battery and accessory power. Furthermore, the electric motor may rotate on its own for such useful purposes as driving mechanical vehicle accessories while the vehicle and engine are stopped by disengaging both clutches C 1  and C 2 .  
         [0038]      FIG. 3  illustrates that reverse may be achieved on a transmission  14 ′ of powertrain  10 ′. The transmission  14 ′ has a simple planetary gear set represented by lever  20 ′ (having nodes A′, B′ and C′) and a single motor/generator  18 ′. The engine  12  is continuously connected to node B′ via input member  17 . The output member  19  is continuously connected to node A′. A continuously variable operating mode is achieved by situating the motor/generator  18 ′ such that it is connectable to node C′ by a clutch C 5 . Additionally, a reverse fixed speed ratio is achieved by engaging a brake B 5  that grounds the node C′ to a transmission housing  60 ′.  
         [0039]      FIG. 4  illustrates that reverse may be achieved by a transmission  114 ′ represented by a four node lever  120 ′ (i.e., a compounded planetary gear set) of a powertrain  100 ′. The engine  12  is connected to the first node A′ via input member  17  and the output member  19  is connected to a second node B′. A brake B 1  selectively connects the third node C′ to a transmission housing  160 ′. The motor/generator  118 ′ is selectively connectable to the fourth node D′ via clutch C 5  to achieve a continuously variable operating mode. To achieve a fixed reverse speed ratio, a brake B 5  selectively connects the node D′ to the transmission housing  160 ′. By braking the lever  120 ′ between the engine  12  and output  19 , a fixed reverse speed ratio is achieved.  
         [0040]     Referring to  FIG. 5 , a powertrain  200  having a transmission  214  with a five node lever  220  representing two or more interconnected planetary gear sets is shown. The five node lever includes a first node A, a second node B, a third node C, a fourth node D and a fifth node E. An engine  12  is continuously connected to the first node A via input member  17 . An output member  19  is continuously connected with the second node B. A motor/generator  218  is selectively connectable with the third node C via a first torque-transmitting mechanism C 1 . Alternatively, the motor/generator  218  may be selectively connectable to the fourth node D via a second torque-transmitting mechanism C 2 . In an alternative embodiment, the first torque-transmitting mechanism C 1  connects the motor/generator  218  to the fifth node E instead of the third node C. This alternative arrangement would require that the motor/generator  218  be capable of generating more torque than with the selective connections shown in  FIG. 5 . A first brake B 1  selectively connects the fifth node E to the transmission housing  260 . A second brake B 2  selectively connects the third node C to the transmission housing  260  and a third brake B 3  selectively connects the fifth node D to the transmission housing  260 .  
         [0041]     A first continuously variable operating mode is established with the engagement of C 1  and a second continuously variable operating mode is established with the engagement of C 2 . A shift between the operating modes may be done by engaging C 2  while disengaging C 1 , or vice versa. In addition to the two continuously variable operating modes, the transmission  214  may achieve four fixed forward speed ratios. The first forward fixed speed ratio is established with the engagement of the brake B 1 , and either the clutch C 1  or the clutch C 2  may be used to connect the motor/generator to rotate with the input and output. A second fixed forward speed ratio is established with the engagement of the brake B 2 , and the clutch C 2  may be used to connect the motor/generator. A third fixed forward speed ratio is established with the engagement of both clutch C 1  and C 2 . A fourth fixed forward speed ratio is established with the engagement of the brake B 3 , and the clutch C 1  may be used to connect the motor/generator.  
         [0042]     Referring to  FIG. 6 , the powertrain  200  of  FIG. 5  is modified slightly to achieve a powertrain  200 ′ with a transmission  214 ′ including a five node lever  220 ′ by adding a third clutch C 3  and a fourth clutch C 4  which allow a reverse fixed forward speed ratio as described below. Clutch C 1  selectively connects motor/generator  218 ′ with node C. Clutch C 2  selectively connects motor/generator  218 ′ with node D. Brake B 1  selectively connects node E with transmission housing  260 ′. Brake B 2  selectively connects motor/generator  218 ′ with node C. Brake B 3  selectively connects node D with the transmission housing  260 ′. If C 3  is engaged, the engine  12  is operatively connected to node A via input member  17  and the first and second continuously variable operating modes as well as the three fixed forward speed ratios described above with respect to  FIG. 5  may be achieved by engaging the other torque-transmitting mechanisms required to establish such respective speed ratios as described above. However, if C 3  is disengaged and C 4  is engaged along with B 1 , a reverse fixed speed ratio is achieved. A continuously variable operating mode is achievable by engaging C 2  and C 4 .  
         [0043]     Referring to  FIG. 7A , a powertrain  310  having an electromechanical transmission  314  including a first planetary gear set  320  interconnected with a second planetary gear set  330  to form a four node lever (as will be discussed below) is illustrated. An engine  12  is connected to an input member  17 . An electric motor/generator  318  includes a rotor portion  381  connected for rotation with a shaft  350  as well as a stator portion  382  grounded to a transmission housing  360 . An electronic control unit (ECU)  380  is operatively connected to both a power inverter  384  and an electric storage device or battery  386 . The electronic control unit  380  and the inverter  384  communicate via transfer conductors  388 A. The electronic control unit  380  and the battery  386  communicate via transfer conductors  388 B. The battery  386  is operatively connected to the motor/generator  318  through the power inverter  384  which is connected to the motor/generator  318  and the battery  386  via transfer conductors  388 C and  388 D, respectively. The ECU  380  may also communicate with the motor/generator  318  or other vehicle electrical components (not shown), such as electric power steering and electric power braking systems, etc.  
         [0044]     The ECU  380  responds to a variety of input signals including vehicle speed, operator demand, the level at which the battery  386  is charged, the power being applied by the engine  12 , and vehicle speed, to regulate the flow of power between the motor/generator  318  and the battery  386 . The ECU  380  can manipulate the motor/generator  318  by means of the inverter  384  to act as either a motor or a generator. The ECU  380  also regulates the flow of power into and out of the battery  386  to the motor  318  via the power inverter  384 .  
         [0045]     The first planetary gear set  320  includes a sun gear member  322 , a ring gear member  324  and a carrier member  329  which includes a plurality of pinion gears  327  that meshingly engage with both the sun gear member  322  and the ring gear member  324 .  
         [0046]     The second planetary gear set  330  includes a sun gear member  332 , a ring gear member  334  and a carrier member  339 . The carrier member  339  includes a plurality of pinion gears  337  that meshingly engage with both the sun gear member  332  and the ring gear member  334 . The carrier member  339  is continuously connected with the ring gear member  324  via an interconnecting member  370  and the ring gear member  334  is continuously connected with the carrier member  329  via an interconnecting member  372 .  
         [0047]     Gears  352  and  356  rotate about the shaft  350 . A first torque-transmitting mechanism, clutch C 1 , is selectively engageable to operatively connect gear  352  with shaft  350 . Gear  352  intermeshes with gear  354  which is connected to and rotates with the sun gear member  332 . A second torque-transmitting mechanism C 2  is selectively engageable to operatively connect gear  356  with shaft  350 . Gear  356  intermeshes with gear  358  which is connected for rotation with intermediate shaft  361 .  
         [0048]     A transfer gear  362  is connected for rotation with the ring gear member  324  and carrier member  339  and intermeshes with a transfer gear  364  which is connected for rotation with first transfer shaft  366 . Second transfer gear  368  is also connected for rotation with first transfer shaft  366  and intermeshes with a differential or final drive mechanism  390  which is operatively connected to output member  19  or drive axle as is understood by those skilled in the art.  
         [0049]     A first brake B 1  is selectively engageable to connect gear  354  with the transmission housing  360  and a second brake B 2  is selectively engageable to connect gear  358  with the transmission housing  360 .  
         [0050]     With respect to the lever diagram of  FIG. 2 , the interconnected ring gear member  334  and carrier member  329  function as first node A. The interconnected ring gear member  324  and carrier member  339  function as a second node B. The sun gear member  332  functions as third node C. The sun gear member  322  functions as fourth node D.  
         [0051]     Referring to  FIG. 7B , a truth table indicates the five operating modes achievable by the transmission  314  of  FIG. 7A . Engaged torque-transmitting mechanisms are labeled “ON.” Specifically, a first continuously variable operating mode V 1  allows a first variable range of speed ratios and is achieved by engagement of the first clutch C 1 . With the engagement of the first clutch C 1 , the motor is operatively connected through the shaft  350  to the sun gear member  332 . The engine  12  is operatively connected through the input member  17  to the carrier member  229 . The speed of the output member  19  is a combination of the speed of the input member  17  and the speed of the motor/generator shaft  350 . Power flow from the motor/generator meets with power added by the engine  12  at the carrier member  339  and is provided to the output member  19  through the intermeshing gears  362 ,  364 ,  368  and the final drive mechanism  390 . If the motor/generator  318  is controlled to operate as a generator in the first continuously variable operating mode V 1 , some of the power flowing from the engine  12  is directed from the carrier member  339  through the sun gear member  332  to the motor/generator  318  through the engaged clutch C 1  and shaft  350 , and then to the battery  386  under the control of the ECU  380 .  
         [0052]     Referring again to  FIG. 7B , a second continuously variable operating mode V 2  is achieved by engagement of the clutch C 2 . The speed of the output member  19  is a combination of the speed of the input member  17  and the speed of the motor/generator shaft  350 . To shift from the first variable operating mode V 1  to the second variable operating mode V 2 , the clutch C 1  is disengaged as clutch C 2  is engaged. Similarly, to shift from mode V 2  to mode V 1 , clutch C 2  is disengaged as clutch C 1  is engaged. In the second continuously variable operating mode V 2 , power flows from the engine  12  to the carrier member  329 . Power flows from the motor/generator  318  through the shaft  350  to intermeshing gears  356  and  358  via engaged clutch C 2 . Power then flows along intermediate shaft  361  to the sun gear member  322  where it is added to power flowing from the engine  12  at the carrier member  329 . The power then flows from the ring gear member  324  to the carrier member  339  via the interconnecting member  370 . Power is transferred through gears  362 ,  364 ,  368  and final drive mechanism  390  to output member  19 . If the motor/generator  318  is controlled to operate as a generator in the mode V 2 , then power added by the engine  12  is directed from the carrier member  329  through the sun gear member  322 , gears  358  and  356 , and shaft  350  to the motor/generator and under the control of the ECU  380  to the battery  386 .  
         [0053]     As indicated in  FIG. 7B , a first fixed forward speed ratio Fl is established with the engagement of the brake B 1  and clutch C 2 . The engine  12  provides power to the input member  17  which is connected to the carrier member  329 . The carrier member  329  rotates at the same speed as the ring gear member  334 . The motor/generator  318  is operatively connected to the sun gear member  322 , and rotates at a fixed speed ratio relative to the input member  17  or output member  19 . Its torque and power are not necessary to regulate the speed ratio through the transmission  314  but it may be used as a motor to assist the engine  12  by using power from the battery  386  or as a generator to take power from the output member  19  and store it in the battery  386 . The sun gear member  332  is grounded to the stationary housing  360  via the brake B 1 . The carrier member  339  and the ring gear member  324  are connected through the gears  362 ,  364 ,  368  and  370  to the output member  19 . The ring gear/sun gear tooth ratios of both of the planetary gear sets  320  and  330  affect the numerical value of the fixed speed ratio.  
         [0054]     A second fixed forward speed ratio F 2  is established with the engagement of the clutches C 1  and C 2 . The clutches C 1  and C 2  connect the gear members  352  and  356 , respectively, with the shaft  350 . By doing so, the sun gear members  322  and  332  are interconnected to rotate at a fixed speed ratio with one another, although not at the same speed if the ratio of gear member  356  to gear member  358  is different from the ratio of gear member  352  to gear member  354 . Because the carrier member  327  is interconnected with the ring gear member  334 , the carrier member  339  is continuously connected with the ring gear member  324  and the sun gear members  322  and  332  are operatively connected, a fixed forward speed ratio is achieved, as will be well understood by those skilled in the art.  
         [0055]     A third fixed forward speed ratio is established with the engagement of the clutch C 1  and the brake B 2 . The brake B 2  grounds the sun gear member  322  to the transmission housing  360  by grounding gear  358 . The engine  12  is connected to the carrier member  329  via the input member  17 . The motor/generator  318  is connected to the sun gear member  332  via the shaft  350  and gears  352  and  354  due to engagement of the clutch C 1 . The motor/generator  318  rotates at a fixed speed ratio relative to the input member  17  or output member  19 . Its torque and power are not necessary to regulate the speed ratio through the transmission  314  but it may be used as a motor to assist the engine  12  by using power from the battery  386  or as a generator to take power from the output member  19  and store it in the battery  386 . The output member  19  is operatively connected to the ring gear member  324  as described above. The ring gear member  324  rotates at the same speed as the carrier member  329 . The carrier member  339  rotates at the same speed as the ring gear member  324 . This arrangement results in a fixed speed ratio between the input member  17  and the output member  19 .  
         [0056]     The interconnected carrier member  329  and ring gear member  334  function as the first node A of the four node lever  120  of  FIG. 2 . The interconnected ring gear member  324  and carrier member  339  function as the second node B of  FIG. 2 . The sun gear member  332  functions as the third node C of  FIG. 2  and the sun gear member  322  functions as the fourth node D of  FIG. 2 .  
         [0057]     Referring to  FIG. 8A , a powertrain  410  has an electromechanical transmission  414  including a first planetary gear set  420  interconnected with a second planetary gear set  430  to form a four node lever (as will be discussed below). An engine  12  is connected to an input member  17 . An electric motor/generator  418  includes a rotor portion  481  connected for rotation with a sleeve shaft  450  as well as a stator portion  482  grounded to a transmission housing  460 . An ECU  480  is operatively connected to both an inverter  484  and an electric storage device or battery  486 . The ECU  480  and the inverter  484  communicate via transfer conductors  488 A. The ECU  480  and the battery  486  communicate via transfer conductors  488 B. The battery  486  is operatively connected to the motor/generator  418  through a power inverter  484  which is connected to the motor/generator  418  and the battery  486  via transfer conductors  488 C and  488 D, respectively. The ECU  480  may also communicate with the motor/generator  418  or other vehicle electrical components (not shown), such as electric power steering and electric power brake systems, etc.  
         [0058]     The ECU  480  responds to a variety of input signals including vehicle speed, operator demand, the level at which the battery  486  is charged and the power being applied by the engine  12  to regulate the flow of power between the motor/generator  418  and the battery  486 . The ECU  480  can manipulate the motor/generator  418  by means of the inverter  484  to act as either a motor or a generator. The ECU  480  also regulates the flow of power into and out of the battery  486  to the motor  418  via the power inverter  484 .  
         [0059]     The first planetary gear set  420  includes a sun gear member  422 , a ring gear member  424  and a carrier member  429  which includes a plurality of pinion gears  427  that meshingly engage with both the sun gear member  422  and the ring gear member  424 .  
         [0060]     The planetary gear set  430  includes a sun gear member  432 , a ring gear member  434  and a carrier member  439 . The carrier member  439  includes a plurality of pinion gears  437  that meshingly engage with both the sun gear member  432  and the ring gear member  434 . The carrier member  439  is continuously connected with the ring gear member  424  via an interconnecting member  470  and the ring gear member  434  is continuously connected with the carrier member  429  via an interconnecting member  472 .  
         [0061]     A first torque-transmitting mechanism, clutch C 1 , is selectively engageable to operatively connect the motor/generator  418  with the sun gear member  432  by connecting the sleeve shaft  450  with the sun gear member  432 .  
         [0062]     A second torque-transmitting mechanism, clutch C 2 , is selectively engageable to operatively connect the motor/generator  418  with the inner shaft  461  and thereby to the sun gear member  422  which is connected for rotation with the inner shaft  461 . The inner shaft  461  is coaxially aligned with the sleeve shaft  450 . When engaged, the clutch C 2  connects the sleeve shaft  450  with the inner shaft  461  for common rotation.  
         [0063]     A first brake B 1  is selectively engageable to connect the sun gear member  432  with the transmission housing  460 . A second brake B 2  is selectively engageable to connect the sun gear member  422  with the transmission housing  460  by grounding the inner shaft  461  to the transmission housing  460 .  
         [0064]     With respect to the four node lever diagram of  FIG. 2 , the interconnected ring gear member  434  and carrier member  429  function as the first node A. The interconnected carrier member  439  and ring gear member  424  function as the second node B. The sun gear member  432  functions as the third node C. The sun gear member  422  functions as a fourth node D.  
         [0065]     Referring to  FIG. 8B , a truth table indicates five operating modes achieved by the transmission  414  of  FIG. 8A . In the truth table  8 B, engaged torque-transmitting mechanisms are labeled “ON.” Specifically, a first continuously variable operating mode V 1  allows a first variable range of speed ratios and is achieved by the engagement of the first clutch C 1 . A second continuously variable operating mode V 2  is achieved by engagement of the clutch C 2 . To shift from the first variable operating mode V 1  to the second variable operating mode V 2 , the clutch C 1  is disengaged as the clutch C 2  is engaged. Similarly, to shift from mode V 2  to mode V 1 , clutch C 2  is disengaged as clutch C 1  is engaged. A first fixed forward speed ratio F 1  is established with the engagement of the brake B 1  and the clutch C 2 . A second fixed forward speed ratio F 2  is established with the engagement of both the clutches C 1  and C 2 . A third fixed forward speed ratio is established with the engagement of the brake B 2  and the clutch C 1 . As in the first and third fixed speed ratios described with respect to the transmission  314  of  FIG. 7A , the clutches C 1  and C 2 , respectively, may be engaged to allow the motor/generator  418  to add or remove power, but without affecting the speed ratio. Those skilled in the art will understand the power flow from the engine  12  and the motor/generator  418  to establish each of the continuously variable operating modes V 1  and V 2  and power flow from the engine  12  to establish the fixed ratio modes F 1 , F 2  and F 3 , in light of the description of power flow in the various modes of the transmission  314  of  FIG. 7A  described above. For instance, in the first continuously variable operating mode V 1 , power flowing from the engine  12  through the carrier member  429  to the ring gear member  434  through the interconnecting member  472  is added at the carrier member  439  to power flowing from the motor/generator  418  to the sun gear member  432  via engaged clutch C 1 . The power then flows from the carrier member  439  to the output member  19  via the interconnecting member  470 , as is apparent from the schematic illustration in  FIG. 8A . Power may be transferred from output member  19  via a belt or chain to offset transfer shafts to power wheels of a vehicle having the transmission  414  (vehicle not shown). If the motor/generator  418  is controlled to operate as a generator in the first variable mode V 1 , then some of the power supplied by the engine  12  to carrier member  439  is relayed to the motor/generator  418  by the intermeshing pinion gear member  437  and sun gear member  432  through the sleeve shaft  450 . Those skilled in the art will readily understand how the additional modes V 2 , F 1 , F 2  and F 3  are achieved in the transmission  414  of  FIG. 8A .  
         [0066]     Referring to  FIG. 9A , a powertrain  510  has an electromechanical transmission  514  that includes a first planetary gear set  520  interconnected with a second planetary gear set  530  to form a five node lever (as will be discussed below). An engine  12  is connected with an input member  17 . An electric motor/generator  518  includes a rotor portion  581  connected for rotation with a sleeve shaft  550  as well as the stator portion  582  mounted to a transmission housing  560 . An ECU  580  is operatively connected to both a power inverter  584  and an electric storage device or battery  586 . The ECU  580  and the inverter  584  communicate via transfer conductor  588 A. The ECU  580  and the battery  586  communicate via transfer conductors  588 B. The battery  586  is operatively connected to a motor/generator  518  through a power inverter  584  which is connected to the motor/generator  518  and the battery  586  via transfer conductors  588 C and  588 D, respectively. The ECU  580  may also communicate with the electric motor/generator  518  or other vehicle electrical components (not shown), such as electric power steering and electric power braking systems, etc. The ECU  580  responds to a variety of input signals including vehicle speed, operator demand, the level which the battery  586  is charged and the power being applied by the engine  12  to regulate the flow of power between the motor/generator  518  and the battery  586 . The ECU  580  can manipulate the motor/generator  518  by means of the inverter  584  to act as either a motor or a generator. The ECU  580  also regulates the flow of power into and out of the battery  586  to the motor/generator  518  via the power inverter  584 .  
         [0067]     The first planetary gear set  520  includes a sun gear member  522 , a ring gear member  524  and a carrier member  529  which rotatably supports both a first set of pinion gears  527  and a second set of pinion gears  528  (as indicated by dashed line). The first set of pinion gears  527  meshingly engages with the sun gear member  522 , the ring gear member  524 , and a second set of pinion gears  528 . A second planetary gear set  530  includes a sun gear member  532  and a ring gear member  534 . The second set of pinion gears  528  meshingly engages with the sun gear member  532 , the ring gear member  534  and the first set of pinion gears  527 . The common carrier member  529  is a double pinion type carrier having both the first and second set of pinion gears  527  and  528  and, because the first and second sets meshingly engage with each other and with other members of the first planetary gear set  520  and the second planetary gear set  530 , the gear sets  520  and  530  are compounded. With respect to the five node lever diagram of  FIG. 5 , the ring gear member  524  functions as the first node A. The carrier member  529  functions as the second node B. The sun gear member  522  functions as the third node C. The sun gear member  532  functions as the fourth node D. The ring gear member  534  functions as the fifth node E.  
         [0068]     A first torque-transmitting mechanism, clutch C 1 , is selectively engageable to operatively connect the motor/generator  518  with the sun gear member  522  by connecting a sleeve shaft  550  on which the motor/generator  518  is connected for common rotation with an inner shaft  561  that is coaxial with the sleeve shaft  550  and is connected for common rotation with the sun gear member  522 .  
         [0069]     A second torque-transmitting mechanism, clutch C 2 , is selectively engageable to operatively connect the motor/generator  518  with the sun gear member  532  by connecting the sleeve shaft  550  with the sun gear member  532  for common rotation. A first brake B 1  is selectively engageable to ground the ring gear member  534  with the transmission housing  560 . A second brake B 2  is selectively engageable to ground the sun gear member  522  with the transmission housing  560 . A third brake B 3  is selectively engageable to ground the sun gear member  532  to the transmission housing  560 . A fourth brake B 4  is selectively engageable to ground the input shaft  17  to the transmission housing  560 .  
         [0070]     Referring to  FIG. 9B , a truth table indicates seven operating modes achievable by the transmission  514  of  FIG. 9A . Engaged torque-transmitting mechanisms in each of the operating modes are labeled “ON”, or “OR” if alternate engagements are possible. Specifically, an electric-only mode, El, is established with the engagement of the brake B 4  and the clutch C 1 . With the engagement of the brake B 4 , the input member  17  is grounded to the transmission housing  560 . Thus, the output member  19  is powered by the motor/generator  518  alone, creating an electric-only mode.  
         [0071]     A first continuously variable operating mode V 1  is established with the engagement of the clutch C 1 . With the engagement of the clutch C 1 , when the motor/generator  518  is controlled to function as a motor, power from the motor/generator  518  is added at sun gear member  532  to power from the engine  12  which is added at the ring gear member  524  by the input member  17 . Thus, the power from the two sources, the engine  12  and the motor/generator  518 , flows through the pinion gears  527  and  528  to the carrier member  529  and thereby to the output member  19 .  
         [0072]     The second continuously variable operating mode V 2  is established with the engagement of the clutch C 2 . With the clutch C 2  engaged, power from the motor/generator  518  is added to the transmission gearing at the sun gear member  532 . Power from the engine  12  is added at the ring gear member  524 . Power from the two respective sources, the engine  12  and motor/generator  518 , is thus added through the intermeshing sets of pinion gears  527  and  528  to the carrier member  529  and thereby to the output member  19 .  
         [0073]     Four fixed forward speed modes are also achievable by the transmission  514 . Specifically, a first fixed forward speed ratio is established with the engagement of the brake B 1  and either engagement of clutch C 1  or of clutch C 2 . A second fixed forward speed ratio F 2  is established with the engagement of the brake B 2  and the clutch C 2 . Engagement of the clutch C 2  allows the motor/generator  518  to add or remove power but does not affect speed ratio. A third fixed forward speed ratio is established with the engagement of both of the clutches C 1  and C 2 . A fourth fixed forward speed ratio is established with the engagement of the brake B 3  and the clutch C 1 . Engagement of the clutch C 1  allows the motor/generator  518  to add or remove power but does not affect speed ratio. Those skilled in the art will readily understand power flow through the transmission  514  in each of the modes indicated in the truth table of  FIG. 9B .  
         [0074]      FIG. 10  illustrates a method  600  of operating an electromechanical transmission having a single motor/generator such as described in any of the embodiments of  FIGS. 1 through 8 B above. An ECU, such as is shown in  FIGS. 7A, 8A  and  9 A, carries out the steps of the method  600 . The method  600  begins with step  601  in which an ECU determines the value of preselected vehicle operating characteristics at a current time when the vehicle is characterized by a current operating state. The current operating state includes the mode, in which the transmission is currently functioning, whether a continuously variable operating mode or a fixed ratio mode, as well as the specific current speed ratio of the transmission. The current operating state may also include a specific current motor speed and motor torque. The vehicle operating characteristics may include vehicle speed, vehicle load and vehicle operator input such as acceleration, steering and braking input. The vehicle operating characteristics may be relayed to the ECU in order for the ECU to accomplish the determining step by sensors located at relevant points in the vehicle such as on the wheels, the engine, the motor/generator, etc., as will be well understood by those skilled in the art.  
         [0075]     Next, the method  600  includes an analyzing step  602  in which the ECU analyzes the determined values to identify a target operating state. The target operating state identified includes one of the continuously variable modes achievable by the transmission as well as a specific speed ratio within the range of the identified continuously variable mode. The analysis may be done according to a stored algorithm or by comparison of the determined values with stored reference values to thereby identify the target operating state. The target operating state is the most efficient mode and speed ratio in which the transmission can operate given the motor speed and motor torque available and under the other vehicle operating characteristics such as vehicle speed, vehicle load and vehicle operator input, the latter three of which are assumed to remain constant throughout the method  600 . The electronic control unit then determines in step  603  whether the current operating state is different than the identified target operating state. If the states are the same, then the transmission continues running in the current operating state and the method  600  returns to step  601 . However, if the states are different, then in step  604 , the ECU changes the engaged torque-transmitting mechanisms to establish the mode of the target operating state. This involves engaging all of the torque-transmitting mechanisms that establish the operating mode of the target operating state that are not engaged in the current operating state and disengaging all of the torque-transmitting mechanisms that establish the first operating state and that are not engaged in the operating mode of the target operating state, to thereby establish the operating mode of the target operating state. Once torque-transmitting mechanisms are engaged according to step  604 , in step  605  the ECU controls power flow between the battery connected with the motor/generator (such as battery  386  of  FIG. 7A ) to target the speed ratio of the target operating state. In other words, power flows either from the battery to the motor/generator (if the motor/generator is required to operate as a motor in the target operating state) or from the motor/generator to the battery (if the motor/generator is required to operate as a generator in the target operating state). Under this control scheme, motor speed will adjust so that the transmission approaches and preferably achieves the target speed ratio of the target operating state. Depending on the charge in the battery when step  605  is undertaken, the target speed ratio may not be achieved. That is, the battery may reach a maximum charge level before the target speed ratio is achieved or may reach a minimum charge level before the target speed ratio is achieved. When either the maximum or minimum charge level is reached in the battery, power flow from or to the motor/generator is no longer possible and the transmission operates in a fixed ratio mode defined by the torque-transmitting mechanisms engaged in step  604 .  
         [0076]     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.