Abstract:
A reduced cost, compact parallel hybrid transmission having only a single motor/generator is provided. The transmission utilizes a reduced number of components, preferably only three interconnecting members and four torque-transmitting mechanisms, to provide a reverse speed mode and seven forward speed modes (i.e., operating states achieved by engagement of a particular torque-transmitting mechanism or torque-transmitting mechanisms, whether encompassing a continuous range of speed ratios or only a fixed speed ratio). At least five of the forward modes are fixed speed ratios.

Description:
TECHNICAL FIELD 
     The present invention relates to electrically variable transmissions having parallel power flow and a single electric unit. 
     BACKGROUND OF THE INVENTION 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 
     A reduced cost, compact parallel hybrid transmission having only a single motor/generator is provided. The transmission utilizes a reduced number of components, preferably only three interconnecting members and four torque-transmitting mechanisms, to provide a reverse speed mode and seven forward speed 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. At least five of the forward modes are fixed speed ratios. 
     The hybrid electro-mechanical transmission includes an input member for receiving power from a power source and an output member for delivering power from the transmission. Only a single motor/generator is used. An energy storage device is used for interchanging electrical power with the motor/generator. Three planetary gear sets are utilized each having a first, second and third member. Preferably, the first, second and third planetary gear sets are coaxially aligned and the motor/generator annularly circumscribes at least one of the planetary gear sets. First, second and third interconnecting members each continuously interconnect a different one of the members of one of the planetary gear sets with another different one of the members of another of the planetary gear sets. Four torque-transmitting mechanisms are selectively engagable alone or in pairs to provide a reverse mode powered only by the motor/generator, a launch mode powered only by the motor/generator and at least five modes of fixed forward speed ratios powered by the power source and optionally the motor/generator. That is, the four torque-transmitting mechanisms are operable to provide at least five fixed forward speed ratios whether or not power flows through the motor/generator. 
     In one aspect of the invention, the first member of the first planetary gear set is continuously interconnected with the input member. The first member of the third planetary gear set is continuously connected with the output member. The first interconnecting member continuously connects the second member of the first planetary gear set with the first member of the second planetary gear set. The second interconnecting member continuously connects the third member of the first planetary gear set with the second member of the second planetary gear set. The third interconnecting member continuously connects the third member of the second planetary gear set with the second member of the third planetary gear set. Preferably, the interconnecting members are concentric. 
     In another aspect of the invention, a control unit is provided for regulating electrical power interchange between the energy storage device and the motor/generator. 
     In another aspect of the invention, the first, second and third members of each of the planetary gear sets include a ring gear member, a sun gear member and a carrier member. The ring gear member of the first planetary gear set is continuously connected with the input member and the carrier member of the third planetary gear set is continuously connected with the output member. The carrier member of the first planetary gear set is continuously connected with the carrier member of the second planetary gear set via the first interconnecting member. The sun gear member of the first planetary gear set is continuously connected with the ring gear member of the second planetary gear set via the second interconnecting member. The sun gear member of the third planetary gear set is continuously connected with the motor/generator and is continuously connected to the sun gear member of the second planetary gear set via the third interconnecting member. 
     In still another aspect of the invention, the first torque-transmitting mechanism selectively connects the ring gear member of the first planetary gear set with the ring gear member of the second planetary gear set. The second torque-transmitting mechanism selectively connects the sun gear member of the first planetary gear set with the stationary member. The third torque-transmitting mechanism selectively connects the ring gear member of the third planetary gear set with the stationary member. The fourth torque-transmitting mechanism selectively connects the carrier members of the first and second planetary gear sets with the carrier member of the third planetary gear set. 
     This single motor/generator parallel hybrid transmission has a lower overall transmission cost because a second motor/generator is not used nor is a second power inverter necessary, as is typically required for an additional motor/generator. Additionally, at cruise during the seventh forward speed mode, no power (i.e., substantially zero) flows through the motor/generator yet the motor/generator provides significant torque at a very low speed. 
     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 
         FIG. 1A  is a schematic representation of an electro-mechanical transmission having a single motor/generator embodying the concepts of the present invention; 
         FIG. 1B  is a chart depicting various operating conditions of the electro-mechanical transmission of  FIG. 1A ; 
         FIG. 2  is a graphical representation of the speed in rotations per minute (rpm) of the motor/generator as well as the engine and the output member relative to the speed of the vehicle in miles per hour (mph) during transient pull conditions; 
         FIG. 3  is a graphical representation of horsepower (hp) of the motor/generator, the engine, the energy storage device and the output member relative to the speed of the vehicle in miles per hour (mph) during transient pull conditions; 
         FIG. 4  is a graphical representation of the speed in rotations per minute (rpm) of the motor/generator, the engine and the output member relative to the speed of the vehicle in miles per hour (mph) during cruise conditions; and 
         FIG. 5  is a graphical representation of horsepower (hp) of the motor/generator, the engine and the output member relative to the speed of the vehicle in miles per hour (mph) during cruise conditions. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One representative form of an electro-mechanical transmission having a single motor/generator embodying the concepts of the present invention is depicted in  FIG. 1A , and is designated generally by the numeral  10 . The hybrid transmission  10  has an input member  17  that may be in the nature of a shaft which may be directly driven by an engine  12 . The engine  12  may be a fossil fuel engine, such as an internal combustion engine or a diesel engine, which is readily adapted to provide its available power output delivered at a constant number of revolutions per minute (rpm). A pump  18  may be driven off of the input member  17  for providing lubrication and cooling fluid throughout the transmission  10 . Power flows from the input member  17  through the transmission  10  as will be described below to be delivered at an output member  19  for powering a final drive  16 . 
     The transmission  10  includes a first planetary gear set  20  that includes a sun gear member  22 , a ring gear member  24  circumscribing the sun gear member  22  and a planet carrier assembly member  26  including a plurality of pinion gear  27  rotatably mounted on a carrier member  29  in meshingly engaging with both the ring gear member  24  and the sun gear member  22 . The input member  17  is continuously connected with the ring gear member  24  for providing power thereto. 
     The transmission  10  further includes a second planetary gear set  30  including a sun gear member  32 , a ring gear member  34  circumscribing the sun gear member  32  and a planet carrier assembly member  36  including a plurality of pinion gears  37  rotatably supported on a carrier member  39  and meshing engaging both the ring gear member  34  and the sun gear member  32 . The carrier member  29  is continuously connected with the carrier member  39  via an interconnecting member  70  which is an interconnecting shaft. The sun gear member  22  is continuously connected with the ring gear member  34  via an interconnecting member  72 , which is a rotatable sleeve shaft and is concentrically arranged about interconnecting member  70 . 
     Furthermore, the transmission  10  includes a third planetary gear set  40  including a sun gear member  42 , a ring gear member  44  circumscribing the sun gear member  42  and a planet carrier assembly member  46  including a plurality of pinion gears  47  rotatably mounted on a carrier member  49  and meshingly engaging with both the ring gear member  44  and the sun gear member  42 . The sun gear member  32  is continuously connected with the sun gear member  42  via an interconnecting member  74  which is another rotatable sleeve shaft concentrically arranged about interconnecting member  70 . The carrier member  49  is continuously connected with the output member  19 . 
     The input member  17  and output member  19  are aligned to form an axis of rotation therethrough. A single motor/generator  80  (which maybe referred to herein as an electric unit) is concentrically disposed about the common axis of rotation formed by the input member  17  and output member  19  for rotation thereabout. As will be well understood by those skilled in the art, the motor/generator  80  includes a stator secured to a stationary member such as a transmission housing  78  as well as a rotatable rotor. The rotor of the motor/generator  80  is secured to the sun gear members  32  and  42  for common rotation therewith via a hub  76  and interconnecting member  74 . 
     As should be apparent from the foregoing description, the transmission  10  selectively receives power from the engine  12 . The hybrid transmission may also receive power from an energy storage device  84  such as a battery pack. Other electric storage devices that have the ability to store electric power and dispense electric power may be used in place of the battery pack without altering the concepts of the present invention. The battery pack  84  may include one or more batteries. The power output of the battery pack  84  is not critical to the invention, but for the purpose of affecting a clear understanding of the hybrid transmission  10  an output power of about  35  horsepower (hp) from the battery pack  84  will be assumed for description of the transmission  10 . The battery pack  84  will be sized depending on regenerative requirements, regional issues such as grade and temperature, and other requirements such as emissions, power assist and electric range. 
     The battery pack  84  communicates with an electrical control unit (ECU)  82  by transfer conductors  88 A and  88 B. The ECU  82  communicates with the motor/generator  80  by transfer conductors  88 C and  88 D. Additionally, the ECU  82  communicates with other vehicle electrical components  86 , such as electrical power steering, and electrical power braking systems, etc., via transfer conductors  88 E and  88 F. Preferably, the maximum electrical power requirements of the other electrical components  86  is such that no more than  2  horsepower (hp) is required to power these components. 
     The ECU  82  responds to a variety of input signals including vehicle speed, operator demand, the level to which the battery pack  84  is charged and the power being applied by the engine  12 , to regulate the flow of power between the motor/generator  80  and the battery pack  84 . The ECU  82  can manipulate the motor/generator  80  to act as either a motor or a generator. The ECU  82  also regulates the flow of power via transfer conductors  88 G and  88 H between the battery pack  84  and the motor/generator  80  through power inverter  89  to convert between direct current power utilized by the battery pack  84  and alternating current power utilized by and/or generated by the motor/generator  80 . 
     A first torque-transmitting mechanism  60 , which is a rotating clutch torque-transmitting mechanism, selectively connects the ring gear member  24  with the sun gear member  22  and also with the ring gear member  34  via the interconnecting member  72 . Thus, when the torque-transmitting mechanism  60 is engaged, the ring gear member  24  and the sun gear member  22  rotate at the same speed, causing the entire planetary gear set  20  to rotate at the speed of the input member  17 . A second torque-transmitting mechanism  62 , which is a brake, selectively engages the sun gear member  22  and the ring gear member  34  with the stationary transmission housing  78 . A third torque-transmitting mechanism  64 , which is also a brake, selectively engages the ring gear member  44  with the transmission housing  78 . Finally, a fourth torque-transmitting mechanism  66 , which is a rotating type clutch, selectively engages the carrier members  29  and  39  with the carrier member  49  and also the output member  19  via interconnecting member  70 . The engagement schedule of the torque-transmitting mechanisms  60 ,  62 ,  64  and  66  is provided in  FIG. 1B . As maybe seen from the truth table of  FIG. 1B , a reverse battery powered mode, a first battery powered forward mode, as well as second, third, fourth, fifth, sixth and seventh forward speed ratio modes are provided by the transmission  10 . The speed ratios shown in  FIG. 1B  are for purposes of example only and are achieved with tooth ratio counts as follows. In the planetary gear set  20 , the ring gear member  24  has 91 teeth and the sun gear member  22  has 35 teeth; in the planetary gear set  30  the ring gear member  34  has 91 teeth and the sun gear member  32  has 45 teeth; and in the planetary gear set  40 , the ring gear member  44  has 91 teeth and the sun gear member  42  has 23 teeth. 
     Reverse Mode 
     To establish a reverse mode with a reverse direction of drive, the torque-transmitting mechanism  64  is engaged to connect the ring gear member  44  with the transmission housing  78 . The engine  12  may not be utilized in the reverse mode because the rotation of the input member  17  is necessarily in the same direction of rotation as the output member  19  in the transmission  10 . Accordingly, the motor/generator  80  acts as a motor to power the output member  19  in a reverse direction when clutch  64  is engaged and the electronic control unit  82  determines that reverse is required by operator demand. Electronic control unit  82  signals the battery pack  84  to power the motor/generator  80  via electrical power routed through the inverter  89  along transfer conductors  88 G and  88 H. Accordingly, assuming that a clockwise direction of rotation of the output member  19  is able to power the vehicle in a forward direction, the motor/generator is powered to act as a motor by rotating in counterclockwise direction, therefore turning the sun gear member  42  in counterclockwise direction. Because the ring gear member  44  is held stationary by the brake  64 , the pinion gears  47  rotate in a clockwise direction and the carrier member  49 , and therefore the output member  19  rotates in a counterclockwise direction. 
     Referring to  FIG. 2 , operating speeds during sample transient pull conditions are illustrated (i.e., conditions in which the vehicle is subjected to heavy loading or acceleration). The reverse operation is illustrated in the portion of the graph with negative vehicle speeds. The speed of the engine is illustrated by the plot  90 , the speed of the electric unit is illustrated by the plot  92 , and the speed of the transmission output member is illustrated by the plot  94 . When the vehicle speed is negative, the electric unit and transmission output member both rotate in the same direction. The speed of the engine is shown at plot  90 ; however, power is not added by the engine  12  during the reverse operating mode. 
       FIG. 3  illustrates the power of various components during the transient pull conditions that result in the speeds of  FIG. 2 . Horsepower of the engine is denoted by plot  100 , horsepower of the electrical unit  80  is denoted by plot  102 , horsepower of the battery pack  84  is denoted by plot  104  and horsepower at the transmission output member  19  is denoted by plot  106 . When the vehicle has a negative speed, the power at the output member  19  is the same as the power of the motor/generator  80 , and therefore the plots  102  and  106  overlay one another in the reverse operating mode (i.e., when vehicle speeds are negative). Power through the engine  12 , shown at plot  100 , is zero in this range. Additionally, power through the battery is at its maximum level,  35  horsepower (hp). 
     Launch/First Forward Mode 
     When the transmission  10  is used to launch the vehicle, the torque-transmitting mechanism  64  is engaged just as it was in the reverse mode. The motor/generator  80  is the prime mover in the first/launch mode, acting as a motor via power received from the battery pack  84  under the control of the electronic control unit  82 . The rotor portion of the motor/generator is controlled to rotate in a clockwise direction, thereby rotating the interconnecting member  74  and the sun gear member  42  in a clockwise direction. Because the ring gear member  44  is braked, with the sample tooth count discussed above, the carrier member  49  and output member  19  will be driven in a clockwise or forward direction of rotation for powering the final drive  16 . The engine  12  may be utilized to add power during launch if the torque-transmitting mechanism  60  is engaged at a slip (i.e. less than full engagement) such that the entire planetary gear set  20  rotates in the same direction as the input member  17  to thereby add driving power to the ring gear member  34 . 
     The operating characteristics of transmission components such as the engine  12 , the motor/generator  80  and the output member  19  in the first forward mode are depicted in  FIGS. 2 and 3  between the Y-axis and the vertical line A for transient pull conditions. Operating speeds of various components during continuous cruise conditions are illustrated at  FIGS. 4 and 5 . The speed of the engine  12  is illustrated by plot  190 , the speed of the electric unit  80  is illustrated by plot  192  and the speed of the transmission output member  19  is illustrated by plot  194 .  FIG. 5  illustrates the power of various components during the continuous cruise conditions that result in the speeds of  FIG. 4 . Horsepower of the engine  12  is denoted by plot  200 , horsepower of the electrical unit  80  is denoted by plot  202 , and horsepower at the transmission output member  19  is denoted by plot  206 . Operating characteristics in the first forward mode during cruising conditions are between the Y-axis and vertical line A′ in both  FIGS. 4 and 5 . In transient pull conditions, for all forward vehicle speeds, the motor/generator  80  operates as a motor to add torque and power to the transmission  10  while in the continuous cruise condition of  FIGS. 4 and 5 , the motor/generator  80  operates as a generator to power the battery pack  84 , or other vehicle electrical components  86 . 
     Second Forward Mode 
     The second forward mode indicated in  FIG. 1B  is established with the engagement of the torque-transmitting mechanisms  60  and  64  and establishes a first fixed forward speed ratio. A “fixed speed ratio” is an operating condition in which the power input to the transmission is transmitted mechanically to the output member, and no power flow is necessary in the motor/generators. An electrically variable transmission that may selectively achieve fixed speed ratios for operation near full engine power can be smaller and lighter for a given maximum capacity. Fixed ratio operation may also result in lower fuel consumption when operating under conditions where the engine speed can approach its optimum without using the motor/generators. A variety of fixed speed ratios and variable ratio spreads can be realized by suitably selecting the tooth ratios of the planetary gear sets or gear members in a transmission. 
     The torque-transmitting mechanism  60  connects the input member  17  and ring gear member  24  with the sun gear member  22  and the torque-transmitting mechanism  64  connects the ring gear member  44  with the transmission housing  78 . The planetary gear sets  20  and  30  as well as the sun gear member  42  rotate at the same speed as the input member  17 . The ring gear member  44  does not rotate. The carrier member  49  rotates at the same speed as the output member  19 . The carrier member  49 , and therefore the output member  19 , rotates at a speed determined from the speed of the sun gear member  42  and the ring gear/sun gear tooth ratio of the planetary gear set  40 . The operating characteristics of the transmission components in the second forward mode/first fixed speed ratio are depicted at  FIGS. 2 and 3  between the vertical lines A and B for transient pull conditions and at  FIGS. 4 and 5  between vertical lines A′ and B′ (for cruising conditions). 
     Third Forward Mode 
     A third forward mode is established with the engagement of the torque-transmitting mechanisms  62  and  64  and results in a second fixed forward speed ratio as indicated in  FIG. 1B . The torque-transmitting mechanism  62  grounds the sun gear member  22  to the transmission housing  78  and the torque-transmitting mechanism  64  grounds the ring gear member  44  to the transmission housing  78 . The ring gear member  24  rotates at the same speed as the input member  17 . The carrier members  29  and  39  rotate at the same speed. The sun gear member  22 , the ring gear member  34  and the ring gear member  44  do not rotate. The carrier member  29  rotates at a speed determined from the speed of the ring gear member  24  and the ring gear/sun gear tooth ratio of the planetary gear set  20 . The sun gear member  32  rotates at the same speed as the sun gear member  42 . The sun gear member  32  rotates at a speed determined from the speed of the carrier member  39 , and the ring gear/sun gear tooth ratio of the planetary gear set  30 . The carrier member  49  rotates at the same speed as the output member  19 . The carrier member  49 , and therefore the output member  19 , rotates at a speed determined from the speed of the sun gear member  42 , and the ring gear/sun gear tooth ratio of the planetary gear set  40 . The operating characteristics of transmission components in the third forward mode are depicted at  FIGS. 2 and 3  between vertical lines B and C for transient pull conditions and at  FIGS. 4 and 5  between vertical lines B′ and C′ (for cruising conditions). 
     Fourth Forward Mode 
     A fourth forward mode is established with the engagement of the torque-transmitting mechanisms  64  and  66  and results in a third fixed forward speed ratio, as indicated in  FIG. 1B . The torque-transmitting mechanism  64  grounds the ring gear member  44  to the transmission housing  78  and the torque-transmitting mechanism  66  connects the carrier members  39  and  49  with one another. The ring gear member  24  rotates at the same speed as the input member  17 . The carrier members  29 ,  39  and  49  rotate at the same speed as the output member  19 . The sun gear member  22  rotates at the same speed as the ring gear member  44 . The carrier member  29 , and therefore the output member  19 , rotates at a speed determined from the speed of the sun gear member  22 , the speed of the ring gear member  24  and the ring gear/sun gear tooth ratio of the planetary gear set  20 . The operating characteristics of the transmission components in the fourth forward mode are depicted in  FIGS. 2 and 3  between vertical lines C and D (for transient pull conditions) and at  FIGS. 4 and 5  between vertical lines C′ and D′ (for cruising conditions). 
     Fifth Forward Mode 
     A fifth forward mode is established with the engagement of the torque-transmitting mechanisms  62  and  66  and establishes a fourth fixed forward speed ratio. The torque-transmitting mechanism  62  connects the sun gear member  22  with the transmission housing  78  and the torque-transmitting mechanism  66  connects the carrier member  39  with the carrier member  49 . The ring gear member  24  rotates at the same speed as the input member  17 . The carrier members  29 ,  39  and  49  rotate at the same speed as the output member  19 . The sun gear member  22  does not rotate. The carrier member  29 , and therefore the output member  19 , rotates at a speed determined from the speed of the ring gear member  24  and the ring gear/sun gear tooth ratio of the planetary gear set  20 . Operating characteristics of transmission components in the fifth forward mode are depicted at  FIGS. 2 and 3  between vertical lines D and E (for transient pull conditions) and at  FIGS. 4 and 5  between vertical lines D′ and E′ (for cruising conditions). 
     Sixth Forward Mode 
     A sixth forward mode is established with the engagement of the torque-transmitting mechanisms  60  and  66  and results in a direct drive ratio of 1.0 with the input member  17  and output member rotating  19  at the same speed, as indicated in  FIG. 1B . This direct drive ratio is established with the engagement of the torque-transmitting mechanisms  60  and  66 . The torque-transmitting mechanism  60  connects the ring gear member  24  with the sun gear member  22  and the torque-transmitting mechanism  66  connects the carrier members  39  and  49  with one another. Each of the planetary gear sets  20 ,  30  and  40  rotate at the same speed as the input member  17 . Operating characteristics of the transmission components in the sixth forward mode are depicted in  FIGS. 2 and 3  between vertical lines E and F (for transient pull conditions) and at  FIGS. 4 and 5  between vertical lines E′ and F′ (for cruising conditions). 
     Seventh Forward Mode 
     A seventh forward mode is established with the engagement of the torque-transmitting mechanism  66 . The ring gear member  24  rotates at the same speed as the input member  17 . The carrier members  29 ,  39  and  49  are interconnected to rotate at the same speed as the output member  19 . The sun gear member  22  rotates at the same speed as the ring gear member  24 . The carrier member  29 , and therefore the output member  19 , rotates at a speed determined from the speed of the sun gear member  22  and the ring gear/sun gear tooth ratio of the planetary gear set  20 . Operating characteristics of the transmission components in the seventh mode are depicted at  FIGS. 2 and 3  to the right of vertical line F (for transient pull conditions) and at  FIGS. 4 and 5  to the right of vertical line F′ (for cruising conditions). As is apparent from  FIGS. 2 and 3 , in the overdrive seventh range, in order for the motor/generator to operate as a motor, it must rotate in a direction opposite that of the engine and output member  19 . It rotates at a low speed such that very little power goes through the motor/generator  80 , but significant torque is added. When the motor/generator  80  is controlled to act as a motor in the seventh forward mode, power flows from the engine  12  and input member  17  through the ring gear member  24  and the intermeshing pinion gears  27  to the carrier member  29 . Power flows from the carrier member  29  to both carrier members  39  and  49  via interconnecting member  70 . Power flowing to the carrier member  49  is transmitted to the output member  19 . Power flows from the battery pack  84  to cause the motor/generator  80  (and interconnecting member  74 ) to rotate in an opposite direction as the input member  17  and output member  19 , adding power to the planetary gear set  30  which flows through the sun gear member  32 , the pinion gears  37  and to the ring gear member  34 , where it is added to the power delivered from the planetary gear set  20  to the carrier member  39 . This power then circulates back to the planetary gear set  20  via the interconnecting member  72 , to be added to the power delivered from the input member  17  at the carrier member  29 . 
     During continuous operation, as depicted in  FIGS. 4 and 5 , in the seventh forward mode the motor/generator  80  rotates slowly in the same direction as engine  12  and output member  19  because the motor/generator must operate as the generator. Only a relatively small amount of electric power goes through the motor/generator and the speed of the motor/generator is low; however, torque of the motor/generator is high. When the motor/generator  80  is controlled to act as a generator in the seventh forward mode, power flows from the engine  12  and input member  17  through the first planetary gear set  20  to both carrier members  39  and  49  and to the output member  19  in the same manner as described above. However, power flowing to the carrier member  39  is split by the planetary gear set  30 , with some of the power flowing through the pinion gears  37  and sun gear member  32  to power the motor/generator  80 . Power flows to the battery pack  84  or other electrical components  86  to cause the motor/generator  80  (and interconnecting member  74 ) to rotate in the same direction as the input member  17  and output member  19 . The rest of the power flowing to the carrier member  39  circulates back to the planetary gear set  20  via the pinion gears  37 , the ring gear member  34  and the interconnecting member  72 . The power circulated back to the planetary gear set  20  then flows through the sun gear member  22 , the pinion gears  27  and the carrier member  29 . 
     As is evidenced in  FIGS. 3 and 5 , power at the transmission output member  19  is lowest in any given forward mode just after the shift to that mode. This “droop” in engine power may be supplemented by operating the motor/generator  80  as a motor to add torque and therefore power during the period of drop in engine power. 
     Thus, the transmission  10  provides fuel economy and emissions benefits by utilizing an electrically (battery) powered reverse and first forward mode while still providing five fixed forward ratios and a seventh, overdrive mode in which the motor/generator  80  may add power to the transmission  10  or may act as a generator to recharge the-energy storage device  84  while operating at a very low speed. 
     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.