Abstract:
A transmission is provided with a reverse input split mode and, preferably, a reverse low fixed speed ratio which provides sufficient reverse grade performance while allowing motor size and planetary and transmission ratios to be optimized for fuel economy or other design criteria. Engine-on reverse performance is improved, reducing dependence on the battery and electric motors to meet reverse grade performance requirements.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to electrically variable transmissions with selective operation both in power split variable speed ratio ranges and fixed speed ratios, having three planetary gear sets, two motor/generators and a plurality of torque-transmitting mechanisms to achieve enhanced reverse performance and an efficient electric forward cruise mode that enhances regenerative braking capability.  
       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.  
         [0011]     A successful substitute for the series hybrid transmission is the two-range, input-split and compound-split electrically variable transmission now produced for transit buses, as disclosed in U.S. Pat. No. 5,931,757, issued Aug. 3, 1999, to Michael Roland Schmidt, commonly assigned with the present application, and hereby incorporated by reference in its entirety. Such a transmission utilizes an input means to receive power from the vehicle engine and a power output means to deliver power to drive the vehicle. First and second motor/generators are connected to an energy storage device, such as a battery, so that the energy storage device can accept power from, and supply power to, the first and second motor/generators. A control unit regulates power flow among the energy storage device and the motor/generators as well as between the first and second motor/generators.  
         [0012]     Operation in first or second variable-speed-ratio modes of operation may be selectively achieved by using clutches in the nature of first and second torque transfer devices. In the first mode, an input-power-split speed ratio range is formed by the application of the first clutch, and the output speed of the transmission is proportional to the speed of one motor/generator. In the second mode, a compound-power-split speed ratio range is formed by the application of the second clutch, and the output speed of the transmission is not proportional to the speeds of either of the motor/generators, but is an algebraic linear combination of the speeds of the two motor/generators. Operation at a fixed transmission speed ratio may be selectively achieved by the application of both of the clutches. Operation of the transmission in a neutral mode may be selectively achieved by releasing both clutches, decoupling the engine and both electric motor/generators from the transmission output. The transmission incorporates at least one mechanical point in its first mode of operation and at least two mechanical points in its second mode of operation.  
         [0013]     U.S. Pat. No. 6,527,658, issued Mar. 4, 2003 to Holmes et al, commonly assigned with the present application, and hereby incorporated by reference in its entirety, discloses an electrically variable transmission utilizing two planetary gear sets, two motor/generators and two clutches to provide input-split, compound split, neutral and reverse modes of operation. Both planetary gear sets may be simple, or one may be individually compounded. An electrical control member regulates power flow among an energy storage device and the two motor/generators. This transmission provides two ranges or modes of electrically variable transmission (EVT) operation, selectively providing an input-power-split speed ratio range and a compound-power-split speed ratio range. One fixed speed ratio can also be selectively achieved.  
         [0014]     Hybrid systems may improve vehicle fuel economy in a variety of ways. For instance, the engine may be turned off at idle, during periods of deceleration and braking, and during periods of low speed or light load operation to eliminate efficiency losses due to engine drag. Captured braking energy (via regenerative braking) or energy stored by one of the motors acting as a generator during periods when the engine is operating is utilized during these engine off periods. Transient demand for engine torque or power is supplemented by the motor/generators during operation in engine-on, electrically variable modes, allowing for downsizing the engine without reducing apparent vehicle performance. Additionally, the engine may be operated at or near the optimal efficiency point for a given power demand. The motor/generators are able to capture vehicle kinetic energy during braking, which is used to keep the engine off longer, supplement engine torque or power and/or operate at a lower engine speed, or supplement accessory power supplies. Additionally, the motor/generators are very efficient in accessory power generation and electric power from the battery serves as an available torque reserve allowing operation at a relatively low transmission numerical speed ratio.  
       SUMMARY OF THE INVENTION  
       [0015]     A transmission is provided with a reverse input split mode and, preferably, a reverse low fixed speed ratio which provides sufficient reverse grade performance while allowing motor size and planetary and transmission ratios to be optimized for fuel economy or other design criteria. A key advantage of this design over other EVT designs for reverse is that the electrical power flow is forward (non-circulating) in both forward and reverse modes. Electrical circulating power in an EVT refers to a condition where the mechanical path carries more than 100% of the output power. Under normal forward electrical power flow conditions, the engine power is split with some portion transmitted electrically and the remainder transmitted mechanically. When a typical EVT operates in reverse, the direction of the electrical power flow is reversed, so the mechanical path must carry the full output power plus the electrical power. Under this condition, the electrical power is said to be circulating in the system. Therefore, the electrical path torque and power must be sized for greater than 100% of the output torque and power in order to accommodate the circulating power. Maximum output torque of a typical EVT is obtained with the engine not producing torque, using battery power. Maximum output torque of the electrically variable transmission of the present invention is obtained with the engine on, yielding more robust performance. Due to the improved reverse performance, the typical requisite increase in motor size and/or higher transmission or planetary ratios is not required in order to achieve sufficient reverse grade performance.  
         [0016]     Accordingly, an electrically variable transmission includes an input member to receive power from an engine, an output member, as well as first and second motor/generators. First, second and third planetary gear sets each have first, second and third members and have the input member and the output member each continuously connected to a different one of the members. A first interconnecting member continuously connects a member of the first planetary gear set with a member of either the second or third planetary gear set that is continuously connected with the second motor/generator. A second and a third interconnecting member each continuously connect a different respective one of the members of the second planetary gear set with a different respective one of the members of the third planetary gear set.  
         [0017]     In referring to the first, second and third gear sets in this description, and in the claims, these sets may be counted “first” to “third” in any order in the drawings (i.e., left to right, right to left, etc.). Additionally, the first, second or third members of each gear set may be counted “first” to “third” in any order in the drawings (i.e., top to bottom, bottom to top, etc.) for each gear set.  
         [0018]     At least three torque-transmitting mechanisms are provided, including a first torque-transmitting mechanism operable for selectively connecting a member of the first planetary gear set that is continuously connected with the input member with a member of the second planetary gear set that is selectively connectable with a stationary member via a second torque-transmitting mechanism. A third torque-transmitting mechanism is operable for selectively connecting a member of the second or third planetary gear set that is not interconnected with any of the other planetary gear sets with the stationary member. The first motor/generator is continuously connected with the member of the first planetary gear set that is not connected with the input member or with the other planetary gear sets. The third torque-transmitting mechanism is selectively engageable to provide an input split, first electrically variable forward mode, and the first torque-transmitting mechanism is selectively engageable to provide a compound split, second electrically variable forward mode.  
         [0019]     The output member is preferably continuously connected to a member of the second or third planetary gear set that is not continuously connected with the member of the first planetary gear set and is not selectively connectable with the stationary member.  
         [0020]     The torque-transmitting mechanisms are engageable to provide an input split, electrically variable reverse mode and an electric forward cruise mode. The electric forward cruise mode, i.e., in which the engine is off, allows the motors to operate a higher speeds and lower torques to achieve the same output torque, resulting in improved efficiency. Regenerative braking is more efficiently performed during the electric forward cruise mode than during an electrically variable mode in which both motors must supply torque and the speeds of the motors are low. Additionally, engine drag (which further reduces efficiency) is eliminated in the electric forward cruise mode.  
         [0021]     The electrically variable transmission may be described in terms of a lever diagram. For example, the first, second and third members of the first planetary gear set are representable by a first lever of a lever diagram having a first, second and third node corresponding with the first, second and third members. Additionally, two of the members of the second planetary gear set are continuously connected with two of the members of the third planetary gear set (e.g., via the second and third interconnecting members described above). Therefore, the second and third planetary gear sets are representable by a second, compounded lever in the lever diagram. The second lever has a fourth, fifth, sixth and seventh node corresponding with the second and third planetary gear sets. The first node is continuously connected with the fourth node. The input member is continuously connected with the second node. The first motor/generator is continuously connected with the third node and the second motor/generator is continuously connected with the fourth node. The first torque-transmitting mechanism is operable for selectively connecting the first node with the fifth node, and the second torque-transmitting mechanism is operable for selectively connecting the fifth node with the stationary member. The output member is continuously connected with the sixth node and the third torque-transmitting mechanism is operable for selectively connecting the seventh node with the stationary member.  
         [0022]     In one aspect of the invention, the second torque-transmitting mechanism is selectively engageable to provide the input split, electrically variable reverse mode and the first and second torque-transmitting mechanisms are selectively engageable to provide the electric forward cruise mode. Torque of the first motor/generator is added to torque of the second motor/generator in the electric forward cruise mode. Furthermore, the first and third torque-transmitting mechanisms are selectively engageable to provide a fixed forward speed ratio.  
         [0023]     Optionally, a fourth torque-transmitting mechanism may be added to the transmission. The fourth torque-transmitting mechanism is operable for selectively connecting any two members of the first planetary gear set, causing all elements of the first planetary gear set to spin at the same speed (thereby causing the first planetary gear set to be “locked” and inactive in that the tooth ratios of the first planetary gear set do not affect the overall transmission ratio). The second and fourth torque-transmitting mechanisms are engageable to provide a fixed reverse speed ratio. The fixed reverse speed ratio allows the efficiency of a purely mechanical power flow path in the reverse direction. The first torque-transmitting mechanism and the fourth torque-transmitting mechanism are engageable to provide a fixed forward speed ratio and the third torque-transmitting mechanism and the fourth torque-transmitting mechanism are selectively engageable to provide another fixed forward speed ratio. Thus, with the first, second, third and fourth torque-transmitting mechanisms, three fixed forward speed ratios are provided.  
         [0024]     In terms of the lever diagram described above, the fourth torque-transmitting mechanism is operable for selectively connecting any two of the first, second and third nodes with one another.  
         [0025]     Additional torque-transmitting mechanisms may be employed to achieve additional fixed forward speed ratios. For instance, a fifth torque-transmitting mechanism operable for selectively connecting the second motor/generator with the stationary member may be employed. The first torque-transmitting mechanism and the fifth torque-transmitting mechanism are selectively engageable to provide a fixed forward speed ratio, thereby creating with the first, second, third and fourth torque-transmitting mechanisms a total of four fixed forward speed ratios. Additionally, a sixth torque-transmitting mechanism operable for selectively connecting the first motor/generator with the stationary member may be employed. The first and sixth torque-transmitting mechanisms are selectively engageable to provide a fixed forward speed ratio and the third and sixth torque-transmitting mechanisms are selectively engageable to provide another fixed forward speed ratio. Accordingly, with all six of the torque-transmitting mechanisms, six fixed forward speed ratios may be achieved. Additionally, the second and sixth torque-transmitting mechanisms are selectively engageable to provide a fixed reverse speed ratio.  
         [0026]     Specific embodiments of the transmission may be described with respect to the first, second and third members of each gear set being a ring gear member, a planet carrier member and a sun gear member. For instance, in some embodiments, the first motor/generator is continuously connected with the sun gear member of the first planetary gear set and the second interconnecting member continuously connects the ring gear member of the second planetary gear set with the carrier member of the third planetary gear set. In some embodiments, the third interconnecting member continuously connects the carrier member of the second planetary gear set with the ring gear member of the third planetary gear set.  
         [0027]     In some embodiments, the first interconnecting member continuously connects the ring gear member of the first planetary gear set with the sun gear member of the second planetary gear set.  
         [0028]     In some embodiments, a member of the first planetary gear set may be continuously connected with a member of the second or third planetary gear set via the first interconnecting member where the first interconnecting member connects to the second motor/generator. The member of the second or third planetary gear set is also continuously connected with the second motor/generator; therefore, the first interconnecting member continuously interconnects the member of the first planetary gear set with the member of the second or third planetary gear set via the second motor/generator.  
         [0029]     The transmission also provides the efficiency of regenerative braking. An energy storage device operable for supply power to or receiving power from the first and second motor/generators is provided. A controller operable for controlling power transfer between the energy storage device and the first and second motor/generators is further provided. The controller causes at least one of the first and second motor/generators to function as a generator to convert rotational energy of the output member to power stored in the energy storage device during braking. Preferably, regenerative braking occurs during the electric forward cruise mode. This may be more efficient than performing regenerative braking during the second electrically variable forward mode since the engine drag is eliminated when the engine is shut off and both motors need not necessarily supply torque at low speeds as they must during the second electrically variable forward mode.  
         [0030]     The arrangement of the torque-transmitting mechanisms within the transmission and the engagement schedule thereof allows for a relatively low numerical top fixed gear ratio in relation to typical hybrid transmission designs, which may improve highway fuel economy.  
         [0031]     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  
       [0032]      FIG. 1  is a schematic lever diagram illustration of an electrically variable transmission of the present invention;  
         [0033]      FIG. 2  is a schematic lever diagram illustration of one embodiment of a transmission within the scope of the lever diagram of  FIG. 1 ;  
         [0034]      FIG. 3  is a schematic lever diagram illustration of a second embodiment of a transmission within the scope of the lever diagram of  FIG. 1 ;  
         [0035]      FIG. 4  is a schematic lever diagram illustration of a third embodiment of a transmission within the scope of the lever diagram of  FIG. 1 ;  
         [0036]      FIG. 5  is a schematic stick diagram illustration of a fourth embodiment of a transmission corresponding with the lever diagram of  FIG. 2 ;  
         [0037]      FIG. 6  is a schematic stick diagram illustration of a fifth embodiment of a transmission corresponding with the lever diagram of  FIG. 2 ;  
         [0038]      FIG. 7  is a schematic stick diagram illustration of a sixth embodiment of a transmission corresponding with the lever diagram of  FIG. 4 ; and  
         [0039]      FIG. 8  is a schematic stick diagram illustration of a seventh embodiment of a transmission corresponding with the lever diagram of  FIG. 4 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]     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 electrically variable transmission (EVT) 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 serves as an input member  17  of the transmission  14 . A final drive unit  16  is operably connected to the transmission  14  via an output member  19 . The transmission  14  includes three planetary gear sets represented in lever diagram form in  FIG. 1 , as will be readily understood by those skilled in the art. A lever or first planetary gear set  20  includes a first, second and third node A, B, C, respectively. The nodes A, B and C represent a first, second and third member of the first planetary gear set  20 , preferably a ring gear member, a carrier member and a sun gear member, although not necessarily in that order.  
         [0041]     The transmission  14  also includes a second lever  30 ,  40 , consisting of two compounded planetary gear sets, a second planetary gear set  30  and a third planetary gear set  40 . The planetary gear sets  30  and  40  also have three members which can be a ring gear member, a sun gear member and a planet carrier member. The planetary gear sets  30  and  40  are compounded in that two members of the second planetary gear set  30  are continuously connected with two members of the planetary gear set  40 . In some embodiments, an interconnected pair of members may be replaced by a single member functioning in both planetary gear set  30  and planetary gear set  40 . In all instances, the compounded planetary gear sets  30 ,  40  may be represented by the second, four node lever,  30 ,  40  having a fourth node D, a fifth node E, a sixth node F and a seventh node G. As illustrated and described below with respect to  FIGS. 2 through 4 , the compounded planetary gear sets  30 ,  40  may be represented by two separate lever diagrams for the gear sets  30  and  40 ; however, in either instance two members of the planetary gear set  30  are continuously connected with two members of the planetary gear set  40 , and those skilled in the art will recognize that such a compounded planetary gear set may be shown schematically as a single lever or as two separate levers. The two interconnected pairs of members of planetary gear sets  30  and  40  are represented by the fifth and sixth nodes E, F. In the lever diagram of  FIGS. 2 through 4  in which the compounded planetary gear sets  30 ,  40  are illustrated with two separate levers, the interconnected nature of the nodes E and F will be apparent.  
         [0042]     A first interconnecting member  70  continuously interconnects the first node A with the fourth node D. The input member  17  is continuously connected with the second node B. The second node B is also selectively connectable with the fifth node E via a first torque-transmitting mechanism  50 . A second torque-transmitting mechanism  52  selectively connects the fifth node E with a stationary member  60 , such as the transmission housing. The third node C is continuously connected with a first motor/generator  80 . A second motor/generator  82  is continuously connected with the fourth node D of the compounded planetary gear sets  30 ,  40 . The first and second motor/generators  80 ,  82  may also be referred to herein as Unit A and Unit B, respectively. The sixth node F of the second lever  30 ,  40  is continuously connected with the output member  19 . Finally, the seventh node G is selectively connectable with the transmission housing  60  via a third torque-transmitting mechanism  54 .  
         [0043]     Three optional torque-transmitting mechanisms may also be employed to achieve various operating states, as will be described below. For instance, a fourth torque-transmitting mechanism  56  (shown in phantom) selectively connects the second node B, i.e., the node continuously connected with the input member  17 , with the first and fourth nodes A, D, respectively, via the first interconnecting member  70 . Within the scope of the invention, the fourth torque-transmitting mechanism  56  may have alternative locations, but always selectively connects any two members of the first planetary gear set to cause all three members of the first planetary gear set to rotate at the same speed (i.e., the fourth torque-transmitting mechanism  56  acts as a lockup clutch). Additionally, a fifth torque-transmitting mechanism  58  selectively connects the second motor/generator  82  with the transmission housing  60 . Finally, a sixth torque-transmitting mechanism  59  selectively connects the first motor/generator  80  with the transmission housing  60 . As will be described below, the torque-transmitting mechanisms are selectively engageable to provide a variety of fixed forward speed ratios, an input split and a compound split first and second electrically variable forward mode, an input split reverse mode and a mechanical reverse mode as well as an electric forward cruise mode. As will be understood by those skilled in the art, the first and second motor/generators  80 ,  82  each have a stator and a rotor (not shown), the rotor being rotatable and the stator being continuously grounded by the transmission housing  60 .  
         [0044]     Each embodiment of the transmission within the scope of the invention has an electric power source which is operatively connected to the motor/generators such that the motor/generators may transfer power to or receive power from the power source. A controller or ECU is operatively connected to the electric power source to control the distribution of power from or to the power source. An electric power source may be one or more batteries. Other electric power sources, such as fuel cells, have the ability to provide, or store and dispense, electric power and may be used in place of batteries without altering the concepts of the present invention. An electric power source and controller is shown and described with respect to the embodiments of FIGS.  5  through  8 . The embodiments of  FIGS. 1 through 4  which are represented by lever diagrams also incorporate an electric power source and controller, although not shown, which are operatively connected to the motor/generators in like manner as shown in  FIGS. 5 through 8 .  
         [0000]     Operational Description  
         [0045]     Electrically Variable Reverse Mode  
         [0046]     The transmission  14  provides an electrically variable reverse mode (characterized by a range of reverse speed ratios) which is capable of launching a vehicle (not shown) in reverse either with the engine  12  off of with the engine  12  running to power the vehicle. The second torque-transmitting mechanism  52  is engaged to establish the electrically variable reverse mode. If the engine  12  is off, the torque-transmitting mechanism  52 , which is a stationary type torque-transmitting mechanism such as a brake, grounds node E to the transmission housing  60 , which provides reaction torque. The second motor/generator  82  is used to launch the vehicle through a reverse reduction gear ratio provided by the compounded second and third planetary gear sets represented by the second lever  30 ,  40 . For electric reverse operation with engine off, engine  12  remains at zero speed, the second motor/generator  82  is at positive speed, and the first motor/generator  80  is at negative speed. To start the engine, the first motor/generator  80  decelerates to zero speed while the second motor/generator  82  provides reaction torque as well as torque to drive the vehicle. This enables acceleration of the engine  12  to a speed where it may be fueled. Once the engine  12  is running, engine power is split through the first planetary gear set represented by the first lever  20  and the first motor/generator  80 , which generates power while the second motor/generator  82  acts as the motor. Thus, power is transmitted to the output member  19  through both a mechanical path and an electrical path. Electrical power flow is in the forward direction as long as the first motor/generator  80  has positive speed. When the first motor/generator  80  decelerates to a negative speed, a second motor/generator  82  acts as a generator to supply power to the first motor/generator  80  to provide engine reaction torque.  
         [0047]     Fixed Reverse Speed Ratio  
         [0048]     If the optional fourth torque-transmitting mechanism  56  is provided, it may be engaged synchronously when the engine  12  and motor/generators  80 ,  82  are operating at speeds that create a transmission speed ratio (i.e., (speed of the input member  17 )/(speed of the output member  19 )) equivalent with a mechanical transmission gear ratio provided by engagement of the torque-transmitting mechanism  56 . As used herein, the terms gear ratio and fixed speed ratio have the same meaning. Alternatively, if the optional sixth torque-transmitting mechanism  59  is provided, it may be engaged synchronously when the engine  12  and motor/generators  80 ,  82  are operating at speeds that create a transmission ratio equivalent with a mechanical transmission ratio provided by engagement of the torque-transmitting mechanism  59 . In the fixed reverse speed ratio, the motor/generators  80 ,  82  are not needed to transmit torque but may be used for an acceleration boost to supplement the engine  12 , or as generators.  
         [0049]     The electrically variable reverse mode and the fixed reverse speed ratio allow the ring gear/sun gear tooth ratio of the planetary gear sets represented by the first and second levers  20  and  30 ,  40 , respectively, as well as the size of the first and second motor/generators  80 ,  82 , respectively, to be optimized for efficient fuel economy or other design criteria. Because the engine  12  is not off during the electrically variable input split reverse mode, reverse grade requirements may be met without increasing the size of the second motor/generator  82  and/or using higher planetary gear ratios than would otherwise be required for optimum fuel economy.  
         [0050]     First Forward Mode  
         [0051]     The transmission is capable of providing an electrically variable first forward mode characterized by a range of forward speed ratios. A vehicle may be launched by the transmission  14  with the engine  12  off or with the engine  12  running in the electrically variable first forward mode. To establish the electrically variable first forward mode, the third torque-transmitting mechanism  54  is engaged to ground node G of the second lever  30 ,  40  to the transmission housing  60 . If the engine  12  is off with the third torque-transmitting mechanism  54  engaged, the second motor/generator  82  is used to launch the vehicle through the reduction gear ratio provided by the compounded second and third planetary gear sets represented by the second lever  30 ,  40 . Initially, the engine  12  remains at zero speed and the first motor/generator  80  spins in a reverse direction. To start the engine  12 , the first motor/generator  80  decelerates to zero speed while the second motor/generator  82  provides reaction torque as well as torque to drive the vehicle. This enables acceleration of the engine  12  to a speed where it may be fueled. Once the engine  12  is running, engine power provided through the input member  17  is split through the first planetary gear set represented by the first lever  20  and the first motor/generator  80 , which generates power while the second motor/generator  82  acts as a motor. Power is transmitted to the output member  19  to drive the vehicle through both a mechanical power path (i.e., through the first planetary gear set  20  and the interconnecting member  70 ) and an electrical power path (i.e., through the first motor/generator  80  to the second motor/generator  82 ). Power flows in the forward direction as long as the first motor/generator  80  has positive speed. When the speed of the first motor/generator  80  becomes negative, the second motor/generator  82  acts as a generator to supply power to the first motor/generator  80 . Regenerative braking is accomplished using the second motor/generator  82 , which is characterized by a direct speed ratio to the output member  19 .  
         [0052]     Electrically Variable Second Forward Mode  
         [0053]     For operation in a second electrically variable forward mode characterized by a lower range of numeric speed ratios, the transmission  14  provides a compound, split mode in which the first torque-transmitting mechanism  50  is engaged and the third torque-transmitting mechanism  54  is released. In this lower range of forward speed ratios, power flows in the forward direction as long as the first and second motor/generators  80 ,  82 , respectively, have positive speed. In this lower range, the second motor/generator  82  acts as a generator and the first motor/generator  80  acts as a motor. If the speed of the second motor/generator  82  becomes negative, the first motor/generator  80  acts as generator to supply power to the second motor/generator  82 . If the speed of the first motor/generator  80  is negative, the second motor/generator  82  becomes a generator to supply power to the first motor/generator  80 . Regenerative braking may be accomplished in the electrically variable second forward mode by balancing torque of the engine  12  and the first and second motor/generators  80 ,  82 , respectively, to provide the desired deceleration rate of the output member  19 .  
         [0054]     Fixed Forward Speed Ratios  
         [0055]     Within both the first and second electrically variable forward modes, the torque-transmitting mechanisms of the transmission  14  may be utilized to provide multiple fixed forward speed ratios. When the transmission ratio reaches a ratio equivalent to that which may be provided mechanically by engagement of two of the torque-transmitting mechanisms, the appropriate torque-transmitting mechanisms are engaged to provide the fixed ratio. When the transmission  14  is operating in a fixed forward speed ratio, the motor/generators  80 ,  82  are not utilized to transmit torque from the engine  12  but may be used for an acceleration boost or for regenerative braking. If the optional fourth, fifth and sixth torque-transmitting mechanisms  56 ,  58  and  59  are provided, up to six forward speed ratios are provided by the transmission  14 . Three fixed forward speed ratios are available during the electrically variable first forward mode when the third torque-transmitting mechanism  54  is engaged. By engaging the fourth torque-transmitting mechanism  56 , a first fixed speed ratio is provided. At a lower speed ratio, the sixth torque-transmitting mechanism  59  may be engaged to establish a second fixed forward speed ratio. The sixth torque-transmitting mechanism  59  is then disengaged to allow an increase in transmission ratio, in the electrically variable first forward mode. At a yet lower speed ratio, the first torque-transmitting mechanism  50  is engaged while the third torque-transmitting mechanism  54  remains engaged to establish a third fixed forward speed ratio. The first torque-transmitting mechanism  50  is then disengaged to allow the electrically variable first forward mode to resume and provide lower speed ratios. To achieve transmission ratios at the electrically variable second forward mode, the third torque-transmitting mechanism  54  is disengaged while the first torque-transmitting mechanism  50  is engaged. During the electrically variable second forward mode, three additional fixed forward speed ratios may be achieved. First, the sixth torque-transmitting mechanism  59  may be engaged to establish a fourth fixed forward speed ratio. At a lower speed ratio, the fourth torque-transmitting mechanism  56  may be engaged to establish a fifth forward fixed speed ratio. At a still lower speed ratio, the fifth torque-transmitting mechanism  58  may be engaged to establish a sixth forward speed ratio. The sixth fixed forward speed ratio provided by engagement of the first torque-transmitting mechanism  50  and the fifth torque-transmitting mechanism  58  permits top gear ratio as low as 0.66 (sample planetary gear set tooth ratios set forth in paragraph [0078] achieve a sixth fixed gear ratio of 0.661), which is significantly lower than that achieved in overdrive by typical hybrid electrically variable transmissions, and more closely mimics the highway fuel economy of an automatic transmission having a lower numeric ratio. The availability of multiple fixed forward speed ratios allows the transmission  14  to be operated in mechanical mode at a variety of speed ratios which, as is readily apparent to those skilled in the art, increases system efficiency.  
         [0056]     Electric Forward Cruise (Regenerative Braking) Mode  
         [0057]     During the electrically variable second forward mode, the transmission  14  provides an electric forward cruise mode. The electric forward cruise mode is established by engaging the second torque-transmitting mechanism  52  while the first torque-transmitting mechanism  50  remains engaged and fuel to the engine is cut off so that the engine  12  is stopped. In this arrangement, the motor/generators  80 ,  82  drive the output member  19  at high ratios of motor speeds to output speed. Additionally, torque from the first and second motor/generators  80 ,  82  is additive. In this mode, both of the motor/generators  80 ,  82  are spinning at a high speed relative to the output member  19  and both decelerate in order to start the engine  12 . Accordingly, energy of a battery connected to the motor/generators  80 ,  82  (battery not shown but connected to the motor/generator  80 ,  82  in like manner as shown and described with respect to the batteries and motor/generator of  FIGS. 5 through 8 ) is augmented by stored kinetic energy of the motor/generators  80 ,  82  during starting of the engine  12 . The kinetic energy of both motor/generators  80 ,  82  is higher in electric cruise mode than in electrically variable second forward mode. Therefore, during the transition, some of this kinetic energy is available, at the discretion of the control strategy, to either help propel the vehicle or increase the speed of the engine  12 . The net effect is that less battery power is required than would otherwise be needed if both motor/generators  80 ,  82  did not decrease in speed.  
         [0058]     The transmission  14  improves regenerative braking efficiency in a mid-speed ratio range. At relatively high numeric transmission speed ratios, the transmission  14  provides efficient regenerative braking because the second motor/generator  82  is directly coupled to the output member  19 . Likewise, at low numeric transmission speed ratios, the transmission  14  may operate in the sixth fixed forward speed ratio described above, providing efficient regenerative braking because the first motor/generator  80  is directly coupled to the output member  19 . However, when vehicle speed drops below a point in which the sixth fixed forward speed ratio may be utilized, the transmission  14  operates in the electrically variable second forward mode, which is not as efficient for regenerative braking as the sixth fixed forward speed ratio since both of the motor/generators  80 ,  82  must supply torque and the speeds of the motor/generators are relatively low. If the engine  12  is off, the second motor/generator  82  torque must be negative in order to balance the regenerative braking torque applied to the output member  19  and the negative torque of the first motor/generator  80 . However, the second motor/generator  82  will also have negative speed, resulting in positive power flow; hence, there is circulating electrical power in that the first motor/generator  80  generating power will exceed the power flow to the battery. Ideally, each of the motor/generators  80 ,  82  should carry a fraction of the regenerative braking power of between zero and one, with the sum of the fractions being one. Even if the engine  12  is not off and the second motor/generator  82  has positive speed, the speeds of the motor/generators  80 ,  82  are relatively low, and there is relatively small mechanical advantage since (neglecting engine drag) the torque of the motor/generators  80 ,  82  must sum to the transmission output torque. Therefore, if the first motor/generator  80  has a large mechanical advantage, the second motor/generator  82  will have a small mechanical advantage, or vice versa. By incorporating an additional torque reaction point to ground in the lever at node E, the mechanical advantage of both motor/generators  80 ,  82  is increased. Additionally, if the engine  12  is running during the electrically variable second forward mode, efficiency is further reduced due to engine drag and lower motor speeds. By providing the electric cruise mode with the engine  12  off, the motor/generators  80 ,  82  operate at higher speeds and lower torques to achieve the same torque at the output member  19 , resulting in improved efficiency.  
       First Preferred Embodiment  
       [0059]     Referring to  FIG. 2 , a first completed preferred embodiment of a powertrain  110  having a transmission  114  within the scope of the invention is illustrated in lever diagram form. The transmission  114  utilizes three differential gear sets, preferably in the nature of planetary gear sets  120 ,  130  and  140 . The planetary gear set  120 , represented in lever diagram form, employs a ring gear member  124 , a planet carrier member  129  and a sun gear member  122 . The ring gear member  124  circumscribes the sun gear member  122 . The planet carrier member  129  rotatably supports a plurality of planet gears that meshingly engage both the ring gear member  124  and the sun gear member  122 . The input member  17  is secured to the carrier member  129 . A first motor/generator  180  is continuously connected with the sun gear member  122 . The planetary gear set  120  may be represented by the lever  20  of  FIG. 1 .  
         [0060]     The second planetary gear set  130  represented in lever diagram form employs a ring gear member  134  which circumscribes a sun gear member  132 . A planet carrier member  139  rotatably supports a plurality of planet gears that meshingly engage both the ring gear member  134  and the sun gear member  132 .  
         [0061]     The planetary gear set  140  employs a ring gear member  144  which circumscribes a sun gear member  142 . A planet carrier member  149  rotatably supports a plurality of planet gears that meshingly engage both the ring gear member  144  and the sun gear member  142 . The output member  19  is continuously connected with the carrier member  149 .  
         [0062]     The ring gear member  124  is continuously connected with the sun gear member  132  by an interconnecting member  170 . The sun gear member  132  is continuously connected with the sun gear member  142  and a second motor/generator  182  via interconnecting member  172  which, as illustrated, may be more than one component. The ring gear member  134  is continuously connected with the carrier member  149  via an interconnecting member  174 .  
         [0063]     The ring gear member  124  corresponds with the first node A of  FIG. 1 . The carrier member  129  corresponds with the second node B of  FIG. 1 . The sun gear member  122  corresponds with the third node C of  FIG. 1 . Because the second and third planetary gear sets  130 ,  140  have two pairs of members connected via two separate interconnecting members  172  and  174 , the planetary gear sets  130  and  140  are compounded and are represented by the second lever  30 ,  40  of  FIG. 1 . The connected sun gear member  132  and sun gear member  142  are together represented by corresponding fourth node D of  FIG. 1 . The carrier member  139  corresponds with the fifth node E of  FIG. 1 . The connected ring gear member  134  and carrier member  139  correspond with the sixth node F of  FIG. 1 . The ring gear member  144  corresponds with the seventh node G of  FIG. 1 .  
         [0064]     The first torque-transmitting mechanism  150  is selectively engageable to connect the carrier member  129  with the carrier member  139 . The second torque-transmitting mechanism  152  is selectively engageable to ground the carrier member  139  to the transmission housing  160 . The third torque-transmitting mechanism  154  is selectively engageable to ground the ring gear member  144  with the transmission housing  160 . The torque-transmitting mechanisms  150 ,  152  and  154  are engageable in like manner as corresponding torque-transmitting mechanisms  50 ,  52  and  54 , respectively, of  FIG. 1  to establish first and second electrically variable forward modes, a fixed forward speed ratio (corresponding with the third fixed forward speed ratio described with respect to  FIG. 1 ) an electric cruise mode, and an input split, electrically variable reverse mode.  
       Second Alternative Preferred Embodiment  
       [0065]     Referring to  FIG. 3 , a second specific preferred embodiment of a powertrain  210  having a transmission  214  within the scope of the invention is illustrated. Transmission  214  utilizes three differential gear sets, preferably in the nature of planetary gear sets  220 ,  230  and  240 , represented in lever diagram form. The planetary gear set  220  employs a ring gear member  224  which circumscribes the sun gear member  222 . Carrier member  229  rotatably supports a plurality of planet gears that meshingly engage both the ring gear member  224  and the sun gear member  222 . The input member  17  is secured to the carrier member  229 . A first motor/generator  280  is continuously connected to the sun gear member  222 .  
         [0066]     The planetary gear set  230  has a ring gear member  234  which circumscribes the sun gear member  232 . A carrier member  239  includes a plurality of planet gears that meshingly engage both the sun gear member  232  and the ring gear member  234 . A second motor/generator  282  is continuously connected to the sun gear member  232 .  
         [0067]     The planetary gear set  240  includes a ring gear member  244  that circumscribes a sun gear member  242 . A carrier member  249  includes a plurality of pinion gears that meshingly engage both the sun gear member  244 .  
         [0068]     An interconnecting member  270  continuously connects the ring gear member  224  with the sun gear member  232 . An interconnecting member  272  continuously connects the carrier member  239  with the ring gear member  244 . An interconnecting member  274  continuously connects the ring gear member  234  with the carrier member  249 .  
         [0069]     The ring gear member  224  corresponds with the first node A of  FIG. 1 . The carrier member  229  corresponds with the second node B. The sun gear member  222  corresponds with the third node C. The sun gear member  232  corresponds with the fourth node D. The interconnected carrier member  239  and ring gear member  244  correspond with the fifth node E. The interconnected ring gear member  234  and carrier member  249  correspond with the sixth node F. The sun gear member  242  corresponds with the seventh node G. Because the planetary gear sets  230  and  240  have two interconnections via interconnecting members  272  and  274 , they may be represented by the single second lever  30 ,  40  of  FIG. 1 . The planetary gear set  220  may be represented by the lever  20  of  FIG. 1 .  
         [0070]     A first torque-transmitting mechanism  250  selectively connects the carrier member  229  with the carrier member  239 . A second torque-transmitting mechanism  252  selectively connects the ring gear member  244  with the transmission housing  260 . A third torque-transmitting mechanism  254  selectively connects the sun gear member  242  with the transmission housing  260 . The torque-transmitting mechanisms  250 ,  252  and  254  are engageable in like manner as corresponding torque-transmitting mechanisms  50 ,  52  and  54 , respectively, as described above with respect to  FIG. 1  to establish first and second electrically variable forward modes, an electric forward cruise mode, a fixed forward speed ratio and an input split, electrically variable reverse mode.  
       Third Preferred Alternative Embodiment  
       [0071]     Referring to  FIG. 4 , a third specific preferred embodiment of a powertrain  310  having a transmission  314  within the scope of the invention is illustrated. The transmission  314  utilizes three differential gear sets, preferably in the nature of planetary gear sets  320 ,  330  and  340 , represented in lever diagram form. The planetary gear set  320  employs a ring gear member  324  which circumscribes the sun gear member  322 . The carrier member  329  rotatably supports a plurality of planet gears that meshingly engage both the ring gear member  324  and the sun gear member  322 . The input member  17  is continuously connected with the carrier member  329 . A first motor/generator  380  is continuously connected with the sun gear member  322 .  
         [0072]     The planetary gear set  330  has a ring gear member  334  which circumscribes the sun gear member  332 . A carrier member  339  includes a plurality of pinion gears that meshingly engage both the ring gear member  334  and the sun gear member  332 . A second motor/generator  382  is continuously connected with the sun gear member  332 .  
         [0073]     The planetary gear set  340  has a ring gear member  344  which circumscribes the sun gear member  342 . A carrier member  349  rotatably supports a plurality of planet gears that meshingly engage both the ring gear member  344  and the sun gear member  342 . The output member  19  is continuously connected with the carrier member  349 .  
         [0074]     An interconnecting member  370  continuously connects the ring gear member  324  with the sun gear member  332 . An interconnecting member  372  continuously connects the carrier member  339  with the ring gear member  344 . An interconnecting member  374  continuously connects the ring gear member  334  with the carrier member  349 .  
         [0075]     Because the planetary gear sets  330  and  340  have two pairs of interconnected members via the interconnecting members  372  and  374 , they may be represented by the single lever  30 ,  40  of  FIG. 1 . The planetary gear set  320  may be represented by the lever  20  of  FIG. 1 . The ring gear member  324  corresponds with the first node A of  FIG. 1 . The carrier member  329  corresponds with the second node B. The sun gear member  322  corresponds with the third node C. The sun gear member  332  corresponds with the fourth node D. The interconnected carrier member  339  and ring gear member  344  correspond with the fifth node E. The interconnected ring gear member  334  and carrier member  349  correspond with the sixth node F. The sun gear member  342  corresponds with the seventh node G.  
         [0076]     A first torque-transmitting mechanism  350  is selectively engageable to connect the carrier member  329  with the carrier member  339 . A second torque-transmitting mechanism  352  is selectively engageable to ground the ring gear member  344  to the transmission housing  360 . A third torque-transmitting mechanism  354  is selectively engageable to ground the sun gear member  342  to the transmission housing  360 . A fourth torque-transmitting mechanism  356  is selectively engageable to connect the carrier member  329  with the interconnected ring gear member  324  and sun gear member  332 . A fifth torque-transmitting mechanism  358  is selectively engageable to ground the sun gear member  332  and the second motor/generator  382  to the transmission housing  360 .  
         [0077]     The torque-transmitting mechanism  350 ,  352  and  354  are engageable in like manner as corresponding torque-transmitting mechanisms  50 ,  52  and  54 , respectively, of  FIG. 1  to establish first and second electrically variable forward modes, a fixed forward speed ratio, a forward electric cruise mode and an input split, electrically variable reverse speed mode. The additional torque-transmitting mechanisms  356  and  358  are selectively engageable as described above with respect to  FIG. 1  in like manner as corresponding torque-transmitting mechanisms  56  and  58  to establish three additional fixed forward speed ratios and a fixed reverse speed ratio. Although not illustrated in the specific embodiments of  FIGS. 2 through 8 , a sixth torque-transmitting mechanism may be added to ground the first motor/generator  80  (Unit A) to the transmission housing to establish two additional fixed forward speed ratios, as described with respect to torque-transmitting mechanism  59  of  FIG. 1 , for a total of six fixed forward speed ratios.  
       Fourth Alternative Preferred Embodiment  
       [0078]     Referring to  FIG. 5 , a fourth specific preferred embodiment of a powertrain  410  having a transmission  414  within the scope of the invention is illustrated. The transmission  414  utilizes three differential gear sets, preferably in the nature of planetary gear sets  420 ,  430  and  440 . The transmission  414  is shown in a stick diagram rather than a lever diagram form. Planetary gear set  420  employs a ring gear member  424  which circumscribes a sun gear member  422 . A carrier member  429  rotatably supports a plurality of pinion gears that meshingly engage both the ring gear member  424  and the sun gear member  422 . The input member  17  is continuously connected with the carrier member  429 . A first motor/generator  480  is continuously connected with the sun gear member  422 .  
         [0079]     The planetary gear set  430  includes a ring gear member  434  which circumscribes a sun gear member  432 . A carrier member  439  includes a plurality of planet gears that meshingly engage both the ring gear member  434  and the sun gear member  432 . The output member  19  is continuously connected with the ring gear member  434 . As will be readily understood by those skilled in the art, the transmission  414  is appropriate for a front wheel drive application, as the output member  19  is in a location well suited for transverse arrange usage.  
         [0080]     The planetary gear set  440  includes a ring gear member  444  that circumscribes a sun gear member  442 . The carrier member  449  includes a plurality of pinion gears that meshingly engage both the ring gear member  444  and the sun gear member  442 . The second motor/generator  482  is continuously connected with the sun gear member  442 .  
         [0081]     An interconnecting member  470  continuously connects the ring gear member  424  with the sun gear member  442 . The second motor/generator  482  is thereby also continuously connected with the ring gear member  424 . The interconnecting member  470  may be one component or separate components. An interconnecting member  472  continuously the ring gear member  434  with the carrier member  449 . An interconnecting member  474  continuously connects the sun gear member  432  with the sun gear member  442 . Thus, two members of the planetary gear set  430  are continuously connected with the two members of the planetary gear set  440  via two interconnecting members  472  and  474 . Accordingly, the planetary gear sets  430  and  440  may be represented in lever diagram formed by the compound lever  30 ,  40  of  FIG. 1 . The planetary gear set  420  may be represented by the lever  20  of  FIG. 1 .  
         [0082]     The ring gear member  424  corresponds with the first node A of  FIG. 1 . The carrier member  429  corresponds with the second node B of  FIG. 1 . The sun gear member  422  corresponds with the third node C of  FIG. 1 . The continuously connected sun gear members  432  and  442  correspond with the fourth node D of  FIG. 1 . The carrier member  439  corresponds with the fifth node E of  FIG. 1 . The continuously connected ring gear member  434  and carrier member  449  correspond with the sixth node F of  FIG. 1 . The ring gear member  444  corresponds with the seventh node G of  FIG. 1 .  
         [0083]     The first torque-transmitting mechanism  450  is selectively engageable to connect the carrier member  429  with the carrier member  439  and also connect the input member  17  with the carrier member  439 . The second torque-transmitting mechanism  452  is selectively engageable to ground the carrier member  439  to the transmission housing  460 . The third torque-transmitting mechanism  454  is selectively engageable to ground the ring gear member  444  to the transmission housing  460 . The torque-transmitting mechanisms  450 ,  452  and  454  are engageable in like manner as corresponding torque-transmitting mechanisms  50 ,  52  and  54 , respectively, of  FIG. 1  to establish first and second electrically variable forward mode, an electric forward cruise mode, a fixed forward speed ratio and an input split, electrically variable reverse mode.  
         [0084]     Preferably, each of the planetary gear sets  420 ,  430  and  440  has a ring gear/sun gear tooth ratio (N R /S R ) of 1.954, although other tooth ratios may also be employed within the scope of the invention. If the optional fourth, fifth and sixth torque-transmitting mechanisms are employed as set forth in  FIG. 1  (i.e., a fourth torque transmitting mechanism such as torque-transmitting mechanism  56  of  FIG. 1  selectively connects the ring gear member  424  with the carrier member  429 ; a fifth torque-transmitting mechanism such as torque-transmitting mechanism  58  of  FIG. 1  selectively grounds the second motor/generator  482  with the transmission housing  460 ; and a sixth torque-transmitting mechanism such as torque-transmitting mechanism  59  of  FIG. 1  selectively grounds the first motor/generator  480  with the transmission housing  460 ), six fixed forward gear ratios and a fixed reverse gear ratio are achieved as follows. A first fixed forward gear ratio of 2.954 is achieved by engagement of the third and fourth torque-transmitting mechanisms  454 ,  56 . A second fixed forward gear ratio of 1.954 is achieved by engagement of the third and sixth torque-transmitting mechanisms  454 ,  59 . A third fixed forward gear ratio of 1.661 is achieved by engagement of the third and first torque-transmitting mechanisms  454 ,  50 . A fourth fixed forward gear ratio of 1.355 is achieved by engagement of the first and sixth torque-transmitting mechanisms  450 ,  59 . A fifth fixed forward gear ratio of 1.0 is achieved by engagement of the first and fourth torque-transmitting mechanisms  450 ,  56 . A sixth fixed forward gear ratio of 0.661 is achieved by engagement of the first and fifth torque-transmitting mechanisms  450 ,  58 . Finally, a fixed reverse gear ratio of −1.954 is achieved by engagement of the second and fourth torque-transmitting mechanisms  452 ,  56 .  
         [0085]     It is apparent from  FIG. 5  in the foregoing description that the transmission  414  selectively receives power from the engine  12 . The hybrid transmission  414  also receives power from or transfers power to an electrical power source  486 , which is operatively connected to a controller or ECU  488 . The electric power source  486  is operatively connected to the motor/generator  480 ,  482  via the controller  488 . The electrical power source  486  may be one or more batteries. Other electrical power sources, such as fuel cells, have the ability to provide, or store and dispense, electrical power and may be used in place of batteries without altering the concepts of the present invention.  
       Fifth Alternative Preferred Embodiment  
       [0086]     Referring to  FIG. 6 , a fifth specific preferred embodiment of a powertrain  510  of a transmission  514  within the scope of the invention is illustrated. The transmission  514  utilizes three differential gear sets, preferably in the nature of planetary gear sets  520 ,  530  and  540 . The transmission  514  is illustrated in stick diagram rather than lever diagram form. The planetary gear set  520  employs a ring gear member  524  which circumscribes the sun gear member  522 . A carrier member  529  rotatably supports a first set of pinion gears  527  and a second set of pinion gears  528 . The first set of pinion gears  527  meshingly engages with the sun gear member  522  and the second set of pinion gears  528 . The second set of pinion gears  528  meshingly engage with the first set of pinion gears  527  and with the ring gear member  524 . The input member  17  is continuously connected with the ring gear member  524 . A first motor/generator  580  is continuously connected with the sun gear member  522 .  
         [0087]     The second planetary gear set  530  has a ring gear member  534  that circumscribes the sun gear member  532 . A carrier member  539  rotatably supports a plurality of pinion gears which meshingly engage with both the sun gear member  532  and the ring gear member  534 . The output member  19  is continuously secured to the ring gear member  534 . As will be readily understood by those skilled in the art, the transmission  514  is appropriate for a front wheel drive application, as the output member  19  is in a location well suited for transverse arrange usage.  
         [0088]     The planetary gear set  540  includes a ring gear member  544  which circumscribes a sun gear member  542 . A carrier member  549  includes a plurality of pinion gears which meshingly engage with both the sun gear member  542  and the ring gear member  544 . The second motor/generator  582  is continuously connected with the sun gear member  542 .  
         [0089]     A first interconnecting member  570  continuously connects the carrier member  529  with the sun gear member  542 . The interconnecting member  570  may be one component or separate components and also continuously connects the second motor/generator  582  with the carrier member  529 . A second interconnecting member  572  continuously connects the ring gear member  534  with the carrier member  549 . An interconnecting member  574  continuously connects the sun gear member  532  with the sun gear member  542 .  
         [0090]     The carrier member  529  corresponds with the first node A of  FIG. 1 . The ring gear member  524  corresponds with the second node B. The sun gear member  522  corresponds with the third node C. The interconnected sun gear members  532  and  542  correspond with the fourth node D. The carrier member  539  corresponds with the fifth node E. The interconnected ring gear member  534  and carrier member  549  correspond with the sixth node F. The ring gear member  544  corresponds with the seventh node G. Because the planetary gear sets  530  and  540  have two pairs of interconnected members via the interconnecting members  572  and  574 , they may be represented by the second lever  30 ,  40  of  FIG. 1 . The planetary gear set  520  is represented by the first lever  20  of  FIG. 1 .  
         [0091]     The torque-transmitting mechanism  550  is selectively engageable to connect the ring gear member  524  with the carrier member  539 . The carrier member  539  is also thereby continuously connected with the input member  17 . The second torque-transmitting mechanism  552  is selectively engageable to ground the carrier member  539  to the transmission housing  560 . The third torque-transmitting mechanism  554  is selectively engageable to ground the ring gear member  544  to the transmission housing  560 . The torque-transmitting mechanisms  550 ,  552  and  554  are engageable in like manner as corresponding torque-transmitting mechanisms  50 ,  52  and  54 , respectively, of  FIG. 1  to establish first and second electrically variable forward modes, a fixed forward speed ratio, an electric forward cruise mode and an input split, electrically variable reverse mode.  
         [0092]     It is apparent from  FIG. 6  and the foregoing description that the transmission  514  selectively receives power from the engine  12 . The hybrid transmission  514  also receives power from an electrical power source  586 , which is operatively connected to a controller or ECE  588 . The electrical power source  586  may be one or more batteries. Other electrical power sources, such as fuel cells, may also be used. The battery  586  and controller  588  are operatively connected to the first and second motor/generators  580  and  582  for transferring power to the motor/generators  580 ,  582  or receiving power therefrom.  
       Sixth Alternative Preferred Embodiment  
       [0093]     Referring to  FIG. 7 , a sixth specific preferred embodiment of a powertrain  610  having a transmission  614  within the scope of the invention is illustrated. The transmission  614  utilizes three differential gear sets, preferably in the nature of planetary gear sets  620 ,  630  and  640 . The planetary gear set  620  employs a ring gear member  624  which circumscribes a sun gear member  622 . A carrier member  629  includes a plurality of planet gears that meshingly engage both the ring gear member  624  and the sun gear member  622 . The input member  17  is continuously connected with the carrier member  629  and a first motor/generator  680  is continuously connected with the sun gear member  622 .  
         [0094]     The planetary gear set  630  has a ring gear member  634  that circumscribes the sun gear member  632 . A carrier member  639  includes a plurality of pinion gears that meshingly engage both the ring gear member  634  and the sun gear member  632 . A second motor/generator  682  is continuously connected with the sun gear member  632 .  
         [0095]     The planetary gear set  640  also has a ring gear member  644  that circumscribes a sun gear member  642 . A carrier member  649  includes a plurality of pinion gears that meshingly engage both the ring gear member  644  and the sun gear member  642 . The output member  19  is continuously connected with the carrier member  649 .  
         [0096]     An interconnecting member  670  continuously connects the ring gear member  624  with the second motor/generator  682 , thereby continuously connecting the ring gear member  624  with the sun gear member  632 , as the sun gear member  632  is also continuously connected with the second motor/generator  682 . A second interconnecting member  672  continuously connects the ring gear member  634  with the carrier member  649 . A third interconnecting member  674  continuously connects the carrier member  639  with the ring gear member  644 .  
         [0097]     Although the transmission  614  is represented schematically in stick diagram form in  FIG. 7 , those skilled in the art will recognize that the planetary gear set  620  may be represented by the lever  20  of  FIG. 1 . The ring gear member  624  corresponds with the first node A of  FIG. 1 . The carrier member  629  corresponds with the second node B and the sun gear member  622  corresponds with the third node C. Because the planetary gear sets  630  and  640  have two pairs of interconnected members via the interconnecting members  672  and  674 , they may be represented by the compound lever  30 ,  40  of  FIG. 1 . The sun gear member  632  corresponds with the fourth node D. The interconnected carrier member  639  and ring gear member  644  correspond with the fifth node E. The interconnected ring gear member  634  and carrier member  649  correspond with the sixth node F. The sun gear member  642  corresponds with the seventh node G.  
         [0098]     The first torque-transmitting mechanism  650  selectively connects the carrier member  629  with the carrier member  639 . The second torque-transmitting mechanism  652  selectively grounds the carrier member  639  and the interconnected ring gear member  644  with the transmission housing  660 . The torque-transmitting mechanism  654  selectively grounds the sun gear member  642  with the transmission housing  660 . The fourth torque-transmitting mechanism  656  selectively connects the carrier member  629  with the first motor/generator  680  and thereby with the sun gear member  622  which is continuously connected with the first motor/generator  680 . The fifth torque-transmitting mechanism  658  selectively grounds the sun gear member  632  to the transmission housing  660 , thereby also grounding the second motor/generator  682  and ring gear member  624 .  
         [0099]     The torque-transmitting mechanism  650 ,  652 ,  654 ,  656  and  658  are engageable in like manner as corresponding torque-transmitting mechanisms  50 ,  52 ,  54 ,  56  and  58 , respectively, of  FIG. 1  to establish a first and a second electrically variably forward mode, four fixed forward speed ratios, a forward electric cruise mode, an input split, electrically variable reverse mode and a fixed reverse speed ratio.  
         [0100]     Preferably, the planetary gear sets  620  and  640  each have a ring gear/sun gear tooth ratio (N R /S R ) of 1.954 and the planetary gear set  630  has a ring gear/sun gear tooth ratio (N R /S R ) of 2.333, although other tooth ratios may also be employed within the scope of the invention. If the optional fourth, fifth and sixth torque-transmitting mechanisms are employed as set forth in  FIG. 1  (i.e., a fourth torque transmitting mechanism such as torque-transmitting mechanism  56  of  FIG. 1  selectively connects the ring gear member  624  with the carrier member  629 ; a fifth torque-transmitting mechanism such as torque-transmitting mechanism  58  of  FIG. 1  selectively grounds the second motor/generator  682  with the transmission housing  660 ; and a sixth torque-transmitting mechanism such as torque-transmitting mechanism  59  of  FIG. 1  selectively grounds the first motor/generator  680  with the transmission housing  660 ), six fixed forward gear ratios and a fixed reverse gear ratio are achieved as follows. A first fixed forward gear ratio of 2.71 is achieved by engagement of the third and fourth torque-transmitting mechanisms  654 ,  56 . A second fixed forward gear ratio of 1.79 is achieved by engagement of the third and sixth torque-transmitting mechanisms  654 ,  59 . A third fixed forward gear ratio of 1.51 is achieved by engagement of the third and first torque-transmitting mechanisms  654 ,  50 . A fourth fixed forward gear ratio of 1.28 is achieved by engagement of the first and sixth torque-transmitting mechanisms  650 ,  59 . A fifth fixed forward gear ratio of 1.0 is achieved by engagement of the first and fourth torque-transmitting mechanisms  650 ,  56 . A sixth fixed forward gear ratio of 0.70 is achieved by engagement of the first and fifth torque-transmitting mechanisms  650 ,  58 . Finally, a fixed reverse gear ratio of −2.33 is achieved by engagement of the second and fourth torque-transmitting mechanisms  652 ,  56 .  
         [0101]     It is apparent from  FIG. 7  and the foregoing description that the transmission  614  selectively receives power from the engine  12 . The hybrid transmission  614  also receives power from an electric power source  686 , which is operably connected to a controller or ECU  688 . The electric power source  686  may be one or more batteries, or may be fuel cells or other electric power sources which have the ability to provide, or store and dispense, electric power without altering the concepts of the present invention. The battery  686  and controller  688  are operatively connected to the first and second motor/generators  680  and  682  for transferring power to the motor/generators  680 ,  682  or receiving power therefrom. The configuration of the transmission  614  of  FIG. 7  is appropriate for a rear wheel drive longitudinal application.  
       Seventh Preferred Alternative Embodiment  
       [0102]     Referring to  FIG. 8 , a seventh specific preferred embodiment of a powertrain  710  having a transmission  714  within the scope of the present invention is illustrated. The transmission  714  utilizes three differential gear sets, preferably in the nature of planetary gear sets  720 ,  730  and  740 . The planetary gear set  720  employs a ring gear member  724  which circumscribes the sun gear member  722 . A carrier member  729  includes a first and a second set of pinion gears  727 ,  728 , respectively. The first set of pinion gears  727  meshingly engage the sun gear member  722  and the second set of pinion gears  728 . The second set of pinion gears  728  meshingly engages with the first set of pinion gears  727  and the ring gear member  724 . The input member  17  is continuously connected with the ring gear member  724 . The first motor/generator  780  is continuously connected with the sun gear member  722 .  
         [0103]     The planetary gear set  730  has a ring gear member  734  which circumscribes the sun gear member  732 . A carrier member  739  includes a plurality of pinion gears which meshingly engage with both the ring gear member  734  and the sun gear member  732 . The second motor/generator  782  is continuously connected with the sun gear member  732 .  
         [0104]     The planetary gear set  740  has a ring gear member  744  which circumscribes the sun gear member  742 . A carrier member  749  includes a plurality of pinion gears that meshingly engage the ring gear member  744  and the sun gear member  742 . The output member  19  is continuously connected with the carrier member  749 .  
         [0105]     An interconnecting member  770  continuously connects the carrier member  729  with the second motor/generator  782  and therefore with the sun gear member  732  which is also continuously connected with the second motor/generator  782 . A carrier member  772  continuously connects the ring gear member  734  with the carrier member  749  and thereby with the output member  19 . An interconnecting member  774  continuously connects the carrier member  739  with the ring gear member  744 .  
         [0106]     Although the transmission  714  of  FIG. 8  is illustrated in stick diagram form, those skilled in the art will readily understand that the planetary gear set  720  is represented by the first lever  20  of  FIG. 1  and the compounded planetary gear sets  730  and  740  are represented by the second lever diagram  30 ,  40  of  FIG. 1 . Carrier member  729  corresponds with the first node A of  FIG. 1 . The ring gear member  724  corresponds with the second node B. The sun gear member  722  corresponds with the third node C. The sun gear member  732  corresponds with the fourth node D. The interconnected carrier member  739  and ring gear member  744  correspond with the fifth node E. The interconnected ring gear member  734  and carrier member  749  correspond with the sixth node F. The sun gear member  742  corresponds with the seventh node G.  
         [0107]     A first torque-transmitting mechanism  750  selectively connects the ring gear member  724  with the carrier member  739 . A second torque-transmitting mechanism  752  selectively grounds the carrier member  739  and the ring gear member  744  with the transmission housing  760 . A third torque-transmitting mechanism  754  selectively grounds the sun gear member  742  with the transmission housing  760 . A fourth torque-transmitting mechanism  756  selectively connects the ring gear member  724  with the carrier member  729 , and with the second motor/generator  782  and thereby with the sun gear member  732  which is continuously connected with the second motor/generator  782 . A fifth torque-transmitting mechanism  758  selectively grounds the sun gear member  732  to the transmission housing  760 , thereby also grounding the second motor/generator  782  and carrier member  729 . The torque-transmitting mechanisms  750 ,  752 ,  754 ,  756  and  758  are engageable in like manner as corresponding torque-transmitting mechanisms  50 ,  52 ,  54 ,  56  and  58 , respectively, of  FIG. 1  to establish a first and a second electrically variable forward mode, four fixed forward speed ratios, an electric forward cruise mode, an input split electrically variable reverse mode and a fixed reverse speed ratio. If a sixth torque-transmitting mechanism were added to the transmission  714  of  FIG. 8  to ground the first motor/generator  780  to the transmission housing  760 , two additional fixed forward speed ratios for a total of six fixed forward speed ratios would be achieved.  
         [0108]     It is apparent from  FIG. 8  and the foregoing description that the transmission  714  selectively receives power from the engine  12 . The hybrid transmission  714  also receives power from an electric power source  786 , which is operably connected to a controller or ECU  788 . The electric power source  786  may be one or more batteries, or may be fuel cells or other electric power sources which have the ability to provide, or store and dispense, electric power without altering the concepts of the present invention. The battery  786  and controller  788  are operatively connected to the first and second motor/generators  780  and  782  for transferring power to the motor/generators  780 ,  782  or receiving power therefrom. The configuration of the transmission  714  of  FIG. 8  is appropriate for a rear wheel drive longitudinal application.  
         [0109]     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.