PATENT DOCUMENT

Abstract:
The electrically variable transmission family of the present invention provides low-content, low-cost electrically variable transmission mechanisms including first and second differential gear sets, a battery, two electric machines serving interchangeably as motors or generators, and four or five selectable torque-transfer devices. The selectable torque transfer devices are engaged singly or in combinations of two to yield an EVT with a continuously variable range of speeds (including reverse) and four mechanically fixed forward speed ratios. The torque transfer devices and the first and second motor/generators are operable to provide five operating modes in the electrically variable transmission, including battery reverse mode, EVT reverse mode, reverse and forward launch modes, continuously variable transmission range mode, and fixed ratio mode.

Full Description:
TECHNICAL FIELD 
     The present invention relates to electrically variable transmissions with selective operation both in power-split variable speed ratio ranges and in fixed speed ratios, and having two planetary gear sets, two motor/generators and four or five torque transmitting mechanisms. 
     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 the emissions of pollutants, 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. 
     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. 
     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. 
     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. 
     SUMMARY OF THE INVENTION 
     The present invention provides a family of electrically variable transmissions offering several advantages over conventional automatic transmissions for use in hybrid vehicles, including improved vehicle acceleration performance, improved fuel economy via regenerative braking and electric-only idling and launch, and an attractive marketing feature. An object of the invention is to provide the best possible energy efficiency and emissions for a given engine. In addition, optimal performance, capacity, package size, and ratio coverage for the transmission are sought. 
     The electrically variable transmission family of the present invention provides low-content, low-cost electrically variable transmission mechanisms including first and second differential gear sets, a battery, two electric machines serving interchangeably as motors or generators, and four or five selectable torque-transfer devices (two clutches and two or three brakes). Preferably, the differential gear sets are planetary gear sets, but other gear arrangements may be implemented, such as bevel gears or differential gearing to an offset axis. 
     In this description, the first and second planetary gear sets may be counted left to right or right to left. 
     Each of the planetary gear sets has three members. The first, second or third member of each planetary gear set can be any one of a sun gear, ring gear or carrier. 
     Each carrier can be either a single-pinion carrier (simple) or a double-pinion carrier (compound). 
     The input shaft is continuously connected with at least one member of the planetary gear sets. The output shaft is continuously connected with another member of the planetary gear sets. 
     An interconnecting member continuously connects a first member of the first planetary gear set and a first member of the second planetary gear set. 
     A first torque transfer device selectively connects a member of the first planetary gear set with another member of the first or second planetary gear set. 
     A second torque transfer device selectively connects a member of the second planetary gear set with another member of the first or second planetary gear set, this pair of members being different from the ones connected by the first torque transfer device. 
     A third torque transfer device selectively connects a member of the first or second planetary gear set with a stationary member (transmission case). 
     A fourth torque transfer device is implemented as a brake connected in parallel with one of the motor/generators for braking rotation thereof. An optional fifth torque transfer device may be implemented as a brake connected in parallel with the other one of the motor/generators for braking rotation thereof. 
     The first motor/generator is mounted to the transmission case (or ground) and is continuously connected to a member of the first or second planetary gear set. 
     The second motor/generator is mounted to the transmission case and is continuously connected to a member of the first or second planetary gear set, this member being different from the member connected with the first motor/generator. 
     The four or five selectable torque transfer devices are engaged singly or in combinations of two to yield an EVT with a continuously variable range of speeds (including reverse) and four mechanically fixed forward speed ratios. A “fixed speed ratio” is an operating condition in which the mechanical power input to the transmission is transmitted mechanically to the output, and no power flow (i.e. almost zero) is present in the motor/generators. An electrically variable transmission that may selectively achieve several 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 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. 
     Each embodiment of the electrically variable transmission family disclosed has an architecture in which neither the transmission input nor output is directly connected to a motor/generator. This allows for a reduction in the size and cost of the electric motor/generators required to achieve the desired vehicle performance. 
     The first, second, third and fourth (and optional fifth) torque transfer devices and the first and second motor/generators are operable to provide five operating modes in the electrically variable transmission, including battery reverse mode, EVT reverse mode, reverse and forward launch modes, continuously variable transmission range mode, and fixed ratio mode. 
     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. 1   a  is a schematic representation of a powertrain including an electrically variable transmission incorporating a family member of the present invention; 
         FIG. 1   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 1   a;    
         FIG. 2   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
         FIG. 2   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 2   a;    
         FIG. 3   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
         FIG. 3   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 3   a;    
         FIG. 4   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
         FIG. 4   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 4   a;    
         FIG. 5   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
         FIG. 5   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 5   a;    
         FIG. 6   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
         FIG. 6   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 6   a;    
         FIG. 7   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
         FIG. 7   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 7   a;    
         FIG. 8   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
         FIG. 8   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 8   a;    
         FIG. 9   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
         FIG. 9   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 9   a;    
         FIG. 10   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
         FIG. 10   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 10   a;    
         FIG. 11   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
         FIG. 11   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 11   a;    
         FIG. 12   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
         FIG. 12   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 12   a;    
         FIG. 13   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
         FIG. 13   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 13   a;    
         FIG. 14   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; and 
         FIG. 14   b  is an operating mode table and fixed ratio mode table depicting some of the operating characteristics of the powertrain shown in  FIG. 14   a.    
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG. 1   a , a powertrain  10  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission (EVT), designated generally by the numeral  14 . Transmission  14  is designed to receive at least a portion of its driving power from the engine  12 . As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  14 . A transient torque damper (not shown) may also be implemented between the engine  12  and the input member  17  of the transmission. 
     In the embodiment depicted the engine  12  may be a fossil fuel engine, such as a diesel engine which is readily adapted to provide its available power output typically delivered at a constant number of revolutions per minute (RPM). 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member  17  is operatively connected to a planetary gear set  20  in the transmission  14 . 
     An output member  19  of the transmission  14  is connected to a final drive  16 . 
     The transmission  14  utilizes two differential gear sets, preferably in the nature of planetary gear sets  20  and  30 . The planetary gear set  20  employs an outer gear member  24 , typically designated as the ring gear. The ring gear  24  circumscribes an inner gear member  22 , typically designated as the sun gear. A carrier  26  rotatably supports a plurality of planet gears  27  such that each planet gear  27  meshingly engages both the outer, ring gear member  24  and the inner, sun gear member  22  of the first planetary gear set  20 . The input member  17  is secured to the carrier  26  of the planetary gear set  20 . 
     The planetary gear set  30  also has an outer gear member  34 , often also designated as the ring gear, that circumscribes an inner gear member  32 , also often designated as the sun gear. A plurality of planet gears  37  are also rotatably mounted in a carrier  36  such that each planet gear member  37  simultaneously, and meshingly, engages both the outer, ring gear member  34  and the inner, sun gear member  32  of the planetary gear set  30 . 
     An interconnecting member  70  continuously connects the ring gear  24  of the planetary gear set  20  with the sun gear  32  of the planetary gear set  30 . 
     The first preferred embodiment  10  also incorporates first and second motor/generators  80  and  82 , respectively. The stator of the first motor/generator  80  is secured to the transmission housing  60 . The rotor of the first motor/generator  80  is secured to the sun gear  22 . 
     The stator of the second motor/generator  82  is also secured to the transmission housing  60 . The rotor of the second motor/generator  82  is secured to the ring gear  24 . 
     A first torque transfer device, such as a clutch  50 , selectively connects the ring gear  24  of the planetary gear set  20  to the carrier  26  of the planetary gear set  20 . A second torque transfer device, such as clutch  52 , selectively connects the sun gear  32  of the planetary gear set  30  with the carrier  36  of the planetary gear set  30 . A third torque transfer device, such as brake  54 , selectively connects the ring gear  34  of the planetary gear set  30  with the transmission housing  60 . That is, the ring gear  34  is selectively secured against rotation by an operative connection to the non-rotatable housing  60 . A fourth torque transfer device, such as brake  55 , selectively brakes the rotor of the motor/generator  80 . The first, second, third and fourth torque transfer devices  50 ,  52 ,  54  and  55  are employed to assist in the selection of the operational modes of the hybrid transmission  14 , as will be hereinafter more fully explained. 
     The output drive member  19  of the transmission  14  is secured to the carrier  36  of the planetary gear set  30 . 
     Returning now to the description of the power sources, it should be apparent from the foregoing description, and with particular reference to  FIG. 1  a, that the transmission  14  selectively receives power from the engine  12 . The hybrid transmission also receives power from an electric power source  86 , which is operably connected to a controller  88 . The electric power source  86  may be one or more batteries. Other electric power sources, such as fuel cells, that have the ability to provide, or store, and dispense electric power may be used in place of batteries without altering the concepts of the present invention. 
     General Operating Considerations 
     One of the primary control devices is a well known drive range selector (not shown) that directs an electronic control unit (the ECU  88 ) to con figure the transmission for either the park, reverse, neutral, or forward drive range. The second and third primary control devices constitute an accelerator pedal (not shown) and a brake pedal (also not shown). The information obtained by the ECU from these three primary control sources is designated as the “operator demand.” The ECU also obtains information from a plurality of sensors (input as well as output) as to the status of: the torque transfer devices (either applied or released); the engine output torque; the unified battery, or batteries, capacity level; and, the temperatures of selected vehicular components. The ECU determines what is required and then manipulates the selectively operated components of, or associated with, the transmission appropriately to respond to the operator demand. 
     The invention may use simple or compound planetary gear sets. In a simple planetary gear set a single set of planet gears are normally supported for rotation on a carrier that is itself rotatable. 
     In a simple planetary gear set, when the sun gear is held stationary and power is applied to the ring gear of a simple planetary gear set, the planet gears rotate in response to the power applied to the ring gear and thus “walk” circumferentially about the fixed sun gear to effect rotation of the carrier in the same direction as the direction in which the ring gear is being rotated. 
     When any two members of a simple planetary gear set rotate in the same direction and at the same speed, the third member is forced to turn at the same speed, and in the same direction. For example, when the sun gear and the ring gear rotate in the same direction, and at the same speed, the planet gears do not rotate about their own axes but rather act as wedges to lock the entire unit together to effect what is known as direct drive. That is, the carrier rotates with the sun and ring gears. 
     However, when the two gear members rotate in the same direction, but at different speeds, the direction in which the third gear member rotates may often be determined simply by visual analysis, but in many situations the direction will not be obvious and can only be accurately determined by knowing the number of teeth present on all the gear members of the planetary gear set. 
     Whenever the carrier is restrained from spinning freely, and power is applied to either the sun gear or the ring gear, the planet gear members act as idlers. In that way the driven member is rotated in the opposite direction as the drive member. Thus, in many transmission arrangements when the reverse drive range is selected, a torque transfer device serving as a brake is actuated frictionally to engage the carrier and thereby restrain it against rotation so that power applied to the sun gear will turn the ring gear in the opposite direction. Thus, if the ring gear is operatively connected to the drive wheels of a vehicle, such an arrangement is capable of reversing the rotational direction of the drive wheels, and thereby reversing the direction of the vehicle itself. 
     In a simple set of planetary gears, if any two rotational speeds of the sun gear, the planet carrier, and the ring gear are known, then the speed of the third member can be determined using a simple rule. The rotational speed of the carrier is always proportional to the speeds of the sun and the ring, weighted by their respective numbers of teeth. For example, a ring gear may have twice as many teeth as the sun gear in the same set. The speed of the carrier is then the sum of two-thirds the speed of the ring gear and one-third the speed of the sun gear. If one of these three members rotates in an opposite direction, the arithmetic sign is negative for the speed of that member in mathematical calculations. 
     The torque on the sun gear, the carrier, and the ring gear can also be simply related to one another if this is done without consideration of the masses of the gears, the acceleration of the gears, or friction within the gear set, all of which have a relatively minor influence in a well designed transmission. The torque applied to the sun gear of a simple planetary gear set must balance the torque applied to the ring gear, in proportion to the number of teeth on each of these gears. For example, the torque applied to a ring gear with twice as many teeth as the sun gear in that set must be twice that applied to the sun gear, and must be applied in the same direction. The torque applied to the carrier must be equal in magnitude and opposite in direction to the sum of the torque on the sun gear and the torque on the ring gear. 
     In a compound planetary gear set, the utilization of inner and outer sets of planet gears effects an exchange in the roles of the ring gear and the planet carrier in comparison to a simple planetary gear set. For instance, if the sun gear is held stationary, the planet carrier will rotate in the same direction as the ring gear, but the planet carrier with inner and outer sets of planet gears will travel faster than the ring gear, rather than slower. 
     In a compound planetary gear set having meshing inner and outer sets of planet gears the speed of the ring gear is proportional to the speeds of the sun gear and the planet carrier, weighted by the number of teeth on the sun gear and the number of teeth filled by the planet gears, respectively. For example, the difference between the ring and the sun filled by the planet gears might be as many teeth as are on the sun gear in the same set. In that situation the speed of the ring gear would be the sum of two-thirds the speed of the carrier and one third the speed of the sun. If the sun gear or the planet carrier rotates in an opposite direction, the arithmetic sign is negative for that speed in mathematical calculations. 
     If the sun gear were to be held stationary, then a carrier with inner and outer sets of planet gears will turn in the same direction as the rotating ring gear of that set. On the other hand, if the sun gear were to be held stationary and the carrier were to be driven, then planet gears in the inner set that engage the sun gear roll, or “walk,” along the sun gear, turning in the same direction that the carrier is rotating. Pinion gears in the outer set that mesh with pinion gears in the inner set will turn in the opposite direction, thus forcing a meshing ring gear in the opposite direction, but only with respect to the planet gears with which the ring gear is meshingly engaged. The planet gears in the outer set are being carried along in the direction of the carrier. The effect of the rotation of the pinion gears in the outer set on their own axis and the greater effect of the orbital motion of the planet gears in the outer set due to the motion of the carrier are combined, so the ring rotates in the same direction as the carrier, but not as fast as the carrier. 
     If the carrier in such a compound planetary gear set were to be held stationary and the sun gear were to be rotated, then the ring gear will rotate with less speed and in the same direction as the sun gear. If the ring gear of a simple planetary gear set is held stationary and the sun gear is rotated, then the carrier supporting a single set of planet gears will rotate with less speed and in the same direction as the sun gear. Thus, one can readily observe the exchange in roles between the carrier and the ring gear that is caused by the use of inner and outer sets of planet gears which mesh with one another, in comparison with the usage of a single set of planet gears in a simple planetary gear set. 
     The normal action of an electrically variable transmission is to transmit mechanical power from the input to the output. As part of this transmission action, one of its two motor/generators acts as a generator of electrical power. The other motor/generator acts as a motor and uses that electrical power. As the speed of the output increases from zero to a high speed, the two motor/generators  80 ,  82  gradually exchange roles as generator and motor, and may do so more than once. These exchanges take place around mechanical points, where essentially all of the power from input to output is transmitted mechanically and no substantial power is transmitted electrically. 
     In a hybrid electrically variable transmission system, the battery  86  may also supply power to the transmission or the transmission may supply power to the battery. If the battery is supplying substantial electric power to the transmission, such as for vehicle acceleration, then both motor/generators may act as motors. If the transmission is supplying electric power to the battery, such as for regenerative braking, both motor/generators may act as generators. Very near the mechanical points of operation, both motor/generators may also act as generators with small electrical power outputs, because of the electrical losses in the system. 
     Contrary to the normal action of the transmission, the transmission may actually be used to transmit mechanical power from the output to the input. This may be done in a vehicle to supplement the vehicle brakes and to enhance or to supplement regenerative braking of the vehicle, especially on long downward grades. If the power flow through the transmission is reversed in this way, the roles of the motor/generators will then be reversed from those in normal action. 
     Specific Operating Considerations 
     Each of the embodiments described herein has sixteen functional requirements (corresponding with the  16  rows of each operating mode table shown in the Figures) which may be grouped into five operating modes. These five operating modes are described below and may be best understood by referring to the respective operating mode table accompanying each transmission stick diagram, such as the operating mode tables of  FIG. 1   b ,  2   b ,  3   b , etc. 
     The first operating mode is the “battery reverse mode” which corresponds with the first row (Batt Rev) of each operating mode table, such as that of  FIG. 1   b . In this mode, the engine is off and the transmission element connected to the engine is not controlled by engine torque, though there may be some residual torque due to the rotational inertia of the engine. The EVT is driven by one of the motor/generators using energy from the battery, causing the vehicle to move in reverse. Depending on the kinematic configuration, the other/motor/generator may or may not rotate in this mode, and may or may not transmit torque. If it does rotate, it is used to generate energy which is stored in the battery. In the embodiment of  FIG. 1   b , in the battery reverse mode, the brake  54  is engaged, the motor/generator  80  has zero torque, the motor/generator  82  has a torque of −1.00 units. A torque ratio of −2.78 is achieved, by way of example. In each operating mode table an (M) next to a torque value in the motor/generator columns  80  and  82  indicates that the motor/generator is acting as a motor, and the absence of an (M) indicates that the motor/generator is acting as generator. An “X” in these columns illustrates that the respective motor is braked, such as by the brake  55 . 
     The second operating mode is the “EVT reverse mode” which corresponds with the second row (EVT Rev) of each operating mode table, such as that of  FIG. 1   b . In this mode, the EVT is driven by the engine and by one of the motor/generators. The other motor/generator operates in generator mode and transfers 100% of the generated energy back to the driving motor. The net effect is to drive the vehicle in reverse. Referring to  FIG. 1   b , for example, in the EVT reverse mode, the brake  54  is engaged, the generator  80  has a torque of −0.31 units, the motor  82  has a torque of −3.69 units, and an output torque of −8.33 is achieved, corresponding to an engine torque of 1 unit. 
     The third operating mode includes the “reverse and forward launch modes” (also referred to as “torque converter reverse and forward modes”) corresponding with the third and fourth rows (TC Rev and TC For) of each operating mode table, such as that of  FIG. 1   b . In this mode, the EVT is driven by the engine and one of the motor/generators. A selectable fraction of the energy generated in the generator unit is stored in the battery, with the remaining energy being transferred to the motor. In  FIG. 1 , this fraction is approximately 99%. The ratio of transmission output speed to engine speed (transmission speed ratio) is approximately ±0.001 (the positive sign indicates that the vehicle is creeping forward and negative sign indicates that the vehicle is creeping backwards). Referring to  FIG. 1   b , in the reverse and forward launch modes, the brake  54  is engaged, and the motor/generator  80  acts as a generator (with −0.31 units of torque), the motor/generator  82  acts as a motor (with −3.21 or 0.99 units of torque), and a torque ratio of −7.00 or 4.69 is achieved. 
     The fourth operating mode is a “continuously variable transmission range mode” which includes the Range 1.1, Range 1.2, Range 1.3, Range 1.4, Range 2.1, Range 2.2, Range 2.3 and Range 2.4 operating points corresponding with rows 5–12 of each operating point table, such as that of  FIG. 1   b . In this mode, the EVT is driven by the engine as well as one of the motor/generators operating as a motor. The other motor/generator operates as a generator and transfers 100% of the generated energy back to the motor. The operating points represented by Range 1.1, 1.2 . . . , etc. are discrete points in the continuum of forward speed ratios provided by the EVT. For example in  FIG. 1   b , a range of torque ratios from 4.69 to 1.86 is achieved with the brake  54  engaged, and a range of ratios 1.36 to 0.54 is achieved with the clutch  52  engaged. 
     The fifth operating mode includes the “fixed ratio” modes (F 1 , F 2 , F 3  and F 4 ) corresponding with rows  13 – 16  of each operating mode table (i.e. operating mode table), such as that of  FIG. 1   b . In this mode the transmission operates like a conventional automatic transmission, with two torque transfer devices engaged to create a discrete transmission ratio. The clutching table accompanying each figure shows only 4 fixed-ratio forward speeds but additional fixed ratios may be available. Referring to  FIG. 1   b , in fixed ratio F 1  the clutch  50  and brake  54  are engaged to achieve a fixed torque ratio of 2.78. In fixed ratio F 2 , the brakes  54  and  55  are engaged to achieve a fixed ratio of 1.94. Accordingly, each “X” in the column of motor/generator  80  in  FIG. 1   b  indicates that the brake  55  is engaged and the motor/generator  80  is not rotating. In fixed ratio F 3 , the clutches  50  and  52  are engaged to achieve a fixed ratio of 1.00. In fixed ratio F 4 , the clutch  52  and brake  55  are engaged to achieve a fixed ratio of 0.70. 
     The transmission  14  is capable of operating in so-called single or dual modes. In single mode, the engaged torque transfer device remains the same for the entire continuum of forward speed ratios (represented by the discrete points: Ranges 1.1, 1.2, 1.3 and 1.4). In dual mode, the engaged torque transfer device is switched at some intermediate speed ratio (e.g., Range 2.1 in  FIG. 1 ). Depending on the mechanical configuration, this change in torque transfer device engagement has advantages in reducing element speeds in the transmission. 
     In some designs, it is possible to synchronize clutch element slip speeds such that shifts are achievable with minimal torque disturbance (so-called “cold” shifts). For example, the transmissions of  FIGS. 3   a ,  4   a ,  5   a  and  7   a  have cold shifts between ranges 1.4 and 2.1. This also serves as an enabler for superior control during double transition shifts (two oncoming clutches and two off-going clutches). 
     As set forth above, the engagement schedule for the torque transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 1   b .  FIG. 1   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 1   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  20  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  30 . Also, the chart of  FIG. 1   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the step ratio between first and second fixed forward torque ratios is 1.43, the step ratio between the second and third fixed forward torque ratios is 1.94, the step ratio between the second and third fixed forward torque ratios is 1.43, and the ratio spread is 3.97. 
     Description of a Second Exemplary Embodiment 
     With reference to  FIG. 2   a , a powertrain  110  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission, designated generally by the numeral  114 . Transmission  114  is designed to receive at least a portion of its driving power from the engine  12 . 
     In the embodiment depicted the engine  12  may also be a fossil fuel engine, such as a diesel engine which is readily adapted to provide its available power output typically delivered at a constant number of revolutions per minute (RPM). As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  14 . A transient torque damper (not shown) may also be implemented between the engine  12  and the input member  17  of the transmission. 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member  17  is operatively connected to a planetary gear set in the transmission  114 . An output member  19  of the transmission  114  is connected to a final drive  16 . 
     The transmission  114  utilizes two differential gear sets, preferably in the nature of planetary gear sets  120  and  130 . The planetary gear set  120  employs an outer gear member  124 , typically designated as the ring gear. The ring gear  124  circumscribes an inner gear member  122 , typically designated as the sun gear. A carrier  126  rotatably supports a plurality of planet gears  127  such that each planet gear  127  meshingly engages both the outer, ring gear  124  and the inner, sun gear member  122  of the first planetary gear set  120 . 
     The planetary gear set  130  also has an outer gear member  134 , often also designated as the ring gear, that circumscribes an inner gear member  132 , also often designated as the sun gear. A plurality of planet gears  137  are also rotatably mounted in a carrier  136  such that each planet gear member  137  simultaneously, and meshingly, engages both the outer, ring gear  134  and the inner, sun gear member  132  of the planetary gear set  130 . 
     The transmission input member  17  is connected with the carrier  126  of the planetary gear set  120 , and the transmission output member  19  is connected with the carrier  136  of the planetary gear set  130 . An interconnecting member  170  continuously connects the ring gear  124  of the planetary gear set  120  with the sun gear  132  of the planetary gear set  130 . 
     The transmission  114  also incorporates first and second motor/generators  180  and  182 , respectively. The stator of the first motor/generator  180  is secured to the transmission housing  160 . The rotor of the first motor/generator  180  is secured to the sun gear  122  of the planetary gear set  120 . 
     The stator of the second motor/generator  182  is also secured to the transmission housing  160 . The rotor of the second motor/generator  182  is secured to the ring gear member  124 . 
     A first torque transfer device, such as a clutch  150 , selectively connects the carrier  126  of the planetary gear set  120  to the sun gear  122  of the planetary gear set  120 . A second torque transfer device, such as clutch  152 , selectively connects the carrier  126  of the planetary gear set  120  with the ring gear  134  of the planetary gear set  130 . A third torque transfer device, such as brake  154 , selectively connects the ring gear  134  of the planetary gear set  130  with the transmission housing  160 . That is, the ring gear  134  is selectively secured against rotation by an operative connection to the non-rotatable housing  160 . A fourth torque transfer device, such as the brake  155 , is connected in parallel with the motor/generator  180  for selectively braking rotation of the motor/generator  180 . The first, second, third and fourth torque transfer devices  150 ,  152 ,  154  and  155  are employed to assist in the selection of the operational modes of the hybrid transmission  114 . 
     Returning now to the description of the power sources, it should be apparent from the foregoing description, and with particular reference to  FIG. 2   a , that the transmission  114  selectively receives power from the engine  12 . The hybrid transmission also exchanges power with an electric power source  186 , which is operably connected to a controller  188 . The electric power source  186  may be one or more batteries. Other electric power sources, such as fuel cells, that have the ability to provide, or store, and dispense electric power may be used in place of batteries without altering the concepts of the present invention. 
     As described previously, each embodiment has sixteen functional requirements (corresponding with the  16  rows of each operating mode table shown in the Figures) which may be grouped into five operating modes. The first operating mode is the “battery reverse mode” which corresponds with the first row (Batt Rev) of the operating mode table of  FIG. 2   b . In this mode, the engine is off and the transmission element connected to the engine is effectively allowed to freewheel, subject to engine inertia torque. The EVT is driven by one of the motor/generators using energy from the battery, causing the vehicle to move in reverse. The other motor/generator may or may not rotate in this mode. As shown in  FIG. 2   b , in this mode the brake  154  is engaged, the motor/generator  180  has zero torque, the motor  182  has a torque of −1.00 unit and an output torque of −2.78 is achieved, by way of example. 
     The second operating mode is the “EVT reverse mode” which corresponds with the second row (EVT Rev) of the operating mode table of  FIG. 2   b . In this mode, the EVT is driven by the engine and by one of the motor/generators. The other motor/generator operates in generator mode and transfers 100% of the generated energy back to the driving motor. The net effect is to drive the vehicle in reverse. In this mode, the brake  154  is engaged, the generator  180  has a torque of −0.36 units, the motor  182  has a torque of −3.63 units, and an output torque of −8.33 is achieved, corresponding to an input torque of 1 unit. 
     The third operating mode includes the “reverse and forward launch modes” corresponding with the third and fourth rows (TC Rev and TC For) of each operating mode table, such as that of  FIG. 2   b . In this mode, the EVT is driven by the engine and one of the motor/generators. A selectable fraction of the energy generated in the generator unit is stored in the battery, with the remaining energy being transferred to the motor. In this mode, the brake  154  is engaged, and the motor/generator  180  acts as a generator (with −0.36 units of torque in reverse and forward), the motor/generator  182  acts as a motor (with −3.16 or 1.04 units of torque), and a torque ratio of −7.00 or 4.69 is achieved. For these torque ratios, approximately 99% of the generator energy is stored in the battery. 
     The fourth operating mode includes the “Range 1.1, Range 1.2, Range 1.3, Range 1.4, Range 2.1, Range 2.2, Range 2.3 and Range 2.4” modes corresponding with rows 5–12 of the operating mode table of  FIG. 2   b . In this mode, the EVT is driven by the engine as well as one of the motor/generators operating as a motor. The other motor/generator operates as a generator and transfers 100% of the generated energy back to the motor. The operating points represented by Range 1.1, 1.2 . . . , etc. are discrete points in the continuum of forward speed ratios provided by the EVT. For example in  FIG. 2   b , a range of ratios from 4.69 to 1.86 is achieved with the brake  154  engaged, and a range of ratios from 1.36 to 0.54 is achieved with the clutch  152  engaged. 
     The fifth operating mode includes the fixed “ratio” modes (F 1 , F 2 , F 3  and F 4 ) corresponding with rows 13–16 of the operating mode table of  FIG. 2   b . In this mode the transmission operates like a conventional automatic transmission, with two torque transfer devices engaged to create a discrete transmission ratio. In fixed ratio F 1  the clutch  150  and brake  154  are engaged to achieve a fixed ratio of 2.78. In fixed ratio F 2 , the brakes  154  and  155  are engaged to achieve a fixed ratio of 1.78. In fixed ratio F 3 , the clutches  150  and  152  are engaged to achieve a fixed ratio of 1.00. In fixed ratio F 4 , the clutch  152  is engaged and the motor/generator  180  is braked by brake  155  to achieve a fixed ratio of 0.83. 
     As set forth above, the engagement schedule for the torque transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 2   b .  FIG. 2   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 2   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  120  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  130 . Also, the chart of  FIG. 2   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the step ratio between first and second fixed forward torque ratios is 1.56, the step ratio between the second and third fixed forward torque ratios is 1.78, and the step ratio between the third and fourth fixed forward torque ratios is 1.20. 
     Description of a Third Exemplary Embodiment 
     With reference to  FIG. 3   a , a powertrain  210  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission, designated generally by the numeral  214 . The transmission  214  is designed to receive at least a portion of its driving power from the engine  12 . As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  214 . A transient torque damper (not shown) may also be implemented between the engine  12  and the input member  17  of the transmission  214 . 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member is operatively connected to a planetary gear set in the transmission  214 . An output member  19  of the transmission  214  is connected to a final drive  16 . 
     The transmission  214  utilizes two differential gear sets, preferably in the nature of planetary gear sets  220  and  230 . The planetary gear set  220  employs an outer gear member  224 , typically designated as the ring gear. The ring gear  224  circumscribes an inner gear member  222 , typically designated as the sun gear. A carrier  226  rotatably supports a plurality of planet gears  227  such that each planet gear  227  meshingly engages both the outer, ring gear member  224  and the inner, sun gear member  222  of the first planetary gear set  220 . 
     The planetary gear set  230  also has an outer ring gear member  234  that circumscribes an inner sun gear member  232 . A plurality of planet gears  237  are also rotatably mounted in a carrier  236  such that each planet gear  237  simultaneously, and meshingly, engages both the outer ring gear member  234  and the inner sun gear member  232  of the planetary gear set  230 . 
     The transmission input member  17  is connected with the ring gear  224 , and the transmission output member  19  is connected to the carrier  226 . An interconnecting member  270  continuously connects the carrier  226  of the planetary gear set  220  with the carrier  236  of the planetary gear set  230 . 
     The transmission  214  also incorporates first and second motor/generators  280  and  282 , respectively. The stator of the first motor/generator  280  is secured to the transmission housing  260 . The rotor of the first motor/generator  280  is secured to the sun gear  222  of the planetary gear set  220 . 
     The stator of the second motor/generator  282  is also secured to the transmission housing  260 . The rotor of the second motor/generator  282  is secured to the sun gear  232 . 
     A first torque-transfer device, such as clutch  250 , selectively connects the ring gear  224  of the planetary gear set  220  with the sun gear  232  of the planetary gear set  230 . A second torque-transfer device, such as clutch  252 , selectively connects the sun gear  222  of the planetary gear set  220  with the ring gear  234  of the planetary gear set  230 . A third torque-transfer device, such as a brake  254 , selectively connects the ring gear  234  of the planetary gear set  230  with the transmission housing  260 . A fourth torque transfer device, such as the brake  255 , is connected in parallel with the motor/generator  282  for selectively braking rotation of the motor/generator  282 . The first, second, third and fourth torque-transfer devices  250 ,  252 ,  254  and  255  are employed to assist in the selection of the operational modes of the hybrid transmission  214 . 
     The hybrid transmission  214  receives power from the engine  12 , and also from electric power source  286 , which is operably connected to a controller  288 . 
     The operating mode table of  FIG. 3   b  illustrates the clutching engagements, motor/generator conditions and output/input ratios for the five operating modes of the transmission  214 . These modes include the “battery reverse mode” (Batt Rev), “EVT reverse mode” (EVT Rev), “reverse and forward launch modes” (TC Rev and TC For), “range 1.1, 1.2, 1.3 . . . modes” and “fixed ratio modes” (F 1 , F 2 , F 3 , F 4 ) as described previously. 
     As set forth above the engagement schedule for the torque-transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 3   b .  FIG. 3   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 3   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  220  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  230 . Also, the chart of  FIG. 3   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the step ratio between the first and second fixed forward torque ratios is 2.42, the step ratio between the second and third fixed forward torque ratios 1.51, and the step ratio between the third and fourth fixed forward torque ratios is 1.23. Each of the single step forward shifts between fixed ratios is a single transition shift. 
     Description of a Fourth Exemplary Embodiment 
     With reference to  FIG. 4   a , a powertrain  310  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission, designated generally by the numeral  314 . The transmission  314  is designed to receive at least a portion of its driving power from the engine  12 . 
     As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  314 . A transient torque damper (not shown) may also be implemented between the engine  12  and the input member  17  of the transmission. 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member  17  is operatively connected to a planetary gear set in the transmission  314 . An output member  19  of the transmission  314  is connected to a final drive  16 . 
     The transmission  314  utilizes two planetary gear sets  320  and  330 . The planetary gear set  320  employs an outer ring gear member  324  which circumscribes an inner sun gear member  322 . A carrier  326  rotatably supports a plurality of planet gears  327  such that each planet gear  327  meshingly engages both the outer ring gear member  324  and the inner sun gear member  322  of the first planetary gear set  320 . 
     The planetary gear set  330  also has an outer ring gear member  334  that circumscribes an inner sun gear member  332 . A plurality of planet gears  337  are also rotatably mounted in a carrier  336  such that each planet gear member  337  simultaneously, and meshingly engages both the outer, ring gear member  334  and the inner, sun gear member  332  of the planetary gear set  330 . 
     The transmission input member  17  is connected with the carrier  326  of the planetary gear set  320 , and the transmission output member  19  is connected with the carrier  336  of the planetary gear set  330 . An interconnecting member  370  continuously connects the sun gear  322  of the planetary gear set  320  with the sun gear  332  of the planetary gear set  330 . 
     The transmission  314  also incorporates first and second motor/generators  380  and  382 , respectively. The stator of the first motor/generator  380  is secured to the transmission housing  360 . The rotor of the first motor/generator  380  is secured to the ring gear  324  of the planetary gear set  320 . The stator of the second motor/generator  382  is also secured to the transmission housing  360 . The rotor of the second motor/generator  382  is secured to the sun gear  332  of the planetary gear set  330 . 
     A first torque-transfer device, such as the clutch  350 , selectively connects the carrier  326  with the ring gear  324 . A second torque-transfer device, such as the clutch  352 , selectively connects the carrier  336  with the ring gear  324 . A third torque-transfer device, such as brake  354 , selectively connects the ring gear  334  with the transmission housing  360 . A fourth torque transfer device, such as the brake  355 , is connected in parallel with the motor/generator  380  for selectively braking rotation of the motor/generator  380 . The first, second, third and fourth torque-transfer devices  350 ,  352 ,  354  and  355  are employed to assist in the selection of the operational modes of the transmission  314 . 
     The hybrid transmission  314  receives power from the engine  12 , and also exchanges power with an electric power source  386 , which is operably connected to a controller  388 . 
     The operating mode table of  FIG. 4   b  illustrates the clutching engagements, motor/generator conditions and output/input ratios for the five operating modes of the transmission  314 . These modes include the “battery reverse mode” (Batt Rev), the “EVT reverse mode” (EVT Rev), “reverse and forward launch modes” (TC Rev and TC For), “continuously variable transmission range modes” (Range 1.1, 1.2, 1.3 . . . ) and “fixed ratio modes” (F 1 , F 2 , F 3 , F 4 ) as described previously. 
     As set forth above, the engagement schedule for the torque-transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 4   b .  FIG. 4   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 4   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  320  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  330 . Also, the chart of  FIG. 4   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the step ratio between first and second fixed forward torque ratios is 1.82, the step ratio between the second and third fixed forward torque ratios is 2.20, and the step ratio between the third and fourth fixed forward torque ratios is 1.67. Each of the single step forward shifts between fixed ratios is a single transition shift. 
     Description of a Fifth Exemplary Embodiment 
     With reference to  FIG. 5   a , a powertrain  410  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission, designated generally by the numeral  414 . The transmission  414  is designed to receive at least a portion of its driving power from the engine  12 . 
     As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  414 . A transient torque damper (not shown) may also be implemented between the engine  12  and the input member  17  of the transmission. 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member  17  is operatively connected to a planetary gear set in the transmission  414 . An output member  19  of the transmission  414  is connected to a final drive  16 . 
     The transmission  414  utilizes two planetary gear sets  420  and  430 . The planetary gear set  420  employs an outer ring gear member  424  which circumscribes an inner sun gear member  422 . A carrier  426  rotatably supports a plurality of planet gears  427  such that each planet gear  427  meshingly engages both the outer ring gear member  424  and the inner sun gear member  422  of the first planetary gear set  420 . 
     The planetary gear set  430  also has an outer ring gear member  434  that circumscribes an inner sun gear member  432 . A plurality of planet gears  437  are also rotatably mounted in a carrier  436  such that each planet gear member  437  simultaneously, and meshingly engages both the outer, ring gear member  434  and the inner, sun gear member  432  of the planetary gear set  430 . 
     The transmission input member  17  is continuously connected with the carrier  426 , and the transmission output member  19  is continuously connected with the carrier  436 . An interconnecting member  470  continuously connects the sun gear  422  with the sun gear  432 . 
     The transmission  414  also incorporates first and second motor/generators  480  and  482 , respectively. The stator of the first motor/generator  480  is secured to the transmission housing  460 . The rotor of the first motor/generator  480  is secured to the ring gear  424 . 
     The stator of the second motor/generator  482  is also secured to the transmission housing  460 . The rotor of the second motor/generator  482  is secured to the sun gear  432 . 
     A first torque-transfer device, such as a clutch  450 , selectively connects the ring gear  424  with the carrier  426 . A second torque-transfer device, such as clutch  452 , selectively connects the ring gear  424  with the ring gear  434 . A third torque-transfer device, such as brake  454 , selectively connects the ring gear  434  with the transmission housing  460 . A fourth torque transfer device, such as the brake  455 , is connected in parallel with the motor/generator  480  for selectively braking rotation of the motor/generator  480 . The first, second, third and fourth torque-transfer devices  450 ,  452 ,  454  and  455  are employed to assist in the selection of the operational modes of the transmission  414 . The hybrid transmission  414  receives power from the engine  12  and also from an electric power source  486 , which is operably connected to a controller  488 . 
     The operating mode table of  FIG. 5   b  illustrates the clutching engagements, motor/generator conditions and output/input ratios for the five operating modes of the transmission  414 . These modes include the “battery reverse mode” (Batt Rev), the “EVT reverse mode” (EVT Rev), “reverse and forward launch modes” (TC Rev and TC For), “continuously variable transmission range modes” (Range 1.1, 1.2, 1.3 . . . ) and “fixed ratio modes” (F 1 , F 2 , F 3 , F 4 ) as described previously. 
     The transmission  414  is a single mode transmission providing ratios between 4.69 and 0.54. 
     As set forth above, the engagement schedule for the torque-transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 5   b .  FIG. 5   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 5   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  420  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  430 . Also, the chart of  FIG. 5   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the step ratio between first and second fixed forward torque ratios is 3.01, the step ratio between the second and third fixed forward torque ratios is 1.33, and the step ratio between the third and fourth fixed forward torque ratios is 1.49. Each of the single step forward shifts between fixed ratios is a single transition shift. 
     Description of a Sixth Exemplary Embodiment 
     With reference to  FIG. 6   a , a powertrain  510  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission, designated generally by the numeral  514 . The transmission  514  is designed to receive at least a portion of its driving power from the engine  12 . 
     As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  514 . A transient torque damper (not shown) may also be implemented between the engine  12  and the input member  17  of the transmission. 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member  17  is operatively connected to a planetary gear set in the transmission  514 . An output member  19  of the transmission  514  is connected to a final drive  16 . 
     The transmission  514  utilizes two planetary gear sets  520  and  530 . The planetary gear set  520  employs an outer ring gear member  524  which circumscribes an inner sun gear member  522 . A carrier  526  rotatably supports a plurality of planet gears  527  such that each planet gear  527  meshingly engages both the outer ring gear member  524  and the inner sun gear member  522  of the first planetary gear set  520 . 
     The planetary gear set  530  also has an outer ring gear member  534  that circumscribes an inner sun gear member  532 . A plurality of planet gears  537  are also rotatably mounted in a carrier  536  such that each planet gear member  537  simultaneously, and meshingly engages both the outer, ring gear member  534  and the inner, sun gear member  532  of the planetary gear set  530 . 
     The transmission input member  17  is continuously connected with the carrier  526 , and the transmission output member  19  is continuously connected with the carrier  536 . An interconnecting member  570  continuously connects the sun gear with the sun gear  532 . 
     The transmission  514  also incorporates first and second motor/generators  580  and  582 , respectively. The stator of the first motor/generator  580  is secured to the transmission housing  560 . The rotor of the first motor/generator  580  is secured to the sun gear  522 . 
     The stator of the second motor/generator  582  is also secured to the transmission housing  560 . The rotor of the second motor/generator  582  is secured to the ring gear  524 . 
     A first torque-transfer device, such as a clutch  550 , selectively connects the ring gear  524  with the carrier  526 . A second torque-transfer device, such as a clutch  552 , selectively connects the carrier  526  with the ring gear  534 . A third torque-transfer device, such as a brake  554 , selectively connects the ring gear  534  with the transmission housing  560 . A fourth torque transfer device, such as the brake  555 , is connected in parallel with the motor/generator  580  for selectively braking rotation of the motor/generator  580 . The first, second, third and fourth torque-transfer devices  550 ,  552 ,  554  and  555  are employed to assist in the selection of the operational modes of the hybrid transmission  514 . 
     The hybrid transmission  514  receives power from the engine  12 , and also exchanges power with an electric power source  586 , which is operably connected to a controller  588 . 
     The operating mode table of  FIG. 6   b  illustrates the clutching engagements, motor/generator conditions and output/input ratios for the five operating modes of the transmission  514 . These modes include the “battery reverse mode” (Batt Rev), the “EVT reverse mode” (EVT Rev), “reverse and forward launch modes” (TC Rev and TC For), “continuously variable transmission range modes” (Range 1.1, 1.2, 1.3 . . . ) and “fixed ratio modes” (F 1 , F 2 , F 3 , F 4 ) as described previously. 
     As set forth above, the engagement schedule for the torque-transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 6   b .  FIG. 6   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 6   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  520  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  530 . Also, the chart of  FIG. 4   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the step ratio between first and second fixed forward torque ratios is 1.43; the step ratio between the second and third fixed forward torque ratios is 1.94, and the step ratio between the third and fourth fixed forward torque ratios is 1.43. 
     Description of a Seventh Exemplary Embodiment 
     With reference to  FIG. 7   a , a powertrain  610  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission, designated generally by the numeral  614 . The transmission  614  is designed to receive at least a portion of its driving power from the engine  12 . 
     As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  614 . A transient torque damper (not shown) may also be implemented between the engine  12  and the input member  17  of the transmission. 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member  17  is operatively connected to a planetary gear set in the transmission  614 . An output member  19  of the transmission  614  is connected to a final drive  16 . 
     The transmission  614  utilizes two planetary gear sets  620  and  630 . The planetary gear set  620  employs an outer ring gear member  624  which circumscribes an inner sun gear member  622 . A carrier  626  rotatably supports a plurality of planet gears  627  such that each planet gear  627  meshingly engages both the outer ring gear member  624  and the inner sun gear member  622  of the first planetary gear set  620 . 
     The planetary gear set  630  also has an outer ring gear member  634  that circumscribes an inner sun gear member  632 . A plurality of planet gears  637  are also rotatably mounted in a carrier  636  such that each planet gear member  637  simultaneously, and meshingly engages both the outer, ring gear member  634  and the inner, sun gear member  632  of the planetary gear set  630 . 
     The transmission input member  17  is continuously connected with the ring gear  624 , and the transmission output member  19  is continuously connected with the carrier  636 . An interconnecting member  670  continuously connects the sun gear  622  with the ring gear  634 . 
     The transmission  614  also incorporates first and second motor/generators  680  and  682 , respectively. The stator of the first motor/generator  680  is secured to the transmission housing  660 . The rotor of the first motor/generator  680  is secured to the sun gear  632 . 
     The stator of the second motor/generator  682  is also secured with the transmission housing  660 . The rotor of the second motor/generator  682  is secured to the carrier  626 . 
     A first torque-transfer device, such as a clutch  650 , selectively connects the ring gear  624  with the sun gear  632 . A second torque-transfer device, such as a clutch  652 , selectively connects the carrier  626  with the carrier  636 . A third torque-transfer device, such as a brake  654 , selectively connects the ring gear  634  with the transmission housing  660 . A fourth torque transfer device, such as the brake  655 , is connected in parallel with the motor/generator  680  for selectively braking rotation of the motor/generator  680 . The first, second, third and fourth torque-transfer devices  650 ,  652 ,  654  and  655  are employed to assist in the selection of the operational modes of the hybrid transmission  614 . 
     The hybrid transmission  614  receives power from the engine  12 , and also exchanges power with an electric power source  686 , which is operably connected to a controller  688 . 
     The operating mode table of  FIG. 7   b  illustrates the clutching engagements, motor/generator conditions and output/input ratios for the five operating modes of the transmission  614 . These modes include the “battery reverse mode” (Batt Rev), the “EVT reverse mode” (EVT Rev), “reverse and forward launch modes” (TC Rev and TC For), “continuously variable transmission range modes” (Range 1.1, 1.2, 1.3 . . . ) and “fixed ratio modes” (F 1 , F 2 , F 3 , F 4 ) as described previously. 
     As set forth above, the engagement schedule for the torque-transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 7   b .  FIG. 7   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 7   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  620  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  630 . Also, the chart of  FIG. 7   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the step ratio between first and second fixed forward torque ratios is 3.31, the step ratio between the second and third fixed forward torque ratios is 1.21, and the step ratio between the third and fourth fixed forward torque ratios is 1.43. Each of the single step forward shifts between fixed ratios is a single transition shift. 
     Description of an Eighth Exemplary Embodiment 
     With reference to  FIG. 8   a , a powertrain  710  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission, designated generally by the numeral  714 . The transmission  714  is designed to receive at least a portion of its driving power from the engine  12 . 
     As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  714 . A transient torque damper (not shown) may also be appointed between the engine  12  and the input member  17  of the transmission. 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member  17  is operatively connected to a planetary gear set in the transmission  714 . An output member  19  of the transmission  714  is connected to a final drive  16 . 
     The transmission  714  utilizes two planetary gear sets  720  and  730 . The planetary gear set  720  employs an outer ring gear member  724  which circumscribes an inner sun gear member  722 . A carrier  726  rotatably supports a plurality of planet gears  727  such that each planet gear  727  meshingly engages both the outer ring gear member  724  and the inner sun gear member  722  of the first planetary gear set  720 . 
     The planetary gear set  730  also has an outer ring gear member  734  that circumscribes an inner sun gear member  732 . A plurality of planet gears  737  are also rotatably mounted in a carrier  736  such that each planet gear member  737  simultaneously, and meshingly engages both the outer, ring gear member  734  and the inner, sun gear member  732  of the planetary gear set  730 . 
     The transmission input member  17  is continuously connected with the carrier  726 , and the transmission output member  19  is continuously connected with the ring gear  724 . An interconnecting member  770  continuously connects the ring gear  724  with the carrier  736 . 
     The transmission  714  also incorporates first and second motor/generators  780  and  782 , respectively. The stator of the first motor/generator  780  is secured to the transmission housing  760 . The rotor of the first motor/generator  780  is secured to the sun gear  722 . 
     The stator of the second motor/generator  782  is also secured to the transmission housing  760 . The rotor of the second motor/generator  782  is secured to the sun gear  732 . 
     A first torque-transfer device, such as a clutch  750 , selectively connects the sun gear  722  with the sun gear  732 . A second torque-transfer device, such as a clutch  752 , selectively connects the carrier  726  with the sun gear  732 . A third torque-transfer device, such as the brake  754 , selectively connects the ring gear  734  with the transmission housing  760 . A fourth torque transfer device, such as the brake  755 , is connected in parallel with the motor/generator  780  for selectively braking rotation of the motor/generator  780 . The first, second, third and fourth torque-transfer devices  750 ,  752 ,  754  and  755  are employed to assist in the selection of the operational modes of the hybrid transmission  714 . 
     The hybrid transmission  714  receives power from the engine  12 , and also exchanges power with an electric power source  786 , which is operably connected to a controller  788 . 
     The operating mode table of  FIG. 8   b  illustrates the clutching engagements, motor/generator conditions and output/input ratios for the five operating modes of the transmission  714 . These modes include the “battery reverse mode” (Batt Rev), the “EVT reverse mode” (EVT Rev), “reverse and forward launch modes” (TC Rev and TC For), “continuously variable transmission range modes” (Range 1.1, 1.2, 1.3 . . . ) and “fixed ratio modes” (F 1 , F 2 , F 3 , F 4 ) as described previously. 
     As set forth above, the engagement schedule for the torque-transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 8   b .  FIG. 8   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 8   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  720  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  730 . Also, the chart of  FIG. 8   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the step ratio between first and second fixed forward torque ratios is 2.27, the step ratio between the second and third fixed forward torque ratios is 1.76, and the step ratio between the third and fourth fixed forward torque ratios is 1.33. 
     Description of a Ninth Exemplary Embodiment 
     With reference to  FIG. 9   a , a powertrain  810  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission, designated generally by the numeral  814 . The transmission  814  is designed to receive at least a portion of its driving power from the engine  12 . 
     As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  814 . A transient torque damper (not shown) may also be implemented between the engine  12  and the input member  17  of the transmission. 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member  17  is operatively connected to a planetary gear set in the transmission  814 . An output member  19  of the transmission  814  is connected to a final drive  16 . 
     The transmission  814  utilizes two planetary gear sets  820  and  830 . The planetary gear set  820  employs an outer ring gear member  824  which circumscribes an inner sun gear member  822 . A carrier  826  rotatably supports a plurality of planet gears  827  such that each planet gear  827  meshingly engages both the outer ring gear member  824  and the inner sun gear member  822  of the first planetary gear set  820 . 
     The planetary gear set  830  also has an outer ring gear member  834  that circumscribes an inner sun gear member  832 . A plurality of planet gears  837  are also rotatably mounted in a carrier  836  such that each planet gear member  837  simultaneously, and meshingly engages both the outer, ring gear member  834  and the inner, sun gear member  832  of the planetary gear set  830 . 
     The transmission input member  17  is continuously connected with the carrier  826 , and the transmission output member  19  is continuously connected with the ring gear  824 . An interconnecting member  870  continuously connects the ring gear  824  with the carrier  836 . 
     The transmission  814  also incorporates first and second motor/generators  880  and  882 , respectively. The stator of the first motor/generator  880  is secured to the transmission housing  860 . The rotor of the first motor/generator  880  is secured to the ring gear  834 . 
     The stator of the second motor/generator  882  is also secured to the transmission housing  860 . The rotor of the second motor/generator  882  is secured to the sun gear  832 . 
     A first torque-transfer device, such as a clutch  850 , selectively connects the sun gear  822  with the sun gear  832 . A second torque-transfer device, such as clutch  852 , selectively connects the carrier  826  with the sun gear  832 . A third torque-transfer device, such as brake  854 , selectively connects the sun gear  822  with the transmission housing  860 . A fourth torque transfer device, such as the brake  855 , is connected in parallel with the motor/generator  880  for selectively braking rotation of the motor/generator  880 . A fifth torque transfer device, such as the brake  857 , is connected in parallel with the motor/generator  882  for selectively braking rotation of the motor/generator  882 . The first, second, third, fourth and fifth torque-transfer devices  850 ,  852 ,  854 ,  855  and  857  are employed to assist in the selection of the operational modes of the hybrid transmission  814 . 
     The hybrid transmission  814  receives power from the engine  12 , and exchanges power with an electric power source  886 , which is operably connected to a controller  888 . 
     The operating mode table of  FIG. 9   b  illustrates the clutching engagements, motor/generator conditions and output/input ratios for the five operating modes of the transmission  814 . These modes include the “battery reverse mode” (Batt Rev), the “EVT reverse mode” (EVT Rev), “reverse and forward launch modes” (TC Rev and TC For), “continuously variable transmission range modes” (Range 1.1, 1.2, 1.3 . . . ) and “fixed ratio modes” (F 1 , F 2 , F 3 , F 4 ) as described previously. 
     As set forth above, the engagement schedule for the torque-transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 9   b .  FIG. 9   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 9   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  820  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  830 . Also, the chart of  FIG. 9   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the step ratio between first and second fixed forward torque ratios is 2.27, the step ratio between the second and third fixed forward torque ratios is 1.76, and the step ratio between the third and fourth fixed forward torque ratios is 1.33. Each of the single step forward shifts between fixed ratios is a single transition shift. 
     Description of a Tenth Exemplary Embodiment 
     With reference to  FIG. 10   a , a powertrain  910  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission, designated generally by the numeral  914 . The transmission  914  is designed to receive at least a portion of its driving power from the engine  12 . 
     As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  914 . A transient torque damper (not shown) may also be implemented between the engine  12  and the input member  17  of the transmission. 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member  17  is operatively connected to a planetary gear set in the transmission  914 . An output member  19  of the transmission  914  is connected to a final drive  16 . 
     The transmission  914  utilizes two planetary gear sets  920  and  930 . The planetary gear set  920  employs an outer ring gear member  924  which circumscribes an inner sun gear member  922 . A carrier  926  rotatably supports a plurality of planet gears  927  such that each planet gear  927  meshingly engages both the outer ring gear member  924  and the inner sun gear member  922  of the first planetary gear set  920 . 
     The planetary gear set  930  also has an outer ring gear member  934  that circumscribes an inner sun gear member  932 . A plurality of planet gears  937  are also rotatably mounted in a carrier  936  such that each planet gear member  937  simultaneously, and meshingly engages both the outer, ring gear member  934  and the inner, sun gear member  932  of the planetary gear set  930 . 
     The transmission input member  17  is continuously connected with the carrier  926 . The transmission output member  19  is continuously connected with the carrier  936 . An interconnecting member  970  continuously connects the sun gear  922  with the sun gear  932 . 
     A first torque-transfer device, such as a clutch  950 , selectively connects the sun gear  922  with the carrier  926 . A second torque-transfer device, such as a clutch  952 , selectively connects the ring gear  924  with the carrier  936 . A third torque-transfer device, such as brake  954 , selectively connects the carrier  936  with the transmission housing  960 . A fourth torque transfer device, such as the brake  955 , is connected in parallel with the motor/generator  980  for selectively braking rotation of the motor/generator  980 . A fifth torque transfer device, such as the brake  957 , is connected in parallel with the motor/generator  982  for selectively braking rotation of the motor/generator  982 . The first, second, third, fourth and fifth torque-transfer devices  950 ,  952 ,  954 ,  955  and  957  are employed to assist in the selection of the operational modes of the hybrid transmission  914 . 
     The hybrid transmission  914  receives power from the engine  12 , and also exchanges power with an electric power source  986 , which is operably connected to a controller  988 . 
     The operating mode table of  FIG. 10   b  illustrates the clutching engagements, motor/generator conditions and output/input ratios for the five operating modes of the transmission  914 . These modes include the “battery reverse mode” (Batt Rev), the “EVT reverse mode” (EVT Rev), “reverse and forward launch modes” (TC Rev and TC For), “continuously variable transmission range modes” (Range 1.1, 1.2, 1.3 . . . ) and “fixed ratio modes” (F 1 , F 2 , F 3 , F 4 ) as described previously. 
     As set forth above, the engagement schedule for the torque-transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 10   b .  FIG. 10   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 10   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  920  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  930 . Also, the chart of  FIG. 10   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the step ratio between first and second fixed forward torque ratios is 2.11, the step ratio between the second and third fixed forward torque ratios is 1.90, and the step ratio between the third and fourth fixed forward torque ratios is 1.43. Each of the single step forward shifts between fixed ratios is a single transition shift. 
     Description of an Eleventh Exemplary Embodiment 
     With reference to  FIG. 11   a , a powertrain  1010  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission, designated generally by the numeral  1014 . The transmission  1014  is designed to receive at least a portion of its driving power from the engine  12 . 
     As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  1014 . A transient torque damper (not shown) may also be implemented between the engine  12  and the input member  17  of the transmission. 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member  17  is operatively connected to a planetary gear set in the transmission  1014 . An output member  19  of the transmission  1014  is connected to a final drive  16 . 
     The transmission  1014  utilizes two planetary gear sets  1020  and  1030 . The planetary gear set  1020  employs an outer ring gear member  1024  which circumscribes an inner sun gear member  1022 . A carrier  1026  rotatably supports a plurality of planet gears  1027  such that each planet gear  1027  meshingly engages both the outer ring gear member  1024  and the inner sun gear member  1022  of the first planetary gear set  1020 . 
     The planetary gear set  1030  also has an outer ring gear member  1034  that circumscribes an inner sun gear member  1032 . A plurality of planet gears  1037  are also rotatably mounted in a carrier  1036  such that each planet gear member  1037  simultaneously, and meshingly engages both the outer, ring gear member  1034  and the inner, sun gear member  1032  of the planetary gear set  1030 . 
     The transmission input member  17  is continuously connected with the carrier  1026 , and the transmission output member  19  is continuously connected with the ring gear  1024 . An interconnecting member  1070  continuously connects the ring gear  1024  with the carrier  1036 . 
     The transmission  1014  also incorporates first and second motor/generators  1080  and  1082 , respectively. The stator of the first motor/generator  1080  is secured to the transmission housing  1060 . The rotor of the first motor/generator  1080  is secured to the sun gear  1032 . 
     The stator of the second motor/generator  1082  is also secured to the transmission housing  1060 . The rotor of the second motor/generator  1082  is secured to the ring gear  1034 . 
     A first torque-transfer device, such as a clutch  1050 , selectively connects the carrier  1026  with the ring gear  1034 . A second torque-transfer device, such as a clutch  1052 , selectively connects the sun gear  1022  with the sun gear  1032 . A third torque-transfer device, such as brake  1054 , selectively connects the carrier  1036  with the transmission housing  1060 . A fourth torque transfer device, such as the brake  1055 , is connected in parallel with the motor/generator  1080  for selectively braking rotation of the motor/generator  1080 . A fifth torque transfer device, such as the brake  1057 , is connected in parallel with the motor/generator  1082  for selectively braking rotation of the motor/generator  1082 . The first, second, third, fourth and fifth torque-transfer devices  1050 ,  1052 ,  1054 ,  1055  and  1057  are employed to assist in the selection of the operational modes of the hybrid transmission  1014 . 
     The hybrid transmission  1014  receives power from the engine  12 , and also exchanges power with the electric power source  1086 , which is operably connected to a controller  1088 . 
     The operating mode table of  FIG. 11   b  illustrates the clutching engagements, motor/generator conditions and output/input ratios for the five operating modes of the transmission  1014 . These modes include the “battery reverse mode” (Batt Rev), the “EVT reverse mode” (EVT Rev), “reverse and forward launch modes” (TC Rev and TC For), “continuously variable transmission range modes” (Range 1.1, 1.2, 1.3 . . . ) and “fixed ratio modes” (F 1 , F 2 , F 3 , F 4 ) as described previously. 
     As set forth above, the engagement schedule for the torque-transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 11   b .  FIG. 11   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 11   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  1020  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  1030 . Also, the chart of  FIG. 11   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the step ratio between first and second fixed forward torque ratios is 2.27, the step ratio between the second and third fixed forward torque ratios is 1.76, and the step ratio between the third and fourth fixed forward torque ratios is 1.33. Each of the single step forward shifts between fixed ratios is a single transition shift. 
     Description of a Twelfth Exemplary Embodiment 
     With reference to  FIG. 12   a , a powertrain  1110  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission, designated generally by the numeral  1114 . The transmission  1114  is designed to receive at least a portion of its driving power from the engine  12 . 
     As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  1114 . A transient torque damper (not shown) may also be implemented between the engine  12  and the input member  17  of the transmission. 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member  17  is operatively connected to a planetary gear set in the transmission  1114 . An output member  19  of the transmission  1114  is connected to a final drive  16 . 
     The transmission  1114  utilizes two planetary gear sets  1120  and  1130 . The planetary gear set  1120  employs an outer ring gear member  1124  which circumscribes an inner sun gear member  1122 . A carrier  1126  rotatably supports a plurality of planet gears  1127  such that each planet gear  1127  meshingly engages both the outer ring gear member  1124  and the inner sun gear member  1122  of the first planetary gear set  1120 . 
     The planetary gear set  1130  also has an outer ring gear member  1134  that circumscribes an inner sun gear member  1132 . A plurality of planet gears  1137  are also rotatably mounted in a carrier  1136  such that each planet gear member  1137  simultaneously, and meshingly engages both the outer, ring gear member  1134  and the inner, sun gear member  1132  of the planetary gear set  1130 . 
     The transmission input member  17  is continuously connected with the sun gear  1122 , and the transmission output member  19  is continuously connected with the carrier  1136 . The interconnecting member  1170  continuously connects the ring gear  1124  with the ring gear  1134 . 
     The transmission  1114  also incorporates first and second motor/generators  1180  and  1182 , respectively. The stator of the first motor/generator  1180  is secured to the transmission housing  1160 . The rotor of the first motor/generator  1180  is secured to the carrier  1126 . 
     The stator of the second motor/generator  1182  is also secured to the transmission housing  1160 . The rotor of the second motor/generator  1182  is secured to the sun gear  1132 . 
     A first torque-transfer device, such as a clutch  1150 , selectively connects the ring gear  1124  with the carrier  1126 . A second torque-transfer device, such as clutch  1152 , selectively connects the carrier  1126  with the carrier  1136 . A third torque-transfer device, such as the brake  1154 , selectively connects the ring gear  1124  with the transmission housing  1160 . A fourth torque transfer device, such as the brake  1155 , is connected in parallel with the motor/generator  1182  for selectively braking rotation of the motor/generator  1182 . The first, second, third and fourth torque-transfer devices  1150 ,  1152 ,  1154  and  1155  are employed to assist in the selection of the operational modes of the transmission  1114 . 
     The hybrid transmission  1114  receives power from the engine  12 , and also exchanges power with the electric power source  1186 , which is operably connected to a controller  1188 . 
     The operating mode table of  FIG. 12   b  illustrates the clutching engagements, motor/generator conditions and output/input ratios for the five operating modes of the transmission  1114 . These modes include the “battery reverse mode” (Batt Rev), the “EVT reverse mode” (EVT Rev), “reverse and forward launch modes” (TC Rev and TC For), “continuously variable transmission range modes” (Range 1.1, 1.2, 1.3 . . . and “fixed ratio modes” (F 1 , F 2 , F 3 , F 4 ) as described previously. 
     As set forth above, the engagement schedule for the torque-transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 12   b .  FIG. 12   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 12   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  1120  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  1030 . Also, the chart of  FIG. 12   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the step ratio between first and second fixed forward torque ratios is 1.43, the step ratio between the second and third fixed forward torque ratios is 1.94, and the step ratio between the third and fourth fixed forward torque ratios is 1.43. 
     Description of a Thirteenth Exemplary Embodiment 
     With reference to  FIG. 13   a , a powertrain  1210  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission, designated generally by the numeral  1214 . The transmission  1214  is designed to receive at least a portion of its driving power from the engine  12 . 
     As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  1214 . A transient torque damper (not shown) may also be implemented between the engine  12  and the input member  17  of the transmission. 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member  17  is operatively connected to a planetary gear set in the transmission  1214 . An output member  19  of the transmission  1214  is connected to a final drive  16 . 
     The transmission  1214  utilizes two planetary gear sets  1220  and  1230 . The planetary gear set  1220  employs an outer ring gear member  1224  which circumscribes an inner sun gear member  1222 . A carrier  1226  rotatably supports a plurality of planet gears  1227  such that each planet gear  1227  meshingly engages both the outer ring gear member  1224  and the inner sun gear member  1222  of the first planetary gear set  1220 . 
     The planetary gear set  1230  also has an outer ring gear member  1234  that circumscribes an inner sun gear member  1232 . A plurality of planet gears  1237  are also rotatably mounted in a carrier  1236  such that each planet gear member  1237  simultaneously, and meshingly engages both the outer, ring gear member  1234  and the inner, sun gear member  1232  of the planetary gear set  1230 . 
     The transmission input member  17  is continuously connected with the sun gear  1222 , and the transmission output member  19  is continuously connected with the ring gear  1234 . An interconnecting member  1270  continuously connects the ring gear  1224  with the sun gear  1232 . 
     The transmission  1214  also incorporates first and second motor/generators  1280  and  1282 , respectively. The stator of the first motor/generator  1280  is secured to the transmission housing  1260 . The rotor of the first motor/generator  1280  is secured to the carrier  1226 . The stator of the second motor/generator  1282  is also secured to the transmission housing  1260 . The rotor of the second motor/generator  1282  is secured to the ring gear  1224 . 
     A first torque-transfer device, such as a clutch  1250 , selectively connects the sun gear  1222  with the carrier  1236 . A second torque-transfer device, such as clutch  1252 , selectively connects the carrier  1226  with the carrier  1236 . A third torque-transfer device, such brake  1254 , selectively connects the carrier  1236  with the transmission housing  1260 . A fourth torque transfer device, such as the brake  1255 , is connected in parallel with the motor/generator  1280  for selectively braking rotation of the motor/generator  1280 . A fifth torque transfer device, such as the brake  1257 , is connected in parallel with the motor/generator  1282  for selectively braking rotation of the motor/generator  1282 . The first, second, third, fourth and fifth torque-transfer devices  1250 ,  1252 ,  1254 ,  1255  and  1257  are employed to assist in the selection of the operational modes of the hybrid transmission  1214 . 
     The hybrid transmission  1214  receives power from the engine  12 , and also exchanges power with an electric power source  1286 , which is operably connected to a controller  1288 . 
     The operating mode table of  FIG. 13   b  illustrates the clutching engagements, motor/generator conditions and output/input ratios for the five operating modes of the transmission  1214 . These modes include the “battery reverse mode” (Batt Rev), the “EVT reverse mode” (EVT Rev), “reverse and forward launch modes” (TC Rev and TC For), “continuously variable transmission range modes” (Range 1.1, 1.2, 1.3 . . . ) and “fixed ratio modes” (F 1 , F 2 , F 3 , F 4 ) as described previously. 
     As set forth above, the engagement schedule for the torque-transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 13   b .  FIG. 13   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 13   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  1220  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  1230 . Also, the chart of  FIG. 13   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the step ratio between first and second fixed forward torque ratios is 1.73, the step ratio between the second and third fixed forward torque ratios is 1.68, and the step ratio between the third and fourth fixed forward torque ratios is 1.56. Each of the single step forward shifts between fixed ratios is a single transition shift. 
     Description of a Fourteenth Exemplary Embodiment 
     With reference to  FIG. 14   a , a powertrain  1310  is shown, including an engine  12  connected to one preferred embodiment of the improved electrically variable transmission, designated generally by the numeral  1314 . The transmission  1314  is designed to receive at least a portion of its driving power from the engine  12 . 
     As shown, the engine  12  has an output shaft that serves as the input member  17  of the transmission  1314 . A transient torque damper (not shown) may also be implemented between the engine  12  and the input member  17  of the transmission. 
     Irrespective of the means by which the engine  12  is connected to the transmission input member  17 , the transmission input member  17  is operatively connected to a planetary gear set in the transmission  1314 . An output member  19  of the transmission  1314  is connected to a final drive  16 . 
     The transmission  1314  utilizes two planetary gear sets  1320  and  1330 . The planetary gear set  1320  employs an outer ring gear member  1324  which circumscribes an inner sun gear member  1322 . A carrier  1326  rotatably supports a plurality of planet gears  1327  such that each planet gear  1327  meshingly engages both the outer ring gear member  1324  and the inner sun gear member  1322  of the first planetary gear set  1320 . 
     The planetary gear set  1330  also has an outer ring gear member  1334  that circumscribes an inner sun gear member  1332 . A plurality of planet gears  1337 ,  1338  are also rotatably mounted in a carrier  1336  such that each planet gear member  1337  meshingly engages the inner, sun gear member  1332  of the planetary gear set  1330 , and each planet gear member  1338  meshingly engages the outer, ring gear member  1334 . 
     The transmission input member  17  is continuously connected with the ring gear  1324 , and the transmission output member  19  is continuously connected with the carrier  1346 . An interconnecting member  1370  continuously connects the ring gear  1324  with the sun gear  1332 . 
     The transmission  1314  also incorporates first and second motor/generators  1380  and  1382 , respectively. The stator of the first motor/generator  1380  is secured to the transmission housing  1360 . The rotor of the first motor/generator  1380  is secured to the sun gear  1322 . 
     The stator of the second motor/generator  1382  is also secured to the transmission housing  1360 . The rotor of the second motor/generator  1382  is secured to the ring gear  1324 . 
     A first torque-transfer device, such as a clutch  1350 , selectively connects the carrier  1326  with the ring gear  1324 . A second torque-transfer device such as clutch  1352  selectively connects the ring gear  1334  with the sun gear  1332 . A third torque-transfer device, such as brake  1354 , selectively connects the carrier  1336  with the transmission housing  1360 . A fourth torque transfer device, such as the brake  1355 , is connected in parallel with the motor/generator  1380  for selectively braking rotation of the motor/generator  1380 . The first, second, third and fourth torque-transfer devices  1350 ,  1352 ,  1354  and  1355  are employed to assist in the selection of the operational modes of the transmission  1314 . 
     The hybrid transmission  1314  receives power from the engine  12 , and also exchanges power with an electric power source  1386 , which is operatively connected to a controller  1388 . 
     The operating mode table of  FIG. 14   b  illustrates the clutching engagements, motor/generator conditions and output/input ratios for the five operating modes of the transmission  1314 . These modes include the “battery reverse mode” (Batt Rev), the “EVT reverse mode” (EVT Rev), “reverse and forward launch modes” (TC Rev and TC For), “continuously variable transmission range modes” (Range 1.1, 1.2, 1.3 . . . ) and “fixed ratio modes” (F 1 , F 2 , F 3 , F 4 ) as described previously. 
     As set forth above, the engagement schedule for the torque-transfer devices is shown in the operating mode table and fixed ratio mode table of  FIG. 14   b .  FIG. 14   b  also provides an example of torque ratios that are available utilizing the ring gear/sun gear tooth ratios given by way of example in  FIG. 14   b . The N R1 /N S1  value is the tooth ratio of the planetary gear set  1320  and the N R2 /N S2  value is the tooth ratio of the planetary gear set  1330 . Also, the chart of  FIG. 14   b  describes the ratio steps that are attained utilizing the sample of tooth ratios given. For example, the ratio step between first and second fixed forward torque ratios is 1.43, the ratio step between the second and third fixed forward torque ratios is 1.94, and the ratio step between the third and fourth fixed forward torque ratios is 1.43. 
     In the claims, the language “continuously connected” or “continuously connecting” refers to a direct connection or a proportionally geared connection, such as gearing to an offset axis. 
     While various preferred embodiments of the present invention are disclosed, it is to be understood that the concepts of the present invention are susceptible to numerous changes apparent to one skilled in the art. Therefore, the scope of the present invention is not to be limited to the details shown and described but is intended to include all variations and modifications which come within the scope of the appended claims.