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, up to six selectable torque-transfer devices and possibly a dog clutch. The selectable torque transmitting devices are engaged to yield an EVT with a continuously variable range of speeds (including reverse) and at least four mechanically fixed forward speed ratios. The torque transmitting 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.

Description:
TECHNICAL FIELD 
       [0001]    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 up to six torque transmitting devices. 
       BACKGROUND OF THE INVENTION 
       [0002]    Internal combustion engines, particularly those of the reciprocating piston type, currently propel most vehicles. Such engines are relatively efficient, compact, lightweight, and inexpensive mechanisms by which to convert highly concentrated energy in the form of fuel into useful mechanical power. A novel transmission system, which can be used with internal combustion engines and which can reduce fuel consumption and the emissions of pollutants, may be of great benefit to the public. 
         [0003]    The wide variation in the demands that vehicles typically place on internal combustion engines increases fuel consumption and emissions beyond the ideal case for such engines. Typically, a vehicle is propelled by such an engine, which is started from a cold state by a small electric motor and relatively small electric storage batteries, then quickly placed under the loads from propulsion and accessory equipment. Such an engine is also operated through a wide range of speeds and a wide range of loads and typically at an average of approximately a fifth of its maximum power output. 
         [0004]    A vehicle transmission typically delivers mechanical power from an engine to the remainder of a drive system, such as fixed final drive gearing, axles and wheels. A typical mechanical transmission allows some freedom in engine operation, usually through alternate selection of five or six different drive ratios, a neutral selection that allows the engine to operate accessories with the vehicle stationary, and clutches or a torque converter for smooth transitions between driving ratios and to start the vehicle from rest with the engine turning. Transmission gear selection typically allows power from the engine to be delivered to the rest of the drive system with a ratio of torque multiplication and speed reduction, with a ratio of torque reduction and speed multiplication known as overdrive, or with a reverse ratio. 
         [0005]    An electric generator can transform mechanical power from the engine into electrical power, and an electric motor can transform that electric power back into mechanical power at different torques and speeds for the remainder of the vehicle drive system. This arrangement allows a continuous variation in the ratio of torque and speed between engine and the remainder of the drive system, within the limits of the electric machinery. An electric storage battery used as a source of power for propulsion may be added to this arrangement, forming a series hybrid electric drive system. 
         [0006]    The series hybrid system allows the engine to operate with some independence from the torque, speed and power required to propel a vehicle, so the engine may be controlled for improved emissions and efficiency. This system allows the electric machine attached to the engine to act as a motor to start the engine. This system also allows the electric machine attached to the remainder of the drive train to act as a generator, recovering energy from slowing the vehicle into the battery by regenerative braking. A series electric drive suffers from the weight and cost of sufficient electric machinery to transform all of the engine power from mechanical to electrical in the generator and from electrical to mechanical in the drive motor, and from the useful energy lost in these conversions. 
         [0007]    A power-split transmission can use what is commonly understood to be “differential gearing” to achieve a continuously variable torque and speed ratio between input and output. An electrically variable transmission can use differential gearing to send a fraction of its transmitted power through a pair of electric motor/generators. The remainder of its power flows through another, parallel path that is all mechanical and direct, of fixed ratio, or alternatively selectable. 
         [0008]    One form of differential gearing, as is well known to those skilled in this art, may constitute a planetary gear set. Planetary gearing is usually the preferred embodiment employed in differentially geared inventions, with the advantages of compactness and different torque and speed ratios among all members of the planetary gear set. However, it is possible to construct this invention without planetary gears, as by using bevel gears or other gears in an arrangement where the rotational speed of at least one element of a gear set is always a weighted average of speeds of two other elements. 
         [0009]    A hybrid electric vehicle transmission system also includes one or more electric energy storage devices. The typical device is a chemical electric storage battery, but capacitive or mechanical devices, such as an electrically driven flywheel, may also be included. Electric energy storage allows the mechanical output power from the transmission system to the vehicle to vary from the mechanical input power from the engine to the transmission system. The battery or other device also allows for engine starting with the transmission system and for regenerative vehicle braking. 
         [0010]    An electrically variable transmission in a vehicle can simply transmit mechanical power from an engine input to a final drive output. To do so, the electric power produced by one motor/generator balances the electrical losses and the electric power consumed by the other motor/generator. By using the above-referenced electrical storage battery, the electric power generated by one motor/generator can be greater than or less than the electric power consumed by the other. Electric power from the battery can sometimes allow both motor/generators to act as motors, especially to assist the engine with vehicle acceleration. Both motors can sometimes act as generators to recharge the battery, especially in regenerative vehicle braking. 
         [0011]    A successful substitute for the series hybrid transmission is the two-range, input-split and compound-split electrically variable transmission now produced for transit buses, as disclosed in U.S. Pat. No. 5,931,757, issued Aug. 3, 1999, to Michael Roland Schmidt, commonly assigned with the present application. Such a transmission utilizes an input means to receive power from the vehicle engine and a power output means to deliver power to drive the vehicle. First and second motor/generators are connected to an energy storage device, such as a battery, so that the energy storage device can accept power from, and supply power to, the first and second motor/generators. A control unit regulates power flow among the energy storage device and the motor/generators as well as between the first and second motor/generators. 
         [0012]    Operation in first or second variable-speed-ratio modes of operation may be selectively achieved by using clutches in the nature of first and second torque transfer devices. In the first mode, an input-power-split speed ratio range is formed by the application of the first clutch, and the output speed of the transmission is proportional to the speed of one motor/generator. In the second mode, a compound-power-split speed ratio range is formed by the application of the second clutch, and the output speed of the transmission is not proportional to the speeds of either of the motor/generators, but is an algebraic linear combination of the speeds of the two motor/generators. Operation at a fixed transmission speed ratio may be selectively achieved by the application of both of the clutches. Operation of the transmission in a neutral mode may be selectively achieved by releasing both clutches, decoupling the engine and both electric motor/generators from the transmission output. The transmission incorporates at least one mechanical point in its first mode of operation and at least two mechanical points in its second mode of operation. 
         [0013]    U.S. Pat. No. 6,527,658, issued Mar. 4, 2003 to Holmes et al, commonly assigned with the present application, 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 
       [0014]    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. 
         [0015]    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 up to six selectable torque-transmitting devices. Additionally, a dog clutch may be provided. Preferably, the differential gear sets are planetary gear sets, such as simple or compound (including Ravigneaux) gear sets, but other gear arrangements may be implemented, such as bevel gears or differential gearing to an offset axis. 
         [0016]    In this description, the first or second planetary gear sets may be counted first to second in any order (i.e., left to right, right to left). 
         [0017]    Each of the two 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 member, or alternatively a pinion. 
         [0018]    Each carrier member can be either a single-pinion carrier member (simple) or a double-pinion carrier member (compound). 
         [0019]    The input shaft is continuously or selectively connected with at least one member of the planetary gear sets. The output shaft is continuously connected with at least one member of the planetary gear sets. 
         [0020]    An optional first interconnecting member continuously connects the first member of the first planetary gear set with the first member of the second planetary gear. 
         [0021]    A first torque transmitting device selectively connects a member of the first planetary gear set with the input member or with a member of the second planetary gear set. 
         [0022]    A second torque transmitting device selectively connects a member of the second planetary gear set with a member of the first planetary gear set, this pair of members being different than the ones connected by the first torque transmitting device. 
         [0023]    A third torque transmitting device selectively connects a member of the first planetary gear set with a member of the second planetary gear set or with the input member. 
         [0024]    A fourth torque transmitting device selectively connects a member of the first planetary gear set with a stationary member (transmission housing/casing). 
         [0025]    A fifth torque transmitting device selectively connects a member of the second planetary gear set with a stationary member (transmission housing/casing). 
         [0026]    An optional sixth torque transmitting device is connected in parallel with one of the motor/generators for selectively preventing rotation of the motor/generator. 
         [0027]    The torque transmitting devices which are brakes may be implemented as conventional friction-based brakes, dog clutches, one-way clutches, or selectable one-way clutches, as appropriate. The rotating clutches may be implemented as conventional friction-based clutches, dog clutches, one-way clutches, or selectable one-way clutches, as appropriate. 
         [0028]    The first motor/generator is mounted to the transmission case and is connected either continuously with a member of the first planetary gear set or selectively via a dog clutch to a member of the first or second planetary gear set. The first motor/generator may also incorporate offset gearing. The dog clutch, if present, allows the first motor/generator to be switched between a pair of members on the first or second planetary gear sets. The dog clutch may reduce clutch spin losses, and allows motor/generator operation at low speeds throughout the operating range of the transmission. The dog clutch may be replaced by a pair of conventional torque transfer devices. 
         [0029]    The second motor/generator is mounted to the transmission case and is connected continuously to a member of the first or second planetary gear set. The second motor/generator connection may incorporate offset gearing. 
         [0030]    The selectable torque transmitting devices are engaged in combinations to yield an EVT with a continuously variable range of speeds (including reverse) and at least 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. This fixed ratio operation is useful for meeting reverse gradeability requirements and also for cold-weather operating when the electrical torque assist may not be available due to poor battery operation. A variety of fixed speed ratios and variable ratio spreads can be realized by suitably selecting the tooth ratios of the planetary gear sets. 
         [0031]    The electrically variable transmissions of the present invention have a compound-split and input-split architecture. Compound-split means neither the transmission input nor output is directly connected to a motor/generator. This allows a reduction in the size and cost of the electric motor/generators required to achieve the desired vehicle performance. In input-split designs, one of the motor/generators is directly connected to the output. 
         [0032]    The proposed designs are all two- or three-mode designs (or two- or three-range) in which one torque transmitting device is disengaged and another one engaged during forward EVT operation to change modes or ranges. The multi-mode designs enable a switch between different operating modes (e.g., compound-split to output-split, etc.) to better match operating requirements while minimizing electrical component loads/speeds and energy used. 
         [0033]    The torque transmitting 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. 
         [0034]    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 
         [0035]      FIG. 1   a  is a schematic representation of a powertrain including an electrically variable transmission incorporating a family member of the present invention; 
           [0036]      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;    
           [0037]      FIG. 2   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
           [0038]      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;    
           [0039]      FIG. 3   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; 
           [0040]      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;    
           [0041]      FIG. 4   a  is a schematic representation of a powertrain having an electrically variable transmission incorporating another family member of the present invention; and 
           [0042]      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.    
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]    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. 
         [0044]    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). 
         [0045]    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  14 . An output member  19  of the transmission  14  is connected to a final drive  16 . 
         [0046]    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 member  24  circumscribes an inner gear member  22 , typically designated as the sun gear. A carrier member  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 . 
         [0047]    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 member. A plurality of planet gears  37  are also rotatably mounted in a carrier member  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 . 
         [0048]    A first interconnecting member  70  continuously connects the ring gear member  24  of the planetary gear set  20  with the sun gear member  32  of the planetary gear set  30 . 
         [0049]    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 member  22  of the planetary gear set  20 . 
         [0050]    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 sun gear member  32  of the planetary gear set  30 . 
         [0051]    A first torque transmitting device, such as input clutch  50 , selectively connects the carrier member  26  of the planetary gear set  20  with the input member  17 . A second torque transmitting device, such as clutch  52 , selectively connects the sun gear member  22  of the planetary gear set  20  with the ring gear member  34  of the planetary gear set  30 . A third torque transmitting device, such as input clutch  54 , selectively connects the sun gear member  22  with the input member  17 . A fourth torque transmitting device, such as the brake  55 , selectively connects the ring gear member  34  with the transmission housing  60 . A fifth torque transmitting device, such as brake  57 , selectively connects the carrier member  26  with the transmission housing  60 . A sixth torque transmitting device, such as brake  58 , is connected in parallel with the motor/generator  82  for selectively braking rotation thereof. The first, second, third, fourth, fifth and sixth torque transmitting devices  50 ,  52 ,  54 ,  55 ,  57  and  58  are employed to assist in the selection of the operational modes of the hybrid transmission  14 , as will be hereinafter more fully explained. 
         [0052]    The output drive member  19  of the transmission  14  is secured to carrier member  36  of the planetary gear set  30 . 
         [0053]    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 
       [0054]    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 configure 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. 
         [0055]    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 member that is itself rotatable. 
         [0056]    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 member in the same direction as the direction in which the ring gear is being rotated. 
         [0057]    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 member rotates with the sun and ring gears. 
         [0058]    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. 
         [0059]    Whenever the carrier member 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 member 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. 
         [0060]    In a simple set of planetary gears, if any two rotational speeds of the sun gear, the planet carrier member, 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 member 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 member 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. 
         [0061]    The torque on the sun gear, the carrier member, 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 member 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. 
         [0062]    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 member in comparison to a simple planetary gear set. For instance, if the sun gear is held stationary, the planet carrier member will rotate in the same direction as the ring gear, but the planet carrier member with inner and outer sets of planet gears will travel faster than the ring gear, rather than slower. 
         [0063]    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 member, 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 member and one third the speed of the sun. If the sun gear or the planet carrier member rotates in an opposite direction, the arithmetic sign is negative for that speed in mathematical calculations. 
         [0064]    If the sun gear were to be held stationary, then a carrier member 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 member 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 member 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 member. 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 member are combined, so the ring rotates in the same direction as the carrier member, but not as fast as the carrier member. 
         [0065]    If the carrier member 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 member 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 member 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. 
         [0066]    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. 
         [0067]    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. 
         [0068]    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 
       [0069]    Each of the embodiments described herein has seventeen functional requirements (corresponding with the 17 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. 
         [0070]    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 clutch  50  and brake  55  are engaged, the generator  80  has zero torque, the motor  82  has a torque of −1.00, and a torque ratio of −2.88 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. 
         [0071]    The second operating mode is the “EVT reverse mode” (or mechanical 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 clutch  50  and brake  55  are engaged, the generator  80  has a torque of −0.35 units, the motor  82  has a torque of −3.55 units, and an output torque of −8.33 is achieved, corresponding to an engine torque of 1 unit. 
         [0072]    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 TC Reverse mode, the clutch  50  and brake  55  are engaged, the motor/generator  80  acts as a generator with −0.35 units of torque, the motor/generator  82  acts as a motor with −3.09 units of torque, and a torque ratio of −7.00 is achieved. In the TC Forward mode, the clutch  50  and brake  55  are engaged, the motor/generator  80  acts as a generator with −0.35 units of torque, the motor/generator  82  acts as a motor with 0.98 units of torque, and a torque ratio of 4.69 is achieved. 
         [0073]    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 clutch  50  and brake  55  engaged. A range or torque ratios from 1.36 to 0.54 is achieved with the clutches  50  and  52  engaged. 
         [0074]    The fifth operating mode includes the “fixed ratio” modes (R 1 , F 1 , F 2 , F 3  and F 4 ) corresponding with rows  13 - 17  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 three torque transmitting devices engaged to create a discrete transmission ratio. The clutching table accompanying each figure shows only four forward fixed-ratio and one reverse fixed-ratio speeds but additional fixed ratios may be available. Referring to  FIG. 1   b , in fixed ratio R 1 , a reverse fixed ratio, the clutch  54  and brakes  55 ,  57  are engaged to achieve a fixed torque ratio of −5.32. In fixed ratio F 1  the clutches  50 ,  54  and brake  55  are engaged to achieve a fixed torque ratio of 2.86. In fixed ratio F 2 , the clutches  50 ,  52  and brake  55  are engaged to achieve a fixed ratio of 1.86. In fixed ratio F 3 , the clutches  50 ,  52  and  54  are engaged to achieve a fixed ratio of 1.00. In fixed ratio F 4 , the clutches  50 ,  52  and brake  58  are engaged to achieve a fixed ratio of 0.53. Accordingly, each “X” in the column of motor/generator  82  in  FIG. 1   b  indicates that the brake  58  is engaged and the motor/generator  82  is not rotating. 
         [0075]    The powertrain  10  may also operate in a “charge-depleting mode”. For purposes of the present invention, a “charge-depleting mode” is a mode wherein the vehicle is powered primarily by an electric motor/generator such that the battery  86  is depleted or nearly depleted when the vehicle reaches its destination. In other words, during the charge-depleting mode, the engine  12  is only operated to the extent necessary to ensure that the battery  86  is not depleted before the destination is reached. A conventional hybrid vehicle operates in a “charge-sustaining mode”, wherein if the battery charge level drops below a predetermined level (e.g., 25%) the engine is automatically run to recharge the battery. Therefore, by operating in a charge-depleting mode, the hybrid vehicle can conserve some or all of the fuel that would otherwise be expended to maintain the 25% battery charge level in a conventional hybrid vehicle. It should be appreciated that the vehicle powertrain is preferably only operated in the charge-depleting mode if the battery  86  can be recharged after the destination is reached by plugging it into an energy source (not shown). 
         [0076]    Also, the engine  12  may be powered using various types of fuel to improve the efficiency and fuel economy of a particular application. Such fuels may include, for example, gasoline; diesel; ethanol; dimethyl ether; etc. 
         [0077]    The transmission  14  is capable of operating in so-called single or dual modes (or ranges). In single mode, the engaged torque transmitting 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 transmitting device is switched at some intermediate speed ratio (e.g., Range  2 . 1  in  FIG. 1   b ). The transmission of  FIG. 4   a  includes three-mode capability. In three-mode, the engaged torque transmitting devices are switched at an additional intermediate speed ratio (e.g., Range  3 . 1  in  FIG. 4   b ). Depending on the mechanical configuration, this change in torque transmitting device engagement has advantages in reducing element speeds in the transmission. 
         [0078]    As set forth above, the engagement schedule for the torque transmitting 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 the fixed reverse and first fixed forward torque ratios is −1.86, the step ratio between the first and second fixed forward torque ratios is 1.54, the step ratio between the second and third fixed forward torque ratios is 1.86, the step ratio between the third and fourth fixed forward torque ratios is 1.86, and the ratio spread is 5.40. 
       Description of a Second Exemplary Embodiment 
       [0079]    With reference to  FIG. 2   a , a powertrain  110  is shown, including an engine  12  connected to another 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 . 
         [0080]    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. 
         [0081]    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 . 
         [0082]    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 member  124  circumscribes an inner gear member  122 , typically designated as the sun gear. A carrier member  126  rotatably supports a plurality of planet gears  127  such that each planet gear  127  meshingly engages both the outer, ring gear member  124  and the inner, sun gear member  122  of the first planetary gear set  120 . 
         [0083]    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 member  136  such that each planet gear member  137  simultaneously, and meshingly, engages both the outer, ring gear member  134  and the inner, sun gear member  132  of the planetary gear set  130 . 
         [0084]    The transmission input member  17  is connected with the carrier member  126  of the planetary gear set  120 . The transmission output member  19  is connected with the carrier member  136  of the planetary gear set  130 . 
         [0085]    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 member  122  of the planetary gear set  120 . 
         [0086]    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 sun gear member  132  of the planetary gear set  130 . 
         [0087]    A first torque transmitting device, such as clutch  150 , selectively connects the sun gear member  122  of the planetary gear set  120  with the ring gear member  134  of the planetary gear set  130 . A second torque transmitting device, such as clutch  152 , selectively connects the ring gear member  124  of the planetary gear set  120  with the sun gear member  132  of the planetary gear set  130 . A third torque transmitting device, such as clutch  154 , selectively connects the sun gear member  122  of the planetary gear set  120  with the sun gear member  132  of the planetary gear set  130 . A fourth torque transmitting device, such as the brake  155 , selectively connects the ring gear member  134  of the planetary gear set  130  with the transmission housing  160 . A fifth torque transmitting device, such as the brake  157 , selectively connects the ring gear member  124  of the planetary gear set  120  with the transmission housing  160 . The first, second, third, fourth and fifth torque transmitting devices  150 ,  152 ,  154 ,  155  and  157  are employed to assist in the selection of the operational modes of the hybrid transmission  114 . 
         [0088]    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. 
         [0089]    As described previously, each embodiment has seventeen functional requirements (corresponding with the 17 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 brake  155  is engaged, the generator  180  has zero torque, the motor  182  has a torque of −1.00 units and an output torque of −2.88 is achieved, by way of example. 
         [0090]    The second operating mode is the “EVT reverse mode” (or mechanical 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 clutch  152  and brake  155  are engaged, the generator  180  has a torque of −0.35 units, the motor  182  has a torque of −3.55 units, and an output torque of −8.33 is achieved, corresponding to an input torque of 1 unit. 
         [0091]    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 TC Rev, the clutch  152  and brake  155  are engaged, the motor/generator  180  acts as a generator with −0.35 units of torque, the motor/generator  182  acts as a motor with −3.09 units of torque, and a torque ratio of −7.00 is achieved. In TC For, the clutch  152  and brake  155  are engaged, the motor/generator  180  acts as a generator with −0.35 units of torque, the motor/generator  182  acts as a motor with 0.98 units of torque, and a torque ratio of 4.69 is achieved. For these torque ratios, approximately 99% of the generator energy is stored in the battery. 
         [0092]    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 clutch  152  and brake  155  engaged. and a range of ratios from 1.36 to 0.54 is achieved with the clutches  150  and  152  engaged. 
         [0093]    The fifth operating mode includes the fixed “ratio” modes (F 1 , F 2 , F 3 , F 4  and F 5 ) corresponding with rows  13 - 17  of the operating mode table of  FIG. 2   b . In this mode the transmission operates like a conventional automatic transmission, with one torque transmitting device engaged to create a discrete transmission ratio. In fixed ratio F 1  the clutches  152 ,  154  and brake  155  are engaged to achieve a fixed ratio of 2.82. In fixed ratio F 2 , the clutches  150 ,  152  and brake  155  are engaged to achieve a fixed ratio of 1.74. In fixed ratio F 3 , the clutches  150 ,  152  and  154  are engaged to achieve a fixed ratio of 1.00. In fixed ratio F 4 , the clutches  150 ,  152  and brake  157  are engaged to achieve a fixed ratio of 0.60. In fixed ratio F 5 , the clutches  150 ,  154  and brake  157  are engaged to achieve a fixed ratio of 0.39. 
         [0094]    As set forth above, the engagement schedule for the torque transmitting 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 1.63, and the ratio spread is 7.32. 
       Description of a Third Exemplary Embodiment 
       [0095]    With reference to  FIG. 3   a , a powertrain  210  is shown, including an engine  12  connected to another 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 . 
         [0096]    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 . 
         [0097]    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 member  224  circumscribes an inner gear member  222 , typically designated as the sun gear. A carrier member  226  rotatably supports a plurality of planet gears  227 ,  228  such that each planet gear  227  meshingly engages inner, sun gear member  222  and each planet gear  228  simultaneously and meshingly engages both the outer, ring gear member  224  and the respective planet gear  227  of the first planetary gear set  220 . 
         [0098]    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 member  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 . 
         [0099]    The transmission output member  19  is connected to the carrier member  236 .
       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 member  222 . 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 member  232 .   A first torque transmitting device, such as input clutch  250 , selectively connects ring gear member  224  with the input member  17 . A second torque transmitting device, such as clutch  252 , selectively connects the sun gear member  222  with the ring gear member  234 . A third torque transmitting device, such as input clutch  254 , selectively connects the sun gear member  222  with the input member  17 . A fourth torque transmitting device, such as the brake  255 , selectively connects the ring gear member  234  with the transmission housing  260 . A fifth torque transmitting device, such as brake  257 , selectively connects the ring gear member  224  with the transmission housing  260 . A sixth torque transmitting device, such as brake  258 , is connected in parallel with the motor/generator  282  for selectively braking rotation thereof. The first, second, third, fourth, fifth and sixth torque transmitting devices  250 ,  252 ,  254 ,  255 ,  257  and  258  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 .       
 
         [0103]    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” (R 1 , F 1 , F 2 , F 3  and F 4 ) as described previously. 
         [0104]    As set forth above the engagement schedule for the torque transmitting 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 1.54, and the ratio spread is 5.40. 
       Description of a Fourth Exemplary Embodiment 
       [0105]    With reference to  FIG. 4   a , a powertrain  310  is shown, including an engine  12  connected to another 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 . 
         [0106]    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. 
         [0107]    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 . 
         [0108]    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 member  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 . 
         [0109]    The planetary gear set  330  also has an outer ring gear member  334  that circumscribes an inner sun gear member  332 . A carrier member  336  rotatably supports a plurality of planet gears  337 ,  338  such that each planet gear  337  meshingly engages the outer ring gear member  324  and the each planet gear  338  simultaneously, and meshingly engages both the inner sun gear member  332  and the respective planet gear  337  of the planetary gear set  330 . 
         [0110]    The transmission output member  19  is connected with the ring gear member  334 . 
         [0111]    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 selectively connectable with the ring gear member  334  or the sun gear member  322  via dog clutch  392 , alternating between positions A and B, respectively. The rotor of the first motor/generator  380  is connected to the dog clutch  392  via offset gearing  394 . Within the scope of the invention, a pair of torque transmitting devices could be utilized to accomplish the selective engagement as achieved by dog clutch  392 , as is understood by those skilled in the art. 
         [0112]    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 member  332 . 
         [0113]    A first torque transmitting device, such as input clutch  350 , selectively connects the carrier member  326  with the input member. A second torque transmitting device, such as clutch  352 , selectively connects the sun gear member  322  with the carrier member  336 . A third torque transmitting device, such as input clutch  354 , selectively connects the sun gear member  322  with the input member  17 . A fourth torque transmitting device, such as brake  355 , selectively connects the carrier member  336  with the transmission housing  360 . A fifth torque transmitting device, such as brake  357 , selectively connects the carrier member  326  with the transmission housing  360 . A sixth torque transmitting device, such as the brake  358 , is connected in parallel with the motor/generator  382  for selectively braking rotation thereof. The first, second, third, fourth, fifth and sixth torque transmitting devices  350 ,  352 ,  354 ,  355 ,  357  and  358  and the dog clutch  392  are employed to assist in the selection of the operational modes of the transmission  314 . 
         [0114]    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 . 
         [0115]    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” (R 1 , F 1 , F 2 , F 3  and F 4 ) as described previously. 
         [0116]    As set forth above, the engagement schedule for the torque transmitting 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 1.54, and the ratio spread is 5.40. 
         [0117]    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. Also, the “stationary member” or “ground” may include the transmission housing (case) or any other non-rotating component or components. Also, when a torque transmitting mechanism is said to connect something to a member of a gear set, it may also be connected to an interconnecting member which connects it with that member. It is further understood that different features from different embodiments of the invention may be combined within the scope of the appended claims. 
         [0118]    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.