Abstract:
An infinitely variable transmission includes a clutch, a brake and a first planetary gear set, including first and second components, and a double planetary gear set, including input and output components and additional third and fourth components. The first component receives power from an engine. The double planetary set sums mechanical power from the first planetary set and an infinitely variable power source (“IVP”). The third component receives mechanical power from a IVP. The second component directly transmits power to the input component. The clutch directly controls power transmission between the first and second components. The brake engages the fourth component to stop its rotation. The output component receives mechanical power directly from the input component and the fourth component. During operation of the engine, controlled actuation of the brake and the clutch causes the output component to be powered by the infinitely variable power source but not by the engine.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     Not applicable. 
     STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     FIELD OF THE DISCLOSURE 
     This disclosure relates to infinitely variable transmissions, including transmissions for the operation of vehicles in multiple powered modes. 
     BACKGROUND OF THE DISCLOSURE 
     It may be useful, in a variety of settings, to utilize both a traditional engine (e.g., an internal combustion engine) and an infinitely variable power source (e.g., an electric or hydrostatic motor, a variable chain drive, and so on) to provide useful power. For example, a portion of engine power may be diverted to drive a first infinitely variable machine (e.g., a first electric machine acting as a generator), which may in turn drive a second infinitely variable machine (e.g., a second electric machine acting as a motor using electrical power from the first electrical machine). In certain configurations, power from both of types of sources (i.e., an engine and an infinitely variable power source) may be combined for final power delivery (e.g., to a vehicle axle) via an infinitely variable transmission (“IVT”) or continuously variable transmission (“CVT”). This may be referred to as “split-mode” or “split-path mode” operation because power transmission may be split between the mechanical path from the engine and the infinitely variable path. Split-mode operation may be attained in various known ways. For example, a planetary gear set may be utilized to sum rotational power from an engine and from an electric machine, with the summed power transmitted downstream within an associated power train. This may allow for delivery of power (e.g., to vehicle wheels) with an infinitely variable effective gear ratio. Various issues may arise, however, including limitations relating to the maximum practical speed of variable power sources. 
     SUMMARY OF THE DISCLOSURE 
     An infinitely variable transmission is disclosed. According to one aspect of the disclosure, an infinitely variable transmission includes a first planetary gear set including a first transmission component and a second transmission component, and a double planetary gear set having an input component, an output component, a third transmission component, and a fourth transmission component. The infinitely variable transmission also includes a clutch and a brake. The first transmission component receives a first mechanical power input for the first planetary gear set from an engine. The third transmission component receives a second mechanical power input for the double planetary gear set from an infinitely variable power source. The second transmission component directly transmits power to the input component of the double planetary gear set. The clutch is configured to engage the first transmission component and the second transmission component in order to control power transmission between the first transmission component and the second transmission component. The brake is configured to engage the fourth transmission component in order to stop rotation of the fourth transmission component. The output component is configured to receive mechanical power directly from the input component and the fourth transmission component. The double planetary gear set is configured to sum mechanical power from the engine and the infinitely variable power source and provide the summed power to the output component. During operation of the engine, controlled actuation of one or more of the first brake and the clutch causes the output component to be powered by the infinitely variable power source but not by the engine. 
     One or more of the following features may also be included in the disclosed transmission. The first transmission component may include a first planet gear carrier supporting one or more first planet gears. The second transmission component may include a first sun gear. The input component may include a first ring gear. The third transmission component may include a second sun gear. The fourth transmission component may include a second ring gear. The output component of the double planetary gear set may include a second planet gear carrier supporting one or more second planet gears, the one or more second planet gears being meshed with one or more of the input component and the fourth transmission component. The output component may transmit mechanical power to a gear box including one or more gears. The infinitely variable power source may include one or more of a pair of electric machines and a hydrostatic machine. 
     According to another aspect of the disclosure, an infinitely variable transmission includes a first planetary gear set including a first input component, a first output component, and a first transmission component. The infinitely variable transmission includes a double planetary gear set, including a second input component, a second transmission component, a third transmission component, and a second output component. The infinitely variable transmission also includes a first clutch, a second clutch, and a brake. The first input component receives a first mechanical power input for the first planetary gear set from an engine. The second input component receives mechanical power directly from the first output component. One or more of the first clutch, the second clutch and the brake are configured to engage one or more of the first input component, the first output component, and the first transmission component, in order to control mechanical power transmission between the engine and the double planetary gear set. One or more of the second and third transmission components receive a second mechanical power input for the double planetary gear set from an infinitely variable power source. The double planetary gear set is configured to sum mechanical power from the engine and the infinitely variable power source and provide the summed power to the second output component. During operation of the engine, controlled actuation of one or more of the first clutch, the second clutch, and the brake causes the second output component to be powered by the infinitely variable power source but not by the engine. 
     The first input component may include a first sun gear, the first clutch being configured to engage the first sun gear to control mechanical power transmission between the first sun gear and the engine. The first transmission component may include a first ring gear, the brake being configured to engage the first ring gear in order to stop rotation of the first ring gear. The first output component may include a first planet gear carrier, the second clutch being configured to engage the first planet gear carrier and the first ring gear in order to control mechanical power transmission between the first planet gear carrier and the first ring gear. The second input component may include a second planet gear carrier, the second planet gear carrier configured to receive mechanical power directly from both the second transmission component and a fourth transmission component included in the double planetary gear set. The second transmission component may include a second sun gear. The fourth transmission component may include a second ring gear. The second input component may directly transmit mechanical power to a third ring gear included in the double planetary gear set. The third transmission component may include a third sun gear. A third planet gear carrier supporting one or more planet gears may be included in the double planetary gear set, the one or more planet gears being meshed with the third ring gear and the third sun gear. 
     The infinitely variable transmission may further include a third clutch configured to engage the second output component and a fourth clutch configured to engage the second output component. Controlled actuation of the third and fourth clutch may control a flow path of mechanical power through the double planetary gear set to the second output component. The third clutch may be configured to engage a third planet gear carrier for transmission of mechanical power between the third planet gear carrier and the second output component. The second control clutch may be configured to controllably engage a second ring included in the double planetary gear set for transmission of mechanical power between the second ring gear and the second output component. 
     According to another aspect of the disclosure, an infinitely variable transmission includes a double planetary gear set including a first input component, a second input component, and an output component. The infinitely variable transmission includes a clutch, a first brake, and a second brake. The first input component is configured to receive a first mechanical power input for the double planetary gear set from an engine. The clutch is configured to engage the first input component in order to control mechanical power transmission between the first input component and an engine. The second input component receives a second mechanical power input for the double planetary gear set from an infinitely variable power source. The double planetary gear set is configured to sum mechanical power from the engine and the infinitely variable power source and provide the summed power to the output component. During operation of the engine, controlled actuation of one or more of the clutch, the first brake and the second brake causes the output component to be powered by the infinitely variable power source but by not the engine. 
     The double planetary gear set may include a first sun gear, a second sun gear, a first ring gear, a second ring gear, a first planet gear carrier supporting one or more first planet gears, and a second planet gear carrier supporting one or more second planet gears. The first input component may include the first sun gear. The second input component may include the second sun gear. The output component may include the second planet gear carrier. The one or more second planet gears are meshed with the second sun gear and the first ring gear. The first ring gear may be integral with the first planet gear carrier. The one or more first planet gears are meshed with the first sun gear and the second ring gear. The first brake may be configured to engage one or more of the second ring gear and the first planet gear carrier in order to control relative motion of the second ring gear and the first planet gear carrier. The second brake may be configured to engage the second ring gear in order to stop rotation of the second ring gear. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of an example vehicle that may include an infinitely variable transmission; 
         FIG. 2  is a schematic view of a power train of the vehicle of  FIG. 1 ; 
         FIG. 3  is a schematic view of an infinitely variable transmission that may be included in the power train of  FIG. 2 ; 
         FIG. 4  is a graphical representation of infinitely variable power source speeds and vehicle wheel speeds for various modes of operation of the infinitely variable transmission of  FIG. 3 ; 
         FIG. 5  is a schematic view of another infinitely variable transmission that may be included in the power train of  FIG. 2 ; 
         FIG. 6  is a graphical representation of infinitely variable power source speeds and vehicle wheel speeds for various modes of operation of the infinitely variable transmission of  FIG. 5 ; 
         FIG. 7  is a schematic view of another infinitely variable transmission that may be included in the power train of  FIG. 2 ; and 
         FIG. 8  is a graphical representation of variable power source speeds and vehicle wheel speeds for various modes of operation of the infinitely variable transmission of  FIG. 7 . 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     The following describes one or more example embodiments of the disclosed multi-mode infinitely variable transmission (“MIVT”), as shown in the accompanying figures of the drawings described briefly above. Various modifications to the example embodiments may be contemplated by one of skill in the art. 
     In various known configurations, one or more planetary gear sets may be utilized to combine the power output of an infinitely variable power source (“IVP”) and an engine (e.g., an internal combustion engine). For example, in a planetary gear set a first component of the gear set (e.g., a ring gear) may receive power from the engine, a second component of the gear set (e.g., a sun gear) may receive power from the IVP, and a third component of the gear set (e.g., a planet gear carrier) may sum the power from the engine and the IVP at the output of the gear set. (For convenience of notation, “component” may be used herein, particularly in the context of a planetary gear set, to indicate an element for transmission of power, such as a sun gear, a ring gear, or a planet gear carrier.) It will be understood that such a configuration may allow for an essentially infinite (and continuous) number of gear ratios for the planetary gear set. For example, for a fixed engine speed, a particular gear ratio may be set by varying the speed of the IVP with respect to the engine speed. 
     In certain instances, it may be useful to facilitate a powered-zero mode for a vehicle (or other machinery), in which the output speed of the vehicle wheels (or other machinery output) reaches zero speed without stopping the engine or releasing torque at the wheels. In this way, for example, vehicle power may be utilized to hold a vehicle stationary. Such a state may be obtained, for example, with a planetary gear set configured as described above. For example, if an engine is spinning a sun gear at a first positive speed and a IVP is directed to spin a ring gear at an equivalent negative speed, an associated planet gear carrier (which may, for example, be connected to a differential drive shaft) may not spin at all. Further, if the electric motor spins at a slightly different (and opposite) speed from the engine, the vehicle may enter a “creeper” mode, in which the vehicle moves very slowly but with high wheel torque. The powered-zero and creeper modes are particularly useful for heavy-duty work vehicles, such as the tractor shown in  FIG. 1 , used in the agricultural, construction and forestry industries. With increasing wheel speed, the vehicle may then, eventually, enter a normal drive mode. In traditional configurations, each of these modes may be split-path modes, in which power transmission is split between a purely mechanical path from the engine and the mixed path through the IVP. 
     One issue relating to infinitely variable power trains may concern the relative efficiency of power transmission in various modes. It will be understood, for example, that mechanical transmission of power from an engine to a gear set (i.e., mechanical path transmission) may be a highly efficient mode of power transmission, whereas transmission of power through a IVP may be less efficient (e.g., because the mechanical power must be converted to electrical or hydraulic power by a first machine, transmitted to a second machine, and then converted back to mechanical power). Accordingly, there may exist significant motivation to utilize the mechanical path more heavily than the IVP path (e.g., by increasing the speed of the engine). However, this heavier utilization of the mechanical path may also drive up the required IVP speed for powered-zero and creeper modes, because these modes may require near or actual speed matching between the IVP and engine speeds. This may lead to increased wear on related gears and other parts (e.g., a planetary gear component receiving power from the IVP and associated bearings), even to the point of part failure. Further, to attain appropriate speeds, the size and power of a relevant IVP may need to be significantly increased from a preferred size and power. Among other advantages, the MIVT disclosed herein may address these issues. For example, through selective use of clutches and/or brakes, an MIVT may allow heavier utilization of a mechanical path, while avoiding the need for excessive IVP speeds in powered-zero and creeper modes. 
     As will become apparent from the discussion herein, a MIVT may be used advantageously in a variety of settings and with a variety of machinery. For example, referring now to  FIG. 1 , a MIVT may be included in the power train  12  of vehicle  10 . In  FIG. 1 , vehicle  10  is depicted as a tractor. It will be understood, however, that other configurations may be possible, including configuration of vehicle  10  as a different kind of tractor, as a log skidder, as a grader, or as one of various other work vehicle types. It will further be understood that the disclosed IVT may also be used in non-work vehicles and non-vehicle applications (e.g., fixed-location power trains). 
     As also noted above, one advantage of the disclosed MIVT is that it may allow operation of a vehicle in a variety of powered modes (e.g., powered-zero mode, creeper mode, and split-path drive mode), which may utilize various combinations of engine and IVP power. For example, through the use of various clutches and/or brakes associated with one or more planetary gear sets, an MIVT may permit engine power to be disconnected from a IVT output, even while the engine continues to operate. For example, if a IVP drives a first component of a planetary gear set and an engine drives a second component of the planetary gear set, in certain embodiments and modes a clutch may disconnect the operating engine from the second component and a brake may stop rotation of a third component of the gear set, thereby allowing delivery of power solely from the IVP through the gear reduction of the planetary gear set. In this way, for example, only electrical power (or hydraulic power, and so on) may be utilized to drive (or hold) vehicle  10  in certain modes, while combined electrical and engine power may be utilized to drive (or hold) vehicle  10  in other modes. As such, among other benefits, an MIVT may avoid certain previous limitations on the fraction of power that may diverted from an engine through an electric path (or hydraulic path, and so on). 
     Referring now to  FIG. 2 , various components of an example power train  12  are depicted. For example, engine  14  may provide mechanical power (e.g., via a rotating shaft) to MIVT  16 . Engine  14  may also provide mechanical power to IVP  18 , which may include one or more IVP machines (e.g., an electric motor and generator, or hydrostatic machine having a hydrostatic motor and associated pump). MIVT  16  may additionally receive mechanical power from IVP  18 . 
     MIVT  16  may include various clutches  20  and brakes  22 , which may be controlled by various actuators  24 . Actuators  24 , in turn, may be controlled by transmission control unit (“TCU”)  26 , which may receive various inputs from various sensors or devices (not shown) via a CAN bus (not shown) of vehicle  10 . MIVT  16  may include one or more output shafts  28   a  for transmission of mechanical power from MIVT  16  to various other components (e.g., a differential drive shaft). In certain embodiments, additional gear sets (e.g., a set of range gears) may be interposed between MIVT  16  and other parts of vehicle  10  (e.g., a differential drive shaft). In certain embodiments, IVP  18  may also provide power directly to other parts of vehicle  10  (e.g., via direct IVP drive shaft  28   b ). 
     Referring now to  FIG. 3 , various internal components of an embodiment of MIVT  16  are presented. It should be noted that the schematic representations of the transmission shown in  FIG. 3  (and also the transmissions shown in  FIGS. 5 and 7 ) illustrate example implementations in simplified form for clarity, and thus may not depict all of the components associated with the represented transmission. Engine  14  may include internal combustion engine  14   a , which may provide mechanical power directly to shaft S 1 . (As used herein, “direct” power transmission may include transmission of power by direct physical connection, integral formation, or via a simple intervening element such as an idler gear or planet gear. In contrast, for example, power transmission between a ring gear of a planetary gear set and a sun gear of the planetary gear set via a planet gear carrier (and associated planet gears) of the planetary gear set may not be considered “direct.”) IVP  18  may include electric generator  30  and electric motor  32 . Electric generator  30  may receive mechanical power via gear  36  and gear  34 , attached to shaft S 1 , and may generate electrical power for transmission to electric motor  32 . Electric motor  32  may convert the received electrical power to mechanical power and thereby rotate shaft S 2 . 
     Although specific terms such as “generator” and “motor” may be used herein to describe various example configurations, it will be understood that these (and similar) terms may be used to refer generally to an electrical machine that may be capable of operating either as a generator or as a motor. For example, electric generator  30  may sometimes operate as an electric motor, and electric motor  32  may sometimes operate as a generator. Likewise, it will be understood that the actual operating modes of other infinitely variable power sources may similarly vary from those explicitly described herein. 
     In certain embodiments, MIVT  16  may include planetary gear set  38  and double planetary gear set  40 . In certain embodiments, planetary gear set  38  and double planetary gear set  40  may be configured to sum mechanical power from engine  14   a  and IVP  18 . Through the use of one or more associated clutches and/or brakes, MIVT  16  may provide an output, in certain modes, that utilizes only power from IVP  18 . 
     Planetary gear set  38  may include, for example, planet gear carrier  42  holding planet gears  44 , which may be meshed with sun gear  46  and ring gear  48 . Drive clutch  50  may be configured to engage planet gear carrier  42  and sun gear  46  (e.g., based upon signals from TCU  26 ) in order to control power transmission between these gears. For example, in a fully engaged state, drive clutch  50  may lock planet gear carrier  42  to sun gear  46 . As depicted in  FIG. 3 , engine  14   a  may directly drive planet gear carrier  42  via shaft S 1 . Accordingly, engagement of clutch  50  may effectively lock sun gear  46  to shaft S 1  and the output of engine  14   a . Reverse brake  52  may be anchored to a fixed housing of MIVT  16  (or another feature) and may be configured to engage to stop the rotation of ring gear  48 . 
     In certain embodiments, an output component of planetary gear set  38  may directly transmit power to an input component of double planetary gear set  40 . For example, sun gear  46  may be integrally connected with ring gear  54 , thereby directly connecting an output of planetary gear set  38  (i.e., sun gear  46 ) to an input to double planetary gear set  40  (i.e., ring gear  54 ). 
     Double planetary gear set  40  may also receive power input from IVP  18 . For example, electric motor  32  may drive the rotation of shaft S 2 , along with attached gear  56 . Gear  56  may be meshed with gear  58 , mounted to shaft S 1 , and gear  58  may directly transmit power to (e.g., may be integrally formed with) sun gear  60  of double planetary gear set  40 . Sun gear  60  may mesh with planet gears  62  (one shown), which may be directly connected with planet gears  64  (one shown), both sets of planet gears  62  and  64  being carried by planet gear carrier  66 . Each of planet gears  64  may mesh with one of planet gears  78 , which in turn may mesh with ring gear  68 . Planet gear carrier  66  connect to ring gear  68  (e.g., via planet gears  64  and  78 ), and creeper brake  70  may be anchored to a fixed housing of MIVT  16  (or another feature) and configured to engage ring  68  to stop the rotation of that component. 
     Planet gear carrier  66  may provide a mechanical power output from double planetary gear set  40  for transmission of mechanical power to various parts of vehicle  10 . For example, planet gear carrier  66  may be integrally connected with output gear  72 , which may be meshed with a gear along idler shaft S 3 . In certain embodiments, additional gear box  74  (e.g., a range gear box) may be interposed between MIVT  16  and other parts of vehicle  10  (e.g., a differential drive shaft (“DDS”)) or may be included as part of MIVT  16 . In this way, for example, various gear shifts may be implemented over the baseline infinitely variable gear ratio provided by MIVT  16 . 
     In certain modes of operation, MIVT  16  (as configured in  FIG. 3 ) may provide for powered-zero and creeper modes in which only power from IVP  18  is provided to the wheels of vehicle  10 . For example, drive clutch  50  may be disengaged and brake  70  may be engaged with ring gear  68  (or, in certain configurations, with ring gear  54  (not shown)). This may, accordingly, disconnect engine  14   a  from double planetary gear set  40 , while providing a fixed gear (e.g., ring gear  68 ) around which the components of double planetary gear set  40  may rotate. Mechanical power from IVP  18  may be provided to sun gear  60 , which may drive planet carrier  66  around ring gear  68 . This may, in turn, cause rotation of output gear  72 , driven by IVP  18  but not engine  14   a , which may allow for driving of the wheels of vehicle  10  (e.g., via gear box  74 ) using only power from IVP  18 . 
     Next, in order to shift the vehicle out of this IVP-only mode, a reverse process to that described above may be executed. For example, drive clutch  50  may be engaged, thereby connecting engine  14   a  to sun gear  46  and ring gear  54 . At the same time (or nearly the same time), creeper brake  70  may be disengaged, thereby allowing double planetary gear set  40  to provide an output at gear  72  that represents a sum of the power from IVP  18  and engine  14   a . It will be understood that this selective use of two of a set of friction elements (e.g., clutches and brakes) may generally facilitate transition between various operating modes for vehicle  10 . 
     In certain embodiments, it may be beneficial to effect a transition between modes (e.g., between an all-IVP creeper mode and a combined drive mode) in particular ways. For example, with drive clutch  50  engaged, it may be possible to spin sun gear  60  (via IVP  18 ) at a speed such that ring gear  68  essentially stops, even without use of brake  70 . In order to provide for more seamless shifting between modes, it may be beneficial to shift between drive and creeper mode at such a point. In this way, for example, brake  70  may be engaged and clutch  50  may be disengaged with minimal disruption to vehicle operation. A similar seamless shift point may also be obtained for shifts from creeper to drive modes, and may represent a target point for those shift operations (and others). It will be understood, however, that in certain embodiments ramped (or other) modulation of clutch  50  (or other components) may be utilized. 
     In certain applications, it may be desirable to operate vehicle  10  in reverse, whether in creeper mode, drive mode, or otherwise. In MIVT  16  as depicted in  FIG. 3 , for example, it may be possible to engage reverse brake  52  for this purpose. 
     Referring now to  FIG. 4 , a graph is presented of the relationship between vehicle wheel speed (in kilometers per hour) and the speed of electric motor  32  (in revolutions per minute) for the configuration of MIVT  16  in  FIG. 3 . Various curves are presented for operation of vehicle  10  with various range gears (not shown) engaged within gear box  74 . It will be understood that the quantities represented in  FIG. 4  should be viewed as examples only. 
     Line  80 , for example, may represent operation of the vehicle in a creeper mode (e.g., under electrical power only). It can be seen that at zero motor speed there may be zero vehicle speed, with non-zero motor speed directly proportional to vehicle speed. In creeper mode (e.g., with brake  70  engaged, drive clutch  50  disengaged, and an A range gear (not shown) in gear box  74  engaged), vehicle  10  may accelerate to a transition point. For example, as described above, vehicle  10  may accelerate to a point at which, based on the engine speed and relevant gear ratios, ring  68  may be relatively stationary even without engagement of brake  70 . At this point (or another), brake  70  may be disengaged and clutch  50  engaged, thereby shifting the vehicle into split-mode drive relatively seamlessly. Motor  32  may then begin to decelerate along line  82 , with vehicle speed (now driven in split-path mode by both motor  32  and engine  14   a ) increasing even as the speed of motor  32  changes direction (i.e., passes from positive rotation to negative rotation). 
     Continuing, vehicle  10  may be shifted from the A range gear in gear box  74  to a higher B range gear (not shown). To continue acceleration of vehicle  10 , it may now be appropriate to switch the direction of the rotation of motor  32 , thereby jumping from negative rotation and line  82  to positive rotation and line  84 . Motor  32  may then be decelerated again, followed by a further shift to a higher C range gear in gear box  74  and a corresponding jump, for motor  32 , from line  84  to  86 . By modulating the rotation of motor  32  in this way, shifts between various range gears of gear box  74  may be accomplished with the same reduction ratio at the start of the shift (e.g., at the end of A range driving) as at the end of the shift (e.g., at the beginning of B range driving). (It will be understood that a reduction ratio may be the product of the gear ratios of the planetary gear sets  38  and  40  and the engaged gear (e.g., the A range gear) of gear box  74 .) 
     Various benefits may obtain from the configuration of  FIG. 3  (and others contemplated by this disclosure). For example, in the configuration of  FIG. 3  (and other configurations) transmission  74  may be located downstream of planetary gear sets  38  and  40 . This may allow the use of the full range of torques and speeds resulting at the output of MIVT  16  (i.e., as may result from the various combinations of the power of engine  14   a  and motor  32 ) with each range or gear of transmission  74 . For example, an electric-only mode (or any of a variety of split-path modes) may be utilized with each range or gear of transmission  74 . This may provide significant flexibility during vehicle operation. 
     Additionally, in the configuration of  FIG. 3  (and other configurations) split-mode drive may be implemented using a relatively simple planetary path, which may decrease wear, improve life, and decrease costs for MIVT  16 , among other benefits. This may be particularly useful, for example, for applications in which a majority of operating time is expected to be spent in split-path mode (e.g., for various agricultural operations conducted with vehicle  10 ). In split-path mode, for example, power from engine  14   a  may be provided through clutch  50  to ring gear  54 , and power from motor  32  being provided to sun gear  60 . These components (i.e., ring gear  54  and sun gear  60 ) may together cause rotation of planet carrier  66  (via planet gears  62 ), which in turn may cause rotation of gear  72  and the corresponding transfer of power into transmission  74 . In contrast, in an electric-only mode, power from motor  32  may be provided to sun gear  60  and then, in turn, to planet gears  62 , planet gears  64  (which may be directly connected to or integrally formed with gears  62 ), and planet gears  78 . With ring gear  68  locked by brake  70 , power may then flow from planet gears  62 ,  64  and  78  to planet carrier  66 , and so on. In this way, it will be understood, fewer gear meshes may be utilized in the split-path power mode than in the electric-only mode, which may represent a relative improvement in power transfer efficiency and may also result in a relative decrease in part wear. 
     Referring now also to  FIG. 5 , an additional example embodiment of MIVT  16  is presented. As depicted in  FIG. 5 , MIVT  16  may include planetary gear set  98  and double planetary gear set  100 . Internal combustion engine  14   a  may directly drive both a hydrostatic drive (e.g., pump  102  and motor  104 ) and shaft S 4 , and hydrostatic drive motor  104  may drive shaft S 5 . Planetary gear set  98  may include sun gear  106 , planet gear carrier  108 , and ring gear  110 . Drive clutch  112  may be configured to engage with shaft S 4  in order to connect the output of engine  14   a  to sun gear  106 . Creeper clutch  114  may be configured to engage both planet gear carrier  108  and ring gear  110 , thereby potentially locking planet gear carrier  108  and ring gear  110  together. Reverse brake  116  may be configured to engage ring gear  110 . In certain configurations, accordingly, reverse brake  116  may be utilized to reverse the output of planetary gear set  98  with respect to the output of engine  14   a.    
     Planetary gear set  98  may include an output that is directly connected (e.g., directly geared to or integral with) an input to double planetary gear set  100 . For example, as depicted in  FIG. 5 , planet gear carrier  108  may be an output component for planetary gear set  98  and may be directly geared (i.e., via gears  118  and  120 ) to planet gear carrier  122  of double planetary gear set  100 . Further, in certain configurations, this input to gear set  100  may rotate directly with another component of gear set  100 . For example, planet gear carrier  122  may be formed as an integral component with ring gear  124 , such that both components rotate in unison. 
     Motor  104  may provide an additional input to double planetary gear set  100 . For example, via shaft S 5 , motor  104  may provide input power to both of sun gears  126  and  128 . Double planetary gear set may also include, for example, ring gear  130 , and planet gear carrier  134 . 
     In this configuration, similar to the discussion above regarding the embodiment of  FIG. 3 , various clutches and brakes associated with MIVT  16  may be utilized to switch between various operating modes for vehicle  10 . For example, when drive clutch  112  is disengaged power may not be transmitted from operating engine  14   a  to planetary gear set  98  or double planetary gear set  100 . Further, with creeper clutch  114  engaged and reverse brake  116  engaged, gear  118  may be locked. Accordingly, engagement of creeper clutch  114  and reverse brake  116  may prevent rotation of both ring gear  124  and planet gear carrier  122  (although planet gears  132  may still rotate around carrier  122 ). In this way, even though engine  14   a  may be operating, double planetary gear set  100  may transmit to output gear  140  only power from motor  104  (e.g., in either a forward or a reverse creeper-mode). 
     In certain embodiments, additional transmission components may be provided to facilitate various types of vehicle operation and operational modes. For example, low clutch  136  and high clutch  138  may be included within double planetary gear set  100 , with high clutch  138  configured to engage both ring gear  130  and output gear  140 , and with low clutch  136  configured to engage both planet gear carrier  134  and output gear  140 . Accordingly, in creeper or other modes, clutches  136  and  138  may be selectively activated in order to adjust the effective total gear ratio of the two planetary gear sets  98  and  100 . 
     In certain embodiments, gear box  142  may be interposed between double planetary gear set  100  and other parts of vehicle  10  (e.g., a DDS), and may include various gears (e.g., range gears). Also in certain embodiments, the configuration represented in  FIG. 5  may allow transition between fixed gear ratios within gear box  142  (and in the context of the infinitely variable gear ratio provided by hydrostatic machine  102 ,  104 ) without necessarily changing the direction of rotation for motor  104 . For example, vehicle  10  may start operation at zero speed, with engine  14   a  disconnected from the transmission (via clutch  112 ) and with clutch  114  and brake  116  engaged. Motor  104 , accordingly, may provide the sole power to the output gear  140  (and gear box  142 ). Motor  104  may be started in the positive direction (for positive-direction creeper mode operation) or negative direction (for negative-direction creeper mode operation). Assuming, for example, an initial positive direction of travel, rotation of motor  104  (and thereby shaft S 5 ) may accelerate in the positive direction, causing sun gears  126 ,  128  to also accelerate. Initially, for example, low clutch  136  may be engaged, whereby power may be transferred from sun gear  128 , via planet gear carrier  134  to output gear  140 . Within gear box  142 , a first low range gear may be engaged, thereby completing the power transmission path from motor  104  to other parts of vehicle  10  (e.g., a differential drive shaft). 
     At a particular speed of motor  104 , depending on the particular associated gear ratios, ring gear  110  may tend to be relatively stationary, even when brake  116  is not engaged. As also noted above, this may provide a useful point at which to transition between operation modes (e.g., creeper mode and split-path mode) or various gears (e.g., range gears within gear box  142 ). Accordingly, continuing the example above, once motor  104  has accelerated through creeper mode to such a speed-matched point (or at various other times), reverse brake  116  may be disengaged and drive clutch  112  may be engaged. This may provide a mechanical transmission path for power from engine  14   a  to double planetary gear set  100 . At the same time (or nearly the same time), low clutch  136  may also be disengaged and high clutch  138  may be engaged. However, due to the configuration represented in  FIG. 5 , it may not be necessary at this point to reverse the rotational direction of motor  104  in order to continue forward acceleration of vehicle  10  (as it may be, for example, for the configuration represented in  FIG. 3 ). In certain embodiments, after engagement of clutch  112  (i.e., entry into a split-path mode), the rotational speed of motor  104  may simply be decelerated from the rotational speed at the time of the transition, with vehicle  10  accelerating accordingly. 
     Referring now to  FIG. 6 , for example, a graph is presented of the relationship between vehicle wheel speed (in kilometers per hour) and the speed of motor  104  (in revolutions per minute) for the configuration of MIVT  16  in  FIG. 5 . Various curves are presented for operation of vehicle  10  with various gears (e.g., range gears) engaged within gear box  142 . It will be understood that the quantities represented in  FIG. 6  should be viewed as examples only. 
     Line  150 , for example, may represent operation of vehicle  10  in a creeper mode (e.g., under hydrostatic power only). It can be seen that at zero motor speed there may be zero vehicle speed, with non-zero motor speed being directly proportional to vehicle speed. In creeper mode (e.g., with reverse brake  116  and creeper clutch  114  engaged, drive clutch  112  disengaged, and an A range gear (not shown) in gear box  142  engaged), the vehicle may accelerate to a transition point. In certain embodiments, this may be a point at which, based on the engine speed and relevant gear ratios, ring gear  110  may be relatively stationary even without engagement of brake  116 . At this transition point (or another), brake  116  may be disengaged and clutch  112  engaged, thereby shifting the vehicle into split-mode drive. Motor  104  may then begin to decelerate along line  152 , with vehicle speed (now driven by both motor  104  and engine  14   a ) increasing even as the speed of motor  104  changes direction (i.e., passes from positive rotation to negative rotation). 
     Continuing, the vehicle may be shifted from the former A range gear in gear box  142  to a higher B range gear (not shown). To continue acceleration of vehicle  10 , it may again be appropriate to switch the direction acceleration of the rotation of motor  104  (but not, immediately, the direction of rotation of motor  104 ), and engage an appropriate B range (with or without switching among clutches  136  and  138 ). Motor  104  may then accelerate along line  154 , with vehicle  10  accelerating accordingly. 
     Referring now to  FIG. 7 , an additional example embodiment of MIVT  16  is presented. As depicted in  FIG. 7 , internal combustion engine  14   a  may provide mechanical power to electric generator  172 , which may provide electrical power to electric motor  174  via power cable  176 . Motor  174  may (e.g., via direct gearing) drive rotation of sun gear  182  of double planetary gear set  178 . Gear set  178  may also be configured to receive mechanical power from engine  14   a  via shaft S 7 , with drive clutch  196  configured to engage both shaft S 7  and sun gear  180 . Planet gear carrier  184 , including planet gears  192  may be directly connected to (e.g., integral with) ring gear  190 , which may itself be configured to receive power from sun gear  182  via planet gear carrier  186 . Ring gear  188  may be meshed with planet gears  192 . Further, planet gear carrier  186  may form an output component of gear set  178  and may, for example, be directly connected to (e.g., integrally formed with) an input component of gear box  202 . 
     As in other embodiments discussed herein, a number of clutches and brakes within MIVT  16  (e.g., as represented in  FIG. 7 ) may allow for useful transition between various operating modes, including a creeper mode powered only by motor  174  and a split-path mode powered by both motor  174  and engine  14   a . For example, clutch  196  may engage with shaft S 7  and sun gear  180  in order to transmit power from engine  14   a  to double planetary gear set  178 . Likewise, clutch  198  may engage both ring gear  188  and planet gear carrier  184  in order to lock these components together. Finally, reverse brake  200  may engage ring gear  188  in order to stop rotation of that gear. 
     In this light, it will be understood that clutch  198 , brake  200  and clutch  196  may be selectively engaged (and disengaged) in order to provide for various modes of operation. For example, with clutch  196  disengaged and both clutch  198  and reverse brake  200  engaged, vehicle  10  may be driven under the power only of motor  174 . Likewise, other operational modes may be possible with various other configurations (e.g., various combinations in which two of clutch  198 , brake  200 , and clutch  196  are engaged). 
     Referring now also to  FIG. 8 , for example, a graph is presented of the relationship between vehicle wheel speed (in kilometers per hour) and the speed of motor  174  (in revolutions per minute) for the configuration of MIVT  16  in  FIG. 7 . Various curves are presented for operation of vehicle  10  with various gears (e.g., range gears) engaged within gear box  202 . It will be understood that the quantities represented in  FIG. 8  should be viewed as examples only. 
     Line  212 , for example, may represent operation of vehicle  10  in a creeper mode (e.g., under electrical power only). It can be seen that at zero motor speed there may be zero vehicle speed, with non-zero motor speed relating proportionally to vehicle speed. In creeper mode (e.g., with reverse brake  200  and clutch  198  engaged, drive clutch  196  disengaged, and an A range gear (not shown) in gear box  202  engaged), vehicle  10  may accelerate to a transition point. For example, vehicle  10  may accelerate to a point at which, based on the engine speed and relevant gear ratios, ring gear  188  may be relatively stationary even without engagement of brake  200 ). At this point (or another), clutch  198  may be disengaged and clutch  196  engaged, thereby shifting the vehicle into split-mode drive. At this time (or near this time) motor  174  may then reverse its direction of rotation, thereby transitioning from line  212  to  214 . Vehicle  10 , accordingly, may continue to accelerate (now driven by both motor  174  and engine  14   a ), with vehicle speed increasing even as the speed of motor  174  changes direction (i.e., passes from negative rotation to positive rotation). Similar shifts may also be effected, for example, into a B range gear (not shown) from the A range gear (not shown) by transitioning motor  174  from line  214  to line  216 , and so on. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that any use of the terms “comprises” and/or “comprising” in this specification specifies the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. Explicitly referenced embodiments herein were chosen and described in order to best explain the principles of the disclosure and their practical application, and to enable others of ordinary skill in the art to understand the disclosure and recognize many alternatives, modifications, and variations on the described example(s). Accordingly, various other implementations are within the scope of the following claims.