Abstract:
A variable power split device having radially inner and outer races, each comprising at least two axially spaced parts. A plurality of planetary members are arranged for rolling contact between the races and a planet follower carrier engages the planetary members. A first rotatable power element spindle connects with the planet follower to couple power between the planet follower carrier and a first power element. A second rotatable power element spindle connects with the inner race to couple power between the inner race and a second power element. A third rotatable power element spindle connects with the outer race to couple power between the outer race and a third power element. Means for adjusting axial separation adjust separation of the axially spaced parts of at least one of the races to vary a power split ratio between the first, second and third rotatable power element spindles.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of and claims priority to PCT Application No. PCT/GB2008/003017 titled Power Split Device and Method, filed Sep. 5, 2008, which claims priority to Great Britain Application No. 0717354.5, filed Sep. 7, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power split device and method. 
     2. Related Art 
     In multiple-power source devices, such as, for example, hybrid vehicles, arrangements exist to distribute power between elements of the vehicle. For example, it is known in a so-called “parallel” hybrid vehicle to provide a planetary gear power train which links an internal combustion engine, the driven road wheels and any electric machines. The planetary gear hybrid power train provides two power paths between the internal combustion engine and the driven road wheels. The first power path may be a mechanical coupling between the internal combustion engine and the driven wheels, whilst the second power path may be via a motor-generator and battery arrangement. 
     This approach enables the two power paths to be utilized under different conditions to improve the overall efficiency of the vehicle. However, whilst the planetary gear power train is simple and fairly efficient it suffers from a number of limitations. Accordingly, it is desired to provide an improved power split device. 
     SUMMARY OF THE INVENTION 
     According to a first aspect to the present invention there is provided a variable ratio power split device, comprising: radially inner and outer races, each comprising at least two axially spaced parts; a plurality of planetary members arranged for rolling contact between the inner and outer races; a planet follower carrier engaging with the planetary members; a first rotatable power element spindle connected with the planet follower and operable to couple power between the planet follower carrier and a first power element; a second rotatable power element spindle connected with the inner race and operable to couple power between the inner race and a second power element; a third rotatable power element spindle connected with the outer race and operable to couple power between the outer race and a third power element; and means for adjusting an axial separation of the axially spaced parts of at least one of the races to vary a power split ratio between the first, second and third rotatable power element spindles. 
     The first aspect recognizes that a limitation with the planetary gear hybrid power train mentioned above is that the elements coupled with the planetary gear hybrid power train are often not operating efficiently. For example, the internal combustion engine must always operate when the driven road wheels rotate above certain speeds which limits the potential of the vehicle to reduce emissions. Furthermore, the speed of the motor generators is directly dependent upon the speed of the internal combustion engine and/or the driven road wheels. Hence, it is unlikely that for any particular operating condition, the elements coupled with the power train can be operated efficiently. This is because the planetary gear hybrid power provides is a fixed power split ratio between the elements. 
     Accordingly, a variable ratio power split device is provided having inner and outer races, planetary members and a planet follower carrier is provided. Rotatable power element spindles are connected to each of the races and the planet follower carrier to couple power with respective power elements. By adjusting the axial separation of one of the races, the ratio of power distributed between the power element spindles is varied which improves the operability of the device. 
     In one embodiment, each of the first, second and third power elements have predetermined efficiency characteristics under predetermined operating conditions and the variable ratio power split device comprises: at least one sensor operable to determine current operating conditions; and a set-point unit operable to determine, with reference to stored data indicative of the predetermined efficiency characteristics, an axial separation of the axially spaced parts of at least one of the races to provide a power split ratio which improves an operating efficiency of at least one of the first, second and third power elements under the current operating conditions. 
     The sensors enable the current conditions to be established. By using knowledge of the characteristics of the power elements, an appropriate relationship of power distribution between the power elements can be set by adjusting the axial separation of the races. In this way, the differing operating requirements of the power element under the current operating conditions can be better balanced to improve the efficiency of at least one of the power elements. 
     In one embodiment, the device comprises: a plurality of the sensors and the set-point unit is operable to determine, with reference to the stored data indicative of the predetermined efficiency characteristics, an axial separation of the axially spaced parts of at least one of the races to provide a power split ratio which improves an operating efficiency of more than one of the first, second and third power elements under the current operating conditions. 
     Accordingly, the axial separation may be varied to provide an optimized efficiency of more than one of the power elements for the current operating conditions. It will be appreciated that in doing so the absolute optimum efficiency of one of the power elements may need to be reduced slightly in order to provide a significantly improved efficiency of one of the other power elements and thereby improve the overall efficiency of power elements coupled to the variable ratio power split device. 
     In one embodiment the set-point unit is operable to determine, with reference to the stored data indicative of the predetermined efficiency characteristics, an axial separation of the axially spaced parts of at least one of the races to provide a power split ratio which causes substantially no power to be coupled to one of the first, second and third power elements under the current operating conditions. 
     Accordingly, the axial separation may be adjusted to enable minimal power to be coupled to any of the first, second or third power elements. Power may then be distributed between the remaining two power elements without any power being provided to the third. 
     In one embodiment, the first, second and third power elements each comprise one of a prime mover, a vehicle transmission assembly and a power transmission assembly. 
     It will be appreciated that the vehicle transmission assembly may be a vehicle drive train. 
     In one embodiment, at least one of the power transmission assembly and the vehicle transmission assembly is operable to store power. 
     It will be appreciated that these assemblies may store power in a variety of ways such as, for example, mechanically, kinetically, chemically and hydraulically. 
     In one embodiment, the at least one of the power transmission assembly and the vehicle transmission assembly is operable to reapply the stored power. 
     Hence, the stored energy may be recovered from these assemblies and reused subsequently. 
     In one embodiment, the variable ratio power split device comprises a further power coupling and wherein the at least one of the power transmission assembly and the vehicle transmission assembly is operable to reapply the stored power via the further power coupling. 
     Accordingly, a separate path may exist whereby any power stored by these assemblies may be applied to each other, other than via the variable ratio power split device. For example, the power transmission assembly may store power and apply this directly to the vehicle transmission assembly via a power coupling other than by way of the planetary members to enable the power stored to be directly applied to the vehicle transmission assembly. 
     In one embodiment, the first power element comprises an internal combustion engine, the second power element comprises a vehicle transmission assembly and the third power element comprises a regenerative power assembly. 
     In one embodiment, the set-point unit is operable in any one of a number of modes and, when in a regenerative mode, is operable to determine, with reference to the stored data indicative of the predetermined efficiency characteristics, an axial separation of the axially spaced parts of at least one of the races to provide a power split ratio which causes minimal power to be coupled to the internal combustion engine and power from the vehicle transmission assembly to be provided to the regenerative power assembly at a speed which improves an operating efficiency of the regenerative power assembly under current operating conditions. 
     When in a regenerative or power storing mode, energy from the vehicle transmission assembly is diverted to the regenerative power assembly, typically to slow a vehicle, and the kinetic energy of the vehicle is then stored as potential energy in the regenerative power assembly. Typically, in such a mode it is desirable for minimal energy to be provided by the internal combustion engine, which may be inactivated during such braking. Also, it is desirable to operate the regenerative power assembly at a speed which maximizes the efficiency of this power storage. Accordingly, the axial separation of the races is adjusted in order to minimize any power being provided by or to the internal combustion engine and to operate the regenerative power assembly at a near constant efficient speed as the speed of the vehicle and hence the speed of the vehicle transmissions assembly reduces. 
     In one embodiment, the set-point unit is operable in any one of a number of modes and, when in a moving, high state of charge mode, is operable to determine, with reference to the stored data indicative of the predetermined efficiency characteristics, an axial separation of the axially spaced parts of at least one of the races to provide a power split ratio which causes power to be coupled from the internal combustion engine and the regenerative power assembly to the vehicle transmission assembly at a speed which improves an operating efficiency of at least one of the internal combustion engine and the regenerative power assembly under current operating conditions. 
     When the regenerative power assembly is in a high state of charge, there is little requirement to store any further energy in the regenerative power assembly. Accordingly, power can be utilized from both the internal combustion engine and the regenerative power supply, and applied to the vehicle transmission assembly to propel the vehicle as required. Hence, the axial separation of the races is adjusted to operate either and/or both the internal combustion engine and the regenerative power supply at a speed which improves their operating efficiency as the speed of the vehicle changes. 
     In one embodiment, the set-point unit is operable in any one of a number of modes and, when in a moving, low state of charge mode, is operable to determine, with reference to the stored data indicative of the predetermined efficiency characteristics, an axial separation of the axially spaced parts of at least one of the races to provide a power split ratio which causes power to be coupled from the internal combustion engine to the regenerative power assembly and the vehicle transmission assembly at a speed which improves an operating efficiency of at least one of the internal combustion engine and the regenerative power assembly under current operating conditions. 
     When the regenerative power assembly is in a low state of charge, any excess energy from the internal combustion energy may be converted to improve the state of charge of the regenerative power assembly. Accordingly, the axial separation of the races is adjusted to enable power to be supplied to the regenerative power supply at a speed which improves the operating efficiency of the regenerative power supply and/or the internal combustion engine. 
     In one embodiment, the set-point unit is operable in any one of a number of modes and, when in a zero emissions mode, is operable to determine, with reference to the stored data indicative of the predetermined efficiency characteristics, an axial separation of the axially spaced parts of at least one of the races to provide a power split ratio which causes minimal power to be coupled to the internal combustion engine and power from the regenerative power assembly to be provided to the vehicle transmission assembly at a speed which improves an operating efficiency of the regenerative power assembly under current operating conditions. 
     Hence, when it is desired to emit no emissions from the internal combustion engine, the axial separation of the races is set such that minimal power is provided from the internal combustion energy and the power for the vehicle transmission assembly is provided by the regenerative power assembly. Hence, the regenerative power assembly is operated at a speed which maximizes the efficiency of the power provided by the regenerative power assembly based on the speed of the vehicle transmission assembly, whilst minimizing any power from the internal combustion energy, which may be switched off. 
     In one embodiment, the set-point unit is operable in any one of a number of modes and, when in a stationary, low state of charge mode, is operable to determine, with reference to the stored data indicative of the predetermined efficiency characteristics, an axial separation of the axially spaced parts of at least one of the races to provide a power split ratio which causes minimal power to be provided to the vehicle transmission assembly and power from the internal combustion engine to be provided to the regenerative power assembly at a speed which improves an operating efficiency at least one of the internal combustion engine and the regenerative power assembly under current operating conditions. 
     When the regenerative power assembly is in a low state of charge and the vehicle is not moving, energy from the internal combustion energy may be converted to improve the state of charge of the regenerative power assembly. Accordingly, the axial separation of the races is adjusted to enable power to be supplied to the regenerative power supply at a speed which improves the operating efficiency of the regenerative power supply and/or the internal combustion engine. 
     In one embodiment, the set-point unit is operable in any one of a number of modes and, when in a moving, low state of charge mode, is operable to determine, with reference to the stored data indicative of the predetermined efficiency characteristics, an axial separation of the axially spaced parts of at least one of the races to provide a power split ratio which causes minimal power to be provided to the regenerative power assembly and power from the internal combustion engine to be provided to the vehicle transmission assembly at a speed which improves an operating efficiency of the internal combustion engine under current operating conditions. 
     When the regenerative power assembly is in a low state of charge, there is excess energy available in the regenerative power assembly. Accordingly, power can only be utilized from the internal combustion engine and applied to the vehicle transmission assembly to propel the vehicle as required. Hence, the axial separation of the races is adjusted to operate the internal combustion engine at a speed which improves its operating efficiency as the speed of the vehicle changes. 
     In one embodiment, the variable ratio power split device comprises: a transmission component and wherein at least one of the first, second and third rotatable power element spindles are connected with the transmission component. 
     Accordingly, a component may be provided between the power element spindles and the power elements. 
     In one embodiment, the transmission component comprises at least one of a gear train, a clutch and a brake. 
     In one embodiment, the first and second power element spindles are concentrically rotatable. 
     Providing concentrically rotatable spindles achieves a simple and compact construction of the variable ratio power split device whilst enabling power to be distributed between each the three power elements. 
     In one embodiment, the variable ratio power split device comprises: the first power element connected with the first rotatable power element spindle; the second power element connected with the second rotatable power element spindle; and the third power element connected with the first third power element spindle. 
     According to a second aspect of the present invention, there is provided a method of varying power, comprising the steps of: arranging a plurality of planetary members for rolling contact between radially inner and outer races, each race comprising at least two axially spaced parts; engaging a planet follower carrier with the planetary members; connecting the planet follower carrier, the inner race and the outer race with a respective one of a first power element, a second power element and a third power element; and adjusting an axial separation of the axially spaced parts of at least one of the races to vary a power split ratio between the first, second and third rotatable power element spindles. 
     In embodiments, there are provided method steps performed by the corresponding features of the first aspect. 
     Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  illustrates a hybrid vehicle incorporating a power split device according to one embodiment; 
         FIG. 2  illustrates the mechanical arrangement of the power split device of  FIG. 1  in more detail; 
         FIG. 3  illustrates a set-point unit of the power split device; 
         FIGS. 4   a  to  4   f  illustrate schematically power flows of the power split device when operating in different modes; 
         FIG. 5  is a graph showing an example relationship between the rotational speed of the components of the power split device when operating at any one of a number of different gearing ratios; 
         FIG. 6  illustrates an example power flow on each component of the power split device; 
         FIG. 7  shows an example motor efficiency characteristic a typical permanent magnet motor/generator; 
         FIG. 8  shows an example generator efficiency characteristics of the typical permanent magnet motor/generator; and 
         FIG. 9  illustrates an example efficiency characteristic of a typical internal combustion engine. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, numerous specific details are set forth in order to provide a more thorough description of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well-known features have not been described in detail so as not to obscure the invention. 
       FIG. 1  illustrates power elements of a typical hybrid vehicle, generally  10 . The hybrid vehicle  10  is powered by an internal combustion engine  20  and a regenerative power assembly comprising a first motor generator  40 , a battery  50  and a second motor generator  60 . The internal combustion engine  20  is coupled with a power split device  30 . Also coupled with the power split device  30  is the regenerative power assembly and a vehicle drive unit  80 , such as the road wheels. The first motor generator  40  is coupled with the power split device  30  and the battery  50 . The battery  50  is coupled with the second motor generator  60 . The second motor generator  60  is coupled via a further power transfer path with the vehicle drive unit  80 . 
     The power split device  30  controls the distribution of power between the internal combustion engine  20 , the first motor generator  40  and the vehicle drive unit  80  by varying an axial separation of race components of the power split device  30 , as will be explained in more detail below. The power split device  30  is controlled by a set-point unit  130  which determines the axial separation of the race components and thus the power split ratio between the power elements. The power split device  30  is operated in any one of a number of different operating modes which are selected based upon the current operating condition of the vehicle and the demands of the driver, as will also be explained in more detail below. The set-point unit  130  controls the power distribution of the power split device  30  in order to maximize the overall efficiency of the hybrid vehicle  10  by operating the power elements as near to their optimal efficiency for the current conditions as is possible. Hence, the power split device  30  can be considered to be analogous to an epicyclic transmission that can vary the ratio between each of the three components to allow each of these components to operate nearer to their respective optimal efficiencies. In other words, the power split device  30  operates as a floating three element epicyclic transmission having a variable ratio. Utilizing the power split device  30  as an epicyclic transmission allows the operating envelope to be extended due to the variable ratio capacity of the power split device. Also, the combination of the internal combustion engine  20 , the vehicle drive unit  80 , the regenerative power assembly and the power split device  30  can be considered to be analogous to an infinitely variable transmission. 
     Although the power elements of the hybrid vehicle  10  have been shown schematically as being coupled with each other, it will be appreciated that transmission components may be provided therebetween in order to provide for further power transmission control. 
       FIG. 2  schematically illustrates in more detail the mechanical configuration of key components of the power split device  30 . The power split device  30  comprises a radially inner race  100 , a radially outer race  110  and, typically, three planetary members in rolling contact with both the inner race  100  and outer race  110 . The planetary members each engage with a planet follower (not shown). The inner race  100  is comprised of two axially spaced components which are axially moveable relative to each other. Similarly, the outer race  110  is composed of two axially spaced components, also axially moveable relative to each other. Varying the axial separation of the components of the inner race  100  and/or the outer race  110  causes the planetary members to move radially within these races and varies the gear ratio of the power split device. Such an arrangement is shown generally in WO 99/35417. However, in the present arrangement, the planets followers (not shown) are coupled via a engine spindle  125  with the internal combustion engine  20 , the inner race  100  is coupled via a motor generator spindle  105  with the motor generator  40  and the outer race  110  is coupled via a drive spindle  115  with the vehicle drive unit  80 . Hence, each of the inner race, outer race and planets are free to rotate, rather than having at least one fixed component. In this arrangement, the engine spindle  125  and the motor generator spindle  105  are arranged to rotate concentrically. This provides a particularly compact and efficient arrangement. Also, as described in more detail below, the spindles  105 ,  11 ,  125  may be coupled with its associated power element via a transmission component such as a gear train, a clutch and/or a brake. 
     The power split device  30  also comprises an actuator  180  operable to vary the axial separation of the components of the outer race  110 . The components of the inner race  100  are resiliently sprung to vary their axial separation in response to pressure from the planetary members  120 , which varies the gear ratio of the power split device  30 . In particular, the inner race  100  comprises two race components  100 A,  100 B which are engaged to the motor generator spindle  105  by means of a coupling comprising a helical interengagement in the form of a screw threaded engagement. The two race components  100 A,  100 B have oppositely handed threads so that a relative rotation of the motor generator spindle  105  and two race components  100 A,  100 B in one directional sense will cause the two components to be displaced towards one another whereas axial separation of the two race components  100 A,  100 B of the inner race  100  occurs where there is relative rotation between them and the motor generator spindle  105  in the opposite directional sense. The actuator  180  controls the axial separation of the components in response to a set-point signal provided by the set-point unit  130  and described in more detail below. 
     Hence, it can be seen that power can be distributed by the races  100 ,  110  and planetary members  120  of the power-split device  30  between the internal combustion engine  20 , the motor generators  40  and the drive unit  80  via their respective spindles. Varying the axial separation of the components varies the ratio of power distribution between these components. 
       FIG. 3  illustrates the set-point unit  130  in more detail. The set-point unit  130  is typically implemented as a microprocessor having associated memory or as a state machine. The set-point unit  130  receives a number of inputs from sensors within the hybrid vehicle  10  and outputs a set-point signal which controls the axial separation of the components of the outer race  110 , an internal combustion engine control signal which controls the load of the internal combustion engine  20  and a motor generator engine control signal which controls the load of the motor generator  40  to maximize the efficiency of the hybrid vehicle  10 . Among the sensory inputs provided to the set-point unit  30  include the speed of the internal combustion engine  20 , the speed of the motor generator  40 , and the speed of the hybrid vehicle  10 . Also provided to the set-point unit is the current state of charge of the battery  50 , as well as the current engine fuelling arrangements for the internal combustion engine  20 . Additionally, the set-point unit  130  is provided with details of the current demand being made by the driver of the hybrid vehicle  10 , such as whether the driver is requesting more, the same or less power, as well as whether the driver wishes to slow the vehicle by braking, these signals are typically from accelerator pedal position sensors and brake pedal force sensors. 
     The set-point unit  130  executes an algorithm which determines an optimal axial separation of the components of the outer race  110 , together with an internal combustion engine loading and/or a motor generator engine loading, where appropriate, to improve the efficiency of the hybrid vehicle  10  under the current operating conditions. 
       FIGS. 4   a  to  4   d  illustrate different operating modes of the power split device  30 . The operating mode is determined based on the sensor information provided to the set-point unit  130 . 
       FIG. 4   a  illustrates the power flow during a regenerative braking mode. This mode is sensed when the driver demand input to the set-point unit  130  indicates that the driver wishes to slow the vehicle and the engine fuelling demand is at a minimum. When in this mode, the set-point unit  130  determines the current rotation speed of the drive spindle  115  based on the vehicle speed information and utilizes an algorithm to determine an axial separation of the components of the outer race  110  to provide a gear ratio which provides a substantially zero rotation speed for the engine spindle  125  whilst driving the motor generator spindle  105  at a speed which maximizes the generator efficiency based on the efficiency characteristics as shown in, for example,  FIG. 8 . The set-point unit  130  outputs a set-point signal to the actuator  180  to achieve this axial separation. As the vehicle slows the algorithm constantly adjusts the axial separation of the race components and in so doing adjusts the gear ratio to best satisfy these demands as closely as possible. In this way, substantially no power is provided to the internal combustion engine  20  during braking and the power from the vehicle drive unit  80  is transferred to the motor generator  40  at a speed which optimizes the efficiency of the motor generator  40 . Hence, during regenerative braking minimal power is provided to the internal combustion engine  20  and instead maximum power is transferred to the motor generator  40  for storage in the battery  50 . 
       FIG. 4   b  illustrates the power flow during a moving, high state of charge mode. This mode is sensed when the driver demand input to the set-point unit  130  indicates that the driver wishes power to be applied to the vehicle drive unit  80 , the vehicle speed information indicates that the vehicle speed is relatively high and the battery state of charge information indicates excess energy is available from the battery  50 . When in this mode both the internal combustion engine  20  and the motor generator  40  are utilized to provide power to the drive unit  80 . Hence, the set-point unit  130  determines the internal combustion engine speed and the motor generator speed as well as the vehicle speed and utilizes an algorithm to optimize the efficiency of the internal combustion engine  20  and the motor generator  40  to achieve the desired vehicle speed. This is achieved by varying the axial separation of the race components of the outer race  110  in order to vary the power provided by both the internal combustion engine  20  and the motor generator  40 , together with varying the load of the internal combustion engine  20  and the load of the motor generator  40  in order to operate these at close to their optimal efficiency. The set-point unit  130  outputs a set-point signal to the actuator  180  to achieve this axial separation, together with an internal combustion engine control signal to achieve the desired load of the internal combustion engine  20  and/or a motor generator engine control signal to achieve the desired load of the motor generator  40 , where appropriate. The internal combustion engine control signal is utilized by an internal combustion engine control unit (not shown) to control the internal combustion engine load using its throttle, fuel injection of other means depending on its type. The motor generator control signal is utilized by a motor generator control unit (not shown) to control the motor generator load, typically by controlling motor current. By varying the axial separation of the components of the outer race  110  the operating speed of the internal combustion engine  20  and the motor generator  40  can be changed such that they operate closer to the most efficient speeds. Clearly, where the demands are such that both cannot possibly be operated at their most efficient then the algorithm may apply weightings to favor operating either the internal combustion engine  20  or the motor generator  40  at their most efficient speeds. In general, where high fuel efficiency is desired, the algorithm will favor operating the internal combustion engine  20  at its most efficient point. 
       FIG. 4   c  illustrates the power flow during a moving, low state of charge mode. This mode is sensed when the driver demand input to the set-point unit  130  indicates that the driver wishes power to be applied to the vehicle drive unit  80  and the battery state of charge information indicates the battery  50  is low on energy. When in this mode the internal combustion engine  20  is utilized to provide power to the drive unit  80  and the motor generator  40 . Accordingly, the set-point unit  130  changes the axial separation of the race components of the outer race  110  to operate the internal combustion engine  20  at its most efficient speed and to divert some of the excess power away from the drive unit  80  and into the motor generator  40 . This is achieved by varying the axial separation of the race components of the outer race  110  in order to vary the power provided by the internal combustion engine  20  to the vehicle drive unit  80  and the motor generator  40 , together with varying the load of the internal combustion engine  20 . The set-point unit  130  outputs a set-point signal to the actuator  180  to achieve this axial separation, together with an internal combustion engine control signal to achieve the desired load of the internal combustion engine  20 . Although the algorithm will seek to drive the motor generator  40  at the most efficient speed possible, once again a weighting will typically be applied to favor operating the internal combustion engine  20  at its most efficient speed. In this way, it can be seen that the axial separation of the race components of the outer race  110  of the power split device  30  can be varied to divert power from the internal combustion engine  20  when operating at its most efficient point and into the regenerative power assembly. 
       FIG. 4   d  illustrates the power flow during a zero emission mode. This mode is sensed when the driver demand input to the set-point unit  130  indicates that the driver wishes power to be applied to the vehicle drive unit  80 , the vehicle speed information indicates that the vehicle speed is relatively low and the battery state of charge information indicates excess energy is available from the battery  50 . When in this mode the motor generator  40  is utilized to provide power to the drive unit  80 . Accordingly, the set-point unit  130  changes the axial separation of the components of the outer race  110  to enable the internal combustion engine  20  to be switched off and power to be supplied from the motor generator  40  instead. The set-point unit  130  outputs a set-point signal to the actuator  180  to achieve this axial separation, together with a motor generator engine control signal to achieve the desired load of the motor generator  40 . The axial separation will be set to attempt to operate the motor generator  40  at the most efficient speed possible for the current conditions. The variable ratio provided by the power split device  30 , enable the motor generator  40  to be utilized to propel the hybrid vehicle  10  for a much wider range of speeds than would otherwise be possible. 
       FIG. 4   e  illustrates the power flow during a stationary, low state of charge mode. This mode is sensed when the driver demand input to the set-point unit  130  indicates that the vehicle  10  is stationary and the battery state of charge information indicates the battery  50  is low on energy. When in this mode the internal combustion engine  20  is utilized to provide power to the motor generator  40 . Accordingly, the set-point unit  130  changes the axial separation of the race components of the outer race  110  to operate the internal combustion engine  20  at its most efficient speed and supply the excess power into the motor generator  40  with substantially no power being supplied to the vehicle drive unit  80 . This is achieved by varying the axial separation of the race components of the outer race  110  in order to vary the power provided by the internal combustion engine  20  to the vehicle drive unit  80  and the motor generator  40 , together with varying the load of the internal combustion engine  20 . The set-point unit  130  outputs a set-point signal to the actuator  180  to achieve this axial separation, together with an internal combustion engine control signal to achieve the desired load of the internal combustion engine  20 . Although the algorithm will seek to drive the motor generator  40  at the most efficient speed possible, once again a weighting will typically be applied to favor operating the internal combustion engine  20  at its most efficient speed. In this way, it can be seen that the axial separation of the race components of the outer race  110  of the power split device  30  can be varied to provide power from the internal combustion engine  20  when operating at its most efficient point into the regenerative power assembly. 
       FIG. 4   f  illustrates the power flow during a moving, low state of charge mode. This mode is sensed when the driver demand input to the set-point unit  130  indicates that the driver wishes power to be applied to the vehicle drive unit  80 , the vehicle speed information indicates that the vehicle speed is relatively high and the battery state of charge information indicates no excess energy is available from the battery  50 . When in this mode only the internal combustion engine  20  is utilized to provide power to the drive unit  80 . Hence, the set-point unit  130  determines the internal combustion engine speed as well as the vehicle speed and utilizes an algorithm to optimize the efficiency of the internal combustion engine  20  to achieve the desired vehicle speed. This is achieved by varying the axial separation of the race components of the outer race  110  in order to vary the power provided by the internal combustion engine  20 , together with varying the load of the internal combustion engine  20  in order to operate this at close to its optimal efficiency. The set-point unit  130  outputs a set-point signal to the actuator  180  to achieve this axial separation, together with an internal combustion engine control signal to achieve the desired load of the internal combustion engine  20 . By varying the axial separation of the components of the outer race  110 , the operating speed of the internal combustion engine  20  can be changed such that it operates closer to its most efficient speed. 
       FIG. 5  illustrates the speed ratio relationship between elements of a typical power split device  30 . A traditional floating epicyclic gear train has 3 elements that rotate about the principle axis of the transmission, the sun, the carrier and the annulus. The speed of these elements are related to each other by the following relationship:
 
ω sun =ω carrier (1 +i )− iω   annulus,  
 
where the epicyclic ratio is specified as:
 
     
       
         
           
             i 
             = 
             
               
                 
                   D 
                   a 
                 
                 
                   D 
                   s 
                 
               
               . 
             
           
         
       
     
     For a typical existing hybrid vehicle, the epicyclic ratio is around 78/30=2.6. Similarly the torque relationship is as follows: 
               Tq   sun     =       Ft   sun     ⁢     R   sun     ⁢     N   planets                     Ft   annulus     =     Ft   sun                   Tq   annulus     =       Ft   sun     ⁢     R   annulus     ⁢     N   planets                     Tq   carrier     =       -     (     2   ⁢           ⁢     Ft   sun       )       ⁢     (         R   annulus     +     R   sun       2     )     ⁢     N   planets             
where N planets  is the number of planet elements, Ft is the tooth/traction force and R subscripts  are the radii of the specific geometry described by the subscript.
 
     Hence: 
     
       
         
           
             
               
                 Tq 
                 sun 
               
               
                 R 
                 sun 
               
             
             = 
             
               
                 
                   Tq 
                   annulus 
                 
                 
                   R 
                   annulus 
                 
               
               = 
               
                 
                   - 
                   
                     Tq 
                     carrier 
                   
                 
                 
                   
                     R 
                     annulus 
                   
                   + 
                   
                     R 
                     sun 
                   
                 
               
             
           
         
       
     
     For the power split device  30 , the equation relating the speed of each element may be derived as: 
                 ω   carrier     =       (         R     cont   ,   in       ⁢     ω   in       +           R     planet   ,   in       ⁢     R     cont   ,   out           R     planet   ,   out         ⁢     ω   outer         )       (       R     cont   ,   in       +         R     planet   ,   in       ⁢     R     cont   ,   out           R     planet   ,   out           )         ,         
where the values of radius all vary depending on the specific design geometry and current instantaneous speed ratio of the power split device  30 . The torque relationships on each element of the power split device  30  are as follows:
 
     
       
         
           
             
               Tq 
               in 
             
             = 
             
               
                 Ft 
                 in 
               
               ⁢ 
               
                 R 
                 
                   cont 
                   , 
                   in 
                 
               
               ⁢ 
               
                 N 
                 planets 
               
             
           
         
       
       
         
           
             
               Ft 
               out 
             
             = 
             
               
                 R 
                 
                   planet 
                   , 
                   out 
                 
               
               = 
               
                 
                   Ft 
                   in 
                 
                 ⁢ 
                 
                   R 
                   
                     planet 
                     , 
                     in 
                   
                 
               
             
           
         
       
       
         
           
             
               Tq 
               out 
             
             = 
             
               
                 Ft 
                 out 
               
               ⁢ 
               
                 R 
                 
                   cont 
                   , 
                   out 
                 
               
               ⁢ 
               
                 N 
                 planets 
               
             
           
         
       
       
         
           
             
               Tq 
               carrier 
             
             = 
             
               
                 - 
                 
                   ( 
                   
                       
                   
                   ⁢ 
                   
                     
                       Ft 
                       in 
                     
                     + 
                     
                       Ft 
                       out 
                     
                   
                   ) 
                 
               
               ⁢ 
               
                 R 
                 orbit 
               
               ⁢ 
               
                 N 
                 planets 
               
             
           
         
       
       
         
           
             
               
                 Tq 
                 in 
               
               
                 R 
                 
                   cont 
                   , 
                   
                       
                   
                   ⁢ 
                   in 
                 
               
             
             = 
             
               
                 
                   
                     Tq 
                     out 
                   
                   ⁢ 
                   
                     R 
                     
                       planet 
                       , 
                       out 
                     
                   
                 
                 
                   
                     R 
                     
                       cont 
                       , 
                       out 
                     
                   
                   ⁢ 
                   
                     R 
                     
                       planet 
                       , 
                       in 
                     
                   
                 
               
               = 
               
                 
                   - 
                   
                     Tq 
                     carrier 
                   
                 
                 
                   
                     R 
                     orbit 
                   
                   ⁡ 
                   
                     ( 
                     
                       1 
                       + 
                       
                         
                           R 
                           
                             planet 
                             , 
                             
                               i 
                               ⁢ 
                               n 
                             
                           
                         
                         
                           R 
                           
                             planet 
                             , 
                             out 
                           
                         
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
     It will be appreciated that utilizing the power split device  30  as an epicyclic transmission allows the operating envelope of the hybrid vehicle to be extended further due to the variable ratio capability of the power split device  30 . 
     Each of the planes of the graph in  FIG. 5  shows the relationship at one particular discrete ratio. The graph shows how the iso-ratio conditions pass through each other when all the components become synchronous. The upper and lower planes of the graph show a complete envelope of relationships possible at different axial separations of the races. The ratio of the power split device  30  dictates the tilt angle of each of the iso-ratio planes. This information is stored by the set-point unit  130  and is utilized by its algorithms. It will be appreciated that a fixed ratio epicyclic gear train would only be able to achieve one of these planes, rather than the operating envelope contained within the upper and lower bounding planes. 
       FIG. 6  illustrates the power flow on each component of the power split device  30  assuming a unity torque applied to the inner race  100 . Clearly the power split device  30  acts as a summing junction for power transmitted (as would be the case with a fixed ratio epicyclic), although the variable ratio nature of the power split device  30  allows this power split to be varied significantly thus allowing more or less engine power to be delivered to either the electric elements or mechanical elements of the hybrid vehicle  10 . Once again, this information is stored by the set-point unit  130  and utilized by its algorithms. 
       FIG. 7  illustrates typical motor efficiency characteristics or the motor generator  40 . There is a rapid degradation in electric machine efficiency it is operated away from ideal operating points. Significant overall vehicle efficiency may be achieved by controlling the electric machines to operate at speed and torque conditions that improve their efficiencies. The variable ratio provided by the power split device  30  allows this improved control to be achieved. These characteristics are also stored by the set-point unit  130  and utilized by its algorithms. 
       FIG. 8  illustrates generator efficiency characteristics of the motor generator  40 . This information is stored by the set-point unit  130  and utilized by its algorithms. 
       FIG. 9  illustrates typical efficiency characteristics of the internal combustion engine  20 . The efficiency is described by Brake specific fuel consumption contours. As can be seen the internal combustion engine  20  operates near its peak efficiency when at low speed and high load. Again, this information is stored by the set-point unit  130  and utilized by its algorithms. 
     As can be seen, this arrangement can be utilized to enable a hybrid vehicle  10  to operate at higher road speeds without having to activate the internal combustion engine  20  through the use of the variable gearing provided by the power split device  30 . This reduces the amount of carbon emissions made by the vehicle. Also, the variable ratio nature of the power split device  30  enables the internal combustion engine  20  and machine generator  40  to be operated under conditions which better match each units individual characteristics and improve their efficiency. 
     Although particular embodiments have been described herein it would be apparent that the invention is not limited thereto and that many modifications and additions may be made within the scope of the invention as defined in the claims. For example, various combinations of features from the following dependent claims could be made with features of the independent claims without departing from the scope of the present invention.