Patent Publication Number: US-2013237360-A1

Title: Power split electric all-wheel drive

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a drivetrain for an automobile, and more particularly to a drivetrain for an all-wheel drive vehicle. 
     High-performance automobiles often utilize a rear-wheel drive drivetrain or powertrain. A conventional manual-shift rear-wheel drive drivertrain may include a flywheel on the rear of the engine, a clutch, a transmission, a driveshaft, a differential and then axles to the wheels. A conventional automatic-shift rear-wheel drive drivertrain will typically not include a flywheel or a clutch, but instead will include a flex plate and torque converter. Many high performance automobiles include an engine capable of producing torque that far exceeds the tractive limit of the rear tires. 
     To improve overall performance, a number of high-performance automobiles have been provided with all-wheel drive or four-wheel drive. Four-wheel drive has the potential to improve handling by increasing traction between the automobile and the road. In one conventional system, four-wheel drive is incorporated into an automobile by providing a transfer case that receives power from the transmission and sends it to both the front and rear wheels. Although effective in providing four-wheel drive, a transfer case adds weight and cost to automobile. Further, a transfer case and the related drivetrain components, such as the driveshaft/propeller, can be rather difficult to package into an automobile. In view of these disadvantages, some systems have been developed that do not include a transfer case. For example, in one existing system, an optional connectable four-wheel drive is added to an otherwise rear-wheel drive vehicle. In this system, a generally conventional rear-wheel drive drivetrain is mounted to the rear of the engine (coupled to the rear end of the crank) to provide power to the rear wheels. In addition, an optionally connectable front drivetrain is mounted to the front of the engine (coupled to the front end of the crank) to drive the front wheels when desired. The front drivetrain includes a clutch, a gear drive and a front differential. The clutch allows the front drivetrain to be disconnected from engine when it is desirable to operate the automobile in rear-wheel drive mode. 
     There is continued growth in the integration of electric machines into automobile applications. At present time, there are a variety of existing systems in which one or more electric machine are incorporated into an automobile to, among other things, provide power to the drivetrain, engine stop-start, cold start assist, regenerative braking and/or through-the-road regeneration. For example, in the context of two-wheel drive hybrid vehicles, an electric machine may be coupled to the engine drivetrain by a power split arrangement, such as a planetary gear set. The power split arrangement allows the electric machine to, among other things, drive the two wheels or add supplemental power to the two driven wheels. In another existing system, an electric machine is coupled to the crank shaft by a belt. In this type of system, the electric machine may be used to start the engine, assist the engine during cold starts, assist the engine while driving and function as a generator. 
     Electric machines are also used to implement four-wheel drive in some applications. For example, in at least one existing system, the engine and associated drivetrain are used to supply power to the rear wheels and a pair of electric machine is used to supply power to each of the front wheels. 
     SUMMARY OF THE INVENTION 
     The present invention provides a front drivetrain capable of providing, among other things, all-wheel drive to a rear-wheel drive automobile. The front drivetrain is primarily intended for use with north/south oriented engines in which the rear drivetrain is coupled to and driven by the rear end of the crank shaft. The front drivetrain is coupled to the front end of the crank shaft and includes a power split planetary gear arrangement that couples an electric machine or hydraulic motor pump to the drivetrain. In one embodiment, the planetary gear arrangement includes a sun gear coupled to the crank shaft, a planetary gear set with a carrier coupled to the driveshaft, a ring gear coupled to the electric machine. The driveshaft may be connected to left and right front axles by a bevel gear set and a differential. In an alternative embodiment, sun gear may be coupled to the electric machine and the ring gear can be coupled to the crank shaft. In this alternative embodiment, the planetary gear set remain coupled to the driveshaft. 
     In one embodiment, a decoupler is disposed between the crank shaft and the planetary gear arrangement. The decoupler may be a spring coupler. 
     In one embodiment, a disconnecting clutch may be disposed between the crank shaft and the planetary gear arrangement. The clutch may be operated to selectively disconnect the front drivetrain from the engine. The disconnecting clutch may be combined with a decoupler. For example, in one embodiment, the front drivetrain includes a spring coupler with an integrated disconnecting clutch. 
     In one embodiment, the crank shaft is connected to the sun gear, the electric machine is coupled to the ring gear and the planetary carrier is coupled to the bevel gear to drive the front wheels. In such embodiments, the system may include a ring gear grounding clutch that enables a direct mechanical drive from the engine to the front wheels. This mechanism may be used to provide the vehicle with “off-road” or “crawling” capabilities. In “off-road” mode, the ring gear grounding clutch may be engaged and the transmission may be placed in neutral to allow the engine to drive only the front wheels. 
     In one embodiment, the front drivetrain may include a differential disconnect. The differential disconnect may include a clutch incorporated into the differential. The differential disconnect can be operated with the disconnecting clutch to isolate a majority of the front drivetrain from the engine and the road. 
     In one embodiment, the crank shaft is connected to the ring gear, the electric machine is coupled to the sun gear and the planetary carrier is coupled to the bevel gear to drive the front wheels. In such embodiments, the system may include an engine disconnect and a ring gear grounding clutch that enables an electric vehicle mode in which the electric machine is capable of directly powering the front wheels. In EV mode, the ring gear grounding clutch may be engaged and the engine disconnect may be disconnected, thereby allowing the electric machine to power the vehicle with the engine off and the transmission in neutral. 
     In one embodiment, the present invention provides a series mode configuration in which power is supplied to the electric machine by an alternator or generator powered by the engine. In this embodiment, the system may include an alternator coupled to the crank by a belt. The output of the alternator may be supplied to the electric machine to operate the electric machine. 
     The present invention provides a simple and effective front drivetrain capable of providing four-wheel drive to an otherwise rear-wheel drive automobile. Advantageously, speed synchronization between the front wheels and the rear wheels is passively controlled by the crank and the front axle. The electric machine can be operated to provide active torque control. In embodiments that include a decoupler (e.g. a spring coupler) between the crank and the planetary gear arrangement, the crank shaft is torsionally isolated to help to prevent or minimize movement of the nodal point of the crank shaft. The present invention allows for elimination of a transfer case, front prop shaft and starter motor. The power split arrangement of the present invention provides a simple architecture and allows for the use of a smaller electric machine and/or a less powerful internal combustion engine. The present invention allows incorporation of an electric machine in various configurations that may, among other things, provide active torque control to the front wheels, starter/generator functionality, regenerative braking and regeneration through the road. In various embodiments, the present invention may also provide “off-road” mode, electric vehicle mode and series mode where it is desirable not to include a traction battery. 
     These and other features of the invention will be more fully understood and appreciated by reference to the description of the embodiments and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation showing a front drivetrain in accordance with an embodiment of the present invention. 
         FIG. 2  is a schematic representation of an alternative front drivetrain having a decoupler. 
         FIG. 3  is a schematic representation of an alternative front drivetrain having a decoupler and a disconnecting clutch. 
         FIG. 4  is a schematic representation of an alternative front drivetrain having a decoupler, a disconnecting clutch and a ring-gear grounding clutch. 
         FIG. 5  is a schematic representation of an alternative front drivetrain having a decoupler, a disconnecting clutch and a differential disconnect. 
         FIG. 6  is a schematic representation of an alternative front drivetrain in which the ring gear is coupled to the crank shaft and the sun gear is coupled to the electric machine. 
         FIG. 7  is a schematic representation of an alternative front drivetrain capable of providing electric vehicle mode similar to  FIG. 6  including a disconnecting clutch and a ring-gear grounding clutch. 
         FIG. 8  is a schematic representation of an alternative front drivetrain capable of providing series mode operation. 
         FIG. 9  is a planetary lever diagram of torque within an embodiment of the present invention. 
         FIG. 10  is a planetary lever diagram of speeds within an embodiment of the present invention. 
     
    
    
     Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited to the details of operation or to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention may be implemented in various other embodiments and of being practiced or being carried out in alternative ways not expressly disclosed herein. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items and equivalents thereof. Further, enumeration may be used in the description of various embodiments. Unless otherwise expressly stated, the use of enumeration should not be construed as limiting the invention to any specific order or number of components. Nor should the use of enumeration be construed as excluding from the scope of the invention any additional steps or components that might be combined with or into the enumerated steps or components. 
     DESCRIPTION OF CURRENT EMBODIMENTS 
     The present invention relates to a front drivetrain capable of being incorporated into a rear-wheel drive architecture to provide an automobile with all-wheel drive. The automobile may include a north/south oriented engine and may include a rear drivetrain that is coupled to the rear end of the crank shaft and provides power to the rear wheels. The present invention includes a power-split planetary gear arrangement that allows the front drivetrain to receive power from the engine and an electric machine. As described in more detail below, the present invention may be implemented in a variety of alternative embodiments. Depending on the embodiment, the planetary gear arrangement and/or the electric machine may provide the automobile with a variety of optional capabilities. For purposes of disclosure, the present invention is described in connection with a front engine layout, but the present invention may alternatively be incorporated into other north/south engine layouts, such as mid-engine layout. 
     A front drivetrain in accordance with an embodiment of the present invention is shown schematically in  FIG. 1  and generally designated  10 .  FIG. 1  shows the front drivetrain  10  incorporated into a front engine layout having a north/south oriented engine E. The engine E includes a crank shaft C that is operatively coupled to the rear drivetrain R. The rear drivetrain may be essentially any drivetrain. However, the rear drivetrain R of  FIG. 1  includes a transmission T coupled to the rear end of the crank shaft C. Although not shown, the transmission T may be operatively connected to the rear wheels using essentially any desired drivetrain components. For example, the transmission T may be coupled to the rear wheels by a driveshaft, a differential and then axles to the wheels. 
     In the embodiment of  FIG. 1 , the front drivetrain  10  generally includes a planetary gear arrangement  12  coupled to the front end of the crank shaft C, an electric machine  14  operatively engaged with the planetary gear arrangement  12 , a planetary gear output shaft  16 , a bevel gear arrangement  18  receiving power from the output shaft  16  and a pair of front axles  20 ,  22  for supplying power to the front wheels. In the embodiment of  FIG. 1 , the front axles  20 ,  22  may be joined by a differential (not shown in  FIG. 1 ) that allows the two front wheels to turn at different speeds, for example, when corning. 
     In the embodiment of  FIG. 1 , the planetary gear arrangement  12  includes a sun gear  24  coupled to the front end of the crank shaft C, a planetary gear set  26  having a carrier  28  coupled to the planetary gear output shaft  16  and a ring gear  32  coupled to the electric machine  14 . The sun gear  24  may be coupled to the front end of the crank shaft C in essentially any way. For example, the front end of the crank shaft C may be coupled to the sun gear  24  by a planetary gear input shaft (not shown). The input shaft may be welded, splined or otherwise mechanically joined to the front end of the crank shaft C. Similarly, the input shaft may be welded, splined or otherwise mechanically joined to the sun gear  24 . The planetary gear set  26  may be any of a wide variety of any planetary gear sets. For example, the planetary gear set  26  may include a plurality of planetary gears (e.g. four) engaged with the sun gear  24  at radially symmetric locations. The planetary gears may be tied together by the carrier  28  so that movement of the planetary gears results in movement of the carrier  28 . The carrier  28  may be coupled to the output shaft  16  in essentially any way. For example, the carrier  28  and output shaft  16  may be fixed together, such as by welding, or they may be coupled together mechanically, such as by splines or pins. As in a typical planetary gear arrangement, the ring gear  32  is disposed about the planetary gear set  26  and includes internal teeth that engage the external teeth of the planetary gears. In this embodiment, the ring gear  32  is also coupled to the electric machine  14 . These components may be coupled in any of a wide variety of ways. In one embodiment, the ring gear  32  may include internal teeth that interface with the planetary gear set  26  and external teeth that interface with the electric machine  14 . For example, the ring gear  32  may include an annular body with internal teeth and external teeth on opposite sides of the body. As another example, the ring gear  32  may include two joined annular bodies—one with internal teeth and the other with external teeth. 
     The front drivetrain subassembly downstream from the planetary gear arrangement  12  may be essentially any assembly of drivetrain components capable of driving the front wheels in the desired manner. In the illustrated embodiment, the subassembly includes planetary output shaft  16  which is coupled to bevel gear arrangement  18  (or miter gear arrangement) that translates rotation of the north/south oriented output shaft  16  into rotation of the east/west oriented front axles  20 ,  22 . The front drivetrain subassembly may include alternative power translating components, as desired. As noted above, the front drivetrain subassembly may include a differential (See  FIG. 5 ). For example, the differential may be integrated into the bevel gear arrangement. 
     The front drivetrain  10  may be operated in a variety of different modes. In all-wheel drive mode, the electric machine  14  may be used to provide active torque control to the front drivetrain  10 . The planetary gear arrangement  12  allows the front wheels to receive torque from the crank shaft C and the electric machine  14 .  FIG. 9  is a lever diagram the represents how a planetary gear arrangement acts a torque summing device, adding the torque of the crank shaft/sun  24  with the torque of the ring  32 /electric machine  14 . As a result, the electric machine  14  may be operated as desired to add torque to the front drivetrain  10 . 
     It is also worthwhile to note that, in this embodiment, speed synchronization is passively controlled by the crank shaft C and the front axles  20 ,  22 . More specifically, in this embodiment, the ring gear  32  is free to travel at the speed required to provide synchronization between the speed of the crank shaft C and the speed of the front axles  20 ,  22 . The speed of the sun gear  24  is dictated by the speed of the crank shaft C (i.e. the speed of the engine) and the speed of the planetary gear set  26  is dictated by the speed that the front wheels are travelling over the road. Front wheel speed is communicated back to the planetary gear set  26  through the axles  20 ,  22 , bevel gear arrangement  18 , output shaft  16  and carrier  28 . As a result, the speed of the ring gear  32  and the electric machine  14  is dictated the speed of the engine E and the front wheels.  FIG. 10  is a lever diagram showing the relationship between the various elements of the planetary gear arrangement  12  in the context of speed synchronization. As shown, the planetary gear arrangement  12  can be modeled as a lever with the sun  24 , carrier  28  and ring  32  being represented by different points along the lever. The lever can be placed on a graph in which is represented in the x-y direction. In this diagram, the speed of the ring  32  can be determined when the lever is oriented with the sun  24  and the carrier  28  at their respective speeds. The leftmost lever diagram shows the system when the vehicle is stationary and the engine is idling. If engine/sun  24  speed is increased without change the speed of the front wheels/carrier  28 , the speed of the ring  32  will be reduced. The rightmost lever diagram includes an alternative example of an up-shift in the transmission. In this example, the system before the up-shift is represented by a solid line and the system after the shift is represented by a broken line. As can be seen, the up-shift does not change the speed of the front wheels/carrier  28 , but it does reduce the speed of the crank shaft/sun  24 . As a result, the up-shift causes an increase in the speed of the ring  32  that is relative to the decrease in the speed of the crank shaft/sun  24 . 
     The electric machine  14  of  FIG. 1  can also function in different modes of operation. For example, the electric machine  14  may be used for regenerative braking on the front axles  20 ,  22 . It may also be used for “through-the-road” regeneration. The electric machine  14  may also be used to start or stop the engine or to assist the engine during cold starts or to reduce emissions. 
     An alternative embodiment of the present invention is shown in  FIG. 2 . This embodiment is essentially identical to the embodiment of  FIG. 1 , except that it includes a decoupler  40  interposed between the crank shaft C and the sun gear  24 . The decoupler  40  may be a spring coupler. In this embodiment, the spring coupler torsionally isolates the crank shaft from the front drivetrain to prevent or minimize movement of the nodal point of the crank shaft C. 
     Another alternative embodiment is shown in  FIG. 3 . In this embodiment, the front drivetrain  10  includes a disconnecting clutch  42  positioned between the crank shaft C and the sun gear  24 . The disconnecting clutch  42  allows the front drivetrain  10  to be completely disconnected from the engine E. This can reduce parasitic losses. In the illustrated embodiment, the disconnecting clutch  42  is integrated into the decoupler  40 . 
     Yet another alternative embodiment is shown in  FIG. 4 . This embodiment of the drivetrain  10  is essentially identical to the front drivetrain of  FIG. 3 , except that it also includes a ring gear grounding clutch  44 . The ring gear grounding clutch  44  may be selectively operated to ground the ring gear  32 , thereby preventing its rotation. In addition to providing the functionality discussed above in connection with the front drivetrain  10  of  FIG. 1 , this embodiment provides an “off road” or “crawling” capability. To operate in “off road” mode, the transmission T may be placed in neutral to disconnect the engine E from the rear wheels, the disconnecting clutch  42  may be operated to couple the crank shaft C to the sun gear  24  and the ring gear grounding clutch  44  may be operated to lock the ring gear  32 . At this point, the crank shaft C is coupled directly to the front wheels via the front drivetrain  10  so that operation of the engine E drives the front wheels. In this embodiment, the planetary gear assembly  12  provides a direct mechanical drive to the front wheels at a fixed gear ratio dictated by the characteristics of the gears in the planetary gear assembly  12 . 
     A further alternative embodiment is shown in  FIG. 5 . This embodiment of the drivetrain  10  is essentially identical to the front drivetrain of  FIG. 3 , except that it also includes a front differential disconnect  46 . The front differential disconnect  46  is shown schematically in the enlarged portion of the illustration. The front differential disconnect  46  may be selectively operated to disconnect the differential, front wheels and front axles  20 ,  22  from the bevel gear arrangement  18 . This allows the drive components upstream from the differential to be isolated from the road. In use, the disconnecting clutch  42  and the front differential disconnect  46  may be operated simultaneously to completely isolate the planetary gear assembly  12  and the electric machine  14  from the engine E and the road, which may eliminate spin losses. Although this embodiment includes a disconnect incorporated into the differential, the differential disconnect  46  may be replaced by other disconnect mechanism located in other positions along the drivetrain  10 . For example, as an alternative to the differential disconnect  46 , the drivetrain may include a disconnecting clutch disposed between the carrier  28  and the bevel gear arrangement  18 , such as in output shaft  16 . As another alternative, the system may include a pair of clutches that disconnect the front axles  20 ,  22  from the differential. In addition to reducing losses, this configuration may used when it is desirable to provide an electric machine power take off for driving electric accessories as discussed in more detail below. 
     The embodiments of  FIGS. 1-5  includes a planetary gear arrangement  12  in which the engine E is coupled to the sun gear  24  and the electric machine  10  is coupled to the ring gear  24 . The present invention may be implemented in other planetary gear configurations. For example,  FIG. 6  shows an alternative embodiment of the front drivetrain  10 ′. In this embodiment, the crank shaft C is connected to the ring gear  32 ′, the electric machine  14 ′ is coupled to the sun gear  24 ′ and the planetary carrier  28 ′ is coupled to the bevel gear arrangement  18 ′ to drive the front wheels. As shown, this alternative embodiment may include a decoupler  40 ′ disposed between the crank shaft C and the ring gear  32 ′. As with the previously described embodiments, the decoupler  40 ′ may be a spring coupler that torsionally isolates the crank shaft C. 
     Referring now to  FIG. 7 , the alternative front drivetrain  10 ′ of  FIG. 6  may be provided with a disconnecting clutch  42 ′ for disconnecting the planetary gear arrangement  12 ′ from the crank shaft C. For example, the disconnecting clutch may be a generally conventional clutch disposed along the driveshaft between the crank shaft C and the ring gear  32 ′. The disconnecting clutch  42 ′ may be operated to isolate the front drivetrain  10 ′ from the engine E. The disconnecting clutch  42 ′ may be integrated into or otherwise combined with the decoupler  40 ′. For example, as shown in  FIG. 7 , the front drivetrain  10 ′ may include a spring coupler with an integrated disconnecting clutch. 
     Referring again to  FIG. 7 , the alternative front drivetrain  10 ′ may include a ring gear grounding clutch  44 ′ for selectively locking the ring gear  32 ′. The ring gear grounding clutch  44 ′ enables an electric vehicle (“EV”) mode in which the electric machine  14 ′ is capable of directly powering the front wheels. In EV mode, the transmission T may be placed in neutral, the engine disconnecting clutch  42 ′ may be operated to disconnect the engine E from the ring gear  32 ′ and the ring gear grounding clutch  44 ′ may be engaged to lock the ring gear  32 ′. Once in EV mode, the electric machine  14 ′ may be operated to directly drive the automobile through the front wheels. In this way, the automobile can be driven by the electric machine  14 ′ with the engine E off. 
     The present invention can be incorporated into a series mode architecture, for example, when it is desirable to eliminate the need for a heavy and expensive traction battery.  FIG. 8  shows a schematic representation of a front drivetrain  10 ″ configured in a series mode architecture.  FIG. 8  includes reference numerals that correspond with the reference numerals of  FIG. 2 , except that each reference numeral include in  FIG. 8  is denoted with a double prime symbol. In this embodiment, the present invention can be configured so that the electric machine  14 ″ receives power from an alternator/generator  50 ″ that is driven by the engine E. In the illustrated embodiment, the system may include an alternator/generator  50 ″ coupled to the crank shaft C by a belt  52 ″. Although this embodiment includes a belt driven alternator/generator  50 ″, alternative drive configurations may be used. As in a conventional series mode architecture, the output of the alternator/generator  50 ″ may be supplied to the electric machine  14 ″ to operate the electric machine  14 ″ when desired. The system may include power conditioning circuitry, as desired. 
     Although not shown, the various embodiments of the present invention may be provided with additional features found in conventional engine, electric and hybrid architectures. For example, as noted briefly above, the present invention may be provided with an additional accessory drive that allows the electric machine to drive electrical accessories. The accessory drive may include power take off integrated into or otherwise coupled with the electric machine so that the electric machine can operate the accessory drive belt. In some applications, it may be possible to use the electric machine to affect a dampening function. For example, in some applications, the electric machine may be capable of providing active engine dampening. 
     Although described primarily in the context of an electric machine, the present invention may also be implemented with a hydraulic motor-pump in place of the electric machine. For example, the system may be provided with a conventional hydraulic motor-pump system having a hydraulic motor-pump, a working fluid, reservoir and an accumulator. 
     Directional terms, such as “front,” “rear,” “vertical,” “horizontal,” “top,” “bottom,” “upper,” “lower,” “inner,” “inwardly,” “outer” and “outwardly,” are used to assist in describing the invention based on the orientation of the embodiments shown in the illustrations. The use of directional terms should not be interpreted to limit the invention to any specific orientation(s). 
     The above description is that of current embodiments of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. This disclosure is presented for illustrative purposes and should not be interpreted as an exhaustive description of all embodiments of the invention or to limit the scope of the claims to the specific elements illustrated or described in connection with these embodiments. For example, and without limitation, any individual element(s) of the described invention may be replaced by alternative elements that provide substantially similar functionality or otherwise provide adequate operation. This includes, for example, presently known alternative elements, such as those that might be currently known to one skilled in the art, and alternative elements that may be developed in the future, such as those that one skilled in the art might, upon development, recognize as an alternative. Further, the disclosed embodiments include a plurality of features that are described in concert and that might cooperatively provide a collection of benefits. The present invention is not limited to only those embodiments that include all of these features or that provide all of the stated benefits, except to the extent otherwise expressly set forth in the issued claims. Any reference to claim elements in the singular, for example, using the articles “a,” “an,” “the” or “said,” is not to be construed as limiting the element to the singular.