Abstract:
A power-split hybrid transaxle uses a chain-only final drive. In addition to transferring power from the primary axis to the differential axis, the chain provides about a 2.5:1 torque multiplication. Eliminating the planetary final drive gear set traditionally associated with a chain axis transfer reduces that axial length of the transaxle and provides more space for a power take-off unit.

Description:
TECHNICAL FIELD 
     This disclosure relates to the field of vehicle transmissions. More particularly, the disclosure pertains to an arrangement of components in a hybrid transaxle. 
     BACKGROUND 
     Hybrid powertrains include energy storage devices such as batteries which are utilized to reduce fuel consumption by capturing braking energy and by permitting more efficient use of an internal combustion engine. The engine may be shut off while the vehicle is stationary. Also, the engine may be operated at higher power setting at which it is typically more efficient and then shut off a portion of the time that the vehicle is moving. 
     One type of hybrid powertrain is an electric power-split hybrid. At low speed, a planetary gear set divides the mechanical power generated by the internal combustion engine into two power flow paths. A portion of the power is conveyed to the drive wheels by gears, chains, or other mechanical power transfer components. The remaining power is directed to an electric machine and converted into electrical power. This electric machine is typically referred to as a generator although it may also be capable converting electrical power into mechanical power. A second electric machine drives the drive wheels. This second machine is typically referred to as a traction motor although it may be capable of converting mechanical power into electrical power. In some operating modes, all electrical power from the generator flows to the traction motor. In other operating modes, some electrical power may be diverted to a battery. In yet other operating modes, the battery may supplement the electrical power. 
     In a front wheel drive hybrid transaxle, the engine crankshaft rotates about an axis that is offset from and substantially parallel to an axle axis. The transaxle includes a differential on the axle axis which divides the power between left and right half-shafts that may rotate at slightly different speeds as the vehicle turns a corner. The space available for the transaxle is restricted by the size of the engine compartment and the space occupied by the engine. 
     SUMMARY OF THE DISCLOSURE 
     A hybrid transaxle includes a planetary gear set, first and second sprockets, a chain, and a differential. The planetary gear set includes a sun gear driveably connected to a first motor, a carrier driveably connected to an input shaft, and a ring gear driveably connected to a second motor and fixedly coupled to a first sprocket. The ring gear may be driveably connected to the second motor by a second planetary gear set having a second sun gear fixedly coupled to the second motor, a second carrier fixedly coupled to the ring gear, and a second ring gear fixedly held against rotation. The second sprocket is supported for rotation about a differential axis. The chain engages the first and second sprockets. A carrier of the differential is fixedly coupled to the second sprocket. The differential may be a bevel gear differential having left and right beveled side gears coupled to left and right half shafts and meshing with beveled planet gears supported for rotation with respect to the differential carrier. 
     In another embodiment, a hybrid transaxle includes a planetary gear set, first and second sprockets, a chain, and a differential. One element of the planetary gear set, such as a sun gear, is fixedly coupled to a first motor. A second element of the planetary gear set, such as a carrier, is fixedly coupled to an input shaft. A third element of the planetary gear set, such as a ring gear, is fixedly coupled to a first sprocket. The ring gear may be driveably connected to the second motor by a second planetary gear set having a second sun gear fixedly coupled to the second motor, a second carrier fixedly coupled to the ring gear, and a second ring gear fixedly held against rotation. The differential is axially aligned with the first planetary gear set and fixedly coupled to the second sprocket. The chain engages the first and second sprockets. The differential may be a bevel gear differential having left and right beveled side gears coupled to left and right half shafts and meshing with beveled planet gears supported for rotation with respect to the differential carrier. 
     In yet another embodiment, a hybrid transaxle includes a first planetary gear set, first and second sprockets, a chain, and a differential. The first planetary gear set includes a first sun gear fixedly couple to a first motor, a first carrier fixedly coupled to an input shaft, and a first ring gear fixedly coupled to the first sprocket. A differential carrier is fixedly coupled to the second sprocket. The differential carrier may be axially aligned with the first planetary gear set. The differential may be a bevel gear differential having left and right beveled side gears coupled to left and right half shafts and meshing with beveled planet gears supported for rotation with respect to the differential carrier. The chain engages the first and second sprockets. A second planetary gear set may include a second sun gear fixedly coupled to a second motor, a second carrier fixedly coupled to the first sprocket, and a second ring gear fixedly held against rotation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a power-split hybrid transaxle having a chain and planetary final drive. 
         FIG. 2  is a schematic diagram of a power-split hybrid transaxle having a chain-only final drive. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
     A group of rotatable elements are fixedly coupled to one another if they are constrained to have the same rotational speed about the same axis in all operating conditions. Rotatable elements can be fixedly coupled by, for example, spline connections, welding, press fitting, or machining from a common solid. Slight variations in rotational displacement between fixedly coupled elements can occur such as displacement due to lash or shaft compliance. In contrast, two or more rotatable elements are selectively coupled by a shift element when the shift element constrains them to have the same rotational speed about the same axis whenever it is fully engaged and they are free to have distinct speeds in at least some other operating condition. Two rotatable elements are driveably connected if they are connected by a fixed power flow path that constrains their rotation speeds to be proportional with a predetermined speed ratio. 
       FIG. 1  is a schematic illustration of a first hybrid transaxle. Power is received mechanically from an internal combustion engine via input shaft  10 . Power is conveyed to left and right front vehicle wheels via half-shafts  12  and  14 . Input shaft  10  is fixedly connected to the carrier  16  of the power split planetary gear set  18 , which is axially located near the front of the transaxle. The sun gear  20  of the power split planetary gear set is fixedly coupled to the rotor of generator  22 , which is located at the back of the transaxle. The ring gear  24  of the power split planetary gear set is fixedly coupled to a first sprocket  26 . A second sprocket  28  is supported for rotation about the differential axis and is fixedly driveably connected to the first sprocket  26  by a chain  30 . The second sprocket  28  is driveably connected to a differential  32  by a final drive planetary gear set  34 . 
     The sun gear  34  of the final drive planetary gear set is fixedly coupled to second sprocket  28 . The carrier  36  of the final drive planetary gear set is fixedly coupled to the input of differential  32 . The ring gear  38  of the final drive planetary gear set is fixedly coupled to the transmission housing  40 . Within differential  32 , a set of bevel planet gears  42  are supported for rotation with respect to the differential input, which may be called a differential carrier. The planet gears mesh with bevel side gears  44  and  46  which are fixedly coupled to half shafts  12  and  14  respectively. 
     Traction motor  48  is fixedly driveably connected to first sprocket  26  via a torque multiplication planetary gear set  50 . The sun gear  52  of the torque multiplication planetary gear set is fixedly coupled to traction motor  48 . The carrier  54  of the torque multiplication planetary gear set is fixedly coupled to the first sprocket  26 . The ring gear  56  of the torque multiplication planetary gear set is fixedly coupled to the transmission housing  40 . 
     Placing sprocket  26  and the power split planetary gear set  18  near the front of the transmission (near the engine) provides several advantages. First, there are no components on the differential axis opposite generator  22  and traction motor  48 . This permits use of large diameter electric machines. For a given torque and power capacity, a larger diameter electrical machine tends to have a shorter axial length. Minimizing the axial length of the electrical machines minimized the overall axial length of the transaxle. Second, the power split planetary gear set  18  is opposite the final drive planetary gear set  34 , such that it does not impacts the total axial length of the transaxle. The axial length to the front side of the sprocket and chain (right side in  FIG. 1 ) is determined by the widths of final drive planetary gear set  34  and differential  32 . 
     In the transmission of  FIG. 1 , each planetary ring gear has approximately twice as many teeth as the respective sun gear. The ratio of ring gear teeth to sun gear teeth is called the beta ratio of the planetary gear set. These planetary gear sets multiply the torque provided by the engine and the traction motor. If power split gear set  18  has a beta ratio of 2, then 2/3 of the torque provided by the engine is transmitted to sprocket  26  by ring gear  24 . If torque multiplication gear set  50  has a beta ratio of 2, the carrier  54  transmits 3 times the torque generated by traction motor  48  to sprocket  26 . In the transmission of  FIG. 1 , Sprocket  26  and  28  have approximately the same number of teeth such that the sprocket and chain assembly contributes little to the torque multiplication. If the final drive planetary gear set  34  has a beta ratio of 2, it multiplies the torque delivered to sprocket  28  by a factor of 3 before transmitting the torque to differential  30 . Overall, the engine torque is multiplied by a factor of 2 and the traction motor torque is multiplied by a factor of 9. 
     Park gear  58  is fixedly coupled to ring gear  24 . A park mechanism (not shown) includes a parking pawl that selectively engages park gear  58  to hold the vehicle against motion. 
       FIG. 2  is a schematic illustration of a second hybrid transaxle. Power is received mechanically from an internal combustion engine via input shaft  10 . Power is conveyed to left and right front vehicle wheels via half-shafts  12  and  14 . Input shaft  10  is fixedly connected to the carrier  16  of the power split planetary gear set  18 , which is axially located near the front of the transaxle. The sun gear  20  of the power split planetary gear set is fixedly coupled to the rotor of generator  22 , which is located at the back of the transaxle. The ring gear  24  of the power split planetary gear set is fixedly coupled to a first sprocket  26 ′. A second sprocket  28 ′ is supported for rotation about the differential axis and is fixedly driveably connected to the first sprocket  26 ′ by a chain  30 ′. The second sprocket  28 ′ is fixedly coupled to differential carrier  34 ′. 
     The transmission of  FIG. 2  does not utilize a final drive planetary gear set between the driven sprocket  28 ′ and the differential  32 . Instead, sprocket  28 ′ is approximately 2.5 times larger than sprocket  26 ′ such that the chain and sprocket assembly provides substantial torque multiplication. The beta ratio of torque multiplication gear set  50 ′ may be increased to make up the difference. If torque multiplication gear set  50 ′ has a beta ratio of 3, then 10 times the torque of traction motor  48  is transmitted to differential  32 . If power split gear set  18  has a beta ratio of 2, then 5/3 of the torque provided by the engine is transmitted to differential  32 . 
     Eliminating final drive planetary gear set  34  reduces the axial length of the transaxle along the differential axis. This is particularly advantageous in small front wheel drive vehicles that tend to have very narrow engine compartments. This is also particularly advantageous in all wheel drive vehicles because more space is available for a power take-off unit to divert torque from differential  32  to the rear wheels. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.