Abstract:
A motor vehicle drive train assembly for achieving multiple effective ratios from a single ratio transaxle enables application of a fixed ratio transaxle, such as used in electric vehicle drive trains, to a multi-speed requirement, for example, in airport ground support equipment. Multiple configurations are enabled by varying the relative speed of the outputs through the differential of the transaxle, such as bringing one of the outputs to zero, or by using another motor to control the speed of one of the outputs.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates generally to the field of vehicles, and particularly to vehicle drive trains. 
   2. Description of the Related Art 
   Airlines use push-back tractors to move aircraft around at airports. Such tractors need to operate at low speed and high torque in order to move aircraft. However, when the tractors uncouple from the aircraft, it is important for them to be able to get back to the gate area as fast as possible, which requires operating at high speed and low torque. Tractors, and other vehicles powered by internal combustion engines, employ multi-speed gear boxes that enable an operator to deal with this two-mode operation simply by shifting gears. However, this two-mode operation remains a problem for vehicles having electric power trains because a typical gear box configuration for an electric power train application is a single speed (i.e., gear ratio) without a clutch or a gear shift. 
   BRIEF SUMMARY 
   Disclosed are methods and apparatus directed to providing a motor vehicle drive train assembly that enables application of a fixed ratio transaxle, such as used in electric vehicle drive trains, to a multi-speed requirement, especially in airport ground support equipment. 
   In one aspect, a drive train for a vehicle includes at least a first integrated power train unit comprising a motor, a first integrated power train differential assembly, a first output shaft and a second output shaft, the motor drivingly coupled to the first integrated power train differential assembly to transfer torque to each of the first and the second output shafts; at least a first axle differential assembly drivingly coupled to the first output shaft of the first integrated power train differential assembly; and a brake assembly adapted to apply a braking force at the second output shaft of the first integrated power train differential assembly. 
   In another aspect, the drive train assembly may further include a second axle differential assembly; a clutch assembly selectively operable to couple the second output shaft of the first integrated power train differential assembly to the second axle differential assembly in a disengaged condition corresponding to a four-wheel drive mode and to uncouple the second output shaft of the first integrated power train differential assembly and the second axle differential assembly in an engaged condition corresponding to a two-wheel drive mode. 
   In a further aspect, the drive train assembly may further include a second integrated power train unit comprising a second motor, a second integrated power train differential assembly, a first output shaft and a second output shaft, the second motor drivingly coupled to the second integrated power train differential assembly, wherein the first vehicle axle shaft differential assembly is drivingly coupled to the first output shaft of the second integrated power train differential assembly via the first integrated power train differential assembly. 
   In yet another aspect, a method of operating a vehicle having at least a first integrated power train unit having a differential assembly includes in a first mode, causing at least the first integrated power train unit to drive only a first of at least two axle; and in a second mode, causing at least the first integrated power train unit to drive at least the first and a second axle of the at least two axles. Causing at least the first integrated power train unit to drive only a first of at least two axle assemblies may include engaging a clutch to uncouple the second axle and the differential assembly of at least the first integrated power train unit, and applying a brake to apply a braking force between the clutch and the differential assembly of at least the first integrated power train unit. Causing at least the first integrated power train unit to drive at least the first and a second axle of the at least two axles may include disengaging a clutch to couple the second axle to the differential assembly at least the first integrated power train unit, and releasing a brake to remove a braking force from between the clutch and the differential assembly of at least the first integrated power train unit. 
   In a further aspect, a drive train for a vehicle may include means for causing at least a first integrated power train unit to drive only a first of at least two axle in a first mode; and means for causing at least the first integrated power train unit to drive at least the first and a second axle of the at least two axles in a second mode. 
   Additional novel features and advantages will be set forth in part in the description which follows, and in part will become more apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
     In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve drawing legibility. Further, the particular shapes of the elements as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the drawings. 
       FIG. 1  is a block diagram of a vehicle including an electric drive train utilizing a single electric motor/transaxle unit according to one illustrated embodiment of the present invention. 
       FIG. 2  is a block diagram of a vehicle including an electric drive train utilizing two electric motor/transaxle units for the CVT mode according to another illustrated embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following description, certain specific details are set forth in order to provide a through understanding of various embodiments of the invention. However, one skilled in the art will understand that the invention may be practiced without these details. In other instances, well-known structures associated with have not been shown or described in detail to avoid unnecessarily obscuring descriptions of the embodiments of the invention. 
   Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including, but not limited to.” 
   The headings provided herein are for convenience only and do not interpret the scope or meaning of the claimed invention. 
   The below teachings address the dilemma of two-mode operation in an electric drive train application with a conventional one speed gear box configuration without a gear shift. The teachings rely on the use of one or more differentials. A feature of differentials is that when torque is applied to an input of the differential, there are at least two outputs. The differential splits the torque from the drive motor between the outputs, so that each wheel or set of wheels receives a portion of the torque from the drive motor. All things being equal, the torque is split evenly between each wheel in the set of wheels, or between each of the set of wheels. 
     FIG. 1  shows a vehicle  4  having rear wheels  6 , front wheels  8 , and an electric drive train  10  that utilizes a single electric motor/transaxle unit  12 . It is to be understood that the housing for the electric motor/transaxle unit  12  is grounded or non-rotating with respect to the vehicle  4 . 
   The drive train  10  includes the motor/transaxle unit  12 , a first output shaft  14 , a rear differential  16 , a second output shaft  20 , and a front differential  22 . The first output shaft  14  is coupled to the rear differential  16  to transmit power to the rear wheels  6  of the vehicle  4  via a rear axel  18 . The second output shaft  20  is coupled to the front differential  22  to transmit power to the front wheels  8  of the vehicle  4  via a front axle  24 . In some embodiments, the drive train  10  may also include the rear axle  18  and/or the front axle  24 , while in other embodiments the rear axle  18  and/or front axle  24  are considered part of the vehicle  4  rather than the drive train  10 . The drive train  10  also includes a brake  26 , and a clutch  28  for selectively coupling the second output shaft  20  to the front differential  22 . The motor/transaxle unit  12  is a commercially available type that includes, for example, an electric motor and typical components of a transaxle, including, for example, transmission, axle, clutch, gearbox, and differential combined in a single unit. 
   In one embodiment, two mode operation is achieved by applying a braking force to one of the output shafts  14 ,  20  via the brake  26 . While illustrated as being applied to the second output shaft  20 , the braking force alternatively, or additionally can be applied to the first output shaft  14 . This takes advantage of the ability of the differential to distribute the power between output shafts  14 ,  20  as required. Application of a braking force to the second output shaft  20  increases the speed of the first output shaft  14 . For example, where the braking force is sufficient to stop the second output shaft  20 , the first output shaft  14  which is not restrained by the brake  26 , may travel at up to twice the speed as when the brake  26  was not applied. 
   Thus, multiple configurations are possible utilizing the fact that by varying the relative speed of the outputs through the differential, either bringing one output shaft  20  to zero, or by using another motor to control the speed of the output shaft  20 , the ratios can be varied to one axle, for example the rear axle  18 , carrying one set of wheels, for example rear wheels  6 . In other words, one output shaft  20  of the differential of the motor/transaxle unit  12  can either, and or both, be connected via the clutch  28  to an axle  24  or locked by the brake  26  to force the other output shaft  14  to operate at double speed. This provides either high torque at low speed or lower torque at higher speed, which suits certain applications such as aircraft push-back tractors. 
   Typically, the differential of a vehicle, such as rear wheel drive car or a truck, is disposed at the rear end of the car or truck with a drive element going into the differential. A drive shaft is coupled to the differential to provide torque, and two shafts extend from the differential that are coupled to the wheels of the car or truck. The amount of torque applied to the input of the differential is typically twice the amount of torque that goes to each wheel, and each wheel receives one-half the amount of torque that is applied to the input assuming that the vehicle is proceeding in a straight line and there is no slippage of the wheels. Minor variations that occur when proceeding around a turn are insignificant factors that are not important to the present discussion. 
   If one of the two wheels is brought to a stop, the other wheel, when lifted off the supporting surface, turns twice as fast. In essence, this important feature of differentials is they can change speed without changing any parts. By stopping one wheel and lifting the other, nothing is structurally changed except the conditions under which the differential was operating. That is different from an automatic or a manual transmission in which it is necessary to structurally change the arrangements of parts to adjust the torque. 
   Typical vehicles must perform at a large variety of different loads and speeds, whether the vehicle is powered by an internal combustion engine and/or an electric motor. However, where the vehicle is a push-back tractor, there are only two specific modes in the duty cycle. A high torque and low speed mode is required to push back an aircraft, and a low torque and high speed mode is needed to return to the terminal area after pushing back the aircraft. Certain other vehicles face similar demands, for example, forklifts. 
   In typical vehicles, whether front or rear wheel drive, there is a differential designed to equally split the torque to the ground. The differential also allows the wheels to travel different distances, for example, as the vehicle goes around corners. Currently, the transaxle is used in what is referred to as an east/west configuration. The east/west configuration means that the differential is disposed between the two rear wheels  6  or between the two front wheels  8  of the vehicle. In such an arrangement, the differential of the transaxle is designed to split the torque equally to each of the driving wheels. 
   Referring to  FIG. 1 , the same transaxle, incorporated in the motor/transaxle unit  12  is turned north/south (i.e., between the rear and front axles  18 ,  24 ). The transaxle  12 , which is an integrated power train, has it own differential. From each of the two output shafts  14 ,  20  of the transaxle that ordinarily would have gone to the vehicle wheels, the transaxle feeds into a second differential  16  and  22 . One output shaft  20  of the transaxle feeds through the clutch  28  and brake  26  to another differential  22 , for example, to the front axle  24  of the vehicle. The other output shaft  20  of the transaxle feeds, for example, the rear axle  18  of the vehicle. 
   Thus, the arrangement for utilizes three differentials, namely the front  22  and rear  16  differentials, and the central or north/south differential that is an integrated component of the motor/transaxle unit  12 . Varying the speed on one of the output shafts, for example the front output shaft  20  of the integrated power train  12  changes the torque and the speed output at the output shaft on the opposite side of the integrated power train  12 , for example, output shaft  14 . By engaging the clutch  28  to uncouple one output shaft  20  from the front differential  22  and applying the brake  26  to bring the speed of the uncoupled output shaft  20  to zero, the axle  18  and wheels  30  of the differential  16  coupled to the other output shaft  14  can be driven at twice the speed. 
   This structural arrangement helps makes possible the two mode operation. For example, the vehicle can be operated at a relatively low speed and high torque in a four-wheel drive mode by disengaging the clutch  28  and releasing the brake  26 . Disengaging the clutch  28  couples the front differential  22  from front output shaft  20 , while releasing the brake  26  frees the output shaft  20  to rotate unhindered. Alternatively, the vehicle can be operated at relatively high speed (e.g., twice the speed) and low torque (e.g., half the torque) in a two wheel drive by engaging the clutch  28  and applying the brake  26 . Engaging the clutch  28  uncouples the front differential  22  from front output shaft  20 , while applying the brake  26  hinders the output shaft  20  from rotating, causing the differential in the motor/transaxle  12  to allocate more power to driving the rear differential  16 . This operation solves the duty cycle issue with push-back tractors and enables on operator to travel back to the gate twice as fast and yet still employ full torque when pushing airplanes. 
     FIG. 2  shows another embodiment of the drive train assembly that enables operation in a continuously variable transmission (CVT) mode, utilizing a multiple integrated power train arrangement. This embodiment operates on the same principle as the previously described embodiment, but employs at least two electric motor/transaxle units. 
   In particular, an electric drive train  32  for a vehicle  4  comprises a first motor/transaxle unit  12 , a first output shaft  14 , a rear differential  16 , a second output shaft  20 , a front differential  22 , a second motor/transaxle unit  34 , a rear output shaft  36 , a front output shaft  38 , and a coupling  40 . It is to be understood that the housings for both motor/transaxle units  12 ,  34  are grounded or non-rotating with respect to the vehicle  4 . The front output shaft  20  of the first motor/transaxle unit  12  is coupled to the rear output shaft  36  via the coupling  40 . The rear output shaft  14  of the first motor/transaxle unit  12  is coupled to the rear wheels  6  of the vehicle  4  through the rear differential  16  and rear axle  18 . The front output shaft  38  of the second motor/transaxle unit  34  is coupled to the front wheels  8  of the vehicle  4  via the front differential  22  and front axle  24 . The electric drive train  32  also includes a brake  26 , and a clutch  28  for selectively coupling the front output shaft  38  of the second motor/transaxle unit  34  to the front differential  22 . 
   In this embodiment, when the clutch  28  is disengaged all four wheels  6 ,  8  are operating, and the motors of both motor/transaxle units  12 ,  24  can be turning in the same direction to provide twice the power and twice the torque to all four wheels  6 ,  8  with respect to the power provide by a single one of the motor/transaxle units  12 . 
   Alternatively, when the clutch  28  is engaged and the brake  26  is applied (i.e., in a two wheel drive CVT mode), the speed and torque applied by the output shaft  14  to the rear axle  18  can be continuously changed or varied by varying the speed of the motor/transaxle unit  34  with the brake  26  applied to the front output shaft  36 , depending on the speed of the motor/transaxle unit  34 . While the brake  26  is applied, as the speed of the second motor/transaxle unit  34  is slowed, the speed of the output shaft  14  to the rear axle  18  is increased up to twice its speed when driven solely by the first motor/transaxle unit  12 . 
   Depending on the duty cycle desired for a particular application, various operating methods may apply. For example, the electric drive train  32  may be operated with the clutch  28  disengaged and the brake  26  released (i.e., open or disengaged) to drive the front  24  and rear axles  18  with the same ratio, or with the clutch  28  engaged and the brake  26  applied (i.e., closed or engaged) to drive the rear axle  18  at twice the standard speed and half the standard torque. 
   The two electric motor/transaxle units  12 ,  34  can be used as a two speed gearbox with combined power with the second unit  34  doubling the power. For example, with the clutch  28  engaged and the brake  26  applied, the second motor/transaxle unit  34  can serve as a variable speed reactor to provide a CVT action in the main motor/transaxle unit  12 . This structural configuration provides high torque and low speed with four wheel drive and higher speed and low torque in two wheel drive. 
   Various embodiments of the invention have been described in fulfillment of the various objects of the invention. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the present invention. For example, the EV vehicle  4  can take any of a variety of forms including, but not limited to, push-tractors, forklifts, automobiles, trucks, buses, or boats. 
   All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, including but not limited to U.S. Provisional Application No. 60/319,076, filed Jan. 16, 2002, are incorporated herein by reference, in their entirety.