Abstract:
A system and method of controlling a hybrid electric vehicle powertrain is provided. The hybrid-electric powertrain includes an engine, a battery, an electric motor, an electric generator and a transmission with a planetary gear unit. The planetary gear unit mechanically couples the engine, the electric motor and the electric generator to effect power delivery to vehicle traction wheels. Stored electrical power is delivering to the electric motor to drive the traction wheels in an electric-only operation mode. A first amount of torque is applied to the planetary gear unit with the electric generator in order to reduce the amount of wear to the planetary gear unit in the electric-only operation mode.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a hybrid electric vehicle powertrain having transmission gearing with gearing elements for establishing separate power flow paths from two power sources to vehicle fraction wheels 
       BACKGROUND 
       [0002]    A known hybrid electric vehicle powertrain with dual power flow paths between an engine and vehicle traction wheels and between an electric motor and vehicle traction wheels will permit the vehicle to operate with maximum performance by managing power distribution from each power source. This includes managing the operating state of the engine, the electric motor, a generator and a battery. 
         [0003]    The battery, the generator and the motor are electrically coupled. A vehicle system controller is interfaced with a transmission control module to ensure that power management for optimum performance and drivability is maintained. 
         [0004]    The powertrain may comprise gearing that defines a parallel power flow configuration in which motor torque and engine torque are coordinated to meet a wheel torque command. The vehicle system controller may cause the engine to be shut down under certain operating conditions, such as during a steady-state highway cruising mode for the vehicle, so that the vehicle may be powered solely by the electric motor. At this time, the battery acts as a power source for the motor. If the battery state-of-charge becomes reduced below a calibrated threshold value during the all-electric drive mode, the engine may be started to charge the battery and to provide a mechanical power source to complement the electric motor torque. 
         [0005]    An example of a hybrid electric vehicle powertrain of this type may include a planetary gear set that is used to direct engine power to either an electric power flow path or a mechanical power flow path. Such a powertrain is disclosed, for example, in U.S. Pat. No. 7,268,442 is assigned to the assignee of this invention. That powertrain includes a planetary gear set wherein the sun gear of the planetary gear set is drivably connected to the generator, the engine is drivably connected to the carrier of the planetary gear set and the motor is drivably connected to the ring gear of the planetary gear set. The power flow path is split by the planetary gear set when both the engine and the motor are active. 
         [0006]    If the hybrid electric vehicle powertrain is a so-called “plug-in” powertrain, the motor will be operated for a significant period of a total driving event while the engine is off. A battery charge depletion strategy then is used to supply electrical energy to the motor until a battery state-of-charge depletion threshold is reached. The battery, following charge depletion, then may be charged by a public utility electric power grid in preparation for a subsequent driving event. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram of a hybrid electric vehicle powertrain with divided power flow paths; 
           [0008]      FIG. 2  is a schematic diagram of the planetary gear set of  FIG. 1 ; 
           [0009]      FIG. 3  is a lever analogy diagram that will be used to describe the function of the planetary gear set when the engine on; 
           [0010]      FIG. 4  is a lever analogy diagram for the planetary gear set when the engine is off; and 
           [0011]      FIG. 5  is a flowchart illustrating the control strategy of the hybrid electric vehicle powertrain of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely examples of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may 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. 
         [0013]    A schematic representation of the architecture for a hybrid electric vehicle powertrain is shown in  FIG. 1 . It includes an electric motor  10  with a rotor  12  and a stator  14 . Rotor  12  is drivably connected to gear  16 , which meshes with countershaft gear  18 . A companion countershaft gear  20  engages drivably gear  22  of a differential-and-axle assembly  24 , which in turn drives the vehicle fraction wheels. Engine  26 , which may be an internal combustion engine or any other suitable vehicle engine (e.g., spark-ignition or diesel) is connected to power input shaft  28  for a planetary gear unit  30 . A transmission oil pump  31  can be geared to the shaft  28 . 
         [0014]    The planetary gear unit  30  includes ring gear  32 , sun gear  34  and a planetary carrier  36 . Sun gear  34  is connected drivably to the rotor  38  of generator  40 . As illustrated in more detail in  FIG. 2 , the planetary gear unit  30  may include a plurality of planetary gears  35  which are mounted to the planetary carrier  36 . The planetary gears  35 , which are mounted to the planetary carrier  36 , have a speed ω c  and a torque τ c , as illustrated in  FIG. 2 . Likewise, the sun gear  34  has a speed a speed ω s  and a torque τ s  and the ring gear  32  has a speed ω r  and a torque τ r , also shown in  FIG. 2 . 
         [0015]    Turning back to  FIG. 1 , a stator  42  for the generator  40  is electrically coupled to a high voltage inverter  44  and a DC/DC high voltage converter  46 , the latter in turn being electrically coupled to the battery, as shown. A battery control module, designated BCM, is also illustrated in  FIG. 1 . A high voltage inverter  48  is coupled to the stator  14  of motor  10 . 
         [0016]    The engine  26  is connected drivably to shaft  28  through a damper assembly  52 . The differential-and-axle assembly  24  is drivably connected to vehicle traction wheels. 
         [0017]    The power flow elements are under the control of a transmission control module (TCM), which is under a supervisory control of a vehicle system controller (VSC). The TCM and VSC are part of a control area network (CAN). Input variables for the VSC may include a driver operating range selector (PRNDL) signal, an accelerator pedal position (APP) signal and a brake pedal signal (BPS). When the generator  40  is commanded to assist the engine  26  during a forward drive vehicle launch, it may be controlled to function as a motor, whereby the planetary carrier  36  turns in a vehicle driving direction. When the generator  40  is acting as a generator to charge the battery, the generator  40  acts as a reaction element as electric power is used to complement engine  26  power. When the generator  40  is used to crank the engine  26  when the vehicle is moving, the generator  40  is controlled to function as a generator, which causes the torque delivered to the sun gear  34  to slow down the sun gear. This results in an increase in planetary carrier  36  speed and engine  26  speed as ring gear  32  speed increases. The electric motor  10  also provides torque to drive the ring gear  32  at this time. Some of the electric power then is used to crank the engine  26 . If the ring gear  32  speed is high enough, the planetary carrier  36  speed reaches an engine  26  ignition speed before the generator  40  speed slows down to zero. If the vehicle speed is low, it is possible that the engine  26  speed will not reach the ignition speed even when the generator  40  speed has decreased to zero. In this case, the generator  40  is controlled to function as a motor. 
         [0018]    When the transmission architecture of  FIG. 1  is used in a so-called “plug-in” hybrid vehicle, the electric motor  10  is used for a considerable percentage of the total operating time for any given driving event with the engine off. At this time, a direct mechanical connection exists between the electric motor  10  and the generator  40 . The generator  40  speed thus becomes high when the vehicle speed is at moderate or high levels. 
         [0019]    When the engine  26  speed equals zero during all-electric drive, the generator  40  will move at a speed that is a multiple of the motor  10  speed, depending upon the overall gear ratio of the planetary gear unit  30 . This may create a problem related to durability of the bearings for the planetary gear unit  30  as well as the generator  40 . This problem may limit the road speed of the vehicle to a value that is less than optimum. This also may reduce available torque needed to start the engine when the battery state-of-charge falls below a predetermined threshold during a given driving event before an opportunity exists for recharging the battery using the utility power grid. A need thus exists for a powertrain architecture that would be designed to avoid over-speeding of the generator during operation in an all-electric drive mode. 
         [0020]    The engine on and off conditions are illustrated by the lever analogy diagrams shown in  FIGS. 3 and 4 , respectively.  FIG. 3  shows speed and torque vectors that exist during motor  10  drive with the engine  26  on for the powertrain illustrated in  FIG. 1 . In  FIG. 3 , ω r  is the ring gear  32  speed, the ring gear  32  being connected to the traction motor  10  through gears  60 ,  18  and  20 . The symbol ω e  is the engine  26  speed, the engine  26  being connected to the planetary gear carrier  36 . The symbol ω g  is the generator  40  speed, the generator  40  being connected to the sun gear  34  so that the generator  40  speed ω g  is generally equal to the sun gear  34  speed ω s . The symbol τ r  in  FIG. 3  represents ring gear  32  torque. The symbol τ e  represents the engine  26  torque which is generally equal to the planetary carrier  36  torque τ c . Likewise, the symbol τ g  represents generator  40  torque, which is generally equal to the sun gear  34  torque τ s  during operation with the engine on. 
         [0021]    If the engine  26  is off and the powertrain is powered solely by the motor  10  in an electric-only drive mode, as in the case of a plug-in hybrid powertrain, a public electric utility grid is used to charge the battery, and the battery is designed to have a significantly increased capacity. This makes possible much greater use of the electric-only drive mode. 
         [0022]    The direct geared connection of the generator  40  to the wheels, which is indicated in  FIG. 1 , causes the generator  40  to turn as the vehicle moves with the engine  26  off. Upon an increase in vehicle speed, the generator  40  speed ω g  may become excessively high and the torque available to start the engine  26  is lowered. This condition is illustrated in  FIG. 4  where ω g  is the generator speed. The ring gear  32  is driven in the opposite direction when the engine  26  is off from the direction indicated in  FIG. 3  when the engine is on. The engine speed, ω e  , of course, is zero when the engine is off, as indicated in  FIG. 4 . The ring gear  32  speed at this time is ω r , which is equal in value to the value for ring gear  32  speed ω r  in  FIG. 3 . When the vehicle is electric-only operation mode the engine  26  and generator  40  provide a drag torque that counter-acts the back-driving torque coming through the ring gear  32 . Since the drag torque from the engine  26  and generator  40  passing through the planetary gear unit  30  represents only parasitic losses, the torque passing through the gear unit  30  from the engine  26  and generator  40  is small resulting in minimal load to the planetary bearings in the planetary gear unit  30 . 
         [0023]    Therefore, in the plug-in hybrid vehicles, the planetary gear unit  30  is running unloaded at high speeds when the vehicle is being driven in electric-only operation mode. Consequently, the higher the speed of the vehicle in electric-only operation mode, the higher the planetary gear unit  30  speed, and consequently, the higher the generator  40  speed. Hence, the planetary gear unit  30  is running unloaded at high speeds when the vehicle is being driven in electric-only operation mode which can cause degradation of the planetary gear unit  30  pinion bearings which are operating at low loads with a tendency to skid and wear. Consequently, this may also limit the speed at which a vehicle may drive in electric-only operation mode and may cause greater hydrocarbon emissions when the engine  26  is required to turn on. 
         [0024]    In order to provide a small amount of biasing load to the planetary pinion bearings in the planetary gear unit  30 , the generator  40  may apply an amount of torque to the planetary gear unit  30 . By applying torque to the planetary gear unit  30 , a load is placed on the bearings of the planetary gear unit  30 . The amount of torque applied by the generator  40  may be a small amount of torque that is less than or equal to the torque that, when applied to the engine  26 , does not result in spinning the engine  26 . In one embodiment, the torque applied to the planetary gear unit  30  by the generator  40  is generally equal to the friction torque in the engine  26 . In another embodiment, the torque applied by the generator  40  to the planetary gear unit  30  is not greater than the amount of torque required to spin the engine  26 . The engine  26  friction torque for a typical warm engine is approximately 10 Newton-meters (Nm) at the engine  26  and approximately 3 Nm at the generator  40 . For a cold engine  26 , the engine  26  friction torque may be approximately 30 Nm at normal temperatures. Therefore, the engine  26  friction torque, and the generator torque  40  may vary depending on temperature or engine architecture, as well as other factors. 
         [0025]    A control system may command the generator  40  to only apply torque to the planetary gear unit  30  when the vehicle is in electric-only operation mode. In an alternate embodiment, the generator  40  only applies torque to the planetary gear unit  30  when the planetary gear unit  30  reaches a threshold speed during electric-only operation mode. The control system for applying torque to the planetary gear unit  30  may be controlled by a closed loop controller  70 . The closed loop controller  70  may be part of the vehicle system control (VSC) module, as shown in  FIG. 1 . Alternatively, the closed loop controller may be part of the transmission control module (TCM) or other vehicle control module. In another embodiment, the controller  70  may be any other stand-alone controller. 
         [0026]      FIG. 5  illustrates a flow chart of a control system for the hybrid electric vehicle powertrain. As those of ordinary skill in the art will understand, the functions represented by the flowchart blocks may be performed by software and/or hardware. Also, the functions may be performed in an order or sequence other than that illustrated in  FIG. 5 . Similarly, one or more of the steps or functions may be repeatedly performed although not explicitly illustrated. Likewise, one or more of the representative steps of functions illustrated may be omitted in some applications. In one embodiment, the functions illustrated are primarily implemented by software instructions, code, or control logic stored in a computer-readable storage medium and if executed by a microprocessor based computer or controller such as the controller  70 . 
         [0027]    As illustrated in  FIG. 5 , a control system monitors the hybrid electric vehicle powertrain in step  110 . In a second step  112 , the controller determines if the vehicle is in electric-only operation mode. If the vehicle is not in electric-only operation mode, the vehicle continues to operate under normal mode  114 . If the vehicle is in electric-only operation mode, the control system then determines if the speed of the planetary gear or generator is greater than a predetermined amount X in step  116 . If the planetary gear speed or generator speed is less than the predetermined amount X, the vehicle continues to operate under normal mode  114 . However, if the speed of the planetary gear or generator is greater than the predetermined amount X, then the control system commands the generator to apply torque to the planetary gear in step  118 . The generator applies torque such that the reaction torque to the input shaft of the engine is less than or equal to a predetermined amount Y. 
         [0028]    In the next step  120 , the control system monitors the engine movement. As shown in  FIG. 1 , the engine movement may be monitored by a device  80 . In one embodiment, the engine movement is monitored by monitoring the engine  26  speed. By monitoring the engine speed, a closed loop control system is essentially monitoring whether or not there is movement of the engine  26  as a result of the torque applied to the planetary gear unit  30  by the generator  40 . In another embodiment, engine  26  motion is measured by detecting displacement of the engine  26 . The device  80  may be any acceptable method of measuring motion of the engine  26  such as a crank position sensor. In another embodiment, the motion of the engine  26  may be detected by an engine tachometer output pulse or any other engine sensor adapted for detecting engine speed and/or displacement. 
         [0029]    If any engine movement is detected in step  122 , the control system then determines the direction of engine movement in step  124 . If the speed or displacement of the engine is in the positive direction, the amount of torque applied by the generator is decreased so that the torque by the generator applied to the planetary gear unit is less than the predetermined value Y in step  126 . If the speed or displacement of the engine is in the reverse direction, the torque applied by the generator to the planetary gear set may be increased so that the torque is greater than the predetermined value Y in step  128 . 
         [0030]    The amount of torque applied by the generator may be a constant torque value or the torque may be a pre-described random pattern with a nominal torque value equal to a predetermined amount. In the situation where the engine movement is in the reverse direction, the increase of torque by the generator may only increase the nominal torque applied in the pre-described random pattern in step  128 . By increasing or decreasing the amount of torque applied by the generator, the movement of the engine is minimized so that the engine speed and the displacement of the engine is generally equal to zero so that emissions and vehicle drivability are not adversely affected. 
         [0031]    As illustrated in the  FIG. 5 , the generator  40  may only apply torque to the planetary gear unit  30  when the planetary gear unit  30  reaches a threshold speed during electric-only operation mode. In another embodiment, it is also contemplated that the control system may apply torque to the planetary gear unit  30  whenever the vehicle is in electric-only operation mode. 
         [0032]    The control system may also be employed to maintain a desired traction wheel torque. In one embodiment, the generator  40  is connected to the sun gear  34  and the electric motor  10  is connected to the ring gear  32 . Therefore, a reaction torque is applied to the motor  10  as a result of the torque applied to the sun gear  34  by the generator  40 . The control system may determine a required motor  10  torque to apply to the traction wheels in order to maintain the desired traction wheel torque in response to the reaction torque applied to the motor  10  by the generator  40  through the planetary gear unit  30 . The traction motor  10  torque should be applied to cancel out any reaction effects on the wheel torque as a result of the generator torque applied to the planetary gear unit  30 . The control system may employ an algorithm to determine the reaction torque to the motor  10  based on the toque applied by the generator  40  to the planetary gear unit  30  as well as other static and dynamic operating factors. 
         [0033]    While various embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.