Abstract:
A system and method for controlling a hybrid vehicle having an engine, a battery powered traction motor, and an automatic step-ratio transmission selectively coupled in series by a clutch include engaging the clutch to couple the engine and the motor and controlling motor torque to provide braking torque through the transmission to substantially maintain a cruise control speed. In one embodiment, a hybrid electric vehicle includes a battery powered traction motor connected to a transmission, an engine selectively coupled in series with the motor by a clutch, and a controller communicating with the traction motor and the engine and configured to control the motor to provide braking torque when the clutch is engaged and engine braking torque is insufficient to maintain cruise control speed of the vehicle as the vehicle travels downhill.

Description:
TECHNICAL FIELD 
     The present invention relates to cruise control in a hybrid vehicle powertrain. 
     BACKGROUND 
     A hybrid electric vehicle powertrain includes an engine and an electric motor, wherein torque (or power) produced by the engine and/or by the motor can be transferred through a transmission to the vehicle drive wheels to propel the vehicle. A traction battery supplies energy to the motor for the motor to produce the (positive) motor torque for propelling the vehicle. The motor may provide negative motor torque to the transmission (for example, during regenerative braking of the vehicle) and thereby act as a generator to the battery. The engine may also provide negative engine torque to the transmission to provide engine braking for braking the vehicle. 
     In a modular hybrid transmission (“MHT”) configuration, the engine is connectable to the motor by a disconnect clutch and the motor is connected to the transmission. The engine, the disconnect clutch, the motor, and the transmission are connected sequentially in series. 
     SUMMARY 
     Embodiments of the present invention are directed to a controller and a control strategy for a hybrid electric vehicle having an engine, an electric motor, and a transmission in which the motor is connected to the transmission and the engine is connectable to the transmission via the motor and a disconnect clutch. The controller and the control strategy control motor torque to maintain cruise control speed of the vehicle. 
     Various embodiments include a system and method for controlling a hybrid vehicle having an engine, a battery powered traction motor, and an automatic step-ratio transmission selectively coupled in series by a clutch include engaging the clutch to couple the engine and the motor and controlling motor torque to provide braking torque through the transmission to substantially maintain a cruise control speed. In one embodiment, a hybrid electric vehicle includes a battery powered traction motor connected to a transmission, an engine selectively coupled in series with the motor by a clutch, and a controller communicating with the traction motor and the engine and configured to control the motor to provide braking torque when the clutch is engaged and engine braking torque is insufficient to maintain cruise control speed of the vehicle as the vehicle travels downhill. The motor torque may be controlled such that the transmission maintains a gear ratio with the cruise control speed being maintained as the vehicle travels over a hill. The motor braking torque is a negative motor torque to maintain the cruise control speed of the vehicle as the vehicle travels downhill. The motor torque is a positive motor torque to maintain the cruise control speed of the vehicle as the vehicle travels uphill. 
     The method may further include transferring engine braking torque to the transmission from the engine via the motor to attempt to maintain the cruise control speed of the vehicle as the vehicle travels downhill. In this case, transferring motor torque to the transmission includes transferring motor braking torque from the motor to the transmission which together with the engine braking torque maintains the cruise control speed whereby shifting of the engine to a lower gear to maintain the cruise control speed is avoided. The motor braking torque may include two components: an initial motor braking torque component which is transferred to the transmission in conjunction with the engine braking torque to maintain the cruise control speed when the engine can provide enough engine braking torque to maintain the cruise control speed without having to shift to a lower gear; and an additional motor braking torque component which is transferred in conjunction with the engine braking torque to maintain the cruise control speed when the engine alone would have to shift to a lower gear to provide additional engine braking torque. 
     The method may further include transferring engine propelling torque to the transmission from the engine via the motor to attempt to maintain the cruise control speed of the vehicle as the vehicle travels uphill. In this case, transferring motor torque to the transmission includes transferring motor propelling torque from the motor to the transmission which together with the engine propelling torque maintains the cruise control speed whereby a downshifting engagement to maintain the cruise control speed is avoided. The motor propelling torque may include two components: an initial motor propelling torque component which is transferred to the transmission in conjunction with the engine propelling torque to maintain the cruise control speed when the engine can provide enough engine propelling torque to maintain the cruise control speed without a downshifting engagement; and an additional motor propelling torque component which is transferred in conjunction with the engine propelling torque to maintain the cruise control speed when a downshifting engagement of the engine alone would have to occur for the engine to provide additional engine propelling torque. 
     In an embodiment, a system is provided. The system includes a controller configured to transfer motor torque to a transmission from a motor in series with an engine to maintain cruise control speed of a vehicle as road load varies, such as when the vehicle travels over a hill. 
     In an embodiment, a hybrid electric vehicle is provided. The vehicle includes a motor connected to a transmission, an engine connected to the transmission via the motor, and a controller. The controller is configured to transfer motor torque from the motor to the transmission to maintain cruise control speed of the vehicle as the vehicle travels over a hill. 
     Various embodiments include associated advantages. For example, using negative motor torque in combination with engine braking torque may reduce transmission shifts when operating in cruise control and road load decreases, such as when traveling downhill. Reducing or minimizing transmission shifting in cruise control according to various embodiments of the present disclosure may improve vehicle efficiency and drivability. Additional objects, features, and advantages of embodiments of the present invention will become more readily apparent from the following detailed description when taken in conjunction with the drawings, wherein like reference numerals refer to corresponding parts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a block diagram of an exemplary hybrid vehicle powertrain in accordance with an embodiment of the present invention; 
         FIG. 2A  illustrates a flowchart describing operation of a control strategy for adjusting motor torque of the motor to compensate for downhill demands during cruise control in a hybrid electric vehicle in accordance with an embodiment of the present invention; and 
         FIG. 2B  illustrates a flowchart describing operation of a control strategy for adjusting motor torque of the motor to compensate for uphill demands during cruise control in a hybrid electric vehicle in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary 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. 
     Referring now to  FIG. 1 , a block diagram of an exemplary powertrain system  10  for a hybrid electric vehicle in accordance with an embodiment of the present invention is shown. Powertrain system  10  includes an engine  20 , an electric machine such as an electric motor/generator  30  (“motor”), a traction battery  36 , and a multiple step-ratio automatic transmission  50 . 
     Engine  20  and motor  30  are drive sources for the vehicle. Engine  20  is connectable in series to motor  30  through a disconnect clutch  32 . Motor  30  is connected to the input side of transmission  50 . For example, motor  30  may be connected to transmission  50  via a torque converter between motor  30  and the input side of transmission  50 . The input side of transmission  50  is connected in series with both engine  20  and motor  30  when engine  20  is connected to motor  30  via disconnect clutch  32 . In this case, transmission  50  is connected to motor  30  while at the same time being connected to engine  20  via motor  30 . On the output side, transmission  50  is connected to the drive wheels  60  of the vehicle. The driving force applied from engine  20  and/or motor  30  is transmitted through transmission  50  to drive wheels  60  thereby propelling the vehicle. 
     Engine  20  has an engine shaft  22  connectable to an input shaft  24  of motor  30  through disconnect clutch  32 . Although disconnect clutch  32  is described and illustrated as a hydraulic clutch, other types of clutches may be used. Motor  30  has an output shaft  42  connected to the input side of transmission  50 . 
     Transmission  50  includes multiple discrete gear ratios automatically selectable by a vehicle controller in response to vehicle operating conditions and a driving mode selected by the operator. The output side of transmission  50  includes an output shaft  54  that is connected to a differential  56 . Drive wheels  60  are connected to differential  56  through respective axles  66 . With this arrangement, transmission  50  transmits a powertrain output torque  68  to drive wheels  60 . 
     Engine  20  is a one source of power for powertrain system  10 . Engine  20  is an internal combustion engine such as a gasoline, diesel, or natural gas powered engine, for example. Engine  20  generates an engine power having an engine torque  76  that is supplied to transmission  50  when engine  20  and motor  30  are connected via disconnect clutch  32 . The engine power corresponds to the product of engine torque  76  and the engine speed of engine  20 . To drive the vehicle with engine  20 , at least a portion of engine torque  76  passes from engine  20  through disconnect clutch  32  to motor  30  and then from motor  30  to transmission  50 . 
     Traction battery  36  is another source of power for powertrain system  10 . Motor  30  is linked to battery  36  through wiring  53 . Depending on the particular operating mode of the vehicle, motor  30  either converts electric energy stored in battery  36  into a motor power having a motor torque  78  or sends a corresponding amount of electrical power to battery  36  when operating as a generator. The motor power corresponds to the product of motor torque  78  and the motor speed of motor  30 . To drive the vehicle with motor  20 , motor torque  78  is transmitted from motor  30  to transmission  50 . When generating electrical power for storage in battery  36 , motor  30  obtains power either from engine  20  in a driving mode or from the inertia in the vehicle as motor  30  acts as a brake when operating in a regenerative braking mode. 
     As described, engine  20 , disconnect clutch  32 , motor  30 , and transmission  50  are connectable sequentially in series as illustrated in  FIG. 1 . As such, powertrain system  10  represents a modular hybrid transmission (“MHT”) configuration in which engine  20  is connected to motor  30  by disconnect clutch  32  with motor  30  being connected to transmission  50 . 
     The state or mode of disconnect clutch  32  being engaged or disengaged determines which input torques  76  and  78  are transferred to transmission  50 . For example, if disconnect clutch  32  is disengaged, then only motor torque  78  is supplied to transmission  50 . If disconnect clutch  32  is engaged/locked, then both engine torque  76  and motor torque  78  are supplied to transmission  50 . Of course, if only engine torque  76  is desired for transmission  50 , disconnect clutch  32  is engaged/locked, but motor  30  is not energized such that engine torque  76  is only supplied to transmission  50 . Depending on the particular application and implementation, disconnect clutch  32  may be operated in a limited slip mode. 
     Transmission  50  includes clutches, bands, gears, and the like, and may include one or more planetary gear sets to selectively effect different discrete gear ratios by selective engagement of friction elements to establish the torque flow paths and provide the corresponding desired multiple step-ratios. The friction elements are controllable through a shift schedule within controller  80  or a dedicated transmission controller that connects and disconnects certain elements of the planetary gear sets to control the ratio between the transmission input and the transmission output. Transmission  50  is automatically shifted from one ratio to another based on the needs of the vehicle. Transmission  50  then provides powertrain output torque  68  to output shaft  54  which ultimately drives drive wheels  60 . The kinetic details of transmission  50  can be established by a wide range of transmission arrangements. Transmission  50  is an example of a transmission arrangement for use with embodiments of the present invention. Any multiple ratio transmission that accepts input torque(s) from an engine and/or a motor and then provides torque to an output shaft at the different ratios is acceptable for use with embodiments of the present invention. 
     Powertrain system  10  further includes a vehicle system controller  80 . Powertrain system  10  further includes an accelerator pedal  92  and a brake pedal  94 . Accelerator pedal  92  and brake pedal  94  are in communication with controller  80 . 
     The driver of the vehicle depresses accelerator pedal  92  to propel the vehicle. In response, a total drive command based on the positioning of accelerator pedal  92  is provided to controller  80 . Controller  80  apportions the total drive command between the engine power and the motor power to be provided to transmission  50  for propelling the vehicle. In particular, controller  80  apportions the total drive command between (i) an engine torque signal  100  (which represents the amount of engine torque  76  to be provided from engine  20 , operating at a corresponding engine speed, to transmission  50  for propelling the vehicle) and (ii) a motor torque signal  98  (which represents the amount of motor torque  78  to be provided from motor  30 , operating at a corresponding motor speed, to transmission  50  for propelling the vehicle). In turn, engine  20  generates the engine power having engine torque  76  and motor  30  generates the motor power having motor torque  78  for propelling the vehicle. Both engine torque  76  and motor torque  78  are supplied to transmission  50  (assuming that engine  20  is connected to motor  30  via disconnect clutch  32 ) such that the vehicle is propelled. Such engine torque  76  and motor torque  78  for propelling the vehicle are referred to herein as “positive” torques. Those of ordinary skill in the art will recognize that the positive/negative naming convention is used for ease of description only. 
     The driver of the vehicle depresses brake pedal  94  to slow or brake the vehicle. In response, a total brake command based on the positioning of brake pedal  94  is provided to controller  80 . Controller  80  apportions the total brake command between (i) powertrain braking power to be provided by engine  20  and/or motor  30  to transmission  50  for braking the vehicle and (ii) friction braking power to be applied by friction brakes  70  to drive wheels  60  for braking the vehicle. The powertrain braking power represents the amount of “negative” powertrain power to be provided by engine  20  and/or motor  30  to transmission  50  for braking the vehicle. Controller  80  apportions the powertrain braking power between (i) an engine torque signal  100  (which in this case represents the amount of negative engine torque  76  to be provided from engine  20 , operating at a corresponding engine speed, to transmission  50  for braking the vehicle) and (ii) a motor torque signal  98  (which in this case represents the amount of negative motor torque  78  to be provided from motor  30 , operating at a corresponding motor speed, to transmission  50  for braking the vehicle). In turn, engine  20  generates the engine power having negative engine torque  76  and motor  30  generates the motor power having negative motor torque  78  for braking the vehicle. Both engine torque  76  and motor torque  78  are supplied to transmission  50  (assuming that engine  20  is connected to motor  30  via disconnect clutch  32 ) to brake the vehicle. Controller  80  further generates a friction braking torque signal  96  (which represents the amount of torque to be obtained through friction brakes  70 ). In turn, friction brakes  70  apply the friction braking torque to drive wheels  60  to brake the vehicle. 
     Referring now to  FIGS. 2A and 2B , with continual reference to  FIG. 1 , operation of control strategies for adjusting motor torque  78  of motor  30  to compensate for variations in road load, such as associated with downhill and uphill travel demands, for example, during cruise control in a vehicle having the MHT vehicle configuration in accordance with embodiments of the present invention will be described.  FIG. 2A  illustrates a flowchart  200  describing operation of a control strategy for adjusting motor torque  78  to compensate for downhill demands during cruise control of the vehicle.  FIG. 2B  illustrates a flowchart  300  describing operation of a control strategy for adjusting motor torque  78  to compensate for uphill demands during cruise control in the vehicle. 
     Briefly, cruise control allows the driver of the vehicle to set a desired speed and maintain the speed of the vehicle within a specified range of the set speed. Controller  80  then controls the powertrain (e.g., engine  20 , motor  30 , transmission  50 , etc.) to keep the vehicle at or near the set speed without any further driver input. However, it is also desirable to reduce or minimize changes in engine speed and transmission shifting or ratio hunting to provide desired driveability and vehicle fuel/energy efficiency. When the vehicle is travelling downhill while in cruise control, some downhill grades are too steep for engine  20  to have enough engine braking capability to hold the set speed without shifting to a lower gear. When this happens, engine noise increases and the cruise control experience is a less comfortable one. Likewise, when the vehicle is travelling uphill while in cruise control, occasionally a downshift is required. When a downshift occurs, engine RPM increases (i.e., tachometer shifts) can become relatively large and disturb the driver or passengers in the vehicle. 
     As explained below, control strategies in accordance with embodiments of the present invention take advantage of the MHT vehicle configuration described herein, in which engine  20  is connected to motor  30  and motor  30  is connected to transmission  50 , by adjusting motor torque  78  from motor  30  to smooth out the demands of travelling downhill and uphill while in cruise control. 
     Turning now to  FIG. 2A , flowchart  200  describes control strategy operation for the case of travelling downhill with disconnect clutch  32  locked and the cruise control set as indicated in block  202 . In this case, negative motor torque  78  from motor  30  is provided to transmission  50  as needed to brake the vehicle to maintain the cruise control speed as indicated in block  204 . The control strategy for the downhill scenario takes advantage of the MHT vehicle configuration in which motor  30  is in series with engine  20 . Motor  30  being in series with engine  20  allows the powertrain braking torque (i.e., the combined braking torque from engine  20  and motor  30 ) to be significantly greater than the braking torque available from engine  20  alone. This is because braking torque from engine  20  is inherently much more limited than braking torque from motor  30  such that motor  30  makes a bigger difference to the powertrain braking torque. In this mode, downshifts may be avoided and cruise control operation is enhanced. Negative motor torque may be provided by operating the motor as a generator to charge battery  36  in a regenerative braking mode. In one embodiment, negative motor torque may also be provided by reversing the driving current polarity to the motor  30  using stored energy from battery  36  to provide additional negative braking torque. 
     The downhill control strategy operation may come into play with engine  20  providing engine braking torque  76  to transmission  50  to attempt to maintain the cruise control speed. Once controller  80  becomes aware that engine  20  does not have enough engine braking capability to hold the cruise control speed without shifting to a lower gear, motor  30  applies a sufficient amount of motor braking torque  78  to transmission  50  in order to maintain the cruise control speed. 
     Turning now to  FIG. 2B , flowchart  300  describes control strategy operation for the case of travelling uphill with the cruise control set as indicated in block  302 . In this case, positive motor torque  78  from motor  30  is provided to transmission  50  as needed to propel the vehicle to maintain the cruise control speed as indicated in block  304 . The control strategy here also takes advantage of the MHT vehicle configuration in which motor  30  is in series with engine  20 . Motor  30  being in series with engine  20  allows torque to be drawn from motor  30  when needed to meet the demands of travelling uphill. As such, when travelling uphill with the cruise control set, instead of changing transmission gears, motor  30  is activated and the current gear position is maintained if possible. If the gear position is not able to be maintained, then gear shifting is at least kept to a minimum. 
     The uphill control strategy operation may come into play with engine  20  providing engine torque  78  to transmission  50  for propelling the vehicle. Motor  30  may or may not be providing motor torque  78  to transmission  50  for propelling the vehicle. In either case, once controller  80  becomes aware that engine  20  is not providing sufficient engine torque  76  to hold the cruise control speed without a downshift being engaged, motor  30  applies a sufficient amount of motor torque  78  to transmission  50  in order to maintain the cruise control speed. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the present 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 present invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the present invention.