Abstract:
A method for controlling a hybrid vehicle having a traction motor between an engine and a step ratio automatic transmission during a startup of the hybrid vehicle. The method includes performing at least one of adjusting a clutch or an oil pressure to change a transmission tie-up force or downshifting the transmission in response to an actuation rate of a vehicle driver demand.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an engine restart torque spike management system for a hybrid vehicle. 
       BACKGROUND 
       [0002]    A hybrid vehicle powertrain includes an engine and an electric motor. Torque, which is produced by the engine and/or by the motor, may be transferred to the vehicle drive wheels through a transmission. A traction battery connected to the motor supplies energy to the motor for the motor to produce motor torque. The motor may provide a negative motor torque to the transmission (for example, during regenerative braking). Under such conditions, the motor acts as a generator to the battery. 
         [0003]    A hybrid vehicle may have a parallel configuration, a series configuration, or combination thereof. In a parallel configuration (i.e., 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 motor may be connected to the transmission via a torque converter having a torque converter clutch. The engine, the disconnect clutch, the motor, the torque converter, and the transmission are connected sequentially in series. 
       SUMMARY 
       [0004]    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, a torque converter with a torque converter clutch, and a transmission. The controller and the control strategy manage an engine restart torque spike during start up by adjusting a transmission tie-up force and/or downshifting the transmission in response to a rate of actuation of an actuator causing the start up. Further, the performing step can be coordinated and timed based on the timing and rate of application of a disconnect clutch. 
         [0005]    Advantageously, the controller and the control strategy can be utilized as a mechanism to mitigate engine restart torque spike. It should be understood the start up may refer to engine start up or electrical motor start up. Under certain conditions, it may be desirable to automatically start and stop a vehicle. Stopping the engine can reduce fuel consumption, especially when the vehicle is stopped for longer periods of time, in stop-and-go traffic for example. However, supplying the driver with a desired amount of wheel torque shortly after restart may be difficult because of torque management within a transmission. For example, if transmission clutches are in an open state during start up, a wheel torque response delay may be perceived by the driver. Also, when the engine restarts because of medium to high gas pedal actuation, a disconnect clutch is applied as quickly as possible. Under such conditions, when the engine restarts, torque spike is generated because the manifold is at atmospheric pressure and any engine speed spike is multiplied by the torque converter and by the transmission gear ratio. 
         [0006]    In a typical scenario, in the case of heavy tip-in, such as heavy application of the acceleration pedal, the disconnect clutch is connected quickly to provide maximum torque as smoothly as possible. However, by connecting the disconnect clutch quickly, powertrain disturbances are caused because engine torque spikes during an engine restart and the motor speed may not match the engine speed, particularly if the electric machine is used to start the engine. In order to reduce disturbances, ramping of the disconnect clutch can be used until the engine speed substantially equals the electrical motor speed, and then quickly locking the clutch. 
         [0007]    Approaches have been advised to mitigate this torque spike. As described above, the disconnect clutch application is delayed or slowed. However, under these conditions, delayed and/or less than uniform launch may result. As a second approach, a transmission tie-up force can be adjusted with the use of a higher gear combination in combination with a rapid lock up of the disconnect clutch. This approach is disclosed in US 2012/0010045, which is incorporated by reference herein in its entirety. 
         [0008]    In contrast to the typical operation occurring as set forth above, a controller and the control strategy in accordance with embodiments of the present invention coordinate a transmission tie-up force event and a downshift event with the disconnect clutch lock-up event to allow the torque capacity of the disconnect clutch to increase to ensure a smooth drive away event. If the input is a light pedal, then a slow ramp of the disconnect clutch is applied. As a result, the transmission tie-up force is dropped quickly because the torque spike is mitigated by the disconnect clutch. If the input is a medium to high pedal, then a fast ramp of the disconnect clutch is applied. As a result, the transmission tie-up force application is delayed to absorb the initial torque spike and provide a monotonic torque rise. This control strategy can be advantageously used to minimize engine control spike during relatively high restart engine speeds, such as a restart due to battery state of charge, when the driver does not expect any torque to be delivered to the wheels. 
         [0009]    In an embodiment, a method is provided. The method includes stopping a vehicle, and during a vehicle start, performing at least one of adjusting a transmission tie-up force or downshifting a transmission in response to a rate of actuation of an actuator causing the vehicle start. 
         [0010]    In an embodiment, a system is provided. The system includes a controller configured to stop a vehicle, and during a vehicle start, perform at least one of adjusting a transmission tie-up force or downshifting a transmission in response to a rate of actuation of an actuator causing the vehicle start. 
         [0011]    In an embodiment, a hybrid electric vehicle is provided. The vehicle includes an engine, an electric motor, a torque converter having a bypass clutch, a transmission, and a controller. The controller is configured to stop a vehicle, and during a vehicle start, perform at least one of adjusting a transmission tie-up force or downshifting a transmission in response to a rate of actuation of an actuator causing the vehicle start. 
         [0012]    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 
         [0013]      FIG. 1  illustrates a block diagram of an exemplary hybrid vehicle powertrain in accordance with an embodiment of the present invention; 
           [0014]      FIG. 2  illustrates a flowchart describing operation of a control strategy for controlling the motor to manage engine restart torque spike with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    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. 
         [0016]    Referring now to  FIG. 1 , a block diagram of an exemplary powertrain system  100  for a hybrid electric vehicle in accordance with one or more embodiments is shown. Powertrain system  100  includes an engine  102 , an electric machine such as an electric motor and generator  104  (otherwise referred to as a “motor”), a traction battery  106 , a disconnect clutch  108 , a torque converter  110 , and a multiple-ratio automatic transmission  112 . 
         [0017]    Engine  102  and motor  104  are drive sources for the vehicle. Engine  102  is connectable to motor  104  through a disconnect clutch  108  whereby engine  102  and motor  104  are connected in series. Motor  104  is connected to torque converter  110 . Torque converter  110  is connected to engine  102  via motor  104  when engine  102  is connected to motor  104  via disconnect clutch  108 . The disconnect clutch  108  is controllable through disconnect clutch valve  109 , which is connected to powertrain control unit  142 . Transmission  112  is connected to the drive wheels  114  of the vehicle. The driving force applied from engine  102  and/or motor  104  is transmitted through torque converter  110  and transmission  112  to drive wheels  114  thereby propelling the vehicle. 
         [0018]    Torque converter  110  includes an impeller rotor fixed to output shaft  116  of motor  104  and a turbine rotor fixed to the input shaft  118  of transmission  112 . The turbine of torque converter  110  can be driven hydro-dynamically by the impeller of torque converter  110 . Thus, torque converter  110  may provide a “hydraulic coupling” between output shaft  116  of motor  104  and the input shaft  118  of transmission  112 . 
         [0019]    Torque converter  110  further includes a torque converter clutch (e.g., a bypass clutch). The torque converter clutch is controllable across a range between an engaged position (e.g., a lock-up position, an applied position, etc.) and a disengaged position (e.g. an unlocked position, etc.). In the engaged position, the converter clutch mechanically connects the impeller and the turbine of torque converter  110  thereby discounting the hydraulic coupling between these components. In the disengaged position, the converter clutch permits the hydraulic coupling between the impeller and the turbine of torque converter  110 . 
         [0020]    When the torque converter clutch is disengaged, the hydraulic coupling between the impeller and the turbine of torque converter  110  absorbs and attenuates unacceptable vibrations and other disturbances in the powertrain. The source of such disturbances includes the engine torque applied from engine  102  for propelling the vehicle. However, fuel economy of the vehicle is reduced when the converter clutch is disengaged. Thus, it is desired that the converter clutch be engaged when possible. 
         [0021]    The torque converter clutch may be controlled through operation of a clutch valve  120 . In response to a control signal, clutch valve  120  pressurizes and vents the converter clutch to engage and disengage. The operation of torque converter  110  can be controlled such that converter clutch is neither fully engaged nor fully disengaged and instead is modulated to produce a variable magnitude of slip in torque converter  110 . The slip of torque converter  110  corresponds to the difference in the speeds of the impeller and the turbine of torque converter  110 . The slip of torque converter  110  approaches zero as converter clutch approaches the fully engaged position. Conversely, the magnitude of the slip of torque converter  110  becomes larger as the converter clutch moves toward the disengaged position. 
         [0022]    When operated to produce a variable magnitude of slip, torque converter  110  can be used to absorb vibrations (for example, when gear ratio changes are being made, when the driver releases pressure from the accelerator pedal, etc.) by increasing the slip, thus causing a greater portion of the engine torque to be passed from the impeller to the turbine of torque converter  110  through hydro-dynamic action. When chance of objectionable vibration and disturbance is absent, the converter clutch can be more fully engaged so that fuel economy is enhanced. However, again, as noted above, it is desired that the converter clutch be engaged when possible as the fuel economy of the vehicle is increased when the converter clutch is engaged. 
         [0023]    As indicated above, engine  102  is connectable to motor  104  through disconnect clutch  108 . In particular, engine  102  has an engine shaft  122  connectable to an input shaft  124  of motor  104  through disconnect clutch  108 . As further indicated above, output shaft  116  of motor  104  is connected to the impeller of torque converter  110 . The turbine of torque converter  110  is connected to the input shaft of transmission  112 . 
         [0024]    Transmission  112  includes multiple gear clutches that may be engaged as needed to activate a plurality of fixed transmission gear ratios. Transmission  112  includes an output shaft  126  that is connected to a differential  128 . Drive wheels  114  are connected to differential  128  through respective axles  130 . With this arrangement, transmission  112  transmits a powertrain output torque  132  to drive wheels  114 . The transmission  112  is controllable through one or more solenoids  115 , which are connected to powertrain control unit  142 . 
         [0025]    Engine  102  is a primary source of power for powertrain system  100 . Engine  102  is an internal combustion engine such as a gasoline, diesel, or natural gas powered engine. Engine  102  generates an engine torque  134  that is supplied to motor  104  when engine  102  and motor  104  are connected via disconnect clutch  108 . To drive the vehicle with engine  102 , at least a portion of engine torque  134  passes from engine  102  through disconnect clutch  108  to motor  104  and then from motor  104  through torque converter  110  to transmission  112 . 
         [0026]    Traction battery  106  is a secondary source of power for powertrain system  100 . Motor  104  is linked to battery  106  through wiring  136 . Depending on the particular operating mode of the vehicle, motor  124  either converts electric energy stored in battery  106  into a motor torque  138  or sends power to battery  106  through wiring  136 . To drive the vehicle with motor  104 , motor torque  138  is also sent through torque converter  110  to transmission  112 . When generating electrical power for storage in battery  106 , motor  104  obtains power either from engine  102  in a driving mode or from the inertia in the vehicle as motor  104  acts as a brake in what is referred to as a regenerative braking mode. 
         [0027]    As described, engine  102 , disconnect clutch  108 , motor  104 , torque converter  110 , and transmission  112  are connectable sequentially in series as illustrated in  FIG. 1 . As such, powertrain system  100  represents a parallel or modular hybrid transmission (“MHT”) configuration in which engine  102  is connected to motor  104  by disconnect clutch  108  with motor  104  being connected to transmission  112  through torque converter  110 . 
         [0028]    Depending on whether disconnect clutch  108  is engaged or disengaged determines which input torques  134  and  138  are transferred to transmission  112 . For example, if disconnect clutch  108  is disengaged, then only motor torque  138  is supplied to transmission  112 . If disconnect clutch is engaged, then both engine torque  134  and motor torque  138  are supplied to transmission  112 . Of course, if only engine torque  134  is desired for transmission  112 , disconnect clutch  108  is engaged, but motor  104  is not energized such that engine torque  134  is only supplied to transmission  112 . 
         [0029]    Transmission  112  includes planetary gear sets (not shown) that are selectively placed in different gear ratios by selective engagement of friction elements (not shown) in order to establish the desired multiple drive ratios. The friction elements are controllable through a shift schedule that connects and disconnects certain elements of the planetary gear sets to control the ratio between the transmission output and the transmission input. Transmission  112  is automatically shifted from one ratio to another based on the needs of the vehicle. Transmission  112  then provides powertrain output torque  140  to output shaft  126  which ultimately drives drive wheels  114 . The kinetic details of transmission  112  can be established by a wide range of transmission arrangements. Transmission  112  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. 
         [0030]    A mechanical pump (not shown) or an electric pump  113  can be utilized to pump transmission fluid into transmission  112 . If the torque converter clutch is applied, then electric pump  113  is utilized to provide transmission fluid because the input shaft would not rotate, and therefore, the mechanical pump may not be operational. 
         [0031]    Powertrain system  100  further includes a powertrain control unit  142 . Control unit  142  constitutes a vehicle system controller. Based on repositioning an accelerator pedal, the driver of the vehicle provides a total drive command when the driver wants to propel the vehicle. The more the driver depresses pedal, the more drive command is requested. Conversely, the less the driver depresses pedal, the less drive command is requested. When the driver releases the pedal, the vehicle begins to coast. 
         [0032]    Control unit  142  apportions the total drive command between an engine torque signal (which represents the amount of engine torque  134  to be provided from engine  102  to transmission  112 ) and a motor torque signal  146  (which represents the amount of motor torque  138  to be provided from motor  104  to transmission  112 ). In turn, engine  102  generates engine torque  134  and motor generates motor torque  138  for transmission  112  in order to propel the vehicle. Such engine torque  134  and motor torque  138  for propelling the vehicle are “positive” torques. However, both engine  102  and motor  104  may generate “negative” torques for transmission  112  in order to brake the vehicle. 
         [0033]    Control unit  142  is further configured to control clutch valve  120  in order to control operation of the torque converter clutch of torque converter  110 . Control unit  142  controls the operation of torque converter  110  such that the converter clutch is modulated across a range between the engaged and disengaged positions to produce a variable magnitude of slip in torque converter  110 . Again, the slip of torque converter  110  corresponds to the difference between the input rotational speed and the output rotational speed of torque converter  110 . The output rotational speed approaches the input rotational speed as the converter clutch approaches the engaged position such that the slip is zero when the converter clutch is in the fully engaged position. Conversely, the output rotational speed lags the input rotational speed as the converter clutch approaches the disengaged position such that the magnitude of the slip becomes larger. A rotation sensor is configured to sense the slip of torque converter  110  and provide information indicative of the slip to control unit  142 . 
         [0034]    Referring now to  FIG. 2 , with continual reference to  FIG. 1 , a flowchart  200  describing operation of a control strategy for mitigating engine restart torque spike during engine restart in accordance with an embodiment of the present invention is shown. 
         [0035]    At block  202 , operating conditions are determined. Non-limiting examples of operating conditions include engine conditions, ambient conditions, torque converter input speed, torque converter output speed, vehicle brake pressure, transmission oil temperature and pressure, and fuel type. After block  202 , the flowchart proceeds to block  204 . 
         [0036]    At block  204 , the flowchart resolves whether or not a start request has been made after a vehicle stop. A start may be an engine restart or a motor restart. An engine start request may be made by a driver&#39;s act. Non-limiting examples of a driver&#39;s act are changing the position of an actuator such as a brake pedal or accelerator pedal. The start request may also be made by a controller that automatically requests an engine start in response to vehicle operating conditions. 
         [0037]    At block  206 , a pedal condition is tracked. The pedal condition can describe the user&#39;s actuation of an actuator such as a brake pedal or accelerator pedal, and the rate of such actuation. Non-limiting examples of pedal conditions are light pedal, medium pedal and heavy pedal. The rate of actuation of the pedal increases from light to medium to heavy. After tracking the pedal condition, then the flowchart  200  moves onto block  208 . 
         [0038]    At block  208 , an adjustment is made to a tie-up force applied to the transmission and/or a downshift event is applied. One or both actions are made in response to the rate of actuation of the start-up actuator. A transmission actuator such as a transmission clutch or a transmission pressure control valve can be actuated to adjust (e.g., reduce) the tie-up force applied to the transmission. For example, the force applied by a gear clutch may be reduced to reduce the transmission tie-up force. By reducing the transmission tie-up force some of the torque input to the transmission may be directed to the transmission output and vehicle wheels. In one or more embodiments, while the force applied to transmission tie-up clutches is being reduced, the transmission remains tied up. Regarding the downshift event, a non-synchronous downshift can be utilized using a single clutch as a control unit, such as a 3 to 1 or 4 to 1 in a 6 speed transmission. This allows a reduction in the torque spike by the gear ratio. For example, 3 to 1 is a factor of 2. 
         [0039]    As described above, a controller and the control strategy in accordance with embodiments of the present invention coordinate a transmission tie-up force event and downshift event with the disconnect clutch lock-up event to allow the torque capacity of the disconnect clutch to increase to ensure a smooth drive away event. If the input is a light pedal, then a slow ramp of the disconnect clutch is applied. As a result, the transmission tie-up force is dropped quickly because the torque spike is mitigated by the disconnect clutch. If the input is a medium to high pedal, then a fast ramp of the disconnect clutch is applied. As a result, the transmission tie-up force application is delayed to absorb the initial torque spike and provide a monotonic torque rise. 
         [0040]    Depending on the use case, a different torque mitigation scheme can be used. If an engine restart is generated by the system requirement such as low state of charge of the traction battery, for example, and the driver does not expect any torque, then transmission tie up can be used to mitigate any jerk when the engine starts and the disconnect clutch is connected. If the restart is prompted by a light pedal, then no torque mitigation in the transmission is utilized, but slow ramping of the disconnect clutch is utilized because the driver expects slow launch. If the restart is prompted by a heavy tip-in, then a downshift with a slipping tie-up is utilized as described above. If the restart is wide open pedal, then a downshift is utilized so that partial torque is delivered to the driving wheels and the vehicle starts moving right away but the disconnect clutch apply is masked by the initial lower gear ratio. 
         [0041]    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.