Abstract:
A hybrid electric vehicle having a motor and an engine that are selectively connected on a driveline and controlled by a controller. The controller is configured to schedule additional motor torque to compensate for engine inertia drag based upon a clutch pressure value and a clutch slip speed value during a period of clutch engagement. The controller is also configured to maintain vehicle acceleration using a proportional integral controller to adjust the motor torque during a period of clutch engagement.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to adjustments made to motor torque during the transient period of clutch engagement to counteract engine inertia drag, engine starting disturbances, and clutch lock-up. 
       BACKGROUND 
       [0002]    Hybrid vehicle architecture may take several forms for operatively connecting a battery, electric traction motor and a combustion engine together in the driveline of the vehicle. One proposed architecture in development by the assignee of this application is a Modular Hybrid Transmission (MHT). A key enabling technology of the MHT is the Electric Converter-Less Transmission (ECLT). To replicate the torque converter function of a conventional automatic transmission, the MHT powertrain relies on active controls of a starter/alternator and a disconnect clutch before the electric motor and a launch clutch after the electric motor. 
         [0003]    Removal of the torque converter improves the powertrain efficiency, however, the drivability of the MHT must meet comparable targets to production automatic transmissions. A major control challenge of the MHT is to absorb clunks, pulsations and vibrations in the driveline during engine starts and clutch engagement, creating a quieter, stressfree driving experience. 
         [0004]    Without the torque converter, new challenges arise as to the coordination of the clutch, engine and motor, especially during the complicated clutch engagement transients. All the friction element control, pressure control, and the motor toque control have to be integrated seamlessly for delivering smooth wheel torque. In addition, converter-less disconnect clutch engagement is very sensitive to the clutch pressure and it is a challenging task to achieve the proper damping and smoothness during the clutch engagement. 
         [0005]    The engine in a MHT must start smoothly and quickly and every start is accompanied by a transient clutch engagement process that results in substantial inertia drags and torque disturbances that are transferred to the driveline. The difficulty and uncertainty of estimating the engine and clutch torque caused by complicated transient dynamics make the motor torque compensation a challenging task. 
         [0006]    During the MHT clutch engagement transient for engine starts, there are problems of oscillations arising from the excitation of the mechanical resonance by various disturbances. This resultant oscillation phenomenon is due to low damping in the driveline due to the absence of a torque converter. The electric motor torque generates torque ripples with frequencies that are motor speed dependent. 
         [0007]    The above problems and other problems are addressed by the present disclosure as summarized below. 
       SUMMARY 
       [0008]    This disclosure proposes a method to improve hybrid motor torque compensation utilizing active countermeasures to directly react and compensate for the torque disturbances during clutch engagement for engine start. The disclosed algorithm actively adjusts motor torque based on the clutch dynamics and the vehicle response. 
         [0009]    According to one aspect of the disclosure, a hybrid vehicle is disclosed that comprises a motor, a engine, a battery for supplying power to the motor, and a controller. The controller is configured to provide a base motor torque command based upon a driver torque demand and an engine torque command; detect a period of clutch engagement after and engine start command is provided by the controller, and schedule additional motor torque to compensate for engine inertia drag based upon a clutch pressure value and a clutch slip speed value. 
         [0010]    According to another aspect of the disclosure, a hybrid vehicle is disclosed that comprises a motor, an engine, a battery for supplying power to the motor, and a controller. The controller is configured to provide a base motor torque command based upon a driver torque demand and an engine torque command; detect a period of clutch engagement after and engine start command is provided by the controller; and maintaining vehicle acceleration using a proportional integration controller to adjust the motor torque. 
         [0011]    According to another aspect of the disclosure, a method is disclosed for operating a hybrid vehicle having an engine that is selectively connected to a driveline by a disconnect clutch and a secondary power source. The method comprises the step of obtaining a base motor torque command, detecting a period of clutch engagement after an engine start command that ends upon full clutch engagement, scheduling additional motor torque to compensate for engine inertia drag based upon a clutch pressure value and a clutch slip speed value, and maintaining vehicle acceleration using a proportional integration controller to adjust the motor torque. 
         [0012]    Other aspects of the disclosure will be better understood in view of the attached drawings and the following detailed description of the illustrated embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1A  is a diagrammatic view of a modular hybrid transmission system for a hybrid vehicle that does not include a torque converter; 
           [0014]      FIG. 1B  is a diagrammatic view of an alternative embodiment of a modular hybrid transmission system for a hybrid vehicle that includes a torque converter; 
           [0015]      FIG. 2  is a control diagram illustrating a torque compensation algorithm that compensates for engine drag and for maintaining vehicle acceleration during engine start; 
           [0016]      FIG. 3  is a control diagram for a transient clutch engagement detection system; 
           [0017]      FIG. 4  is a an expanded control diagram illustrating a torque compensation algorithm that compensates for engine drag and for maintaining vehicle acceleration during engine start in greater detail; and 
           [0018]      FIG. 5  is a graphical representation of a torque compensation system compensating for engine drag and for maintaining vehicle acceleration during engine start. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    A detailed description of the illustrated embodiments of the present invention is provided below. The disclosed embodiments are 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. The specific structural and functional details disclosed in this application are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art how to practice the invention. 
         [0020]    Referring to  FIGS. 1A and 1B , a modular hybrid transmission  10  is shown in a diagrammatic form. An engine  12  is operatively connected to a starter  14  that is used to start the engine  12  when additional torque is needed. A motor  16 , or electric machine, is operatively connected to a driveline  18 . A disconnect clutch  20  is provided on the driveline  18  between the engine  12  and the electric machine  16 . A transmission  22 , or gear box, is also provided on the driveline  18 . Torque transmitted from the engine  12  and motor  16  is provided to the driveline  18  to the transmission  22  that provides torque to the wheels  24 . A launch clutch  26  is provided between the transmission  22  and the engine  12  and/or motor  16  to provide torque through the transmission  22  to the wheels  24 . As shown in  FIG. 1A , launch clutch  26 A is provided between the transmission  22  and the engine  12  and/or motor  16  to provide torque through the transmission  22  to the wheels  24 . As shown in  FIG. 1B , a torque converter  26 B is provided between the transmission  22  and the engine  12  and/or motor  16  to provide torque through the transmission  22  to the wheels  24 . While elimination of the torque converter is an advantage of the embodiment of  FIG. 1A , the present disclosure is also advantageous in reducing vibrations in systems having a torque converter  26 B like that shown in the embodiment of  FIG. 1B . 
         [0021]    The vehicle includes a vehicle system control (VSC) for controlling various vehicle systems and subsystems and is generally represented by block  27  in  FIG. 1 . The VSC  27  includes a plurality of interrelated algorithms which are distributed amongst a plurality of controllers within the vehicle. For example, the algorithms for controlling the MHT powertrain are distributed between an engine control unit (ECU)  28  and a transmission control unit (TCU)  29 . The ECU  28  is electrically connected to the engine  12  for controlling the operation of the engine  12 . The TCU  29  is electrically connected to and controls the electric machine  14  and the transmission  22 . The ECU  28  and TCU  29  communicate with each other and other controllers (not shown) over a hardline vehicle connection using a common bus protocol (e.g., CAN), according to one or more embodiments. Although the illustrated embodiment depicts the VSC  27  functionality for controlling the MHT powertrain as being contained within two controllers (ECU  28  and TCU  29 ) other embodiments of the HEV include a single VSC controller or more than two controllers for controlling the MHT powertrain. 
         [0022]    Referring to  FIG. 2 , an overview of the control algorithm  30  is illustrated. The VSC  27  includes a torque control algorithm  30 , or strategy, disconnect clutch  20  and launch clutch  26  that permit the modular hybrid transmission  10  to function without a torque converter to obtain additional operating efficiency. The control algorithm may be contained within the TCU  29  according to one or more embodiments, or may be incorporated in hardware or software control logicas described in detail below. A base torque determination strategy  32  is developed in a torque control system which controls operation of the engine  12  and motor  16  (shown in  FIG. 1 ) and provides a raw motor torque command output signal  36 . A clutch engagement detection algorithm  38  sets a flag signal  40  in the control system when the disconnect clutch  20  is in the process of becoming engaged with the driveline. The flag is removed when the clutch is fully engaged that may be indicated by comparing the speed of rotation of the engine  12  and the motor  16 . When the speed of rotation of the engine  12  and the motor  16  are equal to each other within a specified tolerance the clutch is determined to be fully engaged. 
         [0023]    A feed forward adjustment algorithm  42  is provided to compensate for engine drag that occurs when the engine  12  is started upon actuation of the starter  14 . When the starter  14  engages the engine  12  negative engine torque occurs. To compensate for the negative engine torque, the torque output of the motor  16  is ramped up before the clutch engagement period. The feed forward adjustment algorithm  42  provides an engine drag torque adjustment signal  44  that is indicative of engine drag to request that the torque output of the motor be ramped up before and during the clutch engagement period. 
         [0024]    A feedback adjustment algorithm  48  is provided to maintain vehicle acceleration during the period of clutch engagement. When the vehicle is accelerating prior to the clutch engagement period, motor torque may be adjusted to maintain the same acceleration and thereby enhance vehicle driveability. Acceleration of the vehicle before the clutch engagement period is recorded by the controller. A filtered vehicle acceleration signal is feedback to the controller in a closed loop and an acceleration feedback signal  50  is provided. 
         [0025]    The torque adjustment for engine drag signal  44  and the acceleration feedback signal  50  are added and filtered at block  54  to provide a motor torque adjustment signal  56 . When the clutch adjustment flag is set to “true” the motor torque adjustment signal is provided as a signal at  58  to be added to the raw motor torque command output signal  36  at block  60  and a motor torque command is provided at  62  to the motor  16 . 
         [0026]    Referring to  FIG. 3 , the clutch engagement detection circuit  38  is shown in greater detail. The clutch engagement detection algorithm begins by starting a timer at  66 . The system determines the time boosting value at  68  based upon inputs including a hydraulic oil temperature signal  70  and a hydraulic line pressure signal  72 . Other signals may also be used to more closely approximate the time required to boost the clutch fluid pressure prior to beginning clutch engagement. The temperature signal  70  and line pressure signal  72  are used to determine the time boosting factor in systems where if fully disengaged the clutch pressure is permitted to fall below a stroke pressure value to zero and thereby further improve system efficiency. 
         [0027]    In systems where the stroke pressure is always maintained by the hydraulic pump that provides hydraulic oil under pressure to the disconnect clutch  20  (as shown in  FIG. 1 ), the step of determining the time boosting factor may be omitted. However, in a system where time boosting is required to compensate for delays relating to filling and pressurizing the disconnect clutch  20 , the time T corresponding to the start of the timer when the stroke pressure is applied at  66  is compared to the time boosting value at  74 . If the time T is less than the time boosting factor, the flag for clutch engagement is set to equal false at  76 . Alternatively, if the time T is not less than the time boosting factor at  74 , the algorithm proceeds to  78  where it is determined whether the clutch is engaged by taking the absolute value of the difference between engine speed (ω e ) and motor speed (ω m ). If the absolute value is less than a specified tolerance value (∂), the flag is set to clutch engagement true at  80 . When the flag is set at  80 , block  84  (shown in  FIG. 4 ) enables the motor torque adjustment signal to be used as will be explained below with reference to  FIG. 4 . 
         [0028]    The engagement detection algorithm  38  first detects the beginning of the contact point at which the clutch force begins to drag the engine up to overcome engine inertia. The clutch travel distance and boosting time (Time boosting ) before the clutch transmits torque are approximately predictable and may be derived based upon a stored value table. The duration of Time boosting  can be inferred from the line pressure command alone assuming that the impact of the temperature of the hydraulic oil is negligible. The relationship of Time boosting  and line pressure can be captured in a calibration table that may be construed empirically based upon clutch engagement experimentation testing. The timing of the contact point may be inferred from the known Time boosting  value and the known timing of the clutch pressure command. The ending point of the engagement when the clutch is fully engaged can be detected by measuring the difference between the engine and motor speeds. Clutch engagement is completed when the engine speed signal and motor speed signal are equal or within a predetermined difference. 
         [0029]    In systems where a minimum stroke pressure is always maintained by the hydraulic system of the clutch, the clutch engagement detection may begin with application of the stroke pressure without requiring the calculation of a Time boosting  timing factor. In such systems, the clutch engagement flag is immediately set upon application of the stroke pressure to the clutch and terminates when the engine and motor speeds are close enough or equal as indicated previously. 
         [0030]    Referring to  FIG. 4 , the clutch engagement detection algorithm  38  is shown to include inputs for engine speed at  86  and for motor speed at  88  that are used to determine if the clutch is engaged at  78  in  FIG. 3 . 
         [0031]    In the feedback adjustment algorithm  48 , an acceleration pre-engagement signal  90  is filtered at block  92  and is maintained as the set point for a PI controller  94 . A vehicle acceleration signal  96  is filtered at block  98  and is provided as feedback to the PI controller  94  for closed-loop control. 
         [0032]    The feedback adjustment algorithm  48  also receives a driver power command  100  that is evaluated by a change of mind detection algorithm at block  102 . A change of mind determination may be indicated if the driver “tips out” by removing pressure from the accelerator pedal or by applying the brakes of the vehicle. One approach for detecting a change of mind “tip out” event is to determine whether the driver power command changes from dP drv /dt&gt;0 to d drv /dt &lt;=0. If a change of mind is detected at block  102  a flag is set and NOR gate  104  is set and provides a signal  106  immediately cancelling the torque feedback adjustment. If no change of mind is determined, the feedback adjustment algorithm  48  provides motor torque adjustment signal  56 . 
         [0033]    In the feed forward adjustment algorithm  42 , negative engine torque during engine start is anticipated. Motor torque is ramped up based upon a clutch pressure signal  110  that is adjusted in a calibration table at block  112  to determine a value Kp for gain scheduling that is provided to P controller  114 . The P controller  114  also receives the engine speed input signal  86  and the motor speed input signal  88  that are provided to a subtractor  116 . The P controller  114  provides a feed forward value that is filtered at  118  and provided as the drag torque adjustment signal  44  to a block  120  to be summed with the acceleration feedback signal  50 . The output of the block  120  is filtered at block  54  and the motor torque adjustment  56  is gated through the block  84 . The output of the block  84  is combined with the raw motor torque command  36  at block  60  to provide a motor torque command  62  to the motor  16 . 
         [0034]    Referring to  FIG. 5 , feed forward and feedback adjustment of the motor torque is illustrated. The line illustrating the disconnect clutch pressure  124  begins at a point at which the engine is not operational and the vehicle is being powered by the electric motor. The disconnect clutch pressure  124  in systems where the stoke pressure is permitted to drop to zero is presumed to be at zero. A starter signal  126  indicates that in the initial period the starter is stopped, but upon initiation of engine operation, the starter motor  14  is initiated as indicated by the elevated portion of line  126 . Upon initial start-up, maximum pressure is provided to fill the clutch  20  with hydraulic fluid. Upon filling, pressure within the clutch  20  is permitted to drop to the system stoke pressure level just prior to the time that the clutch force begins to drag the engine  12 . The engine speed, shown by line  128 , is initially zero, but begins to increase shortly after the initial starting command. At this point, the starter  14  has started the engine  12  and fuel is provided to the engine  12  and engine speed  128  increases as the result of the beginning of the combustion process. Engine speed  128  continues to increase until it reaches the motor speed indicated by line  130 . Upon the engine speed  130  reaching the motor speed  128 , a determination is made that the clutch is fully engaged. 
         [0035]    Referring to line  136 , representing the motor torque, motor torque remains relatively constant throughout the pre-starting and clutch engagement process. The engine torque, shown by line  138  is initially negative when the starter/motor begins providing starter torque as shown by line  140 . Engine torque increases rapidly after the engine starts at which point the engine rotation is being assisted by both the motor torque, as shown by line  136 , and by the engine torque, as shown by line  138 . Transmission of engine torque through the clutch is shown by line  142  that indicates initial engine torque transmitted to the clutch  142  is negative, but as the engine torque  138  increases, the engine torque transmitted to the clutch likewise increases as shown by line  142 . Full engagement of the clutch is reached at dotted line  132 . 
         [0036]    With continued reference to  FIG. 5 , the feed forward motor torque adjustment is represented by line  144 . At the beginning of the window Time prep  motor torque is rapidly increased just prior to beginning the clutch engagement process. The additional engine torque compensates for the negative engine torque caused by inertia drag. The increase in engine torque is gradually reduced to zero when full engagement of the clutch is achieved at  132 . 
         [0037]    Feedback adjustment to maintain vehicle acceleration is illustrated by line  148  in  FIG. 5 . It is estimated that starting when the clutch force begins to drag the engine, a gradual increase in motor torque is provided that adjusts the motor torque based upon the feedback. As the engine starts to produce positive torque, the request for increased motor torque peaks and is then gradually phased out. However, it should be understood that the actual shape of the torque response curve may vary from the illustrated curve. When the clutch is fully engaged and Flag engagement  is set to false, the feedback acceleration adjustment is terminated. 
         [0038]    While exemplary 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.