Abstract:
A system and method for controlling a hybrid vehicle including a transmission having a torque converter with a bypass clutch include controlling the slip between the impeller and the turbine of the torque converter in slip mode operation to regulate the converter torque ratio and maintain substantially constant torque at the turbine. Controlled slip uses the hydrodynamic coupling of the torque converter to balance the desired and delivered torque while damping torque disturbances transmitted through the driveline to manage noise, vibration and harshness (NVH).

Description:
TECHNICAL FIELD 
     The present invention relates to control systems and methods for vehicles utilizing a hybrid powertrain, such as a modular hybrid transmission configuration with a traction motor between an engine and a transmission having a torque converter. 
     BACKGROUND 
     Conventional automatic vehicles may include a transmission having a torque converter to provide a hydrodynamic coupling with torque multiplication. The hydrodynamic coupling allows the engine to continue running while connected to the transmission when the vehicle is stationary. In addition, the torque converter provides torque multiplication to assist vehicle launch and provides damping of driveline torque disturbances. The torque multiplication or torque ratio varies with the speed difference or slip between the torque converter input element (impeller) and output element (turbine). A torque converter clutch or bypass clutch may be provided to mechanically or frictionally couple the impeller and the turbine to eliminate the slip and associated losses to improve efficiency. However, driveline torque disturbances are then more easily transmitted to the vehicle cabin and may result in noise, vibration, and harshness (NVH) and reduce vehicle driveability. As such, the torque converter bypass clutch is usually disengaged or released when the vehicle operating conditions are likely to produce driveline torque disturbances. 
     Various hybrid vehicle configurations have been developed that utilize both an engine and a motor to drive a vehicle through a transmission, which may be implemented by various types of transmissions that may or may not include a torque converter depending on the particular application. For example, a continuously variable transmission (CVT) or automated manual transmission (AMT) may not include a torque converter whereas a step-ratio automatic transmission having a torque converter may be used to provide similar advantages as in a conventional powertrain as previously described. 
     Hybrid vehicles generally include an electrical drive mode where the motor is used to power the vehicle and the engine is off. Applications having a torque converter bypass clutch may engage or lock the bypass clutch in the electrical drive mode to improve efficiency. Another hybrid vehicle operation mode uses both the engine and the motor to power the vehicle. A rolling engine start may be used when the vehicle is moving to transition from the electrical drive mode to the hybrid drive mode. The bypass clutch is typically disengaged during engine start to mitigate associated driveline torque disturbances. However, this reduces efficiency as previously described. As a rolling engine start event happens, the traditional control approach does not address the complexity of the power transition and its impact on the driveability. 
     SUMMARY 
     Embodiments according to the present disclosure include systems and methods for controlling a hybrid vehicle having a traction motor disposed between an engine and a transmission having a torque converter including an impeller and a turbine. In one embodiment, a method for controlling a hybrid vehicle includes controlling slip speed between the impeller and the turbine of the torque converter to maintain turbine torque substantially constant during starting of the engine. 
     In one embodiment, a method for controlling a hybrid vehicle having an engine selectively coupled by a disconnect clutch to a traction motor coupled to an automatic transmission having a torque converter with an impeller, a turbine, and a torque converter clutch, includes operating the vehicle to provide a driver demanded torque in an electric drive mode using only the traction motor with the disconnect clutch disengaged and the torque converter clutch locked with zero slip speed between the impeller and the turbine; engaging the disconnect clutch to start the engine using traction motor torque; controlling torque converter clutch apply pressure to control the slip speed and provide an associated converter torque ratio that maintains a substantially constant turbine torque to compensate for the traction motor torque used while starting the engine; controlling torque converter clutch apply pressure to control the slip speed and provide an associated converter torque ratio based on a combined engine torque and traction motor torque to provide the driver demanded torque after starting the engine; and controlling torque converter clutch apply pressure to lock the torque converter clutch and reduce the slip speed to zero after starting the engine. 
     Embodiments may also include a hybrid electric vehicle having an engine, an automatic transmission including a torque converter with an impeller, a turbine, and a bypass clutch, a traction motor coupled to the impeller and selectively coupled to the engine by a disconnect clutch, and a controller configured to control the bypass clutch to vary a speed difference between the impeller and the turbine to maintain turbine torque substantially constant while starting the engine using traction motor torque. In one embodiment, the controller is configured to control the bypass clutch to provide a desired converter torque ratio based on the speed difference, with the desired converter torque ratio being based on a driver demanded torque, current transmission gear ratio, transmission losses, and traction motor torque used to start the engine. In one embodiment, the controller is configured to modulate apply pressure of the bypass clutch to control the speed difference between the impeller and the turbine. The controller may also be configured to lock the bypass clutch to reduce the speed difference to zero after starting the engine. In various embodiments, the controller is configured to increase the speed difference while starting the engine by reducing the apply pressure of the bypass clutch. 
     The present invention resolves various challenges associated with prior engine start strategies by providing a modular hybrid transmission (MHT) configuration and a control system in a production hybrid vehicle. The modular hybrid transmission configuration includes an automatic transmission having a torque converter that couples input from one or both driving sources, in the form of an engine and a motor, to the transmission as determined by a powertrain controller. 
     In a modular hybrid transmission vehicle, that emphasizes both fuel economy and driveability, the control systems operate the engine, the motor, and clutches including the torque converter bypass clutch so that the driver feels that the vehicle operates smoothly and effectively in response to drive demands and to reduce noise, vibration and harshness (NVH) often attendant to mechanically connected driveline parts. 
     In one embodiment of the present invention, the transmission output may be coupled to a rear driveline including a geared differential. Such systems may require greater torque handling capacity than previously known front wheel drive hybrid vehicles. The powertrain controller includes computer-based processing of data, analyzing of sensed signals, and providing actuator signals. When fuel economy is emphasized by the powertrain controller, the torque converter is locked under most conditions, but unlocks to a controlled slip mode upon demand, and is controlled in accordance with a control algorithm to dampen driveline torque disturbances during engine starting while improving energy efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be more clearly understood by reference to the following detailed description of a preferred embodiment, when read in conjunction with the accompanying drawing figures, in which like reference characters refer to like parts throughout the views, and in which: 
         FIG. 1  is a diagrammatic view of a hybrid vehicle driveline with a modular hybrid transmission incorporating a control system operation according to the present invention; and 
         FIG. 2  is a flow chart of the control algorithm operating a driveline control according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As required, 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 first to  FIG. 1 , a vehicle  10  is shown comprising a hybrid driveline  12  with a first power source in the form of internal combustion engine  14  and a second power source in the form of a battery  16  that powers a traction motor  18 . One or more of the power sources may be coupled together and disengaged from each other in the driveline by means of a disconnect clutch  20 . The disconnect clutch  20  may also be used to rotate the input shaft of the motor  18  by the clutch  20  so that operation of the engine  14  serves to charge the battery  16  as the motor  18  acts as a generator. 
     In the present embodiment, the disconnect clutch  20  disconnects the motor  18  and the engine  14  in the electrical drive mode whereby only the motor  18  is available to power the driveline. In the hybrid drive mode, the disconnect clutch  20  couples the engine  14  with the motor  18  when a rolling engine start command is generated from the powertrain controller  40 . The powertrain controller  40  generates a response when the need for more drive demand or the system demand is sensed. For example, a driver&#39;s manipulation of the actuator  42 , or the sensing of system demand, such as insufficient motor support for a demand due to the status of the battery  16  (that may include battery state of charge or SOC), may generate such a response. 
     The engine  14  and the motor  18  have an output coupled to a transmission mechanism  24  through the torque converter  26 . In the preferred embodiment, a modular hybrid transmission (MHT)  22  includes mechanical and hydraulic controls for a system of multiple, stepped ratio gears arranged for multiple forward speeds, reverse speed and a neutral position. In addition, MHT  22  includes a torque converter  26 . The torque converter  26  includes an impeller, and a turbine that rotates in response to fluid flow from the impeller to the turbine. A bypass clutch  27  provides a frictional coupling between the impeller and turbine of torque converter  26  and is controlled by the powertrain controller  40 . 
     The bypass clutch  27  manages fluid pressure between the impeller and turbine of the torque converter to provide three modes of bypass clutch operation, and torque multiplication may occur depending on the amount of slip between the impeller and turbine sides. In open mode, a maximum amount of fluid is carried by the torque converter housing, separating the impeller from the turbine. In a locked mode, the minimum fluid pressure is carried in the torque converter so the pressure does not separate the impeller from the turbine and they become frictionally or mechanically locked together to eliminate slip and associated losses to improve energy efficiency. In a slip mode, a target amount of slip may be employed between the impeller and the turbine, whereby the fluid may provide the target torque ratio for the torque multiplication, in addition to NVH damping, but fuel economy is reduced due to the heat generated as a result of a slipping. 
     At the rear portion of the driveline, a drive shaft output  28  is linked to a differential  30  in the well-known manner of engine powered production vehicle systems, and in turn drives both of the rear wheels  32 . 
     In accordance with a control system of one embodiment of the present invention, a powertrain controller  40  can include a distributed or consolidated set of operating systems including an engine control module (ECM), a transmission control module (TCM) and a vehicle systems controller (VSC), for example. In the representative embodiment illustrated, an input demand actuator  42 , such as an accelerator pedal, is linked either electronically, mechanically or by other systems to the powertrain controller  40 . In particular, the actuator  42  permits the driver to control powertrain power to the vehicle and governs performance of the vehicle. The present invention improves driveability, the driveline&#39;s ability to react with reduced noise, vibration or harshness (NVH) that may be perceived by the driver and affect the driver&#39;s sense of complete and accurate control of the numerous operations being conducted throughout the drivetrain  12  including a modular hybrid transmission with control methods of the present invention. 
     In a typical electrical drive mode at a higher selected stepped ratio transmission gear number (or lower gear ratio), a torque converter bypass clutch  27  is normally fully locked to improve the fuel economy. As a result, driveline torque disturbances resulting in noise, vibration and harshness may be felt as the engine  14  starts in a rolling start as the powertrain controller  40  reacts to the drive demand or the system demand and commands the power transition from the motor  18  only to the engine  14  or a combination of both the motor  18  and the engine  14 . According to various embodiments of the present invention, the bypass clutch  27  is controlled to set torque converter  26  in a controlled slip mode around a target slip during the rolling engine start event. This target slip will generate a speed differential between the impeller and turbine to provide a corresponding target converter torque ratio for the torque multiplication to adjust delivered torque towards a desired torque resulting from the torque demanded at the transmission output shaft, and compensating for the loss of the electrical motor torque due to the amount of torque used to assist in the quick rolling engine start ( 58 ,  FIG. 2 ). While in the controlled slip mode, the torque converter  26  provides damping of torque disturbances to reduce or eliminate any noise, vibration and harshness from engine start and power transition. In addition, controlled slip operation improves energy (battery and fuel) efficiency relative to completely disengaging the bypass clutch  27 . 
     Due to the nature of the two power sources of a conventional combustion engine  16 , and battery  18  powering electrical motor  20 , in the vehicle  10 , the driveability is a concern associated with the transition from one power source to the other power source. In a typical electrical drive mode, the vehicle is moving with a certain torque demand only supplied by the electrical motor  20  powered by the battery  18  with the torque converter  26  in locked mode to improve the fuel economy. When there is a need for engine power, the engine  14  has to quickly start and become a power source. During this rolling engine start event, the driveline  12  has to maintain substantially the same wheel torque while in the meantime robustly and quickly starting the engine  14 , and seamlessly completing the power transition through the driveline from one to the other, or combining both, power sources. In order to improve the driveability, the improvement is to control the torque converter  26 &#39;s slip speed and associated target torque ratio by operation of the bypass clutch  27  in the slip mode such that the torque converter  26  of the MHT  22  is used to maintain the wheel torque (and associated turbine torque) substantially constant while damping the noise, vibration and harshness during the power transition. 
     In contrast to various prior art control strategies that start the engine with the bypass clutch locked, or fully disengaged, when transitioning from electric mode to hybrid mode, embodiments of the present invention provide a controlled slip mode to reduce driveline torque disturbances while improving efficiency. Torque converter  26  is controlled by controlling the bypass clutch  27  in slip mode, with the target torque ratio calculated as discussed below. The converter torque ratio is controlled by controlling bypass clutch apply pressure to adjust the delivered torque towards the desired torque, which may be based on the driver demand as indicated by the accelerator pedal, or in response to a system demand, such as when the motor cannot produce enough power due to the status of the battery. Controlling or managing the target torque ratio of the torque converter  26  compensates for the loss of the electrical motor torque due to the motor assisting in the quick rolling engine start event. While in the slip mode, the torque converter  26  damps the noise, vibration and harshness from engine start and power transition. 
     The process of control according to a representative embodiment is more clearly illustrated in  FIG. 2 . The terminologies and equations to calculate a hybrid desired torque ratio and associated slip speed for a representative operating scenario are described below. For example, when the actuator  42  is depressed by a driver as more vehicle torque is demanded (see step  54 ), from electrical drive mode steady state shown at  50  with the locked torque converter at  52 , the powertrain controller  40  analyzes the drive demand and may request an engine rolling start as represented at  54 . Then the powertrain controller calculates the requested turbine torque in response to the drive demand, adjusting for the transmission inefficiency losses, as indicated in the following equation: 
                   Tq_TurbineRequested   =       (     Tq_TransOutDemand   +   Tq_TransOutLoss     )         GearRatio   ⁡     (   Gear   )       -   TransLossRatio               1   )               
The torque converter is then operated in slip mode as shown at  56  to maintain a substantially constant turbine torque by controlling apply pressure of the bypass clutch to control the slip, speed ratio, and torque ratio of the torque converter as illustrated in the following equations. By hydraulic design, the torque converter torque ratio is associated with the speed ratio and the speed ratio or slip speed is controlled by modulation of the bypass clutch apply pressure. Therefore, by controlling the torque converter target slip, the torque converter torque ratio can be adjusted to maintain a substantially constant turbine torque while using the motor torque to assist with starting the engine.
 
[1] In one embodiment, a target slip speed, speed ratio, and torque ratio are determined according to the following equations:
 
                     Converter_TargetSlip   =       Spd_Impeller   -     Spd_turbine   .     
     ⁢   Converter_TargetSpdRatio       =       Spd_Turbine   Spd_Impeller     =       Spd_Impeller   -   Converter_TargetSlip     Spd_Impeller           ⁢     
     ⁢       Converter_TargetTqRatio   =     f   ⁢           ⁢   TransConverterRatio   ⁢           ⁢     (   Converter_TargetSpdRatio   )         ,             2   )               
The desired impeller torque is directly related to the requested turbine torque and the torque converter torque ratio, in addition to transmission pumping inefficiency losses, as illustrated by the following equation:
 
 Tq _Impeller Desired= Tq _TurbineRequested÷Converter_Target Tq Ratio+Tq_TransPumpLoss  3)
 
As one of ordinary skill in the art will recognize, during the rolling engine start, the delivered impeller torque is the delivered motor torque, which is offset by the amount of torque used to assist in the engine start, as illustrated by the following equation:
 
 Tq _ImpellerDelivered= Tq _MotorDelivered− Tq _EngineStart_Assist  4)
 
After the completion of the rolling engine start, the delivered impeller torque is the sum of the delivered engine torque and the delivered motor torque, as illustrated by the following equation:
 
 Tq _ImpellerDelivered= Tq _EngineDelivered+ Tq _MotorDelivered  5)
 
Through controlling the bypass clutch and slip speed to make the torque converter generate the target torque ratio, the delivered impeller torque should generate the desired turbine torque based on the torque ratio of the converter before, during, and after the rolling engine start as illustrated by the following equations:
 
 Tq _ImpellerDelivered= Tq _Impeller Desired; therefore
 
Converter_Target Tq Ratio= Tq _TurbineRequested÷( Tq _ImpellerDelivered− Tq _TransPumpLoss)  6)
 
When the engine start is completed, the disconnect clutch  20  may be fully engaged or locked to frictionally or mechanically link the engine  14  with the motor  18 , the vehicle will drive in the hybrid drive mode combining engine and motor torque.
 
     As such, various embodiments of the present invention control the bypass clutch  27  to operate the torque converter  26  in a controlled slip mode ( 56 ). Appropriate control of the bypass clutch to control slip to a target slip (target slip ratio) regulates the associated converter torque ratio which can be used to adjust the delivered torque towards the desired torque at the impeller and turbine. At the same time, the slipping torque converter  26  is a very good damping device to reduce the vibration and harshness from engine start and power transition. After the rolling engine start, the power source may change to the engine  14  as needed, or the combination of both the engine and the electrical motor. In order to improve the fuel economy, the controller will lock the torque converter ( 68   FIG. 2 ) once the demand has been met, as further economy may be realized by adjusting the engine torque and the motor torque contribution to the total powertrain torque. 
     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.