Abstract:
A method for controlling vehicle regenerative braking includes decreasing regenerative braking, provided a converter clutch is locked, such that regenerative braking torque reaches zero before a converter clutch opens due to vehicle speed reaching a reference speed; decreasing regenerative braking, provided the converter clutch is scheduled to open, such that regenerative braking torque reaches zero before the converter clutch opens due to vehicle speed reaching the reference speed; and braking the vehicle using wheel brakes.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a method for controlling a regenerative braking event in a hybrid electric vehicle wherein blend-out control is coordinated with torque converter control. 
         [0003]    2. Description of the Prior Art 
         [0004]    In order to maximize fuel economy in a hybrid electric vehicle, energy saving through regenerative braking with the electrical motor during a brake event is an integral part of the system. A high voltage battery can be charged through regenerative braking while the vehicle is slowing down. On the other hand, to maintain vehicle safety, conventional friction braking is also used when regenerative braking by itself cannot meet the driver demand. At low speed before the regenerative braking becomes unavailable or inefficient; regenerative braking torque should be ramped down to zero gradually and the conventional friction braking torque should ramped up to the driver demand to facilitate smooth brake torque transition. This process is referred as regenerative braking torque blend-out. Thus the driver brake demand can be satisfied with acceptable vehicle drivability, fuel efficiency and vehicle safety. 
         [0005]    In a modular hybrid electric vehicle, a step ratio transmission and torque converter are located between an electrical motor and the driving axle with wheels. During a regenerative braking event, through the transmission and the torque converter the vehicle kinetic energy charges the high voltage battery by rotating the electrical machine. The torque converter clutch must be locked in order to fully transmit the vehicle kinetic energy to the electrical motor. If the torque converter must be open, it would transmit much less vehicle kinetic energy. In this case, the regenerative braking torque blend-out should be completed before the torque converter opens to prevent stalling the electrical motor. 
       SUMMARY OF THE INVENTION 
       [0006]    A method for controlling vehicle regenerative braking includes decreasing regenerative braking, provided a converter clutch is locked, such that regenerative braking torque reaches zero before a converter clutch opens due to vehicle speed reaching a reference speed; decreasing regenerative braking, provided the converter clutch is scheduled to open, such that regenerative braking torque reaches zero before the converter clutch opens due to vehicle speed reaching a the reference speed; and braking the vehicle using wheel brakes. 
         [0007]    Fuel economy, drivability and vehicle safety are major concerns of hybrid electric vehicle control. The coordination between the torque converter and regenerative braking system control provides dynamic, real-time regenerative braking blend-out control that will benefit the fuel economy, the drivability and the vehicle safety. Firstly, the prediction and notification of the torque converter clutch open state allows regenerative braking system to fully utilize the opportunity to capture the kinetic energy as much as possible until the torque converter clutch is about to open. Secondly, the ramp curve supports a smooth torque transition. 
         [0008]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0010]      FIG. 1  is a schematic diagram showing a modular hybrid electric powertrain for a motor vehicle; 
           [0011]      FIGS. 2A and 2B  comprise a flow diagram of an algorithm for controlling regenerative brake torque blend-out in coordination with operation of the torque converter clutch; and 
           [0012]      FIG. 3  comprises graphs that illustrate regenerative brake torque blend-out control coordinated with a torque converter control timing chart. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]    Referring first to  FIG. 1 , a parallel hybrid electric powertrain  10  includes an internal combustion engine  12 , engine disconnect clutch  14 , electric machine or motor/generator  16 , transmission hydraulic pump  18 , torque converter  20 , torque converter lock-up clutch  22 , transmission gearing  24 , final drive gearing  26 , shafts  28 ,  29 , and driven wheels  30 . A low voltage starter  32 , powered by a low voltage battery  34 , cranks the engine while starting the engine  12  and producing sustained combustion. A high voltage battery  36  powers the electric motor/generator  16 . 
         [0014]    The torque converter  20  is a hydraulic coupling that produces a hydrokinetic drive connection between an impeller, which is driveably connected to the engine  12  when clutch  14  is closed, and a turbine, which is driveably connected to the driven wheels  30 . 
         [0015]    The torque converter lock-up clutch  22  alternately opens and closes a drive connection between the torque converter&#39;s turbine and the shaft  38 . 
         [0016]    A vehicle equipped with this powertrain  10  can produce electric drive and hybrid electric drive and can charge the battery  36  either by regenerative braking, i.e., recovering and converting kinetic energy of the vehicle during a braking event to electric energy that can be stored in battery  36 , or by using the engine to charge battery  36 . 
         [0017]    The fuel economy benefit in a hybrid electric vehicle results mainly from its ability to perform regenerative braking. In powertrain  10  motor  16  is coupled to the wheels through the torque converter  20 , transmission gearing  24  and final drive  26 . The torque converter  20  transmits torque through the combination of the hydraulic path and the mechanic path, provided the torque converter clutch  22  is slipping. If the torque converter clutch  22  is fully open, torque can only be transmitted through the hydraulic path. If the clutch  22  is fully locked, the torque can only be transmitted through the mechanical path. 
         [0018]    During regenerative braking, torque is transmitted from the wheels  30  to the electric machine  16 . If clutch  22  is open, the torque converter&#39;s ability to transmit torque in the reverse direction is very limited. To recoup most of the kinetic energy using regenerative braking, the torque converter clutch  22  should be kept locked while the vehicle is slowing down. 
         [0019]    The torque converter clutch  22 , however, must be opened for various reasons. When impeller speed is low, clutch  22  must be open so that the electric motor and/or engine  12  do not stall. When clutch  22  is open, the hydraulic path serves as coupling to deliver torque smoothly to the wheels  30 . 
         [0020]    The control coordinates regenerative braking system control and torque converter control. During regenerative braking operation, the vehicle control system coordinates the regenerative braking portion of the brake torque and the conventional friction portion of the brake torque to make sure the total brake torque meets the driver brake demand and the vehicle deceleration performance. If for any reason, the regenerative brake torque must be reduced, it must be controlled such that it reduces smoothly and the conventional friction brake torque fills in the gap concurrently. This control process is called Regen torque blend-out. Regenerative torque blend-out control should be transparent to the vehicle regenerative torque blend out operator regarding the source of the brake torque and the change in contribution to brake torque from the two sources, while maintaining an uncompromised overall vehicle deceleration performance and safety. 
         [0021]    During regenerative torque blend-out control, the regenerative portion of the brake torque is reduced relative to the magnitude of total brake torque. This reduction is called regenerative torque ramp out. 
         [0022]    The flow diagram of  FIGS. 2A and 2B  illustrates steps of a control algorithm  40  for controlling regenerative brake torque blend-out in coordination with operation of the torque converter clutch  22 . Control algorithm  40  is divided into three phases. In phase 1 the torque converter control predicts whether to open the torque converter within a period of time. If the torque converter is going to open, the control predicts the vehicle speed at which the torque converter opens. In phase 2, the torque converter control commands that the torque converter clutch  22  open and continues to monitor the state of clutch  22  during any normal and emergency torque converter open events. In phase 3, the control determines and reports the torque converter actual open state. 
         [0023]    During regenerative braking operation, the vehicle control system coordinates the regenerative braking portion of the brake torque and the conventional friction portion of the brake torque to ensure that the total brake torque meets the driver brake demand and the vehicle deceleration performance. If for any reason the regenerative brake torque must be reduced, it must be controlled such that its reduction is smooth and the conventional friction brake torque fills in the gap concurrently. This control process is called regenerative brake torque blend out. Successful regenerative brake torque blend out control is transparent to the vehicle operator in terms of the brake torque source and contribution change and produces uncompromised overall vehicle deceleration performance and safety requirement. 
         [0024]    During regenerative braking blend out control, the regeneration portion of the brake torque reduces its contribution to the total brake torque. This reduction of the regenerative braking portion of the brake torque is called regeneration torque ramp out. 
         [0025]    Regenerative brake torque blend-out control will be aligned with the torque converter open control during all three phases. The regenerative brake torque blend-out is controlled through a ramp ratio which is designed to decrease monotonically during each phase, with different calibratable time durations, until the blend-out is completed. In phase 1 the blend-out time duration will be aligned with the time used in the torque converter open prediction. In phase 2, the time duration will be aligned with the torque converter opening actuation time under normal or emergency opening events. In phase 3, the regenerative brake torque will be reduced to zero immediately due to torque converter clutch state being open. To make sure the smooth torque transition, in phase 1 and 2, the ramp ratio change rate is calculated based on the expected time to complete the event and the difference of the target ramp ratio and the ramp ratio at the moment of the last torque converter phase change. 
         [0026]    At step  42  the vehicle is running normally with no regenerative braking and the converter clutch  22  fully locked. 
         [0027]    At step  44  a test is performed to determine whether regenerative braking is occurring. If the result of test  44  is logically false, control returns to step  42 . 
         [0028]    If the result of test  44  is logically true, at step  46  a test is performed to determine whether converter clutch  22  is locked, opening or open. If converter clutch  22  is open, Phase 3 control begins at step  48  by setting any regenerative brake torque request to zero. Then control advances to step  50  where conventional brake torque, i.e., wheel brake torque produced by friction, is increased to meet the entire vehicle braking torque demand. 
         [0029]    If test  46  indicates that converter clutch  22  is locked, at step  52  the current vehicle speed and vehicle deceleration rate are determined based on the output of a suitable speed sensor or sensors. 
         [0030]    At step  54  the predicted vehicle speed based on the current vehicle speed and vehicle deceleration rate within a calibratable predicted period of time is determined. 
         [0031]    At step  56  a test is performed to determine whether the predicted vehicle speed is lower than a reference torque converter unlocking speed. If the result of test  56  is negative, control returns to step  42  after ensuring at step  58  that the vehicle is operating with converter clutch  22  locked. 
         [0032]    If the result of test  56  is positive, at step  60  Phase 1 control begins because torque converter clutch  22  is going to open within the predicted period. 
         [0033]    At step  62 , a regenerative brake torque ramp-out ratio is established based on the length of the predicted period. 
         [0034]    At step  64 , the regenerative brake torque request is decreased based on the ramp-out ratio until the regenerative brake torque request is zero. Control then advances to step  50 . 
         [0035]    If test  46  indicates that converter clutch  22  is opening, at step  66  a test is performed to determine whether torque converter clutch  22  is opening under a normal event or an emergency event? 
         [0036]    If the result of test  66  indicates that an emergency event is causing converter clutch  22  to open, at step  68  Phase 2a begins because torque converter clutch  22  is opening under an emergency event. 
         [0037]    At step  70 , a regenerative brake torque ramp-out ratio is established based on the length of the emergency opening period. 
         [0038]    At step  72 , the regenerative brake torque request is decreased based on the ramp-out ratio until the regenerative brake torque request is zero. Control then advances to step  50 . 
         [0039]    But if the result of test  66  indicates that a normal event is causing converter clutch  22  to open, at step  74  Phase 2b begins because torque converter clutch  22  is opening under a normal event. 
         [0040]    At step  76 , a regenerative brake torque ramp-out ratio is established based on the length of the normal opening period. 
         [0041]    At step  78 , the regenerative brake torque request is decreased based on the ramp-out ratio until the regenerative brake torque request is zero. Control then advances to step  50 . 
         [0042]      FIG. 4  illustrates a typical regenerative braking operation event in a MHT hybrid electric vehicle. Regenerative braking occurred while the torque converter was locked  100  and the vehicle operator initiated a brake demand typically by stepping on the brake pedal  102 . The total negative brake torque, which includes both regenerative braking torque and conventional friction brake torque, met the driver brake torque demand. The regenerative braking torque was used in the electrical motor  16  to charge high voltage battery  36 . 
         [0043]    During regenerative braking, the vehicle decelerates while vehicle speed decreases at a certain deceleration rate  104 . Based on the vehicle speed and the vehicle deceleration rate, a predicted vehicle speed  106  within a calibrateable predicted period window  108  is calculated. When this predicted vehicle speed  106  crosses below the torque converter unlock speed threshold  110 , the torque converter clutch  22  is predicted to open within the predicted period  108 . The negative regenerative braking torque  112  is then ramped-out  114  during and based on the predicted period  108 . 
         [0044]    When the actual vehicle speed  104  crosses below the torque converter unlock speed threshold  110  at  116 , the torque converter clutch  22  is in opening phase  118 . The regenerative braking torque is continually ramped out based on the opening time if not ramping out to zero yet. 
         [0045]    When the torque converter clutch  22  is actually open  120 , the regenerative braking torque would be set to zero immediately at  122  if not ramping out to zero yet. This would ensure no regenerative braking while torque converter clutch  22  opened. 
         [0046]    In the meantime, throughout the regenerative braking event, the conventional friction brake torque  124  is controlled to fill in any gap due to the regenerative braking torque change. Thus the total negative brake torque  126  always meets the driver brake torque demand. 
         [0047]    Gear ratio changes may occur during the regenerative braking event at  128 ,  130  and  132 . 
         [0048]    In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.