Abstract:
A method for controlling restart of an engine in a hybrid electric powertrain, includes engaging a gear of a transmission, releasing a brake pedal, maintaining fluid pressure at an adaptively determined magnitude in a wheel brake, initiating a restart the engine, and reducing fluid pressure in the wheel brake when the engine restarts.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a powertrain and brake system for a hybrid electric vehicle (HEV) and, more particularly, to its control during an engine restart. 
         [0003]    2. Description of the Prior Art 
         [0004]    A HEV is a vehicle that combines a conventional propulsion system, which includes an internal combustion engine and a transmission, a rechargeable energy storage system that includes an electric motor and electric storage battery to improve fuel economy over a conventional vehicle. 
         [0005]    Motor vehicles can be designed to employ certain aspects of hybrid electric technology, but without use of a hybrid electric powertrain. Certain vehicles having a conventional powertrain but no electric machine for driving the wheels, called micro-HEVs, shutdown the engine at idle speed to reduce fuel consumption and reduce emissions while the vehicle is stopped. 
         [0006]    During normal vehicle operation many instances arise where the vehicle must stop: at traffic signals, cross-walks, stop signs and the like. In micro-HEVs the engine is shut down if no power is required, e. g. while waiting at a traffic light. As soon as power is requested, the engine is automatically restarted. By avoiding an unnecessary engine idling event, the vehicle&#39;s fuel economy is improved. For this purpose, it is desirable to shut down the engine function as much as possible when certain engine stop conditions are satisfied. 
         [0007]    An engine restart occurs while the transmission is in gear and the gear selector is in the Drive position. During an engine restart in a micro-HEV equipped with an automatic transmission and torque converter, a torque surge occurs substantially in phase with the engine startup speed peak. 
         [0008]    During the time of engine automatic stop the intake manifold pressure has increased from the throttled vacuum of normal running condition to a higher pressure very close or equal to atmospheric pressure. At time of engine restart, this higher atmospheric pressure in the manifold leads to a very large air charge to the combustion cylinders. To achieve the emissions requirements, the proper engine out exhaust “feedgas” fed to the catalyst shall not have an abundance of either oxygen or incompletely combusted fuel by-products. In this respect, the large air charge at restart is matched by a proportionately large stoichiometric fuel injection mass, with the result being a large torque spike on the first several restart combustion events. After the manifold pressure is reduced to a throttled vacuum by nature of the mostly closed throttle and air being withdrawn by cylinders, all of the air charge, fuel injection, torque production, and engine speed decay back to usual idle conditions. 
         [0009]    The torque spike, which is generated by the engine and amplified by the torque converter during the engine restart, is transmitted to the driven wheels if the engine is restarted with the transmission in-gear. The torque spike produces a forward acceleration pulse. Such an uncomfortable vehicle jerk adversely affects the driver&#39;s acceptance to the stop/start function. 
         [0010]    A need exists for a technique that suppresses the torque surge during an engine restart event in a micro-HEV powertrain. 
       SUMMARY OF THE INVENTION 
       [0011]    A method for controlling restart of an engine in a hybrid electric powertrain, includes engaging a gear of a transmission, releasing a brake pedal, maintaining fluid pressure in a wheel brake, initiating an engine restart, and reducing fluid pressure in the wheel brake when the engine restarts. 
         [0012]    A system for controlling the restart in a hybrid electric powertrain, includes a transmission engaged in a gear, a brake pedal, and a controller configured to maintain fluid pressure at a adaptively determined magnitude in a wheel brake, to initiate an engine restart after the brake pedal is released, and to reduce fluid pressure in the wheel brake when the engine restarts. 
         [0013]    The Wheel Torque Disturbance Suppression (WTDS) control requires no new device or hardware modification or improvement, and can be realized with an existing vehicle brake system with only software development. The WTDS control does not consume additional energy; it uses the brake line pressure input by the vehicle driver or by any active brake function to achieve restart assistance to the engine stop/start process and for powertrain torque disturbance suppression. 
         [0014]    The powertrain torque oscillation and jerk are not transmitted to the vehicle wheels thereby providing a smooth vehicle drive-off process. The brake based technology is simpler in control complexity and control actuation method than engine based or transmission based control methods, and the resulting controlled performance is more consistent and robust. 
         [0015]    Brake line pressure provides brake reaction torque that is able to hold the wheel from any motion caused by engine torque or other disturbance torque acting on the vehicle at standstill. The brake action isolates the unexpected powertrain disturbance torque from the vehicle. As the brake pressure ramps down and the powertrain torque increases, vehicle creep or drive-off are performed in response to the driver&#39;s manual control of the accelerator pedal. 
         [0016]    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 
         [0017]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0018]      FIG. 1  is a schematic diagram of a micro-HEV powertrain; 
           [0019]      FIG. 2  is schematic diagram showing a portion of the WTDS control system; 
           [0020]      FIG. 3  are graphs that illustrate the change of certain powertrain variables during a Drive-Reverse shifter engagement and the control of an engine restart; and 
           [0021]      FIG. 4  illustrates a logic flow diagram of the steps of an algorithm for controlling the engine restart. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    Referring now to the drawings, the micro-HEV powertrain  10  of  FIG. 1  includes a power source  12 , such as an internal combustion engine; an enhanced engine starter motor  14 ; an automatic transmission  16 , whose input shaft  17  is connected through a torque converter to the engine; a transmission output  22 ; final drive mechanism  23  connected to the output  22 ; an electric auxiliary hydraulic pump (EAUX)  24 , whose output pressurizes the hydraulic system of the transmission; an electric storage battery  26 , which supplies electric power to the pump  24 , ABS module  27 ; and axle shafts  28 ,  29 , driveably connect to the driven wheels  30 ,  31 . 
         [0023]    A transmission control module (TCM)  42  receives and sends signals to the pump  24  and transmission  16  and receives input signals from the battery  26  and a gear shifter  44 , which moves among P, R, N, D, L positions in an automatic mode channel  46  and between upshift (+) and downshift (−) positions in a manual mode channel  48 . An engine control module (ECM)  50 , which communicates through a CAN with a brake control module  27 , receives and sends signals to the starter  14  and engine  12  and receives input signals from the battery  26  and an accelerator pedal  52 . Signals representing movement of brake pedal  54  are transmitted through a brake actuator  55  to the brake control module  27 , which controls fluid pressure in brake lines  76 ,  77  and wheel brakes  78 ,  79 . 
         [0024]    While the micro-HEV is stopped, at least one wheel brake maintains brake force on at least one of the wheels  30 ,  31  on the driven axles  28 ,  29  and the non-driven wheels  34 ,  35 . After the vehicle operator releases the service brake, wheel torque disturbance suppression (WTDS) control maintains brake pressure in the brake system at a magnitude necessary for powertrain disturbance torque suppression. WTDS control releases the brake line pressure in coordination with the engine torque output once engine speed reaches and passes a predetermined speed. 
         [0025]    The brake release is activated when an engine started flag is set, which is determined when either (i) a certain engine speed reference speed has occurred and the engine restart speed peak has been detected, or (ii) a certain high engine speed threshold has been passed for a certain calibration period and a certain level of calibrated engine speed gradient occurs. 
         [0026]    In addition to normal brake pressure release conditions, brake pressure is released when (i) the brake pedal is reapplied, or (ii) the accelerator pedal is displaced, or (iii) the WTDS timer expires. 
         [0027]    WTDS only functions during engine restart during a predetermined period measured by a WTDS timer. If the engine does not restart properly, WTDS remains disabled after a WTDS timer expires. During the engine restart event, the brake force is neither reduced too early, in which case more powertrain torque surge is transmitted to the driven wheels, nor too late, in which case loss of drivability and degraded performance will result as the brakes drag and prevent the vehicle accelerating from the stop. When the engine stalls and engine restart is attempted, WTDS will extend its timer and wait for the engine to restart. If the engine is down without trying to restart, or if the engine fails attempting to restart, WTDS will give up and exit after its timer expires without further extension. 
         [0028]      FIG. 2  illustrates schematically a portion of a WTDS control system. WTDS control is preferably entered when the micro-HEV is at standstill, engine  16  is stopped, accelerator pedal  52  is released, brake pedal  54  is applied, the park brake is released, the PRNDL gear selector  44  is in Drive range or Low range, and WTDS control is enabled. 
         [0029]    The brake system controls  60 , which include a WTDS brake actuator  62 , receive input signals from and transmit commands to brake actuators and sensors  64 . The ECM  50  includes a start-stop scheduler (SSAT)  66 , which transmits engine start/stop requests and requests to activate starter  14  to a chassis and powertrain coordinator (CPTC)  68 , which may incorporated the TCM  42  and contains a WTDS algorithm  70 . The final WTDS brake pressure/torque level is adaptively determined in CPTC based on engine and transmission control states and efforts as well as the brake states. The brake system controls  62  and CPTC  68  communicate through a high speed communications area network (HS-CAN)  72 . 
         [0030]    When the driver releases brake pedal  54  in preparation to accelerate the vehicle, pressure in the brake lines  76 ,  77  connected to wheel brakes  78 ,  79  is automatically maintained if the master cylinder pressure P_MC drops to a CPTC adaptively determined magnitude P_WTDS. Such a pressure level will enable the service brakes  78 ,  79  to suppress the worse case torque spike produced by the powertrain. In general, P_WTDS can be determined empirically based on the powertrain configuration, or it can be adaptively calculated with reference to additional driveline and vehicle information, such as the gear ratio in which the vehicle will be accelerated, engine restart strategy, service brake condition, vehicle inertia, powertrain inertia, tolerance factor, etc. 
         [0031]      FIG. 3  contains graphs showing the change of certain powertrain variables during WTDS control of an engine restart. During period A, the vehicle is stopping and the engine is initially stopped before the engine restart is initiated at  80  by a restart request. During period B, engine combustion occurs and becomes sustained. During period C, the vehicle creeps ahead and launches as vehicle speed increases. 
         [0032]    Graph  82  represents the application and gradual release of the brake pedal  54  while vehicle is stopped. Graph  84  represents the application of the accelerator pedal  52 . 
         [0033]    Graph  86  represents pressure in brake lines  76 ,  77 . The brake pressure can be held above the brake line. It can also be held at individual brake chamber/caliper level at each wheel. Line  88  represents the sufficiently high brake pressure level. 
         [0034]    Line  90  represents the adaptively determined brake pressure level P_WTDS, which is a combination of a predetermined brake pressure plus an adaptive brake pressure correction that accounts for the powertrain states, predicted residual powertrain wheel torque, road gradient load torque compensation, and tolerance compensation 
         [0035]    Graph  92  represents the master cylinder pressure P_MC. WTDS control is disabled if no engine stop has occurred, or transmission  16  is not in gear during the engine restart event. 
         [0036]    Graph  94  shows that WTDS control is enabled after engine  12  is stopped and the transmission  16  is in gear. Graph  96  represents active WTDS control after release of the brake pedal  54  and brake system pressure decreasing to the reference pressure P_WTDS. Graph  98  represents active WTDS control becoming inactive after engine  12  restarts or the WTDS timer  100  expires. Graph  102  shows that WTDS control is disabled by setting an engine-running flag. 
         [0037]    The method releases fluid pressure in the wheel brake to allow the engine to accelerate the vehicle in response to depressing accelerator pedal  52 . A technique for producing this result is illustrated in graph  104 , which shows resetting the WTDS timer  100  for about 500 ms when a request to restart the engine  12  occurs at  80 . The countdown  106  of timer  100  occurs after its resetting. Brake system pressure  86  decreases immediately after the engine restart is initiated at  80 , regardless of whether the timer  100  has expired. If timer  100  expires before the engine restarts, brake system pressure  86  decreases immediately. 
         [0038]    Graph  108  represents vehicle speed increasing from zero after sustained engine combustion occurs at  98 . 
         [0039]    Graph  110  represents an engine restart request  67  from SSAT  66  as the brake pedal  54  is released and the gear shifter  44  is in the DRIVE range or LOW range. 
         [0040]    Graph  112 , which represents engine speed, shows an increase in engine speed beginning at the start of the engine restart  80  when the starter  14  cranks the engine  12 . Engine speed continues to increase following the first engine combustion  114 , remains relatively steady at idle speed  116  during the period while engine combustion is sustained, and increases further  118  as engine torque increases. 
         [0041]    Graph  120  represents wheel torque T WHL , which is the sum of engine crankshaft torque at the wheels  30 ,  31  in the current gear T CRANK     —     WHL , brake torque T BRK  and road load at the wheels T RL , which remains zero, in this example, during the restart event. Graph  122  represents crankshaft torque at the wheels  30 ,  31  in the current gear T CRANK     —     WHL . Graph  124  represents brake torque T BRK . During period B, the positive torque spike of engine crankshaft torque at the wheels  30 ,  31  T CRANK     —     WHL  is suppressed by the negative brake torque T BRK , and wheel torque T WHL  is substantially constant. Following period B, wheel torque T WHL  is delayed for at most 300-500 ms following the restart request  110 , depending on how quickly engine  12  can be restarted. 
         [0042]    An engine restart flag varies among stopped  130 , starting  132 , engine speed peak passed, and engine running  134 . 
         [0043]    WTDS control is canceled whenever the accelerator pedal travel is over a certain distance or when the driver reapplies the brake pedal up to a certain level, thereby superseding WTDS control. Thereafter, the vehicle accelerated under creep torque or by a driver acceleration command 
         [0044]      FIG. 4  illustrates a logic flow diagram of the steps of algorithm  70  for controlling the engine restart. At step  140  a test is made to determine whether the brake system control  60  and engine stop-start control are enabled. If the result of test  130  is logically false, at step  142  the WTDS control is inhibited, i.e., turned off. 
         [0045]    If the result of test  140  is logically true, at step  144  a test is made to determine whether an ADS automated engine stop has occurred. If the result of test  144  is logically false, control returns to step  144 . 
         [0046]    If the result of test  144  is true, at step  144  a test is made to determine whether transmission  16  is in gear. If the result of test  146  is false, control returns to step  142 . 
         [0047]    If the result of test  146  is true, at step  148  WTDS control is enabled. 
         [0048]    At step  150  the adaptive load is estimated and WTDS pressure P_WTDS is determined. 
         [0049]    At step  152  a test is made to determine whether the WTDS pressure adaptively determined brake pressure P_WTDS is equal to or greater than master cylinder pressure P_MC. If the result of test  152  is false, control returns to step  148 . 
         [0050]    If the result of test  152  is true, at step  154  WTDS control is activated and brake pressure is maintained at the P_WTDS magnitude. 
         [0051]    At step  156  an engine restart may be initiated either by the vehicle operator or by the WTDS control. 
         [0052]    At step  158  a test is made to determine whether an engine restart has been initiated. If the result of test  158  is false, at step  160  a test is made to determine whether the brake pedal  54  has been reapplied. 
         [0053]    If the result of test  160  is false, control returns to step  154 . If the result of test  160  is true, at step  162  WTDS control is deactivated, brake pressure is released, and control returns to step  152 . 
         [0054]    At step  164  the WTDS timer  100  is set to a reference period length and the countdown  106  occurs at step  166 . 
         [0055]    At step  168  a test is made to determine whether the accelerator pedal  52  has been applied or whether brake pedal  54  has been reapplied. 
         [0056]    At step  170  engine speed is monitored to determine the occurrence of a peak in engine speed. 
         [0057]    If the result of test  168  is false, at step  172  a test is made to determine whether the WTDS timer  100  has expired or whether a torque surge has ended as indicated by the occurrence of a peak in engine speed. 
         [0058]    If the result of test  172  is false, at step  172  a test is made to determine whether an engine stall or automatic engine restart has occurred. If the result of test  174  is true, control returns to step  164 . If the result of test  174  is false, true control returns to step  166 . 
         [0059]    At step WTDS control is deactivated and disabled, brake pressure is released, and control returns to step  140 . 
         [0060]    The proposed brake pressure control does not exclude the case where individual brake circuit pressure or wheel chamber pressure will be used for control action determination rather than the brake master cylinder pressure. 
         [0061]    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.