Abstract:
A method for controlling restart of an engine in a hybrid electric powertrain includes stopping the vehicle and engine, initiating the restart, estimating time required to restart the engine after the restart is initiated, filling and stroking launch elements of a transmission, when the estimated time substantially equals a second estimated time required to fill and stroke said launch elements, and increasing the torque capacity of the launch elements to accelerate the vehicle.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a powertrain for a hybrid electric vehicle (HEV) and, more particularly, to control of a transmission friction control element during an engine restart event. 
     2. Description of the Prior Art 
     A hybrid electric vehicle (HEV) is a vehicle configured with a hybrid propulsion system that utilizes at least two different sources of torque for propelling the vehicle. As one non-limiting example, a hybrid propulsion system may combine a conventional propulsion system that includes an internal combustion engine and a stepped-ratio change automatic transmission with an electric propulsion system that includes one or more electric motors and a rechargeable energy storage device, such as a battery, that can power the electric motors or store energy to improve fuel economy over the conventional vehicle. A hybrid electric vehicle typically provides different powertrain operating modes with the engine running or shutdown depending on the vehicle operating conditions, battery conditions, and driver&#39;s propulsion request. Hence, one of the major functions that an HEV provides is the ability to start or stop the engine during certain conditions. When the engine is running, the electric portion of the propulsion system may be used to assist the engine in providing the required vehicle propulsion. During the conditions when the engine is shutdown, the driver&#39;s propulsion request can be provided entirely by the electric motor. 
     Motor vehicles can be designed to employ certain aspects of hybrid electric vehicle technology to reduce fuel consumption, but without use of a hybrid drivetrain. In such vehicles, called micro-HEVs, shutting down the engine during conditions where the engine operates at idle speed will be used to reduce fuel consumption in a conventional powertrain that includes an internal combustion engine and a stepped-ratio change automatic transmission, but no electric machine for driving the wheels. The primary condition that is checked by the micro-HEV powertrain control system before stopping the engine is that the driver has applied the brakes and the vehicle is stopped since the engine would typically be idling during these conditions in a conventional vehicle. Once the driver releases the brake pedal indicating a request for vehicle propulsion, the powertrain control system will automatically restart the engine. 
     In a micro-HEV application using an internal combustion engine with an enhanced starter motor for engine start/stop and an automatic transmission it is important to provide vehicle propulsion upon an engine restart in a responsive, consistent, and predictable manner. Delays due to transmission engagement and clutch torque capacity application will directly result in wheel torque delays and poor vehicle propulsion response. 
     Premature clutch torque capacity application can also lead to driveline torque oscillations and potential engine stall while restarting. In addition, poor vehicle performance will be sensed by the driver if the transmission engagement feel is too harsh during or after the engine restart. Temperature and other environmental conditions further amplify these issues. 
     If the engine is started while the transmission is fully engaged in gear, driveline excitation can occur as a result of engine start transients transmitting to the wheels. Furthermore, in automatic transmission applications with a torque converter, having the transmission geartrain fully engaged with the torque converter unlocked unnecessarily loads the engine while starting. An engine start strategy for which the transmission is fully engaged in gear requires auxiliary hydraulic line pressure during engine stops for electro-hydraulically operated automatic transmissions. This requires an auxiliary pump and results in energy consumption while the engine is off. 
     A powertrain control system for a micro-HEV powertrain must provide an immediate response to a request for vehicle propulsion when the engine is restarted. A strategy is needed to coordinate clutch filling during an engine start event while minimizing energy consumption in order to provide responsive, smooth, consistent and predictable vehicle propulsion performance. 
     SUMMARY OF THE INVENTION 
     A method for controlling the restart of an engine in a powertrain includes stopping the vehicle and engine, initiating the restart, estimating time required to restart the engine after the restart is initiated, stroking launch friction elements of a transmission when the estimated time substantially equals a second estimated time required to stroke said launch friction elements, and increasing the torque capacity of the launch friction elements to accelerate the vehicle. This invention is applicable, but not limited to conventional torque-converter automatic transmissions and dual-clutch transmissions. 
     The control strategy provides smooth driveline propulsion performance and minimal energy consumption during an engine restart, instead of using an in-gear approach during an engine restart. By predicting the time until the end of an engine start, the transmission clutch filling and stroking is initiated such that, by the completion of the engine start, the launch friction elements, such as clutches, will be fully stroked. Since the clutch filling and stroking time can change with operating and environmental conditions in addition to wear over time, the adaptive clutch filling information used by the shift execution controls can be used for clutch filling and stroking coordination during the engine start. 
     This ensures that premature clutch torque capacity application will not occur so that engine restarts can be performed smoothly without interaction and potential engine stall due to clutch torque loading. Furthermore, the operating time for relying on auxiliary hydraulic line pressure provided by an electric pump can be minimized, thus reducing energy consumption. 
     Optimum vehicle performance is provided consistently and predictably regardless of the vehicle operating conditions throughout the high frequency of engine starts in a micro-HEV. 
     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 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a micro-HEV powertrain; 
         FIG. 2  is schematic diagram showing torque and electric power flow in the powertrain of  FIG. 1 ; 
         FIGS. 3A-3D  are a schematic diagrams showing the operating modes used during an engine start event; and 
         FIG. 4  are graphs that illustrate the change of powertrain variables during the coordinated filling of the launch elements during the engine restart event. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, the micro-HEV powertrain  10  of  FIG. 1  includes a power source  12 , such as an internal combustion engine; engine starter motor  14 ; automatic transmission  16 ; transmission input shaft  18 ; driveably connected to the engine  12 ; a transmission output  24 ; final drive mechanism  26 , connected to the output; an electric auxiliary hydraulic pump (EAUX)  28 , whose output pressurizes the hydraulic system of the transmission  16 ; an electric storage battery  30 , which supplies electric power to the pump  28  starter  14  and a microprocessor-based controller  55 ; and axle shafts  32 ,  33 , driveably connect to the driven wheels  34 ,  35  through the output and final drive mechanism. 
     A gear shifter  40  is moved manually by the vehicle operator among (P)ark, (R)everse, (N)eutral, and (D)rive range positions in an automatic mode channel  42  and between upshift (+) and downshift (−) positions in a manual mode channel  44 . 
     Accelerator and brake pedals  50 ,  52 , controlled manually by the vehicle operator, provide input demands to a control system for changes in engine wheel torque and changes in brake force, respectively. 
     Located within transmission  16  are friction control elements, i.e., clutches and brakes, whose state of coordinated engagement and disengagement produce the forward gears and reverse gear. The first forward gear, low gear, is produced when at least one, but preferably two of the control elements  54 ,  56  are engaged concurrently. The transmission friction control elements, whose engagement produces the desired gear in which the vehicle will be launched, are referred to as launch elements  54 ,  56 . Hydraulic line pressure produced by the electric auxiliary pump  28  while the engine  12  is shutdown is used to fill and stroke the launch elements  54 ,  56 , thereby preparing the transmission  16  for responsive torque transmission once the engine restart is completed. Stroking the launch control elements  54 ,  56  takes up clearances between the servo pistons and a pack of friction plates in the control elements, and clearances among the friction plates. The launch elements  54 ,  56  have substantially no torque transmitting capacity when stroke pressure is present in the servo cylinders that actuate the launch elements. 
     Transmission  16  also contains a hydraulic pump  53 , such as a gerotor pump, whose output is used to produce pressure in the transmission&#39;s hydraulic circuit, through which the control elements  54 ,  56  are pressurized to a state of full engagement in coordination with the engine restart method. 
     A microprocessor-based controller  55 , accessible to a restart control algorithm, communicates through electronic signals transmitted on a communication bus with the engine  12 , starter  14 , transmission  16 , gear selector  40 , auxiliary pump  28 , gear shifter  40 , and the accelerator and brake pedals  50 ,  52 . 
       FIG. 2  shows that engine output torque T ENG , torque T STARTER  produced by the starter motor  14  are combined at a junction  36  to produce engine cranking torque T CRK . Transmission output torque TOUT is transmitted from the transmission  16  to the final drive  24 , which includes an interwheel differential mechanism. Electric power from battery  30  P BAT  is supplied to a junction  38 , from which electric power P EAUX  is distributed to the EAUX  28  and starter motor power P STARTER  is distributed to the starter  14 . Torque TOUT at the transmission output  24  is transmitted to the final drive and differential  26  which drive wheels  34 ,  35 . 
     During the engine and transmission shut down mode illustrated in  FIG. 3A , engine  12  is initially shutdown and transmission  16  is fully disengaged, i.e., the transmission is then disposed to produce no forward or reverse gear. Electric auxiliary pump  28  is not activated and does not provide and hydraulic pressure to transmission  16 . 
     During the engine restart mode, illustrated in  FIG. 3B , engine  12  is being cranked using torque T STARTER  produced by the starter motor  14 , while transmission  16  remains fully disengaged with the clutches destroked. 
     During the clutch filling control mode, illustrated in  FIG. 3C , while engine  12  is restarting, filling of the launch elements  54 ,  56  is initiated as the estimated time at which sustained engine combustion occurs reaches the period required for the launch elements to become filled with pressurized hydraulic fluid. The launch elements  54 ,  56  are filled with fluid as electric auxiliary pump  28  provides hydraulic line pressure to the transmission  16  hydraulic circuit. The electric auxiliary pump  28  uses electric power P EAUX  supplied by electric battery  30 . During the clutch filling control mode, the launch elements  54 ,  56  do not have any torque capacity sufficient to produce vehicle propulsion, and engine  12  is being cranked using torque T STARTER  produced by the starter motor  14 . 
     In order to provide vehicle propulsion after the engine  12  has sustained combustion and begins to run during the engine running mode, illustrated in  FIG. 3D , closed-loop slip control of the launch elements  54 ,  56  is provided by modulating the torque capacity of the launch elements until slip across the launch elements is nearly zero. Thereafter, their torque capacity is increased in an open-loop manner until the transmission is fully engaged in the launch gear. 
       FIG. 4  contains graphs showing the change of certain powertrain variables and transmission launch clutch control during an engine restart event. 
     Graph  60  represents the initial release of brake pedal  52 , and the distance through which the brake pedal is depressed while the vehicle and engine are stopped. The engine restart at  84  is initiated in response to release of the brake pedal  52  at  62 . 
     Graph  64  represents position of the gear shift lever  40  which is in the (D)rive position in the automatic range during the entire event. Graph  66  represents the change of position of the accelerator pedal  50  following the engine restart at  84 . 
     Graph  68 , which represents vehicle speed, shows vehicle speed is zero before the engine restart, vehicle creep at  70  following the engine restart and vehicle launch acceleration  72  once the accelerator pedal  50  is depressed as shown on graph  66  once the engine is running. 
     Graph  74  represents engine speed while starter  14  cranks the engine  12 , graph  76  represents engine speed while engine combustion occurs as the engine is starting, and graph  78  represents engine speed after the restart. 
     Graph  80  represents the estimated length of a time period before sustained engine combustion occurs at  82 . Engine cranking begins at  84 . Clutch fluid filling of the launch elements  54 ,  56  begins at  86 , i.e., when the estimated length of the period before sustained engine combustion occurs is substantially equal to the period length required to fill the launch elements. Clutch filling initiated at  86  would begin earlier if the period to fill the launch elements is increased due to cold temperature or if the time to restart the engine  12  is shorter. 
     Graph  88 , which represents hydraulic pressure in the launch friction elements  54 ,  56 , shows a pressure increase greater than stroke pressure level  90  during the fill period  92 , a decrease to stroke pressure  90 , followed by a pressure increase while the launch elements are engaging at  94 , and a further pressure increase  96  above holding pressure to lock the launch elements so that the transmission remains engaged in gear. 
     Graph  98  shows the variation of slip across the launch elements  54 ,  56 . 
     Graph  100  shows that transmission hydraulic line pressure increases at  102  to pressure  104  provided by EAUX pump  28  during the launch element fill period  92 . Thereafter, hydraulic line pressure increases due to engine  12  restarting and driving the pump  53  located in the transmission, to pressure  106 , at which the transmission pump  53  provides the hydraulic pressure, and not the electric auxiliary pump  28 . 
     Graph  110  represents the on-off status of the EAUX pump  28  At  86  when the clutch filling of launch elements  54 , 56  is initiated, the EAUX pump  28  is turned on and remains on during the engine start and is turned off after the engine is running at  82   
     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.