Abstract:
A method for controlling transmission engagement during restart of an engine in a hybrid electric powertrain includes moving a gear lever between a forward position and reverse position, using electric power to drive an auxiliary pump whose output causes a transmission to engage a gear that corresponds to the lever position and to stroke, to zero torque capacity, an oncoming clutch that corresponds to said gear, initiating the restart, and discontinuing use of the auxiliary pump.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a powertrain for a hybrid electric vehicle (HEV) and, more particularly, to control of a transmission that switches from drive-to-reverse or reverse-to-drive preparatory to an engine restart event. 
         [0003]    2. Description of the Prior Art 
         [0004]    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 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. 
         [0005]    Motor vehicles can be designed to employ certain aspects of hybrid electric 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 and reduce emissions in a conventional powertrain that includes an internal combustion engine and a stepped-ratio 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. 
         [0006]    In a micro-HEV application using an internal combustion engine with an enhanced starter motor for engine start/stop and a dual-clutch (DCT) automatic transmission, as shown in  FIGS. 1 and 2 , 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 poor vehicle propulsion response. 
         [0007]    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 may further amplify these issues. 
         [0008]    A powertrain control system for a micro-HEV powertrain must also provide an immediate response to a request for vehicle propulsion when the engine is restarted after the gear shift lever was moved from the Drive position to the Reverse position, or from Reverse to Drive during the time while the engine was off. 
         [0009]    For example, during an engine stop event while the gear shifter is in Drive and the brake pedal is depressed, the driver can move the shifter into Reverse while continuing to depress the brake pedal, thereby keeping the engine off while the vehicle is stopped. Given this condition, if the transmission is not engaged into reverse gear while the engine is off and the vehicle is stopped at the time the driver moves the gear shifter to Reverse position, there will be an excessive delay in vehicle propulsion once the driver releases the brake and the engine is started. The excessive vehicle propulsion delay is due to delaying the transmission engagement into reverse gear until the engine is started. In DCT applications in which the first and reverse gears are applied on the same shaft (e.g. odd shaft), if the transmission is engaged into reverse at the time of the engine start rather than at the time that the gear shifter was selected to Reverse, the gearbox shift synchronizers within the DCT must be disengaged from first gear, engaged into reverse gear, and then the clutch must be filled and stroked before any clutch torque can be applied. In DCT applications in which the first and reverse gears are applied on different shafts (e.g. 1 st  gear on odd, reverse on even), the gearbox shift synchronizer must be engaged into reverse gear and then the clutch must be filled and stroked before applying any clutch torque. 
         [0010]    Since vehicle propulsion cannot be provided until the clutch torque is applied, the delays in gearbox engagement and clutch actuation directly result in wheel torque delays and poor vehicle propulsion response. 
         [0011]    A powertrain control strategy is needed to avoid this problem in a micro-HEV with a dual-clutch transmission. 
       SUMMARY OF THE INVENTION 
       [0012]    A method for controlling restart of an engine in a hybrid electric powertrain includes (a) moving a gear lever from a forward position to a reverse position or moving a gear lever from a reverse position to a forward position; (b) disengaging an offgoing clutch and an offgoing forward or reverse gear; (c) using electric power to drive an auxiliary pump whose output causes a transmission to engage a reverse or forward gear and to stroke, to zero torque capacity, an oncoming clutch that connects the engine and the reverse or forward gear; (d) initiating the restart; (e) discontinuing use of the auxiliary pump after the engine is started (idling), and (f) increasing the oncoming clutch torque capacity to connect the engine and reverse or forward gear following the engine start. 
         [0013]    The invention contemplates also a hybrid electric powertrain that includes a gear selector; a transmission including an offgoing clutch, an oncoming clutch, first and second gears; brake and accelerator pedals; a source of electric power; an auxiliary pump driven from the source and connected to the transmission; a starter motor driven from the source and connected to the engine; and a controller configured to stop the engine, disengage the offgoing clutch and the first gear, actuate the the auxiliary pump, use output produced by the auxiliary pump to engage the second gear and stroke the oncoming clutch, and actuate the starter to start the engine. 
         [0014]    The control strategy strokes the oncoming input clutch and engages the oncoming gear before the restart is initiated. Consequently, the control system provides a substantially immediate response to a request for vehicle propulsion when the engine is restarted after the gear shift lever was moved from the Drive position to the Reverse position, or from Reverse to Drive while the engine was off. 
         [0015]    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 
         [0016]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0017]      FIG. 1  is a schematic diagram of a micro-HEV powertrain; 
           [0018]      FIG. 2  is schematic diagram showing torque and electric power flow in the powertrain of  FIG. 1 ; 
           [0019]      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; 
           [0020]      FIG. 4  illustrates a logic flow diagram of the steps of an algorithm for controlling the engine restart; and 
           [0021]      FIG. 5  is a schematic diagram showing kinematic details of a dual input clutch powershift transmission. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    Referring now to  FIGS. 1 and 2 , 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 ; a dual clutch automatic transmission  16 , connected by an input shaft  17  and clutches  18 ,  20  to the engine;a shaft  36  containing odd gears 1 st , 3 rd , 5 th  and reverse gears; a shaft  37  containing even gears 2 nd , 4 th , and 6 th  gears; a transmission output  22 ; final drive mechanism  24 , connected to the output  22 ; an electric auxiliary hydraulic pump (EAUX)  25 , whose output pressurizes the hydraulic system of the transmission  16 ; an electric storage battery  26 , which supplies electric power to the pump  25  and starter  14 ; and axle shafts  28 ,  29 , driveably connect to the driven wheels  30 ,  31 . 
         [0023]      FIG. 2  shows that engine output torque T ENG  and torque T STARTER  produced by the enhanced starter motor  14  are combined at a junction  34  to produce the engine crankshaft torque T CRK . Transmission output torque T OUT  is transmitted from the transmission  16  to the final drive and differential  24 , which includes an interwheel differential mechanism. Electric power from battery  26  P BAT  is supplied to a junction  38 , from which electric power P EAUX  is distributed to the EAUX  25  and starter power P STARTER  is distributed to the starter  14 . Electric power P ALT  output by alternator  40  due to engine torque T ALT  is delivered to junction  38 . 
         [0024]    A transmission control module (TCM)  42  is powered by battery  26  and receives and sends signals to the EAUX pump  25  and transmission  16  and receives input signals from a gear shifter  44 , which moves among (P)ARK, (R)REVERSE, (N)EUTRAL, (D)RIVE 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  is powered by battery  26 , receives and sends signals to the starter  14  and engine  12  and receives input signals from an accelerator and brake pedals  52 ,  54 . 
         [0025]    The example described next involves movement of the gear shifter  44  from the (D)RIVE position to the (R)EVERSE position while the engine  12  is shutdown and before an engine restart request is made. Rather than waiting until an engine restart request occurs to shift the transmission into reverse gear, the transmission  16  is shifted into reverse gear when the gear shifter  44  is moved manually by the vehicle operator from the D position to the R position while the engine is shutdown. Hydraulic line pressure produced by the electric auxiliary pump (EAUX) 25  while the engine is shutdown actuates the synchronizers that disengage a forward gear and engage a reverse gear. Furthermore, while the engine is shutdown and once the gearbox is shifted into the desired reverse gear, the input clutch  18  or  20  associated with transmitting power from the engine  12  to the reverse gear path in the transmission is immediately filled with hydraulic fluid and stroked thus preparing the transmission for responsive torque transmission once the engine restart request is initiated. Stroking the clutch  18 ,  20  takes up clearances between the clutch piston and the pack of clutch plates and clearance among the clutch plates so that torque capacity can immediately increased when commanded by the TCM  42  without any additional delays. 
         [0026]      FIG. 3  contains graphs showing the change of certain powertrain variables during a Drive-Reverse shifter engagement while the engine is shutdown followed by an engine restart initiated by the vehicle operator. 
         [0027]    Graph  54  represents the application of the brake pedal  52  and its release at  56 , which initiates vehicle creep in reverse gear. Graph  58  represents the change of position of the gear shift lever  44  from its D to R positions while engine  12  is off. Graph  60 , which represents vehicle speed, shows vehicle deceleration before the engine is stopped at  62 , and vehicle creep  64  as the input clutch torque capacity increases following the engine restart. Graph  66  represents start, stopped and restarted requested states of engine  12 . 
         [0028]    Graph  68 , which represents engine speed, shows a decrease from idle to zero speed as the engine is stopped at  62  and an increase in engine speed beginning at the engine restart  70  when the starter  14  cranks the engine. Engine speed continues to increase to the first engine combustion  74  and remains relatively steady during the period  76  while engine combustion is sustained and the engine idles. 
         [0029]    Graph  78  represents the speed of transmission input  17  and the speed of the oncoming input clutch  18 ,  20  that is associated with reverse gear. Oncoming clutch speed increase during a period  80  occurs as the clutch gains torque capacity and remains steady thereafter. Slip across the input clutch is shown at  82 . 
         [0030]    Graph  84  represents pressure in the oncoming input clutch  18 ,  20 . The oncoming clutch pressure is low during a period  86  while the clutch is stroked and clutch torque capacity is zero and the transmission is engaged in first gear. The clutch is destroked at  88  and remains at zero pressure while the transmission gearbox is disengaged from 1 st  and engaging into reverse gear. Oncoming clutch pressure, thereafter, increases to fill the clutch and falls to low pressure during a period  90  while the clutch is being stroked and clutch torque capacity is zero during the engine restart at  70  and  74 . Oncoming clutch pressure increases during a period  92  while clutch torque capacity increases to produce vehicle creep  64  in reverse gear. 
         [0031]    Graph  94  represents transmission hydraulic line pressure,  96  represents the maximum pressure produced by the electric auxiliary pump  25 . Transmission line pressure  94  decreases during period  98  to a level only provided by the electric auxiliary pump sufficient to keep the clutch stroked while the engine is stopped. Transmission line pressure increases to maximum electric auxiliary pump pressure  96  during period  100  when the pump duty cycle reaches &gt;90 percent at  102 , thereby actuating the transmission to engage into reverse gear and the oncoming clutch to be filled. Transmission line pressure  94  falls to  104  when the duty cycle of pump  25  reaches 20 percent at  106 , thereby stroking the oncoming clutch. Then transmission line pressure  94  increases to a constant magnitude  98  produced by a mechanical pump located in the transmission  16  and driven by the engine  12  upon its restarting. At  110 , following the restart and sustained engine combustion, the electric auxiliary (EAUX) pump  25  is turned off. 
         [0032]    Graph  112  represents the ON-OFF status and PWM (pulse-width modulated) control duty cycle of the electrix auxiliary (EAUX) pump  25 , which produces a low magnitude of pressure during period  114  when the percent duty cycle is at 20 percent while engine  12  is off and the clutch is stroked. At  116 , pressure produced by the pump  25  increases in response to increasing the electric auxiliary pump duty cycle greater than 90 percent, thereby allow the transmission to engage into reverse gear and fill the oncoming clutch while the engine is off. 
         [0033]    Graph  120  shows that transmission  16  shifts from first gear to reverse gear on the odd shaft  36  during the period  122  in response to the gear shift lever  44  being moved from the D position to the R position while the engine is off. Graph  126  shows that the shaft  37  containing the even numbered gears are not affected by D-R movement of the shift lever  44  and the engine restart control strategy for this dual-clutch transmission example. 
         [0034]      FIG. 4  illustrates a logic flow diagram of the steps of an algorithm for controlling the dual clutch transmission engagement and electric auxiliary pump before, during, and after an engine restart. At step  130  a test is made to determine whether the engine  12  is running. If the result of test  130  is logically true, at step  132  the EAUX pump  25  is turned off and control returns to step  130 . If the result of test  130  is logically false, at step  134  the EAUX pump  25  is turned on if it has not already been turned on. 
         [0035]    At step  136  a test is made to determine whether the engine is stopped or stopping. If the result of test  136  is true, at step  138  a test is made to determine whether the gear shift lever  44  is moved from D to R position or from R to D position. If the result of test  138  is true, at step  140  the percent duty cycle of the EAUX pump  25  is increased to its maximum duty cycle so that the maximum pressure produced by the pump is provided. 
         [0036]    At step  142 , the oncoming input clutch  18 ,  20  is destroked and fully disengaged. 
         [0037]    At step  144  a test is made to determine whether the oncoming input clutch  18 ,  20  is destroked and fully disengaged. If the result of test  144  is false, control returns to step  142 . 
         [0038]    If the result of test  144  is true, at step  146  the transmission shifts from first gear to reverse gear, provided the gear selector lever  44  has been moved from the D position to the R position. Alternatively, at step  146  the transmission shifts from reverse gear to first gear, provided the gear selector lever  44  has been moved from the R position to the D position. 
         [0039]    At step  148  a test is made to determine whether the transmission gear change commanded at step  146  has been completed. If the result of test  148  is false, control returns to step  146 . If the result of test  148  is true, control continues to step  150 . 
         [0040]    At step  150  the oncoming input clutch  18 ,  20  is filled and stroked. 
         [0041]    If the result of test  136  is false, or the result of test  138  is false, or upon executing step  150 , at step  152  the transmission electric auxiliary pump  25  pressure is controlled to a level used to stroke the oncoming input clutch  18 ,  20 . 
         [0042]    Following step  152 , at step  154  stroke pressure is maintained in the oncoming input clutch  18 ,  20  and its torque capacity is maintained at substantially zero. 
         [0043]    Although the control strategy is described with respect to a D-R or R-D shift of the gear selector  44 , the strategy can be applied to any gear shifter engagement request, i.e., movement of the shifter  44  between any of its positions including, but not limited to, D-R, R-D, R-L, L-R, while the engine is shutdown. For example, the same steps can be applied for R-D movement of the shift lever  44  while the engine is shutdown, in which case the transmission would be shifted into first gear from reverse gear. 
         [0044]      FIG. 5  illustrates details of a dual input clutch, powershift transmission  16  that includes the first input clutch  18 , which selective connects the input  17  of the transmission alternately to the odd-numbered forward gears  36  and reverse gear  298  associated with a first layshaft  244 , and a second input clutch  20 , which selective connects the input  17  alternately to the even-numbered gears  37  associated with a second layshaft  249 . 
         [0045]    Layshaft  244  supports pinions  260 ,  262 ,  264 , which are each journalled on shaft  244 , and couplers  266 ,  268 ,  302  which are secured to shaft  244 . Pinions  260 ,  262 ,  264  are associated respectively with the first, third and fifth gears. Coupler  266  includes a sleeve  270 , which can be moved leftward to engage pinion  260  and driveably connect pinion  260  to shaft  244 . Coupler  268  includes a sleeve  272 , which can be moved leftward to engage pinion  262  and driveably connect pinion  262  to shaft  244  and can be moved rightward to engage pinion  264  and driveably connect pinion  264  to shaft  244 . 
         [0046]    Layshaft  249  supports pinions  274 ,  276 ,  278 , which are each journalled on shaft  249 , and couplers  280 ,  282 , which are secured to shaft  249 . Pinions  274 ,  276 ,  278  are associated respectively with the second, fourth and sixth gears. Coupler  280  includes a sleeve  284 , which can be moved leftward to engage pinion  274  and driveably connect pinion  274  to shaft  249 . Coupler  282  includes a sleeve  286 , which can be moved leftward to engage pinion  276  and driveably connect pinion  276  to shaft  249  and can be moved rightward to engage pinion  278  and driveably connect pinion  278  to shaft  249 . 
         [0047]    Transmission output  22  supports gears  288 ,  290 ,  292 , which are each secured to output shaft  24 . Gear  288  meshes with pinions  260  and  274 . Gear  290  meshes with pinions  262  and  276 . Gear  292  meshes with pinions  264  and  278 . 
         [0048]    A reverse pinion  296 , journalled on layshaft  244 , meshes with an idler  298 , which meshes with a reverse gear  300  secured to output shaft  22 . A coupler  302  selectively connects reverse pinion  296  to layshaft  244 . 
         [0049]    Couplers  266 ,  268 ,  280 ,  282  and  302  may be synchronizers, or dog clutches or a combination of these. 
         [0050]    During an engine restart following D-R movement of gear selector  44 , clutch  18  is initially the offgoing clutch as first gear is disengaged and is also the oncoming clutch after reverse gear has been engaged since both first and reverse gears can be drivably connected on the same layshaft  244 . During an engine restart following R-D movement of gear selector  44 , clutch  18  is initially the offgoing clutch as reverse gear is disengaged and is also the oncoming clutch after first gear has been engaged since both first and reverse gears can be drivably connected on the same layshaft  244 . 
         [0051]    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.