Abstract:
A method for controlling an engine restart in a hybrid electric powertrain, including the steps of stopping the engine, using an electric power source and an electro-mechanical actuator connected to the source to engage a gear and stroke to zero torque capacity a dry clutch of a dual-clutch transmission, initiating an automatic engine restart, and increasing the torque capacity of the clutch to a desired torque capacity during the engine restart.

Description:
This application is a continuation-in-part of pending U.S. application Ser. No. 12/434,114, filed May 1, 2009. 
    
    
     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 a powertrain with a dry dual-clutch automatic transmission and its control 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. 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. 
     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. In a micro-HEV with an automatic transmission, the primary condition that is checked by the 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 with a dry dual-clutch automatic transmission, coordinated transmission control before, during, and after an engine start is critical to acceptable vehicle performance. Specifically, the control of transmission clutch torque capacity during the engine start directly controls the amount of engine torque transferred to the wheels for vehicle propulsion. Moreover, any delays in clutch torque capacity application directly result in non-responsive vehicle propulsion and poor vehicle performance. Furthermore, in vehicle applications using a dual-clutch (powershift) automatic transmission without a torque converter, any excessive clutch torque capacity during an engine restart can lead to engine stall and/or rough creep and launch performance. 
     One method commonly applied to address these issues is simply keeping the transmission in a neutral state during engine shutdown and restarts. Once the engine is running, the transmission clutch torque can be increased so that torque is transmitted to the wheels. A problem with this approach is that poor vehicle response will be perceived by the driver since vehicle propulsion is not provided until the engine has been started and clutch torque capacity has been increased. In conventional and wet-clutch dual-clutch automatic transmission applications which are electro-hydraulically actuated through a pump mechanically driven by the engine, an electric auxiliary pump would be used to provide hydraulic pressure for any clutch actuation while the engine is not running. 
     A dry dual-clutch automatic transmission is an automatic transmission whose clutches&#39; torque transmitting capacity varies in response to electro-mechanical actuation rather than by pressurized hydraulic fluid. 
     A strategy is needed to coordinate transmission and engine control 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 an engine restart in a hybrid electric powertrain, including the steps of stopping the engine, using an electric power source and an electro-mechanical actuator connected to the source to engage a gear and control a dry clutch of a dual-clutch transmission to a touch point position with zero torque capacity, initiating an automatic engine restart, and increasing the torque capacity of the clutch to a desired torque capacity. 
     The invention contemplates also a powertrain that includes an engine, a dry dual-clutch automatic transmission including two clutches and first and second gears, an electro-mechanical actuator for actuating the clutches and engaging the gears, and a controller configured to stop the engine, engage a gear and control the first clutch to a touch point position with zero torque capacity using the electro-mechanical actuator, initiate an automatic engine restart, and increase the torque capacity of the clutch to a desired torque capacity. 
     The control method and system provide a substantially immediate response to a request for vehicle propulsion when the engine is restarted by taking advantage of the electro-mechanical actuation capabilities of a dry dual-clutch automatic transmission. The control strategy uses a vehicle system controller to coordinate engine and transmission control before, during, and after an engine start event. It produces smooth and responsive driveline propulsion performance. 
     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 ; 
         FIG. 3  is a schematic diagram showing the engine and transmission shutdown operating mode; 
         FIG. 4  is a schematic diagram showing power flow in the transmission preparation operating mode with the engine shutdown; 
         FIG. 5  is a schematic diagram showing power flow while the transmission is transmitting torque during the engine restart operating mode; 
         FIG. 6  is a schematic diagram showing power flow in the engine running operating mode; 
         FIG. 7  is a series of graphs illustrating change in powertrain variables during an engine restart event; and 
         FIG. 8  is a schematic diagram showing kinematic details of a dual input clutch powershift transmission. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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 to the engine by an input shaft  17  and dry clutches  18 ,  20 ; an electro-mechanical actuator  19 , which varies the torque transmitting capacity of the clutches and engages and disengages gears of first and second layshafts  36 , 37 ; a first layshaft  36  containing odd gears 1 st , 3 rd , 5 th  and reverse gears; a second layshaft  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 storage battery  26 , which supplies electric power to the starter motor  14  and electro-mechanical actuator  19 ; and axle shafts  28 ,  29 , driveably connect to the driven wheels  30 ,  31 . 
       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 a net engine crankshaft torque T CRK . Transmission output torque T OUT  is transmitted from the transmission  16  to the final drive mechanism  24 , which includes an interwheel differential and is driveably connected to axle shafts  28 ,  29  and driven wheels  30 ,  31 . Electric power P BAT  from battery  26  and electric power P ALT  produced by an alternator  40  is supplied to a junction  38 , from which electric power P STARTER  and P TRANS  is transmitted to the starter motor  14  and to the electromechanical actuator  19  for controlling dry input clutches  18 ,  20  and gear engagements of layshafts  36 , 37  of transmission  16 , respectively. 
     Referring to  FIG. 1 , a transmission control module (TCM)  42  and an engine control module ECM  50  communicates through electronic signals mutually and with battery  26 , transmission  16 , the electro-mechanical actuator  19  of clutches  18 ,  20 , and 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 . The 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 pedal  52  and brake pedal  54 . The engine control module (ECM)  50  also contains the vehicle system controller for coordinating overall powertrain control. 
     Referring now to  FIGS. 3-6 , during a first powertrain operating mode illustrated in  FIG. 3 , the engine  12  is initially shutdown and the transmission  16  is disengaged in a neutral state, i.e., the dry input clutches  18 ,  20 , whose coordinated engagement and disengagement produce the various transmission gear ratios, are fully disengaged and destroked and transmission  16  produces no output torque. This operating mode is primarily used when the gear lever position is in PARK/NEUTRAL and while the engine is stopped with the ignition key off. 
       FIG. 4  illustrates a second powertrain operating mode of the control strategy, a standby mode while engine  16  is shutdown, during which the dry dual-clutch transmission  16  is prepared to transmit engine torque during the next engine restart. Transmission  16  is controlled to an “active neutral” state where the clutches  18 ,  20  are controlled to a touchpoint position with zero torque capacity but ready to transmit torque once clutch torque capacity is commanded. While transmission  16  is in this standby mode, it transmits no torque, but the oncoming clutch  18 ,  20  that corresponds to the gear in which the vehicle will be launched is stroked at the touchpoint with zero torque capacity. This is possible while engine  16  is shutdown since the dry-clutches  18 ,  20  are electro-mechanically actuated. The gearbox of transmission  16  is also engaged in the launch gear and in a preselected gear on the non-active layshaft. 
     Unlike the clutches of a conventional automatic transmission or a wet clutch, dual-clutch (i.e. powershift) transmission, which are electro-hydraulically actuated, the dry-clutch DCT can be engaged in any gear with full clutch actuation while the engine is not running since the clutches and gears are electro-mechanically controlled by electric motors. The vehicle system controller, comprising ECM  50 , initiates this second powertrain operating mode after the gear lever  44  is shifted into a vehicle driving position (DRIVE, REVERSE, MANUAL, etc.) while the engine is stopped since vehicle propulsion is required upon an engine restart. 
     The second powertrain operating mode can also be used to improve engine restarts while the engine is shutdown and the vehicle is moving. 
     Once the vehicle system controller  50  determines that the engine  16  is to be automatically restarted, e.g. in response to the brake pedal  54  being released or the accelerator pedal  52  being depressed while the vehicle is stopped, the third powertrain operating mode shown in  FIG. 5  is activated. In the third operating mode, responsive vehicle propulsion is provided during the engine restart by increasing torque capacity of the oncoming clutch  18 ,  20  that corresponds to the gear in which the vehicle will be launched. Preferably, the oncoming clutch  18 ,  20  is slipping and is never locked, i.e., full engaged, while increasing the torque capacity during this operating mode in order to prevent stalling the engine  16 . By producing some torque capacity in the oncoming clutch  18 ,  20  during the engine restart, vehicle propulsion delays are minimized. 
     Once engine  16  is restarted and running, the vehicle system controller  50  changes to the fourth operating mode illustrated in  FIG. 6 , wherein transmission  16  transmits the engine torque to the wheels  30 ,  31 . The transmission  16  may be fully engaged in this operating mode. 
       FIG. 7  contains graphs showing the change of certain powertrain variables during while the engine is shutdown followed by an automatic engine restart. 
     Graphs  60 ,  62  represent, respectively, release of the brake pedal  54  application of accelerator pedal  52 , which are required to launch the vehicle. The engine restart is initiated at  64  when the brake pedal  54  returns substantially to its released state. The engine restart may also be initiated earlier before the brake pedal is fully released. 
     Graph  66  shows that the gear shifter  44  remains in its (D)RIVE position while engine  12  is stopped and after it restarts. 
     Graph  68 , which represents vehicle speed, shows the vehicle beginning to creep  70  as the torque capacity of the oncoming clutch  18 ,  20  increases during the engine restart initiated at  64 . 
     Graph  72  represents an engine stop request, followed by an automatically produced engine start request at  64 , which is initiated by release of the brake pedal  54 . 
     Graph  74 , which represents engine speed, shows zero engine speed at  76  while the engine is stopped followed by an increase beginning at the engine restart  64  while starter  14  cranks engine  12 . Engine speed  74  continues a rapid, irregular increase after the first engine combustion occurs at  78  and remains relatively steady during the period  80 , while engine combustion is sustained and the engine idles. 
     Graph  82  shows the output speed (i.e. layshafts  36 ,  37 ) of the oncoming clutch  18 ,  20  increasing as the oncoming clutch gains torque capacity, remains steady for a period thereafter while slip across the clutch occurs, followed by locking the clutch at  84 , whereupon clutch slip is reduced to zero. 
     Graph  86  the variation of torque transmitted by the oncoming clutch  18 ,  20  in response to its electro-mechanical actuation. Clutch torque is zero during a period  88 , while the clutch is stroked at touchpoint, i.e., its torque capacity is zero and clearances among components of the clutch and its actuation system are reduced to substantially zero, thereby preparing the clutch immediately to increase its torque capacity when the clutch is so activated. If the vehicle launch begins in first gear, clutch  18  is stroked at the touchpoint position at  90  during period A, its torque capacity is controlled during periods B, C, D and E, and transmission  16  connects input  17  to output  22  through the first gear at  92  while the engine is stopped, restarted and running. Clutch  20  remains stroked at the touchpoint position at  94  during periods A-E, and transmission  16  is prepared for an upshift to an even-numbered gear, preferably by preselecting and engaging second gear on layshaft  37  at  96 . 
     Graph  98  represents a period during which an engine stop command is present  100  and a period when the stop command is terminated and an engine start command occurs  102 . Engine  12  is in starting mode  103  after the restart is initiated at  64  until sustained engine combustion occurs at  80  and running mode thereafter. 
     Referring to graph  99 , before the restart is initiated at  64 , transmission  16  is in the preparation operating mode  104  in an “active neutral” state, in which the clutches  18 ,  20  are stroked at the touchpoint position and the launch gear is engaged and the upshift gear is also engaged. Immediately upon the restart being initiated at  64  and for a period  108  thereafter, the transmission is commanded to transmit engine torque. During the period  108 , which begins at  106  while engine  12  is being cranked, at  110  the transmission  16  transmits engine torque to the output  22  in the launch gear, preferably first gear. 
       FIG. 8  illustrates details of a dual input clutch, powershift transmission  16 , which includes the first input clutch  18 , which selective connects the input  17  of the transmission alternately to the first layshaft  36  associated with odd-numbered forward gears and reverse gear  298 , and a second input clutch  20 , which selective connects the input  17  alternately to the even-numbered gears. 
     Layshaft  36  supports pinions  260 ,  262 ,  264 , which are each journalled on shaft  36 , and couplers  266 ,  268 ,  302  which are secured to shaft  36 . 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  36 . Coupler  268  includes a sleeve  272 , which can be moved leftward to engage pinion  262  and driveably connect pinion  262  to shaft  36  and can be moved rightward to engage pinion  264  and driveably connect pinion  264  to shaft  36 . 
     Layshaft  37  supports pinions  274 ,  276 ,  278 , which are each journalled on shaft  37 , and couplers  280 ,  282 , which are secured to shaft  37 . 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  37 . Coupler  282  includes a sleeve  286 , which can be moved leftward to engage pinion  276  and driveably connect pinion  276  to shaft  37  and can be moved rightward to engage pinion  278  and driveably connect pinion  278  to shaft  37 . 
     Transmission output  22  supports gears  288 ,  290 ,  292 , which are each secured to output shaft  22 . Gear  288  meshes with pinions  260  and  274 . Gear  290  meshes with pinions  262  and  276 . Gear  292  meshes with pinions  264  and  278 . 
     A reverse pinion  296 , journalled on layshaft  36 , 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  36 . 
     Couplers  266 ,  268 ,  280 ,  282  and  302  may be synchronizers, or dog clutches or a combination of these. 
     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 driveably connected on the same layshaft  36 . 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 driveably connected on the same layshaft  36 . 
     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.