Abstract:
A vehicle includes an engine that shuts down and restarts at idle, a dual-clutch transmission (DCT) having a controller and two input clutches for connecting the engine to respective evenly and oddly numbered gears of the DCT. The controller has a shift algorithm for shifting the DCT to achieve a predetermined engine load state during an engine autostart/autostop cycle, with the load state being unloaded or partially loaded. A DCT for the vehicle includes the input clutches and controller. A method includes shifting the DCT to the predetermined engine load state while the engine is off and an engine restart is commanded, and restarting the engine in the engine load state. The method may also include maintaining the clutches at a first point relative to a kiss point of the input clutches when the load state is unloaded, and at a second point when the load state is partially loaded.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method and apparatus for shift control in a vehicle having a dual-clutch transmission (DCT) and an engine start-stop powertrain. 
       BACKGROUND OF THE INVENTION 
       [0002]    Some vehicle powertrains have the capability to selectively shut down the engine whenever the vehicle is stopped in order to minimize idle fuel consumption. Such functionality is commonly referred to as “autostop/autostart”, and is widely present in hybrid vehicles, although it is not limited to such powertrains. Control of the vehicle transmission during engine restart and vehicle launch typically occurs via shift algorithms programmed within a transmission controller, which may be stand alone or distributed within a larger vehicle control architecture. 
         [0003]    A dual-clutch transmission (DCT) is an automated, manual-like transmission having a gearbox with two independently-operated torque transfer mechanisms or clutches. Associated electronic and hydraulic clutch control devices control the shift operation of the DCT. In a DCT, one input clutch controls the oddly-numbered gears, e.g., first, third, fifth, and reverse in a 7-speed transmission, while another input clutch controls the evenly-numbered gears, e.g., second, fourth, and sixth in the same 7-speed transmission. Using this arrangement, the gears of a DCT can be shifted without totally interrupting power flow from the engine. 
       SUMMARY OF THE INVENTION 
       [0004]    Accordingly, a method and an apparatus are provided herein for use with a vehicle having a dual-clutch transmission (DCT) and engine autostart/stop functionality as described above. The method, which may be embodied as an algorithm and executed by a vehicle transmission controller, allows the engine to completely shut down and restart in either an unloaded or a partially-loaded state. Shutting down of the engine reduces idle fuel consumption, as is well understood in the art and noted above. However, an unloaded engine shutdown sequence may result in an extended duration of engine spin. Therefore, within the scope of the present invention, a partial loading of the crankshaft may occur under certain operating conditions to expedite braking of the engine. Upon restart of the engine, the engine may be at least partially loaded while actively “spinning up” to shorten the time to launch, without adversely shocking the initial forward acceleration of the vehicle body. The present invention uses the “kiss-point” of a given input clutch of the DCT in order to determine precisely when and how to unload the input clutch. As used herein, the term “kiss-point” refers to the point in time at which an input clutch just begins to make frictional contact during its engagement, i.e., the point of incipient capacity. 
         [0005]    In particular, a vehicle includes an engine adapted to automatically shut down and restart during an engine autostop/autostart cycle to thereby minimize idle fuel consumption, a DCT having a pair of input clutches, one being selectively engageable to connect the engine to oddly numbered gears of the DCT, and the other being selectively engageable to connect the engine to evenly numbered gears of the DCT; and a controller. The controller has a shift algorithm adapted for shifting the DCT to achieve a predetermined engine load state during the engine autostart/autostop cycle. The predetermined engine load state is one of a fully unloaded state and an at least partially loaded state. 
         [0006]    A DCT is also provided for a vehicle, the vehicle having an engine adapted to automatically shut down and restart during a vehicle idle condition during an engine autostop/autostart cycle. The DCT includes a controller and a pair of input clutches. One input clutch is selectively engageable to connect the engine to oddly numbered gears of the DCT, and the other input clutch is selectively engageable to connect the engine to evenly numbered gears of the DCT. The controller has a shift algorithm adapted for shifting the DCT to achieve a predetermined engine load state during the engine autostart/autostop cycle. The predetermined engine load state is one of an unloaded state and an at least partially loaded state. 
         [0007]    A method for shifting the DCT includes determining when the engine is shut down, shifting the DCT to a predetermined engine load state while the engine is in an off state, the predetermined engine load state being one of an unloaded state and an at least partially loaded state, and restarting the engine in the predetermined engine load state. The method may also include maintaining the input clutches of the DCT at a first predetermined point relative to a kiss point of the input clutches when the predetermined engine load state is the unloaded state, and at a second predetermined point relative to a kiss point of the input clutches when the predetermined engine load state is the partially loaded state. 
         [0008]    The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a lever diagram of a vehicle having a dual-clutch transmission (DCT), engine autostart-stop functionality, and a shift control algorithm in accordance with the present invention; 
           [0010]      FIG. 2A  is a plot of engine speed vs. time for the vehicle shown in  FIG. 1 ; 
           [0011]      FIG. 2B  is a plot of clutch capacity vs. time for the vehicle shown in  FIG. 1 ; 
           [0012]      FIG. 2C  is a plot of body acceleration vs. time for the vehicle shown in  FIG. 1 ; and 
           [0013]      FIG. 3  is a flow chart describing a shift control algorithm for the DCT of the vehicle shown in  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0014]    Referring to the drawings, wherein like reference numbers correspond to like or similar components throughout the several figures, a vehicle  10  is shown in  FIG. 1  that includes a transmission controller (C)  12  having a shift control algorithm  100  in accordance with the invention, as described below with reference to  FIGS. 2A-3 . The controller  12  is adapted for executing the algorithm  100  to control actuation of one or both of a pair of input clutches  14 A,  14 B of a dual-clutch transmission (DCT)  16 . As shown, the DCT  16  is a 7-speed transmission, although one may configure the DCT as desired, e.g., 5-speed, 6-speed, 7-speed, 8-speed, or more. 
         [0015]    Vehicle  10  includes an engine (E)  18  having a crankshaft  15  baring an input torque (T IN ). The engine  18  is adapted to shut down and restart in a partially-loaded or a fully unloaded state, as determined by the algorithm  100 . That is, the engine  18  has autostart/stop functionality as described above, i.e., the engine may be selectively shut down or turned off to reduce idle fuel consumption during an engine autostop/autostart cycle. A motor (not shown) may be used to crank and restart the engine  18 , as understood in the art. The DCT  16  includes an output shaft  20  connected to a set of road wheels (not shown). The output shaft  20  ultimately carries a transmission output torque (T OUT ) from various gear sets  24  of DCT  16  to thereby propel the vehicle  10 . The gear sets  24  may be selectively actuated using electrical and/or hydraulic controls, according to signals  11  communicated by controller  12 . 
         [0016]    The DCT  16  may include, according to one embodiment, a first shaft  25 , a second shaft  27 , and gear sets  24 A-H, each having a plurality of nodes  26 , of which one in each gear set is grounded to a stationary member  28 , e.g., a transmission case or housing. First shaft  25  is connected to and drives the odd gear sets, e.g., gear sets  24 A-D in a 7-speed transmission as shown. First shaft  25  is therefore labeled in  FIG. 1  as the “odd shaft”. Second shaft  27  is connected to and drives the even gear sets, e.g., gear sets  24 E-H in the 7-speed transmission shown in  FIG. 1 , with gear set  24 H providing the required gearing for the reverse mode. Second shaft  27  is therefore labeled in  FIG. 1  as the “even shaft”. DCT  16  further includes an upper and lower main shaft  30 ,  32 , respectively, which are connected to final drive gear sets  34 A,  34 B. Final drive gear sets  34 A,  34 B are in turn connected to the output shaft  20  to provide any required final gear reduction. Throughout  FIG. 1 , the trapezoidal shaped symbols represent clutch synchronizers, as is well understood in the art. 
         [0017]    Still referring to  FIG. 1 , data used by algorithm  100  may reside within or may be accessible by the controller  12 , and may be sampled or processed thereby during transmission states such as 1 st  gear or higher, neutral (N), and park (P). Vehicle data that may be sampled in order to determine appropriate shift conditions for input clutches  14 A,  14 B may include, but are not necessarily limited to, vehicle output speed, throttle level, braking level, a PRNDL setting, onboard diagnostics, etc. 
         [0018]    The controller  12  may be configured as a microprocessor-based device having such common elements as a microprocessor or CPU, memory including but not limited to: read only memory (ROM), random access memory (RAM), electrically-erasable programmable read-only memory (EEPROM), etc., and circuitry including but not limited to: a high-speed clock (not shown), analog-to-digital (A/D) circuitry, digital-to-analog (D/A) circuitry, a digital signal processor or DSP, and the necessary input/output (I/O) devices and other signal conditioning and/or buffer circuitry. However configured, the controller  12  is operable for executing at least the algorithm  100  of  FIG. 3  as needed to provide the required shift control of DCT  16 . 
         [0019]    Referring to  FIGS. 2A-C , a set of vehicle performance plots are shown for the vehicle  10  of  FIG. 1 , including an engine speed plot  21  of  FIG. 2A , a clutch capacity plot  23  of  FIG. 2B , and a vehicle body acceleration plot  29  of  FIG. 2C . In  FIG. 2A , an engine speed trace  31  is shown via plot  21 . At t 1 , during engine shutdown, engine speed rapidly declines relative to an unloaded crankshaft of engine  18 , as shown by trace  31 . The unloaded engine speed trace  31 A is also plotted for comparison in plot  21 . 
         [0020]    In  FIG. 2B , also at t 1 , the clutch capacity time plot  23  shows that a clutch capacity trace  33  for the input clutches  14 A,  14 B is held above the kiss point relative to an unloaded engine condition, as represented by the clutch capacity trace  33 A. Note that engine speed trace  31 A and clutch capacity trace  33 A correspond to an engine speed “sail on” condition, or the extended duration of engine spin noted above. 
         [0021]    At t 2 , restart of engine  18  is initiated, e.g., when brake capacity and/or brake pedal position reaches a calibrated threshold. Note in  FIG. 2A  the rise in speed trace  31 . Clutch control may commence at approximately t 3 , and a suitable engine run flag, for example “0”, may be set at this point. Clutch capacity rises at t 3 , as shown in  FIG. 2B , and is held above the kiss point. As shown in body acceleration plot  29  in  FIG. 2C , acceleration trace  35  begins to rise at t 3 , indicating a launch of the vehicle  10 . Trace  35 A indicates a launch profile for a too heavily loaded launch, i.e., a launch occurring with excessive clutch capacity, or without a properly shaped engine torque profile at start up. At t 4 , a suitable engine run flag, for example a flag of “1”, may be set to indicate the latest possible entry point for clutch control. Engine speed trace  31  flattens out in a zero-throttle condition after t 4 , otherwise rising as indicated by engine speed trace  31 A. 
         [0022]    Referring to  FIG. 3  in conjunction with the structure of vehicle  10  shown in  FIG. 1 , execution of algorithm  100  by the controller  12  allows the engine  18  to be shut down and restarted in either an unloaded or an at least partially-loaded state. Algorithm  100  begins with step  102  during a coast-down of the vehicle  10 , wherein the engaged input clutch  14 A or  14 B is held at or just below the kiss point at the final moments prior to the vehicle reaching a stop. After engine shut down, the clutch  14 A or  14 B is held just below the kiss point. Algorithm  100  proceeds to step  104 . 
         [0023]    At step  104 , the algorithm  100  determines whether a restart is commanded by the controller  12 , or by a separate controller. If so, the algorithm  100  proceeds to step  108 , and if not, the algorithm proceeds to step  106 . 
         [0024]    At step  106 , the clutch  14 A or  14 B is held at the kiss point (P K ), and the algorithm  100  returns to step  104  as explained above. 
         [0025]    At step  108 , the algorithm  100  determines whether the input clutch  14 A or  14 B is held below the kiss point (P K ) after engine shut down, or if the clutch is held at or above the kiss point. If at or above, the algorithm  100  proceeds to step  110 . If below, the algorithm  100  proceeds to step  112 . 
         [0026]    At step  110 , the crankshaft  15  is partially loaded, and the algorithm  100  proceeds to step  114 . 
         [0027]    At step  112 , the crankshaft  15  is unloaded, and the algorithm  100  proceeds to step  114 . 
         [0028]    At step  114 , restart of engine  18  is initiated. The algorithm  100  proceeds to step  116 A if the crankshaft  15  is unloaded, and to step  116 B if the crankshaft is partially loaded, as determined above at steps  110  and  112 , respectively. 
         [0029]    At step  116 A, the clutch  14 A or  14 B is held at or slightly below the kiss point (P K ) until the engine  18  fully restarts. The algorithm  100  then proceeds to step  118 . 
         [0030]    At step  116 B, the clutch  14 A or  14 B is held at or slightly above the kiss point (P K ) until the engine  18  fully restarts. The algorithm  100  then proceeds to step  118 . 
         [0031]    At step  118 , the controller  12  moves to the kiss point or beyond until the vehicle  10  begins to creep or pull away during launch. The algorithm  100  then proceeds to step  120 . 
         [0032]    At step  120 , clutch control is reapplied at a predetermined point in the restart, e.g., based on the speed of the engine  18 . 
         [0033]    Accordingly, execution of the algorithm  100  using the controller  12  allows the engine  18  to shut down and restart in an unloaded or a partially loaded state in a DCT-equipped vehicle, e.g., the vehicle  10  of  FIG. 1 . Execution of the algorithm  100  may provide a more optimal driveline feel during engine restart and shutdown, while eliminating idle fuel consumption. 
         [0034]    While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.