Abstract:
When torque transmission through the transmitting torque variable mechanism (e.g., an assist clutch) is changed to torque transmission through a meshing gearing after the gearshift, a controlled variable per unit time when the transmitting torque variable mechanism is released is determined or varied according to the vehicle operating condition after a gearshift has been completed. A degraded gearshift feel as a result of axle vibration at the end of the gearshift can thereby be suppressed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a control method and a control system for an automatic transmission.  
         [0002]     A motor vehicle with a manual transmission offers better fuel economy as compared with a motor vehicle mounted with a transmission using a torque converter. It is, however, difficult in such a motor vehicle as that with the manual transmission to manipulate a clutch and an accelerator in a coordinated manner when getting the motor vehicle started. If the coordinated operations of the clutch and the accelerator do not go well when the vehicle is started, a large shock occurs as the clutch is engaged. If there is a short supply of a clutch pressure, the engine speed rises sharply, which is commonly referred to as a “revving up phenomenon.” If the clutch is quickly engaged when the engine speed is yet to reach a sufficient level, or when the vehicle is started on an uphill slope, the engine can stall.  
         [0003]     To solve these problems, a system that automates the clutch and a gearshift using a manual transmission mechanism has been developed. The system is what is called an automated manual transmission, or automated MT.  
         [0004]     The conventional automated MT has the disadvantage that there is an interruption of a driving torque occurring from release and engagement operations of the clutch during control for gearshifts. This can at times give passengers of the vehicle a sense of discomfort.  
         [0005]     A vehicle provided with an automatic transmission has been proposed to avoid the torque interruption during the gearshift. A known approach to the problem is an assist clutch serving as a transmitting torque variable mechanism provided for the conventional automated MT (see, for example, Japanese Patent No. 2703169). The assist clutch provides control, during a gearshift, for synchronizing rotations and transmitting torque properly for the gearshift. In a vehicle such as that described above, an assist clutch torque release control is provided. In the assist clutch torque release control, after the gearshift to a new meshing gearing has been completed, torque transmitted by the assist clutch is reduced, while torque transmitted by the new meshing gearing after the gearshift is increased.  
         [0006]     When the torque transmission through the assist clutch is changed to the torque transmission through the meshing gearing after the gearshift, torque vibration in a transmission output can occur due to a torque step depending on how the assist clutch is controlled. The assist clutch, on the other hand, transmits torque through slippage. This presents a durability problem arising from a quantity of heat generated.  
       SUMMARY OF THE INVENTION  
       [0007]     In view of the foregoing, it is therefore an object of the present invention to suppress torque vibration occurring during a gearshift, and at the same time to extend a service life of a clutch.  
         [0008]     To achieve the foregoing object, the present invention determines or varies a controlled variable per unit time according to a vehicle operating condition, when the transmitting torque variable mechanism is released after the gearshift has been completed.  
         [0009]     When the torque transmission through the transmitting torque variable mechanism (e.g., an assist clutch) is changed to the torque transmission through the meshing gearing after the gearshift, torque vibration in the transmission output can occur due to a torque step depending on how the transmitting torque variable mechanism is controlled. At this time, the controlled variable may be set or varied so that the transmitting torque variable mechanism is released at a mild pace according to the vehicle operating condition. If this is done, a degraded gearshift feel as a result of axle vibration at the end of the gearshift can be suppressed.  
         [0010]     The pace at which the transmitting torque variable mechanism is released may be increased or decreased according to a vehicle operating condition, for example, the quantity of heat generated by the transmitting torque variable mechanism. This can suppress generation of heat from the transmitting torque variable mechanism, leading to enhanced durability. 
     
    
     BRIEF DESCRIPTION OF THE INVENTION  
       [0011]     Other objects and advantages of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings in which:  
         [0012]      FIG. 1  is a diagram showing an overall configuration of an automated manual transmission according to a preferred embodiment of the present invention;  
         [0013]      FIG. 2  is a block diagram showing input/output (I/O) signals transferred in the control system according to the preferred embodiment of the present invention shown in  FIG. 1 ;  
         [0014]      FIG. 3  is a flowchart showing control operations performed by the transmission control means according to the preferred embodiment of the present invention shown in  FIG. 1 ;  
         [0015]      FIG. 4  is a flowchart showing operations of an assist clutch transmitting torque release control timer shown in  FIG. 3 ;  
         [0016]      FIG. 5  is a flowchart showing control provided in a first assist clutch transmitting torque release control shown in  FIG. 3 ;  
         [0017]      FIG. 6  is a table of a transmitting torque release amount of  FIG. 5 ;  
         [0018]     FIGS.  7 (A 1 ) through (G 1 ) and (A 2 ) through (G 2 ) are timing charts of signals during an upshift from a 1st gearshift position to a 2nd gearshift position with varying input torque values when the control shown in  FIGS. 5 and 6  is applied;  
         [0019]     FIGS.  8 (A 1 ) through (G 1 ) and (A 2 ) through (G 2 ) are timing charts of signals during an upshift from the 1st gearshift position to the 2nd gearshift position, and from the 2nd gearshift position to a 3rd gearshift position when the control shown in  FIGS. 5 and 6  is applied;  
         [0020]      FIG. 9  is a flowchart showing control provided in a second assist clutch torque release control shown in  FIG. 3 ;  
         [0021]     FIGS.  10 (A 1 ) through (G 1 ) are timing charts of signals during an upshift from the 1st gearshift position to the 2nd gearshift position when the control shown in  FIG. 9  is applied;  
         [0022]      FIG. 11  is a flowchart showing control provided in a third assist clutch torque release control shown in  FIG. 3 ;  
         [0023]      FIG. 12  is a table of a target transmitting torque of  FIG. 11 ;  
         [0024]     FIGS.  13 (A 1 ) through (G 1 ) are timing charts of signals during an upshift from the 1st gearshift position to the 2nd gearshift position when the control shown in  FIGS. 11 and 12  is applied;  
         [0025]      FIG. 14  is a second table of a target transmitting torque of  FIG. 11 ; and  
         [0026]     FIGS.  15 (A 1 ) through (G 1 ) are timing charts of signals during an upshift from the 1st gearshift position to the 2nd gearshift position when the control shown in  FIGS. 11 and 14  is applied. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     Preferred embodiments of the present invention will be described with reference to the accompanying drawings.  
         [0028]     A first arrangement of a motor vehicle control system according to a preferred embodiment of the present invention will be described with reference to  FIG. 1 .  
         [0029]      FIG. 1  is a skeleton diagram showing a first system configuration of a motor vehicle control system according to a preferred embodiment of the present invention. The system includes an engine  1  as a driving power source, an engine speed sensor (not shown) measuring a speed of the engine  1 , a device regulating an engine torque (not shown; for example, an electronic controlled throttle), and a fuel injection system (not shown) for injecting the amount of fuel corresponding to the amount of intake air. Controlling the amount of intake air, the amount of fuel, ignition timing, and the like using an engine control unit  101  allows the torque of the engine  1  to be controlled accurately. The driving power source is not limited only to a gasoline engine as in the above example. A diesel engine, a natural gas engine, an electric motor, or the like perfectly serves the purpose.  
         [0030]     An input shaft clutch input disc  2  is coupled to the engine  1 . The torque of the engine  1  can be transmitted to, or shut off from, a transmission input shaft  10  by engaging the input shaft clutch input disc  2  with, or releasing the same from, an input shaft clutch output disc  3 . A dry type single plate clutch is generally used as an input shaft clutch. Any type of friction transmission mechanisms can still be used, including a wet type multiple disc clutch and an electromagnetic clutch. The transmission input shaft  10  is provided with a first drive gear  4 , a second drive gear  5 , a third drive gear  6 , a fourth drive gear  7 , a fifth drive gear  8 , a reverse drive gear (not shown), and a seventh drive gear  201 .  
         [0031]     A hydraulically driven actuator  22  is used for controlling a thrust force (an input shaft clutch torque) between the input shaft clutch input disc  2  and the input shaft clutch output disc  3 . Regulating the thrust force (the input shaft clutch torque) allows an output from the engine  1  to be transmitted to, or shut off from, the transmission input shaft  10 .  
         [0032]     The first drive gear  4 , the second drive gear  5 , the third drive gear  6 , the fourth drive gear  7 , the fifth drive gear  8 , and the reverse drive gear are secured in position. The seventh drive gear  201 , on the other hand, is rotatably mounted on the transmission input shaft  10 . There is also provided a sensor  29  for detecting the speed of the transmission input shaft  10 . The sensor  29  functions as a system for detecting an input shaft speed.  
         [0033]     A transmission output shaft  18  is provided with a first driven gear  12 , a second driven gear  13 , a third driven gear  14 , a fourth driven gear  15 , a fifth driven gear  16 , and a reverse driven gear (not shown), rotatably mounted thereon. A seventh driven gear  202  is fixed onto the transmission output shaft  18 . The first driven gear  12  is in mesh with the first drive gear  4 . The second driven gear  13  is in mesh with the second drive gear  5 . The third driven gear  14  is in mesh with the third drive gear  6 . The fourth driven gear  15  is in mesh with the fourth drive gear  7 . The fifth driven gear  16  is in mesh with the fifth drive gear  8 . The reverse driven gear (not shown) is in mesh with the reverse drive gear through a reverse rotation gear (not shown). The seventh driven gear  202  is in mesh with the seventh drive gear  201 .  
         [0034]     A first meshing gearing  19  is provided as a meshing gearing between the first driven gear  12  and the second driven gear  13 . The first meshing gearing  19  causes the first driven gear  12  to be engaged with the transmission output shaft  18 . Or, the first meshing gearing  19  causes the second driven gear  13  to be engaged with the transmission output shaft  18 .  
         [0035]     Rotating torque transmitted from the first drive gear  4  or the second drive gear  5  to the first driven gear  12  or the second driven gear  13  is therefore transmitted to the first meshing gearing  19 . The rotating torque is thereby transmitted to the transmission output shaft  18  through the first meshing gearing  19 .  
         [0036]     A second meshing gearing  20  is provided as a meshing gearing between the third driven gear  14  and the fourth driven gear  15 . The second meshing gearing  20  causes the third driven gear  14  to be engaged with the transmission output shaft  18 . Or, the second meshing gearing  20  causes the fourth driven gear  15  to be engaged with the transmission output shaft  18 .  
         [0037]     Rotating torque transmitted from the third drive gear  6  or the fourth drive gear  7  to the third driven gear  14  or the fourth driven gear  15  is therefore transmitted to the second meshing gearing  20 . The rotating torque is thereby transmitted to the transmission output shaft  18  through the second meshing gearing  20 .  
         [0038]     A third meshing gearing  21  is provided as a meshing gearing between the fifth driven gear  16  and the reverse driven gear (not shown). The third meshing gearing  21  causes the fifth driven gear  16  to be engaged with the transmission output shaft  18 . Or, the third meshing gearing  21  causes the reverse driven gear to be engaged with the transmission output shaft  18 .  
         [0039]     Rotating torque transmitted from the fifth drive gear  8  or the reverse drive gear to the fifth driven gear  16  or the reverse driven gear is therefore transmitted to the third meshing gearing  21 . The rotating torque is thereby transmitted to the transmission output shaft  18  through the third meshing gearing  21 .  
         [0040]     The meshing gearing  19 ,  20 ,  21  described above may be a constant-mesh type. The gearing may even be a clutch provided with a friction transmission mechanism that is used to accomplish rotation synchronization for meshing (what is called a synchromesh).  
         [0041]     To transmit the rotating torque of the transmission input shaft  10  to the first meshing gearing  19 , the second meshing gearing  20 , or the third meshing gearing  21 , the following engagement must be effected. Specifically, either one of the first meshing gearing  19 , the second meshing gearing  20 , and the third meshing gearing  21  is moved in an axial direction of the transmission output shaft  18 , thereby bringing the meshing gearing into engagement with any one of the first driven gear  12 , the second driven gear  13 , the third driven gear  14 , the fourth driven gear  15 , the fifth driven gear  16 , and the reverse driven gear. To bring any one of the first driven gear  12 , the second driven gear  13 , the third driven gear  14 , the fourth driven gear  15 , the fifth driven gear  16 , and the reverse driven gear into engagement with the transmission output shaft  18 , either the first meshing gearing  19 , the second meshing gearing  20 , or the third meshing gearing  21  is to be moved. To move either the first meshing gearing  19 , the second meshing gearing  20 , or the third meshing gearing  21 , a shift/select mechanism  27  is operated using a shift first actuator  23 , a shift second actuator  24 , a select first actuator  25 , and a select second actuator  26 .  
         [0042]     Bringing either one of the first meshing gearing  19 , the second meshing gearing  20 , and the third meshing gearing  21  into engagement with any one of the first driven gear  12 , the second driven gear  13 , the third driven gear  14 , the fourth driven gear  15 , the fifth driven gear  16 , and the reverse driven gear allows the rotating torque of the transmission input shaft  10  to be transmitted to the transmission output shaft  18  through either the first meshing gearing  19 , the second meshing gearing  20 , or the third meshing gearing  21 . In addition, there is provided a sensor  30  for detecting the speed of the transmission output shaft  18 . The sensor  30  functions as a system for detecting an output shaft speed.  
         [0043]     The shift first actuator  23  and the shift second actuator  24 , and the select first actuator  25  and the select second actuator  26  may be configured using a solenoid valve, an electric motor, or the like.  
         [0044]     The shift/select mechanism  27  may be configured using a shifter rail, a shifter fork, and the like. The shift/select mechanism  27  may even be formed into a drum type. The shift/select mechanism  27  is also provided with a position holding mechanism (not shown) for holding gear positions, thereby preventing gears from coming off position during running.  
         [0045]     An assist clutch  203 ,  204  is also provided as one type of the transmitting torque variable mechanism. An assist clutch input disc  203  and an assist clutch output disc  204  are engaged with each other when the seventh drive gear  201  is connected to the assist clutch input disc  203  and the transmission input shaft  10  is connected to the assist clutch output disc  204 . This allows the torque of the seventh driven gear  202  to be transmitted to the transmission output shaft  18 .  
         [0046]     A hydraulically operated actuator  205  is used for controlling a thrust force (or an assist clutch torque) between the assist clutch input disc  203  and the assist clutch output disc  204 . The output from the engine  1  can be transmitted or shut down by regulating this thrust force (the assist clutch torque).  
         [0047]     The transmitting torque variable mechanism may be formed using a friction transmission mechanism, a motor-generator, or the like. The friction transmission mechanism refers to a mechanism that generates a frictional force using a thrust force of a friction surface, thereby transmitting torque. A typical application of the friction transmission mechanism is a friction clutch. The friction clutch is available in several different varieties. The varieties include the dry type single plate clutch, a dry type multi-plate clutch, the wet type multiple disc clutch, an electromagnetic clutch, and the like.  
         [0048]     The control system in accordance with the preferred embodiment of the present invention uses the wet type multiple disc clutch as the friction transmission mechanism for the assist clutch  203 ,  204 . Any other type of transmitting torque variable mechanism can be used.  
         [0049]     As explained in the foregoing, the rotating torque of the transmission input shaft  10  is transmitted from the first drive gear  4 , the second drive gear  5 , the third drive gear  6 , the fourth drive gear  7 , the fifth drive gear  8 , the reverse drive gear, and the seventh drive gear  201  through the first driven gear  12 , the second driven gear  13 , the third driven gear  14 , the fourth driven gear  15 , the fifth driven gear  16 , the reverse driven gear, and the seventh driven gear  202  to the transmission output shaft  18 . The rotating torque is then transmitted to an axle (not shown) through a differential gear (not shown) connected to the transmission output shaft  18 .  
         [0050]     The input shaft clutch actuator  22  controls the thrust force (the input shaft clutch torque) between the input shaft clutch input disc  2  and the input shaft clutch output disc  3 . The assist clutch actuator  205  controls the thrust force (the assist clutch torque) between the assist clutch input disc  203  and the assist clutch output disc  204 . A hydraulic pressure control unit  102  controls the hydraulic pressure for the input shaft clutch actuator  22  and the assist clutch actuator  205  as detailed in the following. Specifically, the hydraulic pressure control unit  102  controls a current flowing through a solenoid valve (not shown) provided for each actuator. A stroke of a hydraulic cylinder (not shown) provided for each actuator is thereby adjusted to achieve an intended level of hydraulic pressure for each actuator. The transmitting torque of each clutch is thus controlled.  
         [0051]     The hydraulic pressure control unit  102  also controls a current flowing through a solenoid valve (not shown) provided for the select first actuator  25  and the select second actuator  26 . A stroke of a hydraulic cylinder (not shown) provided for each of these actuators is thereby adjusted to achieve an intended level of hydraulic pressure for each actuator. Either the first meshing gearing  19 , the second meshing gearing  20 , or the third meshing gearing  21  is thus selected and moved as necessary.  
         [0052]     Further, the hydraulic pressure control unit  102  controls a current flowing through a solenoid valve (not shown) provided for each of the shift first actuator  23  and the shift second actuator  24 . A stroke of a hydraulic cylinder (not shown) provided for each of these actuators is thereby adjusted to achieve an intended level of hydraulic pressure for each actuator. A load for operating the first meshing gearing  19 , the second meshing gearing  20 , and the third meshing gearing  21  can thus be controlled.  
         [0053]     The control system in accordance with the preferred embodiment of the present invention uses hydraulic actuators for the shift first actuator  23  and the shift second actuator  24 , and the select first actuator  25  and the select second actuator  26 , all used for driving the shift/select mechanism  27 . Use of an electric actuator operated by an electric motor or the like is nonetheless possible.  
         [0054]     A single actuator may be used instead of the shift first actuator  23  and the shift second actuator  24 . Further, a single actuator may also be used instead of the select first actuator  25  and the select second actuator  26 . A shifter rail, a shifter fork, and the like may be used to form a mechanism for operating the first meshing gearing  19 , the second meshing gearing  20 , and the third meshing gearing  21 . Or a drum type, or any other type of mechanism may be formed for moving the dog clutch  19 ,  20 ,  21 .  
         [0055]     The control system in accordance with the preferred embodiment of the present invention uses hydraulic actuators for the input shaft clutch actuator  22  and the assist clutch actuator  205 . Use of an electric actuator operated by an electric motor or the like is nonetheless possible.  
         [0056]     Controlling the amount of intake air, the amount of fuel, ignition timing, and the like using the engine control unit  101  allows torque of the engine  1  to be controlled accurately. A power train control unit  100  controls the hydraulic pressure control unit  102  and the engine control unit  101 . The power train control unit  100 , the engine control unit  101 , and the hydraulic pressure control unit  102  exchange information between each other using communications means  103 .  
         [0057]     The control system in accordance with the preferred embodiment of the present invention uses hydraulic actuators. This results in the hydraulic pressure control unit  102  being used for controlling the hydraulic actuators. With the electric actuator operated by the electric motor or the like, however, an electric motor control unit is to be used instead of the hydraulic pressure control unit  102 .  
         [0058]      FIG. 2  is a block diagram showing input/output (I/O) signals sent and received by the power train control unit  100 , the engine control unit  101 , and the hydraulic pressure control unit  102  through the communications means  103 . The power train control unit  100  is formed as a control unit including an input portion  100   i , an output portion  100   o , and a computer  100   c . Similarly, the engine control unit  101  is formed as a control unit including an input portion  101   i , an output portion  101   o , and a computer  101   c . The hydraulic pressure control unit  102  is also formed as a control unit including an input portion  102   i , an output portion  102   o , and a computer  102   c . An engine torque command value tTe is sent from the power train control unit  100  to the engine control unit  101  using the communications means  103 . To achieve tTe, the engine control unit  101  controls the amount of intake air, the amount of fuel, ignition timing (not shown), and the like of the engine  1 . There are provided inside the engine control unit  101  means (not shown) for detecting an engine torque that serves as an input torque for the transmission. The engine control unit  101  detects a speed Ne of the engine  1  and an engine torque Te generated by the engine  1 . The engine control unit  101  then sends signals representing the information to the power train control unit  100  using the communications means  103 . A torque sensor may be used for the engine torque detection means. Another possible approach for the engine torque detection means is to use estimation means making an estimate based on engine parameters, such as an injector injection pulse width, an intake pipe vacuum, the engine speed, and the like.  
         [0059]     The power train control unit  100  sends signals representing the following information to the hydraulic pressure control unit  102 : an input shaft clutch target torque TTqSTA, a target shift load Fsft, a target select position tpSEL, and an assist clutch target torque TTqa. The hydraulic pressure control unit  102  controls the input shaft clutch actuator  22  so as to achieve the input shaft clutch target torque TTqSTA. The input shaft clutch input disc  2  and the input shaft clutch output disc  3  are thereby engaged with, or released from, each other.  
         [0060]     To achieve the target shift load Fsft and the target select position tpSEL, the hydraulic pressure control unit  102  controls the shift first actuator  23 , the shift second actuator  24 , the select first actuator  25 , and the select second actuator  26 , thereby operating the shift/select mechanism  27 . A shift position or a select position are thereby controlled to eventually engaged or release the first meshing gearing  19 , the second meshing gearing  20 , and the third meshing gearing  21 . In addition, to achieve the assist clutch target torque TTqa, the hydraulic pressure control unit  102  controls the assist clutch actuator  205  to engage or release the assist clutch input disc  203  and the assist clutch output disc  204 .  
         [0061]     The hydraulic pressure control unit  102  detects a position signal rpSTA, a shift position signal rpSFT, and a select position signal rpSEL indicating that the input shaft clutch is engaged or released. The hydraulic pressure control unit  102  transmits these signals to the power train control unit  100 .  
         [0062]     Signals representing an input shaft speed Ni and an output shaft speed No are applied to the power train control unit  100  from the input shaft speed sensor  29  and the output shaft speed sensor  30 , respectively. In addition to these signals, a range position signal RngPos indicating a shift lever position such as a P range, an R range, an N range, a D range, or the like, a signal representing an accelerator pedal depression amount Aps, and an ON/OFF signal Brk from a brake switch for detecting whether or not a brake pedal is depressed are applied to the power train control unit  100 . If, for example, a driver places the shift lever in the D range or the like and depresses an accelerator pedal, the power train control unit  100  determines that the driver intends to start a vehicle or accelerate the vehicle. If, for example, the driver depresses the brake pedal, the power train control unit  100  determines that the driver intends to decelerate or stop the vehicle. The power train control unit  100  then sets the engine torque command value tTe, the input shaft clutch target torque TTqSTA, the target shift load Fsft, and the target select position tpSEL so as to achieve the driver&#39;s intention.  
         [0063]     Further, a gearshift position is set from a vehicle speed Vsp calculated from the output shaft speed No and the accelerator pedal depression amount Aps. To execute a gearshift operation into the gearshift position thus set, the engine torque command value tTe, the input shaft clutch target torque TTqSTA, the target shift load Fsft, the target select position tpSEL, and the assist clutch target torque TTqa are then set.  
         [0064]     Specific control operations performed during the gearshift in the motor vehicle control system according to the preferred embodiment of the present invention will be described with reference to  FIGS. 3 through 13 .  
         [0065]     Overall control operations performed during the gearshift in the motor vehicle control system according to the preferred embodiment of the present invention will first be described with reference to  FIG. 3 .  
         [0066]      FIG. 3  is a flowchart showing control operations performed during the gearshift in the motor vehicle control system according to the preferred embodiment of the present invention. Specific operations of the gearshift control described hereunder that have previously been programmed in the computer  100   c  of the power train control unit  100  are executed repeatedly at a predetermined cycle. That is, operations of steps  301  through  311  given in the following are executed by the power train control unit  100 . In step  301 , the power train control unit  100  reads parameters. In step  302 , a gearshift operation is started by setting the gearshift position based on the vehicle speed Vsp and the accelerator pedal depression amount Aps. In step  303  (a release control phase), release control is executed to release the gear. In step  304 , it is determined whether or not the release control is completed. If it is determined that the release control is completed, the control operation proceeds to step  305 . If it is determined that the release control is yet to be completed, step  303  is re-executed. In step  305  (a rotation synchronization control phase), the assist clutch torque is controlled so as to make the input speed synchronized with a speed (a target speed) corresponding to the next gearshift position. In step  306 , it is determined whether or not the rotation synchronization control is completed. If it is determined that the rotation synchronization control is completed, the control operation proceeds to step  307 . If it is determined that the rotation synchronization control is yet to be completed, step  305  is re-executed. In step  307  (an engagement control phase), gear engagement control is executed. In step  308 , it is determined whether or not the engagement control is completed. If it is determined that the engagement control is completed, the control operation proceeds to step  309 . If it is determined that the engagement control is yet to be completed, step  307  is re-executed. In step  309  (an assist clutch release phase), an assist clutch torque release control is executed. In step  310 , it is determined whether or not the assist clutch torque release control is completed. If it is determined that the assist clutch torque release control is completed, the control operation proceeds to step  311 . If it is determined that the assist clutch torque release control is yet to be completed, step  310  is re-executed. The assist clutch torque release control is completed if the assist clutch target torque is 0.  
         [0067]     In step  311  (a gearshift termination phase), the gearshift control is terminated.  
         [0068]     Details of a timer indicating an elapsed time of specific control operations performed during the gearshift in the motor vehicle control system according to the preferred embodiment of the present invention will be described with reference to  FIG. 4 .  
         [0069]      FIG. 4  is a flowchart showing details of the timer indicating an elapsed time of specific control operations performed during the gearshift in the motor vehicle control system according to the preferred embodiment of the present invention.  
         [0070]     Specific details of the timer described hereunder that have previously been programmed in the computer  100   c  of the power train control unit  100  are executed repeatedly at a predetermined cycle. That is, operations of steps  401  and  402  given in the following are executed by the power train control unit  100 .  
         [0071]     In step  401 , it is determined whether or not the assist clutch torque release control is being provided. If it is determined that the assist clutch torque release control is being provided, the control operation proceeds to step  402 . In step  402 , an assist clutch torque release control timer Tmr_tof counts up. If it is determined that the assist clutch torque release control is not being provided, the control operation proceeds to step  403 . In step  403 , the assist clutch torque release control timer Tmr_tof is reset.  
         [0072]     Operations of a first control in step  309  (the assist clutch release control phase) performed as part of the gearshift control in the motor vehicle control system according to the preferred embodiment of the present invention will be described with reference to  FIGS. 5 and 6 .  
         [0073]      FIG. 5  is a flowchart showing control provided in step  309  (the assist clutch torque release control phase) of  FIG. 3 . In step  501 , parameters are read. In step  502 , it is determined whether or not the current operation is immediately after the start of the assist clutch torque release control phase. If the assist clutch torque release control phase timer Tmr_tof is 0, the control operation proceeds to step  503 . If the assist clutch torque release control phase timer Tmr_tof is not 0, the control operation proceeds to step  504 . In step  503 , an assist clutch torque release speed dTTqa is calculated. The assist clutch torque release speed dTTqa in step  503  is a function of a transmission input torque Tqin. In step  504 , an assist clutch target torque at assist clutch torque release is calculated.  
         [0074]      FIG. 6  shows typical setting values for the function f 1  of step  503  shown in  FIG. 5 . It is desirable that the setting value for the function f 1  of step  503  be set to a greater value as the transmission input torque Tqin becomes greater. It is also desirable that the function f 1  be set uniquely for each of different gearshift positions. In step  503 , the assist clutch torque release speed dTTqa is calculated from the transmission input torque Tqin. Instead of using the transmission input torque Tqin, the assist clutch torque release speed dTTqa may be calculated from an accelerator opening, a vehicle acceleration, a step in a transmission output torque as calculated using transmission input torque×reduction gear ratio after gearshift−transmission input torque×assist gear ratio, a quantity of heat generated by the assist clutch, and the like.  
         [0075]     FIGS.  7 (A 1 ) through (G 2 ) are timing charts showing conditions of the first control performed as part of the gearshift control shown in  FIGS. 5 and 6 .  
         [0076]      FIG. 7 (A 1 ) and  FIG. 7 (A 2 ) represent a transmission input torque transmitted to the transmission input shaft  10  shown in  FIG. 1 .  FIG. 7 (B 1 ) and  FIG. 7 (B 2 ) represent the speed of the transmission input shaft  10  shown in  FIG. 1 .  FIG. 7 (C 1 ) and  FIG. 7 (C 2 ) represent a position (a shift position) of the first meshing gearing  19  shown in  FIG. 1 .  FIG. 7 (D 1 ) and  FIG. 7 (D 2 ) represent a command current applied to the hydraulic pressure control unit  102  driving the assist actuator  205  shown in  FIG. 1 .  FIG. 7 (E 1 ) and  FIG. 7 (E 2 ) represent an actual hydraulic pressure for driving the assist actuator  205  shown in  FIG. 1 .  FIG. 7 (F 1 ) and  FIG. 7 (F 2 ) represent the assist clutch torque.  FIG. 7 (G 1 ) and  FIG. 7 (G 2 ) represent the output torque of the transmission output shaft  18  shown in  FIG. 1 . The abscissa represents time.  
         [0077]     FIGS.  7 (A 1 ) through  7 (G 1 ) show the transmission input torque, the input shaft speed, the shift position, the command current, the actual hydraulic pressure, the assist clutch torque, and the transmission output torque, respectively, at different timings during a gearshift from the combination of gears  4  and  12  (hereinafter referred to as a “1st speed”) to the combination of gears  5  and  13  (hereinafter referred to as a “2nd speed”).  
         [0078]     Similarly, FIGS.  7 (A 2 ) through  7 (G 2 ) show the transmission input torque, the input shaft speed, the shift position, the command current, the actual hydraulic pressure, the assist clutch torque, and the transmission output torque, respectively, at different timings during the gearshift from the 1st speed to the 2nd speed. The values of the transmission input torque shown in FIGS.  7 (A 2 ) through  7 (G 2 ) differ from those shown in FIGS.  7 (A 1 ) through  7 (G 1 ).  
         [0079]     When a gearshift command to the 2nd speed is issued at time t 1  during running in the 1st speed, gearshift control is started. When the command current is gradually increased as shown in a period of time from time t 1  to time t 2  of  FIG. 7 (D 1 ) and  FIG. 7 (D 2 ), the actual hydraulic pressure gradually increases as shown in the period of time from time t 1  to time t 2  of  FIG. 7 (E 1 ) and  FIG. 7 (E 2 ). The assist clutch torque also gradually increases as shown in the period of time from time t 1  to time t 2  of  FIG. 7 (F 1 ) and  FIG. 7 (F 2 ).  
         [0080]     At this time, the transmission output torque gradually decreases as shown in the period of time from time t 1  to time t 2  of  FIG. 7 (G 1 ) and  FIG. 7 (G 2 ). Then at time t 2 , the first meshing gearing  19 , which has so far been engaged with the 1st speed side, is set into a state to be released. This is because torque transmitted by the gears  201  and  202  causes torque transmitted by the gears  4  and  12  to be decreased to a value that allows the first meshing gearing  19  to be released.  
         [0081]     When the first meshing gearing  19  is in the state to be released, the actuator  27  is controlled so as to release the first meshing gearing  19 , which has been engaged with the  1 st speed side, bringing the first meshing gearing  19  into a neutral position to initiate an actual gearshift, as shown in a period of time from time t 2  to time t 3  in  FIG. 7 (C 1 ) and  FIG. 7 (C 2 ).  
         [0082]     When the first meshing gearing  19  is in the neutral position, the assist clutch torque is controlled to bring the input shaft speed to a level corresponding to the 2nd speed. When the input shaft speed reaches a level corresponding to the 2nd speed as shown at time t 4  of  FIG. 7 (B 1 ) and  FIG. 7 (B 2 ), the first meshing gearing  19  is allowed to engage with the 2nd speed side as shown in a period of time from time t 4  to time t 5  of  FIG. 7 (B 1 ) and  FIG. 7  (B 2 ).  
         [0083]     When the command current is gradually decreased as shown in a period of time from time t 5  to time t 6 , during which the first meshing gearing  19  is engaged in the 2nd speed, the actual hydraulic pressure gradually decreases as shown in the period of time from time t 5  to time t 6  of  FIG. 7 (E 1 ) and  FIG. 7 (E 2 ). The assist clutch torque also gradually decreases as shown in the period of time from time t 5  to time t 6  of  FIG. 7 (F 1 ) and  FIG. 7  (F 2 ). At this time, the transmission output torque gradually increases as shown in the period of time from time t 5  to time t 6  of  FIG. 7 (G 1 ) and  FIG. 7 (G 2 ). When the release of the assist clutch torque is completed at time t 6 , torque is then transmitted only with the 2nd speed gear.  
         [0084]     The command current applied to the hydraulic pressure control unit  102  that drives the assist actuator  205  of  FIG. 1  is controlled so that the assist clutch torque release speed changes as shown in a portion encircled in  FIG. 7 (F 1 ) and in a portion encircled in  FIG. 7 (F 2 ). This reduces axle vibration at the end of the gearshift in each of different cases of varying values of the transmission input torque.  
         [0085]     FIGS.  8 (A 1 ) through (G 2 ) are timing charts showing conditions of a second control performed as part of the gearshift control shown in  FIGS. 5 and 6 .  
         [0086]     FIGS.  8 (A 1 ) and  FIG. 8 (A 2 ) represent the transmission input torque transmitted to the transmission input shaft  10  shown in  FIG. 1 .  FIG. 8 (B 1 ) and  FIG. 8 (B 2 ) represent the speed of the transmission input shaft  10  shown in  FIG. 1 .  FIG. 8 (C 1 ) represents the position (the shift position) of the first meshing gearing  19  shown in  FIG. 1 .  FIG. 8 (C 2 ) represents the position (the shift position) of the first meshing gearing  19  and the second meshing gearing  20  shown in  FIG. 1 .  FIG. 8 (D 1 ) and  FIG. 8 (D 2 ) represent the command current applied to the hydraulic pressure control unit  102  driving the assist actuator  205  shown in  FIG. 1 .  FIG. 8 (E 1 ) and  FIG. 8 (E 2 ) represent the actual hydraulic pressure for driving the assist actuator  205  shown in  FIG. 1 .  FIG. 8 (F 1 ) and  FIG. 8 (F 2 ) represent the assist clutch torque.  FIG. 8 (G 1 ) and  FIG. 8 (G 2 ) represent the output torque of the transmission output shaft  18  shown in  FIG. 1 . The abscissa represents time.  
         [0087]     FIGS.  8 (A 1 ) through  8 (G 1 ) show the transmission input torque, the input shaft speed, the shift position, the command current, the actual hydraulic pressure, the assist clutch torque, and the transmission output torque, respectively, at different timings during a gearshift from the 1st speed to the 2nd speed.  
         [0088]     FIGS.  8 (A 2 ) through  8 (G 2 ) show the transmission input torque, the input shaft speed, the shift position, the command current, the actual hydraulic pressure, the assist clutch torque, and the transmission output torque, respectively, at different timings during the gearshift from the 2nd speed to the combination of gears  6  and  14  (hereinafter referred to as a “3rd speed”).  
         [0089]     When a gearshift command to the 2nd speed is issued at time t 1  during running in the 1st speed, gearshift control is started. If a gearshift command to the 3rd speed is issued at time t 1  during running in the 2nd speed, gearshift control is started. When the command current is gradually increased as shown in a period of time from time t 1  to time t 2  of  FIG. 8  (D 1 ) and  FIG. 8 (D 2 ), the actual hydraulic pressure gradually increases as shown in the period of time from time t 1  to time t 2  of  FIG. 8 (E 1 ) and  FIG. 8 (E 2 ). The assist clutch torque also gradually increases as shown in the period of time from time t 1  to time t 2  of  FIG. 8 (F 1 ) and  FIG. 8 (F 2 ).  
         [0090]     At this time, the transmission output torque gradually decreases as shown in the period of time from time t 1  to time t 2  of  FIG. 8 (G 1 ) and  FIG. 8 (G 2 ). Then at time t 2 , the first meshing gearing  19 , which has so far been engaged with the  1 st speed side, is set into a state to be released during the gearshift from the 1st speed to the 2nd speed. During the gearshift from the 2nd speed to the 3rd speed, the first meshing gearing  19 , which has so far been engaged with the 2nd speed side, is set into a state to be released.  
         [0091]     This is because torque transmitted by the gears  201  and  202  causes torque transmitted by the gears  4  and  12  to be decreased, during the gearshift from the 1st speed to the 2nd speed, to a value that allows the first meshing gearing  19  to be released. In addition, during the gearshift from the 2nd speed to the 3rd speed, on the other hand, the torque transmitted by the gears  201  and  202  causes torque transmitted by the gears  5  and  13  to be decreased to the value that allows the first meshing gearing  19  to be released.  
         [0092]     When the first meshing gearing  19  and the second meshing gearing  20  are in the state to be released, the actuator  27  is controlled, during the gearshift from the 1st speed to the 2nd speed, so as to release the first meshing gearing  19 , which has been engaged with the 1st speed side. The first meshing gearing  19  is thereby brought into a neutral position as shown in a period of time from time t 2  to time t 3  in  FIG. 8 (C 1 ). Then, the assist clutch torque is controlled to bring the input shaft speed to a level corresponding to the 2nd speed. During the gearshift from the 2nd speed to the 3rd speed, on the other hand, the actuator  27  is controlled so as to release the first meshing gearing  19 , which has been engaged with the 2nd speed side, thereby bringing the first meshing gearing  19  into a neutral position as shown in a period of time from time t 2  to time t 3  in  FIG. 8 (C 2 ). Then, the assist clutch torque is controlled to bring the input shaft speed to a level corresponding to the 3rd speed.  
         [0093]     During the gearshift from the 1st speed to the 2nd speed, when the input shaft speed reaches a level corresponding to the 2nd speed as shown at time t 4  of  FIG. 8 (B 1 ), the first meshing gearing  19  is allowed to engage with the 2nd speed side as shown in a period of time from time t 4  to time t 5  of  FIG. 8 (B 1 ). During the gearshift from the 2nd speed to the 3rd speed, when the input shaft speed reaches a level corresponding to the 3rd speed as shown at time t 4  of  FIG. 8 (B 2 ), the second meshing gearing  20  is engaged with the 3rd speed side as shown in a period of time from time t 4  to time t 5  of  FIG. 8 (B 2 ).  
         [0094]     During the gearshift from the 1st speed to the 2nd speed, the command current is gradually decreased as shown in a period of time from time t 5  to time t 6 , during which the first meshing gearing  19  is engaged with the 2nd speed. During the gearshift from the 2nd speed to the 3rd speed, on the other hand, the command current is gradually decreased as shown in a period of time from time t 5  to time t 6 , during which the second meshing gearing  20  is engaged with the 3rd speed. The actual hydraulic pressure then gradually decreases as shown in the period of time from time t 5  to time t 6  of  FIG. 8 (E 1 ) and  FIG. 8 (E 2 ). The assist clutch torque also gradually decreases as shown in the period of time from time t 5  to time t 6  of  FIG. 8 (F 1 ) and  FIG. 8 (F 2 ). At this time, the transmission output torque gradually increases as shown in the period of time from time t 5  to time t 6  of  FIG. 8 (G 1 ) and  FIG. 8  (G 2 ). When the release of the assist clutch torque is completed at time t 6 , torque is then transmitted only with the 2nd speed gear.  
         [0095]     The command current applied to the hydraulic pressure control unit  102  that drives the assist actuator  205  of  FIG. 1  is controlled so that the assist clutch torque release speed changes as shown in a portion encircled in  FIG. 8 (F 1 ) and in a portion encircled in  FIG. 8 (F 2 ). This reduces axle vibration at the end of the gearshift in each of different cases of varying reduction gear ratios at the end of the gearshift.  
         [0096]     Operations of a second control in step  309  (the assist clutch torque release control phase) performed as part of the gearshift control in the motor vehicle control system according to the preferred embodiment of the present invention will be described with reference to  FIG. 9 .  
         [0097]      FIG. 9  is a flowchart showing control provided in step  309  (the assist clutch torque release control phase) of  FIG. 3 . In step  901 , parameters are read. In step  902 , an actual transmission output torque RTqo is calculated. The actual transmission output torque RTqo may be directly detected using a torque sensor or estimated based on changes in the output shaft speed or the like. In step  903 , it is determined whether or not the current operation is immediately after the start of the assist clutch torque release control phase. If the assist clutch torque release control phase timer Tmr_tof is 0, the control operation proceeds to step  904 . If the assist clutch torque release control phase timer Tmr_tof is not 0, the control operation proceeds to step  906 . In step  904 , transmission output torque at the end of gearshift TTqo is calculated using the transmission input torque Tqin and a reduction gear ratio after the gearshift Gro. In step  905 , the assist clutch target torque at assist clutch torque release TTqa is set to 0. In step  906 , a transmission output torque deviation at the end of gearshift ETqo is calculated using the transmission output torque at the end of gearshift TTqo and the actual transmission output torque RTqo. In step  907 , a feedback amount TTqaFB is calculated based on the transmission output torque deviation at the end of gearshift ETqo. In step  908 , the assist clutch target torque at assist clutch torque release TTqa is set to TTqaFB.  
         [0098]     FIGS.  10 (A 1 ) through (G 1 ) are timing charts showing conditions of the gearshift control shown in  FIG. 9 .  FIG. 10 (A 1 ) represents transmission input torque transmitted to the transmission input shaft  10  shown in  FIG. 1 .  FIG. 10 (B 1 ) represents the speed of the transmission input shaft  10  shown in  FIG. 1 .  FIG. 10 (C 1 ) represents a position (a shift position) of the first meshing gearing  19  shown in  FIG. 1 .  FIG. 10 (D 1 ) represents a command current applied to the hydraulic pressure control unit  102  driving the assist actuator  205  shown in  FIG. 1 .  FIG. 10 (E 1 ) represents an actual hydraulic pressure for driving the assist actuator  205  shown in  FIG. 1 .  FIG. 10 (F 1 ) represents the assist clutch torque.  FIG. 10  (G 1 ) represents the output torque of the transmission output shaft  18  shown in  FIG. 1 . The abscissa represents time.  
         [0099]     FIGS.  10 (A 1 ) through  10 (G 1 ) show the transmission input torque, the input shaft speed, the shift position, the command current, the actual hydraulic pressure, the assist clutch torque, and the transmission output torque, respectively, at different timings during a gearshift from the 1st speed to the 2nd speed.  
         [0100]     When a gearshift command to the 2nd speed is issued at time t 1  during running in the 1st speed, gearshift control is started. When the command current is gradually increased as shown in a period of time from time t 1  to time t 2  of  FIG. 10 (D 1 ), the actual hydraulic pressure gradually increases as shown in the period of time from time t 1  to time t 2  of  FIG. 10  (E 1 ). The assist clutch torque also gradually increases as shown in the period of time from time t 1  to time t 2  of  FIG. 10 (F 1 ). At this time, the transmission output torque gradually decreases as shown in the period of time from time t 1  to time t 2  of  FIG. 10 (G 1 ). Then at time t 2 , the first meshing gearing  19 , which has so far been engaged with the 1st speed side, is set into a state to be released. This is because torque transmitted by the gears  201  and  202  causes torque transmitted by the gears  4  and  12  to be decreased to a value that allows the first meshing gearing  19  to be released.  
         [0101]     When the first meshing gearing  19  is in the state to be released, the actuator  27  is controlled so as to release the first meshing gearing  19 , which has been engaged with the 1st speed side. The first meshing gearing  19  is thereby brought into a neutral position to initiate an actual gearshift, as shown in a period of time from time t 2  to time t 3  in  FIG. 10 (C 1 ). When the first meshing gearing  19  is in the neutral position, the assist clutch torque is controlled to bring the input shaft speed to a level corresponding to the 2nd speed.  
         [0102]     When the input shaft speed reaches a level corresponding to the 2nd speed as shown at time t 4  of  FIG. 10 (B 1 ), the first meshing gearing  19  is allowed to engage with the 2nd speed side as shown in a period of time from time t 4  to time t 5  of  FIG. 10 (B 1 ). When the command current is decreased as shown in a period of time from time t 5  to time t 6 , during which the first meshing gearing  19  is engaged with the 2nd speed, the actual hydraulic pressure decreases as shown in the period of time from time t 5  to time t 6  of  FIG. 10 (E 1 ). The assist clutch torque also decreases as shown in the period of time from time t 5  to time t 6  of  FIG. 10 (F 1 ). At this time, the transmission output torque gradually increases as shown in the period of time from time t 5  to time t 6  of  FIG. 10 (G 1 ). When the release of the assist clutch torque is completed at time t 6 , torque is then transmitted only with the 2nd speed gear. This eliminates torque vibration in the transmission output, thereby reducing axle vibration at the end of the gearshift.  
         [0103]     Operations of a third control in step  309  (the assist clutch torque release control phase) performed as part of the gearshift control in the motor vehicle control system according to the preferred embodiment of the present invention will be described with reference to  FIGS. 11 and 12 .  
         [0104]      FIG. 11  is a flowchart showing control provided in step  309  (the assist clutch torque release control phase) of  FIG. 3 . In step  1101 , parameters are read. In step  1102 , it is determined whether or not the current operation is immediately after the start of the assist clutch torque release control phase. If the assist clutch torque release control phase timer Tmr_tof is 0, the control operation proceeds to step  1103 . If the assist clutch torque release control phase timer Tmr_tof is not 0, the control operation proceeds to step  1105 . In step  1103 , the assist clutch target torque at assist clutch torque release TTqa is calculated. In step  1104 , the assist clutch target torque at assist clutch torque release calculation is preformed. The assist clutch target torque at assist clutch torque release TTqa is a function of the transmission input torque Tqin and the assist clutch torque release control phase timer Tmr_tof.  
         [0105]      FIG. 12  shows a typical first setting value for the function f 2  of step  1104  shown in  FIG. 11 . The first setting value of the function f 2  in step  1104  is the same as that of the assist clutch target torque at assist clutch torque release calculated in steps  902  through  908  of  FIG. 9 .  
         [0106]     This eliminates the need for detecting the actual transmission output torque. The same effect can therefore be produced even without using an expensive torque sensor. The approach lightens calculation load, since there is no need of calculating the feedback amount as calculated in steps  1406  and  1407  of  FIG. 9 .  
         [0107]     It is desirable that the first setting value of the function f 2  in step  1104  be set to a greater value as the transmission input torque Tqin becomes greater. It is also desirable that the function f 2  be set uniquely for each of different gearshift positions when releasing the assist gear clutch torque.  
         [0108]     FIGS.  13 (A 1 ) through (G 1 ) are timing charts showing conditions of the gearshift control shown in  FIGS. 11 and 12 .  FIG. 13 (A 1 ) represents transmission input torque transmitted to the transmission input shaft  10  shown in  FIG. 1 .  FIG. 13 (B 1 ) represents the speed of the transmission input shaft  10  shown in  FIG. 1 .  FIG. 13 (C 1 ) represents a position (a shift position) of the first meshing gearing  19  shown in  FIG. 1 .  FIG. 13 (D 1 ) represents a command current applied to the hydraulic pressure control unit  102  driving the assist actuator  205  shown in  FIG. 1 .  FIG. 13 (E 1 ) represents an actual hydraulic pressure for driving the assist actuator  205  shown in  FIG. 1 .  FIG. 13 (F 1 ) represents the assist clutch torque.  FIG. 13  (G 1 ) represents the output torque of the transmission output shaft  18  shown in  FIG. 1 . The abscissa represents time.  
         [0109]     FIGS.  13 (A 1 ) through  13 (G 1 ) show the transmission input torque, the input shaft speed, the shift position, the command current, the actual hydraulic pressure, the assist clutch torque, and the transmission output torque, respectively, at different timings during a gearshift from the 1st speed to the 2nd speed.  
         [0110]     When a gearshift command to the 2nd speed is issued at time t 1  during running in the 1st speed, gearshift control is started. When the command current is gradually increased as shown in a period of time from time t 1  to time t 2  of  FIG. 13 (D 1 ), the actual hydraulic pressure gradually increases as shown in the period of time from time t 1  to time t 2  of  FIG. 13  (E 1 ). The assist clutch torque also gradually increases as shown in the period of time from time t 1  to time t 2  of  FIG. 13 (F 1 ). At this time, the transmission output torque gradually decreases as shown in the period of time from time t 1  to time t 2  of  FIG. 13 (G 1 ). Then at time t 2 , the first meshing gearing  19 , which has so far been engaged with the 1st speed side, is set into a state to be released. This is because torque transmitted by the gears  201  and  202  causes torque transmitted by the gears  4  and  12  to be decreased to a value that allows the first meshing gearing  19  to be released.  
         [0111]     When the first meshing gearing  19  is in the state to be released, the actuator  27  is controlled so as to release the first meshing gearing  19 , which has been engaged with the  1 st speed side. The first meshing gearing  19  is thereby brought into a neutral position to initiate an actual gearshift, as shown in a period of time from time t 2  to time t 3  in  FIG. 13 (C 1 ).  
         [0112]     When the first meshing gearing  19  is in the neutral position, the assist clutch torque is controlled to bring the input shaft speed to a level corresponding to the 2nd speed. When the input shaft speed reaches a level corresponding to the 2nd speed as shown at time t 4  of  FIG. 13 (B 1 ), the first meshing gearing  19  is allowed to engage with the 2nd speed side as shown in a period of time from time t 4  to time t 5  of  FIG. 13 (B 1 ).  
         [0113]     When the command current is decreased as shown in a period of time from time t 5  to time t 6 , during which the first meshing gearing  19  is engaged with the 2nd speed, the actual hydraulic pressure decreases as shown in the period of time from time t 5  to time t 6  of  FIG. 13 (E 1 ). The assist clutch torque also decreases as shown in the period of time from time t 5  to time t 6  of  FIG. 13 (F 1 ).  
         [0114]     At this time, the transmission output torque gradually increases as shown in the period of time from time t 5  to time t 6  of  FIG. 13 (G 1 ). When the release of the assist clutch torque is completed at time t 6 , torque is then transmitted only with the 2nd speed gear. This eliminates torque vibration in the transmission output, thereby reducing axle vibration at the end of the gearshift.  
         [0115]      FIG. 14  shows a typical second setting value for the function f 2  of step  1104  shown in  FIG. 11 .  
         [0116]     The second setting value for the function f 2  of step  1104  is a setting value obtained by adjusting the first setting value for the function f 2  of step  1104  in consideration of response of the actual hydraulic pressure for driving the assist actuator  205  of  FIG. 1 . It is desirable that the second setting value of the function f 2  in step  1104  be set to a greater value as the transmission input torque Tqin becomes greater. It is also desirable that the function f 2  be set uniquely for each of different gearshift positions when releasing the assist gear clutch torque.  
         [0117]     FIGS.  15 (A 1 ) through (G 1 ) are timing charts showing conditions of the gearshift control shown in  FIGS. 11 and 14 .  FIG. 15 (A 1 ) represents a transmission input torque transmitted to the transmission input shaft  10  shown in  FIG. 1 .  FIG. 15 (B 1 ) represents the speed of the transmission input shaft  10  shown in  FIG. 1 .  FIG. 15 (C 1 ) represents a position (a shift position) of the first meshing gearing  19  shown in  FIG. 1 .  FIG. 15 (D 1 ) represents a command current applied to the hydraulic pressure control unit  102  driving the assist actuator  205  shown in  FIG. 1 .  FIG. 15 (E 1 ) represents an actual hydraulic pressure for driving the assist actuator  205  shown in  FIG. 1 .  FIG. 15 (F 1 ) represents the assist clutch torque.  FIG. 15  (G 1 ) represents the output torque of the transmission output shaft  18  shown in  FIG. 1 . The abscissa represents time.  
         [0118]     FIGS.  15 (A 1 ) through  13 (G 1 ) show the transmission input torque, the input shaft speed, the shift position, the command current, the actual hydraulic pressure, the assist clutch torque, and the transmission output torque, respectively, at different timings during a gearshift from the 1st speed to the 2nd speed.  
         [0119]     When a gearshift command to the 2nd speed is issued at time t 1  during running in the 1st speed, gearshift control is started. When the command current is gradually increased as shown in a period of time from time t 1  to time t 2  of  FIG. 15 (D 1 ), the actual hydraulic pressure gradually increases as shown in the period of time from time t 1  to time t 2  of  FIG. 15  (E 1 ). The assist clutch torque also gradually increases as shown in the period of time from time t 1  to time t 2  of  FIG. 15 (F 1 ). At this time, the transmission output torque gradually decreases as shown in the period of time from time t 1  to time t 2  of  FIG. 15 (G 1 ). Then at time t 2 , the first meshing gearing  19 , which has so far been engaged with the 1st speed side, is set into a state to be released. This is because torque transmitted by the gears  201  and  202  causes torque transmitted by the gears  4  and  12  to be decreased to a value that allows the first meshing gearing  19  to be released.  
         [0120]     When the first meshing gearing  19  is in the state to be released, the actuator  27  is controlled so as to release the first meshing gearing  19 , which has been engaged with the 1st speed side. The first meshing gearing  19  is thereby brought into a neutral position to initiate an actual gearshift, as shown in a period of time from time t 2  to time t 3  in  FIG. 15 (C 1 ). When the first meshing gearing  19  is in the neutral position, the assist clutch torque is controlled to bring the input shaft speed to a level corresponding to the 2nd speed. When the input shaft speed reaches a level corresponding to the 2nd speed as shown at time t 4  of  FIG. 15 (B 1 ), the first meshing gearing  19  is allowed to engage with the 2nd speed side as shown in a period of time from time t 4  to time t 5  of  FIG. 15 (B 1 ).  
         [0121]     When the command current is decreased as shown in a period of time from time t 5  to time t 6 , during which the first meshing gearing  19  is engaged in the 2nd speed, the actual hydraulic pressure decreases as shown in the period of time from time t 5  to time t 6  of  FIG. 15  (E 1 ). The assist clutch torque also decreases as shown in the period of time from time t 5  to time t 6  of  FIG. 15 (F 1 ). At this time, the transmission output torque increases as shown in the period of time from time t 5  to time t 6  of  FIG. 15 (G 1 ). When the release of the assist clutch torque is completed at time t 6 , torque is then transmitted only with the 2nd speed gear. This eliminates torque vibration in the transmission output, thereby reducing axle vibration at the end of the gearshift.  
         [0122]     In accordance with the present invention, release control of the transmitting torque variable mechanism can be optimally provided according to vehicle operating conditions.  
         [0123]     While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.