Patent Publication Number: US-11378024-B2

Title: Internal combustion engine control method and internal combustion engine control device

Description:
TECHNICAL FIELD 
     This invention relates to a control method for an internal combustion engine, and a control device for the internal combustion engine. 
     BACKGROUND ART 
     It is known to improve a fuel economy by inertia travel by stopping an internal combustion engine when an accelerator is in an OFF state (accelerator OFF state) during driving of the vehicle. 
     For example, a patent document 1 discloses an art to stop the engine (the internal combustion engine) after the interruption of the transmission of the engine brake torque by disengaging the clutch when the inertia traveling is sensed, to control the engine speed so that a rotation speed difference between the engine speed and the rotation speed of the driving system becomes a predetermined rotation speed difference when the engine is again connected to the driving system, and then to engage the clutch. 
     However, for example, in a high vehicle speed region in which a transmission gear ratio of a transmission of the driving system is highest, a time period necessary for synchronizing the rotation speeds of forward and rearward of the clutch becomes long. 
     Accordingly, when the clutch is engaged after the engine speed has the predetermined rotation speed difference with respect to the rotation speed of the driving system, the time period from the restart of the engine to the engagement of the clutch becomes long. With this, the unnatural feeling may be provided to the driver. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         
           
             Japanese Patent Application Publication No. 2004-44800 
           
         
       
    
     SUMMARY OF THE INVENTION 
     An internal combustion engine comprises: when the internal combustion engine which is automatically stopped in a state where the clutch is disengaged is restarted, performing a torque down control to decrease a target torque of the internal combustion engine when the clutch is engaged; setting a predetermined torque release time period determined in accordance with a driving state; and ending the torque down control at a timing at which the torque release time period is elapsed from an engagement command of the clutch which is generated during the torque down control. 
     In the present invention, the torque release time period at the clutch engagement is set in accordance with the driving state. With this, it is possible to ensure the response characteristic (the acceleration characteristic) of the vehicle at the restart of the internal combustion engine which is automatically stopped, and to suppress the engagement shock at the clutch engagement. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an explanation view schematically showing an outline of a control device of an internal combustion engine according to the present invention. 
         FIG. 2  is a timing chart of a torque down control of the internal combustion engine in the present invention. 
         FIG. 3  is a timing chart of a torque down control of a first comparative example. 
         FIG. 4  is a timing chart of a torque down control of a second comparative example. 
         FIG. 5  is a flowchart showing one example of a flow of a control of the internal combustion engine in the present invention. 
         FIG. 6  is a flowchart showing one example of a flow of a control of the internal combustion engine in the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an outline of one embodiment of the present invention is explained in detail with reference to the drawings. 
       FIG. 1  is an explanation view schematically showing an outline of a control device of an internal combustion engine  1  according to the present invention. 
     The internal combustion engine  1  is a driving source for a vehicle. The internal combustion engine  1  is connected through a torque converter  2  including a lockup mechanism, to a CVT (continuously variable transmission)  3  which is a transmission. 
     The lockup mechanism is a mechanical clutch installed in the torque converter  2 . The lockup mechanism is arranged to connect the internal combustion engine  1  and the CVT  3  through the torque converter  2 , by a lockup clutch disengagement. Moreover, the lockup mechanism is arranged to directly connect an output shaft  1   a  of the internal combustion engine, and a CVT input shaft  3   a  by lockup clutch engagement. This lockup mechanism is arranged to be controlled among the engagement, a slip engagement, and the disengagement by an LU actual hydraulic pressure produced based on an LU command pressure from a TCU  30  described later. 
     The CVT  3  is arranged to transmit the power through a final speed reduction device (not shown) to driving wheels  4 , like a normal automobile. Moreover, in this embodiment, a forward clutch  5  is disposed between the torque converter  2  and the CVT  3 . 
     That is, the internal combustion engine  1 , the torque converter  2 , the forward clutch  5 , the CVT  3 , and the driving wheels  4  are disposed in this order in series with each other in a power transmitting path by which the driving force by the internal combustion engine  1  is transmitted to the driving wheels  4 . 
     The driving force is transmitted from the engine  1 , through the lockup clutch of the lockup mechanism of the torque converter  2 , and the forward clutch  5  to the driving wheels  4  of the vehicle. 
     The internal combustion engine  1  is arranged to drive a motor  7 , a water pump  8 , and a compressor  9  for an air conditioner through a belt  6 . 
     The motor  7  is arranged to provide the driving force to the internal combustion engine  1 , and to generate the electric power. 
     Moreover, the internal combustion engine  1  is provided with a starter motor  10  used at the start of the internal combustion engine  1 , in addition to the motor  7 . Besides, in a case where the motor  7  is used for the start of the internal combustion engine  1 , it is possible to omit the starter motor  10 . 
     The CVT  3  includes a primary pulley  11 , a secondary pulley  12 , and a V belt  13  wound around V grooves of the primary pulley  11  and the secondary pulley  12 . The primary pulley  11  includes a primary hydraulic cylinder  11   a . The secondary pulley  12  includes a secondary hydraulic cylinder  12   a . A width of the V groove of the primary pulley  11  is varied by adjusting the hydraulic pressure supplied to the primary hydraulic cylinder  11   a . A width of the V groove of the secondary pulley  12  is varied by adjusting the hydraulic pressure supplied to the secondary hydraulic cylinder  12   a.    
     In the CVT  3 , the widths of the V grooves are varied by controlling the hydraulic pressures supplied to the primary hydraulic cylinder  11   a  and the secondary hydraulic cylinder  12   a , so that the contact radii between the V belt  13 , and the primary pulley  11  and the secondary pulley  12  are varied. Consequently, the transmission gear ratio is continuously varied. 
     The hydraulic pressure is supplied to the CVT  3  by a mechanical oil pump (not shown) which is a first oil pump, and which is driven by the internal combustion engine  1 , and an electric oil pump  14  which is a second oil pump. That is, the hydraulic pressure is supplied from the mechanical oil pump or the electric oil pump  14  to the primary hydraulic cylinder  11   a  and the secondary hydraulic cylinder  12   a . The electric oil pump  14  is arranged to be driven when the internal combustion engine  1  is automatically stopped during the driving of the vehicle by an idling stop and so on. That is, the electric oil pump  14  is operated when the mechanical oil pump is stopped. 
     Besides, the hydraulic fluid is supplied to the torque converter  2  and the forward clutch  5  by the mechanical oil pump or the electric oil pump  14 . That is, the mechanical oil pump or the electric oil pump  14  is a supply source of the hydraulic fluid for the lockup clutch of the lockup mechanism of the torque converter  2  and the forward clutch  5 . 
     The forward clutch  5  is a clutch disposed between the internal combustion engine  1  and the driving wheels  4 . The forward clutch  5  is arranged to disconnect the internal combustion engine  1  and the CVT  3  in a disengagement state. The forward clutch  5  is provided to the CVT input shaft  3   a . The forward clutch  5  is arranged to be in an engagement state so that the power can be transmitted between the internal combustion engine  1  and the driving wheels  4 . The forward clutch  5  is arranged to be in the disengagement state so that the power (torque) cannot be transmitted between the internal combustion engine  1  and the driving wheels  4 . That is, when the forward clutch  5  is disengaged, the internal combustion engine  1  and the driving wheels  4  are disconnected. Moreover, when the forward clutch  5  is disengaged, the internal combustion engine  1  and the CVT  3  is disconnected. 
     The internal combustion engine  1  is controlled by an ECU (engine control unit)  20 . The ECU  20  is a known digital computer including a CPU, a ROM, a RAM, and an input and output interface. 
     The ECU  20  receives detection signal of various sensors such as a crank angle sensor  21  arranged to sense a crank angle of a crank shaft (not shown) of the internal combustion engine  1 , an accelerator opening degree sensor  22  arranged to sense a depression amount of an accelerator pedal (not shown), a brake switch  23  arranged to sense an operation of a brake pedal (not shown), a vehicle speed sensor  24  arranged to sense a vehicle speed, and an acceleration sensor  25  arranged to sense an acceleration of the vehicle. The crank angle sensor  21  is arranged to sense an engine speed Re of the internal combustion engine  1 . 
     The ECU  20  is configured to appropriately control an injection amount and an injection timing of a fuel injected from a fuel injection valve (not shown) of the internal combustion engine  1 , an ignition timing and an intake air amount of the internal combustion engine  1 , and so on, based on the detection signal of the various sensors. Moreover, the ECU  20  is configured to appropriately control the motor  7  and the starter motor  10 . 
     Besides, the ECU  20  receives information relating to a battery SOC and so on of a battery mounted on the vehicle. 
     The CVT  3  is controlled by a TCU (transmission control unit)  30 . The TCU  30  is a known digital computer including a CPU, a ROM, a RAM, and an input and output interface. 
     The ECU  20  and the TCU  30  are connected by a CAN communication line  31 . The data can be exchanged between the ECU  20  and the TCU  30  by the CAN communication line  31 . 
     The TCU  30  receives the detection signal of the above-described accelerator opening degree sensor  22 , the brake switch  23 , and the vehicle speed sensor  24  through the CAN communication line  31 . 
     Moreover, the TCU  30  receives detection signal of various sensors such as a primary rotation speed sensor  32  arranged to sense a rotation speed Rp of the primary pulley  11  which is an input side rotation speed of the CVT  3 , a secondary pulley rotation speed sensor  33  arranged to sense a rotation speed of the secondary pulley  12  which is an output side rotation speed of the CVT  3 , a hydraulic pressure sensor  34  arranged to sense the hydraulic pressure of the hydraulic fluid supplied to the CVT  3 , and an inhibitor switch  35  arranged to sense a position of a select lever arranged to select a traveling range. 
     The TCU  30  is configured to appropriately control the transmission gear ratio of the CVT  3 , the torque converter  2 , and the forward clutch  5  based on the inputted detection signal of the various sensors. Moreover, the TCU  30  controls the driving of the electric oil pump  14 . 
     When a predetermined automatic stop condition is satisfied during the traveling of the vehicle, the internal combustion engine  1  is automatically stopped by the stop of the fuel supply. Then, when a predetermined automatic restart condition is satisfied during the automatic stop of the internal combustion engine  1 , the internal combustion engine is restarted by the restart of the fuel supply. 
     The automatic stop of the internal combustion engine  1  during the traveling is a coast stop and a sailing stop. 
     The coast stop is performed when a coast stop execution condition which is the automatic stop condition is satisfied during the traveling of the vehicle. The internal combustion engine  1  in the coast stop state is restarted when a coast stop cancel condition which is the automatic restart condition is satisfied. 
     The coast stop execution condition is satisfied, for example, in a case where the battery SOC is equal to or greater than a predetermined value during the deceleration in a state where the brake pedal is depressed. In the specification, the state where the brake pedal is depressed is an ON state of the brake switch  23 . 
     The coast stop cancel condition is satisfied, for example, in a case where the accelerator pedal is depressed, in a case where the brake pedal is not depressed, or in a case where an electric power of the vehicle is needed to be ensured when the battery SOC becomes equal to or smaller than a predetermined value, and so on. In the specification, the state where the accelerator pedal is depressed is the ON state of the accelerator. Moreover, in the specification, the state where the brake pedal is not depressed is a state where the foot is apart from the brake pedal, that is, the OFF state of the brake switch  23 . 
     In this embodiment, the coast stop state is defined by a state where the internal combustion engine  1  is automatically stopped during the deceleration in the depressed state of the brake pedal at the low vehicle speed. At the coast stop, the forward clutch  5  is engaged. The lockup clutch of the lockup mechanism of the torque converter  2  is disengaged. 
     The sailing stop is performed when a sailing stop execution condition which is the automatic stop condition is satisfied during the traveling of the vehicle. The internal combustion engine  1  in the sailing stop state is restarted when a sailing stop cancel condition which is the automatic restart condition is satisfied. 
     The sailing stop execution condition is satisfied, for example, in a case where the battery SOC is equal to or greater than the predetermined value when the accelerator pedal is switched from the depressed state to the undepressed state during the traveling of the vehicle. That is, the sailing stop condition is satisfied when there is no driving force request. In the specification, the undepressed state of the accelerator pedal is the state where the foot is apart from the accelerator pedal, that is, the OFF state of the accelerator. 
     The sailing stop cancel condition is satisfied, for example, in a case where the accelerator pedal is depressed, in a case where the brake pedal is not depressed, or in a case where the electric power of the vehicle is needed to be ensured when the battery SOC becomes equal to or smaller than the predetermined value, and so on. 
     In this embodiment, the sailing stop state is defined by a state where the internal combustion engine  1  is automatically stopped during an inertia traveling in which the brake pedal is not depressed in a middle or high vehicle speed. At the sailing stop, the forward clutch  5  is disengaged. The lockup clutch of the lockup mechanism of the torque converter  2  is engaged. 
     In a case where the vehicle is accelerated by the restart of the internal combustion engine  1  during the coast stop or the sailing stop, the disengaged clutch is needed to be engaged. When the disengaged clutch is engaged, a torque down control (torque decrease control) is performed to decrease a target torque of the internal combustion engine  1 . 
     In this embodiment, the target torque of this torque down control is set to be equal to or greater than a predetermined torque lower limit value Tmin determined in accordance with the driving state. Moreover, a timing of the end of the torque down control is defined by a predetermined torque release time period t trq  according to the driving state. The torque release time period t trq  is a time period from a timing at which a rotation speed difference between the internal combustion engine  1  and the primary pulley  11  becomes a first predetermined value A during the torque down control, to a timing of the end of the toque down control. That is, the torque release time period t trq  is a time period from the engagement command of the clutch (the lockup clutch or the forward clutch  5 ) which is generated during the torque down control, to the end of the torque down control. 
     The torque lower limit value Tmin is set to compensate for (cover) the traveling resistance of the vehicle, and the resistance of the power train of the vehicle. 
     Specifically, the torque lower limit value Tmin is set to be greater as the vehicle speed is higher. Moreover, the torque lower limit value Tmin is set to be greater as the accelerator opening degree is greater. That is, when the vehicle speed or the accelerator opening degree is large, the torque lower limit value Tmin is set to be greater than that when the vehicle speed or the accelerator opening degree is small. 
     The torque lower limit value Tmin is calculated, for example, by using the vehicle speed and the accelerator opening degree. For example, the ECU  20  or the TCU  30  stores a torque lower limit value calculation map showing the torque lower limit value Tmin corresponding to the vehicle speed and the accelerator opening degree. With this, it is possible to calculate the torque lower limit value Tmin. Besides, it is optional to calculate the torque lower limit value Tmin from a predetermined equation (expression) by using the vehicle speed and the accelerator opening degree. 
     The torque release time period t trq  is set to compensate for (cover) the traveling resistance and the resistance of the power train of the vehicle. 
     Specifically, the torque release time period t trq  is set to be shorter as the vehicle speed during the torque down control is higher. Moreover, the torque release time period t trq  is set to be shorter as the accelerator opening degree during the torque down control is greater. That is, when the vehicle speed or the accelerator opening degree during the torque down control is large, the torque release time period t trq  is set to be shorter than that when the vehicle speed or the accelerator opening degree during the torque down control is small. 
     The torque release time period t trq  is calculated, for example, by using the vehicle speed and the accelerator opening degree. For example, the ECU  20  or the TCU  30  stores a torque release time period calculation map showing the torque release time period t trq  corresponding to the vehicle speed and the accelerator opening degree. With this, it is possible to calculate the torque release time period t trq . Besides, it is optional to calculate the torque release time period t trq  from a predetermined equation (expression) by using the vehicle speed and the accelerator opening degree. 
     In this embodiment, the ECU  20  and the TCU  30  are linked with each other. Accordingly, it is possible to consider the ECU  20  and the TCU  30  as a CU (control unit)  40 . Accordingly, in this embodiment, the CU  40  including the ECU  20  and the TCU  30  corresponds to a torque down control section configured to perform the torque down control when the lockup clutch of the lockup mechanism of the torque converter  2  or the forward clutch  5  is engaged, a torque lower limit value calculation section configured to calculate the torque lower limit value Tmin, and a torque release time period calculation section configured to calculate the torque release time period t trq . Besides, the CU  40  is configured to automatically stop the internal combustion engine  1  when the automatic stop condition is satisfied. 
       FIG. 2  is a timing chart for explaining the torque down control of the internal combustion engine  1  in this embodiment, by exemplifying the sailing stop. 
     A characteristic line C 1  shown by a solid line in  FIG. 2  represents an acceleration Ga in the forward and rearward directions of the vehicle. 
     A characteristic line C 2  shown by a broken line in  FIG. 2  represents a target torque Tv of the internal combustion engine  1  when the torque down control is not performed. A characteristic line C 3  shown by a solid line in  FIG. 2  represents a target torque Tt of the internal combustion engine  1  when the torque down control is performed. 
     A characteristic line C 4  shown by a solid line in  FIG. 2  represents a target pressure Pt of the hydraulic fluid supplied to the forward clutch  5 . A characteristic line C 5  shown by a broken line in  FIG. 2  represents an actual pressure Pa of the hydraulic fluid supplied to the forward clutch  5 . 
     A characteristic line C 6  shown by a broken line in  FIG. 2  represents a rotation speed Rp of the primary pulley  11 . A characteristic line C 7  shown by a solid line in  FIG. 2  represents an engine speed Re of the internal combustion engine  1 . 
     Time t 1  is a timing of the accelerator ON. The internal combustion engine  1  starts the cranking at this time t 1 . At time t 1 , the sailing stop cancel condition is satisfied. The internal combustion engine  1  starts the cranking at this time t 1 . That is, the internal combustion engine  1  is restarted at time t 1 . 
     Time t 2  is a timing at which a pre-charge is performed to suppress a delay of the hydraulic response of the forward clutch  5 . Time t 2  is a timing at which a predetermined time period is elapsed from the timing of the accelerator ON. After the pre-charge, the hydraulic pressure of the forward clutch  5  is controlled to be smaller than the hydraulic pressure by which the torque transmission is started, until the engagement command of the forward clutch  5  is outputted. 
     Time t 3  is a timing at which the engine speed Re of the internal combustion engine  1  is increased to be closer to the rotation speed Rp of the primary pulley  11  so that the rotation speed difference between the internal combustion engine  1  and the primary pulley  11  becomes a second predetermined value B. When the rotation speed difference between the internal combustion engine  1  and the primary pulley  11  becomes the second predetermined value B, the torque down control is started. That is, the torque down control is performed when the rotation speed difference between the internal combustion engine  1  and the primary pulley  11  becomes equal to or smaller than the second predetermined value B. 
     When the torque down control is started, the target torque Tt of the internal combustion engine  1  is limited to the torque lower limit value Tmin. 
     Time t 4  is a timing at which the rotation speed difference between the internal combustion engine  1  and the primary pulley  11  becomes the first predetermined value A. 
     When the rotation speed difference between the internal combustion engine  1  and the primary pulley  11  becomes the first predetermined value A, the engagement command of the forward clutch  5  is outputted to increase the target pressure Pt of the hydraulic pressure supplied to the forward clutch  5 . The actual pressure Pa of the hydraulic pressure supplied to the forward clutch  5  is increased in accordance with the increase of the target pressure Pt of the hydraulic fluid supplied to the forward clutch  5 , so that the forward clutch  5  is engaged. The first predetermined value A is smaller than the second predetermined value B. 
     The driving torque of the internal combustion engine  1  is transmitted to the primary pulley  11  by the engagement of the forward clutch  5  after the engagement command of the forward clutch  5 . Then, the acceleration (the forward and rearward G) of the vehicle becomes a positive value when the vehicle is started to be accelerated. 
     Moreover, at time t 4 , a timer to measure a timing of the end of the torque down control is started. That is, the timer is started at a timing at which the engagement command of the forward clutch  5  during the torque down control is outputted. That is, the timer is started to count at a timing at which the clutch engagement command is outputted. 
     In case of the coast stop, the timer is started at a timing at which the engagement command of the lockup clutch is outputted during the torque down control. 
     Time t 5  is a timing at which the torque release time period t trq  is elapsed from time t 4 . The torque down control is finished at a timing (time t 5 ) at which the torque release time period t trq  is elapsed from a timing at which the rotation speed difference between the internal combustion engine  1  and the primary pulley  11  becomes the first predetermined value A during the toque down control. That is, the torque down control is finished at a timing (time t 5 ) at which the torque release time period t trq  is elapsed from the engagement command of the forward clutch  5  which is generated during the torque down control. 
     Besides, the torque down control in case of the coast stop is finished at a timing at which the torque release time period t trq  is elapsed from the engagement command of the lockup clutch which is generated during the torque down control. 
     The toque release time period t trq  is sequentially calculated during the torque down control. At time t 5 , the internal combustion engine  1  is released from the torque limitation in which the target torque Tt is limited to the torque lower limit value Tmin. 
     The acceleration feeling and the deceleration feeling sensed by the driver at the engagement of the forward clutch  5  and the lockup clutch of the lockup mechanism of the torque converter  2  is not generally problematic. This acceleration feeling and the deceleration feeling are dissolved during a relatively short time period. However, these may provide the unnatural feeling to the driver. 
       FIG. 3  is a timing chart for explaining the torque down control in a first comparative example, by exemplifying the sailing stop. A system configuration of the first comparative example is identical to that of the above-described embodiment of the present invention. Accordingly, the same constitution components have the same symbols. The repetitive explanations are omitted. 
     A characteristic line C 8  shown by a solid line in  FIG. 3  represents an acceleration Gc 1  in the forward and rearward directions of the vehicle in the first comparative example. A characteristic line C 9  shown by a broken line in  FIG. 3  represents an acceleration Gc 0  when the torque of the internal combustion engine  1  during the torque down control is set to the torque lower limit value Tmin, like the above-described embodiment. 
     A characteristic line C 10  shown by a broken line in  FIG. 3  represents a rotation speed Rp of the primary pulley  11  in the first comparative example. A characteristic line C 11  shown by a solid line in  FIG. 3  represents an engine speed Re of the internal combustion engine  1  in the first comparative example. 
     A characteristic line C 12  shown by a solid line in  FIG. 3  represents a target torque Tt 1  of the internal combustion engine  1  in the first comparative example. A characteristic line C 13  shown by a broken line in  FIG. 3  represents a target torque Tt when the torque of the internal combustion engine  1  during the torque down control is set to the torque lower limit value Tmin like the above-described embodiment. 
     A characteristic line C 14  shown by a solid line in  FIG. 3  represents a target pressure Pt of the hydraulic fluid supplied to the forward clutch  5 . 
     A characteristic line C 15  shown by a solid line in  FIG. 3  represents a torque Tc 1  inputted to the CVT  3  in this first comparative example. A characteristic line C 16 Tc shown by a broken line in  FIG. 3  represents a torque Tc inputted to the CVT  3  in the above-described embodiment. 
     Time t 1  in  FIG. 3  is a timing of the accelerator ON. Time t 2  in  FIG. 3  is a timing at which a pre-charge is performed to suppress a delay of the hydraulic response of the forward clutch  5 . Time t 3  in  FIG. 3  is a timing at which the torque down control is started. Time t 4  in  FIG. 3  is a timing at which the engagement command of the forward clutch  5  is outputted. Time t 5  in  FIG. 3  is a timing at which the torque down control is finished. 
     In this first comparative example, the target torque Tt 1  of the internal combustion engine  1  during the torque down control is excessive. That is, in the first comparative example, the target torque Tt 1  of the internal combustion engine during the torque down control is set to be greater than the target torque Tt of the internal combustion engine during the torque down control in the above-described embodiment. 
     Accordingly, the sudden torque variation is transmitted to the CVT  3  at the engagement of the forward clutch  5 , so that the shock is generated. This shock is appeared as the variation of the forward and rearward acceleration. 
     That is, in a case where the target torque Tt 1  of the internal combustion engine  1  is high during the torque down control like the first comparative example, the driver may feel, as the unnatural feeling, the acceleration feeling sensed at the engagement of the forward clutch  5  when the torque step (torque level difference) becomes large at the engagement of the forward clutch  5 . 
       FIG. 4  is a timing chart for explaining the torque down control in a second comparative example, by exemplifying the sailing stop. A system configuration of the second comparative example is identical to that of the above-described embodiment of the present invention. Accordingly, the same constitution components have the same symbols. The repetitive explanations are omitted. 
     A characteristic line C 17  shown by a solid line in  FIG. 4  represents an acceleration Gc 2  in the forward and rearward directions of the vehicle in the second comparative example. A characteristic line C 9  shown by a broken line in  FIG. 4  represents an acceleration Gc 0  when the torque of the internal combustion engine  1  during the torque down control is set to the torque lower limit value Tmin, like the above-described embodiment. 
     A characteristic line C 18  shown by a broken line in  FIG. 4  represents a rotation speed Rp of the primary pulley  11  in the second comparative example. A characteristic line C 19  shown by a solid line in  FIG. 4  represents an engine speed Re of the internal combustion engine  1  in the second comparative example. 
     A characteristic line C 20  shown by a solid line in  FIG. 4  represents a target torque Tt 2  of the internal combustion engine  1  in the second comparative example. A characteristic line C 13  shown by a broken line in  FIG. 4  represents a target torque Tt when the torque of the internal combustion engine  1  during the torque down control is set to the torque lower limit value Tmin like the above-described embodiment. 
     A characteristic line C 14  shown by a solid line in  FIG. 4  represents a target pressure Pt of the hydraulic fluid supplied to the forward clutch  5 . 
     A characteristic line C 21  shown by a solid line in  FIG. 4  represents a torque Tc 2  inputted to the CVT  3  in this second comparative example. A characteristic line C 16  shown by a broken line in  FIG. 4  represents a torque Tc inputted to the CVT  3  in the above-described embodiment. 
     Time t 1  in  FIG. 4  is a timing of the accelerator ON. Time t 2  in  FIG. 4  is a timing at which a pre-charge is performed to suppress a delay of the hydraulic response of the forward clutch  5 . Time t 3  in  FIG. 4  is a timing at which the torque down control is started. Time t 4  in  FIG. 4  is a timing at which the engagement command of the forward clutch  5  is outputted. Time t 5  in  FIG. 4  is a timing at which the torque down control is finished. 
     In this second comparative example, the target torque Tt 2  of the internal combustion engine  1  during the torque down control is deficient. That is, in the second comparative example, the target torque Tt 2  of the internal combustion engine during the torque down control is set to be smaller than the target torque Tt of the internal combustion engine during the torque down control in the above-described embodiment. 
     When the torque of the internal combustion engine  1  is deficient during the torque down control, the traveling resistance and the resistance of the power train of the vehicle are not compensated by the torque (the driving force) of the internal combustion engine  1  at the engagement of the forward clutch  5 . 
     Accordingly, the sudden torque variation is transmitted to the CVT  3  at the engagement of the forward clutch  5 , so that the shock is generated. This shock is appeared as the variation of the forward and rearward acceleration. 
     That is, in a case where the target torque Tt 2  of the internal combustion engine  1  is low during the torque down control like the second comparative example, the driver may feel, as the unnatural feeling, the deceleration feeling sensed at the engagement of the forward clutch  5  when the torque step (torque level difference) becomes large at the engagement of the forward clutch  5 . 
     Therefore, in the above-described embodiment, in the high vehicle speed, the torque release time period t trq  is set to be relatively short so as to prioritize the followability, and to dissolve the unnatural feeling of the driver by below-described reasons. 
     1) It does not feel the shock due to the ambient noise by the high vehicle speed. 
     2) When the transmission gear ratio of the CVT  3  is highest, the shock at the engagement of the clutch which is transmitted to the vehicle body becomes substantially a quarter of that when the transmission gear ratio of the CVT  3  is lowest. Accordingly, when the transmission gear ratio of the CVT  3  is highest, the shock is remarkably decreased.
 
3) In the super high speed (for example, 100 km/h), the sudden followability is needed at the engagement of the clutch to increase the rotation speed of the CVT input shaft  3   a.  
 
     Moreover, in the above-described embodiment, a case of the low vehicle speed is contrary to a case of the high vehicle speed. Accordingly, the torque release time period t trq  is set so as not to be extremely short to suppress the acceleration feeling of the driver, and thereby to decrease the unnatural feeling of the driver. 
     When the accelerator opening degree is large, the acceleration request of the driver is high. Accordingly, the driver is difficult to feel the unnatural feeling by the acceleration and the deceleration. Therefore, the torque release time period t trq  is set to be shorter to prioritize the followability. 
     When the accelerator opening degree is small, it is contrary to a case of the above-described large accelerator opening degree. Accordingly, the torque release time period t trq  is set so as not to be extremely short to suppress the acceleration feeling of the driver, and thereby to decrease the unnatural feeling of the driver. 
     In this way, in the above-described embodiment, the torque down control is finished at a timing at which the predetermined torque release time period t trq  is elapsed from the timing at which the rotation speed difference between the engine speed Re of the internal combustion engine  1  and the input side rotation speed of the CVT  3  (the rotation speed Rp of the primary pulley  11 ) becomes the first predetermined value A. With this, it is possible to control the end timing of the torque down control. That is, in the above-described embodiment, the torque down control is finished at a timing at which the predetermined torque release time period t trq  is elapsed from the engagement command of the lockup clutch of the lockup mechanism of the torque converter  2 , or the forward clutch  5 . With this, it is possible to control the end timing of the torque down control. 
     With this, it is possible to ensure the response characteristic (the acceleration characteristic) of the vehicle at the restart of the internal combustion engine  1  which is automatically stopped, and to suppress the engagement shock at the engagement of the lockup clutch and the forward clutch  5 . 
     Moreover, in the above-described embodiment, the torque release time period t trq  is set in accordance with the vehicle speed and the accelerator opening degree. With this, it is possible to set the torque release time period t trq  so as to compensate for the traveling resistance (the air resistance and the rolling resistance), and the resistance of the power train of the vehicle. 
     When the vehicle is in the high vehicle speed, the torque release time period t trq  is set to be relatively short so as to recover (retrieve) the delay by the rotation increase. With this, it is possible to suppress the deterioration of the response characteristic (the acceleration characteristic) of the vehicle at the restart of the internal combustion engine  1  which is automatically stopped. 
     When the vehicle is in the low vehicle speed, the traveling resistance of the vehicle is relatively small. The transmission gear ratio of the CVT  3  is the low side. Accordingly, the torque release time period t trq  is set to be relatively long. With this, it is possible to decrease the unnecessary acceleration feeling generated at the engagement of the lockup clutch and the forward clutch  5 . 
     Moreover, when the vehicle is in the low vehicle speed, it is possible to set the torque release time period t trq  to the relatively long value, and to finish the torque down control after waiting for the full engagement of the lockup clutch and the forward clutch  5 . In this case, it is possible to further decrease the engagement shock generated at the engagement of the lockup clutch and the forward clutch  5 . 
     When the accelerator opening degree is large, the torque release time period t trq  is set to be relatively short. With this, it is possible to improve the response characteristic (the acceleration characteristic) of the vehicle at the restart of the internal combustion engine  1  which is automatically stopped. 
     When the accelerator opening degree is small, the torque release time period t trq  is set to be relatively long. With this, it is possible to decrease the unnecessary acceleration feeling at the engagement of the lockup clutch and the forward clutch  5 . 
     Moreover, when the accelerator opening degree is small, it is possible to set the torque release time period t trq  to be the relatively long value, and to finish the torque down control after waiting for the full engagement of the lockup clutch and the forward clutch  5 . In this case, it is possible to further decrease the engagement shock generated at the engagement of the lockup clutch and the forward clutch  5 . 
       FIG. 5  and  FIG. 6  are flowcharts showing a flow of the control of the internal combustion engine according to the present invention.  FIG. 5  is a flowchart showing one example of the flow of the control at the restart of the internal combustion engine.  FIG. 6  is a flowchart showing one example of the flow of the control when the torque lower limit value Tmin and the torque release time period t trq  are calculated. 
     Firstly,  FIG. 5  is explained. 
     At step S 1 , it is judged whether or not the internal combustion engine  1  is automatically stopped during the traveling. When it is judged that the internal combustion engine  1  is automatically stopped during the traveling at step S 1 , the process proceeds to step S 2 . When it is judged that the internal combustion engine  1  is not automatically stopped during the traveling at step S 1 , this routine is finished. 
     At step S 2 , it is judged whether or not the automatic restart condition is satisfied. When it is judged that the automatic restart condition is satisfied at step S 2 , the process proceeds to step S 3 . When it is judged that the automatic restart condition is not satisfied at step S 2 , this routine is finished. 
     At step S 3 , the internal combustion engine  1  is started. 
     At step S 4 , it is judged whether or not the rotation speed difference between the engine speed Re of the internal combustion engine  1  and the rotation speed Rp of the primary pulley  11  of the CVT  3  becomes the second predetermined value B. When it is judged that the rotation speed difference between the engine speed Re of the internal combustion engine  1  and the rotation speed Rp of the primary pulley  11  of the CVT  3  becomes the second predetermined value B at step S 4 , the process proceeds to step S 5 . When it is judged that the rotation speed difference between the engine speed Re of the internal combustion engine  1  and the rotation speed Rp of the primary pulley  11  of the CVT  3  does not become the second predetermined value B at step S 4 , the process proceeds to step S 3 . 
     At step S 5 , the torque down control is started. 
     At step S 6 , the torque lower limit value Tmin which is the target torque in the torque down control is read. This torque lower limit value Tmin is calculated by using the vehicle speed and the accelerator opening degree. The torque lower limit value Tmin is varied in accordance with the driving state during the torque down control. That is, the torque lower limit value Tmin is varied in accordance with the vehicle speed and the accelerator opening degree during the torque down control. 
     At step S 7 , it is judged whether or not the rotation speed difference between the engine speed Re of the internal combustion engine  1  and the rotation speed Rp of the primary pulley  11  of the CVT  3  becomes the first predetermined value A. The first predetermined value A is set to be smaller than the second predetermined value B. When it is judged that the rotation speed difference between the engine speed Re of the internal combustion engine  1  and the rotation speed Rp of the primary pulley  11  of the CVT  3  becomes the first predetermined value A at step S 7 , the process proceeds to step S 8 . When it is judged that the rotation speed difference between the engine speed Re of the internal combustion engine  1  and the rotation speed Rp of the primary pulley  11  of the CVT  3  does not become the first predetermined value A at step S 7 , the process proceeds to step S 5 . 
     At step S 8 , the clutch engagement is started. That is, the engagement of the forward clutch  5  is started at the recovery from the sailing stop. The engagement of the lockup clutch is started at the recovery from the coast stop. 
     At step S 9 , the timer to measure Actually, this timer is started from the timing at which the rotation speed difference between the engine speed Re and the rotation speed Rp of the primary pulley  11  becomes the first predetermined value A. 
     At step S 10 , the torque release time period t trq  is read. This torque release time period t trq  is calculated by using the vehicle speed and the accelerator opening degree. The torque release time period t trq  is varied in accordance with the driving state during the torque down control. That is, the torque release time period t trq  is varied in accordance with the vehicle speed and the accelerator opening degree during the torque down control. 
     At step S 11 , it is judged whether or not the torque release time period t trq  is elapsed from the start of the timer. When it is judged that the torque release time period t trq  is elapsed from the start of the timer at step S 11 , the process proceeds to step S 12 . When it is judged that the torque release time period t trq  is not elapsed from the start of the timer at step S 11 , the process proceeds to step S 10 . 
     At step S 12 , the torque down control is finished. 
     Next,  FIG. 6  is explained. 
     At step S 21 , it is judged whether or not the torque down control is started. When it is judged that the torque down control is started (performed) at step S 21 , the process proceeds to step S 22 . When it is judged that the torque down control is not started (performed) at step S 21 , this routine is finished. 
     At step s 22 , the vehicle speed and the accelerator opening degree are read. 
     At step S 23 , the torque lower limit value Tmin is calculated by using the vehicle speed and the accelerator opening degree. 
     At step S 24 , the torque release time period t trq  is calculated by using the vehicle speed and the accelerator opening degree. 
     The current torque lower limit value Tmin calculated at step S 23  is read at step S 6  of  FIG. 5 . 
     The current torque release time period t trq  calculated at step S 24  is read at step S 10  of  FIG. 5 . 
     Besides, the above-described embodiment relates to the control method and the control device for the internal combustion engine. 
     Moreover, the present invention is applicable to the restart of the internal combustion engine  1  which is in the sailing stop state, and the restart of the internal combustion engine  1  which is in the coast stop state.