Coast stop vehicle and control method thereof

An oil pump driven by an engine generates a hydraulic pressure supplied to a frictional engagement element. An accumulator provided at an intermediate position of an oil path supplies the hydraulic pressure generated by the oil pump to the frictional engagement element. A controller stops the engine after a hydraulic pressure reducing condition to reduce the hydraulic pressure supplied to the frictional engagement element holds and the hydraulic pressure supplied to the frictional engagement element is reduced.

FIELD OF THE INVENTION

The present invention relates to a coast stop technology for automatically stopping an engine during the travel of a vehicle.

BACKGROUND OF THE INVENTION

JP2006-170295A discloses a coast stop technology for automatically stopping an engine during the travel of a vehicle for the purpose of reducing a fuel consumption amount. At a coast stop, fuel supply to the engine is stopped and an automatic transmission is set in a neutral state, thereby completely stopping the rotation of the engine.

The coast stop is also possible in a vehicle including a v-belt continuously variable transmission mechanism (hereinafter, referred to as a “variator”) as an automatic transmission. To perform a coast stop, fuel supply to the engine may be stopped and a frictional engagement element (clutch or brake) arranged before or after the variator to switch transmission and non-transmission of power may be released.

SUMMARY OF THE INVENTION

Since an oil pump driven by the engine also stops when the engine stops, a supply pressure to pulleys of the variator, i.e. a belt clamping pressure is also reduced. At this time, if the frictional engagement element is kept engaged with the release thereof delayed, when a torque is input from drive wheels such as due to braking, this torque is transmitted to the variator via the frictional engagement element and slippage occurs between the belt and the pulleys. Thus, the frictional engagement element needs to be released simultaneously with or before the stop of the engine.

However, there are cases where an accumulator for preventing the occurrence of a shock caused by sudden engagement of a clutch such as when an N-D select is being made is provided at an intermediate position of an oil path for supplying a hydraulic pressure to a frictional engagement element. If such an accumulator is provided, the release of the frictional engagement element is delayed until a hydraulic pressure accumulated in the accumulator is drained and the frictional engagement element cannot be released simultaneously with or before the stop of an engine even if an instruction to release the frictional engagement element is given simultaneously with an instruction to stop fuel supply to the engine.

The present invention was developed in view of the above technical problem and aims to suppress belt slippage at the time of performing a coast stop in a vehicle including a belt-type continuously variable transmission mechanism (variator).

According to an aspect of the present invention, a coast stop vehicle which stops an engine during the travel of the vehicle, includes a continuously variable transmission with a variator including a pair of pulleys and a belt mounted between the pulleys and capable of continuously changing a speed ratio and a frictional engagement element which is provided before or after the variator and enables power transmission when being engaged while disabling power transmission when being released, the continuously variable transmission shifting and outputting output rotation of the engine to drive wheels; an oil pump driven by the engine to generate a hydraulic pressure supplied to the frictional engagement element; an accumulator provided at an inter mediate position of an oil path for supplying the hydraulic pressure generated by the oil pump to the frictional engagement element; and a coast stop control unit which stops the engine after a hydraulic pressure reducing condition to reduce the hydraulic pressure supplied to the frictional engagement element holds and the hydraulic pressure supplied to the frictional engagement element is reduced.

According to another aspect of the present invention, a coast stop method for stopping an engine during the travel of a vehicle. An the vehicle includes a continuously variable transmission with a variator including a pair of pulleys and a belt mounted between the pulleys and capable of continuously changing a speed ratio and a frictional engagement element which is provided before or after the variator and enables power transmission when being engaged while disabling power transmission when being released, the continuously variable transmission shifting and outputting output rotation of the engine to drive wheels; an oil pump driven by the engine to generate a hydraulic pressure supplied to the frictional engagement element; and an accumulator provided at an intermediate position of an oil path for supplying the hydraulic pressure generated by the oil pump to the frictional engagement element. The method includes a coast stop control step of stopping the engine after a hydraulic pressure reducing condition to reduce the hydraulic pressure supplied to the frictional engagement element holds and the hydraulic pressure supplied to the frictional engagement element is reduced.

Embodiments and advantages of this invention will be described in detail below with reference to the attached figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention are described with reference to the accompanying drawings. In the following description, a “speed ratio” of a certain transmission is a value obtained by dividing an input rotation speed of this transmission by an output rotation speed thereof. A “lowest speed ratio” means a maximum speed ratio of this transmission and a “highest speed ratio” means a minimum speed ratio thereof.

FIG. 1is a schematic construction diagram of an idle stop vehicle according to a first embodiment of the present invention. This vehicle includes an engine1as a driving source, and output rotation of the engine1is transmitted to drive wheels7via a torque converter2with a lock-up clutch, a first gear train3, a continuously variable transmission (hereinafter, merely referred to as a “transmission4”), a second gear train5and a final reduction unit6. The second gear train5includes a parking mechanism8for mechanically locking an output shaft of the transmission4in a parked state so as not to be able to rotate.

The transmission4includes a mechanical oil pump10m(“MP” in the figure) to which the rotation of the engine1is input and which is driven by utilizing a part of power of the engine1and an electrical oil pump10e(“EP” in the figure) which is driven upon receiving the supply of power from a battery13. The electrical oil pump10eis composed of an oil pump main body and an electric motor and a motor driver for driving and rotating the oil pump main body, and can control a driving load to an arbitrary load or in multiple stages. The transmission4includes a hydraulic control circuit11for adjusting a hydraulic pressure (hereinafter, referred to as a “line pressure PL”) from the mechanical oil pump10mor the electrical oil pump10eand supplying the adjusted hydraulic pressure to the respective parts of the transmission4.

The transmission4includes a belt-type continuously variable transmission mechanism (hereinafter, referred to as a “variator20”) and a sub-transmission mechanism30provided in series with the variator20. “To be provided in series” means that the variator20and the sub-transmission mechanism30are provided in series in a power transmission path from the engine1to the drive wheels7. The sub-transmission mechanism30may be directly connected to an output shaft of the variator20as in this example or may be connected via another transmission or power transmission mechanism (e.g. gear train). Alternatively, the sub-transmission mechanism30may be connected to a stage (input shaft side) before the variator20.

The variator20includes a primary pulley21, a secondary pulley22and a V-belt23mounted between the pulleys21and22. Each of the pulleys21,22includes a fixed conical plate, a movable conical plate arranged with a sheave surface faced toward the fixed conical plate and forming a V-groove between the fixed conical plate and the movable conical plate, and a hydraulic cylinder23a,23bprovided on the back surface of the movable conical plate for displacing the movable conical plate in an axial direction. When hydraulic pressures supplied to the hydraulic cylinders23a,23bare adjusted, the width of the V-groove changes to change contact radii of the V-belt23and the respective pulleys21,22, whereby a speed ratio of the variator20continuously changes.

The sub-transmission mechanism30is a transmission mechanism with two forward speeds and one reverse speed. The sub-transmission mechanism30includes a Ravigneaux-type planetary gear mechanism31in which carriers of two planetary gears are coupled, and a plurality of frictional engagement elements (low brake32, high clutch33, reverse brake34) which are connected to a plurality of rotation elements constituting the Ravigneaux-type planetary gear mechanism31to change coupled states of these rotation elements. If the supply of hydraulic pressures to the respective frictional engagement elements32to34are adjusted to change engaged and released states of the respective frictional engagement elements32to34, a gear position of the sub-transmission mechanism30is changed.

For example, the sub-transmission mechanism30is set to a first gear position if the low brake32is engaged and the high clutch33and the reverse brake34are released. The sub transmission mechanism30is set to a second gear position with a gear ratio smaller than in the first gear position if the high clutch33is engaged and the low brake32and the reverse brake34are released. The sub-transmission mechanism30is set to a reverse gear position if the reverse brake34is engaged and the low brake32and the high brake33are released. In the following description, a case where the sub-transmission mechanism30is in the first gear position is expressed by that “the transmission4is in a low-speed mode” and a case where the sub-transmission mechanism is in the second gear position is expressed by that “the transmission4is in a high-speed mode”.

The frictional engagement elements are provided before or after the variator20in a power transmission path and any of them enables power transmission of the transmission4when being engaged while disabling power transmission of the transmission4when being released.

An accumulator35is connected at an intermediate position of an oil path for supplying a hydraulic pressure to the low brake32. The accumulator35is for causing the supply and discharge of the hydraulic pressure to and from the low brake32to be delayed, and suppresses a sudden increase of the supply hydraulic pressure to the low brake32by accumulating the hydraulic pressure when an N-D select is being made, thereby preventing the occurrence of a shock caused by sudden engagement of the low brake32.

A controller12is the one for controlling the engine1and the transmission4in a comprehensive manner and includes a CPU121, a memory device122composed of a RAM/ROM, an input interface123, an output interface124and a bus125which connects these components to each other as shown inFIG. 2.

To the input interface123are input an output signal of an accelerator pedal opening sensor41for detecting an accelerator pedal opening APO which is an operated amount of an accelerator pedal, an output signal of a rotation speed sensor42for detecting an input rotation speed of the transmission4(=rotation speed of the primary pulley21, hereinafter, referred to as a “primary rotation speed Npri”), an output signal of a vehicle speed sensor43for detecting a vehicle speed VSP, an output signal of a line pressure sensor44for detecting the line pressure PL, an output signal of an inhibitor switch45for detecting the position of a select lever, output signals of a brake fluid pressure sensor46for detecting a brake fluid pressure and an inclination sensor47for detecting the inclination of the vehicle road surface gradient) and like output signals.

A control program of the engine1, a shift control program of the transmission4, and various maps and tables used in these programs are stored in the memory device122. The CPU121reads a program stored in the memory device122and implements it, performs various computations on various signals input via the input interface123to generate a fuel injection amount signal, an ignition timing signal, a throttle opening signal, a shift control signal and a drive signal of the electrical oil pump10e, and outputs the generated signals to the engine1, the hydraulic control circuit11and the motor driver of the electrical oil pump10evia the output interface124. Various values used in the computations by the CPU121and computation results are stored in the memory device122.

The hydraulic control circuit11includes a plurality of flow passages and a plurality of hydraulic control valves. In accordance with the shift control signal from the controller12, the hydraulic control circuit11controls the plurality of hydraulic control valves to switch supply paths of the hydraulic pressure, prepares a necessary hydraulic pressure from a hydraulic pressure produced in the mechanical oil pump10mor the electrical oil pump10e, and supplies this hydraulic pressure to the respective components of the transmission4. In this way, the speed ratio of the variator20and gear position of the sub-transmission mechanism30are changed to shift the transmission4.

FIG. 3shows an example of a shift map stored in the memory device122. The controller12controls the variator20and the sub-transmission mechanism30according to a driving state of the vehicle (vehicle speed VSP, primary rotation speed Npri, accelerator pedal opening APO) in accordance with this shift map.

In this shift map, an operating point of the transmission4is defined by the vehicle speed VSP and the primary rotation speed Npri. The inclination of a line connecting the operating point of the transmission4and a zero point at the lower left corner of the shift map corresponds to the speed ratio of the transmission4(overall speed ratio obtained by multiplying the speed ratio of the variator20by that of the sub-transmission mechanism30, hereinafter, referred to as a “through speed ratio”). In this shift map, a shift line is set for each accelerator pedal opening APO similar to a shift map of a conventional belt-type continuously variable transmission, and the transmission4is shifted in accordance with the shift line selected based on the accelerator pedal opening APO. For simplicity, only a full load line (shift line when the accelerator pedal opening APO=8/8), a partial load line (shift line when the accelerator pedal opening APO=4/8) and a coast line (shift line when the accelerator position APO=0/8) are shown inFIG. 3.

When being in the low-speed mode, the transmission4can be shifted between a low-speed mode lowest line obtained by setting the speed ratio of the variator20to the lowest speed ratio and a low-speed mode highest line obtained by setting the speed ratio of the variator20to the highest speed ratio. In this case, the operating point of the transmission4moves in areas A and B. On the other hand, when being in the high-speed mode, the transmission4can be shifted between a high-speed mode lowest line obtained by setting the speed ratio of the variator20to the lowest speed ratio and a high-speed mode highest line obtained by setting the speed ratio of the variator20to the highest speed ratio. In this case, the operating point of the transmission4moves in areas B and C.

The speed ratio of each gear position of the sub-transmission mechanism30is so set that the speed ratio corresponding to the low-speed mode highest line (low-speed mode highest speed ratio) is smaller than the speed ratio corresponding to the high-speed mode lowest line (high-speed mode lowest speed ratio). By this, a range of the through speed ratio of the transmission4that can be set in the low-speed mode (“low-speed mode ratio range” inFIG. 3) and that of the through speed ratio of the transmission4that can be set in the high-speed mode (“high-speed mode ratio range” inFIG. 3) partly overlap, and the transmission4can be selectively set in either one of the low-speed mode or the high-speed mode if the operating point of the transmission4is in the area B defined between the high-speed mode lowest line and the low-speed mode highest line.

On this shift map, a mode switch shift line at which the sub-transmission mechanism30is shifted is so set as to overlap the low-speed mode highest line. The through speed ratio corresponding to the mode switch shift line (hereinafter, referred to as a “mode switch speed ratio mRatio”) is set at a value equal to the low-speed mode highest speed ratio. The mode switch shift line is set in this way because an input torque to the sub-transmission mechanism30decreases as the speed ratio of the variator20decreases and a shift shock at the time of shifting the sub-transmission mechanism30is suppressed.

When the operating point of the transmission4crosses the mode switch shift line, i.e. an actual value of the through speed ratio (hereinafter, referred to as an “actual through speed ratio” Ratio”) changes over the mode switch speed ratio mRatio, the controller12performs a synchronization shift described below to switch between the high-speed mode and the low-speed mode.

In the synchronization shift, the controller12shifts the sub-transmission mechanism30and changes the speed ratio of the variator20in a direction opposite to a direction in which the speed ratio of the sub-transmission mechanism30is changed. At this time, an inertia phase in which the speed ratio of the sub-transmission mechanism30actually changes and a period during which the speed ratio of the variator20changes are synchronized. The speed ratio of the variator20is changed in the direction opposite to the direction in which the speed ratio of the sub-transmission mechanism30is changed to prevent a change in the input rotation caused by a step in the actual through speed ratio Ratio from giving a sense of incongruity to a driver.

Specifically, when the actual through speed ratio Ratio of the transmission4changes from a low side to a high side over the mode switch speed ratio mRatio, the controller12changes the gear position of the sub-transmission mechanism30from the first gear position to the second gear position (1-2 shift) and changes the speed ratio of the variator20to the low side.

Conversely, when the actual through speed ratio Ratio of the transmission4changes from the high side to the low side over the mode switch speed ratio mRatio, the controller12changes the gear position of the sub-transmission mechanism30from the second gear position to the first gear position (2-1 shift) and changes the speed ratio of the variator20to the high side.

The controller12executes a coast stop control described below to suppress a fuel consumption amount.

The coast stop control is a control for suppressing the fuel consumption amount by automatically stopping the engine1during the travel of the vehicle in a low speed range (coast stop). The coast stop control is common to a fuel-cut control executed when an accelerator is off in that fuel supply to the engine1is stopped, but differs therefrom in that the power transmission path between the engine1and the drive wheels7is cut off to completely stop the rotation of the engine1by releasing the lock-up clutch and the low brake32.

Upon performing the coast stop, the controller12first judges conditions (i) to (iv) listed below:(i) accelerator pedal is not depressed at all (accelerator pedal opening APO=0)(ii) brake pedal is depressed (brake fluid pressure is equal to or higher than a predetermined value)(iii) vehicle speed is a predetermined low speed (e.g. 15 km/h) or lower(iv) lock-up clutch is released.
These conditions are, in other words, conditions to judge whether or not a driver has an intension to stop the vehicle.

The lock-up clutch is released in the case of crossing a lock-up release line (not shown) set on a shift map from a high-speed side or high rotation side to a low speed side or low rotation side. The controller12judges that a coast stop condition holds when all of these conditions (i) to (iv) hold.

When the coast stop condition holds, the controller12subsequently reduces an instruction hydraulic pressure to the low brake32to zero and causes the hydraulic pressure accumulated in the accumulator35to be drained. In terms of reducing the supply hydraulic pressure to the low brake32, the holding of the coast stop condition means the holding of a condition to reduce the supply hydraulic pressure to the low brake32(hydraulic pressure reducing condition). After all the hydraulic pressure accumulated in the accumulator35is drained, the controller12performs the coast stop.

At the coast stop, fuel supply to the engine1is stopped to automatically stop the engine1. When the engine1stops, the mechanical pump10mdriven by the power of the engine1also stops and a discharge pressure thereof becomes zero, whereby the low brake32is completely released. Since all the hydraulic pressure accumulated in the accumulator35is drained beforehand as described above, the low brake32is released substantially simultaneously with the stop of the engine1and the mechanical pump10m.

When the supply hydraulic pressure from the mechanical oil pump10mto the hydraulic cylinders23a,23bof the pulleys21,22becomes zero and the low brake32is released to free the variator20in a rotating direction, a speed ratio of the variator20is changed toward a lowest speed ratio by return springs arranged in the hydraulic cylinders23a,23b.

When the mechanical oil pump10mstops, the drive of the electrical oil pump10eis started and a hydraulic pressure generated in the electrical oil pump10eis supplied to the hydraulic cylinders23a,23bto change the speed ratio of the variator20to the lowest speed ratio.

The hydraulic pressure supplied to the hydraulic cylinders23a,23bis only for clamping the belt23by the pulleys21,22and not sufficient to transmit power. However, since the low brake32is released and the sub-transmission mechanism30is in a neutral state, even if a torque is input from the drive wheels7such as due to braking, this torque is not transmitted to the variator20via the sub-transmission mechanism30and slippage of the belt23is prevented.

After the low brake32is released, the controller12increases the supply hydraulic pressure to the low brake32to a hydraulic pressure (hereinafter, referred to as a “zero-point hydraulic pressure”) at which a clearance between an input-side element and an output-side element of the low brake32is zero and a torque capacity (transmittable torque) of the low brake32is zero. This is to quickly increase the torque capacity of the low brake32and improve re-acceleration responsiveness at the time of re-acceleration by maintaining the low brake32in a state immediately before engagement during the coast stop.

When the engine1is restarted, the rotation speed of the engine1settles at a steady rotation speed after revving up. The controller12maintains the supply hydraulic pressure to the low brake32at the zero-point hydraulic pressure until the revving-up of the engine1settles so as to prevent transmission of the revved-up rotation to the drive wheels7via the low brake32.

Judgment as to whether or not the above conditions (i) to (iv) hold is continued also during the coast stop. If even one of them no longer holds, the coast stop condition does not hold and the controller12resumes fuel supply to the engine1to restart the engine1and stops the electrical oil pump10ewhen the mechanical oil pump10mcomes to generate a sufficient hydraulic pressure.

FIG. 4is a flow chart showing the content of the coast stop control executed by the controller12. The coast stop control is further described with reference toFIG. 4. The contents of the respective steps indicate operations of the variator20or the sub-transmission mechanism30performed in response to the process of the controller12in addition of the content of the process of the controller12.

In S11, the controller12judges whether or not the coast stop condition holds. It is judged that the coast stop condition holds when all of the above conditions (i) to (iv) hold, whereas it is judged that the coast stop condition does not hold when even one of them does not hold. The process proceeds to S12when it is judged that the coast stop condition holds.

In S12, the controller12starts reducing the supply hydraulic pressure to the low brake32by reducing the instruction hydraulic pressure to the low brake32to zero. By this, the hydraulic pressure accumulated in the accumulator35is discharged.

In S13, the controller12judges whether or not a predetermined time has elapsed after giving the instruction to reduce the supply hydraulic pressure to the low brake32in S12. The predetermined time is a time necessary to drain all the hydraulic pressure accumulated in the accumulator35. The process proceeds to S14when it is judged that the predetermined time has elapsed and all the hydraulic pressure accumulated in the accumulator35has been drained.

In S14, the coast stop is performed. Specifically, the controller12stops fuel supply to the engine1to stop the engine1. Then, the drive of the electrical oil pump10eis started.

When the engine1is stopped, the mechanical oil pump10malso stops and the discharge pressure thereof becomes zero (S15). Then, the supply hydraulic pressure to the low brake32also becomes zero and the low brake32is released (S16). Since the hydraulic pressure in the accumulator35is drained beforehand, the operations from the stop of fuel supply to the engine1in S14to the release of the low brake32in S16are quickly performed.

When the discharge pressure of the mechanical oil pump10mbecomes zero, the supply hydraulic pressure to the hydraulic cylinders23a,23bof the pulleys21,22becomes zero and the low brake32is released to free the variator20in the rotating direction, the speed ratio of the variator20is changed toward the lowest speed ratio by the return springs provided in the hydraulic cylinders23a,23b(S17).

When the electrical oil pump10estarts and a discharge pressure thereof increases (S18), the controller12increases the instruction hydraulic pressure to the low brake32to the zero-point hydraulic pressure to increase the supply hydraulic pressure to the low brake32to the zero-point hydraulic pressure (S19). By this, the low brake32is maintained in the state immediately before engagement, thereby eliminating a delay in the engagement of the low brake32and realizing good re-acceleration responsiveness when the engine1is restarted and re-accelerated.

Then, the controller12maintains the supply hydraulic pressure to the low brake32at the zero-point hydraulic pressure until the revving-up at the time of restarting the engine1settles so as to prevent revved-up rotation at the time of restarting the engine1from being transmitted to cause a shock.

On the other hand, the controller12also supplies the discharge pressure of the electrical oil pump10eto the hydraulic cylinders23a,23bof the pulleys21,22to change the speed ratio of the variator20to the lowest speed ratio (S20). Thereafter, the controller12maintains the speed ratio of the variator20at the lowest speed ratio (S21) to realize good acceleration responsiveness at the time of re-acceleration, coupled with the above standby state of the low brake32at the zero-point hydraulic pressure.

FIG. 5is a graph showing a change of a low brake pressure when the above coast stop control is executed to perform the coast stop. The operation when the coast stop is performed is further described with reference toFIG. 5.

When the coast stop condition holds at time t1, the instruction hydraulic pressure to the low brake32is reduced to zero to reduce the supply hydraulic pressure to the low brake32.

At time t2after the elapse of a predetermined time from time t1, all the hydraulic pressure in the accumulator35(“accumulated pressure” inFIG. 5) is drained to perform the coast stop. At the coast stop, fuel supply to the engine1is stopped to stop the engine1, the discharge pressure of the mechanical oil pump10mbecomes zero and the supply hydraulic pressure to the low brake32also becomes zero. Since all the hydraulic pressure in the accumulator35is drained beforehand, the stop of the engine1and the mechanical oil pump10mand the release of the low brake32are substantially simultaneously performed.

Thus, according to the above coast stop control, there is no likelihood that only the supply hydraulic pressure to the hydraulic cylinders23a,23bis reduced too early with the low brake32kept engaged. The low brake32is substantially simultaneously released when the supply hydraulic pressure to the hydraulic cylinders23a,23bis reduced. In other words, the torque capacity of the variator20does not become smaller than that of the low brake32during the coast stop.

By this, even if a torque is input from the drive wheels such as due to braking during the coast stop, this torque is not transmitted to the variator20via the sub-transmission mechanism30, wherefore it is possible to prevent a reduction in the durability of the belt23caused by slippage between the belt23and the pulleys21,22. To reduce the supply hydraulic pressure to the low brake32, it is suitable to zero the instruction hydraulic pressure to the low brake32as in this embodiment. Thus, the hydraulic pressure accumulated in the accumulator35is drained and the hydraulic pressure of the low brake32can be quickly reduced.

At time t3, the instruction hydraulic pressure to the low brake32is increased to the zero-point hydraulic pressure to increase the supply hydraulic pressure to the low brake32to the zero-point hydraulic pressure. By maintaining the supply hydraulic pressure to the low brake32at the zero-point hydraulic pressure, the low brake32can be engaged (torque capacity is generated) without delay and good re-acceleration responsiveness can be realized when the engine1is restarted to reaccelerate the vehicle in response to a re-acceleration request or intension such as the depression of the accelerator pedal and the release of the brake pedal.

The supply hydraulic pressure to the low brake32is maintained at the zero-point hydraulic pressure until the revving-up at the time of restarting the engine1settles at a steady rotation, whereby it can be prevented that the revved-up rotation at the time of restarting the engine1is transmitted to the drive wheels7via the low brake32to cause a shock.

Although not shown, the speed ratio of the variator20is changed to the lowest speed ratio by the action of the return springs and the supply hydraulic pressure from the electrical oil pump10eduring the coast stop. By this, good re-acceleration responsiveness can be realized, coupled with the standby state of the low brake32at the zero-point hydraulic pressure.

The embodiment of the present invention has been described above. The above embodiment is merely an illustration of an application example of the present invention and not of the nature to limit the technical scope of the present invention to the specific construction of the above embodiment. Various changes can be made without departing from the gist of the present invention.

For example, in the above embodiment, the reduction of the supply hydraulic pressure to the low brake32is started when the coast stop condition holds. However, a condition to reduce the supply hydraulic pressure to the low brake32(hydraulic pressure reducing condition) may be judged separately from the coast stop condition and the reduction of the supply hydraulic pressure to the low brake32may be started when the hydraulic pressure reducing condition holds.

For example, when it takes time to drain the hydraulic pressure in the accumulator35, a vehicle speed condition of the hydraulic pressure reducing condition is set to be higher than that of the coast stop condition (e.g. holds at 20 km/h or lower), thereby setting a timing at which the hydraulic pressure reducing condition holds and the reduction of the supply hydraulic pressure to the low brake32is started earlier than a timing at which the coast stop condition holds. InFIG. 5, this means the start of the reduction of the supply hydraulic pressure to the low brake32before time t1. By this, a time until the accumulated pressure is completely drained and the coast stop is performed after the coast stop condition holds can be shortened.

In terms of simplifying the control, it is preferable to start the reduction of the supply hydraulic pressure to the low brake32when the coast stop condition holds as in the above embodiment, i.e. to set hydraulic pressure reducing condition=coast stop condition.

Although the frictional engagement element that is engaged/released at the time of the coast stop is the low brake32of the sub-transmission mechanism30in the above embodiment, such a frictional engagement element is not limited to the low brake32of the sub-transmission mechanism30.

For example, such a frictional engagement element may be a forward clutch of a forward/reverse switching mechanism in the case of a transmission including the forward/reverse switching mechanism before or after the variator20. Alternatively, such a frictional engagement element may be a lock-up clutch in the case of a transmission including a torque converter with the lock-up clutch.

Although the variator20includes the V-belt23as a power transmitting member in the above embodiment, it may include a chain belt instead of the V-belt23.

This application claims priority based on Japanese Patent Application No. 2010-195531, filed with the Japan Patent Office on Sep. 1, 2010, the entire content of which is incorporated into this specification by reference.