Hydraulic control unit

A hydraulic control unit configured to improve energy efficiency is provided. The hydraulic control unit is comprised of: a feeding passage that delivers fluid from at least any of an oil pump and an accumulator storing hydraulic pressure to an actuator; a draining passage that discharge the fluid from the actuator to a drain spot; and a discharging means that is configured to selectively discharge the fluid from the actuator to the accumulator through the feeding passage, if the fluid has to be discharged from the actuator and a pressure of the accumulator is lower than that of the actuator.

CROSS-REFERENCE TO RELATED APPLICATION

This is a national phase application based on the PCT International Patent Application No. PCT/JP2012/065249 filed Jun. 14, 2012, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a hydraulic control unit for delivering and draining hydraulic fluid to/from an actuator.

BACKGROUND ART

A speed ratio and a transmission torque capacity of a vehicle transmission are customarily altered by a hydraulic pressure. For example, Japanese Patent Laid-Open No. 2010-151240 describes a hydraulic control device of a belt-driven continuously variable transmission. The hydraulic control device is comprised of a low pressure circuit to which low pressure fluid regulated by lowering an initial pressure of a pump is delivered, and a high pressure circuit to which high pressure fluid higher than said low pressure is delivered. The low pressure circuit includes friction sites of the continuously variable transmission and lubrication sites such as a bearing. The high pressure circuit includes hydraulic chambers of a drive pulley and a driven pulley to which a drive belt is applied, and an accumulator storing high pressure fluid. The drive pulley is provided with an intensifier valve for supplying the fluid to the hydraulic chamber, and a depressurization valve for draining the fluid from the hydraulic chamber. Also, the driven pulley is provided with an intensifier valve for supplying the fluid to the hydraulic chamber, and a depressurization valve for draining the fluid from the hydraulic chamber. A communication between the accumulator and the pump is provided through a check valve. According to the teachings of Japanese Patent Laid-Open No. 2010-151240, the hydraulic control device is configured to deliver the fluid from the accumulator to the chambers of pulleys to alter a speed ratio moderately. By contrast, the fluid is delivered from the pump to the chambers of pulleys to alter a speed ratio rapidly.

Japanese Patent Laid-Open No. 2009-97677 describes a control device for variable displacement pump motor type transmission. The control device is comprised of a pair of hydraulic pump motors, and a closed circuit formed by connecting an inlet side passage connecting inlet ports with an outlet side passage connecting outlet ports. A first relief valve is disposed on a communication passage connecting the inlet side passage and the outlet side passage. The first relief valve is opened when the fluid pressurized higher than a predetermined level leaks from the inlet port of one of the pump motors to discharge the highly pressurized fluid from the closed circuit. That is, the first relief valve maintains the pressure of the inlet side passage to the predetermined pressure. A second relief valve is disposed on another communication passage connecting the inlet side passage and the outlet side passage. The second relief valve is opened when the fluid pressurized higher than a predetermined level leaks from the outlet port of one of the pump motors to discharge the highly pressurized fluid from the closed circuit. That is, the second relief valve maintains the pressure of the outlet side passage to the predetermined pressure. Additionally, inlet side passage is connected with the accumulator though a check valve. According to the teachings of Japanese Patent Laid-Open No. 2009-97677, therefore, the highly pressurized fluid is accumulated in the accumulator by closing the first relief valve without discharging from the closed circuit when the pressure of the inlet side passage is raised.

As described, according to the hydraulic control device taught by Japanese Patent Laid-Open No. 2010-151240, the intensifier valve for drive pulley is opened when upshifting is carried out to reduce the speed ratio, so that the fluid is delivered from the accumulator or the pump to the chamber. Consequently, a groove width of the drive pulley is narrowed to increase a running radius of the belt. At the same time, the depressurized valve for the driven pulley is opened to widen a groove width of the driven pulley so that the fluid in the chamber of the driven pulley is discharged to a drain spot. Consequently, the hydraulic pressure in the chamber of the driven pulley is adjusted in accordance with the torque transmitting capacity thereof. Thus, according to the hydraulic control device taught by Japanese Patent Laid-Open No. 2010-151240, the highly pressurized fluid is discharged to the drain spot on the occasion of speed change operation. As a result of thus discharging the high pressure fluid during the speed change, an energy loss may be caused to worsen fuel efficiency of the vehicle.

DISCLOSURE OF THE INVENTION

In order to solve the foregoing technical problems, it is therefore an object of this invention to provide a hydraulic control unit for improving energy efficiency.

The present invention is applied to a hydraulic control unit comprised of a feeding passage that delivers fluid from at least any of an oil pump and an accumulator storing hydraulic pressure to an actuator, and a draining passage that discharge the fluid from the actuator to a drain spot. In order to achieve the above-explained objective, according to the present invention, the hydraulic control unit is provided with a discharging means that is configured to selectively discharge the fluid from the actuator to the accumulator through the feeding passage, if the fluid has to be discharged from the actuator and a pressure of the accumulator is lower than that of the actuator.

The hydraulic control unit of the present invention is further comprised of: a feeding valve that is disposed on the feeding passage to deliver the fluid from the accumulator to the actuator; a draining valve that is disposed on the draining passage to discharge the fluid from the actuator to the drain spot; and a first switching valve that is disposed on the feeding passage to selectively connect to the accumulator to the actuator. In addition, the discharging means may also be configured to connect the accumulator to the actuator by the first switching valve while opening the feeding valve and closing the draining valve, if the fluid has to be discharged from the actuator and the pressure of the accumulator is lower than that of the actuator.

The hydraulic control unit of the present invention is further comprised of: a passage connecting the discharging valve to the accumulator; and a second switching valve that is disposed on said passage to selectively connect the actuator to at least any one of the accumulator and the drain spot. In addition, the discharging means may also be configured to connect the accumulator to the actuator by the second switching valve while opening the draining valve and closing the feeding valve, if the fluid has to be discharged from the actuator and the pressure of the accumulator is lower than that of the actuator.

The discharging means may also be configured to open the draining valve, at least in any of cases: that a pressure difference between the actuator and the accumulator is smaller than a predetermined threshold value; and that a drop in the pressure of the actuator after discharging the fluid therefrom is smaller than a predetermined value.

The discharging means may also be configured to deliver the pressure of the actuator to a site where a relatively low pressure is required, if the fluid has to be discharged from the actuator and the pressure of the accumulator is lower than that of the actuator.

The hydraulic control unit of the present invention is further comprised of a third switching valve. An operating state of the third switching valve is selectively switched between: a first operating state where the hydraulic pressure established by the oil pump is delivered to the actuator and the accumulator, and a second operating state where the hydraulic pressure established by the oil pump is delivered only to the actuator without being delivered to the accumulator.

For example, a mechanical oil pump driven by an internal combustion engine, and an electric oil pump driven by an electric motor may be used as the oil pump of the present invention.

Specifically, a hydraulic chamber of a pulley of a belt-driven continuously variable transmission serves as the actuator of the present invention to which the hydraulic pressure is applied to narrow a groove width.

Thus, the hydraulic control unit selectively delivers the hydraulic fluid discharged from the actuator to the accumulator via the feeding passage given that the fluid has to be discharged from the actuator and a pressure of the accumulator is lower than that of the actuator. According to the present invention, therefore, the energy efficiency can be improved as compared to that of the case in which the hydraulic fluid discharged from the actuator is drained to the drain spot.

Specifically, if it is necessary to discharge the fluid from the actuator and the pressure thereof is higher than that of the accumulator, the first switching valve provide a communication between the accumulator and the actuator. In this case, the feeding valve is opened and the draining valve is closed. Consequently, the fluid in the actuator is allowed to be delivered to the accumulator via the feeding passage and the feeding valve. By contrast, if the pressure difference between the actuator and the accumulator is smaller than a predetermined threshold value, the draining valve is opened so that the fluid in the actuator can be discharged rapidly. The draining valve is also opened in case a drop in the pressure of the actuator is smaller than a predetermined threshold value to discharge the fluid from the actuator.

In case the fluid has to be discharged from the accumulator and the pressure of the actuator is higher than that of the accumulator, the accumulator may also be connected to the actuator by the second switching valve. In this case, the feeding valve is closed and the draining valve is opened so that the fluid in the actuator can be delivered to the accumulator via the draining valve and the passage on which the second switching valve is disposed.

As mentioned above, the hydraulic fluid in the actuator whose pressure is higher than that of the accumulator can be delivered to lubricate the site not requiring such high pressure.

According to the present invention, the fluid in the accumulator or the pump is allowed to be delivered to the actuator by bringing the third switching valve into the first operating state. In this case, the fluid can be not only delivered to the actuator but also returned to the accumulator by increasing a discharging amount of the oil pump. By contrast, given that the third switching valve is brought into the second operating state, the fluid can be delivered only to the actuator just as required even if the required amount of the fluid or the hydraulic pressure to be applied to the actuator is increased abruptly. In this case, the fluid will not be returned to the actuator. Therefore, the deficiency in the amount and pressure of the fluid in the actuator can be prevented or suppressed.

As described, both mechanical oil pump and motor-driven oil pump may be employed as the pump in the hydraulic control unit of the present invention

As also described, the hydraulic control unit according to the present invention may be applied to control the pressure of the pulleys of the belt-driven continuously variable transmission. In this case, energy efficiency of the belt-driven continuously variable transmission can be improved by preventing a leakage of highly pressurized fluid.

BEST MODE FOR CARRYING OUT THE INVENTION

The hydraulic control unit (abbreviated as “HCU” in the drawings) of the present invention may be applied not only to a transportation carrier such as an automobile and an air craft but also to a various kinds stationary industrial machineries.FIG. 1shows a preferred example of applying the hydraulic control unit to a conventional belt-driven continuously variable transmission1commonly used in automobiles. A structure of the continuously variable transmission1will be briefly explained hereinafter. The continuously variable transmission1is comprised of a drive pulley2and a driven pulley3. Specifically, the drive pulley2is comprised of a fixed sheave2aand a movable sheave2ballowed to reciprocate toward and away from the fixed sheave2a, and a V-groove is formed between those sheaves2aand2b. Likewise, the driven pulley3is comprised of a fixed sheave3aand a movable sheave3ballowed to reciprocate toward and away from the fixed sheave3a, and a V-groove is formed between those sheaves3aand3b. A torque transmission between those pulleys2and3is provided by a not shown drive belt running in the V-grooves of those pulleys. Accordingly, a speed ratio of the continuously variable transmission1can be varied continuously by altering a running radius of the drive belt. To this end, the drive pulley2is provided with a hydraulic chamber2cto axially reciprocate the movable sheave2b, and the driven pulley3is provided with a hydraulic chamber3cto axially reciprocate the movable sheave3b. According to the preferred example, a hydraulic pressure for altering the belt running radius to change a speed ratio is applied to the hydraulic chamber2c, and a hydraulic pressure for clamping the drive belt by the pulleys2and3is applied to the hydraulic chamber3c.

A C1 clutch4is arranged to selectively deliver torque to an input side or an output side of the continuously variable transmission1. For example, a wet-type multiple plate clutch is used as the C1 clutch4, and a torque transmitting capacity of the C1 clutch4is changed according to the hydraulic pressure applied thereto. Specifically, hydraulic pressure to achieve a required torque capacity to propel the vehicle is applied to the continuously variable transmission1and the C1 clutch4. To this end, relatively high pressures to transmit the torque are applied to the hydraulic chambers2c,3c, and the C1 clutch4. Accordingly, the hydraulic chambers2c,3c, and the C1 clutch4serves as an actuator of the present invention.

Next, here will be explained a structure for applying hydraulic pressure to the hydraulic chambers2c,3C, and the clutch C1. Although not illustrated in detail, there is provided a pump5driven by an engine or an electric motor, and a passage6provides a communication between the pump5and the hydraulic chamber2c. A regulator valve7is connected to the passage6to regulate the pressure of the fluid discharged from the pump5to a predetermined operating pressure. Specifically, the operating pressure is a line pressure as an initial pressure of the hydraulic control system. In a vehicle, specifically, the operating pressure is established in accordance with a drive demand represented by an opening degree of an accelerator. The regulator valve7is used to regulate the fluid pressure, i.e., the operating pressure in the passage6in accordance with the signal pressure. For example, a conventional primary regulator valve adapted to establish the line pressure of an automatic transmission of automobiles may be employed as the regulator valve7.

In order to alter the operating pressure depending on an operating condition of the hydraulic control unit and other external requirements, the signal pressure delivered to the regulator valve7has to be varied arbitrarily. For this purpose, a signal pressure establishing valve8is arranged in the hydraulic control unit. Specifically, the signal pressure establishing valve8is a linear solenoid valve adapted to output the signal pressure in response to a current applied to a solenoid. In order to control the hydraulic control unit, an electronic control unit (abbreviated as ECU)9serving as the controller of the invention is connected thereto. The ECU9is comprised mainly of a microcomputer, which is configured to carry out a calculation on the basis of data inputted thereto and preinstalled data, and to send a calculation result in the form of a control signal to the signal pressure establishing valve8. In the example shown inFIG. 1, data about an opening degree of an accelerator, a vehicle speed, a fluid temperature etc. are inputted to the ECU9.

A discharging outlet of the pump5is connected to a switching valve10for delivering the fluid selectively to the transmission. The switching valve10is comprised of an input port10aconnected with the discharging outlet of the oil pump5, a pair of output ports10band10c, a not shown spool selectively connecting the input port10awith the output port10bor the output port10c, and a spring10dpushing the spool elastically in the predetermined direction. Although not illustrated in detail, a pilot pressure is applied to the spool against the elastic force of the spring. The output port10bis connected to the passage6, and the output port10cis connected to the passage11.

According to the example shown inFIG. 1, a hydraulic pressure of the passage6is applied as a pilot pressure to the switching valve10. Given that the pilot pressure is not applied to the switching valve10or the pilot pressure applied thereto is relatively low, the switching valve10is brought into a first operating state where the input port10ais connected with the output port10b. By contrast, given that the pilot pressure applied thereto is relatively high, the switching valve10is brought into a second operating state where the input port10ais connected with the output port10c. Accordingly, the switching valve10serves as the third switching valve of the present invention. The first operating state will also be an “ON-state”, and the second operating state will also be an “OFF-state” in the following description.

The output port10bis connected with an accumulator13through a check valve12. Specifically, the check valve12is a one-way valve that is opened by the fluid flowing from the pump5toward the accumulator13, and closed by the fluid flowing in the opposite direction. The accumulator13is a conventional accumulator having a container holding a not shown piston sustained by an elastic member such as a spring or a member elastically expanded by a gas encapsulated therein. That is, a capacity of the accumulator13is varied elastically to store the fluid applied thereto at a predetermined pressure level.

A discharging outlet of the accumulator13is connected to a switching valve14of the feeding side. The switching valve14is comprised of a pair of input ports14aand14b, an output port14c, a not shown spool selectively connecting the input port14aor14bwith the output port14c, and a spring14delastically pushing the spool in the predetermined direction. Although not illustrated in detail, a pilot pressure is applied to the spool against the elastic force of the spring. According to the example ofFIG. 1, a hydraulic pressure of the passage11is applied as a pilot pressure to the switching valve14. In addition, the input port14ais connected to the discharging outlet of the accumulator13, the input port14bis connected to the output port10cof the switching valve10, and the output port14cis connected to the passage6.

Specifically, the switching valve14is, under the OFF-state where the pilot pressure is not applied thereto or the pilot pressure applied thereto is relatively low, the input port14ais connected with the output port14c. To the contrary, under the ON-state where the pilot pressure is applied thereto is relatively high, the input port14bis connected with the output port14c. That is, the switching valve14is brought into OFF-state when the switching valve10is in the first operating state so that the accumulator13is connected with the hydraulic chambers2c,3c. In contrast, the switching valve14is brought into ON-state when the switching valve10is in the second operating state so that the discharging outlet of the pump5is connected with the hydraulic chambers2c,3c. Accordingly, the switching valve14serves as the first switching valve of the present invention.

As shown inFIG. 1, a feeding passage15branching out from the passage6is connected to a hydraulic chamber2cof the drive pulley2. A feeding solenoid valve16disposed on the feeding passage15is controlled electrically to deliver the fluid selectively to the hydraulic chamber2c. Likewise, a feeding passage17also branching out from the passage6is connected to a hydraulic chamber3cof the driven pulley3. A feeding solenoid valve18disposed on the feeding passage17is controlled electrically to deliver the fluid selectively to the hydraulic chamber3c. Further, a feeding passage19also branching out from the passage6is connected to a not shown hydraulic chamber of the C1 clutch4. A feeding solenoid valve20is disposed on the feeding passage19and controlled electrically to selectively deliver the fluid to the hydraulic chamber of the C1 clutch4.

A draining passage21extends from the feeding passage15between the feeding solenoid valve16of and the hydraulic chamber2c, and a draining solenoid valve22controlled electrically is disposed the draining passage21to selectively drain the fluid from the hydraulic chamber2cto a drain spot such as an oil pan. Likewise, a draining passage23extends from the passage17between the feeding solenoid valve18and the hydraulic chamber3c, and a draining solenoid valve24controlled electrically is disposed on the draining passage23to selectively drain the fluid from the hydraulic chamber3cto a drain spot such as an oil pan. Further, a draining passage25extends from the passage19between the feeding solenoid valve20and a not shown hydraulic chamber of the C1 clutch4, and a draining solenoid valve26controlled electrically is disposed on the draining passage25is to selectively drain the fluid from the hydraulic chamber of the C1 clutch4.

As described, a solenoid valve electrically controlled to open and close a port thereof is individually used as each feeding solenoid valve16,18and20and each draining solenoid valve22,24and26. When the solenoid valve is not energized (i.e., under OFF-state), the ports thereof is closed in a manner to prevent fluid leakage therefrom. For this reason, the current speed ratio and transmission torque can be maintained by confining the fluid in the hydraulic chamber2c,3cand the C1 clutch4even in case a power distribution to the valves is interrupted accidentally. In addition, the example shown inFIG. 1is further provided with an input pressure sensor27to detect a pressure Pacc of the fluid from the accumulator13, a drive side control pressure sensor28to detect a pressure Pri of the fluid from the hydraulic chamber2c, a driven side control pressure sensor29to detect a pressure Pd of the fluid from the hydraulic chamber3c, and an input pressure sensor30to detect a pressure Pc of the fluid from the clutch C1. The detected value of those pressure sensors27,28,29and30are sent to the ECU9in the form of detection signals to electrically control the valves16,18,20,22,24and26according to the detection signal.

Here will be briefly explained an action of the hydraulic control unit of the present invention. For example, in case of carrying out an upshifting of the belt-driven continuously variable transmission1shown inFIG. 1in a moderate manner, the switching valve10is brought into the first operating state and the switching valve14is brought into the OFF-state. In case of carrying out a downshifting in a moderate manner, the switching valve10is also brought into the first operating state and the switching valve14is also brought into the OFF-state.

To the contrary, in case of carrying out an upshifting in an rapid manner, the switching valve10is brought into the second operating state and the switching valve14is brought into the ON-state. In case of carrying out a downshifting in a rapid manner, the switching valve10is brought into the first operating state and the switching valve14is brought into the ON state.

Given that at least one of the operating pressure Pri, Pd and Pc of the fluid draining from the pulley2,3and the C1 clutch4is higher than the accumulator pressure Pacc, the hydraulic control unit of the present invention performs the following controls.FIG. 2is a flowchart explaining the first control example, and the routine shown therein is repeated at predetermined cycle. The first control example is carried out under the condition that the operating pressure Pd of the driven pulley3is higher than the accumulator pressure Pacc, and the fluid has to be drained from the driven pulley3.

First of all, it is determined whether or not a current operating pressure Pd of the driven pulley3is higher than the accumulator pressure Pacc (at step S1). As described, the accumulator pressure Pacc is a pressure stored in the accumulator13that is detected by the pressure sensor27, the operating pressure Pd of the driven pulley3is detected by the pressure sensor29, and the operating pressure Pri of the drive pulley2is detected by the pressure sensor28. The belt-driven continuously variable transmission1shown inFIG. 1is required to transmit larger torque in accordance with an increase in the opening degree of the accelerator. To this end, both the operating pressure Pri of the drive pulley2and the operating pressure Pd of the driven pulley3are increased to increase the torque transmitting capacity of the transmission1. A required operating pressure Pri of the drive pulley2and a required operating pressure Pd of the driven pulley3can be calculated based on the opening degree of the accelerator and a vehicle speed. More specifically, the required pressures Pri and Pd are determined with reference to preinstalled maps determining a relation between the pressure Pri of the drive pulley2and those parameters, and a relation between the pressure Pd of the driven pulley3and those parameters.

If the answer of step S1is YES, it is determined whether or not the hydraulic pressure has to be applied to the driven pulley3(at step S2). At step S2, specifically, it is determined whether or not a current clamping pressure is lower than a required clamping pressure. If the current clamping pressure is higher than the required clamping pressure so that the answer of step S2is NO, it is determined whether or not the hydraulic pressure applied to the hydraulic chamber3chas to be lowered (at step S3). At step S3, specifically, it is determined whether or not the current clamping pressure is higher than the required clamping pressure or higher than an upper limit of acceptable range of the clamping pressure.

If the current clamping pressure is higher than the required clamping pressure or higher than the upper limit of the acceptable range of the clamping pressure so that the answer of step S3is YES, the switching valve14is brought into the OFF-state so as to lower the belt clamping pressure (at step4). In specific, the ECU9increases the signal pressure established by the signal pressure establishing valve8to raise a regulating level of the regulator valve7thereby increasing a line pressure. Consequently, the switching valve10is brought into the second operating state so that the highly pressurized fluid is delivered to the passage11as the line pressure. In the example shown inFIG. 1, the hydraulic pressure in the passage11is used as the pilot pressure so that the switching valve14is brought into the second operating state with an increase in the pressure in the passage11. In this situation, the input port14aand the output port14care connected to each other as described above.

After or simultaneously with carrying out the control of step S4, the feeding solenoid valve18of the driven pulley3is opened (at step S5). Although not illustrated in detail, the draining solenoid valve24is opened on the occasion of carrying out the operation of step S5. In this situation, both the feeding solenoid valve16of the drive pulley2and the draining solenoid valve22are opened. Consequently, the accumulator13is connected with the hydraulic chamber3cthrough the feeding solenoid valve18so that the relatively higher pressure Pd of the hydraulic chamber3cis allowed to be applied to the accumulator13in which the pressure is relatively lower pressure through the passages17and6. As a result, the current belt clamping pressure is reduced. Then the routine shown inFIG. 2is returned. Accordingly, the feeding passage17and the passage6serve as the feeding passage of the present invention.

If the answer of step S1is NO, the switching valve14is brought into OFF-state (at step S6). After or simultaneously with carrying out the control of step S6, the feeding solenoid valve18and the draining solenoid valve24are electrically controlled to deliver the fluid from the accumulator13to the hydraulic chamber3c, or to drain the fluid from the hydraulic chamber3c, so as to achieve the required belt clamping pressure (at step S7). Then the routine shown inFIG. 2is returned.

If the answer of step S2is YES, the switching valve10is brought into the second operating state and the switching valve14is opened (at step S8). Consequently, the passage connecting the accumulator13with the solenoid valves16and18is opened. After or simultaneously with carrying out the step S8, the feeding solenoid valve18is opened and the draining solenoid valve24is closed (at step S9). As a result, the pressure of the hydraulic chamber3cis increased so that the belt clamping pressure is increased to the required level. Then the routine shown inFIG. 2is returned.

If the answer of step S3is NO, the switching valve14is brought into OFF-state (at step S10). After or simultaneously with carrying out the control of step S10, both the feeding solenoid valve18and the draining solenoid valve24are closed (at step S11). Those steps S10and S11are carried out to confine the pressure in the hydraulic chamber3cand to maintain the pressure confined therein. In this case, since the switching valve14is in the OFF-state the hydraulic fluid can be delivered promptly from the accumulator13to the hydraulic chambers2cand3cto change the speed ratio. Then the routine shown inFIG. 2is returned.

Thus, according to the control example ofFIG. 2, relatively high pressure Pd of the hydraulic chamber3ccan be applied to the accumulator13so that energy efficiency of the hydraulic control unit of the present invention can be improved.

According to the example shown inFIG. 2, it would be difficult to drain the fluid from the hydraulic chamber3cif a pressure difference between the operating pressure Pd and the accumulator pressure Pacc is small.FIG. 3shows a flowchart for draining the fluid promptly from the hydraulic chamber3c. Here, in the flowchart ofFIG. 3, common numbers are allotted to the steps identical to those inFIG. 2.

According to the example shown inFIG. 3, after step S5, it is determined whether or not the fluid in the hydraulic chamber3ccan be drained (at step S12). To this end, it is judged whether or not the pressure difference between the operating pressure Pd and the accumulator pressure Pacc is larger than a predetermined threshold value. Alternatively, the determination at step S12may also be made by determining whether or not a drop in the operating pressure Pd detected by the pressure sensor29is smaller than a predetermined threshold value. If such pressure difference is larger than the predetermined threshold value, the fluid can be drained promptly from the hydraulic chamber3ctoward to the accumulator13. To the contrary, if the pressure difference is smaller than the predetermined threshold value, the fluid may not be drained from the hydraulic chamber3csmoothly.

If the answer of step S12is NO, the draining solenoid valve24is opened (at step S13). Specifically, both the feeding solenoid valve18and the draining solenoid valve24are opened to lower the operating pressure Pd of the hydraulic chamber3cpromptly. To the contrary, if the answer of t step S12is YES, the fluid is allowed to be drained promptly from the hydraulic chamber3ctoward the accumulator13. Then, the routine shown inFIG. 3is returned.

Thus, according to the control example ofFIG. 3, the draining solenoid valve24is opened in case the pressure difference between the operating pressure Pd and the accumulator pressure Pacc is small and the fluid is therefore drained from the hydraulic chamber3cslowly. For this reason, the fluid is allowed to be drained promptly from the hydraulic chamber3cso that the control response of the hydraulic control unit can be improved.

As described, the hydraulic control unit shown inFIG. 1is configured to apply the operating pressure Pd to the accumulator13through the feeding solenoid valve18in case the operating pressure Pd of the fluid discharged from the hydraulic chamber3cof the driven pulley3is higher than the accumulator pressure Pacc. Instead, as shown inFIG. 4, it is also possible to apply the relatively high operating pressure Pd to the accumulator13through the draining solenoid valve24.

According to another example shown inFIG. 4, a switching valve31of the draining side is disposed on the draining passage21of the drive pulley2between the draining solenoid valve22and a drain spot. The switching valve31is comprised of an input port31aconnected to the draining solenoid valve22, and a pair of output ports31bconnected to the drain spot and31cconnected to the passage32, a not shown spool that selectively provide a connection between the input port31aand the output port31bor the output port31c, and a spring31dthat elastically pushes the spool in the predetermined direction. In the example shown inFIG. 4, the operating pressure Pri of the drive pulley2is applied as a pilot pressure to the spool not only in the direction to increase the elastic force of the spring31dbut also in a direction against the elastic force of the spring31d. That is, the pilot pressure delivered from the hydraulic source, and the pilot pressure discharged from the hydraulic chamber2care applied to the spool of the switching valve31. Specifically, under the OFF-state where a total pressure of the elastic force of the spring31dand the pilot pressure from the accumulator13is higher than the pressure discharged from the hydraulic chamber2c, the input port31ais communicated with the output port31b. To the contrary, under the ON-state where the total pressure is lower than the pilot pressure discharged from the hydraulic chamber2c, the input port31ais communicated with the output port31c. Thus, given that operating pressure Pri of the hydraulic chamber2cis higher than the accumulator pressure Pacc, the switching valve31is brought into ON-state.

A switching valve33of the draining side is disposed on the draining passage23of the driven pulley3between the discharging solenoid valve24and a drain spot. The switching valve33is comprised of an input port33aconnected to the discharging solenoid valve24, and a pair of output ports33bconnected to the drain spot and33cconnected to the passage34, a not shown spool that selectively provide a connection between the input port33aand the output port33bor the output port33c, and a spring33dthat elastically pushes the spool in the predetermined direction. In the example shown inFIG. 4, the operating pressure Pd of the driven pulley3is applied as a pilot pressure to the spool not only in the direction to increase the elastic force of the spring33dbut also in a direction against the elastic force of the spring33d. That is, the pilot pressure delivered from the hydraulic source, and the pilot pressure discharged from the hydraulic chamber3care applied to the spool of the switching valve33. Specifically, under the OFF-state where a total pressure of the elastic force of the spring33dand the pilot pressure from the accumulator13is higher than the pressure discharged from the hydraulic chamber3c, the input port33ais communicated with the output port33b. To the contrary, under the ON-state where the total pressure is lower than the pilot pressure discharged from the hydraulic chamber3c, the input port33ais communicated with the output port33c. Thus, given that operating pressure Pd of the hydraulic chamber3cis higher than the accumulator pressure Pacc, the switching valve33is brought into ON-state.

Likewise, a switching valve35of the draining side is disposed on the draining passage25of the C1 clutch4between the discharging solenoid valve26and a drain spot. The switching valve35is comprised of an input port35aconnected to the discharging solenoid valve26, and a pair of output ports35bconnected to the drain spot and35cconnected to the passage6, a not shown spool that selectively provide a connection between the input port35aand the output port35bor the output port35c, and a spring35dthat elastically pushes the spool in the predetermined direction. In the example shown inFIG. 4, the operating pressure Pc of the C1 clutch4is applied as a pilot pressure to the spool not only in the direction to increase the elastic force of the spring35dbut also in a direction against the elastic force of the spring35d. That is, the pilot pressure delivered from the hydraulic source, and the pilot pressure discharged from the C1 clutch4are applied to the spool of the switching valve35. Specifically, under the OFF-state where a total pressure of the elastic force of the spring35dand the pilot pressure from the accumulator13is higher than the pressure discharged from the C1 clutch4, the input port35ais communicated with the output port35b. To the contrary, under the ON-state where the total pressure is lower than the pilot pressure discharged from the C1 clutch4, the input port35ais communicated with the output port35c. Thus, given that operating pressure Pc of the C1 clutch4is higher than the accumulator pressure Pacc, the switching valve35is brought into ON-state. Accordingly, the switching valves31,33and35serve as the second switching valve.

Referring now toFIG. 5, there is shown the third control example carried out by the hydraulic control unit shown inFIG. 4. The routine shown therein is repeated at predetermined cycle. Here, in the flowchart shown inFIG. 5, common numbers are allotted to the steps identical to those inFIG. 2. If the answer of step S3is YES, the switching valve14is brought into the OFF-state as the example shown inFIG. 2, and the switching valve33is also brought into the OFF-state (at step S14). That is, at the step S14, the accumulator13is connected with the discharging solenoid valve24through the passages6and34. In this case, since the pilot pressure discharged from the hydraulic chamber3cis higher than the total pressure of the elastic force of the spring33dand the pilot pressure applied from the accumulator13, that is, since the operating pressure Pd is sufficiently higher than the accumulator pressure Pacc, the switching valve33is brought into the ON-state to provide a communication between the input port33aand the output port33c.

After or simultaneously with carrying out of step S14, the discharging solenoid valve24is opened (at step S15). Although not illustrated in detail, the feeding solenoid valve18is opened when the operation of the step S15is carried out. In this situation, the feeding solenoid valve16of the drive pulley2is opened, and the draining solenoid valve22is closed. Consequently, the accumulator13in which the pressure is relatively lower is connected to the hydraulic chamber3cso that the relatively higher pressure of the hydraulic chamber3cis applied to the accumulator13through the passages34and6. As a result, the operating pressure Pd of the hydraulic chamber3cis lowered so that the belt clamping pressure is lowered. Then the routine shown inFIG. 5is returned.

According to the hydraulic control unit shown inFIGS. 1 and 4is configured to apply the operating pressure Pd to the accumulator13in case the operating pressure Pd of the fluid discharged from the hydraulic chamber3cof the driven pulley3is higher than the accumulator pressure Pacc. Instead, as shown inFIG. 6, it is also possible to apply the operating pressure Pd of the fluid discharged from the hydraulic chamber3cof the driven pulley3to a site where a relatively lower pressure is required.

FIG. 6illustrates still another example of the hydraulic control unit of the present invention. The output port31cof the switching valve31is connected to the passage37connected to the feeding passage19of the C1 clutch4. The output port33cof the switching valve33is connected to the passage38connected to the passage37. As shown inFIG. 6, the operating pressure Pc of the C1 clutch4is applied as the pilot pressure to the switching valves31and33. In this example, the operating pressure Pc of the C1 clutch4is relatively lower than the operating pressures Pri and Pd of the pulleys2and3.

FIG. 7is a flowchart explaining a control example of the hydraulic control unit structured as shown inFIG. 6, and the routine shown therein is repeated at predetermined cycle. The control shown inFIG. 7is carried out under the condition where the operating pressure Pd of the driven pulley3is higher than the accumulator pressure Pacc, and the operating pressure Pd has to be discharged. Here, in the flowchart ofFIG. 7, common numbers are allotted to the steps identical to those inFIG. 2andFIG. 5. After or simultaneously with carrying out the control of step S14, the discharging solenoid valve24of the driven pulley3is opened (at step S16). Although not illustrated in detail, at step S16, the feeding solenoid valve18is closed, the feeding solenoid valve16of the drive pulley2is opened, and the draining solenoid valve22is closed. Consequently, the hydraulic chamber3cis communicated with the C1 clutch4so that the relatively higher operating pressure Pd is allowed to be delivered to the C1 clutch4where the pressure is relatively lower through the passages37and38. As a result, the operating pressure Pd of the hydraulic chamber3cis lowered so that the clamping force for belt is reduced. Then the routine shown inFIG. 7is returned.

Thus, according to the control example shown inFIG. 7, the operating pressure Pd of the hydraulic chamber2cis applied to the C1 clutch4so that efficiency of the hydraulic control unit of the present invention can be improved.

Referring now toFIG. 8, there is shown a time chart indicating a change in the pressure in each chamber under the situation where the hydraulic control unit executes the foregoing controls. In the example shown inFIG. 8, a power switch is pressed manually at point t1to send a signal for launching the vehicle or starting the engine to the ECU9. At point t1, a speed ratio of the belt-driven continuously variable transmission1is adjusted to a ratio possible to launch the vehicle. Then, at point t2, the fluid is delivered from the accumulator13to the hydraulic chambers2cand3cand the hydraulic chamber of the C1 clutch4. Consequently, the accumulator pressure Pacc is lowered and the operating pressures Pri, Pd and Pc are raised.

Meanwhile, a signal pressure established by the signal pressure establishing valve8is applied to the regulator valve7so that a line pressure is regulated in accordance with a drive demand such as an accelerator opening. At point t3, the switching valve10is brought into the second operating state so that the fluid is delivered from the pump5to the hydraulic chambers2cand3cand hydraulic chamber of the C1 clutch4. Then, the speed ratio of the belt-driven continuously variable transmission1is increased to the maximum ratio. In order to launch the vehicle, large torque capacities of the pulleys2and3are required. Therefore, each operating pressure Pri and Pd is increased as indicated inFIG. 8to launch the vehicle. When the vehicle speed starts increasing, the operating pressure Pri and Pd are adjusted according to a drive demand or a vehicle speed to achieve a required torque capacity. Specifically, as indicated inFIG. 8, the operating pressure Pri and Pd is lowered gradually. At point t4, an upshifting is commenced, and the fluid is delivered to the hydraulic chamber2cand discharged from the hydraulic chamber3c. In this situation, the operating pressure Pd of the driven pulley3is higher than the accumulator pressure Pacc. As described, the operating pressure Pd is applied to the accumulator13according to the examples show inFIGS. 2, 3, and 5, and applied to the C1 clutch4according to the example shown inFIG. 7. Then, the upshifting is terminated at point t5, and the foregoing routines are returned.

Thus, according to the control examples shown inFIGS. 2, 3, 5 and 7, the operating pressure Pd of the driven pulley3is applied to the accumulator13when carrying out the upshifting. Alternatively, those control examples may also be carried out on the occasion of downshifting to apply the operating pressure Pc of the C1 clutch4to the accumulator13instead of the operating pressure Pd.

Here will be explained a relation between the foregoing examples and the present invention. The functional means of steps S1to S5and steps S12to S16serve as the discharging means of the present invention.