WO2008068590A2

WO2008068590A2 - Hydraulic pressure control apparatus for belt-type continuously variable transmissions

Info

Publication number: WO2008068590A2
Application number: PCT/IB2007/003751
Authority: WO
Inventors: Yoshinobu Soga
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2006-12-05
Filing date: 2007-12-04
Publication date: 2008-06-12
Also published as: JP2008138820A; WO2008068590A3

Abstract

A hydraulic pressure control apparatus (200) for a belt-type CVT includes: line pressure controlling means (L2) for controlling a line pressure so as to substantially equal the higher of a target drive-pulley hydraulic pressure and a target driven-pulley hydraulic pressure, which are set in accordance with the target speed ratio of the belt-type CVT; and hydraulic pressure switching means (L3, IA) for directly applying the line pressure, without reducing it, to a hydraulic chamber of the drive pulley or the driven pulley, where the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure is to be applied.

Description

HYDRAULIC PRESSURE CONTROL APPARATUS FOR BELT-TYPE

CONTINUOUSLY VARIABLE TRANSMISSIONS

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to a hydraulic pressure control apparatus for a belt-type continuously variable transmission for motor vehicles. In particular, the invention relates to a technology that, in order to reduces the pump loss (the loss at an oil pump), lowers the line pressure that is used as the base pressure to produce the hydraulic pressures for actuating the drive and driven pulleys (primary and secondary pulleys) of a belt-type continuously variable transmission.

2. Description of the Related Art

[0002] There are known belt-type continuously variable transmissions mounted on the output side of a motor vehicle engine, such as the one described in Japanese Patent Application Publication No. 9-217819 (JP-A-9-217819).

[0003] The belt-type continuously variable transmission described in Japanese Patent Application Publication No. 9-217819 has two shafts provided in parallel, a primary pulley provided on one of the shafts, and a secondary pulley provided on the other of the shafts. The primary pulley and the secondary pulley are each constituted of a stationary sheave and a movable sheave that are arranged in combination so as to form a V-shaped groove therebetween. The movable sheaves can move toward and away from the stationary sheaves.

[0004] A belt is wound around the V-shaped grooves of the primary and secondary pulleys. Each of the primary and secondary pulleys has a hydraulic chamber used to produce the force to drive the movable sheave in the axial direction. As such, the distance between the movable sheave and the stationary sheave of each pulley (will be referred to as "belt-groove width") is changed by controlling the hydraulic pressure to each hydraulic chamber, whereby the pitch diameter of the belt at each pulley changes and thus the speed ratio of the belt-type continuously variable transmission changes as needed.

[0005] Belts used in belt-type continuously variable transmissions of the above kind are typically constituted of a number of metal pieces (also called "elements" and "blocks") that are fit in series on rings made of steel or the like (also called "hoops" or "bands") such that the metal pieces can move relative to each other. The pulleys are mated with the side faces of each metal piece such that the metal pieces are cramped between the movable sheaves and the stationary sheaves while the belt runs on the pulleys.

[0006] In the hydraulic pressure control circuit that independently controls the hydraulic pressures to the hydraulic chambers, the line pressure, which is the hydraulic pressure at the discharge side of the oil pump, is used as the based pressure. That is, the hydraulic pressures for actuating the primary pulley and the secondary pulley are obtained by adjusting the base pressure as needed. More specifically, the primary-side hydraulic pressure is obtained by reducing the line pressure by a predetermined amount using a primary-side pressure reducing valve, or the like, and the secondary-side hydraulic pressure is obtained by reducing the line pressure by a predetermined amount using a secondary-side pressure reducing valve, or the like.

[0007] For example, Japanese Patent Publication No. 06-6974 describes driving an oil pump so as to produce a line pressure that is higher than a primary-side hydraulic pressure (will be referred to also as "required primary-side hydraulic pressure" or "target primary-side hydraulic pressure") and a secondary-side hydraulic pressure (will be referred to also as "required secondary-side hydraulic pressure" or "target secondary-side hydraulic pressure") that are set in accordance with the target speed ratio of the belt-type continuously variable transmission, or the like. More specifically, when the target speed ratio is higher than "1", the primary-side hydraulic pressure is made higher than the secondary-side hydraulic pressure, and when the target speed ratio is lower than "1", the secondary-side hydraulic pressure is made higher than the primary-side hydraulic pressure. In this case, when the target speed ratio is higher than "1", the line pressure is made slightly higher than the primary-side hydraulic pressure, and when the target speed ratio is lower than "1", the line pressure made slightly higher than the secondary-side hydraulic pressure.

[0008] In the hydraulic pressure control circuits of conventional belt-type continuously variable transmissions, typically, the line pressure is set to a relatively high level, as described above, in order to improve the reliability of the operation of the movable sheave of each pulley and the shift response even in the case where there axe variations in the estimated hydraulic pressures required to be applied to the hydraulic chambers of the respectively pulleys. That is, the line pressure is maintained at a relatively high level so that the required hydraulic pressures can be produced even when there are variations in the estimated required hydraulic pressures, and the required primary-side hydraulic pressure and the required secondary-side hydraulic pressure are obtained by reducing the line pressure using pressure reducing valves, or the like.

[0009] In order to maintain the line pressure at a relatively high level, however, the discharge pressure of the oil pump also needs to be maintained at a relatively high level. Typically, the higher the discharge pressure of an oil pump, the larger the pump loss (the loss at the oil pump). Therefore, in the case of the hydraulic pressure control circuit described above, it is difficult to reduce the pump loss while securing a sufficient reliability of the shifting operation of the belt-type continuously variable transmission, and in particular, when the oil pump is driven by the drive force of the engine, it causes a large loss torque.

SUMMARY OF THE INVENTION

[0010] In view of the above, the invention has been made to provide a hydraulic control apparatus that reduces the pump loss while securing the hydraulic pressures required in the

hydraulic pressure control circuit of a belt-type continuously variable transmission.

[0011] -Principal of Solutions-

The principal employed in the invention to achieve the above objective is as follows. That is, the line pressure is first adjusted such that it substantially equals the higher of the hydraulic pressure for actuating the drive pulley of a belt-type continuously variable transmission and the hydraulic pressure for actuating the driven pulley and the adjusted line pressure is then directly applied, without being reduced, to the hydraulic chamber of the drive pulley or the hydraulic chamber of the driven pulley to which the higher actuating hydraulic pressure is to be applied. According to this hydraulic pressure control, the line pressure needs to be adjusted just to the minimum necessary level, and therefore the discharge pressure of the oil pump can be reduced and thus the pump loss can be reduced accordingly.

[0012] -Solutions-

A first aspect of the invention relates to a hydraulic pressure control apparatus for a belt-type continuously variable transmission having a drive pulley, a driven pulley, and a belt wound around the drive pulley and the driven pulley so that drive force is transmitted between the drive pulley and the driven pulley via the belt, wherein the speed iatio of the belt-type continuously variable transmission is changed by independently adjusting, using a line pressure as a base pressure, a drive-pulley hydraulic pressure for changing the belt-groove width at the drive pulley and a driven pulley hydraulic pressure for changing the ^"belt-groove width at the driven pulley. The hydraulic pressure control apparatus has line pressure controlling means and hydraulic pressure switching means. The line pressure controlling means controls the line pressure such that the line pressure substantially equals the higher of a target drive-pulley hydraulic pressure and a target driven-pulley hydraulic pressure that are set in accordance with the target speed ratio of the belt-type continuously variable transmission. The hydraulic pressure switching means causes the line pressure to be directly applied, without being reduced, to a hydraulic chamber of the drive pulley or a hydraulic chamber of the driven pulley to which the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure is to be applied.

[0013] According to the hydraulic control apparatus described above, when the belt-type continuously variable transmission is being shifted, the belt-groove width at the drive pulley is changed by the drive pulley hydraulic pressure applied to the hydraulic chamber of the drive pulley, and the belt-groove width at the driven pulley is changed by the driven pulley hydraulic pressure applied to the hydraulic chamber of the driven pulley, whereby the point at which the belt makes contact with the drive pulley and the point at which the belt makes contact with the driven pulley shift in the radial directions of the respective pulleys, so that the drive force is transmitted at a desired speed ratio. The target hydraulic pressures to be applied to the hydraulic chambers of the drive and driven pulleys (will be referred to as "target drive-pulley hydraulic pressure" and "target driven-pulley hydraulic pressure") are each set in accordance with the target speed ratio of the belt-type continuously variable transmission, or the like. For example, in the speed reduction range where the target speed ratio (the drive-pulley rotation speed / the driven-pulley rotation speed) of the belt-type continuously variable transmission is relatively high, the target driven-pulley hydraulic pressure is set higher than the target drive-pulley hydraulic pressure, and in the speed increase range where the target speed ratio of the belt-type continuously variable transmission is relatively low, the target drive-pulley hydraulic pressure is set higher than the target driven-pulley hydraulic pressure. According to the hydraulic pressure control apparatus described above, the line pressure is controlled such that it substantially equals the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure, and the thus controlled line pressure is then directly applied, without being reduced, to the hydraulic chamber of the drive pulley or the hydraulic chamber of the driven pulley to which the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure is to be applied. As such, the line pressure does not need to be increased more than necessary in advance, and therefore the discharge pressure of the oil pump can be lowered and thus the pump loss can be reduced accordingly.

[0014] The above-described hydraulic pressure control apparatus may further have a drive-pulley hydraulic line via which hydraulic pressure is applied to the hydraulic chamber of the drive pulley and a driven-pulley hydraulic line via which hydraulic pressure is applied to the hydraulic chamber of the driven pulley. The drive-pulley hydraulic line may incorporate pressure reducing means for reducing the line pressure input to the pressure reducing means and drive-pulley hydraulic pressure detecting means for detecting the drive pulley hydraulic pressure, and the driven-pulley hydraulic line may incorporate pressure reducing means for reducing the line pressure input to the pressure reducing means and driven-pulley hydraulic pressure detecting means for detecting the driven pulley hydraulic pressure. In this case, the hydraulic pressure switching means may cause the line pressure to be applied to the hydraulic chamber of the drive pulley or the hydraulic chamber of the driven pulley to which the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure is to be applied, by placing the pressure reducing means on the hydraulic line leading to the hydraulic chamber in a non-pressure reducing state. Further, the line pressure controlling means may perform feedback control of the line pressure such that the hydraulic pressure detected by the drive-pulley hydraulic pressure detecting means or the driven-pulley hydraulic pressure detecting means corresponding to the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure substantially equals the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure.

[0015] According to these structures, for example, when the target driven-pulley hydraulic pressure is set higher than the target drive-pulley hydraulic pressure, the pressure reducing means on the driven-pulley hydraulic line is placed in the non-pressure reducing state (controlled not to reduce the hydraulic pressure), whereby the line pressure is directly applied to the hydraulic chamber of the driven pulley. During this, the driven-pulley hydraulic pressure is detected by the driven-pulley hydraulic pressure detecting means, and the feedback control of the line pressure is performed such that the driven-pulley hydraulic pressure substantially equals the target driven-pulley hydraulic pressure. On the other hand, when the target drive-pulley hydraulic pressure is set higher than the target driven-pulley hydraulic pressure, the pressure reducing means on the drive-pulley hydraulic line is placed in the non-pressure reducing state, whereby the line pressure is directly applied to the hydraulic chamber of the drive pulley. During this, the drive~pulley hydraulic pressure is detected by the drive-pulley hydraulic pressure detecting means, and the feedback control of the line pressure is performed such that the drive-pulley hydraulic pressure substantially equals the target drive-pulley hydraulic pressure. As such, the solution presented above reduces the pump loss while securing the hydraulic pressures required to actuate the drive pulley and the driven pulley, thus improving the practicality of the hydraulic pressure control apparatus accordingly.

[0016] Further, the above-described hydraulic pressure control apparatus may be such that the pressure reduction by each pressure reducing means is accomplished through a hydraulic pressure adjustment by a duty control or by a control of the flowrate of hydraulic fluid to be supplied.

[0017] The type of the line pressure control by the line pressure controlling means may be either of the following two types.

[0018] The first type is such that the line pressure controlling means controls the line pressure such that the line pressure is always substantially equal to the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure.

[0019] In this case, the line pressure is always maintained at the minimum necessary level (Le., the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure), and therefore the effect of reducing the pump loss can be exerted continuously.

[0020] The second type is such that: when the belt-type continuously variable transmission is not being shifted, the line pressure controlling means controls the line pressure such that the line pressure substantially equals the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure, and when the belt-type continuously variable transmission is being shifted, the line pressure controlling means controls the line pressure such that the line pressure temporarily becomes higher than the higher of the target drive-pulley hydraulic pressure and the target diiven-pulley hydraulic pressure. In this case, when the belt-type continuously variable transmission is not being shifted, the hydraulic pressure switching means may cause the line pressure to be directly applied, without being reduced, to the hydraulic chamber of the drive pulley or the hydraulic chamber of the driven pulley to which the higher of the target drive-pulley hydraulic pressure and the target diiven-pulley hydraulic pressure is to be applied, and when the belt-type continuously variable transmission is being shifted, the hydraulic pressure switching means may cause the temporarily increased line pressure to be reduced and then applied to the hydraulic chamber of the drive pulley or the hydraulic chamber of the driven pulley to which the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure is to be applied.

[0021] According to this structure, when the belt-type continuously variable transmission is not being shifted, the line pressure is maintained at the minimum necessary level and therefore the pump loss decreases accordingly. On the other hand, when the belt-type continuously variable transmission is being shifted, the line pressure is temporarily increased, and the hydraulic pressure to be applied to the corresponding hydraulic chamber is obtained by reducing the line pressure as needed. This increases the reliability in achieving the target hydraulic pressure and thus improves the operation reliability of each pulley and the shift response.

[0022] Further, the above-described hydraulic pressure control apparatus may be such that in a state where the belt-type continuously variable transmission is repeatedly shifted in a short time period, the line pressure is controlled such that the line pressure continues to be higher than the higher of the target drive-pulley hydraulic pressure and the target driven-puiley hydraulic pressure.

[0023] According to the invention, thus, the line pressure is controlled such that it substantially equals the higher of the hydraulic pressure (target hydraulic pressure) for actuating the drive pulley of a belt-type continuously variable transmission and the hydraulic pressure (target hydraulic pressure) for actuating the driven pulley, and the thus controlled line pressure is directly applied, without being reduced, to the hydraulic chamber of the corresponding pulley. As such, the line pressure needs to be maintained just at the minimum necessary level, and therefore the discharge pressure of the oil can be lowered and thus the pump loss can be reduced accordingly.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The foregoing and/or further objects, features and advantages of the invention will become more apparent from the following description of preferred embodiment with reference to the accompanying drawings, in which like numerals are used to represent like elements and wherein:

FIG. 1 is a skeleton view showing the configuration of a transaxle incorporating a belt-type continuously variable transmission according to the invention;

FIG. 2 is a hydraulic circuit diagram schematically showing the connections between an oil pump, a hydraulic pressure control circuit, a primary-side hydraulic actuator, and a secondary-side hydraulic actuator in the first example embodiment of the invention;

FIG. 3A is a chart illustrating the variations of the set values of the target primary-side hydraulic pressure and the target secondary-side hydraulic pressure that are set according to the target speed ratio of a belt-type continuously variable transmission according to the invention;

FIG. 3B is a chart illustrating the variations of the set values of the target primary-side hydraulic pressure and the target secondary-side hydraulic pressure that are set according to the target speed ratio of a belt-type continuously variable transmission according to the invention;

FIG. 4 is a view schematically showing the configuration of a primary-side hydraulic pressure control circuit according to the second example embodiment of the invention; and

FIG. 5 is a chart illustrating the variations of the set values of the target primary-side hydraulic pressure and the target secondary-side hydraulic pressure that are set according to the target speed ratio of a belt-type continuously variable transmission according to the third example embodiment of the invention.

DETAIUED DESCRIPTION OF THE EMBODIMENTS

[0025] Hereinafter, example embodiments of the invention will be descried with reference to the accompanying drawings. In the following example embodiments, the invention has been applied to a belt-type CVT (Continuously Variable Transmission) for motor vehicles, more specifically, to a belt-type CVT mounted on a FF (Front-engine Front-drive) vehicle.

[0026] -First Example Embodiment- (Overall Configuration of Transaxle) First, the overall configuration of the transaxle incorporating a belt-type CVT will be described.

[0027] FIG. 1 is a skeleton view illustrating a FF vehicle transaxle incorporating a belt-type CVT according to the invention. Referring to FIG 1, an engine 1 serves as the drive force source for propelling the vehicle. Although the engine 1 is not limited to any specific type, in the following description, it is assumed, for descriptive convenience, that the engine 1 is a gasoline engine.

[0028] A transaxle 3 is provided on the output side of the engine 1. The transaxle 3 has a transaxle housing 4 that is attached to the output side of the engine 1 (on one side of the vehicle), a transaxle case 5 that is attached to the opening of the transaxle housing 4 on the side opposite from the engine 1, and a transaxle rear cover 6 that is attached to the opening of the transaxle case 5 on the side opposite from the transaxle housing 4. A torque converter (TfC) 7 is provided in the transaxle housing 4. A forward-reverse drive switching mechanism 8, a belt-type CVT 9, a final speed reduction mechanism 10 having a differential mechanism, etc. are provided in the transaxle case 5 and the transaxle rear cover 6.

[0029] In the transaxle housing 4, an input shaft 11 is provided coaxially with a crankshaft 2. A turbine runner 13 is attached to the engine 1 side end of the input shaft 11. A front cover 15 is coupled with one end of the crankshaft 2 via a drive plate 14. A pump impeller 16 is coupled with the front cover 15. The turbine runner 13 and the pump impeller 16 are arranged so as to face each other. A stator 17 is provided on the inner sides of the turbine runner 13 and the pump impeller 16. An oil pump 20 is provided between the torque converter 7 and the forward-reverse drive switching mechanism 8.

[0030] With regard to the operation of the torque converter 7, as the crankshaft 2 is turned by the engine 1, the pump impeller 16 is turned via the drive plate 14 and the front cover IS, and the turbine runner 13 is turned by the flow of the hydraulic fluid from the oil pump 20. When the rotation speed difference between the pump impeller 16 and the turbine runner 13 is large, the stator 17 changes the flow direction of hydraulic fluid so as to assist the rotation of the pump impeller 16.

[0031] When the speed of the vehicle reaches a predetermined speed after the vehicle starts running, a lock-up clutch 18 is actuated so that the force that has been transferred from the engine 1 to the front cover 15 is directly transferred to the input shaft 11. A damper mechanism 19 absorbs the fluctuation of torque transferred from the front cover 15 to the input shaft 11.

[0032] The forward-reverse drive switching mechanism 8 is provided on the drive force path between the input shaft 11 and the belt-type CVT 9. The forward-reverse drive switching mechanism 8 is constituted of a double-pinion type planetary gearset 24. The planetary gearset 24 has: a sun gear 25 provided on the input shaft 11; a ring gear 26 provided coaxially around the sun gear 25; inner pinion gears 27 provided around the sun gear 25 and meshed with the sun gear 25; outer pinion gears 28 meshed with the respective inner pinion gears 27 and the ring gear 26; and a carrier 29 supporting the pinion gears 27, 28 such that each pinion gear 27, 28 can rotate about its own axis and each pair of the pinion gears 27, 28 can revolve together about the sun gear 25. The carrier 29 and a primary shaft (transmission input shaft) 30 of the belt-type CVT 9, which will be described later, are coupled with each other. The forward-reverse drive switching mechanism 8 is also provided with a forward clutch CL that connects and disconnects the drive force path between the carrier 29 and the input shaft 11 and a reverse brake BR that controls the ring gear 26 to be rotated or held stationary. The forward-reverse drive switching mechanism 8 switches the drive power direction between the forward drive direction (normal direction) and the reverse drive direction (reverse direction) by changing the drive force path by controlling the forward clutch CL and the reverse brake BR.

[0033] The belt-type CVT 9 has the primary shaft (drive shaft) 30 that is coaxial with the input shaft 11 and a secondary shaft (driven shaft) 31 that is provided in parallel with the primary shaft 30. The primary shaft 30 is rotatably supported by bearings 32, 33, and a secondary shaft 31 is rotatably supported by bearings 34, 35.

[0034] A primary pulley (drive pulley) 36 is provided on the primary shaft 30, and a secondary pulley (driven pulley) 37 is provided on the secondary shaft 31.

[0035] The primary pulley 36 has a stationary sheave 38 that is integrally formed on the primary shaft 30 and a movable sheave 39 that is arranged on the primary shaft 30 such that it can move in the axial direction of the primary shaft 30. A V-shaped groove 40 is formed between the stationary sheave 38 and the movable sheave 39.

[0036] A hydraulic actuator 41 drives the movable sheave 39 in the axial direction of the primary shaft 30 so that the movable sheave 39 and the stationary sheave 38 approach or move away form each other as needed. More specifically, a given hydraulic pressure is applied to a hydraulic chamber in the hydraulic actuator 41 so that the movable sheave 39 moves toward or away from the stationary sheave 38.

[0037] Likewise, the secondary pulley 37 has a stationary sheave 42 that is integrally formed on the secondary shaft 31 and a movable sheave 43 that is arranged on the secondary shaft 31 such that it can move in the axial direction of the secondary shaft 31. A V-shaped groove 44 is formed between the stationary sheave 42 and the movable sheave 43. A hydraulic actuator 45 drives the movable sheave 43 in the axial direction of the secondary shaft 31 so that the movable sheave 43 and the stationary sheave 42 approach and move away from each other. More specifically, a given hydraulic pressure is applied to a hydraulic chamber in the hydraulic actuator 45 so that the movable sheave 43 moves toward or away from the stationary sheave 42. [0038] A belt 46 is wound around the V-shaped groove 40 of the primary pulley 36 and the V-shaped groove 44 of the secondary pulley 37. The belt 46 is a so-called pressed-type metal belt constituted of a number of metal pieces (also called "elements" or "blocks") and a plurality of steel rings (also called "hoops" or "bands), such as those described in Japanese Patent Application Publication No. 2000-220697 (JP-A-2000-220697) and in Japanese Patent Application Publication No. 2002-257200 (JP-A-2002-257200).

[0039] The point at which the belt 46 makes contact with the primary pulley 36 and the point at which the belt 46 makes contact with the secondary pulley 37 shift as the movable sheave 39 and the movable sheave 43 moved forward or backward, whereby the drive force (torque) is transmitted at a given speed ratio.

[0040] A counter-drive gear 47 is coupled with the secondary shaft 31 and is supported by bearings 48, 49. Further, the bearing 35 is provided on the transaxle rear cover 6 side, and a parking gear PG is provided between the bearing 35 and the secondary pulley 37.

[0041] Further, on the drive force path between the counter-drive gear 47 of the belt-type CVT 9 and the final speed reduction mechanism 10, an intermediate shaft 50 is provided in parallel with the secondary shaft 31. The intermediate shaft 50 is supported by bearings 51, 52. A counter-driven gear 53 and a final drive gear 54 are provided on the intermediate shaft 50. The counter-driven gear 53 is meshed with the counter-drive gear 47.

[0042] On the other hand, the final speed reduction mechanism 10 has a hollow deferential case 55 that is rotatably supported by bearings 56, 57, and a final ring gear 58 meshed with the final drive gear 54 is provided at the outer periphery of the deferential case 55. In the deferential case 55 are provided a pinion shaft 59 on which two pinion gears 60 are provided. Each pinion gear 60 is meshed with two side gears 61 at both ends. The side gears 61 are coupled with wheels 63 via corresponding drive shafts 62.

[0043] A speed sensor 71 detects the rotation speed of the primary pulley 36, and a speed sensor 72 detects the rotation speed of the secondary pulley 37. [0044] The hydraulic actuator 41 of the primary pulley 36 and the hydraulic actuator 45 of the secondary pulley 37 are supplied, via a hydraulic pressure control circuit 200, with hydraulic pressures that are produced by the oil pimp 20 and then controlled as needed.

[0045] A hydraulic pressure sensor 73 detects a control hydraulic pressure Ppri applied to the hydraulic actuator 41 of the primary pulley 36, and a hydraulic pressure sensor 74 detects a control hydraulic pressure Psec applied to the hydraulic actuator 45 of the secondary pulley 37.

[0046] More specifically, the pressure of the hydraulic fluid that has been pumped up from an oil pan and discharged from the oil pimp 20 is adjusted as a line pressure PL using a pressure adjusting valve that is controlled through duty control. The hydraulic fluid having the line pressure PL is adjusted to the control hydraulic pressure Ppri via a primary-side pressure reducing valve, which is controlled through duty control, and then applied to the hydraulic actuator 41. Further, the hydraulic fluid having the line pressure PL is adjusted to the control hydraulic pressure Psec via a secondary-side pressure reducing valve, which is also controlled through duty control, and then applied to the hydraulic actuator 45. The structure of the hydraulic circuit accomplishing this hydraulic pressure adjustment will be described later.

[0047] A CVT ECU (CVT Electronic Control Unit) 300 controls the transaxle 3, and an E/G ECU (Engine Electronic Control Unit) 400 controls the engine 1. The CVT ECU 300 and the E/G ECU 400 are each constituted of a microcomputer having, as its main components, a calculation unit (CPU or MPU), data storages (RAM and ROM), and input/output interfaces.

[0048] The CVT ECU 300 receives the signals from the speed sensors 71, 72 and the hydraulic pressure sensors 73, 74 and various parameters indicating the state of the transaxle 3 (e.g., the torque ratio at the torque converter 7, the vehicle speed V). The E/G ECU 400 receives various parameters indicating the operation state of the engine 1 (e.g., the engine speed, the accelerator operation amount, the signals from the throttle sensor) and performs various calculations using the received parameters. The results of the calculations are sent to the CVT ECU 300 when needed. Thus, the CVT ECU 300 receives various calculation results and sensor signals, such as the rotation speed Np of the primary shaft 30 detected by the speed sensor 71, the rotation speed Ns of the secondary shaft 31 detected by the speed sensor 72, the vehicle speed V, and sets the control hydraulic pressures Ppri, Psec based on empirically determined maps, or the like, so as to achieve a target speed ratio γ (= Np/Ns) and a target belt cramping force.

[0049] (Hydraulic Pressure Control Circuit 200)

Next, the hydraulic pressure control circuit 200 will be described in detail. FIG. 2 is a hydraulic circuit diagram schematically showing the connections between the oil pump 20, the hydraulic pressure control circuit 200, the primary-side hydraulic actuator 41, and the secondary-side hydraulic actuator 45.

[0050] Referring to FIG. 2, the hydraulic pressure control circuit 200 is configured to control the pressure of the hydraulic fluid that has been pumped up from the oil pan 21 and then discharged from the oil pump 20. More specifically, the oil pimp 20 independently controls the pressure at which to supply hydraulic fluid to the primary-side hydraulic actuator 41 and the pressure at which to supply hydraulic fluid to the secondary-side hydraulic actuator 45 such that the primary pulley 36 and the secondary pulley 37 operate as needed (the movable sheaves 39, 43 move forward or backward as needed).

[0051] More specifically, the hydraulic pressure control circuit 200 has a discharge line Ll through which the hydraulic fluid discharged from the oil pimp 20 flows. The hydraulic pressure control circuit 200 also has a line pressure control circuit L2, a primary-side hydraulic pressure control circuit 13, and a secondary-side hydraulic pressure control circuit IA, which are all connected to the discharge line Ll.

[0052] The line pressure control circuit L2 is provided with a pressure adjusting valve Vl that is a relief valve controlled through duty control using a duty solenoid SOLI. Thus, the line pressure PL, which is the hydraulic pressure at the discharge line Ll, is adjusted by the pressure adjusting valve Vl.

[0053] A primary-side pressure reducing valve V2 through which the line pressure PL, which is the hydraulic pressure at the discharge line Ll, is applied to the primary-side hydraulic actuator 41 directly, or after being reduced, is provided on the primary-side hydraulic pressure control circuit 13. The primary-side pressure reducing valve V2 is controlled through duty control using a duty solenoid SOL2 such that the line pressure PL is adjusted to a target hydraulic pressure to be applied to the primary-side hydraulic actuator 41 (target hydraulic pressure determined based on the target speed ratio of the belt-type CVT 9, etc.). The hydraulic pressure to the primary-side hydraulic actuator 41 is adjusted so as to control the speed ratio of the belt-type CVT 9 to the target speed ratio, whereby the speed increasing rate or the speed reducing rate is controlled.

[0054] Likewise, a secondary-side pressure reducing valve V3 through which the line pressure PL, which is the hydraulic pressure at the discharge line Ll, is applied to the secondary-side hydraulic actuatOT 45 directly, or after being reduced, is provided on the secondary-side hydraulic pressure control circuit L4. The secondary-side pressure reducing valve V3 is controlled through duty control using a duty solenoid SOL3 such that the line pressure PL is adjusted to a target hydraulic pressure to be applied to the secondary-side hydraulic actuator 45 (target hydraulic pressure determined based on the target speed ratio of the belt-type CVT 9, etc.). The hydraulic pressure to the secondary-side hydraulic actuator 45 is used to adjust the belt clamping pressure and therefore it is adjusted such that the belt 46, the primary pulley 36 and the secondary pulley 37 are not damaged but the belt 46 does not slip.

[0055] In the first example embodiment, the hydraulic pressure sensor 73 is provided on the downstream side (the primary-side hydraulic actuator 41 side) of the primary-side pressure reducing valve V2 on the primary-side hydraulic pressure control circuit L3, and the hydraulic pressure sensor 74 is provided on the downstream side (the secondary-side hydraulic actuator 45 side) of the secondary-side pressure reducing valve V3 on the secondary-side hydraulic pressure control circuit L4. That is, the hydraulic pressure sensor 73 is arranged to detect the hydraulic pressure applied to the primary-side hydraulic actuator 41 via the primary-side pressure reducing valve V2, and the hydraulic pressure sensor 74 is arranged to detect the hydraulic pressure that is applied to the secondary-side hydraulic actuator 45 via the secondary-side pressure reducing valve V3. The hydraulic pressures detected by the hydraulic pressure sensors 73, 74 are transmitted to the CVT ECU 300.

[0056] One of the features of the first example embodiment is that the pressure adjustment operation of the pressure adjusting valve Vl is controlled (i.e., the line pressure PL is controlled) in accordance with the operation state of the belt-type CVT 9 and the pressure reduction operations of the primary-side pressure reducing valve V2 and the secondary-side pressure reducing valve V3 are also controlled in accordance with the operation state of the belt-type CVT 9, as will be described in detail below.

[0057] In the following description, "speed increase" refers to the state where the belt-groove width at the primary pulley 36 is small (i.e., the pitch diameter of the belt 46 at the primary pulley 36 is large) and the belt-groove width at the secondary pulley 37 is large (i.e., the pitch diameter of the belt 46 at the secondary pulley 37 is small) and therefore the rotation speed of the secondary pulley 37 is higher than the rotation speed of the primary pulley 36, and "speed reduction" refers to the state where the belt-groove width at the primary pulley 36 is large (i.e., the pitch diameter of the belt 46 at the primary pulley 36 is small) and the belt-groove width at the secondary pulley 37 is small (i.e., the pitch diameter of the belt 46 at the secondary pulley 37 is large) and therefore the rotation speed of the secondary pulley 37 is lower than the rotation speed of the primary pulley 36.

[0058] FIG 3A illustrates example variations of the target primary-side hydraulic pressure and the target secondary-side hydraulic pressure that are set in accordance with the target speed ratio of the belt-type CVT 9, assuming that the rotation speed and torque input to the belt-type CVT 9 are constant. In FIG. 3A, the dashed curve represents the variation of the set value of the target primary-side hydraulic pressure, while the single-dotted curve represents the variation of the set value of the target secondary-side hydraulic pressure.

[0059] As is evident from FIG 3A, in the speed increase range (the left side in FIG 3A) where the target speed ratio of the belt-type CVT 9 is lower than γ, the target primary-side hydraulic pressure is set higher than the target secondary-side hydraulic pressure, and the difference therebetween increases as the target speed increase rate increases. On the other hand, in the speed reduction range (the right side in FIG. 3A) where the target speed ratio of the belt-type CVT 9 is higher than γ, the target secondary-side hydraulic pressure is set higher than the target primary-side hydraulic pressure, and the difference therebetween increases as the target speed reduction rate increases. That is, the order of the set value of the target primaτy-side hydraulic pressure and the set value of the target secondary-side hydτaulic pressure reverses as the — target-speed ratio changes (i.e., at the speed ratio γ). Note that the target speed ratio γ is set to, for example, 1.

[0060] When the target speed ratio of the belt-type CVT 9 is in the speed increase range and thus the target primary-side hydraulic pressure is set higher than the target secondary-side hydraulic pressure, the line pressure PL is directly applied, without being reduced, to the primary-side hydraulic actuator 41 by placing the primary-side pressure reducing valve V2, which is provided on the primary-side hydraulic pressure control circuit L3, in a non-pressure reducing state (fully-open state) (direct application of the line pressure by hydraulic pressure switching means). During this time, the hydraulic pressure acting on the primary-side hydraulic actuator 41 is detected by the hydraulic pressure sensor 73, and the feedback control of the line pressure PL, which is the hydraulic pressure at the discharge line Ll, is performed through the pressure adjusting operation of the pressure adjusting valve Vl, which is controlled through the duty control using the duty solenoid SOLI, such that the hydraulic pressure detected by the hydraulic pressure sensor 73 substantially equals the target primary-side hydraulic pressure (line pressure control by line pressure controlling means).

[0061] As such, the line pressure PL is made substantially equal to the target primary-side hydraulic pressure (PL = Ppri) and then directly applied to the primary-side hydraulic actuator 41 without being reduced.

[0062] At this time, on the secondary pulley 37 side, the line pressure PL is reduced to the target secondary-side hydraulic pressure through the duty control of the secondary-side pressure reducing valve V3, which is provided on the secondary-side hydraulic pressure control circuit L4, using the duty solenoid SOL3 and then it is applied to the secondary-side hydraulic actuator 45.

[0063] Conversely, when the target speed ratio of the belt-type CVT 9 is in the speed reduction range and the target secondary-side hydraulic pressure is therefore set higher than the target primary-side hydraulic pressure, the line pressure PL is directly applied, without being reduced, to the secondary-side hydraulic actuator 45 by placing the secondary-side pressure reducing valve V3, which is provided on the secondary-side hydraulic pressure control circuit L4, in a non-pressure reducing state (fully-open state) (direct application of the line pressure by the hydraulic pressure switching means). During this time, the hydraulic pressure acting on the secondary-side hydraulic actuator 45 is detected by the hydraulic pressure sensor 74, and the feedback control of the line pressure PL₅ which is the hydraulic pressure at the discharge line Ll, is performed through the pressure adjusting operation of the pressure adjusting valve Vl, which is controlled through the duty control using the duty solenoid SOLI, such that the hydraulic pressure detected by the hydraulic pressure sensor 74 substantially equals the target secondary-side hydraulic pressure (line pressure control by the line pressure controlling means).

[0064] As such, the line pressure PL is made substantially equal to the target secondary-side hydraulic pressure (PL = Psec) and is directly applied to the secondary-side hydraulic actuator 45 without being reduced.

[0065] At this time, on the primary pulley 36 side, the line pressure PL is reduced to the taTget primary-side hydraulic pressure through the duty control of the primary-side pressure reducing valve V2, which is provided on the primary-side hydraulic pressure control circuit L3, using the duty solenoid SOL2, and then it is applied to the primary-side hydraulic actuator 41.

[0066] As such, in the first example embodiment, the line pressure PL needs to be adjusted just to the higher of the target primary-side hydraulic pressure and the target secondary-side hydraulic pressure. In FIG 3B, the solid curve indicates the variation of the line pressure PL that is set in accordance with the target speed ratio of the belt-type CVT 9. The line pressure PL indicated by this solid curve is made substantially equal to the target primary-side hydraulic pressure when the target speed ratio of the belt-type CVT 9 is in the speed increase range and is made substantially equal to the target secondary-side hydraulic pressure when the target speed ratio of the belt-type CVT 9 is in the speed reduction range. In FIG. 3B, the dashed curve indicates the variation of the target primary-side hydraulic pressure when the target speed ratio of the belt-type CVT 9 is in the speed reduction range, and the single-dotted curve indicates the variation of the target secondary-side hydraulic pressure when the target speed ratio of the belt-type CVT 9 is in the speed increase range.

[0067] The above-described hydraulic pressure control in the first example embodiment eliminates the necessity of setting the line pressure PL higher than necessary and therefore the discharge pressure of the oil pump 20 can be lowered and thus the pumping loss can be reduced accordingly. In particular, in the case where the oil pimp 20 is driven by the drive force of the engine 1 (the rotational force of the crankshaft) as in the first example embodiment, the loss torque can be significantly reduced. [0068] -Second Example Embodiment- Next, the second example embodiment of the invention will be described. The second example embodiment differs from the first example embodiment in the structure of the primary-side hydraulic pressure control circuit 13 in the hydraulic pressure control circuit 200, and other structures of the second example embodiment are the same as those of the first example embodiment. In the following, therefore, only the structure of the primary-side hydraulic pressure control circuit L3 will be described.

[0069] FIGL 4 is a hydraulic circuit diagram of the primary-side hydraulic pressure control circuit L3 of the second example embodiment. Referring to FIG 4, in the second example embodiment, a flowrate control valve V4 is provided on the primary-side hydraulic pressure control circuit L3 instead of the primary-side pressure reducing valve V2. The flowrate control valve V4 is controlled by the duty solenoid SOL4 to adjust the amount of hydraulic fluid supplied to the primary-side hydraulic actuator 41, whereby the primary pulley 36 is controlled to operate as needed (the movable sheave 39 is controlled to move forward or backward as needed).

[0070] Detail on the operation of the flowrate control valve V4 will be described in the following. When the target speed ratio of the belt-type CVT 9 is in the speed increase range and thus the target primary-side hydraulic pressure is set higher than the target secondary-side hydraulic pressure, the line pressure PL is directly applied to the primary-side hydraulic actuator 41 by placing the flowrate control valve V4 in a non-pressure reducing state (fully-open state) as indicated in FIG 4. During this time, as in the first example embodiment described earlier, the hydraulic pressure acting on the primary-side hydraulic actuator 41 is detected by the hydraulic pressure sensor 73, and the feedback control of the line pressure PL, which is the hydraulic pressure at the discharge line Ll, is performed through the pressure adjusting operation of the pressure adjusting valve Vl, which is controlled through the duty control using the duty solenoid SOLI, such that the hydraulic pressure detected by the hydraulic pressure sensor 73 substantially equals the target primary-side hydraulic pressure. As such, the line pressure PL is made substantially equal to the target primary-side hydraulic pressure and is directly applied to the primary-side hydraulic actuator 41 without being reduced,

[0071] Conversely, when the target speed ratio of the belt-type CVT 9 is in the speed reduction range and the target secondary-side hydraulic pressure is therefore set higher than the target primary-side hydraulic pressure, the flowrate control valve V4 is switched so that the line pressure PL is reduced to the primary-side hydraulic pressure, and the reduced line pressure PL is then applied to the primary-side hydraulic actuator 41. In this case, the line pressure PL is directly applied to the secondary-side hydraulic actuator 45 by placing the secondary-side pressure reducing valve V3 in a non-pressure reducing state (fully-open state). During this time, the hydraulic pressure acting on the secondary-side hydraulic actuator 45 is detected by the hydraulic pressure sensor 74, and the feedback control of the line pressure PL, which is the hydraulic pressure at the discharge line Ll, is performed through the pressure adjusting operation of the pressure adjusting valve Vl, which is controlled through the duty control using the duty solenoid SOLI, such that the hydraulic pressure detected by the hydraulic pressure sensor 74 substantially equals the target secondary-side hydraulic pressure. As such, the line pressure PL is made substantially equal to the target secondary-side hydraulic pressure and is directly applied to the secondary-side hydraulic actuator 45 without being reduced.

[0072] In the second example embodiment, as in the first example embodiment described above, the line pressure PL needs to be adjusted just to the higher of the target primary-side hydraulic pressure and the target secondary-side hydraulic pressure. Thus, the line pressure PL does not need to be made higher than necessary, and therefore the discharge pressure of the oil pimp 20 can be lowered and thus the pumping loss can be reduced accordingly.

[0073] -Third Example Embodiment- Next, the third example embodiment of the invention will be described. In the foregoing example embodiments, the line pressure PL is always adjusted to the higher of the target primary-side hydraulic pressure and the target secondary-side hydraulic pressure. In the third example embodiment, the line pressure PL is temporarily made higher than the higher of the target primary-side hydraulic pressure and the target secondary-side hydraulic pressure in accordance with the operation state of the belt-type CVT 9. Other structures and controls in the third example embodiment are the same as those in the foregoing example embodiments, and therefore the control of the line pressure PL in the third example embodiment will be mainly described in the following.

[0074] In the third example embodiment, when the target speed ratio of the belt-type CVT 9 is constant (when the vehicle is running at a constant speed), the line pressure PL is made substantially equal to the higher of the target primary-side hydraulic pressure and the target secondary-side hydraulic pressure, and the line pressure PL is directly applied, without being reduced, to the hydraulic actuator 41 or 45 corresponding to the higher of the target primary-side hydraulic pressure and the target secondary-side hydraulic pressure.

[0075] For example, as descried above, when the target speed ratio of the belt-type CVT 9 is in the speed increase range and thus the target primary-side hydraulic pressure is set higher than the target secondary-side hydraulic pressure, the line pressure PL is directly applied, without being reduced, to the primary-side hydraulic actuator 41 by placing the primary-side pressure reducing valve V2, which is provided on the primary-side hydraulic pressure control circuit L3, in a non-pressure reducing state (fully-open state). During this time, the hydraulic pressure acting on the primary-side hydraulic actuator 41 is detected by the hydraulic pressure sensor 73, and the feedback control of the line pressure PL, which is the hydraulic pressure at the discharge line Ll, is performed through the pressure adjusting operation of the pressure adjusting valve Vl, which is controlled through the duty control using the duty solenoid SOLI, such that the hydraulic pressure detected by the hydraulic pressure sensor 73 substantially equals the target primary-side hydraulic pressure.

[0076] Conversely, when the target speed ratio of the belt-type CVT 9 is in the speed reduction range and the target secondary-side hydraulic pressure is therefore set higher than the target primary-side hydraulic pressure, the line pressure PL is directly applied, without being reduced, to the secondary-side hydraulic actuator 45 by placing the secondary-side pressure reducing valve V3, which is provided on the secondary-side hydraulic pressure control circuit L4, in a non-pressure reducing state (fully-open state). During this time, the hydraulic pressure acting on the secondary-side hydraulic actuator 45 is detected by the hydraulic pressure sensor 74, and the feedback control of the line pressure PL, which is the hydraulic pressure at the discharge line Ll, is performed through the pressure adjusting operation of the pressure adjusting valve Vl, which is controlled through the duty control using the duty solenoid SOLI, such that the hydraulic pressure detected by the hydraulic pressure sensor 74 substantially equals the target secondary-side hydraulic pressure.

[0077] When the belt-type CVT 9 is being shifted or when the a shift-request signal is transmitted to the CVT ECU 300, the line pressure PL is made temporarily higher than the higher of the target primary-side hydraulic pressure and the target secondary-side hydraulic pressure.

[0078] For example, referring to FIG. 5, when the speed ratio of the belt-type CVT 9 is in the speed reduction range and is being changed from γl to γ2 (i.e., when the belt-type CVT 9 is being shifted to reduce its speed ratio), the line pressure PL is increased (by Pl in FIG. 5, for example, by approx. 10%) during the time period over which the speed ratio of the belt-type CVT 9 is changed from γl to γ2. When the shifting of the belt-type CVT 9 is completed, the line pressure PL is changed back to the previous level (the higher of the target primary-side hydraulic pressure and the target secondary-side hydraulic pressure).

[0079] During the shifting of the belt-type CVT 9, the temporarily-increased line pressure PL is reduced and then applied to the hydraulic actuator 41, 45. In the example illustrated in FIG 5, because the target speed ratio of the belt-type CVT 9 is in the speed reduction range, the line pressure PL is set to a pressure that is slightly higher than the target secondary-side hydraulic pressure. In this case, on the secondary pulley 37 side, the line pressure PL is reduced to the target secondary-side hydraulic pressure through the pressure reduction operation of the secondary-side pressure reducing valve V3 provided on the secondary-side hydraulic pressure control circuit L4, and the resultant hydraulic pressure is then applied to the secondary-side hydraulic actuator 45. On the primary pulley 36 side, the amount by which to reduce the line pressure PL at the primary-side pressure reducing valve V2 is increased by an amount corresponding to the temporary increase in the line pressure PL, and the thus reduced hydraulic pressure is applied to the primary-side hydraulic actuator 41.

[0080] While the foregoing description refers to the example in which the target speed ratio of the belt-type CVT 9 is in the speed reduction range and the belt-type CVT 9 is shifted to reduce its speed ratio, the line pressure PL may be temporarily increased also when the target speed ratio of the belt-type CVT 9 is in the speed reduction range and the belt-type CVT 9 is shifted to increase its speed ratio, when the target speed ratio of the belt-type CVT 9 is in the speed increase range and the belt-type CVT 9 is shifted to reduce its speed ratio, and when the target speed ratio of the belt-type CVT 9 is in the speed increase range and the belt-type CVT 9 is shifted to increase its speed ratio. Further, the line pressure PL may be temporarily increased when the belt-type CVT 9 is being shifted to change its speed ratio from the speed reduction range to the speed increase range or from the speed increase range to the speed reduction range.

[0081] In the case where the line pressure PL is controlled as described above, the line pressure PL, as in the foregoing example embodiments, needs to be maintained just at the minimum necessary level when the belt-type CVT 9 is not being shifted, and thus the pumping loss decreases accordingly. Further, when the belt-type CVT 9 is being shifted, the line pressure PL is temporarily increased, and the increased line pressure PL is then reduced to the target hydraulic pressures and applied to the hydraulic actuators. As such, each target hydraulic pressure can be achieved more reliably, and therefore the operation reliability and the response of each pulley improve accordingly.

[0082] In the third example embodiment, the line pressure PL is temporarily increased only when the belt-type CVT 9 is being shifted. The invention, however, is not limited to this feature. For example, in the case where the belt-type CVT 9 is repeatedly shifted within a short time period (e.g., when the vehicle is being accelerated from a standstill), the line pressure PL may be increased continuously until the vehicle comes into a constant-speed running state. By doing so, the line pressure PL can be prevented from being changed frequently.

[0083] -Other Example Embodiments-

In the above-described example embodiments, the invention is applied to the belt-type CVT 9 for motor vehicles. However, the invention is not limited to this, but it may be applied to belt-type CVTs for various applications other than motor vehicles. Further, the invention may be applied to various other motor vehicles, such as motor vehicles using a diesel engine as a drive force source and hybrid vehicles, as well as motor vehicles using a gasoline engine as a drive force source.

[0084] Further, while the hydraulic pressure control circuit 200 having the pressure adjusting valve Vl, which is a relief valve and used as means for adjusting the line pressure PL, is incorporated in each of the example embodiments described above, the invention is not limited to this. For example, a pilot-valve-type hydraulic circuit having linear solenoid valves may alternatively be incorporated. [0085] Further, the oil pimp 20 is not limited to an oil pump that is driven by the drive force of the engine 1 (e.g., the rotational force of the crankshaft 2), but it may alternatively be an oil pump that is driven by an electric motor.

[0086] While the invention has been described with reference to exemplary embodiments thereof, it should be understood that the invention is not limited to the exemplary embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the exemplary embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.

Claims

1. A hydraulic pressure control apparatus for a belt-type continuously variable transmission having a drive pulley, a driven pulley, and a belt wound around the drive pulley and the driven pulley so that drive force is transmitted between the drive pulley and the driven pulley via the belt, wherein the speed ratio of the belt-type continuously variable transmission is changed by independently adjusting, using a line pressure as a base pressure, a drive-pulley hydraulic pressure for changing the belt-groove width at the drive pulley and a driven pulley hydraulic pressure for changing the belt-groove width at the driven pulley, the hydraulic pressure control apparatus being characterized by comprising: line pressure controlling means for controlling the line pressure such that the line pressure substantially equals the higher of a target drive-pulley hydraulic pressure and a target driven-pulley hydraulic pressure that are set in accordance with the target speed ratio of the belt-type continuously variable transmission; and hydraulic pressure switching means for causing the line pressure to be directly applied, without being reduced, to a hydraulic chamber of the drive pulley or a hydraulic chamber of the driven pulley to which the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure is to be applied.

2. The hydraulic pressure control apparatus according to claim 1, characterized by further comprising: a drive-pulley hydraulic line which incorporates pressure reducing means for reducing the line pressure input to the pressure reducing means and drive-pulley hydraulic pressure detecting means for detecting the drive pulley hydraulic pressure and via which hydraulic pressure is applied to the hydraulic chamber of the drive pulley; and a driven-pulley hydraulic line which incorporates pressure reducing means for reducing the line pressure input to the pressure reducing means and driven-pulley hydraulic pressure detecting means for detecting the driven pulley hydraulic pressure and via which hydraulic pressure is applied to the hydraulic chamber of the driven pulley, wherein: the hydraulic pressure switching means causes the line pressure to be applied to the hydraulic chamber of the drive pulley or the hydraulic chamber of the driven pulley to which the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure is to be applied, by placing the pressure reducing means on the hydraulic line leading to the hydraulic chamber in a non-pressure reducing state, and the line pressure controlling means performs feedback control of the line pressure such that the hydraulic pressure detected by the drive-pulley hydraulic pressure detecting means or the driven-pulley hydraulic pressure detecting means corresponding to the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure substantially equals the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure.

3. The hydraulic pressure control apparatus according to claim 2, wherein: the pressure reduction by each pressure reducing means is accomplished through a hydraulic pressure adjustment that is performed by a duty control or by a control of the flowxate of hydraulic fluid to be supplied.

4. The hydraulic pressure control apparatus according to any claims 1 to 3, wherein: the line pressure controlling means controls the line pressure such that the line pressure is always substantially equal to the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure.

5. The hydraulic pressure control apparatus according to any claims 1 to 3, wherein: when the belt-type continuously variable transmission is not being shifted, the line pressure controlling means controls the line pressure such that the line pressure substantially equals the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure, and when the belt-type continuously variable transmission is being shifted, the line pressure controlling means controls the line pressure such that the line pressure temporarily becomes higher than the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure; and when the belt-type continuously variable transmission is not being shifted, the hydraulic pressure switching means causes the line pressure to be directly applied, without being reduced, to the hydraulic chamber of the drive pulley or the hydraulic chamber of the driven pulley to which the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure is to be applied, and when the belt-type continuously variable transmission is being shifted, the hydraulic pressure switching means causes the temporarily increased line pressure to be reduced and then applied to the hydraulic chamber of the drive pulley or the hydraulic chamber of the driven pulley to which the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure is to be applied.

6. The hydraulic pressure control apparatus according to claim 5, wherein: in a state where the belt-type continuously variable transmission is repeatedly shifted in a short time period, the line pressure is controlled such that the line pressure continues to be higher than the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure.

7. A hydraulic pressure control apparatus for a belt-type continuously variable transmission having a drive pulley, a driven pulley, and a belt wound around the drive pulley and the driven pulley so that drive force is transmitted between the drive pulley and the driven pulley via the belt, wherein the speed ratio of the belt-type continuously variable transmission is changed by independently adjusting, using a line pressure as a base pressure, a drive-pulley hydraulic pressure for changing the belt-groove width at the drive pulley and a driven pulley hydraulic pressure for changing the belt-groove width at the driven pulley, the hydraulic pressure control apparatus comprising: a line pressure controlling portion that that controls the line pressure such that the line pressure substantially equals the higher of a target drive-pulley hydraulic pressure and a target driven-pulley hydraulic pressure that are set in accordance with the target speed ratio of the belt-type continuously variable transmission; and a hydraulic pressure switching portion that causes the line pressure to be directly applied, without being reduced, to a hydraulic chamber of the drive pulley or a hydraulic chamber of the driven pulley to which the higher of the target drive-pulley hydraulic pressure and the target driven-pulley hydraulic pressure is to be applied.