Abstract:
The present invention provides a hydraulic control system in which hydraulic pressure by an oil pump is supplied to a pressure/damper clutch controller, a pressure reducer, and a shift controller, wherein the pressure reduced by the pressure reducer and the hydraulic pressure supplied as a result of port conversion of the sift controller are supplied to the hydraulic pressure controller, and hydraulic pressure controlled by the hydraulic pressure controller is supplied to a hydraulic pressure distributor and directly to some friction elements.

Description:
[0001]    This application is a divisional of co-pending application Ser. No. 09/667,592, filed on Sep. 22, 2000, the entire contents of which are hereby incorporated by reference and for which priority is claimed under 35 U.S.C. § 120; and this application claims priority of Application No. 99-63713 filed in Korea on Dec. 28, 1999 under 35 U.S.C. § 119. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a hydraulic control system for an automatic transmission, and more particularly, to a hydraulic control system applied to a powertrain utilizing two single pinion planetary gearsets, four clutches, two one-way clutches, and two brakes to obtain four forward speeds and one reverse speed.  
           [0004]    2. Description of the Related Art  
           [0005]    Conventional automatic transmissions used in vehicles include a torque converter, a multi-stage gear shift mechanism connected to the torque converter, and a plurality of friction elements actuated by hydraulic pressure for selecting one of the gear stages of the gear shift mechanism, the gear shift mechanism being realized through a planetary gearset. The friction elements are controlled to engaged and disengaged states by a hydraulic control system, which controls pressure generated by an oil pump, to change shift ratios of the planetary gearset.  
           [0006]    The friction elements are selectively operated by a plurality of valves, which undergo port conversion to change the flow of hydraulic pressure, and actuators supplying hydraulic pressure to the valves. Further, a manual valve, indexed with a driver-operated shift selector to realize port conversion, is connected to a plurality of lines to supply hydraulic pressure from the oil pump to each valve and actuator.  
           [0007]    Solenoid valves are operated to ON and OFF states in different combinations to realize control into the various speeds and shift modes. That is, as described above, the solenoid valves are operated to ON and OFF states such that the supply of hydraulic pressure to the valves is controlled. This, in turn, controls the supply of hydraulic pressure to specific friction elements to control the same, thereby ultimately controlling the multi-stage gear shift mechanism for control into the different shift speeds and modes.  
           [0008]    However, in the prior art hydraulic control system, an engine brake is applied frequently at unsuitable times such that drive performance is reduced. Also, fail-safe means in case the transmission malfunctions is not provided.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention has been made in an effort to solve the above problems.  
           [0010]    It is an object of the present invention to provide a hydraulic control system applied to a powertrain utilizing two single pinion planetary gearsets, four clutches, two one-way clutches, and two brakes to obtain four forward speeds and one reverse speed, the hydraulic control system operating an engine brake at only a high stage of each range such that drive performance is improved and which provides a fail-safe mode by holding a fourth speed in a drive D range and a third speed in a low D 2  range such that drive performance is improved.  
           [0011]    To achieve the above object, the present invention provides a hydraulic control system in which hydraulic pressure generated by the operation of an oil pump is supplied to pressure/damper clutch control means, pressure reducing means, and shift control means, pressure reduced by the pressure reducing means and hydraulic pressure supplied as a result of port conversion of the shift control means being supplied to hydraulic pressure control means, and hydraulic pressure controlled by the hydraulic pressure control means being supplied to hydraulic pressure distribution means and directly to selective friction elements to engage the same.  
           [0012]    The shift control means includes a manual valve which is indexed with a driver-controlled select lever to supply hydraulic pressure through five different lines to the hydraulic pressure control means, the hydraulic pressure distribution means, and the friction elements, the hydraulic pressure control means includes first, second and third pressure control valves which are controlled by pressure controlled by first, second and third solenoid valves, the first pressure control valve controlling hydraulic pressure to be directly supplied to a first clutch operating in forward shift modes, the second pressure control valve controlling hydraulic pressure to be supplied to a second brake operating in second and fourth speeds of a drive D range, and the third pressure control valve controlling hydraulic pressure to be supplied to a first brake operating in a low L range and a reverse R range and to a third clutch operating in third and fourth speeds of the drive D range such that hydraulic pressure is supplied to two valves of the hydraulic pressure distribution means.  
           [0013]    The hydraulic pressure distribution means comprises a low control valve for supplying L-range pressure to another valve as control pressure in the low L range such that control pressure of the third pressure control valve is supplied to the first brake; an N-R control valve for supplying reverse pressure to the first brake by reduced pressure supplied via the pressure control valve when shifting into the reverse R range; a line pressure control switch valve controlled by D-range pressure and hydraulic pressure supplied to a second clutch, the line pressure control switch valve supplying a part of the hydraulic pressure supplied to the second clutch to a pressure regulator valve; a first fail-safe valve controlled by control pressure supplied from the low control valve and from the second pressure control valve, the first fail-safe valve undergoing port conversion to supply hydraulic pressure supplied from the third pressure control valve to the second clutch in the third and fourth speeds of the drive D range, and hydraulic pressure supplied from the third pressure control valve to the first brake in the low L range; a second fail-safe valve controlled by reverse pressure, hydraulic pressure supplied to a fourth clutch, and hydraulic pressure supplied to the third clutch, the second fail-safe valve supplying hydraulic pressure supplied from the second pressure control valve to the second brake in the second and fourth speeds of the drive D range; and an engine brake control switch valve for supplying D-range pressure to the fourth clutch in the first and third speeds of the drive D range.  
           [0014]    According to a feature of the present invention, the manual valve is connected to a reverse R range pressure line directly communicated with the third clutch; a forward pressure line connected to the pressure regulator valve; a drive D range pressure line connected to the first, second and third pressure control valves, the second fail-safe valve, and the engine brake control switch valve; a low D 2  range pressure line connected to the fourth clutch with a 3-way valve interposed therebetween; and a low L range pressure line connected to the low control valve.  
           [0015]    According to another feature of the present invention, the first, second and third solenoid valves are 3-way valves which maintain a closed state when controlled to OFF.  
           [0016]    According to yet another feature of the present invention, a valve body of the first pressure control valve includes a first port for receiving hydraulic pressure reduced by a reducing valve; a second port for receiving hydraulic pressure from the manual valve; a third port for supplying the hydraulic pressure received by the second port to the first clutch; and a fourth port for receiving control pressure from the first solenoid valve; a valve spool of the first pressure control valve slidably provided in the valve body of the first pressure control valve includes a first land on which the hydraulic pressure supplied through the first port acts, the first land having a relatively small diameter; a second land on which the hydraulic pressure supplied through the first port acts to selectively open and close the second port; and a third land which, together with the second land, selectively communicates the second port and the third port; and an elastic member is disposed between the third land and the valve body.  
           [0017]    According to still yet another feature of the present invention, a valve body of the second pressure control valve includes a first port for receiving hydraulic pressure reduced by a reducing valve; a second port for receiving hydraulic pressure from the manual valve; a third port for supplying the hydraulic pressure supplied to the second port to the first fail-safe valve and the engine brake control switch valve; a fourth port for receiving control pressure from the second solenoid valve; and a fifth port for supplying as control pressure the control pressure supplied to the fourth port; a valve spool of the second pressure control valve slidably provided in the valve body of the second pressure control valve includes a first land on which the hydraulic pressure supplied through the first port acts, the first land having a relatively small diameter; a second land on which the hydraulic pressure supplied through the first port acts to selectively open and close the second port; and a third land which, together with the second land, selectively communicates the second port and the third port; and an elastic member is disposed between the third land and the valve body.  
           [0018]    According to still yet another feature of the present invention, a valve body of the third pressure control valve includes a first port for receiving hydraulic pressure reduced by a reducing valve; a second port for receiving hydraulic pressure from the manual valve; a third port for supplying the hydraulic pressure supplied to the second port to the first and second fail-safe valves; and a fourth port for receiving control pressure from the third solenoid valve; a valve spool of the third pressure control valve slidably provided in the valve body of the third pressure control valve includes a first land on which the hydraulic pressure supplied through the first port acts, the first land having a relatively small diameter; a second land on which the hydraulic pressure supplied through the first port acts to selectively open and close the second port; and a third land which, together with the second land, selectively communicates the second port and the third port; and an elastic member is disposed between the third land and the valve body.  
           [0019]    According to still yet another feature of the present invention, a valve body of the low control valve includes a first port receiving low L range pressure; a second port for supplying to the first fail-safe valve the hydraulic pressure supplied to the first port; a third port for receiving from the first fail-safe valve the hydraulic pressure supplied to the second clutch; and a fourth port for receiving as control pressure a part of the hydraulic pressure supplied to the first brake; a valve spool of the low control valve slidably provided in the valve body of the low control valve includes a first land on which the control pressure supplied to the third port acts to selectively open and close the first port; and a second land for selectively communicating the second port with an exhaust port; and an elastic member is disposed between the second land and the valve body.  
           [0020]    According to still yet another feature of the present invention, a valve body of the N-R control valve includes a first port communicated with the second pressure control valve; a second port communicated with a reverse R range pressure line; and a third port for selectively supplying the hydraulic pressure supplied to the second port to the first brake via the first fail-safe valve; a valve spool of the N-R control valve slidably provided in the valve body of the N-R control valve includes a first land on which the hydraulic pressure supplied to the first port acts; and a second land for opening and closing the second and third ports; and an elastic member is disposed between the second land and the valve body.  
           [0021]    According to still yet another feature of the present invention, a valve body of the line pressure control switch valve includes a first port for receiving drive D range pressure; a second port for receiving second clutch pressure; and a third port for supplying the hydraulic pressure supplied to the second port as variable line pressure to the pressure regulator valve; and a valve spool of the line pressure control switch valve slidably provided in the valve body of the line pressure control switch valve includes a first land on which the hydraulic pressure supplied to the first port acts; and a second land for selectively communicating the second and third ports.  
           [0022]    According to still yet another feature of the present invention, a surface area of the second land on which hydraulic pressure acts is larger than a surface area of the first land on which hydraulic pressure acts.  
           [0023]    According to still yet another feature of the present invention, a valve body of the first fail-safe valve includes a first port for receiving control pressure from the low control valve; a second port for receiving hydraulic pressure from the third pressure control valve; a third port for receiving hydraulic pressure from the N-R control valve; a fourth port for selectively supplying the hydraulic pressure supplied to the second port to the second clutch; fifth and sixth ports for supplying the hydraulic pressure supplied to the third port to the first brake and the low control valve; and a seventh port for receiving control pressure from the second pressure control valve; a valve spool of the first fail-safe valve slidably provided in the valve body of the first fail-safe valve includes a first land on which the control pressure received through the first port acts; a second land selectively communicating the second port and the fourth port, and, together with the first land, communicating the third port with the fifth and sixth ports; and a third land on which the control pressure received through the seventh port acts; and an elastic member is disposed between the third land and the valve body.  
           [0024]    According to still yet another feature of the present invention, a valve body of the second fail-safe valve includes first, second, third and fourth ports connected respectively to a reverse R range pressure line, the fourth clutch, the third pressure control valve and a drive D range pressure line; a fifth port for receiving hydraulic pressure from the second pressure control valve; and a sixth port for supplying the hydraulic pressure supplied to the fifth port to the second brake; a valve spool of the second fail-safe valve slidably provided in the valve body of the second fail-safe valve includes first, second and third lands on which the hydraulic pressure supplied respectively through the first, second and third ports acts; a fourth land selectively communicating the sixth port with an exhaust port and the fifth port; a fifth land selectively communicating the fifth port with the sixth port; and a sixth land on which control pressure received through the fourth port acts, the sixth land being surrounded by a sleeve of the valve body.  
           [0025]    According to still yet another feature of the present invention, a valve body of the engine brake control switch valve includes a first port for receiving control pressure from the second pressure control valve; a second port connected to a low D 2  range pressure line, the low D 2  range pressure line being connected to the manual valve; and a third port for supplying the hydraulic pressure supplied to the second port to the fourth clutch; a valve spool of the engine brake control switch valve slidably provided in the valve body of the engine brake control switch valve includes a first land on which the hydraulic pressure supplied through the first port acts; and a second land selectively communicating the second port and the third port; and an elastic member is disposed between the second land and the valve body.  
           [0026]    According to still yet another feature of the present invention, the fourth clutch receives D-range pressure either via the engine brake control switch valve or directly from a low D 2  range pressure line.  
           [0027]    According to still yet another feature of the present invention, the engine brake control switch valve and the low D 2  range pressure line are connected to the fourth clutch with a 3-way valve interposed therebetween.  
           [0028]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention:  
         [0030]    [0030]FIG. 1 is a schematic view of a powertrain to which the present invention is applied;  
         [0031]    [0031]FIG. 2 is a chart illustrating engaged and disengaged states of friction elements of the powertrain of FIG. 1 according to various speeds and shift modes;  
         [0032]    [0032]FIG. 3 is a hydraulic circuit diagram in a neutral N range of a hydraulic control system according to a preferred embodiment of the present invention;  
         [0033]    [0033]FIG. 4 is a schematic sectional view of a manual valve of the hydraulic control system of FIG. 3;  
         [0034]    [0034]FIG. 5 is a schematic sectional view of elements used to control hydraulic pressure in the hydraulic control system of FIG. 3;  
         [0035]    [0035]FIG. 6 is a schematic sectional view of elements used to control the distribution of hydraulic pressure in the hydraulic control system of FIG. 3;  
         [0036]    [0036]FIG. 7 is a hydraulic circuit diagram in a first speed of a drive D range of the hydraulic control system of FIG. 3;  
         [0037]    [0037]FIG. 8 is a hydraulic circuit diagram in a second speed of the drive D range of the hydraulic control system of FIG. 3;  
         [0038]    [0038]FIG. 9 is a hydraulic circuit diagram in a third speed of the drive D range of the hydraulic control system of FIG. 3;  
         [0039]    [0039]FIG. 10 is a hydraulic circuit diagram in a fourth speed of the drive D range of the hydraulic control system of FIG. 3;  
         [0040]    [0040]FIG. 11 is a hydraulic circuit diagram in a second speed of a low D 2  range of the hydraulic control system of FIG. 3;  
         [0041]    [0041]FIG. 12 is a hydraulic circuit diagram in a first speed of a low L range of the hydraulic control system of FIG. 3; and  
         [0042]    [0042]FIG. 13 is a hydraulic circuit diagram in a reverse R range of the hydraulic control system of FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]    Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.  
         [0044]    [0044]FIG. 1 shows a schematic view of a powertrain to which the present invention is applied.  
         [0045]    The powertrain utilizes two single pinion planetary gearsets to obtain first, second, third and fourth forward speeds and a reverse speed. In more detail, torque from an engine  2  is transmitted to an input shaft  6  through a torque converter  4 , and the input shaft  6  transmits the engine torque to first and second single pinion planetary gearsets  8  and  10 . Shifting into the different ranges and speeds is realized by the operation of the first and second pinion planetary gearsets  8  and  10 , after which output is realized via a transfer drive gear  14 .  
         [0046]    The first single pinion planetary gearset  8  includes a first sun gear  16 , a first planet carrier  12  and a first ring gear  18 . The second single pinion planetary gearset includes a second sun gear  20 , a second planet carrier  22  and a second ring gear  24 . Output through the transfer drive gear  14  is realized by the connection of the transfer drive gear  14  to the first planet carrier  12  of the first single pinion planetary gearset  8 .  
         [0047]    In a state where the first planet carrier  12  is fixedly connected to the second ring gear  24 , the first sun gear  16  is connected to the input shaft  6  with a first clutch C 1  interposed therebetween, the first clutch C 1  being engaged in all forward speeds. The second planet carrier  22  is connected to the input shaft  6  with a second clutch C 2  interposed therebetween, the second clutch C 2  being engaged in the forward third and fourth speeds. Also, the second sun gear  20  is connected to the input shaft  6  with a third clutch C 3  interposed therebetween, the third clutch C 3  being engaged in a reverse R range.  
         [0048]    Further, the second planet carrier  22  is connected to a transmission housing  26  with a first brake B 1  and a first one-way clutch F 1  interposed therebetween, the first brake B 1  and the first one-way clutch F 1  being mounted in parallel. The second planet carrier  22  is also connected to the first ring gear  18  with a fourth clutch C 4  and a second one-way clutch F 2  interposed therebetween, the fourth clutch C 4  and the second one-way clutch F 2  being mounted in parallel. In addition, the second sun gear  20  is connected to the transmission housing  26  with a second brake B 2  interposed therebetween.  
         [0049]    The friction elements of the powertrain described above are engaged and disengaged to effect shifting as shown in FIG. 2.  
         [0050]    That is, to realize shifting into the forward first speed, the first clutch C 1  and the first and second one-way clutches F 1  and F 2  are engaged such that the first sun gear  16  operates as an input element, and the first ring gear  18  and the second planet carrier  22  operate as reaction elements. To realize shifting into the forward second speed from the forward first speed, the second brake B 2  is additionally engaged such that the first sun gear  18  acts as an input element and the second sun gear  20  acts as a reaction element.  
         [0051]    Further, to effect shifting into the forward third speed from the forward second speed, the second clutch is engaged and the second brake B 2  is disengaged such that the first and second pinion planetary gearsets  8  and  10  are directly connected, thereby realizing output that is of the same rotational speed as input. From the third speed, shifting into the forward fourth speed is realized by engaging the second brake B 2  such that the second sun gear  20  acts as a reaction element. This results in an overdrive state.  
         [0052]    To realize shifting into the reverse R range, the third clutch C 3  and the first brake B 1  are engaged such that the second sun gear  20  acts as an input element and the second planet carrier  22  acts as a reaction element, resulting in shifting into the reverse R range.  
         [0053]    [0053]FIG. 3 is a hydraulic circuit diagram in a neutral N range of the hydraulic control system according to the present invention. Like reference numerals are used for the elements described above.  
         [0054]    The hydraulic control system includes the torque converter  4  which acts as a fluid link between the engine  2  and a transmission, and an oil pump  100  which generates hydraulic flow such that hydraulic pressure is created in the hydraulic control system. The created hydraulic pressure is supplied to pressure/damper clutch control means, pressure reducing means, and shift control means.  
         [0055]    The pressure/damper clutch control means includes a pressure control valve  104  for controlling to a predetermined level the hydraulic pressure created by the operation of the oil pump  100 ; a torque converter control valve  106  for supplying the hydraulic pressure received from the pressure control valve  104  to the torque converter  4  and to locations to be used for lubrication; and a damper clutch control valve  108  for controlling a damper clutch such that the power transmission efficiency of the torque converter  4  is increased.  
         [0056]    The pressure reducing means includes a reducing valve  110  for reducing hydraulic pressure passing therethrough to a level lower than line pressure. A part of the hydraulic pressure reduced by the reducing valve  110  is supplied as control pressure of the damper clutch control valve  108 . A part of the hydraulic pressure reduced by the reducing valve is also supplied to hydraulic pressure control means, the hydraulic pressure control means including first, second and third pressure control valves  112 ,  114  and  116  for forming hydraulic pressure for use as shift range control pressure, and including first, second and third solenoid valves S 1 , S 2  and S 3  for controlling the first, second and third pressure control valves  112 ,  114  and  116 , respectively.  
         [0057]    The shift control means includes a manual valve  118  which is indexed with a driver-controlled select lever to undergo port conversion. According to the shift range selected by the driver, hydraulic pressure supplied to the manual valve  118  is (a) controlled by the hydraulic pressure control means; (b) supplied directly to a low control valve  120  and an N-R control valve  122 , a line pressure control switch valve  124  for controlling line pressure, first and second fail-safe valves  126  and  128 , and a brake control switch valve  130 , the low control valve  120 , the N-R control valve  122 , the line pressure control switch valve  124 , the first and second fail-safe valves  126  and  128 , and the brake control switch valve  130  comprising hydraulic pressure distribution means; or (c) supplied directly to the friction elements.  
         [0058]    The manual valve  118 , as shown in FIG. 4, is connected to a reverse R range pressure line  132 , a forward pressure line  134 , a drive D range pressure line  136 , a low D 2  range pressure line  138 , and a low L range pressure line  140 . Hydraulic pressure supplied from the oil pump  100  is selectively supplied to these lines according to the shift range selected by the driver. The reverse R range pressure line  132  is directly communicated with the third clutch C 3 , which is engaged in the reverse R range; the forward pressure line  134  is connected to a pressure regulator valve  102 ; the drive D range pressure line  136  is connected to the first, second and third pressure control valves  112 ,  114  and  116 , the line pressure control switch valve  124 , the second fail-safe valve  128 , and the brake control switch valve  130 ; the low D 2  range pressure line  138  is connected to the fourth clutch C 4  with a 3-way valve  142  interposed therebetween; and the low L range pressure line  140  is connected to the low control valve  120 .  
         [0059]    As shown in FIG. 5, a valve body of the first pressure control valve  112  of the hydraulic pressure control means includes a first port  150  for receiving the hydraulic pressure reduced by the reducing valve  110 ; a second port  152  for receiving hydraulic pressure from the manual valve  118 ; a third port  154  for supplying the hydraulic pressure received by the second port  152  to the first clutch C 1 ; and a fourth port  156  for receiving control pressure from the first solenoid valve S 1 .  
         [0060]    A valve spool is slidably provided in the valve body of the first pressure control valve  112 . The valve spool includes a first land  158  on which the hydraulic pressure supplied through the first port  150  acts, the first land  158  having a relatively small diameter; a second land  160  on which the hydraulic pressure supplied through the first port  150  acts to selectively open and close the second port  152 ; and a third land  162  which, together with the second land  160 , selectively communicates the second port  152  and the third port  154 . Further, an elastic member  164  is disposed between the third land  162  and the valve body, the elastic member  164  providing a biasing force to the valve spool in a leftward direction (in the drawing).  
         [0061]    As shown in FIGS. 3 and 5, the first solenoid valve S 1 , which controls the first pressure control valve  112 , is a 3-way valve. When the first solenoid valve S 1  is controlled to on, the hydraulic pressure supplied as control pressure to the first pressure control valve  112  is exhausted in a state where the supply of reduced pressure to the first pressure control valve  112  is blocked. On the other hand, when the first solenoid valve Si is controlled to off, an exhaust port of the first solenoid valve Si is closed and a passage to enable to supply of reduced pressure to the first pressure control valve  112  is formed. A more detailed description of the first solenoid valve S 1  will not be provided herein as the structure and operation of the first solenoid valve S 1  are well known in the art.  
         [0062]    Accordingly, when the first solenoid valve S 1  is controlled to on, the valve spool of the first pressure control valve  112  is displaced to the right (in the drawing) such that the second port  152  is closed. However, if the first solenoid valve S 1  is controlled to off, control pressure is supplied to the first pressure control valve  112  such that the valve spool of the same is displaced to the left (in the drawing), thereby resulting in the communication of the second port  152  and the third port  154 . This results in hydraulic pressure being supplied to the first clutch C 1 .  
         [0063]    Again with reference to FIG. 5, a valve body of the second pressure control valve  114  of the hydraulic pressure control means includes a first port  170  for receiving hydraulic pressure reduced by the reducing valve  110 ; a second port  172  for receiving hydraulic pressure from the manual valve  118 ; a third port  174  for supplying the hydraulic pressure supplied to the second port  172  to the first fail-safe valve  126  and the brake control switch valve  130 ; a fourth port  176  for receiving control pressure from the second solenoid valve S 2 ; and a fifth port  178  for supplying as control pressure the control pressure supplied to the fourth port  176 .  
         [0064]    A valve spool is slidably provided in the valve body of the second pressure control valve  114 . The valve spool includes a first land  180  on which the hydraulic pressure supplied through the first port  170  acts, the first land  180  having a relatively small diameter; a second land  182  on which the hydraulic pressure supplied through the first port  170  acts to selectively open and close the second port  172 ; and a third land  184  which, together with the second land  182 , selectively communicates the second port  172  and the third port  174 . Further, an elastic member  186  is disposed between the third land  184  and the valve body, the elastic member  164  providing a biasing force to the valve spool in a leftward direction (in the drawing).  
         [0065]    With the above structure, when the second solenoid valve S 2  is controlled to on, the valve spool of the second pressure control valve  114  is displaced to the right (in the drawing) such that the second port  172  is closed. However, if the second solenoid valve S 2  is controlled to off, control pressure is supplied to the second pressure control valve  114  such that the valve spool of the same is displaced to the left (in the drawing), thereby resulting in the communication of the second port  172  and the third port  174 .  
         [0066]    A valve body of the third pressure control valve  116 , as shown in FIG. 5, includes a first port  190  for receiving hydraulic pressure reduced by the reducing valve  110 ; a second port  192  for receiving hydraulic pressure from the manual valve  118 ; a third port  194  for supplying the hydraulic pressure supplied to the second port  192  to the first and second fail-safe valves  126  and  128 ; and a fourth port  196  for receiving control pressure from the third solenoid valve S 3 .  
         [0067]    A valve spool is slidably provided in the valve body of the third pressure control valve  116 . The valve spool includes a first land  198  on which the hydraulic pressure supplied through the first port  190  acts, the first land  198  having a relatively small diameter; a second land  200  on which the hydraulic pressure supplied through the first port  190  acts to selectively open and close the second port  192 ; and a third land  202  which, together with the second land  200 , selectively communicates the second port  192  and the third port  194 . Further, an elastic member  204  is disposed between the third land  202  and the valve body, the elastic member  204  providing a biasing force to the valve spool in a leftward direction (in the drawing).  
         [0068]    With the above structure, when the second solenoid valve S 3  is controlled to on, the valve spool of the third pressure control valve  116  is displaced to the right (in the drawing) such that the second port  192  is closed. However, if the third solenoid valve S 3  is controlled to off, control pressure is supplied to the third pressure control valve  116  such that the valve spool of the same is displaced to the left (in the drawing), thereby resulting in the communication of the second port  192  and the third port  194 .  
         [0069]    The low control valve  120  is either controlled by hydraulic pressure supplied to the second clutch C 2  in third and fourth speeds or by a part of the hydraulic pressure supplied to the first brake B 1  in the reverse R range, and acts to supply the hydraulic pressure supplied from the manual valve  118  as control pressure to the first fail-safe valve  126  in the low L range.  
         [0070]    A valve body of the low control valve  120 , as shown in FIG. 6, includes a first port  210  receiving low L range pressure; a second port  212  for supplying to the first fail-safe valve  126  the hydraulic pressure supplied to the first port  210 ; a third port for receiving from the first fail-safe valve  126  the hydraulic pressure supplied to the second clutch C 2 ; and a fourth port  216  for receiving as control pressure a part of the hydraulic pressure supplied to the first brake B 1 .  
         [0071]    A valve spool is slidably provided in the valve body of the low control valve  120 . The valve spool includes a first land  218  on which the control pressure supplied to the third port  214  acts to selectively open and close the first port  210 ; and a second land  220  for selectively communicating the second port  212  with an exhaust port. Further, an elastic member  222  is disposed between the second land  220  and the valve body, the elastic member  222 , together with the control pressure supplied through the fourth port  216 , providing a biasing force to the valve spool in a leftward direction (in the drawing).  
         [0072]    The N-R control valve  122  is controlled in the reverse R range by the control pressure supplied to the second pressure control valve  114  and acts to supply the hydraulic pressure supplied to the manual valve  118  to the first brake B 1 . A valve body of the N-R control valve  122 , with reference to FIG. 6, includes a first port  230  communicated with the second pressure control valve  114 ; a second port  232  communicated with the reverse R range pressure line  132 ; and a third port  234  for selectively supplying the hydraulic pressure supplied to the second port  232  to the first brake B 1  via the first fail-safe valve  126 .  
         [0073]    A valve spool is slidably provided in the valve body of the N-R control valve  122 . The valve spool includes a first land  236  on which the hydraulic pressure supplied to the first port  230  acts; and a second land  238  for opening and closing the second and third ports  232  and  234 . Further, an elastic member  240  is disposed between the second land  238  and the valve body, the elastic member  240  providing a biasing force to the valve spool in a rightward direction (in the drawing).  
         [0074]    A valve body of the line pressure control switch valve  124 , with reference to FIG. 6, includes a first port  250  for receiving drive D range pressure; a second port  252  for receiving second clutch pressure; and a third port  254  for supplying the hydraulic pressure supplied to the second port  252  as variable line pressure to the pressure regulator valve  104 . A valve spool is slidably provided in the valve body of the line pressure control switch valve  124 . The valve spool includes a first land  256  on which the hydraulic pressure supplied to the first port  250  acts; and a second land  258  for selectively communicating the second and third ports  252  and  254 .  
         [0075]    Accordingly, variations in line pressure are realized in the third and fourth speeds of the drive D range. At this time, although identical levels of hydraulic pressure enter through the first and second ports  250  and  252 , since a surface area of the second land  258  on which hydraulic pressure acts is greater than a surface area of the first land  256  on which hydraulic pressure acts, the valve spool is displaced to the left (in the drawing) such that the second and third ports  252  and  254  are communicated. As a result, line pressure is able to be controlled.  
         [0076]    The first fail-safe valve  126  is controlled by control pressure supplied from the low control valve  120  and by control pressure supplied from the second pressure control valve  114 . In the third and fourth speeds of the drive D range, the first fail-safe valve  126  undergoes port conversion to supply the hydraulic pressure supplied from the third pressure control valve  116  to the second clutch C 2 , and in the low L range, the first fail-safe valve  126  undergoes port conversion to supply the hydraulic pressure supplied from the third pressure control valve  116  to the first brake B 1 .  
         [0077]    A valve body of the first fail-safe valve  126 , with reference to FIG. 6, includes a first port  270  for receiving control pressure from the low control valve  120 ; a second port  272  for receiving hydraulic pressure from the third pressure control valve  116 ; a third port  274  for receiving hydraulic pressure from the N-R control valve  122 ; a fourth port  276  for selectively supplying the hydraulic pressure supplied to the second port  272  to the second clutch C 2 ; fifth and sixth ports  278  and  280  for supplying the hydraulic pressure supplied to the third port  274  to the first brake B 1  and the low control valve  120 ; and a seventh port  282  for receiving control pressure from the second pressure control valve  114 .  
         [0078]    A valve spool is slidably provided in the valve body of the first fail-safe valve  126 . The valve spool includes a first land  284  on which the control pressure received through the first port  270  acts; a second land  286  selectively communicating the second port  272  and the fourth port  276 , and, together with the first land  284 , communicating the third port  274  with the fifth and sixth ports  278  and  280 ; and a third land  288  on which the control pressure received through the seventh port  282  acts. Further, an elastic member  290  is disposed between the third land  288  and the valve body, the elastic member  290  providing a biasing force to the valve spool in a leftward direction (in the drawing).  
         [0079]    The second fail-safe valve  128  selectively supplies hydraulic pressure supplied from the second pressure control valve  114  to the second brake B 2 . A valve body of the second fail-safe valve  128 , with reference to FIG. 6, includes first, second, third and fourth ports  300 ,  302 ,  304  and  306  connected respectively to the reverse R range pressure line  132 , the fourth clutch C 4 , the third pressure control valve  116  and the drive D range pressure line  136 ; a fifth port  308  for receiving hydraulic pressure from the second pressure control valve  114 ; and a sixth port for supplying the hydraulic pressure supplied to the fifth port  308  to the second brake B 2 .  
         [0080]    A valve spool is slidably provided in the valve body of the second fail-safe valve  128 . The valve spool includes first, second and third lands  312 ,  314  and  316  on which the hydraulic pressure supplied respectively through the first, second and third ports  300 ,  302  and  304  acts; a fourth land  318  selectively communicating the sixth port  310  with an exhaust port and the fifth port  308 ; a fifth land  320  selectively communicating the fifth port  308  with the sixth port  310 ; and a sixth land  322  on which control pressure received through the fourth port  306  acts, the sixth land  322  being surrounded by a sleeve  324  of the valve body.  
         [0081]    The brake control switch valve  130  supplies hydraulic pressure supplied from the manual valve  118  to the fourth clutch C 4  in the first and third speeds of the drive D range and in the low L range. A valve body of the brake control switch valve  130 , with reference to FIG. 6, includes a first port  330  for receiving control pressure from the second pressure control valve  114 ; a second port  332  connected to the low D 2  range pressure line  138 , the low D 2  range pressure line  138  being connected to the manual valve  118 ; and a third port  334  for supplying the hydraulic pressure supplied to the second port  332  to the fourth clutch C 4 . Also, the 3-way valve  142  is provided between the low D 2  range pressure line  138  and the brake control switch valve  130 .  
         [0082]    A valve spool is slidably provided in the valve body of the brake control switch valve  130 . The valve spool includes a first land  336  on which the hydraulic pressure supplied through the first port  330  acts; and a second land  338  selectively communicating the second port  332  and the third port  334 . An elastic member  340  is disposed between the second land  338  and the valve body, the elastic member  340  providing a biasing force to the valve spool in a leftward direction (in the drawing).  
         [0083]    In the neutral N range of the hydraulic control system of the present invention structured as in the above, as shown in FIG. 3, the hydraulic pressure generated as a result of the hydraulic flow created by the oil pump  100  is controlled to a predetermined level of hydraulic pressure by the pressure control valve  104 . The hydraulic pressure is then reduced by passing through the reducing valve  110  then supplied to each the damper clutch control valve  108 , and the first, second and third pressure control valves  112 ,  114  and  116 . At this time, the first and second solenoid valves S 1  and S 2  are controlled to OFF by the ECU, thereby resulting in the flow of hydraulic pressure as shown in FIG. 3.  
         [0084]    In the first speed of the drive D range, as shown in FIG. 7, the hydraulic pressure supplied from the manual valve  118  is supplied to the pressure control valve  104  via the forward pressure line  134 , and, at the same time, is supplied via the drive D range pressure line  136  to the brake control switch valve  130  and the first, second and third pressure control valves  112 ,  114  and  116 . At this time, since the first solenoid valve S 1  is duty controlled, the hydraulic pressure is controlled by the first pressure control valve  112  and supplied to the first clutch C 1 , and the hydraulic pressure supplied to the brake control switch valve  130  is supplied to the fourth clutch C 4  as a result of the leftward displacement of the valve spool of the brake control switch valve  130 .  
         [0085]    If vehicle speed and throttle opening are increased in the first speed of the drive D range, shifting into the second speed of the drive D range is performed. That is, as shown in FIG. 8, the second solenoid valve S 2 , which is controlled to ON by the ECU in the first speed of the drive D range, is duty controlled such that the control pressure of the second pressure control valve  114  is supplied to the first and second fail-safe valves  126  and  128 , and to the brake control switch valve  130 . At this time, the valve spool of the second fail-safe valve  128  is displaced to the left by the hydraulic pressure supplied from the manual valve  118  such that the pressure of the second pressure control valve  114 , supplied to the second fail-safe valve  128 , is supplied to the second brake B 2 . Also, the valve spool of the brake control switch valve  130  is displaced to the right by the pressure of the second pressure control valve  114  such that the supply of hydraulic pressure to the fourth clutch C 4  is discontinued, thereby realizing shifting into the second speed of the drive D range.  
         [0086]    If vehicle speed and throttle opening are increased in the second speed of the drive D range, shifting into the third speed of the drive D range is performed. That is, as shown in FIG. 9, the second solenoid valve S 2  is controlled to ON and the third solenoid valve S 3  is controlled to OFF by the ECU. Accordingly, the hydraulic pressure supplied from the second pressure control valve  114  is discontinued such that the second brake B 2  is disengaged, and, at the same time, the control pressure supplied to the brake control switch valve  130  is released. This results in the valve spool of the brake control switch valve  130  being displaced to the left such that the hydraulic pressure supplied from the manual valve  118  is supplied to the fourth clutch C 4 .  
         [0087]    Further, the pressure of the third pressure control valve  116  passes through the first fail-safe valve  126  to be supplied to the second clutch C 2 , thereby realizing shifting into the third speed. At this time, part of the hydraulic pressure supplied to the second clutch C 2  passes through the line pressure control switch valve  124  and is supplied to the pressure control valve  104  such that line pressure control is realized.  
         [0088]    If vehicle speed and throttle opening are increased in the third speed of the drive D range, shifting into the fourth speed of the drive D range is performed. That is, as shown in FIG. 10, the second solenoid valve S 2  is controlled to OFF such that hydraulic pressure is supplied to the second brake B 2  after passing through the second fail-safe valve  128 . Also, part of the pressure of the second pressure control valve  114  is supplied as control pressure to the brake control switch valve  130  such that the hydraulic pressure supplied to the fourth clutch C 4  is discontinued, thereby realizing shifting into the fourth speed.  
         [0089]    Line pressure control also occurs during the above control, and since the first, second and third solenoid valves S 1 , S 2  and S 3  are all controlled to OFF in the fourth speed of the drive D range, if a malfunction occurs in the drive D range, the fourth speed is held by the OFF control of the first, second and third solenoid valves S 1 , S 2  and S 3 .  
         [0090]    In the low D 2  range, with reference to FIG. 11, hydraulic flow is realized identically as in the second speed of the drive D range. At this time, hydraulic pressure supplied to the low D 2  range pressure line  138  from the manual valve  118  is supplied to the fourth clutch C 4  via the 3-way valve  142  while the engine brake is operated. If a malfunction occurs in the transmission while in the low D 2  range, the third solenoid valve S 3  is controlled to OFF such that hydraulic pressure of the third solenoid valve S 3  is supplied to the second clutch C 2 , and, at the same time, controls the second fail-safe valve  128 . As a result, the hydraulic pressure supplied to the second brake B 2  is discontinued, thereby holding the third speed of the drive D range.  
         [0091]    In the present invention, by holding the third speed in the low D 2  range and the fourth speed in the drive D range when a malfunction occurs in the transmission, overall stability and drive performance are improved.  
         [0092]    In the first speed of the low L range, as shown in FIG. 12, the first and third solenoid valves S 1  and S 3  are controlled to OFF such that the hydraulic pressure of the first pressure control valve  112  is supplied to the first clutch C 1  and the hydraulic pressure of the third pressure control valve  116  is supplied to the first fail-safe valve  126 . At this time, since the hydraulic pressure of the manual valve  118  is supplied to the first fail-safe valve  126  via the low control valve  120 , the valve spool of the low control valve  120  is displaced to the right such that the hydraulic pressure of the third pressure control valve  116  is supplied to the first brake B 1 . Also, line pressure of the manual valve  118  is supplied to the fourth clutch C 4  via the brake control switch valve  130 .  
         [0093]    In the reverse R range, as shown in FIG. 13, part of the hydraulic pressure of the reverse R range pressure line  132  of the manual valve  118  is supplied directly to the third clutch C 3 , and that portion of hydraulic pressure is controlled by the N-R control valve  122  and supplied to the first brake B 1  via the first fail-safe valve  126 . At this time, the hydraulic pressure supplied to the first brake B 1  is controlled while the N-R control valve  124  is controlled by the control pressure of the second solenoid valve S 2 .  
         [0094]    In addition to the above shift operations, downshifting from the fourth to the third speeds, from the third to the second speeds, and from the second to the first speeds, in addition to skip shifting from the fourth to the second speeds is also possible. However, since such shift operations fall within the scope of the present invention presented above, a detailed description will not be provided herein.  
         [0095]    In the hydraulic control system of the present invention applied to a powertrain utilizing two single pinion planetary gearsets, four clutches, two one-way clutches, and two brakes to obtain four forward speeds and one reverse speed, an engine brake is operated at only a high stage of each range such that drive performance is improved. Also, a fail-safe mode is provided by the hydraulic control system of the present invention by holding the fourth speed in the drive D range and the third speed in the low D 2  range such that drive performance is improved.  
         [0096]    Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.