Patent Publication Number: US-2006005789-A1

Title: Flow control valve for engine cooling water

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is based on Japanese Patent Application No. 2004-205097 filed on Jul. 12, 2004, the disclosures of which is incorporated herein by reference.  
     FIELD OF THE INVENTION  
      The present invention relates to a flow control valve to be used in an engine cooling system for a water-cooled engine, in which engine cooling water is circulated to the engine after having been cooled at a radiator. In particular, the present invention relates to a flow control valve for optimizing a temperature of the engine cooling water, by adjusting a flow amount of the engine cooling water flowing through the radiator and a flow amount of the engine cooling water bypassing the radiator, depending on an operational condition of the engine.  
     BACKGROUND OF THE INVENTION  
      An engine cooling system for a water-cooled engine is known in the art as a system for cooling an engine mounted in a vehicle having a radiator, in which engine cooling water is circulated into the engine after having been cooled at the radiator. A thermostat is provided in such an engine cooling system, so that the engine cooling water bypasses the radiator by an operation of the thermostat when the temperature of the cooling water is lower than a predetermined value and is circulated back to the engine through a water pump.  
      In recent years, two opposing requirements for an engine, such as a higher engine output and a lower fuel consumption ratio, have been increased. An engine cooling system, which could meet such requirements for the engine, is accordingly desired. Namely, it is necessary to increase a cooling efficiency at the engine by decreasing the cooling water temperature, and to maintain the temperature at every portion of the engine at such a temperature lower than a durability limit temperature with respect to a thermal load, in order to realize the higher engine output. On the other hand, it is necessary to increase a combustion efficiency at a combustion chamber of the engine, by increasing the cooling water temperature, in order to achieve the lower fuel consumption ratio. As above, such a engine cooling system is required, which could control the cooling water temperature in accordance with various operational conditions of the engine, for example, a high-speed high-load operation of the engine which is a high-speed running of the vehicle with the high engine output, a low-speed high-load operation during the vehicle is running on an up-hilling road, a low-speed low-load operation or a normal operation which is an operation for the low fuel consumption ratio, a re-starting operation of the engine after an engine stop (an engine idling stop operation) for the purpose of a lower harmful emission and the low fuel consumption ratio, and so on.  
      An engine cooling system for the water-cooled engine has been proposed in the art, in which a flow control valve is provided to make it possible to control the cooling water temperature depending on the various operational conditions of the engine. According to such a cooling system, a flow control valve is provided at an interfluent portion of a cooling water circuit and a bypass circuit, so that a flow amount of the cooling water to the radiator (hereinafter also called as “the radiator flow amount”) and a flow amount of the cooling water bypassing the radiator (hereinafter also called as “the bypass flow amount”) are precisely controlled. The flow control valve can control the cooling water temperature more precisely than the control valve operated by the thermostat, and thereby the lower fuel consumption ratio is achieved.  
      However, an extremely high fluid pressure is applied by a water pump to a valve body of the flow control valve provided in the engine cooling system, when the valve body is operated by an actuator, independently whether the engine operation is in the high-load operation or in the normal operation. Accordingly, a large operating force or driving torque is necessary for a driving shaft of the actuator to move the valve body. The actuator becomes larger in its size or higher in cost, when it is necessary to provide a reduction device (a gear reduction device) between the driving shaft of the actuator and a moving shaft of the valve body, for reducing a rotational speed of the driving shaft of the actuator to a certain reduction ratio.  
      In view of the above problem, another flow control valve is proposed, for example, as disclosed in Japanese Patent Publication No. 2003-286843 (which corresponds to U.S. Pat. No. 6,837,193 B2), in which the driving torque required for the actuator is decreased to achieve a small sized actuator. More specifically, a driving load applied to the flow control valve is canceled by a pressure difference between a radiator flow pressure and a bypass flow pressure, to decrease the driving load for the actuator. In the above flow control valve, a first valve body and a first valve seat for controlling the radiator flow amount (of the cooling water flowing through the engine and the radiator and returning to the water pump) and a second valve body and a second valve seat for controlling the bypass flow amount (of the cooling water flowing through the engine, bypassing the radiator, and returning to the water pump) are provided, wherein the first valve body and the second valve body are integrally formed as a single valve body which is then driven by the actuator.  
      In the above flow control valve, however, the radiator flow pressure and the bypass flow pressure may be largely differed from each other, during the high-load operation of the engine during which only the first valve body is opened, or in a case that a difference between fluid flow resistances appears due to a difference of passage diameters between a radiator side passage and a bypass passage. Then it would become difficult to cancel the driving load applied to the flow control valve the pressure difference between the radiator flow pressure and the bypass flow pressure. As a result, the above flow control valve can not sufficiently achieve the effect for decreasing the driving load to the actuator.  
     SUMMARY OF THE INVENTION  
      It is, therefore, an object of the present invention to provide a flow control valve, in which a load to a driving operation by an actuator is minimized, independently of a closing or opening state of a radiator flow control valve and a bypass flow control valve and independently of fluid pressure of the radiator flow or the bypass flow, by canceling a pressure load to the radiator flow control valve and/or bypass flow control valve during the valve (or valves) is moved in its axial direction.  
      It is a further object of the present invention to provide a flow control valve, which is smaller in size and which can eliminate a reduction device.  
      According to a feature of the present invention, a flow control valve comprises a first and a second valve, which are respectively and movably housed in a valve housing, and an actuator for directly or indirectly driving the first and second valves, wherein the first valve controls a radiator flow amount of engine cooling water flowing through a radiator and the second valve controls a bypass flow amount of the engine cooling water bypassing the radiator. The radiator flow amount and the bypass flow amount are independently controlled by the actuator depending on an operational condition of an engine. As a result, a temperature of the engine cooling water can be controlled at a desired value corresponding to the respective operational conditions of the engine.  
      According to another feature of the present invention, the flow control valve further comprises pressure adjusting passages for the respective first and second valves for communicating with each other spaces formed at both sides of the respective valves, so that the fluid pressure at both sides of the respective valves are equalized. As a result, a pressure load to the first and second valve is cancelled during the valve (or valves) is moved in its axial direction.  
      According to a further feature of the present invention, the second valve is arranged that an axial direction of the second valve is almost perpendicular to an axial direction of the first valve, so that fluid pressure applied to the second valve does not adversely influence on an axial movement of the first valve, and vice versa.  
      According to a still further feature of the present invention, a cam face is formed at an outer surface of the first valve, and the second valve is arranged that its axial direction is almost perpendicular to the axial direction of the first valve and a forward end of the second valve is brought into contact with the cam face. As a result, the second valve can be moved in its axial direction in accordance with the axial movement of the first valve, so that a desired characteristic of the bypass flow amount can be obtained by suitably designing a shape of the cam face.  
      According to a still further feature of the present invention, the flow control valve can be used as a control valve for controlling a heater flow amount, in addition to the radiator flow amount and the bypass flow amount. For the purpose of controlling the heater flow amount of the engine cooling water (hot water) for heating air to be blown into a passenger room of a vehicle, a third valve is movably provided in the flow control valve.  
      According to the further feature of the present invention, another cam face is likewise formed at the outer surface of the first valve, and the third valve is arranged that its axial direction is almost perpendicular to the axial direction of the first valve and a forward end of the third valve is brought into contact with the cam face. As a result, the third valve can be moved in its axial direction in accordance with the axial movement of the first valve, so that a desired characteristic of the heater flow amount can be obtained by suitably designing a shape of the cam face. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:  
       FIG. 1A  is a schematic view showing an engine cooling system for a water-cooled engine according to the present invention;  
       FIG. 1B  is a graph showing characteristics of a radiator flow amount and a bypass flow amount with respect to a rotational angle of an actuator;  
       FIG. 2  is a vertical cross sectional view of a flow control valve, according to a first embodiment of the present invention, showing a starting condition of the valve operation;  
       FIGS. 3A and 3B  are also vertical cross sectional views of the flow control valve, respectively showing a valve condition during a normal operation and a valve condition during a high-load operation;  
       FIGS. 4A  to  4 C are cross sectional views, respectively taken along lines IVA-IVA, IVB-IVB, and IVC-IVC in  FIG. 2 ;  
       FIGS. 5A and 5B  are horizontal cross sectional views respectively showing a main portion of a flow control valve according to a second embodiment;  
       FIGS. 5C and 5D  are vertical cross sectional views respectively showing further modifications of the flow control valve according to the second embodiment;  
       FIG. 6  is a vertical cross sectional view of a flow control valve according to a third embodiment of the present invention;  
       FIG. 7  is a vertical cross sectional view of a flow control valve according to a fourth embodiment of the present invention;  
       FIGS. 8A and 8B  are respectively a vertical and a horizontal cross sectional views of a flow control valve according to a fifth embodiment and its modification of the present invention;  
       FIG. 9A  is a schematic view showing an engine cooling system for a water-cooled engine according to the fifth embodiment; and  
       FIG. 9B  is a graph showing characteristics of a radiator flow amount, a bypass flow amount and a heater flow amount with respect to a rotational angle of an actuator of the fifth embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       FIGS. 1A  to  4 C show a first embodiment of the present invention, wherein  FIG. 1A  shows a schematic view showing an engine cooling system for a water-cooled engine, and  FIG. 1B  is a graph showing characteristics of a radiator flow amount and a bypass flow amount with respect to a rotational angle of an actuator.  
      A flow control system according to the present invention comprises an engine cooling system for a water-cooled engine  1  having a cooling water circuit, a flow control valve  2  provided in the cooling water circuit, an electronic control unit (not shown and called hereinafter as ECU) for electronically controlling an opening degree of the flow control valve  2  depending on operational conditions of the engine  1 . The flow control valve  2  comprises, as shown in  FIG. 2 , an actuator  3  to be electronically operated by the ECU, a valve housing  4  which also forms a part of an interfluent portion of the water cooling circuit, a valve body  5  (also referred to as a first spool valve) for controlling a flow amount of the engine cooling water flowing through a radiator  9 , and a valve body  6  (also referred to as a second spool valve) for controlling a flow amount of the engine cooling water flowing through a bypass circuit  11  (also referred to as a bypass passage).  
      The ECU comprises a microcomputer having a well known structure and components, which are CPU for signal processing and calculation, memory devices, such as ROM and RAM, for storing programs and data, an input circuit, an output circuit, a power supply circuit, and so on. Sensor signals from various sensors are inputted into the microcomputer after those signals are processed by A/D converters. Connected to the microcomputer are a crank angle sensor, an acceleration sensor, an intake air flow sensor (an airflow meter, etc.), an intake air temperature sensor, an intake air pressure sensor, a throttle opening sensor, a cooling water temperature sensor on an engine side, a cooling water temperature sensor on a radiator side, a cooling water temperature sensor on a bypass circuit side, a cooling water temperature sensor on a water pump side, and so on. A starter motor drive circuit is connected to the ECU, for controlling a driving current to a starter motor for starting up an engine operation.  
      When an ignition key is inserted into a key cylinder of a vehicle and turned to a position of “ST”, a starter switch (not shown) is turned on (ST:ON) and a starter relay (not shown) provided in the starter motor drive circuit is turned on. The engine  1  is cranked up to start its operation. When the ignition key is turned back to a position of “IG” and thereby an ignition switch (not shown) is turned on (IG:ON) after the engine  1  has started its operation, the ECU starts its electronic controls to various actuators, such as the flow control valve  2 , in accordance with the control programs stored in the memory device. The ECU stops its electronic control when the ignition switch is turned off (IG:OFF).  
      The engine cooling system comprises the cooling water circuit in which the engine cooling water is circulated to cool the engine  1 . The cooling water circuit comprises a radiator cooling circuit in which the cooling water is circulated from and back to a water pump  8  through the engine  1 , the radiator  9  and the flow control valve  2 . The cooling water circuit further has the bypass circuit in which the cooling water is circulated from and back to the water pump  8  through the engine  1 , the bypass passage  11  and the flow control valve  2 . As the engine cooling water, an antifreeze liquid having ethylene glycol as a main component, or a long life coolant containing the antifreeze liquid, antirust and the like, is used.  
      The water pump  8  is arranged adjacent to an output shaft (e.g. a crankshaft) of the engine  1  and integrally provided to an inlet port of the engine  1 . The water pump  8  is one of engine accessories driven to rotate by the engine  1  via a transmitting device, such as a belt, and to circulate the cooling water. The water pump  8  can be formed by a pump driven by an electric motor. The engine  1  is mounted in an engine room of the vehicle, and it is the water-cooled engine having a water jacket  12  formed in a cylinder head and a cylinder block of the engine  1 , so that the cooling water flows through the engine  1  (through the water jacket). The every portion of the engine  1  is thereby cooled to effectively operate the engine  1 .  
      The radiator  9  is arranged in the engine room and at such a position at which the radiator  9  effectively receives wind during the vehicle running. The radiator  9  comprises an upper tank, a lower tank and a core portion between the upper and lower tanks, wherein the core portion has multiple tubes through which the cooling water flows. In the radiator  9 , a heat exchange is performed between the cooling water flowing through the tubes and cooling air passing through between outer surfaces of the multiple tubes, wherein the cooling air is the wind flowing through the radiator during the vehicle running and the wind blown by a cooling fan (not shown). The radiator  9  is, therefore, a heat exchanger for cooling down the cooling water, the water temperature of which is increased as a result of absorbing waste heat of the engine  1  during the cooling water passes through the water jacket  12  of the engine  1 .  
      The radiator cooling circuit comprises passage portions  13  to  16 , and the passage portions are liquid-tightly connected to the radiator  9 . A downstream side of the passage portion  15  is liquid-tightly connected to an upstream side of the flow control valve  2 . The bypass passage  11  is provided, so that the cooling water flowing out of the engine  1  bypasses the radiator  9 , wherein the bypass passage  11  branches off from a connecting portion of the passage portions  13  and  14  and is liquid-tightly connected to the upstream side of the flow control valve  2 .  
      A structure of the flow control valve  2  according to the embodiment is explained with reference to FIGS.  2  to  4 C.  FIG. 2  shows a valve position of the flow control valve at a start of the operation, whereas  FIGS. 3A and 3B  respectively show the valve positions during the normal operation and at the high-load operation. The flow control valve  2  precisely controls a radiator flow amount of the cooling water flowing in the radiator cooling circuit ( 13 ,  14 ,  9 ,  15 ,  2  and  16 ) as well as a bypass flow amount of the cooling water flowing in the bypass circuit  11 , depending on the various operational conditions of the engine. The flow control valve  2  can control the cooling water temperature more precisely than the control valve operated by the thermostat, and thereby the lower fuel consumption ratio is achieved.  
      The actuator  3  is a driving force generating portion for generating a driving force depending on the engine operation, and comprises a stepping motor for moving the first and second spool valves  5  and  6  in their respective axial directions (in a valve opening or closing direction). A rotor shaft  21  of the actuator  3  is rotationally supported by the valve housing  4 . One end of the rotor shaft  21  protrudes into the inside of the valve housing  4 , and an oil seal (a shaft seal) is provided between the rotor shaft  21  and the valve housing  4 . A male screw portion  23  is formed on an outer periphery of the rotor shaft  21 , which is engaged with a female screw portion  57  ( FIG. 4C ) formed on an inner periphery of the first spool valve  5  movable in its axial direction (in a vertical direction in  FIG. 2 ). A cut-out portion  24  vertically extending (in the axial direction) is formed at the male screw portion  23 , as shown in  FIG. 4C . Instead of the stepping motor, a brushless motor, a DC motor with brushes, an alternating current motor of a three-phase induction motor can be used for the actuator  3 . Further, a solenoid actuator for linearly driving the shaft can be used as the driving force generating portion.  
      The valve housing  4  is made by an aluminum die-casting process, and is arranged at the interfluent portion at which the passage portions  15  and  16  of the radiator cooling circuit and the bypass passage  11  are jointly connected. A cooling water passage is formed in the valve housing  4 . The valve housing  4  has a first cylindrical wall portion  40   a  for movingly (reciprocally) supporting the first spool valve  5 , a circular pipe joint portion  40   b  horizontally extending in the leftward direction in  FIG. 2  ( FIGS. 3A and 3B ) from an outer surface of the first cylindrical wall portion  40   a , a second cylindrical wall portion  40   c  horizontally extending in the rightward direction in  FIG. 2  ( FIGS. 3A and 3B ) from the outer surface of the first cylindrical wall portion  40   a  and movingly (reciprocally) supporting the second spool valve  6 , and a circular pipe joint portion  40   d  vertically extending in the upward direction in  FIG. 2  ( FIGS. 3A and 3B ) from an outer surface of the second cylindrical wall portion  40   c . The circular pipe joint portions  40   b  and  40   d  are respectively connected to the passage portion  16  and the bypass passage  11 .  
      The valve housing  4  further has a circular pipe joint portion  40   e  at a lower end of the first cylindrical wall portion  40   a , and a cylindrical valve case  25  is fixed to the circular pipe joint portion  40   e  by screws or any other fixing means. An O-ring is provided between the pipe joint portion  40   e  and the valve case  25  to prevent leakage of the cooling water. The pipe joint portion  40   e  is connected to the radiator  9  through the passage portion  15 . A mixing chamber  27  is formed in the inside of the first cylindrical wall portion  40   a , at which the low temperature cooling water cooled down at the radiator  9  and the high temperature cooling water having bypassed the radiator  9  flow into and are mixed together.  
      A space  31  defined by an upper end surface of the first wall portion  40   a  of the valve housing  4  and an upper end surface of the first spool valve  5  is a first volume variable space, the inner volume of which is varied in accordance with the movement of the first spool valve  5  in its axial direction. A space  32  defined by a right-hand end surface of the second cylindrical wall portion  40   c  and a right-hand end surface of the second spool valve  6  is a second volume variable space, the inner volume of which is varied in accordance with the movement of the second spool valve  6  in its axial direction. An inner diameter of the pipe joint portion  40   e  is made larger than an inner diameter of the first cylindrical wall portion  40   a . A radiator side passage  34  (a first inlet port) is formed in the inside of the pipe joint portion  40   e , so that the cooling water from the radiator  9  flows into the mixing chamber  27 .  
      A bypass side passage  35  (a second inlet port) is formed in the inside of the pipe joint portion  40   d , so that the cooling water from the bypass circuit  11  flows into the mixing chamber  27 . A pump side passage  37  (an outlet port) is formed in the inside of the pipe joint portion  40   b , so that the cooling water flows out from the mixing chamber  27  to the water pump  8  through the water pump passage portion  16 . The first wall portion (the mixing chamber  27 ) of the valve housing  4  is respectively connected to the three ports ( 34 ,  35  and  37 ) in a form of a T-shape in the vertical cross section of the valve housing.  
      The first cylindrical wall portion  40   a  of the valve housing  4  has a first cylindrical partitioning portion  40   f  for operatively separating the mixing chamber  27  from the radiator side passage  34 . A cylindrical inner surface of the first partitioning portion  50   a  forms a first sliding surface (a first valve seat), on which a first seal portion ( 5   a ) of the first spool valve  5  reciprocally moves in a sliding manner.  
      The second cylindrical wall portion  40   c  of the valve housing  4  likewise has a second cylindrical partitioning portion  40   g  for operatively separating the mixing chamber  27  from the bypass side passage  35 . A cylindrical inner surface of the second partitioning portion  40   g  forms a second sliding surface (a second valve seat), on which a second seal portion ( 6   a ) of the second spool valve  6  reciprocally moves in a sliding manner.  
      Multiple first guide portions  4   a  are integrally formed in the valve housing  4  at a lower end side of the first cylindrical partitioning portion  40   f , for guiding the first seal portion ( 5   a ) when the first spool valve  5  is downwardly moved. Multiple second guide portions  4   b  are likewise integrally formed in the valve housing  4  at a left-hand side of the second cylindrical partitioning portion  40   g , for guiding a protruded small-diameter portion  60   a  of the second seal portion ( 6   a ) when the second spool valve  6  is moved in a left-and right-ward direction.  
      An inside space  41  formed at a lower side of the first cylindrical wall portion  40   a  forms a first valve passage  41 , which is surrounded by the first guide portions  4   a , the outer surface of the first spool valve  5  and the inner surface of the circular pipe joint portion  40   e . The first valve passage  41  is communicated with the mixing chamber  27 ; the radiator side passage  34  is communicated with the mixing chamber  27  through the first valve passage  41  when the first spool valve  5  is downwardly moved and opened, as shown in  FIG. 3B .  
      As in the similar manner to the above first valve passage  41 , an inside space  42  formed at a left-hand side of the second cylindrical wall portion  40   c  forms a second valve passage  42 , which is surrounded by multiple second guide portions  4   b , the outer surface of the second spool valve  6  and the inner surface of the second wall portion  40   c . The second valve passage  42  is communicated with the mixing chamber  27 ; the bypass side passage  35  is communicated with the mixing chamber  27  through the second valve passage  42  when the second spool valve  6  is moved in the rightward direction and opened, as shown in  FIG. 3A .  
      The first spool valve  5  is urged by a return spring  44  in a valve opening direction (a downward direction in  FIG. 2 ), to prevent overheat of the engine at a system failure. The first spool valve  5  is prevented from rotating by a stopper pin  51  fixed to the valve housing  4 , so that the first spool valve  5  can be reciprocally moved in the axial direction (in the upward and downward direction) when the rotor shaft  21  is rotated by the actuator  3 . When the first spool valve  5  is upwardly or downwardly moved upon receiving the driving force from the rotor shaft  21 , an opening degree of the first valve passage  41  is varied to control the radiator flow amount. As above, the first spool valve  5  functions as a control valve for the radiator flow amount.  
      The first spool valve  5  comprises a pair of rand portions  5   a  and  5   b  (each of which has a disc portion, an outer periphery and a sealing portion), a cylindrical portion  5   c  connecting the rand portions  5   a  and  5   b  with each other, and a side wall portion  5   d  which is formed into an arc-shape and between the outer peripheries of the two rand portions  5   a  and  5   b . The side wall portion  5   d  is formed so that it faces to the second spool valve  6 . A pair of ring seal grooves is formed at the outer peripheries of the rand portions  5   a  and  5   b , into which ring seals  47   a  and  47   b  are inserted. The ring seal  47   b  is liquid-tightly in contact with the inner surface of the first cylindrical wall portion  40   a , for separating the first volume variable space  31  from the mixing chamber  27 .  
      The other ring seal  47   a  is likewise liquid-tightly in contact with the inner surface of the first partitioning portion  40   f , for operatively separating the radiator side passage  34  (the first valve passage  41 ) from the mixing chamber  27 . Accordingly, a desired radiator flow amount, as shown in  FIG. 1B , with respect to the rotational angle of the actuator  3  can be obtained, when the dimension of the pair of the rand portions  5   a  and  5   b  as well as the first sliding surface (the longitudinal dimension thereof) are suitably selected.  
      The cylindrical portion  5   c  of the first spool valve  5  has the female screw portion  57  which is formed at its inner periphery and engaged with the male screw portion  23  of the rotor shaft  21 . A space  5   e  is formed between the two rand portions  5   a  and  5   b  at the outer periphery of the cylindrical portion  5   c , wherein the space  5   e  forms wholly or partly the mixing chamber  27 . As above, the first spool valve  5  is moved upwardly or downwardly in response to the rotation of the rotor shaft  21 , wherein the male screw portion  23  and the female screw portion  57  form a driving direction changing device. A thick wall portion is provided at the side wall portion  5   d  of the first spool valve  5 , wherein an insertion hole is formed so that one end of the stopper pin  51  is inserted. A profile  52  is formed at an outer surface of the thick wall portion ( 5   d ), so that the second spool valve  6  is moved in conjunction with the first spool valve  5 . The profile  52  comprises a concave and a convex portion, which are so formed to obtain the desired bypass flow amount, as shown in  FIG. 1B , with respect to the rotational angle of the actuator  3 .  
      The profile  52  is formed as a cam face for driving the second spool valve  6  in its axial direction (i.e. in a direction perpendicular to an axial direction of the first spool valve  5 ). The cam face ( 52 ) comprises a first and a second flat surface portions  52   a  and  52   d  (concave portions) extending in parallel to the axial direction of the first spool valve  5 , a first and a second inclined surface portions  52   b  and  52   c  protruding outwardly from the first and second flat surface portions  52   a  and  52   d  and each having an inclined angle with respect to the axial direction of the first spool valve  5 . The first and second inclined surface portions  52   b  and  52   c  form a convex portion, and the inclined angle of the first inclined surface portion  52   b  (with which a ball  55  of the second spool valve  6  is in contact, when the first spool valve  5  is positioned at its starting position, as shown in  FIG. 2 ) is made larger than that of the inclined surface portion  52   c  (with which the ball  55  is brought into contact, when the first spool valve  5  is moved downwardly to its normal operation or high load operation position, as shown in  FIGS. 3A and 3B ).  
      A first pressure adjusting passage  61  is formed between the inner surface (the female screw portion  57 ) of the cylindrical portion  5   c  of the first spool valve  5  and the cut-out portion  24  of the rotor shaft  21 , as shown in  FIG. 4C , for communicating with each other the spaces formed at outer sides of the pair of the rand portions  5   a  and  5   b . More specifically, the first pressure adjusting passage  61  communicates the first volume variable space  31  with the radiator side passage  34  (i.e. the first valve passage  41 ) for equalizing the fluid pressures in the both spaces.  
      The second spool valve  6  is biased by a set spring  45  toward the first spool valve  5 , so that the second spool valve  6  is brought into contact with the profile  52  of the first spool valve  5  via the ball  55 . The second spool valve  6  is moved in its axial direction (in a direction perpendicular to the axial direction of the first spool valve) in accordance with the movement of the first spool valve  5 . When the first spool valve  5  is downwardly moved, from the position of  FIG. 2  to the position of  FIG. 3A , the second spool valve  6  is moved in its right-hand direction to change the opening degree of the second valve passage  42 , so that the bypass flow amount is controlled. The second spool valve  6  operates as a bypass flow control valve, as above.  
      The second spool valve  6  comprises a pair of rand portions (i.e. the second seal portions)  6   a  and  6   b  which are supported by the second sliding surface in the sliding manner, and a cylindrical portion  6   c  connecting the rand portions  6   a  and  6   b  with each other. The rand portion  6   b  of the second spool valve  6  liquid-tightly separates the second volume variable space  32  from the bypass side passage  35 . The second volume variable space  32  is liquid-tightly closed by a plug member  58 .  
      The other rand portion  6   a  of the second spool valve  6  operatively and liquid-tightly separates the bypass side passage  35  from the second valve passage  42  (and thereby the mixing chamber  27 ). The protruded small-diameter portion  60   a  is formed at the other rand portion  6   a , protruding outwardly (in a leftward direction) from the rand portion  6   a  and in the axial direction of the second spool valve  6 . A recess portion  64  is formed at a forward end of the protruded small-diameter portion  60   a , for holding the ball  55 .  
      Accordingly, a desired bypass flow amount, as shown in  FIG. 1B , with respect to the rotational angle of the actuator  3  can be obtained, when the dimensions of the pair of the rand portions  6   a  and  6   b , the second sliding surface (the longitudinal dimension thereof), and the profile  52  are suitably selected.  
      A space  6   e  is formed between the two rand portions  6   a  and  6   b  and at the outer periphery of the cylindrical portion  6   c , wherein the space  6   e  forms wholly or partly the bypass side passage  35 . A second pressure adjusting passage  62  is formed in the cylindrical portion  6   c , as shown in  FIGS. 2 and 4 A, for communicating with each other the spaces formed at outer sides of the pair of the rand portions  6   a  and  6   b . More specifically, the second pressure adjusting passage  62  communicates the second volume variable space  32  with the mixing chamber  27  through the second valve passage  42  for equalizing the fluid pressures in the both spaces. According to the present embodiment, the protruded small-diameter portion  60   a  is formed at the rand portion  6   a  and the recess portion  64  is formed at its forward end, and therefore the second pressure adjusting passage  62  is formed into an L-shaped passage, as shown in  FIG. 2 .  
      An operation of the above engine cooling system is explained with reference to FIGS.  1  to  4 .  
      The actuator  3  of the flow control valve  2  is controlled by the ECU to change the opening degrees of the first and second spool valves  5  and  6  depending on the operational condition of the water cooled engine  1 , as shown in  FIG. 1B . The first spool valve  5  is moved in its axial direction (in the valve opening or closing direction) upon directly receiving the driving force from the rotor shaft  21 , wherein the rotational movement of the rotor shaft  21  is converted into the linear movement by the screw portion  23 , so that the opening degree of the first valve passage  41  is increased or decreased. As a result, the radiator flow amount of the engine cooling water flowing in the radiator cooling circuit can be controlled in accordance with the engine operational condition.  
      The second spool valve  6  is moved in its axial direction (in the valve opening or closing direction) upon indirectly receiving the driving force from the rotor shaft  21  through the first spool valve  5  and the ball  55  being in contact with the profile  52  formed in the first spool valve  5 , so that the opening degree of the second valve passage  42  is likewise increased or decreased. As a result, the bypass flow amount of the engine cooling water flowing in the bypass circuit can be controlled in accordance with the engine operational condition. As above, the radiator flow amount as well as the bypass flow amount can be precisely controlled in accordance with the engine operational condition, so that the temperature of the engine cooling water flowing through the water jacket  12  can be controlled at such a temperature which is most suitable for the respective engine operation conditions.  
      The first and second spool valves  5  and  6  are controlled in the following manner, in accordance with the engine operational conditions. At a starting period of the engine  1 , the actuator  3  is operated to move the first and second spool valves  5  and  6  to the positions shown in  FIG. 2 . In this position, the rand portion  5   b  of the first spool valve  5  is in the sliding contact with the inner surface of the valve housing  4 , whereas the other rand portion  5   a  is in the sliding contact with first sliding surface (i.e. the first valve seat  40   f ). Accordingly, the first valve passage  41  is separated from the mixing chamber  27  and the pump side passage  37 .  
      As in the same manner to the first spool valve  5 , the rand portion  6   b  of the second spool valve  6  is in the sliding contact with the inner surface of the valve housing  4  (the second sliding surface at the right-hand side), whereas the other rand portion  6   a  is in the sliding contact with the second sliding surface (the second valve seat  40   g ). Accordingly, the bypass side passage  35  is separated from the second valve passage  42  and the mixing chamber  27 .  
      As above, the first and second spool valves  5  and  6  are closed during the period for the engine starting operation, as shown in  FIG. 1B , so that the radiator flow amount of the engine cooling water circulating in the radiator cooling circuit as well as the bypass flow amount of the engine cooling water circulating in the bypass circuit are both zero.  
      When the engine is operated in its normal operation but the temperature of the engine cooling water detected by the temperature sensor on the engine side is below a predetermined value, for example 60 to 78° C., the first spool valve  5  is downwardly moved from its starting position of  FIG. 2 , to a position shown in  FIG. 3A . In this valve position, the rand portion  5   a  of the first spool valve  5  is still in the sliding contact with the first valve seat  40   f , so that the separation between the first valve passage  41  and the mixing chamber  27  (and the pump side passage  37 ) is maintained.  
      On the other hand, the ball  55  of the second spool valve  6  is lifted up (moved in the right-hand direction) by the convex portion of the profile  52  formed in the thick wall portion of the first spool valve  5 , as shown in  FIG. 3A , and thereby the rand portion  6   a  of the second spool valve  6  is separated from the second sliding surface (the second valve seat  40   g ). The bypass side passage  35  is communicated from the second valve passage  42  and the mixing chamber  27 .  
      As a result, since the first spool valve  5  is kept closed whereas the second spool valve  6  is opened, the radiator flow amount of the engine cooling water circulating in the radiator cooling circuit is still zero, whereas the bypass flow amount of the engine cooling water circulating in the bypass circuit is controlled to become such an amount corresponding to the opening degree of the second spool valve  6  (i.e. the amount of the movement of the second spool valve  6 ).  
      When the second spool valve  6  is opened as above, the engine cooling water pumped out from the water pump  8  circulates through the water jacket  12  of the engine  1 , the passage portion  13 , the bypass passage  11 , the bypass side passage  35  of the flow control valve  2 , the second valve passage  42 , the mixing chamber  27 , the pump side passage  37  and the passage portion  16 . During this operation, the temperature of the engine cooling water is gradually increased, due to the engine cooling water flowing through the water jacket  12  of the engine  1 , to reach the predetermined value.  
      When the temperature of the engine cooling water detected by the temperature sensor on the engine side becomes higher than the predetermined value, for example 60 to 78° C., during the normal operation of the engine, the first spool valve  5  is further downwardly moved from its first normal position of  FIG. 3A , toward a position shown in  FIG. 3B . In this valve position (before reaching the position of  FIG. 3B ), the rand portion  5   a  of the first spool valve  5  is separated from the first valve seat  40   f , so that the first valve passage  41  is brought into communication with the mixing chamber  27  and the pump side passage  37 .  
      The contact between the ball  55  and the profile  52  of the first spool valve  5  is changed from the contact with the first inclined surface  52   b  to the contact with the second inclined surface  52   c , and the second spool valve  6  is gradually moved in the leftward direction (in the valve closing direction) by the biasing force of the set spring  45  in proportion to the downward movement of the first spool valve  5 .  
      As above, the first spool valve  5  starts its opening operation whereas the second spool valve  6  is moved to its closing position, when the temperature of the engine cooling water detected by the temperature sensor on the engine side becomes higher than the predetermined value, for example 60 to 78° C., during the normal operation of the engine. Accordingly, as shown in  FIG. 1B , the radiator flow amount of the engine cooling water circulating in the radiator cooling circuit is controlled to be such an amount corresponding to a stroke of the downward movement (i.e. the opening degree) of the first spool valve  5 , whereas the bypass flow amount of the engine cooling water circulating in the bypass circuit is controlled to be such an amount corresponding to a stroke of the leftward movement (i.e. the opening degree) of the second spool valve  6 .  
      When the first spool valve  5  is opened as above, the engine cooling water pumped out from the water pump  8  circulates through the water jacket  12  of the engine  1 , the passage portion  13 , the radiator passage portion  14 , the radiator  9 , the radiator passage portion  15 , the radiator side passage  34  of the flow control valve  2 , the first valve passage  41 , the mixing chamber  27 , the pump side passage  37  and the passage portion  16 . On the other hand, since the second spool valve  6  is still in its opened state, the engine cooling water pumped out from the water pump  8  circulates through the water jacket  12  of the engine  1 , the passage portion  13 , the bypass passage  11 , the bypass side passage  35  of the flow control valve  2 , the second valve passage  42 , the mixing chamber  27 , the pump side passage  37  and the passage portion  16 . Due to the above operation, the temperature of the engine cooling water flowing through the water jacket  12  of the engine  1  is maintained at the predetermined value.  
      When the engine  1  is operated with high load, the first spool valve  5  is further downwardly moved from its second normal position, to the position shown in  FIG. 3B . In this valve position ( FIG. 3B ), the rand portion  5   a  of the first spool valve  5  is further separated from the first valve seat  40   f , so that the opening degree of the first valve passage  41  is made larger than that during the normal operation, whereas the ball  55  is brought into contact with the flat surface portion  52   d  of the profile  52  of the first spool valve  5  and the rand portion  6   a  of the second spool valve  6  is thereby brought into the sliding contact with the second sliding surface ( 40   g ) to close the second valve passage  42 . The bypass side passage  35  is thereby separated from the second valve passage  42  and the mixing chamber  27 . As a result, since the first spool valve  5  is kept opened whereas the second spool valve  6  is closed, as shown in  FIG. 1B , the radiator flow amount of the engine cooling water circulating in the radiator cooling circuit is continuously controlled to be the amount corresponding to the stroke of the downward movement (i.e. the opening degree) of the first spool valve  5 , whereas the bypass flow amount of the engine cooling water circulating in the bypass circuit becomes zero.  
      Since the first spool valve  5  is kept opened as above, the engine cooling water pumped out from the water pump  8  circulates through the water jacket  12  of the engine  1 , the passage portion  13 , the radiator passage portion  14 , the radiator  9 , the radiator passage portion  15 , the radiator side passage  34  of the flow control valve  2 , the first valve passage  41 , the mixing chamber  27 , the pump side passage  37  and the passage portion  16 . In this operation, since a larger amount of the engine cooling water flowing through the water jacket  12  of the engine  1  is cooled down at the radiator, the temperature of the engine cooling water can be maintained at the predetermined value. As indicated in  FIG. 1B , it is not always necessary to completely close the second valve passage  42 . Instead, the opening degree of the second valve passage  42  can be reduced to a smaller amount than that during the normal operation.  
      According to the first embodiment, as described above, since the first volume variable space  31  is communicated with the space formed at the opposite side of the first spool valve  5  (i.e. the first valve passage  41  and the radiator side passage  34 ) through the first pressure adjusting passage  61 , the fluid pressures at both of the longitudinal sides of the first spool valve  5  is equalized (P 1 =P 2 ). As a result, the pressure load for the movement of the first spool valve  5  in its axial direction (in the upward-downward direction in  FIG. 2 ) can be cancelled.  
      As in the same manner to the first spool valve  5 , since the second volume variable space  32  is communicated with the space formed at the opposite side of the second spool valve  6  (i.e. the second valve passage  42  and the mixing chamber  27 ) through the second pressure adjusting passage  62 , the fluid pressures at both of the longitudinal sides of the second spool valve  6  is equalized (P 3 =P 4 ). As a result, the pressure load for the movement of the second spool valve  6  in its axial direction (in the leftward-rightward direction in  FIG. 2 ) can be cancelled.  
      As above, the driving load for the actuator  3  of the flow control valve  2  can be minimized, independently of the valve opening or valve closing positions of the first and second spool valves  5  and  6 , and further independently of fluid pressure in the radiator cooling circuit (more specifically, the fluid pressure in the radiator side passage  34 ) or the fluid pressure in the bypass circuit (more specifically, the fluid pressure in the bypass side passage  35 ). The flow control valve  2  can be therefore made in a smaller size, and made in a lower cost since a reduction device for reducing a rotational speed of the actuator by a predetermined reduction ratio can be eliminated. Since the flow amount characteristics shown in  FIG. 1B  can be freely changed by changing the shape of the profile  52 , the main components of the flow control valve  2  can be commonly used for various types of the flow control valves, independently of different requirements for the different cooling systems or different vehicle models. And therefore, the development cost can be also reduced.  
     Second Embodiment  
      A second embodiment will be explained with reference to  FIGS. 5A  to  5 D, wherein the second embodiment differs from the first embodiment in that the first pressure adjusting passage  61  is modified, instead of providing the cut-out portion  24  at the rotor shaft  21 .  
       FIG. 5A  shows a cross sectional view of the cylindrical portion  5   c  of the first spool valve  5 . A part of the cylindrical portion  5   c  is extended in a radial direction to form the first pressure adjusting passage  61 .  
       FIG. 5B  also shows a cross sectional view of the cylindrical portion  5   c  of the first spool valve  5 . Three parts of the cylindrical portion  5   c  are extended in radial directions to form multiple pressure adjusting passages  61 .  
       FIG. 5C  shows a vertical cross sectional view of the flow control valve  2 , wherein the first pressure adjusting passage  61  is formed in the side wall portion  5   d  of the first spool valve  5 .  
       FIG. 5D  also shows a vertical cross sectional view of the flow control valve  2 , wherein a connecting pipe portion  69  is provided between the rand portions  5   a  and  5   b  and the first pressure adjusting passage  61  is formed in the connecting pipe portion  69 .  
      In the above modifications, the first pressure adjusting passage  61  is formed in the first spool valve  5 . However, although not shown in the drawings, the first pressure adjusting passage can be formed in the valve housing  4 .  
      According to the above second embodiment, the cross sectional area of the first pressure adjusting passage  61  can be freely designed, more specifically the cross sectional area can be made larger than that of the first embodiment, so that the pressure equalization between the first volume variable space  31  and the first valve passage  41  (i.e. radiator side passage  34 ) can be more smoothly performed.  
     Third Embodiment  
      A third embodiment will be explained with reference to  FIG. 6 .  
      The forward end  70  of the protruded small-diameter portion of the second spool valve  6  is formed into a semispherical shape, so that the ball  55  of the first embodiment can be eliminated. According to the third embodiment, the number of parts as well as number of assembling processes for the flow control valve can be reduced to realize a cost down.  
     Fourth Embodiment  
      A fourth embodiment will be explained with reference to  FIG. 7 .  
      A blade portion  71  is formed at the rand portion  5   a  of the first spool valve  5 , for rectifying the fluid flow in the radiator side passage  34  and the first valve passage  41  to reduce fluid resistance for the engine cooling water flowing from the radiator  9  into the flow control valve  2 . As a result, the flow amount can be increased at the full open state of the first spool valve  5 , to maximally bring out the cooling effect of the radiator  9 .  
     Fifth Embodiment  
      A fifth embodiment will be explained with reference to  FIGS. 8A  to  9 B, wherein  FIG. 8A  is a vertical cross sectional view of the flow control valve,  FIG. 8B  is a cross sectional view of a modified flow control valve,  FIG. 9A  is a schematic view showing an engine cooling system, and  FIG. 9B  is a graph showing characteristics of a radiator flow amount, a bypass flow amount and a heater flow amount with respect to a rotational angle of an actuator. The same reference numerals to the first embodiment are those parts which are identical or similar to the first embodiment.  
      The engine cooling system shown in  FIG. 9A  comprises three different flow circuits; a radiator cooling circuit in which the engine cooling water flows from the water pump  8  through the engine  1 , the radiator  9 , the flow control valve  2  and back to the water pump  8 ; a bypass circuit in which the engine cooling water flows from the water pump  8  through the engine  1 , the bypass passage  11 , the flow control valve  2  and back to the water pump  8 ; and a heater circuit in which the engine cooling water (hot water) flows from the water pump  8  through the engine  1 , a hot water type heater  10  for an air conditioning system, the flow control valve  2  and back to the water pump  8 .  
      The hot water type heater  10  is provided in an air duct of a vehicle air conditioning device for air-conditioning a passenger room of a vehicle. The heater  10  comprises a heater core having a pair of tanks and multiple tubes connected between the tanks, so that the engine cooling water (the hot water) flows from one of the tanks to the other tank through the tubes. When air passes by the heater  10 , heat is exchanged between the hot water flowing through the tubes and the air flowing around outer surfaces of the tubes, so that the air cooled down by an evaporator (not shown) is re-heated by the heater  10 . At the same time, the engine cooling water (hot water) heated by the waste heat of the engine  1  can be cooled down by the heater  10 . The heater  10  is connected to the engine  1  through a heater passage  17  and to the flow control valve through another heater passage  18 , which is liquid-tightly connected to the flow control valve  2 .  
      The flow control valve  2  comprises a third spool valve  7 , in addition to the first and second spool valves  5  and  6  which are basically identical to those shown in  FIG. 2 . The third spool valve  7  is also similar in its structure to the second spool valve  6  and is biased by a set spring  46  toward the first spool valve  5 , so that the third spool valve  7  is brought into contact with a profile  53  of the first spool valve  5  via a ball  56 . The third spool valve  7  is moved in its axial direction (in a direction perpendicular to the axial direction of the first spool valve) in accordance with the movement of the first spool valve  5 . When the first spool valve  5  is downwardly moved, the third spool valve  7  is moved in its right-hand direction to change an opening degree of a third valve passage  43 , so that the heater flow amount is controlled. The third spool valve  7  operates as a heater flow control valve, as above. When the third spool valve  7  is opened, a heater side passage  36  is communicated with the third valve passage  43 , the mixing chamber  27  and the pump side passage  37 .  
      The third spool valve  7  comprises a pair of rand portions (i.e. third seal portions)  7   a  and  7   b  which are supported by a third sliding surface in the sliding manner, and a cylindrical portion  7   c  connecting the rand portions  7   a  and  7   b  with each other. The rand portion  7   b  of the third spool valve  7  liquid-tightly separates a third volume variable space  33  from the heater side passage  36 . The third volume variable space  33  is liquid-tightly closed by a plug member  59 . A guide portion  4   c  is integrally formed in the valve housing  4 , for guiding the third spool valve  7 .  
      The other rand portion  7   a  of the third spool valve  7  operatively and liquid-tightly separates the heater side passage  36  from the third valve passage  43  (and thereby the mixing chamber  27 ). The protruded small-diameter portion  70   a  is formed at the other rand portion  7   a , protruding outwardly (in a leftward direction) from the rand portion  7   a  and in the axial direction of the third spool valve  7 . A recess portion  65  is formed at a forward end of the protruded small-diameter portion  70   a , for holding the ball  56 .  
      Accordingly, a desired heater flow amount, as shown in  FIG. 9B , with respect to the rotational angle of the actuator  3  can be obtained, when the dimensions of the pair of the rand portions  7   a  and  7   b , the third sliding surface (the longitudinal dimension thereof), and the profile  53  are suitably selected.  
      A space  7   e  is formed between the two rand portions  7   a  and  7   b  and at the outer periphery of the cylindrical portion  7   c , wherein the space  7   e  forms wholly or partly the heater side passage  36 . A third pressure adjusting passage  63  is formed in the cylindrical portion  7   c , as shown in  FIGS. 8A and 8B , for communicating with each other the spaces formed at outer sides of the pair of the rand portions  7   a  and  7   b . More specifically, the third pressure adjusting passage  63  communicates the third volume variable space  33  with the mixing chamber  27  through the third valve passage  43  for equalizing the fluid pressures in the both spaces. According to the present embodiment, the protruded small-diameter portion  70   a  is formed at the rand portion  7   a  and the recess portion  65  is formed at its forward end, and therefore the third pressure adjusting passage  63  is formed into an L-shaped passage, as shown in  FIG. 8A . The third spool valve  7  is formed in the valve housing  4  in the same vertical line to the second spool valve  6 , as shown in  FIG. 8A . However, it can be formed in the same horizontal line to the second spool valve  6 , as shown in  FIG. 8B , wherein the third spool valve  7  is displaced from the second spool valve  6  by a certain angle. As above, in the case that a further spool valve is formed in the flow control valve, the further spool valve can be formed in the similar manner to the third spool valve  7 .  
      According to the above fifth embodiment, the heater flow amount can be controlled in addition to the radiator flow amount and the bypass flow amount, without largely increasing the driving load to the actuator  3 . The flow control valve  2  can be therefore made in a small size, compared with a case in which a heater valve for controlling the heater flow amount is independently provided.  
      Furthermore, since the heater flow amount can be controlled independently from the control for the radiator flow or the bypass flow, the heater passages  17  and  18  can be communicated by the flow control valve  2  before the radiator cooling circuit and the bypass circuit are opened, when the temperature of the heater  10  is to be rapidly and preferentially increased, as shown in  FIG. 9B , during a period of a heater preferential operation. Accordingly, the waste heat from the engine can be intensively supplied to the heater  10  for quickly warming the passenger room of the vehicle.  
      Furthermore, the multiple spool valves  6  and  7  can be arranged in the valve housing  4  in any manner as desired, it becomes possible to design the flow control valve realizing the best arrangement to the cooling system.