Patent Publication Number: US-11028763-B2

Title: Engine cooling device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2019-170211 filed on Sep. 19, 2019, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an engine cooling device that is equipped with a mechanical water pump and a flow rate control valve. 
     2. Description of Related Art 
     A device described in Japanese Patent Application Publication No. 2013-234605 (JP 2013-234605 A) is conventionally known as a water-cooling engine cooling device that cools an engine by circulating coolant through a water jacket formed inside the engine. The engine cooling device described in Japanese Patent Application Publication No. 2013-234605 (JP 2013-234605 A) is equipped with a mechanical water pump that delivers coolant to a water jacket in response to rotation of an engine, and an electronic control valve that closes to limit the outflow of coolant from the water jacket. Moreover, when the engine has not been warmed up, the warm-up of the engine is accelerated by leaving coolant in the water jacket by closing the electronic control valve. 
     Incidentally, the discharge pressure of the mechanical water pump rises as the engine rotational speed rises. Therefore, when the engine rotational speed becomes high with the electronic control valve closed, the hydraulic pressure of the water jacket may become too high. As a measure against this problem, the foregoing conventional engine cooling device restrains the hydraulic pressure of the water jacket from rising, by forcibly opening the electronic control valve without waiting for the completion of warm-up, in the case where the engine rotational speed becomes equal to or higher than a certain rotational speed when the electronic control valve is closed to accelerate warm-up. 
     SUMMARY 
     However, when the supply voltage of an in-vehicle electric power supply drops, the time needed to open the electronic control valve becomes long, and the hydraulic pressure remains high during the time. Therefore, it may be impossible to sufficiently restrain the hydraulic pressure from rising. 
     An engine cooling device that solves the foregoing problem is equipped with a circulation circuit for coolant flowing through a water jacket formed inside an engine, a mechanical water pump that operates in response to rotation of the engine and that circulates the coolant through the circulation circuit, a flow rate control valve that serves to adjust a flow rate of the coolant flowing through the circulation circuit, that has a valve body driven by an electric actuator operating by being supplied with electric power from an in-vehicle electric power supply, and that allows a flow channel area for the coolant to change depending on an operating position of the valve body, and a control unit that sets an operating position within a prescribed control range as a target operating position in accordance with an operating situation of the engine, and that performs drive control of the actuator to change the operating position of the valve body to the set target operating position. The control unit in the foregoing engine cooling device performs protection control for setting an operating position where a withstanding pressure limit rotational speed is equal to or higher than a current engine rotational speed, as the target operating position. Furthermore, the control unit performs retreat control for reducing the control range to a retreat operation range set in advance, as a range of the operating position including a maximum withstanding pressure operating position, when a supply voltage of the in-vehicle electric power supply has dropped. Incidentally, the withstanding pressure limit rotational speed mentioned herein means a maximum value of the engine rotational speed at which a hydraulic pressure in any region of the circulation circuit is lower than an upper limit of the hydraulic pressure permissible in the region. Besides, the maximum withstanding pressure operating position means an operating position where the withstanding pressure limit rotational speed is highest among operating positions within the control range. 
     In the engine cooling device configured as described above, the mechanical water pump that operates in response to rotation of the engine circulates the coolant through the circulation circuit. Therefore, when the engine rotational speed rises, the hydraulic pressure of the circulation circuit rises. Then, when the hydraulic pressure in any region of the circulation circuit has remained higher than a withstanding pressure limit in the region, namely, an upper limit of the hydraulic pressure permissible in the region as a result, the component members of the circulation circuit cannot withstand the hydraulic pressure, thus causing leakage of the coolant and the like. 
     On the other hand, when the operating position of the valve body of the flow rate control valve is changed to change the flow of the coolant through the circulation circuit, the hydraulic pressure in each region of the circulation circuit changes. In consequence, when the operating position of the flow rate control valve is changed to prevent the hydraulic pressure from becoming higher than the withstanding pressure limit in any region of the circulation circuit even in the case where the engine rotational speed rises, the component members of the circulation circuit can be protected against the hydraulic pressure. Incidentally, the maximum value of the engine rotational speed at which the hydraulic pressure in any region of the circulation circuit is lower than the upper limit of the hydraulic pressure permissible in the region, namely, the withstanding pressure limit rotational speed differs depending on the operating position of the valve body. In consequence, the protection of the component members of the circulation circuit against the hydraulic pressure can be achieved by driving the valve body to the operating position where the withstanding pressure limit rotational speed is equal to or higher than the current engine rotational speed. Therefore, the control unit of the foregoing engine cooling device protects the component members of the circulation circuit against the hydraulic pressure, by performing protection control for setting the operating position where the withstanding pressure limit rotational speed is equal to or higher than the current engine rotational speed, as the target operating position, when the engine rotational speed rises. 
     By the way, in the foregoing engine cooling device, the operating position of the valve body is changed by the electric actuator that operates by being supplied with electric power from the in-vehicle electric power supply. Therefore, when the supply voltage of the in-vehicle electric power supply drops, the speed at which the operating position of the valve body is changed by the actuator drops. Accordingly, when the supply voltage has dropped, the time needed to change the operating position of the valve body in protection control becomes long, and it may become impossible to sufficiently restrain the hydraulic pressure of the circulation circuit from rising. 
     As a measure against this problem, with the foregoing engine cooling device, when the supply voltage of the in-vehicle electric power supply has dropped, retreat control for reducing the control range to the retreat operation range set in advance as the range of the operating position including the maximum withstanding pressure operating position is performed. Then, the operating position of the valve body is thus changed to the operating position within the retreat operation range, namely, into the range that is not very distant from the maximum withstanding pressure operating position. Therefore, even when the amount of change in the operating position of the valve body in the case where protection control is thereafter performed in response to a rise in engine rotational speed has stopped increasing after reaching a certain amount and the speed of change in the operating position of the valve body has dropped in response to a drop in the supply voltage of the in-vehicle electric power supply, the time needed to change the operating position of the valve body in protection control is unlikely to become long. Accordingly, with the foregoing engine cooling device, the time needed to restrain the hydraulic pressure of the circulation circuit from rising when the engine rotational speed rises is unlikely to become long, even when the supply voltage of the in-vehicle electric power supply has dropped. 
     Incidentally, in protection control as described above as well, the hydraulic pressure is insufficiently restrained from rising, unless an appropriate operating position where the withstanding pressure limit rotational speed is equal to or higher than the current engine rotational speed is set as the target rotational position. As a measure against this problem, the foregoing engine cooling device may be provided with a storage unit in which information on the withstanding pressure limit rotational speed at each operating position of the valve body is stored, and the control unit may perform protection control by obtaining an operating position of the valve body where the withstanding pressure limit rotational speed is higher than the current engine rotational speed, based on the information stored in the storage unit, and by setting the obtained operating position as the target operating position. In such a case, the information on the withstanding pressure limit rotational speed at each operating position of the valve body is stored in advance in the storage unit. Therefore, the operating position where the withstanding pressure limit rotational speed is equal to or higher than the current engine rotational speed can be adequately set as the target rotational position, based on the information. 
     In the case where the hydraulic pressure of the circulation circuit cannot be sufficiently restrained from rising even when protection control as described above is performed, it is conceivable to achieve protection of the component members of the circulation circuit by reducing the engine torque to lower the engine rotational speed. A determination on such an additional reduction in engine torque can be made by causing the control unit of the foregoing engine cooling device to make a determination on the necessity to reduce the engine torque by determining that the engine torque needs to be reduced when the current engine rotational speed has remained higher than a withstanding pressure limit rotational speed at the current operating position of the valve body for a prescribed time or more. 
     Immediately after the startup of the engine, the supply voltage of the in-vehicle electric power supply may temporarily drop due to the consumption of electric power for the startup of the engine. This drop in the supply voltage of the in-vehicle electric power supply immediately after the startup of the engine is stopped in a short time. Therefore, the performance of retreat control is often unnecessary as a measure against the drop in supply voltage at this time. Under these circumstances, the control unit of the foregoing engine cooling device may determine that a supply voltage of the in-vehicle electric power supply has dropped when the supply voltage is equal to or lower than a voltage drop determination value, and set a higher voltage as the voltage drop determination value when an elapsed time after the startup of the engine is shorter than a prescribed time than when the elapsed time is equal to or longer than the prescribed time. 
     When the temperature of coolant is low, the viscosity of coolant is high, and the flow resistance of coolant applied to the valve body in changing the operating position is high. Therefore, even when the temperature of coolant is low, the speed at which the operating position of the valve body is changed by the actuator is low. Therefore, the control unit of the foregoing engine cooling device is also desired to perform retreat control when a temperature of the coolant is equal to or lower than a prescribed low coolant temperature determination value. 
     Incidentally, in the case where protection control may be performed in a short time even when the supply voltage of the in-vehicle electric power supply has not dropped, it is desirable to make the completion of the change in the operating position of the valve body in protection control possible in a short time by performing retreat control. In one of such cases, the engine rotational speed has risen to such an extent that protection control needs to be performed due to a subsequent slight rise in engine rotational speed. In consequence, the control unit of the foregoing engine cooling device may also perform the retreat control when the engine rotational speed is equal to or higher than a prescribed retreat start rotational speed. Furthermore, in the case where this engine cooling device is applied to an engine mounted on a vehicle, the control unit may set a lower rotational speed as the retreat start rotational speed when the transmission of motive power between the engine and wheels is shut off than when the transmission of motive power between the engine and the wheels is not shut off. When the transmission of motive power between the engine and the wheels is shut off, the rotational load of the engine is low, so the speed of rise in engine rotational speed tends to be higher than when the foregoing transmission of motive power is not shut off. Therefore, when the foregoing transmission of motive power is shut off, it is desirable to perform retreat control from the engine rotational speed that is lower than when the foregoing transmission of motive power is not shut off. 
     Besides, in the engine mounted on the vehicle, the engine rotational speed may rapidly rise due to a downshift or the like during coasting of the vehicle when the engine is dragged as the wheels rotate. In consequence, in the case where the foregoing engine cooling device is applied to an engine mounted on a vehicle, the control unit is also desired to perform retreat control while the vehicle is coasting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the present disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein: 
         FIG. 1  is a view schematically showing the configuration of an engine cooling device according to one of the embodiments; 
         FIG. 2  is a perspective view of a flow rate control valve provided in the cooling device; 
         FIG. 3  is an exploded perspective view of the flow rate control valve; 
         FIG. 4  is a perspective view of a valve body as a component member of the flow rate control valve; 
         FIG. 5  is a perspective view of a housing as another component member of the flow rate control valve; 
         FIG. 6A  is a graph showing a relationship between a relative angle of the valve body of the flow rate control valve and opening ratios of respective output ports; 
         FIG. 6B  is a graph showing a relationship between the relative angle of the valve body and a withstanding pressure limit rotational speed; 
         FIG. 7  is a flowchart showing part of a processing procedure of a flow rate control valve control routine that is carried out by a control unit provided in the engine cooling device according to the embodiment; and 
         FIG. 8  is a flowchart showing the rest of the processing procedure of the flow rate control valve control routine. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An engine cooling device according to one of the embodiments will be described hereinafter with reference to  FIGS. 1 to 8 . The engine cooling device according to the present embodiment is applied to an engine mounted on a vehicle having an automatic transmission. As shown in  FIG. 1 , the engine cooling device according to the present embodiment is equipped with a circulation circuit  21  through which coolant flowing through a water jacket  111  in a cylinder block  11  of an engine  10  and a water jacket  121  in a cylinder head  12  of the engine  10  circulates. The circulation circuit  21  is provided with a mechanical water pump  22  that discharges coolant toward the water jacket  111  in the cylinder block  11 . Besides, the circulation circuit  21  is provided with three heat exchangers, namely, a radiator  23 , an ATF warmer  24 , and a heater core  25  of an air-conditioner for the vehicle. The radiator  23  cools coolant through the exchange of heat with outside air. The ATF warmer  24  heats up or cools automatic transmission fluid (ATF) as hydraulic oil of an automatic transmission  241  coupled to the engine  10 , through the exchange of heat with coolant. The heater core  25  warms the air blown into a cabin by the air-conditioner, through the exchange of heat with coolant. 
     Incidentally, the water pump  22  is coupled to a crankshaft  101  of the engine  10  via a wrapping transmission mechanism  102 . Thus, the water pump  22  operates in response to rotation of the crankshaft  101  of the engine  10 , and delivers coolant toward the water jacket  111 . 
     The circulation circuit  21  is provided with a flow rate control valve  26  into which the coolant that has flowed out from the water jacket  121  in the cylinder head  12  flows. The flow rate control valve  26  has three ports, namely, a radiator port P 1 , a device port P 2 , and a heater port P 3  as output ports for causing the coolant that has flowed into the flow rate control valve  26  to flow out. The radiator port P 1  is connected to a first coolant channel  271  through which coolant is caused to flow via the radiator  23 . The device port P 2  is connected to a second coolant channel  272  through which coolant is caused to flow via the ATF warmer  24 . The heater port P 3  is connected to a third coolant channel  273  through which coolant is caused to flow via the heater core  25 . Incidentally, the circulation circuit  21  is provided with a coolant temperature sensor  122  that detects a temperature of coolant flowing into the flow rate control valve  26  after flowing out from the water jacket  121  in the cylinder head  12 . 
     Furthermore, the engine cooling device according to the present embodiment is equipped with a control unit  50  as a control unit of the engine cooling device. The control unit  50  is equipped with an arithmetic processing circuit  51  that performs arithmetic processing for controlling the engine cooling device, and a memory  52  in which programs and data for control are stored. Besides, the control unit  50  is provided with a voltage adjusting circuit  54  that adjusts a voltage supplied from an in-vehicle electric power supply  53  through pulse width modulation and that supplies the adjusted voltage to a motor  37  built in the flow rate control valve  26 . Incidentally, various pieces of information on an operating situation of the engine  10  and a running situation of the vehicle are input to the control unit  50 . The pieces of information input to the control unit  50  include the temperature of coolant detected by the coolant temperature sensor  122 , an engine rotational speed NE, the setting of a shift range of the automatic transmission  241 , an operation amount of an acceleration pedal, a supply voltage of the in-vehicle electric power supply  53 , and information on how the cabin is warmed by the air-conditioner. Incidentally, the control unit  50  is connected to an engine control unit  55  as an electronic control unit for engine control, through an in-vehicle communication line. 
     Subsequently, the configuration of the flow rate control valve  26  will be described with reference to  FIGS. 2 to 6B . As shown in  FIG. 2 , the flow rate control valve  26  is equipped with a housing  31  that forms the skeleton of the flow rate control valve  26 . A first connector member  32 , a second connector member  33 , and a third connector member  34  are attached to the housing  31 . The first connector member  32  is provided with a radiator port P 1 . The second connector member  33  is provided with a device port P 2 . The third connector member  34  is provided with a heater port P 3 . Moreover, with the connector members  32  to  34  attached to the housing  31 , the output ports P 1  to P 3  are arranged at different positions. 
     As shown in  FIG. 3 , the flow rate control valve  26  is equipped with a valve body  35  that is accommodated in the housing  31 . A coolant channel is formed in the valve body  35 . Besides, a shaft  36  that extends in an axial direction Z of the housing  31  is coupled to the valve body  35 . Moreover, the valve body  35  rotates around the shaft  36  as indicated by an arrow in  FIG. 3 . When a relative angle ANG of the valve body  35  relative to the housing  31  changes through rotation of the valve body  35 , the degrees to which the coolant channel formed in the valve body  35  overlaps with the output ports P 1  to P 3  change, and the flow channel areas of coolant at the output ports P 1  to P 3  change. That is, the flow of coolant in the circulation circuit  21  can be controlled by changing the rotational phase of the valve body  35  relative to the housing  31 . 
     The motor  37  is accommodated in the housing  31  of the flow rate control valve  26 . Besides, a transmission mechanism  38  is provided in the housing  31 . The transmission mechanism  38  has a plurality of gears  39  that mesh with one another, and transmits an output of the motor  37  to the shaft  36  of the valve body  35  via the gears  39 . 
     A cover  40  is attached to the housing  31  in such a manner as to cover that part of the housing  31  which accommodates the motor  37  and the transmission mechanism  38 . A rotational angle sensor  123  that detects a rotational angle of the motor  37  is installed in the cover  40 . Incidentally, information on the rotational angle of the motor  37  detected by the rotational angle sensor  123  is also input to the control unit  50 . 
     As shown in  FIG. 4 , the valve body  35  assumes a shape that is obtained by, for example, superimposing two barrel-like objects on each other in the axial direction Z of the housing  31 . Two holes  351  and  352  that are aligned in the axial direction Z, namely, the first hole  351  and the second hole  352  are formed through a lateral wall of the valve body  35 . The holes  351  and  352  constitute part of the coolant channel provided in the valve body  35 . The first hole  351  is located above in the drawing, and communicates with the radiator port P 1  when the valve body  35  is within a certain angular range relative to the housing  31 . When the first hole  351  communicates with the radiator port P 1 , the coolant that has flowed into the flow rate control valve  26  flows out from the radiator port P 1 . Besides, the second hole  352  communicates with at least one of the device port P 2  and the heater port P 3  when the valve body  35  is within another angular range relative to the housing  31 . When the second hole  352  communicates with the device port P 2 , the coolant that has flowed into the flow rate control valve  26  flows out from the device port P 2 . Besides, when the second hole  352  communicates with the heater port P 3 , the coolant that has flowed into the flow rate control valve  26  flows out from the heater port P 3 . 
     In the case where an upper wall  353  of the valve body  35  is defined as an upper wall of the valve body  35  in the drawing, the shaft  36  is connected to the upper wall  353 . Besides, the upper wall  353  is provided with a circular groove  355  that extends in such a manner as to surround a root of the shaft  36  in such a manner as to leave a part thereof as an engagement portion  354 . 
       FIG. 5  shows the perspective structure of the housing  31  as viewed in a direction in which the valve body  35  is inserted. In assembling the flow rate control valve  26 , the valve body  35  is inserted into the housing  31  via an accommodation opening  311 . That part of the housing  31  which faces the upper wall  353  of the valve body  35  is provided with a stopper  312  accommodated in the groove  355 . Therefore, when the valve body  35  is accommodated in the housing  31 , the engagement portion  354  of the valve body  35  abuts on the stopper  312  to thereby keep the valve body  35  from rotating relatively to the housing  31 . That is, the range where the engagement portion  354  does not abut on the stopper  312  is a range where the valve body  35  is allowed to rotate relatively to the housing  31 . 
     Coolant flows into the housing  31  of the flow rate control valve  26 , via the accommodation opening  311 . That is, the accommodation opening  311  functions as an input port of the flow rate control valve  26 . Then, the coolant that has flowed into the housing  31  flows through the coolant channel provided in the valve body  35 , and is introduced to the output ports P 1  to P 3 . 
       FIG. 6A  is a graph showing a relationship between the relative angle ANG of the valve body  35  relative to the housing  31  and opening ratios of the output ports P 1  to P 3 . Incidentally, in the present embodiment, the relative angle ANG is used as a state quantity indicating an operating position of the valve body  35  in the flow rate control valve  26 . Each of the opening ratios represents the ratio of the flow channel area of the corresponding one of the output ports on the assumption that the opening ratio is 100% when the output port is fully open. 
     In the flow rate control valve  26 , the relative angle ANG is assumed to be “0°” when all the output ports P 1  to P 3  are closed, and the valve body  35  can be rotated relatively to the housing  31  in both the positive direction and the negative direction until the stopper  312  of the housing  31  and the engagement portion  354  of the valve body  35  abut on each other. The sizes and positions of the holes  351  and  352  of the valve body  35  are set such that the opening degrees of the output ports P 1  to P 3  change as shown in  FIG. 6A  as the relative angle ANG changes. In the present embodiment, when the valve body  35  is rotated relatively to the housing  31  in the positive direction, the relative angle ANG increases. On the other hand, when the valve body  35  is rotated relatively to the housing  31  in the negative direction, the relative angle ANG decreases. 
     In the flow rate control valve  26 , when the valve body  35  is rotated relatively in the positive direction from the position where the relative angle ANG is “0°”, the heater port P 3  first starts opening, and the opening degree of the heater port P 3  gradually increases as the relative angle ANG increases. Then, when the relative angle ANG further increases after the heater port P 3  is fully opens, the device port P 2  then opens. The opening degree of the device port P 2  increases as the relative angle ANG increases. Then, after the device port P 2  fully opens, the radiator port P 1  starts opening. The opening degree of the radiator port P 1  also increases as the relative angle ANG increases. In the case where the relative angle is “β°” when the engagement portion  354  and the stopper  312  abut on each other, the radiator port P 1  fully opens before the valve body  35  reaches a position where the relative angle ANG is “+β°”. Then, the output ports P 1  to P 3  are held fully open even when the relative angle ANG increases, until the valve body  35  reaches the position where the relative angle ANG is “β°”. 
     On the other hand, in the flow rate control valve  26 , when the valve body  35  is relatively rotated in the negative direction from the position where the relative angle ANG is “0°”, the heater port P 3  does not open. In this case, the device port P 2  first starts opening, and the opening degree of the device port P 2  gradually increases as the relative angle ANG decreases. Then, the relative angle ANG further decreases after the device port P 2  fully opens, the radiator port P 1  opens. The opening degree of the radiator port P 1  increases as the relative angle ANG decreases. In the case where the relative angle is “−α°” when the engagement portion  354  and the stopper  312  abut on each other, the radiator port P 1  fully opens before the valve body  35  reaches a position where the relative angle ANG is “−α°”. Then, the radiator port P 1  and the device port P 2  are held fully open even when the relative angle ANG decreases, until the valve body  35  reaches the position where the relative angle ANG is “−α°”. 
     Incidentally, in the engine cooling device configured as described above, coolant is circulated through the circulation circuit  21  by the mechanical water pump  22  that operates in response to rotation of the engine  10 . In this engine cooling device, the discharge pressure of coolant in the water pump  22  rises as the engine rotational speed NE rises. On the other hand, in the foregoing engine cooling device, the flow of coolant through the circulation circuit  21  is changed by the flow rate control valve  26 . In this engine cooling device, the hydraulic pressures at the respective portions of the circulation circuit  21  are determined by the engine rotational speed NE and the relative angle ANG of the valve body  35  of the flow rate control valve  26 . 
     Incidentally, there is an upper limit of the permissible hydraulic pressure for each of component members of the circulation circuit  21 . When the hydraulic pressure remains higher than the upper limit, the leakage of coolant may be caused. In the following description, the upper limit of the permissible hydraulic pressure for each of the component members of the circulation circuit  21  will be referred to as a withstanding pressure limit thereof. Besides, the maximum value of the engine rotational speed NE at which the hydraulic pressure in any region of the circulation circuit  21  is lower than the upper limit of the hydraulic pressure permissible in the region will be referred to as a withstanding pressure limit rotational speed. 
     In the present embodiment, in designing the engine cooling device, a value of the withstanding pressure limit rotational speed for each relative angle ANG of the valve body  35  of the flow rate control valve  26  is obtained through an experiment, a simulation, or the like. Moreover, a map M indicating the value of the withstanding pressure limit rotational speed for each relative angle ANG of the valve body  35  is stored in the memory  52  of the control unit  50 . In the engine cooling device according to the present embodiment, the memory  52  corresponds to the storage unit in which information on the withstanding pressure limit rotational speed for each operating position of the valve body  35  is stored. 
       FIG. 6B  shows a relationship between the relative angle ANG of the valve body  35  and the withstanding pressure limit rotational speed in the engine cooling device according to the present embodiment. When the valve body  35  is located at the position where the relative angle ANG is “0°”, the opening ratios of the output ports P 1  to P 3  are all “0%”, and the flow of coolant is blocked by the flow rate control valve  26 . In the following description, that part of the circulation circuit  21  which is located downstream of the water pump  22  and upstream of the flow rate control valve  26  will be referred to as a pump/valve gap portion. When the engine rotational speed NE and hence the discharge pressure of the water pump  22  are raised with the flow of coolant blocked by the flow rate control valve  26 , the hydraulic pressure at the pump/valve gap portion reaches the withstanding pressure limit. At this time, the withstanding pressure limit rotational speed is the engine rotational speed NE at which the hydraulic pressure at the pump/valve gap portion reaches the withstanding pressure limit. 
     When the valve body  35  is relatively rotated in the positive direction from the position where the relative angle ANG is “0°”, the output ports P 1  to P 3  sequentially open, and coolant is delivered from the output ports P 1  to P 3 . Then, as a result, the hydraulic pressure at the pump/valve gap portion is reduced. Therefore, when the valve body  35  is relatively rotated in the positive direction from the position where the relative angle ANG is “0°”, the withstanding pressure limit rotational speed gradually rises. 
     On the other hand, when the flow rate of coolant delivered to the first coolant channel  271  from the radiator port P 1  increases, the pressure loss of the coolant flowing through the radiator  23  increases, and the hydraulic pressure in that part of the circulation circuit  21  which is located upstream of the radiator  23  in the first coolant channel  271  rises. In the following description, that part of the circulation circuit  21  which is located upstream of the radiator  23  in the first coolant channel  271  will be referred to as a valve/radiator gap portion. 
     When the valve body  35  relatively rotates to the position where the relative angle ANG is “γ°”, the engine rotational speed NE at which the hydraulic pressure at the pump/valve gap portion reaches the withstanding pressure limit becomes equal to the engine rotational speed NE at which the hydraulic pressure at the valve/radiator gap portion reaches the withstanding pressure limit. When the valve body  35  is relatively rotated further in the positive direction from the position where the relative angle ANG is “γ°”, the engine rotational speed NE at which the hydraulic pressure at the pump/radiator gap portion reaches the withstanding pressure limit becomes lower than the engine rotational speed NE at which the hydraulic pressure at the pump/valve gap portion reaches the withstanding pressure limit. In consequence, in the range where the relative angle ANG is larger than “γ°”, the withstanding pressure limit rotational speed is the engine rotational speed NE at which the hydraulic pressure at the valve/radiator gap portion reaches the withstanding pressure limit. Incidentally, when the valve body  35  is relatively rotated in the positive direction from the position where the relative angle ANG is “γ°”, the flow rate of coolant in the first coolant channel  271  also increases as the opening ratio of the radiator port P 1  increases. Therefore, the engine rotational speed NE at which the hydraulic pressure at the valve/radiator gap portion reaches the withstanding pressure limit drops. In consequence, the withstanding pressure limit rotational speed stops rising and starts dropping at the position where the relative angle ANG is “γ°” when the valve body  35  is relatively rotated in the positive direction from the position where the relative angle ANG is “0°”. 
     By the same token, when the valve body  35  is relatively rotated in the negative direction from the position where the relative angle ANG is “0°” as well, the withstanding pressure limit rotational speed rises until the valve body  35  reaches the position where the relative angle ANG is “−δ°”, and starts dropping afterward. In this manner, the withstanding pressure limit rotational speed is locally maximized at each of the relative rotational position of the valve body  35  where the relative angle ANG is “γ°”, and the relative rotational position of the valve body  35  where the relative angle ANG is “−δ°”. Incidentally, the three output ports P 1  to P 3  are all open at the relative rotational position of the valve body  35  where the relative angle ANG is “γ°”. In contrast, among the three output ports P 1  to P 3 , only the radiator port P 1  and the device port P 2  are open at the relative rotational position of the valve body  35  where the relative angle ANG is “−δ°”. Therefore, within the range of relative rotation of the valve body  35  from the position where the relative angle ANG is “−α°” to the position where the relative angle ANG is “β°”, the withstanding pressure limit rotational speed is maximized when the valve body  35  has relatively rotated to the position where the relative angle ANG is “γ°”. In the following description, the relative rotational position of the valve body  35  where the relative angle ANG is “γ°” will be referred to as a maximum withstanding pressure relative rotational position. 
     Subsequently, the control of the flow rate control valve  26  of the engine cooling device according to the present embodiment will be described.  FIGS. 7 and 8  are flowcharts of a flow rate control valve control routine that is carried out by the control unit  50  in controlling the flow rate control valve  26 . The control unit  50  repeatedly performs the process of the routine on a prescribed control cycle during operation of the engine  10 . 
     When the process of the present routine is started, a required relative rotational position is calculated first in step S 100 . In concrete terms, the relative angle ANG of the valve body  35  at which the opening ratios of the output ports P 1  to P 3  satisfy a requirement for the warming and cooling of the engine  10  and the ATF and a requirement for the warming of the cabin by the air-conditioner is calculated as a value of the required relative rotational position. Incidentally, the range of the relative rotational position of the valve body  35  that is set as the required relative rotational position ranges from the position where the relative angle ANG is “−α°” to the position where the relative angle ANG is “β°”. 
     Subsequently, in steps S 110  to S 170 , it is determined whether or not conditions (i) to (v) shown below are fulfilled. The condition (i) is that a shift range for parking (P) or a neutral shift range (N) is set as a shift range of the automatic transmission  241 , and that the engine rotational speed NE is equal to or higher than a prescribed retreat start rotational speed N 1  (YES in S 110 ). Incidentally, as shown in  FIG. 6B , the engine rotational speed NE that is lower than a minimum value of the withstanding pressure limit rotational speed is set as a value of the retreat start rotational speed N 1 . 
     The condition (ii) is that a shift range for running, namely, a shift range for forward running (D) or a shift range for backward running (R) is set as the shift range of the automatic transmission  241 , and that the engine rotational speed NE is equal to or higher than a prescribed retreat start rotational speed N 2  (YES in S 120 ) Incidentally, the engine rotational speed NE that is higher than the retreat start rotational speed N 1  in the condition (i) is set as a value of the retreat start rotational speed N 2  in the condition (ii). 
     The condition (iii) is that the vehicle is coasting (YES in S 130 ). In the present embodiment, it is determined that the vehicle is coasting when the operation amount of the accelerator pedal has remained equal to “ε°” and the engine rotational speed NE has remained equal to or higher than a certain rotational speed for a prescribed time or more. 
     The condition (iv) is that the post-startup elapsed time as an elapsed time after the beginning of the startup of the engine  10  is shorter than a prescribed time T 0  (NO in S 140 ), and that the supply voltage of the in-vehicle electric power supply  53  is lower than a voltage drop determination value V 1  (YES in S 150 ). 
     The condition (v) is that the post-startup elapsed time is equal to or longer than the prescribed time T 0  (YES in S 140 ), and that the supply voltage of the in-vehicle electric power supply  53  is equal to or lower than a voltage drop determination value V 2  (YES in S 160 ). Incidentally, a voltage higher than the voltage drop determination value V 1  is set as the voltage drop determination value V 2 . 
     The condition (vi) is that the temperature of coolant is lower than a prescribed low-temperature determination value (YES in S 170 ). When none of the conditions (i) to (vi) is fulfilled, the value of the required relative rotational position is directly set as the value of the target relative rotational position in step S 180 , and the process is then advanced to step S 210 . As described above, the relative rotational position of the valve body  35  that is set as the required relative rotational position ranges from the position where the relative angle ANG is “−α°” to the position where the relative angle ANG is “β°”. Therefore, the relative rotational position of the valve body  35  that is set as the target relative rotational position at this time also ranges from the position where the relative angle ANG is “−α°” to the position where the relative angle ANG is “β°”. 
     In contrast, in the case where at least one of the conditions (i) to (vi) is fulfilled as well, when the value of the required relative rotational position is equal to or larger than “ε°” (NO in S 190 ), the value of the required relative rotational position is directly set as the value of the target relative rotational position in step S 180 , and the process is then advanced to step S 210 . On the other hand, when at least one of the conditions (i) to (vi) is fulfilled and the value of the required relative rotational position is smaller than “ε°” (YES in S 190 ), “ε°” is set as the value of the target relative rotational position in step S 200 , and the process is then advanced to step S 210 . When at least one of the conditions (i) to (vi) is fulfilled in this manner, the relative rotational position of the valve body  35  that is set as the target relative rotational position ranges from the position where the relative angle ANG is “ε°” to the position where the relative angle ANG is “β°”. 
     The value of the relative rotational position that is located on the positive side from the position where the relative angle ANG is “ε°” is set as the value of the target relative rotational position in the case where at one of the conditions (i) to (vi) is fulfilled in this manner. As shown in  FIG. 6B , “ε°” is the relative angle ANG at an end on the negative side of a retreat operation range set in advance as the range of relative rotation of the valve body  35 , including the relative rotational position of the valve body  35  where the relative angle ANG as the maximum withstanding pressure relative rotational position is “γ°”. Accordingly, when at least one of the conditions (i) to (vi) is fulfilled, the relative angle ANG within the retreat operation range is set as the value of the target relative rotational position. 
     It should be noted herein that the control range of the valve body  35  is defined as the range of the relative rotational position of the valve body  35  that is set as the target relative rotational position. The control range of the valve body  35  in the case where none of the conditions (i) to (vi) is fulfilled is the range from the position where the relative angle ANG is “−α°” to the position where the relative angle ANG is “β°”. In contrast, when at least one of the conditions (i) to (vi) is fulfilled, the control range is reduced to the retreat operation range set in advance as the range of the relative rotational position of the valve body  35  including the maximum withstanding pressure relative rotational position. 
     When the process is advanced to step S 210  subsequently to the setting of the target relative rotational position in step S 180  or step S 200  as described above, a value of a withstanding pressure limit rotational speed NL at the relative angle ANG set as the value of the target relative rotational position is calculated, based on the map M stored in the memory  52 , in step S 210 . Furthermore, subsequently in step S 220 , it is determined whether or not the calculated withstanding pressure limit rotational speed NL is lower than the current engine rotational speed NE. Then, if the withstanding pressure limit rotational speed NL at the target relative rotational position is equal to or higher than the current engine rotational speed NE (NO), the process is directly advanced to step S 240 . In contrast, if the withstanding pressure limit rotational speed NL at the target relative rotational position is lower than the current engine rotational speed NE (YES), the withstanding pressure limit rotational speed is equal to or higher than the current engine rotational speed NE in step S 230 , and the relative angle ANG within the retreat operation range is obtained based on the map M. Then, after the obtained relative angle ANG is further reset as the value of the target relative rotational position in step S 230 , the process is advanced to step S 240 . 
     When the process is advanced to step S 240 , a value of the relative angle ANG at the relative rotational position where the valve body  35  is currently located is acquired in step S 240 . Incidentally, in the following description, the relative angle ANG at the relative rotational position where the valve body  35  is currently located will be referred to as a current relative angle. Incidentally, the current relative angle is obtained from a result of detection of the rotational angle of the motor  37  by the rotational angle sensor  123 . 
     Subsequently in step S 250 , a withstanding pressure limit rotational speed NN at the current relative angle is calculated based on the map M stored in the memory  52 . Then, subsequently in step S 260 , it is determined whether or not the current engine rotational speed NE is higher than the withstanding pressure limit rotational speed NN at the calculated current relative angle. If the withstanding pressure limit rotational speed NN is higher than the current engine rotational speed NE (YES), an operation of incrementing the value of a counter COUNT is performed in step S 270 , and the process is then advanced to step S 290 . On the other hand, if the withstanding pressure limit rotational speed NN is equal to or lower than the current engine rotational speed NE (NO in S 260 ), an operation of clearing the value of the counter COUNT to “0” is performed in step S 280 , and the process of the present routine is then ended. The value of the counter COUNT thus operated represents a time during which the withstanding pressure limit rotational speed NN has remained higher than the current engine rotational speed NE. 
     When the process is advanced to step S 290 , it is determined in step S 290  whether or not the value of the counter COUNT is equal to or larger than a prescribed permissible time determination value. If the value of the counter COUNT at this time is smaller than the permissible time determination value (NO), the process of the present routine on the current cycle is ended immediately. On the other hand, if the value of the counter COUNT at this time is equal to or larger than the permissible time determination value (YES), a request for a reduction in engine torque is output to the engine control unit  55 , and the process of the present routine on the current cycle is then ended. Incidentally, the engine control unit  55  reduces the torque of the engine  10  in accordance with the inputting of the request for the reduction in engine torque. 
     Incidentally, the control unit  50  performs supply control of the motor  13  to relatively rotate the valve body  35  toward the target relative rotational position set in the present routine. That is, when the current relative rotational position of the valve body  35  is located in the negative direction from the target rotational position, the control unit  50  supplies electric power to the motor  37  such that the rotational direction of the motor  37  coincides with the direction in which the valve body  35  is relatively rotated in the positive direction. Besides, when the current relative rotational position of the valve body  35  is located in the positive direction from the target rotational position, the control unit  50  supplies electric power to the motor  37  such that the rotational direction of the motor  37  coincides with the direction in which the valve body  35  is relatively rotated in the negative direction. Then, when the current relative rotational position of the valve body  35  coincides with the target relative rotational position, the control unit  50  stops supplying electric power to the motor  37 . 
     The operation and effect of the present embodiment will be described. In the engine cooling device according to the present embodiment that is equipped with the mechanical water pump  22  as described above, the value of the required relative rotational position is set in accordance with the requirement for the warming and cooling of the engine  10  and ATF and the requirement for the warming by the air-conditioner, and the value of the required relative rotational position is usually set directly as the value of the target relative rotational position. Then, supply control of the motor  37  is performed to change the relative rotational position of the valve body  35  to the set target relative rotational position. 
     On the other hand, in the engine cooling device according to the present embodiment that adopts the mechanical water pump  22  operating in response to rotation of the engine  10 , the discharge pressure of coolant in the water pump  22  rises as the engine rotational speed NE rises. Moreover, the hydraulic pressure of the circulation circuit  21  may become higher than the withstanding pressure limit when the valve body  35  of the flow rate control valve  26  is located at a certain relative rotational position at that time. 
     In contrast, the engine cooling device according to the present embodiment performs protection control for restraining the hydraulic pressure of the circulation circuit  21  from rising above the withstanding pressure limit by resetting the relative rotational position where the withstanding pressure limit rotational speed is equal to or higher than the current engine rotational speed NE, as the value of the target relative rotational position, when the engine rotational speed NE rises. 
     Besides, in the present embodiment, when at least one of the conditions (i) to (vi) is fulfilled, retreat control for resetting the relative rotational position within the retreat operation range set in advance as the range of the relative rotational position of the valve body  35  including the maximum withstanding pressure relative angle, as the target relative rotational position is performed. Thus, the relative rotational position of the valve body  35  is changed to the relative rotational position within the retreat operation range, namely, to the range that is not greatly distant from the maximum withstanding pressure relative rotational position. 
     Incidentally, even in the case where retreat control and protection control as described above are performed, when the engine rotational speed NE remains higher than the withstanding pressure limit rotational speed, a request for a reduction in engine torque is output to the engine control unit  55 , and the engine rotational speed NE is restrained from rising due to the reduction in engine torque corresponding to the request. 
     The engine cooling device according to the present embodiment described above can exert the following effects. (1) In the present embodiment, the foregoing retreat control is performed when the supply voltage of the in-vehicle electric power supply  53  has dropped. When the supply voltage of the in-vehicle electric power supply  53  drops, the speed at which the relative rotational position of the valve body  35  is changed by the motor  37  drops, and the time needed to change the relative rotational position of the valve body  35  in protection control becomes long. In this respect, when the foregoing retreat control is performed prior to the performance of protection control, the amount of change in the relative rotational position of the valve body  35  in the case where protection control is thereafter performed in response to a rise in the engine rotational speed NE does not increase beyond a certain amount. Therefore, even when the supply voltage of the in-vehicle electric power supply  53  drops to cause a drop in the speed at which the relative rotational position of the valve body  35  is changed, the time needed to change the relative rotational position of the valve body  35  in protection control is unlikely to become long. Accordingly, even in the case where the supply voltage of the in-vehicle electric power supply  53  has dropped, the time needed to restrain the hydraulic pressure of the circulation circuit  21  from rising when the engine rotational speed NE rises is unlikely to become long. 
     (2) Information on the withstanding pressure limit rotational speed at each relative rotational position of the valve body  35  is stored in advance in the memory  52 . In protection control, the relative rotational position of the valve body  35  at which the withstanding pressure limit rotational speed obtained based on the information is higher than the current engine rotational speed NE is set as the target relative rotational position. Therefore, in protection control, the appropriate target relative rotational position at which the withstanding pressure limit rotational speed is equal to or higher than the engine rotational speed NE can be set. 
     (3) It is determined whether or not the engine torque needs to be reduced, by determining that the engine torque needs to be reduced when the current engine rotational speed NE has remained higher than the withstanding pressure limit rotational speed at the current relative rotational position of the valve body  35  for the prescribed time or more. Therefore, the hydraulic pressure can be restrained from rising, by making a request for a reduction in engine torque and retraining the engine rotational speed NE from rising when the hydraulic pressure cannot be sufficiently restrained from rising through protection control. 
     (4) Immediately after the startup of the engine, the supply voltage of the in-vehicle electric power supply  53  may temporarily drop due to the consumption of electric power for the startup of the engine. This drop in supply voltage of the in-vehicle electric power supply  53  immediately after the startup of the engine is stopped in a short time, so the performance of retreat control as a measure against the drop in supply voltage on this occasion is often unnecessary. In contrast, according to the present embodiment, when the elapsed time after the startup of the engine is shorter than the prescribed time T 0 , the voltage higher than in the case where the elapsed time is equal to or longer than the prescribed time T 0  is set as the voltage drop determination value, so retreat control is unlikely to be performed unnecessarily. 
     (5) When the coolant temperature is low, the viscosity of coolant is high, and the flow resistance of coolant applied to the valve body  35  in changing the relative rotational position of the valve body  35  is high. Therefore, even when the temperature of coolant is low, the speed at which the relative rotational position of the valve body  35  is changed by the motor  37  is low. In contrast, according to the present embodiment, retreat control is performed even when the coolant temperature is equal to or lower than the prescribed low coolant temperature determination value. Therefore, even in the case where the speed at which the relative rotational position of the valve body  35  is changed by the motor  37  has dropped due to the low coolant temperature, the hydraulic pressure of the circulation circuit  21  is unlikely to be insufficiently restrained from rising when the engine rotational speed NE rises. 
     (6) Retreat control is performed even when the engine rotational speed NE is high to a certain extent and the performance of protection control may be needed in a short time. Therefore, the hydraulic pressure of the circulation circuit  21  can be swiftly restrained from rising when the engine rotational speed NE rises. 
     (7) In setting the shift range for stop or the shift range for neutrality, the transmission of motive power between the engine  10  and the wheels is shut off by the automatic transmission  241 , and that part of a motive power transmission system of the vehicle which is located on the wheel sides from the automatic transmission  241  is disconnected from the engine  10 , so the rotational load of the engine  10  decreases. Therefore, in setting the shift range for stop or the shift range for neutrality, the speed at which the engine rotational speed NE rises tends to be higher than in setting the shift range for running with the transmission of motive power not shut off. In contrast, according to the present embodiment, when the shift range of the automatic transmission  241  is set as the shift range for stop or the shift range for neutrality, retreat control is performed at the engine rotational speed NE that is lower than when the shift range of the automatic transmission  241  is set as the shift range for running. Therefore, even in the case where the transmission of motive power between the engine  10  and the wheels is shut off by the automatic transmission  241  and the speed at which the engine rotational speed NE rises tends to be high, the hydraulic pressure of the circulation circuit is easily restrained from rising when the engine rotational speed NE rises. 
     (8) In the engine  10  mounted on the vehicle, while the vehicle is coasting with the engine  10  dragged as the wheels rotate, the engine rotational speed may rapidly rise through a downshift or the like. In contrast, according to the present embodiment, retreat control is performed even while the vehicle is coasting. Therefore, the hydraulic pressure of the circulation circuit  21  is easily restrained from rising even when the engine rotational speed NE rapidly rises while the vehicle is coasting. 
     Incidentally, according to the present embodiment, the operating position of the valve body  35  in the flow rate control valve  26  is represented by the rotational position of the valve body  35  relative to the housing  31 . In the present embodiment, the target relative rotational position corresponds to the target operating position, and the maximum withstanding pressure relative rotational position corresponds to the maximum withstanding pressure operating position. 
     The present embodiment can be carried out after being modified as follows. The present embodiment and the following modification examples can be carried out in combination with one another within such a range that no technical contradiction occurs. In the foregoing embodiment, the information on the withstanding pressure limit rotational speed at each relative rotational position of the valve body  35  is stored in a recording device  42  as the map M, and the target relative rotational position in protection control is calculated based on the stored information. However, the target relative rotational position in protection control may be calculated according to another method, without storing the aforementioned information. For example, the target relative rotational position in protection control may be fixed to the maximum withstanding pressure operating position or the like. 
     In the foregoing embodiment, when the current engine rotational speed NE has remained higher than the withstanding pressure limit rotational speed at the current relative rotational position of the valve body  35  for the prescribed time or more, it is determined that the engine torque needs to be reduced, and a request for a reduction in engine torque is output to the engine control unit  55 . The determination on the necessity to reduce the engine torque and the outputting of the request for reduction may be omitted. 
     In the foregoing embodiment, when the shift range of the automatic transmission  241  is set as the shift range for stop or the shift range for neutrality to shut off the transmission of motive power between the engine  10  and the wheels, retreat control is performed from the engine rotational speed NE that is lower than in setting the shift range for running with the transmission of motive power not shut off. In a vehicle adopting a manual transmission, the transmission of motive power between an engine and wheels is shut off when a clutch provided between the engine and the manual transmission is disengaged or when the manual transmission is in a neutral state. In consequence, in the vehicle adopting the manual transmission, when at least one of a condition (vii) that the clutch is disengaged and a condition (viii) that the manual transmission is in the neutral state is fulfilled, retreat control may be performed from the engine rotational speed NE that is lower than when both the conditions (vii) and (viii) are not fulfilled. 
     In the foregoing embodiment, when the transmission of motive power between the engine and the wheels is shut off, retreat control is performed from the engine rotational speed NE that is lower than when the transmission of motive power between the engine and the wheels is not shut off. However, retreat control may be performed when the engine rotational speed NE becomes equal to or higher than a certain rotational speed, regardless of whether or not the transmission of motive power is shut off. 
     In the foregoing embodiment, the low-voltage determination value is changed depending on the elapsed time after the startup of the engine. However, a fixed value may be set as the low-voltage determination value, regardless of the elapsed time after the startup of the engine. 
     Retreat control is performed when at least one of the conditions (i) to (vi) is fulfilled. However, one or more of the conditions (i), (ii), (iii), and (vi) may be omitted. 
     The number of output ports of the flow rate control valve  26  and the number of coolant channels leading to the output ports in the circulation circuit may be appropriately changed. The flow rate control valve  26  adopted in the foregoing embodiment has the valve body  35  that rotates relatively to the housing  31 , and the flow channel area of coolant at the output ports changes depending on the relative rotational position of the valve body  35 . However, a flow rate control valve having a valve body that performs an operation other than relative rotation, such as a reciprocating rectilinear motion may be adopted. 
     A flow rate control valve adopting an electric actuator other than the motor  37 , for example, an electromagnetic solenoid, as an actuator for driving the valve body  35  may be adopted.