Patent Publication Number: US-8979474-B2

Title: Variable flow rate pump

Description:
TECHNICAL FIELD 
     The present invention relates to a variable flow rate pump represented by a water pump or the like, for example, that circulates a coolant through an engine. 
     TECHNICAL BACKGROUND 
     A water pump is used conventionally in a cooling device for a water-cooled engine installed in a vehicle or the like, and a cooling performance of the engine is closely related to a flow rate of a coolant circulated by the water pump. This type of cooling device is constituted by a coolant passage including a water jacket, which is provided in an engine main body, and a radiator, a thermostat, the aforesaid water pump, and so on, which are connected to the coolant passage. In this type of cooling device, the water pump is operated when the engine is driven to circulate the coolant through the coolant passage. As the coolant flows through the water jacket, heat exchange is performed with an engine main body, and as a result, the engine is cooled (see Patent Document 1, for example). 
     PRIOR ARTS LIST 
     Patent Document 
     Patent Document 1: Japanese Laid-Open Patent Publication No. H11-336549(A) 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     Incidentally, in an engine cooling device, the engine must be cooled when warm using the coolant circulated by the water pump in order to suppress burning, friction, and so on in the engine. During engine startup from a cold condition, on the other hand, the engine must be warmed quickly from the cold condition, in which thermal efficiency is poor. In a conventional water pump that operates in conjunction with driving of the engine, when a pump rotation speed is maintained at a fixed speed at this time, the coolant is discharged at a fixed flow rate corresponding to a volume of a pump swirl chamber or the like, regardless of a temperature of the circulating coolant. Therefore, during an engine warm-up operation, a discharge flow rate of the water pump increases gradually as the pump rotation speed of the water pump rises in conjunction with the engine such that when the pump rotation speed is maintained at a fixed speed thereafter, the coolant supplied to the engine is likewise discharged at a fixed flow rate (a maximum flow rate) corresponding to a pump capacity, regardless of variation in the temperature of the coolant. As a result, the engine may be cooled, leading to friction and so on in the engine interior and an increase in an amount of CO 2  discharged in exhaust gas due to a reduction in thermal efficiency. 
     The present invention has been designed in consideration of this problem, and an object thereof is to provide a variable flow rate pump with which an improvement in a warm-up performance of an engine can be achieved. 
     Means to Solve the Problems 
     To solve the problem described above, a variable flow rate pump (a water pump  30  according to an embodiment, for example) according to the present invention is provided in a coolant circulation passage to take in a coolant from a suction passage (a coolant passage  7  according to an embodiment, for example) of the circulation passage and supply the coolant to a discharge passage (a coolant passage  8  according to an embodiment, for example), and includes: a housing; an impeller chamber formed in the housing to communicate with the suction passage; a swirl chamber formed in the housing to communicate with the discharge passage and the impeller chamber; an impeller supported to be free to rotate in the impeller chamber so as to take in the coolant from the suction passage and discharge the coolant into the discharge passage via the swirl chamber while rotating; and driving means (an engine  2  according to an embodiment, for example) for rotating the impeller. The swirl chamber is formed to be divided into a main swirl chamber (a first swirl chamber  41  according to an embodiment, for example) that communicates with the discharge passage at all times and a secondary swirl chamber (a second swirl chamber  42  according to an embodiment, for example) that is connected to the discharge passage via a thermostat having a switch valve that can be opened and closed. The thermostat is operated to open and close, thereby connecting and cutting off the secondary swirl chamber and the discharge passage, in accordance with a temperature of coolant delivered from the secondary swirl chamber. 
     In the variable flow rate pump configured as described above, the secondary swirl chamber is preferably further divided to form a plurality of divided swirl chambers (the second swirl chamber  42  and a third swirl chamber  43  according to an embodiment, for example), a plurality of thermostats respectively having switch valves that can be opened and closed to connect and cut off the plurality of divided swirl chambers and the discharge passage in accordance with the temperature of the coolant are preferably disposed between the plurality of divided swirl chambers and the discharge passage, and sensitive temperatures of the plurality of thermostats for connecting and cutting off the plurality of divided swirl chambers and the discharge passage are preferably set at respectively different temperatures. 
     Further, a volume of the main swirl chamber is preferably formed to be smaller than a volume of each of the divided swirl chambers. 
     Advantageous Effects of the Invention 
     With the variable flow rate pump according to the present invention, when the engine is started from a cold condition, coolant is supplied to the engine at a small flow rate only from the main swirl chamber that communicates with the engine at all times, and therefore warm-up of the engine can be promoted while suppressing a thermal load such that the engine can be warmed quickly. When the engine is warm, on the other hand, the thermostat connects the secondary swirl chamber and the discharge passage by a valve opening corresponding to the temperature of the circulating coolant such that coolant is supplied to the engine from the secondary swirl chamber in addition to the coolant from the constantly communicative main swirl chamber. As a result, a sufficient engine cooling effect can be exhibited by the coolant having the increased flow rate, leading to a reduction in friction in the engine interior and a corresponding improvement in fuel efficiency. Moreover, an improvement in the thermal efficiency of the engine can be achieved, enabling a reduction in the amount of CO 2  discharged from the engine in the exhaust gas. 
     In the inventions described above, by further dividing the secondary swirl chamber into the plurality of divided swirl chambers and setting the sensitive temperatures of the plurality of thermostats for connecting and cutting off the plurality of divided swirl chambers and the discharge passage at respectively different temperatures, a pump discharge flow rate can be controlled more finely in response to variation in the temperature of the coolant. Further, by adjusting the discharge flow rate in steps in accordance with variation in the temperature of the coolant, coolant discharge at a flow rate exceeding a required flow rate of the engine can be prevented. As a result, a workload of the water pump can be prevented from becoming excessive, and energy loss can be reduced. 
     Furthermore, in the inventions described above, by forming the volume of the main swirl chamber to be smaller than the volume of the divided swirl chambers, the coolant can be supplied to the engine at a small flow rate when the engine is started from a cold condition, and therefore an engine warm-up time can be reduced even further. When the engine is warm, on the other hand, the coolant can be supplied to the engine at a larger flow rate, and therefore the engine cooling effect can be improved even further such that overheating and the like can be prevented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing an engine cooling device including a variable flow rate pump according to an embodiment of the present invention; 
         FIG. 2  is a sectional view showing the main parts of the variable flow rate pump; 
         FIG. 3  is a pattern diagram showing operation conditions of the variable flow rate pump corresponding to variation in a temperature of a coolant, wherein  FIG. 3A  shows a condition during cold startup,  FIG. 3B  shows a condition in which the coolant temperature is lower than an appropriate coolant temperature, and  FIG. 3C  shows a condition in which the coolant temperature exceeds the appropriate coolant temperature; and 
         FIG. 4  is a graph comparing the variable flow rate pump with a conventional normal pump in terms of a relationship of the coolant temperature to a pump discharge flow rate and a pump workload (a consumed horsepower) at a fixed pump rotation speed. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A preferred embodiment of the present invention will be described below with reference to the drawings. In this embodiment, a variable flow rate pump is disposed on a coolant circulation path of an engine, but before describing the variable flow rate pump according to this embodiment, an engine cooling device to which the variable flow rate pump is applied will be described using  FIG. 1 . 
     An engine cooling device  1  is constituted mainly by an engine  2  formed from a water-cooled internal combustion engine, a radiator  10  for cooling a coolant serving as an engine cooling medium when the coolant is discharged from the engine  2 , a thermostat  20  for controlling circulation of the coolant in accordance with a temperature of the coolant, and a variable flow rate pump (to be referred to in the following description as a “water pump”)  30  for forcibly circulating the coolant. The engine cooling device  1  cools the engine  2  by circulating the coolant through coolant passages  5  ( 5   a ,  5   b ),  6 ,  7 ,  8  connecting the components described above. Note that in  FIG. 1 , a flow of the coolant flowing through the coolant passages  5  to  8  is indicated by solid line arrows. 
     The engine  2  is a water-cooled gasoline engine, for example, and a water jacket (not shown) is provided in the interior thereof as a space formed to cover a cylinder (not shown). The coolant is caused to flow into the water jacket through a coolant introduction port  3 , performs heat exchange with the cylinder and so on while passing through the water jacket, and is then discharged from a coolant discharge port  4 . 
     The radiator  10  is connected to the coolant discharge port  4  of the engine  2  via the coolant passage  5  ( 5   a ), and is configured to cool the coolant passing through the interior thereof by blowing air from an electric fan, not shown in the drawing, such that heat is released to the outside. Hence, a temperature of the coolant, which was raised in the water jacket of the engine  2 , is lowered by heat radiation as the coolant passes through the radiator  10 . 
     The thermostat  20  is connected to the radiator  10  via the coolant passage  6  and connected to the coolant passage  5   b , which is formed as a bypass passage that bifurcates from the coolant passage  5  so as to bypass the radiator  10 . The thermostat  20  is constituted by a coolant-sensitive switch valve that opens and closes in accordance with the temperature of the coolant. Accordingly, when the temperature of the coolant is equal to or lower than a predetermined temperature, the coolant passage (the bypass passage)  5   b  communicates with the coolant passage  7 , and when the temperature of the coolant exceeds the predetermined temperature, the coolant passage  6  communicates with the coolant passage  7 . 
     The water pump  30  is connected to the thermostat  20  via the coolant passage  7 , and a pump rotary shaft thereof is drive-coupled to a crankshaft (not shown) of the engine  2  via a pulley, a belt, and so on. Thus, the water pump  30  operates in conjunction with driving of the engine  2 . The coolant passage  8  is connected to a discharge port of the water pump  30  such that the coolant discharged from the water pump  30  is supplied to the water jacket from the coolant introduction port  3  of the engine  2  through the coolant passage  8 . 
     In the engine cooling device thus configured, the coolant discharged from the water pump  30  flows into the water jacket formed in the interior of the engine  2 , cools the engine  2 , and is then discharged to the outside. The discharged coolant is either cooled by the radiator  10  or caused to flow into the thermostat  20  via the bypass passage  5   b  without passing through the radiator  10 , and then returned to the water pump  30  to be circulated. 
     In the engine cooling device  1  described above, the engine  2  must be cooled when warm to suppress burning, friction, and so on in the engine  2 . When the engine  2  is started up from a cold condition, on the other hand, the engine  2  must be warmed quickly from the cold condition, in which thermal efficiency is poor. In a conventional water pump, however, a discharge flow rate increases as a pump rotation speed rises, and when the pump rotation speed is maintained at a fixed speed, the coolant is discharged at a fixed flow rate corresponding to a volume of a swirl chamber or the like, regardless of variation in the temperature of the coolant. Therefore, when an engine rotation speed increases during a warm-up operation in the engine  2 , the rotation speed of the pump rotary shaft (an impeller) of the water pump  30 , which operates in conjunction with the engine  2  via the crankshaft and so on, also increases, leading to an increase in the flow rate of the coolant supplied to the engine  2 . When the pump rotation speed is maintained at a fixed speed, the coolant supplied to the engine is discharged at a fixed flow rate (a maximum flow rate) corresponding to a pump capacity, regardless of variation in the temperature of the coolant, and as a result, the engine  2  is cooled, thereby impairing the warm-up operation. 
     Hence, in the water pump  30  according to this embodiment, the discharge flow rate of the coolant supplied to the engine  2  is controlled variably in accordance with the temperature of the circulating coolant. The constitution of the water pump  30  will now be described with additional reference to  FIGS. 2 and 3 . Note that  FIG. 2  is a sectional view showing the main parts of the water pump  30 , and  FIG. 3  is a pattern diagram showing operation conditions of the water pump  30  corresponding to variation in the temperature of the coolant. 
     As shown in  FIG. 2 , the water pump  30  is mainly constituted by an impeller chamber  32  formed in a housing  31 , a swirl chamber  40  formed in the housing  31  on an outer peripheral side of the impeller chamber  32  and divided into three chambers, and an impeller  33  attached to the impeller chamber  32  to be free to rotate. 
     The impeller  33  includes a base plate portion  34  formed in an annular plate shape, and a plurality of vanes  35  formed to project at equal intervals on one side face of the base plate portion  34 , and is configured to be capable of rotating in a rotation direction F (a clockwise direction) about a pump rotary shaft  36 , which is drive-coupled to the crankshaft (not shown) of the engine  2  via a pulley, a belt, and so on. 
     A suction passage (not shown) that communicates with the coolant passage  7  is connected to a central portion of the impeller chamber  32 , and the impeller chamber  32  receives a centrifugal force generated when the impeller  33  rotates such that the coolant flowing through the coolant passage  7  is suctioned therein through the suction passage. 
     The swirl chamber  40  is constituted by three swirl chambers, namely a first swirl chamber  41 , a second swirl chamber  42 , and a third swirl chamber  43 , which are disposed at intervals in a circumferential direction on the outer peripheral side of the impeller chamber  32 . In other words, rather than being formed in an integral ring shape around the entire circumference of the outer peripheral side of the impeller chamber  32 , as in the related art, the swirl chamber  40  is divided into three chambers on the outer peripheral side of the impeller chamber  32  in the circumferential direction in respective ranges of angles θ 1 , θ 2 , θ 3 . 
     The first swirl chamber  41  opens onto the outer peripheral side of the impeller chamber  32  on an inner peripheral side thereof such that coolant delivered outwardly in a radial direction from the impeller  33  can flow therein over a circumferential direction range of the angle θ 1 , and a first discharge port  51  serving as an outlet for the inflowing coolant is provided in a terminal end portion thereof so as to communicate with the coolant passage  8  at all times. Hence, the coolant that is delivered into the first swirl chamber  41  is discharged from the first discharge port  51  of the first swirl chamber  41  constantly as the impeller  33  rotates. 
     The second swirl chamber  42  opens onto the outer peripheral side of the impeller chamber  32  on an inner peripheral side thereof such that the coolant delivered outwardly in the radial direction from the impeller  33  can flow therein over a circumferential direction range of the angle θ 2  (θ 2 &gt;θ 1 ), and a second discharge port  52  serving as an outlet for the inflowing coolant is provided in a terminal end portion thereof so as to communicate with the coolant passage  8 . 
     Further, a thermostat S 1  that connects and cuts off the second discharge port  52  and the coolant passage  8  is connected between the second discharge port  52  and the coolant passage  8 . The thermostat S 1  is constituted by a coolant-sensitive switch valve that opens and closes in accordance with the temperature of the coolant discharged from the second discharge port  52 . When the temperature of the coolant is equal to or lower than a predetermined first temperature T 1  (60° C., for example), the thermostat S 1  closes, thereby completely cutting off the second discharge port  52  from the coolant passage  8 , and when the coolant temperature exceeds the first temperature T 1 , the thermostat S 1  begins to open such that the second discharge port  52  communicates with the coolant passage  8  and the coolant introduced into the second swirl chamber  41  is discharged from the second discharge port  52  at a flow rate corresponding to a valve opening. When the temperature of the coolant reaches a predetermined second temperature T 2  (70° C., for example), the thermostat S 1  enters a fully open condition. 
     The third swirl chamber  43  opens onto the outer peripheral side of the impeller chamber  32  on an inner peripheral side thereof such that the coolant delivered outwardly in the radial direction from the impeller  33  can flow therein over a circumferential direction range of the angle θ 3  (θ 3 &gt;θ 1 ), and a third discharge port  53  serving as an outlet for the inflowing coolant is provided in a terminal end portion thereof so as to communicate with the coolant passage  8 . 
     Further, a thermostat S 2  that connects and cuts off the third discharge port  53  and the coolant passage  8  is connected between the third discharge port  53  and the coolant passage  8 . The thermostat S 2  is constituted by a coolant-sensitive switch valve that opens and closes in accordance with the temperature of the coolant discharged from the third discharge port  53 . When the temperature of the coolant is equal to or lower than a predetermined third temperature T 3  (75° C., for example), the thermostat S 2  closes, thereby completely cutting off the third discharge port  53  from the coolant passage  8 , and when the coolant temperature exceeds the third temperature T 3 , the thermostat S 2  begins to open such that the third discharge port  53  communicates with the coolant passage  8  and the coolant introduced into the third swirl chamber  43  is discharged from the third discharge port  53  at a flow rate corresponding to the valve opening. When the temperature of the coolant reaches a predetermined fourth temperature T 4  (85° C., for example), the thermostat S 2  enters a fully open condition, and at this point, the flow rate of the coolant discharged from the respective discharge ports  51 ,  52 ,  53  of the water pump  30  reaches a maximum. 
     The water pump  30  configured as described above introduces the coolant delivered into the respective swirl chambers  41 ,  42 ,  43  by the centrifugal force generated when the impeller  33  rotates into the engine  2  at a discharge flow rate corresponding to the temperature of the coolant. In other words, the water pump  30  varies a volume by which the swirl chamber communicates with the coolant passage  8  by switching between a condition in which the coolant passage  8  communicates with the first swirl chamber  41 , a condition in which the coolant passage  8  communicates with the first and second swirl chambers  41 ,  42 , and a condition in which the coolant passage  8  communicates with the respective swirl chambers  41 ,  42 ,  43  in accordance with the temperature of the coolant at a fixed pump rotation speed. Thus, the water pump  30  variably controls the discharge flow rate of the coolant supplied to the engine  2 . 
     Next, an operation of the water pump  30  having the above constitution will be described with additional reference to  FIG. 4 .  FIG. 4  is a graph comparing the water pump  30  according to this embodiment with a conventional water pump (a normal pump) in terms of a relationship of the coolant temperature to the pump discharge flow rate and a pump workload (a consumed horsepower) at a fixed pump rotation speed (2000 rpm). Note that in the drawing, solid lines indicate the discharge flow rate relative to the coolant temperature, while dotted lines indicate the consumed horsepower relative to the coolant temperature. Further, here, the fourth temperature T 4  (85° C.) is set as an appropriate cooling temperature of the engine  2 . 
     When the pump rotation speed is maintained at the fixed speed (2000 rpm) in the conventional water pump at this time, the discharge flow rate and the workload are held at fixed levels at all times, regardless of variation in the temperature of the coolant. As a result, the warm-up performed while the engine is cold, as described above, is impaired, and even when an engine load is small such that a heat balance is maintained, the coolant may be supplied at a greater flow rate than necessary such that an excessive workload (engine driving force) is used. In the water pump  30 , on the other hand, as shown in  FIG. 4 , the discharge flow rate and the workload are adjusted in accordance with variation in the temperature of the coolant, even when the pump rotation speed is maintained at the fixed speed (2000 rpm). This operation will now be described more specifically. 
     When the engine  2  is started in a vehicle, for example, the impeller  33  of the water pump  30  rotates in the rotation direction F (the clockwise direction) about the pump rotary shaft  36  drive-coupled to the crankshaft (not shown) of the engine  2  via a pulley, a belt, and so on. When the engine  2  is started up from a cold condition at this time, the coolant temperature is low, and therefore the thermostats S 1 , S 2  are both closed, as shown in  FIG. 3A , such that only the first discharge port  51  communicates with the coolant passage  8  for introducing the coolant into the engine  2  while the second and third discharge ports  52 ,  53  are cut off from the coolant passage  8 . Accordingly, the coolant that is suctioned into the impeller chamber  32  from the suction passage by the centrifugal force generated as the impeller  33  rotates is delivered into the respective swirl chambers  41 ,  42 ,  43  by the impeller  33 , whereupon only the coolant delivered into the first swirl chamber  41  is discharged through the first discharge port  51  at a flow rate corresponding to the volume of the swirl chamber and supplied to the engine  2  through the coolant passage  8 . 
     Hence, when the engine  2  is started up from a cold condition, coolant is supplied to the engine  2  at a small flow rate only from the first swirl chamber  41  having a small volume, and therefore an engine cooling effect is suppressed (warm-up of the engine  2  is promoted). Hence, in comparison with a conventional water pump configured such that coolant is discharged from a swirl chamber formed integrally around the entire outer periphery (360°) of the impeller chamber in an amount corresponding to the volume of the swirl chamber, a warm-up time of the engine  2  can be reduced, enabling quick warm-up, under an identical pump rotation speed (engine rotation speed) condition. 
     As warm-up of the engine  2  progresses in this condition, the flow rate at which the coolant is supplied to the engine  2  may become insufficient, causing a partial temperature increase in the engine  2 , and as a result, burning or an increase in friction may occur. Therefore, at a prior stage (T 1  to T 2 : 60° C. to 70° C.) before the coolant rises to the appropriate coolant temperature (T 4 : 85° C.) for the engine  2 , coolant is supplied to the engine  2  from the second swirl chamber  42  in addition to the coolant from the first swirl chamber  41 . More specifically, when the engine  2  is driven such that the temperature of the coolant circulating through the coolant passage increases gradually so as to exceed the predetermined first temperature T 1  (60° C.), the thermostat S 1  begins to open, as shown in  FIG. 3B , whereby the second discharge port  52  communicates with the coolant passage  8 . As the temperature of the coolant transitions from the first temperature T 1  (60° C.) to the second temperature T 2  (70° C.), the valve opening of the thermostat S 1  increases substantially proportionately with the coolant temperature, leading to an increase in the flow rate of the coolant from the second swirl chamber  42 . As a result, the coolant from the second discharge port  52 , the flow rate of which increases in accordance with the valve opening of the thermostat S 1 , and the coolant that is discharged from the first discharge port  51  at all times at a fixed flow rate are delivered into the coolant passage  8  and supplied to the engine  2 . In an environment where a heat balance is maintained in the engine  2  at the flow rate of the coolant from the first swirl chamber  41  and the second swirl chamber  42  (i.e. below a maximum capacity of the water pump  30 ), for example, the engine  2  can be cooled efficiently using a smaller pump workload than that of the related art. 
     When a heat balance is not maintained in the engine  2  and the temperature of the coolant rises further so as to exceed the third temperature T 3  (75° C.), on the other hand, the other thermostat S 2  begins to open such that the third discharge port  53  communicates with the coolant passage  8 , and as a result, the coolant passage  8  communicates with all of the first to third discharge ports  51 ,  52 ,  53 . As the temperature of the coolant transitions from the third temperature T 3  (75° C.) to the fourth temperature T 4  (85° C.), the valve opening of the thermostat S 2  increases substantially proportionately with the coolant temperature, leading to an increase in the flow rate of the coolant from the third swirl chamber  43 . As a result, the coolant from the third discharge port  53 , the flow rate of which increases in accordance with the valve opening of the thermostat S 2 , and the coolant that is discharged from the first and second discharge ports  51 ,  52  at a fixed flow rate are delivered into the coolant passage  8  and supplied to the engine  2 . Therefore, the engine  2  can be cooled even more effectively by the action of the coolant having the even higher flow rate. 
     When the temperature of the coolant reaches the fourth temperature T 4  (85° C.), the valve opening of the thermostat S 2  reaches a maximum, and after exceeding the appropriate coolant temperature, the coolant is discharged from the respective discharge ports  51 ,  52 ,  53  and supplied to the engine  2  at the maximum discharge flow rate of the water pump  30 . In other words, an equal discharge flow rate to that of the conventional water pump is realized in this condition. 
     According to the water pump  30  configured as described above, when the engine  2  is started up from a cold condition, the coolant is supplied to the engine  2  at a small flow rate only from the first swirl chamber  41  that communicates with the engine  2  via the coolant passage  8  at all times, and therefore warm-up of the engine  2  can be promoted while suppressing a thermal load of the engine  2  such that the engine  2  can be warmed quickly. When the engine  2  is warm, on the other hand, the thermostats S 1 , S 2  are opened to a valve opening corresponding to the temperature of the circulating coolant such that the coolant is supplied to the engine  2  from the second and third swirl chambers  42 ,  43  at a flow rate corresponding to the valve opening in addition to the coolant from the first swirl chamber  41 . As a result, a sufficient engine cooling effect can be exhibited, leading to a reduction in friction in the engine  2  and a corresponding improvement in fuel efficiency, and an improvement in the thermal efficiency can be achieved, enabling a reduction in an amount of CO 2  discharged from the engine in exhaust gas. Further, by adjusting the discharge flow rate in steps in accordance with variation in the temperature of the coolant, coolant discharge at a flow rate exceeding the required flow rate of the engine  2  can be prevented. As a result, the workload of the water pump can be prevented from becoming excessive, and energy loss can be reduced. 
     A preferred embodiment of the present invention was described above, but the scope of the present invention is not limited to the above embodiment. For example, in the above embodiment, the swirl chamber  40  of the water pump  30  is divided into the first, second, and third swirl chambers, but the present invention is not limited thereto, and the swirl chamber  40  may be further divided into fourth and fifth swirl chambers. In so doing, the discharge flow rate of the water pump can be varied in more steps, enabling finer control of the flow rate. 
     Further, in the above embodiment, the predetermined temperatures (sensitive temperatures) at which the thermostats S 1 , S 2  open and close are set at the first temperature T 1 , i.e. 60° C., and the third temperature T 3 , i.e. 75° C., respectively, but the present invention is not limited thereto, and the sensitive temperatures may be modified appropriately in accordance with a required cooling performance of the engine. 
     EXPLANATION OF NUMERALS AND CHARACTERS 
       1  engine cooling device 
       2  engine (driving means) 
       7  coolant passage (suction passage) 
       8  coolant passage (discharge passage) 
       30  water pump (variable flow rate pump) 
       31  housing 
       32  impeller chamber 
       33  impeller 
       40  swirl chamber 
       41  first swirl chamber (main swirl chamber) 
       42  second swirl chamber (secondary swirl chamber, divided swirl chamber) 
       43  third swirl chamber (secondary swirl chamber, divided swirl chamber) 
     S 1  thermostat 
     S 2  thermostat