Patent Publication Number: US-9404410-B2

Title: Controller for engine cooling system

Description:
This application is a Divisional of application Ser. No. 13/039,599, filed Mar. 3, 2011 and claims priority from Japanese Patent Applications No. 2010-46588 filed on Mar. 3, 2010, and No. 2010-49177 filed on Mar. 5, 2010, the disclosures of each of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a controller for an engine cooling system. Further, the present invention relates to an air-conditioner for an automobile in which heating is performed by use of engine coolant. 
     BACKGROUND OF THE INVENTION 
     JP-8-144758A shows an engine cooling system in which engine coolant is circulated in order to cool a cylinder head and cylinder block of an engine. A mechanical water pump circulating the engine coolant is driven by a driving force transmitted from a crankshaft of the engine. While the engine is running, the mechanical water pump is also driven in order to circulate the engine coolant. A combustion chamber of the engine is also cooled, so that anti-knocking ability is improved. 
     If the engine is shut down with high temperature, the cylinder head temperature may be greater than a specified temperature when the engine is restarted. This specified temperature is established for improving the anti-knocking ability. If the engine is restarted with the cylinder head of high temperature, fuel consumption efficiency may be deteriorated. 
     Especially, in a vehicle having an idle reduction function and a vehicle having a hybrid engine, the equipped engine is frequently stopped and restarted. Thus, the above problems often occur. 
     U.S. Pat. No. 5,337,704 shows an engine cooling system in which engine coolant passed through a cylinder-head passage flows into a heat exchanger for heating a passenger compartment. 
     EP-1008474A1 shows a heating system which includes two heat exchangers into which engine coolant is respectively introduced. 
     In order to improve the anti-knocking ability, a cylinder head should be positively cooled. 
     Meanwhile, in order to restrict an increase in friction in an engine, a cylinder block should be kept at specified temperature or more. A cylinder-head-passage and a cylinder-block-passage are formed in the system, and coolant flow rate flowing through the cylinder-head passage is made larger than that flowing through the cylinder-block-passage. 
     When the engine coolant passed through only the cylinder head is used as a heat source for heating a passenger compartment, it is likely that the air temperature can not be increased enough. 
     SUMMARY OF THE INVENTION 
     The present invention is made in view of the above matters, and it is an object of the present invention to provide a controller for an engine cooling system, which is capable of improving an anti-knocking ability even when the engine is restarted. Further, it is another object of the present invention to provide an air-conditioner for an automobile which can sufficiently heat a passenger compartment by use of an engine coolant passed through a cylinder head. 
     In an engine cooling system, an electric pump is controlled in such a manner as to circulate a coolant so that a cylinder head of an internal combustion engine is cooled. A controller for an engine cooling system includes: a temperature obtaining means for obtaining a temperature of the coolant; a temperature determination means for determining a target temperature of the coolant at which an anti-knocking ability of the internal combustion engine is improved; and a cooling control means for driving the electric pump to cool the cylinder head even after the internal combustion engine is shut off in a case that the temperature of coolant obtained by the temperature obtaining means exceeds the target temperature determined by the temperature determination means. 
     According to the above configuration, even after the engine is shut off, the cylinder head can be cooled in order to improve an anti-knocking ability. Thus, even if the engine is restarted at arbitrary timing, a cylinder head temperature has been preferably controlled. Even at restarting of the engine, the anti-knocking ability can be improved. 
     According to another aspect of the present invention, the temperature determination means continues to execute a target temperature determination processing even after the internal combustion engine is shut off. Thus, also after the engine is shut off, the cooling control processing can be executed. 
     According to another aspect of the invention, the engine cooling system is applied to an engine cooling system of a hybrid vehicle equipped with both an internal combustion engine and an electric motor. The cooling control means continues to drive the electric pump even when the temperature of the coolant becomes less than the target temperature of the coolant in a case that the internal combustion engine is shut off and a vehicle speed is greater than or equal to a specified value. Even when the engine is shut off, if the vehicle speed greater than a specified value, it is likely that the engine is restarted. That is, when the driver slightly steps on the accelerator pedal, the engine is restarted. Since the electric pump continues to be driven, a rapid increase in temperature of the cylinder head can be restricted. 
     According to another aspect of the invention, the cooling control means includes a first cooling control means for increasing a coolant flow rate in such a manner that the coolant flow rate becomes greater than an reference flow rate in a case that the temperature of the coolant obtained by the temperature obtaining means is greater than the target temperature of the coolant; and a second cooling control means for decreasing the coolant flow rate in such a manner that the coolant flow rate becomes less that the reference flow rate in a case that a difference between the temperature of the coolant obtained by the temperature obtaining means and the target temperature of the coolant is within a specified range. When the difference between the coolant temperature and the target coolant temperature is within a range, the electricity supplied to the electric pump is reduced. Thus, the electric power of the battery can be saved. 
     According to another aspect of the invention, the engine cooling system includes a radiator which cools the coolant by heat-exchanging with ambient air, and the temperature determination means obtains an ambient air temperature and determines the target temperature in such a manner as to be greater than the ambient air temperature. Thereby, even though the coolant temperature supplied to the cylinder head is around the ambient temperature, it can be avoided that the electric pump continues to be driven. 
     According to another aspect of the invention, an radiator is arranged downstream of a refrigerant condenser of an air conditioner, and the temperature determination means determines the target temperature of the coolant so that the target temperature is greater than a specified temperature which is obtained by adding an addition temperature to the ambient air temperature. The addition temperature corresponds to a heat radiation quantity of the refrigerant condenser. According to the above configuration, the electric power saving is further accelerated. 
     According to another aspect of the invention, the engine cooling system includes a radiator and an electric cooling fan. The radiator cools the coolant by heat-exchanging with ambient air. The electric cooling fan introduces the ambient air toward the radiator. The controller further includes a cooling fan control means for driving the electric cooling fan even after the internal combustion engine is shut off in a case that the temperature of the coolant exceeds the target temperature of the coolant. In a case that the internal combustion engine is started while the electric pump is stopped, the cooling control means starts driving the electric pump even though the temperature of the coolant does not exceeds the target temperature of the coolant. In a case that the temperature of the coolant does not exceeds the target temperature of the coolant, the cooling fan control means does not starts the electric cooling fan even though the internal combustion engine is started while the electric cooling fan is stopped. In a case that the temperature of the coolant becomes greater than the target temperature, the electric cooling fan is started. 
     Thus, a rapid increase in temperature of the cylinder head can be easily restricted. Even if the engine is restarted, the electric cooling fan is not started. The electric cooling fan is started when the coolant temperature exceeds the target coolant temperature. The electric power for driving the cooling fan can be saved. It should be noted that the temperature obtaining means obtains a temperature of the coolant in the radiator, a water jacket of the cylinder head, an outlet of the water jacket, or an inlet of the water jacket. Most preferably, the temperature obtaining means obtains the temperature of the coolant in the cylinder head or at outlet of a water jacket of the cylinder head. Thus, the temperature of the coolant can be correctly detected. 
     An air-conditioning system comprises a heat-exchanger for heating an air with a coolant of an internal combustion engine. The internal combustion engine has a first coolant outlet through which the coolant passed through a cylinder head flows out, and a second coolant outlet through which the coolant passed through a cylinder block flows out. The heat-exchanger is comprised of a first exchanging portion and a second exchanging portion. The first exchanging portion receives the coolant from at least the first coolant outlet, and the second exchanging portion receives the coolant from the second coolant outlet of which temperature is higher than that of the coolant flowing into the first exchanging portion. 
     According to the above configuration, the air temperature passed through the second exchanging portion can be increased more than the case where the air is heated by the low temperature coolant discharged from the first coolant temperature or the case where the air is heated by mixture of high-temperature coolant and the low-temperature coolant. Thus, the air which will be introduced into a passenger compartment can be sufficiently heated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become more apparent from the following description made with reference to the accompanying drawings, in which like parts are designated by like reference numbers and in which: 
         FIG. 1  is a schematic chart showing an engine control system; 
         FIG. 2A  is a graph showing a relationship between a head-coolant temperature “Thead”, a block-coolant temperature “Tblock” and a fuel economy; 
         FIG. 2B  is a graph showing a relationship between an ignition timing, a head-coolant temperature “Thead” and a fuel economy; 
         FIG. 3  is a flowchart showing a cooling control processing; 
         FIG. 4  is a flowchart showing a control processing of a first water pump; 
         FIG. 5  is a flowchart showing a control processing of a second water pump; 
         FIG. 6  is a flowchart showing a control processing of a cooling fan; 
         FIGS. 7A to 7F  are time charts for explaining an operation of the cooling fan, the first water pump and the second water pump; 
         FIGS. 8A to 8C  are schematic charts showing another cooling systems; 
         FIG. 9  is a chart schematically showing an entire structure of an air conditioner according to a third embodiment; 
         FIG. 10  is a time chart showing a coolant temperature, a radiation heat quantity of heater cores and air flow rate of cooling fan; 
         FIG. 11  is a graph showing a variation in air temperature passed through the first heater core and the second heater core; 
         FIG. 12  is a chart schematically showing an entire structure of an air conditioner according to a fourth embodiment; 
         FIG. 13  is a chart schematically showing an entire structure of an air conditioner according to a fifth embodiment; 
         FIG. 14  is a chart schematically showing an entire structure of an air conditioner according to a sixth embodiment; 
         FIG. 15  is a chart schematically showing an entire structure of an air conditioner according to a seventh embodiment; 
         FIG. 16  is a chart schematically showing an entire structure of an air conditioner according to an eighth embodiment; 
         FIG. 17  is a chart schematically showing an entire structure of an air conditioner according to a ninth embodiment; 
         FIG. 18  is a chart showing a first heater core and a second heater core according to a tenth embodiment; and 
         FIG. 19  is a chart showing a first heater core and a second heater core according to an eleventh embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Hereinafter, a first embodiment that embodies the present invention will be described with reference to the drawings. In the present embodiment, a vehicle is equipped with a hybrid engine.  FIG. 1  schematically shows an entire configuration of a control system in a first embodiment. 
     A vehicle is equipped with an internal combustion engine  10 . The engine  10  is comprised of a cylinder block  11  and a cylinder head  12 . The cylinder block  11  has a cylinder (not shown) in which a piston is slidably provided. The cylinder head  12  is provided on the cylinder block  11  to define a combustion chamber. 
     When air-fuel mixture is combusted in the combustion chamber, the piston slides downward. An output shaft  13  of the engine  10  is connected to a power distribution portion  14 . The power distribution portion  14  has a planetary gear mechanism including a planetary gear, a sun gear and a ring gear. The planetary gear is connected to the output shaft  13  of the engine  10 , the sun gear is connected to a first shaft  16  for driving a generator  15 , and the ring gear is connected to a second shaft  18  for driving a motor generator  17 . 
     The torque of the engine  10  is distributed to the first shaft  16  and the second shaft  18  through the power distribution portion  14 . The second shaft  18  is connected to wheels  22  through a reduction gear mechanism  21 . The generator  15  generates electricity which is charged in a battery  24  through an inverter  24 . The motor generator  17  is driven receiving electric power from the battery  24 . The torque generated by the motor generator  17  is transmitted to the wheels  22  through the second shaft  18 . 
     When the vehicle is accelerated or the vehicle is running in a high-load condition, both the internal combustion engine  10  and the motor generator  17  generate the torque. When the vehicle is running in low speed, the internal combustion engine  10  is stopped and the motor generator  17  generates torque. Meanwhile, when the vehicle is decelerated, the internal combustion engine  10  is stopped and the motor generator  17  generates electricity by regenerating a running energy, so that the battery  24  is charged. It should be noted that the battery  24  can be charged by driving the engine  10  when the vehicle is stopped. 
     The vehicle is equipped with an air conditioning system  30  for cooling a passenger compartment. The air conditioning system  30  is comprised of a compressor  31 , a condenser  32 , a receiver  33 , an expansion valve  34 , and an evaporator  35 . The compressor  31  is an electric compressor utilizing the electric power charged in the battery  24 . 
     Further, the vehicle is equipped with an engine cooling system  40  for cooling the engine  10 . The engine cooling system  40  has a cylinder-block-passage  41  through which the engine coolant flows in order to cool the cylinder block  11  and a cylinder-head-passage  51  through which the engine coolant flows in order to cool the cylinder head  12 . These passages  41 ,  51  are fluidly isolated from each other. 
     The cylinder-block-passage  41  is fluidly connected to a water jacket  42  of the cylinder block  11 . A first water pump  43  is provided in the cylinder-block-passage  41  to supply the engine coolant toward the water jacket  42  of the cylinder block  11 . The first water pump  43  is an electric water pump utilizing the electric power charged in the battery  24 . Further, a first radiator  44  is arranged in the cylinder-block-passage  41 . The first radiator  44  is for cooling the engine coolant passed through the water jacket  42 . 
     The cylinder-head-passage  51  is fluidly connected to a water jacket  52  of the cylinder head  12 . A second water pump  53  is provided in the cylinder-head-passage  51  to supply the engine coolant toward the water jacket  52  of the cylinder head  12 . The second water pump  53  is also an electric water pump utilizing the electric power charged in the battery  24 . Further, a heater core  54  and a second radiator  55  are provided in the cylinder-head-passage  51 . 
     The engine coolant flows through the heater core  54  before flowing through the second radiator  55 . The heater core  54  is for heating air which will be supplied to the passenger compartment. The temperature in the passenger compartment is controlled by adjusting air flow rate flowing through the heater core  54  and bypassing the heater core  54 . 
     The second radiator  55  is for cooling the engine coolant passed through the water jacket  52 . The first radiator  44  and the second radiator  55  are assembled together and are arranged downstream of the condenser  32  in an introduced outside-air-flow direction. 
     The first radiator  44  is arranged upstream of the second radiator  55  in the introduced outside-air-flow direction. 
     A cooling fan  56  is arranged downstream of the first and second radiators  44 ,  55  to introduce the outside air toward the radiators  44 ,  55 . The cooling fan  56  is an electric fan utilizing electric power charged in the battery  24 . 
     The present control system is provided with an electronic control unit (ECU)  61  and an electronic control unit for air conditioner (AC-ECU)  62 . The ECU  61  and the AC-ECU  62  are mainly constructed of a microcomputer having a CPU, a ROM, a RAM and a backup memory. 
     The AC-ECU  62  receives signals from a room temperature sensor  63  and a user interface  64 . Based on these signals, the AC-ECU  62  controls the compressor  31  based on the received signals. 
     The ECU  61  executes fuel injection control and ignition timing control. Further, the ECU  61  controls the generator  15  and the motor generator  17 . The ECU  61  receives signals from a first coolant-temperature sensor  65 , a second coolant-temperature sensor  66 , a vehicle speed sensor  67 , and an ambient temperature sensor  68 . The first coolant-temperature sensor  65  detects coolant temperature at an outlet or an inlet of the water jacket  52  of the cylinder head  12 . Alternatively, the first coolant-temperature sensor  65  may detect coolant temperature in the water jacket  52  of the cylinder head  12 . The second coolant-temperature sensor  66  detects coolant temperature at an outlet or an inlet of the water jacket  42  of the cylinder block  11  or coolant temperature in the water jacket  42  of the cylinder block  11 . Thus, the temperature of the cylinder head  12  and the cylinder block  11  can be correctly detected. The coolant temperature detected by the first sensor  65  is referred to as head-coolant temperature “Thead”, and the coolant temperature detected by the second sensor  66  is referred to as block-coolant temperature “Tblock”, hereinafter. Also, each of temperature sensor  65 ,  66  may detects coolant temperature in the corresponding radiator  44 ,  55 . Based on the received signals, the ECU  61  controls the first water pump  43 , the second water pump  53  and the cooling fan  56  so as to cool the cylinder block  11  and the cylinder head  12 . Further, the ECU  61  receives various information signals from the AC-ECU  62 . 
     The ambient temperature sensor  68  is provided to detect ambient air temperature around the condenser  32  and the radiators  44 ,  55 . The ECU  61  can be comprised of two units. One of units controls the engine  10  and the other controls the generator  15  and the motor generator  17 . 
     As shown in  FIG. 2A , when the block-coolant temperature “Tblock” is low, friction is increased. Thus, it is preferable that the block-coolant temperature “Tblock” is maintained at high temperature. Specifically, the block-coolant temperature “Tblock” should be maintained at 85° C. Meanwhile, as the head-coolant temperature “Thead” is lower, the anti-knocking ability is improved. Thus, it is preferable that the head-coolant temperature “Thead” is maintained at low temperature. As shown in  FIG. 2B , as the head-coolant temperature “Thead” becomes lower, the ignition timing in trace knock is more advanced, so that the ignition timing comes close to the MBT. 
     Referring to  FIG. 3 , a cooling control processing will be described, in which the block-coolant temperature “Tblock” and the head-coolant temperature “Thead” are suitably controlled. 
     This cooling control processing is executed in a specified cycle by the ECU  61 . It should be noted that this cooling control processing can be executed for a specified time period even after the ignition switch is turned off. 
     In step S 101 , the computer of the ECU  61  reads various signals from sensors, such as the first coolant-temperature sensor  65 , the second coolant-temperature sensor  66 , the vehicle speed sensor  67 , and the ambient temperature sensor  68 . Further, the computer receives information about a cooling requirement. If the cooling requirement exists, the computer receives information about heat radiation quantity of the condenser  32 . The heat radiation quantity of the condenser  32  can be derived by use of a predetermined map. Based on a detection signal of a room temperature sensor  63  and a cooling requirement level, the heat radiation quantity of the condenser  32  is computed according to cooling load (load of air conditioner). Alternatively, the heat radiation quantity can be computed based on a driving condition of the compressor  31 , the refrigerant pressure and the cooling requirement level. 
     Also in step  101 , the computer receives information about a heating requirement. If the heating requirement exists, the computer receives information about a lower limit temperature of the coolant. The information about the lower limit temperature of the coolant can be derived by use of a predetermined map based on the detection signal of the room temperature sensor  63 . The process in step S 101  corresponds to an obtaining means of the present invention. 
     Then, in steps S 102 -S 110 , a coolant temperature threshold α is computed. The processes for computing the threshold α corresponds to a temperature determining means of the present invention. The coolant temperature threshold α is a parameter for switching a driving level of the second water pump  53  and/or the cooling fan  56 . In the case that the head-coolant temperature “Thead” is higher than the threshold α, the second water pump  53  and/or the cooling fan  56  is driven in high driving level. 
     Specifically, in step S 102 , the computer determines whether the cooling requirement is established. When the answer is NO, the procedure proceeds to step S 103  in which “ambient air temperature Tair detected by the sensor  68 +10° C.” is defined as the threshold α. Specifically, the threshold α is between 40° C. and 60° C. Thereby, it is restricted that the driving levels of the second water pump  53  and the cooling fan  56  cooling are maintained at high driving level even though the head-coolant temperature “Thead” is lower than the ambient air temperature “Tair”. 
     Meanwhile, when the answer is YES, the procedure proceeds to step S 104  in which addition temperature β for cooling is computed. This addition temperature β is computed based on the heat radiation quantity of the condenser  32  computed in step S 101  and air velocity flowing toward the condenser  32  and the radiators  44 ,  55 . The air velocity is computed based on the vehicle speed “VS” detected by the vehicle speed sensor  67  and the driving level of the cooling fan  56 . Then, the procedure proceeds to step S 105  in which the threshold α is defined as “ambient air temperature Tair+10° C.+β(° C.)”. Since the heat radiation of the condenser  32  has some effect on the cooling efficiency of the second radiator  55 , the threshold α is determined based on the addition temperature β. In steps S 103  and S 105 , the added temperature value “10° C.” is variable. 
     Then, the procedure proceeds to step S 106  in which the computer determines whether the threshold α is less than 40° C. When the answer is YES in step S 106 , the procedure proceeds to step S 107  in which the threshold α is reset to 40° C. As described above, as the head-coolant temperature “Thead” is decreased, the anti-knocking ability is improved. However, such effect is converged around 40° C., as shown in  FIG. 2B . 
     Meanwhile, the threshold α is a reference to determine whether driving level of the second water pump  53  and the cooling fan  56  should be set higher. As the driving level is set higher, the electric power consumption of the battery  24  is increased. Therefore, the coolant temperature threshold α has a lower limit value. 
     Then, the procedure proceeds to step S 108  in which the computer determines whether the heating requirement is established. When the answer is YES in step S 108 , the procedure proceeds to step S 109  in which the computer determines whether the current coolant temperature threshold α is less than the lower limit value “Tlow” associated with the heating requirement. When the answer is YES in step S 109 , the procedure proceeds to step S 110  in which the coolant temperature threshold α is reset to the lower limit value “Tlow” associated with the heating requirement. When the answer is NO, the procedure proceeds to step S 111 . As described above, the coolant temperature threshold α is established to satisfy the heating requirement. 
     Then, the procedure proceeds to step S 111  in which a first water pump control is executed. In step S 112 , a second water pump control is executed. In step S 113 , a cooling fan control is executed. 
       FIG. 4  is a flow chart showing the first water pump control executed in step S 111 . 
     In step S 210 , the computer determines whether the first water pump  43  is stopped. When the answer is YES, the procedure proceeds to step S 202  in which the computer determines whether the block-coolant temperature “Tblock” is greater than or equal to a start reference temperature “TSAR” (for example, 85° C.). When the answer is NO in step S 202 , the procedure ends. When the answer is YES in step S 202 , the procedure proceeds to step  203  in which the first water pump  43  is driven in low driving level. 
     When the answer is NO in step S 201 , the procedure proceeds to step S 204  in which the computer determines whether the first water pump  43  is driven in high driving level. It should be noted that the discharge quantity of the first water pump  43  per unit time in high driving level is greater than that in low driving level. The first water pump  43  consumes more electricity in high driving level than in low driving level. When the answer is NO in step S 204 , the procedure proceeds to step S 205  in which the computer determines whether the block-coolant temperature “Tblock” is greater than or equal to a high-level reference temperature “THR” (for example, 100° C.). When the answer is YES in step S 205 , the procedure proceeds to step S 206  in which the first water pump  43  is driven in high driving level. 
     When the answer is YES in step S 204 , the procedure proceeds to step S 207  in which the computer determines whether the block-coolant temperature “Tblock” is less than or equal to a low-level reference temperature “TLR” (for example, 95° C.). When the answer is YES in step S 207 , the procedure proceeds to step S 208  in which the first water pump  43  is driven in low driving level. 
     In step S 209 , the computer determines whether the block-coolant temperature “Tblock” is less than or equal to stop reference temperature “TSOR” (for example, 80° C.). When the answer is YES in step S 209 , the procedure proceeds to step S 210  in which the first water pump  43  is stopped. 
     That is, the first water pump  43  is not started until the block-coolant temperature “Tblock” becomes the start reference temperature “TSAR”. After the first water pump  43  is started, the first water pump  43  keeps running until the block-coolant temperature “Tblock” becomes less than or equal to the stop reference temperature “TSOR”. Thereby, the block-coolant temperature “Tblock” is kept around the start reference temperature “TSAR” irrespective of whether the engine  10  is running. The start reference temperature “TSAR” is established in such a manner that friction is restricted and heavy thermal load is not applied to the cylinder block  11 . 
       FIG. 5  is a flow chart showing the second water pump control executed in step S 112 . This control processing corresponds to cooling control means of the present invention. 
     In step S 301 , the computer determines whether the second water pump  53  is stopped. When the answer is YES, the procedure proceeds to step S 302  in which the computer determines whether the engine  10  has been started. When the answer is NO in step S 302 , the procedure ends. When the answer is YES in step S 302 , the procedure proceeds to step  303  in which the second water pump  53  is driven in low driving level. 
     When the answer is NO in step S 301 , the procedure proceeds to step S 304  in which the computer determines whether the engine  10  is stopped and the vehicle speed “VS” is “0”. When the answer is NO in step S 304 , the procedure proceeds to step S 305  in which the computer determines whether the second water pump  53  is driven in high driving level. When the answer is NO in step S 305 , the procedure proceeds to step S 306  in which the computer determines whether the second water pump  53  is driven in middle driving level. A discharge quantity of the second water pump  53  in the middle driving level is greater than that in low driving level and less than that in high driving level. 
     When the answer is NO in step S 306 , that is, when the second water pump  53  is driven in the low driving level, the procedure proceeds to step S 307  in which the computer determines whether the vehicle speed “VS” is greater than or equal to a reference vehicle speed “RVS” (for example, 30 km/h) or whether the head-coolant temperature “Thead” is greater than or equal to the coolant temperature threshold α. When the answer is NO in step S 307 , the procedure ends. When the answer is YES in step S 307 , the procedure proceeds to step S 308  and step S 309 . In step S 308 , the current coolant temperature threshold α is stored as momentum information “MI”. In step S 309 , the driving level of the second water pump  53  is set to the middle driving level. 
     Even though the head-coolant temperature “Thead” is not greater than or equal to the threshold α, when the vehicle speed “VS” is greater than the reference vehicle speed “RVS”, the driving level of the second water pump  53  is changed from the low driving level to the middle driving level. Thus, it is possible to enhance the cooling efficiency based on an estimation of engine start. It can be avoided that the head-coolant temperature “Thead” suddenly exceeds the threshold α. 
     When the answer is YES in step S 306 , the procedure proceeds to step S 310  in which the computer determines whether the head-coolant temperature “Thead” is greater than or equal to an upper limit temperature “TUL” (for example, 70° C.). The upper limit temperature “TUL” is greater than the coolant temperature threshold α. When the answer is NO in step S 310 , the procedure proceeds to step S 311  in which the computer determines whether the current vehicle speed “VS” is less than or equal to “RVS−15” and the head-coolant temperature “Thead” is less than or equal to “MI−10”. When the answer is YES in step S 311 , the procedure proceeds to step S 312  in which the driving level of the second water pump  53  is set to the low driving level. When the answer is YES in step S 310 , the procedure proceeds to step S 313  in which the driving level of the second water pump  53  is set to the high driving level. 
     When the answer is YES in step S 305 , the procedure proceeds to step S 314  in which the computer determines whether the head-coolant temperature “Thead” is less than or equal to “TUL−10”. When the answer is YES in step S 314 , the procedure proceeds to step S 315  in which the momentum information “MI” is stored. In step S 316 , the driving level of the second water pump  53  is changed to the middle driving level. 
     When the answer is NO in step S 301  and the answer is YES in step S 304 , the procedure proceeds to step S 317 . In step S 317 , the computer determines whether the head-coolant temperature “Thead” is greater than or equal to the coolant temperature threshold α. When the answer is NO, the procedure proceeds to step S 318  in which the second water pump  53  is stopped. When the answer is YES in step S 317 , the procedure proceeds to step S 319  in which the computer determines whether the head-coolant temperature “Thead” is greater than or equal to a value obtained by adding a specified value (for example, 10° C.) to the threshold α. 
     When the answer is YES in step S 319 , the procedure proceeds to step S 309  in which the driving level of the second water pump  53  is set to the middle driving level. When the answer is NO in step S 319 , the procedure proceeds to step S 320  in which the driving level of the second water pump  53  is set to the low driving level. 
       FIG. 6  is a flow chart showing the cooling fan control executed in step S 113 . This control processing corresponds to cooling fan control means of the present invention. 
     In step S 401 , the computer determines whether the cooling fan  56  is stopped. When the answer is YES, the procedure proceeds to step S 402  in which the computer determines whether the vehicle speed “VS” is lower than or equal to the reference vehicle speed “RVS”. When the answer is YES in step S 402 , the procedure proceeds to step S 403  in which the computer determines whether the vehicle acceleration “VA” is less than or equal to a reference acceleration “RVA”. The vehicle acceleration “VA” is computed based on the vehicle speed “VS” detected by the vehicle speed sensor  67 . When the answer is YES in step S 403 , the procedure proceeds to step S 404  in which the computer determines whether the head-coolant temperature “Thead” is greater than or equal to the coolant temperature threshold α. 
     When the answer is NO in any one of steps S 402 -S 404 , the procedure ends. When the answer is YES in every step S 402 -S 404 , the procedure proceeds to steps S 405  and S 406 . In step S 405 , the current coolant temperature threshold α is stored as momentum information “MI”. In step S 406 , the cooling fan  56  is started to be driven in high driving level. It should be noted that the momentum information “MI” stored in step S 405  is independent from the momentum information “MI” stored in second water pump control shown in  FIG. 5 . When the answer is NO in step S 401 , the procedure proceeds to step S 407  in which the computer determines whether the engine  10  is stopped and the vehicle speed “VS” is “0”. When the answer is NO in step S 407 , the procedure proceeds to step S 408  in which the computer determines whether the vehicle speed “VS” is less than or equal to a reference vehicle speed “RVS”. When the answer is NO in step S 408 , the procedure proceeds to step S 409  in which the cooling fan  56  is stopped. 
     That is, regardless of whether the head-coolant temperature “Thead” is greater than or equal to the threshold α, when the vehicle speed “VS” is greater than a specified value, the cooling fan  56  is stopped. Thereby, electric power consumption of the battery  24  can be reduced. Alternatively, a timing at which the cooling fan  56  is started to be driven can be retarded relative to a timing at which the head-coolant temperature “Thead” becomes greater than or equal to the threshold α, whereby hunting of the cooling fan  56  can be avoided. 
     When the answer is YES in step S 408 , the procedure proceeds to step S 410  in which the computer determines whether the driving level of the cooling fan  56  is high driving level. When the answer is YES in step S 410 , the procedure proceeds to step S 411  in which the computer determines whether the head-coolant temperature “Thead” is less than or equal to “MI−10”. When the answer is YES in step S 411 , the procedure proceeds to step S 412  in which the driving level of the cooling fan  56  is set to low driving level. The air flow rate per unit time in high driving level is greater than that in low driving level. 
     When the answer is NO in step S 410 , the procedure proceeds to step S 413  in which the computer determines whether the head-coolant temperature “Thead” is greater than or equal to the coolant temperature threshold α. When the answer is YES in step S 413 , the procedure proceeds to step S 414  in which the momentum information “MI” is stored. In step S 415 , the driving level of the cooling fan  56  is changed to the high driving level. 
     When the answer is NO in step S 401  and the answer is YES in step S 407 , the procedure proceeds to step S 416 . In step S 416 , the computer determines whether the head-coolant temperature “Thead” is less than the coolant temperature threshold α. When the answer is YES in step S 416 , the procedure proceeds to step S 417  in which the cooling fan  56  is stopped. 
     Referring to a time chart shown in  FIGS. 7A-7F , operations of the cooling fan  56 , the first water pump  53  and the second water pump  43  will be described, hereinafter.  FIG. 7A  shows the vehicle speed “VS”,  FIG. 7B  shows the engine speed “NE”,  FIG. 7C  shows the coolant temperature “TCL”. In  FIG. 7C , a solid line represents the head-coolant temperature “Thead” and a two-dashed line represents the block-coolant temperature “Tblock”.  FIG. 7D  shows the driving level of the cooling fan  56 ,  FIG. 7E  shows the driving level of the second water pump  53 , and  FIG. 7F  shows the driving level of the first water pump  43 . 
     In a condition where an ignition switch is ON, a driver operates an accelerator pedal at timing t 1 . The motor generator  17  and the internal combustion engine  10  are started. Accordingly, the second water pump  53  is started in the low driving level. 
     Then, at timing t 2 , the head-coolant temperature “Thead” is higher than the coolant temperature threshold α and the cooling fan  56  is started in the high driving level. The driving level of the second water pump  53  is changed from the low driving level to the middle driving level. At timing t 3 , the vehicle speed “VS” exceeds the reference vehicle speed “RVS” and the cooling fan  56  is stopped. 
     At timing t 4 , the engine  10  is shut off. In this moment, the vehicle speed “VS” is not “0” and the head-coolant temperature “Thead” is greater than or equal to the threshold α, the second water pump  53  is driven in the middle driving level. Meanwhile, since the vehicle speed “VS” is greater than or equal to the reference vehicle speed “RVS”, the cooling fan  56  is kept OFF. 
     Then, the vehicle speed “VS” is decelerated and the vehicle speed “VS” becomes lower than the reference vehicle speed “RVS” at timing t 5 . The cooling fan is restarted in high driving level. At timing t 6 , the head-coolant temperature “Thead” becomes lower than the coolant temperature threshold α. At this moment, since the vehicle speed “VS” in not “0”, the cooling fan  56  and the second water pump  53  are driven in the low driving level. At timing t 7 , the vehicle speed “VS” becomes “0” and the cooling fan  56  and the second water pump  53  are stopped. In the above flow, since the block-coolant temperature “Tblock” is not greater than the start reference temperature “TSAR”, the first water pump  43  is kept stopped, so that the block-coolant temperature “Tblock” is increasing. 
     At timing t 8 , the driver operates the accelerator pedal to start the engine  10 , so that the second water pump  53  is started in the low driving level. 
     At timing t 9 , the head-coolant temperature “Thead” exceeds the threshold α and the driving level of the second water pump  53  is changed to the middle driving level. It should be noted that the vehicle acceleration “VA” is greater than the reference acceleration “RVA” at this moment. Thus, the cooling fan  56  is kept stopped. 
     Then, the operation of the engine  10  is continued and the waste heat quantity of the engine  10  increases. At timing t 10 , the block-coolant temperature “Tblock” is greater than the start reference temperature “TSAR” and the first water pump  43  is started in the low driving level. At timing t 11 , the head-coolant temperature “Thead” exceeds the upper limit temperature “TUL”, so that the driving level of the second water pump  53  is changed to the high driving level. 
     The engine  10  is shut off at timing t 12 , so that the increase in head-coolant temperature “Thead” and block-coolant temperature “Tblock” is stopped. At timing t 13 , the head-coolant temperature “Thead” becomes lower than the upper limit temperature “TUL”, so that the driving level of the second water pump  53  is changed to the middle divining level. At timing t 14 , the block-coolant temperature “Tblock” becomes lower than the stop reference temperature “TSOR”, so that the first water pump  43  is stopped. 
     Then, the vehicle speed “VS” is further decelerated and the cooling fan  56  is started in the high driving level at timing t 15 . At timing t 16 , the vehicle speed “VS” becomes “0”. It should be noted that since the head-coolant temperature “Thead” is significantly greater than the coolant temperature threshold α, the cooling fan  56  and the second water pump  53  are kept driven in the current driving level. 
     Then, at timing t 17 , since the head-coolant temperature “Thead” becomes lower than “threshold α+10”, the driving level of the second water pump  53  is changed to the low driving level. At timing t 18 , since the head-coolant temperature “Thead” becomes lower than the threshold α, both the cooling fan  56  and the second water pump  53  are stopped. 
     According to this embodiment explained above, the following advantages are obtained. 
     After the engine  10  is shut off, if the head-coolant temperature “Thead” is greater than the threshold α, the engine coolant is circulated to cool the cylinder head  12 . Thereby, even if the engine  10  is shut off in a condition where the head-coolant temperature “Thead” is high, the temperature of the cylinder head  12  will decrease to the desired value for improving the anti-knocking ability at a time of restarting the engine  10 . Thus, even when the engine  10  is restarted after the idle reduction control, the anti-knocking ability is improved and the fuel consumption efficiency can be enhanced. 
     Even when the engine  10  is shut off, if the vehicle speed “VS” is not “0”, it is likely that the engine  10  is restarted. That is, when the driver slightly steps on the accelerator pedal, the engine  10  is restarted. In such a situation, without respect to the head-coolant temperature “Thead”, the second water pump  53  is continuously driven. Meanwhile, when the engine  10  is off and the vehicle speed “VS” is “0”, the second water pump  53  is stopped according to the head-coolant temperature “Thead”. Thereby, in a situation that the head-coolant temperature “Thead” will increase, the head-coolant temperature “Thead” is decreased prior to a reducing of the power consumption of the battery  24 . In a situation that the head-coolant temperature “Thead” will decrease without circulating the engine coolant, the reducing of the power consumption of the battery has a priority to the decreasing of the head-coolant temperature “Thead”. Therefore, while reducing the power consumption of the battery  24 , the head-coolant temperature “Thead” can be kept low. 
     When the engine  10  is stopped and the vehicle speed “VS” is “0”, the driving level of the second water pump  53  is the middle driving level or the low driving level. Even when the head-coolant temperature “Thead” is greater than or equal to the threshold α, as long as a difference between the head-coolant temperature “Thead” and the threshold α is within a specified range, the second water pump  53  is driven in the low driving level. Thereby, in a situation that the head-coolant temperature “Thead” will decrease without circulating the engine coolant, the power saving of the battery  24  can be achieved. 
     Even after the engine  10  is shut off, if the head-coolant temperature “Thead” is greater than the threshold α, the cooling fan  56  is driven to cool the engine coolant flowing through the cylinder head  12 . Thus, even when the vehicle speed “VS” is low or zero after the engine is shut off, the engine coolant is efficiently cooled by the cooling fan  56 , so that the cylinder head  12  can be cooled rapidly after the engine  10  is shut down. 
     Further, when the engine  10  is restarted with the second water pump  53  stopped, the second water pump  53  is also restarted. Thus, a rapid increase in temperature of the cylinder head  12  can be restricted easier than a case where the second water pump  53  is started when the head-coolant temperature “Thead” exceeds the coolant temperature threshold α. On the other hand, in the case that the head-coolant temperature “Thead” does not exceed the threshold α, even if the engine coolant is cooled, a cooling effect is not high. Even if the engine  10  is restarted, the cooling fan  56  is not started. The cooling fan  56  is started when the head-coolant temperature “Thead” exceeds the coolant temperature threshold α. Therefore, the electric power for driving the cooling fan  56  can be saved. 
     Second Embodiment 
     As shown in  FIG. 8A , the cylinder-block-passage and the cylinder-head-passage can be combined as one passage. 
     Specifically, a flow rate control valve  73  is disposed at a branch portion of the water jackets  42 ,  52 . According to control signals from the ECU  61 , the flow rate control valve  73  controls the flow rate of engine coolant flowing through each water jacket  42 ,  52 . A water temperature sensor is provided to each of outlets of the water jackets  42 ,  52  to detect the head-coolant temperature “Thead” and the block-coolant temperature “Tblock”. The ECU  61  controls the water pump  72  and the flow rate control valve  73  in order to control the head-coolant temperature “Thead” and the block-coolant temperature “Tblock”. 
     A thermostat  74  is provided in the coolant passage. A bypass passage  75  is provided which bypasses the radiator  71 . When the engine coolant temperature is low, the engine coolant flows through the bypass passage  75 . The thermostat  74  is a well known mechanical thermostat or an electrical thermostat. The bypass passage  75  can be provided to the engine cooling system  40  in the first embodiment. 
     Alternatively, as shown in  FIG. 8B , the engine coolant passed through the water jacket  42  can be introduced into the water jacket  52  through a bypass passage  76 . Alternatively, as shown in  FIG. 8C , the engine coolant passed through the water jacket  52  can be introduced into the water jacket  42  through a bypass passage  77 . 
     Other Embodiment 
     The present invention is not limited to the above-mentioned embodiments, for example, may be performed as follows. 
     The first water pump  43  is a mechanical water pump and the second water pump  53  is an electric water pump which can be driven even after the engine  10  is shut off. Since the first water pump  43  is driven by the engine torque, the electric power of the battery  24  can be saved. 
     The coolant temperature threshold α can be varied between when the engine  10  is ON and when the engine  10  is OFF. For example, when the engine is OFF, the threshold α can be set larger by a specified value than that of when the engine is ON. In this case, the threshold α is set for improving the anti-knocking ability. The electric power of the battery  24  can be saved. 
     When the vehicle speed “VS” is greater than “0” and is less than a specified speed (for example, 10 km/h), the second water pump  53  can be stopped in a case that the head-coolant temperature “Thead” is not greater than the threshold α. 
     The engine coolant flowing through the cylinder head  12  can be cooled by the evaporator of the air conditioning system  30 . 
     The driving levels of the first water pump  43 , the second water pump  53  and the cooling fan  56  can be changed continuously instead of stepwise. 
     Even when the engine is OFF, the second water pump  53  can be started according to a variation in head-coolant temperature “Thead”. 
     The present invention can be applied to a hybrid vehicle and a vehicle having a function of idle reduction control. Also, the present invention can be applied to a vehicle equipped with a conventional engine. Further, the present invention can be applied to a vehicle equipped with a supercharger. In such a vehicle, high compression ratio can be obtained. In the above embodiment, the water jacket  42  and the water jacket  52  are fluidly connected in parallel. Alternatively, these water jackets  42 ,  52  are fluidly connected in series. 
     Third Embodiment 
       FIG. 9  schematically shows an entire structure of an air conditioner according to third embodiment. An air-conditioner is provided to a hybrid vehicle. 
     The air-conditioner  101  is provided with a first coolant circuit  110  and a second coolant circuit  120 . The engine coolant passed through a cylinder head  131  flows in the first coolant circuit  110 . The first coolant circuit  110  includes a first heater core  111 , a first water pump  112 , and a first temperature sensor  113 . The engine coolant passed through a cylinder block  132  flows in the second coolant circuit  120 . The second coolant circuit  120  includes a second heater core  121 , a second water pump  122 , and a second temperature sensor  123 . 
     A cylinder block  132  and a cylinder head  131  of the engine  130  have well-known configuration. 
     The cylinder head  131  has a first coolant inlet  131   a  and a first coolant outlet  131   b . The engine coolant flows through a coolant passage formed in the cylinder head  131 . The coolant flows into the coolant passage through the first coolant inlet  131   a , and flows out from the coolant passage through the first coolant outlet  131   b.    
     Similarly, the cylinder block  132  has a second coolant inlet  132   a  and a second coolant outlet  132   b . The engine coolant flows into a coolant passage formed in the cylinder block  132  through the second coolant inlet  132   a , and flow out through the second coolant outlet  132   b.    
     The first heater core  111  and the second heater core  121  have well known configuration comprised of tubes and fins. 
     In this embodiment, the first heater core  111  and the second heater core  121  are fluidly independent from each other. 
     A coolant inlet  111   a  of the first heater core  111  is connected to the first coolant outlet  131   b  through a pipe. A coolant inlet  121   a  of the second heater core  121  is connected to the second coolant outlet  132   b  through a pipe. 
     The first heater core  111  and the second heater core  121  are accommodated in a duct (not shown) of the air-conditioner. The first heater core  111  and the second heater core  121  are arranged in series with respect to an air flow. The second heater core  121  is arranged downstream of the first heater core  111 . 
     A first temperature sensor  113  is disposed between the first coolant outlet  131   b  and the coolant inlet  111   a  of the first heater core  111 , so that the first temperature sensor  113  detects temperature of coolant discharged from the first coolant outlet  131   b . A second temperature sensor  123  is disposed between the second coolant outlet  132   b  and the coolant inlet  121   a  of the second heater core  121 , so that the second temperature sensor  123  detects temperature of coolant discharged from the second coolant outlet  132   b.    
     A first water pump  112  and a second water pump  122  generate coolant flow and adjust coolant flow rate. The first water pump  112  is arranged between the coolant outlet  111   b  of the first heater core  111  and the first coolant inlet  131   a  of the cylinder head  131 . The second water pump  122  is arranged between the coolant outlet  121   b  of the second heater core  121  and the second coolant inlet  132   a  of the cylinder block  132 . 
     The first water pump  112  and the second water pump  122  are electric pumps. In the present embodiment, the first water pump  112  and the second water pump  122  are controlled in such a manner that the coolant flow rate in the cylinder head  131  is greater than that in the cylinder block  132 . 
     In the first coolant circuit  110 , the coolant discharged from the first coolant outlet  131   b  flows into the first heater core  111 , then flows into the engine  130  through the first coolant inlet  131   a.    
     In the second coolant circuit  120 , the coolant discharged from the second coolant outlet  132   b  flows into the second heater core  121 , then flows into the engine  130  through the second coolant inlet  132   a.    
     It should be noted that both the first coolant circuit  110  and the second coolant circuit are fluidly connected to a radiator (not shown). 
     An operation of the air-conditioner  101  will be described hereinafter. 
       FIG. 10  is a time chart showing a coolant temperature, a radiation heat quantity of heater cores  111 ,  121  and air flow rate of cooling fan.  FIG. 10  shows a case where the coolant temperature rises to a minimum temperature necessary for heating and then the coolant temperature is maintained at this temperature. 
     During a starting period, a heating of the passenger compartment has priority. 
     Specifically, from a timing of an engine start until a timing t 3 , the second water pump  122  is stopped and the first water pump  112  is driven to circulate a specified quantity of the coolant in the first coolant circuit  110 . Thereby, the coolant is circulated only in the first coolant circuit  110 . The temperature of coolant flowing into the first core  111  is increased. At this moment, the circulating coolant flow rate is defined in such a manner that the coolant temperature reaches the first specified temperature T 1  and the second specified temperature T 2  as soon as possible. 
     When the coolant temperature detected by the first temperature sensor  113  becomes a first specified temperature T 1  at a timing t 1 , the cooling fan is started. Then, when the coolant temperature becomes a second specified temperature T 2  at timing t 2 , the air flow rate of the cooling fan is increased to a specified value. It should be noted that the second specified temperature T 2  is a minimum temperature necessary for obtaining a target outlet air temperature. The second specified temperature T 2  is a reference temperature on which the computer determines whether the engine should be driven to heat the passenger compartment. Further, the first specified temperature T 1  is temperature at which the air can be introduced into the passenger compartment. 
     At a timing t 3 , the second water pump  122  is started and the first water pump  111  is controlled in such a manner that the coolant flow rate in the cylinder head  131  is increased. 
     At a timing t 4 , a warm-up of the engine  130  is completed. After the timing t 4 , the engine  130  is operated in a stable condition. The computer controls the first and the second water pump  112 ,  122  in such a manner that the coolant flow rate in the cylinder head  131  is greater than that in the cylinder block  132 . 
     Specifically, the first water pump  112  is controlled so that the coolant temperature flowing into the first heater core  111  reaches a third specified temperature T 3 . The third specified temperature T 3  is a target temperature of the coolant passed through the cylinder head  131 , which is established for positively cooling the cylinder head  131 . Further, the computer controls the second water pump  122  is such a manner that the coolant temperature flowing into the second heater core  121  becomes the second specified temperature T 2 . 
     According to the above, the cylinder head  131  is kept at low temperature so that the anti-knocking ability is improved. Also, the cylinder block  132  is kept at high temperature so that viscosity of engine oil is hardly deteriorated. Thus, an increase in friction of the engine can be restricted. 
     The computer controls the cooling fan in such a manner that air flow rate of the cooling fan corresponds to a target air temperature TAO. 
       FIG. 10  also shows a comparative example in which the coolant passed through the cylinder head  131  and the coolant passed through the cylinder block  132  are merged in the engine  130 . The merged coolant flows out from the engine  130  through a single coolant outlet and flows into a single heater core. Further, in the comparative example, a ratio between the coolant flow rate in the cylinder head and the coolant flow rate in the cylinder block is fixed value. When the engine is operated in a stable condition, the coolant flow rate discharged from the engine is the same as the third embodiment. 
     In the comparative example, at a timing t 5 , the coolant temperature reaches the first specified temperature T 1  and the cooling fan is started. At a timing t 3 , the coolant temperature reaches the second specified temperature T 2  and the cooling fan is driven to obtain the air flow rate corresponding to the target air temperature TAO. 
     By comparing the present embodiment with the comparative example as shown in  FIG. 10 , it is apparent that an increase in the coolant temperature flowing into the first heater core  111  can be accelerated, so that heating of a passenger compartment can be early conducted in the present embodiment. 
     Furthermore, according to the present embodiment, when the engine is driven in a stable condition, a radiation heat quantity of the second heater core  121  is greater than that of the comparative example. Consequently, as shown in  FIG. 11 , the air temperature passed through the second heater core  121  can be made greater than that of the comparative example. 
       FIG. 11  is a graph showing a variation in air temperature passed through the first heater core  111  and the second heater core  121 . 
     In the first heater core  111 , the coolant passed through the cylinder head  131  is heat-exchanged with the air passing through the first heater core  111 . Although the coolant temperature passed through the cylinder head  131  is lower that a minimum temperature required for heating the passenger compartment, the passing air can receive a lot of heat from the coolant of which flow rate is larger than that of the coolant passed through the cylinder block  132 . As a result, the temperature of air A 1  passed through the first heater core  111  comes close to the coolant temperature Th 1  before flowing into the first heater core  111 . 
     In the second heater core  121 , the coolant passed through the cylinder block  132  is heat-exchanged with the air A 1  passed through the first heater core  111 . Since the coolant temperature passed through the cylinder block  132  is higher than the coolant temperature passed through the cylinder head  131 , the temperature of air A 2  passed through the second heater core  121  can be made higher that that of the air A 1 . 
     At this time, the computer adjusts the coolant flow rate passing through the second heater core  121  by controlling the second water pump  122 . 
     Advantages of the present embodiment will be described hereinafter. 
     Since the second heater core  121  heats the passing air by heat-exchanging with the high temperature coolant discharged from the second coolant outlet  132   b , the air temperature passed through the second heater core  121  can be increased more than the case where the air is heated by the low temperature coolant discharged from the first coolant outlet  131   b  or the case where the high-temperature coolant and the low-temperature coolant is merged. 
     Furthermore, according to the present embodiment, the air is heated by using of low-temperature coolant passing through the first heater core  111  and high-temperature coolant passing through the second heater core  121 . 
     Energy transfer efficiency from the coolant to the air can be enhanced. 
     Even if the air flow rate of the cooling fan is large, the air can be heated well enough to heat the passenger compartment. 
     It should be noted that the temperature of air A 1  may be greater than the second specified temperature T 2 . 
     Fourth Embodiment 
       FIG. 12  schematically shows an entire structure of an air conditioner according to a fourth embodiment. In the present embodiment, the second coolant circuit  120  has a bypass passage  124  and a flow path selector valve  125 . 
     The bypass passage  124  bypasses the second heater core  121 . The flow path selector valve  125  switches a flow path between the bypass passage  124  and the second hater core  121 . 
     During a starting period of the engine, the engine coolant flows through the bypass passage  124 . 
     The flow path selector valve  125  can be replaced by a flow regulating valve. 
     Fifth Embodiment 
       FIG. 13  schematically shows an entire structure of an air conditioner according to a fifth embodiment. 
     The coolant passed through the first heater core  111  and the coolant passed through the second heater core  121  merges at a confluent portion  141 . Then, the coolant is divided into two flows at a diversion portion  142  toward the first coolant inlet  131   a  and the second coolant inlet  132   a . A single water pump  143  circulates the coolant. 
     In the present embodiment, a hydraulic resistance in the first coolant circuit  110  is set lower than that in the second coolant circuit  120 , so that the coolant flow rate in the cylinder head  131  is greater than that in the cylinder block  132 . For example, the passage sectional area of the coolant passage in the cylinder head  131  is larger than that in the cylinder block  132 . 
     Sixth Embodiment 
       FIG. 14  schematically shows an entire structure of an air conditioner according to a sixth embodiment. The present embodiment is based on the fifth embodiment shown in  FIG. 13 . A bypass passage  124  and a flow regulating valve  126  are added to the fifth embodiment. 
     The computer controls the flow regulating valve  126  in such a manner that the coolant flow rate in the cylinder head  131  is greater than that in the cylinder block  132 . 
     Seventh Embodiment 
       FIG. 15  schematically shows an entire structure of an air conditioner according to a seventh embodiment. In the present embodiment, the fifth embodiment shown in  FIG. 13  is modified as follows. 
     That is, a diversion portion  142  is formed between the first coolant outlet  131   b  and the first heater core  111 . The coolant discharged from the first coolant outlet  131   b  is diverged at the diversion portion  142 . The diverged coolant flows into a radiator  151 , and then merged at a confluent portion  141 . 
     Further, a bypass passage  152  and a thermostat  153  are provided. 
     A diversion portion  145  is formed between the second coolant inlet  132   b  and the second heater core  121 , and a confulent portion  146  is formed between the first coolant outlet  131   b  and the first heater core  111 . A flow regulating valve  147  is provided at the diversion portion  145 . The flow regulating valve  147  adjusts the coolant flow rate which flows toward the second heater core  121  and the confluent portion  146 . 
     When the coolant flow rate flowing toward the confluent portion  146  is “0”, all of the high-temperature coolant discharged from the second coolant outlet  132   b  flows into the second heater core  121 . 
     Alternatively, a part of the high-temperature coolant discharged from the second coolant outlet  132   b  is merged with the low-temperature coolant discharged from the first coolant outlet  131   b  and then flows into the first heater core  111 . The other high-temperature coolant discharged from the second coolant outlet  132   b  flows into the second heater core  121 . 
     Eighth Embodiment 
       FIG. 16  schematically shows an entire structure of an air conditioner according to an eighth embodiment. In the present embodiment, the seventh embodiment shown in  FIG. 15  is modified so that the flow regulating valve  147  is replaced by a thermostat  148 . 
     When the thermostat  148  is opened, a part of the high-temperature coolant discharged from the second coolant outlet  132   b  is merged with the low-temperature coolant discharged from the first coolant outlet  131   b  and then flows into the first heater core  111 . The other high-temperature coolant discharged from the second coolant outlet  132   b  flows into the second heater core  121 . 
     Ninth Embodiment 
       FIG. 17  schematically shows an entire structure of an air conditioner according to a ninth embodiment. The ninth embodiment is a modification of the eighth embodiment. A confluent portion  149  is formed upstream of a radiator  151 . This confluent portion  149  is fluidly connected to a confluent portion  141  downstream of the first and the second heater core  111 ,  121 . The coolant discharged from the radiator  151  flows into a water pump  143 . 
     Since the coolant passed through the first and the second heater core  111 ,  121  flows into the radiator to be cooled, the coolant temperature flowing into the cylinder head  131  can be made low. 
     Tenth Embodiment 
       FIG. 18  shows a first heater core  111  and a second heater core  121 . The first heater core  111  and the second heater core  121  are integrated to one unit. 
     The first heater core  111  has an inlet tank having an inlet  111   a  and a plurality of tubes. Also, the second heater core  121  has an inlet tank having an inlet  121   a  and a plurality of tubes. The first heater core  111  and the second heater core  121  have a common outlet tank  161 . The outlet tank  161  has an outlet  161   b.    
     The low-temperature coolant passed through the first heater core  111  and the high-temperature coolant passed through the second heater core  121  are merged in the outlet tank  161 . 
     Eleventh Embodiment 
       FIG. 19  shows a first heater core  111  and a second heater core  121 . The first heater core  111  and the second heater core  121  are integrated to one unit. 
     The first heater core  111  has an outlet tank having an outlet  111   b  and a plurality of tubes. Also, the second heater core  121  has an inlet tank having an inlet  121   a  and a plurality of tubes. The first heater core  111  and the second heater core  121  have a common header tank  161 . This header tank  161  has an inlet  161   a  which communicates with the first coolant outlet  131   b  of the cylinder head  131 . 
     Thereby, the high-temperature coolant discharged from the second coolant outlet  132   b  flows through the second heater core  121  and flows into the common header tank  161 . In the common header tank  161 , this coolant merges with the low-temperature coolant discharged from the first coolant outlet  131   b  of the cylinder head  131 . The merged coolant flows through the first heater core  111  and flows out from the outlet  111   b  of the heater core  111 . 
     Other Embodiment 
     In the above embodiments, the coolant discharged from the first coolant outlet  131   b  is the coolant which has cooled the cylinder head  131 . Alternatively, the coolant discharged from the coolant outlet  131   b  may include a part of the coolant which has cooled the cylinder block  132 . 
     Also, in the above embodiments, the coolant discharged from the second coolant outlet  132   b  is the coolant which has cooled the cylinder block  132 . Alternatively, the coolant discharged from the second coolant outlet  132   b  may include a part of the coolant which has cooled the cylinder head  131 . The coolant temperature discharged from the second coolant outlet  132   b  is higher than that discharged from the first coolant outlet  131   b.    
     It should be noted that the coolant temperature discharged from the second coolant outlet  132   b  is highest. 
     In the above embodiments, the coolant flowing into the second heater core  121  is the coolant discharged from the second coolant outlet  132   b . This coolant may include a part of the coolant discharged from the first coolant outlet  131   b.    
     It should be noted that the coolant temperature flowing into the second heater core  121  is higher than an average temperature of the coolant discharged from the second coolant outlet  132   b  and the coolant discharged from the first coolant outlet  131   b.    
     The coolant flow rate flowing into the first heater core  111  can be set equal to the coolant flow rate flowing into the second heater core  121 . 
     In the fifth to ninth embodiments, the engine  130  has the first coolant inlet  131   a  and the second coolant inlet  132   a . Alternatively, the engine  130  may have only one coolant inlet. 
     The first heater core  111  and the second heater core  121  may be arranged in parallel. 
     The coolant temperature flowing into the first heater core  111  can be maintained at the third specified temperature T 3 . 
     In the above embodiments, the waste heat of an engine for a hybrid vehicle is utilized as a heat source. Alternatively, waste heat of supercharging engine, a range extender and the like can be utilized as a heat source. 
     The coolant is selected from various kinds of fluid for cooling the engine. Each of the above embodiments can be properly combined.