Patent Publication Number: US-2015068472-A1

Title: EGR Gas Cooling System

Description:
TECHNICAL FIELD 
     The present invention relates to a circulation system of cooling water which circulates in each heat exchanger to be mounted on a motor vehicle and a control for it. 
     BACKGROUND ART 
     As a countermeasure for atmospheric air pollution by exhaust gas of a motor vehicle, the vehicle on which a diesel engine is mounted generally uses an EGR system which circulates the exhaust gas to the engine. 
     However, under a high load state of the engine, the temperature of the exhaust gas rises. Thus, when the exhaust gas is circulated to an air intake system of the engine as it is, combustion temperature rises so that, conversely, a concentration of nitrogen oxide in the exhaust gas is increased. Thus, a heat exchanger (an EGR cooler) is ordinarily provided to cool the exhaust gas by cooling water, and then, circulate the exhaust gas to the engine. 
     The cooling water of the EGR cooler is also used as cooling water of the engine at the same time. 
     A main cooling water circuit forms a passage in which the cooling water that radiates heat to running air by a radiator is returned to the engine, and a part of the cooling water is taken out and allowed to enter the EGR cooler so as to cool the EGR gas by the EGR cooler before the cooling water cools the engine, and then, joins to the cooling water flowing into the engine from the radiator, and a part of the cooling water is allowed again to flow to the EGR cooler and the rest thereof cools the engine and is circulated to the radiator. 
     When the engine is operated, the temperature of the cooling water the heat of which is radiated by the radiator ordinarily exceeds about 80° C., which depends on operation conditions. Accordingly, the temperature of the exhaust gas cooled by the EGR cooler through the cooling water is not the temperature of the cooling water or lower. 
     In recent years, in order to strengthen a regulation to the exhaust gas, a heavy duty of the EGR cooler and a low combustion temperature of the engine are requested to be obtained. 
     To meet a request for the heavy duty of the EGR cooler, the size of the EGR cooler is enlarged or the number of the EGR coolers to be used is increased to a plurality of numbers of EGR coolers. 
     In order to lower the combustion temperature of the engine, the temperature of the exhaust gas (the EGR gas) which is circulated to the engine needs to be lowered. 
     In a conventional technique, there is a supercharged air cooling system in which a sub-radiator is newly installed that more cools cooling water supplied to an intercooler so as to lower a supercharged air temperature for the purpose of lowering a combustion temperature of an engine, and a low temperature cooling water circuit in which the cooling water circulates to the sub-radiator, the intercooler and the engine is provided separately from a main cooling water circuit (Patent Document 1). 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: US-A-2,008,066697 
     SUMMARY OF THE INVENTION 
     Problems that the Invention is to Solve 
     When the above-described supercharged air cooling system is applied to an EGR gas cooling system, for instance, as shown in  FIG. 3 , two EGR coolers  201  and  202  are installed and the second EGR cooler (a secondary EGR cooler  202 ) is arranged in the above-described low temperature cooling water circuit  209 . Thus, in the secondary EGR cooler  202 , EGR gas can be cooled by cooling water of lower temperature. 
     In this EGR gas cooling system, the cooling water the heat of which is radiated in a sub-radiator  210  and passes through the secondary EGR cooler  202  is joined to a main cooling water circuit  206  and returned to an engine  203 . 
     In an operation of the engine  203 , after the engine is started, a warming-up operation is carried out to rapidly raise the temperature of a device and the cooling water is also preferably raised to a proper temperature. 
     Accordingly, the main cooling water circuit  206  is closed by a thermostat  208  when a water temperature is low, so that the cooling water is not supplied to a main radiator  207 . 
     However, the cooling water is always circulated to the low temperature cooling water circuit  209 . Thus, even when the engine is desired to be warmed up, since the cooling water receives heat from the engine  203  and the sub-radiator  210  radiates the heat, the engine  203  is prevented from being warmed up. 
     Especially, under an operating condition that an air temperature is low and a load of the engine is also low, the rise of the temperature of the cooling water is delayed. 
     The present invention is devised by solving the above-described problems and it is an object of the present invention to provide an EGR gas cooling system in which a low temperature cooling water circuit is provided to lower the temperature of cooling water supplied to an EGR cooler and a way of flow of the cooling water in the low temperature cooling water circuit is controlled so that the temperature of the cooling water may be rapidly raised after an engine is started. 
     Means for Solving the Problems 
     According to a first aspect of the invention, there is provided an EGR gas cooling system including: a main cooling water circuit configured to circulate cooling water between an engine, a main radiator that is configured to radiate heat of the cooling water of the engine and a primary EGR cooler that is configured to cool EGR gas by the cooling water; and a low temperature cooling water circuit configured to circulate the cooling water between the engine, a sub-radiator provided integrally with or separately from the main radiator to radiate heat in the cooling water and a secondary EGR cooler that is configured to cool the EGR gas by the cooling water, wherein a flow rate controller, which is configured to control a flow rate of the cooling water, is provided between the engine and the sub-radiator of the low temperature cooling water circuit. 
     According to a second aspect of the invention, the flow rate controller opens the low temperature cooling water circuit when the temperature of the cooling water in the engine is a prescribed temperature or higher. 
     According to a third aspect of the invention, the flow rate controller opens the low temperature cooling water circuit when the temperature of the EGR gas is a prescribed temperature or higher. 
     According to a fourth aspect of the invention, the flow rate controller opens the low temperature cooling water circuit when the EGR gas is supplied to the secondary EGR cooler. 
     According to a fifth aspect of the invention, a bypass passage is provided in the low temperature cooling water circuit, the bypass passage branches from the low temperature cooling water circuit, bypasses the sub-radiator and joins to the low temperature cooling water circuit in an upstream side of the secondary EGR cooler, and the flow rate controller controls a flow of the cooling water of the sub-radiator side and the bypass passage of the low temperature cooling water circuit 
     According to a sixth aspect of the invention, the cooling water is supplied to the bypass passage when the temperature of the cooling water in the engine is lower than a prescribed temperature, and the cooling water is supplied to the sub-radiator when the temperature of the cooling water in the engine is the prescribed temperature or higher. 
     According to a seventh aspect of the invention, the flow rate controller supplies the cooling water to the bypass passage when the EGR gas is not supplied to the secondary EGR cooler, and the flow rate controller supplies the cooling water to the sub-radiator when the EGR gas is supplied to the secondary EGR cooler. 
     According to an eighth aspect of the invention, the flow rate controller is arranged in an upstream side of the sub-radiator. 
     According to a ninth aspect of the invention, the flow rate controller is a passage control valve which is configured to adjust a flow rate of the cooling water depending on an opening degree. 
     Advantageous Effects of the Invention 
     According to a first invention, since the flow rate controller which controls the flow rate of the cooling water is provided between the engine and the sub-radiator of the low temperature cooling water circuit, the low temperature cooling water circuit can be closed by the flow rate controller immediately after the engine is started to rapidly warm up the engine. Further, after a warming-up operation of the engine is finished or when the EGR gas needs to be cooled, the low temperature cooling water circuit can be opened to cool the EGR gas by the secondary EGR cooler. A decrease of temperature rise time of the engine and a cooling performance of the EGR gas may be allowed to be compatible with each other. 
     According to a second invention, since the flow rate controller opens the low temperature cooling water circuit when the temperature of the cooling water in the engine is a prescribed temperature or higher, the low temperature cooling water circuit can be closed by the flow rate controller immediately after the engine is started to rapidly warm up the engine. Further, after a warming-up operation of the engine is finished, the low temperature cooling water circuit can be opened to cool the EGR gas by the secondary EGR cooler. A decrease of temperature rise time of the engine and a cooling performance of the EGR gas may be allowed to be compatible with each other. 
     According to a third invention, since the flow rate controller opens the low temperature cooling water circuit when the temperature of the EGR gas is a prescribed temperature or higher, the low temperature cooling water circuit can be closed by the flow rate controller immediately after the engine is started to rapidly warm up the engine. Further, after a warming-up operation of the engine is finished, the low temperature cooling water circuit can be opened to cool the EGR gas by the secondary EGR cooler. A decrease of temperature rise time of the engine and a cooling performance of the EGR gas may be allowed to be compatible with each other. 
     According to a fourth invention, since the flow rate controller opens the low temperature cooling water circuit when the EGR gas is supplied to the secondary EGR cooler, the low temperature cooling water circuit can be closed by the flow rate controller immediately after the engine is started to rapidly warm up the engine. Further, when the EGR gas is supplied to the secondary cooler, the low temperature cooling water circuit can be opened to cool the EGR gas by the secondary EGR cooler and the secondary EGR cooler can be prevented from being broken due to boiling of the cooling water. 
     Accordingly, especially in a vehicle which begins to circulate the EGR gas even when the temperature of the cooling water is low, the EGR gas can be cooled and the secondary EGR cooler can be prevented from being broken. 
     According to a fifth invention, since in the low temperature cooling water circuit, a bypass passage is provided which branches from the low temperature cooling water circuit, bypasses the sub-radiator and joins to the low temperature cooling water circuit in an upstream side of the secondary EGR cooler and the flow rate controller is provided which controls a flow of the cooling water of the sub-radiator side and the bypass passage of the low temperature cooling water circuit, the cooling water can be supplied to the bypass passage by the flow rate controller immediately after the engine is started to rapidly warm up the engine. Further, after a warming-up operation of the engine is finished or when the EGR gas needs to be cooled, the cooling water can be supplied to the sub-radiator side by the flow rate controller to lower the temperature of the cooling water and cool the EGR gas by the secondary EGR cooler. Thus, a shortening of temperature rise time of the engine and a cooling performance of the EGR gas may be allowed to be compatible with each other. 
     According to a sixth invention, since the flow rate controller supplies the cooling water to the bypass passage when the temperature of the cooling water in the engine is lower than a prescribed temperature, and supplies the cooling water to the sub-radiator when temperature of the cooling water in the engine is the prescribed temperature or higher, the cooling water can be supplied to the bypass passage by the flow rate controller when the temperature of the cooling water is low to rapidly warm up the engine. Further, when the temperature of the cooling water is high, the cooling water can be supplied to the sub-radiator by the flow rate controller to lower the temperature of the cooling water and cool the EGR gas by the secondary EGR cooler. Thus, a shortening of temperature rise time of the engine and a cooling performance of the EGR gas may be allowed to be compatible with each other. 
     Further, since the cooling water is constantly supplied to the secondary EGR cooler and does not stagnate, a fear may be reduced that the cooling water in the secondary EGR cooler is locally boiled by the EGR gas to break the secondary EGR cooler. 
     According to a seventh invention, the flow rate controller supplies the cooling water to the bypass passage when the EGR gas is not supplied to the secondary EGR cooler, and the flow rate controller supplies the cooling water to the sub-radiator when the EGR gas is supplied to the secondary EGR cooler, the cooling water can be supplied to the bypass passage by the flow rate controller when the EGR gas does not need to be cooled to rapidly warm up the engine. When the EGR gas begins to be circulated to the secondary EGR cooler, the cooling water can be supplied to the sub-radiator by the flow rate controller to lower the temperature of the cooling water and cool the EGR gas by the secondary EGR cooler. Further, the secondary EGR cooler can be prevented from being broken due to a boiling of the cooling water. 
     Accordingly, especially in a vehicle which begins to circulate the EGR gas even when the temperature of the cooling water is low, the EGR gas can be cooled and the secondary EGR cooler can be prevented from being broken. 
     Further, according to the structures of an eighth invention or a ninth invention, the present invention may be realized by a simple structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory view showing an EGR gas cooling system according to a first embodiment and a second embodiment of the present invention. 
         FIG. 2  is an explanatory view showing an EGR gas cooling system according to a third embodiment of the present invention. 
         FIG. 3  is an explanatory view showing a usual EGR gas cooling system. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     First Embodiment 
     Now, an EGR gas cooling system according to a first embodiment of the present invention will be described below. 
     The EGR gas cooling system is used for vehicles such as trucks or construction machines using diesel engines of middle or large classes which circulate a large quantity of exhaust gas (EGR) so as to be adapted to an emission regulation of exhaust gas. 
     In order to cool a large quantity of EGR gas, a vehicle has two EGR coolers including a primary EGR cooler  1  and a secondary EGR cooler  2  as shown in  FIG. 1 . The EGR gas passes through a piping from an air exhaust system of an engine  3 , is supplied to the primary EGR cooler  1  and the secondary EGR cooler  2  in order and cooled, and then, circulated to an air intake system of the engine  3  (a piping  14  of the EGR gas). 
     Further, in the vehicle, supercharged air obtained by compressing intake air by a turbo-charger  4  is cooled through running air by an intercooler  5  and supplied to the air intake system of the engine (a supercharged air path  15 ). A part of exhaust gas exhausted from the air exhaust system of the engine  3  is circulated to the engine as the EGR gas. However, the rest thereof is used as energy for compressing the intake air by the turbo-charger  4  and supplying the compressed intake air to the air intake system of the engine  3 . 
     The primary EGR cooler  1  is arranged in a main cooing water circuit  6 . 
     The main cooling water circuit  6  forms a passage in which cooling water that radiates heat to running air by a main radiator  7  is returned to the engine  3 , and a part of the cooling water is taken out and allowed to enter the primary EGR cooler  1  so as to cool the EGR gas by the primary EGR cooler  1  before the cooling water cools the engine  3 , and then, joins to the cooling water flowing into the engine  3  from the main radiator  7 , and a part of the cooling water is allowed again to flow to the primary EGR cooler  1  and the rest thereof cools the engine  3  and is circulated to the main radiator  7 . 
     In the vicinity of an outlet which leads the cooling water to the main radiator  7  from the engine  3 , a main flow rate controller by a thermostat  8  is provided. The thermostat  8  closes a piping to the main radiator  7  when the temperature of the cooling water in the engine  3  is lower than a prescribed temperature, and opens the piping to the main radiator  7  when the temperature of the cooling water is the prescribed temperature or higher. 
     Thus, when the temperature of the engine  3  is low immediately after the engine is started, the thermostat  8  closes the piping to the main radiator  7  from the engine  3 . Thus, the engine  3  is not prevented from being warmed up. At this time, the cooling water does not pass the main radiator  7 , and is supplied as shown by a broken line arrow mark F in  FIG. 1  and circulated in a passage between the engine  3  and the primary EGR cooler  1  to cool the EGR gas in the primary EGR cooler  1 . Further, when the cooling water of the engine  3  is warmed to the prescribed temperature by a warming-up operation, the thermostat  8  is opened so that the cooling water is supplied to the main radiator  7  so as to radiate heat to the running air. 
     The secondary EGR cooler  2  is arranged in a low temperature cooling water circuit  9 . 
     The low temperature cooling water circuit  9  forms a passage in which a part of the cooling water is taken out before the cooling water having the heat radiated to the running air by the main radiator  7  is returned to the engine  3  to cool the engine  3 , heat is radiated to running air by a sub-radiator  10  to lower the temperature of the cooling water and cool the EGR gas in the secondary EGR cooler  2 , and then, the cooling water joins to the main cooling water circuit  6  returned to the engine  3  from the primary EGR cooler  1  and circulated. 
     The cooling water is circulated in the main cooling water circuit  6  and the low temperature cooling water circuit  9  by a water pump  11 . 
     In such a way, when the secondary EGR cooler  2  and the low temperature cooling water circuit  9  are provided, in the secondary EGR cooler  2 , the EGR gas cooled in the primary EGR cooler  1  can be cooled by the cooling water which becomes lower in the sub-radiator  10 , so that a cooling performance of the EGR gas cooling system can be improved. 
     The sub-radiator  10  may be provided separately from the main radiator  7  as shown in  FIG. 1 , or may be provided integrally with the main radiator  7 , and then, internally divided. 
     In the EGR gas cooling system, also in the vicinity of an outlet which leads out the cooling water to the low temperature cooling water circuit  9  from the engine  3 , a thermostat  12  (one example of a flow rate controller) is provided. 
     In the thermostat  12 , wax which expands and contracts depending on a temperature is sealed. The wax reacts to the rise and fall of the temperature of the cooling water of the engine  3 , so that the low temperature cooling water circuit  9  can be opened and closed. 
     Thus, when the temperature of the engine  3  is low immediately after the engine is started, the thermostat  12  closes a piping to the sub-radiator  10  from the engine  3  so that the cooling water is not supplied to the low temperature cooling water circuit  9 , and the cooling water flows as shown by the broken line arrow mark in  FIG. 1 . Thus, the cooling water does not unnecessarily cool the engine  3  to prevent the engine from being warmed up until the engine  3  is warmed and the cooling water reaches the prescribed temperature, so that a temperature rise time can be shortened. 
     On the other hand, when the temperature of the cooling water of the engine  3  is the prescribed temperature or higher, the low temperature cooling water circuit  9  is opened to lower the temperature of the cooling water by the sub-radiator  10  and the EGR gas can be more cooled by the secondary EGR cooler  2 . 
     In such a way, by a relatively inexpensive thermostat  12 , a shortening of the temperature rise time of the engine  3  and a cooling performance of the EGR gas cooling system can be easily allowed to be compatible with each other. 
     Since the thermostat  12  is a mechanism using a thermal expansion of a material, a completely closed state is not instantaneously changed to a completely opened state. Thus, the temperature of the cooling water has a width or range until the low temperature cooling water circuit  9  shifts from the completely closed state to the completely opened state. Accordingly, the temperature of the cooling water at which the thermostat  12  begins to open needs to be set to a temperature several degrees lower than a temperature at which the EGR gas begins to be supplied to the secondary EGR cooler  2  by considering an unevenness of a product accuracy. 
     Second Embodiment 
     Now, an EGR gas cooling system according to a second embodiment of the present invention will be described below. 
     The EGR gas cooling system according to the second embodiment is the same as the first embodiment in view of a point that a main cooling water circuit  6  is provided in which cooling water is circulated between an engine  3 , a main radiator  7  and a primary EGR cooler  1 , and a point that a sub-radiator  10  and a secondary EGR cooler  2  are provided and a low temperature cooling water circuit  9  is provided in which cooling water is circulated thereto. 
     The second embodiment is different from the first embodiment in view of a point that a solenoid valve  13  (one example of a flow rate controller) is provided in the vicinity of an outlet which leads out the cooling water to the low temperature cooling water circuit  9  from the engine  3 . 
     The solenoid valve  13  is controlled by a signal from a controlling computer (not shown in the drawing) to be mounted on a vehicle. 
     In the second embodiment, as an example of a control method of the solenoid valve  13 , the solenoid valve  13  is set to be held in a completely closed state during a warming-up operation after the engine  3  is started. The solenoid valve  13  is set to be completely opened when a circulation of EGR gas to the secondary EGR cooler  2  is started. 
     Since the opening and closing of the solenoid valve  13  are set in such a way as described above, the low temperature cooling water circuit  9  is closed until the EGR gas is supplied. Thus, the engine  3  is prevented from being unnecessarily cooled by the cooling water, so that the engine  3  can be rapidly warmed up. 
     On the other hand, when the EGR gas is supplied to the secondary EGR cooler  2 , the solenoid valve  13  is opened, so that the cooing water is circulated to cool the EGR gas in the secondary EGR cooler  2 . Accordingly, when the EGR gas is supplied to the secondary EGR cooler  2 , a circulation of the cooling water does not stagnate. Further, there is no fear that the cooling water in the secondary EGR cooler  2  is heated and boiled by the EGR gas to break the secondary EGR cooler  2 . 
     When a load of the engine or the temperature of the cooling water corresponds to prescribed conditions, a flow rate control valve (not shown in the drawing) such as an EGR valve provided in a piping  14  of the EGR gas is opened by the controlling computer so that the EGR gas begins to be circulated to the primary EGR cooler  1  and the secondary EGR cooler  2  arranged in series. 
     As another example of the control method of the solenoid valve  13 , the solenoid valve  13  may be set to be held in a completely closed state during a warming-up operation after the engine  3  is started, and the solenoid valve  13  may be set to be opened by a prescribed rate (for instance, about half opened) so as to begin to supply the cooling water to the secondary EGR cooler  2  in accordance with a start of a circulation of the ERG gas. After that, a thermostat  8  of the main cooling water circuit  6  is opened so that a flow of the cooling water to the main radiator  7  is detected by a flow rate sensor or a temperature sensor, and the solenoid valve  13  is set to be completely opened at the same time. A condition under which the solenoid valve  13  is completely opened may be set to other standard or timing which can maximize a cooling performance of the secondary EGR cooler. 
     Further, a flow rate of the EGR gas may be detected by the flow rate sensor and an opening degree of the solenoid valve  13  may be set to be suitably adjusted so that a suitable flow rate of the cooling water may be obtained correspondingly thereto. 
     Further, the temperatures of the cooling water respectively in the engine  3 , the main radiator  7  and the sub-radiator  10  are detected by the temperature sensor and the opening degree of the solenoid valve  13  may be set to be suitably adjusted in accordance with the relation of them. 
     Further, the temperature of the EGR gas of the engine or the secondary EGR cooler is detected. Then, the solenoid valve  13  may be set to be opened when the temperature of the EGR gas is a prescribed temperature or higher. 
     As described above, when the opening degree of the solenoid valve  13  is adjusted in multiple stages, the temperature of the engine  3  can be more efficiently raised and a cooling efficiency of the EGR gas can be improved by the secondary EGR cooler  2 . 
     Further, in the vicinity of the outlet which leads out the cooling water to the low temperature cooling water circuit  9  from the engine  3 , a flow rate controller formed with an actuator type valve may be provided in place of the solenoid valve. 
     A control method of the actuator type valve is the same as the control method of the solenoid valve  13 . 
     As other flow rate controller, an electronic control valve which can adjust an opening degree by an electronically controlled motor may be provided. A control method of the electronic control valve is the same as the control method of the solenoid valve  13   
     Third Embodiment 
     Now, an EGR gas cooling system according to a third embodiment of the present invention will be described below. 
     The EGR gas cooling system is used for vehicles such as trucks or construction machines using diesel engines of middle or large classes which circulate a large quantity of exhaust gas (EGR) so as to be adapted to an emission regulation of exhaust gas. 
     In order to cool a large quantity of EGR gas, a vehicle has two EGR coolers including a primary EGR cooler  101  and a secondary EGR cooler  102  as shown in  FIG. 2 . The EGR gas passes through a piping from an air exhaust system of an engine  103 , is supplied to the primary EGR cooler  101  and the secondary EGR cooler  102  in order and cooled, and then, circulated to an air intake system of the engine  103  (a piping  114  of the EGR gas). 
     In the piping  114  of the EGR gas, a flow rate control valve (not shown in the drawing) such as an EGR valve is provided. When a load of the engine or a temperature of cooling water corresponds to prescribed conditions, the flow rate control valve is opened by a controlling computer (not shown in the drawing) so that the EGR gas begins to be circulated to the primary EGR cooler  101  and the secondary EGR cooler  102  arranged in series. 
     Further, in the vehicle, supercharged air obtained by compressing intake air by a turbo-charger  104  is cooled through running air by an intercooler  105  and supplied to the air intake system of the engine (a supercharged air path  115 ). A part of the exhaust gas exhausted from the air exhaust system of the engine  103  is circulated to the engine as the EGR gas. However, the rest thereof is used as energy for compressing the intake air by the turbo-charger  104  and supplying the supercharged air to the air intake system of the engine  103 . 
     The primary EGR cooler  101  is arranged in a main cooing water circuit  106 . 
     The main cooling water circuit  106  forms a passage in which the cooling water that radiates heat to running air by a main radiator  107  is returned to the engine  103 , and a part of the cooling water is taken out and allowed to enter the primary EGR cooler  101  so as to cool the EGR gas by the primary EGR cooler  101  before the cooling water cools the engine  103 , and then, joins to the cooling water flowing into the engine  103  from the main radiator  107 , and a part of the cooling water is allowed again to flow to the primary EGR cooler  101  and the rest of the cooling water cools the engine  103  and is circulated to the main radiator  107 . 
     In the vicinity of an outlet which leads the cooling water to the main radiator  107  from the engine  103 , a main flow rate controller by a thermostat  108  is provided. The thermostat  108  closes a piping to the main radiator  107  when the temperature of the cooling water in the engine  103  is lower than a prescribed temperature, and opens the piping to the main radiator  107  when the temperature of the cooling water is the prescribed temperature or higher. 
     In the thermostat  108 , wax which expands and contracts depending on a temperature is sealed. The wax reacts to the rise and fall of the temperature of the cooling water of the engine  103 , so that the piping to the main radiator  107  can be opened and closed. 
     Thus, when the temperature of the engine  103  is low immediately after the engine is started, the thermostat  108  closes the piping to the main radiator  107  from the engine  103 . Thus, the engine  103  is not prevented from being warmed up. At this time, the cooling water does not pass the main radiator  107 , and is supplied as shown by a broken line arrow mark F in  FIG. 2  and circulated in a passage between the engine  103  and the primary EGR cooler  101  to cool the EGR gas in the primary EGR cooler  101 . Further, when the cooling water of the engine  103  is warmed to the prescribed temperature by a warming-up operation, the thermostat  108  is opened so that the cooling water is supplied to the main radiator  107  so as to radiate heat to the running air. 
     The secondary EGR cooler  102  is arranged in a low temperature cooling water circuit  109 . 
     The low temperature cooling water circuit  109  forms a passage in which a part of the cooling water is taken out before the cooling water having the heat radiated to the running air by the main radiator  107  is returned to the engine  103  to cool the engine  103 , heat is radiated to running air by a sub-radiator  110  to lower the temperature of the cooling water and cool the EGR gas in the secondary EGR cooler  102 , and then, the cooling water is joined to the main cooling water circuit  106  returned to the engine  103  from the primary EGR cooler  101  and circulated. 
     The cooling water is circulated in the main cooling water circuit  106  and the low temperature cooling water circuit  109  by a water pump  111 . 
     In such a way, since the secondary EGR cooler  102  and the low temperature cooling water circuit  109  are provided, in the secondary EGR cooler  102 , the EGR gas cooled in the primary EGR cooler  101  can be cooled by the cooling water which becomes lower in the sub-radiator  110 , so that a cooling performance of the EGR gas cooling system can be improved. 
     The sub-radiator  110  may be provided separately from the main radiator  107  as shown in  FIG. 2 , or may be provided integrally with the main radiator  107 , and then, internally divided. 
     In the EGR gas cooling system, a bypass passage  112  is provided which branches from the low temperature cooling water circuit  109  in an upstream side of the sub-radiator  110 , bypasses the sub-radiator  110  and joins to the low temperature cooling water circuit  109  in an upstream side of the secondary EGR cooler  102 . 
     Further, in the vicinity of a branching part or a joining part of the low temperature cooling water circuit  109  and the bypass passage  112 , a passage control valve by a solenoid valve  113  is provided so that the cooling water may be supplied from one of the sub-radiator  110  side or the bypass passage  112  which is selected. Further, as another example, the solenoid valve  113  (one example of a flow rate controller, one example of a passage control valve) may be provided in an intermediate part of the bypass passage  112  so as to distribute and supply a large quantity of flow rate of cooling water to one of the sub-radiator  110  side and the bypass passage  112 . 
     The solenoid valve  113  switches a flow of the cooling water by a signal from a controlling computer (not shown in the drawing) to be mounted on the vehicle. 
     As an example of a control method of the solenoid valve  113 , in the present embodiment, the temperature of the cooling water in the engine  103  is detected by a temperature sensor (not shown in the drawing). When the temperature of the cooling water is lower than the prescribed temperature, the cooling water is supplied to the bypass passage  112 . When the temperature of the cooling water is the prescribed temperature or higher, the cooling water is supplied to the sub-radiator  110 . 
     Thus, when the temperature of the engine  103  is low immediately after the engine is started, the solenoid valve  113  closes a piping to the sub-radiator  110  so that the cooling water is supplied to the bypass passage  112 . Thus, the cooling water does not radiate heat in the sub-radiator  110  to prevent the engine from being warmed up until the engine  103  is warmed and the cooling water reaches the prescribed temperature, so that a temperature rise time can be shortened. 
     Further, at this time, the cooing water is continuously circulated to the secondary EGR cooler  102 . Under a state that the EGR gas is supplied to the secondary EGR cooler  102 , when the cooing water of the secondary EGR cooler  102  stagnates, there is a fear that the cooling water may be heated and boiled to break the secondary EGR cooler  102 . However, in the present embodiment, since the cooling water is constantly supplied to the secondary EGR cooler  102 , the cooing water is not boiled until the cooling water of an entire part of the EGR gas cooling system reaches a boiling point. 
     On the other hand, when the temperature of the cooling water of the engine  103  is the prescribed temperature or higher, the solenoid valve  113  closes the bypass passage  112  so that the cooling water may be supplied to the sub-radiator  110  to radiate heat and the EGR gas may be more cooled by the secondary EGR cooler  102 . 
     After a high load is applied to the engine  103  by climbing a slope or the like, when the load is abruptly lowered and the running air to the main radiator  107  is reduced, the temperature of the cooling water suddenly rises. However, in the present embodiment, even in such a case, the cooling water can be supplied to the sub-radiator  110  by the solenoid valve  113 , so that the temperature of the cooling water can be lowered by heat radiation. 
     As another example of a control method of the solenoid valve  113 , a flow rate of the EGR gas in the piping  114  of the EGR gas is detected by a flow rate sensor (not shown in the drawing). Then, the cooling water may be set to be supplied to the bypass passage  112  when a circulation of the EGR gas is stopped. The cooling water may be set to be supplied to the sub-radiator  110  when the EGR gas is supplied to the piping  114 . 
     Further, the cooling water may be set to be supplied to the bypass passage  112  when the flow rate of the EGR gas is lower than a prescribed value. The cooling water may be set to be supplied to the sub-radiator  110  when the flow rate of the EGR gas is the prescribed value or larger. 
     Thus, the cooling water is supplied to the bypass passage  112  until the EGR gas is circulated. Thus, the cooling water is prevented from radiating heat in the sub-radiator  110 , so that the engine  103  can be prevented from being unnecessarily cooled and the engine  103  can be rapidly warmed up. 
     On the other hand, when the EGR gas is supplied to the secondary EGR cooler  102 , the cooing water is supplied to the sub-radiator  110  by the solenoid valve  113  to cool the EGR gas in the secondary EGR cooler  102 . Accordingly, when the EGR gas is supplied to the secondary EGR cooler  102 , the cooling water does not stagnate in the secondary EGR cooler  102 . There is no fear that the cooling water in the secondary EGR cooler  102  is heated and boiled by the EGR gas to break the secondary EGR cooler  102 . 
     Further, as other example of a control method of the solenoid valve  113 , the passage may be switched on the basis of both the temperature of the cooling water and the flow of the EGR gas. 
     Namely, when the temperature of the cooling water in the engine  103  is lower than the prescribed temperature (T1), the cooling water is supplied to the bypass passage  112  by the solenoid valve  113 . 
     Then, when the cooling water is heated to reach T1, and when the EGR gas is supplied to the secondary EGR cooler  102 , the cooling water is supplied to the sub-radiator  110  side by the solenoid valve  113 . When the cooling water is T1 or higher, however, the EGR gas is not supplied to the secondary EGR cooler  102 , the cooling water is supplied to the bypass passage  112 . 
     Further, when the cooling water reaches a prescribed temperature (T2) higher than T1, even if the EGR gas is not supplied to the secondary EGR cooler  102 , the cooling water is supplied to the sub-radiator  110  side by the solenoid valve. 
     Thus, especially when the engine needs to be warmed up, the cooling water is supplied to the bypass passage  112 . Thus, the engine  103  can be rapidly warmed up. On the other hand, when it is highly necessary to radiate heat by the cooling water so as to realize a low temperature, for instance, when the temperature of the cooling water is T1 or higher and the EGR gas is supplied to the secondary EGR cooler  102 , or when the temperature of the cooling water is T2 or higher and sufficiently high, the cooling water can be supplied to the sub-radiator  110  to radiate the heat. 
     Further, as other example of a control method of the solenoid valve  113 , the temperature of the EGR gas exhausted from the engine  103  is detected by a temperature sensor (not shown in the drawing). Thus, the cooling water may be set to be supplied to the bypass passage  112  when the temperature of the EGR gas is lower than a prescribed temperature. The cooling water may be set to be supplied to the sub-radiator  110  when the temperature of the EGR gas is the prescribed temperature or higher. At this time, the temperature of the cooling water may be adjusted by a distribution of a flow rate in such a way that a part of the cooling water is supplied to the bypass passage and the rest thereof is supplied to the sub-radiator. 
     Thus, when the temperature of the EGR gas is low so that the EGR gas does not need to be cooled in the secondary EGR cooler  102 , the cooling water can be supplied to the bypass passage  112 . Thus, the EGR gas can be prevented from being excessively cooled to generate condensate of the exhaust gas in the secondary EGR cooler  102 . 
     Further, as the passage control valve of the cooling water, an actuator type valve may be used in place of the solenoid valve  113 . 
     A control method of the actuator type valve is the same as the control method of the solenoid valve  113 . 
     As other passage control valve, an electronic control valve which can adjust an opening degree by an electronically controlled motor may be provided. 
     A control method of the electronic control valve is the same as the control method of the solenoid valve  113   
     Further, as the passage control valve of the cooling water, a thermostat may be used. 
     The thermostat supplies the cooling water to the bypass passage  112  when the temperature of the cooling water in the branching part is lower than the prescribed temperature. When the temperature of the cooling water is the prescribed temperature or higher, the wax of the thermostat reacts thereto so that the thermostat supplies the cooling water to the sub-radiator  110 . 
     In such a way, by a relatively inexpensive thermostat, a shortening of the temperature rise time of the engine  103  and a cooling performance of the EGR gas cooling system can be easily allowed to be compatible with each other. 
     Further, the bypass passage  112  may be joined to a piping of the low temperature cooling water circuit  109  in the upstream side of the secondary EGR cooler  102  as shown in  FIG. 2 . However, the bypass passage  112  may be directly connected to the secondary EGR cooler  102  so as to be joined to the low temperature cooing water circuit  109 . 
     The present invention is described above in detail by referring to the specific embodiments. However, it is to be understood to a person with ordinary skill in the art that various changes or modifications may be made without departing from the spirit and scope of the present invention. For instance, the structures of the first to the third embodiments may be suitably combined together to rapidly raise the temperature of the cooling water after the engine is started. 
     This application is based on Japanese Patent Application No. 2011-202528 filed on Sep. 16, 2011 and Japanese Patent Application No. 2011-202534 filed on Sep. 16, 2011 and contents thereof are incorporated herein as references. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               1 ,  101 : primary EGR cooler 
               2 ,  102 : secondary EGR cooler 
               3 ,  103 : engine 
               4 ,  104 : turbo-charger 
               5 ,  105 : intercooler 
               6 ,  106 : main cooling water circuit 
               7 ,  107 : main radiator 
               8 ,  108 : thermostat (main cooling water circuit) 
               9 ,  109 : low temperature cooling water circuit 
               10 ,  110 : sub-radiator 
               11 ,  111 : water pump 
               12 : thermostat (low temperature cooing water circuit) 
               112 : bypass passage 
               13 ,  113 : solenoid valve 
               14 ,  114 : piping of EGR gas 
               15 ,  115 : supercharged air path