Patent Publication Number: US-7594483-B2

Title: Internal combustion engine cooling system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to Japanese Patent Application No. 2007-122194, filed on May 7, 2007. The entire disclosure of Japanese Patent Application No. 2007-122194 is hereby incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to an internal combustion engine cooling system. More specifically, the present invention relates to an internal combustion engine cooling system that regulates a temperature of transmission oil using a coolant (cooling medium) that also serves to cool the internal combustion engine. 
   2. Background Information 
   A technology has been proposed for regulating a temperature of transmission oil by heating and cooling the transmission oil using a coolant from an internal combustion engine (see Japanese Laid-Open Patent Publication No. 2004-332583). In this proposed technology, a water-cooled engine cooling system apparatus uses the engine coolant in a single oil heat exchanger to heat and cool the transmission oil in an efficient manner. A thermostat valve is provided between an outlet of a radiator and a water pump. The oil heat exchanger exchanges heat between the coolant and the transmission oil, with a coolant inflow passage carrying the coolant from an outlet side of a water pump to the oil heat exchanger. A first coolant outflow passage returns the coolant exiting the oil heat exchanger back to a position between the radiator and the thermostat valve, and a second coolant outflow passage returns the coolant exiting the oil heat exchanger to a position between the thermostat valve and the water pump. The cooling system executes an inlet coolant temperature control to regulate the temperature transmission oil temperature. 
   In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved internal combustion engine cooling system. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure. 
   SUMMARY OF THE INVENTION 
   With the technology presented in Japanese Laid-Open Patent Publication No. 2004-332583, a thermostat and a bypass passage are provided to return coolant that has circulated through the water jacket to an upstream portion of coolant passage leading from the radiator to the oil heat exchanger. With such a configuration, the coolant flowing to the oil heat exchanger is comparatively warm. Consequently, the temperature of the transmission oil will become high when the load imposed on the engine is high and when it is necessary to aggressively cool the transmission oil with the oil heat exchanger. 
   The present invention was conceived in view of this problem. One object is to provide an internal combustion engine cooling system that can prevent the temperature of the transmission oil from becoming excessively high. 
   In view of the above, an internal combustion engine cooling system is provided that basically comprises an engine water jacket, a coolant circulation passage, a radiator, a thermostat valve, a bypass passage, a bridge passage, a circulation passage resistance generating section and an oil heat exchanger. The coolant circulation passage fluidly connects a water jacket outlet of the engine water jacket to a water jacket inlet of the engine water jacket. The radiator is disposed in the coolant circulation passage between the water jacket outlet and the water jacket inlet. The thermostat valve is disposed in the coolant circulation passage between an inlet side of the radiator and the water jacket outlet to close the coolant circulation passage leading to the radiator when a coolant temperature of the cooling medium is lower than a prescribed temperature and to open the coolant circulation passage leading to the radiator when the coolant temperature of the cooling medium is equal to or higher than a prescribed temperature. The bypass passage branches from the coolant circulation passage at a position located between the water jacket outlet and the thermostat valve, and connects to the coolant circulation passage on an outlet side of the radiator for bypassing the thermostat valve and the radiator. The bridge passage connects an intermediate portion of the bypass passage to an intermediate portion of the coolant circulation passage located downstream of the radiator and upstream of a merging position where the bypass passage merges with the coolant circulation passage for establishing communication between the intermediate portions of the bypass passage and the coolant circulation passage. The circulation passage resistance generating section is arranged in a portion of the coolant circulation passage located downstream of a position where the bridge passage connects to the coolant circulation passage and upstream of the merging position where the bypass passage merges with the coolant circulation passage. The oil heat exchanger is arranged in the bridge passage to exchange heat between the cooling medium and transmission oil passing therethrough. 
   These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the attached drawings which form a part of this original disclosure: 
       FIG. 1  is a simplified block diagram of an internal combustion engine (e.g., a diesel engine) in which an internal combustion engine cooling system is employed in accordance with one embodiment; 
       FIG. 2  is a simplified block diagram of the internal combustion engine cooling system in accordance with the illustrated embodiment for the internal combustion engine illustrated in  FIG. 1 ; 
       FIG. 3  is a block diagram of the internal combustion engine cooling system illustrated in  FIG. 2 , but indicating the coolant flow during engine warming; and 
       FIG. 4  is a block diagram of the internal combustion engine cooling system illustrated in  FIGS. 2 and 3 , but indicating the coolant flow after engine warming is complete. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Selected embodiments of the present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
   Referring initially to  FIG. 1 , a schematic diagram of a direct injection diesel engine is illustrated in which an internal combustion engine cooling system is employed illustrated in accordance with one embodiment. In particular,  FIG. 2  diagrammatically illustrates the internal combustion engine cooling system of the illustrated embodiment. The diesel engines are well known in the art. Since diesel engines are well known in the art, the precise structure of the diesel engine will not be discussed or illustrated in detail herein. 
   The cooling system is a water-cooled internal combustion engine cooling system in which an outlet coolant temperature control is performed. The constituent features will now be explained. An engine water jacket  1  is provided on an engine with a water pump  2  fluidly connected to the water jacket  1  for pumping coolant into the water jacket  1 . The water pump  2  is arranged upstream of the water jacket  1 . A thermostat valve  3  is arranged downstream of the water jacket  1  such that coolant exiting the water jacket  1  flows through the thermostat valve  3 . A radiator  4  is arranged downstream of the thermostat valve  3  for receiving coolant from the water jacket  1 . Coolant that has been cooled in the radiator  4  is returned to the water pump  2  as a cooled cooling medium. 
   Also an exhaust gas recirculation (EGR) apparatus  5  is provided that includes an exhaust gas recirculation (EGR) passage  5 A, an exhaust gas recirculation (EGR) valve  5 B arranged in the EGR passage  5 A, and an exhaust gas recirculation cooling device  6  (hereinafter called “EGR cooler”) provided in the EGR passage  5 A to exchange heat between an exhaust gas flowing through the EGR passage  5 A and the coolant. An exhaust gas recirculation cooling device circulation passage  7  (hereinafter called “EGR cooler circulation passage”) is provided to pass coolant through the EGR cooler  6 . In particular, a portion of the coolant discharged from the water jacket  1  passes through the EGR cooler  6  and a portion passes through a heater core  8  arranged in a heater passage  9  for heating the interior of the vehicle. 
   The cooling system includes an engine coolant circulation passage  10  that carries coolant exiting the engine (water jacket  1 ) through the radiator  4  and back to the engine (water jacket  1 ). The thermostat valve  3  and the radiator  4  are provided in the engine coolant circulation passage  10 . The water pump  2  is driven by a crankshaft (not shown) of the engine. The thermostat valve  3  shuts off the flow of coolant to the radiator  4  when the temperature of the coolant coming from the water jacket  1  is lower than a prescribed temperature and allows (opens) the flow of coolant to the radiator  4  when the temperature of the coolant is equal to or higher than the prescribed temperature. The prescribed temperature is set in advance to a temperature (e.g., 90° C.) lower than a minimum temperature at which there is a possibility that the engine will overheat (temperature will be come excessive) such that the passage leading to the radiator  4  is opened when the coolant temperature is below the minimum temperature. 
   The coolant passages leading to the EGR cooler  6  and the heater core  8  are arranged to branch from a portion of the coolant circulation passage  10  located between the water jacket  1  and the thermostat valve  3 , pass through the EGR cooler  6  and/or the heater core  8 , and return to the upstream side of the water pump  2  through the EGR cooler circulation passage  7 . 
   A bypass passage  11  is also provided which branches from a portion of the coolant circulation passage  10  located between the water jacket  1  and the thermostat valve  3  and carries a portion of the coolant to a portion of the coolant circulation passage  10  located downstream of the radiator  4 , thus bypassing the radiator  4 . 
   The EGR passage  5 A is a passage that directs a portion of the exhaust gas flowing through an exhaust passage of the engine to an air induction passage. The EGR cooler  6  exchanges heat between the coolant and the exhaust gas flowing through the EGR passage  5 A so as to cool the exhaust gas introduced into the air induction passage. When the EGR valve  5 B is opened, a portion of the engine exhaust gas flows through the EGR passage  5 A and into the air induction passage. When the EGR valve  5 B is closed, the EGR passage  5 A is blocked such that engine exhaust gas does not flow therethrough. The EGR apparatus  5  serves to reduce the amount of NOx produced during fuel combustion by directing a portion of the exhaust gas into the intake air. When the amount of oxygen in the combustion chamber is insufficient or when the temperature inside the combustion chamber is too high, the EGR valve  5 B is closed and exhaust gas recirculation is not executed. 
   The heater core  8  exchanges heat between air flowing through the heater passage  9  and coolant that is warmer than the air for heating the vehicle interior. The heated air exiting the heater core  8  is used to heat the vehicle interior or adjust a temperature of an air conditioner. 
   A turbo cooler  12 , an electric water pump  13 , and an orifice  14  are arranged along the bypass passage  11  in order as listed from upstream to downstream. The electric water pump  13  is driven by an electric motor to pump coolant through the bypass passage  11  in the downstream direction. The orifice  14  is provided to set the amount of coolant that will flow through the bypass passage  11 . The orifice  14  constitutes a passage resistance generating section of the bypass passage  11 . Of course, it will be apparent to those skilled in the art from this disclosure that other types of devices can be used for the passage resistance generating section such as a throttling device or a cooling device of an auxiliary machine provided on the internal combustion engine. In other words, the term “passage resistance generating section” refers to any device that can restrict the flow of the coolant or generate a resistance against the flow of the coolant. 
   A bridge passage  15  branches from a portion of the bypass passage  11  located downstream of the orifice  14 . The bridge passage  15  branches from downstream of the orifice  14  and connects to the coolant circulation passage  10  downstream of the radiator  4 , e.g., a passage in which coolant discharged from the radiator  4  flows. An oil heat exchanger or AT cooler  16  exchanges heat between the coolant and the transmission oil. The oil heat exchanger  16  is provided in the bridge passage  15 . An orifice  17  is provided in the coolant circulation passage  10  at a position downstream of where the bridge passage connects to the coolant circulation passage  10 . The orifice  17  constitutes a passage resistance generating section that serves to generate a resistance against flow through the passage  10 . The orifice  17  is contrived to set the amount of coolant that will flow through the bridge passage  15 , as will be explained later. Of course, it will be apparent to those skilled in the art from this disclosure that other types of devices can be used for the orifice  17  as needed and/or desired such as a throttling device or a cooling device of an auxiliary machine provided on the internal combustion engine. 
   When the engine is warming up and the thermostat valve  3  is closed, coolant that has passed through the bypass passage  11  flows into the bridge passage  15  and is discharged into the coolant circulation passage  10 . When warming up has being completed and the thermostat valve  3  is open, coolant flows into the bridge passage  15  from the coolant circulation passage  10  and is discharged into the bypass passage  11 . 
   As diagrammatically shown in  FIG. 2 , the oil heat exchanger (AT cooler)  16  is connected to an oil pipe such that the coolant can exchange heat with the transmission oil. The transmission oil flows from the transmission to the oil heat exchanger  16  and returns to the transmission after passing through the oil heat exchanger. With this arrangement, the transmission oil passing through the oil pipe and the coolant circulating through the bridge passage  15  exchange heat with each other such in the oil heat exchanger  16  that the transmission oil is heated or cooled. 
   The electric water pump  13  is provided when the internal combustion engine is a diesel engine. More specifically, a diesel engine is typically provided with a diesel particulate filter (DPF) for capturing particulate matter contained in the exhaust gas. When the amount of captured particulate matter exceeds a prescribed amount, the diesel particulate filter cannot capture any more particulate matter. Therefore, the diesel particulate filter is regenerated (i.e., the accumulated particulate matter is combusted) on a regular basis or when the amount of captured particulate matter has exceeded the prescribed amount. During regeneration, the internal combustion engine is stopped and, thus, the water pump  2  is not running. In order to prevent the intercooler and other items arranged in the bypass passage  11  from reaching excessively high temperatures, the electric water pump  13  is driven such that the amount of coolant necessary to cool the intercooler is sent through the bypass passage  11 . 
   Another orifice  18  is arranged in the coolant circulation passage  10  at a position between the water pump  2  and the position where the bypass passage  11  merges with the coolant circulation passage  10 . An oil cooler  19  is arranged in parallel with the orifice  18  to exchange heat between the coolant and an engine oil. Coolant vapor resulting from evaporation of the coolant inside the radiator  4  is guided to a reservoir tank  20  where it returns from the vapor state to a liquid state before being returned to the coolant circulation passage  10 . 
   With this illustrated embodiment, when the engine is warming up and the thermostat valve  3  is closed, the cooling medium exits the outlet of the water jacket  1  and returns to the water jacket  1  through the bypass passage  11 , thus accelerating the warming of the engine. A portion of the cooling medium flowing through the bypass passage  11  branches from the bypass passage  11  and enters the bridge passage  15 , thus exchanging heat in the oil heat exchanger  16  before returning to the engine. The amount of cooling medium that enters the bridge passage  15  depends on the passage resistance generated by the orifice  17  (e.g., a passage resistance generating section) arranged in the coolant circulation passage downstream of the oil heat exchanger  16 . Thus, while most of the cooling medium returns to the engine, an appropriate amount can be used to exchange heat in the oil heat exchanger  16 . As a result, the cooling medium can be directed to the oil heat exchanger  16  even when the thermostat valve  3  is closed, thus enabling the oil temperature to be prevented from rising excessively when the engine operates under a very high load while cold. 
   Additionally, after the engine is warm and the thermostat valve  3  is opened, the cooling medium exiting the engine flows to the radiator  4  and a portion of the cooling medium cooled in the radiator  4  branches from the upstream side of the orifice  17  (e.g., a passage resistance generating section) and flows into the bridge passage  15  in the opposite direction as when the engine is warming, thus flowing directly to the oil heat exchanger  16  for the purpose of cooling the automatic transmission oil. As a result, when the thermostat valve  3  is opened, coolant flowing downstream of the radiator  4 , which is the coolest coolant in the system, can be directed to the oil heat exchanger  16 , thus enabling the oil temperature to be prevented from rising excessively when the engine operates under a very high load and enabling the size of the oil heat exchanger to be reduced. 
   The operation of an internal combustion engine cooling system in accordance with this embodiment will now be explained with reference to  FIGS. 3 and 4 . 
   When the engine is warming up and the coolant temperature is low, the thermostat valve  3  is closed such that coolant does not flow downstream of the thermostat valve  3 . Consequently, as indicated with arrows in  FIG. 3 , the coolant pumped through the water jacket  1  by the water pump  2  bypasses the thermostat valve  3  and the radiator  4  and all (100%) of the coolant passes in a parallel fashion through the EGR cooler circulation passage  7 , the heater passage  9  and the bypass passage  11 . The number values (percentages) shown along the passages in  FIG. 3  indicate the amount (percentage) of coolant that flows through each of the passages under certain operating conditions under the assumption that 100% is the total amount of coolant discharged from the water pump  2 . These values are provided as a reference and are not intended to be exact percentages. The flow resistances of the passages can change depending on the operating state of the engine (e.g., the engine speed) and cause the percentage values to change. 
   The coolant passing through the heater passage  9  enters the heater core  8  and releases heat that is used to heat the cabin interior of the vehicle. The coolant existing the heater core  8  then mixes with the un-cooled coolant in the EGR cooler circulation passage  7  before entering and passing through the EGR cooler  6 . The coolant entering the EGR cooler  6  is warmed as it passes through the heat exchanger section of the EGR cooler  6 . Since the EGR valve  5 B is closed during engine warming, the exhaust gas is not recirculated and the coolant does not release as much heat as it otherwise would before returning to the water pump  2 . 
   Meanwhile, the coolant flowing into the bypass passage  11  passes through the turbo cooler  12 , the electric water pump  13 , and the orifice  14 . Then a portion of the coolant branches into the bridge passage  15  and the remainder flows to the downstream portion of the bypass passage  11  and returns to the water pump  2  via the coolant circulation passage  10 . 
   The coolant that branches into the bridge passage  15  passes through the oil heat exchanger (AT cooler)  16  and exchanges heat with the transmission oil that circulates through the transmission. The coolant exiting the oil heat exchanger (AT cooler)  16  flows to the coolant circulation passage  10  on the downstream side of the radiator  4  and passes through the orifice  17  before merging with the coolant flowing from the downstream end of the bypass passage  11  and returning to the water pump  2 . The oil heat exchanger  16  serves to warm the transmission oil when the temperature of the transmission oil is lower than the coolant temperature and to warm the coolant and thus accelerate warming of the engine when the temperature of the transmission oil is higher than the coolant temperature. As a result, the automatic transmission can be prevented from reaching an excessive temperature and the warming of both the engine and the transmission can be accelerated after a cold start. Since warming of both the engine and the transmission after a cold start can be accelerated, friction in the engine and transmission can be reduced earlier when the engine is started under low-temperature conditions. 
   When, for example, a driver suddenly demands high-load operation of the engine by operating the accelerator while the engine is cold or not yet finished warming up, there is the possibility that the temperature of the transmission oil will suddenly rise. With this embodiment, however, the transmission oil can be cooled and an abrupt increase in the transmission oil temperature can be prevented because a portion of the coolant is circulated to the oil heat exchanger  16 . 
   When the engine is cold started, there is a region where use of the EGR apparatus  5  is restricted because the coolant temperature is low. However, with this embodiment, the restriction on the use of the EGR apparatus  5  can be lifted earlier because the warming of the engine is accelerated by the oil heat exchanger  16 . Thus, combustion using EGR gas introduced into the engine can be conducted comparatively early after the engine is started. As a result, the exhaust gas emissions can be reduced and the fuel efficiency can be improved. 
   The amount of coolant that branches into the bridge passage  15  can be adjusted by adjusting the opening surface area of the orifice  17  arranged downstream of the position where the bridge passage  15  branches from the coolant circulation passage  10 . The opening surface area of the orifice  17  controls the flow resistance generated by the orifice  17 . The amount of coolant passing through the bridge passage  15  decreases when the orifice  17  is constricted such that the flow resistance increases, and the amount of coolant passing through the bridge passage  15  increases when the orifice  17  is opened. While the engine is warming up, the rotational speed of the engine is generally comparatively low and, thus, the amount of coolant discharged from the water pump  2  is comparatively small. The amount of coolant passing through the bypass passage  11  and the passage flow resistance caused by the orifice  17  arranged in the coolant circulation passage  10  are also comparatively small. Consequently, the orifice  17  should be adjusted such that the amount of coolant flowing through the bridge passage  15  is approximately one half or slightly less than half of the amount of coolant flowing through the bypass passage  11 . 
   Conversely, when the thermal load is high (e.g., when the outside temperature is high, the engine load is large, and/or the transmission load is large), the temperature of the coolant becomes high. Under such conditions, the thermostat valve  3  is fully open and the coolant pumped out of the water jacketed  1  by the water pump  2  flows as indicated with the arrows shown in  FIG. 4 . More specifically, the coolant flows back to the water pump  2  through the portion of the coolant circulation passage  10  containing the radiator  4 , through the heater passage  9  and EGR cooler circulation passage  7 , and through the bypass passage  11 . The number values (percentages) shown along the passages in  FIG. 4  indicate the amount (percentage) of coolant that flows through each of the passages under certain operating conditions under the assumption that 100% is the total amount of coolant discharged from the water pump  2 . These values are provided as a reference and are not intended to be exact percentages. The flow resistances of the passages can change depending on the operating state of the engine (e.g., the engine speed) and cause the percentage values to change. 
   The coolant circulating through the heater passage  9  and the EGR cooler circulation passage  7  has a high temperature because it has come directly from the water jacket  1  of the engine. The coolant passing through the heated core  8  releases and becomes lower in temperature as it exchanges heat with the cabin air in the heater coil  8 , thus serving to heat the interior of the cabin. The coolant exiting the heater core  8  then merges with higher-temperature coolant that has not passed through the heater core  8  in the EGR cooler circulation passage  7  and flows into the EGR cooler  6 . After the engine has warmed up and the EGR valve  5 B has been opened, a portion of the exhaust gas is circulated to the air induction system through the EGR passage  5 A and the EGR cooler  6 . The coolant passing through the EGR cooler  6  cools the exhaust gas passing through the EGR cooler  6  by absorbing heat from the exhaust gas and returns to the water pump  2  at a higher temperature than it had prior to passing through the EGR cooler  6 . 
   The coolant flowing to the bypass passage  11  passes through the turbo cooler  12 , the electric water pump  13 , and the orifice  14  and returns directly to the water pump  2  after merging with the coolant circulation passage  10 . 
   The coolant in the coolant circulation passage  10  flows through the fully opened thermostat valve  3  and the radiator  4 . Most of the coolant cooled in the radiator  4  passes through the orifice  17  and returns to the water pump  2 . Meanwhile, a portion of the coolant exiting the radiator  4  flows into the bridge passage  15  due to the flow passage resistance set by the orifice  17 . The flow of coolant into the bridge passage  15  in such a case is oriented in the opposite direction as when the thermostat valve  3  is closed. The coolant flowing through the bridge passage  15  in this case passes through the oil heat exchanger (AT cooler)  16  and enters the bypass passage  11  through the portion where the bridge passage  15  merges with the bypass passage  11  downstream of the orifice  14 . The coolant that has passed through the upstream portion of the bypass passage  11  merges with the coolant from the bridge passage  15  downstream of the orifice  17 . The merged coolant flows through the portion of the bypass passage  11  located downstream of the orifice  17 , merges with coolant that has passed through the orifice  17  at the portion where the bypass passage  11  connects to the coolant circulation passage  10 , and returns to the water pump  2 . 
   In this case, the amount of coolant that branches to the bridge passage  15  can adjusted by adjusting the opening surface area of the orifice  17  arranged in the coolant circulation passage  10  downstream of the position where the bridge passage  15  branches from the coolant circulation passage  10 . The opening surface area of the orifice  17  controls the flow resistance generated by the orifice  17 . 
   In this state, coolant flows in both the bypass passage  11  and the portion of the coolant circulation passage  10  downstream of the radiator  4 , and the orifice  17  provided downstream of the radiator  4  causes a portion of the coolant to flow through the bridge passage  15  to the oil heat exchanger  16 . In short, coolant that has just passed through the radiator  4  and coolant that has not passed through any heat exchanging section that would increase its temperature can be directed to the oil heat exchanger  16 . In short, the coolant that has the lowest temperature of any coolant in the system can be sent to the oil heat exchanger  16 . Consequently, coolant can be sent directly to the oil heat exchanger  16  for the purpose of cooling the automatic transmission oil so that the transmission oil can be cooled more efficiently and the transmission oil temperature can be suppressed with a smaller oil heat exchanger  16  even under high load, high coolant temperature conditions. 
   The effects that can be obtained with this embodiment will now be explained. 
   An internal combustion engine cooling system in accordance with this embodiment has the coolant circulation passage  10  configured and arranged to pass a coolant (cooling medium) exiting the water jacket  1  of the internal combustion engine through the radiator  4  and return the coolant to the water jacket  1 . The thermostat valve  3  is arranged between an inlet of the radiator  4  and an outlet of the water jacket  1 , with the bypass passage  11  being configured and arranged to branch from the coolant circulation passage at a position located between the outlet of the water jacket  1  and the thermostat valve  3 . The bypass passage  11  connects to the coolant circulation passage on an outlet side of the radiator  4  so as to bypass the thermostat valve  3  and the radiator  4 . The cooling system apparatus further has the bridge passage  15  configured and arranged to connect an intermediate portion of the bypass passage  11  to a portion of the coolant circulation passage  10  located downstream of the radiator  4 , thus establishing communication between intermediate portions of the bypass passage  11  and the coolant circulation passage  10 . The passage resistance generating section, e.g., an orifice  17 , is arranged in a portion of the coolant circulation passage  10  located downstream of a position where the bridge passage  15  connects to the coolant circulation passage  10  and upstream of the position where the bypass passage  11  merges with the coolant circulation passage  10 . The oil heat exchanger  16  is arranged in the bridge passage  15  to exchange heat between the coolant and a transmission oil passing therethrough. 
   As a result, during engine warming, the warming of the engine can be accelerated by closing the thermostat valve  3  and returning the coolant exiting the water jacket  1  back to the water jacket  1  through the bypass passage  11 . While the thermostat valve  3  is closed, the orifice  17  (which is arranged in the coolant circulation passage  10  downstream of the oil heat exchanger  16 ) is set to generate such a flow passage resistance that a portion of the coolant flowing through the bypass passage  11  branches into the bridge passage  15  with a portion of the coolant flowing through the bypass passage  11  exchanging heat in the oil heat exchanger  16  before returning to the engine. Thus, while most of the coolant returns directly to the engine, an appropriate amount can be used to exchange heat in the oil heat exchanger  16 . As a result, the coolant can be directed to the oil heat exchanger  16  even when the thermostat valve  3  is closed, thus enabling the oil temperature to be prevented from rising excessively when the engine operates under a very high load while cold. 
   Additionally, after a cold start, the warming of both the engine and the transmission can be accelerated while preventing the automatic transmission from reaching an excessive temperature. Since warming of both the engine and the transmission after a cold start can be accelerated, friction in the engine and transmission can be reduced earlier when the engine is started under low-temperature conditions. Furthermore, since warming of the engine can be accelerated, combustion using recirculated exhaust gas can be conducted earlier and the exhaust emissions can be improved earlier. 
   Additionally, after the engine is warm and the thermostat valve  3  is opened, the coolant exiting the engine flows to the radiator  4  and a portion of the coolant cooled in the radiator  4  branches from the upstream side of the orifice  17  (e.g., a passage resistance generating section) and flows into the bridge passage  15  in the opposite direction as when the engine is warming, thus flowing directly to the oil heat exchanger  16  for the purpose of cooling the automatic transmission oil. As a result, when the thermostat valve  3  is opened, coolant flowing downstream of the radiator  4 , which is the coolest coolant in the system, can be directed to the oil heat exchanger  16 , thus enabling the oil temperature to be prevented from rising excessively when the engine operates under a very high load and enabling the size of the oil heat exchanger  16  to be reduced. 
   Since coolant can be passed through the oil heat exchanger  16  in one direction or the opposite direction by opening and closing the thermostat valve  3 , this embodiment can be realized without adding additional valves or making the coolant passages more complex. As a result, the effect described above can be achieved at a low cost. 
   The bypass passage  11  is preferably provided with the turbo cooler  12 , the electric water pump  13 , and the orifice  14  that serve to restrict the bypass passage  11  or increase the flow resistance of the bypass passage  11  at a position upstream of where the bridge passage  15  connects to the bypass passage  11 . As a result, coolant flowing through the bridge passage  15  after the engine is warm and the thermostat valve  3  is opened can be prevented from back flowing upstream into the bypass passage  11  and can be made to merge and flow downstream with the coolant flowing through the bypass passage  11 . 
   Since the cooling system cools the turbo cooler  12  and the electric water pump  13 , which are auxiliary machines provided on the engine and serve to restrict or increase the flow resistance of the bypass passage  11 , the heat absorbed by cooling the auxiliary machines during engine warming serves to accelerate the warming of the engine. 
   The EGR cooler  6  is provided in the EGR passage  5 A that is arranged with one end connected to the exhaust system of the engine and the other end connected to the air induction system of the engine. Coolant flows through the EGR cooler  6  and exchanges heat with the exhaust gas flowing through the EGR passage  5 A, thereby cooling the recirculated exhaust gas. The EGR cooler  6  is provided in the EGR cooler circulation passage  7  that is arranged in parallel with the bypass passage  11  such that coolant flowing therethrough from the water jacket  1  bypasses the thermostat valve  3  and the radiator  4  and returns to the water jacket  1 . As a result, when the engine is warming up and the EGR valve  5 B is closed such that exhaust gas is not recirculated, the coolant passing through the EGR cooler circulation passage  7  does not release as much heat before returning to the water pump  2  as it would if the EGR valve  5 B was open and, thus, the engine warming is accelerated. 
   The cooling system is configured such that a portion of the coolant exiting the water jacket  1  passes through the heater passage  9 , exchanges heat with air in the heater core  8 , and is introduced into the EGR cooler circulation passage  7  upstream of the EGR cooler  6 . As a result, coolant that has released heat in the heater core  8  in order to heat the vehicle interior is added to the coolant passing through the EGR cooler  6 . The introduction of lower-temperature coolant enables the EGR cooler  6  to cool the recirculated exhaust gas more efficiently. 
   The bypass passage  11  connects to the coolant circulation passage  10  at a position downstream of the orifice  17  (e.g., a passage resistance generating section) and a branch passage leading to an oil cooler  19  is arranged downstream of where the bypass passage  11  connects to the coolant circulation passage  10 . As a result, the temperature of the engine oil can be adjusted regardless of whether the engine is warming up or has already warmed up. 
   GENERAL INTERPRETATION OF TERMS 
   In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts. Also as used herein to describe the above embodiment(s), the following directional terms “forward”, “rearward”, “above”, “downward”, “vertical”, “horizontal”, “below” and “transverse” as well as any other similar directional terms refer to those directions of a vehicle equipped with the present invention. Accordingly, these terms, as utilized to describe the present invention should be interpreted relative to a vehicle equipped with the present invention. The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. 
   While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. For example, the size, shape, location or orientation of the various components can be changed as needed and/or desired. Components that are shown directly connected or contacting each other can have intermediate structures disposed between them. The functions of one element can be performed by two, and vice versa. The structures and functions of one embodiment can be adopted in another embodiment. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such feature(s). Thus, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.