Patent Publication Number: US-10309292-B2

Title: Coolant circulation system for vehicle-mounted internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a coolant circulation system for a vehicle-mounted internal combustion engine. 
     Japanese Laid-Open Patent Publication No. 2008-169750 discloses a coolant circulation system that executes circulation stop control for stopping circulation of the coolant after the internal combustion engine starts up, for the purpose of promoting the warm-up of the internal combustion engine. This coolant circulation system changes the period during which the circulation stop control is executed in accordance with the temperature of the coolant detected at the start of the circulation stop control. Specifically, the lower the temperature of the coolant at the start of the circulation stop control, the greater becomes a determination value for terminating the circulation stop control. In addition, the circulation stop control is terminated based on the fact that the time during which the circulation stop control is executed or an accumulated air amount during the circulation stop control has reached a determination value. 
     The lower the temperature of the coolant at the start of the circulation stop control is, the longer becomes the time required for completing the warm-up. For this reason, the coolant circulation system sets a termination condition such that the lower the temperature of the coolant at the start of the circulation stop control is, the longer becomes the period during which the circulation stop control is executed. 
     An internal combustion engine is provided with a liquid temperature sensor for detecting the temperature of the coolant is provided. As described above, if the period during which the circulation stop control is executed is changed in accordance with the temperature of the coolant at the start of the circulation stop control, the coolant may boil in the part of the internal combustion engine with a higher temperature of the coolant than that in the vicinity of the liquid temperature sensor. Consequently, while the circulation stop control is executed in accordance with the temperature of the coolant at the start of the circulation stop control, the temperature of the coolant may reach the boiling point in the part of the internal combustion engine with a higher temperature of the coolant than that in the vicinity of the liquid temperature sensor. 
     For example, to prevent the coolant from boiling even when the temperature of the coolant in the internal combustion engine is not uniform, the determination value may be reduced. In this case, the circulation stop control terminates at a lower temperature. However, in such a case, the circulation stop control may be terminated before the warm-up is performed sufficiently. This may reduce, the effect of promoting the warm-up by the circulation stop control. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a coolant circulation system for a vehicle-mounted internal combustion engine that prevents boiling of coolant and at the same time effectively promote the warm-up. 
     To achieve the foregoing objective and in accordance with a first aspect of the present invention, a coolant circulation system for a vehicle-mounted internal combustion engine is provided. The system includes a coolant circuit including a water jacket of an internal combustion engine, a motor-driven pump, which is provided in a middle of the coolant circuit and moves coolant in the coolant circuit, a liquid temperature sensor, which detects a temperature of the coolant flowing in the coolant circuit, and a controller, which controls the motor-driven pump. The controller executes circulation stop control in which the motor-driven pump is not driven so that circulation of the coolant is kept stopped after the internal combustion engine starts up. The controller changes a period during which the circulation stop control is executed in accordance with a temperature the coolant detected by the liquid temperature sensor at the start of the circulation stop control. The controller executes variation determination control in which the motor-driven pump is driven during a predetermined period after the internal combustion engine starts up to move the coolant in the coolant circuit, thereby determining whether a variation in a temperature of the coolant in the internal combustion engine is equal to or less than a predetermined value based on the temperature of the coolant detected by the liquid temperature sensor. The controller executes the circulation stop control on condition that it is determined in the variation determination control that the variation in the temperature of the coolant is equal to or less than the predetermined value. 
     Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram illustrating the configuration of a diesel engine to which a coolant circulation system for a vehicle-mounted internal combustion engine is applied; 
         FIG. 2  is a schematic diagram illustrating a coolant circulation system for a vehicle-mounted internal combustion engine according to one embodiment; 
         FIG. 3  is a flowchart of a series of processes of variation determination control in the coolant circulation system; 
         FIG. 4  is a timing diagram of the relationship between movement of a drive duty cycle of a motor-driven pump and movement of an outlet liquid temperature in a case in which the variation in the temperature of the coolant is small; and 
         FIG. 5  is a timing diagram of the relationship between movement of the drive duty cycle of the motor-driven pump and movement of the outlet liquid temperature in a case in which the variation in the temperature of the coolant is large. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A coolant circulation system for a vehicle-mounted internal combustion engine according to one embodiment is described below with reference to  FIGS. 1 to 5 . 
     The configuration of a diesel engine  10 , which is a vehicle-mounted internal combustion engine having the coolant circulation system incorporated therein, is described first with reference to  FIG. 1 . 
     As shown in  FIG. 1 , a turbocharger  20  is incorporated in the diesel engine  10 . The diesel engine  10  includes an intake passage  11 , in which an air cleaner  12 , a compressor  21 , an intercooler  41 , and an intake throttle valve  13  are disposed in this order from the upstream side. The air cleaner  12  filters air taken into the intake passage  11 . The compressor  21  includes a compressor wheel therein. The compressor  21  compresses air by rotation of the compressor wheel to feed the compressed air to the downstream side. The intercooler  41  cools the air compressed by the compressor  21 . The intake throttle valve  13  changes the valve opening degree to adjust the flow rate of air flowing in the intake passage  11 , that is, an intake air amount. 
     A combustion chamber  14  is constituted by each cylinder of the diesel engine  10 . The part of the intake passage  11  on the downstream side of the intake throttle valve  13  is connected via an intake port to each of the combustion chambers  14 . A fuel injection valve  15  is disposed in each combustion chamber  14 . Air-fuel mixture of intake air from the intake passage  11  and fuel injected from the fuel injection valve  15  is burned in the combustion chamber  14 . 
     The diesel engine  10  includes an exhaust passage  16 , in which a turbine  22  and an exhaust air cleaner  17  are disposed in this order from the upstream side. Exhaust air generated by the combustion of the air-fuel mixture in the combustion chamber  14  is guided via the exhaust port to the exhaust passage  16  and then discharged to the outside. The turbine  22  includes therein a turbine wheel that is coupled to the compressor wheel by a shaft to be integrally rotational. The turbine  22  and the compressor  21  constitute the turbocharger  20 . The exhaust air cleaner  17  collects particulates in the exhaust air, thus purifying the exhaust air. A fuel addition valve  18  is provided in the part of the exhaust passage  16  upstream from the turbine  22 . The fuel addition valve  18  adds fuel to the exhaust air discharged from the combustion chamber  14 . 
     When the turbine wheel is rotated by the flow of exhaust air in the turbocharger  20 , the compressor wheel is also rotated in cooperation with the rotation of the turbine wheel. Compressed air is thus fed into the combustion chamber  14 , that is, forced-induction is performed. That is, the turbocharger  20  drives the turbine wheel by the flow of the exhaust air to force intake air into the diesel engine  10 . The turbine  22  includes an exhaust blow port that allows for passage of exhaust air blowing against the turbine wheel and a variable nozzle  23  at the exhaust blow port. As the opening degree of the variable nozzle  23  is changed, the opening area of the exhaust blow port is also changed. That is, as the opening degree of the variable nozzle  23  is adjusted, the flow of the exhaust air blowing against the turbine wheel, the pressure of forced intake air, or the forced-induction pressure is also adjusted. 
     In addition, the diesel engine  10  includes an exhaust gas recirculation (EGR) passage (hereinafter, referred to as an EGR passage  31 ). The EGR passage  31  enables the part of the exhaust passage  16  upstream from the turbine  22  to communicate with the part of the intake passage  11  downstream from the intake throttle valve  13 . An EGR cooler  32  and an EGR valve  33  are disposed in the EGR passage  31 . The EGR cooler  32  cools exhaust air that passes through the EGR passage  31  to be recirculated in intake air. As the opening degree of the EGR valve  33  is changed, the amount of the exhaust air recirculated in the intake air is adjusted. A bypass passage  34 , which bypasses the EGR cooler  32  to allow the exhaust air to flow therein, is connected to the EGR passage  31 . An EGR switching valve  35 , which opens or closes the outlet of the bypass passage  34 , is provided in the part of the EGR passage  31  on the downstream side of the EGR cooler  32 . When the EGR switching valve  35  closes the outlet of the bypass passage  34 , exhaust air passes through the EGR cooler  32  and is cooled therein, and then recirculated in the intake air. On the other hand, when the EGR switching valve  35  does not close the outlet of the bypass passage  34 , exhaust air passes through not the EGR cooler  32  but the bypass passage  34  and then is recirculated in the intake air. In this case, the exhaust air is recirculated in the intake air without being cooled in the EGR cooler  32 . 
     The diesel engine  10  is controlled by a controller  100 . Detection signals of various sensors provided in respective parts of the diesel engine  10  are input to the controller  100 . The sensors include an intake air pressure sensor  51 , a crank position sensor  52 , an airflow meter  53 , an outlet liquid temperature sensor  54 , and a vehicle speed sensor  55 . The intake air pressure sensor  51  detects a forced-induction pressure Pim, which is the pressure of intake air in the part of the intake passage  11  downstream from the intake throttle valve  13 . The crank position sensor  52  detects an engine rotational speed NE, which is the rotational speed of the crankshaft functioning as the output shaft of the diesel engine  10 . The airflow meter  53  detects an outside air temperature tha, which is the temperature of intake air in the part of the intake passage  11  upstream from the compressor  21 , and an intake air amount GA. The outlet liquid temperature sensor  54  is a liquid temperature sensor that detects the temperature of the coolant in the coolant circulation system. The outlet liquid temperature sensor  54  detects an outlet liquid temperature ethwout, which is the temperature of the coolant at the outlet of the diesel engine  10 . The vehicle speed sensor  55  detects a vehicle speed SPD, which is the speed of the vehicle having the diesel engine  10  incorporated therein. 
     Next, the coolant circulation system for the diesel engine  10  is described with reference to  FIG. 2 . 
     As shown in  FIG. 2 , the coolant circulation system includes a coolant circuit R 10  including water jackets  36  and  45  of the diesel engine  10 . A motor-driven pump  60  is provided in the middle of the coolant circuit R 10 . The motor-driven pump  60  pumps the coolant into the coolant circuit R 10  to move the coolant in the coolant circuit R 10 . The coolant circuit R 10  includes four passages, that is, a first circulation path R 1 , a second circulation path R 2 , a third circulation path R 3 , and a fourth circulation path R 4 . 
     The first circulation path R 1  includes the block-side water jacket  45  and the head-side water jacket  36 . The block-side water jacket  45  is provided in a cylinder block  40  of the diesel engine  10 , whereas the head-side water jacket  36  is provided in a cylinder head  30  of the diesel engine  10 . An exhaust air cooling portion  36   a  of the head-side water jacket  36  cools the exhaust port. 
     The coolant ejected from the motor-driven pump  60  is first introduced into the block-side water jacket  45 , passes through the block-side water jacket  45 , and then flows into the head-side water jacket  36 . The space between adjacent cylinders in the cylinder block  40  is referred to as an inter-bore region. A drill path DP that connects the block-side water jacket  45  to the head-side water jacket  36  is provided in the inter-bore region. Some of the coolant introduced in the block-side water jacket  45  is guided through the drill path DP to the head-side water jacket  36 . 
     The coolant having passed through the head-side water jacket  36  is guided from the outlet of the cylinder head  30  through pipes to an air conditioner heater  64  and an ATF warmer  65 , which warms up the automatic transmission fluid functioning as the operating oil of the automatic transmission. The outlet is provided at the exhaust air cooling portion  36   a  of the head-side water jacket  36 . The coolant having passed through the water jackets  45  and  36  of the diesel engine  10  is guided from the outlet through pipes to the heater  64  and the ATF warmer  65 . 
     The outlet liquid temperature sensor  54  is provided near the outlet of the exhaust air cooling portion  36   a  in the first circulation path R 1 . The outlet liquid temperature sensor  54  detects the outlet liquid temperature ethwout, which is the temperature of the coolant flowing from the exhaust air cooling portion  36   a  through the outlet. 
     The coolant having passed through the heater  64  and the ATF warmer  65  passes through a thermostat  62  and then returns to the intake port of the motor-driven pump  60 . As described above, the first circulation path R 1  is configured such that the coolant passes through the water jackets  45  and  36 , and the heater  64  and the ATF warmer  65 , and then returns to the motor-driven pump  60 . A first shut-off valve  66  is provided immediately before the heater  64  in the first circulation path R 1 . A second shut-off valve  67  is provided immediately before the ATF warmer  65  in the first circulation path R 1 . Introduction of the coolant into the heater  64  and the ATF warmer  65  is shut off as needed. 
     The second circulation path R 2  branches from the first circulation path R 1  at the part of the cylinder block  40  upstream from the block-side water jacket  45 . The second circulation path R 2  is for guiding the coolant to an oil cooler  63 , which cools the lubricant of the diesel engine  10 . The coolant having passed through the oil cooler  63  is guided through pipes to the turbocharger  20  and the fuel addition valve  18 . The coolant having passed through the turbocharger  20  and the fuel addition valve  18  is introduced into the part of the first circulation path R 1  downstream from the heater  64  and the ATF warmer  65  and upstream from the thermostat  62 . The coolant then returns to the intake port of the motor-driven pump  60 . As described above, the second circulation path R 2  is configured such that the coolant passes through the oil cooler  63 , and the turbocharger  20  and the fuel addition valve  18 , and then returns to the motor-driven pump  60 . 
     The third circulation path R 3  branches from the second circulation path R 2  at the part of the second circulation path R 2  downstream from the cylinder block  40  and upstream from the oil cooler  63 . The third circulation path R 3  is for guiding the coolant to the EGR cooler  32 , the EGR switching valve  35 , and the EGR valve  33 . The coolant having passed through the EGR cooler  32  reaches the EGR valve  33  via the EGR switching valve  35 . The coolant having passed through the EGR valve  33  is guided through pipes to the intake throttle valve  13 . The coolant having passed through the intake throttle valve  13  is introduced into the part of the first circulation path R 1  downstream from the heater  64  and the ATF warmer  65  and then returns to the intake port of the motor-driven pump  60 . Some of the coolant introduced into the EGR cooler  32  is introduced through pipes into the part of the first circulation path R 1  downstream from the heater  64  and the ATF warmer  65  and upstream from the thermostat  62 . The coolant then returns to the intake port of the motor-driven pump  60 . As described above, the third circulation path R 3  is for circulating the coolant through the EGR cooler  32 , the EGR switching valve  35 , the EGR valve  33 , and the intake throttle valve  13 . 
     The fourth circulation path R 4  branches from the first circulation path R 1  at the exhaust air cooling portion  36   a.  The fourth circulation path R 4  is for guiding the coolant to a radiator  61 . The coolant having passed through the radiator  61  passes through the thermostat  62  and returns to the motor-driven pump  60 . A path from the radiator  61  to the motor-driven pump  60  is opened or closed by the thermostat  62 . That is, when the engine is cold in which the temperature of the coolant flowing in the first to third circulation paths and then passing through the thermostat  62  is lower than the valve opening temperature of the thermostat  62 , the fourth circulation path R 4  is closed by the thermostat  62 . In this case, the coolant is not circulated in the fourth circulation path R 4  and the radiator  61  does not radiate heat. The warm-up of the diesel engine  10  is thus promoted. On the other hand, when the temperature of the coolant is increased and the temperature of the coolant flowing in the first to third circulation paths and then passing through the thermostat  62  is equal to or higher than the valve opening temperature of the thermostat  62 , the thermostat  62  is opened. Some of the coolant having passed through the water jackets  45  and  36  then flows in the fourth circulation path R 4  and circulates through the radiator  61 . The heat of the coolant that has passed through the water jackets  45  and  36  and thus has a high temperature is radiated by the radiator  61  and overheating of the diesel engine  10  is prevented. 
     The controller  100  also executes such control of the coolant circulation system. That is, the controller  100  also functions as the controller in the coolant circulation system. For example, the controller  100  opens or closes the first shut-off valve  66  and the second shut-off valve  67  based on the outlet liquid temperature ethwout detected by the outlet liquid temperature sensor  54 . In addition, the controller  100  controls the motor-driven pump  60 , thus controlling the circulation amount of the coolant. 
     Next, the control of the coolant circulation system executed by the controller  100 , in particular, control of the motor-driven pump  60  is described. 
     When the diesel engine  10  has been warmed up, the controller  100  controls the outlet liquid temperature ethwout detected by the outlet liquid temperature sensor  54  to be close to a target temperature. At this time, the controller  100  executes outlet liquid temperature feedback control for feedback-controlling the drive duty cycle of the motor-driven pump  60  in accordance with the outlet liquid temperature ethwout. That is, the controller  100  feedback-controls the drive amount of the motor-driven pump  60  per unit time. The target temperature is higher than the valve opening temperature of the thermostat  62  and lower than the boiling point of the coolant. 
     When the outlet liquid temperature ethwout at the time of the start-up of the internal combustion engine is equal to or lower than a threshold α, the controller  100  basically executes circulation stop control, in which the motor-driven pump  60  is not driven and circulation of the coolant is kept stopped. The threshold α is set to be slightly lower than the valve opening temperature of the thermostat  62 . That is, the controller  100  executes the circulation stop control at the time of cold-start, in which the diesel engine  10  has not been warmed up. With the circulation stop control, the temperature of the coolant in the diesel engine  10  is easily increased according to an operation of the engine and thus the warm-up of the diesel engine  10  is promoted. 
     During the circulation stop control, the coolant is hardly moved in the coolant circuit R 10 , and thus it is impossible to check the progress of the warm-up by the outlet liquid temperature sensor  54 . Thus, the controller  100  estimates the temperature of the coolant in the exhaust air cooling portion  36   a  during the circulation stop control. The controller  100  determines whether the warm-up is completed based on an estimated liquid temperature ethwest, which is the estimated temperature, and terminates the circulation stop control. 
     The controller  100  calculates the estimated liquid temperature ethwest by setting the initial liquid temperature to the outlet liquid temperature ethwout at the start of the circulation stop control. When the controller  100  calculates the estimated liquid temperature ethwest, the controller  100  adds the temperature increase per unit time to the previous estimated liquid temperature ethwest at a predetermined calculation cycle, thus updating the estimated liquid temperature ethwest. In this coolant circulation system, the temperature of the coolant in the exhaust air cooling portion  36   a  is calculated as the estimated liquid temperature ethwest. This is because in the diesel engine  10 , the temperature of the exhaust air cooling portion  36   a  in particular tends to be increased during the operation of the engine. This is for preventing local boiling of the coolant during the circulation stop control. 
     Specifically, the controller  100  calculates a temperature change of the coolant by heat reception per unit time by using the engine rotational speed NE, a fuel injection amount Q, the forced-induction pressure Pim, and an EGR rate. The engine rotational speed NE is correlated with the number of times combustion occurs per unit time. The fuel injection amount Q is correlated with the amount of heat generated in the single occurrence of combustion. The forced-induction pressure Pim and the EGR rate are indexes that show the state of the combustion chamber  14  at the time of combustion. Thus, by using the engine rotational speed NE, the fuel injection amount Q, the forced-induction pressure Pim, and the EGR rate, it is possible to estimate the amount of heat received per unit time. The controller  100  obtains these values and calculates the temperature change of the coolant by heat reception. The forced-induction pressure Pim is an index of the heat capacity of gas in the combustion chamber  14 . The EGR rate is an index of the specific heat of gas in the combustion chamber  14 . 
     In addition, the controller  100  calculates a temperature change of the coolant by heat radiation per unit time based on the difference obtained by subtracting the outside air temperature tha from the estimated liquid temperature ethwest and the vehicle speed SPD. The higher the vehicle speed SPD is, the greater becomes the amount of outside air exposed to the diesel engine  10  per unit time. The amount of heat radiated to the outside air is thus increased. Moreover, the lower the outside air temperature tha is, the greater the amount of heat radiated becomes. The amount of heat radiated per unit time can be estimated by using the vehicle speed SPD and the outside air temperature tha and performing calculation based on the difference obtained by subtracting the outside air temperature tha from the estimated liquid temperature ethwest and the vehicle speed SPD. Thus, the controller  100  obtains the vehicle speed SPD and the outside air temperature tha and calculates the temperature change of the coolant by heat radiation. The controller  100  calculates the temperature change of the coolant by heat radiation by reflecting the surface area of the diesel engine  10  and the heat conductivity of the cylinder block  40  and the cylinder head  30 . 
     The controller  100  calculates a temperature increase of the coolant per unit time from the balance of the calculated temperature change due to the heat reception and the calculated temperature change due to the heat radiation. The controller  100  then adds the calculated temperature increase to the previous estimated liquid temperature ethwest, thus updating the estimated liquid temperature ethwest. 
     When the estimated liquid temperature ethwest is equal to or higher than a predetermined liquid temperature  5 , the controller  100  terminates the circulation stop control. The predetermined liquid temperature δ is a temperature at which it is possible to determine that the cylinder block  40  and the cylinder head  30  have been warmed up based on the fact that the estimated liquid temperature ethwest is equal to or higher than the predetermined liquid temperature δ. Moreover, the predetermined liquid temperature δ is lower than the boiling point of the coolant. 
     After terminating the circulation stop control, the controller  100  executes low flow rate control before executing the outlet liquid temperature feedback control. With the low flow rate control, the motor-driven pump  60  is slowly driven. The coolant is then circulated in the coolant circuit R 10  at a low flow rate so as not to reduce the temperature of the cylinder block  40  and the cylinder head  30  warmed up by the circulation stop control. In the low flow rate control, the motor-driven pump  60  is driven with a drive amount less than that in the outlet liquid temperature feedback control. The coolant in the coolant circuit R 10  is thus stirred little by little while being warmed up by heat generated in the diesel engine  10 . Not only the temperature of the coolant in the water jackets  45  and  36  but also the temperature of the coolant in the coolant circuit R 10  is gradually increased. The coolant is moved in the coolant circuit R 10  during the low flow rate control, and thus it is possible to check the progress of the warm-up by the outlet liquid temperature ethwout detected by the outlet liquid temperature sensor  54 . When the outlet liquid temperature ethwout is equal to or higher than the threshold α, the controller  100  determines that a uniform temperature of the coolant is achieved and then terminates the low flow rate control. The controller  100  then starts the outlet liquid temperature feedback control described above. 
     As described above, in the coolant circulation system, the controller  100  basically executes the circulation stop control when the outlet liquid temperature ethwout at the time of start-up of the internal combustion engine is equal to or lower than the threshold α, and preferentially warms up the cylinder block  40  and the cylinder head  30  through the circulation stop control. When the estimated liquid temperature ethwest is equal to or higher than the predetermined liquid temperature δ, the controller  100  executes the low flow rate control to achieve a uniform temperature of the coolant so as not to cool the cylinder block  40  and the cylinder head  30 . When the temperature of the coolant is made uniform and the outlet liquid temperature ethwout is equal to or higher than the threshold α, the controller  100  determines that the warm-up has been completed, terminates the low flow rate control, and starts the outlet liquid temperature feedback control. 
     However, in the coolant circulation system, the execution of the circulation stop control or the low flow rate control is sometimes prohibited depending on the conditions. For example, when a sensor connected to the controller  100  is abnormal or when the diesel engine  10  is in a high-load operating state, the execution of the circulation stop control and the low flow rate control is prohibited. In addition, when the accumulated fuel injection amount since the start of the circulation stop control is equal to or greater than a termination determination value, the execution of the circulation stop control is prohibited and the low flow rate control is executed. The termination determination value is a threshold for determining whether the coolant is likely to boil. Based on the fact that the accumulated fuel injection amount is equal to or greater than the termination determination value, the controller  100  determines that the accumulated fuel injection amount has been increased to an extent that the amount of heat generated in the diesel engine  10  reaches the amount of generated heat required for boiling the coolant. The controller  100  sets the termination determination value such that the lower the initial liquid temperature, the greater the termination determination value becomes. The controller  100  calculates the accumulated fuel injection amount by accumulating the fuel injection amount Q during the circulation stop control. When the calculated accumulated fuel injection amount is equal to or greater than the termination determination value, the controller  100  terminates the circulation stop control. 
     As described above, the controller  100  calculates the estimated liquid temperature ethwest during the circulation stop control of the coolant circulation system. When the estimated liquid temperature ethwest is equal to or higher than the predetermined liquid temperature δ, the controller  100  terminates the circulation stop control. In this case, the controller  100  calculates the estimated liquid temperature ethwest by setting the initial liquid temperature to the outlet liquid temperature ethwout at the start of the circulation stop control. The controller  100  sets the period during which the circulation stop control is executed such that the lower the outlet liquid temperature ethwout at the start of the circulation stop control, the longer period becomes. That is, the controller  100  changes the period during which the circulation stop control is executed in accordance with the outlet liquid temperature ethwout detected by the outlet liquid temperature sensor  54  at the start of the circulation stop control. 
     When such a configuration is employed, the coolant may boil in the part of the internal combustion engine with a higher temperature of the coolant than that in the vicinity of the liquid temperature sensor. That is, while the circulation stop control is executed according to the temperature of the coolant at the start of the circulation stop control, the coolant may reach the boiling point in the part of the internal combustion engine with a higher temperature of the coolant than that in the vicinity of the liquid temperature sensor. 
     In the case of the coolant circulation system, the estimated liquid temperature ethwest is calculated by setting the initial liquid temperature to the outlet liquid temperature ethwout at the start of the circulation stop control as described above. For this reason, when the temperature of the coolant in the exhaust air cooling portion  36   a  at the start of the circulation stop control deviates largely from the outlet liquid temperature ethwout, the estimated liquid temperature ethwest easily deviates from the temperature of the coolant in the exhaust air cooling portion  36   a.  For example, there is a large variation in the temperature of the coolant in the water jackets  45  and  36  and thus the temperature of the coolant in the exhaust air cooling portion  36   a  at the start of the circulation stop control is sometimes higher than the outlet liquid temperature ethwout. In such a case, the coolant may boil in the exhaust air cooling portion  36   a  before the estimated liquid temperature ethwest reaches the predetermined liquid temperature δ. 
     Thus, the coolant circulation system executes variation determination control for determining the variation in the temperature of the coolant at the time of start-up of the internal combustion engine. In the variation determination control, it is determined whether the variation in the temperature of the coolant in the diesel engine  10  is equal to or less than a predetermined value. The circulation stop control is executed on condition that the variation in the temperature of the coolant is equal to or less than the predetermined value. 
     Next, a series of processes of the variation determination control is described with reference to  FIG. 3 . This series of processes is performed by the controller  100  when the diesel engine  10  starts up. While performing the series of processes, the controller  100  repeatedly obtains the outlet liquid temperature ethwout at a predetermined cycle. 
     As shown in  FIG. 3 , when the series of processes starts, the controller  100  determines at step S 100  whether the outlet liquid temperature ethwout is equal to or lower than the threshold α. If it is determined that the outlet liquid temperature ethwout is equal to or lower than the threshold α (YES at step S 100 ), the controller  100  proceeds process to step S 110 . 
     At step S 110 , the controller  100  drives the motor-driven pump  60 . The controller  100  drives the motor-driven pump  60  at a lower drive duty cycle than that in the low flow rate control. Next, the controller  100  determines at step S 120  whether the circulation amount of the coolant since the drive of the motor-driven pump  60  starts is equal to or greater than a threshold β. The threshold β, is set to be the circulation amount before the coolant in the exhaust air cooling portion  36   a  is moved to the outlet liquid temperature sensor  54 . That is, the circulation amount is based on the capacity of the part of the coolant circuit R 10  from the exhaust air cooling portion  36   a  to the outlet liquid temperature sensor  54 . The controller  100  determines whether the circulation amount of the coolant is equal to or greater than the threshold β based on the drive time since the drive of the motor-driven pump  60  starts. 
     If it is determined that the circulation amount of the coolant since the drive of the motor-driven pump  60  starts is less than the threshold β (NO at step S 120 ), the controller  100  returns the process to step S 110 . If it is determined that the circulation amount of the coolant since the drive of the motor-driven pump  60  starts is equal to or greater than the threshold β (YES at step S 120 ), the controller  100  proceeds to step S 130 . That is, the controller  100  continues to drive the motor-driven pump  60  until the circulation amount of the coolant since the drive of the motor-driven pump  60  starts is equal to or greater than the threshold β. The motor-driven pump  60  is thus driven during the period in which the coolant that is present in the exhaust air cooling portion  36   a  at the time of start-up of the internal combustion engine reaches the outlet liquid temperature sensor  54 . 
     At step S 130 , the controller  100  determines whether the deviation amount ΔTh of the outlet liquid temperature ethwout obtained immediately before the drive of the motor-driven pump  60  starts from the maximum value, which is the highest temperature of the outlet liquid temperatures ethwout obtained while the motor-driven pump  60  is driven, is equal to or less than a threshold γ. Specifically, the controller  100  calculates, as the deviation amount ΔTh, the absolute value of the difference between the maximum value, which is the highest temperature of outlet liquid temperatures ethwout obtained while the motor-driven pump  60  is driven, and the outlet liquid temperature ethwout obtained immediately before the drive of the motor-driven pump  60  starts. The controller  100  then compares the deviation amount ΔTh to the threshold γ. 
     The threshold γ is used to determining whether the execution of the circulation stop control is permitted. Based on the fact that the deviation amount ΔTh is equal to or less than the threshold γ, it is possible to determine that the variation in the temperature of the coolant in the diesel engine  10  is within the range that allows the estimated liquid temperature ethwest to be calculated with an adequate accuracy for executing the circulation stop control. 
     If it is determined that the deviation amount ΔTh is equal to or less than the threshold γ (YES at step S 130 ), the controller  100  proceeds to step S 140  and starts the circulation stop control. If it is determined that the deviation amount ΔTh is greater than the threshold γ (NO at step S 130 ), the controller  100  proceeds to step S 150  and starts the low flow rate control without executing the circulation stop control. 
     Meanwhile, if it is determined that the outlet liquid temperature ethwout is higher than the threshold α (NO at step S 100 ), the controller  100  proceeds to step S 160  and starts outlet liquid temperature feedback control without executing the circulation stop control and the low flow rate control. The controller  100  performs the process at step S 140 , step S 150 , or step S 160  and then terminates the series of processes. 
     The processes at steps S 110  to S 130  correspond to the variation determination control in the coolant circulation system. That is, the controller  100  drives the motor-driven pump  60  in a predetermined period at the time of cold-start of the internal combustion engine to move the coolant in the coolant circuit R 10 . The controller  100  thus executes the variation determination control for determining whether a variation in the temperature of the coolant in the diesel engine  10  is equal to or less than the predetermined value based on the outlet liquid temperature ethwout. If the variation in the temperature of the coolant is equal to or less than the predetermined value, the controller  100  executes the circulation stop control. 
     Next, an operation of the variation determination control is described with reference to  FIGS. 4 and 5 .  FIGS. 4 and 5  are timing diagrams of the relationship between the movement of the drive duty cycle of the motor-driven pump  60  when the outlet liquid temperature ethwout at the time of start-up of the internal combustion engine is equal to or lower than the threshold α and the movement of the outlet liquid temperature ethwout.  FIG. 4  shows the case in which the variation in the temperature of the coolant in the diesel engine  10  is small.  FIG. 5  shows the case in which the variation in the temperature of the coolant in the diesel engine  10  is large. 
     The case in which the variation in the temperature of the coolant is small is described first with reference to  FIG. 4 . When the diesel engine  10  starts up at time t 1 , the variation determination control starts. The motor-driven pump  60  is driven at an extremely low drive duty cycle and the coolant in the coolant circuit R 10  starts to be moved. The outlet liquid temperature ethwout detected by the outlet liquid temperature sensor  54  is also changed. While executing the variation determination control and driving the motor-driven pump  60 , the controller  100  continues to obtain the outlet liquid temperature ethwout. When the circulation amount of the coolant since the drive of the motor-driven pump  60  starts is equal to or greater than the threshold β at time t 2 , the controller  100  determines whether the deviation amount ΔTh of the outlet liquid temperature ethwout obtained immediately before the drive of the motor-driven pump  60  starts from the maximum value of the outlet liquid temperatures ethwout obtained during the drive of the motor-driven pump  60  is equal to or less than the threshold γ. In the example of  FIG. 4 , the deviation amount ΔTh is equal to or less than the threshold γ, and thus the circulation stop control starts and the drive of the motor-driven pump  60  is stopped after the time t 2  (the drive duty cycle is set to be 0%). 
     Next, the case in which the variation in the temperature of the coolant is large is described with reference to  FIG. 5 . When the variation determination control starts at the time t 1 , the outlet liquid temperature ethwout detected by the outlet liquid temperature sensor  54  starts to change. In this case, the variation in the temperature of the coolant in the diesel engine  10  is large and thus the outlet liquid temperature ethwout changes more than that of the example of  FIG. 4 . The controller  100  determines at the time t 2  whether the deviation amount ΔTh is equal to or less than the threshold γ, as in the example of  FIG. 4 . The deviation amount ΔTh is greater than the threshold γ in the example of  FIG. 5  and thus the circulation stop control is not executed and the low flow rate control is instead executed after the time t 2 . After the time t 2 , the motor-driven pump  60  is driven at a higher drive duty cycle than that when the variation determination control is executed. 
     The above-described embodiment achieves the following advantages. 
     (1) When the variation in the temperature of the coolant in the diesel engine  10  is large, that is, when the outlet liquid temperature ethwout detected by the outlet liquid temperature sensor  54  is likely to be inappropriate for starting circulation stop control, the circulation stop control is not executed. It is thus possible to prevent the coolant from boiling. 
     (2) To prevent the coolant from boiling even when the temperature of the coolant in the diesel engine  10  is not uniform, for example, the predetermined temperature δ may be set to be much lower and the circulation stop control may be terminated at a much lower temperature. However, in this case, the circulation stop control is terminated before the warm-up is sufficiently performed, and thus the effect of promoting the warm-up by the circulation stop control is degraded. 
     In the embodiment described above, the circulation stop control is executed only when the variation in the temperature of the coolant in the diesel engine  10  is small and the circulation stop control can be adequately executed according to the outlet liquid temperature ethwout detected by the outlet liquid temperature sensor  54  at the start of the circulation stop control. Thus, it is possible to extend the period during which the circulation stop control is executed as compared to the case in which the circulation stop control is terminated at a lower temperature, as described above. It is thus possible to effectively promote the warm-up by the circulation stop control. 
     (3) With the advantages (1) and (2) described above, it is possible to prevent the coolant from boiling and at the same time, to effectively promote the warm-up. 
     (4) To adequately estimate the variation in the temperature of the coolant in the internal combustion engine in variation determination control, it is preferable to detect the temperature of the coolant in the part with a high temperature and the temperature of the coolant in the part with a low temperature. Regarding this point, the exhaust air cooling portion  36   a  is close to the combustion chamber  14  and cools the exhaust port exposed to high-temperature exhaust air. For this reason, the temperature of the coolant near the exhaust air cooling portion  36   a  tends to be particularly increased. Meanwhile, the outlet of the coolant is disposed on the surface of the diesel engine  10  cooled by outside air. For this reason, in the coolant of the diesel engine  10 , the coolant near the outlet in particular tends to have a low temperature while the internal combustion engine stops. 
     In the coolant circulation system, the temperature of the coolant in the part with a low temperature is detected first by the outlet liquid temperature sensor  54  in the variation determination control. The motor-driven pump  60  is then driven until the temperature of the coolant that is present in the exhaust air cooling portion  36   a  at the time of start-up of the internal combustion engine is detected by the outlet liquid temperature sensor  54 . Thus, it is possible to estimate the variation in the temperature of the coolant by detecting the temperature of the coolant in the exhaust air cooling portion  36   a  and the temperature of the coolant at the outlet, without driving the motor-driven pump  60  until all the coolant in the diesel engine  10  passes through the outlet liquid temperature sensor  54 . 
     That is, it is possible to quickly terminate the variation determination control and proceed to the circulation stop control as compared to the case in which the motor-driven pump  60  is driven until all the coolant in the diesel engine  10  passes through the outlet liquid temperature sensor  54 . Consequently, the effect of promoting the warm-up is not degraded by the movement of the coolant caused by the variation determination control. 
     (5) At the time point when the motor-driven pump  60  is driven until the coolant in the exhaust air cooling portion  36   a  reaches the outlet liquid temperature sensor  54 , it is possible to determine the variation in the temperature of the coolant by using the maximum value of the temperature of the coolant detected during the drive of the motor-driven pump  60 . It is thus possible to determine the variation in the temperature of the coolant by reflecting information about the temperature of the coolant detected during the drive of the motor-driven pump  60  as much as possible. 
     (6) In the case in which the variation in the temperature of the coolant is determined at the time point when the motor-driven pump  60  is driven until the coolant in the exhaust air cooling portion  36   a  reaches the outlet liquid temperature sensor  54 , when it is determined that the variation in the temperature of the coolant is small (YES at step S 130 ), the coolant in the exhaust air cooling portion  36   a  in which the temperature of the coolant is particularly high in the diesel engine  10  has been moved to the outlet liquid temperature sensor  54 . For this reason, the outlet liquid temperature ethwout that is detected when the circulation stop control starts on condition that the variation in the temperature of the coolant is small approximates the temperature of the exhaust air cooling portion  36   a,  which is easily increased particularly during the operation of the engine. In addition, in this cooling system, the estimated liquid temperature ethwest is calculated by setting the initial liquid temperature to the temperature detected at the start of the circulation stop control. It is thus possible to adequately estimate the temperature of the coolant in the exhaust air cooling portion  36   a,  which is easily increased in particular. As the circulation stop control is terminated based on the calculated estimated liquid temperature ethwest, it is possible to execute the circulation stop control as long as possible within the range that prevents the coolant from boiling. 
     (7) The accumulated fuel injection amount is correlated with the total amount of heat generated in the internal combustion engine during the circulation stop control. It is thus possible to estimate the progress of the warm-up and the possibility of boiling by the accumulated fuel injection amount. Regarding this point, if the accumulated fuel injection amount during the circulation stop control is equal to or greater than the termination determination value, the coolant circulation system prohibits the execution of the circulation stop control, temporarily stops the circulation stop control, and executes the low flow rate control. It is thus possible to determine that boiling is likely to occur by using the accumulated fuel injection amount and then to terminate the circulation stop control. 
     (8) It is preferable to execute liquid temperature feedback control after the warm-up for the purpose of preventing overheating of the diesel engine  10 . However, when the motor-driven pump  60  is driven after the circulation stop control to start circulation of the coolant, if the process immediately proceeds to the liquid temperature feedback control, the coolant that has not been warmed up flows into the water jackets  45  and  36  of the diesel engine  10  and cools the diesel engine  10  warmed up during the circulation stop control. It is thus preferable to execute the low flow rate control for driving the motor-driven pump  60  with a drive amount less than that in the liquid temperature feedback control after the circulation stop control, thus circulating the coolant little by little so as not to cool the diesel engine  10 . Regarding this point, after the circulation stop control is terminated, the low flow rate control is executed before the outlet liquid temperature feedback control is executed in the present embodiment. It is thus possible to prevent the diesel engine  10  from being cooled as the process proceeds to the outlet liquid temperature feedback control. 
     (9) In the variation determination control, to determine the variation in the temperature of the coolant in the internal combustion engine, the motor-driven pump  60  is driven to move the coolant and then the temperature of the coolant is detected. At this time, if the drive amount of the motor-driven pump  60  is too large, the coolant is stirred and thus the variation in the temperature of the coolant cannot be determined accurately. Regarding this point, according to the present embodiment, the motor-driven pump  60  is driven with a drive amount much less than that in the low flow rate control in the variation determination control. It is thus possible to prevent the coolant from being stirred by the drive of the motor-driven pump  60 . As a result, it is possible to determine the variation in the temperature of the coolant more accurately. 
     The above-described embodiment may be modified as follows. 
     While the coolant circulation system for the diesel engine  10  has been exemplified, the internal combustion engine to which a configuration similar to the present invention may be applied is not limited to a diesel engine. For example, the present invention may be applied to a coolant circulation system for cooling a gasoline engine. 
     The drive duty cycle of the motor-driven pump  60  in the variation determination control does not need to be lower than the drive duty cycle of the motor-driven pump  60  in the low flow rate control. However, to prevent a coolant from being stirred by the drive of the motor-driven pump  60  and determine a variation in the coolant more accurately, it is preferable to set the drive amount of the motor-driven pump  60  as small as possible in the variation determination control. 
     The liquid temperature sensor is not limited to the outlet liquid temperature sensor. That is, the liquid temperature sensor that detects the temperature of the coolant does not need to be disposed at the outlet of the coolant from the internal combustion engine. For example, the liquid temperature sensor may be disposed at the entrance of the coolant to the internal combustion engine. In this case, however, to determine the variation in the temperature of the coolant in the internal combustion engine by using the temperature of the coolant detected by the liquid temperature sensor, it is necessary to drive the motor-driven pump  60  until the coolant is circulated in the coolant circuit R 10  in the variation determination control. In this case, the coolant tends to be stirred before the coolant in the internal combustion engine reaches the liquid temperature sensor. As a result, it is impossible to accurately determine a variation in the temperature of the coolant. It is thus preferable to dispose the liquid temperature sensor near the outlet of the coolant from the internal combustion engine. 
     A method of calculating an increase in the temperature of the coolant in calculating the estimated liquid temperature ethwest may be changed as necessary. For example, other parameters correlated with the amount of heat received and the amount of heat radiated may be added to the parameters used to calculate the temperature increase. 
     The liquid temperature that is estimated as the estimated liquid temperature ethwest does not need to be the liquid temperature of the coolant in the exhaust air cooling portion  36   a.  However, to prevent boiling, it is preferable to estimate the temperature of the coolant in the part of the internal combustion engine in which the temperature tends to be increased. 
     The same problem as in the present invention may occur when the period during which the circulation stop control is executed is changed in accordance with the temperature of the coolant detected by a liquid temperature sensor at the start of the circulation stop control. The conditions for terminating the circulation stop control can thus be changed as necessary. For example, the circulation stop control is also terminated when the accumulated fuel injection amount during the circulation stop control is equal to or greater than the termination determination value in the embodiment described above, and thus calculation of the estimated liquid temperature ethwest may be omitted. Also in this case, the lower the initial liquid temperature is, the greater the termination determination value is set. The period during which the circulation stop control is executed is thus changed in accordance with the temperature of the coolant detected by a liquid temperature sensor at the start of the circulation stop control. Similar advantages to those of the embodiment described above are obtained if the circulation stop control is executed when it is determined by the variation determination control that the variation in the temperature of the coolant is small. 
     Similarly to the accumulated fuel injection amount, an accumulated intake air amount during the circulation stop control may be an index of the total amount of heat generated in the internal combustion engine during the circulation stop control. Thus, the fact that the intake air amount during the circulation stop control is equal to or greater than the termination determination value may be set as the condition for terminating the circulation stop control. In addition, if the accumulated stop time of the motor-driven pump  60  during the circulation stop control is long, it is possible to estimate that the warm-up is accelerated. Consequently, the fact that the accumulated stop time is equal to or greater than the termination determination value may be set as the condition for terminating the circulation stop control. In both cases, if the termination determination value is set such that the lower the initial liquid temperature, the greater the termination determination value is, when it is determined in the variation determination control that the variation is small, the circulation stop control is executed. As a result, advantages similar to those of the embodiment described above are obtained. Alternatively, the termination determination value may be set by combining these termination conditions as in the embodiment described above. 
     In the variation determination control, whether the variation in the temperature of the coolant is equal to or less than the predetermined value is determined depending on whether the deviation amount ΔTh of the outlet liquid temperature ethwout detected immediately before the drive of the motor-driven pump  60  starts from the maximum value of the outlet liquid temperature ethwout detected during the drive of the motor-driven pump  60  is equal to or less than the threshold δ. The method of calculating the deviation amount used for determination may be adequately changed. For example, instead of the outlet liquid temperature ethwout that is detected immediately before the drive of the motor-driven pump  60  starts, the outlet liquid temperature ethwout detected at the start of the drive of the motor-driven pump  60  and the outlet liquid temperature ethwout detected immediately after the drive of the motor-driven pump  60  starts may be used. Alternatively, instead of the maximum value of the outlet liquid temperature ethwout detected during the drive of the motor-driven pump  60 , the outlet liquid temperature ethwout when the drive of the motor-driven pump  60  is stopped and the outlet liquid temperature ethwout immediately after the drive of the motor-driven pump  60  is stopped may be used. 
     The method of determining whether the variation in the temperature of the coolant is equal to or less than the predetermined value may be adequately changed. For example, whether the variation is equal to or less than the predetermined value may be determined based on the deviation amount between the maximum value and the minimum value that are obtained during the variation determination control. Alternatively, the deviation amount does not need to be used to determine the variation. For example, whether the variation is equal to or less than the predetermined value may be determined based on the standard variation of the temperature of the coolant that is obtained during the variation determination control.