Patent Publication Number: US-10787953-B2

Title: Device for determining abnormalities of cooling water temperature sensors

Description:
TECHNICAL FIELD 
     The present invention relates to a coolant temperature sensor abnormality determination device that determines whether or not a coolant temperature sensor, which detects the temperature of a coolant flowing through a cooling circuit for an engine, has an abnormality. 
     BACKGROUND ART 
     A coolant temperature sensor that detects the temperature of a coolant is arranged in a cooling circuit through which the coolant that cools an engine flows. Patent document 1 discloses an example of an abnormality determination device that determines whether or not such a coolant temperature sensor has an abnormality. The abnormality determination device of patent document 1 is configured to determine whether or not a coolant temperature sensor has an abnormality by, for example, comparing detection values of two coolant temperature sensors that are arranged in the cooling circuit. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 2012-102687 
     SUMMARY OF THE INVENTION 
     Problems that are to be Solved by the Invention 
     In the abnormality determination device of patent document 1, for example, in a state in which the detected value of one of the coolant temperature sensors is fixed at an engine warming completion temperature, when the engine is restarted in an engine warming completion state, the discrepancy is small between the detection values of the two sensors. This results in a normality determination. Thus, it is desirable that the reliability of the determination result be increased in the abnormality determination device that uses the two coolant temperature sensors. 
     It is an object of the present invention to provide a coolant temperature sensor abnormality determination device that increases the reliability of a determination result of whether or not the coolant temperature sensor has an abnormality. 
     Means for Solving the Problem 
     A coolant temperature sensor abnormality determination device that solves the above problem includes an estimated temperature calculation unit configured to calculate an estimated temperature that is an estimated value of a temperature of a coolant that cools an engine and a determination unit configured to determine whether or not two coolant temperature sensors, which are configured to detect the temperature of the coolant, have an abnormality based on detection values of the two coolant temperature sensors and the estimated temperature. The determination unit has a determination permission condition under which a reference temperature is set to the estimated temperature of a present time point and the estimated temperature is then changed from the reference temperature by a determination temperature. The determination unit is configured to determine, when the determination permission condition is satisfied, that the two coolant temperature sensors are functioning normally if a discrepancy between the detection values of the two coolant temperature sensors is less than a normal temperature that is less than or equal to the determination temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing the structure of an engine system including one embodiment of a coolant temperature sensor abnormality determination device. 
         FIG. 2  is a schematic diagram showing the circuit configuration of a cooling circuit for the engine system of  FIG. 1 , in which  FIG. 2A  is a diagram showing the flow of a coolant when a thermostat is closed, and  FIG. 2B  is a diagram showing the flow of the coolant when the thermostat is open. 
         FIG. 3  is a functional block diagram showing the coolant temperature sensor abnormality determination device of the embodiment of  FIG. 1 . 
         FIG. 4  is a flowchart showing an example of procedures executed in an abnormality determination process performed by the abnormality determination device of  FIG. 3 . 
         FIG. 5  is a flowchart showing an example of the procedures executed in a normality determination process performed by the abnormality determination device of  FIG. 3 . 
         FIG. 6  is a timing chart showing the relationship of changes in an estimated temperature estimated by the abnormality determination device of  FIG. 3  and the normality determination process of  FIG. 5 . 
     
    
    
     EMBODIMENTS OF THE INVENTION 
     One embodiment of a coolant temperature sensor abnormality determination device will now be described with reference to  FIGS. 1 to 6 . First, the entire structure of an engine system including the coolant temperature sensor abnormality determination device will be described with reference to  FIG. 1 . 
     Overview of Engine System 
     As shown in  FIG. 1 , the engine system includes a water-cooled engine  10 . A cylinder block  11  includes cylinders  12 . An injector  13  injects fuel into each cylinder  12 . An intake manifold  14  that supplies each cylinder  12  with intake air and an exhaust manifold  15  into which exhaust gas flows from each cylinder  12  are connected to the cylinder block  11 . A member formed by the cylinder block  11  and a cylinder head (not shown) is referred to as the engine block. 
     An intake passage  16  connected to the intake manifold  14  includes, sequentially from an upstream side, an air cleaner (not shown), a compressor  18 , which is an element forming a turbocharger  17 , and an intercooler  19 . An exhaust passage  20  connected to the exhaust manifold  15  includes a turbine  22 , which is an element forming the turbocharger  17 . 
     The engine system includes an EGR device  23 . The EGR device  23  includes an EGR passage  25  that connects the exhaust manifold  15  and the intake passage  16 . The EGR passage  25  includes a water-cooling EGR cooler  26  and an EGR valve  27 , which is located closer to the intake passage  16  than the EGR cooler  26 . When the EGR valve  27  is open, some of the exhaust gas is drawn into the intake passage  16  as EGR gas, and the cylinders  12  are supplied with working gas that is a mixture of exhaust gas and intake air. 
     The engine system includes various sensors. An intake air amount sensor  31  and an intake temperature sensor  32  are located at an upstream side of the compressor  18  in the intake passage  16 . The intake air amount sensor  31  detects an intake air amount Ga, which is a mass flow rate of intake air that flows into the compressor  18 . The intake temperature sensor  32  functions as an ambient temperature sensor and detects an intake temperature Ta, which is the temperature of the intake air, as an ambient temperature. An EGR temperature sensor  34  is located in the EGR passage  25  between the EGR cooler  26  and the EGR valve  27  to detect an EGR cooler outlet temperature T egrc , which is the temperature of the EGR gas that flows into the EGR valve  27 . A boost pressure sensor  36  is located between the intake manifold  14  and a portion of the EGR passage  25  connected to the intake passage  16  to detect a boost pressure Pb, which is a pressure of working gas. A working gas temperature sensor  37  is coupled to the intake manifold  14  to detect a working gas temperature Tim, which is the temperature of the working gas that flows into the cylinders  12 . An engine speed sensor  38  detects an engine speed Ne, which is the speed of a crankshaft  30 . 
     Cooling Circuit 
     The overview of a cooling circuit for the engine system will now be described with reference to  FIG. 2 . 
     As shown in  FIGS. 2A and 2B , a cooling circuit  50  includes a first cooling circuit  51  and a second cooling circuit  52 . The first cooling circuit  51  includes a pump  53  that forcibly moves a coolant using the engine  10  as a power source. The second cooling circuit  52  is connected to an upstream side and a downstream side of the pump  53  of the first cooling circuit  51 . The cooling circuit  50  includes a thermostat  55  located where the first cooling circuit  51  and the second cooling circuit  52  are connected. 
     The first cooling circuit  51  is a circuit including a coolant passage formed in the engine  10  and the EGR cooler  26 . In the first cooling circuit  51 , a coolant is circulated by the pump  53 . The second cooling circuit  52  is a circuit including a radiator  56  that cools the coolant. The thermostat  55  opens and allows the coolant to flow to the radiator  56  when the temperature of the coolant is greater than or equal to an opening temperature. The opening temperature is a temperature that is greater than or equal to an engine warming completion temperature T 1 , at which the warming of the engine  10  is completed. 
     The thermostat  55  is activated so that the heat dissipation amount of the radiator  56  is in equilibrium with various heat absorption amounts. Thus, when the thermostat  55  is open, a coolant is controlled at an equilibrium temperature T cthm . The equilibrium temperature T cthm  is set based on the results of experiments that have been conducted in advance using an actual machine. Further, the cooling circuit  50  includes a coolant temperature detector  44  that detects the temperature of the coolant that passes through the thermostat  55 . The coolant temperature detector  44  includes a first coolant temperature sensor  44   a  that detects a first coolant temperature Tw 1 , which is the temperature of the coolant, and a second coolant temperature sensor  44   b  that detects a second coolant temperature Tw 2 , which is also the temperature of the coolant (refer to  FIG. 3 ). The coolant temperatures Tw 1  and Tw 2  are substantially equal when the coolant temperature sensors  44   a  and  44   b  are functioning normally. 
     Coolant Temperature Sensor Abnormality Determination Device 
     The coolant temperature sensor abnormality determination device (hereinafter referred to as the abnormality determination device) that determines whether or not the coolant temperature sensors have an abnormality will now be described with reference to  FIGS. 3 to 6 . 
     As shown in  FIG. 3 , an abnormality determination device  60  is mainly configured by a microcomputer and can be achieved by, for example, circuitry, that is, one or more dedicated hardware circuits such as an ASIC, one or more processing circuits that operate in accordance with computer programs (software), or a combination thereof. The processing circuit includes a CPU and a memory  63  (for example, ROM and RAM) that stores a program or the like executed by the CPU. The memory  63 , or computer readable medium, includes any usable medium that can be accessed by a versatile or dedicated computer. In addition to a signal from each sensor, the abnormality determination device  60  receives a signal indicating a fuel injection amount Gf, which is a mass flow rate of fuel, from the fuel injection controller  42 , a signal indicating a vehicle speed v from a vehicle speed sensor  45 , and the like. The abnormality determination device  60  determines whether or not the coolant temperature sensors  44   a  and  44   b  have an abnormality based on various programs stored in the memory  63  and various data such as an engine heat absorption amount map  63   c . When a determination unit  62  determines that an abnormality has occurred in the coolant temperature sensors  44   a  and  44   b , the abnormality determination device  60  turns on a malfunction indication lamp (MIL)  65  to notify a driver of the abnormality of the engine system. 
     The abnormality determination device  60  includes an estimated temperature calculation unit  61  (hereinafter referred to as the calculation unit  61 ) that calculates an estimated temperature Tc, which is the estimated value of each of the coolant temperatures Tw 1  and Tw 2 , in predetermined control cycles (infinitesimal time dt). The abnormality determination device  60  also includes the determination unit  62  that determines whether or not the coolant temperature sensors  44   a  and  44   b  have an abnormality based on the estimated temperature Tc and the coolant temperatures Tw 1  and Tw 2 . 
     Estimated Temperature Calculation Unit  61   
     The calculation unit  61  performs a calculation with the following equation (1) based on the signals from the various sensors to calculate the estimated temperature Tc using the coolant equilibrium temperature T cthm  as an upper limit value. The calculation unit  61  sets the first coolant temperature Tw 1  when the engine  10  is started to an initial value of the estimated temperature Tc. In equation (1), T ci−1  is the previous value of the estimated temperature Tc, dq/dt is a calculation result of equation (2) and a heat balance q related to the coolant during the infinitesimal time dt, and C is an added value of a heat capacity of the coolant and a heat capacity of the engine block. In equation (2), a cylinder heat absorption amount q cyl  is the amount of heat transferred from combustion gas to inner walls of the cylinders  12 , and an EGR cooler heat absorption amount q egr  is the heat absorption amount of the coolant in the EGR cooler  26 . An engine heat absorption amount q eng  is a heat absorption amount resulting from, for example, friction between the inner walls and pistons of the cylinders  12 , adiabatic compression of working gas in the cylinders  12 , and the like. A block heat dissipation amount q blk  is the amount of heat dissipated from the engine block to the ambient air. Various calculations performed by the calculation unit  61  will now be described. 
     
       
         
           
             
               
                 
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     Cylinder Heat Absorption Amount q cyl  During Infinitesimal Time dt 
     When calculating the cylinder heat absorption amount q cyl , the calculation unit  61  calculates a working gas amount Gwg, which is a mass flow rate of working gas supplied to the cylinders  12 , and a working gas density ρim, which is the density of the working gas. The calculation unit  61  calculates the working gas amount Gwg and the working gas density ρim by performing a predetermined calculation based on an equation of state P×V=Gwg×R×T using the boost pressure Pb, the engine speed Ne, the displacement D of the engine  10 , and the working gas temperature Tim. 
     Further, the calculation unit  61  calculates an exhaust temperature T exh , which is the temperature of the exhaust gas in the exhaust manifold  15 . As shown by equation (3), the calculation unit  61  calculates a temperature increase value when the mixture of the fuel injection amount Gf/working gas amount Gwg is burned at the engine speed Ne. Then, the calculation unit  61  calculates the exhaust temperature T exh  by adding the working gas temperature Tim to the temperature increase value. The calculation unit  61  calculates a temperature increase value from a temperature increase map  63   a  stored in the memory  63 . The temperature increase map  63   a  is a map that sets a temperature increase value for each engine speed Ne and fuel injection amount Gf/working gas amount Gwg based on the results of experiments and simulations that have been conducted in advance using an actual machine. 
     
       
         
           
             
               
                 
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     In addition, as shown by equation (4), the calculation unit  61  calculates a first heat transfer coefficient h cyl , which indicates how easy combustion gas heat is transferred to the inner walls of the cylinders  12  based on the engine speed Ne, the fuel injection amount Gf, and the working gas density ρim. The calculation unit  61  calculates the first heat transfer coefficient h cyl  from a first coefficient map  63   b  stored in the memory  63 . The first coefficient map  63   b  is a map that sets the first heat transfer coefficient h cyl  for each engine speed Ne, the fuel injection amount Gf, and the working gas density ρim based on the results of experiments and simulations that have been conducted in advance using an actual machine. In equation (4), the engine speed Ne is a parameter of the average speed of each piston, the fuel injection amount Gf is a parameter of fuel injection pressure, and the working gas density ρim is a parameter of an exhaust speed of exhaust gas from the cylinders  12 .
 
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     As shown by equation (5), the calculation unit  61  calculates the cylinder heat absorption amount q cyl  during the infinitesimal time dt by multiplying the first heat transfer coefficient h cyl  and a surface area A cyl  of each cylinder  12  by the temperature difference between the exhaust temperature T exh  and the previous value T ci−1  of the estimated temperature. The cylinder heat absorption amount q cyl  is the amount of heat exchange between the combustion gas and the inner walls of the cylinders  12 . The surface area of each cylinder  12  is the surface area of a cylinder in which the bore diameter of each cylinder  12  is a diameter and the stroke amount of each piston is a height. 
     
       
         
           
             
               
                 
                   
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     EGR Cooler Heat Absorption Amount q egr  During Infinitesimal Time dt 
     When calculating the EGR cooler heat absorption amount q egr , the calculation unit  61  calculates a value obtained by subtracting the intake air amount Ga from the working gas amount Gwg as an EGR amount G egr . As shown by equation (6), the calculation unit  61  calculates the EGR cooler heat absorption amount q egr  during the infinitesimal time dt by multiplying the temperature difference between the exhaust temperature T exh  and the EGR cooler outlet temperature T egrc  by the EGR amount G egr  and a constant-volume specific heat Cv of exhaust gas. 
     
       
         
           
             
               
                 
                   
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     Engine Heat Absorption Amount q eng  During Infinitesimal Time dt 
     As shown by equation (7), the calculation unit  61  calculates the engine heat absorption amount q eng  that uses the engine speed Ne as a parameter. The calculation unit  61  calculates the engine heat absorption amount q eng  during the infinitesimal time dt from the engine heat absorption amount map  63   c  stored in the memory  63 . The engine heat absorption amount map  63   c  is a map that sets the engine heat absorption amount q eng  during the infinitesimal time dt for each engine speed Ne based on the results of experiments and simulations that have been conducted in advance using an actual machine. 
     
       
         
           
             
               
                 
                   
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     Block Heat Dissipation Amount q blk  During Infinitesimal Time dt 
     When calculating the block heat dissipation amount q blk , as shown by equation (8), the calculation unit  61  calculates a second heat transfer coefficient h blk , which indicates how easy heat is transferred between the engine block and the ambient air based on the vehicle speed v. The calculation unit  61  calculates the second heat transfer coefficient h blk  from a second coefficient map  63   d  stored in the memory  63 . The second coefficient map  63   d  is a map that sets the second heat transfer coefficient h blk  for each vehicle speed v based on the results of experiments and simulations that have been conducted in advance using an actual machine. As shown by equation (9), the calculation unit  61  calculates the block heat dissipation amount q blk  during the infinitesimal time dt by multiplying a surface area A blk  of the engine block and the second heat transfer coefficient h blk  by the temperature difference between the previous value T ci−1  of the estimated temperature Tc and the intake temperature Ta. The surface area A blk  of the engine block is the area of a portion of the entire surface of the engine block excluding the portion located at the rear side with respect to the travelling direction. That is, the surface area A blk  is the total area of a front surface portion where the current of air directly strikes and side surface portions along which the current of air flows in a direction opposite to the travelling direction. 
     
       
         
           
             
               
                 
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     The calculation unit  61  that has calculated the various heat amounts described above calculates the estimated temperature Tc by adding a value obtained by dividing the heat balance q by a heat capacity C to the previous value T ci−1  as a temperature change amount in accordance with the above (1). As shown by equation (1), the calculation unit  61  calculates the estimated temperature Tc using the coolant equilibrium temperature T cthm  as an upper limit value. Thus, for example, when the previous value T ci−1  is the equilibrium temperature T cthm , the estimated temperature Tc is maintained at the equilibrium temperature T cthm  when the heat balance q is a positive value, and the estimated temperature Tc is lower than the equilibrium temperature T cthm  when the heat balance q is a negative value. The heat balance q is a positive value when the engine  10  is in a normal drive state. The heat balance q is a negative value, for example, when the engine  10  is idling at a cold location or the engine  10  is in a low-load, low-speed state on a downhill. The state in which the heat balance q is a negative value is hereinafter referred to as the heat dissipation state. 
     Determination Unit  62   
     The determination unit  62  determines whether or not the coolant temperature sensors  44   a  and  44   b  have an abnormality based on the estimated temperature Tc, which is a calculation result of the calculation unit  61 , the coolant temperatures Tw 1  and Tw 2 , and determination data  63   e  stored in the memory  63 . The determination unit  62  performs an abnormality determination process of determining that an abnormality has occurred in the coolant temperature sensors  44   a  and  44   b  in parallel with a normality determination process of determining that the coolant temperature sensors  44   a  and  44   b  are functioning normally. 
     Abnormality Determination Process 
     As shown in  FIG. 4 , in the abnormality determination process, the determination unit  62  obtains the coolant temperatures Tw 1  and Tw 2  and determines whether or not a discrepancy ΔTw (=|Tw 1 −Tw 2 |) is greater than or equal to a normal temperature ΔTn (step S 101 ). The normal temperature ΔTn is a value set in the determination data  63   e  and is, for example, “15° C.,” which is less than or equal to a determination temperature ΔTj (described below). That is, the value (temperature width) serving as the normal temperature ΔTn is set to a value that is less than or equal to the value (change amount) set as the determination temperature ΔTj. When the discrepancy ΔTw is greater than or equal to the normal temperature ΔTn (step S 101 : YES), the determination unit  62  determines that an abnormality has occurred in the coolant temperature sensors  44   a  and  44   b  (step S 102 ) and ends the abnormality determination process. When the discrepancy ΔTw is less than the normal temperature ΔTn (step S 101 : NO), the determination unit  62  obtains the coolant temperature temperatures Tw 1  and Tw 2  again and determines whether or not the discrepancy ΔTw is greater than or equal to the normal temperature ΔTn. 
     Normality Determination Process 
     The normality determination process performed by the determination unit  62  will now be described with reference to  FIG. 5 . The normality determination process is repeatedly performed until the abnormality is determined in the abnormality determination process. Further, the calculation unit  61  calculates the estimated temperature Tc in parallel with the normality determination process. 
     As shown in  FIG. 5 , in step S 201 , the determination unit  62  sets a reference temperature Ts to the estimated temperature Tc of the present time point. When the engine  10  starts, the reference temperature Ts is set to the first coolant temperature Tw 1 , which is the detection value of the first coolant temperature sensor  44   a . Subsequently, the determination unit  62  determines whether or not the estimated temperature Tc has been changed by the determination temperature ΔTj or higher based on the difference between the estimated temperature Tc and the reference temperature Ts (step S 202 ). The determination temperature ΔTj is a value set in the determination data  63   e  and is, for example, “20° C.,” which is higher than the normal temperature ΔTn. 
     When the change amount of the estimated temperature Tc is greater than or equal to the determination temperature ΔTj (step S 202 : YES), the determination unit  62  determines that the determination permission condition has been satisfied and obtains the coolant temperatures Tw 1  and Tw 2  to determine whether or not the discrepancy ΔTw is less than the normal temperature ΔTn (step S 203 ). 
     When the discrepancy ΔTw is less than the normal temperature ΔTn (step S 203 : YES), the determination unit  62  determines that the coolant temperature sensors  44   a  and  44   b  are functioning normally (step S 204 ) and temporarily ends the normality determination process. When the discrepancy ΔTw is greater than or equal to the normal temperature ΔTn (step S 203 : NO), the determination unit  62  ends the normality determination process. Here, the determination unit  62  determines that an abnormality has occurred in the coolant temperature sensors  44   a  and  44   b  in the abnormality determination process performed in parallel with the normality determination process. 
     When the change amount of the estimated temperature Tc is lower than the determination temperature ΔTj (step S 202 : NO), the determination unit  62  determines whether or not a predetermined time has elapsed from when the reference temperature Ts was set (step S 205 ). When the predetermined time has not elapsed (step S 205 : NO), the determination unit  62  determines again in step S 202  whether or not the change amount of the estimated temperature Tc is greater than or equal to the determination temperature ΔTj. When the predetermined time has elapsed (step S 205 : YES), the determination unit  62  updates the reference temperature Ts by resetting the reference temperature Ts to the estimated temperature Tc (step S 206 ) and then determines again in step S 202  whether or not the change amount of the estimated temperature Tc is greater than or equal to the determination temperature ΔTj. 
     Operation 
     The operation of the abnormality determination device  60  when the coolant temperature sensors remain functioning normally from a cold start of the engine  10  will now be described with reference to  FIG. 6 . In  FIG. 6 , “Tw” represents the actual temperature of a coolant. 
     Referring to  FIG. 6 , when the engine  10  starts at time t 1 , a first normality determination process starts. In the first normality determination process, the first coolant temperature Tw 1 , which is the detection value of the first coolant temperature sensor  44   a , is set to an initial value Tc 1  of the estimated temperature Tc and the reference temperature Ts. At time t 2  in which the estimated temperature Tc has been changed from the reference temperature Ts by the determination temperature ΔTj, after the determination permission condition is satisfied, the discrepancy ΔTw between the coolant temperatures Tw 1  and Tw 2  is less than the normal temperature ΔTn. Thus, the normality is determined and the first normality determination process ends. 
     At time t 2 , a second normality determination process starts. In the second normality determination process, the reference temperature Ts is set to the estimated temperature Tc 2  at time T 2 . At time t 3  in which the estimated temperature Tc has been changed by the determination temperature ΔTj, after the determination permission condition is satisfied, the normality is determined and the second normality determination process ends. 
     At time t 3 , a third normality determination process starts. In the third normality determination process, the reference temperature Ts is set to the estimated temperature Tc 3  at time t 3 . However, the estimated temperature Tc is maintained at the coolant equilibrium temperature T cthm , and the estimated temperature Tc has not been changed by the determination temperature ΔTj at time t 4 , which is when a predetermined time has elapsed from time t 3 . Thus, at time t 4 , the reference temperature Ts is updated to an estimated temperature Tc 4  at time t 4 . At time t 5  in which the estimated temperature Tc has been changed from the updated reference temperature Ts by the determination temperature ΔTj, after the determination permission condition is satisfied, the normality is determined and the third normality determination process ends. At time t 5 , an estimated temperature Tc 5  at time t 5  is set to the reference temperature Ts to start a fourth normality determination process. In this manner, the abnormality determination device  60  repeatedly performs the normality determination on the coolant temperature sensors  44   a  and  44   b.    
     The coolant temperature sensor abnormality determination devices of the above embodiment have the advantages described below. 
     (1) The estimated temperature Tc has to be changed by the determination temperature ΔTj for the normality determination to be performed on the coolant temperature sensors  44   a  and  44   b . In other words, when the estimated temperature Tc is changed by the determination temperature ΔTj, the normality is determined on the coolant temperature sensors  44   a  and  44   b . This increases the reliability of the normality determination. As a result, the reliability of the determination result increases. 
     (2) Regardless of whether or not the determination permission condition has been satisfied, when the discrepancy ΔTw between the detection values of the coolant temperature sensors  44   a  and  44   b  is greater than or equal to the normal temperature ΔTn, the abnormality determination device  60  determines that an abnormality has occurred in the coolant temperature sensors  44   a  and  44   b . This allows for quick detection of the occurrence of an abnormality in the coolant temperature sensors  44   a  and  44   b.    
     (3) The abnormality determination device  60  resets the reference temperature Ts when the determination permission condition is not satisfied for the predetermined time. This avoids situations in which the determination that the coolant temperature sensors  44   a  and  44   b  are functioning normally is not performed over a long time. 
     (4) The estimated temperature Tc is calculated based on the heat balance q of the cylinder heat absorption amount q cyl , the EGR cooler heat absorption amount q egr , the engine heat absorption amount q eng , and the block heat dissipation amount q blk . This increases the accuracy of the estimated temperature Tc. 
     (5) The calculation unit  61  calculates the estimated temperature Tc using the equilibrium temperature T cthm  as an upper limit value. In this configuration, there is no need to take into account the amount of heat dissipated from the radiator  56  when the thermostat  55  is open. This decreases the load on the calculation unit  61  for calculating the estimated temperature Tc and eliminates the need for, for example, a configuration that calculates the amount of heat dissipated from the radiator  56 . Thus, the abnormality determination device  60  can be formed by fewer elements. 
     (6) In the above embodiment, the working gas density ρim is used as a parameter of the exhaust speed of exhaust gas from the cylinders  12 . The density of the exhaust gas in the exhaust manifold  15  through which the exhaust gas flows, rather than the working gas density ρim, may be considered as the preferred parameter of the exhaust speed of exhaust gas from the cylinders  12 . However, when the density of exhaust gas in the exhaust manifold  15  is used, an additional sensor having superior durability with respect to the temperature and elements of exhaust gas will be necessary. In this regard, in the above embodiment, the working gas density ρim is used as a parameter of the exhaust speed of exhaust gas from the cylinders  12 . Thus, conventional sensors of the engine system can be used. This allows for the reduction of the components and costs of the abnormality determination device  60 . 
     The above embodiment may be modified as follows. 
     Under the condition in which the coolant temperature Tw is greater than or equal to the opening temperature of the thermostat  55 , the calculation unit  61  may calculate the estimated temperature Tc by calculating the heat dissipation amount in the radiator  56  and taking the calculated value into account. The heat dissipation amount in the radiator  56  can be calculated based on, for example, the change amount of the first coolant temperature Tw 1 , the amount of a coolant, and the heat capacity of the coolant. 
     The calculation unit  61  may calculate the first heat transfer coefficient h cyl  using the density of exhaust gas in the exhaust manifold  15  instead of the working gas density ρim. This configuration increases the accuracy of the first heat transfer coefficient h cyl . As a result, the accuracy of the estimated temperature Tc increases. The density of the exhaust gas can be calculated from, for example, the pressure and temperature of the exhaust manifold  15 . 
     The calculation unit  61  may calculate the EGR cooler heat absorption amount q egr  based on the difference between the EGR cooler outlet temperature T egrc  and the detection value of the temperature sensor that detects the temperature of EGR gas flowing into the EGR cooler  26 . 
     When the EGR cooler  26  is of an air-cooled type, the calculation unit  61  may calculate an added value of the cylinder heat absorption amount q cyl  and the engine heat absorption amount q eng  as a heat absorption amount of a coolant. 
     When the estimated temperature Tc reaches the equilibrium temperature T cthm , the determination unit  62  may set the reference temperature Ts to the equilibrium temperature T cthm . Such a configuration decreases the temperature change amount that is needed when the estimated temperature Tc is changed by the determination temperature ΔTj after reaching the equilibrium temperature T cthm  as compared to a configuration in which the reference temperature Ts is set to the estimated temperature Tc obtained slightly before reaching the equilibrium temperature T cthm . This increases the frequency in which normality determinations are performed on the coolant temperature sensors  44   a  and  44   b.    
     The determination unit  62  may perform normality determination processes in parallel that set the reference temperature Ts to the estimated temperatures Tc at different times. This increases the frequency in which normality determinations are performed on the coolant temperature sensors  44   a  and  44   b.    
     The determination unit  62  may continue the normality determination process after the engine  10  stops. That is, in a process in which the coolant temperature Tw decreases, the determination unit  62  may determine whether or not there is an abnormality based on the discrepancy ΔTw between the coolant temperatures Tw 1  and Tw 2  when the estimated temperature Tc after the engine  10  stops is changed by the determination temperature ΔTj from the reference temperature Ts that is set during the driving of the engine  10 . 
     When detecting an abnormality, the determination unit  62  may detect, as a sensor in which an abnormality has occurred, a sensor detecting a detection value that is further deviated from the estimated temperature Tc of the first and second coolant temperature sensors  44   a  and  44   b.    
     The engine  10  may be a diesel engine, a gasoline engine, or a natural gas engine. Further, the MIL  65  may be, for example, a warning sound generator that generates a warning sound.