Patent Publication Number: US-2023160331-A1

Title: Thermostat leak detection

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 17/395,135, filed Aug. 5, 2021, titled, THERMOSTAT LEAK DETECTION, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD 
     The invention pertains generally to controllers and more particularly to controllers that are configured to detect thermostat faults. 
     BACKGROUND 
     A variety of internal combustion engines employ cooling systems in which a liquid engine coolant is pumped under pressure through the engine and a corresponding radiator in order to remove excess heat from the engine and keep the engine within a normal operating temperature range. Many such cooling systems employ a fan that can be turned on and off as appropriate in order to better control how much heat is being removed from the engine coolant flowing through the radiator. Cooling systems may employ a thermostat in order to control when engine coolant flows through the radiator and when engine coolant does not flow through the radiator. When an engine is below its normal operating temperature range, such as during a cold start, the thermostat may remain closed in order to allow the engine to warm up and more quickly reach its normal operating temperature range. As the engine coolant temperature approaches its normal operating temperature range, the thermostat will start to open, thereby allowing coolant flow through the radiator to enable cooling the engine. 
     Many thermostats are mechanical devices that can get stuck at an inappropriate thermostat position, particularly as thermostats age and/or become corroded. A thermostat that is stuck in an open or partially open position when coolant temperatures would otherwise indicate that the thermostat should be fully closed will cause the engine to run cooler than its normal operating temperature range. This can cause the engine to run inefficiently, burning more fuel, which results in additional pollutants, plug fouling and the like. Accordingly, there is a desire for the engine to reach its normal operating temperature as quickly as possible. A thermostat that is stuck in a closed or partially closed position can easily cause the engine to overheat as coolant evaporates. If carried to an extreme, overheating can also cause substantial engine damage and may cause a driver and/or passengers to become stranded. 
     Accordingly, there is a desire for methods and systems for determining when a thermostat is not working properly and to achieve early warning of possible problems. 
     OVERVIEW 
     The present inventors have recognized, among other things, that a problem to be solved is the need for new and/or alternative approaches to determining if/when a thermostat is not working properly as part of an engine cooling system. The present inventors have determined that particular models that employ both measurable engine parameters as well as predictable engine parameters may be used to determine when a thermostat is malfunctioning before the thermostat malfunction results in an engine breakdown. The present inventors have determined that particular models may be used to provide an estimated coolant temperature and an estimated thermostat position. The estimated coolant temperature can be compared to an actual measured engine coolant temperature to diagnose whether there is a malfunction. Subsequently, based on this comparison, the thermostat position can be estimated and assessed to determine whether there is a risk of the thermostat being stuck at a position. 
     In an example, a cooling system controller is configured to monitor performance of a cooling system that is configured to circulate engine coolant through an engine, the cooling system including a thermostat that controls engine coolant flow through a radiator. The controller includes an input port configured to receive an engine coolant temperature signal representative of an engine coolant temperature from an engine coolant temperature sensor and a controller that is operably coupled to the input port. The controller is configured to periodically execute a closed-loop healthy model, the closed-loop healthy model periodically outputting a healthy case thermostat position estimate and to periodically execute a closed-loop faulty model, the closed-loop faulty model periodically outputting a faulty case thermostat position estimate. The controller is configured to perform a statistical analysis on the periodically outputted healthy case thermostat position estimates and the faulty case thermostat position estimates in order to ascertain whether the thermostat is functioning appropriately and to output a warning signal when the thermostat is not functioning appropriately. An output port is operably coupled to the controller and is configured to provide the warning signal to an engine management system. 
     Alternatively or additionally, the closed-loop healthy model may include utilizing an Extended Kalman Filter (EKF) to estimate the healthy estimated thermostat position. 
     Alternatively or additionally, the closed-loop healthy model may further include utilizing an Extended Kalman Filter (EKF) to estimate a healthy estimated engine coolant temperature. 
     Alternatively or additionally, the closed-loop faulty model may include utilizing an Extended Kalman Filter (EKF) to estimate the faulty estimated thermostat position. 
     Alternatively or additionally, the closed-loop faulty model may further include utilizing an Extended Kalman Filter (EKF) to estimate a faulty estimated engine coolant temperature. 
     Alternatively or additionally, the controller may be further configured to utilize a comparison between the healthy estimated engine coolant temperature and the faulty estimated engine coolant temperature as a further indication of whether the thermostat is functioning appropriately. 
     Alternatively or additionally, the thermostat may be configured to remain fully closed when the engine coolant temperature is below a first coolant temperature, be fully open when the engine coolant temperature is above a second coolant temperature and be partially open when the engine coolant temperature is between the first coolant temperature and the second coolant temperature. The controller may be further configured to confirm that the healthy estimated thermostat position corresponds to fully closed when the engine coolant temperature is below the first coolant temperature, to confirm that the healthy estimated thermostat position corresponds to fully open when the engine coolant temperature is above the second coolant temperature, to confirm that the healthy estimated thermostat position corresponds to a position between fully closed and fully open when the engine coolant temperature is between the first coolant temperature and the second coolant temperature and to output a warning signal when the healthy estimated thermostat position does not correspond to what the thermostat position should be given the engine coolant temperature. 
     Alternatively or additionally, the first coolant temperature and the second coolant temperature may be selectable based on specific engine requirements. 
     Alternatively or additionally, the first coolant temperature may range from about 82 degrees C. to about 91 degrees C. and the second coolant temperature may range from about 93 degrees C. to about 103 degrees C. 
     Alternatively or additionally, the controller may be further configured, prior to issuing the warning signal, to ascertain whether a cumulative mass coolant flow rate through the radiator exceeds a threshold, and if so, to turn on the warning flag. 
     In another example, a method of monitoring performance of a thermostat within an engine cooling system is provided. The engine cooling system includes a radiator and a coolant pump circulating coolant and the thermostat controls flow of coolant through the radiator. The method includes receiving an engine coolant temperature signal from an engine coolant temperature sensor, supplying the engine coolant temperature signal to an Extended Kalman Filter (EKF), the EKF estimating an estimated engine coolant temperature and an estimated thermostat position and comparing the estimated engine coolant temperature with an actual engine coolant temperature as indicated by the engine coolant temperature signal. When the estimated engine coolant temperature is within a temperature range centered on the actual engine coolant temperature, a determination is made that the thermostat position is appropriate. When the estimated engine coolant temperature is outside a temperature range centered on the actual engine coolant temperature, a determination is made that the thermostat position is not appropriate. 
     Alternatively or additionally, when the estimated engine coolant temperature is below the temperature range centered on the actual engine coolant temperature, a determination may be made that the thermostat is stuck open and a warning flag is turned on. 
     Alternatively or additionally, when the estimated engine coolant temperature is above the temperature range centered on the actual engine coolant temperature, a determination may be made that the thermostat is stuck closed and a warning flag is turned on. 
     Alternatively or additionally, the method may further include comparing the estimated thermostat position with what an actual thermostat position should be based on the actual engine coolant temperature, and turning on the warning flag when there is a discrepancy between the estimated thermostat position and the actual thermostat position. 
     In another example, an engine management system is configured to control operation of an engine, the engine including a cooling system having a radiator, a cooling fan and a thermostat that controls flow of engine coolant to the radiator, the thermostat adjustable between a fully closed position and a fully open position. The engine management system includes an engine coolant temperature sensor arranged upstream of the thermostat and a controller that is operably coupled with the engine coolant temperature sensor. The controller is configured to receive one or more model inputs including data related to operation of the engine, and calculate an estimated engine coolant temperature, to receive an engine coolant temperature signal from the engine coolant temperature sensor, to analyze the engine coolant temperature signal and the estimated engine coolant temperature to determine whether a thermostat fault is present and responsive to presence of the thermostat fault, issue a warning signal. 
     Alternatively or additionally, the controller may be configured to determine that a thermostat fault is present when the estimated engine coolant temperature varies by more than 10 degrees C. from an actual engine coolant temperature as indicated by the engine coolant temperature signal. 
     Alternatively or additionally, the controller may be configured to determine that a thermostat fault is present when the estimated engine coolant temperature varies by more than 20 degrees C. from an actual engine coolant temperature as indicated by the engine coolant temperature signal. 
     Alternatively or additionally, the one or more model inputs may include a mass flow rate of coolant through the engine. 
     Alternatively or additionally, the mass flow rate of coolant through the engine may be estimated using a rotational speed of a coolant pump circulating coolant through the engine and an estimated thermostat position. 
     Alternatively or additionally, the engine management system may further include using a reference model and the one or more model inputs to calculate the estimated engine coolant temperature. 
     This overview is intended to provide an introduction to the subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
         FIG.  1    is a schematic block diagram of an engine cooling system; 
         FIG.  2    is a schematic block diagram of an engine control system; 
         FIG.  3    is a flow diagram showing steps that may be carried out by a cooling system controller; 
         FIG.  4    is a schematic view of a model for predicting a thermostat fault; 
         FIG.  5    is a schematic view of a model for predicting a thermostat fault; 
         FIG.  6    is a graphical representation of measured data; 
         FIG.  7    is a schematic view of a particular model for predicting a thermostat fault; 
         FIG.  8    is a flow diagram showing steps that may be carried out by a cooling system controller; 
         FIG.  9    is a flow diagram showing a method of monitoring performance of a thermostat; and 
         FIG.  10    is a graphical representation of heating and cooling curves. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a schematic block diagram showing an engine cooling system  10  for an engine  12 . While the engine cooling system  10  is described with respect to an internal combustion engine such as a gasoline-fueled engine or a diesel engine, this is merely illustrative. The engine cooling system  10  may be used for cooling non-combustion engines such as an electric motor, a fuel cell, or a hydrogen-powered engine. The engine  12  (and hence the engine cooling system  10 ) may be installed within any of a variety of different types of vehicles, such as but not limited to passenger vehicles, light duty pickup trucks, heavy duty pickup trucks, over the road trucks, construction vehicles and the like. The lines included in the schematic block diagram show possible coolant paths. When a thermostat  14  is open or partially open, meaning that the thermostat  14  permits at least some engine coolant to flow through the thermostat  14 , the engine coolant passing through the thermostat  14  passes to and through a radiator  16  where the engine coolant gives up heat as the engine coolant flows through the radiator  16 . 
     In many cases, the radiator  16  is positioned within the vehicle such that air passes through the radiator  16  as a result of the vehicle moving. The cooling system  10  may include a fan  18  that can be turned on or off to increase air flow through the radiator  16  and thus increase heat transfer from the engine coolant as desired. The fan  18  may be an electronic fan, for example, and may include one larger fan or two relatively smaller fans. In some older vehicles, the fan  18  may have a thermostatically controlled clutch and thus be belt driven off the engine  12 . Engine coolant passing through the radiator  16  will then revert back to a pump  20 . The pump  20  may be belt driven off the engine  12 . 
     When the thermostat  14  is fully closed, meaning that no engine coolant is permitted to pass through the thermostat  14  and reach the radiator  16 , the engine coolant will revert back to the pump  20 . In some cases, at least some of the engine coolant circulating through the engine cooling system  10  may be used to heat a passenger space of the vehicle. The engine coolant may pass through a heater  22 , sometimes referred to as a heater core. The heater  22  is essentially another radiator. Hot engine coolant passes through the heater  22  and gives up heat to air being blown through the heater  22 . In this case, however, the air being blown through the heater  22  is being driven by an electrical fan that is used to blow the air through the heater  22  and through a duct system into the passenger space in order to heat the passenger space. In some cases, at least some engine coolant may pass through a cooler  24  which can be used for heat dissipation from engine oil, transmission fluid or exhaust gases. In some cases, the cooler  24  may be an oil cooler, a transmission cooler, a high pressure or low pressure EGR cooler or an exhaust intercooler. 
     The engine cooling system  10  includes a bottle  26 , which may also be referred to in some cases as a coolant recovery tank or a coolant expansion tank. It will be appreciated that engine coolant, which is generally a mix of propylene glycol and water, and minor amounts of various additives, will expand as it becomes hot. The bottle  26  provides a place for the expanded engine coolant to flow into. Because the engine cooling system  10  is pressurized, at least in part in order to increase the effective boiling point of the engine coolant, excess engine coolant may flow into the bottle  26  as the engine coolant heats up and subsequently as the engine coolant cools down, engine coolant may be drawn out of the bottle  26  and back into circulation. 
     An engine coolant temperature sensor  28  is shown adjacent the engine  12 . In some cases, there may be more than one engine coolant temperature sensor  28 , and the one or more engine coolant temperature sensor(s)  28  may be located in other positions. In some cases, placing the engine coolant temperature sensor  28  adjacent to where the engine coolant exits the engine block provides the most accurate indication of engine coolant temperatures and thus the actual temperature of the engine block itself. The engine coolant temperature sensor  28  may output an engine coolant temperature signal that is representative of the engine coolant temperature. The engine coolant temperature signal may be provided to a cooling system controller, as shown for example in  FIG.  2   , as an Extended Kalman Filter needs an observable condition as an input. 
       FIG.  2    is a schematic block diagram of an illustrative cooling system controller  30 . The cooling system controller  30  may be configured to monitor performance of a cooling system such as the cooling system  10 . The cooling system controller  30  includes an input port  32  that is configured to receive an engine coolant temperature signal representative of an engine cooling temperature from an engine coolant temperature sensor such as the engine coolant temperature sensor  28 . The input port  32  may represent a logical input. The input port  32  may represent a wiring terminal or terminals configured to receive one or more wires carrying the engine coolant temperature signal. The input port  32  may be configured to receive a variety of different signals from a variety of different sensors, for example. 
     A controller  34  is operably coupled to the input port  32 . An output port  36  is operably coupled to the controller  34  and is configured to provide a warning signal to an engine management system  38 . In some instances, the cooling system controller  30  may be a standalone controller that is distinct from the engine management system  38 . In some cases, the cooling system controller  30  may be incorporated into the engine management system  38 . The engine management system  38  may represent a collection of control systems that regulate operation of various systems within a vehicle in which the engine management system  38  is installed. The engine management system  38  may represent a compilation of both hardware and software, for example. The controller  34  may be configured to carry out a number of steps in monitoring thermostat performance. Some of these steps are outlined in  FIG.  3   . 
       FIG.  3    is a flow diagram showing an illustrative series  40  of steps that the controller  34  may be configured to carry out. The controller  34  may be configured to periodically execute a closed-loop healthy model, the closed-loop healthy model periodically outputting a healthy case thermostat position estimate, as indicated at block  42 . The controller  34  may be configured to periodically execute a closed-loop faulty model, the closed-loop faulty model periodically outputting a faulty case thermostat position estimate, as indicated at block  44 . The controller  34  may be configured to perform a statistical analysis on the periodically outputted healthy case thermostat position estimates and the faulty case thermostat position estimates in order to ascertain whether the thermostat is functioning appropriately, as indicated at block  46 . The controller  34  may be configured to output a warning signal when the thermostat is not functioning appropriately, as indicated at block  48 . 
     In some instances, the closed-loop healthy model includes utilizing an Extended Kalman Filter (EKF) to estimate the healthy estimated thermostat position. The closed-loop healthy model may further include utilizing the Extended Kalman Filter (EKF) to estimate a healthy estimated engine coolant temperature. In some instances, the closed-loop faulty model may include utilizing an Extended Kalman Filter (EKF) to estimate the faulty estimated thermostat position. The closed-loop faulty model may further include utilizing the Extended Kalman Filter (EKF) to estimate a faulty estimated engine coolant temperature. In some instances, the controller  34  may be further configured to utilize a comparison between the healthy estimated engine coolant temperature and the faulty estimated engine coolant temperature as a further indication of whether the thermostat is functioning appropriately. 
     With reference to  FIG.  4   , the controller  34  may be configured to execute comparison and hypothesis testing. It will be appreciated that a thermostat such as the thermostat  14  may be configured to remain fully closed when the engine coolant temperature is below a first coolant temperature, which can be identified as T c,min  and to be fully open when the engine coolant temperature is above a second coolant temperature, which can be identified as T c,max . The thermostat may be configured to be partially open (at a position between fully closed and fully open) when the engine coolant temperature is between the first coolant temperature T c,min  and the second coolant temperature T c,max . In some cases, the first coolant temperature and the second coolant temperature are selectable based on thermostat characteristics and the particular type of thermostat that the engine  12  has. As an example, the first coolant temperature may range from about 82 degrees C. to about 91 degrees C. and the second coolant temperature may range from about 93 degrees C. to about 103 degrees C. 
       FIG.  4    provides an example  50  of how the controller  34  may ascertain whether the thermostat is functioning properly. The controller  34  may compare and test hypothesis, as indicated at block  52 . In some case, the engine coolant temperature may be below T c,min , as indicated at block  54 . In some cases, the engine coolant temperature may be above T c,max , as indicated at block  56 . In some cases, the engine coolant temperature may be between T c,min  and T c,max  as indicated at block  58 . If the engine coolant temperature is below T c,min , as indicated at block  54 , then all models including the closed-loop healthy model and the closed-loop faulty model should all indicate that the thermostat is fully closed, as indicated at block  60 . If the engine coolant temperature is above T c,max , as indicated at block  56 , then all models should indicate that the thermostat is fully open, as indicated at block  62 . If the engine coolant temperature is between T c,min  and T c,max , as indicated at block  58 , then all of the models should indicate that the thermostat position is somewhere between fully closed and fully open, as indicated at block  64 . If any of these are not true, the controller  34  may perform further checks, such as but not limited to determining whether the cumulative radiator mass flow rate is above a threshold, as indicated at block  66 . If so, the controller  34  may determine that the thermostat is not functioning correctly, as indicated at block  68 . 
     With brief reference to  FIG.  10   , it will be appreciated that a thermostat position u t  is a dynamic model that can be described with the help of a heating curve f t1 (T c ) and a cooling curve f t2 (T c ) where T c  denotes the coolant temperature. Hysteresis may be defined as follows: 
         T   sat ( k )=min( T   c,max ,max( T   c,min   ,T   c ( k ))), in the interval  T   c ∈[ T   c,min   ,T   c,max ].
 
     The hysteresis model of a thermostat opening u t  may be given as a min-max operator over hysteresis curves f t1 (T c ) and f t2 (T c )) and past thermostat opening u T (k−1) as the following: 
         u   t ( k )=max{ f   t1 ( T   sat ( k )),min( f   t1 ( T   sat ( k )), u   t ( k− 1))}. 
     A temperature control model  1  can be created. Assuming, for simplicity, a sampling rate of 1 per second, the combustion dynamic model may be given by two difference equations that abstract the heat transfer from the generated heat to the coolant and to the ambient. The coolant temperature at the engine outlet may be given by the following: 
         C   1 ( T   c,EngOut ( k+ 1)− T   c,EngOut ( k ))= {dot over (Q)}   Comb ( k )− {dot over (m)}   c,eng ( k ) c   p,c ( T   c,EngOut ( k )− T   c,EngIn ( k ))− k   A,c ( T   c,EngOut ( k )− T   amb ( k )),
 
     where C 1  and k A,c  are heat transfer parameters determined experimentally, 
     {dot over (m)} c,eng  is coolant mass flow through the engine, 
     T c,EngIn  and T c,EngOut  are coolant temperatures at the engine inlet and the engine outlet, respectively, 
     T amb  is a measured ambient temperature, and 
     {dot over (Q)} Comb  is a combustion heat generated by the engine. 
     A temperature control model  2  can be created. According to cooling system configuration, the coolant temperature at the engine inlet is given by flow mixing of radiator coolant flow {dot over (m)} c,rad  with temperature T c,RadOut  with the coolant flow {dot over (m)} c,m  and temperature T c,m . This can be seen below: 
         {dot over (m)}   c,Eng ( k ) T   c,EngIn ( k )= {dot over (m)}   c,Rad ( k ) T   c,RadOut ( k )+ {dot over (m)}   c,m ( k ) T   c,m ( k ). 
     Inserting inside the combustion difference equation yields the following first order model: 
     
       
         
           
             
               
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     It will be appreciated that the radiator coolant flow {dot over (m)} c,rad , the engine coolant flow {dot over (m)} c,eng  and other flows through the cooler, heater {dot over (m)} c,m  depend on the thermostat position u t  and revolutions of the pump N pump . Temperatures at the component outlets T c,RadOut  and T c,m  can be modeled based on the engine data and with the help of physical-based models. Other inputs such as combustion heat {dot over (Q)} Comb  can be determined from engine data. 
       FIG.  5    is a schematic block diagram of a method  70  for predicting a thermostat fault. In the method  70 , the controller  34  executes a closed-loop healthy model  72  as well as a closed-loop faulty model  74 . The closed-loop healthy model  72  and the closed-loop faulty model  74  are executed simultaneously. One of the outputs of the closed-loop healthy model is an estimated healthy thermostat opening û t  and one of the outputs of the closed-loop faulty model is an estimated faulty thermostat opening û f . These two values are compared at a block  76 , which leads to a determination of a detected leak, as indicated at block  78 . In this, the thermostat is considered to be leaking if the actual thermostat position is different than expected, based on engine coolant temperatures. In some instances, the thermostat may be considered to be leaking if the thermostat is stuck at any particular position, whether fully open, fully closed or somewhere in between, regardless of engine coolant temperatures. 
     The Extended Kalman Filter (EKF) is a non-linear estimator of the internal dynamical states for a state-space system that is affected by additive noise. The internal model may be augmented with noise, as indicated below: 
     
       
         
           
             
               
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     In this, w k  is the process noise with a covariance E[w k w k   T ]=Q and v k  is the measurement noise with a covariance E[w k w k   T ]=R. 
     The closed-loop model of a healthy system includes measured coolant temperature T c,EngOut  with the Kalman gains L T  and L u , and is described by the following: 
     
       
         
           
             
               
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     The output of EKF (Extended Kalman Filter) is an estimated coolant temperature {circumflex over (T)} c,EngOut  and estimated thermostat position û t . It will be appreciated that estimated radiator flow may be determined from a flow model of engine coolant through the radiator  16  that depends on the estimated thermostat opening û t  and a pump speed N pump . The estimated radiator flow may be indicated as {dot over (m)} c,rad (û t , N pump ). 
     The closed-loop model of a faulty system includes measured coolant temperature T c,EngOut  with the Kalman gains L T  and L u  and the faulty thermostat position û f , and is described by the following: 
     
       
         
           
             
               
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     The output of EKF (Extended Kalman Filter) is an estimated coolant temperature {circumflex over (T)} c,EngOut  and estimated thermostat position û f  in a fault case. Fault case is considered when the thermostat model does not follow the heating and cooling curves at nominal characteristics. It will be appreciated that estimated radiator flow may be determined from a flow model of engine coolant through the radiator  16  that depends on the estimated thermostat position û f  and a pump speed N pump . The estimated radiator flow may be indicated as {dot over (m)} c,rad (û f , N pump ). 
       FIG.  6    is a graphical representation  80  showing an example of measured data. In the graphical representation  80 , the vertical axis denotes number of drive cycles and the horizontal axis denotes the measured cumulative radiator flow per minute. A number of healthy cases are indicated within a healthy case ellipse  82  and a number of faulty cases are indicated within a faulty case ellipse  84 . It can be seen that a separation threshold  86  is defined therebetween. 
       FIG.  7    is a schematic block diagram of a model  88  that may be used to estimate an unmeasured radiator flow via an Extended Kalman Filtering approach. Model inputs  90  are fed to a reference model  92 . An output of the reference model  92  is an estimated temperature  94 . The estimated temperature  94  and a measured temperature  96  from an engine coolant temperature sensor are fed to a summation point  98 , and then to an EKF (Extended Kalman Filter)  100 . An estimated thermostat position is outputted from the EKF  100 . Given the estimated thermostat position and pump speed, the estimated radiator flow can be determined. 
       FIG.  8    is a flow diagram showing an illustrative series  106  of steps that the controller  34  or an engine management system including the functionality of the controller  34  is configured to carry out. The controller  34  may be configured to receive one or more model inputs including data related to operation of the engine, and calculate an estimated engine coolant temperature, as indicated at block  108 . The one or more model inputs may include, for example, a mass flow rate of coolant through the engine. The mass flow rate of coolant through the engine may be estimated using a rotational speed of a coolant pump circulating coolant through the engine and an estimated thermostat position. 
     The controller  34  may be configured to receive an engine coolant temperature signal from the engine coolant temperature sensor, as indicated at block  110 . The controller  34  may be configured to analyze the engine coolant temperature signal and the estimated engine coolant temperature to determine whether a thermostat fault is present, as indicated at block  112 . The controller  34  may be configured to determine that a thermostat fault is present when the estimated engine coolant temperature varies by more than 10 degrees C., or perhaps 20 degrees C., from an actual engine coolant temperature as indicated by the engine coolant temperature signal. The controller  34  may be configured to issue a warning signal responsive to the presence of the thermostat fault, as indicated at block  114 . In some cases, the controller  34  may also be configured to use a reference model and the one or more model inputs to calculate the estimated engine coolant temperature. 
       FIG.  8    is a flow diagram showing an illustrative method  116  of monitoring performance of a thermostat within an engine cooling system, the engine cooling thermostat including a radiator and a coolant pump circulating coolant, the thermostat controlling flow of coolant through the radiator. The method includes receiving an engine coolant temperature signal from an engine coolant temperature sensor, as indicated at block  118 . The engine coolant temperature signal is supplied to an Extended Kalman Filter (EKF), the EKF estimating an estimated engine coolant temperature and an estimated thermostat position, as indicated at block  120 . The estimated engine coolant temperature is compared with an actual engine coolant temperature as indicated by the engine coolant temperature signal, as indicated at block  122 . 
     When the estimated engine coolant temperature is within a temperature range centered on the actual engine coolant temperature, a determination may be made that the thermostat position is appropriate, as indicated at block  124 . When the estimated engine coolant temperature is outside a temperature range centered on the actual engine coolant temperature, a determination may be made that the thermostat position is not appropriate, as indicated at block  126 . In some cases, the method  116  may further include comparing the estimated thermostat position with what an actual thermostat position should be based on the actual engine coolant temperature, and turning on the warning flag when there is a discrepancy between the estimated thermostat position and the actual thermostat position, as indicated at block  128 . 
     In some instances, when the estimated engine coolant temperature is below the temperature range centered on the actual engine coolant temperature, a determination may be made that the thermostat is stuck open and a warning flag is turned on. In some instances, when the estimated engine coolant temperature is above the temperature range centered on the actual engine coolant temperature, a determination may be made that the thermostat is stuck closed and a warning flag is turned on. 
     In the event of inconsistent usages between this document and any documents so incorporated by reference, the usage in this document controls. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” Moreover, in the claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     Method examples described herein can be machine or computer-implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code may form portions of computer program products. Further, in an example, the code can be tangibly stored on one or more volatile, non-transitory, or non-volatile tangible computer-readable media, such as during execution or at other times. Examples of these tangible computer-readable media can include, but are not limited to, hard disks, removable magnetic or optical disks, magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like. 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. 
     The Abstract is provided to comply with 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 
     Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, innovative subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the protection should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.