Abstract:
A method for diagnosing engine thermostat is presented. Indicated engine coolant temperature is estimated based on engine operating conditions, such as engine speed, net engine torque, air flow, fuel-air ratio, net engine torque, etc. This estimate is compared to the reading of the engine coolant temperature sensor in areas below the temperature at which the thermostat starts to open and above in order to detect degradation in the performance of the sensor or the engine thermostat. If the estimate and the reading agree in one area and disagree in the other, then the thermostat performance is degraded. If the estimate and the reading disagree in both temperature ranges, then the coolant sensor performance may be degraded.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority from provisional application U.S. Ser. No. 60/189,647, filed Mar. 15, 2000. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to systems for diagnosing engine thermostat in a vehicle equipped with an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     Vehicle cooling systems typically have a coolant temperature sensor for providing coolant temperature information to the electronic engine controller and a thermostat for providing constant coolant temperature control. The purpose of the cooling system is to remove surplus heat from the engine, maintain an even and efficient heat level, and to bring the cold engine up to the efficient heat level as soon as possible after starting. When the engine is cold, the thermostat closes and confines the coolant to the engine, enabling it to heat up quickly. When the coolant reaches a predetermined temperature, the thermostat starts to open and allows coolant to circulate. The thermostat constantly changes the size of its opening depending on engine heat conditions. If the cooling system performance is degraded, engine performance may be compromised. For example, if the thermostat does not open once the coolant reaches a certain temperature, engine may overheat. Also, if the thermostat is stuck open, the engine will not heat up properly, a rich fuel-air mixture may be supplied longer than necessary, thus potentially degrading emissions and fuel efficiency. Similarly, if the engine coolant temperature sensor is not indicating actual coolant temperature, emissions, fuel efficiency and driver satisfaction will be degraded. One method of diagnosing the engine coolant system is described in U.S. Pat. No. 4,274,381. Engine coolant temperature is inferred from a temperature sensor such as the temperature sensor of the catalytic converter. This inferred value is compared to the value read by the coolant temperature sensor. If the two values are not the same, sensor degradation is indicated. Then, a signal corresponding to the output of the engine coolant temperature sensor under normal engine operating conditions replaces the output of the degraded coolant temperature sensor. 
     The inventor herein has recognized a disadvantage with this approach. In particular, this method does not diagnose the cooling system thermostat. As long as the readings from the two sensors agree, the system is assumed to be functioning properly. However, if engine operating conditions indicate a certain coolant temperature level, and the actual coolant temperature is different, the thermostat may be stuck in an open or closed state. If the thermostat performance is degraded, efficient temperature levels will not be maintained under all operating conditions, and thus, vehicle performance, fuel efficiency and emission control may be degraded. Also, the prior art does not use a cooling system model to estimate what coolant temperature should be based on engine operating conditions. Further, it does not take into account changes in the cooling system based on the open or closed state of the thermostat. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method for diagnosing a cooling system in an internal combustion engine, and in particular to diagnosing the engine thermostat. 
     The above object is achieved and disadvantages of prior approaches overcome by a method for diagnosing a thermostat in an internal combustion engine, the method comprising: estimating an engine coolant temperature based on an operating condition and a characteristic of the thermostat; reading said engine coolant temperature; and determining that the thermostat performance is degraded if said reading agrees with said estimate in a first operating region, and said reading disagrees with said estimate in a second operating region. 
     An advantage of the above object of the invention is that a method of diagnosing the engine thermostat is developed. By determining agreement in one region and disagreement in a second region, it is possible to isolate thermostat degradation from engine coolant temperature sensor degradation. 
    
    
     Other objects, features, and advantages of the present invention will be readily appreciated by the reader of this specification. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The object and advantages of the invention claimed herein will be more readily understood by reading an example of an embodiment in which the invention is used to advantage with reference to the following drawings wherein: 
     FIG. 1 is a block diagram of a vehicle illustrating various components related to the present invention; 
     FIG. 2 is a block diagram of an engine in which the invention is used to advantage; 
     FIGS. 3,  4 ,  5  and  6  are block diagrams of embodiments in which the invention is used to advantage. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an internal combustion engine  10 , further described herein with particular reference to FIG. 2, is shown coupled to the electronic engine controller  12 , and to the cooling system  13 . Cooling system  13  is also coupled to a thermistor type engine coolant temperature sensor  14 , and to a thermostat  15 . The thermostat  15  opens when engine coolant temperature exceeds a predetermined high value to allow coolant to circulate and thus facilitate engine cooling. The coolant temperature sensor  15  is also coupled to the electronic engine controller  12 . The information provided by the coolant temperature sensor is used in a variety of engine control strategies, such as emissions, fuel injection, etc. 
     Electronic engine controller  12  controls internal combustion engine  10  having a plurality of cylinders, one cylinder of which is shown in FIG.  2 . Engine  10  includes combustion chamber  30  and cylinder walls  32  with piston  36  positioned therein and connected to crankshaft  13 . Combustion chamber  30  communicates with intake manifold  44  and exhaust manifold  48  via respective intake valve  52  and exhaust valve  54 . Exhaust gas oxygen sensor  16  is coupled to exhaust manifold  48  of engine  10  upstream of catalytic converter  20 . In a preferred embodiment, sensor  16  is a HEGO sensor as is known to those skilled in the art. 
     Intake manifold  44  communicates with throttle body  64  via throttle plate  66 . Throttle plate  66  is controlled by electric motor  67 , which receives a signal from ETC driver  69 . ETC driver  69  receives control signal (DC) from controller  12 . Intake manifold  44  is also shown having fuel injector  68  coupled thereto for delivering fuel in proportion to the pulse width signal (fpw) from controller  12 . Fuel is delivered to fuel injector  68  by a conventional fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (not shown). 
     Engine  10  further includes conventional distributor-less ignition system  88  to provide ignition spark to combustion chamber  30  via spark plug  92  in response to controller  12 . In the embodiment described herein, controller  12  is a conventional microcomputer including: microprocessor unit  102 , input/output ports  104 , electronic memory chip  106 , which is an electronically programmable memory in this particular example, random access memory  108 , and a conventional data bus. 
     Controller  12  receives various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including: measurements of inducted mass air flow (MAF) from mass air flow sensor  110  coupled to throttle body  64 ; engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling jacket  114 ; a measurement of throttle position (TP) from throttle position sensor  117  coupled to throttle plate  66 ; a measurement of transmission shaft torque, or engine shaft torque from torque sensor  121 , a measurement of turbine speed (Wt) from turbine speed sensor  119 , where turbine speed measures the speed of shaft  17 , and a profile ignition pickup signal (PIP) from Hall effect sensor  118  coupled to crankshaft  13  indicating an engine speed (We). Alternatively, turbine speed may be determined from vehicle speed and gear ratio. 
     Continuing with FIG. 2, accelerator pedal  130  is shown communicating with the driver&#39;s foot  132 . Accelerator pedal position (PP) is measured by pedal position sensor  134  and sent to controller  12 . In an alternate embodiment, throttle plate  66  communicates with the driver&#39;s foot through a mechanical linkage. The position of throttle plate  66  is measured by throttle position sensor  117 , and sent to controller  12 . 
     Referring now to FIG. 3, a routine is described for using the estimated engine coolant temperature value to diagnose the engine coolant temperature sensor and the thermostat. First, in step  500  a determination is made whether the vehicle has just been turned on (engine start-up). If the answer to step  500  is YES, estimated coolant temperature at start-up, TCEST_STRT is calculated in step  570  (see step  710  of FIG.  4 ). The routine then proceeds to step  580  where the value of the engine coolant temperature sensor, ECT, is read. Next, in step  590  a determination is made whether the value read by the sensor exceeds the estimated engine coolant temperature at engine start-up by a value larger than a preselected tolerance, ECT_STRT_DEL. If the answer to step  590  is NO, the engine coolant temperature sensor passes the rationality test and the routine is exited. If the answer to step  590  is YES, the routine proceeds to step  600 , whereupon a decision is made whether the engine coolant temperature sensor reading exceeds a predetermined tolerance level, ECT_HOT. If the answer to step  600  is NO, the sensor passes the rationality test and the routine proceeds to step  630 , whereupon the estimated value of the engine coolant temperature, TCEST, is seeded with the measured coolant temperature, ECT. The routine is exited. If the answer to step  600  is YES, the sensor fails the test and in step  610  the estimated value of the engine coolant temperature is set to be equal to the estimated value of the engine coolant temperature at engine start-up. The routine proceeds to step  620  whereupon a diagnostic code is set, and the routine is exited. 
     If the answer to step  500  is NO, the routine proceeds to step  510  whereupon the estimated value of the engine coolant temperature, TCEST, is calculated. The details of step  510  are described in FIG.  5 . Next, in step  520 , a decision is made whether the above estimated value exceeds the coolant temperature at which the thermostat is supposed to open by more than a predetermined tolerance amount. In other words, a decision is made whether the coolant temperature is high enough for the thermostat to open. If the answer to step  520  is NO, no thermostat test can be performed and the routine is exited. If the answer to step  520  is YES, a decision is made in step  530  whether the value read by the engine coolant temperature sensor exceeds the temperature at which the thermostat is supposed to open, TSTO, by more than a small predetermined tolerance. If the answer to step  530  is NO, the engine coolant temperature sensor does not pass the warm-up test, a diagnostic code is set in step  640  and the routine is exited. In other words, if the estimated engine coolant temperature is at the level at which the thermostat is supposed to open, and the temperature read by the coolant temperature sensor is below that value, a decision is made that either the sensor or the thermostat are not functioning properly, and a diagnostic code is set. 
     If the answer to step  530  is YES, the sensor passes the test, and the routine proceeds to step  540  whereupon a determination is made whether the engine coolant temperature sensor reading exceeds a predetermined tolerance level, ECT_HOT. If the answer to step  540  is NO, the routine exits. If the answer to step  540  is YES, the routine proceeds to step  550  where a determination is made whether the value read by the engine coolant temperature sensor exceeds the estimated value by larger than a small predetermined tolerance, TCEST_ERROR. If the answer to step  550  is YES, i.e., the value read by the sensor is significantly higher than the estimated value, a decision is made that the sensor is not functioning properly, and the routine proceeds to step  620  as described above. If the answer to step  550  is NO, the sensor is functioning properly and the routine proceeds to step  560  whereupon the value of estimated engine coolant temperature, TCEST, is set to be equal to the actual value read by the engine coolant temperature sensor, ECT. The routine then exits. If it is determined that the engine coolant temperature sensor is not functioning properly, the estimated coolant temperature value can be substituted to enable normal vehicle operation until service time. In that way, improved customer satisfaction as well as improved vehicle performance will be achieved. 
     Moving on to FIG. 4, a routine is described for calculating estimated engine coolant temperature at engine start-up. First, in step  700 , a decision is made whether the engine has just started. If the answer to step  700  is YES, estimated engine coolant temperature at start-up, TCEST_STRT, is calculated in step  710  according to the following equation: 
     
       
         TCEST_STRT=(ECT_NVRAM−T 0 )*EXP(−SOAK_TIME/TAU)+T 0 ,  
       
     
     where ECT_NVRAM is the engine coolant temperature stored in non-volatile memory, and corresponds to the engine coolant temperature at shutdown, T 0  is ambient temperature, SOAK_TIME is engine off time, and TAU is an empirically derived time constant. This value is used in step  570  of FIG.  3 . The routine then exits. If the answer to step  700  is NO, the routine proceeds to step  720 , whereupon the value read by the engine coolant temperature sensor is stored in non-volatile memory, and the routine is exited. 
     Referring now to FIG. 5, a routine is described for estimating engine coolant temperature based on the engine thermodynamic model. First, in step  800 , engine parameters, such as air flow, W, fuel flow, WF, exhaust gas temperature, EGT, engine speed, N, net torque, TNET, and inlet air temperature, IAT, are read. Then, in step  810 , heat transferred into the cooling system, QCDOT, is calculated according to the following equation: 
     
       
         QCDOT=WF*HFV−(W*CPA+WF*CPF)*(EGT−IAT)−N*TNET,  
       
     
     where HVF is the lower heating value of the fuel, CPA is the constant pressure specific heat of air, and CPF is the constant pressure specific heat of the fuel. 
     Next, in step  820 , a determination is made whether the estimated value of the engine coolant temperature, TCEST, is larger than the threshold temperature at which the thermostat should start to open, TSTO. The initial value for TCEST comes from steps  620 , FIG.  1 . If the answer to step  820  is NO, i.e., the estimated coolant temperature is below the threshold at which the thermostat is supposed to start opening, the rate of change of coolant temperature, TCDOT, is calculated according to the low coolant temperature model. If the answer to step  820  is YES, the high coolant temperature model is used to estimate TCDOT in step  840 . Once steps  830  or  840  are completed, the routine proceeds to step  850  where TCEST is calculated according to the following equation: 
     
       
         TCEST=TCDOT*DT+TCEST,  
       
     
     where DT is a predetermined time interval. The routine then exits. 
     Referring now to FIG. 6, a routine is described for diagnosing the cooling system thermostat. First, in step  900 , a decision is made whether the estimated engine coolant temperature (TCEST) is greater than the temperature at which the thermostat starts to open (TCSTO) within a small predetermined tolerance, TOL 01 . If the answer to step  900  is YES, the routine proceeds to step  910  whereupon a determination is made whether the difference between the estimated engine coolant temperature and the coolant temperature read by the sensor is less than or equal to a small predetermined constant, TOL 11 . If the answer to step  910  is YES, the routine proceeds to step  960  whereupon a flag FLAG 01  is set to 1. The routine then proceeds to step  970 . If the answer to step  910  is NO, the routine proceeds to step  950 , whereupon a flag FLAG 01  is set to 0. The routine then proceeds to step  970 . 
     If the answer to step  900  is NO, the routine proceeds to step  920  and a decision is made whether the difference between the estimated engine coolant temperature and the coolant temperature read by the sensor is less than or equal to a small predetermined constant, TOL 11 . If the answer to step  920  is YES, the routine proceeds to step  940  whereupon a flag FLAG 10  is set to 1. The routine then proceeds to step  970 . If the answer to step  920  is NO, the routine proceeds to step  930 , whereupon a flag FLAG 10  is set to 0. The routine then proceeds to step  970 . 
     Continuing in step  970 , a decision is made whether FLAG 01  is 1, FLAG 10  is 0, and a counter is greater than or equal to a preselected value C 1 . If the answer to step  970  is YES, i.e., the reading disagrees with the estimate for temperatures below TSTO, and agrees with the estimate for temperatures above TSTO, for a period of time greater than or equal to C 1 , the thermostat is diagnosed as stuck open, a diagnostic code is set in step  1000 , and the routine exits. If the answer to step  970  is NO, the routine proceeds to step  980  whereupon a determination is made whether FLAG 01  is 0, and FLAG 10  is 1, and a counter is greater than or equal to a preselected value C 2 . If the answer to step  980  is YES, i.e., the reading agrees with the estimate for temperatures below TSTO, and disagrees for temperatures above TSTO, for a period of time greater than or equal to C 3 , the thermostat is diagnosed as stuck closed, a diagnostic code is set in step  990 , and the routine exits. If the answer to step  980  is NO, the routine proceeds to step  1020  whereupon a decision is made whether FLAG 01  is 0, and FLAG 10  is 0, and a counter is greater than or equal to a preselected value C 1 . If the answer to step  1020  is YES, i.e., the reading and the estimate disagree both in the region below TSTO and above TSTO for a period of time greater than C 1 , the engine coolant temperature sensor is diagnosed as degraded, a diagnostic code is set in step  1010 , and the routine exits. 
     Engine coolant temperature is estimated from the thermodynamic characteristics of the engine by estimating heat added to the coolant. The heat added to the coolant is used to estimate the rate of change of coolant temperature, which is then integrated over time to estimate engine coolant temperature. The estimated temperature is compared to the temperature reading provided by the engine coolant temperature sensor to diagnose the cooling system. If the estimate agrees with the reading in the region below the temperature at which the thermostat starts to open (TSTO), and disagrees with the estimate in the area above TSTO, the thermostat is diagnosed as stuck closed, and a diagnostic code is set. If the estimate disagrees with the reading for temperatures below TSTO, and agrees with it for temperatures above TSTO, the thermostat is diagnosed as stuck open, and a diagnostic code is set. The readings are taken continuously for a period of time before diagnostics are performed, to allow for system delays. 
     This concludes the description of the invention. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the invention. Accordingly, it is intended that the scope of the invention is defined by the following claims.