Abstract:
A method is presented for diagnosing an engine coolant temperature sensor and an engine thermostat. Engine coolant temperature is estimated based on engine operating conditions, such as engine speed, net engine torque, air flow, fuel-air ratio, exhaust gas temperature, etc., and a characteristic of the thermostat. The estimate is compared to the actual reading of the engine coolant temperature sensor in order to detect degradation in the performance of the sensor or the engine thermostat. If degradation is detected, the estimated engine coolant temperature can be used for various engine control strategies, such as electronic fuel injection, thereby improving vehicle performance, fuel efficiency, and emission control.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to systems for estimating engine coolant temperature in a vehicle equipped with an internal combustion engine, and more particularly, to using this information to determine whether the performance of the cooling system is degraded. 
     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. Coolant temperature is a very important parameter in several engine control strategies, and in particular in an electronically controlled fuel supply system. If the coolant temperature sensor is degraded, fuel consumption and emission strategy may be degraded. For example, if the coolant temperature sensor is indicating that the engine is cold, rather than warmed up, a rich fuel-air mixture may be supplied longer than necessary, thus potentially degrading emissions and fuel efficiency. 
     One method of diagnosing the engine coolant temperature sensor is described in U.S. Pat. No. 4,274,381. Engine coolant temperature is inferred from another 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, 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, there is not a way to determine which one of the above mentioned sensors is degraded. Also, providing a predetermined signal to replace the degraded sensor information is not an accurate representation of the actual operating conditions, especially at high/low ambient temperatures, or at engine start-up. 
     Another disadvantage is that this method does not diagnose the cooling system thermostat. 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. Further, the prior art does not take into account the state of the thermostat (open or closed) when estimating coolant temperature. 
     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 coolant temperature sensor and the thermostat. 
     The above object is achieved and disadvantages of prior approaches overcome by a method for diagnosing a cooling system having an engine coolant temperature sensor and 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 the engine coolant temperature sensor; comparing said estimate with said reading; and determining operability of the system based on said comparison. 
     An advantage of the above object of the invention is that a more precise method of diagnosing the engine coolant temperature sensor is developed. By taking into account a characteristic of the thermostat, it is possible to more accurately estimate coolant temperature since the cooling system performs differently depending on the operation of the thermostat. The electronic engine controller can use a more accurate estimate of the coolant temperature in case the coolant temperature sensor performance is degraded. 
     In another aspect of the present invention, a method for estimating an engine coolant temperature and diagnosing a coolant temperature sensor and a thermostat is developed. This method comprises determining a first estimate of heat added to the coolant based on an engine operating condition; determining a second estimate of coolant temperature based on said first estimate; reading the coolant temperature sensor; comparing said estimate with said reading; determining whether the coolant temperature sensor is functioning properly based on said comparing; and determining whether the thermostat is functioning properly based on said comparing. By using heat added to estimate coolant temperature, an accurate model is obtained to improve estimation. This ability contributes to improved vehicle performance, fuel efficiency and emissions control. 
     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 objects 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 , and  5  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  17 . Cooling system  17  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 (Nt) 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 (Ne). 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 does not pass 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 degraded, 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  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. 
     This concludes the description of the invention. Engine thermodynamic properties, such as net torque, fuel-air ratio, engine speed, exhaust gas temperature, etc., are used to estimate the heat transfer to the cooling system. This estimate is used to estimate the rate of change in engine coolant temperature. Two different models are used depending on the characteristic of the thermostat. If the coolant temperature is above the threshold at which the thermostat is supposed to open, high range coolant temperature change rate is calculated. If the coolant temperature is below the threshold at which the thermostat is supposed to open, low range coolant temperature change rate is calculated. The estimated engine coolant temperature is then calculated by integrating the rate of change of coolant temperature over a period of time. This method provides an accurate estimate of engine coolant temperature by taking into account engine thermodynamic properties, as well as changes in the cooling system due to the characteristic of the thermostat. 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.