Abstract:
In a method for diagnosing a faulty thermostat in a coolant circuit, in particular for an internal combustion engine, having a fan, the faulty thermostat is detected as a function of a measured temperature and a setpoint temperature, the fan being turned on at least temporarily during the diagnosis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to Application No. 10 2010 001 618.7, filed in the Federal Republic of Germany on Feb. 5, 2010, which is expressly incorporated herein in its entirety by reference thereto. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method for diagnosing a thermostat, as well as to a computer program, an electrical memory medium, and a control and regulating device. 
       BACKGROUND INFORMATION 
       [0003]    To comply with the OBDII legislation in the United States, all exhaust-relevant components of a motor vehicle must be diagnosed by the engine control unit. Proof is usually obtained within an exhaust gas test cycle run on a vehicle test stand. Furthermore, a minimum frequency for running the particular diagnostic method must be verified by internal counters within the control unit. 
         [0004]    Specifically this also requires detection of a defective engine temperature sensor as well as a sticking, faultily open coolant thermostat in the coolant circuit of the vehicle. 
         [0005]    Certain conventional methods diagnose the thermostat on the basis of a modeled expected temperature characteristic of the coolant temperature. These functions are based on models having relatively high tolerances. Therefore, a faultily open thermostat is detectable only when the deviation in the temperature characteristic from the modeled value is also relatively great. 
         [0006]    However, specifically in high-power, large-volume engines, there is the problem that they heat up relatively slowly in the exhaust gas test cycle and therefore the temperature difference between faultless operation and operation with a faultily open thermostat does not turn out to be great enough. Reliable diagnosis of a sticking, open thermostat is therefore impossible. 
         [0007]    A trip, during which a diagnostic sequence should be possible according to the statutory minimum requirements, results in a slight heating of the engine under some circumstances, so that a diagnosis is problematic. 
       SUMMARY 
       [0008]    Example embodiments of the present invention provide a method that may ensure an adequate distance between measured and modeled temperatures, in particular during trips resulting in only slight heating of the engine. 
         [0009]    A method for diagnosing a faulty thermostat in a coolant circuit having a fan, in which the fan is turned on at least temporarily during the diagnosis, is particularly advantageous because the difference between the particular cooling effects in the case of a faulty thermostat and a faultless thermostat is particularly great due to the fan being turned on. 
         [0010]    The method may be particularly easy to implement if the thermostat is closed faultlessly. 
         [0011]    In addition, detection of a faulty thermostat is particularly simple if the faulty thermostat is detected as a function of a measured temperature and a setpoint temperature. 
         [0012]    Detection of a faulty thermostat is also particularly simple if the fan is triggered after a temperature increase in the coolant with respect to the ambient temperature which is to be defined. 
         [0013]    The method may be particularly reliable if the setpoint temperature is ascertained as a function of a second temperature sensed and/or as a function of the operating state of the internal combustion engine. 
         [0014]    The method may also be particularly reliable if a faulty thermostat is detected as a function of the curve of the first measured temperature and the curve of the setpoint temperature. It is also particularly expedient if a faulty thermostat is detected when the rise in the first measured temperature is slower than the rise in the setpoint temperature. 
         [0015]    The method may be economical in particular when the fan is turned on for the first time before reaching a predefinable temperature, in particular when this predefinable temperature is selected such that a faultless thermostat is still closed. 
         [0016]    The method may be inexpensive as an implementable measure because it does not require any additional cost. In particular no additional components, for example, sensors, need be provided. 
         [0017]    Further features and aspects of example embodiments of the present invention are described in more detail below with reference to the appended Figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  schematically illustrates an air-cooled coolant circuit having a fan. 
           [0019]      FIG. 2  schematically illustrates time curves of the temperatures with a faultless thermostat and a faultily open thermostat as well as the time curve of the triggering of the engine fan. 
           [0020]      FIG. 3  schematically illustrates the sequence of the diagnostic method. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIG. 1  shows internal combustion engine  1 , a first coolant line  3 , a second coolant line  5  and a thermostat  7 . First coolant line  3  together with a first connection  2  and a second connection  6  forms a first coolant circuit of internal combustion engine  1 . Second coolant line  5  together with first connection  2  and second connection  6 , a cooler  18 , and thermostat  7  forms a second coolant circuit. The first coolant circuit and the second coolant circuit are filled with a coolant such as water. Thermostat  7  switches between first coolant circuit and second coolant circuit. Thermostat  7  is closed at low temperatures and coolant flows through the first coolant circuit and through internal combustion engine  1 . Thermostat  7  is opened at high temperatures and coolant flows through second coolant circuit  5  and through internal combustion engine  1 . 
         [0022]    The first coolant circuit, like internal combustion engine  1 , is in a first area  10 , which is at a first temperature  12 . Second coolant circuit  5  flows through first area  10  as well as a second area  14 , which is at a second temperature  16 . After prolonged operation of internal combustion engine  1  in particular, first temperature  12  is definitely higher than second temperature  16 . 
         [0023]    Thermostat  7  is closed when the value of first temperature  12  is below a thermostat-specific threshold temperature. When thermostat  7  is closed, the coolant circulates in the first coolant circuit. There is no cooling by the first coolant circuit. Thermostat  7  opens when the value of first temperature  12  exceeds the threshold temperature. When thermostat  7  is open, coolant circulates in the second coolant circuit. Cooling of the coolant therefore takes place in cooler  18  due to heat exchange with the air in second area  14 . The temperature of the coolant is now lower than first temperature  12  in first area  10  and thus withdraws heat from first area  10  via a heat exchange. 
         [0024]      FIG. 1  also shows a diagnostic unit  20 , which detects whether thermostat  7  is operating faultlessly. In the illustrated exemplary embodiment, a thermostat  7  which is operating faultlessly would be closed and coolant should flow through the first coolant circuit. However, in this exemplary embodiment, thermostat  7  is faultily opened, i.e., coolant flows through the second coolant circuit. Diagnostic unit  20  detects that thermostat  7  is faultily opened. 
         [0025]    Diagnostic unit  20  includes a calculation unit  22 , a comparator unit  24  and a triggering unit  26 .  FIG. 1  also shows a temperature sensor  30  and an engine fan  32 . Temperature sensor  30  senses first temperature  12  and relays this temperature to calculation unit  22  and comparator unit  24 . The value of this first sensed and relayed temperature  12  is referred to as a measured temperature  31 .  FIG. 1  also shows a temperature sensor  39 , which senses second temperature  16  and relays it to calculation unit  22 . The value of this sensed relayed temperature  16  is referred to as ascertained ambient temperature  41 . 
         [0026]    Triggering unit  26  controls engine fan  32  via a triggering signal  34 . For example, triggering signal  34  may assume “on” and “off” values. If triggering signal  34  is “on,” engine fan  32  is induced to a rotational movement  36 . In this manner, cool air, which is still at second temperature  16 , is conveyed into the vicinity of cooler  18  and thus the cooling effect of coolant flowing through the second coolant circuit is increased. 
         [0027]    In addition to measured temperature  31 , calculation unit  22  receives engine variables  40  from a data unit  38  assigned to internal combustion engine  1 , these variables characterizing the prevailing operating state of internal combustion engine  1 . Such an engine variable may be in particular the air mass combusted in the internal combustion engine. This is a measure of the total heat generated by the internal combustion engine since a reference point in time (for example, the point in time of the start of the internal combustion engine) and is also a measure of the heating of the internal combustion engine. Data unit  38  may be, for example, a sensor, an engine control unit, or a memory unit of an engine control unit. 
         [0028]    Calculation unit  22  ascertains a setpoint temperature  42  from engine variables  40  and measured temperature  31  and relays it to comparator unit  24 . 
         [0029]    Setpoint temperature  42  corresponds to the expected measured temperature when thermostat  7  is operating faultlessly. Comparator unit  24  checks the deviation in measured temperature  31  from setpoint temperature  42  and decides that thermostat  7  is defective if the deviation between measured temperature  31  and setpoint temperature  42  is too great. Setpoint temperature  42  is calculated in calculation unit  22  with the aid of a model of internal combustion engine  1 , for example, which continuously calculates the expected temperature, i.e., setpoint temperature  42 , from engine variables  40  and measured temperature  31 . 
         [0030]    The model first calculates, for example, the heat generated in internal combustion engine  1  by using thermodynamic equations. In a next step, the model calculates the thermal output released by radiation, convection and optionally also thermal conduction and calculates expected setpoint temperature  42  from the heat balance. It is also possible for the model to ascertain expected setpoint temperature  42  with the aid of characteristic curves and characteristic maps, for example, taking into account measured engine variables  40  and measured ambient conditions (for example, ascertained ambient temperature  41 ). 
         [0031]      FIG. 2  shows the curve of measured temperature  31  and setpoint temperature  42  for the case when thermostat  7  should be closed faultlessly but in fact is faultily open. Time t is plotted on the abscissa and temperature T is plotted on the ordinate. The time curve of setpoint temperature  42 , shown in  FIG. 2 , is calculated by calculation unit  22  under the assumption of a faultlessly closed thermostat  7 . The curve of measured temperature  31  with faultily open thermostat  7  and engine fan  32  at rest is labeled with reference numeral  31   a . At a reference point in time  70 , instantaneous setpoint temperature T setpoint  is compared with measured temperature T def . Measured temperature T def  of temperature curve  31   a  is labeled with reference numeral  82 . 
         [0032]    The deviation between T setpoint  and T def  may have various causes. Possible causes include, for example but not exclusively, errors in the model used in calculation unit  22 , measurement errors or inaccuracies in temperature sensor  30  in ascertaining measured temperature  31  or a faultily opened thermostat  7 . The decision that thermostat  7  is defectively open is therefore possible only if measured temperature T def  is lower than setpoint temperature T setpoint  by a minimum temperature difference  84 . At a sufficiently low first temperature  12 , the temperature difference from second temperature  16  is small, the cooling effect of cooler  18  is thus minor, and a defectively open thermostat  7  is not to be differentiated reliably from a correctly closed thermostat  7  by the difference between setpoint temperature T setpoint  and measured temperature T def . This case is illustrated in  FIG. 2   a.    
         [0033]      FIG. 2   b  shows the time curve of triggering signal  34 . At the start of the diagnosis, triggering signal  34  corresponds to the “off” value. Engine fan  32  is thus not in motion. Since thermostat  7  is (faultily) opened, coolant is flowing through the second coolant circuit. At a first point in time  90 , triggering unit  26  switches triggering signal  34  to “on.” Engine fan  32  then begins to move and increases the cooling power of cooler  18 . The particular curve of measured temperature  31  is plotted in  FIG. 2   a  using reference numeral  31   b . In comparison with temperature curve  31   a  when engine fan  32  is at rest, the temperature of temperature curve  31   b  is much lower. The temperature of temperature curve  31   b  is all the lower, the lower ambient temperature  41  is and the greater the volume flow is created by the fan. 
         [0034]    At a second point in time  92 , which may be before or after reference point in time  70 , triggering unit  26  again switches triggering signal  34  from “on” to “off.” At reference point in time  70 , the value of measured temperature  31  indicated using reference numeral  86  is now different from setpoint temperature T setpoint  by more than minimum temperature difference  84 . Therefore, a defectively open thermostat is detected. 
         [0035]    Reference point in time  70 , first point in time  90 , and second point in time  92  may be ascertained by taking into account additional variables, for example, in particular the air mass combusted in the internal combustion engine, integrated over time. Second point in time  92  may also be selected in relation to first point in time  90 , such that the time difference between second point in time  92  and first point in time  90  exceeds a predefinable time difference as a function of the characteristic of fan  32  and a characteristic cooling power of the second coolant circuit. 
         [0036]      FIG. 3  shows as an example the sequence of the diagnostic procedure in diagnostic unit  20 . In step  200 , the internal combustion engine is in the normal operating mode. At a point in time  202 , there is a check as to whether a diagnosis is necessary (for example, because the last diagnosis was more than a predefined period of time in the past) and whether the diagnostic method may be used, for example, because measured temperature  31  is lower than a predefinable temperature. This predefinable temperature should in particular be lower than the threshold temperature of thermostat  7 , so that the thermostat is closed faultlessly. This predefinable temperature may also be reduced in comparison with the threshold temperature, for example, by the maximum increase in measured temperature  31  to be expected through internal combustion engine  1  during the diagnostic procedure or to compensate for exemplary scattering, in particular of the thermostat but also of other components. 
         [0037]    If these conditions are met, the method branches off to step  204 ; if these conditions are not met, the method branches back to step  200 . The diagnostic method begins in step  204  and the model in calculation unit  22  is initialized. Step  206  then follows. 
         [0038]    In step  206  it is checked whether the time is already more advanced than first point in time  90 . Alternatively, it may also check in step  206  whether the difference between measured temperature  31  and ascertained ambient temperature  41  is greater than a predefinable temperature difference. The predefinable temperature difference must be selected as a function of the characteristics of fan  32 , such that in the case of a faulty fan, the difference between setpoint temperature T setpoint  and ascertained temperature  86  becomes large enough. 
         [0039]    If this is the case, the method branches to step  208 . If this is not the case, it jumps back to step  206 . 
         [0040]    In step  208 , triggering unit  26  switches triggering signal  34  to “on.” This is followed by step  210 . In step  210 , it is checked whether the time has already exceeded second point in time  92 . Alternatively, in step  210  it may also be checked whether the integrated air mass combusted in the engine has exceeded an air mass threshold value. This air mass threshold value must be selected such that it ensures that the engine has warmed up sufficiently for the expected difference between setpoint temperature T setpoint  and measured temperature T def  to become large enough in the case of a defectively open thermostat. As another alternative, it may also be checked in step  210  whether temperature difference T setpoint −T def  is already greater than minimum temperature difference  84 . If this is the case, the method branches further to step  212 . If this is not the case, it jumps back to step  210 . 
         [0041]    In step  212 , triggering unit  26  switches triggering signal  34  to “off” and branches further to step  214 . In step  214 , it is checked whether the instantaneous point in time is later than reference point in time  70 . If this is the case, step  216  follows. If this is not the case, the method jumps back to step  214 . In step  216  calculation unit  22  calculates setpoint temperature  42  continuously, for example, and relays it to comparator unit  24 . Temperature sensor  30  likewise relays measured temperature  31  to comparator unit  24 . Comparator unit  24  then calculates the difference between setpoint temperature T setpoint  and measured temperature T def . Step  218  then follows in sequence. In step  218  it is checked whether difference T setpoint  T def  is greater than the predefinable minimum temperature difference. If this is the case, step  220  follows. If this is not the case, step  200  follows again. The diagnostic procedure ends at the end of step  218 . In step  220 , a faultily open thermostat is now diagnosed. Next an error flag for the engine control unit may be set, for example, setting the engine in emergency operation or outputting an acoustic or visual warning for the driver. 
         [0042]    Since reference point in time  70  may be either before or after second point in time  92 , the sequence of steps  210 ,  212  and  214 ,  216  may be swapped. After step  208 , it is thus possible to branch off to step  214 , from step  216  to  210  and from step  212  to step  218 . 
         [0043]    Instead of internal combustion engine  1 , any other heat-generating machine may also be used, for example, a fuel cell or a battery. 
         [0044]    The comparison of measured temperature  31  and setpoint temperature  42  performed in comparator unit  24  may also take into account the temperatures at more than one reference point in time  70 . For example, it is possible to use the integrated difference between measured temperature  31  and setpoint temperature  42  during a comparative period of time to detect a defectively open thermostat. 
         [0045]    It is also possible to diagnose a defectively closed thermostat  7 . In this case, the model of the temperature trend present in calculation unit  22  must also take into account the influence of rotating engine fan  32 . Unlike the exemplary embodiment described here, in which a defectively open thermostat is identified by an unexpectedly high cooling performance, a defectively closed thermostat may be identified by an unexpectedly low cooling performance.