Abstract:
A method of detecting a thermal event is provided that relies not only on monitored exhaust temperatures, but also on temperature gradients propagating in the direction of exhaust flow. Specifically, the method of detecting a thermal event in a vehicle exhaust system includes monitoring at least one operating parameter at multiple locations spaced in exhaust flow of the vehicle exhaust system. The method then includes initiating a protective action if the monitoring indicates that at least one respective predetermined temperature requirement and a respective predetermined temperature gradient requirement are exceeded at two of the multiple temperature sensor locations within a predetermined time period.

Description:
TECHNICAL FIELD 
     The present teachings generally include a method of detecting a thermal event in an exhaust system and an exhaust system having a controller configured to carry out the method. 
     BACKGROUND 
     Vehicle exhaust systems often include exhaust after-treatment devices that filter or otherwise treat the exhaust prior to releasing the exhaust into the environment. The after-treatment devices can be damaged if the exhaust temperatures become too high. Temperature sensors are sometimes placed in the exhaust system, and a controller monitors temperature data received from the temperature sensors. 
     Some control systems rely on data from temperature sensors to detect a thermal event. For example, one system determines that there is a thermal event, and initiates a protective action, when two sensors indicate that a predetermined temperature has been reached for a predetermined amount of time. A “thermal event” is an exhaust system operating condition or set of conditions that have been determined to potentially lead to component damage. Accordingly, it is desirable that control systems anticipate, prevent, or quickly limit the duration of a thermal event. 
     SUMMARY 
     Thermal protection control systems that rely only on temperature readings of temperature sensors may not be completely accurate. For example, if a thermal event causes the circuit of a temperature sensor to open, the temperature sensor reading will default to indicate either a very low temperature or a reading at the highest possible value. Neither of these default values is likely to accurately represent the exhaust temperature. 
     A method of detecting a thermal event is provided that relies not only on monitored exhaust temperatures, but also on temperature gradients propagating in the direction of exhaust flow. Specifically, a method of detecting a thermal event in a vehicle exhaust system includes monitoring at least one operating parameter, which may be exhaust temperature as measured by temperature sensors, at multiple locations spaced in exhaust flow of the vehicle exhaust system. The method then includes initiating a protective action if the monitoring indicates that at least one respective predetermined temperature requirement and a respective predetermined temperature gradient requirement are exceeded at two of the multiple temperature sensor locations within a predetermined time period. The method thus predicts when a thermal event exists that is capable of damage to the exhaust system. The predetermined time period may be a heat transport delay time calibrated for the exhaust system. 
     The predetermined temperature requirement that is monitored may include both a predetermined minimum temperature and a predetermined maximum temperature. For example, in one embodiment, based on the temperature data, for each one of the temperature sensors, the method determines whether exhaust flow temperature exceeds a respective predetermined minimum temperature, and then determines whether a respective predetermined temperature gradient is exceeded if the exhaust flow temperature exceeds the respective predetermined minimum temperature. If the respective predetermined temperature gradient is exceeded, the method then determines whether a respective maximum predetermined temperature is exceeded for a predetermined period of time. A detection flag may be set for the predetermined time period if the respective maximum predetermined temperature is exceeded for the predetermined period of time. If two detection flags are set, the protective action is then initiated. Because two detection flags have been set, the method more reliably indicates that excessive temperatures are propagating through the exhaust system. Typical large temperature gradients and typical high temperatures not warranting a protective action will not cause a false indication of a thermal event, because these generally do not occur at two different temperature sensors within the transport delay period. 
     The protective action taken may be, but is not limited to, providing an alert to the vehicle operator, limiting engine power, limiting accelerator position, or any combination of these. 
     As used herein, monitoring exhaust temperatures or monitoring a temperature gradient in the exhaust can be accomplished using temperature sensors and historical stored data from the sensors. Alternatively, exhaust temperature and exhaust temperature gradients can be monitored based on other operating parameters from which temperature and temperature gradient are determined. e.g., from a stored look-up table in which values of monitored operating parameters correspond with a temperature and temperature gradient. 
     The above features and advantages and other features and advantages of the present teachings are readily apparent from the following detailed description of the best modes for carrying out the present teachings when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an engine with an exhaust system for a vehicle, and a controller configured to detect a thermal event in the exhaust system; 
         FIG. 2  is a schematic illustration of another engine with another exhaust system, and a controller configured to detect a thermal event in the exhaust system, in accordance with an alternative aspect of the present teachings; 
         FIG. 3  is a plot of temperature in degrees Celsius versus time in seconds for various temperature sensors in the exhaust system of  FIG. 1 ; 
         FIG. 4  is a schematic illustration of the controller of  FIGS. 1 and 2 ; and 
         FIG. 5  is a flow diagram of a method of detecting a thermal event in the exhaust system of  FIGS. 1 and 2  as carried out by the controller of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, wherein like reference numbers refer to like components throughout the several views,  FIG. 1  shows a vehicle engine  10  and an exhaust system  12 . The engine  10  has an air and fuel intake system  14  through which intake air  16  flows to the engine  10 . Exhaust flow  18  from the engine  10  enters the exhaust system  12 . In this embodiment, the engine  10  is a diesel engine, and the exhaust system  12  has a diesel oxidation catalyst (DOC)  20 , a selective catalyst reduction (SCR)  22 , and a diesel particulate filter (DPF)  24 . 
     A liquid injector  26 , such as for injecting hydrocarbon fuel, is positioned upstream of mixers  28 ,  30 . A temperature sensor  32  is positioned in communication with the exhaust flow just upstream of an inlet  34  of the DOC  20 . The DOC  20  oxidizes and burns hydrocarbons in the exhaust flow  18  exiting the engine  10 . Another temperature sensor  36  is positioned in communication with the exhaust flow just downstream of an outlet  38  of the DOC  20 . 
     A diesel exhaust fluid (DEF) injector  40  injects diesel exhaust fluid or urea into the exhaust stream, which is then mixed by DEF mixers  42  and  44  before entering the SCR  22 , where the injected liquid aids the SCR  22  in converting at least some of the nitrogen oxides in the exhaust flow into nitrogen and water. A nitrogen oxide sensor  46  is positioned in the exhaust stream downstream of an outlet of the SCR  22 . 
     The exhaust then flows to an inlet  50  of the DPF  24 . A temperature sensor  48  is positioned adjacent the inlet  50  in communication with the exhaust flow. A temperature sensor  52  is positioned adjacent an outlet  54  of the DPF  24  in communication with the exhaust flow. The exhaust exits the exhaust system  12  downstream of the DPF  24 , as indicated by arrow  56 . 
     The DOC  20 , the SCR  22  and the DPF  24  are referred to as exhaust after-treatment devices. One or more of the exhaust after-treatment devices or the temperature sensors  32 ,  36 ,  48 ,  52  or other components of the exhaust system  12  could be damaged if the exhaust temperature rises too high for a prolonged period of time. When operating conditions exist under which such damage may occur, it is referred to as a thermal event. Certain protective actions can be taken to reduce the exhaust temperature, such as limiting the engine power, limiting the vehicle accelerator position, and/or notifying the vehicle operator of excessive temperatures, such as with a notification in an information display. 
     To protect the exhaust system  12  from a thermal event, a controller  60  is operatively connected to the temperature sensors  32 ,  36 ,  48 ,  52 . As shown in more detail in  FIG. 4 , the controller  60  receives signals  62  indicative of vehicle operating conditions, including temperature data from the temperature sensors  32 ,  36 ,  48 ,  52 . The controller  60  has a processor  64  that executes an algorithm  100  (described in greater detail with respect to  FIG. 5 ) to determine the existence of a thermal event and then send a control signal  68  to initiate a protective action  72  through a protection module  70  that takes into account other vehicle operating conditions to determine which protective action should be commanded. The processor  64  references a stored look-up table  74  that correlates the data received from the temperature sensors  32 ,  36 ,  48 ,  52  with reference exhaust temperatures to assist the algorithm  100  in determining a thermal event. 
       FIG. 3  shows plots of temperatures in degrees Celsius of the exhaust flow over time in seconds for some of the temperature sensors of  FIG. 1 . Specifically, curve  80  is the temperature of the exhaust flow substantially at the outlet  38  of the DOC  20  based on data received from the temperature sensor  36 . The curve  82  is the temperature of the exhaust flow substantially at the inlet  50  of the DPF  24  based on data received from the temperature sensor  48 . The curve  84  is the temperature of the exhaust flow substantially at the outlet  54  of the DPF  24  based on data received from the temperature sensor  52 . The controller  60  utilizes the data received from the sensors  32 ,  36 ,  48 ,  52  in carrying out the algorithm  100 . 
     Specifically, referring to  FIG. 5 , the algorithm  100 , which is also referred to as a method of detecting a thermal event in a vehicle exhaust system, such as the exhaust system  12  of  FIG. 1 , begins with step  102 , in which the controller  60  receives signals  62  indicative of exhaust temperature data from the temperature sensors  32 ,  36 ,  48 ,  52 . Based on the data received, the algorithm  100  carries out a number of subsequent steps for each of the temperature sensors in parallel to determined if a thermal event exists. The flow diagram of  FIG. 5  shows the algorithm  100  carrying out the steps for the temperature sensors  36 ,  48  and  52 . Although not shown, the same steps may be carried out for the temperature sensor  32 , and for any other temperature sensors optionally included in various locations in the exhaust flow. 
     After temperature data  62  is received by the controller  60  in step  102 , the algorithm  100  moves to step  104  and determines whether the exhaust temperature at the sensor  36  exceeds a predetermined minimum temperature, such as but mot limited to, 300 degrees Celsius. If the temperature does not exceed the predetermined minimum temperature, the algorithm  100  exits at step  105  and returns to step  102 . In  FIG. 3 , for example, the exhaust temperature at the sensor  36  (curve  80 ) exceeds the predetermined minimum temperature of 300 degrees Celsius until about the time 4060 seconds, except between the time 4037 seconds and 4040 seconds. 
     If the temperature exceeds the predetermined minimum temperature, then the algorithm  100  proceeds to step  106  in which it is determined whether a predetermined temperature gradient is exceeded. The temperature gradient is a predetermined large increase in temperature over time for exhaust flow at the sensor  36  that may be associated with a thermal event. For example, the predetermined temperature gradient may be an increase of 50 degrees Celsius per second. If the temperature gradient does not exceed the predetermined temperature gradient, then the algorithm  100  exits at step  107  and returns to step  102 . 
     If the predetermined temperature gradient is exceeded, then the algorithm proceeds to step  108  in which it is determined whether a maximum predetermined temperature is exceeded for a predetermined period of time. For example, the maximum predetermined temperature may be 800 degrees Celsius and the predetermined period of time may be six seconds. If the temperature of the exhaust as indicted by the sensor  36  does not exceed the maximum predetermined temperature for the predetermined period of time, then the algorithm  100  exits at step  109  and returns to step  102 . It appears from the curve  80  that the exhaust temperature based on data received at the sensor  36  does not exceed 800 degrees Celsius for six seconds, and so the algorithm  100  would exit at step  109  and return to step  102 . 
     Assuming that the temperature of the exhaust as indicated by the sensor  36  does exceed the maximum predetermined temperature for the predetermined period of time, then the algorithm  100  proceeds to step  110 , in which the algorithm  100  sets a flag that remains set for a predetermined time period, which may be a calibratable heat transport delay time. As used herein, as will be readily understood by those skilled in the art, a “flag” is an indicator, that may be set or unset, and is used to indicate a condition in the execution of a computer algorithm. In this instance, the flag set in step  110  is an indicator that the determinations of steps  104 ,  106  and  108  are positive. The calibratable heat transport delay time is the amount of time it takes for heat to propagate in the direction of exhaust flow in the exhaust system  12 , as indicated by testing performed on the exhaust system  12 . For example, the calibratable heat transport delay time may be the amount of time it takes for the predetermined temperature gradient used in step  106  to move from the sensor  36  to the temperature sensor  48 . In the embodiment of  FIG. 1 , the calibratable heat transport delay time may be six seconds. 
     The steps  102 ,  104 ,  106 ,  108 ,  110  and  112  are together referred to as monitoring the exhaust temperature at the various temperature sensor locations of the exhaust system  12 . 
     At the same time that steps  104 ,  106 ,  108  and  110  are being carried out, the algorithm  100  is simultaneously carrying out similar steps  204 ,  206 ,  208  and  210  for the temperature sensor  48 , and steps  304 ,  306 ,  308  and  310  for the temperature sensor  52 , based on the data received in step  102 . In step  204 , it is determined whether the exhaust temperature at the sensor  48  exceeds a predetermined minimum temperature. The predetermined minimum temperature may be the same as that used in step  104 , such as 300 degrees Celsius, or a different predetermined minimum temperature used for sensor  48 . If the temperature does not exceed the predetermined minimum temperature, the algorithm exits at step  205  and returns to step  102 . In  FIG. 3 , for example, the exhaust flow at the sensor  48  (curve  82 ) would exceed the predetermined minimum temperature of 300 degrees Celsius beginning at about time 4044 seconds and throughout the remainder of the plot. 
     If the temperature exceeds the predetermined minimum temperature, then the algorithm  100  proceeds to step  206  in which it is determined whether a predetermined temperature gradient is exceeded. The temperature gradient is a predetermined large increase in temperature over time for exhaust flow at the sensor  48  that may be associated with a thermal event. The predetermined temperature gradient used in step  206  may be the same or different than the predetermined temperature gradient used in step  106 . For example, the predetermined temperature gradient may be an increase of 50 degrees Celsius per second. If the temperature gradient determined in step  206  does not exceed the predetermined temperature gradient, then the algorithm  100  exits at step  207  and returns to step  102 . In  FIG. 3 , it appears that the predetermined temperature gradient of 50 degrees Celsius per second may be exceeded when the minimum temperature of 300 degrees Celsius is exceeded between the times 4047 seconds and 4053 seconds and about 350 degrees Celsius and 900 degrees Celsius. 
     If the predetermined temperature gradient is exceeded, then the algorithm  100  proceeds to step  208  in which it is determined whether a maximum predetermined temperature is exceeded for a predetermined period of time. For example, the maximum predetermined temperature may be 800 degrees Celsius and the predetermined period of time may be six seconds. If the temperature of the exhaust as indicted by the sensor  48  does not exceed the maximum predetermined temperature for the predetermined period of time, then the algorithm  100  exits at step  209  and returns to step  102 . It appears from the curve  82  that the exhaust temperature based on data received at the sensor  48  exceeds 800 degrees Celsius between about time 4051 seconds and 4072 seconds which is more than the predetermined period of time. 
     Assuming that the temperature of the exhaust as indicated by the sensor  48  exceeds the maximum predetermined temperature for the predetermined period of time, then the algorithm  100  proceeds to step  210 , in which the algorithm  100  sets a flag that remains set for a predetermined time period, which may be the calibratable heat transport delay time. That is, after the data indicates a temperature greater than 800 degrees Celsius for six seconds, a flag is set for the predetermined time period, which is also six seconds in this example. Accordingly, at about 4057 seconds, a flag is set that remains set until the time 4063 seconds. 
     The same steps are carried out with respect to sensor  52 . In step  304 , it is determined whether the exhaust temperature at the sensor  52  exceeds a predetermined minimum temperature. The predetermined minimum temperature may be the same as that used in step  104 , such as 300 degrees Celsius, or a different predetermined minimum temperature used for sensor  52 . If the temperature does not exceed the predetermined minimum temperature, the algorithm  100  exits at step  305  and returns to step  102 . In  FIG. 3 , for example, the exhaust flow at the sensor  52  (curve  84 ) would exceed the predetermined minimum temperature of 300 degrees Celsius beginning at about time 4057 seconds until about 4067 seconds. 
     If the temperature exceeds the predetermined minimum temperature, then the algorithm  100  proceeds to step  306  in which it is determined whether a predetermined temperature gradient is exceeded. The temperature gradient is a predetermined large increase in temperature over time for exhaust flow at the sensor  52  that may be associated with a thermal event. The predetermined temperature gradient used in step  306  may be the same or different than the predetermined temperature gradient used in step  106 . For example, the predetermined temperature gradient may be an increase of 50 degrees Celsius per second. If the temperature gradient does not exceed the predetermined temperature gradient, then the algorithm  100  exits at step  307  and returns to step  102 . In  FIG. 3 , it appears that the predetermined temperature gradient of 50 degrees Celsius per second may be exceeded when the minimum temperature of 300 degrees Celsius is exceeded between the times 4057 seconds (approximately 400 degrees Celsius) and 4063 seconds (approximately 1000 degrees Celsius). 
     If the predetermined temperature gradient is exceeded, then the algorithm  100  proceeds to step  308  in which it is determined whether a maximum predetermined temperature is exceeded for a predetermined period of time. For example, the maximum predetermined temperature may be 800 degrees Celsius and the predetermined period of time may be six seconds. If the temperature of the exhaust as indicted by the sensor  52  does not exceed the maximum predetermined temperature for the predetermined period of time, then the algorithm  100  exits at step  309  and returns to step  102 . It appears from the curve  84  that the exhaust temperature based on data received at the sensor  52  exceeds 800 degrees Celsius between about time 4062 seconds and 4068 seconds, which satisfies the six second predetermined period of time. 
     Assuming that the temperature of the exhaust as indicated by the sensor  52  exceeds the maximum predetermined temperature for the predetermined period of time, then the algorithm  100  proceeds to step  310 , in which the algorithm  100  sets a flag for the sensor  52  that remains set for a predetermined time period, which may be the calibratable heat transport delay time. In  FIG. 3 , the vertical line  86  at approximately 4062 seconds indicates the beginning of the predetermined period of time. The line  88  indicates the end of the predetermined period of time. Because the temperature remains above 800 degrees Celsius for the six second predetermined period of time, as indicated by curve  84 , a flag is set at the time 4068 seconds and remains set until 4074 seconds (the end of the predetermined time period, which is also six seconds in this embodiment). 
     Following any of steps  110 ,  210 ,  310 , if a flag was set for any of the sensors  36 ,  48 ,  52 , respectively, in step  112 , the algorithm  100  determines whether at least two flags are concurrently set. Any flag set in step  110 ,  210  or  310  remains set for the predetermined heat transport delay time. In step  112 , if it is determined that two flags are not concurrently set, the algorithm  100  moves to step  114 , in which it is determined whether the predetermined time period has passed since the flag was set. If the predetermined time period has passed, the algorithm exits at step  115  and returns to step  102  to continue monitoring the exhaust system  12  for a thermal event. If the predetermined time period has not passed, the algorithm  100  returns to step  112  and again queries whether the two flags are concurrently set. The algorithm  100  continues to loop through steps  112  and  114  until either the predetermined time period passes without an additional flag being set, or the algorithm  100  recognizes that an additional flag has also been set via one of the steps  110 ,  210  or  310  prior to the predetermined time period expiring since the first of the flags was set. In the latter case, the algorithm  100  recognizes this as a thermal event, and moves to step  116  to initiate a protective action, which may also be referred to as setting a thermal protection fault. The protective action may be one or more of many protective steps taken to protect the exhaust system  12  from damage. 
     For example, referring to  FIG. 3 , the processor  74  will send a signal  69  to the protection control module  70  indicating a thermal event (that is, a positive determination in step  112 ). The protection control module  70  is configured to determine which of many potential protective actions should be taken. A control signal  68  is then sent to initiate the protective action  72 . The protective action  72  may be alerting the vehicle operator of the thermal event, such as by a message on an information display, an audio signal, or the like. The alert may instruct the operator to have the exhaust system serviced. The protective action  72  may be placing a maximum power output on the engine  10  to limit additional heat to the exhaust system  12 . The protective action  72  may be controlling an actuator that limits the maximum position of an accelerator. This in turn has the affect of limiting the maximum engine power. These protective actions may be taken alone or in combination. 
     Referring to  FIG. 2 , an alternative embodiment of an exhaust system  412  can also be protected using the controller  60  and algorithm  100  of  FIGS. 4 and 5 . The exhaust system  412  has many of the same components as shown and described in the exhaust system  12  or  FIG. 1 . These components are labeled with the same reference numbers and function as described with respect to  FIG. 1 . The exhaust system  412  has a nitrogen oxide sensor  46  upstream of the DOC  20  as well as a temperature sensor  32  adjacent the inlet  34  of the DOC  20 . The exhaust system  412  also has a temperature sensor  36  adjacent the outlet  38  of the DOC  20 . A DEF injector  40  and mixers  42 ,  44  are upstream of the SCR  22 . Another nitrogen oxide sensor  46  is downstream of the SCR  22  followed by two mixers  28 ,  30 . Another temperature sensor  33  is positioned adjacent an inlet  35  of a second diesel oxidation catalyst (DOC)  21 . A temperature sensor  48  is placed at the outlet of the DOC  21  and the inlet of a DPF  24 . Another temperature sensor  52  is at the outlet  54  of the DPF  24 . A particulate matter sensor  59  is also placed just prior to the exit of the exhaust system  412 , indicated at arrow  56 . 
     The algorithm  100  can be applied to the exhaust system  412 , with each of the temperature sensors  32 ,  36 ,  33 ,  48  and  52  providing temperature data to the controller  60 . The steps  104 ,  106 ,  108 ,  110  are applied to each of the sensors  32 ,  36 ,  33 ,  48  and  52  in parallel, and the determination of whether two flags are concurrently set in step  112 , leading to a protective action in step  116  is applied to the exhaust system  412 . 
     The steps  102 ,  104  through  112 ,  204  through  112 , and  304  through  112  are together referred to as monitoring the exhaust temperature at the various temperature sensor locations of the exhaust system  12 . This monitoring, together with the protective action of step  116 , allows an identification of a thermal event that necessitates a thermal action based on a thermal gradient propagating through the exhaust system, and relies on identifying thermal indicators at two different sensors spaced in the exhaust system both of which cause flags to be set within a predetermined time period. Under the method  100 , operating conditions most likely to cause damage to the exhaust system  12  or  412  can be alleviated, while normal spikes in temperature that do not warrant protective action are prevented from being identified as a thermal event. 
     While the best modes for carrying out the many aspects of the present teachings have been described in detail, those familiar with the art to which these teachings relate will recognize various alternative aspects for practicing the present teachings that are within the scope of the appended claims.