Abstract:
A method for diagnosing a failure of an EGR circuit of an engine including an EGR exchanger, an EGR valve, a bypass duct of the EGR exchanger, and a bypass flap arranged upstream from the EGR exchanger and the bypass duct to adjust a proportion of exhaust gases flowing therethrough. The EGR circuit can be activated according to a cooled mode, in which the flap is shut, or according to a bypass mode, in which the flat is open. The method carries out, during a diagnosis phase, two activations of the flap, and measures an average of variations of the EGR gas temperature at an outlet of the EGR exchanger during the diagnosis phase.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of U.S. application Ser. No. 12/679,190, filed on Mar. 19, 2010, which is a National Stage of PCT/FR08/051406, filed on Jul. 25, 2008, and is based upon and claims the benefit of priority from the French Patent Application No. 07 57707, filed on Sep. 20, 2007, the entire contents of each of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method of diagnosing a failure of the EGR circuit of an engine, more specifically the blocking of the bypass flap of the EGR exchanger. 
       BACKGROUND OF THE INVENTION 
       [0003]    The bypass flap is a key element in the exhaust gas recirculation (EGR) system. 
         [0004]    Its function is to direct the EGR gases into a bypass circuit of the exchanger in order to exploit hot gases in priming the catalyst. 
         [0005]    The correct operation of the flap therefore makes it possible to guarantee the depollution of current diesel engines. Blocking of the flap in bypass mode or in cooled mode has direct consequences on the pollution emitted from the engine outlet. 
         [0006]    Since pollution thresholds are becoming increasingly strict, it is vitally important, in order to satisfy the next standards, to diagnose such failures of the flap. 
         [0007]    The risk associated with the blocking of the flap is also not linked solely to pollution. In practice, a failure of the flap can have consequences on the reliability of the surrounding components (degradation due to an excessively high temperature of the EGR valve and its mounting) and the integrity of the engine control strategies that use it (such as, for example, clearing the valve and the exchanger, or even priming the catalyst). 
         [0008]    A number of failure diagnosis methods have already been developed, with variable performance levels. 
         [0009]    A first method, disclosed in the document JP2006-291921, uses a temperature sensor situated at the inlet of the intake distributor and makes it possible to diagnose a blocking of the flap by measuring the temperature difference between the cooled mode and the bypass mode. However, this method based on a simple difference between two temperatures, offers little robustness with respect to the dispersions and variations associated with the inertia of the EGR circuit. Furthermore, this method has the drawback of not making it possible to check the correct closure of the flap after actuation: a blocking of the flap in the actuated position will therefore be detected only on the next occurrence of the diagnosis. Moreover, this method appears relatively imprecise because the temperature sensor situated at the inlet of the intake distributor is subject to the influence of the cool air intake. 
         [0010]    Another method, described in the document JP 2003-247459, implements a strategy based on monitoring the air flow rate before and after activation of the bypass flap, the air intake flap and the EGR valve being totally open. The main advantage of this solution is that it simply uses the flow meter situated on the cool air intake duct. However, depending on the technical means employed, this strategy may result in a not-inconsiderable false detection ratio, due to the EGR environment (high temperatures, fouling of the connections) and to the limited reactivity of the flap control. In practice, pressure wave phenomena delay the control through the vacuum of the bypass flap. 
         [0011]    Finally, other applications use a contactor that makes it possible to know the open/closed position of the flap. However, this strategy can also lead to a high false detection ratio, because of the EGR environment. 
         [0012]    One aim of the invention is therefore to define a simple and reliable method that makes it possible to detect any failure of the bypass flap. Another aim of the invention is to diagnose a total loss of the cooling function. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0013]    A first subject of the invention is a method for diagnosing a failure of the EGR circuit of an engine comprising an EGR exchanger, an EGR valve, a bypass duct of the EGR exchanger, and a so-called bypass flap, arranged upstream of the EGR exchanger and of the bypass duct in order to control the proportion of exhaust gases passing through the latter, wherein the EGR circuit can be activated according to a so-called cooled mode, in which the flap is closed, and a so-called bypass mode, in which the flap is open, the method being characterized in that, during a diagnosis phase, two activations of the flap are carried out, and in that an average of the variations of the temperature of the EGR gases is measured at the outlet of the EGR exchanger during the diagnosis phase. 
         [0014]    According to other characteristics of the invention:
       the aim of the first activation is to open the flap, and the aim of the second activation is to close the flap;   the maximum temperature reached during the diagnosis phase is measured;   a first temperature difference is calculated between said maximum temperature and the temperature measured at the moment of the first activation and a second temperature difference is calculated between said maximum temperature and the temperature measured at the end of the diagnosis phase;   the average of the first and second temperature differences is calculated;   the average is compared to a predetermined threshold so that:
           if the average is greater than the threshold, then the flap is functional;   if the average is less than the threshold, then the flap has failed.   
               
 
         [0022]    Another subject of the invention relates to a device for diagnosing a failure of the EGR circuit of an engine comprising an EGR exchanger, an EGR valve, a bypass duct of the EGR exchanger, and a so-called bypass flap, arranged upstream of the EGR exchanger and of the bypass duct in order to control the proportion of exhaust gases passing through the latter, wherein the EGR circuit can be activated according to a so-called cooled mode, in which the flap is closed, and a so-called bypass mode, in which the flap is open, the device comprising:
       a temperature sensor ( 34 ) arranged in such a way as to measure the temperature of the EGR gases leaving the exchanger,   a means of acquiring the temperature measurements,   a means of calculating the averages of the measured temperatures,   a means of comparison with a predetermined threshold (S).       
 
         [0027]    A third subject of the invention relates to a motor vehicle comprising temperature measurement means and calculation means making it possible to implement the diagnosis method according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    Other aims, features and benefits of the invention will become more apparent from reading the following detailed description of the invention, with reference to the appended drawings in which: 
           [0029]      FIG. 1  diagrammatically represents an EGR gas recirculation device to which the invention is applied; 
           [0030]      FIG. 2  is a graph illustrating the operating principle of the invention; 
           [0031]      FIG. 3  is a flow diagram of the implementation of the diagnosis; 
           [0032]      FIG. 4  is a curve of the results of tests carried out with a functional bypass flap; 
           [0033]      FIG. 5  is a curve of results of tests carried out with a blocked bypass flap. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    Referring to  FIG. 1 , the engine  1  comprises a cool air intake circuit  2  comprising an air inlet duct  21  and an air flow meter  22 . The cool air intake into the intake distributor  4  is adjusted by means of an air flap  23 . 
         [0035]    The engine also comprises an EGR gas recirculation circuit  3  comprising a duct  30  for recirculating the EGR gases taken from the exhaust, an EGR exchanger  31  and a valve  33 , called EGR valve, that makes it possible to control the flow rate of EGR gases entering into the intake distributor  4 . A bypass duct  35  is arranged parallel to the exchanger  31 ; a bypass flap  32  is arranged upstream of the latter so that:
       when the flap  32  is closed, the EGR gases pass through the exchanger and are cooled therein (the so-called “cooled mode”), and   when the flap  32  is open, the EGR gases flow through the bypass duct and substantially retain their high temperature (the so-called “bypass” mode).       
 
         [0038]    In  FIG. 1 , the bypass duct  35  is diagrammatically represented as being incorporated in the exchanger  31 , but this representation is not limiting and bypass ducts dissociated from the EGR exchanger can be found. 
         [0039]    A temperature sensor  34  is, furthermore, arranged between the outlet of the exchanger  31  and the EGR valve  33 , in order to measure the temperature (denoted TsEGR) of the EGR gases leaving the exchanger. According to a variant embodiment, the sensor  34  can also be arranged downstream of the EGR valve  33 . 
         [0040]    The diagnosis strategy is based on monitoring the EGR temperature before and after the activation of the exchanger bypass. In practice:
       a high variation in TsEGR between the activation (opening) and closure of the flap indicates that the flap  32  is functional;   conversely, a low variation in TsEGR indicates that the flap  32  is blocked.       
 
         [0043]    The diagnosis principle according to the invention will be described with reference to  FIG. 2 . 
         [0044]    Firstly, it is important to specify the area and the conditions necessary in implementing the diagnosis. Detecting a failure of the EGR bypass flap is in effect possible only when the operating conditions are stabilized, in order to filter the temperature fluctuations and inertias. The operating point of the strategy will be to make a determination as a function of the temperature differences observed on the levels of the European type approval cycle (NEDC): those with the greatest temperature difference will be chosen. 
         [0045]    The diagnosis strategy is based on monitoring the variation in the temperature TsEGR at the outlet of the EGR exchanger  31 , after actuation of the bypass flap  32 . 
         [0046]    In  FIG. 2 , the curve C 1 , in the form of a pulse, represents the trend of the control of the bypass flap  32  during a diagnosis phase. The low value corresponds to the cooled mode (that is to say that the flap  32  is closed so that all the EGR gases pass through the exchanger  31 ); the high value corresponds to the bypass mode (in which the flap  32  is open so that the EGR gases flow through the bypass duct  35 ). 
         [0047]    The curve C 2  represents the theoretical trend of the temperature TsEGR during a diagnosis phase, with the same time scale as that of the curve C 1 , with a functional flap  32 . 
         [0048]    The curve C 3  represents the theoretical trend of the temperature TsEGR during a diagnosis phase, with the same time scale as that of the curve C 1 , with a failed flap  32 . 
         [0049]    When the diagnosis conditions are reached, the bypass flap  32  is actuated, so that the system switches from the cooled mode to the bypass mode (represented by the arrow A 1  on the curve C 1 ). At that instant, a first acquisition (denoted T 1 ) of the temperature TsEGR, is carried out. 
         [0050]    The flap is therefore driven into the open position for a first duration t, which should correspond to the average duration that makes it possible to ensure that the flap is fully open. This time is determined on a per-case basis and controlled in terms of ageing. 
         [0051]    The flap is then actuated (arrow A 2  on the curve C 1 ) so as to switch from the bypass mode to the cooled mode; it is kept closed for a second duration t. At the end of this time period, an acquisition (denoted T 3 ) of the temperature TsEGR is carried out. The duration of the diagnosis phase is therefore 2t. 
         [0052]    Throughout the entire diagnosis phase, the maximum temperature (denoted T 2 ) reached at the outlet of the EGR exchanger is sought. This makes it possible to disregard the thermal inertia of the EGR circuit. 
         [0053]    It is then possible to calculate a first temperature difference: ΔT 1 =T 2 −T 1 . This calculation is carried out between the point  1  (flap opening moment) and the point  2  (maximum TsEGR reached during the diagnosis phase) of the curve C 2 . 
         [0054]    A second temperature difference is also calculated: ΔT 1 =T 2 −T 3 . This calculation is carried out between the point  2  and the point  3  (corresponding to the end of the diagnosis phase and complete closure of the flap) of the curve C 2 . 
         [0055]    The diagnosis criterion is the average of these two differences, which makes it possible to disregard the external disturbances encountered in the routine use of a vehicle (road condition, dispersions, etc.). This diagnosis criterion is designated by ΔTmoy=(ΔT 1 +ΔT 2 )/2. 
         [0056]    ΔT 1  is calculated on the first activation. However, this calculation alone does not make it possible to detect an untimely blocking of the flap while the diagnosis is running. It is therefore important to check whether the flap has indeed reverted to its original position. The second difference ΔT 2  calculated on the second activation makes it possible to detect it. 
         [0057]    Furthermore, the calculation of the second difference increases the detection reliability. 
         [0058]    With a functional flap (curve C 2 ), activating the flap has a visible impact on the temperature TsEGR, which is reflected in relatively high differences ΔT 1  and ΔT 2 , typically of the order of 25° C. for low-efficiency exchangers, possibly ranging up to 100° C. for exchangers that offer high efficiency. 
         [0059]    However, with a failed flap (curve C 3 ), the activation of the flap has no impact on the temperature TsEGR, which is reflected in very low temperature differences, of the order of 0 to 10° C. 
         [0060]    Using statistical studies, a threshold S is determined such that:
       if ΔTmoy=(ΔT 1 +ΔT 2 )/2 is greater than S, then the bypass flap is functional;   if ΔTmoy=(ΔT 1 +ΔT 2 )/2 is less than S, it is considered that the flap has failed, the possible failure causes being mechanical seizing, disconnection of the hose from the bypass solenoid valve, or even a control problem.       
 
         [0063]    Referring to  FIG. 3 , the logical process used to carry out the diagnosis therefore comprises the following steps:
       when the vehicle is started up, the device is initialized (box  101 );   until the conditions have stabilized (box  102 ), the diagnosis is inactive;   when the conditions have stabilized (box  103 ), the bypass control is activated and the temperature TsEGR_ 1  is stored (box  104 );   the bypass control is then deactivated (box  105 );   the temperature TsEGR_ 2  is stored (box  106 );   the temperature TsEGR_ 3  is stored (box  107 );   the following temperature differences are calculated:       
 
         [0000]      ΔT1=|TsEGR — 1=TsEGR — 2|
 
         [0000]      ΔT2=|TsEGR — 2=TsEGR — 3|
        then the average of the differences (ΔTmoy=(ΔT 1 +ΔT 2 )/2) is compared with the predetermined threshold S (box  108 );
           if ΔTmoy&lt;S, a fault is signaled (box  109 );   if ΔTmoy&gt;S, the diagnosis is deactivated (box  102 ).   
               
 
         [0073]    When the fault is confirmed, an information item (called DTC or Diagnostic Trouble Code) is stored in the memory of the computer; a service lamp lights; finally, a degraded mode is activated, which involves closing the EGR valve in order to reduce the temperature at its terminals. 
         [0074]      FIG. 4  shows the results of the measurement of TsEGR with a functional flap. 
         [0075]      FIG. 5  shows the results of the measurement of TsEGR with the bypass control disconnected. 
         [0076]    The diagnosis method that has just been outlined therefore offers the benefit of being based on a very reliable strategy. In practice, tests carried out on an application with an EGR exchanger with very low efficiency have shown the discriminating nature of the criterion ΔTmoy. 
         [0077]    Furthermore, the proposed method makes it possible to check the correct closure of the valve even during the diagnosis phase. 
         [0078]    Finally, measuring the temperature TsEGR entails using a temperature sensor, but this measurement does not make it possible to carry out only diagnosis of the bypass flap but can be used for other diagnoses—diagnosing the EGR exchanger for example, and notably the total loss of the cooling function; failures leading to this loss—water leak for example—nevertheless being more rare.