Abstract:
An apparatus and method for diagnosing the condition of a sensor in an internal combustion engine upstream from the catalytic converter. The diagnosis utilizes a signal from a second non-linear probe downstream from the catalytic converter. This signal is processed to give a signal that is filtered. The filtered signal is in turn compared with maximum and minimum values. The upstream probe is considered to be correct if the signal falls between these two values or faulty if it falls outside the values.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to internal combustion engines of the fuel-injection type equipped with a catalytic exhaust converter preceded by a sensor and, more particularly in such engines, a device and a process for diagnosis of the condition of the sensor disposed upstream from the catalytic converter. 
     2. Discussion of the Background 
     It is known how to use systems for modifying the quantity of fuel injected into an engine as a function of the exhaust-gas composition and, more particularly, of the oxygen content of these gases. To this end, the oxygen content is measured by means of a nonlinear sensor known as the “lambda” sensor or EGO sensor, where EGO is an English-language acronym for “Exhaust Gas Oxygen”. Such a sensor is disposed upstream from the catalytic exhaust converter, and the signal delivered by this sensor is used to modify the quantity of fuel injected into the engine cylinders via a first feedback loop. For this reason, the sensor is also known as a richness-regulating sensor. 
     It is clear that poor condition of this sensor leads to poor operation of the engine and of the catalytic converter, in turn leading to pollutant emissions at abnormally high levels. It is therefore important to determine the condition of this sensor at all times in order to diagnose poor operation thereof when its condition has deteriorated beyond certain limits. The present solutions for diagnosis of the condition of the upstream sensor comprise analyzing the behavior of the sensor in response to richness excitations in open loop or closed loop and monitoring the following parameters: 
     the minimum voltage delivered by the sensor: if too high, a fault is indicated; 
     the maximum voltage delivered by the sensor: if too low, a fault is indicated; 
     the lean-to-rich transition time; if too long, a fault is indicated; 
     the rich-to-lean transition time; if too long, a fault is indicated; 
     the period of the signal delivered by the sensor in closed loop: if too long, a fault is indicated. 
     The diagnosis then comprises declaring failure of the sensor if one or more faults are detected. 
     Such a diagnostic process is based on analysis of the sensor behavior in order co deduce therefrom a sensor condition on the basis of assumed degradation mechanisms. For example, as a sensor ages, its dynamic voltage range is reduced and/or its transition times become longer The disadvantage of such a diagnostic process is that a perfect correlation does not exist between these measurements and the emissions of pollutants. 
     In addition, calibration of fault detection thresholds proves to be very tricky and necessitates: 
     perfect knowledge of the mechanisms of aging of the sensors, 
     numerous tests to establish a relationship between the measured degradations of parameters and their effects on pollutant emissions. 
     In addition, it is not possible in all cases to guarantee that the diagnosis is reliable. For example, a sensor with reduced dynamic voltage range may prove to be good with regard to pollutant emission if only that characteristic is affected. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is therefore to provide, for diagnosis of the condition of a sensor disposed upstream from a catalytic converter associated with an internal combustion engine of the fuel-injection type, a device and a process which do not exhibit the aforesaid disadvantages of the devices and processes of the prior art. 
     Another object of the present invention is also to provide, for diagnosis of the condition of an upstream sensor, a device and a process which does not depend on measurements of intrinsic characteristics of the sensor. The process of the invention is based on monitoring of characteristics of the richness feedback loop which have an influence on pollutant emission, or in other words the mean period and mean richness of the feedback loop. In this way, the condition of the upstream sensor is evaluated on the basis of effects that it produces on the richness feedback loop, or in other words on the emissions of pollutants, and not on the basis of its intrinsic characteristics. 
     The effects of the condition of the upstream sensor are capable of causing pollutant emissions by exceeding the limits of the “window” of good operation of the catalytic converter, this exceeding being due to drift of the mean operating richness and/or to excessively long mean period of the richness loop. 
     To detect drift of the mean operating richness, the invention proposes to provide a second nonlinear sensor disposed downstream from the catalytic converter and constituting an integral part of a second feedback loop, by virtue of which the output voltage V downstream  of the second sensor, called downstream sensor hereinafter, is slaved to a setpoint voltage VC downstream  corresponding to the center of the window of good operation of the catalytic converter. The signal delivered by this loop is used to modify the signal of the first feedback loop containing the upstream sensor. 
     Such a system of richness slaving with double control loop is described in the patent application filed today by the Applicant and entitled: “SYSTEM AND PROCESS WITH DOUBLE CONTROL LOOP FOR INTERNAL COMBUSTION ENGINE”. The invention relates to a device for diagnosis of the condition of a nonlinear sensor disposed upstream from a catalytic converter associated with an internal combustion engine of the fuel-injection type controlled by an electronic computer, the said engine containing a first control loop, including the said nonlinear sensor, to deliver to the computer a first signal KCL for correction of the quantity of fuel injected, and a second control loop, including a second nonlinear sensor disposed downstream from the said catalytic converter, to deliver a second signal KRICH for correction of the quantity of fuel injected, the said diagnostic device being characterized in that it comprises: 
     a filter circuit to which there is applied the second correction signal KRICH in order to deliver a filtered signal KRICH F , 
     a measuring circuit to which there is applied the output signal V upstream  of the upstream sensor in order to determine the mean value T m  of the period of correction of the first control loop, and 
     a logic circuit to determine, as a function of the values of the filtered signal KRICH F  and of the mean period T m , whether the condition DIAG of the upstream sensor is good or defective. 
     In one embodiment of the invention, the logic circuit determines that the upstream sensor is defective if the filtered signal is larger than a maximum value or smaller than a minimum value or else if the mean period is longer than a maximum value. 
     In another embodiment of the invention, the maximum and minimum values of the filtered signal KRICH F  are determined by calibration as a function of the value of the mean period and are stored in a memory. This memory is addressed by the value of the mean period in order to deliver the maximum and minimum values, with which the value of the filtered signal is compared. 
     The invention also relates to a process which comprises the following stages: 
     filtering of the second correction signal KRICH to obtain a filtered signal KRICH F , 
     calculation of the mean value T m  of the period of the output signal V upstream  of the upstream sensor, 
     comparison of the said filtered signal KRICH F  with two values, the maximum KRICH max  and the minimum KRICH min , to determine whether the condition DIAG of the said upstream sensor is correct or defective, according to whether the filtered signal KRICH F  is respectively within the limits defined by the maximum and minimum values or outside the said limits for the value of the mean period T m . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the present invention will become apparent upon reading the following description of a particular embodiment, the said description being made with reference to the attached drawings, wherein: 
     FIG. 1 is a functional diagram of a system for double-loop control of richness to which the invention applies; 
     FIGS. 2-A and  2 -B are diagrams showing how the richness correction is applied with a single feedback loop containing one sensor upstream from the catalytic converter; 
     FIGS. 3-A and  3 -B are diagrams showing one mode of correction of the richness by using a second feedback loop containing a sensor downstream from the catalytic converter; 
     FIG. 4 is a diagram showing the mode of filtering of the correction signal KRICH to obtain a filtered signal KRICH F ; 
     FIG. 5 is a diagram showing an algorithm for calculation of the mean period of the signal of the upstream sensor; 
     FIG. 6 is a diagram showing the curves which define the zones of correct or defective functioning of the upstream sensor, and 
     FIG. 7 is a diagram showing a decision algorithm for determining the condition of the upstream sensor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, an internal combustion engine  10  is controlled in known manner by an electronic computer  12 . The exhaust gases of this engine are filtered by an exhaust muffler  14  of the catalytic converter type, from which they escape to the open air. A first sensor  16  is disposed at the inlet of the exhaust muffler and measures the content of one of the main components of the exhaust gases, this component usually being oxygen. This sensor is of the nonlinear type, and is often called, as indicated hereinabove, a “lambda” sensor or EGO sensor. This sensor delivers at its output terminal an electric signal V upstream  (FIG.  2 -A), which is applied to a comparator circuit  18  in which V upstream  is compared with a threshold voltage VS upstream  to determine the sign of V upstream  relative to that threshold. 
     The threshold value VS upstream  depends on the sensor characteristics and corresponds to the transition voltage of the sensor when the conditions of stoichiometry are satisfied. 
     The output terminal of comparator circuit  18 , which delivers a binary signal  1  or  0 , is connected to the input terminal of a first richness-regulating correction circuit  20  of the proportional-plus-integral type with gains P and I respectively. The correction circuit  20  delivers a signal KCL, which has the shape represented by the diagram of FIG. 2-B. It is this signal KCL which is delivered to computer  12  to control the quantity of fuel to be injected. Thus, as soon as V upstream  becomes smaller than VS upstream , this means that the mixture is lean in fuel and that the quantity of fuel must be increased. This is accomplished by the jump +P (FIG. 2-B) followed by a positive slope of value I until the instant that V upstream  exceeds VS upstream , which means that the mixture has become rich in fuel and that the quantity thereof must be reduced. This is accomplished by a jump −P followed by a negative slope of value I. 
     The correction value KCL delivered by correction circuit  20  is modified by a second correction circuit  22 , which introduces a correction term KRICH before being applied to computer  12 . This correction term KRICH is determined by a circuit  24  on the basis of an output signal V downstream  of a second lambda sensor  26 , which is disposed at the outlet of the catalytic exhaust converter  14 . This circuit  24  substantially comprises a comparator  28 , to which there are applied the signal V downstream  and a setpoint signal denoted by VC downstream , and a third correction circuit  30 , to which there is applied the signal (V downstream −VC downstream ) delivered by comparator circuit  28 . The third correction circuit  30  is, for example, of the proportional plus integral type, and delivers the signal KRICH, which is applied to the second correction circuit  22 . 
     The second correction circuit  22  is able to introduce the correction KRICH by different modes, one of which will be explained with reference to the timing diagrams of FIGS. 3-A and  3 -B. These diagrams are plots of the signal KCL as modified by the second correction circuit  22 , the modified signal KCL being denoted by KCL m . 
     According to the diagrams of FIGS. 3-A and  3 -B, the signal KRICH is applied during lean-to-rich transitions detected by the first sensor, which corresponds to the descending side of the signal KCL. In the case in which KRICH&gt;0 (increasing the richness), the plot of KCL m  is that of FIG. 3-A, while in the case in which KRICH&lt;0 (increasing the leanness), the plot of KCL m  is that of FIG. 3-B. 
     The device for diagnosis of the condition of sensor  16  comprises the elements represented inside the rectangle  40  of the diagram of FIG.  1 . These are a filter  32 , to which there is applied the output signal KRICH of correction circuit  24  of the second loop, as well as a circuit  34  for calculation of the mean period T m  of the signal V upstream  of the upstream sensor  16 . The output terminals of filter  32  and of calculation circuit  34  are connected to a logic circuit  36 , which determines whether the condition of sensor  16  is good or poor as a function of the output signal KRICH F  of filter  32  and of the value T m  of the mean period of the signal V upstream . The binary signal  1  or  0  corresponding to good or poor condition of sensor  16  appears at the output terminal DIAG of logic circuit  36 . 
     The communications delivered by computer  12  are as follows: 
     the engine speed REG, 
     the pressure P of the intake manifold, 
     the state of the first loop: active or inactive, 
     the state of the second loop: active or inactive. 
     Circuits  32  and  34  process the communications listed above and authorize filtering and calculation of T m  only if the following conditions are satisfied simultaneously: 
     REG min &lt;REG&lt;REG max    
     P min &lt;P&lt;P max    
     first loop in active state, 
     second loop in active state, 
     where REG min  and REG max  are respectively the minimum and maximum values of engine speed REG between which the diagnosis can be made; P min  and P max  are respectively the minimum and maximum values of the pressure P of the intake manifold between which the diagnosis can be made. Filter circuit  32  performs the calculation of the filtered richness correction KRICH F  according to the algorithm of FIG.  4 . This calculation (step  42 ) is performed only if the conditions listed above are satisfied (step  44 ) and, in this case, the mean richness KRICH F  is given by: 
     KRICH F =KRICH F +K(KRICH−KRICH F ) 
     where K is a filter factor between  0  and  1 . 
     Calculation circuit  34  performs the calculation of the mean period T m  according to the algorithm of FIG.  5 . This calculation is performed only if the conditions listed above are satisfied (step  50 ). This calculation of the mean period T m  comprises counting the transitions of the voltage V upstream  from a value smaller than the threshold VS upstream  to a value larger than the threshold during a certain time interval T D  and dividing this interval T D  by the number N of transitions that were detected. The algorithm for calculation of the mean period T m  of the first loop is represented by the diagram of FIG.  5 . The first step ( 50 ) comprises verifying whether the diagnostic conditions listed above are satisfied. If the response is “YES”, counting step  52  for time T is started, or in other words the calculation of the mean period T m  begins. As soon as V upstream &gt;VS upstream  (step  54 ) and the sensor&#39;s previous state, STATE A , corresponding to V upstream &lt;VS upstream  (STATE A =0), step  58  comprises storing this new state of the sensor in memory as STATE A =1. The following step  60  comprises verifying whether a transition (TRANS=1) was already detected previously; if the response is positive, this means that a period has elapsed and the count  62  of the number N of periods is incremented by one unit. At the same time, the counter of the duration T D  of the diagnosis is incremented by the value T of the counter  52 . The calculation  66  of the mean period T m =T D /N is then performed with the new values of N and T D . The following step  68  resets counter  52  to zero for a new measurement T of the period in progress. 
     In order that the calculation described in the foregoing can be performed correctly, the following states must be present: 
     TRANS=0, STATE A =1 and T=0, 
     which is accomplished by steps  72 ,  74  and  76  in cascade, which are initialized by the verification (step  50 ) that the diagnostic conditions are not satisfied, which is always the case during starting of the engine. Thus, for the first measurement of the period, the counter  52  is at the value 0 but, since STATE A =1, the calculation cannot begin until this state changes to STATE A =0, in order to be certain of detecting a transition In the desired direction. This is obtained by the detection that V upstream &lt;VS upstream , in which case the change to STATE A =0 takes place (step  78 ). 
     During starting, TRANS=0, and so the condition of step  60  is not satisfied and the period cannot be calculated. Otherwise, step  70  imposes TRANS=1, which resets counter  52  to zero via step  68 , and a new count of T can begin. 
     During starting, STATE A =1, and so the condition of step  56  is not satisfied, in which case the steps of the algorithm begin over again. 
     Logic circuit  36  performs the steps of the algorithm of FIG. 7 in order to compare the value of KRICH F  with values determined as being the limit values beyond which the sensor is considered to be defective, specifically for a determined value T m  of the mean period. 
     These limit values, denoted by KRICH max  for too large richness increase and KRICH min  for too large leanness increase, are determined by calibration with the use of a series of sensors whose aging characteristics are known. 
     This calibration permits plotting of the curves KRICH max  and KRICH min  as a function of the period T m  (FIG.  6 ), and these curves can be stored in memory in the form of two maps or of a single map that consolidates both curves. These maps can be constructed by memories which are addressed by the value of Tm, and the values read are KRICH max  and KRICH min  corresponding to the value of T m  (FIG.  6 ). 
     The first step  80  of the diagnostic algorithm comprises comparing the duration T D  for calculation of the period T m  to a minimum duration T Dmin , shorter than which a diagnosis would not be reliable. If T D &gt;T Dmin , the following step  82  comprises comparing KRICH F  with a value KRICH max  read from the map  88  giving KRICH F =S(T m ). This map is addressed by the value of T m  to obtain a value of KRICH max , which is compared with KRICH F . If the condition is not verified, the sensor is considered to be defective (step  92 ). 
     If the condition is verified, the following step  84  is to compare KRICH F  with the value of KRICH min  for T m  as read from map  86 , in which there are stored the values of the curve KRICH min =S(T m ). If the condition KRICH&gt;KRICH min  is not verified, the sensor is considered to be defective (step  92 ), with DIAG=0. In the opposite case, the sensor is considered to be correct (step  90 ), with DIAG=1. 
     As soon as the sensor is considered to be correct or defective, the diagnosis is terminated (step  94 ) and a new diagnosis can be initiated to obtain a new value of KRICH F  and of T m . 
     When the curves of FIG. 6 are reduced to the form of maps, and the algorithm of FIG. 7 is applied, the sensors considered to be poor (DIAG=0) are in the shaded portion outside the two curves, and the sensors considered to be good (DIAG=1) correspond to the area between the curves. 
     Instead of the two curves of FIG. 6, it is possible to limit the choice to fixed thresholds for KRICH′ max , KRICH′ min  and T′ max , and so it is no longer necessary to have two maps. In this simplified case, the value of KRICH F  is compared with the two chosen thresholds, while the value T m  of the mean value is compared with the threshold T′ max . If KRICH F  is larger than KRICH′ max  or smaller than KRICH′ min  or larger than T′ max , the sensor is considered to be defective. In the opposite case, the sensor is considered to be good. 
     The algorithm of FIG. 7 can be implemented in the form of a software routine or in the form of electronic circuits, in which the comparison steps  80 ,  82  and  84  would be accomplished by digital comparators.