EP0856098B1 - Device and method for diagnosing the condition of a probe upstream from a catalytic converter - Google Patents

Description

The invention relates to internal combustion engines injection type and having an exhaust pipe catalytic preceded by a probe and, more particularly in such motors, a device and a method for diagnosing the condition of the probe arranged upstream of the catalytic converter.

It is known to use systems to modify the amount of fuel that is injected into an engine depending on the composition of the exhaust gases and, more particularly, the oxygen content of these gas. For this purpose, the oxygen content is measured at using a non-linear probe called a "lambda" probe or EGO probe, EGO being the acronym for "Exhaust Gas Oxygen". Such a probe is arranged in upstream of the catalytic converter and the signal supplied by this probe is used to modify the quantity of fuel which is injected into the engine cylinders via a first feedback loop. For this reason, this probe is also called the wealth control probe.

It is clear that the poor condition of this probe leads engine and pot malfunction catalytic, which induces pollutant emissions to abnormally high values. It is therefore important to determine the state of this probe at any time way to diagnose its malfunction when his condition has deteriorated beyond certain limits.

• la tension minimale fournie par la sonde : si elle est trop élevée, il y a défaut ;
• la tension maximale fournie par la sonde : si elle est trop faible, il y a défaut ;
• le temps de basculement pauvre-riche : s'il est trop long, il y a défaut ;
• le temps de basculement riche-pauvre : s'il est trop long, il y a défaut ;
• la période du signal fourni par la sonde en boucle fermée : si elle est trop longue, il y a défaut.

Current solutions for diagnosing the state of the upstream probe consist in analyzing the behavior of the probe in response to richness excitations in open loop or in closed loop and in monitoring the following parameters:

• the minimum voltage supplied by the probe: if it is too high, there is a fault;
• the maximum voltage supplied by the probe: if it is too low, there is a fault;
• poor-rich changeover time: if it is too long, there is a fault;
• the rich-poor changeover time: if it is too long, there is a fault;
• the period of the signal supplied by the closed loop probe: if it is too long, there is a fault.

The diagnosis then consists in declaring the failure of the probe if one or more faults are detected.

Such a diagnostic method is based on the analysis of the behavior of the probe to deduce a state of the probe by presuming degradation modes.

For example, an aged probe has a voltage dynamic reduced and / or extended switching times. The disadvantage of such a diagnostic method is that it there is no perfect bijection between these measurements and pollutant emissions.

• une connaissance parfaite des modes de vieillissement des sondes,
• de nombreux essais pour établir un lien entre les dégradations mesurées des paramètres et leurs effets sur les émissions de polluants.

In addition, the calibration of fault detection thresholds is very delicate and requires:

• a perfect knowledge of the aging modes of the probes,
• numerous attempts to establish a link between the measured degradations of the parameters and their effects on pollutant emissions.

Furthermore, it is not possible to guarantee in in all cases the reliability of the diagnosis. For example, a reduced voltage dynamic probe may prove good vis-à-vis the emission of pollutants if alone this characteristic is affected.

An object of the present invention is therefore to operates a device and a method for diagnosing the state of a probe placed upstream of a pot catalytic associated with an internal combustion engine of the injection type which does not have the disadvantages above listed devices and methods of the art prior.

Another object of the present invention is also to implement a device and a method of diagnosis of the state of an upstream probe which does not call for measurements of intrinsic characteristics of the probe.

The method of the invention is based on monitoring characteristics of the wealth loop which have an influence on pollutant emissions, namely, the average period and the average wealth of the closure. In this way, the state of the upstream probe is evaluated at from the effects it produces on the closure of wealth, that is to say on pollutant emissions, and not from its own characteristics.

The effects of the state of the upstream probe are likely to cause pollutant emissions by exceeding the limits of the "window" of good operation of the catalytic converter, this excess being due to the drift in the average operating wealth and / or the average period of the wealth loop which becomes too long.

To detect the drift in the average operating richness, the invention proposes to implement a second non-linear probe which is arranged downstream of the catalytic converter and which is an integral part of a second feedback loop thanks to which the output voltage V _downstream of the second probe, called the downstream probe, is slaved to a setpoint voltage VC _downstream corresponding to the center of the window for correct operation of the catalytic converter. The signal which is provided by this loop is used to modify the signal of the first feedback loop comprising the upstream probe.

Such a double wealth control system control loop is described in US-A-5,337,555 and in the patent application filed today by the plaintiff and entitled: "DUAL LOOP CONTROL SYSTEM AND METHOD FOR INTERNAL COMBUSTION ENGINE".

• un circuit de filtrage auquel est appliqué le deuxième signal de correction KRICH pour fournir un signal filtré KRICH_F,
• un circuit de mesure auquel est appliqué le signal de sortie V_amont de la sonde amont pour déterminer la valeur moyenne T_m de la période de correction de la première boucle de commande, et
• un circuit logique pour déterminer l'état DIAG bon ou défectueux de la sonde amont en fonction des valeurs du signal filtré KRICH_F et de la période moyenne T_m.

The invention relates to a device for diagnosing the state of a nonlinear probe disposed upstream of a catalytic converter associated with an internal combustion engine of the injection type controlled by an electronic computer, said engine comprising a first loop of control, including said non-linear probe, to supply the computer with a first KCL correction signal of the quantity of fuel injected and a second control loop, including a second non-linear probe arranged downstream of said catalytic converter to provide a second signal KRICH correction of the quantity of fuel injected, said diagnostic device being characterized in that it comprises:

• a filtering circuit to which the second correction signal KRICH is applied to supply a filtered signal KRICH _F ,
• a measurement circuit to which the _upstream output signal V of the upstream probe is applied to determine the average value T _m of the correction period of the first control loop, and
• a logic circuit for determining the good or defective DIAG state of the upstream probe as a function of the values of the filtered signal KRICH _F and of the average period T _m .

In one embodiment of the invention, the logic circuit determines that the upstream probe is defective if the filtered signal is greater than one maximum value or less than a minimum value or again if the average period is greater than one 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 average period and are recorded in a memory. This memory is addressed by the value of the average period to provide the maximum and minimum values to which the value of the filtered signal is compared.

• filtrage du deuxième signal de correction KRICH pour obtenir un signal filtré KRICH_F,
• calcul de la valeur moyenne T_m de la période du signal de sortie V_amont de la sonde amont,
• comparaison dudit signal filtré KRICH_F à deux valeurs maximale KRICH_max et minimale KRICH_min pour déterminer l'état DIAG correct ou défectueux de ladite sonde amont selon que le signal filtré KRICH_F est respectivement à l'intérieur des limites définies par les valeurs maximale et minimale ou à l'extérieur desdites limites pour la valeur de la période moyenne T_m.

The invention also relates to a method which comprises the following steps:

• filtering of the second correction signal KRICH to obtain a filtered signal KRICH _F ,
• calculation of the average value T _m of the period of the _upstream output signal V of the upstream probe,
• comparison of said filtered signal KRICH _F with two maximum values KRICH _max and minimum KRICH _min to determine the correct or defective DIAG state of said upstream probe according to whether the filtered signal KRICH _F is respectively within the limits defined by the maximum values and minimum or outside of said limits for the value of the average period T _m .

• la figure 1 est un schéma fonctionnel d'un système de double boucle de commande de richesse auquel s'applique l'invention ;
• les figures 2-A et 2-B sont des diagrammes montrant comment s'effectue la correction de richesse avec une seule boucle de contre-réaction comportant une sonde en amont du pot catalytique ;
• les figures 3-A et 3-B sont des diagrammes montrant une manière de corriger la richesse en utilisant une deuxième boucle de contre-réaction comportant une sonde en aval du pot catalytique ;
• la figure 4 est un diagramme montrant la manière de filtrer le signal de correction KRICH pour obtenir un signal filtré KRICH_F ;
• la figure 5 est un diagramme montrant un algorithme de calcul de la période moyenne du signal de la sonde amont ;
• la figure 6 est un diagramme montrant les courbes qui déterminent les zones de fonctionnement correct ou défectueux de la sonde amont, et
• la figure 7 est un diagramme montrant un algorithme de décision pour déterminer l'état de la sonde amont.

Other characteristics and advantages of the present invention will appear on reading the following description of a particular embodiment, said description being made in relation to the accompanying drawings, in which:

• Figure 1 is a block diagram of a double wealth control loop system to which the invention applies;
• FIGS. 2-A and 2-B are diagrams showing how the richness correction is carried out with a single feedback loop comprising a probe upstream of the catalytic converter;
• FIGS. 3-A and 3-B are diagrams showing a way of correcting the richness by using a second feedback loop comprising a probe downstream of the catalytic converter;
• Fig. 4 is a diagram showing how to filter the KRICH correction signal to obtain a filtered KRICH _F signal;
• FIG. 5 is a diagram showing an algorithm for calculating the average period of the signal from the upstream probe;
• FIG. 6 is a diagram showing the curves which determine the zones of correct or defective operation of the upstream probe, and
• FIG. 7 is a diagram showing a decision algorithm for determining the state of the upstream probe.

In FIG. 1, an internal combustion engine 10 is controlled, in a known manner, by a computer electronic 12. The exhaust gases from this engine are filtered by a type 14 exhaust catalytic, from which they escape towards the open air. A first probe 16 is disposed at the entrance of the pot exhaust and measures the content of one of main exhaust components, this component usually being oxygen. This probe is of the non-linear type and is often called, like indicated above, "lambda" probe or EGO probe.

This probe provides on its output terminal an _upstream electrical signal V (Figure 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 with respect to this threshold.

The value of the _upstream VS threshold depends on the characteristics of the probe and corresponds to the tilting voltage of the probe when the stoichiometric conditions are met.

The output terminal of the comparator circuit 18, which provides a binary signal 1 or 0 , is connected to the input terminal of a first correction regulator 20 for richness regulation which is of the proportional type of gain P and integral of gain I The corrector circuit 20 supplies a signal KCL which has the form represented by the diagram of FIG. 2-B. It is this signal KCL which is supplied to the computer 12 to control the quantity of fuel to be injected. Thus, as soon as V _upstream is less than VS _upstream , this means that the mixture is poor in fuel and that the quantity of fuel must be increased. This is what is achieved by the jump + P (Figure 2-B) followed by a positive slope of value I until the moment when V _upstream exceeds VS _upstream , which means that the mixture becomes rich in fuel and that 'the quantity must be reduced. This is achieved by a jump -P followed by a negative slope of value I.

The correction value KCL, supplied by the corrector circuit 20, is modified by a second corrector circuit 22, which introduces a corrector term KRICH, before being applied to the computer 12. This corrector term KRICH is determined by a circuit 24 from an output signal V _downstream of a second lambda probe 26 which is disposed at the outlet of the catalytic converter 14. This circuit 24 essentially consists of a comparator 28 to which the signal V _downstream are applied and a so-called _downstream VC setpoint signal and a third corrector circuit 30 to which the signal (V _downstream - VC _downstream ) supplied by the comparator circuit 28 is applied. The third corrector circuit 30 is for example of the proportional and integral type and provides the KRICH signal which is applied to the second correction circuit 22.

The second corrector circuit 22 can introduce the KRICH correction in different ways, one of which will be explained in relation to the time diagrams of FIGS. 3-A and 3-B. These diagrams are plots of the signal KCL as modified by the second correcting circuit 22, the modified signal KCL being called KCL _m .

According to the diagrams of FIGS. 3-A and 3-B, the signal KRICH is applied during the lean-to-rich transitions which are detected by the first probe, which corresponds to the falling edge of the signal KCL. In the case where KRICH> 0 (enrichment), the course of KCL _m is that of the figure 3-A while in the case where KRICH <0 (depletion), the course of KCL _m is that of the figure 3-B .

The device for diagnosing the state of the probe 16 comprises the elements represented inside the rectangle 40 of the diagram in FIG. 1. It is a filter 32 to which the output signal KRICH of the correcting circuit is applied. 24 of the second loop as well as a calculation circuit 34 of the average period T _m of the _upstream signal V of the upstream probe 16. The output terminals of the filter 32 and of the calculation circuit 34 are connected to a logic circuit 36 which determine the good or bad state of the probe 16 as a function of the output signal KRICH _F of the filter 32 and of the value T _m of the mean period of the _upstream signal V. The binary signal 1 or 0 of the good or bad state of the probe 16 appears on the DIAG output terminal of the logic circuit 36.

• le régime moteur REG,
• la pression du collecteur d'entrée P,
• l'état de la première boucle : actif ou non,
• l'état de la deuxième boucle : actif ou non.

The information which is provided by the computer 12 is as follows:

• REG engine speed,
• the pressure of the inlet manifold P,
• the state of the first loop: active or not,
• the state of the second loop: active or not.

• REG_min < REG < REG_max
• P_min < P < P_max
• Première boucle à l'état actif,
• Deuxième boucle à l'état actif,

Circuits 32 and 34 process the information listed above and only allow filtering and calculation of T _m if the following conditions are met simultaneously:

• REG _min <REG <REG _max
• P _min <P <P _max
• First loop in active state,
• Second loop in active state,

REG _min and REG _max being respectively the minimum and maximum values of the REG engine speed between which the diagnosis can be carried out; P _min and P _max being respectively the minimum and maximum values of the pressure P of the inlet manifold between which the diagnosis can be carried out.

Filtering 32 performs the calculation of the filtered richness correction KRICH _F according to the algorithm of FIG. 4. This calculation (step 42) is only carried out if the conditions listed above are fulfilled (step 44) and, in in this case, the average wealth KRICH _F is given by: KRICH F = KRICH F + K (KRICH - KRICH F ) with K a filtering factor between 0 and 1.

The calculation circuit 34 performs the calculation of the average period T _m according to the algorithm of FIG. 5. This calculation is only carried out if the conditions listed above are met (step 50). This calculation of the average period T _m consists in counting the transitions of the _upstream voltage V from a value below the threshold VS _upstream to a value above the threshold during a certain time interval T _D and in dividing this interval T _D by the number N of transitions that have been detected.

The algorithm for calculating the average period T _m of the first loop is represented by the diagram in FIG. 5. The first step (50) consists in checking whether the diagnostic conditions listed above are fulfilled. If the answer is "YES", the step of counting 52 of the time T is started, that is to say that the calculation of the average period T _m begins. As soon as V _upstream > VS _upstream (step 54) and the old state STATE _A of the probe corresponding to V _upstream <VS _upstream (STATE _A = 0), step 58 consists in memorizing this new state of the probe by STATE _A = 1. The next step 60 consists in checking whether a transition (TRANS = 1) has already been detected before; in the event of a positive response, this means that a period has elapsed and the count 62 of the number N of periods is increased by one. At the same time, the counter for the duration T _D of the diagnosis is increased by the value T of the counter 52. The calculation 66 of the average period T _m = T _D / N is then carried out with the new values of N and T _D . The next step 68 resets the counter 52 to zero for a new measurement T of the current period.

For the above calculation to be performed correctly, the following states must be present: TRANS = 0, STATUS AT = 1 and T = 0, which is achieved by steps 72, 74 and 76 in cascade which are initialized by verification (step 50) that the diagnostic conditions are not fulfilled, which is always the case when starting the engine. Thus, for the first measurement of the period, the counter 52 is at the value 0 but as STATE _A = 1, the calculation cannot begin until this state does not pass to STATE _A = 0 so as to be certain to detect a transition in the desired direction. This is obtained by detecting that V _upstream <VS _upstream , in which case we go to STATE _A = 0 (step 78).

At startup, TRANS = 0 so that the condition of step 60 is not completed and there cannot be for calculating the period. If not, step 70 impose TRANS = 1, which resets counter 52 to zero by step 68 and a new count of T can to start.

At startup, STATE _A = 1 so that the condition of step 56 is not fulfilled, in which case the steps of the algorithm are repeated.

The logic circuit 36 performs the steps of the algorithm of FIG. 7 so as to compare the value of KRICH _F with values which have been determined to be limit values beyond which the probe is considered to be defective and this for a determined value T _m of the average period.

These limit values, called KRICH _max for too high richness and KRICH _min for too much depletion, are determined by a calibration using a series of probes whose aging characteristics are known.

This calibration makes it possible to plot the KRICH _max and KRICH _min curves as a function of the period T _m (FIG. 6), curves which can be stored in the form of two cartographic tables or of a single table grouping the two. These cartographic tables can be produced by memories which are addressed by the value of T _m , and the values read are KRICH _max and KRICH _min for the value of T _m (FIG. 6).

The first step 80 of the diagnostic algorithm consists in comparing the duration T _D of the calculation of the period T _m with a minimum duration T _Dmin below which a diagnosis would not be reliable. If T _D > T _Dmin , the next step 82 consists in comparing KRICH _F with a value KRICH _max which is read in the cartographic table 88 giving KRICH _max = S (T _m ). This table is addressed by the value of T _m to give a value of KRICH _max which is compared to KRICH _F. If the condition is not checked, the probe is considered to be defective (step 92).

If the condition is satisfied, the next step 84 is to compare KRICH _F with the value of KRICH _min for T _m as read in table 86 in which the values of the curve KRICH _min = S (T _m ) are recorded. If the condition KRICH> KRICH _min is not checked, the probe is considered to be defective (step 92) with DIAG = 0. Otherwise, the probe is considered to be correct (step 90) with DIAG = 1.

As soon as the probe is considered to be correct or defective, the diagnosis is complete (step 94) and a new diagnosis can be launched to obtain a new value of KRICH _F and of T _m .

With the curves of figure 6 which are placed under the form of tables and by implementing the algorithm of Figure 7, the probes that are considered to be bad (DIAG = 0) are in the hatched part at the outside of the two curves, the probes that are considered good (DIAG = 1) correspond to the surface inside the curves.

Instead of the two curves in Figure 6, it is possible to limit oneself to choosing fixed thresholds for KRICH ' _max , KRICH' _min and T ' _max and it is therefore no longer necessary to have two cartographic tables. In this simplified case, the value of KRICH _F is compared with the two selected thresholds while the value T _m of the average value is compared with the threshold T ' _max . If KRICH _F is greater than KRICH ' _max , or less than KRICH' _min or greater than T ' _max , the probe is considered to be defective. Otherwise, the probe is considered good.

The algorithm of Figure 7 can be performed under the as software or as circuits electronics in which the comparison steps 80, 82 and 84 would be carried out by comparators of numbers.

Claims (8)

1. A diagnostic apparatus for the state of a non-linear sensor (16) disposed upstream of a catalytic converter (14) associated with an internal combustion engine (10) of the type involving injection controlled by an electronic computer (12), said engine comprising a first control loop including said non-linear sensor (16) for supplying the computer (12) with a first correction signal (KCL) for the amount of fuel injected and a second control loop including a second non-linear sensor (26) disposed downstream of said catalytic converter (14) to supply a second correction signal (KRICH) for the amount of fuel injected, the diagnostic apparatus being characterised in that it comprises:

   a filter circuit (32) to which the second correction signal (KRICH) is applied to supply a filtered signal (KRICH_F),

   a measuring circuit (34) to which the output signal (V_upstream) of the upstream sensor is applied to determine the mean value (T_m) of the correction period of the first control loop, and

   a logic circuit (36) for determining the good or defective state (DIAG) of the upstream sensor (16) in dependence on the values of the filtered signal (KRICH_F) and the mean period (T_m).
2. A diagnostic apparatus according to claim 1 characterised in that the filter circuit (32) implements first-order filtering.
3. A diagnostic apparatus according to claim 1 or claim 2 characterised in that the filter circuit (32) is of digital type.
4. A diagnostic apparatus according to one of preceding claims 1 to 3 characterised in that the circuit (34) for calculating the mean value (T_m) of the correction period of the first control loop is of the digital type.
5. A diagnostic apparatus according to any one of preceding claims 1 to 4 characterised in that the logic circuit (36) comprises three comparators of which the first compares the value of the filtered signal (KRICH_F) to a maximum value (KRICH_max) in a first comparator, the second the value of the filtered signal (KRICH_F) to a minimum value (KRICH_min) and the third the value of the mean period to a maximum value T_max, the upstream sensor (16) being considered as defective when the value of the filtered signal (KRICH_F) is higher than the maximum value (KRICH_max) or lower than the minimum value (KRICH_min) or higher than the maximum value (T_max) of the mean period.
6. A diagnostic apparatus according to claim 5 characterised in that the logic circuit (36) comprises at least one table or memory in which are recorded the maximum and minimum values (KRICH_max and KRICH_min respectively) of the filtered signal (KRICH_F) in dependence on the value of the mean period (T_m) and two comparators of which the first compares the value of the filtered signal (KRICH_F) to a maximum value (KRICH_max) which is read in the table and the second the value of the filtered signal (KRICH_F) to a minimum value (KRICH_min) which is read in the table. reading in the table being implemented by means of the mean period (T_m).
7. A diagnostic process for the state of a non-linear sensor (16) disposed upstream of a catalytic converter associated with an internal combustion engine (10) of the type involving injection controlled by an electronic computer (12), said engine comprising a first control loop including said non-linear sensor (16) for supplying the computer (12) with a first correction signal (KCL) for the amount of fuel injected and a second control loop including a second non-linear sensor (26) disposed downstream of said catalytic converter (14) to supply a second correction signal (KRICH) for the amount of fuel injected, the diagnostic process being characterised in that it comprises the following steps:

   filtration (32) of the second correction signal (KRICH) to obtain a filtered signal (KRICH_F),

   calculation (34) of the mean value (T_m) of the period of the output signal (V_upstream) of the upstream sensor (16), and

   comparison of said filtered signal (KRICH_F) to two maximum and minimum values (KRICH_max and KRICH_min respectively) to determine the correct or defective state (DIAG) of said upstream sensor (16) according to whether the filtered signal (KRICH_F) is respectively within the limits defined by the maximum and minimum values or outside said limits for the value of the mean period (T_m).
8. A diagnostic process according to claim 7 characterised in that it further comprises the following steps:

   calibration to determine the maximum and minimum values (KRICH_max and KRICH_min respectively) for a plurality of values of the mean period (T_m),

   recording of said maximum and minimum values and values of the mean period (T_m) in a memory addressable by its content, and

   reading of said memory by means of the mean value (T_m) of the period to obtain the maximum and minimum values (KRICH_max and KRICH_min respectively).

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