EP0856098A1

EP0856098A1 - Device and method for diagnosing the condition of a probe upstream from a catalytic converter

Info

Publication number: EP0856098A1
Application number: EP96934934A
Authority: EP
Inventors: Eric Marcheguet; Vasco Afonso; François RATINET
Original assignee: Renault SAS
Current assignee: Renault SAS
Priority date: 1995-10-18
Filing date: 1996-10-18
Publication date: 1998-08-05
Anticipated expiration: 2016-10-18
Also published as: US6192310B1; FR2740173B1; FR2740173A1; DE69605816D1; JP2000508035A; KR19990064350A; KR100425426B1; DE69605816T2; JP3993891B2; EP0856098B1; WO1997014876A1

Abstract

A device and a method for use in internal combustion injection engines comprising an exhaust pipe catalytic converter and a non-linear probe upstream therefrom. The device and method are useful for diagnosing the condition of the upstream probe (16) using a signal (Vdown) from a second non-linear probe (26) downstream from the catalytic converter (14). For this purpose, the signal from the downstream probe (26) is processed to give a signal (KRICH) that is filtered (32) to provide a signal (KRICHF). Said signal (KRICHF) is in turn compared (40) with maximum and minimum values and the upstream probe is considered to be correct if the signal falls between said values, or faulty if it falls outside the same. Said device and method are useful in injection engines fitted with a catalytic converter and upstream and downstream non-linear probes.

Description

DEVICE AND METHOD FOR DIAGNOSING THE CONDITION OF A PROBE PROVIDED UPSTREAM OF THE CATALYTIC POT.

The invention relates to internal combustion engines of the injection type and comprising a catalytic exhaust system preceded by a probe and, more particularly in such engines, a device and a method for diagnosing the state of the probe arranged in upstream of the catalytic converter.

It is known to use systems to modify the quantity of fuel which is injected into an engine as a function of the composition of the exhaust gases and, more particularly, of the oxygen content of these gases. To this end, the oxygen content is measured using a non-linear probe called the "lambda" probe or EGO probe, EGO being the acronym for "Exhaust Gas Oxygen". Such a probe is arranged 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 a richness regulation probe.

It is clear that the poor condition of this probe leads to a malfunction of the engine and the catalytic converter, which induces emissions of pollutants at abnormally high values. It is therefore important to determine the state of this probe at all times so as to diagnose its malfunction when its state has deteriorated beyond certain limits. Current solutions for diagnosing the state of the upstream probe consist in analyzing the behavior of the probe in response to rich loop excitations or closed loop and to monitor 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;

- the 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 probe failure if one or more faults are detected.

Such a diagnostic method is based on the analysis of the behavior of the probe in order to deduce therefrom a state of the probe by presuming degradation modes. For example, an aged probe has reduced voltage dynamics and / or extended tilt times.

The disadvantage of such a diagnostic method is that there is no perfect bijection between these measurements and the pollutant emissions.

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.

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

An object of the present invention is therefore to implement a device and a method for diagnosing the state of a probe disposed upstream of a catalytic converter associated with an internal combustion engine of the injection type which does not have the above-listed drawbacks of prior art devices and methods. Another object of the present invention is also to implement a device and a method for diagnosing the state of an upstream probe which does not call for measurements of the intrinsic characteristics of the probe. The method of the invention is based on monitoring the characteristics of the wealth loop which have an influence on the pollutant emissions, namely, the average period and the average wealth of the loop. In this way, the state of the upstream probe is evaluated on the basis of the effects it produces on the wealth loop, that is to say on pollutant emissions, and not on the basis of its own characteristics. The effects of the state of the upstream probe are likely to generate pollutant emissions by exceeding the limits of the "window" for proper operation of the catalytic converter, this excess being due to the drift in the average operating richness 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 _ava ι of the second probe, called the downstream probe, is controlled by a reference voltage VC _ava ^ corresponding to the center of the window for the 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 richness control system with double control loop is described in the patent application filed today by the applicant and entitled: "SYSTEM AND METHOD OF DOUBLE CONTROL LOOP FOR INTERNAL COMBUSTION ENGINE".

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 for correcting KRICH of the quantity of fuel injected, said diagnostic device being characterized in that it comprises: - a filtering circuit to which the second KRICH correction signal is applied to supply a filtered signal KRICHp, - a measuring circuit to which the output signal V _t from the upstream probe is applied to determine the average value T of the correction period d e the first control loop, and - a logic circuit to determine the good or defective DIAG state of the upstream probe as a function of the values of the filtered signal KRICHp 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 a maximum value or less than a minimum value or even if the average period is greater than a maximum value. In another embodiment of the invention, the maximum and minimum values of the filtered signal KRICHp 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.

The invention also relates to a method which comprises the following steps: filtering the second correction signal KRICH to obtain a filtered signal KRICHp,

- 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 KRICHp with two values maximum KRICH _maχ and minimum KRICH _m ^ _n to determine the correct or defective DIAG state of said upstream probe according to whether the filtered signal KRICHp is respectively within the limits defined by the maximum values and minimum or outside of said limits for the value of the average period ^τ m- 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;

- Figures 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; - Figures 3-A and 3-B are diagrams showing a way of correcting the richness using a second feedback loop comprising a probe downstream of the catalytic converter;

- Figure 4 is a diagram showing how to filter the correction signal KRICH to obtain a filtered signal KRICHp;

- Figure 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

- Figure 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 an electronic computer 12. The exhaust gases from this engine are filtered by an exhaust pipe 14 of catalytic type, from which they escape towards the open air. A first probe 16 is disposed at the inlet of the exhaust pipe and measures the content of one of the main components of the exhaust gases, this component usually being oxygen. This probe is of the non-linear type and is often called, as 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 _aτnon ^. is compared with a threshold voltage ^v S _aιrtθn -t- to determine the sign of V _aιnont with respect to this threshold.

The value of the threshold ^v S _aιnon t 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 _am0 nt ^is less than ^vs _am have ' ^it 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 d be 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 exhaust 14. This circuit 24 essentially consists of a comparator 28 to which the signal ^v _ava ^ and a so-called reference signal VCa., v ,, al _! 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 supplies the signal KRICH 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 KCL signal as modified by the second corrector circuit 22, the modified KCL signal being called KCL. ,,.

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 plot of KCL ^ is that of figure 3-A while in the case where KRICH <0 (depletion), the plot of KCL ^ is that of 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 signal ^v _amcm t ^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 KRICHp of the filter 32 and of the value T _m of the mean period of the signal ^v _amθ nf ^The binary sig ^nal 1 or O of the good or bad state of probe 16 appears on the DIAG output terminal of logic circuit 36.

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

- the engine speed REG, - 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.

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

- Pmi • n <P ^r <P ^r max

First loop in active state, - Second loop in active state,

REG _min and REG _maχ being respectively the minimum and maximum values of the engine speed REG between which the diagnosis can be carried out; P _m i _n 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. The filtering 32 performs the calculation of the filtered richness correction KRICHp 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 this case, the average wealth KRICHp is given by:

KRICHp = KRICHp + K (KRICH - KRICHp) 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 mean period T ^ is to count the transitions of the voltage V _upstream of a value below the threshold ^vs. upstream ^ ^{a vaj} - ^eur upper threshold for a certain time interval T _D and dividing the interval T _D by the number N of transitions that have been detected.

The algorithm for calculating the average period T 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_ begins. As soon as ^v upstream ^{> vs} araont (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, STATE _A = 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 "= 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 the detection that ^v _aιrιon t ^<vs upstream ' ^aut 3 ^ue l ^{case we} go to STATE _A = 0 (step 78).

On start-up, TRANS = 0 so that the condition of step 60 is not fulfilled and there cannot be a calculation of the period. If this is not the case, step 70 imposes TRANS = 1, which resets counter 52 by step 68 and a new count of T can begin. 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 KRICHp with values which have been determined to be limit values beyond which the probe is considered to be defective and this for a value determined T _m of the mean period. These limit values, called KRICH _maχ for an excessively high richness and KRICH- _j ^ for an excessive impoverishment, are determined by a calibration using a series of probes whose aging characteristics are known. This calibration makes it possible to plot the KRICH _τnaχ and KRICH _m ^ _n curves as a function of the period T _m (FIG. 6), curves which can be stored in the form of two cartographic tables or a single table combining the two. These cartographic tables can be produced by memories which are addressed by the value of T _m , and the values read are KRICrL and KRIŒL _j . ^ 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- _DD with a minimum duration T _Dιn: £ _n below which a diagnosis would not be reliable. If T _D > T _Dm ^ _n , the next step 82 consists in comparing KRICHp to a value KRICH _ιnaχ which is read in the cartographic table 88 giving KRICH_ _aχ = SfT- ..). This table is addressed by the value of T _m to give a value of KRICH _jnaχ which is compared to KRICHp. 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 KRICHp with the value of KRICH ^^ for T ^ as read in table 86 in which the values of the curve are recorded S (T _m ). If the condition KRICH> KRICH ^ 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 KRICHp and of T _m . With the curves of figure 6 which are put in the form of tables and by implementing the algorithm of figure 7, the probes which are regarded as bad (DIAG = 0) are in the hatched part outside the two curves, the probes which 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 ' _maχ and it is therefore no longer necessary to have two cartographic tables. In this simplified case, the value of KRICHp is compared with the two selected thresholds while the value T _m of the average value is compared with the threshold T ' _aχ . If KRICHp is greater than KRICH ' _maχ , or less than KRICH' _m ^ _n or greater than T'_ _aχ , the probe is considered to be defective. Otherwise, the probe is considered good. The algorithm of FIG. 7 can be implemented in the form of software or in that of electronic circuits in which the comparison steps 80, 82 and 84 would be performed by number comparators.

Claims

1. Device for diagnosing the state of a nonlinear probe (16) disposed upstream of a catalytic converter (14) associated with an internal combustion engine (10) of the injection type controlled by an electronic computer (12 ), said motor comprising a first control loop, including said non-linear probe (16), for supplying the computer (12) a first correction signal (KCL) of the quantity of fuel injected and a second control loop, including a second nonlinear probe (26) disposed downstream of said catalytic converter (14) to supply a second correction signal (KRICH) of the quantity of fuel injected, said diagnostic device being characterized in that it comprises: - a filter circuit (32) to which the second correction signal (KRICH) is applied to provide a filtered signal (KRICHp),

a measurement circuit (34) to which the output signal ( ^v _{aιrι ont} ) ^ ^e ^ is applied to the upstream ^probe to determine the average 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 probe (16) as a function of the values of the filtered signal (KRICHp) and of the average period (T_).

2. Diagnostic device according to claim, characterized in that the filtering circuit (32) performs first order filtering.

3. Diagnostic device according to claim 1 or 2, characterized in that the filtering circuit (32) is of digital type.

4. Diagnostic device according to one of the preceding claims 1 to 3, characterized in that the calculation circuit (34) of the average value (T _m ) of the correction period of the first control loop is of the digital type .

5. Diagnostic device according to any one of the preceding claims 1 to 4, characterized in that the logic circuit (36) comprises three comparators which first compare the value of the filtered signal (KRICHp) with a maximum value (KRICH _ιnaχ ) in a first comparator, the second the value of the filtered signal (KRICHp) to a minimum value (KRICH _ιnin) and the third of the mean period value at a maximum T _maχ value, ^has upstream sensor (16) being considered defective when the value of the filtered signal (KRICHp) is greater than the maximum value (KRICH _maχ ) or less than the minimum value (KRICH _m £ _n ) or greater than the maximum value (T _maχ ) of the average period.

6. Diagnostic device according to claim 5, characterized in that the logic circuit (36) comprises at least one table or memory in which the maximum (KRICH _maχ ) and minimum (KRICH _min ) values of the filtered signal (KRICHp) are recorded. according to the value of the average period (T ^) and two comparators which compare the value of the filtered signal first (KRICHp) with a maximum value (KRICH _ιnaχ ) read in the table, and the second the signal value filtered (KRICHp) to a minimum value (KRICH _m ^ _n ) read in the table, the reading in the table being carried out using the average period (T _m ).

7. Method for diagnosing the state of a non-linear probe (16) disposed upstream of a catalytic converter (14) associated with an internal combustion engine (10) of the injection type controlled by an electronic computer (12 ), said engine comprising a first control loop, including said non-linear probe (16), for supplying computer 12 with a first correction signal (KCL) of the quantity of fuel injected and a second control loop, including a second non-linear probe (26) disposed downstream of said catalytic converter (14), to supply a second correction signal (KRICH) of the quantity of fuel injected, said diagnostic method being characterized in that it comprises the following steps:

- filtering (32) of the second correction signal (KRICH) to obtain a filtered signal (KRICHp),

- calculation (34) of the average value (T ^) of the period of the output signal (Vamont) ^{of the} upstream ^probe (16),

- comparison of said filtered signal (KRICHp) with two maximum values (KRICH _maχ ) and minimum (KRICH _m ^ _n ) to determine the correct or defective state (DIAG) of said upstream probe (16) depending on whether the filtered signal (KRICHp) is respectively inside the limits defined by the maximum and minimum values or outside these limits for the value of the average period (T _ro ).

8. A diagnostic method according to claim 7, characterized in that it further comprises the following steps: calibration to determine the maximum (KRICH _maχ ) and minimum (KRICH _min ) values for a plurality of values of the average period (T _m ), recording of said maximum and minimum values as well as values of the average period (T _m ) in a memory addressable by its content, and reading of said memory using the average value (T _m ) of the period to obtain the maximum (KRICH _maχ ) and minimum (KRICH _m ^ _n ) values.