ENGINE COMPRISING A CATALYTIC CONVERTER WITH FAILSAFE OPERATION

Abstract

The invention proposes an engine (11) comprising a catalytic converter (12c), a first probe (120) able to deliver a first signal (Vsp) proportional to the oxygen content of an exhaust gas and installed upstream of the converter, a second oxygen probe (130) downstream of the converter and able to provide a second signal (Vsb), and means (13) for making a diagnosis regarding the operating condition of the converter, characterized in that: the second oxygen probe (130) is of the on/off type and the means (13) are designed to convert the first signal (Vsp) into a third signal (Vspb) of the on/off type and to make the diagnosis on the basis of the second and third signals (Vsb, Vspb).

Description

The invention relates to a method of monitoring the effectiveness of a catalytic converter installed in an exhaust line of an internal combustion engine, particularly a diesel engine.

The catalytic converters at which the invention is aimed are, in particular, the converters designed to trap the oxides of nitrogen NOx present in the gases.

It is known that internal combustion engines produce exhaust gases which contain pollutants such as the aforementioned oxides of nitrogen NOx, unburnt hydrocarbons HC and also carbon monoxide CO.

These pollutants have to be treated so that their emissions meet the criteria dictated by standards such as European standards for example.

To do this, one known technique used for reducing these emissions is to incorporate a catalytic converter into the engine exhaust line.

This converter is able, on the one hand, to store the NOx and then, during a particular phase of operation known as “purging the catalyst”, to reduce these NOx to release them into the atmosphere in the form of nitrogen N₂ and carbon dioxide CO₂.

On the other hand, this catalytic converter has a function of oxidizing the reducing species such as carbon monoxide CO and unburnt hydrocarbons HC.

In order to obtain such results, a catalytic converter that traps NOx comprises, as is known per se, a catalytic phase in the cells of a monolith that are in contact with the gases.

The catalytic phase typically consists of platinum, of palladium, of rhodium, and/or of alkaline earth metals.

One of the problems with catalytic converters is that the catalytic phase loses effectiveness as it ages.

In particular, aging of this phase often results in a reduction in the number of NOx storage sites and therefore in a drop in the effectiveness with which NOx, HC and co can be stored and treated.

There are other factors that lead to the drop in converter performance as the converters age but in any event, particularly given the pollution levels dictated by the standards, use of a catalytic converter with a trap for nitrogen oxides usually entails monitoring the operation of such a converter in order to avoid troublesome deviations.

Monitoring systems have already been proposed.

In general, their function is to detect failure of a component of a piece of pollution-reducing equipment installed in a vehicle and to alert a driver to any failure that will cause the aforementioned emission levels to be exceeded, so that repairs can be carried out promptly.

Thus, document FR 2 866 926 discloses a monitoring system in which use is made of a mixture richness lambda probe installed downstream of a catalytic converter to measure the richness of the exhaust gases at this point and from this make a diagnosis regarding the operating status of the converter.

More specifically, the diagnosis is based on measuring the effective time taken to reduce the stored NOx, this time being a function of a voltage supplied by the lambda probe.

Also, document U.S. Pat. No. 5,228,335 discloses a system for detecting failure of a catalytic converter on the basis of the use of two proportional mixture richness probes (the voltage supplied by the probe is proportional to the richness) installed one upstream and one downstream of the converter.

More specifically, that system makes a diagnosis on the basis of integrating, with respect to time, the difference between the signals supplied by each of the two probes.

This diagnosis therefore relies on calculating an area which is a function of the measurement signals from the probes, and this area is then compared against a threshold value in order to evaluate the operating status (or, which amounts to the same, the degree of aging) of the converter.

Although they have proved most useful, such systems are, however, relatively expensive and tricky to implement.

It is therefore an object of the invention to provide a method and a system for robustly and reliably monitoring the operating status of a catalytic converter, doing so at a lower cost.

To this end, the invention proposes an engine comprising a catalytic converter, a first probe capable of delivering a first signal proportional to an oxygen content in an exhaust gas and installed upstream of the converter, a second oxygen probe downstream of the converter and capable of supplying a second signal, and means for making a diagnosis regarding an operating status of the converter, characterized in that:

• • the second oxygen probe is of the on/off type, and
  • the means are designed to convert the first signal into a third signal of the on/off type and to make the diagnosis on the basis of the second and third signals.

Preferred but nonlimiting aspects of this engine are as follows:

• • the signal conversion means comprise a probe model, such as a map of the on/off type, dependent on the first signal;
  • the model is designed such that the third signal it supplies is in the form of a square wave with predetermined upper and lower values;
  • the model is designed such that the third signal it supplies switches from the upper value to the lower value, and vice versa, when the first signal crosses a first predetermined threshold value, one of the upper and lower values preferably being substantially equal to the first threshold value;
  • the means are also designed to control the temperature of an oxygen-content-sensitive element in the second probe;
  • the control means are designed to decrease the temperature down to a predetermined temperature value, preferably ranging between 400 and 500° C., before the means for making the diagnosis make use of the second and third signals.

The invention also proposes a method of monitoring the operating status of a catalytic converter of an engine, characterized in that it comprises the following steps:

• • obtaining, from a first probe, a first signal proportional to an oxygen content in a gas and situated upstream of the converter,
  • obtaining a second signal from a second probe of the on/off type installed downstream of the converter,
  • converting the first signal into a third signal of the on/off type, and
  • making a diagnosis regarding the converter operating status on the basis of the second and third signals.

Preferred but nonlimiting aspects of this method are as follows:

• • in the step of making the diagnosis, a diagnostic criterion is determined on the basis of an area of a difference between the second and third signals;
  • in the step of making the diagnosis, the diagnostic criterion is compared against three predetermined threshold values that respectively define three ranges of values corresponding to three different converter operating statuses;
  • the method comprises a step in which the running of the diagnostic step is governed by whether the first and/or second signal meets at least one predetermined criterion;
  • the diagnostic criterion is of the following form:

$C\mathrm{diag}={\int }_{t1}^{t2}\frac{\mathrm{Dgaz}}{3.6}\left(\mathrm{Vspb}-\mathrm{Vsb}\right)t$

where Cdiag, t1, t2 and Dgaz respectively denote the diagnostic criterion used, two predetermined moments in time and a rate of flow of exhaust gas through the catalytic converter.

Other aspects, objects and advantages of the invention will become better apparent from reading the following description of the invention, which description is given with reference to the attached drawings, in which:

FIG. 1 illustrates an engine according to a preferred embodiment of the invention,

FIG. 2 shows a flow diagram of a method according to one preferred embodiment of the invention, and

FIG. 3 shows a graphical simulation of the signals emitted by the probes installed in the engine and by the probe model.

FIG. 1 depicts an engine 10 according to a preferred embodiment of the invention.

This engine, in the conventional way, comprises an engine block 11, an exhaust line 12 through which an exhaust gas emitted by the engine block flows, and control means for controlling engine performance.

As illustrated by way of nonlimiting example, the engine block in particular comprises an inlet manifold 10a and an exhaust manifold 11b.

The exhaust line 12 is made up in particular of an exhaust gas recirculation or EGR system 12a (EGR being the widely accepted abbreviation for the English expression “exhaust gas recirculation”), of a turbocharger 12b equipped with a turbine 100 capable of driving a compressor 10, and of a catalytic converter 12c.

The turbine 100 in the conventional way receives the exhaust gas leaving the exhaust manifold 11b, while the compressor receives fresh air from an air filter 12d and compresses it, thus supercharging the inlet manifold 11a.

The catalytic converter 12c is in Communication with an outlet of the turbine.

In particular, it is arranged with respect to this turbine in such a way that it receives the exhaust gas that leaves the turbine after having driven the rotation of this turbine.

The purpose of the catalytic converter is to trap the oxides of nitrogen present in the gas.

The trap is typically obtained by impregnating with a catalytic phase the cells of a monolith (not depicted) of porous structure and having a large surface area for contact with the gas.

As is known per se, the monolith may just as easily be a particulate filter or an oxidation catalyst so that the post-treatment of the gas by reducing the oxides of nitrogen NOx can be coupled with post-treatment of particulates, unburnt hydrocarbons HC and carbon monoxide CO.

The engine further comprises an oxygen probe 120 installed upstream of the converter, preferably at the inlet thereto.

This probe is a probe of the proportional type, that is to say that it delivers a signal proportional to the oxygen content in contact with a sensitive element of which this probe is made.

The engine also comprises a probe 130 of the binary type, that is to say one that delivers a signal of the on/off type according to the oxygen content in contact with its sensitive element.

What “on/off” means here is that the signal delivered generally adopts two perfectly distinguishable values.

These two values may typically be a value close to zero and a value greater than zero, for example 1, respectively.

It must be noted here that, according to the preferred embodiment of the invention, the signals delivered by the two probes 120 and 130 are signals in the form of voltages.

As also illustrated in FIG. 1, the engine further comprises an electronic control unit 13 (ECU), possibly coupled with a computer, not depicted.

The ECU 13 controls the engine in the way known per se by running an appropriate control method.

The control method in particular includes a step of controlling the emissions of pollutants by the engine.

The catalytic converter in this instance is controlled by the ECU through this method.

The control method further comprises a step of monitoring the operating status of the converter.

This step in particular comprises a diagnostic step that we shall describe later on.

In any event, because this monitoring step is based on the use of the two probes 130 and 140, these probes exchange information with the ECU.

In particular, as indicated by the link 140 in the figure, the proportional-type probe 120 delivers its signal to the ECU in the form of a voltage.

To make this text easier to understand, this signal will hereinafter be denoted Vsp.

Likewise, the link 141 indicates that the on/off-type probe 130 delivers its signal to the ECU in the form of a voltage.

To make this text easier to understand, this signal will hereinafter be termed Vsb.

The way in which the engine works, and more specifically the aforementioned monitoring method, will now be described.

According to one feature of the invention, the monitoring method uses the signals Vsp and Vsb delivered by the proportional-type probe 120 and by the binary probe 130.

In particular, and with reference to FIG. 2, this method involves a step 200 of measuring the oxygen content upstream and downstream of the converter, and a step of acquiring the corresponding signals Vsp and Vsb into the ECU.

In a step 201, the signal Vsp from the probe 120 is converted into a signal Vspb of the on/off type.

Then, in a step 202, a diagnosis is made regarding the operation of the converter, using the signals Vsb and Vspb.

Thus, the diagnosis is made on the basis of two signals of the on/off type in the knowledge that one of them (Vspb) is a signal modeled from a signal (Vsp) of proportional type, and therefore from a signal originating from a probe of proportional type.

One advantage is that converting the signal Vsp into Vspb improves the quality of the diagnosis over the diagnosis that would have been obtained had it been based on a combination of a signal of proportional type and a signal of binary type.

In addition, use of a binary probe downstream of the converter makes it possible to reduce costs while at the same time improving the performance of the monitoring method, as indicated hereinabove.

In a step 203, an action is possibly taken on the basis of the results of the diagnostics.

In particular, it will be seen that, when the diagnostics indicate that the converter has reached too great a degree of aging, and is thus operating defectively, the action may be to activate an indicator on an instrument panel in order to warn a driver to stop at a garage so that repairs can be made.

Of course there are other conceivable actions; we shall examine these later.

One preferred embodiment of the method according to the invention will now be described in greater detail.

In the step 200, the signals Vsb and Vsp are delivered to the ECU.

In the step 201, this ECU converts the signal Vsp into a signal Vspb of the on/off type, when it is determined that the converter is in a purge phase.

If it is not in such a phase, the method is deactivated.

It will be noted that, as an alternative, the condition on the purge phase may be applied before the step 200 is even performed.

According to the preferred embodiment, other tests have to be validated before the conversion is performed.

In particular, it is possible to check that the signal Vsp from the probe 120 of the proportional type is not undershooting to less than, for example, 20% of a final steady-state value.

It is also possible to check that the signal Vap has a transition time, between two calibratable voltage values (for example between 0.98 and 0.955 V), of less than a predetermined amount (for example 0.52 seconds).

When the various conditions are satisfied, the method according to the preferred embodiment of the invention performs the conversion at step 201 using a probe model of the on/off type, one input of which is sensitive to the signal Vsp delivered by the probe 120.

By way of nonlimiting example, the model may be designed such that the converted signal Vspb is a square wave with a predetermined upper and lower value.

The switch from the lower value to the upper value occurs for example when the voltage Vsp drops below a calibratable threshold value.

As a preference, provision will be made for the lower value to be equal to zero volts and for the upper value to be close to said threshold value.

As a further preference, the threshold value and the upper value will be substantially equal to 1 V.

In this regard, reference may be made to the nonlimiting example of FIG. 3, which shows that the signal Vspb switches from about zero volts to about 0.93 V when the signal Vsp crosses, in the downward direction, the threshold value of about 0.95 V.

In an alternative form of step 201 of the method, the model allows the signal Vsp to be converted into a binary signal Vspb using a map.

This map provides information to assist with determining the form of the signal Vspb as a function of the signal Vsp.

And as a preference, it provides the signal Vspb directly as a function of Vsp.

In any event, when the ECU has the two signals Vsb and Vspb of the on/off type available, these two signals are used to make the diagnosis in step 202.

Numerous diagnostic criteria may then be employed.

According to one aspect, the criterion may be based on determining the difference between the aforementioned two signals.

However, more specifically, in the preferred embodiment, an area of this difference is determined by calculating an integral.

By way of nonlimiting example, it is possible to determine the following integral:

$C\mathrm{diag}={\int }_{t1}^{t2}\frac{\mathrm{Dgaz}}{3.6}\left(\mathrm{Vspb}-\mathrm{Vsb}\right)t.$

where Cdiag, t1, t2 and Dgaz respectively denote the diagnostic criterion used, two predetermined moments in time and a rate of flow of exhaust gas through the catalytic converter.

As a preference, t1 and t2 correspond to the start and end of purge, respectively.

By way of illustration, reference may be made to FIG. 3 in which hatching has been used to indicate an area A which corresponds to a result of Cdiag.

Whatever the Cdiag criterion chosen, this criterion is then compared against at least one predetermined threshold value in order to evaluate the converter status.

In the preferred embodiment of the invention, the Cdiag criterion is compared against three threshold values.

One advantage of a comparison such as this is that the diagnosis becomes all the more precise.

In particular, if Cdiag is above a first threshold value S1_diag the converter effectiveness is considered to be good.

If Cdiag lies between S1_diag and a second threshold value S2_diag, then the converter effectiveness is considered to have decreased since its initial use, in other words to exhibit a first degree of aging.

However, this loss in effectiveness is considered to be acceptable against an engine pollution-reduction specification.

Finally, if Cdiag lies between S2_diag and a third threshold value S3_diag, then the converter effectiveness is considered to have decreased significantly since its time of first use.

It is displaying a second degree of aging that is considered to be unacceptable.

It will be noted here that the abovementioned three threshold values can be calibrated during an engine testing phase.

By way of nonlimiting example, FIG. 3 shows two curves Vsb1 and Vsb2 each corresponding to the signal Vsb from the binary probe 130 when the converter is respectively displaying the first and second degrees of aging.

It can in fact be seen that the area between the curves Vsb1 and Vspb is smaller than the area between Vsb2 and Vspb.

A person skilled in the art will thus readily understand that the smaller the temporal shift between the curve Vsb from the binary probe 130 and the curve Vspb from the model, the greater the degree of aging of the converter.

In any event, when the result of the comparison has been obtained, it is possible either to proceed directly to step 203 in order to take appropriate action, or to repeat the abovementioned steps 200 to 202 before proceeding to the step 203, in order to make a complex diagnosis based on a series of diagnostics performed during use of the vehicle.

The complex diagnostics may in particular consist in checking that each diagnosis in the series is leading to the same result.

Of course, other complex diagnostics may be defined.

For example, it is possible for the result of the complex diagnostics to be chosen to be the one obtained by a certain percentage, for example 90%, of the diagnostics in the series.

As indicated above, the actions taken at step 203 depend on the result determined in the previous step 202.

In particular, when a loss of converter effectiveness, therefore converter malfunctioning, is detected, the action may be to indicate to the driver that he should stop at a garage so that repairs can be carried out.

This indication is typically given using an indicator lamp positioned on the instrument panel.

It will be noted that an indication such as this may also be activated once the vehicle has been driven a number of times in succession during which malfunctioning was detected.

The action may also consist in storing in a memory the state of the malfunction together with various data items relating to this state.

For example, the total distance covered by the vehicle or other variables representing engine parameters at the time the failure was detected may be recorded in this memory.

A repair shop may then be able, by consulting the memory, to determine easily what has caused the aging of the catalytic converter.

Furthermore, in the case of complex diagnostics, the data relating to each of the diagnostics in the series may be recorded, even if, ultimately, the result of the complex diagnostics is declaring that the converter is in a state that does not entail replacement.

One advantage is that the repair shop carrying out any repair can, by consulting the memory, gain an impression of the life of the converter.

For example, if it determines that the complex diagnostic has 80% of the diagnostics indicating a failure, as against the 90% threshold value indicated earlier by way of example, it may warn the driver that although troublesome failure of the converter has not yet been indicated, it will soon occur and that as a precautionary measure it would therefore be advisable to have the repair carried out.

In order further to improve the monitoring method of the invention, it is possible, according to another feature of the invention, to couple the steps 200 to 203 with a step 300 in which the operation of the binary probe 130 is checked in order to obtain optimal sensitivity and life performance.

Thus, when the monitoring method or system of the invention is not in action, the probe 130 is controlled to make it operate under conventional conditions.

For example, a heating current is sent through the probe 130 before step 200 is performed, so that the temperature Tip of the oxygen-content-sensitive element of this probe is at approximately 700° C.

By contrast, when the monitoring system is in action, the probe is controlled in such a way that said temperature Tip is at a lower temperature, preferably of between 400 and 500° C.

In this temperature range, the probe is supposed to offer better performance in terms of sensitivity.

Nonetheless, because in this temperature range the probe 130 suffers greater degradation than it does at 700° C., particularly as a result of greater soiling, reducing the temperature Tip of the sensitive element in this way is still only done for isolated lengths of time.

At the very most, this reduction lasts as long as one or more cycles of the monitoring method.

In this way, the life of the binary probe 130 is maintained.

According to one aspect, the change in temperature Tip consists essentially in changing a setpoint value for the heating current that heats the probe 130 from the value 700° C. to a value that lies within said range.

According to another aspect, the change may be effected over several steps.

In the preferred embodiment of the invention, use is made of two steps.

In the first step, the temperature Tip is decreased at a predetermined rate leading to said 400-500° C. range.

This rate may be calibrated during said vehicle testing.

In the second step, the temperature is regulated so that it never leaves said range.

It will be noted here that, whatever the control method used, the value of the temperature Tip in said range can be determined to take account of production spread and/or aging of the probe 130.

Of course, the present invention is not in any way restricted to the embodiment described hereinabove and depicted in the drawings.

In particular, the engine of the invention may employ the monitoring method provided that tests other than those discussed have been satisfied.

For example, the method may be implemented during converter purges and on an additional proviso that the vehicle speed is below a predetermined threshold value, for example 30 km/h.

Claims

1. An engine comprising a catalytic converter, a first probe capable of delivering a first signal (Vsp) proportional to an oxygen content in an exhaust gas and installed upstream of the converter, a second oxygen probe downstream of the converter and capable of supplying a second signal (Vsb), and means for making a diagnosis regarding an operating status of the converter, characterized in that:

the second oxygen probe is of the on/off type, and

the means are designed to convert the first signal (Vsp) into a third signal (Vspb) of the on/off type and to make the diagnosis on the basis of the second and third signals (Vsb, Vspb).

2. The engine as claimed in claim 1, characterized in that the signal conversion means comprise a probe model dependent on the first signal.

3. The engine as claimed in claim 2, characterized in that the model is designed so that the third signal it supplies is in the form of a square wave with predetermined upper and lower values.

4. The engine as claimed in claim 3, characterized in that the model is designed so that the third signal it supplies switches from the upper value to the lower value, and vice versa, when the first signal crosses a first predetermined threshold value, one of the upper and lower values being substantially equal to the first threshold value.

5. The engine as claimed in claim 1, characterized in that the means are also designed to control the temperature (Ttip) of an oxygen-content-sensitive element in the second probe.

6. The engine as claimed in claim 5, characterized in that the control means are designed to decrease the temperature down to a predetermined temperature value ranging between 400 and 500° C., before the means for making the diagnosis make use of the second and third signals.

7. A method of monitoring the operating status of a catalytic converter of an engine, characterized in that it comprises the following steps:

obtaining, from a first probe, a first signal (Vsp) proportional to an oxygen content in a gas and situated upstream of the converter,

obtaining a second signal (Vsb) from a second probe of the on/off type installed downstream of the converter,

converting the first signal into a third signal (Vspb) of the on/off type, and

making a diagnosis regarding the converter operating status on the basis of the second and third signals.

8. The method as claimed in claim 7, characterized in that, in the step of making the diagnosis, a diagnostic criterion (Cdiag) is determined on the basis of an area (A) of a difference between the second and third signals.

9. The method as claimed in claim 8, characterized in that, in the step of making the diagnosis, the diagnostic criterion is compared against three predetermined threshold values (S1diag, S2diag, S3diag) that respectively define three ranges of values corresponding to three different converter operating statuses.

10. The method as claimed in claim 7, characterized in that it comprises a step in which the running of the diagnostic step is governed by whether the first and/or second signal meets at least one predetermined criterion.

11. The method as claimed in claim 8, characterized in that the diagnostic criterion is of the following form: C   diag = ∫ t   1 t   2  Dgaz 3.6  ( Vspb - Vsb )    t where cdiag, t1, t2 and Dgaz respectively denote the diagnostic criterion used, two predetermined moments in time and a rate of flow of exhaust gas through the catalytic converter.