FR3106366A1 - A method of monitoring the operational condition of a selective catalytic reduction system for nitrogen oxides for a vehicle. - Google Patents

Abstract

Method for monitoring an operating state of a selective catalytic reduction system for nitrogen oxides for a vehicle. Method for monitoring an operating state of a system for the selective catalytic reduction of nitrogen oxides for an engine connected to an exhaust line, the system being provided with a catalyst, in particular with a catalytic reduction bread , the method comprising:- a step of acquiring exhaust gas temperature data upstream of the selective catalytic reduction system and exhaust gas temperature data downstream of the selective catalytic reduction system, and- determining a value representative of the heat capacity of the selective catalytic reduction system and/or detection of exothermic and/or endothermic reactions occurring in the selective catalytic reduction system. No figure for the abstract.

Description

Method for monitoring an operating state of a selective catalytic reduction system for nitrogen oxides for a vehicle.

The invention relates to a method for controlling an operating state of a system for the selective catalytic reduction of nitrogen oxides for a vehicle comprising an engine connected to an exhaust line. The invention also relates to a control device implementing such a method. The invention also relates to a motor vehicle comprising such a device. The invention also relates to a computer program implementing such a method. The invention finally relates to a recording medium on which such a program is recorded.

Internal combustion engines produce exhaust gases which contain polluting substances such as unburned hydrocarbons (HC), carbon monoxide (CO) or nitrogen oxides (NOx). It is necessary to treat these polluting substances before venting them into the atmosphere. Motor vehicles are equipped for this purpose with exhaust gas post-treatment systems.

FIG. 1 presents an example of an engine having a gas post-treatment system in its exhaust line, for example a diesel engine. Typically, an exhaust gas post-treatment system comprises an oxidation catalyst 101, a selective catalytic reduction system 102 and a particulate filter 103. The oxidation catalyst 101 or DOC (for "Diesel Oxidation Catalyst" in English) handles the treatment of HC and CO. The selective catalytic reduction system 102 of nitrogen oxides or SCR (for “Selective Catalytic Reduction” in English) ensures the treatment of the NOx contained in the exhaust gases and makes it possible to reduce the NOx thanks to the injection of a reducing agent and a catalytic bread. The reducer is injected into the gas exhaust line via an injector. Conventionally, the reducing agent is ammonia (NH ₃ ). Ammonia can be injected using another chemical species such as urea, or AdBlue® which is composed of urea diluted to 30% in water. The particle filter 103 or FAP 103 ensures the treatment of the soot particles contained in the exhaust gases, by retaining these same particles.

During a malfunction of the catalytic reduction system 102, NOx emissions increase. Such an increase in NOx emissions can cause a regulatory emission threshold to be exceeded, such as an on-board diagnosis or OBD (for “On Board Diagnosis”) threshold. In this case, the regulations, in particular the European standard in force "EURO 6" requires that the malfunction of the catalytic reduction system 102 be detected by means of a control of the operating state of the catalytic reduction system 102.

Under these conditions, to control the correct operation of this catalytic reduction system 102, a control method is known in the state of the art providing for the use of temperature and NOx concentration sensors upstream and downstream of the system. , the temperature and the concentrations being subsequently compared with threshold values relating to standards required by OBD diagnostics. Such a control method requires the use of additional and non-selective nitrogen oxide sensors, more precisely sensitive not only to NOx, but also to NH3, which makes it difficult and expensive to implement.

Other control methods described in the patent documents FR2833994, FR2993315 and FR3005104 are known, which provide for the verification of the proper functioning of a catalyst of an exhaust line based on the increase in temperature generated by the activity of a catalytic converter diagnosed following forced excitation of the catalytic converter by a controlled increase in the concentration of reducing agents upstream of the catalytic converter generated by an injection of fuel upstream of the catalytic converter. By causing a late injection of fuel into the cylinders of the engine, the temperature at the outlet of the catalyst in normal operation increases, whereas if the catalyst fails, the temperature at the outlet does not increase.

However, such control methods are intrusive with respect to the normal mode of operation of the engine and also generate additional fuel consumption.

Application FR3053732 describes a method for monitoring an operating state of a system for the selective catalytic reduction of nitrogen oxides. This method uses a diagnostic criterion which is based on a temporal integration of the difference between:
- the temperature difference between upstream and downstream of the catalyst, and
- the absolute value of this temperature difference.

This process still lacks precision.

The object of the invention is to provide a control device and method remedying the above drawbacks and improving the control devices and methods known from the prior art. In particular, the invention makes it possible to produce a simple control device and method which make it possible to detect not only the absence or the presence of a catalyst for the selective reduction of nitrogen oxides, but also damage to such a catalyst. .

According to the invention, a method makes it possible to control an operating state of a system for the selective catalytic reduction of nitrogen oxides for an engine connected to an exhaust line, the system being provided with a catalyst, in particular with a catalytic reduction bread. The process includes:
- a step for acquiring data for the temperature of the exhaust gases Tam upstream of the selective catalytic reduction system and for the temperature of the exhaust gases Tav downstream of the selective catalytic reduction system, and
- a determination of a value representative of the heat capacity of the selective catalytic reduction system and/or a detection of exothermic and/or endothermic reactions occurring in the selective catalytic reduction system.

In the determination, a first criterion C1 can be calculated,
in particular C1=max(Tam-Tav)-min(Tam-Tav),
during a first period.

The first period can be initiated as soon as the engine is started and end when a gradient of said upstream temperature greater than a first threshold is detected upstream of the selective catalytic reduction system and when a first time delay initiated when the engine is started has expired .

The first criterion C1 can be validated if, during the first period, the temperature of the exhaust gases Tam upstream of the selective catalytic reduction system has varied more than a second threshold in a duration less than a third threshold.

The selective catalytic reduction system can be considered as present if the value of the first criterion is greater than a fourth threshold and/or the selective catalytic reduction system can be considered as absent if the value of the first criterion is less than a fifth threshold.

In detection, we can calculate a second criterion C2,
in particular C2=max(Tam-Tav)-min(Tam-Tav),
for a second period.

The second period can be initiated as soon as the engine is started and end when a sixth temperature threshold is reached downstream of the selective catalytic reduction system and when a second time delay initiated when the engine is started has expired.

We can validate the second criterion C2 if, during the second period, we verify that:
- the temperature of the catalyst for the selective reduction of nitrogen oxides is sufficiently low, in particular by checking that a sufficient period of time has elapsed since the last operation of the engine, and
- a limited amount of water is stored in the selective catalytic reduction system at start-up.

The selective catalytic reduction system can be considered as functional if the value of the second criterion is greater than a seventh threshold and/or the selective catalytic reduction system can be considered as non-functional if the value of the second criterion is less than an eighth threshold .

According to the invention, a device makes it possible to control the operating state of a system for the selective catalytic reduction of nitrogen oxides for an engine connected to an exhaust line. The device comprises hardware and/or software elements implementing the method defined above, in particular hardware and/or software elements designed to implement the method the method defined above.

According to the invention, a device makes it possible to control the operating state of a system for the selective catalytic reduction of nitrogen oxides for an engine connected to an exhaust line. The device comprises means for implementing the method defined previously.

According to the invention, a power unit or a motor vehicle comprises a device defined previously.

According to the invention, a computer program product comprises program code instructions recorded on a computer-readable medium to implement the steps of the method defined above when said program runs on a computer.

According to the invention, a computer program product downloadable from a communication network and/or recorded on a data medium readable by a computer and/or executable by a computer, comprises instructions which, when the program is executed by the computer, lead it to implement the method defined above.

According to the invention, a data recording medium, readable by a computer, on which a computer program is recorded comprises program code instructions for implementing the method defined above.

According to the invention, a computer-readable recording medium comprises instructions which, when executed by a computer, lead the latter to implement the method defined previously.

The invention also relates to a signal from a data carrier, carrying the computer program product defined above.

The appended figures represent, by way of example, an embodiment of a vehicle and an embodiment of a control method according to the invention.

FIG. 1 represents a view of an exhaust line of a vehicle comprising an exhaust gas post-treatment system of the state of the prior art.

FIG. 2 represents a schematic view of a vehicle comprising a device for monitoring an operating state of a system for the selective catalytic reduction of nitrogen oxides according to one embodiment of the invention.

FIG. 3 represents a schematic view of the selective catalytic reduction system included in an exhaust line of a vehicle according to the embodiment of the invention.

FIG. 4 is a graph of the temporal evolutions of the temperatures upstream and downstream of a catalyst in good operating condition.

FIG. 5 is a graph of the temporal evolutions of the temperatures upstream and downstream of a failed catalyst.

FIG. 6 is a flowchart of an embodiment of a control method according to the invention.

An embodiment of a vehicle 200, in particular a motor vehicle, is described below with reference to FIGS. 2 and 3. The vehicle comprises an engine or powertrain unit 150 including a heat engine 6, for example of the diesel type, and an exhaust line 7. The engine 6 comprises cylinders, a fresh air intake manifold or distributor and an exhaust manifold. The cylinders are supplied with air via the distributor, itself supplied by a line provided with an air filter. During the operating phase of the engine 6, exhaust gases 14 recovered by the exhaust manifold of the engine 6, come from the combustion and are evacuated to the outside, passing through this exhaust line 7. This engine 6 is also equipped with an exhaust gas post-treatment system 14 defined in the exhaust line 7. This gas post-treatment system 14 comprises in particular the selective catalytic reduction system 2 provided with a catalyst 9 such as a catalytic reduction bread 9. It will be noted that the system 2 can comprise several catalytic breads 9 and that under these conditions the invention makes it possible to control the operating state of each catalytic bread 9 separately.

In the description, the terms upstream and downstream are defined with respect to the normal direction of flow of the exhaust gases, namely from the engine to the outside atmosphere through an exhaust line 7 of a vehicle.

The vehicle 200, in particular the powertrain 150, further comprises an embodiment of a control device 1 of an operating state of a selective catalytic reduction system 2 of nitrogen oxides for this vehicle.

The control device 1 comprises an electronic control unit 3 better known by the acronym ECU (English acronym for: Electronic Control Unit) and which can be an on-board computer, as well as a verification module 4 of the operation of the engine 6 and a control module 5 of system operating parameters 2.

The control unit 3 is connected to these modules in particular via a CAN network (acronym for “Controller Area Network”). Alternatively, these modules 4, 5 can also be included in this control unit 3. This control unit 3 comprises hardware and software resources and more precisely at least one processor cooperating with memory elements. These hardware and software resources are capable of executing instructions for implementing motor control software.

The verification module 4 of the operation of the engine 6 makes it possible to verify in particular the parameters corresponding to an operating point of the engine 6, such as the driver's torque request, the speed of the engine 6, etc. In particular, this verification module 4 also makes it possible to detect an operating phase of the engine 6, in particular a phase during which a first component 15a of an exothermic reaction capable of being carried out within the system 2, is present in the exhaust gases 14.

The control module 5 of the operating parameters of the system 2 notably comprises temperature sensors 8a, 8b making it possible to measure the temperatures Tam and Tav of the exhaust gases 14 respectively upstream and downstream of the system 2 and, in particular, in upstream and downstream of the catalyst 9.

Referring to Figure 3, in this powertrain 150, the selective catalytic reduction system 2 of nitrogen oxides better known by the acronym SCR which means in English "Selective Catalytic Reduction" is intended for the pollution control of exhaust gases. exhaust 14 of engine 6. As mentioned above, this system 2 comprises a catalyst 9 also called a catalytic converter, for example in this embodiment a catalytic bread. The system 2 ensures the treatment of nitrogen oxides (essentially nitrogen monoxide NO and nitrogen dioxide NO ₂ ) generally denoted NOx thereafter, which are contained in these exhaust gases 14 and then makes it possible to reduce the NOx thanks to the injection of a reducing agent 16, or reducing agent, and to the catalyst 9. This system 2 comprises a storage module 18 comprising the reducing agent 16 which is connected to the exhaust line 7 upstream of a module 19 for mixing this exhaust gas 14 with this reducing agent 16. The mixing module 19 is arranged upstream of the catalyst 9 of this system 2.

In FIGS. 4 and 5, catalyst 9 comprises a substrate 10, which may be a ceramic monolith such as cordierite ceramic, provided with channels 12 whose internal walls are coated with an active catalytic phase 11 which can be designate by the Anglo-Saxon term "washcoat". According to the French terminology, we will speak of active phase 11 or of catalytic coating 11 as an equivalent of the term “washcoat”. This catalytic coating 11 is a porous intermediate layer included in each channel 12 of the substrate 10. This layer has a very large specific surface 17 of contact with the exhaust gases 14 and participates in the storage of a reduction agent 16 in sites assets 13 defined at the level of this surface 17 and at the level of which NOx reduction reactions occur.

As mentioned previously, the reduction agent 16 which is injected into the exhaust line 7 by the storage module 18 is mixed with the exhaust gases 14 in the mixing module 19. This reduction agent 16 can be for example hydrogen, hydrocarbons or even ethanol. However, in this embodiment, the reducing agent 16 is preferably an ammoniacal compound which can be either in a liquid form such as AdBlue® which is a solution which can comprise approximately one third of urea and two thirds water, or either in a solid form such as gaseous ammonia stored in the form of solid crystals.

Thus, this reducing agent 16 such as ammonia can then be stored in the active sites 13 of this catalytic coating 11 to reduce the NOx included in the exhaust gases 14 which pass through the channels 12 of the substrate 10 of this catalyst. 9 according to the following main reduction reactions:
4NO + _4NH3 + _O2 → _4N2 + _6H2O
2NO2 + _4NH3 + _O2 → _3N2 + _6H2O
NO2 + NO + _2NH3 → _2N2 + _3H2O

This catalytic coating 11 may consist of a second component 15b which may be a microporous mineral belonging to the group of silicates such as zeolite. This second component 15b is able to cause, with the first component 15a:
- a first exothermic reaction at the origin of a phenomenon called "exotherm" corresponding to a heating of the catalyst 9 due to the release of energy produced by this exothermic reaction in the catalyst 9 and to the adsorption of water gas on the zeolite, when the temperature of the catalyst for the selective reduction of nitrogen oxides is sufficiently low, and
- a second endothermic reaction at the origin of a so-called “endotherm” phenomenon corresponding to a cooling of the catalyst 9 due to the desorption of water from the zeolite.

This constitutes a particular thermal effect, which manifests itself when a catalyst is very degraded (but nevertheless present), and also in the absence of a catalyst, when the temperature of the catalyst for the selective reduction of nitrogen oxides is sufficiently low.

It has been noted that this exothermic phenomenon and/or this endothermic phenomenon is non-existent for a system 2 having a degraded operating state that does not allow treatment of the polluting substances included in the exhaust gases 14 and resulting in particular from a faulty state of the catalyst 9. Indeed, such a state of the catalyst 9 reflects wear and/or aging of the latter, in particular of the catalytic coating 11 consisting of this second component 15b, directly inducing a reduction or even disappearance of the number active sites 13 at which NOx reduction reactions occur. Indeed, over time, the exhaust gases 14 crumble this coating 11 and in particular the active sites 13 with which it is provided. This degradation is all the greater when the flow rate of the exhaust gases 14 is high and when the latter are at very high temperatures. This slow and continuous erosion reduces the active sites 13 in number but also in effectiveness. Thus, each active site 13 no longer captures a reduction agent 16 with as much efficiency as a catalyst 9 exhibiting a functional state.

Furthermore, in the absence of a catalyst on the exhaust line, but less so in the case of a catalyst present but degraded, the temperature difference between the inlet and the outlet of the catalyst is very small compared to this same temperature difference in the presence of a catalyst. This is especially true when the temperature upstream of the catalyst increases or decreases rapidly. In the first case (i.e. increase in the temperature of the gases upstream of the catalyst), the thermal inertia of the catalyst absorbs part of the heat from the gases. A significant reduction in the temperature of the gases between the inlet and the outlet is thus obtained. In the second case (i.e. reduction in the temperature of the gases upstream of the catalyst), part of the heat stored in the catalyst is transferred to the gases. A significant increase in the temperature of the gases between the inlet and the outlet is thus obtained.

This constitutes another thermal effect.

The invention takes advantage of the two thermal effects to diagnose, by two criteria calculated under particular conditions, whether the catalyst is present or absent, and if it is present, whether it is degraded.

The device 1 makes it possible to implement an embodiment of a method for controlling the operating state of the system 2 for the selective catalytic reduction of nitrogen oxides, or a method for diagnosing the system for selective catalytic reduction 2 nitrogen oxides.

Such a method allows in particular that information on a malfunction of the system 2 is given to a user of a vehicle during a malfunction of the system 2.

Such an embodiment of such a method is described below with reference to Figure 6.

The method comprises setting up a control of an operating state of the system 2 based on the measurement of the temperature Tam of the exhaust gases 14 upstream of the system 2 for the selective catalytic reduction of nitrogen oxides, in particular upstream of the catalyst 9, and on the measurement of the temperature Tav of the exhaust gases 14 downstream of the system 2 for the selective catalytic reduction of nitrogen oxides, in particular downstream of the catalyst 9. This control is carried out continuously during phases of operation of the engine 6 which present favorable conditions for carrying out such a check, and which are detailed below.

The method allows reliable and efficient monitoring of the operating status of the selective catalytic reduction system 2 of nitrogen oxides of the vehicle.

To do this, this method comprises a step 20 of detecting an operating phase of the motor 6. The detection of this operating phase authorizes triggering of the control of the operating state of the system 2. Such a step 20 is implemented implemented by the device, in particular by the control unit 3 and in particular by the verification module 4 of the operation of the engine 6. During this step 20, the verification module 4 verifies the operating parameters of the engine 6 and is thus capable of detecting different phases of operation of the latter. Such a verification module 4 is configured to transmit the various operating phases detected to the control unit 3. This control unit 3 is then capable of identifying among these operating phases of the engine 6 received, a phase during which the first component 15a is included in the exhaust gases 14 which will pass through the system 2 and in particular the catalyst 9. This first component 15a included in the exhaust gases 14 participates with the second component 15b included in the catalyst 9 in reactions exothermic and endothermic capable of being produced within the latter, in particular within the catalytic coating 11.

In this context, the first component 15a can be water. Such a component 15a is for example present in the exhaust gases 14, due to the humidity of the air and the combustion in the cylinders of the engine. The fraction of water which is in gaseous form is capable of being adsorbed on the zeolite if the temperature of the catalyst for the selective reduction of nitrogen oxides is sufficiently low. Advantageously, it is checked that this condition is fulfilled during a detected phase which may be a cold start phase of the engine 6. This diagnostic period P may be included, when the operating phase of the engine 6 detected is a phase of cold start, between the start and the expiry of a given time delay. This diagnostic period P is a function of the detected operating phase of the engine 6, of the characteristics of the engine 6, of the characteristics of the catalyst 9, and/or of the environmental conditions in which the engine 6 and the catalyst 9 are included. variations are possible. For example, without limitation, the engine coolant temperature can be checked to be below a threshold, which signals that the engine and the selective nitrogen oxide reduction catalyst have cooled.

In a first step 21, as soon as the engine 6 is started, the device 1 continuously acquires exhaust gas temperature data Tam upstream of the selective catalytic reduction system 2 and exhaust gas temperature data Tav downstream of the selective catalytic reduction system 2. These data are for example stored in a memory. These data are for example acquired at each instant determined by a predetermined and/or constant acquisition or sampling frequency. To do this, the temperature sensors 8a, 8b of the exhaust gases 14 arranged upstream and downstream of the catalyst 9 transmit signals relating to measurements of these temperatures upstream and downstream Tam, Tav to the control unit 3, in particular to the module 5 for controlling the operation of the system 2. In a variant, only the temperature sensor 8b arranged downstream of the catalyst 9 transmits signals relating to measurements of this downstream temperature Tav, the control unit 3 then being capable of determining the temperature Tam upstream of the catalyst 9 from a thermal model of the exhaust gases 14 adapted to determine the temperature Tam of the exhaust gases 14 upstream of the catalyst 9 as a function of operating parameters of the engine 6 and/or catalyst 9. Subsequently, these temperatures Tam, Tav upstream and downstream of catalyst 9 are archived in the memory elements of control unit 3.

The upstream and downstream temperatures Tam, Tav determined and archived in the control unit 3, are illustrated in Figures 4 and 5. For a better understanding of the invention are represented:
- in Figure 4, the temperatures Tam, Tav upstream and downstream of the catalyst 9, determined for this catalyst 9 when it is new and therefore has a correct state of operation, and
- in Figure 5, the temperatures Tam, Tav upstream and downstream of the catalyst 9, determined for this catalyst 9 when it is used and has a faulty state.

In Figure 4, it can be seen that the downstream temperature Tav is significantly higher than the upstream temperature Tam over the period extending between 60 s and 120 s. This is the consequence of an exothermic reaction occurring in the catalyst. Then, it can be seen that the downstream temperature Tav is significantly lower than the upstream temperature Tam over the period extending between 150 s and 370 s. This is the consequence of an endothermic reaction occurring in the catalyst. The sequence of these two successive reactions reflects a state of proper operation of system 2 for the selective catalytic reduction of nitrogen oxides.

In Figure 5, it can be seen that the downstream temperature Tav is significantly lower than the upstream temperature Tam over the period extending between 100 s and 250 s. This is the consequence of an endothermic reaction occurring in the catalyst. On the other hand, there is no phase where the downstream temperature Tav is substantially higher than the upstream temperature Tam. There is therefore no exothermic reaction occurring in the catalyst. This reflects a state of dysfunction of system 2 for the selective catalytic reduction of nitrogen oxides.

In a second step 22, a criterion C is calculated. This criterion is for example defined by the formula C=max(Tam-Tav)-min(Tam-Tav), in which max(Tam-Tav) denotes the maximum value over the acquisition period of the difference between the upstream temperature Tam and the downstream temperature Tav, and min(Tam-Tav) denotes the minimum value over the acquisition period of the difference between the upstream temperature Tam and the downstream temperature Tav.

In a first phase 80, the device determines the heat capacity or a value representative of the heat capacity (thermal inertia) of the selective catalytic reduction system 2.

To do this, in a third step 23, the device calculates a first criterion C1, during a first period. This first criterion C1 is the value of criterion C at the end of the first diagnostic period. The first period is preferably initiated as soon as the engine is started. The first period preferably ends when a temperature gradient greater than a first threshold MAX1, MIN1 is detected upstream of the selective catalytic reduction system 2 and when a first time delay t1 initiated on starting the engine has expired. By temperature gradient is meant here the absolute value of the derivative with respect to time of the temperature upstream of the selective catalytic reduction system 2. A threshold MAX1 can be used for the positive variations in temperature and a threshold MIN1 for the negative temperature variations.

In a fourth step 24, the device validates the calculation of the first criterion C1 if, during the first period, the temperature of the exhaust gases Tam upstream of the system 2 for the selective catalytic reduction of nitrogen oxides has varied more than one second threshold S1 in a duration less than a third threshold S2.

In a fifth step 25, the device compares the value of the criterion C1 with a fourth threshold C12 and/or with a fifth threshold C11 in order to obtain the heat capacity or an image of the heat capacity (or thermal inertia), this is that is to say from the heat capacity, of the selective catalytic reduction system for nitrogen oxides 2 and to deduce therefrom whether the catalyst is present or absent, but it does not make it possible to detect a functional or non-functional catalyst. For example, the selective catalytic reduction system 2 of nitrogen oxides is considered absent if the value of the first criterion is lower than the fifth threshold C11. For example again, the selective catalytic reduction system 2 is considered to be present if the value of the first criterion is greater than the fourth threshold C12.

In a second phase 90, the device detects the existence or the absence of exothermic and/or endothermic reactions occurring in the selective catalytic reduction system 2. This second effect makes it possible to detect not only the presence or the absence of the catalyst , but also to diagnose a faulty catalytic converter.

To do this, in a sixth step 26, the device calculates a second criterion C2, during a second diagnostic period. This second criterion C2 is the value of criterion C at the end of the second period. The second period is preferably initiated as soon as the engine is started. The second period preferably ends when:
- a temperature threshold T8 is reached by the temperature Tav downstream of the selective catalytic reduction system 2, and
- a second time delay t2 initiated on starting the motor has expired.

In a seventh step 27, the device validates the calculation of the second criterion C2 if:
- a first following condition is verified: a sufficient time has elapsed since the last operation of the engine, and
- a second following condition is verified: a limited quantity of water is stored in the selective catalytic reduction system 2 at the time of starting.

Concerning the first condition, it appears that the temperature of the catalyst at the start of the engine is a parameter of first order influence. If the temperature is above a threshold, water adsorption may not occur or heat release may not be sufficient to significantly increase the gas temperature between the catalyst inlet and outlet. Also, ambient conditions could affect the exothermic/endothermal effect. This first condition can be expressed as follows:
[[[|Teau_engine–Tamb|init < ∆T1 OR inactive > tthreshold]
AND (T2 < Teau_motor init < T3)
AND (T4 < Tamb at time of decision < T5)]
OR [T6 < Tam init < T7]]
with
Teau_engine: engine coolant temperature,
Teau_engine init: the temperature of the engine coolant at the start of the second period,
Tamb at the time of the decision: the ambient temperature of the atmosphere surrounding the engine or the vehicle at the time of validation of the calculation of criterion C2,
Tamb init: the ambient temperature of the atmosphere surrounding the engine or vehicle at the start of the second period,
Tam init: the temperature Tam upstream of selective catalytic reduction system 2 at the start of the second period
∆T1, T2, T3, T4, T5, T6 and T7: temperature thresholds,
inactive: the duration of engine inactivity before the start of the second period,
tthreshold: a duration threshold.

Regarding the second condition, it appears that in order to take advantage of the exothermic and endothermic effects in the diagnosis of the selective catalytic reduction system 2, it is important to check that during the preceding journey, all (or at least a significant part of) the water adsorbed by the catalyst (for example in the zeolite) has been desorbed during the previous phase of engine operation. If this is not the case, the catalyst can no longer adsorb a significant quantity of gaseous water during the next operating phase. The catalyst can therefore, in this next phase, only desorb the water already stored. In this case, only the endothermic effect could be observed. An empirical observation of the phenomenon over more than 90 operating phases led to the identification, for example, of a simple criterion making it possible to verify this second condition. This criterion is as follows:
Tav max prev > T8 AND ttotal pre > D1
with
Tav max prev: the maximum Tav temperature downstream of the selective catalytic reduction system 2 during the previous operating phase,
ttotal pre: the duration of motor activity during the previous operating phase,
D1: a duration threshold,
T8: a temperature threshold.

A third condition for validation of the second criterion C2 could for example also be used. This third condition can be a verification that the quantity of fuel used by the engine is sufficient to maximize the quantity of adsorbed water. Indeed, the humidity of the air and the combustion of the fuel bring water into the exhaust line and allow the adsorption of gaseous water and the desorption.

In an eighth step 28, if the calculation of the second criterion C2 is validated in step 27, the device compares the value of the second criterion C2 with a threshold C22 and/or with a threshold C21 in order to obtain endothermic and exothermic reactions occurring in the selective catalytic reduction system 2 and to deduce therefrom whether the catalyst is present or absent or not functional. For example, the selective catalytic reduction system 2 is considered as absent and/or non-functional if the value of the second criterion is lower than the threshold C21. For example again, the selective catalytic reduction system 2 is considered to be present and/or functional if the value of the second criterion is greater than the threshold C22.

The first and second phases can be implemented independently of each other. In particular, in one embodiment of the method, only the first phase can be implemented or only the second phase can be implemented.

However, the first and second phases are advantageously implemented in a preferred embodiment. In these phases, the calculations of the first and second criteria can be executed one after the other or simultaneously or quasi-simultaneously.

Thresholds C11 and C21 may be identical or substantially identical. Alternatively, the thresholds C11 and C21 can be different.

Thresholds C12 and C22 may be identical or substantially identical. Alternatively, the thresholds C12 and C22 can be different.

Preferably, the two criteria C1 and C2 are used to detect the absence of the catalyst and only the criterion C2 to detect a significant loss of efficiency of the catalyst.

The method may further comprise a step 29 of transmitting an alert message, in particular a visual and/or audible alert message to the user of the vehicle when the diagnostic criterion C1 and/or C2 is lower than the threshold C11 and/or C21, in order to warn the user of a malfunction of the catalyst 9. This is determined during a step 30 or 31, following which one passes to step 29 or one loops on the step 20.

In addition, it will be noted that the invention also relates to control software comprising program code instructions for the execution of the steps of this method when said software is executed by the control unit 3, the verification module 4 engine operation and/or the control module 5 system operating parameters 2.

Thus, the invention allows a realization of a control of the operating state of the system 2, in particular of the catalyst 9, which is efficient and robust while exploiting data that is very simple to obtain and very simple to process.

The process allows the use of inaccurate temperature probes. Indeed, even if there is a dispersion of the probes (which is constant or substantially constant over the measurement range), the diagnostic criterion C1 and/or C2 is not affected or is only slightly affected.

Claims (11)

Method for monitoring an operating state of a selective catalytic reduction system (2) of nitrogen oxides for an engine (6) connected to an exhaust line (7), the system (2) being provided with a catalyst (9), in particular a catalytic reduction bread, the method comprising:
- a step for acquiring exhaust gas temperature data (Tam) upstream of the selective catalytic reduction system (2) and exhaust gas temperature data (Tav) downstream of the selective catalytic reduction system (2 ), And
- a determination of a value representative of the heat capacity of the selective catalytic reduction system (2) and/or a detection of exothermic and/or endothermic reactions occurring in the selective catalytic reduction system (2). Method according to Claim 1, characterized in that, in the determination, a first criterion C1 is calculated,
in particular C1=max(Tam-Tav)-min(Tam-Tav),
during a first period. Method according to the preceding claim, characterized in that the first period is initiated as soon as the engine is started and ends when a gradient of said upstream temperature greater than a first threshold (MAX1, MIN1) is detected upstream of the selective catalytic reduction system (2) and when a first time delay (t1) initiated at engine starting has expired. Method according to Claim 2 or 3, characterized in that the first criterion C1 is validated if, during the first period, the temperature of the exhaust gases (Tam) upstream of the selective catalytic reduction system (2) has varied more than a second threshold (S1) in a duration less than a third threshold (S2). Method according to one of Claims 2 to 4, characterized in that the selective catalytic reduction system (2) is considered to be present if the value of the first criterion is greater than a fourth threshold (C12) and/or the selective catalytic reduction system (2) as absent if the value of the first criterion is less than a fifth threshold (C11). Method according to one of the preceding claims, characterized in that, in the detection, a second criterion C2 is calculated,
in particular C2=max(Tam-Tav)-min(Tam-Tav),
for a second period. Method according to the preceding claim, characterized in that the second period is initiated as soon as the engine is started and ends when a sixth temperature threshold (T8) is reached downstream of the selective catalytic reduction system (2) and when a second time delay (t2) initiated at motor starting has expired. Method according to Claim 6 or 7, characterized in that the second criterion C2 is validated if, during the second period, it is verified that:
- the temperature of the catalyst for the selective reduction of nitrogen oxides is sufficiently low, in particular by checking that a sufficient period of time has elapsed since the last operation of the engine, and
- a limited quantity of water is stored in the selective catalytic reduction system (2) at start-up. Method according to one of Claims 6 to 8, characterized in that the selective catalytic reduction system (2) is considered to be functional if the value of the second criterion is greater than a seventh threshold (C22) and/or the selective catalytic reduction system (2) as non-functional if the value of the second criterion is lower than an eighth threshold (C21). Device (1) for monitoring an operating state of a system for the selective catalytic reduction (2) of nitrogen oxides for an engine (6) connected to an exhaust line (7), comprising elements (3 , 5, 8a, 8b) hardware and/or software implementing the method according to one of the preceding claims, in particular hardware elements (3, 5, 8a, 8b) and/or software designed to implement the method according to one of the preceding claims. Motor unit (150) or motor vehicle (200) comprising a device according to the preceding claim.