FR2995008A1 - Method for controlling amount of reductant of system for selective catalytic reduction of nitrogen oxides for internal combustion engine of car, involves performing step of resetting setpoint when difference of quantities exceeds limit - Google Patents

Abstract

The method involves governing an operation of an injector by a control law. A control setpoint is controlled in a control loop (B2) based on a difference between a quantity of reductant to be injected (Qdepoll) and a measured quantity of reductant (Qmeasure). A step of resetting the control setpoint is performed when a difference of an expected quantity and a measured quantity of nitrogen oxides (NOxcible) exceeds a threshold limit, and the difference between the quantity of reductant to be injected and the measured quantity of injected reductant exceeds another threshold limit.

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to the field of selective catalytic reduction of nitrogen oxides. The invention relates more particularly to controlling the amount of reducing agent of a selective catalytic reduction system for nitrogen oxides. BACKGROUND For many years, automotive engine manufacturers have made great efforts to reduce the emission into the atmosphere of environmentally harmful chemical compounds produced by combustion engines during fuel combustion. .

Among these compounds, carbon dioxide 002 as well as nitrogen oxides, mainly NO monoxide and nitrogen dioxide NO2, are found together with the abbreviation NO. It should be noted that the production of nitrogen oxides is greater for diesel engines than for gasoline engines because of their higher combustion temperature.

To limit the emission of nitrogen oxides in the atmosphere, a solution currently used is to place on the exhaust system of the vehicle a NO treatment system, called SCR system (corresponding to the acronym "Selective Catalytic Reduction "), whose function is to chemically reduce the nitrogen oxides to di-nitrogen molecules and water vapor by means of a reducing agent. In practice, the reducing agent is introduced into the exhaust line upstream of a specific catalyst SCR in which the reduction reaction occurs. It is known to use as reducing agent urea in aqueous solution.

The dosage of the amount of reducing agent to be injected into the exhaust gas is crucial. Indeed, an excess dosage of reducing agent in the exhaust gas leads to a discharge of ammonia into the atmosphere while a defect ratio of reducing agent in the exhaust gas leads to a lower reduction nitrogen oxides.

Conventionally, as in the case of the documents EP2226480A1 or US20090301066A1, the dosage of the injection of the reducing agent is regulated by means of a control method based on the determination of a difference between the actual emission and the emission nitrogen oxides or between the actual amount and the amount of reducing agent setpoint, determining a correction of the amount of reducing agent as a function of this difference, and an injection of the amount of corrected reducing agent.

Such a method makes it possible to correct an injection drift due to an abnormal operation of the SCR system, but if this abnormal operation is due to a failure of an organ of the SCR system, such a method does not make it possible to identify the origin of the failure. .

An object of the present invention is to provide a method which makes it possible to overcome the drawbacks of the prior art, in particular to assist in the identification of the failing organ. The invention thus relates to a method for controlling the amount of reducing agent of a selective catalytic reduction system for nitrogen oxides comprising, a reducing injector, a catalyst, means for measuring the quantity of injected reducing agent, process wherein: In a first control loop the amount of reductant to be injected is controlled in a closed loop as a function of a difference between an expected quantity of nitrogen oxides and a measured quantity of nitrogen oxides downstream. of the catalyst, characterized in that, the operation of the injector being governed by a control law defining a quantity of reducer to be injected according to a control setpoint, -On closed loop control, in a second control loop , the control setpoint of the injector, as a function of a difference between the quantity of reducer to be injected and a quantity of gearbox measured, and in that when the difference between the quantity has been nitrogen oxide tension and the measured quantity of nitrogen oxides exceeds a first allowable threshold and that the difference between the amount of reductant to be injected and the amount of injected reducer measured exceeds a second admissible threshold is made of first a step of resetting the piloting law of the injector. Preferably, during the step of resetting the piloting law of the injector, the closed-loop regulation of the first regulation loop is inhibited.

More preferably, the step of resetting the piloting law of the injector comprises the search for a corrective coefficient which makes it possible to converge the difference between the quantity of reducer to be injected and the quantity of injected reducer measured towards the second qualifying threshold. Preferably, if the resetting step made it possible to converge towards the second admissible threshold, but the difference between the expected quantity of nitrogen oxides and the measured quantity of nitrogen oxides always exceeds the first admissible threshold, the method then comprises a step of resetting the catalyst conversion performance due to the aging state of the latter.

More preferably, the step of resetting the performance of conversion of the nitrogen oxides of the catalyst due to the state of aging comprises the search for an optimum curve of suitable conversion of the nitrogen oxides, representative of a state determined aging of the catalyst, which makes it possible to converge the difference between the expected quantity of nitrogen oxides and the measured quantity of nitrogen oxides towards the first admissible threshold. In a variant where the quality of the gearbox may be a source of failure, the method comprises a step of verifying the compliance of a quality criterion of the gearbox between the step of resetting the injector law and the step of the conversion performance due to the aging state of the catalyst. Preferably, if the quality criterion of the reducer is considered to be compliant, the step of resetting the conversion performance due to the aging state of the catalyst is then carried out.

More preferably, if the step of resetting the conversion performance of the nitrogen oxides of the catalyst does not converge towards the first admissible threshold, it is concluded that the catalyst has failed or has failed to determine the oxides of the catalyst. nitrogen upstream of the catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages will appear on reading the following description of a particular embodiment, not limiting of the invention, with reference to the figures in which: - Figure 1 is a diagram a selective catalytic reduction system installed in a heat engine. FIG. 2 is a block diagram of the control of the selective catalytic reduction system. FIG. 3 is a graph showing schematically the evolution of the performance of a catalyst and the adaptation of the catalytic selective catalytic reduction loading setpoint.

DETAILED DESCRIPTION FIG. 1 shows a system 1 for treating nitrogen oxides for eliminating the nitrogen oxides emitted for example by a motor vehicle engine, by chemically reacting them with a reducing agent such as ammonia. or a precursor of the reducing agent such as urea in aqueous solution which upon decomposition releases ammonia. Such a system 1 is also classically designated SCR system according to the English acronym for "selective catalytic reduction". The chemical reactions between the nitrogen oxides and the ammonia take place in a suitable catalyst 2 called selective catalytic reduction also designated SCR catalyst. In the remainder of the present document, the term "reducing agent" will be used indifferently to designate the reducing agent or a precursor of the reducing agent. At the inlet of this catalyst SCR 2, there are exhaust gases present in the exhaust line 3 of the engine (not shown), containing in particular nitrogen oxides.

In order for the selective catalytic reduction reaction to take place in the SCR 2 catalyst, it is necessary to introduce the reducing agent 4 into the SCR 2 catalyst. This reducing agent 4 is stored in a specific reservoir. It is sent, via a reducing feed system 6, to a specific injector 7. This injector 7 then injects the reducer 4 into the exhaust line 3, upstream of the catalyst SCR 2. The upstream and the downstream are here conventionally defined with respect to the direction of the flow of the exhaust gases from the thermal motor.

The feed system 6 in the reducer comprises a suction pump 8 of the reducer in the tank 5. The pump 8 is connected to the injector 7 by at least one pipe 9. The pipes can be heated for example to maintain the reducer liquid in the case of a liquid precursor. For the same reasons, a heating element 10 may be placed in the tank 5. In the case of a liquid precursor, the tank 5 may also be equipped with a level gauge 11 of a reducer and a temperature sensor 12 .

Downstream of the catalyst SCR 2 is a nitrogen oxide sensor 13. The system 1 SCR further comprises means 14 for measuring the amount of injected reducer. The means 14 may comprise a flow meter.

FIG. 2 shows the device for controlling the amount of reducing agent of the system 1 for catalytic selective reduction of nitrogen oxides. In FIG. 2, the block 20 represents means for determining the nitrogen oxides upstream of the catalyst 2. The means for determining the nitrogen oxides can be in the form of a nitrogen oxide sensor or still an estimator. Block 212 represents a module for determining an amount, NOx, of nitrogen oxides expected downstream of catalyst 2. Block 212 may comprise a catalysis model 21 as well as a correction module 22 of the catalysis model. . Block 212 uses, as input, the nitrogen oxides determined upstream of catalyst 2 at block 20. The device for controlling the quantity of injected reducer further comprises a first regulation loop, B1, in which the block is determined. A quantity of reducing agent to be injected, 10depon, as a function of a difference between the amount of nitrogen oxides - expected, NOxcible downstream of the catalyst 2 and the amount of nitrogen oxides measured downstream of the catalyst 2, NOxme're, by the nitrogen oxide sensor 13. The control device further comprises a first diagnosis 24 based on the comparison between the expected amount of nitrogen oxides, NOxcible and the amount of nitrogen oxides measured downstream of the catalyst 2, NOxmesure- This first diagnosis 24 concludes a problem of pollution control when the difference between the amount of expected oxides of nitrogen, NOxcible and the amount of nitrogen oxides measured downstream of the catalyst 2, NOmesure exceeds a pre This first diagnosis 24 can issue an alert when the first admissible threshold Si is exceeded.

Since the operation of the injector 7 is governed by a control law defining a quantity of gearbox to be injected, -depoll, as a function of a control set point, the device controlling the quantity of gearbox injected comprises a second control loop, B2, in which is determined in block 27 the control setpoint, To ', of the injector 7 as a function of the difference between the amount of reducer to be injected, -depoll, and a quantity of measured reducer, 0 -measures- The control device further comprises a second diagnosis 28 based on the comparison between the amount of reducer to be injected, 10 -depom, and the quantity of measured reducer, 10-meter. This second diagnosis 28 concludes that there is a problem of injection when the difference between the quantity of gearbox to be injected, 10 -depom, and the quantity of gearbox measured, the measure exceeds a second admissible threshold S2. This second diagnosis 28 can issue an alert when the second allowable threshold S2 is exceeded. The control setpoint, Touv, corresponds in FIG. 2 to an opening time of the injector, however other modes of control of the injector can be envisaged (pulse train, frequency of opening, etc.). in normal operation: -It regulates in closed loop, in the first control loop, B1, the amount of reducer to be injected, -depoll, as a function of a difference between the quantity of nitrogen oxides expected, NOxcible downstream of the catalyst SCR 2 and the amount of nitrogen oxides measured downstream of the catalyst 2, NOxmesure, -On regulates in a closed loop in the second control loop, B2, the control setpoint of the injector 7 as a function of the the difference between the quantity of reducer to be injected, Qdepoll, and the quantity of gearbox measured, 0 -measure- When abnormal operation due to injection drift is detected, that is to say when the difference between the expected amount of nitrogen oxides, NOxcible, and the measured amount of oxide When the difference between the quantity of injected gearbox I -depoll, and the quantity of gearbox injected measured, it exceeds the second allowable threshold S2, it exceeds the first permissible threshold S2. first performs a step of resetting the control law of the injector. Indeed, we first start by attempting an injection correction step by resetting the control law of the injector because the latter reacts faster. The precision of injection is of order 1 in the performance of the depollution: it also implies that it is the most urgent to correct.

During the step of resetting the piloting law of the injector 7, it is possible to inhibit the closed-loop regulation of the first control loop, B1, otherwise the quantity of gearbox to be injected, -poll, is determined in an open loop. depending on the amount of nitrogen oxides expected, NOxcible downstream of the catalyst 2. This prevents the controller of the closed loop B1 stores erroneous information during the injector registration process, because this process includes subinjections / voluntary over-injections to define the drift of the injector. During the step of resetting the piloting law of the injector 7, it is also possible to inhibit the second diagnosis 28, to prevent it from occurring during the registration step and to provide erroneous information. The step of resetting the piloting law of the injector 7 includes the search for a corrective coefficient which makes it possible to converge the difference between the quantity of reducer to be injected, 10depon, and the quantity of measured injected reducer, Omesure , towards the second threshold - admissible S2. The correction coefficient corresponds to a proportionality coefficient within a given range between a minimum coefficient and a maximum coefficient.

Several situations then arise: If there is no corrective coefficient value of the determined range, it converges to a difference between the quantity of reducer to be injected, and the amount of injected reducer measured, which is measurable, ie, the second allowable threshold S2, and a flow rate is detected while the corrective coefficient is at a minimum, it is then concluded that there is an open leakage or injector problem. If no corrective coefficient value of the determined range makes it possible to converge towards a difference between the quantity of reducer to be injected and the quantity of injected reducer measured, measurement, allowable, and that no flow is detected then that the corrective coefficient is at the maximum, one then concludes with a problem of closure of the pipe or injector blocked closed. In the case where the closed-loop regulation of the first regulation loop, B1, was inhibited during the resetting step, the closed-loop control is reactivated.

In the case where the resetting step has made it possible to converge towards the second admissible threshold S2 and that the difference between the expected quantity of nitrogen oxides, NOxcible and the measured quantity of NOxmesure nitrogen oxides does not exceed the first admissible threshold If, then, the device for controlling the injected reducer quantity continues its normal operation with the injected injector. If the resetting step has converged to the second allowable threshold S2 but the difference between the expected quantity of nitrogen oxides, NOmble and the measured quantity of NOxmeside nitrogen oxides always exceeds the first admissible threshold. If, then, a step of verification of the quality of the gearbox is carried out. This step of verifying the quality of the reducer is to be done only if the quality of the reducer can be a source of problem, for example when the reductant is a precursor in aqueous solution whose concentration can vary, with an effect on the effectiveness of pollution. In the case where the quality of the reductant is not a source of problem, for example because the reducing agent is a pure reducing agent, such as gaseous ammonia, the process of the invention passes directly from the step of adjustment of the piloting law of the injector to a step of resetting the performance of the catalyst 2 detailed below, that is to say in the case where the quality of the reducer can be a source of failure, the method of the invention comprises a step of verifying a quality criterion of the gearbox between the step of resetting the injector law 7 and the step of resetting the performance of the catalyst 2. The quality criterion on which the verification is done is by For example, in the case of a reducing agent in the form of an aqueous solution of urea, the urea concentration of the aqueous solution. The verification of the quality criterion of the gearbox can be based on a measurement or an estimation of the gearbox quality. The control device therefore comprises a module 25 for checking a quality criterion of the gearbox. In order to reduce the calculation load of the method, the module 25 is active only during this correction step. At this stage, if the gearbox quality is considered non-compliant, for example because the difference between the current value (measured or estimated) and a reference value of the quality criterion exceeds a permissible threshold, then a failure is concluded. due to the quality of the reducer. The control device may also include a third diagnosis 26 which can issue an alert when the quality of the gearbox is found to be non-compliant.

If the quality of the gearbox is found to be non-compliant, the method may include an additional step of resetting the injection to compensate for the quality difference of the gearbox. If the quality of the gear unit is deemed to be in conformity, this means that the difference between the expected quantity of nitrogen oxides, NOmble and the measured quantity of NOxmesure nitrogen oxides is not due to a quality problem. reducer. In this situation, then, a step of resetting the conversion performance of the nitrogen oxides due to the aging state of the catalyst is then attempted.

Indeed, in practice, as shown in FIG. 3, a new SCR 2 catalyst has an optimum nitrogen oxide conversion curve as a function of its Tscp temperature and the QAR amount of reducing agent in the catalyst. From this optimum conversion curve can be defined a charge of the catalyst 2, as shown in Figure 3 by the curve 32. Due to the aging of the catalyst, this optimum curve evolves, like the example given by the optimum curve aged 31 in Figure 3. This evolution of the optimum curve leads to change the catalyst load setpoint, as shown in curve 33 of Figure 3. The conversion performance adjustment step due to the aging state of the catalyst comprises the search for an optimum conversion curve adapted nitrogen oxides, representative of a determined aging state of the catalyst, which allows to converge the difference between the difference between the expected quantity of nitrogen oxides, NOxible and the measured quantity of NOxmeside nitrogen oxides, to the first permissible threshold 51.

The search for this optimum adapted conversion curve is carried out with the correction module 22 of the catalysis model, for example from abacuses held in a memory (not shown), for consideration in the catalysis model 21.

The search for the optimum conversion curve as a function of the aging state of the catalyst 2 may be limited to a permissible range in which the state of aging is consistent for example with the life of the vehicle. In the case where the step of resetting the performance made it possible to converge towards the first admissible threshold 51, then the device for controlling the quantity of injected reductant continues its normal operation with the catalysis model recalibrated by taking into account the aging of the catalyst 2.

In the case where the step of resetting the performance does not converge to the first admissible threshold Si, then it is concluded that one is faced with a catalyst failure or a failure of the determination of nitrogen oxides upstream, which may be due to a problem of operation of the engine or the means for determining the nitrogen oxides upstream of the catalyst 2. By means of this invention, the pollution of the oxides of nitrogen is thus improved by limiting the ammonia emissions while optimizing reducer consumption.

The invention makes it possible to strongly help after-sales. Indeed, depending on the state of aging, it is known whether the catalyst is to change or if it can still operate. The invention makes it possible to target after-sales repairs depending on the diagnosis: injection or quality of reducing agent or catalyst.

The advantages of this solution over a SCR system of nitrogen oxides of the prior art are: - saving time in after-sales, - the increased transparency of the SCR system vis-à-vis the customer, - performance of pollution control better maintained over time, The knowledge of aging catalyst makes it possible to adapt and optimize the injection strategies. This will result in better depollution and better control of untimely releases of ammonia.25

Claims (8)

REVENDICATIONS1. A method of controlling the amount of reducing agent of a selective catalytic reduction system (1) for nitrogen oxides comprising, a reducing agent injector (7), a catalyst (2), means (14) for measuring the quantity injected reducer, process in which: -in closed loop, in a first control loop (B1) the amount of reductant to be injected depoll) (0 1 depending on a difference between an expected amount - oxides d nitrogen (NOmble) and a measured amount of nitrogen oxides (NOmble) downstream of the catalyst (2), characterized in that, the operation of the injector being governed by a control law defining a quantity of reducer to inject, depoll) I (0 1 according to a setpoint of - pilot, -On regulates in closed loop, in a second control loop (B2), the control setpoint of the injector (7), as a function of a difference between the quantity of reducer to be injected (0 e a quantity of reducing agent measured ee -depoll, 1 tt (Q measurement), measurement), and in that when the difference between the expected quantity of nitrogen oxides (NOmble) and the measured quantity of nitrogen oxides (NOxible) exceeds one first permissible threshold (Si) and that the difference between the quantity of gearbox to be injected (0 1 and the quantity of measurement) -depoll, measured injected gear (Qm exceeds a second admissible threshold (S2), a first step is first carried out resetting the piloting law of the injector (7). 2. Method according to claim 1, characterized in that during the step of resetting the control law of the injector (7), the closed-loop regulation of the first control loop (B1) is inhibited. 3. Method according to claim 1 or claim 2, characterized in that the step of resetting the control law of the injector comprises the search for a corrective coefficient which makes it possible to converge the difference between the quantity of depoll injection reducer, 1 (0 and the quantity of injection injected measured (Qmesure) to the second threshold - admissible (S2). 4. Method according to claim 3, characterized in that if the resetting step converged to the second allowable threshold (S2) but the difference between the expected amount of nitrogen oxides (NOxcible) and the measured amount of nitrogen oxides (NOxcible) always exceeds the first admissible threshold (Si), the method then comprises a step of resetting the conversion performance of the catalyst (2) due to the aging state of the latter. 5. Method according to claim 4, characterized in that the step of resetting the conversion performance of the nitrogen oxides of the catalyst (2) due to the state of aging comprises the search for an optimum conversion curve adapted nitrogen oxides, representative of a determined state of aging of the catalyst (2), which enables the difference between the expected quantity of nitrogen oxides (NOxcible) and the measured quantity of oxides of nitrogen (N0x, '' e) to the first allowable threshold (S1). 6. Method according to claim 4 or claim 5, characterized in that the quality of the reducer can be a source of failure, said method comprises a step of verifying the compliance of a quality criterion of the gear between the step of adjusting the injector law (7) and the step of resetting the conversion performance due to the aging state of the catalyst (2). 7. Method according to claim 6, characterized in that if the quality criterion of the reducer is considered compliant then the step of resetting the conversion performance due to the aging state of the catalyst (2) is carried out. 8. Process according to any one of Claims 4 to 7, characterized in that if the step of resetting the conversion performance of the nitrogen oxides of the catalyst (2) does not make it possible to converge towards the first admissible threshold ( S1), it is concluded that the catalyst (2) has failed or has failed to determine the nitrogen oxides upstream of the catalyst (2).