WO2013139526A1

WO2013139526A1 - Process for determining the amount of ammonia stored in a catalyst, and corresponding system

Info

Publication number: WO2013139526A1
Application number: PCT/EP2013/052609
Authority: WO
Inventors: Damiano Di-Penta; Pierre-Yves Le-Morvan; Aurelien Ramseyer
Original assignee: Renault S.A.S.
Priority date: 2012-03-22
Filing date: 2013-02-08
Publication date: 2013-09-26
Also published as: EP2827973A1; FR2988305B1; FR2988305A1

Abstract

The invention relates to a process for determining the amount of ammonia stored in a nitrogen oxide reduction catalyst 11. According to the process, the flow rates of nitrogen oxides and of ammonia feeding the catalyst 11 are determined, and, from said flow rates and from a given model, the amount of ammonia stored in the catalyst 11 is estimated. Also estimated, from said flow rates and from the given model, are the flow rates of nitrogen oxides and of ammonia leaving the catalyst 11, the overall flow rate of nitrogen oxides and ammonia is measured downstream of the catalyst 11 and the given model is corrected as a function of the difference between the overall flow rate measured and the flow rates of nitrogen oxides and of ammonia leaving the catalyst that are estimated by the model, the correction of the model depending on the sensitivity S of the measurement of the overall flow rate of nitrogen oxides and ammonia with respect to the amount of ammonia stored in the catalyst.

Description

The present invention relates to a method and a system for the treatment of exhaust gases, in particular a catalyst capable of treating nitrogen oxides (NOx), especially NO and N0 ₂ . More specifically, the invention relates to a method for determining the amount of ammonia stored in the catalyst.

In order to meet the lowering of permissible thresholds for gas emissions, more and more complex gas treatment systems are placed in the exhaust line of lean-burn engines, in particular diesel engines. These aftertreatment systems notably make it possible to reduce the emissions of particles and nitrogen oxides in addition to carbon monoxide and unburned hydrocarbons.

The selective catalytic reduction process (English: selective catalytic reduction S CR) is a known process for the treatment of NOx nitrogen oxides. The process consists of continuous treatment of nitrogen oxide emissions by means of a catalyst in the exhaust line of the engine and a reducing agent injected into the exhaust line. The reducing agent, for example urea, is stored in a tank, in the vehicle, and is injected and mixed with the exhaust gas before entering the catalyst. The catalyst accelerates the reduction reaction of the nitrogen oxides by the reducing agent.

In order to control the reduction reaction and therefore the treatment of the emissions, the amount of reducing agent injected into the exhaust line as well as the amount of reducing agent stored in the catalyst must be precisely controlled: an overdose of the The reducing agent would lead to an increase in consumption unnecessarily and potentially to the release of ammonia (highly odorous and toxic), while underdosing would limit the treatment efficiency of the nitrogen oxides contained in the exhaust gas.

More specifically, the reduction catalyst stores the ammonia of the reducing agent and releases it to reduce oxides. nitrogen contained in the exhaust gas. Thus, to optimize the efficiency of the reduction process, it is necessary to regulate the mass (also called "buffer") of ammonia NH ₃ stored in the catalyst. However, this mass is not measurable in real time, and must therefore be estimated by a model. There are thus models for evaluating the mass of ammonia stored in the catalyst, but the estimate obtained derives with respect to the real value, which leads to an over-injection or an under-injection of the reducing agent. .

It is known to correct the estimate of the amount of ammonia stored in the catalyst using the measurement of a NOx sensor mounted downstream of the catalyst. Indeed, the NOx sensor is not only sensitive to NOx but also NH ₃ which escapes from the catalyst. Such devices are for example described in patent applications US2010 / 024389 and US2009 / 288396.

However, it is difficult to interpret in real time the measurements of the NOx sensor (which correspond to both nitrogen oxides and ammonia) to correct the estimate of the amount of ammonia stored in the catalyst, and the aforementioned models always lead to a drift between the estimate and the amount of ammonia stored in the catalyst.

The present invention aims to solve the technical problems mentioned above. In particular the invention aims to provide a more accurate estimate of the amount of ammonia stored in the catalyst, to allow a better NOx treatment efficiency or to detect a failure of the catalyst or the injector. reducing agent.

In one aspect, there is provided a method for determining the amount of ammonia stored in a nitrogen oxide reduction catalyst for mounting in an exhaust line of an internal combustion engine. According to the method, the flow rates of nitrogen oxides and of ammonia feeding the catalyst are determined, and it is estimated, from said flow rates and a specific model, the quantity of ammonia stored in the catalyst. It is also estimated, from said flows and the determined model, the flow rates of nitrogen oxides and of ammonia leaving the catalyst, the overall flow rate of oxides of nitrogen and ammonia downstream of the catalyst is measured and the model determined is corrected according to the difference between the measured overall flow rate and the flow rates of the oxides of Nitrogen and ammonia from the catalyst estimated by the model, the model correction being dependent on the sensitivity of the measurement of the overall flow of nitrogen oxides and ammonia to the amount of ammonia stored in the catalyst. .

Thus, thanks to a robust criterion based on observer theory, it is possible to obtain a more accurate estimate of the amount of ammonia stored in the catalyst. In particular, the method makes it possible to correct the estimated quantity of ammonia stored as a function of the sensitivity of the sensor (used to correct the estimated quantity) to the quantity of ammonia stored. It thus becomes possible to obtain a more precise estimate of the quantity of ammonia stored, in particular over long operating periods, when the measurement of the sensor depends more and more on the level of ammonia stored in the catalyst.

Preferably, the correction of the model as a function of the sensitivity of the measurement of the overall flow rate of nitrogen oxides and ammonia relative to the quantity of ammonia stored in a catalyst, is non-linear. The correction is thus carried out only when the sensitivity of the measurement of the overall flow rate of nitrogen oxides and ammonia with respect to the quantity of ammonia stored in a catalyst exceeds, in absolute value, a determined value.

The invention also relates, in another aspect, to a method of controlling an exhaust gas treatment system, the treatment system comprising a nitrogen oxide reduction catalyst mounted in an exhaust line of an internal combustion engine, wherein:

the quantity of ammonia stored in the catalyst is determined according to the process described above, and then

- A given ammonia flow rate is injected into the exhaust line, upstream of the catalyst, depending on the amount of ammonia stored in the catalyst. The invention also relates, in another aspect, to an exhaust gas treatment system emitted by an internal combustion engine, comprising a nitrogen oxide reduction catalyst mounted in the exhaust line of the engine, a means for control device and a device for determining the amount of ammonia stored in a nitrogen oxide reduction catalyst, the device comprising a means for determining the flow rates of nitrogen oxides and ammonia feeding the catalyst, and a estimation means capable of estimating, from said flow rates and a given model, the quantity of ammonia stored in the catalyst. According to the invention, the estimation means is also able to estimate, from said flow rates and the determined model, the flow rates of nitrogen oxides and ammonia leaving the catalyst, and the device also comprises a sensor mounted in downstream of the catalyst and able to measure the overall flow rate of nitrogen oxides and ammonia leaving the catalyst, and a correction means adapted to correct the determined model as a function of the difference between the measured overall flow rate and the flow rates of the catalyst. nitrogen oxides and ammonia leaving the catalyst estimated by the model, the correction means being able to correct the model as a function of the sensitivity of the measurement of the overall flow of nitrogen oxides and ammonia compared with the amount of ammonia stored in the catalyst.

Preferably, the correction means is able to correct the model in a non-linear manner as a function of the sensitivity.

The treatment system may also comprise an ammonia injection means in the exhaust line, upstream of the catalyst, controlled by the control means and capable of injecting an ammonia flow rate determined by the control means according to the amount of ammonia stored in the catalyst.

Other advantages and characteristics of the invention will appear on examining the detailed description of an embodiment of the invention which is in no way limitative, and the appended drawings, in which: Figure 1 shows, schematically, an exhaust after-treatment system according to the invention; and

FIG. 2 represents a block diagram illustrating the architecture of a means for determining the quantity of ammonia stored in a reduction catalyst.

FIG. 1 very schematically shows the general structure of an internal combustion engine 1 and an aftertreatment system of the exhaust gases 2. The internal combustion engine 1 comprises, by for example, at least one cylinder 3, an intake manifold 4, an exhaust manifold 5, an exhaust gas recirculation circuit 6 provided with an exhaust gas recirculation valve 7, and a turbo compression system 8.

The exhaust after-treatment system 2 comprises an exhaust line 9 comprising an injector 10 of a reducing agent, for example urea, and a reduction catalyst 11 (in English: Selective Catalytic Reduction). SCR) mounted downstream of the injector 10. The exhaust line 9 may also comprise a mixing means mounted between the injector 10 and the reduction catalyst 11, and for homogenizing the mixture constituted by the exhaust gas and the reducing agent.

The system 2 also comprises a temperature sensor 12 mounted upstream of the reduction catalyst 11 and making it possible to know the temperature of the gases supplying the catalyst 11 during the various phases of treatment of the exhaust gases. The system 2 can also comprise a NOx sensor 13, mounted downstream of the catalyst 11. The sensor 13 makes it possible in particular to measure the flow of nitrogen oxides and ammonia leaving the catalyst 11 in operation.

An electronic control unit 14 processes the various signals and controls the combustion, in particular by sending set values to the cylinder fuel injector. 3 and controlling a device, for example with a valve, controlling the quantity of air supplying the cylinder 3.

The electronic control unit 14 may also control the reducing agent injector 10 to introduce into the exhaust line 9 the desired amount of reducing agent.

The electronic control unit 14 also comprises means for determining the quantity of ammonia stored in a reduction catalyst 11. The determination means 15 receives several data inputs, including the data of the temperature sensor 12 and the sensor. NOx 13, and allows the electronic control unit 14 to know the amount of ammonia stored in the catalyst 11 to determine the amount of reducing agent to be introduced into the exhaust line 9.

Thus, as represented in FIG. 2, the determining means 15 may comprise an estimating means 16 receiving as input: the temperature T values of the gases measured by the sensor 12, the X ™ _H ^ and Χχ _{0χ rates} of ammonia and nitrogen oxides respectively, supplying the reduction catalyst 11 and the exhaust gas flow rates Qch feeding the catalyst. The estimation means 16 then calculates, from a dynamic model based on the ammonia adsorption and desorption reaction mechanisms on the catalyst, reduction of the nitrogen oxides by the adsorbed ammonia and oxidation of ammonia, the levels ^ and X ^™ o _x of ammonia and nitrogen oxides respectively, leaving the reduction catalyst 11 and ammonia niNH3 mass stored in the catalyst 11. The model can include use the following equation system:

X NH3 ^{~ n} NH Moreover, the model used makes it possible to correct a drift of the estimate of the mass of ammonia stored in the catalyst. For this purpose, it is considered that the measurement of the sensor 13 can be written in the following form:

^Λ measurement ~ ^α · ^Λ NOx ⁺ P · ^Λ NH3

where a and β may be constant or dependent on quantities such as temperature or flow rate.

The model of the means 16 is then corrected by the following adaptive model:

dth

= fi ^X NOx> ^X NH3, Û _NH3 , T, Q _ech ) + A dt

X ⁼ h NOx NOx NOx i ^X ^'X NH3' ¾3 ^, Qe _c h)

¾3 '^, Qech)

wherein the variables with a hat are estimated and corrected by the value Δ, and wherein the quantity S defines the sensitivity of ^a - ^ NOx + β · P ^ar ΐ ΧΝ ° relative to the amount of ammonia in the catalyst

11.

The magnitude Δ is the value of the correction loop of the adaptive model. The magnitude Δ is thus reintroduced at the input of the model to correct the values obtained. The magnitude Δ is given by the following equation:

where K is the gain of the observer, and where:

S '= 0 if S _inf <S <S _sup

S '= S otherwise

with Si _nf and S _sup two calibration parameters.

Thus, the determining means 15 includes a gap determining means 17, receiving as input the rates o o _x and

X ™ _H calculated by the estimation means 16, and the values ° " _Jure measured by the sensor 13, and outputting the difference

Thirteenth hyphenation ^~ i ^a - ^ Νθχ ⁺ β - ^ NHi] ■ The determination means 15 also includes one means 1 8 receiving as input the S value calculated by the estimation means 16 and outputting the coefficient K. S to be multiplied away by the means 17 to obtain the magnitude Δ. The means 17 and 18, as well as the multiplying means thus form a correction means 19 for the model of the estimation means 16.

Thus, the determination means 15 makes it possible to determine the amount niN H3 stored in the catalyst 11, taking into account the sensitivity S of the sensor relative to the quantity of ammonia stored in the catalyst 11. The sensitivity S generally presents, as a function of the temperature T and the quantity of ammonia stored m _N H3, three zones:

a first zone where T and m _N H3 have mean values and where S is substantially equal to 0 corresponding to an insensitivity of the NOx and NH ₃ leaving the catalyst relative to the quantity of ammonia stored niNH3: this zone corresponds to a zone in which there is no correction to be made to the model and in which S 'is zero;

a second zone where m _N H3 has values close to zero and where S is negative, corresponding to a loss of processing efficiency of the catalyst 1 1: the adaptive model then corrects the difference determined by the means 17 by lowering the level ammonia stored in the catalyst 11; and

a third zone where T has high values and where S is positive, corresponding to an ammonia leak of the catalyst 1 1: the adaptive model then corrects the difference determined by the means 17 by increasing the level of ammonia stored in the catalyst 1 1. Thus, it is noted that the correction made to the model takes into account, non-linearly, the sensitivity of the measurement of NOx and NH ₃ with respect to the quantity of ammonia stored, in order to obtain a finer estimate. More precisely, when the model overestimates the real value of m _N H3, the calculation of the sensitivity S makes it possible to detect a potential loss of efficiency and to correct the estimate accordingly when the actual value of the quantity of NH ₃ stored in catalyst 11 becomes dangerously low. This correction of the estimation occurs only when the criterion S becomes sufficiently large, but makes it possible to increase the flow of ammonia injection in the exhaust line accordingly or to detect a possible failure of the system.

On the contrary, when the model underestimates the real value of m _N H3, the calculation of the sensitivity S makes it possible to detect a potential ammonia leak and to correct the estimate accordingly when the real value of the quantity of NH ₃ stored in the catalyst 11 becomes dangerously high. Here again, this correction of the estimation occurs only when the criterion S becomes sufficiently large, but makes it possible to reduce the flow rate of ammonia injection in the exhaust line accordingly or to detect a possible system failure.

Thus, by integrating the sensitivity of the measurement of NOx and NH ₃ with respect to the quantity of ammonia stored in the model for estimating the quantity of ammonia stored in the catalyst 11, the invention makes it possible to estimate more reliably and robustly this amount of ammonia, and in particular can detect a decrease or a significant increase in this amount from the desired value.

Claims

A method for determining the amount of ammonia stored in a nitrogen oxide reduction catalyst (11) for mounting in an exhaust line (9) of an internal combustion engine, wherein the flow rates of nitrogen oxides and of ammonia supplying the catalyst (11), and it is estimated, from said flow rates and of a specific model, the quantity of ammonia stored in the catalyst, characterized in that:

the flow rates of nitrogen oxides and of ammonia leaving the catalyst are also estimated from said flow rates and the determined model,

the overall flow rate of oxides of nitrogen and ammonia downstream of the catalyst is measured and

the model determined is corrected as a function of the difference between the measured total flow rate and the flow rates of nitrogen oxides and ammonia leaving the catalyst estimated by the model, the correction of the model depending on the sensitivity (S) of measuring the overall flow rate of nitrogen oxides and ammonia with respect to the amount of ammonia stored in the catalyst.

2. Method according to claim 1, wherein the correction of the model as a function of the sensitivity (S ') of the measurement of the overall flow rate of oxides of nitrogen and of ammonia relative to the quantity of ammonia stored in a catalyst. , is non-linear.

An exhaust gas treatment system (2) emitted by an internal combustion engine, comprising a nitrogen oxide reduction catalyst (11) mounted in the exhaust line (9) of the engine, a means for control and a device for determining the amount of ammonia stored in the catalyst, the device comprising a means for determining the flow rates of nitrogen oxides and ammonia supplying the catalyst, and an estimating means (16) capable of to estimate, from said flows and a given model, the quantity of ammonia stored in the catalyst, characterized in that the estimation means (16) is also able to estimate, from said flow rates and the determined model, the flow rates of nitrogen oxides and ammonia leaving the catalyst, and in that the device also comprises a mounted sensor downstream of the catalyst and capable of measuring the overall flow rate of nitrogen oxides and ammonia leaving the catalyst, and a correction means (19) able to correct the determined model as a function of the difference between the measured overall flow rate and the flow rates of nitrogen oxides and ammonia leaving the catalyst estimated by the model, the correction means (19) being able to correct the model as a function of the sensitivity (S) of the measurement of the overall flow rate of oxides of nitrogen and ammonia relative to the amount of ammonia stored in the catalyst.

4. Treatment system (2) according to the preceding claim, wherein the correction means is adapted to correct the model non-linearly depending on the sensitivity.

5. Treatment system (2) according to claim 3 or 4, also comprising a means for injecting ammonia into the exhaust line, upstream of the catalyst, controlled by the control means and able to inject a flow of ammonia. ammonia determined by the control means as a function of the amount of ammonia stored in the catalyst.