Abstract:
The invention relates to a method for controlling a system for the treatment of exhaust gases in an exhaust line of an internal combustion engine. The method comprises estimating the molar ratio of nitrogen dioxide to nitrogen monoxide at the inlet of an NOx selective reduction catalyst based on cartography that is dependent on the residence time of the exhaust gases in an oxidation catalyst disposed upstream of the gas treatment means leading to a reduction of nitrogen oxides.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a National Stage of International Application No. PCT/FR2010/052206, filed Oct. 18, 2010, which claims priority to French application 0958103, filed on Nov. 17, 2009, the content of which (text, drawings and claims) is incorporated herein by reference. 
       FIELD 
       [0002]    The present invention relates to a method for controlling polluting emissions from a combustion engine. 
       BACKGROUND 
       [0003]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0004]    The use of fossil fuel, such as petroleum or coal, in a combustion system, in particular the fuel in an engine, entails the production of a non-negligible quantity of pollutants which can be discharged through the exhaust and cause damage to the environment. Among these pollutants, nitrogen oxides (called NOx) pose a particular problem since these gases are suspected of being one of the factors contributing to the formation of acid rain and deforestation. Furthermore, NOx are linked to human health problems and are a key element in the formation of “smog” (pollution clouds) in cities. Legislation is imposing ever more strict levels for their reduction and/or their elimination from fixed or mobile sources. 
         [0005]    Among the pollutants that the legislation tends to regulate more strictly are soot or other particulate materials resulting from incomplete combustion of fuel, more particularly when the engine is operated in poor mixture, in other words with excess oxygen (air) relative to the stoichiometry of the combustion reaction. Poor mixtures are used in so-called diesel engines, ignited by compression. 
         [0006]    Various different depollution means and strategies are employed to control these two categories of pollutants. 
         [0007]    To limit the emission of particles, the technology of particle filters is little by little becoming common practice for all vehicles equipped with a diesel engine. This technology consists mainly in forcing the exhaust gas to pass through the porous channels of a ceramic honeycomb structure. The soot filtered in this way is accumulated and then eliminated in a regeneration operation of the filter during which it is burned. To obtain this regeneration, it is however necessary to increase the temperature of the exhaust gas, which is typically obtained by enriching the exhaust gas with fuel (injected directly in the exhaust line or in the combustion chamber of the engine, during the discharge phase of the combustion cycle) and/or by increasing the charge of the engine. A catalytic agent is used to facilitate the combustion of soot. This agent is either permanently deposited in the filter channels, or introduced as an additive with the fuel. This last technology allows for operating at lower temperatures than those required with catalytic filters. 
         [0008]    To limit NOx emissions, the main solution implemented in current vehicles is the reduction of emissions at the source, in other words, operating the engine in such conditions that the rate of NOx produced is less than the limit rate. These conditions are obtained by controlling, in a very precise manner, the different parameters of the engine, starting from the parameters of fuel injection and reinjection at admission of part of the exhaust gas, in order to reduce the oxygen concentration favoring the formation of nitrogen oxides. 
         [0009]    However, it is not possible to drastically reduce the emissions at the source without limiting certain engine performances. For this reason, different solutions have been proposed for denitrifying exhaust gas. One solution which has provided proof of its effectiveness, specifically for heavy trucks, is the chemical conversion by reduction of nitrogen oxides by means of a reducing agent injected directly in the exhaust line. A post-treatment solution which has provided proof of its effectiveness is the use of ammonia (NH 3 ), such as aqueous urea. Ammonia reacts with NOx as a catalyst to form inert nitrogen N 2  and water H 2 O. This solution is mainly known under its English acronym SCR or “Selective Catalytic Reduction”. 
         [0010]    A commonly used reducer is ammonia, stored in the form of urea, whereby the ammonia is obtained by thermolysis/hydrolysis of urea in the exhaust line according to the following reactions: 
         [0000]      (NH 2 ) 2 CO→HNCO+NH 3 : thermolysis at 120° C.  (1)
 
         [0000]      HNCO+H 2 O→CO 2 +NH 3 : hydrolysis at 180° C.  (2)
 
         [0011]    The SCR catalyst serves to facilitate the reduction of NOx by NH 3  according to the 3 following reactions: 
         [0000]      4NH 3 +4NO+O 2 →4N 2 +6H 2 O  (3)
 
         [0000]      2NH 3 +NO+NO 2 →2N 2 +3H 2 O  (4)
 
         [0000]      8NH 3 +6NO 2 →7N 2 +12H 2 O  (5)
 
         [0012]    Since ammonia is considered a toxic gas, it is important that the quantity of injected urea is at all times based on the quantity of nitrogen oxides to be treated. 
         [0013]    A simple closed loop control based on the information provided by a NOx sensor installed downstream of the NOx trap is excluded for an engine operating predominantly at transitory speeds, such as the engine of an automotive vehicle. 
         [0014]    However, the NOx quantity can be estimated on the basis of mapping nitrogen oxide emissions as a function of the engine operating conditions, in other words, essentially as a function of speed and torque requirements. 
         [0015]    In practice however, the precise adjustment of the quantity of urea to be injected poses numerous difficulties. Indeed, the ammonia available for the reaction is the ammonia “stored” at any given time in the catalyst. The higher the temperature of the exhaust gas, the lower the ammonia storing capacity of the catalyst, since a desorption reaction is competing with an adsorption reaction. On the other hand, this temperature increase tends to promote the kinetics of the reaction, and therefore favors reducing reactions. In these conditions, emissions are difficult to control precisely. 
         [0016]    It is therefore desirable to detect in real time abnormal operation of the SCR system, without necessarily considering an instantaneous drift of emissions as a sign of such dysfunction. 
         [0017]    One of the difficulties in estimating the proper operation of the SCR system is that the exact nature of the gas to be treated is not very well known. Indeed, the NOx consists in part of nitrogen monoxide and in part of nitrogen dioxide, to mention only the two principal substances. The treatment capacity depends on the ratio of these two substances. 
         [0018]    This ratio is very difficult to measure even on an engine test bench, since most of the analysis means proceed in the first stage with total oxidation of nitrogen oxides. It is evident then that it is even more difficult to evaluate this ratio in real time in a vehicle, since even the aging, for instance, of certain engine elements can lead to a drift of this ratio. 
       SUMMARY 
       [0019]    Therefore, according to the invention a method is proposed for controlling a system for treatment of exhaust gas present in the exhaust line of an internal combustion engine. The method includes estimating the molar ratio between nitrogen dioxide and nitrogen monoxide at the inlet of a selective NOx reduction catalyst, starting from mapping depending on the sojourn time (i.e., the residence time) of the exhaust gas in an oxidation catalyst installed upstream of the exhaust gas treatment system entailing a reduction of nitrogen oxides. 
         [0020]    In a variant, reduction of nitrogen oxides are accomplished via a reducer injection means and a selective reduction catalyst, downstream of these injection means. 
         [0021]    In a variant, the value estimated starting from the mapping as a function of the sojourn time of the exhaust gas in the diesel oxidation catalyst (DOC) is corrected by a factor depending on the aging state of the DOC catalyst. 
         [0022]    In a variant, the aging factor is defined as the ratio between the accumulated exposure time beyond a first critical temperature causing a degradation of the diesel oxidation catalyst and a reference exposure time to the first critical temperature, for which the degradation of the DOC catalyst is complete. 
         [0023]    In a variant, when the DOC catalyst is exposed to a second critical temperature, higher than the first critical temperature, the exposure times are multiplied by a correction factor greater than 1. 
         [0024]    In a variant, the sojourn time of the exhaust gas in the diesel oxidation catalyst DOC is estimated starting from the temperature and the pressure of the exhaust gas at the outlet of the diesel oxidation catalyst and the flow of the exhaust gas upstream of the diesel oxidation catalyst. 
         [0025]    In a variant, gas treatment means bringing about the reduction of nitrogen oxides are constituted of a particle filter capable of operating in a mode called continuous conversion mode. 
         [0026]    Further areas of applicability of the present teachings will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present teachings. 
     
    
     
       DRAWINGS 
         [0027]    Other details and advantageous characteristics of the invention will become clear from the following detailed description with reference to the attached figures showing: 
           [0028]      FIG. 1  is a schematic diagram of an engine exhaust gas treatment line. 
           [0029]      FIG. 2  is a block diagram illustrating the dependency between the estimating model for the NO 2 /NO ratio at the outlet of the diesel oxidation catalyst. 
           [0030]      FIG. 3  is a block diagram illustrating the calculation of the sojourn time of the exhaust gas in the diesel oxidation catalyst. 
           [0031]      FIG. 4  is a block diagram of the estimating module for the NO 2 /NO ratio at the outlet of the diesel oxidation catalyst. 
           [0032]      FIG. 5  is a block diagram of the calculation module for the efficiency of the SCR system. 
           [0033]      FIG. 6  is a graph illustrating the variation of the efficiency of a SCR catalyst in function of its ammonia charge. 
           [0034]      FIG. 7  is a block diagram of a calculation module for the ammonia charge in the catalyst. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    It should be noted that NOx nitrogen oxides, as used herein, refers to the two nitrogen compositions by which the emissions are regulated, namely nitrogen monoxide and nitrogen dioxide, produced in particular by engines operating with poor mixtures. In other words, engines with excess oxygen relative to the stoichiometry of the combustion reaction of the fuel, for example, diesel engines with ignition by compression. 
         [0036]    For clarity purposes, in the following description the hypothesis is systematically made that the reducing agent is injected as is in the exhaust line, upstream of the SCR catalyst. This is for instance the case if this agent of hydrogen or ammonia is stored in gaseous form or produced in a suitable generator before being introduced in controlled manner in the exhaust line. However, this reducing agent can also be introduced in the form of a precursor, such as the well known example of urea, which after a thermolysis and hydrolysis reaction, is transformed into ammonia (see the equations 1 and 2 shown above). 
         [0037]    In addition, it is assumed that this reducing agent is effectively ammonia, and for clarity purposes, the designation (NH 3 ) is used systematically in the following description, even though the invention is not limited to this embodiment. 
         [0038]      FIG. 1  is a schematic of an engine exhaust gas treatment line. A diesel oxidation catalyst preDOC is installed at the beginning of the line. The primary role of the preDOC is to convert in the combustion chambers of the engine the carbon monoxide, and the fraction of unburned or partially burned gaseous hydrocarbons, in carbon dioxide. Such disposition, close to the engine, therefore in the hottest area of the exhaust line, guarantees maximum efficiency of this catalyst, but is not part of the specific scope of the invention. 
         [0039]    Continuing in the direction of the exhaust gas flow, a temperature sensor AT3 allows for estimating the exhaust gas temperature upstream of the selective catalytic reduction SCR treatment system. 
         [0040]    This SCR treatment system generally includes an injector, connected to a not shown source of reducing agent, and downstream of the injector, a selective reduction catalyst, called SCR catalyst. In well known manner, such SCR catalyst can be constituted of a zeolite deposited on a ceramic support, for instance of cordierite type. This type of zeolite based catalyst hardly promotes the conversion reaction of NO in NO 2 . On the other hand, other catalysts containing platinum or palladium can provide this conversion. 
         [0041]    If the reducing agent is not injected directly in its final form, but under the form of a precursor, for instance an aqueous solution of urea, the SCR system can also comprise a means suitable to facilitate the mixture of precursor in the exhaust gas while allowing for a relatively compact line architecture. 
         [0042]    A particle trap FAP is also present in this line, installed in the present case downstream of the selective reduction catalyst, but it can also be installed upstream of the reducing agent injector. 
         [0043]    Finally, a NOx sensor is provided in order to ensure that the emissions of the vehicle are always less than the prevailing emission norm. 
         [0044]    In the rest of this document we will note with the suffix 0 data at a point upstream of the oxidation catalyst preDOC (but downstream of the branching point exhaust gas recirculation EGR of the gas), and with suffix 1, at a point downstream of the catalyst preDOC and upstream of the SCR catalyst. 
         [0045]    The catalytic reduction of NOx by ammonia in a SCR catalyst consists essentially in a series of reactions numbered (3) to (5) mentioned above, whereby the NOx reacts with ammonia stored in the catalyst at a given time. 
         [0046]    The efficiency of the system can be calculated at any time as, the ratio between, on the one hand, the difference between the NOx quantity emitted by the engine and the NOx quantity emitted at the end of the line, and on the other hand, the NOx quantity emitted by the engine. 
         [0047]    The NOx quantity emitted at the end of the line is estimated by means of the NOx sensor mounted downstream of the treatment devices. 
         [0048]    The NOx quantity emitted by the engine can be obtained starting from a map established on the basis of actual emission measurements at the outlet of the engine, typically on an engine test bench associated with a booth for chemical and quantifying analysis of the exhaust gas. 
         [0049]    During the regulation phase of the engine, a set of parameters can be defined for each operating point of the engine optimizing the performance and the emissions of the vehicle (which can be defined as a request for engine torque at a given engine speed). Among these parameters are, for instance, the quantity of injected fuel, the quantity of fresh air admitted in the engine, the fuel injection conditions, the valve opening timing, the exhaust gas recirculation rate (EGR), etc. These parameters are transmitted to the engine processor by means of a set of maps which take into consideration parameters such as exterior temperature, the altitude (to take into account the rarefaction of oxygen), the preheating state of the engine, etc. Since the regulated emissions of the vehicle are taken into account when defining the engine parameters, an instantaneous NOx mass flow will be obtained for this engine, for each engine operating point and for given exterior conditions. If necessary, maps can be provided for degraded operating conditions, for instance, in the event of absence of gas recirculation due to malfunction of the EGR valve. 
         [0050]    The SCR system operates nominally if the observed efficiency conforms to a theoretical model of this system which defines the maximum conversion potential in the given conditions. 
         [0051]    To create this model, it should be noted that the catalytic reduction capacity of NOx of the SCR catalyst is a function of its temperature and the nature of the gas to be treated, in other words, of the NO 2 /NO ratio noted in the following as R NO2/NO . 
         [0052]    As long as the minimum activation temperature is not reached (called Light-off temperature), the SCR catalyst is essentially inert. Beyond that, its efficiency increases until a zone of optimum operation is reached. This efficiency is not universal but depends certainly on the nature of the chemical substances to be treated, therefore in the specific case of exhaust gas treatment, on the ratio R NO2/NO . 
       A—Determination of the NO 2 /NO Ratio at the Inlet of the SCR Catalyst 
       [0053]    In an embodiment of the invention, the ratio R NO2/NO  can be estimated based on maps established during the regulation of the engine, by analyzing the gas produced in the different operating points of the engine. 
         [0054]    In practice, this analysis is relatively complicated, and most often, it is simply based on determining the different substances, since the NOx fraction, which is a sum of NO 2  and NO is considered as one part (as is the case with the existing regulatory standards, and the gas analysis is performed in simpler manner after complete oxidation of the gas, therefore total conversion of the NO fraction in NO 2 ). For this reason, one of the goals of the present invention is to propose a means for estimating the ratio R NO2/NO  in a simpler manner. 
         [0055]    Between the outlet of the engine and the selective reduction catalyst SCR, oxidation of a portion of the NO is taking place in the oxidation catalyst preDOC. For this reason, one of the goals of the present invention is to propose a model for estimating the ratio R NO2/NO , whereby this model is remarkable in that the ratio is estimated as a function of the sojourn time (i.e., the residence time) of the gas in the oxidation catalyst preDOC, and weighted according to an aging factor of the oxidation catalyst preDOC. 
         [0056]    This model corresponds in essence to the diagram shown in  FIG. 2 , where only 3 input data are shown: the estimated temperature T 1  and pressure P 1  at the outlet of the preDOC catalyst and the exhaust gas flow Flow 0  upstream of this preDOC catalyst are necessary to estimate the ratio R NO2/NO  by means of 3 specific modules which allow for more precise calculation of the aging factor of the oxidation catalyst preDOC, the speed of the exhaust gas in the preDOC catalyst and an estimation module for the R NO2/NO  ratio from the result of these two preceding modules. 
       Aging Factor Module for the Oxidation Catalyst 
       [0057]    According to the invention, the aging factor can be estimated starting from a timer which sums the total elapsed time above a critical temperature beyond which the performance of the catalyst preDOC is irrevocably degraded. Advantageously, when the temperature to which the catalyst preDOC is exposed is particularly high (higher than a second threshold temperature, higher than the critical temperature), the time spent beyond this second threshold temperature is multiplied, for instance, by 1.5. 
         [0058]    A normalized time factor can then be defined as the ratio between this timer and a reference timer, corresponding to the accumulated exposure time beyond the critical temperature resulting in complete degradation of the preDOC catalyst. If this timer reaches or surpasses this reference time, the aging factor will be set equal to  1 . On the other hand, a new preDOC catalyst, never exposed to a temperature beyond the reference temperature will have an aging factor equal to 0. 
       Sojourn Time of the Gas in the Oxidation Catalyst 
       [0059]    A sojourn time module is illustrated in  FIG. 3  and is essentially based on the law of ideal gas, with three input data, the temperature T 1  (expressed in ° Kelvin), and the pressure downstream P 1  of the oxidation catalyst (expressed in Pascals) and the flow of exhaust gas upstream of the catalyst (expressed in g/s), in other words the gas flow produced by the engine, less the flow of gas admitted in the EGR circuit if the engine is equipped with such circuit and if the branching point of the EGR circuit is located upstream of the oxidation catalyst preDOC, in other words if the engine is equipped with a high pressure EGR circuit. 
         [0060]    The calculation additionally requires knowledge of the volume V of the preDOC catalyst (volume accessible to the gas, expressed in liters). In an ideal gas, the relation between the number n of moles of gas in the volume V at pressure P 1  and temperature T 1  is equal to RT/PV, R being the ideal gas constant. In the case of exhaust gas, the molar mass can be approximated, considering that the latter is essentially constituted of products of the combustion of fuel with oxygen in the air (which can be modeled by the reaction C 7 H 16 +11O 2 →7CO 2 +8H 2 O) and that the nitrogen in the air is found in the exhaust gas. From this, the gas mass in the preDOC catalyst is deduced. 
         [0061]    Dividing the gas mass sojourning in the preDOC catalyst by the upstream mass flow, an estimate is obtained of the sojourn time in the preDOC catalyst. Hypothetically, the sojourn time of the gas in the preDOC catalyst will be identical for all gaseous substances present in the exhaust gas, e.g., the NOx. 
       Estimation of the Ratio R NO2/NO  at the Outlet of the Oxidation Catalyst 
       [0062]    During the gas sojourn in the oxidation catalyst preDOC, part of the NOx oxidizes into NO 2 . The reduction reactions that take place in the catalytic reduction catalyst SCR occur with a gas enriched in NO 2 . It is therefore necessary to estimate the NO 2 /NO ratio (R NO2/NO ) at the outlet of the oxidation catalyst preDOC. 
         [0063]    The ratio R NO2/NO  depends on the nature and the dimensions of the oxidation catalyst preDOC, the aging status of the oxidation catalyst preDOC and the sojourn time of the gas in the oxidation catalyst preDOC. It has been shown above how to estimate the aging factor and the sojourn time. 
         [0064]    The authors of the present invention have found that this ratio NO 2 /NO at the outlet of the oxidation catalyst preDOC can be considered as independent of the NO 2 /NO ratio at the inlet of the catalyst preDOC, and depends on the oxidation capacity of the oxidation catalyst preDOC and/or the sojourn time of the gas in the catalyst preDOC. For a given architecture choice, and a given aging state, the only parameter variable as a function of the operating conditions of the engine and affecting this oxidation capacity is the temperature in the catalyst preDOC. 
         [0065]      FIG. 4  illustrates the estimating module of the NO 2 /NO ratio at the outlet of the oxidation catalyst preDOC. 
         [0066]    A first map  31  is used to select a value for the NO 2 /NO ratio according to the sojourn time of the gas in the preDOC catalyst. This value is corrected by a first correction factor, supplied by a second map  32  as a function of the temperature downstream of the preDOC catalyst, this first factor is for instance selected between 0 and 1. 
         [0067]    The aging of the preDOC catalyst affects at least two levels of its performance: on the one hand its priming temperature increases, and on the other hand, beyond this priming temperature, the oxidation capacity is degraded. Therefore, two specific maps can be used to test whether the preDOC catalyst operates in a suitable temperature range, and to assign a second correction factor, again selected between 0 and 1, to take into account this degradation of the oxidation capacity. Advantageously, this second correction factor can be different at low and high temperatures, to take into account that for the low temperatures (just below the priming temperature) the degradation is stronger than for the higher temperatures, so that a lower temperature exacerbates the aging of the catalyst preDOC. 
         [0068]    Finally the ratio is filtered by a first order low-pass filter to smooth the strong dynamics of the sojourn time, caused by variations of the flow in the exhaust line. 
         [0069]    Once the NO 2 /NO ratio is estimated, it is possible to begin modeling the reduction in the SCR catalyst, since it is assumed that this ratio is not varying between the outlet of the oxidation catalyst preDOC and the inlet of the reduction catalyst SCR. 
       B—Determination of the Mass of NH 3  Stored in the SCR Catalyst 
       [0070]    One mole of ammonia injected upstream of the SCR catalyst can eventually be “trapped” by the SCR catalyst, transformed by reacting with the NOx or go through the SCR catalyst without being transformed and end up downstream of the NH SCR catalyst. 
         [0071]    The reduction catalyst SCR has very strong affinity for ammonia NH 3  and beyond a certain minimum threshold of NH 3  charge, the NOx reduction reaction by ammonia is not significantly catalyzed. Beyond a certain maximum threshold of NH 3  charge, the storage capacity is exceeded and some ammonia is released by the SCR catalyst. The ammonia released by the SCR catalyst must be added to the ammonia coming from the injection of reducer to avoid excess ammonia at the end of the exhaust line, adding to the emitted pollutants. 
         [0072]    These minimum and maximum thresholds depend on the temperature of the SCR catalyst and are lower with increasing temperature. 
         [0073]    Each temperature of the SCR catalyst corresponds to an optimum storage instruction, between the minimum and the maximum thresholds, which can be estimated as equal to the median between the minimum and maximum thresholds, if it is desired to minimize the total number of maps accessed. At any time, the quantity of injected reducer can be adjusted in such manner to stabilize the quantity of ammonia stored at this optimal instruction level. 
         [0074]    It should be noted that the ammonia stored in the SCR catalyst is not volatilized following a vehicle stop, so that the modeled mass can easily be memorized at the end of each driving phase to be used as initial mass for the following driving phase. To this end, a non-volatile memory can be used, for instance a programmable and electrically erasable read-only memory of EEPROM type (acronym for “Electrically Erasable Programmable Read-Only Memory”). 
         [0075]    The NH 3  mass in the SCR catalyst depends on the quantity of injected ammonia, and on the NOx quantity treated by the SCR catalyst, in other words, the difference between the NOx flows upstream and downstream of the SCR catalyst. In other terms words, the NH 3  mass in the SCR catalyst is obtained by integrating the storage speed or depletion speed (by reaction of the NOx), while posing as condition to the limits that this mass is minimum equal to 0 g. This storage speed mainly depends on the quantity of ammonia injected in the line. Assuming that the ammonia is injected in the form of an aqueous solution of 32.5% urea (in mass). It can be easily calculated that for each gram of NH 3  solution, the SCR catalyst is loaded with 0.184 g of catalyst, e.g., zeolite deposited on a ceramic support. 
         [0076]    The NH 3  mass in the SCR catalyst is decremented by the NOx mass reacting in it. This reacting NOx mass is a function of the stoichiometric ratio. To obtain the NH 3  mass, it is sufficient to integrate the storage or depletion speed of NH 3  (the minimum mass is 0 g). 
         [0077]      FIG. 7  illustrates in more precise manner how this model can be implemented in a control module. 
         [0078]    The NH 3  mass modeled in the SCR catalyst can also be reinitialized to a new value during a driving phase by the adaptive strategy or by calibration. 
         [0079]    Besides this quantity of ammonia stored in the SCR catalyst, there is also at any given time a certain quantity of ammonia available for the NOx reduction reaction. If the ratio R NO2/NO  is smaller than 0.5, it is estimated according to the invention that one mole of nitrogen oxides (NO or NO2) reacts with one mole of ammonia. If this ratio is greater than 0.5, more than one mole of nitrogen oxide reacts with one mole of ammonia, and a stoichiometric ratio noted R NO2/NO  can be defined as follows: 
         [0000]      If R NO2/NO &lt;0.5 then R NH3/NOx =1 
         [0000]      If not R NH3/NO =(1+8(R NO2/NO −0.50))/(1+6*(R NO2/NO −0.50))
 
       C—Conversion Potential of the SCR Catalyst 
       [0080]    The reduction capacity of the SCR catalyst can only be expressed if the SCR catalyst is effectively at a temperature higher than its priming temperature (e.g., for a zeolite type SCR catalyst), a condition which is not satisfied in the first moments following the start of the engine. 
         [0081]    Finally, this capacity depends on the temperature of the SCR catalyst and on the NO 2 /NO ratio at the inlet of the SCR catalyst. 
         [0082]    Starting from an estimate of the NO 2 /NO ratio and the temperature of the SCR catalyst, a maximum conversion potential can be defined, assuming that the injection instruction complies with the ammonia storage instruction on the one hand, and the R NH3/NOx  ratio on the other hand. 
         [0083]    This maximum conversion potential reflects the actual instantaneous conditions, therefore the gap between the conversion potential and the observed efficiency reflects a dysfunction of the SCR system, a dysfunction that can be taken into account practically in real time because it corresponds with instantaneous conditions which may not be momentarily favorable to proper conversion. 
         [0084]      FIG. 5  illustrates a possible use of this model. The input information consists of 4 elements: the initial ammonia charge  41 , the sojourn time  42  of the exhaust gas in the SCR catalyst, the temperature of the SCR catalyst  43  and the R NO2/NO  ratio. 
         [0085]    The temperature of the SCR catalyst is not homogenous, neither in longitudinal nor transversal direction. On the other hand, the SCR catalyst has a certain thermal inertia and its temperature cannot be confounded with the temperature of the exhaust gas. As far as this model is concerned, it is only important that this temperature is estimated in a given point of the SCR catalyst. 
         [0086]      FIG. 6  shows the variation of the efficiency of a SCR catalyst in function of its ammonia charge. Therefore, the charge  41  (shown in  FIG. 5 ) can be converted by means of a map or a transfer function  45  (shown in  FIG. 5 ) into an estimated conversion potential. 
         [0087]    This value must be corrected a first time to take into account the time that the gas sojourns in the SCR catalyst and the temperature of the latter, to reflect the fact that the kinetics of the reduction reaction is greater with a hotter catalyst and that the reaction requires a certain time lapse, so that if the sojourn time is shortened (higher exhaust gas flow due to higher engine speed) then the efficiency will be less at isothermal temperature. This correction is performed based on a factor between 0 and 1 obtained in  46 . 
         [0088]    As previously indicated, the NOx conversion efficiency depends also on the NO 2 /NO ratio, and another multiplication factor between 0 and 1 is therefore determined in  47 , also starting from an appropriate map. 
         [0089]    The combination in  48  (shown in  FIG. 5 ) of factors  46 ,  47  allows for the modulation of the conversion potential  45 . 
         [0090]    If necessary, as illustrated in  FIG. 5 , an adaptation factor  49  is also taken into account which reflects the fact that it is sometimes advantageous to select an injection quantity of ammonia smaller than the quantity which should give the best results, in order to ensure a minimum time interval between two fillings of the reducer reservoir. 
         [0091]    The module allows also for estimating the conversion potential of the SCR catalyst and comparing it with the observed nominal efficiency, and for decision making, e.g., the immobilization of the vehicle or the engine going into degraded mode, if it is determined that the SCR system does not operate in nominal manner and requires maintenance to limit pollution risk. 
         [0092]    This module can also be used to correct the ammonia charge instruction as proposed in patent application FR2931201, according to which, when an anomaly of the SCR system is detected, whereby the measured efficiency is not conforming to the expectations according to the model and the quantity of reducing agent to be injected is modified, and if the employment of this method leads to a number of successive modifications of the same nature greater than a predetermined value N, the mathematical model is corrected, and the initial mathematical model is replaced by the corrected model.