FR3026432A1 - METHOD OF CONTROLLING SULFUR OXIDE PURGING FROM A DELEGATION MEANS PRESENT IN AN EXHAUST LINE OF A MOTOR VEHICLE - Google Patents

Abstract

The invention relates to a sulfur oxide purge control method of a depollution means present in the exhaust line of a motor vehicle, the depollution means (2) absorbing sulfur oxides contained in the gases. exhaust and the desorbent during a purge, a purge being triggered when the mass of sulfur oxides stored in the pollution control means (2) is greater than a critical mass, the current mass of sulfur oxides stored in the depollution means (2) being regularly estimated, characterized in that the current mass of sulfur oxides stored in the depollution means (2) is estimated according to a model taking into account the kinetic parameters for the absorption and desorption of sulfur oxides in the or of the depollution means (2). Application in the field of motor vehicles.

Description

The present invention relates to a process for controlling the purge of sulfur oxides by means of a means for controlling the sulfur dioxide oxidation. depollution present in an exhaust line of a motor vehicle.  The technical field of the present invention is the pollution control of the exhaust gas of a gasoline or diesel engine of a vehicle.  More particularly, but not limitatively, the present invention is applicable to engines with a pollution control system concerning, in particular, nitrogen oxides in the form of a trap for passive nitrogen oxides and which may comprise at least one particulate filter.  Thus, the sulfur oxide purge control method finds a particularly advantageous application in a process for controlling the pollution of nitrogen oxides in an exhaust line of a motor vehicle, the pollution control system. oxides of nitrogen comprising at least one passive nitrogen oxide trap of the PNA (Passive NOx Adsorber) or LNT (English Lean NOx Trap) type, taking into account the possible stocking of the nitrogen oxide trap by the presence of sulfur oxides from the fuel used by the motor vehicle.  Preferably, a selective catalytic reduction SCR system (SCR - Selective Catalytic Reduction) is also included in the exhaust system of the motor vehicle to contribute to the elimination of nitrogen oxides from the exhaust gases. in combination with the nitrogen oxide trap.  In particular, the SCR system and the nitrogen oxides trap can be switched to clean up or work together.  The first possibility can occur when the RCS tank is empty or at a critical level to ensure a satisfactory clearance in the kilometers to go.  There are indeed two main systems for the cleanup of nitrogen oxides.  The first system is called the RCS system for Selective Catalytic Reduction.  The reduction process operates by injection into the exhaust line of a so-called SC reducing agent, this agent being advantageously but not exclusively urea or a derivative of urea.  A SCR system injects the reducing agent SCR in the exhaust line, this agent neutralizing nitrogen oxides.  The second system of pollution control of nitrogen oxides which may also be referred to hereinafter NOx uses nitrogen oxide traps, for example a nitrogen oxide trap system of the LNT type which eliminates NOx via a brief passage in richness 1 or higher in the engine output gases where the HOs react with the stored NOx and neutralize them by transforming them into N2.  This system is called "Active" because there is a modification to the engine control.  Another "passive" trap system with nitrogen oxides of the PNA type is said to be passive because there is no passage in wealth 1.  These traps allow the retention of nitrogen oxides under unfavorable engine operating conditions of pollution, this NOx trap can release and / or destroy the nitrogen oxides trapped in other conditions more favorable to their destruction.  These different types of traps will be grouped together below under the name of NOx trap.  Such a NOx trap as NOx adsorber can be used in combination with a RCS system.  This makes it possible to increase the removal efficiency of the nitrogen oxides by adsorption of the nitrogen oxides at low temperature and desorption of the oxides once the SCR catalyst is active.  The RCS system is frequently placed downstream of the NOx trap.  [0009] A passive adsorber trap of NOx retains on the sites provided in its interior nitrogen oxides by chemical reaction.  Once the trap is filled with reacted nitrogen oxides, it can be done by a strategy of injecting a surplus of fuel, thus sending an excess of hydrocarbons or HO through the exhaust line, these hydrocarbons reacting with the nitrogen oxides then released and neutralizing them.  The two remediation methods in nitrogen oxides SCR or NOx trap previously mentioned have specific advantages and disadvantages.  The major advantage of an active absorption NOx trap is not to require the presence of a reducing agent reservoir, a heated reducing agent line, a reducing agent injector. etc.  The nitrogen oxide trapping material may be integrated in an oxidation catalyst, for example a Diesel or DOC oxidation catalyst, such a catalyst being mainly used for the decontamination of carbon monoxide or CO or hydrocarbons or HC .  A passive absorption NOx trap therefore has a small footprint and a reduced mass and a lower cost.  Another major advantage of an Active NOx trap is that the operation of the depollution is not affected by a lack of depollution agent in any tank.  This is not the case for a RCS system and an active NOx trap is from this point of view very advantageous compared to a RCS system for a driver who does not have to worry about the filling of a fuel tank. depollution agent for the operation of the trap.  [0013] If an Active NOx trap incidentally has the disadvantage of involving a greater fuel consumption and also increasing the emissions of 002, it has the notorious disadvantage of being liable to be poisoned by sulfur oxides. from the fuel of the motor vehicle and taking the place of NOx in its interior.  Its faculty of retention of NOx is then diminished.  The same problem is also associated with the Passive NOx trap.  The sites that store NOx in NOx traps (active or passive) are very similar so the storage of SOx is also very similar and also very difficult to desorb in both technologies.  The sulfur content of fuels varies from country to country.  If this content can be about 5 to 10 ppm sulfur oxides or SOx in a diesel fuel, it can reach up to 3000 ppm for a gasoline fuel in other countries.  Sulfur oxides or SOx are adsorbed in the NOx trap (passive or active) on the NOx storage sites, which gradually reduces the NOx storage capacity of the trap therefore its depollution capacity, especially at low levels. temperature.  In addition, when a NOx trap (passive or active) is present in the exhaust line together with a RCS system, there is a risk of a depollution agent injection called reducing agent RCS in this system. larger than necessary since the estimated concentration of NOx is distorted by the presence of SOx.  This leads to an undesirable depollution agent expense.  Patent FR-A-2 927 943 discloses a method for controlling a sulfur oxide purge of a catalytic converter with a sulfur content evaluation model for which an instant storage efficiency is defined. sulfur oxides (EFFS0x) in the following manner: EFFSOX = 1 SOx flow at the exit of the exhaust line 3 SOX flow at the engine output 130 and a current mass (SSi) of sulfur oxides is evaluated adsorbed into the pot, at a sampling instant i, using the model: SSi-F1 = SSi + QS0x EFFSOX where QS0x is the flow of sulfur oxides in the engine exhaust.  First, the major problem of such a sulfur oxide control model is that it is well suited to control software, it is not very accurate especially for low concentrations adsorbed and desorbed.  Over a long period of time, deviations in the SOx concentrations stored in place of the NOx thus calculated in relation to reality will lead to a lower efficiency in NOx depollution and over-injection of reducing agent SCR.  Second, the models described in this document do not take into consideration the duration taken for SOx desorption.  Third, the models described in this document relate to diesel or DOC oxidation catalyst technologies and do not take into account NOx storage with an independent passive NOx trap.  This leads to inaccuracies in the management of the RCS system downstream and leads to a reduced NOx decontamination and over-injection of reducing agent SCR.  These three disadvantages also affect the proper functioning of all the pollution control elements present in the exhaust line of the motor vehicle, mainly RCS depollution and the purges of the passive NOx trap.  The problem underlying the present invention, for an exhaust line in a motor vehicle comprising at least one nitrogen oxide decontamination system in the form of a passive NOx trap is better able to evaluate the concentration of sulfur oxides or SOx stored in the trap to better control the operation of the pollution control system when the passive NOx trap desorbs the nitrogen oxides it contains.  To achieve this objective, it is provided according to the invention a sulfur sulfur oxidation purge control method of a depollution means present in the exhaust line of a motor vehicle, the absorbing depollution means of the present invention. sulfur oxides contained in the exhaust gas and the desorbent during a purge, a purge being triggered when the mass of sulfur oxides stored in the depollution means is greater than a critical mass, the current mass of oxides Sulfur stored in the depollution means being regularly estimated, characterized in that the current mass of sulfur oxides stored in the depollution means is estimated according to a model taking into account the kinetic parameters for the absorption and desorption of oxides of sulfur. sulfur in the or of the depollution means.  Advantageously, the current mass of estimated stored sulfur oxides [stored SOx] in the depollution means is estimated according to the following equation: ([stored SOx]] = ([SOx Motor output] * flow rate * Ts) * desorption rate SOx) - [SOx] output in which equation [SOx Motor output] is the estimated mass of sulfur oxides at the motor output, flow rate is the gas flow rate in the exhaust line, Ts is the surface temperature of the air pollution control means, SOx desorption rate is the desorption rate of the sulfur oxides in the depollution means, [SOx] output is the estimated mass of oxides of sulfur at the outlet of the depollution means.  Advantageously, it is estimated a rate of absorption and desorption of sulfur oxides according to the following equations: SOx adsorption rate = K ads, SOx * [SOx] * (1-0S0x) K ads, SOx = Aads, S0x * exp (-Eads, SOx / R * Ts) Desorption rate SOx = K des, SOx * [eS0x] K des, SOx = Ades, S0x * exp (-Edes, SOx * [eS0x] / R * Ts) in which equations: Kads, SOx is the constant adsorption rate of sulfur oxides, [SOx] is the mass of sulfur oxides in the depollution means, OSOx is the adsorption surface coverage for oxides of sulfur, R is the exhaust gas constant, Eads, S0x is the activation energy for adsorption, Ts the surface temperature of the depollution means, Aads, S0x is the first preexponential factor, Kdes, SOx is the constant desorption rate of sulfur oxides, Edes, S0x is the activation energy for desorption of sulfur oxides, Ades, S0x is the second preexponential factor [0026] The invention concerns also a method for controlling the depollution of nitrogen oxides in an exhaust line of a motor vehicle, the exhaust line comprising a pollution control means in the form of or incorporating a nitrogen oxide trap passive absorbing nitrogen oxides during an absorption phase and the desorbent during a desorption phase, this nitrogen oxide trap being capable of being poisoned by sulfur oxides, characterized in that the means of depollution is purged sulfur oxides in accordance with such purge process previously described.  Advantageously, the current mass of nitrogen oxides in the passive nitrogen oxide trap in the form of or integrated in the depollution means is estimated according to a model specific to nitrogen oxides giving a current mass. of nitrogen oxides stored in the nitrogen oxide trap, this stored mass of nitrogen oxides being corrected by subtracting a mass of sulfur oxides stored in the estimated nitrogen oxide trap according to the model takes into account the kinetic parameters for the absorption and desorption of sulfur oxides to obtain a mass of nitrogen oxides actually stored in the nitrogen oxide trap.  Advantageously, it is launched a purge of sulfur oxides in the nitrogen oxide trap when the mass of sulfur oxides stored in the nitrogen oxide trap estimated according to the model taking into account the kinetic parameters for the oxides of nitrogen. Absorption and desorption of sulfur oxides exceeds a predetermined level judged to be detrimental to the effectiveness of the nitrogen oxide decontamination exerted by the nitrogen oxide trap.  Advantageously, the exhaust line having a selective catalytic reduction system of the nitrogen oxides present in the exhaust gas of the line with injection of a quantity of reducing agent in the estimated line as a function of the content of nitrogen oxides to be reduced, account is taken of the mass of sulfur oxides stored in the nitrogen oxide trap when estimating the amount of reducing agent to be injected.  Advantageously, there is provided a possibility of permutation of the selective catalytic reduction system to the nitrogen oxide trap when the amount of reducing agent is below a predetermined minimum threshold value deemed critical to ensure the depollution, a return to the catalytic reduction system being provided as soon as the reducing agent is added.  The invention also relates to an exhaust line of a motor vehicle whose depollution is controlled by a control unit according to such a previously described control method, the line comprising a trap for passive nitrogen oxides, characterized in that the passive nitrogen oxide trap is integrated in an oxidation catalyst.  Advantageously, the oxidation catalyst has an active exchange phase with the exhaust gas flowing through the line, this active phase being impregnated with deposits of at least one chemical element forming the oxide trap. nitrogen reacting with the nitrogen oxides to give a chemically stable compound, the active phase being also capable of reacting with sulfur oxides, conditions of richness greater than one fuel in the exhaust line releasing the oxides of nitrogen and sulfur, the nitrogen oxides being reduced by a selective catalytic reduction system present in the exhaust line downstream of the nitrogen oxide trap.  To achieve this objective, it is provided according to the invention a control method of the pollution control of nitrogen oxides in an exhaust line of a motor vehicle, [0034] The technical effect is, d ' on the one hand, to know precisely the amount of SOx that has accumulated in the NOx trap, for example contained in the oxidation catalyst, to follow the evolution of the storage functions and their possible impact on the catalytic reduction system RCS.  Other features, objects and advantages of the present invention will appear on reading the detailed description which follows and with reference to the accompanying drawings given by way of non-limiting examples and in which: - Figure 1 is a schematic representation of a longitudinal view of an exhaust line that can be controlled according to the method of the present invention, this line having a SCR system, a nitrogen oxide trap integrated in an oxidation catalyst and a filter FIG. 2 is a diagrammatic representation of a diagram for implementing the process for controlling the pollution control of nitrogen oxides according to the present invention; FIG. 3 is a diagrammatic representation of two desulphatation curves in FIG. a function of time to illustrate the effects of absorption and desorption on the release of sulfur oxides over time.  In what follows, NOx denotes nitrogen oxides, SOx denotes sulfur oxides and SCR all that is relative to a selective catalytic reduction.  Figure 1 shows an example of exhaust line provided with pollution control elements with its associated control unit 9.  This exhaust line has one of its ends from the exhaust manifold of the engine 1 thermal vehicle.  In this exhaust line, there is provided as first pollution control element an oxidation catalyst 2 including a passive nitrogen oxide trap or NOx trap followed by a selective catalytic reduction system RCS 3 and a particulate filter 4, present for a diesel engine but increasingly common for gasoline engines, these particles are generally soot particles.  This particulate filter 4 has a filter support for capturing the particles contained in the gases of the exhaust line.  According to this embodiment, the oxidation catalyst 2 incorporates in its interior a nitrogen oxides trapping material, which allows to combine two elements of pollution in a single element.  The oxidation catalyst 2 has an active exchange phase with the exhaust gas flowing through the line, this phase also being called washcoat in English.  This active phase is impregnated by deposits of at least one chemical element forming the NOx trap.  This chemical element reacts with the nitrogen oxides to give a chemically stable compound except for conditions of richness greater than 1 in fuel in the exhaust line, the oxides of nitrogen being then released and reduced by a reducing agent to form nitrogen, resulting in a slightly higher fuel consumption for the desorption operation of a NOx trap.  The active exchange phase of the oxidation catalyst 2 can be composed of Al 2 O 3, TiO 2, ZrO 2, CeO 2, Y 2 O 3 with catalyst elements such as precious metals such as Pt, Pd, Rh, Ru, Re, Au, Ag, transition metals such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, hydrocarbon traps such as clays or zeolites, alkali metals, alkaline rare earths, perovskites including calcium titanate CaTiO3, etc.  For the storage of nitrogen oxides, NOx absorption metals may be provided such as alkali metals, alkaline rare earths, transition metals or oxides of one, two or more oxides. the following elements: zeolites, zeolites doped with preferably oxides of Al, Ce, Zr, Cu, Fe, Mn, W and Cu and mixtures thereof, perovskites, clays, carbon and / or their oxides.  Similarly, other materials such as lanthanide and actinide oxides and their derivatives can be used.  Precious metals can also be used to accelerate the adsorption and desorption rate of nitrogen oxides.  Precious metals and their derivatives such as Au, Ag, Ir, Ru, Rh, Pt, Pd can serve as reduction sites for nitrogen oxides in the presence of reducing agents.  Zeolites may not only serve as adsorption materials for nitrogen oxides but may also serve as hydrocarbon or HO traps.  All these examples are not limiting.  One or more active exchange phases can be used.  Materials for storing nitrogen oxides or platinum group noble metals can also be used.  For the sake of argument, an oxidation catalyst with a NOx trap may have a 1.6 L monolithic cordierite structure with approximately 240 cells per cm 2 and a wall thickness of 0.15 millimeters.  The loading ratio of platinum group metals between platinum, palladium and rhodium can be respectively 2, 1 and 1.  The nitrogen oxide storage material may be a cesium oxide of the CeOx formula with other oxides such as ZrOx or BaOx.  Advantageously, it can be provided a nitrogen oxide probe 7 upstream of the NOx trap in the oxidation catalyst 2 as well as a nitrogen oxide probe 12 downstream of the particulate filter 4, the filter 4 may be impregnated or not with catalytic reduction agent.  The readings of these probes 7, 12 to nitrogen oxides are sent to the control unit 9 comprising a microprocessor for the control of emissions of nitrogen oxides in the exhaust line.  Such an exhaust line, for the pollution control of nitrogen oxides, can comprise simultaneously a passive NOx trap and a selective catalytic reduction system 3, the latter being able to be integrated in a particulate filter 4, which is not the case in Figure 1.  In this selective catalytic reduction system 3, there is also provided a reducing agent liquid reservoir 8, an agent injector 5 in the line.  The amounts of reducing agent injected are controlled by the control unit 9.  With regard to the selective catalytic reduction system 3, as reducing agent used for the reduction of NOx in the exhaust line, it can be used carbon monoxide, hydrocarbons, alcohols preferably glycols or glycerols, esters, acids, ammonia and ammonium hydroxide, urea, guanidine or guanidine salts, individually or in combination.  These reducing elements can be injected by the injector 5 into the exhaust line according to a pre-established injection strategy, which according to the present invention can take into account the presence of SOX in the exhaust gas.  The selective catalytic reduction system 3 and the NOx trap in the catalyst 2 can work together provided that the NOx trap is not full of NOx and is in a state to provide a cleanup NOx.  During desorption of the NOx trap, the selective catalytic reduction system 3 has the role of reducing desorbed NOx, which implies an increase in the amount of reducing agent injected into the line.  Alternatively, there may also be a possibility of permutation of the selective catalytic reduction system to the nitrogen oxide trap by replacing the first by the second or retaining only the second when the amount of reducing agent in the system. RCS is below a minimum predetermined threshold value deemed critical to ensure the depollution, a return to the catalytic reduction system being provided as soon as the addition of reducing agent, so as soon as the reservoir 8 is filled.  Other pollution control elements, for example for hydrocarbons and carbon monoxide, not shown in this figure can also be provided in the exhaust line.  These elements may for example be of the three-way catalyst or reduction catalyst type.  On the exhaust line, it may also be provided other means of measuring emissions, for example a pressure sensor 6 or a temperature sensor.  The present invention essentially relates to two methods, the second of which is dependent on the first.  The first method relates to the control of purge sulfur oxides of a depollution means present in the exhaust line of a motor vehicle while the second concerns the control of the pollution control of nitrogen oxides in a line of exhaust of a motor vehicle.  With reference to FIG. 2, the various steps of a pollution control process including the purge control method are shown.  It should be borne in mind that some of these steps are not essential for the realization of the present invention.  The control unit and control of the depollution of the exhaust line previously referenced 9 in Figure 1 receives the operating parameters 13 of the engine of a motor vehicle for an estimation of nitrogen oxides or NOx and sulfur oxides or SOx at the output of the engine of the motor vehicle this being shown under reference 14 in Figure 2.  It is also possible to measure at least partially the amount of NOx or SOx released by the engine of the vehicle.  The control method can use a SOx absorption and desorption model in the passive NOx trap, this model being referenced 15, a NOx absorption and desorption model in the passive NOx trap. model being referenced 16, a model for reducing NOx and the consumption of reducing agent in a selective catalytic reduction SC system, this model being referenced 17 and finally a reducing agent injection model RCS, this model being referenced 18, to save the reducing agent, especially when it is present in a catalyst impregnation layer, for example the particulate filter.  All these steps, relating to the second method of controlling the pollution of nitrogen oxides, are part of a global model for taking into account the absorption of NOx and SOx and to assess their influence on the injection of reducing agent RCS into the exhaust line of the motor vehicle, this reducing agent being present in a tank of the RCS system and reacting with the NOx.  The concentrations of NOx and SOx stored in the passive NOx trap are a function of time and temperature.  The first rectangle referenced 14 of Figure 1 illustrates a basic model for evaluating NOx and SOx emissions at the output of the engine of the motor vehicle.  These evaluations are based on maps based on the engine torque, engine speed, coolant temperature, vehicle speed, etc., the engine parameters symbolized by the rectangle 13 being transmitted to the control unit. in the exhaust line of the vehicle.  Such maps are known and will not be more fully described below.  Such maps can also be replaced or verified by NOx or SOx sensors.  The rectangle referenced 15 of FIG. 1 illustrates a basic model for evaluating the absorption of SOx in the passive NOx trap by absorption present in the exhaust line of the motor vehicle.  The NOx trap stores the NOx by absorption in its interior and can restore them by desorption under certain operating conditions of the engine of the motor vehicle.  This NOx trap is likely to be poisoned by SOx, which distorts NOx estimates stored in the trap.  In this model for estimating SOx, it is postulated that an adsorbed SOx molecule takes the place of a NOx molecule in the trap absorption site.  Advantageously, the model is divided into sub-models respectively characterizing the adsorption of SOx, the desorption of SOx and SOx at the output of the passive NOx trap.  The sulfur content of the fuel used can be regularly updated with the help of a map based on the geographical area in which the vehicle is traveling and the quality of the fuel available in this area.  For the SOx adsorption model, an adsorption rate is defined according to the following equation: SOx adsorption rate = K ads, SOx * [SOx] * (1-0S0x) K ads, SOx = Aads, S0x * exp (-Eads, SOx / R * Ts) [0062] For the SOx desorption model, a desorption rate is defined according to the following equation: Desorption rate SOx = K des, SOx * [ eS0x] K of, SOx = Ades, S0x * exp (-Edes, SOx * [eS0x] / R * Ts) [0063] By the model, it is estimated a mass of SOx stored according to the following equation: ([SOx stored]) = ([SOx motor output] * flow rate * Ts * SOx desorption rate) - [SOx] output in which equation, [SOx Motor output] is the estimated mass of sulfur oxides at the motor output , rate of flow the rate of flow of gases in the exhaust line, Ts the surface temperature of the air pollution control means, here the NOx trap, desorption rate SOx the desorption rate of sulfur oxides in the NOx trap, [SOx] the estimated mass of sulfur oxides at the outlet of the depollution means, here the NOx trap.  In the above equations, the parameters mentioned are respectively: Kads, SOx is the constant adsorption rate of sulfur oxides, [SOx] is the mass of sulfur oxides in the depollution means, OSOx is the cover adsorption surface area for sulfur oxides, R is the exhaust gas constant, Eads, S0x is the activation energy for adsorption, Ts the surface temperature of the depollution means, Aads, S0x is the first preexponential factor, K des, SOx is the constant rate of desorption of sulfur oxides, Edes, SOx is the activation energy for the desorption of sulfur oxides, Ades, SOx is the second preexponential factor [0065] eliminate the SOx that take the place of NOx in the passive NOx trap, it is necessary to purge the trap regularly.  For this, the present invention relates to a purge sulfur oxide control method of a depollution means present in the exhaust line of a motor vehicle, here the NOx trap.  The depollution means absorbs sulfur oxides contained in the exhaust gas and desorb during a purge, the purge being triggered when the mass of sulfur oxides stored in the depollution means is greater than one. critical mass.  The current mass of sulfur oxides stored in the depollution means is regularly estimated and this mass is estimated according to a model taking into account the kinetic parameters for the absorption and desorption of sulfur oxides in the or of the means of depollution, model that was previously described with the equations that have been stated.  Thus, when the stored SOx exceeds a predetermined limit according to the model, it is proceeded to a regeneration of SOx.  The predetermined limit may be 30% of stored SOx in relation to the total NOx storage capacity.  A purge of the sulfur oxides in the NOx trap is therefore initiated when the mass of SOx stored in the estimated NOx trap according to the model taking into account the kinetic parameters for the absorption and desorption of SOx exceeds a predetermined level judged to be detrimental. the effectiveness of the NOx decontamination carried out by the NOx trap.  SOx desorption modes caused by the purge are known and can include rich phases, specific strategies in fuel supply or exhaust gas recirculation system to increase the fuel wealth, the temperature to inside the NOx trap for desorption of SOx.  When a particulate filter is present in the vehicle exhaust line, which is frequently the case, these desorption modes can take place at the end of the regeneration phase of the particulate filter.  For a nonlimiting example, the temperature may be above 700 ° C for five minutes to recover the NOx storage capacity of the NOx trap operating by adsorption, this preferably in the presence of high levels of hydrocarbons and carbon monoxide such as during a regeneration phase.  Alternatively, a SOx desorption cycle can be started systematically as for particle filter regeneration cycles.  Once all SOx have been desorbed, the SOx content stored in the NOx trap is reset.  For the NOx absorption models in the passive NOx trap illustrated by the rectangle referenced 16 in FIG. 1, such models are known from the state of the art and it is not necessary to detail them. precisely in the present application.  Such models can be divided into sub-models, respectively for NOx adsorption, NOx desorption, NOx content downstream of the passive NOx trap.  According to the present invention, during a control process of the pollution control of nitrogen oxides in an exhaust line of a motor vehicle, the exhaust line comprising a means of depollution in the form of one or comprising a trap for passive nitrogen oxides absorbing nitrogen oxides during a phase and the desorbent during an absorbent phase, this nitrogen oxide trap being capable of being poisoned by SOx, the depollution means is purged with SOx in accordance with the purge process described above.  The contents of NOx stored and at the outlet of the absorber are corrected by the SOx models according to the equation: Real [NOx] Stored = [NOx] Stored uncorrected - [SOx] stored equation based on the postulate that a molecule of SOx can replace a molecule of 20 NOx.  A particularly advantageous application of the present invention is when the exhaust line has a selective catalytic reduction system SCR NOx present in the exhaust gas of the line with injection of a quantity of reducing agent. in the estimated line according to the NOx content to be reduced.  In this case, the mass of SOx stored in the NOx trap is taken into account to estimate the amount of reducing agent to be injected.  As regards the NOx reduction model and the reduction agent consumption 17 and the RCS 18 reducing agent injection model, these models and their method of calculation are already known and do not need to be used. to be further described herein, it is important that these models 17, 18 be indirectly corrected by the mass of SOx estimated to have been stored in the NOx trap at the beginning of the purge.  The main advantage of the present invention is that a kinetic-based SOx estimation model is more accurate than a table-based model mainly because the adsorption and desorption rate of SOx as Depending on the speed of the exhaust gas is taken into account.  The adsorption and desorption of SOx have a non-zero duration and are not instantaneous processes.  This can be seen in Figure 3 showing desulfation as a function of time for two samples A and B.  The kinetic SOx storage model can be combined with a NOx adsorption and desorption model to calculate the actual amount of NOx adsorbed and desorbed in a NOx trap.  As a result, a control of the NOx purge for a NOx trap in accordance with the process according to the invention results in a gain in CO 2 and less release of hydrocarbons as well as, if necessary, by an injection of SC reducing agent. more accurate for selective catalytic reduction systems.  There is then less amount of reducing agent lost and greater NOx removal efficiency.  The losses due to too much injection of reducing agent are thus reduced, which has the additional advantage of reducing odors of unused reducing agent which can be strong especially in the case of use of urea or NH3 as reducing agent.  This also provides a greater optimization of the autonomy of the reducing agent reservoir and avoids on-board diagnostic device problems.  A kinetic SOx storage model also makes it possible to control the SOx purge control time by restoring the NOx storage function to an optimal level and avoiding superfluous dilutions of the engine oil, hydrocarbon losses. and penalties in CO2.  This is valid for any type of passive NOx trap by absorption and desorption.  The invention is not limited to the described and illustrated embodiments that have been given as examples.

Claims (10)

REVENDICATIONS1. Sulfur oxide purge control method of a depollution means present in the exhaust line of a motor vehicle, the depollution means (2) absorbing sulfur oxides contained in the exhaust gas and the desorbent during a purge, a purge being triggered when the mass of sulfur oxides stored in the depollution means (2) is greater than a critical mass, the current mass of sulfur oxides stored in the depollution means (2 ) being regularly estimated, characterized in that the current mass of sulfur oxides stored in the depollution means (2) is estimated according to a model taking into account the kinetic parameters for the absorption and desorption of the sulfur oxides in the or the depollution means (2). 2. Method according to claim 1, wherein the current mass of estimated stored sulfur oxides [stored SOx] in the depollution means (2) is estimated according to the following equation: ([stored SOx]) = ([SOx Motor output] * flow rate * Ts) * SOx desorption rate) - [SOx] output in which [SOx Motor output] equation is the estimated mass of sulfur oxides at the motor output (1), rate of flow is the gas flow rate in the exhaust line, Ts is the surface temperature of the air pollution control means (2), SOx desorption rate is the desorption rate of the sulfur oxides in the means of depollution (2), [SOx] output is the estimated mass of sulfur oxides at the outlet of the depollution means (2). 3. Process according to claim 2, wherein an absorption rate, SOx adsorption rate, and desorption rate, SOx desorption rate, and sulfur oxides are estimated according to the following equations: SOx adsorption rate = K ads, SOx * [SOx] * (1-0S0x) K ads, SOx = Aads, S0x * exp (-Eads, SOx / R * Ts) Desorption rate SOx = K, SOx * [eS0x] K, SOx = Ades, S0x * exp (-Edes, SOx * [eS0x] / R * Ts) in which equations: Kads, SOx is the constant rate of adsorption of sulfur oxides, [SOx] is the mass of sulfur oxides in the depollution means (2), OSOx is the adsorption surface coverage for the sulfur oxides, R is the exhaust gas constant, Eads, S0x is the activation energy for the adsorption, Ts the surface temperature of the depollution means (2), Aads, SOx is the first preexponential factor, K des, SO x is the constant desorption rate of sulfur oxides, Edes, SO x is the activation energy for the desorption of oxides of e sulfur, Ades, S0x is the second preexponential factor 4. A method for controlling the pollution of oxides of nitrogen in an exhaust line of a motor vehicle, the exhaust line comprising a pollution control means (2) in the form of one or integrating an oxide trap of passive nitrogen absorbing the nitrogen oxides during an absorption phase and the desorbent during a desorption phase, this nitrogen oxide trap being liable to be poisoned by sulfur oxides, characterized in that the depollution means (2) is purged of sulfur oxides according to a process according to any one of claims 1 to 3. 5. A control method according to claim 4, wherein the current mass of nitrogen oxides in the nitrogen oxide trap in the form of or integrated in the depollution means (2) is estimated according to a model specific to nitrogen oxides yielding a current mass of nitrogen oxides stored in the nitrogen oxide trap, this stored mass of nitrogen oxides being corrected by subtraction of a mass of sulfur oxides stored in the nitrogen oxide trap Nitrogen oxide trap estimated according to the model taking into account the kinetic parameters for the absorption and desorption of sulfur oxides to obtain a mass of nitrogen oxides actually stored in the nitrogen oxide trap. 6. A control method according to claim 4 or 5, wherein a purge of sulfur oxides is initiated in the nitrogen oxide trap when the mass of sulfur oxides stored in the nitrogen oxide trap estimated according to the model taking into account the kinetic parameters for the absorption and desorption of the sulfur oxides exceeds a predetermined level judged to be detrimental to the efficiency of the nitrogen oxide depollution exerted by the nitrogen oxide trap. 7. Control method according to any one of claims 4 to 6, wherein, the exhaust line having a selective catalytic reduction system (3) of the nitrogen oxides present in the exhaust gas of the line with injection of a quantity of reducing agent into the estimated line as a function of the nitrogen oxide content to be reduced, account is taken of the mass of sulfur oxides stored in the nitrogen oxide trap at the same time. estimating the amount of reducing agent to be injected. 8. A control method according to claim 7, wherein there is provided a possibility of permutation of the selective catalytic reduction system (3) to the nitrogen oxide trap when the amount of reducing agent is below a threshold value. predetermined minimum critical to ensure the depollution, a return to the catalytic reduction system being provided as soon as the addition of reducing agent. 9. Exhaust line of a motor vehicle whose decontamination is controlled by a control unit according to a process according to any one of claims 4 to 8, the line comprising a trap for passive nitrogen oxides, characterized in the passive nitrogen oxide trap is integrated in an oxidation catalyst (2). 10. Exhaust line according to claim 9, wherein the oxidation catalyst (2) has an active exchange phase with the exhaust gas flowing through the line, this active phase being impregnated with deposits of at least one chemical element forming the nitrogen oxide trap reacting with the nitrogen oxides to give a chemically stable compound, the active phase being also capable of reacting with sulfur oxides, conditions of richness greater than one in fuel in the exhaust line releasing the oxides of nitrogen and sulfur, the nitrogen oxides being reduced by a selective catalytic reduction system present in the exhaust line downstream of the nitrogen oxide trap. 25