FR2927943A1 - Sulfur oxides purging controlling method for motor vehicle, involves determining instantaneous storage effectiveness of sulfur oxides from value obtained from formula formed by parameter taking into account ageing of material of converter - Google Patents

Abstract

The method involves evaluating a current mass of sulfur oxides absorbed in a catalytic converter (4), at a sampling instant by using a model constituted by flow of the oxides in an exhaust of an internal combustion engine (1) and instantaneous storage effectiveness of the sulfur oxides in the converter. The effectiveness is determined from a value obtained from a table or formula constituted by a temperature of the converter, filling ratio of the oxides, air-fuel mixture richness, volumetric speed per hour of gas and a parameter taking into account ageing of a material of the converter.

Description

The present invention relates to a method for purging sulfur oxides of a catalytic converter for treating the exhaust gases of an internal combustion engine. In order to meet the lower thresholds for emissions of gaseous pollutants from motor vehicles, catalytic systems, of increasingly complex gas after-treatment, are arranged in the exhaust line of lean-burn engines. These reduce particulate and nitrogen oxide emissions, in addition to carbon monoxide and unburned hydrocarbons. Unlike conventional oxidation catalysts, these systems operate discontinuously or alternatively, that is, in normal operation, they trap pollutants, but only treat them during regeneration phases.

Thus to be regenerated, these traps require specific modes of combustion to ensure the necessary thermal and / or wealth levels. The storage capacity of a catalyst comprising a nitrogen oxide trap (or NOx) is altered over time by the presence of sulfur in the exhaust gas. As it is indeed present in the fuel and the oil, it is found in the exhaust. It is trapped on the reaction sites of the trap, thus decreasing the number of reactive sites for NOx. In other words, the efficiency of a NOx trap decreases with the accumulation of sulfur in it.

As for NOx, it is necessary after having crossed a certain loading threshold, to eliminate (or desorb sulfur) to restore the trap of the efficiency in terms of NOx absorption. This process, called desulfation or deSOx, requires a reducing medium (rich mixture) like the NOx purges, but also a high thermal level in the trap (NOx-Trap) (of the order of 700 ° C). This operation has impacts in terms of mechanical strength of the engine (extreme thermal of the various organs), fuel dilution in oil and consumption. In addition, the operating conditions are difficult to obtain on the vehicle.

Another problem arises in catalysts exposed to high temperatures. This is the aging of the catalytic phase, which mainly results in an irreversible reduction in the specific surface area of the gas treatment and an increase in the initiation temperature of the catalytic reactions. This has a strong impact on the efficiency and loading capacity of NOx, sulfur and oxygen as well as efficiency of regeneration of the trap (NOx reduction and desulfation). A good knowledge of the amount of sulfur stored in loading as desulfation reduces the constraints associated with its elimination. However, there are no onboard sensors to measure sulfur species. This quantity of sulfur must therefore be evaluated by calculation. It has already been proposed by EP-A-0 962 639 to estimate the sulfur poisoning of a catalyst. However, the estimation method evoked is still perfectible, in terms of determining the exact mass of sulfur oxides, costs related to the desulfation of the catalyst, and optimization of the desorption of sulfur in the form of sulfur dioxide. SO2, rather than hydrogen sulphide (H2S) or carbonyl sulphide (COS). These different objects are achieved in accordance with the present invention. Thus, it relates to a process for controlling sulfur oxide (SOx) purge of a catalytic converter for treating the exhaust gases of an internal combustion engine, said pot comprising means for adsorbing oxides of sulfur (SOx) contained in these gases, according to which a purge of the sulfur oxide (SOx) pot is triggered by an increase in the richness (Ri) of an air / fuel supply mixture of the engine, starting from a richness corresponding to a lean or stoichiometric mixture, and which comprises the following steps: a) the current mass (SSi) of sulfur oxides adsorbed in the pot is evaluated at a sampling instant i, using the model: 35 SSi + 1 = SSi + Qsox. EFFSOX where Qsox is the flow of sulfur oxides in the engine exhaust and EFFSOX is the instantaneous storage efficiency of the sulfur oxides in the pot, b) a purge of the pot is triggered when said quantity 5 exceeds a threshold ( Si) predetermined, a function of the temperature (Tcat) of the pot, characterized by the fact that EFFSOX is determined from a value derived from a table or a formula whose inputs are the temperature (Tcat), the fill rate (SS / SSC) in pot sulfur oxides, the richness (Ri) of the air / fuel mixture and at least one of the following additional inputs: the hourly volume velocity of the gases (WH) and a setting parameter account of aging of the constituent material of the catalytic converter (Aging). Thus, by adding one or two additional variables for the determination of EFFSOX, the calculation is improved. Furthermore, according to other advantageous and nonlimiting features: - only one of said additional inputs is taken into account; these two additional inputs are taken into account; In step a), the flow rate of sulfur oxides (downstream QSOx) circulating downstream of the pot is estimated by the following relationship: QSOx downstream = (1-EFFSOx) * QSOX upstream, where QSOX upstream is the flow rate in oxides sulfur circulating upstream of the pot; In step b), ie during a purge of the pot, the variation of the mass of oxides of sulfur (SOx) stored per unit of time dmSOx / dt is evaluated as a function of the temperature catalyst (Tcat), the richness (Ri) of the air / fuel mixture, the hourly volume velocity of the gases (WH) and the mass of instant sulfur oxides (mSox) retained in the pot, ie dmSOx / dt = f (Tcat, Ri, VVH, mSOx); and deduce the total mass (SS) of sulfur oxides stored in the pot at a time i from the relationship SS = mSox; +1 = mSOx; + dmSOx; ; said pot is modeled by subdividing it into n contiguous reactors, n being greater than or equal to 2 and each of them is associated with sulfur oxide (SOx) masses and different temperatures; said temperatures are given by sensors or by temperature models; to evaluate the current mass (SSi) of adsorbed sulfur oxides, at a sampling instant i, of one of the reactors j, j being between 1 and n, the model is used: SSii (j) = mSOx; (j-1) + QSOx; . EFFSOx; where QSOx; is the instantaneous flow rate of sulfur oxides upstream of said reactor, and EFFSOX; is the instantaneous storage efficiency of the sulfur oxides in the pot; In step b), for a reactor j, the variation of the mass of sulfur oxides (SOx) purged per unit of time dmSOx / dt is evaluated as a function of the reactor temperature (Tcati), the richness (Ri) of the air / fuel mixture, the hourly gas volume velocity (VVH) and the instantaneous sulfur oxide mass (mSox) retained in the reactor, is dmSOx; / dt = f (Tcat; , Ri, VVH ;, mSOx;), and that the flow rate of sulfur oxides downstream from this reactor is deduced by the following relationship: QSOx downstream = sum (dmSOx / dt); the so-called dmSOx / dt variation is evaluated by also taking into account either the oxygen concentration of the gases in the reactor, or a binary information related to the presence or absence of oxygen in the reactor gases; the said variation dmSOx / dt is evaluated by also taking into account a parameter related to the aging of the monolith of said reactor. Other features and advantages of the invention will become apparent from the following detailed reading of a preferred embodiment. This description will be made with reference to the accompanying drawings in which: - Figure 1 is a diagram showing a device for carrying out the method according to the invention; FIG. 2 is a diagram showing the modeling of a catalyst in four reactors; FIG. 3 is a block diagram illustrating the various parameters and their interactions in the method according to the invention. FIG. 1 shows schematically an internal combustion engine 35 1 whose operation is controlled by a digital electronic computer 2. The combustion chamber is connected to an exhaust line 3. A catalytic converter 4 for treating gas exhaust is mounted in the line 3, and there is provided a control valve 5 of the air flow entering the engine such as a motorized throttle for example.

The gasoline or diesel engine also comprises a plurality of fuel injectors 6 arranged so as to inject fuel into the combustion chamber that can operate the engine lean (normal operation). The computer 2 controls the operation of the injector 6 and the valve 5 to adjust the composition of the air / fuel supply of the engine, so that the richness of this mixture is momentarily greater than 1 (purge operation of the pot 4) in the combustion chamber. The computer 2 may optionally control an additional injector 7 of fuel placed in the exhaust line 3. A sensor 8 delivered to the computer 2 a signal representative of the temperature Tcat of the catalytic converter 4 or the temperature of the gases entering the pot. Other sensors (not shown) deliver to the computer signals representative of other quantities conventionally used to manage the operation of the engine: engine speed, air intake pressure, etc. The catalytic converter 4 is naturally of the type comprising means for adsorbing nitrogen oxides and sulfur when the engine 25 operates lean air / fuel mixture. The computer 2 is moreover duly programmed in particular to: determine the current value, at a sampling instant i, of the mass of sulfur oxides absorbed by the pot 4, using a model; decide to trigger and stop a purge according to the mass of sulfur oxides stored in the pot 4 and the temperature thereof, this using an expert system, - then order the establishing the richness of the air / fuel mixture at a value corresponding to a stoichiometric or rich mixture, when such a purge is triggered, optionally controlling an increase in the temperature of the exhaust gas. Hereinafter will be described the structure and operation of the means necessary for the execution of the various functions of the system. Operation in storage

This operation is observed when the engine is fed lean mixture. The calculation of the sulfur flow from the engine is based on knowledge of the sulfur contents of the fuel and the oil required for engine lubrication. Since the sulfur contents are known, the estimation of fuel flow rates and oil consumption makes it possible to evaluate the quantity of sulfur contained in the exhaust gas leaving the engine. From this data, it is estimated how much sulfur can be trapped in the NOx trap (NOxTrap). For this purpose, a loading efficiency is defined which is used to characterize the SOx loading of the trap. Thus, according to the above-mentioned EP0962639: An instant efficiency of storage of SOx (EFFSOx) is defined as follows: EFFSOX = 1 - SOx flow at the outlet of the exhaust line 3 SOX flow at the engine outlet 1 A SSISSC filling ratio is further defined by: SSISSC = mass of stored SOx (SS) maximum mass of stored SOx (SSC) with SS = SOx Storage = mSOx, ie the mass of sulfur trapped in the NOxTRAP at time t. SSC = SOx Storage Capacity, that is the SOx storage capacity, NOxTRAP-specific value, identified and mapped, varying with the temperature of the catalyst Tcat. According to the aforementioned patent, EFFSOx storage efficiency is a function of the following parameters: the temperature of the catalyst Tcat, the ratio of the mass of SOx stored SS on the mass of maximum SOx storable SSC and the richness of the fuel Ri. According to the present invention, the residence time of the gases or VVH (hourly volume velocity) is also taken into account, as a function of the flow rate of the gases. This refines the model by this consideration. We have in the end:

EFFSOx = f (T, SSISSC, Ri, WH)

Furthermore, the present invention proposes to also take into account the thermal aging of the monolith in the estimation of the sulfur loading efficiency.

EFFSOx becomes then:

EFFSOx = f (T, SS / SSC, Wealth, WH, Aging) Finally, in loading mode, we have at the moment i: SS = mSOx; +1 = mSOx; + QSOx.EFFSOx

The present invention also makes it possible to estimate the sulfur flow rate downstream of the NOxTRAP, in the loading mode: This flow rate is expressed as follows: QSOxAVAL = (1-EFFSOx) * QSOx

Operation in SO 2 removal The removal of SOx sulfur oxides occurs when the exhaust gas richness in NOxTRAP is greater than or equal to 1. According to the aforementioned patent, the variation of the mass of SOx ( dmSOx / dt) stored per unit of time is a function of the temperature of the catalyst Tcat, the richness Ri and the gas flow rate (or VVH). The present invention adds to it the instantaneous SOx mass dependence contained in the mSOx (or SS) trap. We then obtain in the end: dmSox / dt = f (Tcat, Ri, WH, mSOx) SS = mSOxi + 1 = mSOxi + dmSOxi The invention also makes it possible to estimate the sulfur flow rate downstream of the NOxTRAP in desorption mode: QSOxaval = dmSOx / dt Discretization of the catalytic converter volume in n reactors

The rate of desulfation being characterized in particular by the temperature and by the mass of sulfur stored at this time, the present invention proposes to take into account the distribution of the SOx mass and the temperature heterogeneity throughout the extent of the NOxTrap. Indeed, until now, we modeled the total mass of SOx stored by the catalytic converter considered as consisting of a single volume.

The distribution of this mass was therefore considered homogeneous in the trap. Depending on the volume of NOxTRAP modeled or the expected accuracy, this simplification may be too reductive. This is why, in order to be able to represent the impact of this heterogeneity on the physics of sulfur desorption and its estimation. The invention proposes to divide this single reactor into n reactors allowing to associate to each of them SOx masses and different temperatures. We then obtain a discretized image along a dimension of the distribution of SOx in the NOxTrap. The amount of sulfur in each of the reactors is then determined by the efficiency of each of them, thanks to the knowledge of their temperature, given by sensors or temperature models (themselves discretized). In the illustration of Figure 2, NOxTrap is discretized into 4 reactors. To each of them corresponds a temperature information (T1 to T4) which makes it possible to calculate their mass of clean SOx (mSOx1 to mSOx4). The mass of total SOx is then the sum of the 4 calculated masses, ie SS = Sum (SSi). Of course, the number of reactors can be different, provided that it is greater than or equal to two.

Then, the loading model becomes, for each reactor: SSi = mSOxi, i + 1 = mSOxii + QSOxi.EFFSOxi With QSOxi = flow of sulfur upstream of the reactor. For the first reactor, Q SO x = Q SO x = engine sulfur flow; For the other reactors, QSOxi = QSOxAVAL (ic) = flow of sulfur downstream of the preceding reactor, ie: QSOxAVAL (i) = (1-EFFSOx (i)) * QSOxAVAL (i-1) Furthermore, the desulphatation model becomes, for each reactor: dmSOxi / dt = f (Tcati, Ri, VVHi, mSOxi) and the flow rate of sulfur downstream of the trap becomes QSOxAVAL = Sum (dmSOxi / dt) This taking into account of the distribution allows a better 25 estimation of the desulphatation rate and therefore of the trapped sulfur mass, which allows an optimization of the estimated mass of NOx stored.

Taking into account the presence of O2 in the exhaust qaz 30:

One of the most influential parameters on the desulfation rate is the local temperature in the NOxTrap. According to the embodiment of the rich gaseous medium for the desulfation, there remains a proportion of oxygen in the gases which has not reacted. This oxygen oxidizes part of the reductants present in excess of the NOxTRAP catalytic phase contact and thus locally generates an exotherm strongly involved in the desorption of sulfur. In the presence of catalytic elements in the NOxtTRAP and following the conditions of gas flow temperature, NOxTRAP volume and residual oxygen concentration in the gases, the oxygen may be fully consumed so that the downstream portion of the trap sees only a rich medium and no local exotherm. The present invention proposes to characterize different desulphatation rates as a function or not of the presence of oxygen in the gases in the reactor, this information being able to be provided by an oxygen consumption model. Finally, the desultation model becomes: dmSOx; / dt = f (Tcat ;, Ri, VVH ;, mSOx ;, [021] where [O2] i is either the oxygen concentration of the gases in the reactor, or a binary information informing of the presence or absence of oxygen.

Taking into account aging

The process according to the invention is further improved by taking into account the thermal aging of the catalytic phase in the estimation of the desulfation efficiency. This aging parameter is given by an estimator described in the document FR-A-2 878 961 in the name of the present applicant. We then have: 25 dmSOx; / dt = f (Tcat ;, Ri, VVH ;, mSOx ;, [021, Aging) FIG. 3 shows all the parameters used in the context of the present process and the variables that they allow to determine. The advantages provided by the method according to the invention are the following: the addition of additional parameters allows a better estimate of the mass of SOx contained in the trap. the discretization of the trap into several reactors makes it possible to account for the distribution of the mass of SOx in the length of the trap. 2927943 It - the taking into account of the oxygen of the gases in the desulfation reactions, which allows a more precise estimate of the mass of SOx. aging takes into account the addition of a parameter affecting the efficiency of loading and unloading SOx, and improving the accuracy of the sulfur poisoning estimation model. These improvements make it possible to improve the estimation of the mass of SOx, as well as that of the mass of NOx (directly dependent on the mass of SOx). In addition, a more precise estimation of the SOx and NOx masses makes it possible to limit the costs associated with regenerations, such as dilution and overconsumption, by optimizing the desulfation times.

Finally, the estimation of the sulfur flow rate downstream of the trap, both in loading and in desulphatation, makes it possible to know the flow rate of sulfur entering a downstream post-treatment system.

Claims (11)

1. Sulfur oxide (SOx) purge control method of a catalytic converter (4) for treating the exhaust gases of an internal combustion engine (1), said pot (4) comprising means for adsorption of sulfur oxides (SOx) contained in these gases, according to which a purge of the pot (4) sulfur oxides (SOx) is initiated by increasing the richness (Ri) of an air / fuel mixture of feeding the engine, from a richness corresponding to a lean or stoichiometric mixture, and which comprises the following steps: a) the current mass (SSi) of sulfur oxides adsorbed in the pot (4) is evaluated, a sampling instant i, using the model: SSi- F1 = SSi + Qsox. EFFSOX where Qsox is the flow rate of sulfur oxides in the engine exhaust (1) 15 and EFFSOX is the instantaneous storage efficiency of the sulfur oxides in the pot (4), b) a purge of the pot ( 4) when said quantity crosses a predetermined threshold (Si), a function of the temperature (Tcat) of the pot (4), characterized in that EFFSOX is determined from a value derived from a table or from a formula whose inputs are the temperature (Tcat), the filling ratio (SS / SSC) in sulfur oxides of the pot (4), the richness (Ri) of the air / fuel mixture and at least one of the additional inputs following: the hourly volume velocity of the gases (WH) and a parameter taking into account the aging of the constituent material of the catalytic converter (Aging). 2. Method according to claim 1, characterized in that it takes into account only one of said additional inputs. 3. Method according to claim 1, characterized in that one takes into account said two additional inputs. 4. Method according to one of the preceding claims, characterized in that in step a), the flow rate of sulfur oxides (downstream QSOx) circulating downstream of the pot by the relationship: QSOx downstream = (1 -EFFSOx) * upstream QSOX, where upstream QSOX is the flow rate of sulfur oxides flowing upstream of the pot (4). 5. Method according to one of claims 1 to 4, characterized in that in step b), ie during a purge pot (4), is evaluated the variation of the mass of d sulfur oxides (SOx) stored per unit of time dmSOx / dt depending on the catalyst temperature (Tcat), the richness (Ri) of the air / fuel mixture, the hourly volume velocity of the gases (WH) and the the mass of instant sulfur oxides (mSox) retained in the pot (4), ie dmSOx / dt = f (Tcat, Ri, WH, mSOx), and that the total mass (SS) of oxides is deduced therefrom sulfur stored in the pot from the relation SS = mSox; +1 = mSOx; + dmSOx; 6. Method according to one of the preceding claims, characterized in that one modelises said pot (4) by dividing it into n contiguous reactors, n being greater than or equal to 2 and associated with each of them masses of sulfur oxides (SOx) and different temperatures. 7. Method according to claim 6, characterized in that said temperatures are given by sensors or by temperature models. 8. Process according to claim 6, characterized in that, to evaluate the current mass (SSi) of adsorbed sulfur oxides, at a sampling instant i, of one of the reactors j, j being between 1 and the model is used: SSii (j) = mSOx; (j-1) + Qsox; . EFFSOx; where Qsox; is the instantaneous flow rate of sulfur oxides upstream of said reactor, and EFFSOX; is the instantaneous storage efficiency of the sulfur oxides in the pot (4). 9. Method according to one of claims 6 or 7, characterized in that, in step b), for a reactor j, the variation of the mass of sulfur oxides (SOx) purged by time unit dmSOx / dt as a function of the reactor temperature (Tcati), the richness (Ri) of the air / fuel mixture, the hourly space velocity (VVH) and the instant sulfur oxide mass (mSox) retained in the reactor, ie dmSOx / dt = f (Tcat, Ri, WH, mSOx), and the sulfur oxide flow rate downstream from this reactor is deduced by the following relationship: QSOx downstream = sum (dmSOx) / dt). 10. Process according to claim 9, characterized in that the said variation dmSOx / dt is evaluated by taking into account also the oxygen concentration of the gases in the reactor j, or a binary information related to the presence, respectively absence of oxygen in the reactor gases j. 11. The method of claim 9 or 10, characterized in that one evaluates said dmSOx / dt variation taking into account also a parameter related to the aging of the monolith of said reactor j.