FR2927945A1 - Sulfur oxide purging controlling method for motor vehicle, involves determining variation of mass of sulfur oxides according to temperature of converter, where variation is equal to zero, while quantity of surface oxygen is not zero - Google Patents

Abstract

The method involves activating purging of a catalytic converter (4), when a magnitude of the converter exceeds a preset threshold. A variation of mass of the sulfur oxides purged by a time unit is evaluated according to the temperature of the converter, an air-fuel mixture richness, the magnitude taking into account a quantity or presence of surface oxygen, where the surface oxygen is an oxygen impregnated on a surface of absorption units of the converter, and the variation is equal to zero, while the quantity of surface oxygen is not zero.

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.

A good knowledge of the quantity of stored sulfur makes it possible to reduce the constraints related to its elimination. The current state of the art does not make it possible to measure the amount of sulfur present in the exhaust gas. It is therefore necessary to estimate its quantity and its distribution in the trap by modeling (open loop), its measurement can not be made using sensors. In addition, it is interesting in a cost reduction approach to use modeling rather than a sensor, even when it exists.

Currently, the sulfur entering the NOx Trap is evaluated from the fuel consumption and from the average oil consumption. The amount of sulfur stored is a proportion of the incoming sulfur, this proportion being given by mapping according to the operating points of the engine.

It has already been proposed in EP-A-0 962 639 to estimate the sulfur poisoning of a catalyst. This sulfur oxide (SOx) purge control method of a catalytic converter for treating the exhaust gases of an internal combustion engine, said pot comprising means for adsorbing sulfur oxides (SOx) contained therein 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 poor or stoichiometric mixture, comprises the following steps: a) the current mass (SSi) of adsorbed sulfur oxides in the pot is evaluated at a sampling instant i, using the model: 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 crosses a threshold ( If) predetermined, depending on the temperature (Tcat) of the pot. However, the evoked estimation method is still perfectible, in terms of determination of 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 carbon dioxide. sulfur SO2, rather than hydrogen sulphide (H2S) or carbonyl sulphide (COS). These different objects are achieved in accordance with the present invention.

It is therefore a process as described above, which is essentially remarkable in that in step b), the variation of the sulfur oxide mass is evaluated for said pot ( SOx) purged per unit of time dmSOx / dt depending on the pot temperature (Tcat), the richness (Ri) of the air / fuel mixture, as well as on a "stored mO2" quantity which takes into account the the amount or the presence of surface oxygen (OS), this "surface oxygen" (OS) being the oxygen impregnated on the surface of the adsorption means of said pot, said variation dmSOx / dt being, however, considered to be equal zero until the amount of surface oxygen (OS) is different from zero.

The present invention therefore proposes to add to the parameters making it possible to evaluate the desulfation rate, its dependence on the quantity of surface oxygen, or its mere presence, available in the trap, by assuming that SO2, H2S and COS appear only when the OS is fully consumed.

We then assume that we are dealing with the following two simplified reactions: • In rich mode and in the presence of OS: reducing agents + OS - * No desulfation; • In rich mode and in the absence of OS: reducers without OS - * H2S + COS + S02 with possible conversion of S02 in H2S + COS (this reaction is all the stronger as the time spent in the catalyst by the desorbed sulfur is important). This means that when the conditions of temperature (about 700 ° C) and mixing (richness greater than 1) are reached, then: • As long as the OS is present in the trap, the reducers and the OS will react together but there will be no effective desulfation. • Once there is no more OS in a part of the NOx Trap, the reductants will then react with the sulfur stored in the trap and cause desorption of sulfur in the form of H2S, COS and SO2 as follows. a kinetic law of desorption according to different parameters (for example richness, temperature, VVH ...) • In a generally poor medium (that is to say with presence of bone or poor gas mixture), the sulfur (SO2, H2S, COS) is deposited (or redeposited) on the trap in SO2 form, following a kinetic law of adsorption function of different parameters (for example richness, temperature, WH, sulfur mass already stored ...). Other advantageous and non-limiting characteristics of this process: the variation of the mass of sulfur oxides (SOx) purged per time unit dmSOx / dt is also evaluated for said pot, as a function of the hourly space velocity of the gases (WH) and the mass of instant sulfur oxides (mSox) retained in the reactor, so that this variation is expressed as follows: dmSOx / dt = f (Tcat, Ri, VVH, mSOx, stored mO2). said quantity "stored mO2" is determined from a model of consumption of surface oxygen; said "stored mO2" quantity is determined from a measurement of oxygen probe (s) placed downstream and / or upstream of said pot; said pot is modeled by subdividing it into n contiguous reactors, n being greater than or equal to 1 and each of them is associated with sulfur oxide (SOx) masses and different temperatures; said temperatures are given by sensors or by temperature models.

Other features and advantages of the invention will appear on the detailed reading which follows 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 30 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. 4 represents curves showing, as a function of time, the evolution of the flow rates of sulfur species downstream of the catalytic converter depending on whether surface oxygen is present or not; FIG. 5 is a curve giving, also as a function of time, the evolution of the amount of surface oxygen; FIG. 6 groups curves showing, as a function of time, the desorption kinetics of the various sulfur species; FIG 7 is a schematic view of a catalyst, substantially similar to Figure 2; FIG. 8 is a curve giving, as a function of the reactor of the catalyst of FIG. 7, the evolution of the richness of the mixture passing through these reactors; FIG. 9 is a curve giving, simultaneously, the evolution of the surface oxygen; - Finally, Figure 10 is a curve giving a view of the kinetics in progress on the first two reactors of the catalyst. FIG. 1 shows schematically an internal combustion engine 1, the operation of which is controlled by a digital electronic calculator 2. The combustion chamber is connected to an exhaust line 3. A catalytic converter 4 for the treatment of gases of 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 which can operate the lean-burn engine (normal operation). The computer 2 controls the operation of the injector 6 and the valve 5 to adjust the composition of the air / fuel mixture 30 of the engine supply, so that the richness of this mixture is momentarily greater than 1 (purge operation 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 operates lean air / fuel mixture.

The computer 2 is moreover duly programmed to notably: -determine the current value, at a sampling instant i, of the mass of sulfur oxides absorbed by the pot 4, using a model, -decide triggering and stopping 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, possibly order 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. 25 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 gases 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 instantaneous SOx storage efficiency (EFFSOx) is defined in the following manner: 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: SS / SSC = mass of stored SOx (SS) maximum mass of storable SOx (SSC) with SS = SOx Storage = mSOx, ie the mass of trapped sulfur 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, the storage efficiency EFFSOx is a function of the following parameters: the temperature of the catalyst Tcat, the ratio of the mass of SOx stored SS on the maximum mass of SOx storable SSC and the richness of the fuel Ri. It is possible to take into account, in addition to the aforementioned parameters, the residence time of the gases or WH (hourly volume velocity), a function of the flow rate of the gases. This refines the model by this consideration. We have in the end:

EFFSOx = f (T, SS / SSC, Ri, WH) It may also be necessary to take into account also the thermal aging of the monolith in the estimation of the sulfur loading efficiency.

EFFSOx becomes then:

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

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

Operation in storage of SOx

The SOx sulfur oxides release occurs when the richness of the exhaust gases in the NOxTRAP is greater than or equal to 1. The invention can be summed up in two kinetics (adsorption and desorption) which occur in the following cases: adsorption in the form of SO2, if the medium is poor overall, • desorption in the form of SO2, H2S, COS, if the medium is globally rich. A third kinetic is used, that of the conversion of SO2 into H2S and COS in the presence of catalyst in a generally rich medium. This means that as long as there are OSCs in the NOx Trap, there is no desorption of sulfur products. In other words, the desulfation rate dmSOx / dt is zero. Once the OS has been fully consumed by the reducers, the speed takes a value that depends on the 35 parameters mentioned above. In summary, the desulfation rate dmSOx / dt is expressed as: dmSOx / dt = f (T, richness, WH, mSOx, stored mO2), which also corresponds to the downstream sulfur flow rate mSOxi = mSOxi- 1 + (dmSOx / dt) * DT

Taking into account the OSC will thus make it possible to reflect the two chemical reactions exposed above. Indeed, each chemical reaction has its own kinetics: the species react and form at a rate dependent on the reaction involved and the environment where the reaction is located. We will thus express two different speeds (since involving two different reactions one after the other) depending on the presence (or quantity) of surface oxygen.

This allows a better description of the phenomena involved and therefore a better estimate of the amount of SOx present in the trap at every moment. The information relating to the quantity or the simple presence of oxygen can be provided by a model of consumption of oxygen. surface oxygen such as that described in French Application No. FR0506202, or by the use of oxygen probes downstream and / or upstream of the NOxTRAP volume. Taking this additional parameter into account makes it possible to improve the accuracy of the calculation of the desulfation rate by differentiating the reactions with and without oxygen. This allows a more precise 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 accurate estimation of SOx and NOx masses makes it possible to limit dilution and overconsumption by optimizing the regeneration phases of the catalytic converter and their duration. Finally, this makes it possible to distinguish sulfur species desorbed as SO2 or H2S. It then makes it possible to estimate the nature of the sulfur species desorbed and their flow rate. This makes it possible to provide two useful information for the control of desulfation by fractionation of wealth, instead of just one. Thus, by comparing the curves of FIGS. 4 and 5, it is observed that as long as OS retains a significant value (time period before t1), the sulfur oxides are converted into SO2. After this period of time, they are transformed into H2S and COS.

In parallel, the desorption kinetics of SO2 corresponds to the curve A of FIG. 6 and is zero, when one takes into account the present of OS. It corresponds to the curve B, and the curve C, as soon as the value of the OS is equal to zero, the curve B corresponding to the desorption kinetics of the SO2, while the curve C corresponds to the desorption kinetics of the H2S.

Discretization of the volume of the catalytic converter in n reactors

The rate of desulfation being characterized in particular by the temperature and the mass of sulfur stored at this time, it may be useful to take into account the distribution of SOx mass and temperature heterogeneity throughout the NOxTrap range. Indeed, until now, we modeled the total mass of SOx stored by the catalytic converter considered to consist of a single 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. Therefore, 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 with each of them masses of SOx 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 Figures 2 and 7, NOxTrap is discretized into 4 reactors. To each of them corresponds a temperature information (TI to T4) which makes it possible to calculate their mass of clean SOx (mSOx1 to II 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 one. Then, the loading model becomes, for each reactor: SS; = mSOx;, i + 1 = mSOx; i + QSOx; .EFFSOx; With QSOxi = sulfur flow upstream of the reactor. For the first reactor, QSOxi = QSOx = engine sulfur flow; For the other reactors, QSOxi = QSOxAVAL (; _c) = flow of sulfur downstream of the preceding reactor, ie: QSOxAVAL (i) = (1-EFFSOx (;) * QSOxAVAL 0-1) Furthermore, the desulfation model becomes for each reactor: dmSOx; / dt = f (Tcat; Ri, VVH; mSOx;) and the sulfur flow rate downstream of the trap becomes QSOxAVAL = Sum (dmSOx; / dt) This taking into account of the distribution allows a better estimate of the desulphatation rate and therefore the trapped sulfur mass, which allows an optimization of the estimated mass of NOx stored. Thus, with reference to FIGS. 7 to 10, it is considered that, as long as oxidants (OS) remain to react with the excess reductants of the gases in a reactor, it is believed that the mixture which reaches the following reactors is stoichiometry and without reducers to reduce their bone. After the OS has been consumed on the first reactor, the excess oxygen-associated reductants of the gases react to form SO2 and H2S. However, this sulfur remains on the following reactors as long as they have surface oxygen to reabsorb. Finally, FIG. 3 shows all the parameters used in the context of the present method and the variables that they make it possible to determine. 10 15 20

Claims (7)

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 in step b), the variation of the mass is evaluated for said pot; of sulfur oxides (SOx) purged per unit of time dmSOx / dt as a function of the pot temperature (Tcat), the richness (Ri) of the air / fuel mixture, as well as according to a quantity "mO2 stored "which takes into account the amount or the presence of surface oxygen (OS), this" surface oxygen "(OS) being the oxygen impregnated on the surface of the adsorption means of said pot, said variation of SOx / dt is, however, considered to be zero as long as the amount of surface oxygen (OS) is not zero. 30 2. Method according to claim 1, characterized in that it is evaluated, for said pot, the variation of the mass of sulfur oxides (SOx) purged per unit of time dmSOx / dt also as a function of the hourly volume velocity of gas (VVH) and the mass of oxides of instant sulfur (mSox) retained in the reactor, so that this variation is expressed as follows: dmSOx / dt = f (Tcat, Ri, VVH, mSOx, stored m02 ). 3. Method according to claim 1 or 2, characterized in that the variation of the mass of sulfur oxides (SOx) purged per unit of time dmSOx / dt is also evaluated for said pot, as a function of a parameter taking into account the aging of the constituent material of the catalytic converter (aging). 4. Method according to one of the preceding claims, characterized in that said size "stored mO2" is determined from a surface oxygen consumption model. 5. Method according to one of claims 1 to 4, characterized in that said size "mO2 stored" is determined from a measurement of oxygen sensor (s) placed downstream and / or upstream of said pot. 15 6. Method according to one of the preceding claims, characterized in that one modelises said pot (4) by subdividing it into n contiguous reactors, n being greater than or equal to 1 and associated with each of them masses of sulfur oxides (SOx) and different temperatures. 20 7. Method according to claim 6, characterized in that said temperatures are given by sensors or temperature models.