FR2927944A1 - Sulfur oxide purging control method for motor vehicle, involves determining variation in mass of sulfur oxide relative to temperature of converter, air-fuel mixture richness and magnitude taken into account presence of surface oxygen - Google Patents

Abstract

The present invention relates to a sulfur oxide purge control method of a catalytic converter (4) for treating the gases of an internal combustion engine (1), said pot (4) comprising adsorption means of sulfur oxides (SOx), according to which a purge is initiated by increasing the richness (Ri) of an air / fuel mixture, and which comprises the following steps: a) the current mass (SSi) of sulfur oxides adsorbed in the pot (4), using the model: SSi + 1 = SSi + Qsox. EFFSOXwhere Qsox is the flow rate of sulfur oxides and EFFSOX is the instantaneous storage efficiency of the sulfur oxides, b) a purge of the pot (4) is triggered when said quantity crosses a predetermined threshold (Si), characterized by the fact that in step b), for said pot, the variation of the mass of sulfur oxides (SOx) purged per unit of time dmSOx / dt is evaluated as a function of the pot temperature (Tcat), the richness (Ri) of the air / fuel mixture, as well as a "stored mO2" quantity which takes into account the amount or the presence of surface oxygen (OS).

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.

There are two main difficulties during desulfation: The first is to keep the NOxTrap temperature below a maximum admissible temperature by the trap to prevent its destruction, while maintaining a temperature level sufficient to make possible the desorption of SOx. The second is to control the selectivity of sulfur. Indeed, during the purge SOx, we try to eliminate sulfur in a particular form, SO2, which is an odorless species.

But in some cases (wealth too high, lack of OS (ie stored oxygen or surface) in the NOxTrap, too rich niche time), the sulfur can be eliminated by production of H2S and COS, which are two smelly and toxic species with high concentrations.

The challenge of desulfation control is to desorb sulfur as SO2. To do this, the control of the desulphatation is carried out by fractionation: it is about an alternation of richness of the mixture at the entry of the NOx Trap around the wealth 1, that is to say close to a stoichiometric air / fuel ratio. The poor phase (richness less than 1) reduces the temperature of the NOx Trap (by momentary cessation of desulphatation) and reloads the OSC of NOx Trap (oxygen stored on the trap) by the oxygen contained in the gases. Exhaust (indeed, the selectivity SO2 / H2S depends on the amount of OS). The rich phase (wealth close to 1) makes it possible to achieve an effective deSOx under certain conditions. 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, includes the following steps:

3 a) the current mass (SSi) of sulfur oxides adsorbed in the pot is evaluated at a sampling instant i, using the model

SSi + 1 = SSi + Qsox. EFFSOX where Qsox is the flow rate 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 (Si ) predetermined, depending on the temperature (Tcat) of the pot. 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. It is therefore a process as set out above, which is essentially remarkable in that in step b), the variation of the mass of sulfur oxides ( 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 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. The present invention therefore proposes to add to the parameters making it possible to evaluate the desulfation rate, its dependence on the amount of surface oxygen, or its mere presence, available in the trap. Indeed, according to this amount, the rate of desulfation 30 evolves. Thus, depending on whether the surface oxygen is present or absent, there are the following simplified reactions: • Presence of surface oxygen: reducing agents + SO + SOx desorption of SO2. 35 • Absence of surface oxygen: reducing agents + SOx - * desorption of H2S + COS. This means that as long as there is OS in the NOxTRAP, the sulfur products are desorbed as SO2. Once the surface oxygen is fully consumed, the sulfur is desorbed as H2S and COS.

Each of these reactions corresponds to a kinetics of its own, that is to say that the desorption rate will be different depending on the reaction. According to 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, also as a function of the hourly space velocity 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, WH, mSOx, m02 stored). said "stored mO2" quantity is determined from a surface oxygen consumption model; said quantity "stored mO2" 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. 1 shows schematically an internal combustion engine 1 whose operation is managed by a digital electronic computer 2. The combustion chamber is connected to an exhaust line 3. A catalytic converter 4 for treating the exhaust gas is mounted in line 3, and there is provided a control valve 5 of the air flow entering the engine such as a motorized throttle valve 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 35 operates lean air / fuel mixture. 6

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, by means of 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 to perform 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 content 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, a loading efficiency is defined which serves to characterize the SOx loading of the trap. Thus, according to the above-mentioned EP0962639: An instantaneous SOx storage efficiency (EFFSOx) is defined as follows: EFFSOX = 1 - SOx flow at the outlet of the exhaust line 3 SOX flow at the engine outlet 1 SS / SSC filling ratio is further defined by: SS / SSC = mass of stored SOx (SS) maximum mass of stored SOx (SSC) with SS = SOx Storage = mSOx, ie 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. It is possible to take into account, in addition to the aforementioned parameters, the residence time of the gases or VVH (hourly volume velocity), a function of the gas flow rate.

This refines the model by this consideration. We have in the end:

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

One can 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 25 30

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

The SOx sulfur oxides removal occurs when the exhaust gas richness in the NOxTRAP is greater than or equal to 1. As indicated above, as long as there is presence of NOxTRAP in the OS, the products are desorbed. sulfur in the form of SO2. Once the surface oxygen is fully consumed sulfur is desorbed as H2S. To each of these reactions corresponds a kinetics of its own, that is to say that the desorption rate will be different depending on the reaction. The present invention proposes to take into account a quantity taking into account the quantity or presence of bone, for the determination of the desulfation rate dmSOx / dt (or variation of the mass of sulfur oxides purged per unit of time), which is expressed as dmSOx / dt = f (T, richness, WH, mSOx, stored m02), 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) according to the presence (or quantity) of surface oxygen.5 This allows a better description of the phenomena involved and therefore a best estimate of the amount of SOx present in the trap at each moment The information relating to the quantity or the mere presence of oxygen can be provided by a surface oxygen consumption model such as that described in the French application. No. FR0506202, or by the use of oxygen sensors downstream and / or upstream of the volume of NOxTRAP. 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 the 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 usable information for the control of desulfation by fractionation of wealth, instead of just one. Thus, by comparing the curves of Figures 4 and 5 attached, it is observed that as OS retains a significant value (time period before t1), the sulfur oxides are converted into SO2. After this period of time, they are converted to H2S and COS. In parallel, the desorption kinetics of SO2 corresponds to the curve A of FIG. 6, if we take into account this OS. It corresponds to the curve B, if one takes into account the quantity of oxygen, the part BI corresponding to the desorption of SO2, while the B2 part corresponds to the desorption of H2S + COS.

Discretization of the volume of the catalytic converter in n reactors

Since the desulfation rate is characterized in particular by the temperature and the mass of sulfur stored at this time, it may be useful to take into account the SOx mass distribution and temperature heterogeneity throughout the NOxTrap range. . Indeed, until now, we modeled the total mass of SOx stored by the catalytic converter considered as consisting of a single and 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 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 may be different, provided that it is greater than or equal to one. 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, QSOxi = QSOx = 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: ## EQU1 ## and the flow rate of sulfur downstream of the trap becomes QSOxAVAL = Sum (dmSOx; The distribution allows a better estimate of the desulfation rate and thus the trapped sulfur mass, which allows an optimization of the estimated mass of NOx stored. FIG. 3 shows all the parameters used in the context of the present method and the variables that they make it possible to determine.

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 supply of 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, at a sampling instant i, using the model: SSi + l = SSi + Qsox. EFFSOX where Qsox is the flow rate of sulfur oxides in the engine exhaust (1) 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 a quantity "mO2 stored "which takes into account the amount or presence of surface oxygen (OS), this" surface oxygen "(OS) being the oxygen impregnated on the surface of the adsorption means of said pot. 2. Method according to claim 1, characterized in that the variation of the mass of sulfur oxides (SOx) purged per unit time dmSOx / dt is also evaluated for said pot, as a function of the hourly volume velocity. gases (VVH) 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, m02 stored). 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. 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. 7. Method according to claim 6, characterized in that said temperatures are given by sensors or temperature models.