FR2796102A1 - PROCESS AND DEVICE FOR PURGING SULFUR OXIDES FROM A CATALYTIC PUMP FOR TREATING EXHAUST GASES FROM AN INTERNAL COMBUSTION ENGINE - Google Patents

Abstract

The invention concerns a method for heating the catalytic converter (11), trapping nitrogen oxides, to the required temperature for desulphurizing the converter, which consists in: supplying at least first and second cylinder (1i) assemblies (A, B) of the engine with air/fuel mixtures adjusted such that the exhaust gases of the first assembly (11 to 13) contain an excess of oxidising species whereas the exhaust gases of cylinders (14 to 15) of the second group (B) contain an excess of reducing species. The invention is characterised in that it consists in supplying one (14) of the cylinders of the second group (B) with a mixture of high richness so as to maximise the production of carbon monoxide (CO), whereas the other cylinder (15) of the same group is supplied with a lean or stoichiometric mixture.

Description

 The present invention relates to a process for purging sulfur oxides from a catalytic converter trapped in sulfur oxides. nitrogen oxides, for the treatment of exhaust gases from an internal combustion engine, by adjusting the richness of the air / fuel mixture supplying the engine to a predetermined value and heating the pot to a higher or equal temperature at a predetermined temperature.

We know from French patent application No. 98 06961, filed on June 3, 1998 by the applicant, a method for controlling a purge of sulfur oxides (SOx) from a catalytic converter for treating exhaust gases from an internal combustion engine, this pot comprising means for adsorption, or “trapping”, of the oxides of nitrogen and, incidentally, of sulfur contained in these exhaust gases when the engine is supplied with an air / fuel mixture says "poor", that is to say whose fuel content is less than stoichiometry. A purge of the sulfur oxide pot is triggered by an increase in the fuel content of the air / fuel mixture. According to this process, a quantity representative of the quantity of sulfur oxides stored in the pot is evaluated, using a model, and a purge of the pot is triggered when this quantity crosses a predetermined threshold, depending on the temperature. of the pot. The quantity taken into account may consist of the mass of sulfur oxides stored in the pot, the storage efficiency of sulfur oxides or the filling rate of the pot with sulfur oxides. Reference is made to the aforementioned request for more details concerning the calculation of these quantities using a model. There are also described in this patent application means which allow, during a purge of the pot triggered at a temperature below said predetermined temperature, to heat this pot to this temperature. To do this, it is proposed to delay the ignition time of the air / fuel mixture and / or to burn an additional quantity of fuel, injected during the expansion or exhaust phase of the burnt gases.

Also known from US Pat. No. 5,758,493 is a process for purging sulfur oxides from a catalytic converter of the nitrogen oxide trapping type described above, according to which the temperature of the pot is raised. to that suitable for such a purge by triggering an exothermic redox reaction in the pot. To do this, the cylinders of the engine producing the exhaust gases treated in this pot are divided into two halves, the cylinders of one of these halves being supplied with a rich air / fuel mixture while the cylinders of the other half are supplied with a lean air / fuel mixture, the exhaust gases produced by these two halves being guided separately to the pot, where they mix.

As the exhaust gases in the first half are rich in reducing chemical species (unburnt hydrocarbons HC, carbon monoxide CO) while those in the second half are rich in oxygen, an oxidation reaction takes place in the pot. exothermic reduction which heats the pot to the temperature suitable for purging the pot of sulfur oxides, by temporary increase in the average richness of the air / fuel mixture supplying the engine.

It has been observed that the efficiency of this method of heating the pot decreases when the pot ages or when the flow rate of the exhaust gases increases. Furthermore, according to the method described in US 5,758,493, all the cylinders of the same half are supplied either with a lean air / fuel mixture, or with a rich air / fuel mixture. The described feeding mode of the cylinders does not maximize the formation of carbon monoxide in the cylinders supplied with a rich mixture, whereas it is this reducing chemical species which is the most effective for purging the pot of sulfur oxides.

The present invention specifically aims to provide a process for purging sulfur oxides from a catalytic converter trapping nitrogen oxides, involving heating of the pot by an exothermic redox reaction to bring the pot to the suitable temperature, this method being corrected for the drawbacks explained above of the methods known from the prior art.

The present invention also aims to provide a device for the implementation of this method.

These aims of the invention are achieved, as well as others which will appear on reading the description which follows, with a process for purging sulfur oxides from a catalytic converter trapping nitrogen oxides, for the treatment of the exhaust gases of an internal combustion engine, by adjusting the richness of the air / fuel mixture supplying the engine to a predetermined value (RsoX) and heating the pot to a temperature greater than or equal to a predetermined temperature , method according to which, for this heating, at least first and second groups of engine cylinders are supplied with air / fuel mixtures adjusted so that the exhaust gases of the cylinders of the first group contain an excess of oxidizing chemical species then that the exhaust gases from the cylinders of the second group contain an excess of reducing chemical species, so as to cause an oxidation-reduction reaction ex othermique suitable for heating said pot to said predetermined temperature, this process being remarkable in that it supplies at least one of the cylinders of the second group with an air / fuel mixture of high richness so as to maximize the production of reducing carbon monoxide by this cylinder, while the other cylinder or cylinders of the same group are supplied with lean or stoichiometric mixtures, so as to adjust the average richness of the air / fuel mixture supplying the engine cylinders to said richness value (R ,, ,) predetermined.

As will be seen below in detail, by thus varying the richness of the air / fuel mixture supplying the various cylinders of the second group, producing exhaust gases rich in reducing chemical species, in particular carbon monoxide, the production of this chemical species and therefore the heating of the pot drawn from the redox reaction in which it participates as well as the efficiency of the purging of the pot into sulfur oxides.

According to another characteristic of the process according to the invention, the redox reaction takes place in the catalytic converter trapping nitrogen oxides and in a common conduit for the exhaust gases leaving the first and second groups of cylinders .

According to yet another characteristic of the method according to the invention, at least one of the groups of cylinders comprises first and second sub-groups of cylinders whose exhaust gases are rich in oxidizing chemical species and in reducing chemical species, respectively, so as to allow an exothermic oxidation-reduction reaction in a priming catalytic converter collecting the exhaust gases from the cylinders of said first and second sub-groups, upstream of the common conduit, said exothermic reaction contributing to the heating of said catalytic converter to trapping of nitrogen oxides placed downstream of said conduit.

By thus additional heating of the exhaust gases in the priming pot and this conduit, it is possible to counter the effects of an aging of the pot at trapping of nitrogen oxides or of an increase in the flow of exhaust gas at treat in this jar, as we will see later.

Other characteristics and advantages of the present invention will appear on reading the description which follows and on examining the appended drawing in which - FIG. 1 is a partial diagram of an internal combustion engine and of control means of the operation of this engine, and - Figure 2 is a diagram of a device for implementing the method according to the present invention. There is shown schematically in Figure 1 of the accompanying drawing an internal combustion engine, by one of its cylinders 1 equipped with a piston 2, inlet 3 and exhaust 4 valves and actuators such as a fuel injector 5, a spark plug 6 and a valve 7 for controlling the air flow entering the cylinder, via an intake manifold 8. A computer 9 conventionally manages the operation of the engine by processing information from various sensors, such as the engine speed N, the torque C requested by the driver of a vehicle powered by said engine, etc., etc.

We find the valve 7 and the pipe 8 in the device shown schematically in Figure 2 where the motor M shown comprises a plurality 11,12, ... ln of cylinders, n can be even or odd.

The exhaust gases produced by the combustion of an air / fuel mixture in the cylinders 1i (i from 1 to n) are collected by two exhaust pipes 91, 92 each associated with a group A or B of engine cylinders. .

Thus, in the embodiment shown in FIG. 2 by way of illustrative and nonlimiting example only, the cylinders 11 to 13 (group A) deliver exhaust gases into the manifold 91 while the cylinders 19 to 15 (group B) flow into tubing 92.

The gases thus collected are mixed upstream of a conduit 10 connected to the inlet of a three-way catalytic converter 11 for treating exhaust gases.

 Optionally, the gases collected by the pipes 91 and 92 can first pass through catalytic converters 121, 122 respectively, called "priming" treating, as is well known, the exhaust gases when the main catalytic converter 11 has not not reached a temperature sufficient to operate efficiently, as is the case during a cold start of the engine, for example.

The pot 11 comprises means for storing nitrogen oxides NOx when the engine is supplied with an air / fuel mixture "lean" in fuel, the stored nitrogen oxides being periodically desorbed to avoid saturation of its storage capacity . The desorbed nitrogen oxides are then reduced in the pot 11 to a harmless chemical species (nitrogen).

The sulfur oxides contained in the exhaust gases are also stored in the pot 11, under the same conditions as the nitrogen oxides. They then reduce the storage capacity of the pot 11 in nitrogen oxides and must therefore also be periodically desorbed and treated. To be effective, the purging of the pot of sulfur oxides must take place at a predetermined Rsox purge richness of the air / fuel mixture and at a pot temperature higher than that which allows the purging of the pot of nitrogen oxides. .

The computer 9 is duly programmed to control these purging operations with nitrogen oxides and sulfur oxides. By way of illustrative and non-limiting example only, reference may be made to French patent application no. 97 15775 and to the above-mentioned French patent application no. 98 06961, both filed in the name of the applicant, for more details on strategies for purging nitrogen oxides and sulfur oxides respectively, capable of being implemented by the computer 9.

When the computer 9 initiates a purge of sulfur oxides from the pot, the pot should be brought to a high temperature (above 650 C). If this is not the case, it is necessary to quickly bring the pot to this temperature to make purging possible, by a temporary increase in the Rsox value of the average richness of the air / fuel mixture supplying the engine, controlled by the computer by appropriate adjustment of the opening time of the fuel injectors such as 5 and of the valve 7 for adjusting the air flow admitted into the engine.

To do this, the pot 11 is reheated by selectively causing therein an exothermic redox reaction between an excess of oxidizing chemical species (oxygen) contained in the exhaust gases of the cylinders of group A and an excess of species reducing chemicals (HC, CO) contained in the exhaust gases of the cylinders of group B. To do this, according to the teachings of US Pat. No. 5,758,493, one could adjust the fuel richness of the air / fuel mixture of all the cylinders of group A to a value corresponding to a lean mixture and the corresponding richness for all the cylinders of group B to a value corresponding to a rich mixture.

This method of supplying the cylinders of group B is not favorable for maximum production of carbon monoxide, whereas this is the most effective reducing species for purging the pot of sulfur oxides and for reducing the overconsumption of fuel required by heating the catalytic converter 11.

According to the present invention, this maximum production of carbon monoxide is obtained by supplying one of the cylinders of group B, for example the cylinder 14, with an air / fuel mixture of high or very high fuel richness (between 1.3 and 1 , 5 for example). To compensate for this high enrichment of the air / fuel mixture supplying the cylinder 14 and maintaining the average richness of the air / fuel mixture supplying the engine at the suitable purge value Rsox (between 1 and 1.1 for example) allowing furthermore a correct combustion of the air / fuel mixture, it is naturally necessary to reduce in proportion the richness of the mixture which feeds the other cylinder (15) of group B, then supplied with a lean or stoichiometric mixture, for example.

According to the present invention, the richness of the air / fuel mixtures supplying the engine cylinders and the instants of ignition of these mixtures are controlled by the computer 9 so that the powers produced by the combustions in the different cylinders are identical, that the pot 11 reaches the desired temperature for the purging of sulfur oxides and that the average richness is equal to Rgox, as we saw above. For this, the computer draws the masses of fuel to be injected and the required ignition advances from tables stored in memory, and this cylinder by cylinder. The input parameters of these tables can be the engine speed N and / or the torque C requested by the driver. The latter can be managed by the computer using valve 7.

The exothermic oxidation-reduction reaction obtained as indicated above can develop, of course, not only in the pot 11 but also in the conduit 10 where the oxidizing and reducing chemical species in question are collected, before entering pot 11.

The redox reaction then obtained in line 10 and in pot 11 normally makes it possible to reach the temperature of pot suitable for purging in sulfur oxides, when such a purge is controlled by the computer 9. As we 'seen above, it was observed however that this result was not achieved when the pot 11 has suffered the effects of aging or when the exhaust gas flow rate of the engine is particularly high. Indeed, the pot does not succeed in completely oxidizing the excess reducers from the cylinders of group B.

To compensate for the heat loss then observed in the pot, the present invention makes it possible to preheat the gases which will pass into the pot 11 during their passage through the priming pots 121 and 122 and into the conduit 10.

To do this, the computer 9 selectively controls the supply of certain cylinders of group A with a rich mixture, the average mixture passing through all of the cylinders of this group remaining poor. The excess of reducing chemical species leaving the burning cylinder a rich mixture then mixes, in the priming pot 121, with an excess of oxidizing chemical species coming from the other cylinders, which causes in this pot and, possibly, in the duct 10, an exothermic redox reaction which preheats the exhaust gases before they enter the pot 11.

Likewise, the computer 9 can control the supply of certain cylinders of group B with a lean mixture, the average mixture remaining rich, so as to cause an exothermic oxidation-reduction reaction in the priming pot 122.

It now appears that the invention makes it possible to achieve the announced aim, namely to ensure heating of the catalytic converter to trap nitrogen oxides suitable for purging this pot of sulfur oxides, the latter being facilitated by the described production process of carbon monoxide and being optimized from the point of view of the fuel consumption required by this purge, while also being possibly adapted to an aged catalytic converter or to a particularly high exhaust gas flow rate. In addition, the strategy described above for controlling the supply of the engine, cylinder by cylinder, makes it possible to heat the pot 11 to the required temperature without risk of destroying the priming pots 121,122.

Of course, the invention is not limited to the embodiment described and shown which has been given only by way of example. This is how the catalytic converters 121, 122 could be physically joined, the gas flows passing through them remaining isolated from one another.

Claims (6)

1. Method for purging sulfur oxides (SOx) from a catalytic converter (11) trapping nitrogen oxides (NOx), for the treatment of exhaust gases from an internal combustion engine (M) by adjusting the richness of the air / fuel mixture supplying the engine to a predetermined value (Rgox) and heating the pot to a temperature greater than or equal to a predetermined temperature, method according to which, for this heating, at least first (A) and second (B) groups of cylinders (1i) of the engine with air / fuel mixtures regulated so that the exhaust gases of the cylinders of the first group (A) contain an excess of oxidizing chemical species while the gases of the cylinders of the second group (B) contain an excess of reducing chemical species, characterized in that at least one (1q) of the cylinders of the second group (B) is supplied with an air / fuel mixture of high richness so to maximize the production of reducing carbon monoxide (CO) by this cylinder, while the other cylinder (s) (15) of the same group are supplied with lean or stoichiometric mixtures, so as to adjust the average richness of the air / fuel for supplying the engine cylinders (M) at said predetermined richness value (Rsox). 2. Method according to claim 1, characterized in that said oxidation-reduction reaction takes place in the catalytic converter trapping nitrogen oxides and in a common conduit for the exhaust gases leaving the first and second groups of cylinders. 3. Method according to claim 2, characterized in that at least one of said groups (A, B) of cylinders comprises first and second sub-groups of cylinders whose exhaust gases are rich in oxidizing chemical species and in reducing chemical species, respectively, so as to allow an exothermic redox reaction in a priming catalytic converter (12,12 ') collecting the exhaust gases from the cylinders of said first and second subgroups, upstream of the common conduit (10), said exothermic reaction contributing to the heating of said catalytic converter (11) trapping nitrogen oxides placed downstream of said conduit. 4. Device for implementing the method according to any one of the preceding claims, comprising first (91) and second (92) pipes designed to collect the exhaust gases of a first group (A) and d '' a second group (B) respectively of cylinders (1i) of the engine, a conduit (10) for joining the two streams of exhaust gas thus collected, upstream of the catalytic converter (11), trapping nitrogen oxides , means (5,7,9) for controlling the fuel richness of the air / fuel mixture supplying the cylinders of the two groups (A, B), capable of ensuring the presence, in the exhaust gases of the groups (A) and (B) of cylinders, of an excess of oxidizing chemical species and of an excess of reducing chemical species respectively, said control means further supplying at least one (14) of the cylinders of group (B ) with a high-rich air / fuel mixture that maximizes the production of carbon monoxide (CO) in the exhaust gases of said cylinder (14), while maintaining the average richness of said mixture in the cylinders of group (B) at a predetermined value. 5. Device according to claim 4, characterized in that it further comprises, between each of the pipes (91) and (92) and the pot (11) a priming catalytic pot (121), (122), respectively, said control means (5,7,9) ensuring the supply of the cylinders of the two groups (A) and (B) cylinder by cylinder, so as to selectively cause an exothermic oxidation-reduction reaction in said pots priming (121, 122). 6. Device according to claim 5, characterized in that said redox reaction also develops in a conduit (10) gathering the exhaust gases from the two groups (A) and B) of cylinders to inject them into said pot (11) trapping nitrogen oxides.