FR2932719A1 - Nitrogen oxide reducing device for internal combustion engine, has sensor placed at proximity of part of catalytic block and comprising small storage capacity, where sensor emits signal, when quantity of rejected molecules exceeds threshold - Google Patents

Abstract

The device (100) has a catalytic block (120) placed on a path of an exhaust pipe (16) for storing reducer molecules e.g. ammonia molecules, to reduce nitrogen oxides. The block has two parts (120A, 120B) with different storage capacities to store the molecules. An ammonia sensor (130) is placed at the proximity of the part (120A) and includes small molecule storage capacity in downstream of the block along an exhaust gas flowing direction. The sensor emits a signal, when the quantity of molecules rejected in the gas exceeds a preset threshold.

Description

TECHNICAL FIELD TO WHICH THE INVENTION RELATES The present invention relates to a device for reducing nitrogen oxides contained in exhaust gases of an internal combustion engine, comprising a catalytic bread in which reducing molecules are stored adapted to reducing said nitrogen oxides and which is placed in the path of an exhaust duct in which said exhaust gas flows. BACKGROUND OF THE INVENTION Internal combustion engines emit unburned hydrocarbons, soot particles and pollutant nitrogen oxide molecules in their exhaust gases. In order to limit these pollutant emissions, exhaust gas treatment devices are located on the exhaust line downstream of the combustion chamber. These treatment devices comprise in particular a particulate filter placed downstream of an oxidation catalyst in the direction of flow of the exhaust gas, as well as a device for selective reduction of nitrogen oxides. This selective reduction device for nitrogen oxides comprises in particular a catalytic bread, a device for injecting a reducing chemical compound and a reservoir containing this reducing chemical compound. This reducing chemical compound conventionally contains urea which is injected into an exhaust pipe of the exhaust line, upstream of the catalytic bread, where it reacts to form ammonia. The ammonia thus formed is trapped in the catalytic bread and reacts with the nitrogen oxide molecules to reduce them to clean nitrogen and water molecules that are discharged into the exhaust gas. The amount of urea injected into the exhaust pipe is calculated so that the quantity of ammonia produced and the quantity of nitrogen oxide molecules contained in the exhaust gas are in stoichiometric proportions with respect to the reaction of reduction of the nitrogen oxide molecules by ammonia, so that this reaction consumes all the ammonia molecules formed upstream of or in the catalytic bread and trapped therein. However, if an excess of ammonia is not consumed by the reduction reaction of the nitrogen oxide molecules and can not be stored in the catalytic bread because the storage capacity of it is exceeded, it is evacuated in the exhaust gas downstream of the selective reduction device of nitrogen oxides.

In order to limit the toxic emissions of ammonia, it is known to place an ammonia sensor in said catalytic bread to measure the amount of ammonia trapped in this catalytic bread. If this quantity exceeds a predetermined threshold value, the amount of urea injected upstream of the catalytic bread is decreased or canceled in order to reduce the amount of ammonia produced and to avoid the saturation of the catalyst cake with ammonia. OBJECT OF THE INVENTION The object of the present invention is to limit the amount of ammonia released into the exhaust gas by a new device for selective reduction of nitrogen oxides. The present invention more particularly proposes a device for reducing nitrogen oxides as defined in the introduction, wherein said catalytic bread comprises at least two parts having different storage capacities of said reducing molecules and a sensor of these reducing molecules placed at near the part of the catalytic bread having the lowest storage capacity of said reducing molecules, downstream thereof in the direction of flow of the exhaust gas. The sensor of reducing molecules, for example ammonia, measures the amount of reducing molecules released into the exhaust gases by the part of the storage capacity of the catalytic bread. Thus, as soon as this measured quantity exceeds a predetermined threshold value, the sensor may emit a signal to reduce the quantity of reducing molecules introduced into the catalytic bread, before the other part of the catalytic bread of greater storage capacity also rejects reducing molecules. The quantity of reducing molecules globally rejected is thus reduced. According to other advantageous and non-limiting characteristics of the device for reducing nitrogen oxides according to the invention, - said two parts of said catalytic bread are distinct; said two parts of said catalytic bread are arranged in parallel in the path of the exhaust duct; said two parts of said catalytic bread are arranged in the path of two distinct channels of the exhaust duct; said catalytic roll is in one piece and comprises an inlet or outlet face of the beveled exhaust gases; - It is expected that said at least two parts of the catalytic bread are impregnated with a different amount of metal oxides; - It is expected that said at least two parts of the catalytic bread have different specific surfaces; said reducing molecules are ammonia molecules; it further comprises a reservoir containing urea molecules connected to a device for injecting said urea molecules into the flow of the exhaust gases, arranged upstream of said catalytic bread, said injection device being controlled by a an electronic control unit according to the information received therefrom from said reducing molecule sensor; and it further comprises a catalyst for the hydrolysis of urea molecules placed in the path of the exhaust gas between said injection device and said catalytic bread. The invention also proposes a use of the device for reducing nitrogen oxides as described above, comprising the following steps: the amount of reducing molecules discharged into the exhaust gases downstream from said portion of lower capacity is measured. storing the catalytic bread of said nitrogen oxide reduction device; if this measured quantity of rejection molecules is greater than or equal to a predetermined threshold value, the quantity of reducing molecules introduced into said catalytic bread is reduced to limit the rejection of reducing molecules by the other part of greater storage capacity; catalytic bread. DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT The following description, with reference to the appended drawings, given by way of non-limiting example, will make it clear what the invention consists of and how it can be achieved. In the accompanying drawings: FIG. 1 is a schematic view of various components of an internal combustion engine comprising a device for selective reduction of nitrogen oxides according to a first embodiment of the invention, FIG. a detailed view of an engine comprising a device for selective reduction of nitrogen oxides according to a second embodiment of the invention; FIG. 3 is a detailed view of an engine comprising a device for selective reduction of the nitrogen oxides according to a third embodiment of the invention, and FIG. 4 is a detailed view of an engine comprising a device for selective reduction of nitrogen oxides according to a fourth embodiment of the invention. . For simplicity of the figures, the references designating similar elements of the exhaust line in Figures 1 to 4 have been retained from one figure to another. FIG. 1 shows a supercharged internal combustion engine comprising a combustion chamber 23 supplied with fresh air through an intake line 10 and opening downstream on an exhaust line 40.

The intake line 10 comprises an intake duct 4 in which fresh air circulates. The fresh air flow rate is measured at the inlet of the intake duct 4 by an air flow meter 1. The engine comprises a turbocharger 14 comprising two turbines 2, 9. The driving turbine 9 is placed in an exhaust duct 16 of the exhaust line 40 and drives the driven turbine 2 placed in the intake duct 4 to compress the fresh air flowing there. As this compression has the effect of heating the air, an air cooler 3 is provided on the path of the intake duct 4, which cools the air leaving the turbocharger 14.

The intake duct 4 opens into a distributor 6. It comprises upstream of this distributor 6 an admission flap 5. The orientation of the intake flap 5 relative to the axis of the intake duct 4 controls the Fresh air flow entering the distributor 6. The distributor 6 is connected to an intake valve 21 provided with an intake valve of a cylinder 20 of the engine. The compressed air enters via this intake valve 21 into a combustion chamber 23 of the cylinder 20 and an injector 8 is provided which injects the fuel into the combustion chamber 23. After the combustion, the residual exhaust gases are conveyed out of the combustion chamber 23 through an exhaust valve 22 provided with an exhaust valve in the exhaust duct 16 of the exhaust line 40. Part of this exhaust gas is removed by a recirculation duct 17 which brings them back, after passing through an air cooler 18, to the distributor 6 where they mix with the fresh air arriving from the intake duct 4. The supply of exhaust gas into the distributor 6 is regulated by a valve 19 called EGR (Exhaust Gas Recirculation).

The exhaust gases that are not directed in the recirculation duct 17 flow in the exhaust duct 16 to arrive at the driving turbine 9 of the turbocharger 14. They then pass through an exhaust gas treatment device 11 comprising a an oxidation catalyst and a particulate filter and a device for selective reduction of the nitrogen oxides 100, 200, 300, 400, placed in the path of the exhaust line 40, before being released into the atmosphere. The selective reduction device of the nitrogen oxides 100, 200, 300, 400 shown in FIGS. 1 to 4 is placed downstream of the exhaust gas treatment device 11. However, it can also be considered that it is placed upstream of this treatment device. The selective reduction device of the nitrogen oxides 100, 200, 300, 400 comprises a catalyst roll 120, 220, 320, 420, an injection device 110, 210, 310, 410 of a reducing chemical compound, and a reservoir. containing this reducing chemical compound (not shown). The reducing chemical compound used here is urea. More particularly, it is a 32.5% aqueous solution of urea, such as that marketed under the trademark AdBlue. Under the effect of the exhaust gas heat in the exhaust duct 16, a molecule of urea produces one molecule of ammonia and one molecule of isocyanic acid, which is hydrolyzed to give a second molecule of ammonia and carbon dioxide. Alternatively, the device for selective catalysis of nitrogen oxides may also comprise an exhaust gas homogenization device for homogeneously distributing the ammonia molecules formed in the exhaust gases before they enter the catalytic bread. . This homogenizing device may for example be formed of a grid disposed upstream of the catalytic bread and extending along a section of the exhaust duct.

The device for selective catalysis of nitrogen oxides may also comprise a device for catalyzing the hydrolysis of urea molecules to promote the production of ammonia from urea. The ammonia molecules produced are stored on the surface of catalytic bread 120; 220; 320; 420.

They can then react with the nitrogen oxide molecules contained in the exhaust gas passing through said catalytic bread to reduce them to nitrogen and water.

This catalytic bread 120, 220, 320, 420 comprises, for example, a support matrix made of titanium or aluminum oxide impregnated with metal oxide molecules such as vanadium oxide. Remarkably, the catalytic bread 120, 220, 320, 420 of the nitrogen oxide selective reduction device 100, 200, 300, 400 comprises at least two parts 120A, 120B, 220A, 220B, 320A, 320B, 420A, 420B having different storage capacities of the ammonia molecules and an ammonia sensor 130; 230; 330; 430 placed near the portion 120A; 220A; 320A; 420A of the catalytic bread having the lowest storage capacity, downstream thereof in the flow direction of the exhaust gas in said nitrogen oxide reduction device 100; 200; 300; 400. Storage capacity here means the number of sites capable of trapping ammonia molecules on the surface of the catalytic bread matrix 120; 220; 320; 420. This storage capacity depends on the geometry of the catalytic bread 120, 220, 320, 420 and the material used to produce it. The storage capacity of the catalytic bread 120, 220, 320, 420 is all the greater as its specific surface area and therefore its dimensions are larger and the greater the greater the concentration of metal oxide on the surface of the catalytic bread 120 220, 320, 420 is large.

In a first embodiment of the invention, shown in Figure 1, the catalytic bread 120 comprises two separate parts 120A, 120B of different sizes arranged in parallel in the exhaust duct 16. The two parts 120A, 120B bread catalytic are here made of the same material.

These two parts 120A, 120B for example each have a semi-cylindrical shape so that their meeting globally forms a cylinder. The first portion 120A of the catalytic bread 120, as shown in FIG. 1, has smaller dimensions than the second portion 120B of the catalytic bread 120. Its specific surface area is therefore smaller, and the first portion 120A of the catalytic bread 120 thus has an ammonia storage capacity lower than the second portion 120B of the catalytic bread 120. In a second embodiment of the invention, shown in Figure 2, the catalytic bread 220 is monobloc, and is for example under a generally cylindrical shape of section substantially equal to that of the exhaust duct. This catalytic roll comprises an inlet face 221 through which the exhaust gases enter the catalytic bread and an outlet face 222 through which these exhaust gases emerge from the catalytic bread 220. According to this second embodiment, the two input faces 221 and output 222 are not parallel: here the output face 222 is beveled. The length of the catalytic bread therefore varies according to the profile of this beveled exit face 222. The portion 220A of the catalytic bread being located near the part of the side wall 223 of the shortest length here has a specific surface and therefore a storage capacity lower than the portion 220B of the catalytic bread 220 being located near the part of the side wall 223 of greater length of the catalytic bread 220. Alternatively, the sole face of the catalytic bread can be beveled. Both input and output faces can also be beveled. In a third embodiment of the invention, shown in FIG. 3, the catalytic bread 320 comprises two separate portions 320A and 320B of similar dimensions arranged in parallel in the exhaust duct 16. The storage capacity of a first 320A part of the catalytic bread 320 is less than the storage capacity of the other 320B part, since the first portion 320A of the catalytic bread is impregnated with a smaller amount of metal oxide. For example, the support of the first portion 320A of catalytic bread 320 is made of titanium oxide as the support of the second portion 320B of catalytic bread 320 but has fewer vanadium oxide molecules on its surface.

In a fourth embodiment of the invention, shown in Figure 4, the catalytic bread 420 has two parts 420A and 420B distinct. The exhaust duct here separates locally into two channels 16A, 16B in which the exhaust gases circulate. Each of the two portions 420A, 420B of catalytic bread 420 is placed in the path of one of these channels 16A, 16B of the exhaust line 40. The length and therefore the storage capacity of the ammonia of the part 420A catalytic bread 420 placed in one of the two channels 16A are lower than those of the other portion 420B of the catalytic bread 420, placed in the other channel 16B.

Whatever the embodiment of the selective reduction device of the nitrogen oxides according to the invention, it is used to reduce the nitrogen oxides contained in the exhaust gas in the following manner: - the quantity is measured ammonia released into the exhaust gas downstream of the 120A portion; 220A; 320A; 420A of lower ammonia storage capacity of the catalytic bread 120; 220; 320; 420 of the oxidation reduction device; nitrogen 100; 200; 300; 400; if this measured quantity of ammonia discharged is greater than or equal to a predetermined threshold value, the amount of urea injected into the exhaust gas upstream of the catalytic bread is regulated to limit the release of ammonia by the other party 120B; 220B; 320B; 420B greater storage capacity of catalytic bread 120; 220; 320; 420.

In practice, the engine comprises an electronic control unit (ECU) 30 which receives the measurements of the amount of ammonia recorded by the ammonia sensor 130; 230; 330; 430 placed close to the exit face of the exhaust gases. 120A portion exhaust; 220A; 320A; 420A lower storage capacity of catalytic bread 120; 220; 320; 420. This electronic control unit 30 controls the amount of urea injected by the injection device 110, 210, 310, 410 and the frequency of these injections. If the measured amount of ammonia released at 120A, 320A, 420A, 420A of catalytic bread 120, 320, 420 of lower storage capacity is greater than or equal to the predetermined threshold value, this indicates that the 120A portion; 220A; 320A; 420A lower storage capacity of catalytic bread 120; 220; 320; 420 is saturated with ammonia. It can not store more ammonia molecules. This saturation occurs for example during a change of engine speed: the amount of nitrogen oxides present in the exhaust gas decreases, and the reduction reaction of these nitrogen oxide molecules no longer consumes all the ammonia molecules formed from the urea molecules injected into the exhaust gas. The excess ammonia molecules remain stored in the catalytic bread, until saturation thereof, and they are discharged into the exhaust gas. Part 120A, 220A, 320A, 420A of lower storage capacity of catalytic bread 120, 220, 320, 420 is saturated faster than other portion 120B, 220B, 320B, 420B of catalytic bread 120, 220, 320; 420. When the ammonia sensor 130; 230; 330; 430 measures an amount of ammonia greater than said predetermined threshold value, the electronic control unit 30 then controls a decrease in the amount of urea injected into the exhaust gas. and / or decreasing the frequency of these injections so as to reduce the amount of ammonia released by the portion 120A, 220A, 320A, 420A of the catalytic bread 120, 220, 320, 420 of lower storage capacity. In addition, this avoids any ammonia discharge by the other part 120B; 220B; 320B; 420B of the catalytic bread 120; 220; 320; 420 having a greater storage capacity of the ammonia, since the amount of Ammonia produced is reduced before this other portion 120B; 220B; 320B; 420B is saturated. The overall quantity of ammonia released is therefore reduced.

The electronic control unit 30 also controls the actuation of the intake flap 5 and the EGR valve 13 to regulate the flow of air into the manifold 6 and therefore the amount of air introduced into the combustion chamber. The electronic control unit 30 finally controls the amount of fuel injected by the injector 8 into the combustion chamber as well as the moment of this injection. The present invention is not limited to the embodiments described and shown, but the skilled person will be able to make any variant consistent with his mind.

In particular, the various embodiments described here can be combined with each other.

Claims (11)

REVENDICATIONS1. A device for reducing oxides of nitrogen (100; 200; 300; 400) contained in the exhaust gas of an internal combustion engine comprising a catalytic bread (120; 220; 320; 420) in which molecules are stored; reducers adapted to reduce said nitrogen oxides and which is placed in the path of an exhaust duct (16) in which said exhaust gases circulate, characterized in that said catalytic bread (120; 220; 320; 420 ) comprises at least two parts (120A, 120B, 220A, 220B, 320A, 320B, 420A, 420B) having different storage capacities of said reducing molecules and a sensor (130; 230; 330; 430) of these reducing molecules placed in the vicinity of the portion (120A; 220A; 320A; 420A) of the catalytic bread (120; 220; 320; 420) having the lowest storage capacity of said reducing molecules downstream thereof in the direction of flow; exhaust gas. 2. Device for reducing nitrogen oxides (100; 300; 400) according to the preceding claim, wherein said two parts (120A, 120B; 320A, 320B; 420A, 420B) of said catalytic bread (120; 320; 420) are distinct. 3. Device for reducing nitrogen oxides (100; 300; 400) according to the preceding claim, wherein said two parts (120A, 120B; 320A, 320B; 420A, 420B) of said catalytic bread (120; 320; 420) are arranged in parallel in the path of the exhaust duct (16). A nitrogen oxide reduction device (400) according to claim 2, wherein said two portions (420A, 420B) of said catalytic bread (420) are disposed in the path of two channels (16A, 16B) separate from the duct exhaust (16). The nitrogen oxides reduction device (200) according to claim 1, wherein said catalytic bread (220) is integral and has an inlet (221) or outlet (222) end of the beveled exhaust gas. . A nitrogen oxide reduction device (100; 200; 300; 400) according to one of the preceding claims, wherein said at least two parts (120A, 120B; 220A, 220B; 320A, 320B are provided; 420A, 420B) of the catalytic bread (120; 220; 320; 420) are impregnated with a different amount of metal oxides. The nitrogen oxide reduction device (100; 200; 300; 400) according to one of the preceding claims, wherein said at least two parts (120A, 120B; 220A, 220B; 320A, 320B are provided; 420A, 420B) of the catalytic bread (120; 220; 320; 420) have different specific surfaces. The nitrogen oxide reduction device (100; 200; 300; 400) according to one of the preceding claims, wherein said reducing molecules are ammonia molecules. 9. Device for reducing nitrogen oxides (100; 200; 300; 400) according to the preceding claim, further comprising a reservoir containing urea molecules connected to an injection device (110; 210; 310; 410). ) said urea molecules in the exhaust gas stream, disposed upstream of said catalytic bread (120; 220; 320; 420), said injection device (110; 210; 310; 410) being controlled by a unit; electronic control device (30) according to the information received by said sensor (130; 230; 330; 430) of reducing molecules. 10. Device for reducing nitrogen oxides (100; 200; 300; 400) according to the preceding claim, further comprising a catalyst for the hydrolysis of urea molecules placed in the path of the exhaust gas between said injection device (110; 210; 310; 410) and said catalytic bread (120; 220; 320; 420). 11. Use of the nitrogen oxides reduction device (100; 200; 300; 400) according to one of the preceding claims, comprising the following steps: - the amount of reducing molecules discharged into the exhaust gases is measured; downstream of said portion (120A; 220A; 320A; 420A) of lower storage capacity of the catalytic bread (120; 220; 320; 420) of said nitrogen oxide reducing device (100; 200; 300; 400); if this measured quantity of rejecting reducing molecules is greater than or equal to a predetermined threshold value, reducing the quantity of reducing molecules introduced into said catalytic bread (120; 220; 320; 420) in order to limit the rejection of reducing molecules by the another portion (120B; 220B; 320B; 420B) of greater storage capacity of the catalytic bread (120; 220; 320; 420).