FR3029970A1 - EXHAUST GAS POST-TREATMENT DEVICE OF A COMBUSTION ENGINE - Google Patents

Abstract

The invention relates to a device for post-treatment of the exhaust gases of a combustion engine (1), characterized in that it comprises, from upstream to downstream: • a DOC oxidation catalyst element (3) ; • a mouth (41) of means (4) for introducing a reducing agent or precursor of a reducing agent for the selective catalytic reduction of SCR nitrogen oxides; A catalytic selective catalytic reduction unit SCR of NOx nitrogen oxides (6); A particulate filter member (7) provided with a SCRF selective catalytic reduction catalyst coating of NOx nitrogen oxides; • an ammonia leak treatment unit (8); said catalyst member for selective catalytic reduction of nitrogen oxides (6) being of a length (L1) of between 45 mm and 80 mm, in particular between 50 and 75 mm.

Description

BACKGROUND OF THE INVENTION The invention relates to means for treating pollutants from the exhaust gases of combustion engines. [2] Pollutant emissions from combustion engines in motor vehicles are regulated by standards. Regulated pollutants are, depending on the combustion engine technology considered, carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx, that is NO and NO2), and particles (PM), which are formed during combustion of the fuel in the combustion chamber and then emitted to the exhaust. [3] It is known to employ a number of means of pollution in the exhaust line of combustion engines to limit the emissions of regulated pollutants. This type of device is generally referred to as the "after-treatment" device for the exhaust gases. [4] An oxidation catalyst allows the treatment of carbon monoxide, unburned hydrocarbons, and under certain conditions nitrogen oxides (by storage of NOx); a particulate filter may be employed for the treatment of soot particles. [5] To meet pollution control standards for nitrogen oxide (NOx) emissions, a specific post-treatment system can be introduced into the exhaust system of vehicles, including vehicles equipped with diesel engines. For the treatment of nitrogen oxides (NOx), selective catalytic reduction (SCR) technologies for "Selective Catalytic Reduction" in English are known which consist of reducing NOx by introducing a reducing agent (or a precursor of such a reducing agent) in the exhaust gas by catalyzed reactions. It may for example be a urea solution, the decomposition of which will make it possible to obtain ammonia which will serve as a reducing agent, but also of a reducing agent or precursor of such a reducing agent. in gaseous form. In the remainder of the present document, a "reducing agent" will generally be used to designate a reducing agent or a reducing agent precursor. [6] The reducing agent generated makes it possible to reduce the nitrogen oxides by reaction in an SCR catalyst, that is to say a substrate carrying a catalytic impregnation able to promote the reduction of NOx by the reducing agent. . [7] European standards, in particular, tend to become more and more severe.

5 And the solutions to reduce pollutant emissions at the end of the exhaust line to meet the current standards will prove insufficient in view of the changes in standards expected beyond 2017. [8] Indeed, the first stage of the standard Euro 6b (entry into force in September 2014) led car manufacturers to choose between different options to reduce NOx emissions more specifically: - reduction of NOx "at the source", at the level of the actual operation of the engine, via technologies of the exhaust gas recycling type in the engine, recycling also called EGR according to the acronym of the English term corresponding to "Exhaust Gas Recirculation" high and low pressure, for example; the reduction of NOx at the exhaust line via a sequential catalytic processing technology called NOx trap; the reduction of NOx at the level of the exhaust line also, via a continuous treatment technology called "selective catalytic reduction" as briefly described above (SCR); even by cumulating many of these solutions. [9] While these solutions can satisfy this first step in the evolution of the standard (Euro6b), they are not necessarily capable of satisfying the second stage, which promises to be even more severe (Euro 6c, entry into force foreseen in September 2017), with pollutant measurements on a new rolling cycle called "WLTC" (for "Worldwide Harmonized Light Vehicle Test Cycle" in English, or harmonized test cycle for light vehicles in French), containing more transient phases than the current approval cycle (known as "MVEG" for the Motor Vehicle Emissions Group), but also out-of-cycle measures (known as "RDEs" for Real Driving Emissions or emissions in conditions driving practices) should be introduced. [0010] In particular, to meet the risks of excessively high NOx emissions outside the cycle, various technological solutions and architectures can be envisaged. They have their advantages and disadvantages. But the most efficient nitrogen oxide treatment technology is selective catalytic reduction (SCR) because it is efficient in more extended temperature and gas flow ranges than a NOx trap, the other solution post-processing. In addition, there are additional constraints on the implantation of the post-processing device. In fact, in general, the catalytic systems used are all the more effective as the temperature of the exhaust gases passing through them is high (to a certain extent). They will then start all the faster after starting the engine as the temperature of the exhaust gas will increase rapidly. It is therefore advantageous to implement the after-treatment devices as close to the engine, that is to say closer to the exhaust manifold, under hood, even though this environment is generally very crowded. The post-processing devices must therefore be as compact as possible without affecting their performance. Throughout the present text, the terms "upstream" and "downstream" are understood as a function of the general direction of flow of the exhaust gases in the exhaust line integrating the post-processing units, since the engine to the end cannula of the exhaust line. It is, for example, known from the patent application WO 2011/089330 a post-processing device grouping in the same envelope several bodies that will be successively traversed by the exhaust gas. It is proposed, in particular, a series of organs comprising upstream downstream: - an oxidation catalyst, - a urea reducing agent injector, - a mixer whose role is to intimately mix the droplets of urea injected into the casing through which the gases pass, so as to decompose into ammonia as homogeneously as possible over the entire cross section of the casing, - an organ SCR, - a particulate filter (called FAP by the following). It also proposes an alternative, consisting of replacing the SCR member and the FAP, by a FAP which is impregnated with a NOx reduction catalyst and which thus fulfills both the soot filter and the reduction function. NOx (called SCRF later). However, a dedicated member SCR as described in this document may not start early enough for reasons of adverse thermal, especially in urban driving conditions during which the temperatures in the exhaust line are quite low . However, it is precisely during this type of urban driving 30 that the evolutions of the European standard (in particular) will become binding in terms of reduction of NOx emissions. And the variant incorporating the SCR catalyst in the particulate filter (SCRF) is also not good enough in urban driving conditions, because of the significant thermal inertia of the particulate filter-specific substrate, even if it is positioned very close to the engine. Indeed, the substrate which ensures the filtration of the particles and which is impregnated with the catalyst coating (impregnation coating known in English as "washcoat") is a porous ceramic that consumes a lot of heat to increase in temperature (significant thermal inertia ). The start of the SCR phase could thus be done only after a certain time, making it difficult to respect the future evolutions of the standard. In addition, additional constraints arise when the motor vehicle is a vehicle called "heavy" (more than 1500 kg), whether a vehicle for private or utility type. In fact, under the same driving conditions as a lighter vehicle, the "heavy" vehicle will have higher temperature conditions at the exhaust to be managed, and larger quantities to process NOx generated in the engine. To compensate for these higher NOx emissions, the amounts of reducing agent to be injected into the exhaust line (for example urea decomposing to ammonia) will have to be larger also, since these quantities are dictated by the stoichiometry of the NOx reactions with ammonia. The higher temperatures of the gases at the engine outlet also favor the thermo-desorption of the ammonia stored in the SCR (and / or SCRF) members, and may furthermore contribute to the degradation of their active catalytic phases which can induce a decrease. their ammonia storage capacity. The conjugation of higher temperatures and a larger amount of urea (or ammonia) to be injected onto the line induces an increased risk of ammonia emissions that would not have reacted in the end. exhaust. Ammonia leaks at the end of the exhaust line are smelly, and can be inconvenient, especially if the vehicle is in a confined space of closed parking type. The invention therefore aims to design a post-treatment of engine exhaust gas that overcomes the aforementioned drawbacks. Its purpose is, in particular, to improve the existing devices to enable stricter standards for pollutant emissions to be observed, and more particularly for NOx emissions under unstabilized rolling conditions of the urban rolling type and / or in a wider temperature range, while minimizing possible NOx leakage agent leakage at the end of the exhaust line. Advantageously, it is also intended to obtain a post-processing device that is more efficient and that remains, in addition, compact. The invention firstly relates to a device for post-treatment of the exhaust gas of a combustion engine, which comprises, from upstream to downstream: - an oxidation catalyst member (also called DOC); A mouth of a reducing agent introducing means or reducing agent precursor for the selective catalytic reduction of SCR nitrogen oxides; a catalyst element for selective catalytic reduction SCR of NOx nitrogen oxides; A particulate filter member provided with a catalytic selective catalytic reduction coating SCRF of NOx nitrogen oxides; an ammonia leak treatment unit; said catalyst member for selective catalytic reduction of nitrogen oxides (6) being between 45 mm and 80 mm in length, in particular between 50 and 75 mm. Preferably, said catalytic selective catalytic reduction member of the nitrogen oxides is chosen to be at least twice as long, in particular at least 2, 1 or at least 2.2 to 3 times smaller than the length of the particulate filter. Preferably, the pollution control system comprises only the organs 15 listed above (except for any additional devices not directly involved in the depollution, but rather to its extent, such as sensors for example). This post-processing device architecture has proved extremely favorable in several aspects. [0022] It makes it possible to improve the performance of the device, in particular as regards the reduction of NOx under the least favorable conditions, namely, as mentioned above, in urban taxi conditions (where the temperature of the gases exhaust remains lower than a road or highway type taxi); even conditions of aggressive driving type, with high flow rates of exhaust gas to be treated. [0023] And these very interesting results were obtained by the combination of two SCR organs: one dedicated to this function, the other integrated in the particle filter, with a ratio between the lengths of the two very specific corresponding bodies. to a SCR organ much smaller than the particulate filter. Thus, the NOx reduction function is "distributed" over two successive members, with a dedicated SCR member that is small, and in fact much more thermally efficient than the catalytic phase that impregnates the particulate filter (SCRF). It can thus be considered that the SCR member provides the majority of the NOx reduction of the device, especially under unfavorable driving conditions 3029970 6 as low temperatures and that the SCRF sees its contribution to the reduction of NOx rise with the temperature and the amount of NOx to be treated (the heavy loads in particular require the SCRF, the organ SCR not being sufficient to treat all the emissions of NOx). By avoiding the use of a single SCRF, not only does the device of the invention better thermally primer, but it also attenuates the impact of a possible degradation of the catalytic coating of the FAP which could occur during regenerations which would cause the FAP excessive temperatures (over 1000 ° C, to give an order of magnitude). It has also been shown that this architecture made it possible to limit at most 10 the ammonia discharges / leaks at the end of the exhaust line (also referred to in English as "NH3 slip"), discharges of ammonia from the reducing agent injected upstream of the SCR catalyst carrying member but unreacted), this through the addition of a body for treating these ammonia leaks. This is very advantageous, especially for heavy vehicles which require the injection of a larger amount of reducing agent, thus with an increased risk of ammonia release at the end of the line. The device according to the invention will treat the gaseous and particulate pollutants as they pass through the depollution organs: they thus first enter the first "brick" consisting of the oxidation catalyst, where CO and HC are oxidized to water (H2O) and carbon dioxide (002). Out of this first DOC brick, products of the oxidation of CO and HC namely H20 and 002, as well as nitrogen oxides and particles. These compounds then travel through the SCR catalyst brick (very short / small as detailed below), which reduces NOx to nitrogen (N2) according to various reactions which will be detailed later. Remaining at the outlet of the SCR catalyst residual NOx, the excess of ammonia (NH3) from the catalyst (explained later) and particles. These compounds enter the SCRF brick, which will complete the reduction of NOx by NH3 and remove the particles by storing them before burning them during regenerations. [0028] Preferably, the catalyst element for the selective catalytic reduction of nitrogen oxides has a length of, for example, approximately 50 or 60 mm. So it's really a very small SCR organ, which can be referred to as the SCR "slice", which has been perfectly fulfilling its role when it might have been feared that it would be a 3029970 7 The short length / thickness would ultimately make it inefficient (in particular because of a shortened contact time between the catalyst and the exhaust gases). Preferably, the length of the SCRF member is at least 100 mm, in particular between 100 and 175 mm, in particular about 120 to 130 mm. [0030] Preferably, the total length of the selective catalytic reduction catalytic reduction unit of the nitrogen oxides and of the particulate filter, including any space between them, is at most 250 mm, in particular at most 190 mm, preferably between 170 and 180 mm. The invention therefore maintains a moderate size compared to a solution using only one SCRF member: it does not significantly lengthen the post-treatment device, and thus preserves the compactness of the assembly. Preferably, the total length between the inlet of the oxidation catalyst member and the outlet of the particulate filter is at most 450 mm, in particular at most 400 mm, preferably between 280 and 380 mm. [0032] Finally, the possible excess of ammonia is treated by the ad hoc treatment member 15 at the downstream end of the post-treatment device. According to a first variant, the DOC oxidation catalyst member, the mouthpiece, the selective NOx SCR reduction catalyst member, and the particulate filter member provided with a SCRF catalytic selective reduction catalyst coating. NOx nitrogen oxides are combined in a single envelope, and the ammonia leak treatment member is disposed outside of said single envelope. It is thus possible to distribute the various organs of the post-processing device, depending on the space available under the hood: thus, the bodies except the one treating the ammonia leak remain grouped under the engine output hood, while the The ammonia leak treatment unit can be deported further downstream on the exhaust line and thus be in the underbody area of the vehicle. Optionally, the action of the ammonia leak treatment member is completed (or even replaced) by an ammonia leak treatment coating integrated with the particulate filter, preferably in its downstream portion. According to another variant, said organs and mouthpiece are grouped together in a single envelope. This architecture preserves the compactness of the assembly, which is contained in a single envelope, and which can thus be advantageously housed as close to the exhaust manifold 3029970 8 engine output on the exhaust line, and especially under hood. Preferably, the ammonia leak treatment unit is a catalyst for treating ammonia leakage by oxidation of ammonia to NOx and then reducing said NOx to nitrogen, which can be used in the process. continued from this text refer to ASC. "ASC" is the acronym in English for the term "Ammonia Slip Catalyst" or catalyst for ammonia leaks in French. Alternatively, the ammonia leak treatment unit may be a catalyst for cleaning ammonia leaks by oxidation of ammonia, which may be referred to below as CUC in the following text. . "CUC" is the English acronym for "Clean-Up Catalyst" or cleaning catalyst for the treatment of ammonia leaks. This type of catalyst only oxidizes ammonia to NOx. Advantageously, the oxidation catalyst member may also comprise an adsorber material of nitrogen oxides, called PNA, which is the acronym for "Passive NOx Adsorber" in English. The role of a PNA type material is to be able to store during the cold phases the nitrogen oxides emitted by the engine, as the bodies catalyzing the reduction of NOx (the SCR member and the particulate filter with SCRF catalytic coating) are not yet functional. Indeed, it is necessary to wait 180 to 200 ° C to be able to inject the reducer (urea) in the exhaust line and form the ammonia which will then convert the NOx. With NH3 "pre-stored" in the SCR coating, the conversion of NOx can take place a few tens of degrees before (at about 140 ° C). The NAP works by storing NOx "cold" (thanks, in particular, to the addition, in the "classical" impregnation of an oxidation catalyst, of simple or mixed oxides with a basic character such as, for example, for example, cerium or barium oxides) before returning them to a higher temperature when the SCR is fully operational (between 200 and 300 ° C). To ensure proper operation of the PNA, purge steps are provided to clean its surface which has been sulfated over time in a known manner. Preferably, the post-treatment device according to the invention comprises a mixing device for mixing the exhaust gas and the reducing agent and / or converting the precursor into a reducer between the mouth of the means for introducing the reductant or precursor of a reducing agent for the selective catalytic reduction of SCR nitrogen oxides and the selective catalytic reduction catalyst of nitrogen oxides. The post-treatment device may also comprise a NOx sensor between the particulate filter element provided with a catalytic selective catalytic reduction coating SCRF of NOx nitrogen oxides and the treatment organ for leaks. ammonia, and optionally another NOx sensor upstream of the DOC oxidation catalyst member and / or another NOx sensor downstream of the ammonia leak treatment unit. The "upstream" sensor, upstream of the DOC catalyst, can also be replaced by modeling if necessary. Preferably, the oxidation catalyst member has a catalyst whose quantity of noble metals is adjusted so as to obtain at the outlet of the organ exhaust gas whose NO2 / NOx ratio is equal or similar. of 0.5 (we understand by "neighbor" a variation of for example +/- 15% around this value). It has indeed been observed that, especially when the SCR coating material 15 of the SCR member was selected based on zeolites exchanged with iron, its efficiency was maximized by having a NO2 / NOx ratio close to 0.5. at the input of the SCR organ. The organ with zeolites exchanged with iron works better at low temperature than zeolites exchanged with copper, and also at higher temperatures. On the other hand, zeolites exchanged with copper offer the advantage of being less sensitive to this low NO2 / NOx ratio. This ratio can be adjusted around this value by adjusting the composition of the oxidation catalyst. The formulation of this type of catalyst generally contains predominantly Al 2 O 3 doped alumina, hydrated aluminosilicate zeolites (also known by the abbreviation ZSM5) and not exchanged to trap cold HC, and precious metals such as Platinum (Pt) and Palladium (Pd), with a defined ratio. Indeed, depending on this Pt / Pd ratio, the oxidation catalyst will be more or less able to oxidize carbon monoxide (CO) and unburned hydrocarbons (HC): The more the catalyst contains platinum, the more its capacity to to oxidize NO to NO2 will be great. It should be noted that at the engine outlet, and therefore at the inlet of the oxidation catalyst member, the emissions of 30 NOx are mainly composed of NO (> 90%). The oxidation catalyst will therefore efficiently oxidize NO to NO2 to adjust the NO2 / NOx ratio to the desired value. Preferably, the catalyst of the selective catalytic reduction catalyst member is based on zeolite (s) exchanged (s) iron. In fact, the impregnation coatings based on zeolites exchanged with Fe (Fe) have a lower temperature initiation than those based on zeolites exchanged with Cu (Cu), since the ratio NO 2 / NOx is close to 0.5. When this condition is satisfied, an iron-exchanged zeolite coating can convert NOx as early as 150 ° C. Preferably, the catalyst of the particulate filter is based on zeolite (s) exchanged (s) copper. Indeed, this type of catalyst is particularly suitable for impregnating a particulate filter: it has a better thermal resistance than a catalyst based on zeolites exchanged with iron (it must indeed undergo without degradation of any periodic regeneration of the filter at very high temperature), - the combustion of soot by NO2 (effect called "CRT" (acronym for Coutinuously Regenerating Trap), ie trap with continuous regeneration in French) at temperatures close to 250 ° C to 350 ° C tends to reduce the NO2 / NOx ratio, the formulations based on zeolites exchanged with Copper (Cu) being also better adapted because less sensitive to low temperature at this ratio than those exchanged with iron, - it also has a storage capacity higher NH3. This last characteristic is particularly interesting because the small volume (the small length for an unchanged section) of the SCR organ can cause NH3 leakage. It is therefore very useful that these ammonia leaks can be "captured" properly in the SCRF brick downstream of the SCR member. It should be noted that the copper exchanged zeolites proposed for the SCRF and / or exchanged with iron for the catalyst of the catalytic reduction catalyst member SCR are for example based on zeolites of the chabazite, ferrierite or hydrated aluminosilicate type (ZMSS). ), and may also contain at least one of the following oxides: cerium (Ce) oxide, zirconium (Zr), or at least one of the following metals: niobium (Nb), tungsten (W), titanium (Ti). According to one embodiment, the support of the oxidation catalyst member and / or that of the selective catalytic reduction catalyst member is metallic, and optionally equipped with heating means, for example of resistors type 30 electric. This reduces their duration of temperature rise, and therefore the time from which they begin. Alternatively, one can use for one and / or the other of these bodies a ceramic-type material such as cordierite. The support of the SCRF particulate filter may be, for example, silicon carbide (SiC), cordierite or aluminum titanate. Advantageously, the after-treatment device according to the invention also comprises an exhaust gas mixing device and the gearbox and / or precursor of the gearbox between the mouth of the gearbox introduction means and / or of a reducing agent precursor for the selective catalytic reduction of nitrogen oxides SCR and the selective catalytic reduction catalyst member of nitrogen oxides. This mixer has the function of mixing as well as possible the exhaust gas with the reducing agent or the reducing agent precursor, this being especially very useful when the precursor is of the liquid type, such as urea in aqueous phase. . The invention also applies to the direct injection of the reducing gas, such as ammonia, which feeds the exhaust line from one or more salt cartridges (including SrCl2 type) suitable adsorbing ammonia and releasing it by thermal activation, in a known manner (technology commonly called SCR "solid"), and in this case, the mixer is less necessary. Preferably, the mixer is of a type having a path length for gases passing through it at least twice the length it occupies longitudinally in the envelope. The purpose of the mixer is to homogenize the mixture between the exhaust gas and the reducing agent and, if a precursor of a reducing agent is introduced, to promote the decomposition of the reducing agent precursor into a reducing agent. The use of a mixer imposing on the exhaust gas a relatively long path compared to the length of the mixer, for example of a type imposing gas a substantially helical path with impactor, is particularly suitable for the invention. It makes it possible, by obtaining an exhaust gas travel distance greater than its own dimensions, to use in a compact device a solution based on urea as an ammonia precursor, then The fact that the thermolysis of urea (corresponding to the transformation under the action of heat of urea into isocyanic acid HNCO and ammonia NH3) in the exhaust gases requires a significant time. The mixer may also be, for example, a T-blender using the recirculation of the downstream oxidation catalyst gas in a jacket around the oxidation catalyst with an injection on the exit side of the oxidation catalyst. Preferably, the single envelope is substantially cylinder-shaped provided with an inlet divergent and an outlet convergent (in the form of cone sections), of a total length of at least 30.degree. more than 450 mm, especially at most 400 mm, preferably between 280 and 380 mm, and it therefore has a compactness quite compatible with an implantation in a sub-bonnet of a motor vehicle. [0060] Preferably, the means for introducing the reducing agent is a solenoid or piezoelectric or mechanical or hydropneumatic actuator type injector. The duct between the exhaust manifold and the device according to the invention may further comprise one or more turbocharger turbines in the context of a supercharged engine, and, in particular, the device according to the invention may be connected directly to the casing of a turbocharger, at the outlet of a turbine. The invention also relates to an exhaust line which comprises the post-processing device described above. The invention also relates, in a first embodiment, to a motor vehicle defining a space under the hood, which contains what is usually referred to as the engine compartment, and a space under the box, and comprising 15 a heat engine connected to the previous exhaust line, such that the engine and the aftertreatment device of the exhaust line are arranged in the space under the hood. In this way, all the depollution units are grouped compactly close to the engine. The invention also relates, in a second embodiment, to a motor vehicle defining a space under the hood and an underbody space, and comprising a heat engine connected to the preceding exhaust line, such as the engine, the DOC oxidation catalyst member, the mouthpiece, the NOx SCR selective reduction catalyst member, the particulate filter member provided with a SCRF catalytic selective reduction catalyst coating of the nitrogen oxides NOx and The mouth of the aftertreatment device of the exhaust line are arranged in the space under the hood, and such that the ammonia leak treatment unit is disposed in the underbody space. Thus, the engine is made up of the pollution control members needing a high exhaust gas temperature, and the ammonia leak treatment unit is removed from the engine in order to protect it from too severe thermal conditions. The invention is described in more detail below with reference to the figures relating to a non-limiting embodiment relating to a device for the after-treatment of the exhaust gases of a diesel engine: ## EQU1 ## FIG. 1 schematically represents a motor and its exhaust line of a motor vehicle comprising the post-processing device according to an example 1 of the invention; FIG. 2 is a graph showing the evolution of the ammonia storage capacity in a SCR / SCRF assembly as a function of the temperature; FIG. 3 represents an operating diagram of the ammonia leak treatment catalyst of the post-processing device of FIG. 1. The references taken from one figure to the other designate the same components, and the different components. Components shown are not necessarily scaled. The 10 figures are very schematic for easy reading. In the invention, and as shown in Figure 1, there is provided a device for treating the exhaust gas of a motor 1 according to an example 1. This device is integrated into the exhaust line connected to the manifold (not shown) of the exhaust gases of the engine 1. It comprises, in the same envelope 2 (which may also be referred to as the English term "canning") and, depending on the direction of flow of the gases exhaust (from upstream to downstream, therefore), an oxidation catalyst member 3, a mouth 41 of a reducing agent introducing means 4 (or a reducing agent precursor), a mixer 5, an organ catalyst SCR 6 (selective catalytic reduction catalyst of nitrogen oxides), a SCRF particulate filter provided with an SCR 7 impregnating coating and an ammonia leak treatment catalyst 8. At least one NOx sensor 9 between the filter 7 and the ammonia treatment catalyst 8. [0068 ] The casing 2 is located closer to the exhaust manifold, in particular about 350 mm from its outlet (for example at most 500 mm from its outlet).

It is arranged, in the motor vehicle, in the under-hood space accommodating the engine 1. The dimensional / geometric data are as follows: The casing 2 has a cylindrical section and accommodates the various members 3 , 6,7 and 8, also of substantially cylindrical outer shapes and sections of about 175 cm 2 (or 150 mm diameter). The ends of the casing 2 are in the form of cone sections, to allow connection to the rest of the exhaust line of much smaller section. The length L1 of the SCR member 6 is between 50 and 75 mm, for example 60 mm. The length L 2 of the particle filter 7 is between 4 and 6 inches, ie between 101.6 and 152.4 mm, for example here 5 inches or 127 mm. The length L12 measured 3029970 14 from the upstream face of the SCR member 6 to the downstream face of the particle filter 7 is, knowing that they are separated, about 4 mm, here 191 mm. The length L3 of the oxidation catalyst member is about 70 mm. The length LO from the upstream face of the oxidation catalyst 3 to the downstream face 5 of the member 8 is between 280 and 380 mm. The length of the treatment member 8 of NH3 is between 50 and 75 to 80 mm. The length LO corresponds substantially to the length of the cylindrical portion of the casing 2. The total length LT of the casing 2, including the two connecting cones, is therefore a little greater than LO. The first "brick" of this post-treatment device is the oxidation catalyst 3, which oxidizes the reducing species that are carbon monoxide (CO) and unburned hydrocarbons (HC). The reactions it favors are as follows: CO + 1/2 02 CO2 (R1) Oxidation reaction of carbon monoxide CxHy + (x + y / 4) 02 x CO2 + (y / 2) H2O (R2 Reaction of oxidation of unburned hydrocarbons It consists of a cordierite-type honeycomb support on which a catalytic active phase ("washcoat") is deposited. This phase comprises oxides such as alumina doped with various stabilizers (lanthanum, cerium, zirconium, titanium, silicon, etc.). On these oxides, precious metals (platinum, palladium) are deposited in order to catalyze low temperature oxidation reactions. Acidic compounds such as zeolites are also added. Their ability to store hydrocarbons at low temperatures and remove them from storage at high temperatures can improve the treatment of HC during cold phases. To these functions (oxidation of carbon monoxide and unburned hydrocarbons and storage of these at low temperature) can be added a storage function for nitrogen oxides, NOx also at low temperature. This storage function is ensured by the introduction of materials of simple or mixed oxides type basic type such as, for example, cerium oxides or barium among others. [0075] The urea injector 4 and the mixer 5 (also known as a mixing box), already described and known, are described in detail here, in particular from the aforementioned patent application WO 2011/089330. Just remember that the mixing box 5 fed by an injector 4, itself fed by a gauge-pump module that draws urea in aqueous solution in a tank of about 20 liters (it can contain less since the volume of urea embedded depends on the consumption strategy adopted), ensures a mixture between the drops of urea and the exhaust gas sufficient for the reaction (R3) to be completely thermolyzed and that the reaction ( R4) is partially carried out before being "completed" on the SCR member 6. The reactions (R3) and (R4) are explained below. The SCR member 6 and the SCRF particle filter 7 treat the nitrogen oxides. The principle of the reduction of these NOx by SCR (whether by the dedicated member 6 or by the coating of the particulate filter 7) can be broken down into two main steps: 1> Formation of the reducing agent (NH 3) from Adblue ® which is a mixture of 32.5% urea and water (NH2) 2CO NH3 + HNCO (R3) thermolysis of urea HNCO + H2O NH3 + CO2 (R4) hydrolysis of isocyanic acid 15 The decomposition of the urea, injected by the injector 4 into the mixing box 5, is in two steps: a first called "thermolysis" which forms a molecule of NH3 and a molecule of isocyanic acid (HNCO ) and a second which forms the second molecule of NH3 from the hydrolysis of isocyanic acid. These two steps, but especially the vaporization of the water contained in the mixture, require temperatures of at least 180 to 200 ° C, hence the interest that the injector and the gas-liquid mixing box ( urea) are close to the output of the motor 1. This step makes it possible to form the reducer essential for the operation of the SCR reduction. 2> Selective Catalytic Reduction of NOx by NH3 by SCR Coatings of Organ 6 and 7: 25 4 NO + 02+ 4 NH3 4 N2 + 6 H2O (R5) Standard SCR NO + NO2 + 2 NH3 2 N2 + 3 H2O (R6 ) Fast kinetic SCR 6 NO2 + 8 NH3 7 N2 + 12 H2O (R7) SCR with slow kinetics Several reactions can take place (R5 to R7), but the optimal and sought conversion of NOx is obtained thanks to the reaction (R6) whose kinetics is the fastest but whose stoichiometry imposes a NO2 / NOx ratio close to 0.5, especially at low temperatures (that is to say at most 250 ° C). The catalyst of the organ SCR 6 is based on zeolites exchanged with iron, such as zeolites [3, fer-ferrierite, ZSM5, and the SCR catalyst of the particle filter 7 is based on zeolites. to copper, such as chabazite, zeolite [3, cuferriérite, ZSM5 ... As already mentioned above, it is the best choice, in particular for the catalyst SCR 6 to start as quickly as possible, to "Low" temperature when the NO2 / NOx ratio of the exhaust gas is close to 0.5 at the inlet of the SCR member 6, and so that the catalyst of the particulate filter remains effective even after severe regeneration and that is less sensitive to the non-ideal NO2 / NOx ratio due to the oxidation of soot by NO2. The substrate of the SCR member 6 is rather cordierite, while the porous support of the filter 7 is rather of SiC silicon carbide. With the architecture of example 1 according to the invention, favorable thermal conditions are obtained, which results in a high efficiency in the treatment of NOx: it was measured on a WLTC cycle at the output of the line. exhaust a level of 40mg / km of NOx for a standard at 80mg / km. Thus, with the invention, a very low level of NOx emissions to the atmosphere is obtained, while preserving the compactness of the entire post-treatment device. The SCRF particle filter 7 located downstream (approximately 8 millimeters behind) also plays an important role, since this brick ensures the removal of particles and performs the treatment of NOx which would not have been reduced in the SCR member 6, during heavy engine loads, for example. Let us return now to the operation of the ammonia leakage treatment catalyst 8. In Example 1 discussed, we chose a catalyst member 8 of ASC type, whose operating principle and structure are illustrated in Figure 3. It has two impregnation layers: a layer C2 which performs the oxidation function of NH3 to NOx and a layer C1 which performs the NOx reduction function by NH3. The composition of the ASC 8 member is thus as follows: the upper layer C1 (that which is in contact with the exhaust gas) corresponds to a catalytic coating of SCR type and the lower layer C2 (the one which is in contact with the walls 30 of the catalytic substrate) contains precious metals (preferably palladium in a very small amount, between 0.5 and 5 g / ft 3, ie between 0.5 and 5 g per volume of 38.48 cm 3, ideally from 1 to 2) deposited on Alumina. The operation of the ASC member 8 is as follows: the residual ammonia enters the layer C1 and stores in this layer in part. The remainder of the ammonia passes through this layer C1 and enters the layer C2 whose precious metals (Pd) promote the oxidation of ammonia NH3 to NOx. When the NOx emerge from the catalytic oxidation coating of the layer C2, they necessarily pass through the layer C1 where the NH3 is stored. The NOx reduction reaction with NH3 can then take place. The NOx are thus converted to nitrogen (N2) before emerging from this catalyst 8. [0085] At the outlet of the exhaust line, there is therefore no or almost no risk of ammonia emissions in the atmosphere thanks to this catalyst 8, even if the engine equips a utility vehicle with a strong need for NOx treatment, as explained below: [0086] If we take up all the reactions R3 to R7 Explained above, it is thus understood that there is a storage phase of NH3 in the catalytic coatings SCR 6 and SCRF 7, prior to the conversion of NOx to N2 through the same NH3 generated by the decomposition of urea. This ammonia storage, represented by a criterion called "NH3 storage capacity", varies according to various parameters, such as the intrinsic characteristics of the catalytic coating, its state (new or aged), the intra-catalyst temperature or, to a lesser extent, the residence time of the gases in the catalyst ... There are, for example, SCR catalytic coatings which have more or less NH3 storage capacities: the ammonia storage capacity of the SCR coatings based on for example copper is greater than that of the SCR coatings based on iron. For a given SCR catalytic coating, its state of aging will intervene: the older the coating, the more its storage capacity will decrease. For an SCR catalytic coating and a given aging state, the storage capacity will change as a function of the intra-SCR or intra-SCRF temperature, as shown in FIG. 2. This figure represents a graph with, as abscissa , the temperature in degrees C, and, on the ordinate, the mass of NH3 stored by a SCR coating in milligrams / liter of catalyst. This graph shows a decrease in storage capacity with temperature, a decrease due to the phenomenon of thermodisorption of ammonia. Finally, it can be noted that the oxidation of NH 3 to NO x which is not favorable to the reduction of these latter occurs on this aged SCR coating from 450 to 500 ° C. It is thus understood that there are phases where emissions of NH3 to the cannula (that is to say at the downstream end of the exhaust line) can appear: - the NH3 3029970 18 can be destock the SCR coating of the organ 6 or 7, because of the thermal conditions encountered, without reacting with the NOx; - NH3 may no longer be stored for reasons of saturation of the storage sites (especially if the SCR coating which has aged thermally has lost its storage capacity); - or if the amounts of NH3 5 are too large relative to its storage capacity. The pollution control systems are generally controlled in efficiency by on-board diagnostic tools known as "OBD" for "On Board Diagnosis" throughout the life of the vehicle. Thus, there are various procedures that make it possible to verify whether the pollution control systems are in working order: by under-injecting urea and therefore ammonia, the system must be able to detect a lower efficiency NOx treatment; or, on the contrary, by over-injecting urea and specifically creating NH3 emissions downstream of the SCR system 6, 7, the system must be able to detect them. In the latter case, the system is diagnosed by means of the NOx sensor 9 located downstream of the SCR system 6, 7. The present invention eliminates these ammonia leaks downstream, by the addition of the organ 8 ammonia treatment, and thus to the cannula, at the end of the vehicle exhaust line. The more the vehicle is heavy, the more these phases at risk (injected amounts of urea very important to "respond" to the amounts of NOx produced by the engine, storage capacity of NH3 reduced because of the high thermal conditions, etc. ..) can occur and be responsible for NH3 emissions to the cannula, even with a line of clearance as effective as mentioned above. Alternatively to the ammonia treatment unit 8 of the ASC type, it is possible to use a CUC type catalyst, which has a single function of oxidation of NH 3 to NOx, by means of a catalytic coating containing platinum. (The Pt load is not necessarily important: 10 to 20g / ft3, or 10 to 20 g per volume of 30.48 cm3, which may be sufficient). It is a simple solution but less efficient than the previous one, since it will recreate NOx at the end of the exhaust line. It is then necessary to ensure that the sum of NOx not treated by the SCR systems 6.7 and residual emissions of NH3 converting to NOx by the action of such a CUC type catalyst does not exceed the limit. NOx emission regulations. According to another variant, the catalyst for treating ammonia leaks 8 is positioned further downstream on the exhaust line: it is no longer in the common envelope, closer to the engine in space. under hood, but downstream in the space 35 under crate, in a dedicated envelope further on the line. This implantation is of interest to avoid premature thermal aging of the catalyst for treating ammonia leaks 8. [0094] In conclusion, thanks to the post-treatment device of the invention, it is not only possible to meet the requirements increasing standards, particularly with regard to NOx emission levels, but it is also possible to reduce fuel consumption by displacing, through the control of the heat engine, the CO2 / NOx compromise towards strategies " low CO2 ", while minimizing the risk of ammonia emissions at the end of the exhaust line. 10

Claims (12)

REVENDICATIONS1. Device for the aftertreatment of the exhaust gases of a combustion engine (1), characterized in that it comprises, from upstream to downstream: a DOC oxidation catalyst element (3); a mouth (41) of means for introducing (4) reducing agent or precursor of a reducing agent for the selective catalytic reduction of SCR nitrogen oxides; a catalytic selective catalytic reduction member SCR of NOx nitrogen oxides (6); a particulate filter member (7) provided with a catalytic selective catalytic reduction coating SCRF of nitrogen oxides NO; an ammonia leak treatment unit (8); said catalyst member for catalytic selective reduction of nitrogen oxides (6) being of a length (L1) of between 45 mm and 80 mm, in particular between 50 and 75 mm. 2. post-treatment device according to the preceding claim, characterized in that the DOC oxidation catalyst member (3), the mouth (41), the selective reduction catalyst element SCR (6) NOx, l particulate filter member (7) provided with a catalytic selective catalytic reduction coating SCRF of NOx nitrogen oxides and the mouth (41) are grouped together in a single envelope (2), and in that the ammonia leakage treatment (8) is disposed outside said single envelope. 3. post-treatment device according to one of the preceding claims, characterized in that the action of the ammonia leak treatment member (8) is completed by an ammonia leak treatment coating integrated in the particle filter (7), preferably in its downstream part. 4. post-treatment device according to claim 1, characterized in that said organs and mouthpiece are grouped in a single envelope (2). 5. post-treatment device according to one of the preceding claims, characterized in that the ammonia leak treatment member (8) is a catalyst for treating ammonium ammonia leakage by ammonia oxidation in ammonia. NOx then the reduction of said NOx into nitrogen. 3029970 21 6. post-treatment device according to one of claims 1 to 4, characterized in that the ammonia leak treatment member (8) is a catalyst for cleaning CUC ammonia leaks by oxidation of the ammonia to NOx. 7. A post-treatment device according to one of the preceding claims, characterized in that the oxidation catalyst member (3) comprises a nitrogen oxide adsorber material PNA. 8. post-treatment device according to one of the preceding claims, characterized in that it comprises a mixing member (5) for mixing the exhaust gas and the reducing agent and / or the conversion of the precursor to a reducing agent between 10 the mouth (41) of the reducing agent or precursor introduction means (4) of a reducing agent for the selective catalytic reduction of the nitrogen oxides SCR and the catalyst element for the selective catalytic reduction of the nitrogen oxides (6). ). 9. post-treatment device according to one of the preceding claims, characterized in that it comprises a NOx sensor between the particulate filter member (7) provided with a catalytic coating catalytic selective reduction SCRF oxides NOx nitrogen and the ammonia leak treatment member (8), and optionally another NOx sensor upstream of the DOC oxidation catalyst member (3) and / or another downstream NOx sensor the ammonia leak treatment unit (8). 10. Exhaust line, characterized in that it comprises the after-treatment device according to one of the preceding claims. 11 Motor vehicle delimiting a space under the hood and a space underbody, and having a heat engine connected to the exhaust line according to the preceding claim, characterized in that the engine and the after-treatment device of the exhaust line are arranged in the space under hood. 25 12. A motor vehicle delimiting a space under the hood and a space under the body, and comprising a heat engine connected to the exhaust line according to claim 10, characterized in that the engine, the oxidation catalyst member DOC (3) , the mouthpiece (41), the NOx selective reduction catalyst member SCR (6), the particulate filter member (7) provided with a SCRF selective catalytic reduction catalyst coating of NOx nitrogen oxides and the mouth (41) of the aftertreatment device of the exhaust line are arranged in the space under the hood, and in that the ammonia leak treatment unit (8) is arranged in the space under cash.