FR2888614A1 - Heat engine exhaust gas treating device for motor vehicle, has support, for oxidation catalyst that is located in downstream of nitrogen oxide trap, impregnated with catalytic impregnation for oxidation of hydrocarbons and carbon monoxide - Google Patents

Abstract

The device has a particle filter (3), and a catalytic converter (2) with a support (4) for nitrogen oxide (NOx) trap and a support (5) for an oxidation catalyst. The oxidation catalyst is located in the downstream of the trap. The support (5) is impregnated with a catalytic impregnation, acting as a NOx trap, that favors the oxidation of hydrocarbons and carbon monoxide contents in the exhaust gas of a heat engine. An independent claim is also included for a motor vehicle comprising a heat engine exhaust gas treating device.

Description

2888614 1

  Catalytic treatment device for motor vehicle exhaust.

  The invention relates to the field of catalytic treatment devices for exhaust gases from combustion engines, especially motor vehicles. The invention relates in particular to the field of treatment devices equipped with a particulate filter, capable of being regenerated.

  In a known manner, thermal engine exhaust lines comprise a catalytic converter and a particulate filter. The particulate filter fixes different chemical species of exhaust gases such as solid species (soot, ash), liquid species (hydrocarbons, water) and gaseous (sulfur species etc). Regeneration of particulate filters consists of burning, that is to say oxidation or release in gaseous form these chemical species. This requires that the temperature in the particulate filter exceeds a temperature threshold depending on the chemical species considered for example, 100 C for water, 150 C to 250 C for hydrocarbons and 500 C to 600 C for soot.

  The catalytic converter generally contributes to reducing the amount of nitrogen oxides released into the atmosphere. It is mainly monoxide and nitrogen dioxide, which we will call NOx in the whole demand. Some manufacturers have considered trapping NOx on a solid in the catalytic converter during normal engine operation, then desorbed and reduced by a transient passage of gas rich in reducing components.

  Patent Application FR 2 812 690 has described a system for assisting the regeneration of a particulate filter comprising an oxidation catalyst and a multi-injector supply system capable of injecting fuel into the engine cylinders during their relaxation phase. An increase in the temperature of the exhaust gas in the particulate filter is caused by the injection system. During the regeneration phase of the particulate filter, the exhaust gas contains unburned hydrocarbons. The oxidation catalyst located upstream of the particulate filter promotes the oxidation reaction of hydrocarbons and carbon monoxide to gaseous carbon dioxide. This reaction is exothermic so that the temperature of the exhaust gas increases until soot burns in the particulate filter.

  The patent application FR 2 812 686 has described a system for assisting the regeneration of a particulate filter integrated in an exhaust line of a Diesel type engine. This system comprises a NOx trap, located upstream of the particulate filter, and makes it possible to jointly treat the particles and the nitrogen oxides escaping from a motor vehicle. This system includes three modes of operation, first of all the NOx trap operating mode when the exhaust gases are poor in reducing components. The NOx purge operation occurs when, transiently, the exhaust gases are enriched in hydrocarbons and depleted of oxygen to be rich in reductant. Finally, this system also participates in the regeneration of the particulate filter by injecting hydrocarbons with an excess of oxygen. The NOx trap comprising catalytic compounds promoting the oxidation of hydrocarbons and carbon monoxide which contribute, as in the patent application FR 2 812 690, to raise the temperature sufficient for the exhaust gases to burn the soot in the atmosphere. particle filter.

  The disadvantage of such a system for assisting the regeneration of a particulate filter is that the temperature rise needed to burn the soot takes place in the NOx trap. However, at high temperature, the catalytic impregnation of the NOx trap deteriorates and in particular reduces its NOx storage capacity.

  Furthermore, the patent application JP-83 38 229 (Toyota) combines, in a muffler, a NOx trap and an oxidation catalyst located upstream of the particulate filter. The catalytic impregnation of the NOx trap is deposited either on the oxidation catalyst or on the particulate filter. The regeneration mode of the particulate filter 2888614 described in the Toyota application differs from the regeneration modes recalled previously. An electric heater is inserted on the exhaust line upstream of the oxidation catalyst and the particulate filter so that at higher exhaust gas temperature, the NO2 content absorbed by the NOx trap decreases and the NO2 not trapped by the NOx trap oxidizes the particles fixed by the particulate filter. This document does not provide to impregnate the particulate filter of a catalytic phase to reduce the soot combustion temperature, so that the exhaust pipe has the prior disadvantage of degrading the NOx trap due to the temperature necessary for the regeneration of the particulate filter and also has the disadvantage of discharging into the atmosphere of nitrogen dioxide during the regeneration phase of the particulate filter.

  Patent EP-1 072 763 (Renault) describes a system for treating particles and nitrogen oxides for a combustion engine. The described system also combines a NOx trap and an oxidation catalyst and avoids evacuating nitrogen dioxide by impregnating the particulate filter with a NOx reducing catalyst released from the NOx trap. The oxidation catalyst is exclusively located upstream of the NOx trap. The described system contributes to reducing the degradation of the NOx trap by the fact that the nitrogen oxides, which are strongly oxidizing, promote the combustion of the particles at a lower temperature. The amount of energy to be produced to increase the temperature is thus reduced. Such a system has the drawback of requiring additional catalytic treatment to reduce the NOx released by the NOx trap and passing through the particulate filter.

  The invention proposes a device for treating the exhaust gases of a heat engine, in particular a motor vehicle, solving the above problems and making it possible both to protect the NOx trap against too high temperatures without causing fumes of dioxide nitrogen during the regeneration phase of the particulate filter nor require additional catalytic treatment of the particulate filter.

  According to one embodiment, the device for treating the exhaust gases of a heat engine, particularly a motor vehicle, comprises a catalytic converter and a particulate filter. The catalytic converter comprises a trap for nitrogen oxides (NOx) and a catalytic oxidation means located downstream of the nitrogen oxide trap (NOx).

  In such a device, when the injection means adds unburned hydrocarbons in exhaust gas saturated with oxygen, and therefore poor in reducing agents, the NOx trap not only makes it possible to absorb nitrogen oxides but also oxidize excess hydrocarbons and carbon monoxide. This exothermic reaction begins to raise the temperature of the exhaust gas. The catalytic oxidation means, located downstream of the NOx trap, continues to increase the temperature by continuing to promote the oxidation of hydrocarbons and excess carbon monoxide so that the maximum temperature reached at the inlet of the filter particulate and necessary to burn soot, affects only the oxidation catalyst and not the NOx trap. During the regeneration phase of the particulate filter, the maximum temperature of the NOx trap remains lower than that required to burn the soot. The NOx trap is protected.

  Advantageously, the nitrogen oxide trap (NOx) comprises catalytic coatings capable of promoting the oxidation of hydrocarbons and carbon monoxide.

  Advantageously, the catalytic oxidation means comprises a catalytic coating comprising, as catalyst, at least one metal such as platinum or palladium and one or more transition metal oxides, alkali, alkaline earth, or rare earths.

  According to one variant, the catalytic coatings of the nitrogen oxides trap (NOx) and of the oxidation means are supported by a single ceramic monolith or corderite.

  According to another variant, the catalytic coatings of the nitrogen oxide trap (NOx) and the oxidation means each have a different support, each of the supports being a ceramic monolith or corderite.

  According to another embodiment, the motor vehicle comprises a device for treating the exhaust gases of a heat engine, a nitrogen oxides (NOx) probe placed downstream of the nitrogen oxides trap (NOx), an additional injection means unburned hydrocarbons in the exhaust gas, located upstream of the device, an intake reduction means adapted to reduce the proportion of oxygen in the exhaust gas and a control means designed to purge the nitrogen oxide trap (NOx) by simultaneously operating said additional injection and intake reduction means.

  Advantageously, the motor vehicle comprises a means of partial recycling of the exhaust gas, the control means of the device being able to increase the proportion of gas recirculation to reduce the proportion of oxygen in the exhaust gas.

  According to one variant, the motor vehicle comprises a device for treating the exhaust gases of a heat engine, an intake means capable of increasing the proportion of oxygen in the intake gases beyond the stoichiometric proportion, a additional injection means unburned hydrocarbons in the exhaust gas, located upstream of the device, and control means designed to cause the regeneration of the particle filter by simultaneously actuating said additional injection means and admission.

  Advantageously, the motor vehicle comprises a multiple distribution means to each cylinder of the engine, capable of selectively injecting unburned hydrocarbons during the expansion phase of at least one cylinder.

  Other features and advantages of the invention will appear on reading the detailed description of an embodiment of the device taken as a non-limiting example and illustrated by the appended drawing, in which the figure is a representation. schematic of an embodiment, the device and the vehicle of the invention.

  As illustrated in the figure, the device for treating the exhaust gases of a heat engine 1 comprises a catalytic converter 2 and a particulate filter 3. The catalytic converter 2 successively comprises a nitrogen oxide trap support 4 ( NOx) and an oxidation catalyst support. The two supports 4 and 5 are two ceramic monoliths, the NOx trap holder 4 occupying most of the pot upstream of the catalytic converter 2 is covered with a catalytic impregnation acting nitrogen oxide trap (NOx) . The oxidation catalyst support 5 is impregnated with a catalytic impregnation promoting the oxidation of the hydrocarbons and carbon monoxide contained in the exhaust gases of the heat engine 1.

  In a variant, the two types of catalytic impregnation cover the same monolith, nevertheless, the part acting as a nitrogen oxides trap (NOx) is situated upstream and the part containing only oxidation catalysts is located downstream. .

  In another variant, one or both supports 4, 5 of catalytic coatings are corderite. It is also possible to use metal honeycomb supports. In such an alternative, the catalysts are in the form of surface coatings.

  The chemical content of the catalytic impregnations will now be described. Catalytic impregnation acting as a nitrogen oxides trap (NOx) comprises compounds that promote the storage of nitrogen oxides (NOx) in an oxidizing medium. These catalytic compounds are in the form of nitrates or oxides of an alkaline or alkaline-earth element. The catalytic impregnation of the NOx trap also comprises oxidation compounds, composed of metal oxides or a metal of groups VIA, VIIA, VIII or IB. The catalytic impregnation also comprises a rhodium-based reduction compound, for example. The operation of such a catalyst depends on the proportion of oxygen or reducing compounds in the exhaust gas. When the exhaust gases are poor in reducing, that is to say rich in oxygen, which corresponds to the normal operation of an engine, then the oxidation compounds promote the oxidation of nitrogen monoxide NO in Nitrogen oxides NO2 and storage compounds promote the conversion of NO2 nitrogen oxides to nitrates deposited on the monolith. During this absorption phase, that is during normal engine operation, the oxidation compounds of the catalytic impregnation of the NOx trap also promote the oxidation of the residual hydrocarbons and carbon monoxide to dioxides. carbon dioxide (CO2).

  The catalytic impregnation of the downstream oxidation catalyst comprises platinum or palladium metal oxides and oxides of transition metals of alkaline, alkaline earth or rare earth oxides.

  According to another embodiment, the motor vehicle comprises an air intake means 6, cylinders 7 and a normal fuel injection means 8 in the cylinders 7 when they are in the compression phase. In normal operation, the heat engine 1 compresses in the cylinders 7 a mixture of air and hydrocarbons to a pressure level where the combustion of the mixture takes place by releasing energy. In normal operation, the proportion of oxygen introduced into the cylinder is greater than the stoichiometric proportion so as not to inject unnecessary hydrocarbons into the cylinders 7. The gases recovered by the exhaust manifold 9 and passing through the catalytic converter 2 and the particulate filter 3 normally have an excess of oxygen. The engine also comprises an additional injecting means 10 injecting hydrocarbons during the expansion phase of the cylinders 7 so that these hydrocarbons are injected while the combustion conditions are no longer met, and are evacuated unburned in the exhaust gas. In a variant, the additional injection means 10 could be an injector connected to the exhaust manifold 9 or directly upstream of the catalytic converter 2.

  A nitrogen oxides (NOx) probe 12 is located downstream of the catalytic converter 2. An EGR recycling means 11, intended for partial recycling of the exhaust gases, is inserted between the exhaust manifold 9 and the exhaust means. 6 intake of air. A control means 13 receives a signal from the probe 12, controls the EGR recycling means 11 as a function of the signal received by the probe 12 and can modify the proportion of exhaust gas recirculation. The control means 13 can also control the additional injection means 10.

  We will now describe the behavior of the device and the motor vehicle. In normal operation, only the normal injection means 8 is actuated, the intake mechanism 6 saturates the oxygen intake gases so that almost all the hydrocarbons injected during the compression phase of the cylinders are burned and only a residual amount of hydrocarbons or carbon monoxide escapes with the exhaust gas. The exhaust gases comprise excess oxygen. Normal operation results in absorption of nitrogen oxides by the NOx trap. The nitric oxide (NO) is oxidized to nitrogen dioxide (NO2), the nitrogen dioxide is oxidized to nitrates which are deposited on the support 4 of the NOx trap. The exhaust gases continue their course through the oxidation catalyst support 5 in which any residual hydrocarbons and incompletely burned carbon monoxide are oxidized to gaseous carbon dioxide (CO2). Residual nitric oxide that has passed through the NOx trap can be further oxidized to nitrogen dioxide by the action of the oxidation catalyst. In the particulate filter, the exhaust gases are filtered by fixing in particular soot.

  During the purge phase of the nitrogen oxides, the control means 13 simultaneously actuates the additional injection means 10 and an intake reduction means. The intake reduction means can reduce the intake section of the air. It can also increase the proportion of exhaust gas EGR recycling. In one way or another, the intake reduction means reduces the proportion of oxygen in the inlet gases below the stoichiometric proportion necessary for the combustion of the hydrocarbons injected by the normal injection means 8 Thus, the exhaust gases passing through the catalytic converter 2 and the particulate filter 3 exhibit an oxygen deficiency. In addition, the additional injection means 10 introduced into the exhaust gas a stoichiometric amount of unburned hydrocarbons corresponding to the amount of nitrates trapped in the support 4 of the NOx trap. During this phase of desorption in a medium rich in reducing agents, the nitrates are converted into nitrogen oxides (NO2) which themselves are transformed into nitrogen gas (N2). By passing through the support 5 of the oxidation catalyst, the exhaust gases undergo few transformations, only a residual amount of hydrocarbons or carbon monoxides is converted into gaseous carbon dioxide (CO2). By passing through the particulate filter, the exhaust gases continue to be filtered as in the normal phase in a low gear environment.

  If the injected quantity of hydrocarbons by the additional injection means is greater than the quantity necessary for the desorption of the nitrates, then part of the excess hydrocarbons is oxidized by the oxidation catalyst while passing through the support 5 and the rest passes through the particulate filter and escapes into the atmosphere, causing pollution. Conversely, if the injected quantity of unburned hydrocarbons by the additional injection means 10 is insufficient for the desorption of the nitrates trapped in the support 4, then a residual amount of nitrogen dioxides (NO 2) is not transformed into nitrogen. gas (N2), passes through the particulate filter and causes an increase in pollution. The NOx probe 12 makes it possible to locate when the support 4 is saturated with nitrates and then controls the purge phase of the NOx trap. A temperature probe disposed on the support 5 of the oxidation catalyst makes it possible to identify the end of the NOx purge phase because the oxidation of excess unburned hydrocarbons is an exothermic reaction.

  During the regeneration phase of the particulate filter, the control means 13 acts as the additional injection means 10 but the intake means 6 operates in its normal configuration so that the exhaust gases are saturated with oxygen and comprise unburned hydrocarbons. By passing through the support 4 of the NOx trap, the exhaust gases in this configuration combine both the NOx absorption phase and an oxidation phase of unburned hydrocarbons. All of these two reactions are exothermic so that the temperature of the exhaust gas increases as the exhaust gas passes through the support 4. On reaching the support 5 of the oxidation catalyst, the gases continue to be oxidized so that the temperature of the exhaust gas continues to increase until temperatures of 750 to 850 C are reached, allowing particulate filter soot to be oxidized by excess oxygen present in the exhaust. The maximum temperature of the exhaust gas is reached in the support 5 of the oxidation catalyst and the support 4 of the NOx trap is subjected during this regeneration phase only to temperatures of at least 100 to 200 ° C lower so that the catalytic impregnations of the NOx trap do not degrade.

Claims (1)

11 CLAIMS   1. Apparatus for treating the exhaust gases of a heat engine (1), especially a motor vehicle, comprising a catalytic converter (2) and a particulate filter (3), the catalytic converter (2) comprising a trap nitrogen oxides (NOx) and a catalytic oxidation means, characterized in that the catalytic oxidation means is located downstream of the nitrogen oxide trap (NOx).   2. Device according to claim 1, wherein the nitrogen oxide trap (NOx) comprises catalytic coatings capable of promoting the oxidation of hydrocarbons and carbon monoxide.   3. Device according to claim 1 or 2, wherein the catalytic oxidation means comprises a catalytic coating comprising, as catalyst, at least one metal such as platinum or palladium and one or more transition metal oxides, alkaline, alkaline earth or rare earth elements.   4. Device according to any one of claims 1 to 3, wherein the catalytic coatings of the trap nitrogen oxides (NOx) and the oxidation means are supported by a single ceramic monolith or corderite.   5. Device according to claim 1 to 3, in which the catalytic coatings of the nitrogen oxide trap (NOx) and of the oxidation means each have a different support (4, 5), each of the supports (4,5). being a monolithic ceramic or corderite.   6. A motor vehicle comprising a device according to any one of claims 1 to 5, a nitrogen oxides (NOx) probe placed downstream of the nitrogen oxide trap (NOx), an additional injection means (10). ) of unburned hydrocarbons in the exhaust gas, located upstream of the device, an intake reduction means adapted to reduce the proportion of oxygen in the exhaust gas and a control means (13) designed to cause purging the nitrogen oxide trap (NOx) by simultaneously operating said additional injection means (10) and intake reduction means.   7. Motor vehicle according to claim 6, comprising a means for partial recycling (11) of the exhaust gas, the control means (13) of the device being able to increase the proportion of gas recycle to reduce the proportion of oxygen in the exhaust.   8. Motor vehicle comprising a device according to any one of claims 1 to 5, an intake means (6) adapted to increase the proportion of oxygen in the inlet gas beyond the stoichiometric proportion, a means of additional injection (10) of unburned hydrocarbons in the exhaust gas located upstream of the device, and control means (13) adapted to cause the regeneration of the particulate filter (3) by simultaneously actuating said means for injection (10) and admission (6).   9. Motor vehicle according to one of claims 6 to 8, comprising a multiple distribution means to each cylinder of the engine, adapted to selectively inject unburned hydrocarbons during the expansion phase of at least one cylinder.