FR2910553A1 - METHOD FOR REACTIVATING COMPONENTS CONTAINING PRECIOUS METALS FOR POST-TREATMENT OF EXHAUST GAS OF INTERNAL COMBUSTION ENGINES OPERATING PERMANENTLY IN A POOR REGIME - Google Patents

Abstract

A method of reactivating components (16, 18, 20) containing precious metals of aftertreatment of exhaust gases in exhaust gas plants (12), including at least one component (16, 18, 20) of aftertreatment of exhaust gases from internal combustion engines (10), operating in a lean regime over their entire load range and rotational speed, contains precious metals.For the reactivation of at least one components (16, 18, 20), this is temporarily supplied by an exhaust gas reducing atmosphere which has, for at least a short duration, a lambda value (lambda) of 1.2 maximum, under conditions which are suitable for reducing, at least partially, oxidized precious metal compounds.

Description

The invention relates to a method for the restoration of a

  reactivity of components containing precious metals of aftertreatment of exhaust gases, which are arranged in an exhaust gas installation of internal combustion engines operating continuously in a lean regime over their entire operating range. Many internal combustion engines, including diesel engines (spontaneous ignition), operate today in lean conditions over their entire operating range depending on the load and the speed of rotation. On the other hand, for Otto engines likely to operate in a lean state, NO accumulation catalysts are generally used to respect the legal limit values of NO, which accumulate NO, and must be regenerated in regeneration cycles interposed at steady state. rich. Unlike such Otto engines likely to operate in a lean regime, rich-regime phases, in which the air / fuel ratio lambda is less than 1, and in which an excess of reducing compounds HC and CO exhaust gas exist by definition, are not normally intercalated for diesel engines running continuously in a lean regime.

  Moreover, in the case of modern diesel engines, nitrogen oxide NO emissions are reduced to a compliant level of exhaust gas only by engine-level measurements, such as gas recirculation. EGR exhaust or others. This means that these internal combustion engines operate in a lean regime over their entire lifetime, and therefore with an excess of oxygen (lambda> 1) in the air / fuel ratio. In order to reduce the exhaust gas emissions, in particular unburned HC hydrocarbons, carbon monoxide CO and carbon black particles, the internal combustion engine exhaust systems operating continuously at steady state poor, such as diesel engines, have different aftertreatment components of exhaust gases. These include oxidation catalysts, particulate filters, SCR (selective chemical reduction) catalysts, CRT (continuous regeneration trap) catalysts, NH3 retention catalysts and HC accumulators. These components generally contain a precious metal or a mixture of different precious metals as a catalytic material which causes the chemical conversion of the above-mentioned exhaust gas compounds. The precious metals used are, for example, platinum, palladium, rhodium, iridium and others. With regard to this constellation, however, it has proved problematic that in a permanent poor regime, and consequently permanently poor exhaust gas, oxidation of the precious metal could occur. The higher the temperatures of the components in the lean atmosphere of the exhaust gases, the faster and more persistent the oxidation of the precious metals. However, the noble metal oxides generated do not have a catalytic effect or a reduced catalytic effect, which in the long term does not make it possible to ensure sufficient post-treatment of the exhaust gases. The object of the invention is to provide a method which supports the long-term maintenance of the catalytic activity of precious metal-containing components aftertreatment of exhaust gases of internal combustion engines operating continuously in steady state. poor. This object is achieved by a method of reactivating components containing precious metals of aftertreatment of exhaust gases in exhaust gas systems of internal combustion engines operating continuously in a lean regime, as well as by a control unit described below. The method according to the invention provides that, for the reactivation of at least one of the components containing precious metals post-treatment of exhaust gas, it is fed temporarily by a reducing atmosphere of exhaust gas. which has, at least for a short time, a lambda value (1) of 1.2 at most. This is done under conditions that are suitable for reducing, at least partially, oxidized precious metal compounds. These conditions concern in particular the precise lambda value, the temperature of the aftertreatment components of the exhaust gas concerned, as well as the duration of the rich feed, these parameters being mutually influencing. It is thus possible to invalidate, at least partially, the oxidation of precious metals, namely to convert again, at least partially, the precious metal oxides generated into active elemental precious metals with a zero oxidation state. It has been found here that such reactivation intervals according to the invention can be dimensioned so that the emissions of exhaust gas reducing compounds involved are not significant because of the predominantly poor diet, and do not lead to the exceeding of tolerable limit values. In the context of the present invention, the term "internal combustion engines operating continuously in a lean operating mode" means engines that are usually operating in a lean operating mode over their entire load range and rotational speed. ie which represent themselves very high load demands and / or speeds of rotation without an actively controlled reversion of regime on rich regime (thus with sub-stoichiometric air / fuel ratio with lambda <1). Furthermore, in the spirit of the present invention, "continuous-cycle internal combustion engines operating at steady state" do not have accumulation components for exhaust gas oxidation compounds in their fuel system. exhaust gas, the regeneration of which involves rich intercalated intervals. In particular, they do not include an NO accumulator or a NOS accumulation catalyst. Internal combustion engines operating continuously in a lean regime therefore include not only stationary or non-stationary diesel engines, for example motor vehicles and boats, but also gas turbines or the like, which comprise components containing precious metals of aftertreatment of exhaust gases.

By "reducing exhaust gas atmosphere" it is furthermore to be understood herein as an operating condition in the exhaust gas plant, which is suitable for reducing oxidized precious metals, especially oxides of precious metals.

In the strict sense of the term, "reducing exhaust gas atmosphere" therefore does not mean sub-stoichiometric exhaust gas with a lambda value <1 (although this preferably contributes to a reducing gas atmosphere). exhaust), but all operating conditions, including lambda value, temperature and duration. In a preferred arrangement of the invention, the reducing exhaust gases are represented by the rich regime of the internal combustion engine. The rich regime can be effected by a quenching of the internal combustion engine, namely by a reduction of the admitted air mass, by an increase in an internal or external exhaust gas recirculation rate or by a post fuel injection before and / or after an end of fuel combustion. These measurements can also be carried out in any combination, and are preferably selected or combined with one another so that rotational torque variations due to reactivation measurements are minimized. According to a specific embodiment of the invention, the exhaust gas reducing atmosphere may have an on-stoichiometric lambda value, for example of X ≤ 1.1, in particular of 1 ≤ 1.05. If such a lambda value is set, exhaust gas reducing compounds, such as CO and HC, are certainly contained only to a small degree in the exhaust gas, the reducing effect can, however, by This can be achieved by relatively high temperatures and / or relatively long reactivation times. Ideally, lambda values of the slightly sub-stoichiometric exhaust gas of 1 ≤ 1.0, especially X ≤ 0.98, preferably 15.95, are however set during the reactivation. Depending on the type of the internal combustion engine, values below 0.92 are even tolerable. If the lambda values are significantly lower, for example less than 0.90, the ignition and proper combustion of the mixture is generally no longer ensured. The above lambda values can in principle be set continuously or at intervals during the reactivation. It is also conceivable to represent the preferred lambda values locally locally or in locally limited partial sections of the exhaust system. The exhaust gas reducing atmosphere feed of the at least one aftertreatment component 30 of the exhaust gas can take place for very short durations of for example at least 1 s, in particular of at least 2 s, in particular of at least 5 s. Below these times, the effect of reducing the measurement is generally too low. Ideally, the reactivation is carried out for a duration of at least 10 s, in particular of at least 20 s. It is also conceivable to perform it 2910553 6 for a duration of more than 30 s, even more than 60 s. With these extended periods of time, however, the rich regime of the internal combustion engine will generally have negative repercussions, for example in the form of ignitions missed or overconsumption. Another important parameter for the implementation of the reactivation according to the invention is the temperature of the exhaust gas aftertreatment component concerned. The higher the temperature of the component, the shorter the duration of the reactivation measurement, and the higher the lambda of the exhaust gas can be set to a high value (less rich). As the lower limit for the temperature of the at least one component containing precious metals of aftertreatment of exhaust gas, a value of 200 ° C. has proved its worth. Below this temperature, the reduction of precious metals is usually too slow. According to preferred arrangements of the invention, the supply of exhaust gas in the reducing atmosphere takes place at a temperature of the aftertreatment component of the exhaust gases of at least 250.degree. C., especially at least 300.degree. 0C, preferably at least 350 C. Ideally, however, higher temperatures of at least 400 ° C. are used for carrying out the reactivation, in particular at least 450 ° C., preferably at least 450 ° C. at least 500 ° C., particularly preferably at least 600 ° C. The upper limit of temperature is finally limited only by the high temperature resistance of the aftertreatment component of the exhaust gas.

It is particularly preferred to carry out the reactivation as close as possible after a regeneration of a diesel particulate filter of the exhaust gas plant, because regenerations of a particulate filter are highly exothermic and This ensures high temperatures not only of the particulate filter, but also of the downstream component 2910553. The feed in the exhaust gas reducing atmosphere therefore preferably starts at the latest 1000 s, in particular at the latest 300 s, preferably at the latest 60 s after regeneration of the particulate filter. For the reduction of the oxidation products of the catalytic noble metal, the presence of reducing compounds in the exhaust gas is necessary, especially unburned HC hydrocarbons and CO carbon monoxide.

Preferred exhaust gases contain at least 10 mg / s HC, especially at least 20 mg / s HC, and / or at least 50 mg / s CO, especially at least 100 mg / s CO. The reactivation according to the invention should be carried out as seldom as possible, in order to minimize disadvantages in the combustion and exhaust behavior of the internal combustion engine. On the other hand, the time intervals between two reactivations should not be too great, in order to avoid the formation of irreversible oxidation products of the precious metals, and to be able to carry out their reduction under relatively mild conditions. In this context, sluggish intervals between the reactivations of 3 to 600 minutes, especially 5 to 300 minutes, preferably 10 to 120 minutes, gave good results. Ideally, a reactivation is performed after a lean interval of 20 to 60 minutes. The invention further relates to a control unit, in particular a motor control apparatus for controlling a motor vehicle, comprising a program algorithm for controlling the implementation of the reactivation method according to the invention of components containing precious metals. aftertreatment of exhaust gas from internal combustion engines operating continuously in lean regime.

Other features and advantages of the present invention will become more apparent upon reading the following description by way of example, when taken in conjunction with the accompanying drawing.

The single figure represents an internal combustion engine 10, which is here in particular a diesel engine, which operates in a lean regime over its entire load range and speed of rotation. Exhaust gases from the internal combustion engine 10 are directed to an exhaust gas installation, all of which is identified by 12. The exhaust gas plant 12 comprises a gas pipe 14 exhaust, which contains different components of exhaust aftertreatment. In the flow direction of the exhaust gases, the exhaust gas pipe 14 comprises in particular an oxidation catalyst 16, a particulate filter 18 and an SCR catalyst 20. The oxidation catalyst 16 proceeds with a conversion of carbon monoxide (CO) and unburned hydrocarbons (HC), and for this purpose contains precious metals such as platinum, palladium and / or others. The particulate filter 18 accumulates carbon black particles from the exhaust gas, and is regenerated at certain time intervals, the carbon black particles being oxidatively burned by the catalysis of the precious metals. For this purpose, the particulate filter 18 also contains a suitable precious metal. With regard to the SCR catalyst 20, it is a reduction catalyst which, with the aid of a reducing agent, in this case ammonia (NH 3), converts nitrogen oxides (NOx) exhaust gas in N2 and H2O. For example, the reducing agent NH 3 is injected into the exhaust gas pipe 14 in the form of an aqueous solution of urea as a chemical progenitor for ammonia via a metering unit 22. which is connected to a tank, not shown here. The order of succession and the arrangement of the various components of the exhaust gas plant 12 may differ from those shown here. Alternatively or in addition to the components shown, the exhaust gas installation 12 may comprise a catalyst CRT 5 (continuous regeneration trap "filter with constant regeneration"), which proceeds to a temporary accumulation of particles, and which burns them by oxidation almost continuously with the assistance of a precious metal and NO ,. NH 3 accumulators are also taken into account, in particular downstream of SCR catalysts, which recover a leak of NH 3, and convert it catalytically with the aid of a precious metal. Finally, the exhaust gas plant 12 may also contain an HC accumulator, which certainly contains most of the time a precious metal, but which is usually provided on a common catalyst support by being integrated with the catalyst. other components containing precious metals. The air supply of the internal combustion engine 10 takes place via a suction channel 32, 20 in which is disposed an adjustable throttle valve 34. The control of the internal combustion engine 10, as well as of different components of the exhaust gas plant 12, is effected by means of a motor control apparatus 28, in which usually signals of gas and temperature sensors, not shown, of the installation 12 of exhaust gas. The motor control 36 further detects various operating parameters of the internal combustion engine 10, for example the engine rotational speed, the coolant temperature or a value of the accelerator pedal. Depending on the incoming parameters, the motor control 28 controls the operation of the motor 10, for example the fuel injection quantities and the injection moments, the position of the throttle valve 26 as well as the 2910553 Dosage of reducing agent. The permanent poor regime of the internal combustion engine 10 leads to undesirable oxidation of the precious metals of the aftertreatment components 16, 18, 20, which leads to an alteration of their activity. In order to avoid this, the exhaust gas aftertreatment components 16, 18, 20 are reactivated according to a program algorithm 30, which is stored in the motor controller 28. The program algorithm IO controls the reactivation according to the invention as described hereinafter. Reactivation of the exhaust gas aftertreatment components 16, 18, 20 is initiated after defined lean-hour intervals of a duration which is preferably from 20 to 60 minutes, in particular of the order of 30 minutes. . For reactivation, the components 16, 18, 20 to be reactivated are supplied by an exhaust gas reducing atmosphere, which is defined by the parameters that are the lambda value of the exhaust gas, the temperature of the post-combustion component. exhaust treatment and the duration of reactivation. A slightly sub-stoichiometric lambda value of the exhaust gas with 1 0.98 ... 1.0 is particularly regulated in the present example. In order to achieve this, the internal combustion engine 10 is temporarily set to a rich regime. This is for example effected by a reduction of the suction air by the corresponding closure of the throttle valve. Alternatively or additionally, an exhaust gas recirculation rate may be increased, an exhaust gas recirculation valve (external EGR), not shown, being opened for this purpose, or engine valve overlaps. with internal combustion 10 being enlarged (internal RGE). Another measure that can be used alone or in combination is the post-injection of fuel into the cylinders of the internal combustion engine 10 at a late time, at which complete combustion no longer takes place, the post-injection being able to performed before and / or after an end of combustion. A minimum temperature of the component 16, 18, 20 is required as the other parameter of the reactivation, in order to be able to keep the reactivation time as short as possible and the lambda of the exhaust gases as low as possible. For this purpose, a temperature of at least 450 ° C., in particular at least 500 ° C., is preferably chosen for the component 16, 18, 20 to be activated. In order to achieve this, the reactivation (in the context of the lean-power intervals described above) is carried out as close as possible in time after a successful regeneration of the particulate filter 18. The reactivation is preferably initiated at most. Late 60 seconds after the end of the regeneration of the particulate filter. The slightly sub-stoichiometric exhaust gas aftertreatment components 16, 18, 20 are fed at the indicated minimum temperature for a reactivation time of at least 20 seconds, preferably at least 30 s. Examinations have shown that these exemplary parameters are able to convert, for the most part, the noble metal oxides generated during the steady-state steady state of the internal combustion engine 10 into elemental precious metals, and to re-establish thus their initial activity. Although the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be readily understood by those skilled in the art that changes in form and detail can be made. without departing from the spirit or scope of the invention.

2910553 12 DIGITAL REFERENCE LIST 10 Internal combustion engine 12 Exhaust gas system 5 14 Exhaust gas pipe 16 Oxidation catalyst 18 Particulate filter 20 SCR catalytic converter 22 Dosing unit 10 24 Suction channel 26 Valve throttle 28 Motor control device 30 Program algorithm

Claims (13)

  A method for reactivating components (16, 18, 20) containing precious metals of aftertreatment of exhaust gases in exhaust gas plants (12), including at least one component (16, 18, 20) ) of the exhaust gas aftertreatment of internal combustion engines (10), operating in a lean regime over their entire load range and rotational speed, contains precious metals, characterized in that for the reactivation of at least one of the precious metal-containing exhaust gas aftertreatment components (16, 18, 20), the latter is temporarily supplied with an exhaust gas reducing atmosphere which comprises at least one for a short duration, a lambda value (1) of at most 1.2, under conditions that are suitable for reducing, at least partially, oxidized precious metal compounds.   2. Method according to claim 1, characterized in that the exhaust gas atmosphere is embodied by a rich regime of the internal combustion engine (10).   3. Method according to claim 2, characterized in that the rich regime of the internal combustion engine (10) is performed by at least one of the following measures: start of smothering of the internal combustion engine (10), increase of an exhaust gas recirculation rate, and post-fuel injection before and / or after an end of fuel combustion.   4. Method according to one of the preceding claims, characterized in that the exhaust gas reducing atmosphere comprises an exhaust gas lambda value of X, ≤ 1.1, in particular of 1 5_ 1, 05.   5. Method according to one of the preceding claims, characterized in that the exhaust gas reducing atmosphere comprises a slightly sub-stoichiometric lambda value of the exhaust gas of 1 ≤ 1.0, especially 0, 98, preferably 1≤0.95. 10   6. Method according to claim 5, characterized in that the feed in reducing atmosphere of the exhaust gas is carried out for a duration of at least 5 s, in particular of at least 10 s, preferably at least 20 s. s, particularly preferably at least 30 s.   7. Method according to one of the preceding claims, characterized in that the exhaust gas reducing atmosphere supply is carried out at a temperature of the at least one post-treatment component (16, 18, 20). an exhaust gas containing precious metals of at least 200 ° C, in particular at least 300 ° C, preferably at least 400 ° C, particularly preferably at least 500 ° C.   8. Method according to one of the preceding claims, characterized in that the feed in the exhaust gas reducing atmosphere starts at the latest 1000 seconds, in particular not later than 300 seconds, preferably no later than 60 seconds after regeneration. a particulate filter (18).   9. Method according to one of the preceding claims, characterized in that the reactivation is carried out after lean intervals of 3 to 600 minutes, in particular from 5 to 300 minutes, preferably from 10 to 120 minutes, of particularly preferably from 20 to 60 minutes. 5   10. Method according to one of the preceding claims, characterized in that the exhaust gas reducing atmosphere comprises mass flows of HC of at least 10 g / s of HC, in particular at least 20 g / s of HC. g / s of HC and / or mass fluxes of CO of at least 50 g / s of CO, in particular of at least 100 g / s of CO.   11. Method according to one of the preceding claims, characterized in that the internal combustion engine (10) is a stationary or non-stationary diesel engine.   12. Method according to one of the preceding claims, characterized in that at least one of the exhaust gas aftertreatment components (16, 18, 20) containing precious metals comprises: a catalyst of oxidation (16), - a particulate filter (18), a SCR catalyst (20) - a CRT catalyst, - an NH3 retention catalyst, and an HC trap.   13. Control unit, in particular a motor control device (28) for controlling a motor vehicle, characterized in that it comprises a program algorithm (30) for controlling the implementation of a process according to one of claims 1 to 12.