DE19933736B4 - Arrangement and method for cleaning an exhaust gas of an internal combustion engine - Google Patents

Abstract

The invention relates to an arrangement for purifying an exhaust gas of an internal combustion engine, in particular of motor vehicles, with a catalyst system arranged in an exhaust gas channel for reducing a nitrogen oxide (NO <SUB> x </ SUB>) proportion of the exhaust gas and a particle emission and a method for cleaning an exhaust gas of an internal combustion engine, in particular motor vehicles, by passing the exhaust gas over a catalyst system arranged in an exhaust gas duct. It is provided that the catalyst system (10) in the flow path of the exhaust gas successively a NO <SUB> x </ SUB> memory (20), an oxidation catalyst (22), an HC memory and a NO <SUB> x </ SUB> reduction catalyst (26) and in method (a) a particulate emission downstream of the catalyst system (10) is detected and (b) depending on the particulate emission, a NO <SUB> x </ SUB> emission downstream of the catalyst system (10 ) measure is taken by a working mode of the internal combustion engine (14) by at least temporary Beinflussung at least one operating parameter of the internal combustion engine (14) is set.

Description

The The invention relates to an arrangement for purifying an exhaust gas of a Internal combustion engine, in particular of motor vehicles, with a catalyst system disposed in an exhaust passage and a Process for purifying the exhaust gas by passing the exhaust gas over a is guided in a catalytic converter arranged catalyst system, with those in the preamble of the claims 1 and 17 mentioned features.

During one Combustion of an air-fuel mixture in the internal combustion engine arise in different proportions pollutants, such as soot particles, Reducing or oxidizing agent. It is known that the exhaust gas for Cleaning over to guide the catalyst system arranged in the exhaust duct. The Catalyst system may include different types of catalysts. Thus, by means of an oxidation catalyst, an oxidation of Reducing agents, such as CO, HC or H2, supported with oxygen. Besides is emitted by the internal combustion engine particle mass by an at least partial oxidation of deposited on the particle cores Hydrocarbons allows. On the other hand, reduction catalysts known during the Combustion NOx formed to nitrogen using the Reduce reducing agent.

Known are also storage components that an addition of selected gases by physical and / or Chemiesorption allow. Exemplary here called HC memory or NOx storage. Thereby an absorption takes place until the crossing of one Desorption or reaching a maximum storage capacity instead of.

to Reduction of fuel consumption has proven to be particularly favorable a lean operation of the internal combustion engine as long as possible maintain. In lean operation exceeds an oxygen content a fuel content at the exhaust gas (λ> 1) with the result the existence Proportion of reducing agents, such as CO, HC or H2, decreases. With sinking Reducing agent content is a complete conversion of NOx the NOx reduction catalyst is no longer possible. Nevertheless, a sufficient to provide low NOx emission, exist in principle two possibilities. On the one hand, the NOx is stored in the NOx storage in lean operation. This must then be regenerated in periodic cycles by a change in a regeneration operation (λ ≤ 1) takes place. A NOx desorption temperature is usually so by the Choice of memory components determines that the lean operation as possible can be maintained for a long time. The NOx desorption temperature is therefore as possible high to choose.

To the others can by a targeted Reduktionsmittelindüsung ago the NOx reduction catalyst, for example, by an injection of a fuel in the Exhaust gas channel, the proportion of reducing agents are increased. The disadvantage here is that under circumstances a catalyst temperature is not yet within an active temperature range of the catalyst is such that in particular in a starting phase of the internal combustion engine, the pollutant emissions can only be reduced insufficiently.

EP 0 814 242 A describes an exhaust gas purification system for lean-burn internal combustion engines with a NOx storage and - arranged on the same support - a reduction catalyst. In order to protect the NOx storage from unwanted SOx deposits, the NOx storage is preceded by a SOx storage. In this case, the system is designed such that at substoichiometric exhaust this has a sufficiently high temperature, so that the stored SOx is released and the downstream NOx storage happens without being stored again.

Of the inventive arrangement and the method of the invention the task is based on NOx and particulate emissions as possible Keep low, especially if an exhaust gas temperature still very low.

According to the invention this Object by an arrangement and a method for cleaning the exhaust gas the internal combustion engine with the features of claims 1 and 17 solved. Because that Catalyst system in the flow train the exhaust gas sequentially a NOx storage, an oxidation catalyst and a NOx reduction catalyst, it is easily possible to use the Particle and the To reduce NOx emissions.

In A preferred embodiment of the invention is between the Oxidation catalyst and the NOx reduction catalyst, a HC memory arranged, which has a HC emission downstream of the catalyst system at temperatures below the active temperature range of the oxidation catalyst are prevented.

Furthermore, it is advantageous to additionally arrange a reducing agent injection immediately before the NOx reduction catalytic converter. By such an arrangement, a sufficient reduction of NO _x at the NO _x reduction catalyst can be ensured even under a lean operation of the internal combustion engine.

Furthermore, it is advantageous, the NO _x storage and the oxidation catalyst to a catalyst unit together. In the same way, the HC storage and the NO _x reduction catalyst can be combined to form a second catalyst unit. The storage components and the catalyst components of the catalyst unit are distributed either homogeneously or heterogeneously on a common carrier. A heterogeneous distribution can be realized, for example, by zonewise application of the storage or catalyst component.

The NO _x desorption temperature of the NO _x storage is advantageously chosen such that it is within the active temperature range of the downstream catalyst components. Further, an HC desorption temperature of the HC storage must be within the active temperature range of the NO _x reduction catalyst.

In A preferred embodiment of the invention contains the oxidation catalyst as catalytically active component platinum. It is also advantageous as catalytically active component additionally palladium or rhodium up to a ratio from palladium or rhodium to platinum of not more than 1: 5.

Advantageously, the NO _x reduction catalyst is a low-temperature catalyst, for example based on an Ag-Ga zeolite catalyst. Furthermore, it is advantageous to design the NO _x storage as a low-temperature storage, that is, that NO _x can be stored to a sufficient extent already at relatively low temperatures.

The method for cleaning the exhaust gas of the internal combustion engine can be configured such that first a particle emission downstream of the catalyst system is detected with suitable sensors or estimated by means of suitable models. Subsequently, depending on the particle emission, a NO _x emission is taken downstream of the catalyst system reducing measure. For this purpose, a working mode of the internal combustion engine is adjusted by an at least temporary influencing of at least one operating parameter of the internal combustion engine. Preferably, exhaust gas recirculation is introduced as NO _x -mindernde measure or increased EGR rate.

Further preferred embodiments of the invention will become apparent from the others, in the subclaims mentioned features.

The Invention will be described below in an embodiment with reference to the accompanying drawings explained in more detail. It demonstrate:

1 a schematic representation of an arrangement of a catalyst system in an exhaust passage of an internal combustion engine and

2 a flowchart for controlling a cleaning of an exhaust gas of the internal combustion engine with the arranged in the exhaust gas catalyst system.

The 1 shows a schematic representation of an arrangement of a catalyst system 10 in an exhaust duct 12 an internal combustion engine 14 , In this case, the catalyst system 10 additionally assigned sensors, such as a temperature sensor 16 , These make it possible to determine an exhaust gas temperature, a catalyst temperature or even a proportion of a gas component in the exhaust gas. Location and operation of such sensors are known. It is also known to calculate the parameters characterizing the state of the catalyst (NO _x , HC and O ₂ loading state, catalyst temperature, etc.) by means of suitable models.

Furthermore, the catalyst system 10 a reductant injection 18 assigned. This is usually a fuel in the exhaust duct 12 injected. The mode of operation of such a reducing agent injection 18 is also known and will therefore not be explained here.

In the flow train of the exhaust gas of the internal combustion engine 14 are consecutively according to the embodiment, a NO _x storage 20 , an oxidation catalyst 22 , a HC memory 24 and a NO _x reduction catalyst 26 , To simplify a manufacturing effort, the memory components (NO _x - and HC memory 20 . 24 ) with the catalyst components (oxidation and NO _x reduction catalysts 22 . 26 ) in each case to a first catalyst unit 28 or a second catalyst unit 30 be summarized. Here, the storage component and the catalyst component of the catalyst units 28 . 30 either homogeneously or heterogeneously distributed on a support. A heterogeneous application, for example with a zonal distribution of the storage components or catalyst components, can be realized in terms of production technology, in particular by partial dipping of the support.

The NO _x memory 20 may in particular be a low-temperature storage. That is, it has a sorption characteristic which permits an addition of NO _x already at low temperatures and leads to a desorption of NO _x at already low to medium operating temperatures in the catalyst system.

The oxidation catalyst 22 supports the oxidative conversion of reducing agents, such as HC, CO or H ₂ , with oxygen or NO _x to water and carbon dioxide. In addition, at least partially nitric oxide NO can be oxidized to nitrogen dioxide NO ₂ . It has proved to be advantageous as the catalytically active component of such oxidation catalysts 22 To use platinum. It is particularly advantageous to add palladium or rhodium as a further catalytically active component to the platinum. A ratio of palladium or rhodium to platinum should be at most 1: 5.

The HC memory 24 is preferably prepared on a zeolite basis and has sufficient HC storage capacity even at temperatures well below 180 ° C. An HC desorption temperature is usually around 200 ° C. As NO _x -reduction catalyst 26 Particularly low-temperature catalysts are preferred, for example based on noble metal or else in the form of Ag-Ga zeolite catalysts.

An exemplary mode of operation of such a catalyst system 10 be briefly described below:
Immediately after a start of the internal combustion engine 14 are the exhaust gas temperatures, for example, via the sensor 16 can be detected, still very low (<180 ° C). This is during a combustion process of an air-fuel mixture in the internal combustion engine 14 generated NO _x stored in the NO _x storage. In the case of low-temperature reservoirs, this is made possible essentially by physisorption, so that irreversible sulfate formation can be at least largely avoided as a result of possibly existing sulfur components in the fuel. An NO _x desorption temperature is in NO _x stores 22 based on alkali / alkaline earth metals at about 180 ° C, perovskites in a range of 200 ° C to 350 ° C. It is therefore possible, the NO _x storage 20 the respective circumstances, for example, by combining different memory components to adapt.

The resulting during the combustion process reducing agent, such as CO, HC or H ₂ , are already at low temperatures on the oxidation catalyst 22 reacted with oxygen, so that normally downstream of the oxidation catalyst 22 only very small amounts of reducing agents are present in the exhaust gas. Similarly, soot particles are reduced or destroyed by the oxidation of deposited on the particle cores unburned hydrocarbons. Breakthroughs of incompletely burned hydrocarbons HC, for example due to insufficient temperatures of the oxidation catalyst 22 , can from the HC memory 24 be caught. In this way, a NO _x emission, the particle emission and the reducing agent emission can be kept very low. The reducing agent injection 18 is activated only at temperatures above about 150 ° C to ensure a substantial evaporation of the fuel.

If the exhaust gas temperature rises to approximately 200 ° C, the NO _x desorption temperature is exceeded. The NO _x desorption temperature should be selected such that it is within an active temperature range of the catalyst components 22 . 26 lies. In this way, the NO _x at the oxidation catalyst 22 on the one hand serve as an oxidizing agent for the reducing agent and on the other hand be oxidized at high oxygen contents in the exhaust gas to an increased extent to NO ₂ .

Subsequently, the NO _x in the catalyst unit 30 at least for the most part reduced, by means of the reducing agent injection 18 provided reducing agent or by means of the HC memory 24 absorbed and desorbed at increasing temperatures reducing agent.

During an acceleration process, first the temperatures rise in the region of the first catalyst unit 28 at. The NO _x memory 20 is increasingly emptied and the discharged NO _x partially with the existing HC and Partikelrohemissionen the internal combustion engine 14 on the oxidation catalyst 22 reduced or oxidized by oxygen to _NO2. The NO ₂ -rich exhaust gas strikes the NO _x reduction catalyst 26 then on the injected fuel and the additional of the HC memory 24 desorbed reducing agent. For this purpose, the HC desorption temperature of the HC memory 24 within the active temperature range of the NO _x reduction catalyst 26 lie.

A further reduction in NO _x emission can be achieved via a in the 2 be achieved by a flowchart example illustrated method. For this purpose, in a step S1 first the particle emission downstream of the catalyst system 10 detected with suitable sensors in a known manner or estimated by suitable calculation models. In a step S2, the particle emission is compared with a predefinable threshold value. If the particle emission falls below the threshold value, then in a step S3 further measures which reduce the NO _x emission can be made on the part of the internal combustion engine 14 be taken by a working mode of the internal combustion engine 14 by an at least temporary influencing of at least one operating parameter of the internal combustion engine 14 is set. For example, it is conceivable, an exhaust gas recirculation initiate or increase an EGR rate. Although these measures lead to an increase in the particle emission, but at the same time reduce the NO _x emissions. If the particle emission does not subsequently rise above the predetermined threshold again, this measure can be maintained.

Claims (19)

Arrangement for cleaning an exhaust gas of an internal combustion engine, in particular of motor vehicles, with a catalyst system arranged in an exhaust gas channel for reducing a nitrogen oxide (NO _x ) content of the exhaust gas and a particle emission, characterized in that the catalyst system ( 10 ) in the flow of exhaust gas sequentially a NO _x storage ( 20 ), an oxidation catalyst ( 22 ) and a NO _x reduction catalyst ( 26 ). Arrangement according to claim 1, characterized in that between the oxidation catalyst ( 22 ) and the NO _x reduction catalyst ( 26 ) an HC memory ( 24 ) is arranged. Arrangement according to one of the preceding claims, characterized in that a reducing agent injection ( 18 ) before the NO _x reduction catalyst ( 26 ) is arranged. Arrangement according to claims 1 or 2, characterized in that the NO _x storage ( 20 ) and the oxidation catalyst ( 22 ) a first catalyst unit ( 28 ) form. Arrangement according to claim 2, characterized in that the HC memory ( 24 ) and the NO _x reduction catalyst ( 26 ) a second catalyst unit ( 30 ) form. Arrangement according to Claims 4 and 5, characterized in that a memory component (NO _x or HC memory 20 . 24 ) and a catalyst component (oxidation or NO _x reduction catalyst 22 . 26 ) of the catalyst unit 28 . 30 are homogeneously distributed on a support. Arrangement according to Claims 4 and 5, characterized in that the storage component and the catalyst component of the catalyst unit ( 28 . 30 ) are distributed heterogeneously on the carrier. Arrangement according to claim 7, characterized that the Memory component and the catalyst component in zones the carrier are applied. Arrangement according to one of the preceding claims, characterized in that a NOx desorption temperature of the NOx store ( 20 ) is within an active temperature range of the catalyst components. Arrangement according to claim 2, characterized in that an HC desorption temperature of the HC reservoir ( 24 ) within the active temperature range of the NOx reduction catalyst ( 26 ) lies. Arrangement according to one of the preceding claims, characterized in that the oxidation catalyst ( 22 ) contains as the catalytically active component platinum. Arrangement according to claim 11, characterized in that the oxidation catalyst ( 22 ) as the catalytically active component additionally contains palladium or rhodium. Arrangement according to claim 12, characterized the existence relationship of palladium or rhodium to platinum is at most 1: 5. Arrangement according to one of the preceding claims, characterized in that the NOx reduction catalytic converter ( 26 ) is a low temperature catalyst. Arrangement according to claim 14, characterized in that the NOx reduction catalyst ( 26 ) is an Ag-Ga zeolite catalyst. Arrangement according to one of the preceding claims, characterized in that the NOx storage ( 20 ) is a low-temperature storage. Method for cleaning an exhaust gas of an internal combustion engine, in particular of motor vehicles, by passing the exhaust gas over a catalyst system arranged in an exhaust gas duct, characterized in that (a) a particle emission downstream of the catalyst system ( 10 ) and (b) depending on the particle emission, a NO _x emission downstream of the catalyst system ( 10 ) mitigating action is taken by a working mode of the internal combustion engine ( 14 ) by an at least temporary influencing of at least one operating parameter of the internal combustion engine ( 14 ) is set. Method according to claim 17, characterized in that that the measure when falling below a predefinable threshold for particle emission is initiated. A method according to claims 17 or 18, characterized in that initiated as a measure exhaust gas recirculation or an EGR rate is increased.