DE19933736A1 - Arrangement for purifying an I.C. engine exhaust gas comprises a catalyst system in the exhaust gas flow consisting of a nitrogen oxides store, an oxidation catalyst and a nitrogen oxides reduction catalyst - Google Patents

Abstract

Arrangement for purifying an I.C. engine exhaust gas comprises a catalyst system (10) in the exhaust gas flow consisting of a NOx store (20), an oxidation catalyst (22) and a NOx reduction catalyst. An Independent claim is also included for a process for purifying an I.C. engine exhaust gas comprising determining the particle emissions downstream of the catalyst system, and depending on the particle emissions, taking measures to reduce the NOx emission whilst the operating mode of the engine (14) is adjusted by temporarily influencing the operating parameters of the engine. Preferred Features: A hydrocarbon store (24) is arranged between the oxidation catalyst and the NOx reduction catalyst.

Description

The invention relates to an arrangement for cleaning an exhaust gas Internal combustion engine, especially of motor vehicles, with one in a Ab arranged gas channel catalyst system and a method for cleaning the Exhaust gas by the exhaust gas via a arranged in an exhaust duct Catalyst system is performed with the in the preamble of claims 1 and 17th mentioned features.

During the combustion of an air-fuel mixture in the internal combustion engine, pollutants such as soot particles, reducing agents or oxidizing agents are produced in different proportions. It is known to guide the exhaust gas for cleaning over the catalyst system arranged in the exhaust gas duct. The catalyst system can include different types of catalysts. For example, an oxidation catalyst supports the oxidation of reducing agents, such as CO, HC or H ₂ , with oxygen. In addition, a particle mass emitted by the internal combustion engine is made possible by an at least partial oxidation of hydrocarbons attached to the particle cores. On the other hand, reduction catalysts are known which reduce NO _x formed during the combustion process to nitrogen using the reducing agents.

Storage components are also known which allow the accumulation of selected gases by physical and / or chemical sorption. Examples are CH storage or NO _x storage. In this case, absorption takes place until a desorption temperature is exceeded or a maximum storage capacity is reached.

To reduce fuel consumption, it has proven to be particularly advantageous to maintain lean operation of the internal combustion engine for as long as possible. In lean operation, an oxygen content exceeds a fuel content in the exhaust gas (λ <1) with the result that a proportion of the reducing agent, such as CO, HC or H ₂ , drops. As the proportion of reducing agent decreases, complete conversion of NO _x to the NO _x reduction catalytic converter is no longer possible. In order to guarantee a sufficiently low NO _x emission, there are basically two options. On the one hand, the NO _x stored in the NO _x storage in lean operation. This must then be regenerated in periodic cycles by changing to a regeneration mode (λ ≦ 1). A NO _x desorption temperature is usually determined by the choice of the storage components in such a way that the lean operation can be maintained as long as possible. The NO _x desorption temperature should therefore be chosen as high as possible.

On the other hand, the proportion of reducing agents can be increased by targeted injection of reducing agent upstream of the NO _x reduction catalyst, for example by injecting a fuel into the exhaust gas duct. The disadvantage here is that, under certain circumstances, a catalytic converter temperature is not yet within an active temperature range of the catalytic converter, so that pollutant emissions can only be inadequately reduced, particularly in a starting phase of the internal combustion engine.

The object of the arrangement and method according to the invention is to keep the NO _x and particle emissions as low as possible, in particular even when an exhaust gas temperature is still very low. According to the invention, this object is achieved by an arrangement and a method for cleaning the exhaust gas of the internal combustion engine with the features of claims 1 and 17. Characterized in that the catalyst system in the flow train of the exhaust gas sequentially comprises a NO _x storage, an oxidation catalyst and a NO _x reduction catalyst, it is possible in a simple manner to reduce the particle and NO _x emissions.

In a preferred embodiment of the invention, an HC storage is arranged between the oxidation catalyst and the NO _x reduction catalyst, which prevents HC emission downstream of the catalyst system at temperatures which are below the active temperature range of the oxidation catalyst.

It is also advantageous to additionally arrange a reducing agent injection immediately upstream of the NO _x reduction catalyst. Such an arrangement can ensure a sufficient reduction of NO _x on the NO _x reduction catalytic converter even when the internal combustion engine is operating lean.

It is also advantageous to combine the NO _x store and the oxidation catalyst to form a catalyst unit. In the same way, the HC storage and the NO _x reduction catalytic converter can also be combined to form a second catalytic converter unit. The storage components and the catalyst components of the catalyst unit are distributed either homogeneously or heterogeneously on a common support. A heterogeneous distribution can be achieved, for example, by applying the storage or catalyst component zone by zone.

The NO _x desorption temperature of the NO _x -storage device is advantageously chosen such that it lies within the active temperature range of the downstream catalyst components. Furthermore, an HC desorption temperature of the HC storage must be within the active temperature range of the NO _x reduction catalytic converter.

In a preferred embodiment of the invention, the oxidation catalyst contains as catalytically active component platinum. It is also advantageous as a catalytically active Component additionally palladium or rhodium up to a ratio of palladium or rhodium to platinum of a maximum of 1: 5.

The NO _x reduction catalyst is advantageously a low-temperature catalyst, for example based on an Ag-Ga zeolite catalyst. Furthermore, it is advantageous to design the NO _x store as a low-temperature store, that is to say that NO _{x can} already be stored to a sufficient extent at relatively low temperatures.

The method for cleaning the exhaust gas of the internal combustion engine can be designed such that first a particle emission downstream of the catalyst system is detected with suitable sensors or is estimated using suitable models. Then, depending on the particle emission, a measure to reduce NO _x downstream of the catalyst system is taken. For this purpose, a working mode of the internal combustion engine is set by at least temporarily influencing at least one operating parameter of the internal combustion engine. Exhaust gas recirculation is preferably initiated or an EGR rate is increased as a measure that reduces NO _x .

Further preferred refinements of the invention result from the remaining ones in the features mentioned in the subclaims.

The invention is described in an exemplary embodiment based on the associated Drawings explained in more detail. Show it:

Fig. 1 is a schematic representation of an arrangement of a catalyst system in an exhaust duct of an internal combustion engine and

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

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

Furthermore, a reducing agent injection 18 is assigned to the catalyst system 10 . For this purpose, a fuel is usually injected into the exhaust duct 12 . The mode of operation of such a reducing agent injection 18 is also known and is therefore not explained in more detail here.

In the flow path of the exhaust gas of the internal combustion engine 14 , according to the exemplary embodiment, there are successively an NO _x accumulator 20 , an oxidation catalytic converter 22 , an HC accumulator 24 and a NO _x reduction catalytic converter 26 . To simplify production costs, the storage components (NO _x and HC stores 20 , 24 ) with the catalyst components (oxidation and NO _x reduction catalysts 22 , 26 ) can each be combined to form a first catalyst unit 28 or a second catalyst unit 30 . The storage component and the catalyst component of the catalyst units 28 , 30 can be distributed either homogeneously or heterogeneously on a support. A heterogeneous application, for example with a zone-wise distribution of the storage components or catalyst components, can be achieved in terms of production technology, in particular by partial immersion of the carrier.

The NO _x store 20 can in particular be a low-temperature store. This means that it has a sorption characteristic that allows NO _{x to be deposited} even at low temperatures and leads to 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 form water and carbon dioxide. In addition, nitrogen monoxide NO can be oxidized at least partially to nitrogen dioxide NO ₂ . It has proven advantageous to use 22 platinum as the catalytically active component of such oxidation catalysts. It is particularly advantageous to add palladium or rhodium to the platinum as a further catalytically active component. A ratio of palladium or rhodium to platinum should not exceed 1: 5.

The HC store 24 is preferably produced 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. Low-temperature catalysts are particularly preferred as NO _x reduction catalyst 26 , for example based on noble metal or else in the form of Ag-Ga zeolite catalysts.

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

The reducing agents formed during the combustion process, such as CO, HC or H ₂ , are reacted with oxygen at low temperatures on the oxidation catalytic converter 22 , so that in the normal case downstream of the oxidation catalytic converter 22 there are only very small amounts of reducing agents in the exhaust gas. Likewise, soot particles are reduced or destroyed by the oxidation of unburned hydrocarbons attached to the particle cores. Breakthroughs of incompletely burned hydrocarbons HC, for example as a result of temperatures of the oxidation catalytic converter 22 which are not yet sufficient, can be absorbed by the HC store 24 . In this way, NO _x emissions, particle emissions and reducing agent emissions can be kept very low. The reducing agent injection 18 is only activated at temperatures above approximately 150 ° C. in order to ensure that the fuel largely evaporates.

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

Subsequently, the NO _{x is} at least largely reduced in the catalyst unit 30 , specifically by means of the reducing agents provided by the reducing agent injection 18 or by means of the reducing agents absorbed in the HC store 24 and desorbed again at increasing temperatures.

During an acceleration process, the temperatures in the area of the first catalytic converter unit 28 first rise. The NO _x store 20 is increasingly emptied and the NO _x emitted is partially reduced with the existing HC and particulate emissions from the internal combustion engine 14 on the oxidation catalytic converter 22 or oxidized to NO ₂ by oxygen. The NO ₂ -rich exhaust gas then hits the injected fuel and the reducing agents additionally desorbed from the HC store 24 at the NO _x reduction catalytic converter 26 . For this purpose, the HC desorption temperature of the HC store 24 must lie within the active temperature range of the NO _x reduction catalytic converter 26 .

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

Claims (19)

1. Arrangement for cleaning an exhaust gas of an internal combustion engine, in particular of motor vehicles, with a catalyst system arranged in an exhaust duct for reducing a nitrogen oxide (NO _x ) portion of the exhaust gas and a particle emission, characterized in that the catalyst system ( 10 ) in the flow of the exhaust gas successively comprises a NO _x store ( 20 ), an oxidation catalyst ( 22 ) and a NO _x reduction catalyst ( 26 ). 2. Arrangement according to claim 1, characterized in that an HC memory ( 24 ) is arranged between the oxidation catalyst ( 22 ) and the NO _x reduction catalyst ( 26 ). 3. Arrangement according to one of the preceding claims, characterized in that a reducing agent injection ( 18 ) is arranged upstream of the NO _x reduction catalyst ( 26 ). 4. Arrangement according to claims 1 or 2, characterized in that the NO _x storage ( 20 ) and the oxidation catalyst ( 22 ) form a first catalyst unit ( 28 ). 5. Arrangement according to claim 2, characterized in that the HC storage ( 24 ) and the NO _x reduction catalyst ( 26 ) form a second catalyst unit ( 30 ). 6. Arrangement according to claim 4 and 5, characterized in that a storage component (NO _x - or HC storage 20 , 24 ) and a catalyst component (oxidation or NO _x reduction catalyst 22 , 26 ) of the catalyst unit 28 , 30 homogeneously on one Carriers are distributed. 7. Arrangement according to claim 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. 8. Arrangement according to claim 7, characterized in that the Storage component and the catalyst component in zones on the Carrier are applied. 9. Arrangement according to one of the preceding claims, characterized in that a NO _x desorption temperature of the NO _x store ( 20 ) is within an active temperature range of the catalyst components. 10. Arrangement according to one of the preceding claims, characterized in that an HC desorption temperature of the HC store ( 24 ) is within the active temperature range of the NO _x reduction catalyst ( 26 ). 11. Arrangement according to one of the preceding claims, characterized in that the oxidation catalyst ( 22 ) contains platinum as the catalytically active component. 12. The arrangement according to claim 11, characterized in that the oxidation catalyst ( 22 ) additionally contains palladium or rhodium as a catalytically active component. 13. The arrangement according to claim 12, characterized in that a ratio of Palladium or rhodium to platinum is a maximum of 1: 5. 14. Arrangement according to one of the preceding claims, characterized in that the NO _x reduction catalyst ( 26 ) is a low-temperature catalyst. 15. The arrangement according to claim 14, characterized in that the NO _x - reduction catalyst ( 26 ) is an Ag-Ga zeolite catalyst. 16. Arrangement according to one of the preceding claims, characterized in that the NO _x store ( 20 ) is a low-temperature store. 17. A 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 ) is determined and
• b) depending on the particle emission, a NO _x emission downstream of the catalyst system ( 10 ) is taken by setting a working mode of the internal combustion engine ( 14 ) by at least temporarily influencing at least one operating parameter of the internal combustion engine ( 14 ).

18. The method according to claim 17, characterized in that the measure when falling below a predeterminable threshold value for the particle emission is initiated. 19. The method according to claims 17 or 18, characterized in that as Measure initiated exhaust gas recirculation or increased EGR rate becomes.