DE102016218348A1 - Reduction of desorbed nitrogen oxides in the exhaust gas tract - Google Patents

Abstract

A method is provided for operating the arrangement of an internal combustion engine with a nitrogen oxide storage catalyst and a second catalyst, wherein the second catalyst is designed to reduce nitrogen oxide storage catalyst desorbed nitrogen oxides with which when switching from lean to rich operation of the internal combustion engine by slipping from the nitrogen oxide storage catalyst desorbed nitrogen oxides are removed from the exhaust gas. There is further provided an arrangement for carrying out the method.

Description

The present invention relates to a process for the reduction of nitrogen oxides desorbed from the nitrogen oxide storage catalyst in an arrangement with a nitrogen oxide storage catalyst and a further catalyst.

Nitrogen oxide storage catalysts (also called lean NOx trap, LNT) are used for the temporary adsorption of nitrogen oxides from the exhaust gas of internal combustion engines. In addition, they fulfill their tasks of oxidative aftertreatment of carbon monoxide (CO) and hydrocarbons (HC). In the lean operation of an internal combustion engine resulting nitrogen oxides can be stored in a LNT; For this purpose, the LNT oxidizes the nitrogen monoxide (NO) contained in the lean exhaust gas to nitrogen dioxide (NO ₂ ) and then stores it in the form of nitrates. As adsorbents which are incorporated in the coating of the LNT serve, for. B. barium and / or other oxides. As the nitrogen oxide loading of the LNT increases, its storage capacity is increasingly decreasing. At the same time, the tendency of the LNT to release nitrogen oxides by slipping increases to the same extent. To increase the storage capacity of the LNT again, the LNT must be regenerated. During regeneration, the stored nitrogen oxides are desorbed again and reduced to nitrogen on catalytically active components of the LNT with the aid of the rich exhaust gas constituents (CO, HC). For this purpose, the exhaust gas is enriched, z. B. by operating the internal combustion engine with a rich mixture. The LNT is also regenerated when the exhaust gas is removed by a rich operation of the engine, e.g. B. due to a performance requirement, gets fatter.

The amount of nitrogen oxides introduced by slip depends on the nitrogen oxide load and the storage capacity of the LNT. The storage capacity of the LNT is particularly weak during the transition from lean to rich operation, whereby there is often a release, also called desorption, of nitrogen oxides due to slip during the transition from lean operation to rich operation due to a power requirement or a necessary regeneration of the LNT. This unintentionally lead to nitrogen oxides in the exhaust gas. It is therefore the task of removing existing nitrogen oxides from the exhaust gas downstream of an LNT.

This object is achieved by a method having the features of claim 1. Further advantageous embodiments of the invention will become apparent from the dependent claims and claims, the figures and the embodiments.

• – S1) Betreiben der Brennkraftmaschine mit einem mageren Gemisch, wobei Stickoxide im dem ersten Katalysator gespeichert werden,
• – S2) Umschalten auf ein Betreiben der Brennkraftmaschine mit einem fetten Gemisch,
• – S3) Einleiten einer bestimmten Menge des Reduktionsmittels aus dem Reservoir in den Abgastrakt,
• – S4) Reduktion von desorbierten Stickoxiden im zweiten Katalysator,

und Wiederholen der Verfahrensschritte von Schritt S1 an.A first aspect of the invention relates to a method of regulating nitrogen oxide slip from an exhaust aftertreatment device in an exhaust tract of an internal combustion engine using an assembly comprising a first catalyst formed as LNT, at least one second catalyst comprising at least one precious metal downstream of the first catalyst. and at least one reservoir for at least one reducing agent that can be introduced into the exhaust tract between the first and second catalytic converter, with the steps:

• - S1) operating the internal combustion engine with a lean mixture, wherein nitrogen oxides are stored in the first catalyst,
• S2) switching to operating the internal combustion engine with a rich mixture,
• S3) introducing a certain amount of the reducing agent from the reservoir into the exhaust gas tract,
• S4) reduction of desorbed nitrogen oxides in the second catalyst,

and repeating the process steps from step S1.

The method is advantageous because it allows the immediate removal of nitrogen oxides introduced from the LNT. As a result, less nitrogen oxides get into the exhaust gas derived in the environment. In addition, the immediate introduction of the reducing agent into the exhaust gas immediately after the start of the switching by the lower oxygen concentration causes an efficient reduction of nitrogen oxides.

Reducing agents for the reduction of nitrogen oxides are commercially available and known in the art.

A mixture is referred to in the present application, a fuel-air mixture. The terms lean and rich mixture refer to mixtures with excess air or excess fuel. Accordingly, the operation of an internal combustion engine with a lean mixture is also referred to as lean operation and with a rich mixture also as rich operation.

Preferably, in the method according to the invention, the amount of the introduced reducing agent is dependent on an oxygen content downstream of the first catalyst. This is advantageous because the oxygen content in the exhaust gas changes over the course of the switchover from lean to rich operation.

Preferably, the amount of the introduced reducing agent is dependent on a nitrogen oxide content downstream of the first catalyst. This is advantageous because it feeds an amount that is really required according to a measured nitrogen oxide concentration. The amount of introduced reducing agent may be dependent on both the nitrogen oxide content and the oxygen content.

Also preferably, the amount of the introduced reducing agent is determined on the basis of a model-based sequence when switching to a rich mixture. Ideally, this embodiment is performed when switching due to a power request or torque request. A corresponding model can be created on the basis of empirical values and / or calculations on the basis of determined parameters (eg injected fuel quantity, amount of nitrogen oxide stored in the first catalytic converter). The model-based determination can be combined with the abovementioned embodiments for the dependence of the introduced reducing agent on the oxygen content and / or nitrogen content.

Furthermore, it is preferred if in the method in step S2 a high load is applied to the internal combustion engine. Ideally, this embodiment is performed when switching due to a power request or torque request.

Furthermore, it is preferred if regeneration of the first catalyst is initiated in the method in step S2. The regeneration of the first catalyst is routinely carried out to remove stored nitrogen oxides when the storage capacity is nearly exhausted. For this purpose, the internal combustion engine is operated with a rich mixture until the first catalyst is regenerated.

A second aspect of the invention relates to an arrangement for carrying out the method according to the invention, comprising at least one first, designed as LNT catalyst, downstream of the first catalyst in the flow direction of the exhaust gas at least one reservoir for at least one reducing agent, a reducing agent introduction means in the exhaust tract, and at least one second catalyst, comprising at least one noble metal, which is designed to reduce nitrogen oxides released from the first catalyst.

In the coating of the catalysts catalytically active precious metals are incorporated. For this purpose, especially platinum, rhodium and / or palladium are used. It is preferred that the noble metal of the second catalyst of the inventive arrangement is platinum.

Furthermore, it is preferred if at least one oxygen sensor and at least one nitrogen sensor are arranged in the arrangement.

In the arrangement according to the invention, the oxygen sensor is preferably a universal residual oxygen sensor (UEGO sensor, universal exhaust gas oxygen sensor). This advantageously makes it possible to measure an oxygen concentration because the amount of reducing agent introduced is dimensioned as a function of the oxygen concentration in the exhaust tract downstream of the LNT.

Furthermore, in the arrangement according to the invention, a control device is additionally provided, which is designed to receive and process signals from the oxygen sensor and the nitrogen oxide sensor and to issue control signals to the reservoir for releasing the reducing agent. The control device thus advantageously makes it possible to control the reduction of nitrogen oxides introduced from the LNT by slippage.

A third aspect of the invention relates to a motor vehicle with an inventive arrangement.

The invention will be explained in more detail with reference to FIGS. Show it:

1 an inventive arrangement.

2 a flowchart of an embodiment of the method according to the invention.

3 a diagram for the dependence of the nitrogen oxide output from the reducing agent.

In an arrangement 1 as shown by 1 is an internal combustion engine 2 with an exhaust tract 3 connected. In the exhaust tract 3 is a first catalyst 4 arranged, which is designed as LNT (lean NOx trap, lean NOx trap). In the exhaust tract 3 downstream of the first catalyst 4 is an oxygen sensor 5 arranged. The oxygen sensor 5 is ideally a universal oxygen sensor. Furthermore, in the exhaust system 3 a sensor for nitrogen oxides 6 arranged. There may also be other nitrogen oxide sensors 6 at other points in the exhaust system 3 be arranged, for. B. downstream of a likewise in the exhaust tract 3 arranged second catalyst 7 , There may also be other oxygen sensors 5 be arranged in the exhaust system. Furthermore, in the exhaust system 3 a reservoir for reducing agents 8th arranged. For the emission of exhaust gas behind the second catalyst 7 runs the exhaust tract 3 in an exhaust 9 ,

For controlling a reduction of nitrogen oxides in the second catalyst is a control device 10 in the arrangement 1 available. The control device 10 is formed, measured values from the oxygen sensor 5 and / or the nitrogen oxide sensor 6 to receive, based on which to calculate a necessary amount of reducing agent and a corresponding signal to the reducing agent reservoir 8th to send it to release a certain amount of a reducing agent.

In the in 2 illustrated embodiment of a method for reducing by slip from the first catalyst 4 introduced nitrogen oxides, the internal combustion engine is operated with a lean mixture in a first step S1, wherein nitrogen oxides in the first catalyst 4 get saved. If there is a power or torque requirement of the internal combustion engine 2 , or should the first catalyst 4 routinely regenerated, the mixture is enriched, ie, the internal combustion engine 2 operated with a rich mixture. For this purpose, in a second step S2 to an operation of the internal combustion engine 2 switched with a rich mixture. In a third step S3, the control device immediately after step S3, a signal for releasing the reducing agent from the reservoir 8th such that in step S3 a certain amount of the reducing agent from the reservoir 8th is initiated. The amount of reducing agent introduced may vary depending on a downstream of the first catalyst 4 measured oxygen concentration are measured. In a fourth step S4 desorbed nitrogen oxides in the second catalyst 7 reduced. The cleaned exhaust gas is then through the exhaust 9 issued. Then the process starts again from step S1.

The release of reducing agent causes efficient reduction of slip from the first catalyst 4 escaped nitrogen oxides. In 3 Two diagrams are shown, where in 3a a course without reducing agent and in 3b a course is shown with reducing agents. The upper line refers to the nitrogen oxide concentration upstream of the second catalyst 7 , and the lower line to the nitrogen oxide concentration downstream of the second catalyst 7 , According to 3a Without reducing agent, the concentration of nitrogen oxides in the exhaust gas is not significantly reduced. With reducing agent, the concentration of nitrogen oxides in the exhaust gas is substantially reduced, as in 3b shown.

LIST OF REFERENCE NUMBERS

1

 arrangement

2

 Internal combustion engine

3

 exhaust tract

4

 first catalyst

5

 oxygen sensor

6

 nitrogen sensor

7

 second catalyst

8th

 Reservoir for reducing agent

9

 Exhaust

10

 control device

Claims (9)

A method of controlling nitrogen oxide slip from an exhaust aftertreatment device in an exhaust tract of an internal combustion engine using an assembly comprising a first catalyst formed as LNT, at least one second catalyst comprising at least one precious metal downstream of the first catalyst, and at least one reservoir for at least one Reducing agent that can be introduced into the exhaust tract between the first and second catalyst, comprising the steps: - S1) operating the internal combustion engine with a lean mixture, wherein nitrogen oxides are stored in the first catalyst, S2) switching to operating the internal combustion engine with a rich mixture, S3) introducing a certain amount of the reducing agent from the reservoir into the exhaust gas tract, S4) reduction of desorbed nitrogen oxides in the second catalyst, and repeating the process steps from step S1. The method of claim 1, wherein the amount of reductant introduced depends on an oxygen content downstream of the first catalyst. The method of claim 1 or 2, wherein the amount of the introduced reducing agent is dependent on a nitrogen oxide content downstream of the first catalyst. Method according to one of the preceding claims, wherein the amount of the introduced reducing agent is determined on the basis of a model-based sequence when switching to a rich mixture. Method according to one of the preceding claims, wherein in step S2, a high load is applied to the internal combustion engine. Method according to one of claims 1-4, wherein in step S2, a regeneration of the first catalyst is initiated. Arrangement ( 1 ) for carrying out a method according to one of the preceding claims, comprising: at least one first catalyst designed as LNT ( 4 ), and downstream of the first catalyst ( 4 ) in the flow direction of the exhaust gas at least one reservoir ( 8th ) for at least one reducing agent, an introduction device for the reducing agent into the exhaust gas tract, and at least one second catalyst comprising at least one noble metal ( 7 ) formed of the first catalyst ( 4 ) to reduce released nitrogen oxides. Arrangement ( 1 ) according to claim 7, wherein the noble metal is platinum. Motor vehicle with an arrangement according to claim 8.

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