DE102008047722A1 - Exhaust gas cleaning system operating method for Otto-internal combustion engine of motor vehicle, involves operating engine with air/fuel mixture when operating condition of engine produces exhaust gas temperature below preset temperature - Google Patents

Abstract

The method involves operating an internal combustion engine (2) i.e. Otto-internal combustion engine, with a lean air/fuel mixture, when an operating condition of the internal combustion engine produces an exhaust gas temperature above a predetermined temperature. A reducing agent is supplied to an exhaust gas before a selective catalytic reduction (SCR) catalytic converter (4) during lean operating conditions. The engine is operated with a stoichiometric air/fuel mixture when the operating condition of the engine produces the exhaust gas temperature below the preset temperature. The exhaust gas flows through a nitrogen oxide storage catalytic converter and the selective catalytic reduction (SCR) catalytic converter, where the nitrogen oxide storage catalytic converter is coated as a three-way catalytic converter or an oxidation catalytic converter.

Description

The The invention relates to a method for operating an emission control system with the features of the preamble of claim 1.

• a) abwechselnd mit magerem und fettem Luft-/Kraftstoffgemisch zu betreiben, wenn der Betriebszustand des Motors eine Abgastemperatur unterhalb einer vorgegebenen Temperatur erzeugt und
• b) mit konstant magerem Luft-/Kraftstoffgemisch zu betreiben, wenn der Betriebszustand des Motors eine Abgastemperatur oberhalb der vorgegebenen Temperatur erzeugt und während dieses Betriebszustandes vor dem SCR-Katalysator dem Abgas Ammoniak direkt oder in Form einer zu Ammoniak zersetzlichen Verbindung zuzuführen.

It is based on the international patent application WO 2008/022751 A2 out. From this, a method for operating an exhaust gas purification system on a lean-burn engine of a vehicle is known, which first contains a nitrogen oxide storage catalytic converter and then an SCR catalytic converter in the flow direction of the exhaust gas. The exhaust gas has a dependent on the current operating condition of the engine exhaust gas temperature and contains, among other things, nitrogen oxides as pollutants. It is suggested the engine

• (a) operate alternately with lean and rich air / fuel mixtures when the engine operating condition produces an exhaust gas temperature below a specified temperature and
• b) operate with a constantly lean air / fuel mixture when the operating condition of the engine generates an exhaust gas temperature above the predetermined temperature and supply the exhaust gas ammonia directly or in the form of a compound decomposable to ammonia during this operating state before the SCR catalyst.

at In this method of operation, the lean-burn engine becomes low exhaust gas temperatures operated with alternately lean and rich air / fuel mixture. During lean operation, the gases contained in the exhaust gas Nitrogen oxides stored in the storage catalytic converter. If the storage capacity of the storage catalyst is exhausted, it is by switching the engine regenerated to rich operation. This is in this temperature range a portion of the stored nitrogen oxides reduced to ammonia, the is cached by the downstream SCR catalyst. During the subsequent lean operation, the stored Ammonia for the reduction of not absorbed by the storage catalyst Nitrogen oxides.

Also if this operating procedure for a lean engine no direct disadvantage, the present invention is the Task to reduce the fuel demand again something.

These The object is achieved by the feature in the characterizing part of the claim 1 solved.

In the method according to the invention, in contrast to the prior art, there is no intermittent rich / lean operation. In this case, the NO _x storage catalytic converter (nitrogen oxide storage catalytic converter) assumes the function of storing NO _x until the SCR system is ready for operation (selective catalytic reduction), ie until a temperature in the SCR catalytic converter reaches more than approximately 180 ° C. Due to the close-coupled arrangement of the NO _x storage catalytic converter and its low thermal mass, it achieves the minimum storage temperature of about 250 ° C. much earlier than an SCR catalytic converter or an NO _x storage catalytic converter usually arranged in the underbody. During further operation, the NO _x storage catalyst continues to be charged independently of the rising NO _x slip (nitrogen oxides which are not stored by the NO _x storage catalytic converter). The NO _x slip conversion now takes over the SCR system. After a relatively short period of time, the NO _x storage catalytic converter is fully loaded, ie it can no longer store any nitrogen oxides, so that the entire conversion of the nitrogen oxides takes place via the SCR system. The regeneration of the NO _x storage catalyst takes place in a homogeneous or slightly rich phase (λ = 1, stoichiometric, λ ≦ 1, rich engine operation) after the next cold start. The NO _x storage catalytic converter must be dimensioned so that even after aging there is a storage capacity that can bridge the phase until the SCR system is ready for operation.

The close-coupled NO _x storage catalytic converter also undertakes the conversion of the pollutants during homogeneous operation and is responsible for the conversion of HC and CO (hydrocarbons and carbon monoxide) during lean engine operation (λ> 1). Ideally, the NO _x storage and HC / CO conversion functionalities are represented by a single catalytic coating.

The following conflict of objectives is achieved by the method according to the invention: Increased fuel consumption by long homogeneous operation after a cold start of the internal combustion engine against a NO _x breakthrough by a non-existent operational readiness of the SCR system due to low operating temperature.

The Embodiments according to the subclaims are particularly preferred embodiments.

In the following, the inventive method is based on a single, in 1 illustrated exhaust purification system explained.

1 shows schematically an emission control system for carrying out the method according to the invention.

1 shows an example, schematically an emission control system 1 for carrying out the method according to the invention. To an internal combustion engine 2 , with four cylin circles Z, is a two-piece exhaust manifold 5 arranged. The exhaust manifold 5 directs exhaust gas of the internal combustion engine 2 in a turbine 6 a turbocharger not shown in detail. A construction without turbocharger is possible. After the turbine 6 the exhaust gas first flows through a nitrogen oxide storage catalyst 3 (NO _x storage catalyst) and further through an SCR catalyst 4 (selective catalytic reaction). For this flows the exhaust gas, whose flow direction is shown schematically by two arrows, cleaned of pollutants into the open. This is the basic structure of the emission control system 1 , Wherein the exhaust system can also be executed mehrflutig.

Between the turbine 6 the exhaust gas turbocharger and the nitrogen oxide storage catalyst 3 is a first lambda probe 9 in the emission control system 1 integrated, preferably a lambda broadband probe. In the flow direction of the exhaust gas after the nitrogen oxide storage catalyst 3 are in the order named a second lambda probe 10 , a temperature sensor 11 and a first NO _x sensor 12 in the emission control system 1 integrated. Next is in the flow direction of the exhaust gas after the SCR catalyst 4 a second NO _x sensor 13 arranged. In the flow direction of the exhaust gas after the first NO _x sensor 12 and in front of the SCR catalyst 4 is a reducing agent injection device 8th provided for solid, liquid or gaseous reducing agent from a reducing agent tank 7 is fed. As a reducing agent, for example, an aqueous urea solution can be used or ammonia directly or in the form of a decomposable to ammonia compound. Further, in the flow direction of the exhaust gas after the nitrogen oxide storage catalyst 3 and even before the reductant injection device 8th For example, an exhaust gas recirculation device 14 be provided.

With the in 1 described emission control system 1 can the inventive method for operating an emission control system 1 connected to the lean-burn internal combustion engine 2 , In particular, an Otto internal combustion engine, is arranged for a motor vehicle, wherein the exhaust gas first, the nitrogen oxide storage catalyst 3 and further the SCR catalyst 4 flows through, wherein the exhaust gas one of the current operating state of the internal combustion engine 2 has dependent exhaust gas temperature and contains, among other nitrogen oxides as a pollutant, the internal combustion engine 2 is operated with a lean air-fuel mixture (λ> 1) when the operating state of the internal combustion engine 2 generates an exhaust gas temperature above the predetermined temperature and during this lean operating condition the exhaust gas before the SCR catalyst 4 a reducing agent is supplied, wherein the internal combustion engine 2 is operated with a stoichiometric air / fuel mixture (λ = 1) when the operating condition of the internal combustion engine 2 an exhaust gas temperature below the certain temperature generated to be performed.

Due to the system, the specific temperature is usually between 220 ° and 650 ° C. In a particularly preferred embodiment, the nitrogen oxide storage catalyst is 3 a three-way catalyst or an oxidation catalyst upstream. In a further particularly preferred embodiment, the nitrogen oxide storage catalyst 3 simultaneously coated as a three-way catalyst and / or as an oxidation catalyst.

In the method according to the invention, in contrast to the prior art, there is no intermittent rich / lean operation. In this case, the NO _x storage catalytic converter (nitrogen oxide storage catalytic converter) assumes the function of storing NO _x until the SCR system is ready for operation (selective catalytic reduction), ie until a temperature in the SCR catalytic converter reaches more than approximately 180 ° C. Due to the close-coupled arrangement of the NO _x storage catalytic converter and its low thermal mass, it achieves the minimum storage temperature of about 250 ° C. much earlier than an SCR catalytic converter or NO _x storage catalytic converter usually arranged in the underbody. During further operation, the NO _x storage is further loaded independently of the rising NO _x slip (nitrogen oxides, which are not stored by the NO _x storage catalyst). The NO _x slip conversion now takes over the SCR system. After a relatively short period of time, the NO _x storage catalytic converter is fully loaded, ie it can no longer store any nitrogen oxides, so that the entire conversion of the nitrogen oxides takes place via the SCR system. The regeneration of the NO _x storage catalyst takes place in the homogeneous phase (λ = 1, stoichiometric engine operation) after the next cold start. The NO _x storage catalytic converter must be dimensioned so that even after aging there is a storage capacity that can bridge the phase until the SCR system is ready for operation.

The close-coupled NO _x storage catalytic converter also undertakes the conversion of the pollutants during homogeneous operation and is responsible for the conversion of HC and CO (hydrocarbons and carbon monoxide) during lean engine operation (λ> 1). Ideally, the NO _x storage and HC / CO conversion functionalities are represented by a single catalytic coating.

The following conflict of objectives is solved by the method according to the invention: Increased fuel consumption by long homogeneous operation after one Cold start of the internal combustion engine against a NO _x breakthrough by a non-existent operational readiness of the SCR system due to low operating temperature.

1

emission control system

2

Internal combustion engine

3

Nitrogen oxide storage catalyst

4

SCR catalyst

5

exhaust manifold

6

turbine

7

Reductant tank

8th

Reductant injection device

9

first lambda probe

10

second lambda probe

11

temperature sensor

12

first NO _x sensor

13

second NO _x sensor

14

Exhaust gas recirculation device

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - WO 2008/022751 A2 [0002]

Claims (4)

Method for operating an emission control system ( 1 ), which on a lean operable internal combustion engine ( 2 ), in particular an Otto internal combustion engine, is arranged for a motor vehicle, wherein an exhaust gas, the exhaust gas purification system ( 1 ) first a nitrogen oxide storage catalyst ( 3 ) and further an SCR catalyst ( 4 flows through, wherein the exhaust gas one of the current operating state of the internal combustion engine ( 2 ) has dependent exhaust gas temperature and contains, among other nitrogen oxides as pollutants, wherein the internal combustion engine ( 2 ) is operated with a lean air / fuel mixture (λ> 1) when the operating state of the internal combustion engine ( 2 ) generates an exhaust gas temperature above the certain temperature and during this lean operating state the exhaust gas before the SCR catalyst ( 4 ) a reducing agent is supplied, characterized in that, the internal combustion engine ( 2 ) is operated with a stoichiometric air / fuel mixture (λ = 1) when the operating state of the internal combustion engine ( 2 ) produces an exhaust gas temperature below the determined temperature. Method according to claim 1, characterized that the specific temperature is between 220 ° C and 650 ° C lies. Method according to claim 1 or 2, characterized in that the nitrogen oxide storage catalyst ( 3 ) a three-way catalyst or an oxidation catalyst is connected upstream. Method according to one of the claims 1 to 3, characterized in that the nitrogen oxide storage catalyst ( 3 ) is simultaneously coated as a three-way catalyst or as an oxidation catalyst.