CN110344917B

CN110344917B - Method for operating an exhaust gas aftertreatment system

Info

Publication number: CN110344917B
Application number: CN201910271331.1A
Authority: CN
Inventors: C.奥泽曼; O.A.皮特纳
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2018-04-05
Filing date: 2019-04-04
Publication date: 2023-02-17
Anticipated expiration: 2039-04-04
Also published as: DE102018205132A1; CN110344917A

Abstract

The invention relates to a method for operating an exhaust gas aftertreatment system for an internal combustion engine (1), comprising the following steps: measuring (100) a first temperature (T) upstream of the soot particle filter by means of a first temperature sensor at the beginning ₁ ) And simultaneously measuring (101) a second temperature (T) upstream of the second SCR-catalyst by means of a second temperature sensor ₂ ). If the first temperature (T) ₁ ) Above a first threshold value (S) ₁ ) And the second temperature (T) ₂ ) Above a second threshold value (S) ₂ ) The dosing of the reducing agent solution is interrupted (130) for the first SCR catalytic converter by means of the first dosing valve, and the dosing (140) of the entire reducing agent solution required for the reduction of nitrogen oxides is performed instead by means of the second dosing valve. Finally, the soot particulate filter is passively regenerated (160).

Description

Method for operating an exhaust gas aftertreatment system

Technical Field

The invention relates to a method for operating an exhaust gas aftertreatment system, wherein the regeneration of a soot particulate filter is controlled as a function of the exhaust gas temperature. The invention also relates to a computer program for performing each step of the method when the method is run on a computer and to a machine-readable storage medium on which the computer program is stored. Finally, the invention relates to an electronic control unit designed to carry out the method according to the invention.

Background

Nowadays, various different catalysts are used for exhaust gas aftertreatment systems. Soot particulate filters reduce particulate emissions in exhaust gases generated during combustion. These particles are mostly carbon-based, but mostly also have other agglomerates (agglomerations) and are harmful to health, in particular particles known as fine particles or finest particles or also as fine dust, and have such a small diameter that they can enter the human respiratory tract. These particles are captured by the filter substance in the soot particle filter and adhere (festhalten) to it. A typical carbon black particle filter used today has, as filter material, ceramic modules filled with wall plates (wanddurchflute), in which particles are trapped in a porous ceramic. In order to remove the particles from the soot particle filter, the soot particle filter is regenerated, in which the exhaust gas is heated to such a high temperature (above 500 ℃ for oxygen) that the particles burn. In passive regeneration, nitrogen dioxide is used to combust the carbon-based particles into carbon dioxide and nitric oxide. The process is carried out at a temperature between 250 ℃ and 500 ℃, with a range between 300 ℃ and 350 ℃ being expected to be optimal. The required nitrogen dioxide is generated by components present in the exhaust gas aftertreatment system, such as an oxidation catalyst or a nitrogen oxide storage catalyst, which preferably combines nitrogen dioxide in an unloaded state and forms nitrogen dioxide in a saturated state, in particular by oxidizing the nitrogen monoxide present in the exhaust gas by means of oxygen.

Furthermore, an SCR catalyst is used, which is produced by means of an SCR method (A)Selective Catalytic RReduction) reduces nitrogen oxides (NOx) in the exhaust. DE 103 46 220 A1 describes the basic principle. A32.5% urea-water solution (HWL), also known commercially as AdBlue, is used here ^® -dosing into the exhaust gas. Typically, a dosing system with a dosing module is provided for this purpose in order to dose the HWL into the exhaust gas flow upstream of the SCR catalytic converter. Ammonia is split off by the HWL (abspalten), and then combines on the reactive surface of the SCR catalyst. There, the ammonia combines with the nitrogen oxides, thereby producing water and nitrogen. The HWL is supplied from the reducing agent tank to the dosing module via a pressure line by means of a supply module with a supply pump.

The SCR method reduces the nitrogen oxides available for passive regeneration of the soot particulate filter if the SCR catalyst is configured before the soot particulate filter or if a reactive surface of the SCR catalyst is configured on the soot particulate filter.

Today, especially particulate emissions and nitrogen oxide emissions are subject to strict regulations due to their health-hazardous effects. In order to check and monitor emissions, test methods are used in the vehicle sector, in which emissions are diagnosed during driving. The test methods of importance today include: the Test Cycle "world unified Light load Test Cycle" (WLTC), in which different driving cycles are examined in a standard way, which simulate different driving conditions with different driving speeds; and a checking method "Real Driving Emissions" (RDE) that checks Emissions on different road sections under actual traffic conditions in actual Driving operation.

Disclosure of Invention

The present invention relates to an exhaust gas aftertreatment system for an internal combustion engine, comprising, but not limited to:

a soot particle filter, two separate SCR-catalysts and associated dosing valves and temperature sensors. These components are arranged downstream of the internal combustion engine in the exhaust gas train of the internal combustion engine.

The first SCR catalytic converter is arranged upstream of the soot particulate filter or in the soot particulate filter. In a soot particle filter means: the reactive surface of the SCR catalyst, i.e. the surface on which nitrogen oxides are reduced, is arranged on the surface of the filter structure of the soot particle filter. In other words, such soot particle filters are coated with the surface of the SCR catalyst and are therefore referred to as SCR-coated soot particle filters, for example SDPF (c: (r) ())Selective Catalytic Reduction Diesel Partikel Filter). In particular for ceramic modules filled with wall plates, the catalytic surface of the SCR-catalyst is arranged on the inner surface of the channels extending through the porous material. The first dosing valve is arranged upstream of the first SCR catalytic converter and supplies the reducing agent solution to the first SCR catalytic converter as part of a first dosing module in such a way that it doses the reducing agent solution into the exhaust gas system upstream of the first SCR catalytic converter.

The second SCR catalyst is arranged downstream of the soot particulate filter and reduces nitrogen oxides that have passed the first SCR catalyst. The second metering valve is arranged in the exhaust gas system upstream of the second SCR catalyst and downstream of the first SCR catalyst, preferably also downstream of the soot particle filter. The second dosing valve provides the reducing agent solution to the second SCR catalytic converter as part of a second dosing module, in that it doses the reducing agent solution into the exhaust gas system upstream of the second SCR catalytic converter. The second dosing valve can be controlled independently of the first dosing valve, wherein preferably both dosing valves provide a combined dosing strategy.

The first temperature sensor is arranged upstream of the soot particle filter and preferably also upstream of the first SCR catalytic converter, and measures the temperature of the exhaust gas there. A second temperature sensor is arranged downstream of the soot particle filter and upstream of the second SCR catalytic converter, and preferably upstream of the second dosing valve, and measures the temperature of the exhaust gas there.

A method for operating such an exhaust gas aftertreatment system is proposed. The method comprises the following steps:

a first temperature sensor measures a temperature upstream of the soot particulate filter, and a second temperature sensor measures a second temperature upstream of the second SCR-catalyst. If, on the one hand, the first temperature is above a first threshold value and, on the other hand, the second temperature is above a second threshold value, the dosing of the reducing agent solution is interrupted by the first dosing valve. In other words, if at the same time the first temperature is above the first threshold value and the second temperature is above the second threshold value, the reducing agent solution is not dosed by the first dosing valve. The two thresholds may be different and preferred choices for these thresholds are given below. As a result, no reduction of nitrogen oxides, in particular also of nitrogen dioxide, takes place by the first SCR catalyst.

The dosing of the reducing agent solution required for the reduction of the nitrogen oxides takes place completely via the second dosing valve, so that the nitrogen oxides are reduced solely by the second SCR catalyst. Sufficient reducing agent solution is thus dosed to achieve the desired nitrogen oxide conversion, thus reducing the desired amount of nitrogen oxides and reducing the nitrogen oxide emissions to a predeterminable or predeterminable degree.

Finally, a passive regeneration of the soot particulate filter is carried out. As a result, since the first SCR-catalyst does not reduce nitrogen oxides, a greater mass of nitrogen dioxide is available for regeneration of the soot particulate filter than in conventional passive regeneration, in which the mass of nitrogen oxides is reduced by reduction by the first SCR-catalyst. As a result, the passive regeneration of the soot particle filter is more effective and can be carried out at lower temperatures, but at the same time the nitrogen oxide emissions are kept at a predeterminable or predeterminable level by the second SCR catalyst.

For motor vehicles, an advantage is thereby obtained in relation to the KI factor. The KI factor was calculated as the regeneration (M) of the soot-loaded particle filter in the WLTC test according to equation 1 _pi ) Quotient of the mass of (soot) particles averaged over the period and the mass of (soot) particles averaged over the unloaded particle filter without regeneration:

(equation 1).

The results of the active regeneration during the WLTC test are weighted by the number of WLTC cycles required until the implemented soot loading of the particulate filter is achieved. The KI factor is then computationally (recherisch) directly influencing (einfliessen) the determination of the limit value for the RDE loop. In this case, the limit values specified for the RDE loop (statutory) are reduced by the amount of KI factor (betrg) up to the appropriate limit value. Therefore, the smaller the KI-factor, the more particle-emissions are allowed in the RDE-specification. If passive regeneration is performed more frequently and efficiently, this provides the following advantages: the number of WLTC cycles required until the soot loading of the particle filter is achieved can thus be increased significantly depending on the framework conditions of the exhaust gas aftertreatment system, as a result of which the KI factor can be reduced.

As a further advantage of passive regeneration, which is carried out frequently and efficiently, the share of active (aktiven) regeneration is reduced. More fuel is consumed in active regeneration in order to reach the higher temperatures required for active regeneration. Furthermore, this leads to an increased functional durability of the catalytic components in the exhaust gas aftertreatment system, in particular to an increased durability of the oxidation catalyst, and to a smaller aging of the components. In addition, this also leads to an increase in the durability of the internal combustion engine, since the lubrication of the internal combustion engine or its components can be maintained during passive regeneration, while the lubricating capacity is reduced during active regeneration due to the fact that the lubricating oil is diluted by the subsequent injection of more fuel (in order to increase the temperature).

Furthermore, the carbon dioxide emissions are likewise reduced, since, as already mentioned, less fuel is consumed for increasing the temperature during passive regeneration than during active regeneration, as a result of which carbon dioxide is generated, and passive regeneration is carried out more frequently and more efficiently.

According to one aspect, the nitrogen dioxide concentration upstream of the soot particle filter is increased by means of an oxidation catalyst and/or a saturated nitrogen oxide storage catalyst. The oxidation catalyst oxidizes nitrogen monoxide present in the exhaust gas to nitrogen dioxide. The nitrogen oxide storage catalyst preferably combines nitrogen dioxide in the unloaded state and forms nitrogen dioxide in the saturated state, so that it acts oxidatively. Thus, more nitrogen dioxide is available for passive regeneration of the soot particle filter, without overloading at least the first SCR catalytic converter.

The first threshold value of the first temperature is preferably selected such that the nitrogen dioxide, in particular during the formation in the oxidation catalytic converter and during the passage through the soot particulate filter, is thermally stable and can be passively regenerated. On the other hand, the first threshold value of the first temperature is not so high that the nitrogen dioxide is subjected to a decomposition by heat. For nitrogen dioxide, the critical temperature for thermal decomposition is about 450 ℃. Exceeding the critical temperature is preferably avoided. The first threshold value of the first temperature is therefore preferably in a temperature range between 250 ℃ and 450 ℃, particularly preferably in a temperature range between 350 ℃ and 450 ℃.

The first threshold value of the second temperature is preferably selected such that the reduction of nitrogen oxides can be carried out completely by the second SCR catalyst. The nitrogen oxide conversion of the SCR catalyst is dependent on the temperature of the SCR catalyst. The temperature of the exhaust gas at the second SCR catalyst must be sufficiently high so that the nitrogen oxide conversion of the second SCR catalyst is sufficiently high to reduce the nitrogen oxides to the desired quality. Therefore, the second threshold value of the second temperature is preferably higher than a temperature of 220 ℃.

If the second SCR catalyst cannot fully support the reduction of nitrogen oxides, i.e., if the second SCR catalyst does not achieve the desired nitrogen oxide conversion, the dosing of the reducing agent solution takes place via the first dosing valve, in particular simultaneously with the dosing via the second dosing valve.

The computer program is set up to carry out each step of the method, in particular when the method is executed on a computer or a controller. The method can be implemented in a conventional electronic control unit without structural changes. For this purpose, the computer program is stored on a machine-readable storage medium.

By running the computer program on a conventional electronic control unit, an electronic control unit is obtained which is designed to operate an exhaust gas aftertreatment system.

Drawings

Embodiments of the invention are illustrated in the drawings and will be described in detail in the following description.

Fig. 1 is a schematic illustration of an exhaust gas aftertreatment system for an internal combustion engine, which exhaust gas aftertreatment system can be operated by means of an embodiment of the method according to the invention;

fig. 2 is a flow chart of an embodiment of the method according to the invention.

Detailed Description

Fig. 1 shows a schematic illustration of an exhaust gas aftertreatment system for an internal combustion engine 1 of a motor vehicle, not shown. The internal combustion engine 1 is assigned (zuordnen) at least one injector 11, by means of which fuel is injected into a cylinder 12 of the internal combustion engine 1 in a manner known per se, wherein the fuel then combusts and harmful substances are produced there. The exhaust gas aftertreatment system is arranged in an exhaust gas system 2 of the internal combustion engine 1 and acts on the pollutants emitted by the internal combustion engine 1 (ausgesto beta. Nen). The exhaust gas aftertreatment system comprises a soot particle filter 3, in the specific case a diesel (soot) particle filter (DPF) for a diesel engine, in which a first SCR-catalyst 4 is implemented. More precisely, the reactive surface of the first SCR catalytic converter 4 is arranged on a filter element, not shown here. Both the soot particle filter 3 and the first SCR catalyst 4 therefore act simultaneously on the exhaust gas in the chamber of the soot particle filter 3. In other exemplary embodiments, which are not shown here, the first SCR catalytic converter 4 is designed as a separate component and is arranged upstream of the soot particle filter 3 in the exhaust gas system 2. In order for the SCR catalytic converter to be able to reduce nitrogen oxides, it requires a reducing agent solution which can be dosed into the exhaust gas system 2 upstream of the first SCR catalytic converter 4 by means of a first dosing valve 41.

Upstream of the soot particle filter 3 and the first dosing valve 41, an oxidation catalyst 5 is arranged, in the case of a diesel engine, in particular a Diesel Oxidation Catalyst (DOC), which oxidizes pollutants present in the exhaust gas, in particular carbon monoxide and hydrocarbons. Furthermore, the oxidation catalyst 5 oxidizes nitrogen monoxide into nitrogen dioxide, which is used for passive regeneration of the soot particle filter. Between the oxidation catalytic converter 5 and the soot particulate filter 3, and in this embodiment upstream of the first dosing valve 41, a first temperature sensor 6 is arranged, which measures a first temperature T of the exhaust gas in the vicinity of the inlet of the soot particulate filter 3 ₁ 。

In addition to the first SCR catalytic converter 4, which is also often referred to as a SCR catalytic converter close to the motor because it is arranged at the outlet of the internal combustion engine 1, a second SCR catalytic converter 7 is arranged in the exhaust gas system 2 downstream of the first SCR catalytic converter 4 and the soot particle filter 3. The second SCR catalytic converter 7 is designed, in particular, in terms of its volume, to fully support the reduction of nitrogen oxides. A second dosing valve 71 is associated with the second SCR catalytic converter 7, which is arranged upstream of the second SCR catalytic converter 7 and downstream of the first SCR catalytic converter 4 and via which the reducing agent solution for the second SCR catalytic converter 7 is dosed into the exhaust gas system 2. The second temperature sensor 8 is arranged atDownstream of the soot particle filter 3 and upstream of the second metering valve 71, and a second temperature T of the exhaust gas is measured there ₂ 。

The two temperature sensors 6, 8 are connected to an electronic control unit 9 and measure the temperature T ₁ 、T ₂ To the controller. Furthermore, the electronic control unit is connected to the two dosing valves 41, 71 and to further components of the delivery and dosing system for the reducing agent solution, which are not shown here, and can control the dosing of the reducing agent solution independently of one another via the two dosing valves 41, 71. Finally, the control unit is connected to the injector 11 and can control the quantity of fuel injected. In addition to the power of the internal combustion engine 1 and the exhaust pollutants, the temperature of the exhaust gas can also be varied by the quantity of fuel injected.

Fig. 2 is a flow chart of an embodiment of the method according to the invention. At the beginning, the first temperature T is not only detected by the first temperature sensor 6 ₁ A measurement 100 is made and a second temperature T is measured by a second temperature sensor 8 ₂ A measurement 101 is made. Then the first temperature T ₁ And a first threshold value S ₁ And compared 110. The first threshold S ₁ At a temperature of 250 ℃, at which the nitrogen dioxide formed in the oxidation catalyst 5 is thermally stable and the soot particle filter 3 is passively regenerated, carbon-based soot is combusted under the influence of nitrogen dioxide and oxygen to form nitrogen monoxide and carbon dioxide. A first threshold S higher than 450 DEG C ₁ Thermal decomposition of nitrogen dioxide (Zerfall) results. At the same time, the second temperature T ₂ And a second threshold value S ₂ Are compared 111. Second threshold S ₂ This is selected such that the reduction of the nitrogen oxides can be carried out completely by the second SCR catalyst 7. To this end, a second threshold value S is determined from the model 200 ₂ The model is directed to a desired nitrogen oxide conversion η of at least a second SCR catalyst 7 _w . The nitrogen oxide conversion is generally related to the second temperature T ₂ In this connection, the mode is therefore selected as a function of temperature and in particular taking into account the heat capacity (W228rmekapazit 228), mass and heat losses in the exhaust gas (W228Form 200. Second threshold S ₂ Is selected so as to achieve a desired nitrogen oxide conversion eta _w In this case, the nitrogen oxide emissions are reduced to such an extent that they are within permissible limit values. In this embodiment, the second threshold is at a temperature of 220 ℃.

If the first temperature T or the second temperature T is not the same ₂ Are both simultaneously above the threshold S assigned to them in the

comparison

110 or 111, respectively ₁ Or S ₂ Dispensing is interrupted 130 by the first dispensing valve 41. In other words, the two

comparisons

110 or 111 satisfy a logical and-relationship. If both temperatures are only one of T ₁ Or T ₂ Below the threshold S assigned to them ₁ Or S ₂ Dispensing continues 120 in the usual manner. Nevertheless, regeneration of the particulate filter 3 can take place 121 in the usual manner. If the dosing is interrupted 130 by the first dosing valve 41, the first SCR catalytic converter 4 no longer reduces nitrogen oxides and no nitrogen dioxide is present.

However, to continue the reduction of nitrogen oxides, the second dosing valve 71 completely doses the entire mass of reducing agent solution required for the reduction of nitrogen oxides by the second SCR catalytic converter 7. The quality of the total reducing agent solution required for reducing the nitrogen oxides is determined by the desired nitrogen oxide conversion η _w A determination is made that the nitrogen oxide emissions are reduced to a degree within permissible limit values. Desired nitrogen oxide conversion eta _w From the model 200 described above, in particular as a function of the second temperature T ₂ And other parameters of the second SCR-catalyst 7. The second SCR catalytic converter 7 thus reduces the nitrogen oxides individually, to be precise, so that the nitrogen oxide emissions are within permissible limits.

Subsequently, the soot particulate filter 3 is subjected to a passive regeneration 150, wherein soot particles that have been deposited (ablagern) inside the soot particulate filter 3 and nitrogen dioxide generated in the oxidation catalytic converter 5 are regenerated at a first temperature T ₁ Above a first threshold value S ₁ With oxygen into carbon dioxide and nitrogen monoxide, which is not reduced by the interruption of the dosing by the first dosing valve 41 and the associated non-reduction of the SCR catalyst 4Into the soot particle filter 3, which oxygen has not participated in the combustion in the internal combustion engine 1 and is present in the exhaust gases.

During the dosing 140 of the mass of reducing agent and the regeneration 150 of the soot particulate filter 3 by the second dosing valve 41, the actual nitrogen oxide conversion η of the second SCR catalyst is permanently determined 160 ₂ . During the passive regeneration 150 of the soot particulate filter 3, the actual nitrogen oxide conversion η of the second SCR catalyst is checked in a comparison 170 ₂ Whether or not it is lower than the desired nitrogen oxide conversion eta _w This must be achieved by the second SCR catalytic converter 7 in order to reduce the nitrogen oxide emissions to a level within permissible limits. As already mentioned, this desired nitrogen oxide conversion η _w Is evaluated by the module 200. If the actual nitrogen oxide conversion eta ₂ Less than desired conversion η of nitrogen oxides _w The nitrogen oxide emissions are no longer reduced to such an extent that they are within the permissible limit values. Thus, the dosing 180 is carried out by the first dosing valve 4, so that the desired nitrogen oxide conversion η is achieved overall by the two SCR catalytic converters 4, 7 _w And to reduce the nitrogen oxide emissions to a level within the permissible limit values.

Claims

1. A method for operating an exhaust gas after-treatment system for an internal combustion engine (1), the exhaust gas after-treatment system comprising:

-a soot particle filter (3);

-a first SCR-catalyst (4) arranged upstream of the soot particulate filter (3) or in the soot particulate filter (3);

-a first dosing valve (41) upstream of the first SCR-catalyst (4) providing the first SCR-catalyst (4) with a reducing agent solution;

-a first temperature sensor (6) upstream of the soot particulate filter (3);

-a second SCR-catalyst (7) arranged downstream of the soot particulate filter (3);

-a second dosing valve (71) upstream of the second SCR-catalyst (7) and downstream of the first SCR-catalyst (4), which supplies the second SCR-catalyst (7) with a reducing agent solution independently of the first dosing valve (41); and

-a second temperature sensor (8) downstream of the soot particle filter (4) and upstream of the second SCR-catalyst (7),

the method is characterized by comprising the following steps:

-measuring (100) a first exhaust gas temperature (T) upstream of the soot particulate filter (3) by means of the first temperature sensor (6) ₁ ）；

-measuring (101) a second exhaust gas temperature (T) upstream of the second SCR-catalyst (7) by means of the second temperature sensor (8) ₂ ）；

-if said first exhaust gas temperature (T) is lower ₁ ) Above a first threshold value (S) ₁ ) And the second exhaust gas temperature (T) ₂ ) Above a second threshold value (S) ₂ ) -interrupting (130) the dosing of the reducing agent solution through the first dosing valve (41);

-dosing (140) all the reducing agent solution required for the reduction of nitrogen oxides by means of the second dosing valve (71);

-passively regenerating (160) the soot particulate filter (3);

-permanently ascertaining (160) an actual nitrogen oxide conversion (η) of the second SCR catalyst (7) ₂ ）；

-converting the actual nitrogen oxide conversion (η) of the second SCR-catalyst (7) to ₂ ) With the desired nitrogen oxide conversion (. Eta.) _w ) Performing a comparison (170);

-if the actual nitrogen oxide conversion (η) of the second SCR-catalyst (7) is not sufficient ₂ ) Lower than desired nitrogen oxide conversion (. Eta.) _w ) Then, a reducing agent solution is dosed (180) by means of the first dosing valve (41) in order to achieve a desired nitrogen oxide conversion (eta) overall by means of the two SCR catalysts (4, 7) _w ）。

2. The method as claimed in claim 1, characterized in that the nitrogen dioxide concentration upstream of the soot particle filter is increased by means of an oxidation catalyst (5) and/or a nitrogen oxide storage catalyst.

3. Method according to any of the preceding claims, characterized in that said first threshold value (S) ₁ ) Is selected such that the nitrogen dioxide is thermally stable and enables passive regeneration of the soot particle filter (3).

4. Method according to claim 1 or 2, characterized in that said second threshold value (S) ₂ ) Is selected such that the reduction of nitrogen oxides can be carried out completely by the second SCR catalyst (71).

5. A machine-readable storage medium, on which a computer program is stored, which computer program is set up to carry out each step of the method according to any one of claims 1 to 4.

6. An electronic control unit (9) which is designed to operate an exhaust gas aftertreatment system for an internal combustion engine (1) by means of a method according to one of claims 1 to 4.