CN114439584A

CN114439584A - Method for reducing laughing gas emissions of a combustion engine and exhaust gas aftertreatment system

Info

Publication number: CN114439584A
Application number: CN202111287686.3A
Authority: CN
Inventors: A·卡斯滕森; A·布罗默; S·雷施; A·赫尔; S·保克纳; S·温登堡
Original assignee: Volkswagen Automotive Co ltd
Current assignee: Volkswagen Automotive Co ltd
Priority date: 2020-11-02
Filing date: 2021-11-02
Publication date: 2022-05-06
Also published as: DE102020128786A1

Abstract

The invention relates to a method for reducing laughing gas emissions in an exhaust system (20) of a combustion engine (10). The outlet (18) of the combustion engine (10) is connected to an exhaust system (20). In an exhaust device (20) In the method, a nitrogen oxide storage component (28,30,32) is arranged in the flow direction of the exhaust gas flow of the combustion engine (10), an electrical heating device (34) is arranged downstream of the nitrogen oxide storage component (28,30,32), and an oxidation catalyst (38) is arranged downstream of the electrical heating device (34). A determination of the temperature (T) of the nitrogen oxide storage component (28,30,32) and of the oxidation catalyst (38) is provided_NS,T_OX) Determining the operating state of the combustion engine (10) and at a temperature (T)_NS,T_OX) And the determined operating state of the combustion engine (10) may be such that the electric heating device (34) is activated in anticipation of formation of laughing gas at least one of the exhaust gas aftertreatment components (28,30,32, 38). The invention also relates to an exhaust gas aftertreatment system for carrying out such a method.

Description

Method for reducing laughing gas emissions of a combustion engine and exhaust gas aftertreatment system

Technical Field

The present invention relates to a method for reducing laughing gas emissions of a combustion engine and an exhaust gas aftertreatment system for carrying out such a method according to the preambles of the independent claims.

Background

The kyoto protocol is a common result of international climate policies. Among them, the international society member states promises to impose absolute and legally binding restrictions on greenhouse gas emissions in an international treaty. With the approval of the kyoto protocol, industrial countries are compelling and constraining to reduce the emissions of the most important greenhouse gases — including carbon dioxide (CO2), methane (CH4), and laughing gas (N2O).

Laughing gas is a greenhouse gas that is approximately 300 times as harmful to the climate as carbon dioxide. The main sources of laughing gas emissions are nitrogenous fertilizers in agriculture and agriculture stock farming. Other sources are industrial processes in the chemical industry and stationary and mobile combustion processes.

In the course of further tightening future emission legislation for motor vehicles equipped with combustion engines, also the limit values for laughing gas emissions have to be taken into account. Here, the requirement for further reduction of the consumption and the further tightening of the exhaust gas standards with regard to the permissible nitrogen oxide emissions and laughing gas emissions represent a challenge for the engine developer. In the case of gasoline engines, exhaust gas purification takes place in a known manner by means of a three-way catalyst and further catalysts connected upstream and downstream of the three-way catalyst. In the case of diesel engines, exhaust gas aftertreatment systems are currently used which have an oxidation catalyst, a catalyst for the selective catalytic reduction of nitrogen oxides (SCR catalyst) and a particle filter for separating soot particles and optionally further catalysts. Here, ammonia is preferably used as the reducing agent. Since the treatment with pure ammonia is complicated, it is customary in vehicles to use a synthetic aqueous urea solution which is mixed with the hot exhaust gas stream in a mixing device upstream of the SCR catalyst. By this mixing, the urea aqueous solution is heated, whereby the urea aqueous solution releases ammonia in the exhaust gas passage. Commercial aqueous urea solutions typically consist of 32.5% urea and 67.5% water.

In combustion engines, laughing gas is not primarily generated during combustion of the engine, but rather is formed at certain unfavorable temperature conditions on the catalyst for the exhaust gas aftertreatment.

DE 10242303 a1 discloses an exhaust gas purification device and a method for purifying exhaust gas. In the exhaust system of a combustion engine, an oxidation catalyst, a NOx storage catalyst, a particle filter and an SCR catalyst are arranged in the flow direction of the exhaust gas flow through the exhaust system. A metering valve is provided downstream of the particulate filter and upstream of the SCR catalyst to introduce the reducing agent into the exhaust gas system. It is provided here that the NOx storage catalyst can be heated by means of an electric heating element in order to control the storage behavior of the nitrogen oxides.

Disclosure of Invention

The object of the present invention is now to minimize laughing gas emissions in addition to the already limited exhaust gas constituents (NOx, CO, HC) and thus to further improve the exhaust gas purification or to avoid negative secondary effects (Sekund ä reflekte).

This object is achieved by a method for reducing laughing gas emissions in an exhaust of a combustion engine. The outlet of the combustion engine is connected to an exhaust system. In an exhaust gas system, a nitrogen oxide storage component is arranged in the flow direction of the exhaust gas flow of a combustion engine, an electrical heating device, in particular an electrically heatable catalyst, is arranged downstream of the nitrogen oxide storage component, and an oxidation catalyst is arranged downstream of the electrical heating device. It is provided that the temperatures of the nitrogen oxide storage component and the oxidation catalytic converter are determined, the operating state of the combustion engine is determined, and the electric heating device is activated when the temperatures of the combustion engine and the determined operating state are such that laughing gas is expected to form at the at least one exhaust gas aftertreatment component, in particular at the oxidation catalytic converter.

Especially in the case of the release of nitrogen oxides, with unburned hydrocarbons in the exhaust gas and a catalyst temperature in the range of 200-. Under these conditions, formation of laughing gas increases, and it is therefore desirable to avoid such conditions in order to minimize formation of laughing gas. As long as the nitrogen oxide storage component absorbs nitrogen oxides in the exhaust gas of the combustion engine, there is a lack of important reaction partners for the formation of laughing gas. If an operating state of the combustion engine is detected in which nitrogen oxides can be released or can no longer be absorbed by the nitrogen oxide storage means, while unburned hydrocarbons are present in the exhaust gas and the temperature of the oxidation catalyst lies in the critical range from 200 ℃ to 240 ℃, the electrical heating device is activated in order to heat the oxidation catalyst to such an extent that laughing gas formation is minimized.

By targeted control of the temperature of the oxidation catalyst via the electrically heatable catalyst, the formation of laughing gas on the catalyst in the exhaust system can be minimized. The high combustion temperature and exhaust gas temperature lead to a rapid decomposition of laughing gas and thus to a further minimization of laughing gas emissions.

The method mentioned in the independent claim can be advantageously extended and improved by the features listed in the dependent claims.

In a preferred embodiment of the invention, a load state of a nitrogen oxide storage element, in particular a passive NOx adsorber or a NOx storage catalyst, is determined. By determining the load state, it can be estimated whether an operating state can be expected in which nitrogen oxides are released and there is a risk of formation of laughing gas.

In an advantageous embodiment of the method, it is provided that a first nitrogen oxide concentration is measured upstream of the nitrogen oxide storage component and a second nitrogen oxide concentration is measured downstream of the nitrogen oxide storage component, wherein a load state of the storage component or a nitrogen oxide emission from the nitrogen oxide storage component is determined from the measured nitrogen oxide concentrations. By measuring the nitrogen oxide concentration, the load state of the nitrogen oxide storage element can be calculated and/or the nitrogen oxide emissions from the nitrogen oxide storage element can be ascertained.

Alternatively or additionally, it is advantageously provided that a load state of the nitrogen oxide storage element or a nitrogen oxide emission from the nitrogen oxide storage element is calculated by means of a load model. Alternatively or additionally, it can be calculated by means of a corresponding calculation model whether an increase in the risk of nitrogen oxide emissions from the nitrogen oxide storage component and formation of laughing gas should be expected.

In an advantageous embodiment of the method, it is provided that the unburned hydrocarbon concentration is determined upstream of an oxidation catalyst in the exhaust system. Since the simultaneous presence of unburned hydrocarbons favors the formation of laughing gas, the control of the electrically heatable catalyst can be improved by additionally determining the concentration of unburned hydrocarbons upstream of the oxidation catalyst.

In a further development of the method, it is provided that the electrically heatable catalytic converter is activated when a threshold value of the nitrogen oxide load of the nitrogen oxide storage component is exceeded. Since the risk of releasing nitrogen oxides increases starting from a threshold value, in particular starting from a loading of the nitrogen oxide storage unit with a storage capacity of the nitrogen oxide storage unit of more than 75%, it is advantageous to activate the electrically heatable catalyst when this threshold value is exceeded. By activating the electrical heating device in time, the oxidation catalyst is heated to such an extent that it reaches a temperature of at least 250 ℃, preferably at least 350 ℃, particularly preferably at least 450 ℃, before releasing nitrogen oxide from the nitrogen oxide storage component and thus minimizing formation of laughing gas.

A further aspect of the invention relates to an exhaust gas aftertreatment system for a combustion engine which is connected with its outlet to an exhaust gas system, wherein a nitrogen oxide storage element is arranged in the exhaust gas system in the flow direction of the exhaust gas flow of the combustion engine, an electrical heating device, in particular an electrically heatable catalyst, is arranged downstream of the nitrogen oxide storage element, and an oxidation catalyst is arranged downstream of the electrical heating device, and is connected to a control unit which is provided for carrying out such a method when machine-readable program code is executed by the control unit. By means of such an exhaust gas aftertreatment system, it can be prevented that the oxidation catalyst is operated in a temperature range which is favorable for the formation of laughing gas in critical operating states of the combustion engine for such formation. Laughing gas emissions can thereby be minimized.

In a preferred embodiment of the exhaust gas aftertreatment system, it is provided that the nitrogen oxide storage component is or comprises a NOx storage catalyst. Nitrogen oxides can be stored by means of a NOx storage catalyst. However, in order to regenerate the NOx storage catalyst, a rich phase of the engine or metering of fuel into the exhaust is required in order to convert the stored nitrogen oxides with unburned hydrocarbons. However, since the simultaneous presence of unburned hydrocarbons and nitrogen oxides favors the formation of laughing gas, it is very helpful to heat the oxidation catalyst by the electric heating device so strongly that the temperature of the oxidation catalyst is above the critical temperature range of 200 ℃ to 250 ℃ for laughing gas formation.

In this case, it is particularly preferred to design the NOx storage catalytic converter as a low-temperature NOx storage catalytic converter. A low-temperature NOx storage catalyst is understood in this context to mean a NOx storage catalyst which can store nitrogen oxides already from a temperature of 140 ℃, preferably already from 120 ℃, particularly preferably already from 100 ℃. Such low-temperature NOx storage catalysts can in particular minimize nitrogen oxide emissions in the cold start phase of the combustion engine, in which other exhaust gas aftertreatment components for reducing nitrogen oxides, in particular one or more catalysts for the selective catalytic reduction of nitrogen oxides, have not yet reached their operating temperature.

Alternatively or additionally, it is advantageously provided that the nitrogen oxide storage component is or comprises a passive NOx adsorber. By means of passive NOx adsorbers, nitrogen oxides can already be adsorbed and temporarily stored at exhaust gas temperatures of approximately 80 ℃. If the exhaust gas temperature and the temperature of the passive NOx adsorber exceed the threshold of about 200-250 deg.C, the stored nitrogen oxides are released again and the passive NOx adsorber is regenerated. Therefore, passive NOx adsorbers are particularly suitable for minimizing nitrogen oxide emissions during cold start phases of a combustion engine. Since the release of nitrogen oxides takes place at a temperature level critical for the formation of laughing gas, it is advantageous to heat the oxidation catalyst by means of an electrical heating device, whereby the oxidation catalyst is sufficiently hot at the time of the release of nitrogen oxides to avoid or minimize the formation of laughing gas.

The various embodiments of the invention mentioned in the present application can advantageously be combined with one another, unless otherwise stated in individual cases.

Drawings

The invention is explained in more detail below in the examples with the aid of the attached figures. In this case, identical components or components having the same function are designated by the same reference numerals in the different figures. Wherein:

fig. 1 shows a combustion engine with an exhaust gas aftertreatment system in a schematic view;

FIG. 2 shows a catalyst arrangement of an exhaust gas aftertreatment system in a schematic view;

FIG. 3 shows a temperature profile of an oxidation catalyst over time and a control of an electric heating device at a cold start of a combustion engine; and

fig. 4 shows a flow chart for carrying out the method for reducing laughing gas emission according to the invention.

Detailed Description

Fig. 1 shows a schematic view of a combustion engine 10, which combustion engine 10 is connected with its outlet 18 to an exhaust 20. The combustion engine 10 is implemented as a direct injection diesel engine. The combustion engine 10 has a plurality of combustion chambers 12. One fuel injector 14 is arranged in each case at the combustion chamber 12 for injecting fuel into the respective combustion chamber 12. The combustion chambers are each delimited by a piston 16, which is movably arranged in a cylinder bore of the combustion engine 10. The combustion engine 10 is connected with its inlet to an air supply system, not shown, and with its outlet 18 to an exhaust 20. Combustion engine 10 may have a high pressure exhaust gas recirculation with an exhaust gas recirculation line and a high pressure exhaust gas recirculation valve through which exhaust gas of combustion engine 10 may be recirculated from outlet 18 to the inlet. An intake valve and an exhaust valve are arranged in the combustion chamber 12, with which a fluid connection from an air supply system to the combustion chamber 12 or from the combustion chamber 12 to an exhaust system 20 can be opened or closed.

In the exhaust system 20, a nitrogen oxide storage component 28, in particular a passive NOx adsorber 32, is arranged as a first component of the exhaust gas aftertreatment downstream of the turbine 26 of the exhaust gas turbocharger 24, through the exhaust gas duct 22 of the exhaust system 20 in the flow direction of the exhaust gas flow through the combustion engine 10. The passive NOx adsorber 32 is preferably embodied as a passive NOx adsorber 32 without oxidation components and is used only for temporarily storing nitrogen oxides during the cold start phase of the combustion engine. Downstream of the nox storage section 28, an electric heating device 34 with an electric heating element 36 is arranged, with which the exhaust gas flow of the combustion engine 10 can be heated substantially independently of the operating mode of the combustion engine 10. Further downstream, an oxidation catalyst 38, in particular a diesel oxidation catalyst, is arranged. Alternatively, the nitrogen oxide storage component 28 can also be embodied as a NOx storage catalyst 30, in particular as a low-temperature NOx storage catalyst.

Downstream of the oxidation catalyst 38, further exhaust gas aftertreatment components 40, 42, 44, 46, in particular at least one exhaust gas aftertreatment component 40 for the selective catalytic reduction of nitrogen oxides, preferably an SCR catalyst 42 or a particle filter 44 with an SCR coating 46, can be arranged in the exhaust gas system 20, with which reducing agent can be metered into the exhaust gas line 22. Downstream of the oxidation catalyst 38 and upstream of the exhaust gas aftertreatment component 40 for the selective catalytic reduction of nitrogen oxides, a metering element 58 is arranged, with which metering element 58 a reducing agent, in particular an aqueous urea solution, can be metered into the exhaust gas system 20 of the combustion engine 10.

An exhaust gas mixer may be arranged downstream of the metering element 58 and upstream of the exhaust gas aftertreatment component 40 for selective catalytic reduction of nitrogen oxides in order to improve the mixing of the reducing agent with the exhaust gas flow of the combustion engine 10 before entry into the exhaust gas aftertreatment component 40 for selective catalytic reduction.

Furthermore, a plurality of exhaust gas sensors 48, in

particular NOx sensors

54, 56 or sensors for detecting unburned hydrocarbons, as well as

temperature sensors

50, 52, can be provided in the exhaust system 20. The combustion engine 10 and the

sensors

48, 50, 52, 54, 56 are connected to a control device 60, which controls, in particular, the injection quantity and the injection time of the fuel into the combustion chamber 12 of the combustion engine 10 and the activation or heating power of the electric heating device 34. The control device 60 comprises a computing unit 62 and a memory unit 64, wherein a machine-readable program code 66 is stored in the memory unit 64, which program code 66 can be executed by the computing unit 62 to carry out the method according to the invention.

Fig. 2 shows a schematic representation of a catalyst arrangement of an exhaust gas aftertreatment system. Here, a subsection of the exhaust gas line 22 of the exhaust system 20 is shown. The catalyst arrangement comprises a nitrogen oxide storage component 28, an electrically heatable device 34 in the form of an electrically heatable catalyst, which comprises an electrical heating element 36. The electrical heating device 34 is followed by an oxidation catalyst 38. The nitrogen oxide storage element 28 is preferably formed as a passive NOx adsorber 32, but may alternatively be formed as a NOx storage catalyst 30.

Upstream of the nitrogen oxide storage component 28, a first NOx sensor 54 is arranged in the exhaust gas line 22. Downstream of the NOx storage component 28, a second NOx sensor 56 is arranged, so that the load on the NOx storage component 28 or the release of NOx can be determined from the difference measured between the

NOx sensors

54, 56. A first temperature sensor 50 is arranged at the nox storage component 28, with which first temperature sensor 50 the temperature T of the nox storage component 28 can be measured_NS. A second temperature sensor 52 is arranged at the oxidation catalytic converter 38, with which second temperature sensor 52 the temperature T of the oxidation catalytic converter 38 can be measured_OX。

Fig. 3 shows the temperature profile T of the oxidation catalyst 38 during a cold start of the combustion engine 10_OXA graph of (a). Here, the temperature T is measured_OXThe noncritical, cold first temperature range I, viewed from the point of view of formation of laughing gas, rises into the noncritical, hot third temperature rangeIII, wherein activation a by the electric heating device 34 crosses the critical temperature zone II as quickly as possible and when all nitrogen oxides can be stored in the nitrogen oxide storage means 28 and/or there are no unburnt hydrocarbons in the exhaust gas stream, to minimize formation of laughing gas. In the combustion engine 10, laughing gas is not primarily generated by engine combustion in the combustion chamber 12, but is primarily generated as secondary emissions during exhaust gas aftertreatment. Laughing gas is formed on the oxidation stage of the oxidation catalyst 38 or the NOx storage catalyst 30 only when the reactants required for the formation of laughing gas (nitrogen oxides and unburned hydrocarbons) are present and a laughing gas formation critical temperature range of about 200 ℃ to 250 ℃ prevails at the catalysts 30, 38.

As soon as nitrogen oxide storage element 28 adsorbs nitrogen oxide, reaction partners which are important for the formation of laughing gas are absent at oxidation catalyst 38. The current storage load of the nitrogen oxide storage unit 28 is determined by means of a computer model in the control unit 60 or by measuring the nitrogen oxide concentration in the exhaust gas line 22 at one

NOx sensor

54, 56 upstream and downstream of the nitrogen oxide storage unit 28, respectively, or by a combination of both methods. The maximum storage capacity of the nox storage component is used in the control unit 60. Temperature T of oxidation catalyst 38_OXAnd temperature T of NOx storage component 28_NSDetermined by computational modeling,

temperature sensors

50, 52, or a combination of both methods.

There are three operating conditions of the combustion engine 10 in which nitrogen oxides may occur downstream of the nitrogen oxide storage component 28.

1.) the nitrogen oxide storage component 28, in particular the passive NOx adsorber 32, is heated by the engine exhaust gas up to the desorption temperature range and the stored nitrogen oxides are released by thermal desorption. This can be done either passively during normal operation of the combustion engine 10 or by targeted internal engine heating measures, for example for regenerating a particle filter.

2.) the threshold value of the nox loading of the nox storage section 28 is exceeded, so that the entering nox can no longer be stored completely in the nox storage section 28 and the nox reaches the exhaust system 20 downstream of the nox storage section 28.

3.) exhaust gas temperature T of the nitrogen oxide storage component 28_EGOr temperature T_NSSo high that the incoming nitrogen oxides are not stored in the nitrogen oxide storage component 28 and flow through the nitrogen oxide storage component 28.

When nitrogen oxides are detected downstream of the nitrogen oxide storage component 28 by one of three mechanisms or a combination of several of these mechanisms and at the same time the temperature T of the oxidation catalyst 38 is detected_OXIn the critical range for laughing gas formation, the electric heating element 36 of the electric heating device 34 is activated in order to heat the oxidation catalyst 38 into the temperature zone III in which laughing gas formation is not critical.

Fig. 4 shows a flow chart for carrying out the method for reducing laughing gas emission according to the invention. In the method step<100>In determining the temperature T of the NOx storage component 28_NS. This can be done by means of the temperature sensor 50 or by means of a calculation model stored in the control device 60. In the method step<110>In (1), determining the temperature T of the oxidation catalyst 38_OX. This can be done by means of the temperature sensor 52 or by means of a calculation model stored in the control device 60. In the method step<120>The current operating conditions of the combustion engine 10 are determined. In the method step<130>The concentration of nox in the exhaust 20 upstream of the nox storage component 28 is determined. This can be done by means of the nox sensor 54 or by means of a calculation model stored in the control unit 60 of the combustion engine 10. In addition, in the method step<140>To determine the nox concentration downstream of the nox storage component 28. In the method step<150>Whether or not the prerequisite condition for formation of laughing gas as secondary emissions at the oxidation catalyst 38 is satisfied is calculated. If this is the case, then in a method step<160>The oxidation catalyst 38 is heated to a temperature above the critical temperature zone II by means of the electrical heating device 34 in order to minimize the formation of laughing gas.

List of reference numerals

10 combustion engine

12 combustion chamber

14 piston

16 fuel injector

18 outlet

20 air exhausting device

22 exhaust gas duct

24 exhaust gas turbocharger

26 turbine

28 nitrogen oxide storage component

30 NOx storage catalyst

32 passive NOx adsorber

34 electric heating equipment

36 electric heating element

38 oxidation catalyst

40 exhaust aftertreatment component for selective catalytic reduction

42 SCR catalyst

44 particulate filter

46 SCR coating

48 exhaust gas sensor

50 first temperature sensor

52 second temperature sensor

54 first NOx sensor

56 second NOx sensor

60 control instrument

62 calculation unit

64 memory cell

66 program code

Claims

1. Method for reducing laughing gas emissions in an exhaust system (20) of a combustion engine (10), the outlet (18) of which is connected to the exhaust system (20), wherein in the exhaust system (20) a nitrogen oxide storage means (28,30,32) is arranged in the flow direction of an exhaust gas flow of the combustion engine (10), an electrical heating device (34) is arranged downstream of the nitrogen oxide storage means (28,30,32), and an oxidation catalyst (38) is arranged downstream of the electrical heating device (34), comprising the steps of:

determining nitrogen oxide storageTemperature (T) of the component (28,30,32)_NS)

Determining a temperature (T) of an oxidation catalyst (38)_OX)

Determining an operating state of a combustion engine (10)

At temperature (T)_NS, T_OX) And the determined operating state of the combustion engine (10) may be such that the electric heating device (34) is activated in anticipation of formation of laughing gas at least one of the exhaust gas aftertreatment components (28,30,32, 38).

2. A method according to claim 1, characterized by determining the load state of the nitrogen oxide storage component (28,30, 32).

3. Method according to claim 1 or 2, characterized in that the first nitrogen oxide concentration (K) is measured upstream of the nitrogen oxide storage component (28,30,32)_NOX1) And downstream of the nitrogen oxide storage component (28,30,32), a second nitrogen oxide concentration (K) is measured_NOx2) In which the measured nitrogen oxide concentration (K)_NOX1,K_NOx2) A load state of the nitrogen oxide storage element (28,30,32) or a nitrogen oxide emission from the nitrogen oxide storage element (28,30,32) is determined.

4. A method according to any one of claims 1 to 3, characterized in that the load state of the nitrogen oxide storage (28,30,32) or the nitrogen oxide emission from the nitrogen oxide storage (28,30,32) is calculated by means of a loading model.

5. A method according to any one of claims 1-4, characterised in that the concentration of unburnt hydrocarbons is determined in the exhaust device (20) upstream of the oxidation catalyst (38).

6. Method according to any one of claims 1 to 5, characterized in that a threshold value (T) for the NOx load at the NOx storage component (28,30,32)_BNOX) When overtaken, the electric heating device is activatedAnd (34).

7. Exhaust gas aftertreatment system for a combustion engine (10), which combustion engine (10) is connected with its outlet (18) to an exhaust gas system (20), wherein in the exhaust gas system (20) a nitrogen oxide storage component (28,30,32) is arranged in the flow direction of the exhaust gas flow of the combustion engine (10), an electrical heating device (34) is arranged downstream of the nitrogen oxide storage component (28,30,32) and an oxidation catalyst (38) is arranged downstream of the electrical heating device (34), and is connected with a control device (60), which control device (60) is provided for carrying out the method according to one of claims 1 to 6, when machine-readable program code is implemented by the control device (60).

8. Exhaust gas aftertreatment system according to claim 7, characterized in that the nitrogen oxide storage component (28) is or comprises a NOx storage catalyst (30).

9. The exhaust gas after treatment system according to claim 8, characterized in that the NOx storage catalyst (30) is a low temperature NOx storage catalyst.

10. Exhaust gas aftertreatment system according to any one of claims 7 to 9, characterized in that the nitrogen oxide storage component (28) is or comprises a passive NOx adsorber (32).