U.S. patent application number 14/695732 was filed with the patent office on 2015-08-13 for exhaust gas aftertreatment installation and method.
The applicant listed for this patent is Daimler AG. Invention is credited to Brigitte BANDL-KONRAD, Andreas HERTZBERG, Bernd KRUTZSCH, Arno NOLTE, Markus PAULE, Stefan RENFFTLEN, Norbert WALDBUESSER, Michel WEIBEL, Guenther WENNINGER, Rolf WUNSCH.
Application Number | 20150226100 14/695732 |
Document ID | / |
Family ID | 32519680 |
Filed Date | 2015-08-13 |
United States Patent
Application |
20150226100 |
Kind Code |
A1 |
BANDL-KONRAD; Brigitte ; et
al. |
August 13, 2015 |
Exhaust Gas Aftertreatment Installation and Method
Abstract
An exhaust gas aftertreatment installment and associated exhaust
gas aftertreatment method utilizes a nitrogen oxide storage
catalytic converter and an SCR catalytic converter. A particulate
filter is provided upstream of the nitrogen oxide storage catalytic
converter or between the latter and the SCR catalytic converter or
downstream of the SCR catalytic converter. The time of regeneration
operating phases of the nitrogen oxide storage catalytic converter
can be determined as a function of the nitrogen oxide content of
the exhaust gas downstream of the nitrogen oxide storage catalytic
converter or of the SCR catalytic converter and/or as a function of
the ammonia loading of the latter. Moreover, a desired ammonia
generation quantity can be determined for a respective regeneration
operating phase. The installation and method are adopted for use
for motor vehicle internal combustion engines and other engines
which are operated predominantly in lean-burn mode.
Inventors: |
BANDL-KONRAD; Brigitte;
(Goeppingen, DE) ; HERTZBERG; Andreas;
(Markgroeningen, DE) ; KRUTZSCH; Bernd;
(Denkendorf, DE) ; NOLTE; Arno; (Stuttgart,
DE) ; PAULE; Markus; (Korb, DE) ; RENFFTLEN;
Stefan; (Eislingen, DE) ; WALDBUESSER; Norbert;
(Stuttgart, DE) ; WEIBEL; Michel; (Stuttgart,
DE) ; WENNINGER; Guenther; (Stuttgart, DE) ;
WUNSCH; Rolf; (Malsch, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Daimler AG |
Stuttgart |
|
DE |
|
|
Family ID: |
32519680 |
Appl. No.: |
14/695732 |
Filed: |
April 24, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13616886 |
Sep 14, 2012 |
9057307 |
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14695732 |
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|
12878544 |
Sep 9, 2010 |
8297046 |
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13616886 |
|
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|
11690214 |
Mar 23, 2007 |
7814747 |
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12878544 |
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10541311 |
Jan 24, 2006 |
7210288 |
|
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PCT/EP2003/014313 |
Dec 16, 2003 |
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11690214 |
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Current U.S.
Class: |
423/213.7 ;
422/170 |
Current CPC
Class: |
F01N 2560/06 20130101;
F01N 3/0821 20130101; Y02T 10/12 20130101; F01N 3/035 20130101;
F01N 13/0093 20140601; F01N 3/30 20130101; F01N 13/009 20140601;
F01N 3/0871 20130101; B01D 53/9431 20130101; F01N 3/2013 20130101;
F01N 3/2882 20130101; F01N 3/2066 20130101; F01N 2510/0684
20130101; F01N 2610/02 20130101; F01N 2250/14 20130101; F01N
2610/146 20130101; F01N 3/0842 20130101; Y02A 50/2325 20180101;
F01N 13/011 20140603; Y02T 10/24 20130101; B01D 53/9418 20130101;
B01D 53/9422 20130101; F01N 3/103 20130101; F01N 2250/02 20130101;
B01D 2255/911 20130101; F01N 2250/12 20130101; F01N 2610/03
20130101; F01N 2570/14 20130101; F01N 2560/026 20130101; Y02A 50/20
20180101; B01D 53/9495 20130101; F01N 2560/025 20130101; F01N
2570/18 20130101; B01D 53/9477 20130101; B01D 53/96 20130101; F01N
3/0231 20130101; F01N 3/208 20130101; Y02A 50/2344 20180101; F01N
3/0814 20130101 |
International
Class: |
F01N 3/035 20060101
F01N003/035; F01N 13/00 20060101 F01N013/00; B01D 53/94 20060101
B01D053/94; F01N 3/20 20060101 F01N003/20 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 2, 2003 |
DE |
103 00 298.7 |
Claims
1. An installation for aftertreatment of exhaust gas generated by a
combustion device of a motor vehicle, the installation comprising:
an NO.sub.2 producing catalytic converter arranged in an exhaust
gas line of the motor vehicle so as to receive the exhaust gas
generated by the combustion device; an SCR catalytic converter
arranged in the exhaust gas line downstream of the NO.sub.2
producing catalytic converter, wherein the SCR catalytic converter
is configured to store ammonia and to reduce nitrogen oxides in the
exhaust gas with the stored ammonia; a particulate filter arranged
in the exhaust gas line downstream of the NO.sub.2 producing
catalytic converter and upstream of the SCR catalytic converter,
wherein the particulate filter has a catalytic coating comprising
an SCR catalyst.
2. The installation according to claim 1, further comprising: an
oxidation catalytic converter arranged in the exhaust gas line
downstream of the SCR catalytic converter.
3. The installation according to claim 1, further comprising: a
reducing agent supplying device configured to feed fuel into the
exhaust gas stream upstream of the NO.sub.2 producing catalytic
converter.
4. The installation according to claim 1, wherein the exhaust gas
line comprises an exhaust pipe having an air gap insulation.
5. The installation according to claim 1, further comprising: an
electrical heater arranged in the exhaust gas line upstream of the
NO.sub.2 producing catalytic converter.
6. The installation according to claim 1, wherein the SCR catalyst
coating of the particulate filter is similar to a coating of the
SCR catalytic converter.
7. The installation according to claim 1, wherein the combustion
device is a spark ignition engine.
8. A method for operating an installation for aftertreatment of
exhaust gas generated by a combustion device of a motor vehicle,
the method comprising: receiving, by an NO.sub.2 producing
catalytic converter arranged in an exhaust gas line of the motor
vehicle, the exhaust gas generated by the combustion device;
receiving, by a particulate filter arranged in the exhaust gas line
downstream of the NO.sub.2 producing catalytic converter, the
exhaust gas from the NO.sub.2 producing catalytic converter,
wherein the particulate filter has a catalytic coating comprising
an SCR catalyst; receiving, by an SCR catalytic converter arranged
in the exhaust gas line downstream of the particulate filter, the
exhaust gas from the particulate filter; and storing ammonia and
reducing nitrogen oxides in the exhaust gas with the stored ammonia
by the SCR catalytic converter.
9. The method according to claim 8, further comprising: receiving,
by an oxidation catalytic converter arranged in the exhaust gas
line downstream of the SCR catalytic converter, the exhaust gas
from the SCR catalytic converter.
10. The method according to claim 8, further comprising: feeding
fuel, by a reducing agent supplying device, into the exhaust gas
stream upstream of the NO.sub.2 producing catalytic converter.
11. The method according to claim 8, wherein the exhaust gas line
comprises an exhaust pipe having an air gap insulation.
12. The method according to claim 8, further comprising:
electrically heating, by an electrical heater arranged in the
exhaust gas line upstream of the NO.sub.2 producing catalytic
converter, the exhaust gas.
13. The method according to claim 8, wherein the SCR catalyst
coating of the particulate filter is similar to a coating of the
SCR catalytic converter.
14. The method according to claim 8, wherein the combustion device
is a spark ignition engine.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 13/616,886 filed Sep. 14, 2012, which is a continuation of U.S.
application Ser. No. 12/878,544 filed Sep. 9, 2010, which is a
continuation of U.S. application Ser. No. 11/690,214, filed Mar.
23, 2007, which is a continuation of U.S. application Ser. No.
10/541,311 filed Jan. 24, 2006, which is a national stage of
PCT/EP2003/014313, filed Dec. 16, 2003, and claims priority of
German patent application no. 103 00 298.7, filed Jan. 2, 2003 the
entire disclosure of which is incorporated herein by reference.
BACKGROUND AND SUMMARY OF THE INVENTION
[0002] The invention relates to an exhaust gas aftertreatment
installation for the aftertreatment of an exhaust gas generated by
a combustion device, in particular a motor vehicle internal
combustion engine, having a nitrogen oxide storage catalytic
converter and an SCR catalytic converter with the ability to store
ammonia, which is arranged downstream of the nitrogen oxide storage
catalytic converter or integrated with the latter in a common
catalytic converter unit. The invention also relates to an exhaust
gas aftertreatment method which can be carried out using an
installation of this type. Installations and methods of this type
are used in particular for the aftertreatment or purification of
exhaust gases from internal combustion engines operated
predominantly in lean-burn mode in motor vehicles.
[0003] The use of nitrogen oxide storage catalytic converters, also
known as NO.sub.x storage catalytic converters or NO.sub.x adsorber
catalytic converters, or NSC for short, is generally known for
lowering the levels of nitrogen oxides downstream of internal
combustion engines operated in lean-burn mode. Lean operating
phases of the internal combustion engine correspond to adsorption
phases of the nitrogen oxide storage catalytic converter, in which
the nitrogen oxide storage catalytic converter oxidizes nitrogen
monoxide (NO) to form nitrogen dioxide (NO.sub.2), which it
temporarily stores in the form of nitrates. During brief, periodic
regeneration or desorption phases, the stored nitrates are removed
from the nitrogen oxide storage catalytic converter by the nitrates
being converted into nitrogen dioxide and then nitrogen monoxide.
The latter is then reduced to form nitrogen by suitable reducing
agents.
[0004] One known technique used to provide the required reducing
agents consists in switching the combustion device, which is
operated predominantly in lean-burn mode and the exhaust-gas from
which is being aftertreated, to rich-burn mode for a brief period
of time, with the result that hydrogen (H.sub.2), carbon monoxide
(CO) and unburnt hydrocarbons (HC) are present in the exhaust gas
as reducing agents. Various specific measures for controlling the
air/fuel ratio, also known as the air ratio .lamda. for short, have
already been proposed for this purpose, cf. for example the
laid-open specifications EP 0 560 991 A1 and DE 196 26 835 A1. The
nitrogen oxide reduction may also take place in a downstream
catalytic converter known as a deNOx catalytic converter; it is
also possible for hydrocarbons to be metered in downstream of the
engine in order to provide the reducing agents, cf. for example the
laid-open specifications EP 0 540 280 A1 and EP 0 573 672 A1.
[0005] A number of problem points need to be borne in mind during
this alternating adsorption/desorption operation. For example,
considerable quantities of the polluting gas ammonia (NH.sub.3) may
form through reaction of hydrogen with nitrogen monoxide and/or
nitrogen dioxide may form in the regeneration phase, depending on
the catalytic converter temperature, the exhaust gas composition
and the material composition of the nitrogen oxide storage
catalytic converter. When switching from a lean exhaust gas
atmosphere to a rich exhaust gas atmosphere, there is the risk of
an undesirable breakthrough of nitrogen oxides on account of sudden
decomposition of nitrates if suitable quantities of reducing agent
are not provided quickly enough. When switching from a rich exhaust
gas atmosphere to a lean exhaust gas atmosphere, the nitrogen oxide
storage catalytic converter may be heated as a result of exothermic
combustion reactions, with the result that nitrates which have
already formed can be decomposed again and can predominantly no
longer be stored, which can cause undesirable nitrogen oxide
slippage. With this NO.sub.x storage catalytic converter
technology, efficient lowering of the levels of nitrogen oxides is
restricted to a relatively narrow temperature range, approximately
between 200.degree. C. and 400.degree. C., since at lower
temperatures it is difficult to oxidize NO to form NO.sub.2, and at
higher temperatures the nitrates formed can no longer be stably
stored in significant quantities, and the thermodynamic equilibrium
between NO and NO.sub.2 increasingly shifts toward nitrogen
monoxide.
[0006] A further problem when using sulfur-containing fuels is what
is known as sulfur poisoning of the NO.sub.x storage catalytic
converter as a result of the accumulation of sulfates, which are
more stable than the nitrates and do not decompose during the
NO.sub.x regeneration phases. Therefore, special desulfating phases
at elevated exhaust-gas temperature and with a rich exhaust gas
composition are carried out from time to time in order to remove
sulfates, cf. for example laid-open specification DE 198 27 195 A1.
During desulfating, the polluting gas hydrogen sulfide (H.sub.2S)
may form; emission of this gas should be avoided. For this purpose,
for example in patent DE 100 25 044 C1, it is proposed that
secondary air be fed into the exhaust train during the desulfating
phases, in order to oxidize the hydrogen sulfide in a downstream
oxidation catalytic converter.
[0007] Another known exhaust gas aftertreatment method is the
process known as selective catalytic reduction, or SCR for short.
In this process, a selectively acting reducing agent, typically
ammonia, is added to the exhaust gas in order to reduce nitrogen
oxides. The ammonia is temporarily stored in a suitable deNOx
catalytic converter, also known as an SCR catalytic converter for
short, and is used by the latter to catalytically reduce nitrogen
oxides (NO.sub.x) contained in the exhaust gas to form nitrogen and
water. The effectiveness of SCR catalytic converters is highly
dependent on the NO/NO.sub.2 ratio at low temperatures, with a
maximum efficiency at an NO.sub.2 level of approx. 50% for
temperatures below 200.degree. C. and a greatly reduced efficiency
if the NO.sub.2 level is lower. At higher temperatures above
approx. 400.degree. C., the nitrogen oxide reduction is limited by
oxidation of ammonia, and moreover the ammonia storage capacity of
the SCR catalytic converter decreases as the temperature rises. The
overall result for SCR systems of this type is that the temperature
window which is suitable for efficiently lowering the levels of
nitrogen oxides is from approximately 250.degree. C. to
approximately 550.degree. C. SCR catalytic converters are subject
to thermal aging and should not be exposed to temperatures of over
approximately 700.degree. C. to 750.degree. C. At low temperatures,
SCR catalytic converters may also temporarily store unburnt
hydrocarbons, and even when the exhaust gas composition is rich,
given a suitable design, they may oxidize hydrocarbons, in
particular if they contain vanadium oxide (V.sub.2O.sub.5) as a
catalytic material.
[0008] To provide the ammonia in the exhaust gas, it is known to
introduce urea, from which ammonia is formed through hydrolysis or
thermolysis. One problem in this context, in particular for vehicle
applications, is that a corresponding supply of urea has to be
carried around. Therefore, as an alternative, internal generation
of ammonia has already been proposed. For this purpose, it is
proposed in laid-open specifications WO 97/17532 A1 and DE 199 49
046 A1 that an ammonia-generating catalytic converter and,
downstream of the latter, an ammonia storage and nitrogen oxide
reduction catalytic converter be provided, preferably with the
addition of an oxygen storage catalytic converter between the two
catalytic converters mentioned above or downstream of the ammonia
storage catalytic converter and the nitrogen oxide reduction
catalytic converter.
[0009] Laid-open specification EP 0 878 609 A1 has described an
exhaust gas aftertreatment installation of the generic type, in
which an NO.sub.x storage catalytic converter and, downstream of
the latter, an SCR catalytic converter are arranged in the exhaust
train. As an alternative or in addition to a three-way catalytic
converter, the NO.sub.x storage catalytic converter can be designed
to form ammonia when the engine is briefly operated under rich-burn
conditions, which is realized by an afterinjection of fuel into at
least some of the engine combustion chambers. With this type of
internal generation of ammonia, there is a risk of the quantity of
ammonia which is generated exceeding the ammonia storage capacity
of the SCR catalytic converter, resulting in undesirable ammonia
slippage.
[0010] The exhaust gas aftertreatment measures described above do
not lower the levels of particles, in particular of carbon
particulates. It is known to use particulate filters to do this.
Carbon particulates which have collected in the particulate filter
can be burnt off at elevated temperature in the presence of oxygen.
A standard measure for heating the particulate filter consists in
introducing fuel into the exhaust gas, for example by an
afterinjection, and burning this fuel in an oxidation catalytic
converter connected upstream of the particulate filter. What are
known as CRT systems with continuous particulate filter
regeneration are also known, cf. for example U.S. Pat. No.
4,902,487. In these systems, nitrogen dioxide is formed from NO at
the oxidation catalytic converter and oxidizes the carbon
particulates which have collected in the particulate filter.
[0011] To lower the levels of particulate and nitrogen oxides,
laid-open specification DE 199 21 974 A1 describes an exhaust gas
aftertreatment installation having an oxidation catalytic
converter, a downstream NO.sub.x storage catalytic converter and a
particulate filter downstream of the NO.sub.x storage catalytic
converter or between the oxidation catalytic converter and the
NO.sub.x storage catalytic converter. In the case of the former
arrangement, the oxidation catalytic converter promotes the
function of the NO.sub.x storage catalytic converter through the
formation of NO.sub.2, but it is not possible to achieve a CRT
effect for the particulate filter, since the nitrogen oxides are
already being reduced to nitrogen upstream of the particulate
filter. If the oxidation catalytic converter is used to heat the
particulate filter by combustion of afterinjected fuel, this
arrangement results in a high thermal loading of the NO.sub.x
storage catalytic converter and a relatively high fuel consumption.
In the other arrangement described, the exhaust gas upstream of the
NO.sub.x storage catalytic converter predominantly contains NO and
only a small amount of NO.sub.2, since the latter is converted into
NO by the CRT effect, with the result that the storage
characteristics of the NO.sub.x storage catalytic converter
deteriorate. If the oxidation catalytic converter in this
arrangement is used for desulfating-heating of the NO.sub.x storage
catalytic converter through combustion of afterinjected fuel, the
high heat capacity and the heat transfer in the intervening exhaust
train section means that a relatively high temperature has to be
reached at the oxidation catalytic converter, which can lead to
thermal aging effects in the latter.
[0012] The invention is based on the technical problem of providing
an exhaust gas aftertreatment installation of the type described in
the introduction and an associated exhaust gas aftertreatment
method with which as many of the following demands as possible can
be satisfied to a high degree with relatively little outlay:
effective reduction of nitrogen oxides in a wide temperature range,
no need for an additional reducing agent operating medium,
avoidance of ammonia and hydrogen sulfide emissions, minimal
particulate emissions, particulate oxidation through NO.sub.2
reaction, minimal CO and HC emissions, relatively low thermal
loading of all the components used to purify the exhaust gas,
minimal increased fuel consumption and a low demand for
installation space.
[0013] The invention solves this problem by providing an exhaust
gas aftertreatment installation for the aftertreatment of an
exhaust gas generated by a combustion device, in particular a motor
vehicle internal combustion engine, having a nitrogen oxide storage
catalytic converter and an SCR catalytic converter with the ability
to store ammonia, which is arranged downstream of the nitrogen
oxide storage catalytic converter or integrated with the latter in
a common catalytic converter unit.
[0014] The invention also solves this problem by providing a method
for the aftertreatment of an exhaust gas generated by a combustion
device, in particular a motor vehicle internal combustion engine,
in which nitrogen oxides contained in the exhaust gas are
temporarily stored in a nitrogen oxide storage catalytic converter
during adsorption operating phases of the latter and are released
again from the nitrogen oxide storage catalytic converter during
regeneration operating phases of the latter, with ammonia being
generated, and ammonia which is generated is temporarily stored in
a downstream SCR catalytic converter and is used for nitrogen oxide
reduction, characterized in that the recorded nitrogen oxide
content of the exhaust gas downstream of the SCR catalytic
converter and/or downstream of the nitrogen oxide storage catalytic
converter and/or the ammonia loading of the SCR catalytic converter
is used as a criterion for the instant at which a regeneration
operating phase is triggered for the nitrogen oxide storage
catalytic converter can further include the following step wherein
nitrogen oxides contained in the exhaust gas are temporarily stored
in a nitrogen oxide storage catalytic converter during adsorption
operating phases of the latter and are released again from the
nitrogen oxide storage catalytic converter during regeneration
operating phases of the latter, with ammonia being generated, and
ammonia which is generated is temporarily stored in a downstream
SCR catalytic converter and is used for nitrogen oxide reduction,
characterized in that a desired ammonia generation quantity which
is to be generated during a current regeneration operating phase of
the nitrogen oxide storage catalytic converter is determined, and
the subsequent regeneration operating phase is carried out as a
function of the desired ammonia generation quantity determined.
[0015] In addition to a nitrogen oxide storage catalytic converter
and an SCR catalytic converter connected downstream of it, the
exhaust gas aftertreatment installation additionally includes a
particulate filter and/or an NO.sub.2-producing catalytic converter
arranged upstream of the SCR catalytic converter.
[0016] The nitrogen oxide storage catalytic converter allows the
levels of nitrogen oxides to be effectively lowered, in particular
even in the case of lean-burn internal combustion engines. By
virtue of its NH.sub.3 storage capacity, the downstream SCR
catalytic converter prevents undesired emission of ammonia
generated by the NO.sub.x storage catalytic converter. At the same
time, the SCR catalytic converter is able to use stored ammonia to
reduce any nitrogen oxide which may still be present in the exhaust
gas downstream of the NO.sub.x storage catalytic converter, with
the ammonia being oxidized at the same time. This effect can be
exploited to deliberately form ammonia at the nitrogen oxide
storage catalytic converter, in order for this ammonia to be used
as reducing agent in the SCR catalytic converter. As a result, the
effective lowering of the levels of nitrogen oxides can be
maintained even during periods in which the temperature of the
NO.sub.x storage catalytic converter is temporarily outside its
range in which it has a conversion activity, for example as a
result of the exothermicity following a transition from a lean
exhaust gas composition to a rich exhaust gas composition.
[0017] If nitrogen oxides are still present in the exhaust gas
downstream of the NO.sub.x storage catalytic converter, these
nitrogen oxides are generally in the form of NO; the NO.sub.2
content is much lower, amounting, for example, to at most 20%.
However, at low temperatures below 300.degree. C., the efficiency
of the SCR catalytic converter is highest approximately at an
NO.sub.2 level of 50%, and significantly reduced where the NO.sub.2
level is lower. To increase the efficiency of the SCR catalytic
converter, the NO.sub.2-producing catalytic converter may be
connected upstream of it. This catalytic converter may have a
relatively small volume and may have a coating which, inter alia,
contains a precious metal (e.g. platinum) and is able to increase
the NO.sub.2 level in the NO.sub.x emission to at least
approximately 50% at least in a temperature range from
approximately 200.degree. C. to 350.degree. C. One particular
feature of the coating of the NO.sub.2-producing catalytic
converter is its property of deliberately not oxidizing ammonia
which is generated in the NO.sub.x storage catalytic converter when
operating at .lamda.<1, but rather allowing this ammonia to pass
through in unchanged form. This can be achieved, for example, by
the coating not containing an oxygen-storing component.
[0018] In a corresponding control unit, which can also be used, for
example, to control the combustion device, such as an internal
combustion engine, it is preferable to implement functions which
decide on the need for and feasibility of targeted generation of
NH.sub.3 and preset the operating parameters, in particular the
duration and extent of enrichment during NSC regeneration, in a
suitable way. The formation of NH.sub.3 can typically be boosted by
a lower air ratio and a longer regeneration time, provided that the
temperature of the NO.sub.x storage catalytic converter is in the
range in which NH.sub.3 formation is possible. Furthermore,
operation of the combustion device during NSC regeneration can be
set in a manner known per se such that high untreated NO.sub.x
emissions from the combustion device result, and as a result the
formation of NH.sub.3 at the NO.sub.x storage catalytic converter
is boosted further.
[0019] It has been found that the SCR catalytic converter can also
be used to avoid H.sub.2S emission, produced, for example, during
desulfating. Tests have shown that an SCR catalytic converter, on
account of its specific properties, can oxidize hydrogen sulfide
formed during the desulfating to form SO.sub.2 even at a rich
exhaust gas composition (.lamda.<1). This makes it possible to
avoid unpleasant odor pollution.
[0020] As a further particular feature, SCR catalytic converters,
provided that they contain vanadium oxide, can even oxidize unburnt
hydrocarbons (HC) during rich conditions (.lamda.<1). This makes
it possible to reduce the extent to which reducing agent breaks
through during NSC regeneration. In particular, by way of example,
it is possible to lower the emission levels of possibly
carcinogenic hydrocarbons, such as benzene, toluene, ethylbenzene
and xylene, which can form under rich conditions at the NO.sub.x
storage catalytic converter. On account of its ability to store
hydrocarbons even at low temperatures, the SCR catalytic converter
can additionally make a contribution to lowering the levels of HC
emissions following a cold start. The HC which have been stored at
low temperatures are released again at higher temperatures and can
be oxidized at the SCR catalytic converter or a downstream
oxidation catalytic converter.
[0021] A particulate filter can be used for lowering the levels of
particulates downstream of the engine. This particulate filter
highly efficiently retains the particulates emitted. As is
customary, the particulates which have collected can be burnt off
at regular intervals by increasing the temperature to over
600.degree. C. If the exhaust gas reaching the particulate filter
contains NO.sub.2, particulate oxidation also takes place even in
the temperature range between approximately 250.degree. C. and
400.degree. C. as a result of reaction with NO.sub.2 (CRT effect).
The particulate filter may generally be catalytically coated, in
which case the coating may contain components such as a precious
metal (e.g. platinum) and a washcoat.
[0022] The maximum thermal loading of the individual components can
be adapted to the specific requirements by using a suitable
arrangement of the components. Moreover, by using a suitable
arrangement it is possible to ensure that the temperatures of the
individual components are within a range which is expedient for the
corresponding function when the vehicle is driving. The rich-burn
operation which is required for the regeneration of the NO.sub.x
storage catalytic converter can be realized by engine-internal
measures or additional introduction of reducing agent (e.g. fuel or
hydrogen) downstream of the engine.
[0023] The heating of the NO.sub.x storage catalytic converter for
desulfating purposes and of the particulate filter for thermal
regeneration purposes can be effected by engine-internal measures,
including afterinjection of fuel. In addition to the deliberately
increased exhaust gas temperature, incompletely burnt hydrocarbons
which remain in the exhaust gas lead to additional exothermicity at
a catalytic converter optionally arranged close to the engine, with
the result that the exhaust gas temperature is increased further.
In addition or as an alternative, it is also possible for reducing
agent (e.g. fuel or hydrogen) to be supplied in the exhaust train
directly upstream of the component(s) to be heated or upstream of
an oxidation catalytic converter connected upstream of these
components. This has the advantage that the heat losses involved in
heating further upstream components and heat losses resulting from
cooling in the exhaust pipe are reduced. This restricts the outlay
on energy and therefore the increased fuel consumption for the
heating to a minimum. A further advantage is that in this way
further upstream components are not subjected to high exhaust gas
temperatures, and consequently the thermal aging of these
components can be restricted to a minimum. Moreover, this prevents
further upstream components, e.g. an upstream NO.sub.x storage
catalytic converter, from leaving the temperature window required
for high efficiency as a result of the heating.
[0024] In the case of a catalytically coated particulate filter, a
further advantage is that the conversion of fuel, on account of the
high heat capacity of the particulate filter, continues to be
possible even, for example, after prolonged overrun phases of the
internal combustion engine with a low exhaust gas temperature. By
contrast, when using a conventional catalytic converter, there is a
risk that, on account of the low heat capacity, the temperature
will drop below the light-off temperature under comparable
conditions, meaning that catalytic conversion of the hydrocarbons
will no longer be possible. In general terms, other heating methods
are possible as an alternative to supplying reducing agent (e.g.
hydrogen or fuel) upstream of a catalytic converter. These other
methods are not expressly mentioned below but can be used instead
of supplying reducing agent downstream of the engine. In this
context, by way of example, mention may be made of an electrically
heated catalytic converter, electrical heating of the particulate
filter or the use of a burner as standard conventional
measures.
[0025] The supply of reducing agent downstream of the engine and
upstream of the NO.sub.x storage catalytic converter can also be
utilized to set rich conditions for NSC regeneration when the
engine is operating with lean exhaust gas. This is preferably
implemented when the engine is operating between .lamda.=1.0 and
.lamda.=1.2, since otherwise the quantity of reducing agent to be
supplied is too great. This results in the advantage that high
untreated NO.sub.x emission usually occurs in the range between
.lamda.=1.0 and .lamda.=1.2, whereas the level of untreated
NO.sub.x emissions is considerably lower at air ratios of
.lamda.<1. Consequently, this method can be used to achieve a
high level of NO.sub.x emission and therefore extensive formation
of NH.sub.3 during NSC regeneration.
[0026] To avoid high emissions of CO and HC during NSC regeneration
operations with .lamda.<1, if necessary secondary air can be
blown in ahead of a downstream oxidation catalytic converter. The
secondary air can, for example, be provided by an electrically
driven secondary air pump or a compressor or, in the case of
supercharged engines can be removed downstream of the
compressor.
[0027] It is possible to considerably reduce the overall space
taken up by optionally combining or integrating two of the
abovementioned functionalities in one component, for example, by
applying a catalytic coating to a particulate filter.
[0028] In further configurations, the exhaust gas aftertreatment
installation contains one or more NO.sub.x sensors downstream of
the NO.sub.x storage catalytic converter and/or the SCR catalytic
converter, and/or means for recording the temperature of one or
more of the exhaust gas purification components and/or means for
recording the NH.sub.3 loading of the SCR catalytic converter.
[0029] Various possible implementations are possible for the order
in which the NO.sub.x storage catalytic converter, SCR catalytic
converter, particulate filter and NO.sub.2-producing catalytic
converter are arranged in the exhaust train, each of these
implementations having specific properties and advantages; the
options include multi-flow arrangements.
[0030] The basic method described above allows relatively accurate,
model-based control of the time at which a regeneration of the
nitrogen oxide storage catalytic converter is initiated.
[0031] The second method described above allows targeted control of
the generation of ammonia during a respective regeneration phase of
the nitrogen oxide storage catalytic converter taking account of
the current conditions, in particular with regard to the
temperatures of the ammonia-generating NO.sub.x storage catalytic
converter and of the SCR catalytic converter and/or the ammonia
loading of the SCR catalytic converter; a variable quantity of
ammonia which is to be generated during the current regeneration
phase of the NO.sub.x storage catalytic converter can be
predetermined as a function of the operating state which has been
ascertained.
[0032] In one configuration of this method, the results of the
determination as to whether the exhaust gas air ratio downstream of
the NO.sub.x storage catalytic converter has dropped below a
threshold value, which is dependent on the quantity of ammonia
which it is desired to form, can be used as a criterion for
terminating a regeneration phase of the NO.sub.x storage catalytic
converter.
[0033] In a further configuration of the method according to the
invention, an external supply of reducing agent, e.g. downstream of
the engine, into the exhaust train can be provided for during the
NSC regeneration, in order for the combustion device also to be
operated under lean-burn conditions during this period, resulting
in high untreated NO.sub.x emissions from the combustion
device.
[0034] In a further configuration of the method, it is possible to
provide for secondary air to be fed into the exhaust train at a
suitable location during the rich-burn operating phases, in order
to oxidize any NH.sub.3, H.sub.2S, CO and HC which may be present
during these operating phases and thereby to prevent corresponding
emissions of pollutants.
[0035] Advantageous embodiments of the invention are illustrated in
the drawings and described below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 shows a schematic block diagram illustration of an
internal combustion engine of a motor vehicle with connected
single-flow exhaust gas aftertreatment installation, which
includes, connected in series, a first oxidation catalytic
converter, a particulate filter, an NO.sub.x storage catalytic
converter, an NO.sub.2-producing catalytic converter, an SCR
catalytic converter and a second oxidation catalytic converter,
[0037] FIG. 2 shows a schematic block diagram illustration in
accordance with FIG. 1, but with a modified exhaust gas
aftertreatment installation which includes, connected in series, an
NO.sub.x storage catalytic converter, an NO.sub.2-producing
catalytic converter, a particulate filter, an SCR catalytic
converter and an oxidation catalytic converter,
[0038] FIG. 3 shows a schematic block diagram illustration in
accordance with FIG. 1, with a modified exhaust gas aftertreatment
installation, which includes, connected in series, an NO.sub.x
storage catalytic converter, an NO.sub.2-producing catalytic
converter, an SCR catalytic converter, an oxidation catalytic
converter and a particulate filter,
[0039] FIG. 4 shows a schematic block diagram illustration in
accordance with FIG. 1, but with a modified exhaust gas
aftertreatment installation, which includes a first oxidation
catalytic converter, an integrated nitrogen oxide storage and SCR
catalytic converter, a second oxidation catalytic converter and a
particulate filter, and
[0040] FIG. 5 shows a schematic block diagram illustration in
accordance with FIG. 1, but for a modified internal combustion
engine with a two-flow exhaust train and associated two-flow
exhaust gas aftertreatment installation.
DETAILED DESCRIPTION OF THE DRAWINGS
[0041] The exhaust gas aftertreatment installation illustrated in
FIG. 1 includes, downstream of an internal combustion engine 1, as
seen in the direction of flow of the exhaust gas, in an exhaust
train 3, the following successive components: an oxidation
catalytic converter 4, a particulate filter 5, an NO.sub.x storage
catalytic converter 6, an NO.sub.2-producing catalytic converter
13, an SCR catalytic converter 7 and a further oxidation catalytic
converter 4 as exhaust gas purification components. A control unit
2 is used to control the internal combustion engine 1, which may,
for example, be a standard diesel or spark-ignition engine, and the
exhaust gas aftertreatment installation. Furthermore, there are
temperature sensors 8, NO.sub.x sensors 9, lambda sensors 10,
devices 11 for supplying reducing agent downstream of the engine, a
device 12 for supplying secondary air and pressure sensors 14
provided at suitable locations in the exhaust train 3, as
shown.
[0042] The internal combustion engine 1 delivers exhaust gas
containing, inter alia, NO.sub.x particulates, CO and HC. During
standard lean-burn operation of the internal combustion engine 1,
CO and HC are oxidized at the oxidation catalytic converter 4 to
form CO.sub.2 and H.sub.2O. Moreover, some of the NO contained in
the exhaust gas is oxidized to form NO.sub.2. The particulates
which are present in the exhaust gas are retained in the
particulate filter 5. Some of the carbon particulates which have
collected in the particulate filter 5 are oxidized by reaction with
NO.sub.2, with NO.sub.2 being reduced to NO. The nitrogen oxides
contained in the exhaust gas are stored in the NO.sub.x storage
catalytic converter 6. If nitrogen oxides are still present in the
exhaust gas downstream of the NO.sub.x storage catalytic converter
6, these nitrogen oxides are generally in the form of NO; the
NO.sub.2 level is significantly lower, amounting, for example, to
at most 20%. However, the efficiency of the SCR catalytic converter
7 at low temperatures below 300.degree. C. is highest at an
NO.sub.2 level of approximately 50% and is significantly reduced if
the level of NO.sub.2 is lower. Therefore, the NO.sub.2-producing
catalytic converter 13 is connected upstream of the SCR catalytic
converter 7. This catalytic converter 13 has a coating which
contains, inter alia, precious metal (e.g. platinum) and is able to
increase the level of NO.sub.2 in the total emission of NO.sub.x to
at least approximately 50%, at least in a temperature range from
approximately 200.degree. C. to 350.degree. C. One particular
feature of the coating of the NO.sub.2-producing catalytic
converter is its ability of deliberately not oxidizing ammonia
generated in the NO.sub.x storage catalytic converter during
operation at .lamda.<1 but rather allowing this ammonia to pass
through in unchanged form. This can be achieved, for example, by
the coating not containing any oxygen-storing components. The SCR
catalytic converter 7 is able to reduce the nitrogen oxide with the
aid of NH.sub.3 stored therein. In this case, the efficiency at
temperatures below 300.degree. C. is increased by the
NO.sub.2-producing catalytic converter 13 connected upstream.
[0043] Heating measures can be employed to ensure that sufficient
temperatures are reached at all the components, in particular at
the NO.sub.x storage catalytic converter 6 and SCR catalytic
converter 7, even during low-load operation, and thereby to ensure
optimum lowering of the levels of NOR. These heating measures may
be engine-internal, e.g. a shift towards the late position for the
main injection or an afterinjection, or may take place downstream
of the engine, as a result of reducing agent being supplied
upstream of the NO.sub.x storage catalytic converter, in order to
generate exothermicity, provided that the NO.sub.x storage
catalytic converter 6 has reached a sufficient temperature to
convert the reducing agent. Further measures for increasing the
exhaust gas temperature may include, increasing the idling speed,
lengthening the afterglow time, connecting up additional electrical
consumers or increasing the EGR rate. The abovementioned heating
measures can be controlled, for example, by the control unit 2 as a
function of the incoming temperature sensor signals or by means of
a model. The exhaust pipe 3 may be thermally insulated in order to
minimize heat losses from the exhaust gas. By way of example, it is
possible to use an air gap insulation.
[0044] NSC regenerations are required at regular intervals. The
times for regeneration operations are determined with the aid of
the NO.sub.x sensor 9 downstream of the SCR catalytic converter.
The signal from the NO.sub.x sensor 9 is recorded and assessed in
the control unit 2. If the breakthrough of NO.sub.x is unacceptably
high, for example if it exceeds a certain proportion of the
untreated NO.sub.x emissions (e.g. NO.sub.x content downstream of
NO.sub.x storage catalytic converter >10% of the untreated
NO.sub.x emissions), the control unit 2 requests an NSC
regeneration. If criteria which are taken into account in the
control unit 2 and are a precondition on the engine side for an NSC
regeneration to be realized are satisfied, for example, operation
in a specific part of the operating range of the engine 1 in which
NSC regeneration can be effected, the NSC regeneration is
initiated.
[0045] Models for the untreated NO.sub.x emission, the NO.sub.x
storage capacity of the NO.sub.x storage catalytic converter 6, the
NH.sub.3 formation at the NO.sub.x storage catalytic converter and
the NH.sub.3 storage in the SCR catalytic converter 7 are stored in
the control unit 2. These models can be used to assess the NO.sub.x
sensor signal downstream of the SCR catalytic converter and can
also be used for diagnostic purposes. The models can be adapted to
the current state of aging of the catalytic converters on the basis
of the sensor signals. Alternatively, the aging state can be
diagnosed by comparing the sensor signals from the NO.sub.x sensors
9 and/or the lambda sensors 10 with the modeled behavior of the
catalytic converters, without, however, adapting the model.
[0046] As an alternative to considering the NO.sub.x signal
downstream of the SCR catalytic converter, or as an additional
option, it is possible to request an NSC regeneration when the
modeled SCR loading with NH.sub.3 drops below a relative value,
e.g. 5% of the possible NH.sub.3 loading, or an absolute value,
e.g. 0.1 g of NH.sub.3.
[0047] As an alternative to considering the NO.sub.x signal
downstream of the SCR catalytic converter, it is also possible to
consider the signal from the NO.sub.x sensor 9 downstream of the
NO.sub.x storage catalytic converter. This can be assessed in a
similar way to the procedure described above and can be used as a
criterion to demand an NSC regeneration.
[0048] Alternatively the signal from the NO.sub.x sensor 9
downstream of the NO.sub.x storage catalytic converter can be used
as an input variable for the NSC model in the control unit 2. The
nitrate loading of the NO.sub.2 storage catalytic converter 6 can
be calculated with the aid of the model, thereby allowing the
NH.sub.3 formation to be estimated taking the aging into account.
Since the NH.sub.3 loading of the SCR catalytic converter 7 is
likewise calculated by means of a model, it is possible to request
an NSC regeneration if the modeled SCR loading with NH.sub.3 drops
below a relative value, e.g. 5% of the possible NH.sub.3 loading,
or an absolute value, e.g. 0.1 g of NH.sub.3.
[0049] In principle, it is also possible to use both NO.sub.x
sensors 9, in which one sensor, as described above, delivers a
criterion for requesting an NSC regeneration and the second
NO.sub.x sensor is used to diagnose and adapt the catalytic
converter model in the control unit 2. In addition, an NH.sub.3
sensor (not shown in FIG. 1) can be provided downstream of the
NO.sub.x storage catalytic converter and also downstream of the SCR
catalytic converter; the signal from this NH.sub.3 sensor can be
used to adapt the models of the NO.sub.x storage catalytic
converter 6 and the SCR catalytic converter 7 or to control the
regeneration parameters.
[0050] The control unit 2 includes functions which decide upon the
need for and feasibility of controlled generation of NH.sub.3
during an impending NSC regeneration and predetermine the operating
parameters, in particular the duration and degree of enrichment,
accordingly. An important criterion in this respect is the
temperatures of the NO.sub.x storage catalytic converter 6 and of
the SCR catalytic converter 7. The temperatures are determined by
the temperature sensors 8 in or downstream of the respective
components. Alternatively, the temperature of the SCR catalytic
converter 7 can be calculated on the basis of the measured
temperature downstream of the NO.sub.x storage catalytic converter,
with the aid of a model in the control unit 2, so that there is no
need for the sensor 8 downstream of the SCR catalytic converter. In
addition to the temperatures of the NO.sub.x storage catalytic
converter 6 and of the SCR catalytic converter 7, it is also
possible, for example, for the current NH.sub.3 loading, calculated
by model means, of the SCR catalytic converter 7 to be used as a
criterion.
[0051] If the temperatures of the NO.sub.x storage catalytic
converter 6 and of the SCR catalytic converter 7 are within
predetermined ranges, the current NH.sub.3 loading of the SCR
catalytic converter 7 is low and any further conditions are
satisfied, maximum formation of NH.sub.3 is desired. If the
temperature of the NO.sub.x storage catalytic converter 6 is
outside the predetermined range, which may, for example, be between
230.degree. C. and 370.degree. C., formation of NH.sub.3 is not
possible or is only possible to a limited extent and is therefore
also not aimed for. This avoids unnecessarily increased fuel
consumption and high levels of HC and CO emissions on account of
the regeneration lasting too long. If the temperature of the SCR
catalytic converter 7 is outside the predetermined range, which
may, for example, be between 200.degree. C. and 450.degree. C.,
formation of NH.sub.3 is likewise not aimed for, or only a small
amount is aimed for, since either NH.sub.3 cannot be sufficiently
stored in the SCR catalytic converter 7 and therefore would be
emitted or conversion of NO.sub.x at the SCR catalytic converter 7
is scarcely possible. The same applies to the situation in which
the current NH.sub.3 loading of the SCR catalytic converter 7 is so
high that it is impossible for further NH.sub.3 to form.
[0052] In general, in addition to the current temperatures, in
particular in the case of the SCR catalytic converter 7, it is also
possible to take account of temperature gradients, e.g. a rapid
rise, and/or to implement a temperature prediction. The predicted
temperature is then taken into account in addition to the current
temperature. By way of example, reduced formation of NH.sub.3 is
aimed for if a strong increase in the temperature of the SCR
catalytic converter 7 is predicted, since otherwise there is a risk
of subsequent NH.sub.3 desorption in the SCR catalytic converter 7.
The state of aging of the NO.sub.x storage catalytic converter 6 is
likewise taken into account when deciding on the formation of
NH.sub.3 which is aimed for, since the reduction of NO.sub.x in the
SCR catalytic converter 7 becomes increasingly important with
progressive aging of the NO.sub.x storage catalytic converter
7.
[0053] The optimum air ratio is defined in the control unit 2 as a
function of the NH.sub.3 which is aimed for. If it is aimed to form
high levels of NH.sub.3 an air ratio for maximum formation of
NH.sub.3 is preset. If this is not the case, an air ratio for the
formation of small amounts or no NH.sub.3 is preset. Intermediate
values for the formation of medium amounts of NH.sub.3 are also
possible. The air ratio can also be varied continuously or in steps
during the NSC regeneration as a function of the temperatures of
NO.sub.x storage catalytic converter 6 and SCR catalytic converter
7 and further parameters. By way of example, a lower air ratio can
be set at the start and a higher air ratio can be set as
regeneration progresses, in order to form high levels of NH.sub.3
combined, at the same time, with low emissions of HC and CO during
the NSC generation.
[0054] The duration of the NSC regenerations is likewise set as a
function of the desired formation of NH.sub.3. The gradual drop in
the air ratio downstream of the NO.sub.x storage catalytic
converter 6 to values of lower than .lamda.=1 is used to this end
as a criterion for terminating the NSC regeneration. If it is aimed
to form high levels of NH.sub.3, the regeneration is terminated as
soon as the air ratio determined by the lambda sensor 10 downstream
of the NO.sub.x storage catalytic converter drops below a threshold
value .lamda.1. It is optionally also possible to continue a
predetermined time or number of working cycles of the engine 1
beyond this point. If the aim is for no NH.sub.3 to be formed, the
regeneration is terminated immediately as soon as the air ratio
downstream of the NO.sub.x storage catalytic converter drops below
a threshold value .lamda.2, where .lamda.2 is greater than
.lamda.1. As a result, breakthroughs of HC/CO are substantially
avoided and a low increased fuel consumption is achieved. The
threshold values can be varied as a function of the NSC aging and
further parameters. Intermediate values are also possible in order
to achieve the formation of a medium level of NH.sub.3.
[0055] As soon as the aim is to form high levels of NH.sub.3,
operation of the engine during NSC regeneration is additionally set
in such a way that the untreated NO.sub.x emissions from the engine
1 are as high as possible. This can be effected in combination with
a supply of reducing agent downstream of the engine and upstream of
the NO.sub.x storage catalytic converter 6. This further boosts the
formation of NH.sub.3 at the NO.sub.x storage catalytic converter
6. If the aim is for no NH.sub.3 formation, by contrast, the engine
operation is set in such a way that the untreated NO.sub.x
emissions from the engine are as low as possible.
[0056] The device 11 arranged upstream of the NO.sub.x storage
catalytic converter 6 for supplying reducing agent downstream of
the engine is used to heat up the NO.sub.x storage catalytic
converter 6 for a desulfating operation. This measure is carried
out in addition to measures taken at the engine for heating. The
boost provided by the supply of reducing agent downstream of the
engine makes it possible to reduce the extent of the measures taken
at the engine, so that the thermal aging of the oxidation catalytic
converter is reduced. Heating of the NO.sub.x storage catalytic
converter 6 exclusively by supplying reducing agent downstream of
the engine is not desirable, since the large quantity of reducing
agent would generate considerable exothermicity at the NO.sub.x
storage catalytic converter 6, which would lead to thermal aging.
The quantity of reducing agent supplied is controlled, inter alia,
as a function of the following parameters: the desired temperature
of the NO.sub.x storage catalytic converter 6, the actual
temperature downstream of the particulate filter, the actual
temperature downstream of the NO.sub.x storage catalytic converter
and the mass flow of exhaust gas.
[0057] Furthermore, at least one of the two devices illustrated in
FIG. 1 for supplying reducing agent downstream of the engine can be
used to maximize the untreated NO.sub.x emissions from the engine 1
during the NSC regeneration. For this purpose, the engine 1 is
operated with an air ratio .lamda.>1 at which a high NO.sub.x
emission level is reached, preferably between .lamda.=1.0 and
.lamda.=1.2. The supply of reducing agent downstream of the engine
allows the air ratio upstream of the NO.sub.x storage catalytic
converter to be lowered to .lamda.<1.
[0058] Thermal regenerations of the particulate filter 5 are
required at regular intervals. This may be the case, for example,
if a high pressure drop at the particulate filter 5 is recorded by
the pressure sensors 14 upstream and downstream of the particulate
filter, implying unacceptably high loading of the particulate
filter 5. The device 11 for supplying reducing agent downstream of
the engine which is arranged upstream of the particulate filter 5
and the oxidation catalytic converter 4 connected upstream of the
particulate filter 5 can be used to heat the particulate filter 5.
This measure is carried out in addition to engine-internal measures
for raising the exhaust gas temperature. The boost provided by the
supply of reducing agent downstream of the engine makes it possible
to reduce the extent of engine-internal measures. It is not
desirable for the oxidation catalytic converter 4 and therefore the
particulate filter 5 to be heated exclusively by supplying reducing
agent downstream of the engine, since the large quantity of
reducing agent would generate extensive exothermicity at the
oxidation catalytic converter 4, which would lead to thermal aging.
The quantity of reducing agent supplied is controlled, inter alia,
as a function of the following parameters: the desired temperature
of the particulate filter 5, the actual temperature upstream of the
particulate filter, the actual temperature downstream of the
particulate filter and the mass flow of exhaust gas.
[0059] The device 12 for supplying secondary air and the further
oxidation catalytic converter 4 are connected downstream of the SCR
catalytic converter 7. This oxidation catalytic converter 4 may be
significantly smaller and also, for example, have a lower precious
metal loading than the oxidation catalytic converter 4 upstream of
the particulate filter. The device 12 for supplying secondary air
is activated whenever the engine 1 is being operated with
.lamda.<1. Undesirable emissions, such as NH.sub.3, H.sub.2S and
CO and HC, may form during these phases. These components can be
oxidized in the oxidation catalytic converter 4 connected
downstream of the device 12 for supplying secondary air, so that
emission of these undesirable components is avoided.
[0060] For this purpose, the quantity of secondary air supplied is
set in such a way that, despite the engine operating at
.lamda.<1, a lean exhaust gas composition (.lamda.>1) is
achieved at the downstream oxidation catalytic converter 4. Inter
alia, the signal from at least one lambda sensor 10 and the mass
flow of exhaust gas are used in the control unit 2 to control the
quantity of secondary air supplied. If the device 12 for supplying
secondary air is able to provide sufficient quantities of secondary
air only after a certain delay time, it is in each case activated a
corresponding time before the changeover to operation with
.lamda.<1.
[0061] It is advantageous in this exemplary embodiment that the SCR
catalytic converter 7 is protected from high temperatures
downstream of the particulate filter during the thermal
regeneration of the particulate filter 5, since the length of the
exhaust pipe means that the exhaust gas has been considerably
cooled by the time it reaches the SCR catalytic converter 7, and
moreover, the NO.sub.x storage catalytic converter 6 acts as a heat
sink. In this way, the thermal aging of the SCR catalytic converter
7 can be kept at a low level. As a further advantage, the high heat
capacity of the particulate filter 5 stabilizes the temperatures of
the downstream components even when the vehicle driving conditions
are not in a steady state. This ensures that the NO.sub.x storage
catalytic converter 6 and the SCR catalytic converter 7 are
generally in a favorable temperature range even when the vehicle
driving conditions are not in a steady state, and therefore the
system operates highly efficiently. Moreover, the risk of
undesirable desorption of stored NH.sub.3 at the SCR catalytic
converter 7 as a result of a rapid increase in temperature is
minimized.
[0062] The particulate filter 5 may be catalytically coated. The
catalytic coating of the particulate filter 5 may be similar to the
coating of one of the oxidation catalytic converters 4. As a
result, the upstream oxidation catalytic converter 4 can be made
smaller or provided with a lower precious metal content, which
leads to space and/or cost benefits. If appropriate, the oxidation
catalytic converter 4 connected upstream of the particulate filter
5 can also be dispensed with altogether. In this case, the
temperature sensor 8 upstream of the particulate filter 5 is
likewise removed. Therefore, it is no longer the temperature
upstream of the particulate filter, but rather the temperature
downstream of the particulate filter which is used as control
variable when heating the particulate filter 5 for thermal
regeneration. However, the catalytic coating of the particulate
filter 5 may also be similar to the coating of the NO.sub.x storage
catalytic converter 6. This allows the downstream NO.sub.x storage
catalytic converter 6 to be of smaller design, which leads to space
and/or cost benefits. If appropriate, it is also possible for the
separate NO.sub.x storage catalytic converter 6 to be dispensed
with altogether, i.e. for it to be integrated in the particulate
filter 5.
[0063] It should be noted that at least some of the temperature
sensors 8 can also be dispensed with. It is likewise also possible
to eliminate some or all of the NO.sub.x sensors 9. In this case,
the components contained in the exhaust gas aftertreatment system
are modeled in the control unit 2 in such a way that the NO.sub.x
concentrations or temperatures which occur at the respective
positions are available as output variables of the model. It is
also possible for one or both of the devices 11 for supplying
reducing agent downstream of the engine to be dispensed with. The
device 12 for supplying secondary air and the downstream oxidation
catalytic converter 4 can likewise be dispensed with.
[0064] The NO.sub.2-producing catalytic converter 13 may, as
illustrated in FIG. 1, be designed as a separate component.
Alternatively, it is also possible for it to be integrated at the
outlet end of the NO.sub.x storage catalytic converter 6 or at the
inlet end of the SCR catalytic converter 7. Alternatively the
NO.sub.2-producing catalytic converter 13 can also be dispensed
with.
[0065] The exemplary embodiment shown in FIG. 2 differs from that
shown in FIG. 1 in that the NO.sub.x storage catalytic converter 6
forms the first exhaust gas purification component, which is then
followed in the downstream direction by the NO.sub.2-producing
catalytic converter 13, the particulate filter 5, the SCR catalytic
converter 7 and the downstream oxidation catalytic converter 4, in
the order listed, while the other, upstream oxidation catalytic
converter is no longer present. Functionally equivalent components
are denoted by the same reference designations in FIG. 2 as in FIG.
1. The following explanations of the exemplary embodiment shown in
FIG. 2 can be restricted to the differences which result from the
different order of the exhaust gas purification components;
otherwise, the properties and advantages which have been explained
above in connection with the exemplary embodiment shown in FIG. 1
apply accordingly.
[0066] Therefore, in the example shown in FIG. 2, the NO.sub.x
storage catalytic converter 6 performs the function of oxidizing CO
and HC to form CO.sub.2 and H.sub.2O during standard lean-burn
operation of the internal combustion engine 1 and to temporarily
store a significant proportion of the nitrogen oxides contained in
the exhaust gas.
[0067] In this case the NO.sub.2-producing catalytic converter 13
is connected directly upstream of the particulate filter 5, in
order to increase the level of NO.sub.2 in the nitrogen oxides
which are still present in the exhaust gas downstream of the
NO.sub.x storage catalytic converter 6 depending on the operating
state, typically from at most 20% to at least approximately 50%.
This makes it possible to promote the oxidation of carbon
particulates in the particulate filter 5 by reaction with NO.sub.2
(CRT effect) and to increase the efficiency of the downstream SCR
catalytic converter 7. These effects can be boosted further by
admitting a maximum NO.sub.x slippage at the NO.sub.x storage
catalytic converter 6, so that the NO.sub.x reduction potential of
the SCR catalytic converter 7 can be exploited to its maximum
extent. At temperatures below 300.degree. C., the efficiency is
also increased by the NO.sub.2-producing catalytic converter 13
connected upstream of the particulate filter 5.
[0068] In the arrangement shown in FIG. 2, once again, the device
11 for supplying reducing agent downstream of the engine, which is
in this case positioned directly upstream of the NO.sub.2-producing
catalytic converter 13, can be used to heat the particulate filter
5, making it possible to reduce the extent of engine-internal
measures required for this purpose, thereby reducing aging effects
in the NO.sub.x storage catalytic converter 6. It is not desirable
to heat the NO.sub.2-producing catalytic converter 13 purely by the
supply of reducing agent downstream of the engine, on account of
thermal aging effects in this catalytic converter. The quantity of
reducing agent supplied is controlled inter alia as a function of
the following parameters: the desired temperature of the
particulate filter 5, the actual temperature upstream of the
NO.sub.2-producing catalytic converter and downstream of the
NO.sub.x storage catalytic converter, the actual temperature
downstream of the particulate filter and the mass flow of exhaust
gas.
[0069] An advantage of the exemplary embodiment shown in FIG. 2 is
that the NO.sub.x storage catalytic converter 6, on account of
being positioned close to the engine, reaches the required
operating temperature soon after a cold start. This means that few
measures are required to erase the exhaust gas temperature. This
reduces the increased fuel consumption resulting from heating
measures. Furthermore, in this exemplary embodiment, the SCR
catalytic converter 7 is protected from high temperatures during
the desulfating of the NO.sub.x storage catalytic converter 6,
since the length of the exhaust pipe results in considerable
cooling of the exhaust gas by the time it reaches the SCR catalytic
converter 7, and in addition, the particulate filter 5 and the
NO.sub.2-producing catalytic converter 13 act as heat sinks. In
this way, the thermal aging of the SCR catalytic converter 7 can be
kept at a low level. As a further advantage, the high heat capacity
of the particulate filter 5 leads to stabilization of the
temperature of the downstream SCR catalytic converter 7 even when
the vehicle driving operation is not in a steady state. This
ensures that the SCR catalytic converter 7 is generally in a
suitable temperature range, and therefore works with a high level
of efficiency, even when the vehicle driving operation is not in a
steady state. In addition, the risk of undesirable desorption of
stored NH.sub.3 at the SCR catalytic converter 7 as a result of a
rapid increase in temperature is minimized.
[0070] The particulate filter 5 may be catalytically coated. The
catalytic coating of the particulate filter 5 may be similar to the
coating of the NO.sub.2-producing catalytic converter 13. As a
result, the upstream NO.sub.2-producing catalytic converter 13 can
be made smaller or given a lower precious metal content, which
leads to space and/or cost benefits. If appropriate, it is also
possible for the NO.sub.2-producing catalytic converter connected
upstream of the particulate filter 5 to be dispensed with
altogether.
[0071] However, the catalytic coating of the particulate filter 5
may also be similar to the coating of the NO.sub.x storage
catalytic converter 6. This allows the NO.sub.x storage catalytic
converter 6 arranged close to the engine to be of smaller design,
which leads to space and/or cost benefits. If appropriate, it is
also possible for the separate NO.sub.x storage catalytic converter
6 to be dispensed with altogether, i.e. for its function to be
integrated in the particulate filter 5. In this case, it is also
possible for both the NO.sub.2-producing catalytic converter 13 and
the device 11 for supplying reducing agent downstream of the engine
and upstream of the NO.sub.2-producing catalytic converter 13 to be
dispensed with. However, the pressure sensor 14 illustrated
upstream of the particulate filter is retained, and the NO.sub.x
sensor 9, illustrated downstream of the NO.sub.x storage catalytic
converter 6 in FIG. 2, and the lambda sensor 10 are then positioned
downstream of the particulate filter 5.
[0072] Furthermore, the catalytic coating may also be similar to
the coating of the downstream SCR catalytic converter 7. This
allows the SCR catalytic converter 7 to be of smaller design, which
leads to space and/or cost benefits. If appropriate, the SCR
catalytic converter as a separate component can also be dispensed
with altogether. In this case, the temperature sensor 8 illustrated
in FIG. 2 between particulate filter and SCR catalytic converter
can be dispensed with, but the pressure sensor 14 illustrated
downstream of the particulate filter is retained.
[0073] Instead of being arranged upstream of the particulate filter
5, the NO.sub.2-producing catalytic converter 13 may also be
integrated at the outlet end of the NO.sub.x storage catalytic
converter 6. Alternatively, the NO.sub.2-producing catalytic
converter 13 may also be arranged upstream of the SCR catalytic
converter 7. As a further possible option, it is also possible for
the NO.sub.2-producing catalytic converter 13 to be integrated at
the inlet end of the SCR catalytic converter 7. Alternatively, the
NO.sub.2-producing catalytic converter 13 can also be dispensed
with altogether.
[0074] Although this is not illustrated in FIG. 2, it is
additionally possible for a further oxidation catalytic converter
to be connected upstream of the NO.sub.x storage catalytic
converter 6, which can further lower the emissions of HC and CO in
particular during a cold start.
[0075] As a further variant, FIG. 3 shows an exhaust gas
aftertreatment installation which differs from that shown in FIG. 2
by virtue of the fact that the positions of particulate filter 5
and SCR catalytic converter 7 are switched, and the oxidation
catalytic converter 4 is connected directly upstream of the
particulate filter 5. Moreover, the other components, i.e. the
various sensors 8, 9, 10, 14, the devices 11 for supplying reducing
agent downstream of the engine and the secondary air feed device 12
are arranged at modified positions suitably matched to the
associated exhaust gas purification components 4 to 7, 13 in the
exhaust train 3, in the same way as was the case in the example
shown in FIG. 2 with respect to that shown in FIG. 1. As in the
exemplary embodiment explained above with reference to FIG. 2, the
following text will deal only with those measures and resulting
effects of the example shown in FIG. 3 which differ from those of
the examples shown in FIGS. 1 and 2, while otherwise reference can
be made, in connection with the corresponding functions and
properties to the statements made above in connection with the
examples illustrated in FIGS. 1 and 2.
[0076] The NO.sub.2-producing catalytic converter 13, which in the
example shown in FIG. 3 is connected directly upstream of the SCR
catalytic converter 7, once again allows the efficiency of the SCR
catalytic converter 7 to be noticeably increased, in particular in
the temperature range below 300.degree. C.
[0077] In this case, the device 11 for supplying reducing agent
downstream of the engine which is connected upstream of the
oxidation catalytic converter 4 before the particulate filter 5 can
be used to heat the particulate filter 5. In this arrangement, the
NO.sub.x storage catalytic converter 6 and the SCR catalytic
converter 7 are not thermally loaded by this measure. Heating of
the particulate filter exclusively by the supply of reducing agent
downstream of the engine is not desirable, in order not to cause
excessive thermal aging of the oxidation catalytic converter 4. The
quantity of reducing agent supplied is controlled, inter alia, as a
function of the following parameters: the desired temperature of
the particulate filter 5, the actual exhaust gas temperature
upstream of the oxidation catalytic converter 4 or downstream of
SCR catalytic converter 7, the actual exhaust gas temperature
downstream of the particulate filter 5 and the mass flow of exhaust
gas.
[0078] An advantage of the example shown in FIG. 3 is that in
addition to the NO.sub.x storage catalytic converter 6, the SCR
catalytic converter 7, by virtue of its position downstream of the
NO.sub.x storage catalytic converter 6, reaches the required
operating temperature relatively quickly after a cold start. A
further advantage is that there is no need for an additional
oxidation catalytic converter as a final component of the system in
order to oxidize the undesirable exhaust gas components by blowing
in secondary air. Moreover, in this exemplary embodiment the SCR
catalytic converter 7 is protected from high temperatures during
the thermal regeneration of the particulate filter 5, since it is
positioned upstream of the particulate filter 5.
[0079] The particulate filter 5 may be catalytically coated, in
which case the catalytic coating may be similar to that of the
upstream oxidation catalytic converter 4, so that the latter can be
made smaller or given a lower precious metal content, which leads
to space and/or cost benefits. It is optionally also possible for
the oxidation catalytic converter connected upstream of the
particulate filter 5 to be dispensed with altogether.
[0080] The NO.sub.2-producing catalytic converter 13 may, as shown
in FIG. 3, be designed as a separate component. Alternatively, it
may be integrated into the relevant component at the outlet end of
the NO.sub.x storage catalytic converter 6 or at the inlet end of
the SCR catalytic converter 7 or may be dispensed with altogether.
In an embodiment which is not illustrated, it is possible for a
further oxidation catalytic converter to be connected upstream of
the NO.sub.x storage catalytic converter 6, which can further
reduce the HC and CO emissions in particular during a cold
start.
[0081] FIG. 4 shows a further variant, in which the exhaust gas
aftertreatment installation has a combined SCR and nitrogen oxide
storage catalytic converter 15, which combines the functions of the
NO.sub.x storage catalytic converter 6 and the SCR catalytic
converter 7 from the examples shown in FIGS. 1 to 3. An oxidation
catalytic converter 4 is connected directly upstream of both this
integrated catalytic converter 15 and the downstream particulate
filter 5. The other components in accordance with the examples
shown in FIGS. 1 to 3 have been arranged suitably for the
positioning of these exhaust gas purification components 4, 5, 15,
as shown.
[0082] The integrated catalytic converter 15 can be realized, for
example, by using an SCR catalytic converter which is produced as
an unsupported extrudate, with the catalytic material of the SCR
catalytic converter serving as a support for a further catalytic
coating, specifically a nitrogen oxide storage catalyst coating.
Alternatively, it is also possible, in the case of an SCR catalytic
converter which is not produced as an unsupported extrudate, to
apply a nitrogen oxide storage catalyst coating in addition to the
SCR coating. In any case, it is advantageous that this measure
considerably reduces the space required. Moreover, both functional
components reach the required operating temperature very soon after
a cold start, which means that scarcely any additional heating
measures, which would increase the fuel consumption, are
required.
[0083] During standard lean-burn operation of the internal
combustion engine 1, CO and HC are oxidized at the oxidation
catalytic converter 4 close to the engine, to form CO.sub.2 and
H.sub.2O. A significant proportion of the nitrogen oxide contained
in the exhaust gas is temporarily stored in the integrated nitrogen
oxide storage and SCR catalytic converter 15, while the remainder
is reduced by ammonia which is temporarily stored therein.
[0084] In the example shown in FIG. 4, the times of NSC
regenerations are determined with the aid of the NO.sub.x sensor 9
downstream of the integrated, combined catalytic converter 15, or
alternatively by one of the other methods described above. In this
case, the models stored in the control unit 2 comprise a model
relating to the nitrogen oxide storage properties, the ammonia
storage properties and the ammonia generation properties of the
combined catalytic converter 15.
[0085] In the example shown in FIG. 4, the temperature of the
combined catalytic converter 15, which can be recorded by the
downstream temperature sensor 8, is an important criterion for the
operating parameters of an impending NSC regeneration and targeted
generation of ammonia. In addition, it is also possible, for
example, for the current ammonia loading, determined on the basis
of a model, of the combined catalytic converter 15 to be used as a
further criterion. Maximum formation of NH.sub.3 may be aimed for,
for example, if the temperature of the combined catalytic converter
15 is within a predetermined range of, for example, between
230.degree. C. and 370.degree. C. and the current NH.sub.3 loading
of the combined catalytic converter 15 is low, and also any further
conditions are satisfied.
[0086] The device 11 for supplying reducing agent downstream of the
engine which is arranged upstream of the oxidation catalytic
converter 4 close to the engine can be used to heat the integrated,
combined catalytic converter 15 for the purpose of desulfating of
the nitrogen oxide storage catalyst coating, with the support of
additional engine heating measures, as explained above. The
quantity of reducing agent supplied is controlled, inter alia, as a
function of the following parameters: the desired temperature of
the combined catalytic converter 15, the actual temperature of the
combined catalytic converter 15 and the mass flow of exhaust
gas.
[0087] The oxidation catalytic converter 4 connected upstream of
the particulate filter 5 and the device 11 for supplying reducing
agent downstream of the engine which is connected upstream of this
oxidation catalytic converter 4 can be used to heat the particulate
filter. In this arrangement, the combined nitrogen oxide storage
and SCR catalytic converter 15 once again remains free of the
thermal loading caused by the particulate filter heating. The
quantity of reducing agent supplied is controlled, inter alia, as a
function of the following parameters: the desired temperature of
the particulate filter 5, the actual exhaust gas temperature
upstream of the oxidation catalytic converter 4, the actual exhaust
gas temperature downstream of the particulate filter 5 and the mass
flow of exhaust gas.
[0088] It is advantageous in the exemplary embodiment shown in FIG.
4 that the combined nitrogen oxide storage and SCR catalytic
converter 15 is protected from high temperatures during the thermal
regeneration of the particulate filter 5 and there is no need for
an additional oxidation catalytic converter as a final exhaust gas
purification system component for oxidizing the undesirable exhaust
gas components when secondary air is blown in. The particulate
filter 5 may be catalytically coated, in particular with a
catalytic coating similar to that of the upstream oxidation
catalytic converter 4. In this case the upstream oxidation
catalytic converter 4 can be designed to be smaller or with a lower
precious metal content, which leads to corresponding space and/or
cost benefits. It is optionally also possible for the oxidation
catalytic converter 4 connected upstream of the particulate filter
5 to be dispensed with altogether.
[0089] The SCR catalyst coating and the nitrogen oxide storage
catalyst coating can be applied to a support in suitably mixed form
in the combined catalytic converter 15. Alternatively, it is
possible for the two coatings to be applied alternately in the
direction of flow of the exhaust gas, so that the exhaust gas
firstly flows through a region comprising nitrogen oxide storage
catalyst coating, then a region comprising SCR catalyst coating,
then another region comprising nitrogen oxide storage catalyst
coating, etc. This embodiment can also be realized by alternately
arranging disks of a nitrogen oxide storage catalytic converter and
disks of an SCR catalytic converter in series in a housing. The
repeatedly alternating arrangement in the direction of flow of the
exhaust gas has the advantage over a mixed combination of the two
catalyst materials of resulting in a wider temperature window in
which the system operates efficiently. For example, temperature
peaks in the front region of the system have a less pronounced
effect, for example during the formation of ammonia, if nitrogen
oxide storage catalytic converter and SCR catalytic converter are
in a repeatedly alternating arrangement than if an arrangement
comprising a nitrogen oxide storage catalytic converter and a
separate, downstream SCR catalytic converter is used.
[0090] FIG. 5 shows an exemplary embodiment in which the internal
combustion engine 1 has a two-flow section of the exhaust train 3,
with a suitably assigned exhaust gas aftertreatment installation.
The latter comprises an NO.sub.x storage catalytic converter 6, an
NO.sub.2-producing catalytic converter 13 and an SCR catalytic
converter 7, as well as associated sensor means 8, 9, 10 and device
11 for supplying reducing agent downstream of the engine, at
suitable positions in accordance with the examples shown in FIGS. 1
to 4, arranged identically and symmetrically in each of the two
parallel exhaust train sections. The two parallel exhaust trains
are combined downstream of the two SCR catalytic converters 7 to
form a subsequent single exhaust train section, in which the
particulate filter 5, as well as an oxidation catalytic converter 4
connected directly upstream of it and suitable sensor means 8, 9,
14, device 11 for supplying reducing agent downstream of the engine
and device 12 for supplying secondary air, are arranged, as
shown.
[0091] A two-flow arrangement of this type may be appropriate, for
example, for engines whereof the cylinders are in a V arrangement.
It is a required condition that if exhaust gas turbo-charging is
being used, a separate turbocharger be employed for each cylinder
bank, since otherwise the two flows have to be combined upstream of
the turbine of the turbocharger. The operating strategy for this
exhaust gas aftertreatment system is such that only minor
modifications are required compared to the operating mode of a
single-flow system. This is important, since otherwise expensive
new development of additional functions would be required in the
control unit 2 of the internal combustion engine 1. Moreover, this
approach allows the number of sensors used, and therefore the
increased costs for the system as a whole, to be limited. The basic
concept is for the two parallel trains to be considered as a single
train, so that in principle the same operations take place in both
trains. This is only possible if the two trains do not
fundamentally differ with regard to untreated engine emissions,
catalytic converter types and catalytic converter volumes, etc.
More complex approaches, such as for example trains which are
operated completely independently of one another and also, for
example, time-offset phases of rich exhaust gas, so that lean
exhaust gas is always present downstream of the point where the two
trains are combined, are alternative options.
[0092] The operating mode of this two-flow system substantially
corresponds to that of the single-flow system illustrated in FIG.
3, in which the exhaust gas purification components 4 to 7 and 13
are arranged in the same order, and consequently in this respect,
reference can be made to the explanations given above in the
example shown in FIG. 3. The corresponding processes in each case
take place independently and in parallel in the two parallel
exhaust train sections. A simple functional structure can be
maintained for the control unit 2 by virtue of the fact that in the
stored model the fundamentally identical catalytic converters 6 and
7, which are in each case arranged in parallel, of the two parallel
exhaust train sections are in each case combined to form a single
catalytic converter of double the volume. In one advantageous
realization, if differences are detected between the two train
sections with regard to the demand for NSC regeneration, it is
possible to provide for an NSC regeneration to be initiated even if
NO.sub.x emissions which exceed the predetermined threshold value
are detected at only one of the two NO.sub.x storage catalytic
converters 6. Alternatively, the signals from the NO.sub.x sensors
9 downstream of the NO.sub.x storage catalytic converters 6 can be
used in averaged or other weighted form as a representative
nitrogen oxide concentration downstream of the NO.sub.x storage
catalytic converter for the NSC model in the control unit 2.
[0093] When the aim is for high levels of ammonia to be formed, one
of the two lambda sensors 10 downstream of the two parallel
NO.sub.x storage catalytic converters 6 measuring a value below the
associated threshold value .lamda..sub.1 can be used as a criterion
for terminating NSC regenerations. If the aim is for no ammonia to
be formed, the NSC regeneration is, for example, terminated as soon
as the air ratio downstream of one of the two parallel NO.sub.x
storage catalytic converters 6 drops below the threshold value
.lamda.2.
[0094] An advantage of the exemplary embodiment shown in FIG. 5 is
that, despite the geometry of the exhaust system differing
significantly, the operating strategy remains similar to that used
for the single-flow system shown in FIG. 3, and the properties and
effects explained above in connection with this single-flow system
are achieved in the same way, without any significant additional
functions having to be provided for the control unit 2.
[0095] As an alternative to the example shown, it is also possible
for the two parallel exhaust train sections to be combined to form
a subsequent single-flow train section immediately downstream of
the two parallel NO.sub.x storage catalytic converters 6; in this
case, there is then only one SCR catalytic converter 7 with
upstream optional NO.sub.2-producing catalytic converter 13,
located in this single-flow train section. In further alternative
embodiments, it is possible to use integrated nitrogen oxide
storage and SCR catalytic converters in each of the plurality of
train sections, in accordance with the example shown in FIG. 4,
and/or to use a different order of the exhaust gas purification
components, in particular in accordance with FIGS. 1, 2 and 4.
[0096] Whereas the exemplary embodiments shown refer to an internal
combustion engine of a motor vehicle, it will be understood that
the exhaust gas aftertreatment installation according to the
invention and the exhaust gas aftertreatment method according to
the invention, as illustrated and explained above with reference to
the examples shown, can also be used for other, for example
stationary combustion devices which generate an exhaust gas which
needs to be purified.
* * * * *