WO2004111401A1

WO2004111401A1 - Device and method for purifying the exhaust gas of an internal combustion engine

Info

Publication number: WO2004111401A1
Application number: PCT/DE2004/000592
Authority: WO
Inventors: Johannes Schaller; Wolfram Veigel; Hartmut Lueders; Christian Walz
Original assignee: Robert Bosch Gmbh
Priority date: 2003-05-27
Filing date: 2004-03-23
Publication date: 2004-12-23
Also published as: DE10324013A1

Abstract

Disclosed are a device and a method for purifying the exhaust gas of an internal combustion engine, in which the exhaust gas flows through a storage catalytic converter in a first step and an arrangement for performing selective catalytic reduction in another step. The arrangement for performing selective catalytic reduction is provided with means for supplying a reducing agent downstream of the engine. The storage catalytic converter (111) is configured so as to be able to at least partly store nitrogen oxides contained in the exhaust gas as long as the arrangement (6) for performing selective catalytic reduction has not reached its operating temperature, especially during a cold start of the internal combustion engine, while releasing previously stored nitrogen oxides or allowing to pass nitrogen oxides which newly arrive from the internal combustion engine no later than the time that the arrangement for performing selective catalytic reduction has reached its operating temperature such that both the previously stored nitrogen oxides and the newly arriving nitrogen oxides are reduced upon leaving the storage catalytic converter (111).

Description

Device for cleaning the exhaust gas of an internal combustion engine and method therefor

The invention relates to a device for cleaning the exhaust gas of an internal combustion engine with an arrangement for selective catalytic reduction. The invention further relates to a method for cleaning exhaust gases from an internal combustion engine, in which an exhaust gas stream is passed through a device for selective catalytic reduction.

State of the art

To reduce the nitrogen oxide content in oxygen-rich exhaust gas, as is the case in particular

Diesel internal combustion engines and is emitted by internal combustion engines with gasoline direct injection, it is known to introduce a reducing agent into an exhaust tract. A suitable reducing agent is, for example, NH ₃ , which can be introduced as a gas into the exhaust gas stream. In this so-called selective catalytic reduction (SCR, “selective catalytic reduction”), the ammonia is contained in the exhaust gas

Nitrogen oxides selectively converted to molecular nitrogen and water. One problem the insufficient activity of the known SCR system is to be considered at exhaust temperatures below about 25O ⁰ C. A prescreening of an oxidation catalyst the one hand ensures a reduction in the proportions of deactivating acting hydrocarbons and on the other hand for oxidation of NO to NO _2, which overall leads to a substantial increase in _NOx conversion at exhaust gas temperatures above about 150 ⁰ C. In particular, when used in cars but phases occur with even lower exhaust temperatures relatively frequently, which have an average catalyst temperature of less than 18O ⁰ C in known MVEG test cycle (MVEG: Motor Vehicles Emissions Expert Group; an expert group of the European Commission).

In order to ensure a good distribution of the reducing agent on the SCR catalytic converter, a mixing distance of approx. 40 cm can be provided, which may be with a

Mixing device is provided. Such a mixing device for an exhaust gas cleaning system is described in the older German patent application with the file number 101 31 803.0. Here, a mixing body arranged in the exhaust pipe has a gas impact surface and a beam impact surface, so that exhaust gas flowing from the internal combustion engine onto the gas impact surface and transversely to the

Exhaust gas feed reductant can hit the jet impact surface.

DE 197 40 702 provides, by connecting an adsorption catalyst upstream of a catalyst for selective catalytic reduction, to further reduce the nitrogen oxide emission, in particular when the internal combustion engine is warming up, than is the case with conventional ones

Systems for selective catalytic reduction is possible. The adsorption catalyst is regenerated by supplying fuel after the engine.

Advantages of the invention

The device according to the invention and the method according to the invention have the advantage, in contrast, that denitrification of the exhaust gas is ensured with simple means even at low exhaust gas temperatures, the storage catalytic converter regenerating itself without additional means in a regulating manner depending on the temperature of the exhaust gas and the nitrogen oxides released can be eliminated by the existing arrangement for selective catalytic reduction. The storage catalytic converter thus effectively buffers nitrogen oxides as long as the arrangement for selective catalytic reduction has not yet reached its working temperature.

The measures listed in the dependent claims are advantageous

Further developments and improvements of the devices or methods specified in the independent claims are possible.

It is particularly advantageous to additionally provide an oxidation catalyst. With such a configuration of the oxidation and the so-called SCR catalyst (Arrangement for selective catalytic reduction), a reduction in the NO _x emission limit values can be achieved below an amount of NO _x emitted, which ensures compliance with the permissible exhaust gas standards during the MVEG test cycle. Such a reduction in the NO _x emission can be achieved in that the temperature-resistant one that is already present and used for nitrogen oxidation

Oxidation catalyst is used in addition to NOx reduction during a cold start phase. In a close-fitting position of the oxidation catalyst has reached a temperature of more than 100 ⁰ C after about 50 seconds, which is a NO _x for reduction by means of NH3 or an NH3-releasing reducing agent is sufficient. Oxidation catalysts mainly have as an active component

Precious metals such as platinum. This favors oxidation reactions of hydrocarbons, carbon monoxide and nitrogen monoxide even at low temperatures. If NH3 is injected as a reducing agent, these catalysts show a relatively strong De-NO _x activity even at temperatures below 100 ^° C.

If a configuration with a changeover device is chosen instead of a separate feed device for the oxidation catalyst, this can reduce the construction effort. In the same way, however, the invention comprises an embodiment with separate and separately controllable supply devices for reducing agents.

The switchover device for optionally supplying the reducing agent into the exhaust gas flow upstream or into the oxidation catalytic converter or into the SCR catalytic converter can be designed as a valve, in particular as a 3/2-way valve. In this way, the reducing agent can be supplied either to the oxidation catalytic converter or to the SCR catalytic converter, depending on which temperature levels they have reached while driving.

One embodiment of the invention provides that the switching device is designed as a mixing valve. In this way, simultaneous loading of the oxidation and SCR catalysts can take place during a transition period

Reducing agents take place. With such a mixing valve, an abrupt switchover can be avoided, so that an optimal cleaning effect can be achieved depending on the operating temperatures of the catalysts. The switching device is preferably temperature-controlled, so that during a cold start phase with still low exhaust gas temperatures, the oxidation catalytic converter and after a warm-up phase, the SCR catalytic converter can be charged with reducing agent.

The feed device preferably comprises in each case a metering device for metering quantities and nozzles for distributing or atomizing the reducing agent in the exhaust gas stream.

The at least one oxidation catalyst is preferably in the immediate vicinity of one

Exhaust gas outlet of the internal combustion engine arranged so that it reaches relatively high temperatures and thus a high cleaning effect after only a short time.

An ammonia-containing or ammonia-releasing substance, for example, can be used as the reducing agent, which can bring about NO _x reduction. Urea or ammonium carbamate are examples of such substances.

In a method according to the invention for cleaning exhaust gases of an internal combustion engine, in which an exhaust gas stream is passed through at least one oxidation catalytic converter arranged in the exhaust gas duct and a device for selective catalytic reduction (SCR catalytic converter) connected downstream thereof, a reducing agent is supplied to the exhaust gas stream, which according to the invention the exhaust gas flow is optionally supplied upstream of or within the at least one oxidation catalytic converter. Optionally, the reducing agent is fed to both catalysts or only one of the catalysts at a time. The

Reducing agent is preferably distributed or atomized by means of a nozzle.

One embodiment of the method according to the invention provides a temperature-controlled supply of the reducing agent into the oxidation catalyst and / or into the device for selective catalytic reduction.

If NH3 is applied to the oxidation catalyst, it shows a relatively pronounced de-NO _x activity at temperatures above 100 ^° C. However, the usable temperature window for NO _x reduction is relatively limited, because above con about 250 ° C to 300 ⁰ C there is no more nitrogen oxide reduction, but an additional Nitrogen oxide production is observed through an oxidation of ammonia. It must therefore be ensured that the oxidation catalyst is only exposed to the reducing agent in a starting phase (in the MVEG test only up to about 350 s). Preferably, the reducing agent into the oxidation catalytic converter at exhaust gas temperatures of less than about 15O ⁰ C to 200 ⁰ C is fed in the oxidation catalyst.

After such a period of time, the SCR catalytic converter has normally also reached its working temperature and the reducing agent injection is switched over to the SCR catalytic converter. This can be done at temperatures of about 150 ⁰ C to 200 ⁰ C in the SCR catalyst. Reducing agent metering onto the oxidation catalytic converter is in principle possible at operating points with a low exhaust gas temperature - i.e. not only during a cold start - and provides a very effective NOx reduction potential where the SCR catalytic converter achieves insufficient activity. With an injection in front of the oxidation catalyst up to a time of about 600 s, a significant increase in sales of the exhaust gas cleaning system can be achieved. A suitable meaningful switching point of the temperature-controlled switching valve can at 100 to 200 ° C, preferably at 130 to 180 ⁰ C.

A practical embodiment of the system can, for example, provide a 3/2-way changeover valve, which depends on the catalyst temperatures and the

Engine operating point is operated. Such an upgrade of an existing system is relatively simple and can be accomplished with little effort. The catalyst system, the temperature sensors and the dosing system are already available, although these components do not have to be modified. Only the changeover valve and the reducing agent supply line upstream of the oxidation catalytic converter need to be retrofitted. A suitable dosing strategy can effectively reduce nitrogen oxides over an entire test cycle (MVEG cycle). In the NEDC test, a _NOx -Umsatzsteigerung of about 40% can be achieved, so that with reduced raw emissions itself a fulfillment of the relatively strict US standards mö - ¹ Gol I ^1.

In a preferred embodiment, the oxidation catalyst can be designed as a catalytically coated particle filter. The catalytic coating of the particle filter has a similar effect to the coating of a known oxidation catalyst. Furthermore, between the oxidation catalyst and the SCR catalyst, a separate particle filter can be provided, which effects a filtering of the soot particles.

Furthermore, the oxidation catalytic converter can be omitted and the additional injection position is in front of the storage catalytic converter. In this arrangement, the platinum-containing storage catalytic converter takes over the function of the oxidation catalytic converter.

The invention is explained in more detail below with reference to the accompanying drawing. The only FIG. 1 shows a schematic illustration of an internal combustion engine with an exhaust gas aftertreatment unit in an exhaust gas duct.

In an exhaust gas duct 28 of an internal combustion engine 2, an oxidation catalytic converter 4 and a device for selective catalytic reduction, referred to as an SCR catalytic converter 6, are arranged. The internal combustion engine 2 has an inlet duct 21 for the supply of fresh gas 22 and outlet ducts 26 which are in a collector 27 to the exhaust duct

28 are merged. An exhaust gas turbine 24 of an exhaust gas turbocharger 23 is arranged in the exhaust gas duct and is coupled via a shaft 25 to a compressor (not shown here). The exhaust gas turbocharger 23 is optional and serves to improve the performance and exhaust gas behavior of the internal combustion engine 2.

The internal combustion engine 2 is preferably a diesel internal combustion engine with auto-ignition or a gasoline engine with direct fuel injection. Both types of engine each emit a relatively oxygen-rich exhaust gas. The exhaust gas stream 29 successively passes through a storage catalytic converter 111, the oxidation catalytic converter 4 and the SCR catalytic converter 6 and leaves the exhaust gas cleaning system as cleaned

Exhaust gas 14, which is passed into the open via a silencer (not shown).

The exhaust gas cleaning system furthermore has a feed device 8 for feeding a reducing agent 81 into the exhaust gas stream 29. The feed device 8 comprises a switchover device 83 as well as a first connecting line 84 connected to a first nozzle 85 and a second connecting line 86 connecting to a second nozzle 87. The first nozzle 85 is arranged and serves in the exhaust gas duct 28 upstream of the oxidation catalytic converter 4 for fine distribution or atomization of the reducing agent 81 upstream of the oxidation catalytic converter 4. The second nozzle 87 is upstream of the SCR catalytic converter 6 and downstream of the oxidation catalytic converter 4 arranged and serves to supply reducing agent 81 into the exhaust gas stream 29 upstream of the SCR catalytic converter 6.

First and second connecting lines 84, 86 open into the switching device 83, which are used for an optional distribution of the reducing agent to the first and / or second

Connection line 84, 86 can provide. The control of the switching device 83 is preferably temperature-dependent, so that the oxidation catalytic converter 4 can be acted upon in a cold-running phase and, after a certain temperature has been reached, the reducing agent 81 can be applied to the SCR catalytic converter 6.

The storage catalytic converter located between the exhaust gas turbocharger 23 and the nozzle 85, ie in front of the catalytic converter arrangement 4, can in particular be arranged close to the engine. A storage catalytic converter provided in this way stores the nitrogen oxide emissions of the engine in the low temperature range below 100 ^° C. After a certain time, the storage capacity of the storage catalytic converter is exhausted and this allows the nitrogen oxide molecules to pass through. If the storage catalytic converter is heated further, its nitrogen oxide storage capacity decreases and the previously stored amount of nitrogen oxides is released again. The storage catalytic converter arranged close to the engine thus causes a delayed release of the nitrogen oxides. The time delay here is given the appropriate dimensioning of the

Storage catalyst preferably at least 300 seconds, so that after exhausting the storage capacity of the storage catalyst, the downstream system for selective catalytic reduction has reached its working temperature and can reduce the nitrogen oxides to nitrogen and water. The storage catalytic converter should have the following properties:

Nitrogen oxide storage even at low temperatures; Decrease in the storage capacity already at temperatures of approx. 300 ^° C. in order to ensure that the storage catalytic converter is driven empty towards the end of a driving cycle during normal driving operation and is available for nitrogen oxide storage at the following cold start; good regenerability solely due to heating, especially good

Desulfation properties. A storage catalytic converter that contains A12O3, CeO2 and ZrO2 and contains platinum in a concentration of approx. 90 g per cubic foot can be used to meet the required properties. In a cold-running phase of the exhaust gas purification system, only the first nozzle 85 is acted on with reducing agent by appropriate setting of the switching device 83

Subsequently, when the catalytic converters have already reached a predetermined operating temperature, the second connecting line 86 and the second nozzle 87 are acted on with reducing agent 81.

A typical transition temperature can be approximately 100 ^° C. to 200 ^° C., preferably approximately 130 to 180 ^° C., above which a switchover to

The reducing agent 81 can be applied to the SCR catalytic converter 6. Switching can advantageously also take place by means of a mixing valve, which can ensure simultaneous action on the oxidation catalytic converter 4 and the SCR catalytic converter 6 in the region of the transition temperature. The choice of the transition temperature is explained by a clever use of the different operating temperatures of the oxidation catalytic converter and the SCR catalytic converter. This fact is already described in Figures 4 to 6 of the German patent application with the file number 10162383.6; this description is hereby incorporated into the present patent application.

The invention also encompasses an alternative configuration which provides two separate feed devices for the oxidation catalytic converter and for the SCR catalytic converter.

In an alternative embodiment, the oxidation catalyst 4 can be a catalytically coated particle filter which, due to its catalytic coating, has the same effect as a known oxidation catalyst. In addition to the configuration shown, a separate particle filter can be arranged between the oxidation catalytic converter 4 and the SCR catalytic converter 6. This brings about a further improvement in the cleaning effect of the exhaust gases.

In a further alternative embodiment, the storage catalytic converter and the oxidation catalytic converter or the storage catalytic converter and the catalytically coated filter can be integrated in a single component, so that the nozzle 85 is arranged in front of this single component. This can be done by using an oxidation catalyst Nitric oxide storage capacity. The simultaneous integration of the particle filter takes place via catalytic coating of a particle filter.

In a simple embodiment, the oxidation catalyst can also be omitted.

Claims

Expectations

1. Device for cleaning the exhaust gas of an internal combustion engine, in particular an internal combustion engine with auto-ignition and / or with direct fuel injection, in which the exhaust gas flows through a storage catalytic converter in a first step and an arrangement for selective catalytic reduction in a further step, the arrangement for selective catalytic reduction Reduction means for the post-engine supply of a reducing agent, characterized in that the storage catalytic converter (111) is designed such that it can at least partially store nitrogen oxides contained in the exhaust gas, in particular when the internal combustion engine is cold started, as long as the

Arrangement (6) for selective catalytic reduction has not yet reached its working temperature, and that the storage catalytic converter (111) releases stored nitrogen oxides at the latest when the arrangement for selective catalytic reduction has reached its working temperature, or nitrogen oxides newly flowing in from the internal combustion engine pass through leaves, so that both the nitrogen oxides stored up to that point and the newly flowing nitrogen oxides are reduced after leaving the storage catalytic converter (111).

2. Device according to claim 1, characterized in that an oxidation catalytic converter (4) is arranged between the storage catalytic converter (11 1) and the arrangement (6).

3. Device according to claim 2, that the means for the post-motor reducing agent supply have a switching device (83), so that the Reducing agent can optionally be introduced into the exhaust gas stream upstream of the at least one oxidation catalytic converter.

4. The device according to claim 2, that the means for the post-motor reducing agent supply have a switching device (83), so that the

Reducing agent can optionally be introduced into the at least one oxidation catalyst.

5. The device according to claim 3 or 4, characterized in that the switching device is designed as a valve.

6. The device according to claim 5, characterized in that the valve is a 3/2-way valve.

7. The device according to claim 5, characterized in that the valve as

Mixing valve is formed.

8. Device according to one of claims 3 to 7, characterized in that the switching device is temperature controlled.

9. The device according to claim 2, characterized in that the at least one oxidation catalyst is designed as a catalytically coated particle filter.

10. The device according to claim 2, characterized in that a particle filter is provided between the at least one oxidation catalyst and the arrangement.

11. Device according to one of the preceding claims, characterized in that the means for the post-motor reducing agent supply have a switching device, so that the reducing agent] optionally in the

Exhaust gas flow can be introduced before the storage catalyst.

12. Device according to one of the preceding claims, characterized in that the means for the post-motor reducing agent supply Have switching device so that the reducing agent can optionally be introduced into the storage catalyst.

13. A method for cleaning the exhaust gas of an internal combustion engine, in particular an internal combustion engine with auto-ignition and / or with direct fuel injection, in which the exhaust gas flows through a storage catalytic converter (111) in a first step and an arrangement (6) for selective catalytic reduction in a further step, wherein the arrangement for selective catalytic reduction has means for the post-engine supply of a reducing agent, characterized in that the storage catalytic converter, in particular during a cold start of the

Internal combustion engine, at least partially stores nitrogen oxides contained in the exhaust gas as long as the arrangement (6) for selective catalytic reduction has not yet reached its working temperature, and that the storage catalytic converter has stored nitrogen oxides by then at the latest when the arrangement for selective catalytic reduction has reached its working temperature releases or allows new nitrogen oxides flowing in from the internal combustion engine to pass, so that both the nitrogen oxides stored up to that point and the newly flowing nitrogen oxides are reduced after leaving the storage catalytic converter.