Method and Device for Purifying an Exhaust Gas Flow of a Lean-Burning Internal Combustion Engine

Abstract

A method and device for purifying an exhaust gas flow of a lean-burning internal combustion engine, with a particle filter which stores particles both in rich and lean operation of the internal combustion engine and which has at least one catalytically active component on which in lean operation of the internal combustion engine defined exhaust gas components, in particular carbon monoxides and/or hydrocarbons are oxidized, the particle filter furthermore having at least one catalytically active component on which in rich operation of the internal combustion engine defined nitrogen oxides are converted into ammonia using hydrogen present in the exhaust gas flow, the ammonia being temporarily stored in at least one SCR catalytic converter which is connected downstream to the particle filter in the exhaust gas flow direction, and the SCR catalytic converter having at least one catalytically active component on which in lean operation of the internal combustion engine defined nitrogen oxides are reduced using the ammonia which has been temporarily stored in the SCR catalytic converter by selective catalytic reduction (SCR).

Description

The invention relates to a method and device for purifying an exhaust gas flow of a lean-burning internal combustion engine, in particular a diesel internal combustion engine.

BACKGROUND OF THE INVENTION

For adherence to future emission boundary values, future generations of motor vehicles, in particular diesel motor vehicles, must meet still more rigid requirements than is currently the case. In particular, this relates to oxidation of exhaust gas components from incomplete combustion, removal of soot particles from the exhaust gas flow and the reduction of nitrogen oxides.

In order to meet these requirements, DE 699 16 312 T2 discloses, in conjunction with a diesel internal combustion engine, providing an exhaust gas purification device which has a housing on whose inlet end there is a catalytic converter which is designed as a low-temperature starter oxidation catalytic converter. This catalytic converter is designed to satisfy emission regulations with reference to CO and HC and is intended to, moreover, convert at least 70% of the nitrogen monoxides (NO) contained in the exhaust gas flow into nitrogen dioxide (NO₂). The exhaust gas leaving this catalytic converter then flows into a soot filter in which the nitrogen dioxide and excess oxygen oxidize the soot at a temperature of approximately 250° C. The gas leaving the soot filter then flows by way of an injector by means of which a NO_x-specific reactant, specifically ammonia, is metered into the exhaust gas flow from a storage tank. This mixture then enters a NO_x absorber which in lean operation of the internal combustion engine (operation with an air excess or lambda greater than 1) and without metering of ammonia is to be able to remove all the flowing NO_x. But for the case in which ammonia is metered, the NO_x is converted into N₂. This NO_x—, O₂— and ammonia-containing exhaust gas stream then flows into a SCR catalytic converter in which these reactants are brought essentially into chemical equilibrium with the pollutant-free gases. In this manner of operation, in particular; the metering of ammonia as a reducing agent results in the danger that unwanted ammonia breakthroughs will occur and thus ammonia will be released into the environment. Since ammonia is a toxic gas, this process is of little practicability without additional control mechanisms for ammonia breakthroughs.

DE 600 01 421 D2 discloses a similar one with an oxidation catalytic converter, a soot filter and a SCR catalytic converter in which ammonia is metered as a reducing agent directly upstream from the SCR catalytic converter.

A similar structure is also disclosed by WO 01/96717 A1 in which in a housing there are to be an oxidation catalytic converter, a particle filter and a NO_x absorber or SCR catalytic converter in this sequence relative to the main flow direction of the exhaust gas. By way of a line which is routed to the exhaust line upstream from the oxidation catalytic converter, depending on whether a NO_x absorber or a SCR catalytic converter is located in the structure, either a regeneration agent for regeneration of the NO_x absorber or ammonia for selective catalytic reduction on the SCR catalytic converter is added. A fundamentally similar structure is furthermore also known from WO 03/054364 A1.

These latter named processes managements also have the disadvantages referred to above in conjunction with DE 699 16 312 T2.

Conversely, it is the object of this invention to make available a method and a device for purifying an exhaust gas flow of a lean-burning internal combustion engine, in particular a diesel internal combustion engine, by means of which carbon monoxides, hydrocarbons and nitrogen oxides as well as carbon-containing particles, in particular soot particles, can be removed from the exhaust gas flow with high operating reliability in a manner which favors construction and thus production technology.

SUMMARY OF THE INVENTION

According to the invention, there is a particle filter which stores particles both in rich and also in lean operation of the internal combustion engine and which has at least one catalytically active component on which in lean operation of the internal combustion engine defined exhaust gas components, in particular carbon monoxides (CO) and/or hydrocarbons (HC), are oxidized. Furthermore, the particle filter has at least one catalytically active component on which in rich operation of the internal combustion engine defined nitrogen oxides are converted into ammonia using the hydrogen present in the exhaust gas flow. This ammonia is temporarily stored in at least one SCR catalytic converter connected downstream to the particle filter in the exhaust gas flow direction, the SCR catalytic converter having at least one catalytically active component on which in lean operation of the internal combustion engine defined nitrogen oxides are reduced using the ammonia which has been temporarily stored in the SCR catalytic converter by selective catalytic reduction (SCR).

According to the invention, with this process and with this structure the four essential, limited exhaust gas components (soot particles, hydrocarbons, carbon monoxides, nitrogen oxides) can be easily and economically eliminated or effectively reduced since reliable reduction or elimination of these exhaust gas components can be achieved by switching of operating modes which is easy to manage by the engine control, in particular between rich and lean operation. This is also clue to the circumstance that such catalytically active materials which promote the chemical reaction or conversion in conjunction with the exhaust gas components CO, HC, NO_x and soot particles (4-way principle) form on the surface layer on the particle filter. Thus, for example, unoxidized or only partially oxidized exhaust gas components are converted with an excess of oxygen, i.e., thus in lean operation according to the two equations below on an oxidizing, catalytically active component of the particle filter:

2 CO+O₂→2 CO₂   (1)

4 C_XH_Y+4 X+Y O₂→2 Y H₂O+4 X CO₂   (2)

The carbon-containing particles, in particular soot and/or ash articles, are continuously stored or attached in the particle filter and continuously oxidized using the following reaction

C+2 NO₂→2 NO+CO₂   (3)

For this purpose the particle filter preferably has at least one catalytically active component on which the carbon-containing particles which have been stored or attached in the particle filter can be continuously or permanently oxidized. In addition, there can also be cyclic burn-up of carbon-containing particles, in particular soot particles, which takes place according to the following equation:

C+2 O→CO₂   (4)

For this purpose it is advantageous to heat up the exhaust gas flow by supplying heat in order to raise the efficiency of oxidation and thus the efficiency with respect to the burn-up of soot particles. This heat can be supplied in a number of ways. In particular, preferably viewed in the exhaust gas flow direction upstream from the particle filter a given amount of fuel can be metered into the exhaust gas flow (controlled afterinjection). Upstream from the particle filter there is an oxidation catalytic converter in which defined exhaust gas components are at least partially exothermally oxidized, in particular hydrocarbons, carbon monoxides and nitrogen monoxides, and, in the absence of oxygen, hydrogen. Such an oxidation catalytic converter can thus be used at the same time in an advantageous double function for heating of the exhaust gas flow and in its traditional function for oxidation of certain exhaust gas components, in particular of nitrogen monoxide into nitrogen dioxide.

With the solution according to the invention a four-way particle filter with a SCR catalytic converter is thus made available, by means of which the four critical exhaust gas components (HC, CO, NO_x, and soot particles) can be eliminated in a manner which favors construction and thus production technology. One special advantage can be seen in particular in that according to this invention in rich operation of the internal combustion engine the ammonia is converted in the particle filter by the reaction of nitrogen oxides with the hydrogen present in the exhaust gas flow, the ammonia which has been formed in this way then again being temporarily stored in the downstream SCR catalytic converter for selective catalytic reduction in lean operation of the internal combustion engine. This is due to the circumstance that in this process and this structure the necessity of additional metering of ammonia from a separate storage tank can thus be greatly reduced or entirely eliminated, so that there is no risk of ammonia breakthroughs with the disadvantages described above in connection with the introductory part of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of the method of he present invention

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The single figure schematically shows an internal combustion engine 1 (ICE), from which the outgoing exhaust gas flow with the components carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NO_x), hydrogen (H₂) and soot particles (C) is routed in the direction to an oxidation catalytic converter 2 shown here by a broken line, in which, for example, nitrogen monoxides are oxidized by means of oxygen, i.e., in particular in lean operation with an air excess, into nitrogen dioxide. The oxidation catalytic converter 2 is shown here by a broken line; this means that it also potentially need not be provided. Furthermore, the functions of the oxidation catalytic converter can also be divided, for example, such that a smaller oxidation catalytic converter is provided upstream from the turbocharger.

A particle filter 3 in the flow direction of the exhaust gas indicated by the arrows is connected downstream from the oxidation catalytic converter 2 and it is coated in given particle filter regions with such a catalytically active component on which in lean operation (lambda greater than 1) of the internal combustion engine hydrocarbons (HC) and carbon monoxides (CO) are oxidized using oxygen into H₂O and CO₂.

Furthermore, the particle filter 3 is coated with this catalytically active component, in turn specifically in given regions on which nitrogen oxides (NO_x) are converted into ammonia (NH₃) using the hydrogen (H₂) then present in the exhaust gas flow in lean operation (lambda less than 1) of the internal combustion engine.

This ammonia which has been formed in the particle filter 3 then flows into a SCR catalytic converter 4 in which it is temporarily stored until the engine control device switches again to lean operation of the internal combustion engine so that in conjunction with a correspondingly catalytically active component on the SCR catalytic converter 4, nitrogen oxides (NO_x) are reduced using the temporarily stored ammonia (NH₃) into nitrogen (N₂) and water (H₂O).

With this structure of the exhaust gas line of an internal combustion engine of a motor vehicle according to the invention or with this process according to the invention thus a four-way particle filter 3 together with the SCR catalytic converter 4 is made available in which the limited exhaust gas components can be easily and economically eliminated or effectively reduced.

This is to say that here, on the one hand, nitrogen oxides can be converted in the particle filter with a lack of oxygen and the resulting ammonia is used to reduce the remaining nitrogen oxides with an oxygen excess in the SCR catalytic converter 4.

Claims

1. A method for purifying an exhaust gas flow of a lean-burning internal combustion engine, with a particle filter which stores particles both in rich and lean operation of the internal combustion engine and which has at least one catalytically active component on which in lean operation of the internal combustion engine defined exhaust gas components, in particular carbon monoxides and/or hydrocarbons are oxidized, the particle filter furthermore having at least one catalytically active component on which in rich operation of the internal combustion engine defined nitrogen oxides are converted into ammonia using hydrogen present in the exhaust gas flow, the ammonia being temporarily stored in at least one SCR catalytic converter which is connected downstream to the particle filter in the exhaust gas flow direction, and the SCR catalytic converter having at least one catalytically active component on which in lean operation of the internal combustion engine defined nitrogen oxides are reduced using the ammonia which has been temporarily stored in the SCR catalytic converter (4) by selective catalytic reduction (SCR).

2. The method according to claim 1 wherein the particle filter has at least one catalytically active component on which the carbon-containing particles which have been stored or attached in the particle filter (3) are continuously or permanently oxidized.

3. The method according to claim 1 wherein an oxidation catalytic converter is connected upstream from the particle filter and in it defined exhaust gas components are at least partially oxidized, in particular nitrogen monoxide (NO) is oxidized to nitrogen dioxide (NO2) by means of oxygen (O2).

4. The method according to claim 2 wherein the carbon-containing particles are continuously oxidized according to the following equation: C+2 NO2→2 NO+CO2 with C=carbon, NO2=nitrogen dioxide, NO=nitrogen oxide, CO2=carbon dioxide.

5. The method according claim 1 wherein in the exhaust gas flow direction upstream from the particle filter a given amount of fuel is metered into the exhaust gas flow at given times for oxidation, in particular for rapid oxidation of the carbon-containing particles.

6. The method according to of claim 3 to in the exhaust gas flow direction upstream from the oxidation catalytic converter fuel is metered into the exhaust gas flow in a defined manner.

7. The method according to claim 1 wherein the exhaust gas flow supplied to the particle filter is heated up to a given temperature by means of heat supply at given times, in particular by exothermal reaction of metered fuel on the oxidation catalytic converter.

8. The method according to claim 4 wherein the controlled raising of the exhaust gas temperature oxidation of the carbon-containing particles takes place according to the following equation: C+O2→CO2 with C=carbon, O2=oxygen, CO2=carbon dioxide.

9. The method according to claim 1 wherein lean operation of the internal combustion engine in an oxidation catalytic converter and/or in a particle filter carbon monoxides (CO) and hydrocarbons (CxHy) are oxidized according to the following equations: 2 CO+O2→C2   (1)4 CxHy+4 x+y O2→2 y H2O+4 x CO2   (2)with O2=oxygen, CO2=carbon dioxide, H2O=water.

10. The method according to claim 1 wherein rich operation of the internal combustion engine in the particle filter nitrogen oxides, in particular nitrogen monoxide (NO), are converted according to the following equation into ammonia (NH3): NO+NO2+2 CO 4 H2→2 NH3+H2O+2 CO2 with NO2=nitrogen dioxide, H2=hydrogen, CO=carbon monoxide, H2O=water, CO2=carbon dioxide.

11. The method according to claim 1 wherein selective catalytic reduction on the SCR catalytic converter takes place according to the following equation: 2 NM3+NO+NO2→4 N2+3 H2O

12. A device for purification of an exhaust gas flow of an internal combustion engine, in particular for executing the method according to claim 1, with at least one particle filter which is located in the exhaust gas line of the internal combustion engine and which has at least one catalytically active component on which in lean operation of the internal combustion engine defined exhaust gas components, in particular carbon monoxides and/or hydrocarbons, can be oxidized,the particle filter furthermore having at least one catalytically active component on which in rich operation of the internal combustion engine defined nitrogen oxides can be converted into ammonia using the hydrogen present in the exhaust gas flow,at least one SCR catalytic converter in which ammonia can be temporarily stored is connected downstream to at least one particle filter andthe SCR catalytic converter having at least one catalytically active component on which in lean operation of the internal combustion engine defined nitrogen oxides can be reduced by selective catalytic reduction (SCR) using the ammonia which has been temporarily stored in the SCR catalytic converter.

13. The device according to claim 12 wherein the particle filter has at least one catalytically active component on which the carbon-containing particles attached or stored in the particle filter can be continuously or permanently oxidized.

14. The device according to claim 12 wherein an oxidation catalytic converter in which defined exhaust gas components can be at least partially oxidized is connected upstream from the particle filter.

15. The device according to claim 12 wherein there is a fuel metering device by means of which fuel can be metered into the exhaust gas flow upstream from at least one particle filter.

16. The device according to claim 12 wherein a controlled temperature rise can be set by means of the metered fuel.

17. A method of treating an exhaust gas of an internal combustion engine, comprising: routing said gas through a particle filter provided with a first catalytically active component functional under lean operating conditions of said engine to oxidize carbon monoxides and hydrocarbons, and a second catalytically active component functional under rich operating conditions of said engine to produce ammonia;routing said gas through a downstream convertor provided with a third catalytically active component functional under lean operating conditions of said engine to reduce nitrogen oxides using ammonia; androuting said ammonia produced in said particle filter to said downstream converter, and temporarily storing it therein for reaction with said nitrogen oxides.

18. The method of claim 17 including oxidizing carbon containing particles in said particle filter by means of a fourth catalytically active component.

19. An assembly for treating the exhaust gas of an internal combustion engine comprising: means for routing a flow of such gas from said engine;a particulate filter disposed in said routing means, provided with a first catalytically active component functional under lean operating conditions of said engine to oxidize carbon monoxide and hydrocarbons, and a second catalytically active component functional under rich operating conditions of said engine to produce ammonia;a converter disposed in said routing means downstream of said particle filter, provided with a third catalytically active component functional under lean operating conditions of said engine to reduce nitrogen oxides using ammonia; andmeans for routing ammonia produced in0 said particle filter to said converter, and temporarily storing and ammonia therein for reaction with said nitrogen oxides.

20. An assembly according to claim 19 wherein said particle filter includes a fourth catalytically active compound functional to oxidize carbon containing particles.