EXHAUST AFTERTREATMENT SYSTEM AND METHOD FOR REGENERATING A PARTICULATE FILTER

Abstract

The invention relates to an exhaust aftertreatment system for an internal combustion engine that, at its outlet, is connected to an exhaust gas system. In the exhaust gas system, a three-way catalytic converter close to the engine, an electrically heatable three-way catalytic converter downstream from the three-way catalytic converter close to the engine, and further downstream, a particulate filter or a four-way catalytic converter are situated in the flow direction of an exhaust gas of the internal combustion engine through an exhaust duct of the exhaust gas system. In addition, the exhaust aftertreatment system includes a secondary air system with a secondary air pump and a secondary air line, which opens into the exhaust duct at an introduction point downstream from the three-way catalytic converter close to the engine and upstream from the electrically heatable three-way catalytic converter. The invention further relates to a method for regenerating a particulate filter or the four-way catalytic converter by use of such an exhaust aftertreatment system.

Claims

1. An exhaust aftertreatment system for an internal combustion engine having an exhaust gas system, comprising: a three-way catalytic converter close to the engine, a particulate filter or four-way catalytic converter situated downstream from the three-way catalytic converter close to the engine, and an electrically heatable three-way catalytic converter situated downstream from the three-way catalytic converter and upstream from the particulate filter or the four-way catalytic converter, wherein an introduction point for introducing secondary air into an exhaust duct of the exhaust gas system is provided downstream from the three-way catalytic converter close to the engine, and upstream from the electrically heatable three-way catalytic converter.

2. The exhaust aftertreatment system according to claim 1, wherein the particulate filter has a design that is free of a catalytically active coating.

3. The exhaust aftertreatment system according to claim 1, wherein the electrically heatable three-way catalytic converter has a design that is free of an oxygen store.

4. The exhaust aftertreatment system according to claim 1, further comprising a hydrogen catalytic converter situated downstream from the introduction point and upstream from the electrically heatable three-way catalytic converter.

5. The exhaust aftertreatment system according to claim 4, wherein. in the exhaust gas system, a first lambda sensor is situated upstream from the three-way catalytic converter close to the engine, and a second lambda sensor is situated downstream from the hydrogen catalytic converter and upstream from the electrically heatable three-way catalytic converter.

6. The exhaust aftertreatment system according to claim 3, wherein the particulate filter is designed with a three-way catalytically active coating as a four-way catalytic converter, and wherein, in the exhaust gas system a first lambda sensor is situated upstream from the three-way catalytic converter close to the engine, and a second lambda sensor is situated downstream from the electrically heatable three-way catalytic converter and upstream from the four-way catalytic converter.

7. The exhaust aftertreatment system according to claim 1, wherein the secondary air is provided by a secondary air system that includes a secondary air pump and a secondary air line that connects the secondary air pump to the introduction point, a secondary air valve being situated in the secondary air line.

8. A method for regenerating a particulate filter or a four-way catalytic converter in an exhaust gas system of an internal combustion engine, wherein the internal combustion engine at its outlet is connected to the exhaust gas system in which a three-way catalytic converter close to the engine, an electrically heatable three-way catalytic converter downstream from the three-way catalytic converter close to the engine, and a particulate filter or a four-way catalytic converter downstream from the electrically heatable three-way catalytic converter are situated in the flow direction of an exhaust gas through the exhaust gas system, and having a secondary air system with which secondary air is introducible into an exhaust duct of the exhaust gas system downstream from the three-way catalytic converter close to the engine and upstream from the electrically heatable three-way catalytic converter, the method comprising the following steps: activating the electrically heatable three-way catalytic converter when a regeneration of the particulate filter or of the four-way catalytic converter is requested, heating the electrically heatable three-way catalytic converter to its light-off temperature (T.sub.LO), adjusting the combustion air ratio of the internal combustion engine from a stoichiometric combustion air ratio (.sub.E=1) to an understoichiometric combustion air ratio (.sub.E<1), and at the same time, and blowing secondary air into the exhaust gas system downstream from the three-way catalytic converter close to the engine, resulting in an overstoichiometric exhaust gas-air ratio (.sub.EG>1) at the inlet of the particulate filter or the four-way catalytic converter.

9. The method according to claim 8, wherein the regeneration of the particulate filter or four-way catalytic converter is ended when the pressure difference across the particulate filter or the four-way catalytic converter drops below a threshold value.

10. The method according to claim 8, wherein the introduction of secondary air into the exhaust gas system starts with a time delay after the beginning of heating of the electrically heatable three-way catalytic converter.

11. The method according to claim 8, wherein the introduction of secondary air and the electrical heating of the electrically heatable three-way catalytic converter are ended simultaneously when the particulate filter or the four-way catalytic converter is completely regenerated.

12. The method according to claim 8, wherein, in a heating phase of the particulate filter, the internal combustion engine is operated with an understoichiometric combustion air ratio (<1), in this heating phase secondary air being blown in in such a way that stoichiometric exhaust gas results upstream from the particulate filter or the four-way catalytic converter, and the combustion air ratio and/or the secondary air blowing being adapted during the regeneration of the particulate filter or the four-way catalytic converter in such a way that overstoichiometric exhaust gas results in the exhaust duct upstream from the particulate filter or the four-way catalytic converter during the regeneration of the particulate filter or the four-way catalytic converter.

13. The method according to claim 8, wherein the electrical heating of the electrically heatable three-way catalytic converter is ended when the electrically heatable three-way catalytic converter has reached its light-off temperature, and the introduction of secondary air is not started until after conclusion of the heating operation for the electrically heatable three-way catalytic converter.

14. The method according to claim 8, wherein the regeneration of the particulate filter or of the four-way catalytic converter takes place intermittently, the electrically heatable three-way catalytic converter being electrically heated in each case when the temperature of the particulate filter or of the four-way catalytic converter has dropped below a threshold temperature.

15. The method according to claim 14, wherein the introduction of secondary air takes place continuously during the intermittent regeneration of the particulate filter or the four-way catalytic converter until the particulate filter or the four-way catalytic converter is completely regenerated.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The invention is explained below in exemplary embodiments, with reference to the associated drawings, which show the following:

[0033] FIG. 1 shows a preferred exemplary embodiment of an internal combustion engine with an exhaust aftertreatment system according to the invention;

[0034] FIG. 2 shows another exemplary embodiment of an internal combustion engine with an exhaust aftertreatment system according to the invention, with provision of an additional hydrogen catalytic converter in the exhaust gas system;

[0035] FIG. 3 shows another exemplary embodiment of an internal combustion engine with an exhaust aftertreatment system according to the invention, with the particulate filter designed as a four-way catalytic converter;

[0036] FIG. 4 shows a diagram for illustrating a method according to the invention for regenerating a particulate filter or four-way catalytic converter in the exhaust gas system of an internal combustion engine;

[0037] FIG. 5 shows a second diagram for illustrating an alternative method for regenerating a particulate filter;

[0038] FIG. 6 shows a third diagram for illustrating an alternative method for regenerating a particulate filter;

[0039] FIG. 7 shows a fourth diagram for illustrating an alternative method for regenerating a particulate filter; and

[0040] FIG. 8 shows another diagram for illustrating a method according to the invention for regenerating a particulate filter.

DETAILED DESCRIPTION OF THE INVENTION

[0041] FIG. 1 shows an internal combustion engine 10 that is spark-ignited by means of spark plugs 14. The internal combustion engine has a plurality of combustion chambers 12. The internal combustion engine 10 at its outlet 16 is connected to an exhaust gas system 20. The exhaust gas system 20 includes an exhaust duct 18 in which a turbine 32 of an exhaust gas turbocharger 30, downstream from the turbine 32 a first three-way catalytic converter 22 close to the engine, and downstream from the first three-way catalytic converter 22 an electrically heatable three-way catalytic converter 24, are situated in the flow direction of an exhaust gas through the exhaust duct 18. A particulate filter 26 is situated downstream from the electrically heatable three-way catalytic converter 24. The particulate filter 26 preferably has a design that is free of a catalytic coating. Alternatively, the particulate filter 26 may be designed as a four-way catalytic converter 27 as illustrated in FIG. 3, i.e., a particulate filter 26 with a three-way catalytically active coating. In the exhaust duct 18, a first lambda sensor 36, preferably a broadband lambda sensor, is situated downstream from the outlet 16, and upstream from the first three-way catalytic converter 22, preferably downstream from the turbine 32 of the exhaust gas turbocharger 30 and upstream from the first three-way catalytic converter 22. In the exhaust duct 18, a second lambda sensor 38, in particular a further broadband sensor or a trim lambda sensor, is situated downstream from the first three-way catalytic converter 22 and upstream from the electrically heatable three-way catalytic converter 24. A first pressure sensor is provided in the exhaust duct 18, downstream from the electrically heatable three-way catalytic converter 24 and upstream from the particulate filter 26. A second pressure sensor is provided downstream from the particulate filter 26, so that a differential pressure measurement may take place across the particulate filter 26. In addition, at least one temperature sensor may be provided in the exhaust gas system 20 in order to determine an exhaust gas temperature and/or at least one component temperature of an exhaust aftertreatment component 22, 24, 26, in particular a temperature of the particulate filter 26. The exhaust aftertreatment system also includes a secondary air system 40 with a secondary air pump 42 and a secondary air line 46, which opens into the exhaust duct 18 of the exhaust gas system 20 at an introduction point 48 directly downstream from the three-way catalytic converter 22 close to the engine. This results in a preferably long mixing path 52 for the exhaust gas of the internal combustion engine 10 and the secondary air prior to entry into the electrically heatable three-way catalytic converter 24 and the particulate filter 26. A secondary air valve 44 with which the secondary air feed into the exhaust duct 18 may be controlled is provided in the secondary air line 46. The secondary air pump 42 and the secondary air valve 44 are controllable via an engine control unit 50 of the internal combustion engine 10. The combustion air ratio .sub.E of the internal combustion engine 10 in combination with the lambda sensors 36, 38 may be regulated via this control unit 50.

[0042] FIG. 2 illustrates an alternative exemplary embodiment of an exhaust aftertreatment system according to the invention. With essentially the same design as in FIG. 1, a hydrogen catalytic converter 28 is connected upstream from the electrically heatable three-way catalytic converter 24; at the hydrogen catalytic converter, the hydrogen in the exhaust gas is oxidized to form water vapor to avoid distortion of the measuring signal at the second lambda sensor 38 by the hydrogen.

[0043] FIG. 4 illustrates a flow chart for a method according to the invention for regenerating a particulate filter 26 or a four-way catalytic converter 27. In the starting situation, the internal combustion engine 10 is operated with a stoichiometric combustion air ratio .sub.E=1. The electrical heating E of the electrically heatable three-way catalytic converter 24 is started at start time I. This operating state is maintained until the electrically heatable three-way catalytic converter 24 has reached its light-off temperature T.sub.LO. At time II the combustion air ratio of the internal combustion engine 10 is adjusted to an understoichiometric combustion air ratio .sub.E<1, and at the same time the blowing in of secondary air S is activated. This results in overstoichiometric exhaust gas at the second lambda sensor 38 prior to entry into the electrically heatable three-way catalytic converter 24. Chemical heating takes place in parallel with the electrical heating of the electrically heatable three-way catalytic converter 24, since the unburned exhaust gas components are exothermically reacted with the secondary air at the electrically heatable three-way catalytic converter 24. This operating state is maintained until the particulate filter 26 or the four-way catalytic converter 27 is heated to its regeneration temperature T.sub.REG. When the particulate filter 26 or the four-way catalytic converter 27 has reached its regeneration temperature T.sub.REG, the retained soot is oxidized with the oxygen from the secondary air, and the particulate filter 26 or the four-way catalytic converter 27 is regenerated in this way. The regeneration is depicted by reference symbol X in the diagram. When the regeneration X of the particulate filter 26 or the four-way catalytic converter 27 is concluded, the electrical heating E and the secondary air feed S are switched off at time IV. The internal combustion engine 10 is concurrently switched from the understoichiometric operation back to a stoichiometric operation.

[0044] FIG. 5 illustrates an alternative exemplary embodiment of a method according to the invention for regenerating the particulate filter 26 or the four-way catalytic converter 27. With essentially the same process as described for FIG. 4, in this exemplary embodiment the secondary air is initially controlled at time II in such a way that stoichiometric exhaust gas results, and not until time III is the secondary air increased in such a way that overstoichiometric exhaust gas results and the secondary air system 40 supplies the oxygen necessary for regenerating the retained soot. The gaseous emissions are thus minimized in the heating phase, since the electrically heatable three-way catalytic converter 24 is still acted on with a stoichiometric exhaust gas in the heating phase, and thus contributes to efficient conversion of the gaseous pollutants.

[0045] FIG. 6 illustrates another method according to the invention for regenerating the particulate filter 26 or the four-way catalytic converter 27. With essentially the same process as described for FIGS. 4 and 5, the regeneration X of the particulate filter 26 or four-way catalytic converter 27 is divided here into two phases X.sub.I and X.sub.II. Since excess oxygen must always be present for oxidizing soot, complete conversion of the emissions during the regeneration of the particulate filter 26 or four-way catalytic converter 27 would not be ensured. To shorten or prevent this phase, it is possible to carry out the heating operation with stoichiometric exhaust gas, and during the regeneration phases X.sub.I, X.sub.II to operate the internal combustion engine 10 with a stoichiometric combustion air ratio .sub.E=1. It is thus ensured in each phase of the method that a stoichiometric exhaust gas flows through at least one of the catalytic converters 22, 24. However, with this method the particulate filter 26 or four-way catalytic converter 27 cools during the regeneration, and an additional heating step must be initiated at time V. The electrical heating is maintained until the particulate filter 26 or four-way catalytic converter 27 has once again reached its regeneration temperature T.sub.REG, so that the second regeneration phase X.sub.II follows at time VI. As soon as the particulate filter 26 or four-way catalytic converter 27 has once again reached the regeneration temperature T.sub.REG, prior to entry into the particulate filter 26 or four-way catalytic converter 27 the exhaust gas-air ratio may once again be adjusted in such a way that a slightly overstoichiometric exhaust gas-air ratio of 1.05<.sub.EG<1.2 results in order to allow the oxidation of the soot. This intermittent regeneration operation is carried out until the particulate filter 26 or the four-way catalytic converter 27 is completely regenerated.

[0046] FIG. 7 illustrates another method according to the invention for regenerating the particulate filter 26 or four-way catalytic converter 27. The electrically heatable three-way catalytic converter 24 is electrically heated only until this catalytic converter 24 has reached its light-off temperature T.sub.LO. Starting at time II, the further heating of this catalytic converter 24 takes place via an exothermic chemical reaction of the unburned exhaust gas components with the oxygen from the secondary air. Electrical energy may be saved in this way.

[0047] FIG. 8 illustrates another exemplary embodiment of a method according to the invention. In this embodiment variant, the internal combustion engine 10 is continuously operated with a stoichiometric combustion air ratio .sub.E=1, and the necessary temperature increase of the particulate filter 26 or the four-way catalytic converter 27 is achieved solely by electrically heating the electrically heatable three-way catalytic converter 24. However, this method requires higher electrical heating power compared to the other described methods, which possibly may not be sufficient in all operating points to provide a regeneration of the particulate filter 26 or the four-way catalytic converter 27. However, this approach is characterized by the lowest emissions.

LIST OF REFERENCE SYMBOLS

[0048] 10 internal combustion engine [0049] 12 combustion chamber [0050] 14 spark plug [0051] 16 outlet [0052] 18 exhaust gas duct [0053] 20 exhaust gas system [0054] 22 three-way catalytic converter [0055] 24 electrically heatable three-way catalytic converter [0056] 26 particulate filter [0057] 27 four-way catalytic converter [0058] 28 hydrogen catalytic converter [0059] 30 exhaust gas turbocharger [0060] 32 turbine [0061] 34 compressor [0062] 36 first lambda sensor [0063] 38 second lambda sensor [0064] 40 secondary air system [0065] 42 secondary air pump [0066] 44 secondary air valve [0067] 46 secondary air line [0068] 48 introduction point [0069] 50 control unit [0070] 52 mixing path [0071] E electrical heating of the electrically heatable three-way catalytic converter [0072] S secondary air blowing [0073] X regeneration of the particulate filter [0074] T temperature [0075] T.sub.E-TWC temperature at the electrically heatable three-way catalytic converter [0076] T.sub.LO light-off temperature [0077] T.sub.OPF temperature of the particulate filter [0078] T.sub.REG regeneration temperature of the particulate filter [0079] .sub.E combustion air ratio of the internal combustion engine [0080] .sub.II exhaust gas-air ratio at the inlet of the electrically heatable three-way catalytic converter 10