REGENERATION OF A PARTICULATE FILTER OR FOUR-WAY CATALYTIC CONVERTER IN AN EXHAUST SYSTEM OF AN INTERNAL COMBUSTION ENGINE

Abstract

In a method for the regeneration of a particulate filter or of a four-way catalytic converter in an exhaust system of an internal combustion engine, an increase in the nitrogen oxide emissions during the regeneration of the particulate filter or of the four-way catalytic converter can be prevented or at least reduced. A particulate filter or a four-way catalytic converter is arranged in the exhaust system of an internal combustion engine. The fuel injection and the ignition are switched off in response to a request for the internal combustion engine to be turned off. Due to mass inertia, the internal combustion engine transitions from the switch-off rotational speed to a standstill whereby, during this phase, oxygen-rich air is conveyed into the exhaust passage. A partial regeneration of the filter or of the catalytic converter takes place with the oxygen contained in this fresh air, whereby the particulate mass discharged from the filter or the catalytic converter is determined by means of a computational model.

Claims

1. A method for the after-treatment of the exhaust gas of an internal combustion engine (10) in whose exhaust system (12) a particulate filter (14) or a four-way catalytic converter (22) is arranged, said method encompassing the following steps: running the internal combustion engine (10) in a normal mode of operation with a stoichiometric air-fuel ratio, whereby the soot particles formed during the combustion are captured in the exhaust system (12) by the particulate filter (14) or by the four-way catalytic converter (22), determining the particle entrainment into the particulate filter (14) or into the four-way catalytic converter (22) by means of a computational model, transmitting a request for the internal combustion engine (10) to be switched off to a control unit (24) of the internal combustion engine (10), switching off the fuel injection into the combustion chambers (34) of the internal combustion engine (10), whereby the fuel injection is switched off when the rotational speed (n) of the internal combustion engine (10) is above a threshold value (S.sub.1), regenerating the particulate filter (14) or the four-way catalytic converter (22) by means of the residual oxygen conveyed into the exhaust system (12) when the internal combustion engine (10) is winding down after the fuel injection has been switched off, and determining the soot discharge from the particulate filter (14) or from the four-way catalytic converter (22) by means of a computational model.

2. The method for the after-treatment of the exhaust gas according to claim 1, wherein, before the engine is switched off, the rotational speed (n) of the internal combustion engine (10) is raised to a rotational speed (n.sub.A) above the threshold value (S.sub.1) if the internal combustion engine (10) is idling at the point in time of the request for it to be switched off.

3. The method for the after-treatment of the exhaust gas according to claim 1, wherein the threshold value (S.sub.1) for the rotational speed (n) is above the usual idling speed (n.sub.I) of the internal combustion engine (10).

4. The method for the after-treatment of the exhaust gas according to claim 3, wherein the threshold value (S.sub.1) is within the range from 1100 rpm to 1800 rpm.

5. The method for the after-treatment of the exhaust gas according to claim 1, wherein the fuel injection into the combustion chambers (34) of the internal combustion engine (10) is only switched off once the temperature (T.sub.PF) of the particulate filter (14) or of the four-way catalytic converter (22) or the temperature (T.sub.EG) of the exhaust gas is above a threshold temperature (S.sub.T).

6. The method for the after-treatment of the exhaust gas according to claim 5, wherein the threshold temperature (S.sub.T) is within the range from 550 C. to 750 C.

7. The method for the after-treatment of the exhaust gas according to claim 1, wherein a throttle valve (36) installed in the intake duct (38) of the internal combustion engine (10) is completely opened in response to a request for the internal combustion engine (10) to be switched off.

8. The method for the after-treatment of the exhaust gas according to claim 1, wherein the switch-off signal of the internal combustion engine (10) is triggered by a start-stop system (40) of the internal combustion engine (10).

9. The method for the after-treatment of the exhaust gas according to claim 1, wherein the method is started when the load of the particulate filter (14) or of the four-way catalytic converter (22) is above a threshold value (S.sub.L) for the load of the particulate filter (14) or of the four-way catalytic converter (22).

10. The method for the after-treatment of the exhaust gas according to claim 1, wherein the temperature (T.sub.EG) of the exhaust gas is raised when it is recognized that a request for the internal combustion engine (10) to be switched off is imminent.

11. A device for the after-treatment of the exhaust gas of an internal combustion engine (10) having an exhaust system (12) in which a particulate filter (14) or a four-way catalytic converter (22) is arranged, comprising a control unit (24) with a machine-readable program code, whereby, when the program code is executed, the control unit (24) is configured to carry out a method according to claim 1.

12. The device for the after-treatment of the exhaust gas according to claim 11, wherein the particulate filter (14) or the four-way catalytic converter (22) is arranged near the engine as the first component of the exhaust after-treatment system.

13. The device for the after-treatment of the exhaust gas according to claim 11, wherein the internal combustion engine (10) is associated with a start-stop system (40).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The invention will be explained below on the basis of embodiments with reference to the accompanying drawings. The following is shown:

[0032] FIG. 1 is an internal combustion engine having an intake system and an exhaust system with which the emissions during the regeneration of the particulate filter can be reduced within the scope of a method according to the invention;

[0033] FIG. 2 is an alternative embodiment of the exhaust system according to the invention in which, instead of a particulate filter, a four-way catalytic converter is arranged near the engine;

[0034] FIG. 3 is a diagram showing the sequence of the inventive method for the after-treatment of the exhaust gas of an internal combustion engine.

DETAILED DESCRIPTION OF THE INVENTION

[0035] FIG. 1 shows a schematic view of an internal combustion engine 10 having an exhaust system 12 connected to an outlet 32 of the internal combustion engine 10. A particulate filter 14 is arranged in the exhaust system 12 near the engine in the flow direction of an exhaust gas of the internal combustion engine 10 through the exhaust system 12. In this context, the term near the engine refers to a position in the exhaust system corresponding to a distance of less than 80 cm, preferably less than 50 cm, that the exhaust gas has to travel from the outlet 32 of the internal combustion engine 10. Downstream from the particulate filter 14, especially in the undercarriage of a motor vehicle, there is a three-way catalytic converter 16 and, further downstream, a NO.sub.x storage catalytic converter 18, which are connected to each other by an exhaust passage 20 of the exhaust system 12. The amount of fuel injected into the combustion chambers 34 of the internal combustion engine 10 and thus the fuel-air ratio .sub.E of the internal combustion engine 10 can be regulated by means of a control unit 24. Several lambda sensors 26, 28, 30 are arranged in the exhaust passage 20 in order to regulate the fuel-air ratio .sub.E of the internal combustion engine 10. An exhaust-gas turbocharger 42 whose turbine 44 is powered by an exhaust-gas stream of the internal combustion engine 10 is provided downstream from the outlet 32 of the internal combustion engine 10 and upstream from the particulate filter 14. The internal combustion engine 10 also comprises an intake system having an intake duct 38 in which there is a throttle valve 36 that serves to control the amount of air fed to the combustion chambers 34 of the internal combustion engine 10. Moreover, the intake system has a compressor 46 that is powered by the turbine 44 of the exhaust-gas turbocharger 42 and that compresses the fresh air fed to the internal combustion engine 10. The internal combustion engine 10 has a start-stop system 40 with which the internal combustion engine 10 is switched off when the vehicle comes to a standstill and which is only restarted in response to a start signal, for instance, the release of the depressed brake pedal.

[0036] With a structure essentially identical to the one depicted in FIG. 1, the (uncoated) particulate filter 14 shown in FIG. 2 is replaced with a particulate filter having a three-way catalytically active coating, a so-called four-way catalytic converter 22. Here, the four-way catalytic converter 22 combines the functions of a particulate filter and of a three-way catalytic converter. Owing to the arrangement of the additional three-way catalytic converter 16 in the undercarriage of the motor vehicle, the four-way catalytic converter can be configured so as to be relatively small in order to allow a quick warm-up to the operating temperature following a cold start of the internal combustion engine 10. As an alternative, thanks to the four-way catalytic converter 22, it is also possible to dispense with an additional three-way catalytic converter 16, especially a three-way catalytic converter 16 in the undercarriage of the motor vehicle.

[0037] During the operation of the internal combustion engine 10, the particles that are formed in the exhaust gas of the internal combustion engine 10 during the combustion can be captured by the particulate filter 14 or by the four-way catalytic converter 22. In this process, the particulate filter 14 or the four-way catalytic converter 22 is loaded with soot in the known manner. This loading can cause effects such as elevated fuel consumption, power loss and misfiring if the exhaust-gas counter-pressure rises above a given threshold value S.sub.L because of the loading of the particulate filter 14 or of the four-way catalytic converter 22. As a result, the particulate filter 14 or the four-way catalytic converter 22 has to be regenerated cyclically or as a function of the loading. In order to regenerate the particulate filter 14 or the four-way catalytic converter 22, it is not only necessary for a regeneration temperature to be reached but also for residual oxygen to be present in the exhaust system so that the soot particles captured in the particulate filter 14 or in the four-way catalytic converter 22 can be oxidized. Due to the over-stoichiometric operation of the internal combustion engine 10, the three-way catalytic converter 16 and the four-way catalytic converter 22 lose their conversion capacity for nitrogen oxides since there is no longer any reducing agent present to reduce the nitrogen oxide to elementary nitrogen.

[0038] In order to prevent over-stoichiometric operation of the internal combustion engine 10 as well as the increase in the nitrogen oxide emissions associated with this, it is provided for the oxygen needed to regenerate the particulate filter 14 or the four-way catalytic converter 22 to be conveyed into the exhaust system in that, when the internal combustion engine 10 is being switched off, first of all, the fuel injection into the combustion chambers 24 of the internal combustion engine 10 is switched off and then the residual rotational speed of the internal combustion engine 10 until it comes to a standstill is used to convey oxygen-rich fresh air into the exhaust system 12.

[0039] FIG. 3 shows an engine diagram that explains a method according to the invention for the after-treatment of the exhaust gas of the internal combustion engine 10. In this context, the rotational speed n of the internal combustion engine is plotted over the time t. In a first phase I of the method, a check is carried out as to whether the rotational speed n of the internal combustion engine 10 is above a threshold value S.sub.1 for the rotational speed n. If the rotational speed n is below this threshold value S.sub.1, especially at the usual idling speed n.sub.I, first of all, the rotational speed n of the internal combustion engine is increased to a rotational speed n above the threshold value S.sub.1, especially to a rotational speed of at least 1100 rpm, especially preferably at least 1200 rpm. If the rotational speed n of the internal combustion engine 10 is above this threshold value S.sub.1, then no additional measures are undertaken. In a second method step, the injection of fuel into the combustion chambers 34 of the internal combustion engine 10 is switched off at a point in time T.sub.1 and the ignition is turned off.

[0040] As a result, the rotational speed n of the internal combustion engine 10 drops from an operating rotational speed to 0 so that the internal combustion engine 10 comes to a stop. During the switch-off procedure, the inertia of the internal combustion engine until it comes to a standstill conveys fresh air into the exhaust passage 20. In this process, an excess of oxygen >>1 is briefly established in the exhaust system 12 during a second phase II, so that the soot captured in the particulate filter 14 or in the four-way catalytic converter 22 is oxidized and is then discharged in the form of carbon dioxide (CO.sub.2) from the particulate filter 14 or from the four-way catalytic converter 22. The soot continues to be discharged for as long as the conditions needed for oxidation of the soot are present. In this context, FIG. 3 shows the particle mass P.sub.pm discharged from the particulate filter 14 or from the four-way catalytic converter 22 plotted over the time t. If the internal combustion engine 10 comes to a stop, then fresh air is no longer conveyed into the exhaust passage 20, so that the regeneration of the particulate filter 14 or of the four-way catalytic converter 22 comes to a halt. In this manner, the proposed method also offers protection against an uncontrolled burn-off of soot on the particulate filter 14 or on the four-way catalytic converter 22, since the excess of oxygen for the oxidation of the soot is always available only for a very limited period of time amounting to just a few seconds. If the internal combustion engine 10 is restarted after it has been at a standstill, then the particulate filter 14 or the four-way catalytic converter 22 is once again loaded with soot particles, whereby the entrainment of soot into the particulate filter 14 or into the four-way catalytic converter 22 is ascertained by means of a computational model.

LIST OF REFERENCE NUMERALS

[0041] 10 internal combustion engine

[0042] 12 exhaust system

[0043] 14 particulate filter

[0044] 16 three-way catalytic converter

[0045] 18 NO.sub.x storage catalytic converter

[0046] 20 exhaust passage

[0047] 22 four-way catalytic converter

[0048] 24 control unit

[0049] 26 first lambda sensor

[0050] 28 second lambda sensor

[0051] 30 third lambda sensor

[0052] 32 outlet

[0053] 34 combustion chamber

[0054] 36 throttle valve

[0055] 38 intake duct

[0056] 40 start-stop system

[0057] 42 exhaust-gas turbocharger

[0058] 44 turbine

[0059] 46 compressor

[0060] n rotational speed of the internal combustion engine

[0061] n.sub.A switch-off rotational speed

[0062] n.sub.I usual idling speed

[0063] P.sub.pm particle mass discharged from the particulate filter

[0064] S.sub.1 threshold value

[0065] S.sub.T threshold temperature

[0066] S.sub.L threshold value for the load of the particulate filter

[0067] T.sub.EG temperature of the exhaust gas

[0068] rpm rotations per minute

[0069] .sub.E fuel-air ratio of the internal combustion engine