METHOD AND DEVICE FOR THE EXHAUST GAS AFTERTREATMENT OF AN INTERNAL COMBUSTION ENGINE

Abstract

The invention relates to a method for exhaust treatment of an internal combustion engine, in particular for regeneration of a particle filter in the exhaust gas duct of an internal combustion engine, wherein the particle filter becomes loaded with soot particles during normal operation of the internal combustion engine. To reach the regeneration temperature of the particle filter, the internal combustion engine is operated with a rich fuel mixture, while secondary air is being introduced into the exhaust gas duct at the same time, and the unburned fuel components are reacted exothermically with the secondary air on the particle filter until the particle filter has heated up to the regeneration temperature. Once the regeneration temperature has been reached, the internal combustion engine is operated at a stoichiometric combustion air ratio, and secondary air is injected into the exhaust gas duct for oxidation of the soot particles retained in the particle filter, wherein control of the amount of secondary air is accomplished by means of a lambda probe downstream from an introduction point for the secondary air and upstream from the particle filter.

Claims

1. A method for exhaust gas aftertreatment of an internal combustion engine having an exhaust gas duct and a three-way catalyst arranged in the exhaust gas duct, a particle filter arranged downstream from the three-way catalyst, and a secondary air supply, comprising the following steps: operating the internal combustion engine at a stoichiometric combustion air ratio (.sub.E=1), wherein the soot particles formed by combustion are retained in the particle filter, determining a load status of the particle filter, initiating regeneration of the particle filter when the need for regeneration of the particle filter is detected on determining the charge state, raising the exhaust gas temperature (T.sub.EG) in a heating phase by operating the internal combustion engine with a rich combustion air ratio (.sub.E<1) which is below the stoichiometric ratio while at the same time introducing secondary air into the exhaust gas duct upstream from the particle filter, wherein the unburned fuel components are exothermically reacted with the secondary air in the exhaust gas duct or on the particle filter until achieving a regeneration temperature (T.sub.R); regenerating the particle filter, wherein the internal combustion engine is operated at a stoichiometric combustion air ratio (.sub.E=1), and secondary air is introduced into the exhaust gas duct, so that an exhaust gas (.sub.M>1), which exceeds the stoichiometric ratio, is formed downstream from the secondary air inlet, and regulating the amount of secondary air that is introduced into the exhaust gas duct by a lambda probe downstream from an introduction point for the secondary air and upstream from the particle filter.

2. The method according to claim 1, further comprising establishing a stoichiometric air mixing ratio (.sub.M=1) in the exhaust gas duct downstream from the three-way catalyst and upstream from the particle filter by introduction of secondary air during the heating phase.

3. The method according to claim 1, further comprising determining a temperature (T.sub.PF) of the particle filter, and during the regeneration phase, keeping the temperature (T.sub.PF) above the regeneration temperature (T.sub.R) of the particle filter.

4. The method according to claim 1, further comprising stopping the introduction of secondary air upon reaching an upper threshold temperature (T.sub.SO).

5. The method according to claim 3, further comprising increasing or decreasing the amount of secondary air injected into the exhaust gas duct as a function of a change in temperature (T.sub.PF) of the particle filter.

6. The method according to claim 1, further comprising increasing the amount of secondary air injected into the exhaust gas duct with an increase in regeneration of the particle filter.

7. The method according to claim 1, wherein, for regeneration of the particle filter, the particle filter is switched between the heating phase and the regeneration phase several times in alternation.

8. The method according to claim 1, further comprising keeping the temperature of the particle filter during regeneration within a temperature window between the regeneration temperature (T.sub.R) and an upper threshold temperature (T.sub.SO).

9. The method according to claim 8, wherein the temperature window is in the range of 600 C. to 750 C.

10. The method according to claim 1, further comprising regulating the amount of secondary air so that, during regeneration of the particle filter, an air mixing ratio (.sub.M) of 1.05 to 1.4, preferably of 1.1 to 1.25, is established upstream from the particle filter.

11. The method according to claim 1, further comprising regulating the amount of secondary air so that a stoichiometric exhaust gas is established downstream from the particle filter.

12. The method according to claim 1, further comprising ending the heating phase only when the particle filter has reached a temperature of at least 30 C. above the regeneration temperature of the particle filter.

13. The method according to claim 1, wherein the secondary air is obtained from an intake duct of the internal combustion engine downstream from a compressor and is introduced into the exhaust gas duct.

14. A control unit for an internal combustion engine, having a computer-readable program algorithm for controlling the method according to claim 1.

15. A device for exhaust gas aftertreatment of an internal combustion engine having an exhaust gas duct, a three-way catalyst arranged in the exhaust gas duct and a particle filter arranged downstream from the three-way catalyst in the exhaust gas duct and having a secondary air supply, wherein an introduction point for the secondary air from the secondary air source is provided between the three-way catalyst and the particle filter, and also having a first lambda probe arranged upstream from the three-way catalyst and a second lambda probe arranged downstream from the introduction point and upstream from the particle filter, wherein the device is equipped to carry out a method according to claim 1.

Description

[0031] The invention will now be explained in detail in the embodiments based on the accompanying drawings, in which:

[0032] FIG. 1 shows an internal combustion engine having a three-way catalyst and a particle filter arranged downstream from the three-way catalyst as well as a secondary air supply for carrying out a method according to the invention,

[0033] FIG. 2 shows the exhaust gas duct of an internal combustion engine as well as the lambda sensor system for controlling a method according to the invention, and

[0034] FIG. 3 shows a flowchart of a method according to the invention for regeneration of the particle filter.

[0035] FIG. 1 shows an internal combustion engine 10 in the form of a gasoline engine charged with a turbocharger 32, having an intake duct 26 and an exhaust gas duct 12. A compressor 28, a throttle valve 34 and a charge air cooler 36 are arranged in the intake duct 12. A turbine 38 of the turbocharger 32, which drives the compressor 28 of the turbocharger 32 by means of a drive shaft 40, is arranged in the exhaust gas duct 12 in the direction of flow of the exhaust gas of the internal combustion engine 10. Alternatively, the compressor 28 may also be designed as a mechanically driven compressor or as an electric compressor.

[0036] A three-way catalyst is arranged in the exhaust gas duct 12 downstream from the turbine 38 in the direction of flow of the exhaust gas of the internal combustion engine 10 through the exhaust gas duct 12. The three-way catalyst 14 here is preferably arranged near the engine to permit rapid heating of the three-way catalyst 14 to a light-off temperature and thus efficient conversion of pollutants. The phrase an arrangement near the engine is understood to refer to an arrangement having a central exhaust pathway of max. 50 cm, in particular max. 30 cm, after the outlet of the internal combustion engine 10. Downstream from the three-way catalyst 14, an introduction point 20 for introducing the secondary air into the exhaust gas duct 12 is provided. A secondary air supply 18, comprising a secondary air valve 42 and a secondary air line 44, is connected to the introduction point 20, such that the secondary air line 44 connects a portion of the intake duct 26 downstream from the compressor 28 to the exhaust gas duct 12. A secondary air pump 48, with which an elevated pressure in comparison with the pressure in the exhaust gas duct 12 can be generated, is provided on the secondary air line 44. Alternatively, the secondary air line 44 can also connect the surroundings to the exhaust gas duct 12. In doing so, the secondary air line 44 opens at the secondary air valve 42 and/or the introduction point 20 downstream from the three-way catalyst 14 and upstream from the particle filter 16 in the exhaust gas duct 12. Upstream from the three-way catalyst 14, a first lambda probe 22 with which the combustion air ratio .sub.E of the internal combustion engine 10 is regulated, is provided in the direction of flow of the exhaust gas of the internal combustion engine 10 through the exhaust gas duct 12. A second lambda probe 24, with which the amount of secondary air introduced into the exhaust gas duct 12 can be controlled by the secondary air valve 42, is provided downstream from the introduction point 20 and upstream from the particle filter 16. In doing so, the first lambda probe 22, the second lambda probe 24 and the secondary air valve 42 are connected via signal lines 46 to a control unit 30 of the internal combustion engine 10 in order to enable regulation of the amount of secondary air injected into the exhaust gas duct 12.

[0037] In FIG. 2, the internal combustion engine 10 with the exhaust gas duct 12 is shown in a further simplified diagram. A first lambda probe 22 for controlling the combustion air ratio in internal combustion engine 10 is arranged downstream from the internal combustion engine 10 and upstream from the three-way catalyst 14 in the exhaust gas duct 12. A control circuit for the introduction of secondary air into the exhaust gas duct 12 is situated downstream from the three-way catalyst, wherein the control circuit comprises at least one secondary air supply 18 and a second lambda probe 24 arranged downstream from the introduction point 20 of the secondary air supply 18 and upstream from a particle filter 16.

[0038] Soot formed during operation of the internal combustion engine is retained by the particle filter 16, causing the particle filter 16 to become loaded with soot particles from the internal combustion engine 10. A method for regenerating the particle filter 16 is initiated when an established soot loading threshold for the particle filter 16 is detected, which may take place based on a differential pressure measurement upstream and downstream from the particle filter 16, for example, or by means of a model-based calculation. To do so, first the exhaust gas temperature of the internal combustion engine 10 is raised to a regeneration temperature T.sub.R of at least 600 C. before entering the particle filter 16. The particle filter 16 preferably has a catalytic coating for exothermically oxidizing unburned hydrocarbons, carbon monoxide and/or hydrogen on the surface of the particle filter 16. First it is necessary to verify whether the particle filter 16 has reached a so-called light-off temperature of approx. 350 C. This ensures that unburned constituents of the fuel can be exothermically oxidized out of the exhaust gas of the internal combustion engine 10 on the particle filter 16. If the light-off temperature of the particle filter 16 has been reached, the particle filter 16 is heated further up to the regeneration temperature T.sub.R. This is done by operating the internal combustion engine 10 with a rich mixture, which preferably has a combustion air ratio lambda .sub.E of approx. 0.9. The unburned constituents of the mixture, in particular carbon monoxide, hydrocarbons and hydrogen, are introduced into the exhaust system 12 together with the combustion products. These unburned constituents of the fuel can be converted exothermically on the downstream particle filter by means of an air intake in the exhaust gas duct 26 downstream from the compressor 28 and by introducing this air into the exhaust gas duct 12 through the secondary air line 44 and the secondary air valve 42. High levels of exhaust gas enthalpy can be introduced into the particle filter 16 through the external air supply via the secondary air supply 18 and by operating the engine with a rich fuel mixture. A complex sensor system, comprising a pressure sensor, a temperature sensor and the second lambda probe 24, is necessary for monitoring and/or regulating this exhaust gas enthalpy. The combustion air ratio .sub.E of the internal combustion engine 10 can be adjusted by precontrol, so that the desired target temperature is set. At the same time, the air mixing ratio .sub.m is measured by the second lambda probe 24 from the combustion air ratio of the internal combustion engine 10 and the secondary air that is introduced downstream from the introduction point 20 and upstream from the particle filter 16. During the heating phase of the particle filter 16, this air mixing ratio is regulated at .sub.m=1, so that emissions can be converted on the catalytic coating of the particle filter 16, and the particle filter can achieve its optimum exhaust gas purifying effect.

[0039] Once the heating phase is concluded and a temperature above the regeneration temperature T.sub.R of the particle filter 18 has been reached, the system switches to a regeneration phase. To do so, the internal combustion engine 10 is again operated at a stoichiometric combustion air ratio .sub.E=1. Therefore, all the pollutants of the exhaust gas of the internal combustion engine 10 can be reacted completely on the three-way catalyst 14 during the regeneration phase. To supply the oxygen for regeneration of the particle filter 16, secondary air is again introduced into the exhaust gas duct 12. The desired mixing ratio, for example, .sub.m=1.1, can be adjusted through corresponding regulation of the secondary air through the secondary air valve 42. This ensures that the conversion rate of the soot retained in the particle filter 16 does not become too high, for example, which could otherwise result in thermal damage to the particle filter 16. If the temperature at the entrance to the particle filter 16 drops during regeneration, then the amount of secondary air is increased to increase the conversion rates of soot retained on the particle filter and thus to increase the exhaust gas temperature T.sub.EG. Such regeneration of the particle filter 16 according to the invention is illustrated in FIG. 3. Regeneration of the particle filter 16 is continued until the particle filter 16 is completely regenerated, which can be ascertained by means of a differential pressure measurement or also by means of a calculation model for the soot input and soot output. After complete regeneration of the particle filter 16, the secondary air supply 18 is switched off in a renewed loading phase of the particle filter 16 and the engine as well as the entire exhaust system is again operated with a stoichiometric combustion air mixture .sub.E=1.

[0040] In a first phase I, the internal combustion engine 10 is operated at a stoichiometric combustion air ratio .sub.E=1, and soot particles are retained in the particle filter 16. Then the secondary air supply 18 is switched off and no further oxygen is introduced into the exhaust gas duct 12 of the internal combustion engine 10. In a second phase II, which is also referred to as the heating phase, the external air supply through the secondary air supply 18 is activated, and the internal combustion engine 10 is operated at a rich combustion air ratio .sub.E<1, which is below the stoichiometric ratio, so that a stoichiometric air mixing ratio .sub.M=1 is established downstream from the introduction point 20.

[0041] In a third phase III, which is also referred to as the regeneration phase, the internal combustion engine 10 is operated at a stoichiometric combustion air ratio .sub.E=1, and the introduction of secondary air is increased continuously until the particle filter 16 is completely regenerated. In doing so, a lean air mixing ratio .sub.M>1, which is greater than the stoichiometric ratio, is established in the exhaust gas duct 12 downstream from the introduction point 20. Once the particle filter 16 has been regenerated completely, the secondary air supply 18 is shut down again and the internal combustion engine 10 is again operated at a stoichiometric combustion air ratio .sub.E=1 in a renewed loading phase I. The combustion air ratio .sub.E is represented by a solid line in FIG. 3, the air mixing ratio .sub.M downstream from the introduction point 20 is represented by a dotted line, and the secondary air introduced through the secondary air supply 18 is represented by a dashed line.

LIST OF REFERENCE NUMERALS

[0042] 10 internal combustion engine [0043] 12 exhaust gas duct [0044] 14 three-way catalyst [0045] 16 particle filter [0046] 18 secondary air supply [0047] 20 introduction point [0048] 22 first lambda probe [0049] 24 second lambda probe [0050] 26 intake duct [0051] 28 compressor [0052] 30 control unit [0053] 32 turbocharger [0054] 34 throttle valve [0055] 36 charge air cooler [0056] 38 turbine [0057] 40 drive shaft [0058] 42 secondary air valve [0059] 44 secondary air line [0060] 46 signal line [0061] 48 secondary air pump [0062] .sub.E combustion air ratio of the internal combustion engine [0063] .sub.M air mixing ratio in the exhaust gas duct downstream from the introduction of secondary air [0064] T.sub.EG temperature of the exhaust gas [0065] T.sub.PF temperature of the particle filter [0066] T.sub.R regeneration temperature of the particle filter [0067] T.sub.SO upper threshold temperature [0068] T.sub.PF change in temperature of the particle filter during regeneration