DE10010031B4 - Method and device for carrying out a NOx regeneration of an arranged in an exhaust passage of an internal combustion engine NOx storage catalyst - Google Patents

Abstract

Method for carrying out a NO _x regeneration of an NO _x storage catalytic converter (16) arranged in an exhaust duct (12) of an internal combustion engine (10), wherein the internal combustion engine (10) can be operated in different operating modes and exhaust gas recirculation in at least one of the operating modes an intake air of the internal combustion engine (10) takes place and wherein the NO _x regeneration is carried out in a stoichiometric or rich operation of the internal combustion engine and the NO _x regeneration is carried out during at least part of its regeneration period at a reduced exhaust gas recirculation rate or without exhaust gas recirculation, characterized the exhaust gas recirculation rate is increased or switched on for 5 to 0.5 s before the completion of the NO _x regeneration.

Description

The invention relates to a method and a device for carrying out a NO _x regeneration of an arranged in an exhaust passage of an internal combustion engine NO _x storage catalytic converter with the features mentioned in the preamble of claims 1 and 8.

In order to carry out a post-treatment of exhaust gases of internal combustion engines, in particular of temporarily lean internal combustion engines, it is known to arrange catalysts with NO _x storage function (NO _x storage catalysts) in exhaust gas trains of internal combustion engines. The catalyst thereby reduces emissions of pollutants by promoting conversion of exhaust constituents such as carbon monoxide, unburned hydrocarbons and nitrogen oxides into less environmentally relevant compounds. Currently, the most sustainable operation of internal combustion engines in a lean operating mode with λ> 1, that is, with an excess of oxygen in a supplied air-fuel mixture, sought. Under these conditions, fuel consumption is reduced. On the other hand, in lean mode, no complete catalytic reduction of NO _x can be achieved because of the oxygen excess. Instead occurs in lean operation, absorption of NO _x in the NO _x storage. Because of a limited NO _x storage capacity of the catalyst, it must be periodically subjected to a NO _x regeneration, for which purpose it is usually charged with a rich exhaust gas atmosphere, so that absorbed NO _x is now present in excess in the exhaust reducing agent (HC, CO) can be converted.

It is also known to operate an internal combustion engine with an exhaust gas recirculation, wherein a portion of the exhaust gas is admixed to the internal combustion engine to be supplied air-fuel mixture. By this measure, a combustion temperature and, associated therewith, the NO _x formation can be lowered. For this purpose, an exhaust gas recirculation valve arranged in a return line is usually activated as a function of an operating point of the internal combustion engine.

The regulation of the NO _x regeneration of NO _x storage catalytic converters and the exhaust gas recirculation usually takes place independently of one another, so that an exhaust gas recirculation during a NO _x regeneration is permitted. This raises various problems. First, a duration of the lean-to-rich operation of the internal combustion engine, which elapses from the timing of presetting a rich air-fuel mixture to the actual presence of a rich exhaust, is undesirably prolonged. This is due to the presence of oxygen-rich exhaust gas in the exhaust gas recirculation line, which is still supplied to the combustion chamber of the engine, while a rich mode of the internal combustion engine is already requested. Another problem arises from the switching from a shift to a homogeneous operation of the internal combustion engine, which is usually associated with the switch to the rich mode. Associated with this, in particular shortly after the change of operation, there are combustibility problems and misfiring when the mixture is too lean to ignite despite increasing the fuel supply. This problem is exacerbated by the return of the initially lean exhaust gas. Finally, the duration of a NO _x regeneration is considerably extended by the exhaust gas recirculation. This is due to the reduced exhaust gas mass flow and the associated reduced reducing agent flow, which is necessary for the reaction of the released nitrogen oxides in the NO _x storage catalytic converter. An extension of the regeneration period leads to a considerable increase in fuel consumption.

Out DE 198 24 915 C1 a method for switching an internal combustion engine between a lean stratified operation and a rich homogeneous operation is known, which either takes place depending on the load point or due to a necessary NO _x regeneration of the NO _x storage catalytic converter. Here, in order to improve the combustion and ignition behavior, the rich homogeneous operation of the internal combustion engine is operated with or without reduced exhaust gas recirculation rate and the EGR rate is lowered shortly before switching the internal combustion engine from the lean stratified operation to the rich homogeneous operation. On the other hand, no specific procedure is disclosed for the re-switching of the internal combustion engine into the lean stratified operation.

The invention has for its object to provide a method and an apparatus by which the problems described in the implementation of a NO _x regeneration of a NO _x storage catalytic converter according to the prior art are overcome. In addition, the transfer of business at the end of the regeneration operation should be improved.

This object is achieved by the method and the device having the features mentioned in the independent claims 1 and 8. By performing the NO _x regeneration at a reduced exhaust gas recirculation rate or without exhaust gas recirculation and increasing the exhaust gas recirculation rate 5 to 0.5 seconds before the completion of the NO _x regeneration Ensures a quick and misfire-free switching of the internal combustion engine from lean to rich operation, shortens the regeneration times and smooths the transition of the operating modes at the end of the regeneration. Further, due to the rapid switching, an NO _x desorption peak observed when the lambda value passes from λ> 1 to λ ≤ 1 is reduced. By increasing the exhaust gas recirculation rate before the completion of the NO _x regeneration, the transition when switching the internal combustion engine from the regeneration to a lean mode is made more fluid, so that virtually no change in the operating behavior of the motor vehicle occurs.

In a preferred embodiment of the method, the NO _x regeneration is carried out during at least part of its regeneration period at an exhaust gas recirculation rate of 0 to 15% by volume, in particular 0 to 10% by volume. Currently common return rates range from 25 to 35% by volume, depending on the engine type. In this case, the volume fraction refers to a portion of the recirculated exhaust gas volume at a total of the internal combustion engine to be supplied air volume. Within the scope of advantageous specifications, the exhaust gas recirculation rate can be varied stepwise or continuously during regeneration.

It is further preferred that the exhaust gas recirculation rate before the end of the NO _x regeneration to 5 to 25 vol .-%, in particular 15 vol .-%, is increased. The exhaust gas recirculation rate to be reached at the end of the NO _x regeneration can in particular be made dependent on the recirculation rates to be set after the NO _x regeneration has ended.

A further advantageous embodiment provides to increase the exhaust gas recirculation rate gradually or continuously before the end of the NO _x regeneration. In this way, the transition can be further leveled.

According to another embodiment, the exhaust gas recirculation rate is increased 2 s before the completion of the NO _x regeneration. In this context, it can be further advantageously provided that the time of increasing the exhaust gas recirculation rate before the end of the NO _x regeneration is measured as a function of the before and / or after the increase of the exhaust gas recirculation rate. For example, with a large difference in the exhaust gas recirculation rates before and after the increase, an increase in the exhaust gas recirculation rate should be started relatively early before the completion of the NO _x regeneration.

Means are provided in the device according to the invention for carrying out a NO _x regeneration with which the method steps of performing the NO _x regeneration at a reduced exhaust gas recirculation rate or without exhaust gas recirculation and increasing an exhaust gas recirculation rate before completion of the NO _x regeneration are executable.

In a preferred embodiment, these means comprise a control unit in which a procedure for controlling the method steps for performing a NO _x regeneration and the NO _x storage catalytic converter is stored in digital form, wherein the control unit can be integrated in an engine control unit.

Further advantageous embodiments of the invention are the subject of the remaining dependent claims.

The invention will be explained in more detail in embodiments with reference to the accompanying drawings. Show it:

1 a schematic arrangement of an internal combustion engine with an exhaust passage and

2 a time course of lambda during a NO _x regeneration with and without exhaust gas recirculation.

1 shows a schematic representation of an internal combustion engine 10 with one of these associated exhaust duct 12 , Downstream of the internal combustion engine 10 are a small volume precatalyst 14 and a NO _x storage catalyst 16 in the exhaust duct 12 arranged. A lambda probe 18 serves to detect an oxygen concentration in the exhaust gas and is still in front of the catalyst system 14 . 16 in the exhaust duct 12 , The lambda probe 18 gives a signal to an engine control unit 20 again, which this and other measurement signals and operating parameters of the internal combustion engine 10 processed. Integrated in the engine control unit 20 is a control unit 22 in which the inventive procedure for performing a NO _x regeneration of the NO _x storage catalytic converter 16 is stored in digital form. Depending on all incoming signals, the engine control unit controls 20 or the control unit 22 the operating mode of the internal combustion engine 10 by, for example, influencing an air-fuel mixture to be fed. For this purpose, a sucked fresh air volume flow by position of a throttle valve 24 in an intake pipe 26 regulated. Furthermore, the engine control unit controls 20 or the control unit 22 one in a return line 28 arranged exhaust gas recirculation valve 30 at.

Will now during a lean phase of the internal combustion engine 10 a need for regeneration of the NO _x storage catalytic converter 16 determined, so causes the control unit 22 the switching of the internal combustion engine 10 in a homogeneous, stoichiometric or rich operating mode. The control unit gives this 22 For example, a more closed position of the throttle 24 before, so that the oxygen content in the supplied air-fuel mixture is reduced. According to the invention, the exhaust gas recirculation is reduced and / or completely prevented during at least part of the regeneration period. This is also done by a corresponding control of the exhaust gas recirculation valve 30 through the engine control unit 20 or the control unit 22 ,

2 shows a simplified representation of a time course of a before the catalyst system 14 . 16 with the lambda probe 18 measured lambda value during a NO _x regeneration of the NO _x storage catalytic converter 16 , It represents the broken line 32 a Lambdaverlauf during a regeneration with connection of an exhaust gas recirculation according to current practice (recirculation rate about 30 vol .-%), while the solid line 34 shows a corresponding lambda curve without exhaust gas recirculation. During an initial lean phase both courses show 32 . 34 a constant level according to a Lambdaagvorrgabe λ _m on. After detection of a regeneration need and switching one of the internal combustion engine 10 supplied air-fuel mixture to a rich, that is fuel-rich composition, at a time t ₀ remains the measured at the same time exhaust gas recirculation signal 32 even for a certain delay period at the lean lambda λ _m . This can be attributed to the oxygen-rich exhaust gas, which from the previous lean phase is still in the exhaust gas recirculation line 28 is located and the sucked fresh air is supplied. After this delay, a relatively shallow drop in the signal sets 32 until a time t ₁ at which a value corresponding to the lambda preset value λ _{f has been} reached. The flatness of the previous decrease in the lambda value is also the result of the exhaust gas feed into the fresh air sucked in. During the following regeneration period, which extends up to a time t ₂ , the Lambda fat preset λ _{f is} maintained and stored nitrogen oxides NO _{x of} the NO _x storage catalytic converter 16 reduced with the reducing agents present in the exhaust gas CO and HC. After emptying memory at time t ₂ , the internal combustion engine 10 switched back to the lean mode, so that the lambda value is back to the Lambdamagervorgabe λ _m .

The lambda curve measured according to the invention without exhaust gas recirculation 34 (solid line) shows significant deviations from the above-explained, measured at connected exhaust gas recirculation course 32 , Without exhaust gas recirculation has the lambda curve 34 a nearly immediately after switching the internal combustion engine 10 in the rich operating mode at the time t ₀ incipient and relatively steeply sloping course, so that the lambda fat default λ _{f is} already reached at a time t ₁ '. Furthermore, the regeneration period is sufficient for a complete emptying of the memory of the NO _x storage catalytic converter 16 must be applied, considerably shorter than with approved exhaust gas recirculation, so that the end of the NO _x regeneration already at a time t ₂ 'is reached. The very long regeneration period with simultaneous exhaust gas recirculation (course 32 is a result of the continuous exhaust gas removal, so that only a reduced reducing agent mass flow is available to convert the nitrogen oxides of the NO _x storage catalytic converter.

Overall, the regeneration period without exhaust gas recirculation is shortened by approximately half compared to the regeneration time required with simultaneous exhaust gas recirculation. The inventively preferred exhaust gas recirculation rates of a maximum of 15% by volume have lambda profiles, which represent an intermediate state between the explained courses.

By switching on the exhaust gas recirculation or increasing the exhaust gas recirculation rate before the end of the NO _x regeneration, although the regeneration period is slightly increased. This disadvantage is offset by the advantage of a largely torque-neutral transition in the operating mode of the internal combustion engine.

LIST OF REFERENCE NUMBERS

10

Internal combustion engine

12

exhaust duct

14

precatalyzer

16

NO _x storage catalyst

18

lambda probe

20

Engine control unit

22

control unit

24

throttle

26

intake

28

Exhaust gas recirculation line

30

Exhaust gas recirculation valve

32

Lambda curve with exhaust gas recirculation

34

Lambda curve without exhaust gas recirculation

λ _f

Lambda fat default

λ _m

Lambda skimmed default

Claims (10)

Method for carrying out a NO _x regeneration of one in an exhaust gas duct ( 12 ) one Internal combustion engine ( 10 ) arranged NO _x storage catalytic converter ( 16 ), wherein the internal combustion engine ( 10 ) can be operated in different operating modes and in at least one of the operating modes exhaust gas recirculation into an intake air of the internal combustion engine ( 10 ) and wherein the NO _x regeneration is performed in a stoichiometric or rich operation of the internal combustion engine and the NO _x regeneration is performed during at least part of its regeneration period at a reduced exhaust gas recirculation rate or without exhaust gas recirculation, characterized in that the exhaust gas recirculation rate is 5 to 0 , Is increased or switched on 5 seconds before the completion of the NO _x regeneration. A method according to claim 1, characterized in that the NO _x regeneration of an exhaust gas recirculation rate of 0 to 15 vol .-%, in particular at 0 to 10 vol .-% is performed. Method according to one of claims 1 or 2, characterized in that the exhaust gas recirculation rate before the end of the NO _x regeneration to 5 to 25 vol .-%, in particular 15 vol .-%, is increased. Method according to one of the preceding claims, characterized in that the exhaust gas recirculation rate is increased stepwise or continuously before the end of the NO _x regeneration. Method according to one of the preceding claims, characterized in that the exhaust gas recirculation rate is increased 2 s before completion of the NO _x regeneration. A method according to claim 5, characterized in that the time of increasing the exhaust gas recirculation rate is measured before the end of the NO _x regeneration depending on the existing before and / or after the increase in the exhaust gas recirculation rate. Method according to one of the preceding claims, characterized in that the NO _x regeneration is carried out in a homogeneous operation of the internal combustion engine. Device for carrying out a NO _x regeneration of one in an exhaust gas duct ( 12 ) an internal combustion engine ( 10 ) arranged NO _x storage catalytic converter ( 16 ), wherein the internal combustion engine ( 10 ) is operable in different operating modes and in at least one of the operating modes an exhaust gas recirculation into an intake air of the internal combustion engine ( 10 ), wherein means are provided, with which the method steps performing the NO _x regeneration in a stoichiometric or rich operation of the internal combustion engine and at a reduced exhaust gas recirculation rate or without exhaust gas recirculation and increasing the exhaust gas recirculation rate 5 to 0.5 s before ending the NO _x - Regeneration are executable. Device according to claim 8, characterized in that the means comprise a control unit ( 22 ) in which a procedure for controlling the process steps for performing an NO _x regeneration of an NO _x storage catalytic converter ( 16 ) is stored in digital form. Apparatus according to claim 9, characterized in that the control unit ( 22 ) in an engine control unit ( 20 ) is integrated.

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