DE102015203777A1 - Method for exhaust aftertreatment, and exhaust aftertreatment arrangement - Google Patents

Abstract

The invention relates to a method for exhaust aftertreatment and an exhaust aftertreatment arrangement, wherein the exhaust aftertreatment arrangement with at least one NOx storage catalyst (LNT) (130) for storing in the exhaust of an internal combustion engine (10) nitrogen oxides in operating phases with lean exhaust gas mixture and for the conversion of nitrogen oxides in operating phases with having rich exhaust gas mixture. In operating phases with a rich exhaust gas mixture, the oxygen content in the exhaust gas leaving the internal combustion engine (10) is controlled at least temporarily by varying at least one combustion parameter characteristic of the operation of the internal combustion engine (10).

Description

The invention relates to a method for exhaust aftertreatment and an exhaust aftertreatment arrangement.

The introduction of increasingly stringent NO _x limit values has led to the development of various exhaust aftertreatment technologies in order to control the NO _x emissions (= nitrogen oxide emissions) in the exhaust gas of a diesel engine.

One of these solutions is the so-called NO _x storage catalytic converter (LNT), whose operating principle is based on first storing nitrogen oxides (NO _x ) under lean exhaust conditions and then setting a rich, substoichiometric reducing exhaust gas mixture in a regeneration phase (which typically lasts a few seconds). into harmless components, especially nitrogen, carbon dioxide and water vapor, to convert. The frequency with which this regeneration phase is carried out is essentially determined by the NO _x emissions and the storage capacity of the NO _x storage catalytic converter, this storage capacity in turn being dependent on the exhaust gas temperature.

A second technology is the selective catalytic reduction in a so-called SCR catalyst (SCR = "Selective Catalytic Reduction"), wherein the nitrogen oxides contained in the exhaust gas are reduced to nitrogen (N ₂ ) on the SCR catalyst by means of ammonia , Moreover, the SCR catalyst is capable of storing, for example, the ammonia fed directly to the exhaust gas stream upstream of the SCR catalyst at low exhaust gas temperatures.

A NO _x storage catalytic converter (LNT) has to be regenerated under operating conditions with rich exhaust gas mixture in order to convert the stored nitrogen oxides (NO _x) and recording capacity for more to be stored nitrogen oxides (NO _x) to provide.

In this case, an effective regeneration can take place only in a defined temperature window. However, if the temperature is too low, some of the stored nitrogen oxides (NO _x ) will be desorbed from the system without being converted. On the other hand, if the temperature during the regeneration is too high, a massive or pronounced NO _x desorption takes place, but likewise a large proportion of the nitrogen oxides (NO _x ) is not reduced.

During regeneration with a rich exhaust gas mixture, the exhaust gases emanating from the internal combustion engine still contain a certain amount of oxygen. This oxygen must be converted into carbon dioxide (CO ₂ ) and water (H ₂ O) before the nitrogen oxides (NO _x ) can be reduced. These conversions are exothermic reactions in which heat is generated. This generated heat results in a further rise in the temperature of the NO _x storage catalyst (LNT), resulting in a change in the reaction kinetics. This change in the reaction kinetics can in turn mean that the operating conditions no longer correspond to the optimal conditions for the nitrogen oxide conversion.

Furthermore, the temperature increase associated with the above exothermic reactions can release a larger amount of oxygen from the existing oxygen storage components (OSC). This results in an increased demand for reducing agents to produce rich exhaust gas operating conditions within the NO _x storage catalyst (LNT).

It is an object of the present invention to provide a method for exhaust aftertreatment and an exhaust aftertreatment arrangement which allow for an improved reduction of nitrogen oxides in the exhaust gas.

This object is achieved by the method according to the features of the independent claim 1 and the exhaust aftertreatment arrangement according to the features of the independent claim 7.

In a method according to the invention for exhaust aftertreatment, with an exhaust gas aftertreatment arrangement, which has at least one NO _x storage catalytic converter (LNT) for storing nitrogen oxides present in the exhaust gas of an internal combustion engine in operating phases with lean exhaust gas mixture and for converting nitrogen oxides into operating phases with rich exhaust gas mixture, is in operation phases with rich exhaust gas mixture, the oxygen content in the exhaust gas leaving the internal combustion engine is controlled at least temporarily by varying at least one combustion parameter characteristic of the operation of the internal combustion engine.

The present invention is based in particular on the concept of specifically controlling the amount of oxygen which emanates from the internal combustion engine in operating phases with a rich exhaust gas mixture. When the temperature of the NO _x storage catalyst is relatively low, the amount of oxygen in operating phases with rich exhaust gas mixture is set to a relatively high value. This results in an increase in temperature in the NO _x storage catalytic converter, so that an approach to the nitrogen oxides (NO _x ) stored to reduce them occurs in optimum conditions, since higher temperatures lead to higher temperatures Conversion rates in the nitrogen oxide conversion lead. Since a larger amount of stored oxygen (O ₂ ) is released, the duration of the operating phase with rich exhaust gas mixture can be extended accordingly.

If the temperature of the NO _x storage catalyst is relatively high, the amount of oxygen in operating phases with rich exhaust gas mixture is set to a relatively low value in order to avoid exothermic reactions as possible, so that a deviation from the optimal conditions for reducing stored nitrogen oxides is largely prevented.

In one embodiment of the invention, control of the amount of oxygen (O ₂ ) in the exhaust gas emanating from the internal combustion engine takes place under operating conditions with rich exhaust gas mixture by varying the exhaust gas recirculation rate. In one embodiment, the braking torque and the lambda value are kept constant while the exhaust gas recirculation rate ("EGR rate") for adjusting or adjusting the amount of oxygen is varied. In this case, the exhaust gas recirculation rate can be set to a high value in order to increase the oxygen (O ₂ ) amount and to promote exothermic reactions. On the other hand, the exhaust gas recirculation rate can be set to a low level in order to reduce the oxygen (O ₂ ) amount and to suppress exothermic reactions on the catalyst as much as possible.

In other embodiments, other combustion parameters (boost pressure, injection timing, injection quantity, injection pressure, etc.) for controlling the amount of oxygen may also be varied. Possible are z. B. also several injections with z. B. different injection quantities.

Further embodiments of the invention are described in the description and the dependent claims.

The invention will be explained in more detail below with reference to an embodiment shown in the accompanying drawing.

Show it:

1 a schematic representation of an exhaust aftertreatment device according to an embodiment of the invention; and

2 Diagrams for explaining the effect of a variation of the exhaust gas recirculation rate according to an embodiment of the method according to the invention.

1 shows only a schematic representation of an exhaust aftertreatment arrangement according to an embodiment of the invention. According to 1 come from an internal combustion engine 10 Outgoing exhaust gases, in particular through a NO _x storage catalytic converter (LNT) 130 , wherein based on the exhaust gas flow further exhaust aftertreatment components 120 respectively. 140 upstream or downstream of the NO _x storage catalytic converter (LNT) 130 can be provided.

According to the invention takes place in a regeneration phase or an operating phase with a rich air-fuel mixture, an active control in which from the internal combustion engine 10 Exhaust emissions contained in the exhaust gas, exothermic reactions associated with an increase in the exhaust gas temperature, depending on a current operating condition and in particular the currently existing exhaust gas temperature of the internal combustion engine 10 either to promote or suppress exiting exhaust gas.

Concretely, at a comparatively low exhaust gas temperature, the oxygen amount in a rich air-fuel mixture operating phase is set to a relatively high value, whereas at a comparatively high exhaust gas temperature, the oxygen amount is set to a relatively low value in rich air-fuel mixture operating phases Avoid exothermic reactions as much as possible.

The realization of the control of the oxygen content is carried out by varying at least one for the operation of the internal combustion engine 10 characteristic combustion parameter, wherein in one embodiment, the exhaust gas recirculation rate ("EGR rate") is varied to adjust the amount of oxygen.

According to 2 For example, the brake torque and the mean effective pressure (BMEP) and the lambda value are kept constant, while the exhaust gas recirculation rate ("EGR rate") for adjusting or adjusting the amount of oxygen is varied. In this case, the exhaust gas recirculation rate can be set to a high value in order to increase the oxygen (O ₂ ) amount and to promote exothermic reactions. On the other hand, the exhaust gas recirculation rate can be set to a low level in order to reduce the oxygen (O ₂ ) amount and to suppress exothermic reactions on the catalyst as much as possible.

In other embodiments, other combustion parameters such. B. in the intake of the engine 10 set boost pressure and / or injection timing, injection quantity and / or injection pressure of a fuel injection for adjusting the amount of oxygen can be varied.

As a result, according to the invention, by actively controlling the oxygen content in the engine 10 exiting exhaust gas, an effective regeneration of the NO _x storage catalytic converter and thus an effective reduction of nitrogen oxides in the exhaust gas can be achieved.

Claims (8)

A method for exhaust aftertreatment, with an exhaust gas aftertreatment device, which at least one NO _x storage catalytic converter (LNT) ( 130 ) for storing in the exhaust gas of an internal combustion engine ( 10 ) existing nitrogen oxides in operating phases with lean exhaust gas mixture and for converting nitrogen oxides in operating phases with rich exhaust gas mixture, characterized in that in operating phases with rich exhaust gas mixture at least temporarily the oxygen content in the from the internal combustion engine ( 10 ) exiting exhaust gas by varying at least one for the operation of the internal combustion engine ( 10 ) characteristic combustion parameter is controlled. A method according to claim 1, characterized in that the control of the oxygen content in the from the internal combustion engine ( 10 ) exiting exhaust gas as a function of the exhaust gas temperature of the engine ( 10 ) exiting exhaust gas takes place. A method according to claim 2, characterized in that with increasing exhaust gas temperature of the engine ( 10 ) exiting the exhaust gas based on the control of the oxygen content based setpoint for the oxygen content is reduced. Method according to one of claims 1 to 3, characterized in that the at least one combustion parameter is a rate of an exhaust gas recirculation (during operation of the engine 10 ) carried out exhaust gas recirculation. Method according to one of the preceding claims, characterized in that the at least one combustion parameter during operation in the intake tract of the internal combustion engine ( 10 ) includes set boost pressure. Method according to one of the preceding claims, characterized in that the at least one combustion parameter comprises an injection time, an injection amount and / or an injection pressure of a fuel injection. Method according to one of claims 1 to 5, characterized in that the at least one combustion parameter comprises a plurality of injection timing, different injection quantities and / or a plurality of injection pressures of a fuel injection. Exhaust gas aftertreatment arrangement, which comprises at least one NO _x storage catalytic converter (LNT) ( 130 ) for storing in the exhaust gas of an internal combustion engine ( 10 ) existing nitrogen oxides in operating phases with lean exhaust gas mixture and for the conversion of nitrogen oxides in operating phases with rich exhaust gas mixture, characterized in that it is adapted to perform a method according to any one of the preceding claims.

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