DE10038458B4 - Apparatus and method for exhaust gas purification - Google Patents

Abstract

Apparatus for exhaust gas purification for an internal combustion engine (10) with at least two NOx storage catalysts (41, 42) arranged in parallel, each of which is followed by at least one sensor (51, 52) Branch (25) is distributed to the NOx storage catalytic converters (41, 42), and a control means is arranged in the branch (25), by means of which the distribution of the exhaust gas flow to the NOx storage catalytic converters (41, 42) can be controlled, and with a control device (60) to which signals from the sensors (51, 52) are fed and which is set up to control the control means as a function of the signals, characterized in that the control unit (60) is further set up to control the control means as a function thereof to control which NOx storage catalytic converter (41, 42) is in one or more of the previously lean phases of the internal combustion engine (10) n NOx breakthrough occurred first and / or in which NOx storage catalytic converter (41, 42) a reducing agent breakthrough occurred first in one or more of the chronologically preceding rich phases.

Description

The invention relates to an apparatus and a method for exhaust gas purification according to the preambles of the independent claims.

From the EP 580 389 A1 and EP 0585 900 A1 So-called NO _x storage catalytic converters are known for use in the exhaust gas flow of lean-burn internal combustion engines. The lean operation of internal combustion engines has the advantage that this fuel consumption can be significantly reduced, the disadvantage is that during the lean operation in the raw exhaust a relatively large amount of nitrogen oxides (NO _x ) is present. To solve this problem, it is known to arrange a NO _x storage catalytic converter in the exhaust gas stream of the internal combustion engine, on the surface of which the nitrogen oxides are deposited and chemically bound, for example in accordance with the following reaction: 2BaO + O ₂ + 4NO ₂ → 2Ba (NO ₃ ) ₂

As a result, the amount of nitrogen oxides is significantly reduced in the exhaust stream of the catalyst. At certain intervals, such a storage catalytic converter must be regenerated because its storage capacity is exhausted. This happens because the internal combustion engine is operated rich for a certain period of time, so that in the exhaust gas downstream of the NO _x catalyst unburned hydrocarbons and carbon monoxide is. By means of these reducing agents, the storage catalyst is regenerated, the nitrogen oxides or the corresponding nitrates being reduced to nitrogen by the available reducing agents, for example with the following reaction: Ba (NO ₃ ) ₂ + 5CO → BaO + N ₂ + 5CO ₂

Ideally, these reducing agents are fully implemented so that neither nitrogen oxides nor unburned hydrocarbons or carbon monoxide are present in the final exhaust gas during the regeneration phase.

NOx storage catalysts are mainly used in direct injection systems, which make significant fuel savings possible in the so-called stratified charge mode. This enables safe combustion of extremely lean mixtures at idle and part load with fuel savings of> 40% at idle and approximately 15% over the entire Eurotest cycle MVEG. The fuel is injected via electromagnetic nozzle valves directly into the combustion chamber. In this case, the engine is operated with almost fully open throttle, which in addition charge exchange losses are avoided. Only after a certain demanded power, the engine must be operated again in conventional operation, in which the throttle position determines the amount of air / fuel mixture sucked. During the stratified charge operation, the NOx storage catalyst is operated in the absorption mode. In the homogeneous operation of the engine, however, a stoichiometric or for the regeneration of the NOx storage catalyst, a rich air / fuel mixture is supplied.

Usually, the NO _x storage catalytic converter is preceded by a 3-way catalytic converter, so that the entire exhaust gas purification system consists of two catalysts connected in series.

If, for reasons of packaging, the use of a single NOx storage catalytic converter is not possible, an exhaust gas line is divided behind the oxidation catalytic converter and the exhaust gas is fed to two parallel NOx storage catalytic converters.

Such a solution is used, for example, in four-wheel drive vehicles in which the propeller shaft makes the arrangement of a single cylindrical NOx catalyst impossible. On the other hand, in turbocharged engines with a single turbocharger, the arrangement of two completely self-sufficient exhaust strands from two pre-catalysts and two NOx storage catalysts is not possible.

When more than one NOx storage catalytic converter is used, its uniform application of NOx can not be assumed, since the function and efficiency of a NOx storage catalytic converter depend on a large number of influencing factors. However, it is not possible to detect the operating state, for example the NOx loading state or the end of the NOx regeneration of the individual NOx storage catalytic converters via sensor means arranged in the mixed exhaust gas downstream of the NOx storage catalytic converter, for example a downstream NOx sensor or a lambda probe ,

Cheaper is therefore the arrangement of sensor means downstream of each NOx storage catalyst. In the control of the NOx regeneration, the problem thereby arises of determining the end time of the regeneration mode under the condition that the NOx regeneration by the exhaust gas necessarily occurs simultaneously with several NOx storage catalytic converters connected in parallel. In particular, it must be avoided here that one or more of the catalysts are permanently only partially regenerated, which would increasingly limit their NOx storage capacity.

On the other hand, there is a risk that a catalyst regenerated rapidly relative to the other NOx storage catalysts suffers reductant breakdown, resulting in release of hydrocarbons into the environment.

Out WO 98/45582 A1 An exhaust gas purification device is known in which the exhaust gas flow is first distributed through a common passage section and then by means of a branching control means into two parallel passage sections to two parallel NOx absorbers, which may also have a catalytic component. The main exhaust gas flow is directed to the first NOx absorber until it has to be regenerated. Then, the main exhaust gas flow is switched to the second NOx absorber, while regenerated by feeding a reducing agent into the subsidiary exhaust gas flow upstream of the first NOx absorber. The timing of the regeneration end may be detected by means of an oxygen or NOx sensor disposed downstream of the regenerating NOx absorber. A similar principle is in DE 198 26 836 A1 described.

The object of the invention is therefore to specify a method and a device for purifying exhaust gas in an internal combustion engine with at least two parallel downstream of a branch of an exhaust line arranged regenerable NOx storage catalytic converters, which allow a complete regeneration of all NOx storage catalytic converters and at the same time a high degree of exhaust gas purification Afford.

This object is achieved with a device having the features of claim 1 and by a method having the features of claim 8.

The basic idea of the invention is to provide a control means in the branch which divides the exhaust gas flow, by means of which the distribution of the exhaust gas flow to the two NO _x storage catalytic converters can be influenced. It can thereby be achieved that both catalysts at least approximately exploit their maximum NO _x storage capacity and that both catalysts are completely or at least almost completely eliminated in the regeneration phase. This has the advantage that it allows the length of the lean phases to be maximized, and that a voiding of both catalysts in the regeneration phase can be ensured without this leading to reductant breakthroughs.

According to claim 3, a sensor is preferably mounted behind the two NO _x storage catalysts. By means of the signals generated by this sensor, the control means is controlled. Thus, the setting of the control means is influenced by the actual states of the catalysts.

According to claim 9, the behavior of the catalysts in one or more preceding lean and / or rich phases is taken into account in the control of the control means. This creates a feedback control loop with which very good results can be achieved with regard to the best possible utilization of the existing catalysts and a good adaptation to the current states of the catalysts is achieved.

The invention will now be explained in more detail by means of embodiments with reference to the figures. Show it:

1 : a schematic diagram of the arrangements according to the invention,

2 : a schematic sketch of the branch with built-in flap.

In the 1 and 2 the device according to the invention is shown schematically. The of an internal combustion engine 10 coming raw exhaust gases are by means of a first exhaust gas line 11 a precatalyst 20 fed. This precatalyst 20 is usually a 3-way catalyst. The through the pre-catalyst 20 Pre-cleaned exhaust gases pass through the second exhaust gas line 22 to the turnoff 25 , the the exhaust system in the third exhaust gas line 33 and the fourth exhaust gas line 34 Splits. In the turnoff 25 a finely adjustable or continuously adjustable flap is arranged, by means of which the distribution of the exhaust gas flow between the third exhaust gas line 33 and the fourth exhaust gas line 34 can be controlled. In the third exhaust gas line 33 is the first NO _x storage catalyst 41 arranged. Downstream of it is the first sensor 51 , It can either be a lambda probe, in particular a step response lambda probe, or a NO _x sensor. In the fourth exhaust gas line 34 is the second NO _x storage catalytic converter 42 , Downstream of the second NO _x storage catalyst is the second sensor 51 , It too is either a lambda probe, in particular a step response lambda probe, or a NO _x sensor. The signals of the two sensors 51 . 52 be the controller 60 fed. This in turn controls the one in the branch 25 arranged, serving as a control means flap 27 and gives signals to the engine controller 70 further. control unit 60 and engine control 70 Of course, they can also be integrated in a single device.

The type of control of the flap 27 can be done according to different schemes. Three such schemes are shown below. Which of these options is chosen in each individual case depends on the respective technical conditions. The three options presented are particularly preferred, but not exhaustive.

First embodiment

These are the two sensors 51 . 52 around step response lambda probes. The control of the flap 27 takes place relatively simply in this embodiment: In the lean phases is observed in which of the NO _x storage catalysts takes place first NO _x breakthrough. In the next lean phases then the flap 27 adjusted so that the exhaust gas flow is reduced in the first broken through NO _x storage catalytic converter. The adjustment of the flap 27 can be done, for example, by a fixed amount. The adjustment of the plate 27 in the fat takes place in an analogous manner. During the regeneration, it is observed in which of the two NO _x storage catalysts first a reducing agent breakthrough occurs, that is, behind which NO _x storage catalytic converter, the lambda value first drops. During the subsequent regeneration phase, the flap in 27 adjusted so that less exhaust gas flows to the first broken through NO _x storage catalytic converter.

In this simple embodiment, the influence of the flap 27 completely decoupled with respect to the lean and fatty phases. The influence is also only with regard to the immediately preceding lean or fat phase.

Second embodiment

Again, these are the two sensors 51 . 52 around step response lambda probes. However, unlike the first embodiment described above, only the reductant breakthroughs in the regeneration phases are observed here. However, one needs the behavior of the last and penultimate regeneration phase. In detail, the control of the flap happens 27 according to the following matrix: Reductant breakthrough in the penultimate regeneration phase first at the first NO _x storage catalyst 41 ? Yes No Reductant breakthrough in the last regeneration phase first at the first NO _x storage catalyst 41 ? Yes In the lean: flap towards A In the fat: flap towards A In the lean: flap towards A In the fat: flap towards B No In the lean: flap towards B In the fat: flap towards A In the lean: flap towards A In the fat: flap towards B

Here, "flap in direction A" means the flap 27 is adjusted so that less exhaust gas is the first NO _x storage catalyst 41 accrues. Accordingly, "flap in direction B" means that the flap is adjusted so that more exhaust gas is added to the first NO _x storage catalyst 41 accrues.

With this type of control, the storage capacity of the catalysts is taken into account. Likewise, a possible catalyst damage is considered, which can occur in a known manner Reduktionsmitteldurchbrüche before a complete NO _x regeneration.

Third embodiment

In this embodiment, the sensors are 51 . 52 around NO _x sensors. The NO _x breakdown behavior in lean operation and the reducing agent breakdown behavior in rich operation during the last cycle are considered. The control of the flap 27 then takes place according to the following matrix: Reductant breakthrough in the last regeneration phase first at the first NO _x storage catalyst 41 ? Yes No NO _x breakthrough in the last lean phase, first at the first NO _x storage catalyst 41 ? Yes In the lean: flap towards A In the fat: flap towards A In the lean: flap towards A In the fat: flap towards B No In the lean: flap towards B In the fat: flap towards A In the lean: flap towards A In the fat: flap towards B

The definition of the direction of the flap movements corresponds to the definition given above.

There are also other sensor arrangements or control schemes conceivable. In particular, it is conceivable that one of the two sensors is a NO _x sensor and the other sensor is a lambda probe.

In all embodiments, the signals in the sensors 51 . 52 also indirectly or directly to the engine control 70 passed. In particular, the information about the reductant breakthroughs is needed at the end of the regeneration phases for the engine control. As with previous systems, the engine will return to lean operation once a reductant breakthrough has been detected on either of the two NO _x storage catalysts, otherwise there would be an unnecessary environmental burden of carbon monoxide and incompletely burned hydrocarbons.

It is conceivable in all embodiments that the deflection of the flap 27 is limited to maximum values, if a complete shut-off of an exhaust gas line with severely damaged NOx catalyst is undesirable. Furthermore, status information may be passed to the diagnostic devices when the door is open 27 has a constantly strong deflection in one direction or very different positions in lean or rich operation. These operating conditions indicate a severely damaged catalyst.

The proposed devices and the proposed method are also suitable for structurally and / or production-related uneven loading of the two NO _x storage catalysts and to compensate for differences in performance of the catalysts within the series production.

LIST OF REFERENCE NUMBERS

10

internal combustion engine

11

first exhaust gas line

20

precatalyzer

22

second exhaust gas line

25

diversion

27

flap

33

third exhaust gas line

34

fourth exhaust gas line

41

first NO _x storage catalyst

42

second NO _x storage catalyst

51

first sensor

52

second sensor

60

control unit

70

motor control

Claims (9)

Device for exhaust gas purification for an internal combustion engine ( 10 ) with at least two parallel arranged NO _x storage catalysts ( 41 . 42 ), each having at least one sensor ( 51 . 52 ), whereby that of the internal combustion engine ( 10 ) coming exhaust gas flow via a branch arranged in the exhaust line ( 25 ) to the NO _x storage catalysts ( 41 . 42 ) and in the branch ( 25 ) a control means is arranged, by means of which the distribution of the exhaust gas flow to the NO _x storage catalytic converters ( 41 . 42 ) and with a control unit ( 60 ), which signals the sensors ( 51 . 52 ) and which is arranged to control the control means in dependence on the signals, characterized in that the control unit ( 60 ) is further arranged to control the control means in dependence on which NO _x storage catalytic converter ( 41 . 42 ) in one or more of the advanced lean phases of the internal combustion engine ( 10 ) NO _x breakthrough first occurred and / or in which NO _x storage catalyst ( 41 . 42 ) a reductant breakthrough first occurred in one or more of the fat phases preceding in time. Device according to claim 1, characterized in that the control means comprises a flap ( 27 ). Apparatus according to claim 1 or 2, characterized in that the sensors ( 51 . 52 ) Are lambda probes. Apparatus according to claim 1 or 2, characterized in that the sensors ( 51 . 52 ) NO _x sensors are. Device according to one of the preceding claims, characterized in that between combustion engine ( 10 ) and branch ( 25 ) an oxidation catalyst ( 20 ) is arranged. Apparatus according to claim 5, characterized in that the oxidation catalyst ( 20 ) is a 3-way catalyst. Process for exhaust gas purification in an internal combustion engine ( 10 ) with at least two parallel in the exhaust gas flow of an alternately lean and fat-operated internal combustion engine ( 10 ) arranged NO _x storage catalysts ( 41 . 42 ), wherein the two NO _x storage catalysts ( 41 . 42 ) in the exhaust gas flow downstream of a branching device having a control means ( 25 ) and behind each of the two NO _x storage catalytic converters ( 41 . 42 ) a sensor ( 51 . 52 ), and wherein the control of the control means taking into account that of the sensors ( 51 . 52 ), characterized in that the control of the control means depends on which NO _x storage catalytic converter ( 41 . 42 ) an NO _x breakthrough first occurred in one or more of the earlier lean phases and / or in which NO _x storage catalyst ( 41 . 42 ) a reductant breakthrough first occurred in one or more of the fat phases preceding in time. A method according to claim 7, characterized in that only the last lean and rich phase is taken into account in the control of the control means. A method according to claim 7, characterized in that in each case the last and penultimate fat phase is taken into account in the control of the control means.

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