EP1252420A1

EP1252420A1 - Device and method for controlling a nox regeneration of a nox storage catalyst

Info

Publication number: EP1252420A1
Application number: EP01942402A
Authority: EP
Inventors: Ekkehard Pott; Sören HINZE
Original assignee: Volkswagen AG
Current assignee: Volkswagen AG
Priority date: 2000-01-14
Filing date: 2001-01-10
Publication date: 2002-10-30
Anticipated expiration: 2021-01-10
Also published as: WO2001051778A1; EP1252420B1; DE50108080D1; DE10001310A1

Abstract

The invention relates to a method for controlling an NOX regeneration of an NOX storage catalyst that is situated in the exhaust gas system of an internal combustion engine of a vehicle, comprising the following steps: (a) in each lean operating phase of the internal combustion engine, at least one status parameter of the NOX storage catalyst is determined using a measured or calculated NOX crude emission (mroh) of the internal combustion engine and an NOX breakdown emission (m) detected downstream in the exhaust gas by an NOX-sensitive measuring device; (b) a deviation of the status parameter of a current lean operating phase from the status parameters of a preceding lean operating phase is determined and (c) in the event that regeneration is necessary, an NOX regeneration is carried out n times according to the deviation (multiple NOX regeneration). The invention also relates to a device which has means for carrying out the steps of the inventive method.

Description

Device and method for controlling a NO _x regeneration of a NO _x storage catalytic converter

The invention relates to a device and a method for controlling a NO _x regeneration of a NO _x storage catalytic converter arranged in the exhaust line of an internal combustion engine for motor vehicles with the features mentioned in the independent claims

During the combustion process of an air-fuel mixture in the internal combustion engine, pollutants such as carbon monoxide CO, unburned hydrocarbons HC or nitrogen oxides NO _x are produced to different extents. To purify the exhaust gas, it is known to pass the exhaust gas through catalysts arranged in the exhaust line HC with oxygen on so-called oxidation catalysts converts to carbon dioxide and water. NO _x, on the other hand, is reduced by the reducing agents CO and HC on the NO _x storage catalyst. In lean operation, in particular in the consumption-optimized area of gasoline engines, with an air ratio of λ approximately 1.1, the reducing agent mass flows are sufficient no longer sufficient to ensure complete conversion of the NO Therefore, the catalyst is assigned a so-called NO _x storage device which absorbs the NO _x under these conditions (summarized with the catalyst component to form the NO _x storage catalyst).

A storage capacity of the NO _x storage catalytic converter is naturally limited, so that a NO _x regeneration must be initiated at regular intervals by changing to a rich atmosphere. A rich target value is usually specified for this purpose, until a downstream of the NO _x Lambda probe arranged below a predetermined rich threshold value Then normal operation of the internal combustion engine is resumed

After the change to a rich atmosphere, as described, the desorption of the stored NO _x and simultaneous reduction of the same with the now available reducing agents on the catalyst component of the NO _x storage catalytic converter takes place. At the beginning of the NO _x regeneration, this process proceeds in a kinetically controlled manner. there at an interface between the exhaust gas and the NO _x storage catalyst Sufficient NO _x is present. However, with increasing duration of the fat phase, diffusion processes take place to an ever greater extent, which are significantly slower. Such diffusion inhibition increases with increasing NO _x loading level and with increasing layer thickness of the NO _x storage. At the end of the fat phase, the NO _x stored in the form of nitrate must first diffuse from deeper layers of the NO _x storage in the direction of the interface. Diffusion inhibition is not taken into account in the conventional methods for controlling NO _x regeneration, because it is assumed here that complete NO _x regeneration is already given after the rich threshold value has been reached. In particular at high loading rates, however, the NO _x regeneration is actually only incomplete, so that a subsequent lean phase is shortened and the fuel consumption is usually increased as a result.

The object of the present invention is therefore to provide an apparatus and a method with which the NO _x regeneration can be controlled in such a way that a complete emptying of the NO _x store is ensured.

The described disadvantages of the prior art can be overcome by the method according to the invention and the device according to the invention for controlling the NO _x regeneration of the NO _x storage catalytic converter with the features mentioned in the independent claims. According to the procedure

(a) in each lean operating phase of the internal combustion engine, at least one state parameter of the NO _x storage catalytic converter on the basis of a measured or calculated _NOx -Rohemission the internal combustion engine and a downstream by an NO _x -sensitive measuring device detected NO _x in -Durchbruchsemission

Exhaust gas determined,

(b) a deviation of the state parameters of a current lean operating phase from the state parameters of a previous lean operating phase is determined and

(c) if there is a need for regeneration as a function of the deviation, the NO _x regeneration is carried out n times (multiple NO _x regeneration).

Thus, selected in compliance with state parameter of the _NOx - storage catalyst in certain operating situations, a multiple-NO _x regeneration initiated that cause the complete emptying of the _NOx catalyst. For this purpose, the device has means with which the aforementioned method steps can be carried out. These means can include a control device in which a procedure for controlling the multiple NO _x regeneration is stored in digitized form. The control device can be integrated into an already frequently existing engine control device become

In a preferred embodiment of the method are used as state parameters

a NO _x absorber function of the NO _x storage catalyst for a given NO _x raw material and / or

a NO _x breakthrough emission at a given NO _x raw emission and / or

a time interval, starting with a complete regeneration (NO _x or SO _x regeneration) until reaching a predetermined NO _x breakthrough emission and / or

a NO _x crude at a given NO _x absorptive function

determined

The sizes mentioned allow a particularly reliable determination of the need to initiate the multiple NO _x regeneration. In this way, an unnecessary multiple NO regeneration and thus an unnecessary additional consumption of fuel can be avoided

The multiple NO _x regeneration is preferably implemented in that a NO _x storage catalytic converter is initially charged n times with an exhaust gas in accordance with a rich target specification (rich phases). The rich phase ends when a lambda value in the downstream of the NO Spe storage catalytic converter Range of λ = 0.999 to 0.95 (rich threshold value) is reached With the end of the rich phases, the NO _x storage catalyst (n-1) times to n times is exposed to an exhaust gas in accordance with a lean target specification, for as long as until a lambda value in the range of λ = 1, 001 to 1, 2 (lean threshold value) is reached downstream of the NO _x storage catalytic converter Air conditions as they exist in normal operation in a consumption-optimized lean phase, so that only very small additional NO _x emissions can be expected. This short lean phase is already canceled when the lambda value has reached the lean threshold value, that is to say largely also an oxygen loading of the NO _x storage catalytic converter is completed. As an alternative to this, the lean target specification can also be specified as a function of the value of the rich target specification. Such a lean target specification is preferably to be selected to be at least 0.04 units larger than the rich target specification.

The number (n) of fat phases can in principle be predefined. However, it has proven to be advantageous to determine this as a function of the extent of the deviation in the state parameters. If, for example, the absorbency of the current lean phase has dropped significantly compared to a previous lean phase, the number is consequently increased. In order to prevent an increase against infinity, this number can preferably be limited by specifying a maximum value.

As an alternative or in combination with the aforementioned procedure when determining the number of fat phases, the success of previous multiple NO _x regenerations can also be taken into account. This can be done, for example, by comparing the state parameters before and after the previous multiple NO _x regeneration. If the multiple NO _x regeneration has not led to an improvement in the storage properties of the NO _x storage catalytic converter to the desired extent, the number of rich phases for the next multiple NO _x regeneration can be increased. Such a determination can be implemented in a particularly preferred and simple manner by specifying threshold values for the success of the previous multiple NO _x regeneration. If these threshold values are undershot, the number (n) is then increased. A reduction in the number (n) of the fat phases can of course take place in the same way per se, in that the number is reduced again in the case of previous "successful multiple NO _x regeneration", of course only up to a minimum number of two fat phases.

Further preferred embodiments of the invention result from the other features mentioned in the subclaims.

The invention is explained in more detail below in exemplary embodiments with reference to the associated drawings. Show it:

Figure 1 is a schematic diagram of an internal combustion engine with an exhaust gas purification system that includes a NO _x storage catalyst; FIG. 2 shows a curve of an absorption capacity as a function of a NO _x .

Raw emission,

FIG. 3 shows a course of a NO _x breakthrough emission as a function of time,

FIG. 4 shows the course of the air conditions downstream and upstream of the NO _x .

Storage catalyst during a multiple NO _x regeneration and

Figure 5 is a flowchart for controlling the multiple NO _x regeneration

FIG. 1 schematically shows an internal combustion engine 10 with an exhaust gas cleaning system 14 arranged in the exhaust line 12. The exhaust gas cleaning system 14 comprises a pre-catalytic converter 16, for example in the form of a three-way catalytic converter and a NO _x storage catalytic converter 18 arranged downstream thereof. Furthermore, the exhaust line 12 is equipped with a sensor This allows the local air conditions, gas compositions and temperatures to be recorded.To this end, up to two temperature sensors 20, 22 as well as two gas sensors 24, 26 are integrated in the exhaust line 12.Gas sensor 24 is, for example, a lambda probe, which then provides information about an air ratio directly after the internal combustion engine 10 there is the gas sensor 26 carries a NO _x sensor measuring device, so that NO _x emissions downstream of the NO _x storage catalytic converter 18 (NO _x breakthrough emissions) can be detected. Such a NO _x sensor usually also enables a g simultaneous determination of the air conditions, thus provides a lambda value downstream of the NO _x storage catalytic converter 18

The signals detected by the Sensonk are usually passed on to an engine control unit 28. The engine control unit 28 can, among other things, store procedures in digital form that allow control of a combustion process in the internal combustion engine 10 Intake volume in an intake manifold 32 with a throttle valve 34 and an injection system (not shown here) are influenced in such a way that rich, stoichiometric or lean mixtures are set up. Such a control is known and is therefore not to be explained in more detail here

Furthermore, there is a control device 36 - integrated here in the engine control device 28 - in which the signals detected by the sensors are also provided in the control device 36 is a procedure for controlling a multiple NO _x regeneration of the NO _x storage catalytic converter 18 in digital form

During operation of the internal combustion engine 10 under lean conditions, in particular from a consumption- _optimized point of view, a raw NO _x emission is usually increased. The reducing agents carbon monoxide CO and unburned hydrocarbons HC required for converting the NO _x are not sufficiently available under such conditions To remedy this, the catalyst has a NO _x storage device which binds NO _x as nitrate. Diffusion causes the nitrate to migrate into deeper layers of the NO storage with increasing duration of the lean phase. For complete NO _x regeneration, the nitrate bound in the deeper layers must be therefore first diffuse in the direction of an interface between the exhaust gas and the NO _x storage catalyst 18. Since this process is much slower than the reaction with the reducing agents CO and HC, there is a diffusion inhibition at the end of the NO _x regeneration To ensure constant emptying of the NO _x storage device, proceed as follows

First, in each lean operating phase of the internal combustion engine 10, at least one state parameter of the NO _x storage catalyst 18 is determined on the basis of the measured or calculated NO _x raw emissions m _ro h and the NO _x breakthrough emissions m detected by the NO _x sensitive measuring device Deviation of the state parameters of a current lean operating phase and the state parameters in a previous lean operating phase are determined and, if there is a need for regeneration of the NO _x storage catalyst 18, the NO _x regeneration is carried out n times as a function of the deviation (multiple NO _x regeneration)

A NO _x absorption factor A of the NO _x storage catalytic converter 18 with a predetermined NO _x raw emission m _r0 h / \ is suitable as the state parameter, for example. The determination of a deviation ΔA _j can be seen in FIG. 2. The NO _x absorption capacity A also decreases increasing raw emission m _ron At the beginning, that is to say after the end of a previous regeneration, the NO _x absorption factor A can ideally reach a value of 1, that is to say the entire raw emission is bound in the NO _x storage catalytic converter 18 and no breakthrough emission m With increasing loading, the absorption capacity A drops, since the nitrate bound to the interface must first diffuse into deeper layers of the NO _x storage. Overall, a NO _x storage capacity is already due to sulfurization or incomplete NO _x regeneration, the NO _x absorption factor A decreases more sharply. Examples of this are three curves 40, 42, 44. Curve 40 shows a curve of absorption capacity A with a fresh and unloaded NO _x storage catalytic converter 18 Curve 42 shows the course of the absorption capacity A in a NO _x storage catalytic converter 18, the storage capacity of which is already reduced compared to the fresh state, for example by thermal damage or sulfur loading. The curve 44 has been recorded in time following the curve 42, and here is the absorption capacity further reduced In intersections 46, 48, 50 with the given raw emission m _ron , the values AQ, A 'and A "for the absorption capacity are available. By forming the difference between the current size A" and the size A' results from the previous lean phase there is a deviation ΔA ^

In the same way, a time interval t can serve as the state parameter. FIG. 3 shows the time course of the breakthrough emission m of a fresh storage catalytic converter (curve 52), a course in a current lean phase (curve 56) and a course in a previous lean phase (curve 54) The time interval t begins with a complete regeneration (NO _x - or SO _x regeneration) and ends when a predetermined breakthrough emission m ^ is reached. The intersections 58, 60, 62 with the curves 52, 54, 56 therefore make it possible to determine the time intervals Determine tg, t 'and t "By forming the difference, one obtains a deviation Δt- _j between the time intervals t' and t"

In addition to the aforementioned state parameters, it should also be pointed out here that other parameters characterizing the storage capacity of the NO _x storage catalytic converter 18 can of course also be used. For example, a breakthrough emission m for a given raw emission m _{ron m} or a raw emission m _{ron can be} used a given absorption capacity A _{m roή} to increase the accuracy, the subsequently determined deviations of the entering condition parameters can be combined with each other. For a better comparison, the detected condition parameters can be standardized with the condition parameters of a fresh storage catalytic converter

Depending on the previously determined deviations ΔA < _| or Δt- | can now be determined whether the conditions for initiating the multiple NO _x regeneration are met. For this purpose, the deviations ΔA- _j , Δt- _j can be read into a map whose output size is a number n of the fat phases of the multiple NO _x regeneration is at very small deviations ΔA-], Λt- | n is set to the value 1, so that the NO _x regeneration is carried out in a manner known per se

FIG. 4 shows a curve of the lambda values upstream and downstream of the NO _x storage catalytic converter 18 during a multiple NO _x regeneration with two rich phases (n = 2). The solid bold line represents the lambda value in front of the NO _x storage catalytic converter 18, and the dashed line shows the course of the lambda value downstream of it

First of all, the internal combustion engine 10 is operated under lean conditions, the state parameters being continuously ascertained in the aforementioned manner. After a need for regeneration has been ascertained, a time T- _| a change to a first rich phase by regulating the internal combustion engine 10 to a rich target specification SVf. With a time delay caused by a dead volume, the lambda value downstream of the NO _x storage catalytic converter 18 drops to λ = 1. During this time, the absorbed NO _{x is converted} in the area the interface and the interface of nearby layers of the NO _x storage instead of From a point in time T2 λ continues to decrease. The reducing agents CO and HC formed to an increased extent by the change in a rich atmosphere are therefore no longer used in full to reduce NO _x

After falling below a rich threshold value SWf (time T3), a lean exhaust gas is again provided on the engine side, namely in accordance with a lean target value SV _m - _j. The rich threshold value is particularly in the range from λ = 0.999 to 0.95 and the lean target value SV _m <] in a range from λ = 1, 05 to 1, 5 Under the latter conditions the NO _x -Rohemιssιon m _ro f- emitted by the internal combustion engine 10 is relatively low or can be largely compensated for by the reducing agents CO, HC still present, so that a new absorption in the NO _x storage takes place only to a very small extent

In accordance with the change in atmospheric conditions, the lambda value downstream of the NO _x storage catalytic converter also changes. It initially rises to a value of λ = 1 and remains there until oxygen loading of the NO _x storage device is complete (time T4) when the If the lambda value exceeds a predetermined lean threshold value SW _m 1 at a time T5, the air-fuel mixture is set again on the engine side in accordance with the rich target specification SVf. The lean threshold value SW _m -j is in a range from λ = 1, 001 to 1, 2 If to one If the lambda value downstream of the NO _x storage catalytic converter 18 falls again below the threshold value SWf, normal operation of the internal combustion engine 10 is activated again. Under certain circumstances, it may be sensible to first replace the storage catalytic converter 18 corresponding to the lean one instead of a direct transition to normal operation Target SV _m - _| to be applied and only to resume normal operation from a point in time when the lean threshold value SW _m -j is exceeded. The number of lean and rich target values SV _m < _| , SVf is then the same

In addition to the preferred range described above for the value of the lean target specification SV _m - | it is also conceivable to determine the value as a function of the position of the rich target specification SVf. A lean target specification SV _m 2 exceeds the rich target specification SVf by at least 0.04, so that SV _m 2 may still be in the rich range, of course For the latter case, a new threshold value SW _m 2 must also be defined, which lies between the two target values SV _m 2, SVf

A number n of the fat phases during the multiple NO _x regeneration can preferably be determined in such a way that a value is output with the aid of the characteristic diagram as a function of a height of the deviations (for example ΔA-], Δt _η ). Alternatively or in combination can the number n depending on the state parameters A, m, t, ^m raw ^{before ur |} ^{A corresponding} multiple NO _x regeneration is to be determined from FIG. 5. A corresponding flowchart can be seen in FIG. 5. First, in a step S1, the extent to which the state parameters of a current lean phase differ from a previous lean phase is recorded If there is only a slight deviation, the number n is set to 1 (step S2) and a NO _x regeneration of a conventional type is carried out. With larger deviations, the number n is set to a value greater than 1 (step S3)

In a subsequent query, it is checked whether a previous multiple NO _x regeneration has been successfully carried out (step S4). For this purpose, the state parameters before and after the multiple NO _x regeneration are compared, with an increasing success, in principle, with increasing positive deviation , ie a larger NO _x storage capacity is available for the next lean phase. Such a query can be carried out in such a way that threshold values S \ N j \, SW | \ / | _m , SWM, t ^unc '^^ M, m, raw are specified if the number is undershot, the number n is increased to n = n + 1 If the deviation lies above the last-mentioned threshold values, a query S6 checks whether the number n can be reduced by 1 without the value becoming less than 2. If the answer is affirmative, the number n is set to n = n − 1 (step S8). Otherwise, the number n is set to n = 2 (step S7). If necessary, the determined value is still limited to a maximum value n _max (step S9).

Claims

PAT EN TA NSPRÜ CHE

1. A method for controlling an NO _x regeneration of an exhaust gas system in an internal combustion engine arranged for motor vehicles NO _x -

Storage catalytic converter, in which

(a) in each lean operating phase of the internal combustion engine, at least one state parameter of the NO _x storage catalytic converter on the basis of a measured or calculated _NOx -Rohemission (m _ro h) of the internal combustion engine and a downstream by an NO _x -sensitive measuring device detected NO _x -

Breakthrough emission (m) is determined in the exhaust gas,

(b) a deviation of the state parameters of a current lean

Operating phase is determined by the state parameters of a previous lean operating phase and

(c) if there is a need for regeneration depending on the deviation, the NO _x regeneration is carried out n times (multiple NO _x -

Regeneration).

2. The method according to claim 1, characterized in that as a state parameter

- A NO _x absorption capacity (A) of the NO _x storage catalytic converter for a given raw NO _x emission (m _ro [- * /) and / or

- A NO _x breakthrough emission (m) for a given NO _x raw emission ( ^m raw, m) and / or

a time interval (t) starting with a complete regeneration (NO _x - or SOχ regeneration) until reaching a predetermined NO _x breakthrough emission (m ^) and / or

- a raw NO _x emission (m _ron ) for a given NO _x absorption capacity

(Am, raw) be determined.

3. The method according to claim 2, characterized in that the determined state parameters (A, m, t, m _ro h) are standardized to the state parameters of a fresh NO _x storage catalytic converter (AQ, mg, tg, mroh.O).

4. The method according to any one of the preceding claims, characterized in that during the multiple NO _x regeneration of the NO _x storage catalyst

- An exhaust gas is applied n times in accordance with a rich target specification (SVf) (rich phases) until a lambda value in the range of λ = 0.999 to 0.95 (rich threshold value SWf) is reached downstream of the NO _x storage catalytic converter and

- Starting with the end of the rich phases of the NO _x storage catalytic converter (n-1) - times to n times with an exhaust gas according to a lean target (SV _m , l. SV _m 2) is applied until downstream of the NO _x storage catalytic converter a lambda value in the range from λ = 1, 001 to 1, 2 (lean threshold value SW _m - |, SW _m 2) is reached.

5. The method according to claim 4, characterized in that the lean target specification (SV _m ι) is in the range of λ = 1.05 to 1.5.

6. The method according to claim 4, characterized in that the lean target specification (SV _m 2) has a lambda value that is at least 0.04 above the rich

Target specification (SVf) lies.

7. The method according to any one of the preceding claims, characterized in that the number (n) of the fat phases is determined as a function of a height of the deviation of the state parameters (A, m, t, m _ron ).

8. The method according to claim 7, characterized in that the number (n) is limited by a predetermined maximum value (n _max ).

9. The method according to any one of the preceding claims, characterized in that the number (n) of the fat phases in dependence on the state parameters (A, m, t, m _ro h) is determined before and after a previous multiple NO _x regeneration.

10. The method according to claim 9, characterized in that the number (n) is increased if a deviation of the state parameters (A, m, t, m _ron ) before the multiple

NO _x regeneration and after it falls below a predetermined threshold value (SW ^ A, SW _{M m} , SW _{M t} , SW _{M m raw} ).

11. Device for controlling a NO _x regeneration of a NO _x storage catalytic converter arranged in the exhaust line of an internal combustion engine for motor vehicles, in which means are available with which

(a) in each lean operating phase of the internal combustion engine is a state parameter of the NO _x storage catalytic converter on the basis of a measured or calculated _NOx -Rohemission (m _ron) of the internal combustion engine and a downstream by an NO _x -sensitive measuring device detected NO _x - Druchbruchsemission (m) in Exhaust gas can be determined,

(b) a deviation of the state parameters of a current lean operating phase from the state parameters of a previous lean one

Operating phase can be determined and

(c) if there is a need for regeneration depending on the deviation, the NO _x regeneration can be carried out n times (multiple NO _x regeneration).

12. The device according to claim 11, characterized in that these means comprise a control device (36) in which a procedure for controlling the multiple NO _x regeneration is stored in digitized form.

13. The apparatus according to claim 12, characterized in that the control unit (36) is part of an engine control unit (28).