EP1188915A2 - Method for regulating the regeneration of a NOx storage catalyst - Google Patents

Abstract

A process for controlling nitrogen oxide (NOx) regeneration in a NOx storage catalyst, located in the exhaust gas of a lean burn combustion engine, comprises establishing the catalyst damage and then controlling the flow of a reducing agent during regeneration. Regeneration is stopped once a voltage threshold in an exhaust gas probe downstream of the catalyst has been reached.

Description

The invention relates to a method for controlling a NO _x regeneration of a NO _x storage catalytic converter with the features mentioned in the preamble of claim 1.

For cleaning an exhaust gas from internal combustion engines, it is known to arrange the same catalyst systems in the exhaust line, which decompose the pollutant components of the exhaust gas into less environmentally relevant products. If the pollutant components are reducing agents such as carbon monoxide CO or incompletely burned hydrocarbons, they can, if there is sufficient oxygen, be oxidized to carbon dioxide and water in so-called oxidation catalysts. It is also known to convert nitrogen oxides NO _x formed during the combustion process back to nitrogen using the reducing agents. For this purpose, reduction catalysts have been developed which, if the reducing agents are provided to a sufficient extent, allow the NO _x to be converted almost completely.

A mass flow of reducing agent as well as an extent of NO _x formation during the combustion process depend strongly on the mixture ratios prevailing during the combustion. In phases of rich or stoichiometric operation, a largely complete implementation on the NO _{x is} guaranteed. If, however, the system is switched to lean operation, the NO _x reduction is reduced due to an excess of oxygen. Since lean operation has proven to be particularly economical, but high NO _x emissions should be avoided for environmental reasons, NO _x storage components have been developed to remedy this. Such storage components sorb NO _x in phases of lean operation and store this as nitrate. When changing to rich or stoichiometric operation, the NO _x is desorbed again. The catalyst component for reducing the NO _x and the storage component can be combined to form a so-called NO _x storage catalyst.

A storage capacity of the NO _x storage catalytic converter is naturally limited. Such a NO _x storage catalytic converter must therefore be regenerated at regular intervals. The NO _x regeneration takes place by changing to stoichiometric or rich operation, the stored nitrate being desorbed again as NO _x and reduced to nitrogen on the catalyst component with the aid of the reducing agents.

The NO _x regeneration in NO _x storage catalytic converter systems is initiated, for example, after reaching a predefined NO _x loading state or after exceeding a predefined NO _x concentration downstream of the NO _x storage catalytic converter. If there is a need for regeneration, a mass flow of reducing agent is increased by a motor intervention, for example in the area of the throttle valves or the injection system. The increase is usually achieved by lowering a lambda value upstream of the NO _x storage catalytic converter to a predetermined rich setpoint. The NO _x regeneration is generally terminated when the lambda value, or a lambda proportional voltage correlating therewith, on a gas probe arranged downstream of the NO _x storage catalytic converter exceeds a predetermined threshold value (regeneration termination voltage threshold). Both the regeneration termination voltage threshold and the rich target value for lambda during NO _x regeneration are defined in the prior art regardless of the catalytic converter state. This is associated with the risk that the NO _x regeneration of aged, ie damaged, NO _x storage catalytic converters is not carried out as required. With very large reducing agent mass flows and low regeneration termination voltage thresholds, only a partial regeneration of the NO _x storage catalytic converter can be expected. The high reducing agent mass flow is only partially used for NO _x regeneration and thus shortly after the start of the NO _x regeneration, a slightly rich exhaust gas passes through the NO _x storage catalytic converter and leads to the termination of the NO _x regeneration.

It is therefore an object of the present invention to provide a generic method with which the NO _x regeneration can be carried out as required, taking into account the state of the catalyst.

According to the invention, this object is achieved by the method for regulating a NO _x regeneration of a NO _x storage catalytic converter arranged in the exhaust line of a lean-burn internal combustion engine with the features mentioned in claim 1. Characterized in that a damage state of the NO _x storage catalytic converter is detected and, depending on the damage condition, a reducing agent mass flow during the NO _x regeneration and / or a regeneration termination voltage threshold of a determining lambda gas probe arranged downstream of the NO _x storage catalytic converter are specified. the parameters determining the NO _x regeneration can be adapted to the actual catalyst state and an almost complete NO _x regeneration can be ensured.

According to a preferred embodiment of the method, the damage state of the NO _x storage catalytic converter is assessed on the basis of a current NO _x storage capacity in comparison to an NO _x storage capacity of an undamaged NO _x storage catalytic converter. The NO _x storage capacity can be determined using known models on the basis of the signals of the exhaust gas cleaning system provided by the sensors. Here, variables such as a current NO _x loading state and a remaining NO _x storage capacity can be taken into account.

As the degree of damage to the NO _x storage catalytic converter increases, the reducing agent mass flow can then be changed by regulating the lambda value upstream of the NO _x storage catalytic converter, in particular the lambda value can be shifted in the direction λ = 1 during NO _x regeneration. On the other hand, the regeneration termination voltage threshold can be increased. The former measure reduces the likelihood of reducing agent mass breakthroughs, since an almost complete conversion in the NO _x storage catalytic converter is still ensured even at high space velocities of the exhaust gas. By increasing the regeneration termination voltage threshold, a higher NO _x breakthrough emission can be tolerated in addition or in combination.

Further preferred refinements of the invention result from the others in the Characteristics mentioned subclaims.

Figur 1

eine schematische Anordnung eines NO_x-Speicherkatalysators im Abgasstrang einer magerlauffähigen Verbrennungskraftmaschine und

Figur 2

Spannungsverläufe einer Lambdasonde während einer NO_x-Regeneration eines frischen und eines geschädigten NO_x-Speicherkatalysators sowie Sollvorgaben für Lambda stromauf des NO_x-Speicherkatalysators und Regenerationsabbruch-Spannungsschwellen.

The invention is explained in more detail in an exemplary embodiment with reference to the accompanying drawings. Show it:

Figure 1

a schematic arrangement of a NO _x storage catalyst in the exhaust line of a lean-burn internal combustion engine and

Figure 2

Voltage curves of a lambda probe during a NO _x regeneration of a fresh and a damaged NO _x storage catalytic converter as well as target values for lambda upstream of the NO _x storage catalytic converter and regeneration termination voltage thresholds.

1 shows a schematic arrangement of a lean-burn internal combustion engine 10 with an exhaust gas purification system 12. The exhaust gas purification system 12 comprises at least one NO _x storage catalytic converter 14 and a suitable sensor system for detecting operating parameters in the exhaust line 16. This sensor system includes downstream and upstream of the NO _x Storage catalytic converter 14 arranged gas probes, here the lambda probes 18, 20 and a NO _x sensitive measuring device 22.

The signals provided by the sensors are fed into an engine control unit 24 imported and evaluated there using stored algorithms. About the Engine control unit 24 can then perform a control intervention to change the during a Combustion process of the internal combustion engine 10 prevailing conditions respectively. A throttle valve 26 in an intake duct is an example of an actuator here 28 shown. A change can then be made by changing a throttle valve angle Intake volume of the internal combustion engine 10 are influenced. Are conceivable also interventions in the combustion process - especially one not here shown injection system - by varying injection parameters, such as one Injection duration, injection angle, injection volume and the like. Procedure and Devices for carrying out such control interventions are well known and are therefore not explained in more detail here. All that remains to be said is that in this way, a reducing agent mass flow is specifically increased or decreased can be.

The engine control unit 24 is also integrated into a control unit 30, with which one Adaptation of a regeneration termination voltage threshold and / or the Reductant mass flow depending on a catalyst state can. For this purpose, a corresponding algorithm is stored in the control unit 30. The Control unit 30 can of course also be implemented as an independent unit become.

The catalyst state can also be quantified using a model and made available to the further process, for example, as a percentage degree of damage. A current NO _x storage capacity, that is to say the NO _x mass, which can be maximally sorbed by the NO _x storage catalytic converter 14, serves in particular as the evaluation criteria. The NO _x storage capacity can in turn be determined with the aid of known models in which a NO _x mass flow upstream of the NO _x storage catalytic converter 14 is detected and a NO _x breakthrough emission downstream of the NO _x storage catalytic converter is checked. Ultimately, the current catalytic converter state can then be characterized, for example, as 85% of a fresh NO _x storage catalytic converter.

FIG. 2 shows, for a fresh and a damaged NO _x storage catalytic converter 14, on the one hand a voltage curve at the downstream lambda probe 20 and on the other hand target values SV _i for lambda upstream of the NO _x storage catalytic converter 14 and regeneration termination voltage thresholds S _i . The voltage curve at the lambda probe 20 during the NO _x regeneration of a fresh NO _x storage catalytic converter is given by the curve 40 (bold solid line) and for the already damaged NO _x storage catalytic converter by the curve 42 (dashed line). First of all, there is a need for regeneration for aged NO _x storage catalysts at a time T _R. This can be determined, for example, using the NO _x -sensitive measuring device 22 in a known manner on the basis of a detected NO _x breakthrough emission. The internal combustion engine 10 then changes to a rich working mode, so that the NO _x storage catalytic converter 14 is subjected to an increased mass flow of reducing agent. The target specifications SV _i , which are specified during this rich regeneration phase of the internal combustion engine 10 by suitably influencing the actuators for the lambda value upstream of the NO _x storage catalytic converter 14, are dependent on the catalytic converter state. The same applies to the regeneration termination voltage thresholds S _i , when they reach lambda probe 20, lean operation of internal combustion engine 10 is resumed.

A curve 44 shows the target profile of lambda during NO _x regeneration of a fresh NO _x storage catalytic converter 14 (double, solid line). In order to carry out the NO _x regeneration as quickly as possible, a relatively low setpoint SV ₁ , for example at λ = 0.85, is selected. Towards the end of the NO _x regeneration, the curve 40 for the voltage signal at the lambda probe 20 shows a rapid increase, which results from the sudden increase in the reducing agent breakthrough emission. If the voltage reaches a regeneration termination voltage threshold S ₁ , for example at 650 mV (time T _{E, 1} ), the internal combustion engine 10 is switched back to lean operation.

If the NO _x storage catalytic converter 14 is already damaged, the mass flow of reducing agent is reduced. This is done by appropriately specifying a target specification SV ₂ for the rich regeneration phase - here, for example, to a lambda value of approximately 0.92. As a result of this, the mass flow of reducing agent also drops, so that it can be prevented that a slightly rich exhaust gas passes through the NO _x storage catalytic converter 14 shortly after the start of the NO _x regeneration and that the NO _x regeneration is possibly incompletely terminated. Furthermore, the regeneration termination voltage threshold S _{2 is also raised} for this catalyst state in the present case. This also extends the rich regeneration phase, so that an almost complete NO _x regeneration of the NO _x storage catalytic converter 14 is ensured and a change to the lean operation takes place only at a time T _{E, 2} (curve 46; double, dashed line).

LIST OF REFERENCE NUMBERS

10

Internal combustion engine

12

emission control system

14

NO _x storage catalytic converter

18

exhaust gas line

18, 20

lambda probes

22

NO _x sensitive measuring device

24

Engine control unit

26

throttle

28

intake port

30

control unit

40

Curve for the voltage curve during the NO _x regeneration of a fresh NO _x storage catalytic converter

42

Curve for the voltage curve during the NO _x regeneration of a damaged NO _x storage catalytic converter

44

Curve for the target values of lambda during the NO _x regeneration of a fresh NO _x storage catalytic converter

46

Curve for the target values of lambda during the NO _x regeneration of a damaged NO _x storage catalytic converter

SV _i

Target specification for lambda in rich working mode

S _i

Regeneration termination voltage threshold

T _R

Time of the need for regeneration

T _{E, i}

Times of the existence of the regeneration termination voltage thresholds

Claims (6)

Method for regulating a NO _x regeneration of a NO _x storage catalytic converter arranged in the exhaust line of a lean-burn internal combustion engine, characterized in that a damage state of the NO _x storage catalytic converter (14) is detected and, depending on the damage condition, a reducing agent mass flow during the NO _x Regeneration and / or a regeneration termination voltage threshold (S _i ) can be specified on an ascertaining exhaust gas probe (20) arranged downstream of the NO _x storage catalytic converter (14). Method according to Claim 1, characterized in that the damage state of the NO _x storage catalytic converter (14) is assessed on the basis of a current NO _x storage capacity in comparison with an NO _x storage capacity of an undamaged NO _x storage catalytic converter. Method according to one of claims 1 or 2, characterized in that the reducing agent mass flow is influenced by regulating a lambda value upstream of the NO _x storage catalytic converter (14). Method according to Claim 3, characterized in that the lambda value is shifted in the direction λ = 1 during the NO _x regeneration with increasing degree of damage to the NO _x storage catalytic converter (14). Method according to one of the preceding claims, characterized in that the regeneration termination voltage threshold (S _i ) is increased with increasing degree of damage to the NO _x storage catalytic converter (14). Method according to one of the preceding claims, characterized in that the exhaust gas probe (20) is a lambda probe and / or a NO _x sensor.

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