DE10114523B4 - Method for controlling a NOx regeneration of an arranged in the exhaust line of an internal combustion engine NOx storage catalyst - Google Patents

Abstract

A method for controlling a NO _x regeneration of an in the exhaust line (14) of an internal combustion engine (10) arranged NO _x storage catalytic converter (22), wherein the NO _x storage catalytic converter (22) during the NO _x regeneration with a rich exhaust gas (λ <1) and a lambda value is detected by means of an oxygen-sensitive measuring device (16) arranged downstream of the NO _x storage catalytic converter (22) and provided in an evaluation and control unit (18), characterized in that an end time (t _S ) the NO _x regeneration with reaching a first threshold value (S ₁ ) for the time derivative of the lambda value (dU / dt) and / or reaching a second threshold value for a change in the time derivative of the lambda value (d ² U / dt ² ) determined becomes.

Description

The invention relates to a method for controlling a NO _x regeneration of an arranged in the exhaust line of an internal combustion engine NO _x storage catalytic converter with the features mentioned in the preamble of claim 1.

Methods and emission control systems for internal combustion engines are known from the prior art, with which an emission of environmentally relevant exhaust gas components can be reduced. Depending on the design, the emission control systems include particulate filters and catalyst systems that convert the pollutants produced during the combustion of an air-fuel mixture into less environmentally friendly products. Known catalyst systems include on the one hand catalyst components that allow oxidation of pollutants such as carbon monoxide CO and incompletely burned hydrocarbons HC, with air-oxygen and on the other hand allow a reduction of nitrogen oxides NO _x with the reducing agents CO, HC. Furthermore, it is known to integrate gas-specific storage components in such catalyst systems. For example, it is possible to selectively absorb HC or NO _x . The latter storage component can be advantageously combined with the reductive and oxidative catalyst components to form a so-called NO _x storage catalyst.

An absorption of NO _x in the NO _x storage catalyst always takes place when a reducing agent mass flow for catalytic reduction is not sufficient - so in particular when the internal combustion engine is in a lean phase of operation. However, the NO _x storage process is limited due to a finite NO _x storage capacity of the NO _x storage catalytic converter, so that in order to prevent NO _x breakthrough at regular intervals regeneration must be initiated. For this purpose, the internal combustion engine is briefly driven under a stoichiometric or rich operating mode, so that the necessary reducing agent mass flow can be made available. A regulation of the composition of the air-fuel mixture (lambda control) for setting the respectively required operating mode is well known and will therefore not be explained in detail at this point.

It is also known to regulate the NO _x regeneration by specifying certain regeneration parameters. For example, it is possible to determine via such regeneration parameters under which circumstances the NO _x regeneration is to be initiated. A NO _x regeneration need may be determined such that a limit value for a NO _x emission measured downstream of the catalyst or a remaining NO _x storage capability of the NO _x storage catalytic converter is formed. If the NO _x storage capacity falls below the limit value, the NO _x regeneration is necessarily initiated. Furthermore, the regeneration parameters also include directly the parameters that must be influenced to perform the regeneration. Thus, for example, a specific lambda value, a catalyst temperature or even specific injection parameters that regulate a post-injection, for example, can be specified. The abundance of conceivable measures in the field of monitoring, maintenance and operation optimization of the emission control system can be coordinated in a known manner by means of a common evaluation and control unit - possibly as part of an existing engine control unit.

A regeneration end is usually determined according to the known methods such that a jump signal of an oxygen-sensitive measuring device downstream of the NO _x storage catalytic converter is evaluated. The jump signal results from the change in the lambda value taking place at the end of the NO _x regeneration downstream of the NO _x storage catalytic converter. Although with the intake of NO _x regeneration of the NO _x storage catalyst is exposed to a rich or stoichiometric exhaust gas, however, the reducing agents are largely consumed by the stored NO _x , so that initially no significant lambda change is detected downstream of the NO _x storage catalytic converter. Only when the NO _x regeneration is largely completed, the atmosphere changes around said measuring device. In controlled methods, the detection of the jump signal leads to the resumption of normal operation and thus in most cases to the resumption of lean operation of the internal combustion engine. The disadvantage of such an approach is that this always a certain override must be taken into account, as is still stoichiometric or rich mixture in the exhaust system between the internal combustion engine and the NO _x storage and also each control intervention can be implemented only with a certain time delay , As a result, at the end of the regeneration, the emission of the likewise environmentally relevant reducing agents increases significantly (reducing agent breakthrough) and fuel consumption is increased due to the unnecessarily prolonged regeneration period.

DE 198 23 921 A1 describes a method for checking the efficiency of a NOx storage catalytic converter, in which the exceeding of a predetermined threshold value for the signal of an O ₂ -sensitive sensor arranged downstream of the NOx catalyst indicates the end time of the regeneration phase.

The object of the present invention is to provide a method with which an end time of the NO _x regeneration can be advanced without endangering the completeness of the regeneration.

According to the invention this object is achieved by a method for controlling a NO _x regeneration of an arranged in the exhaust line of an internal combustion engine NO _x storage catalytic converter according to the features mentioned in claim 1. Characterized in that the end time of the NO _x regeneration upon reaching a first threshold value (S ₁ ) for the time derivative of the lambda value (dU / dt) and / or with reaching a second threshold value for a change in the time derivative of the lambda value (d ² U / dt ² ) is determined, the regeneration period can be meaningfully shortened. The stoichiometric or rich mixture which is still present between the internal combustion engine and the NO _x storage catalytic converter at this time is normally sufficient to allow a largely complete regeneration of the storage catalytic converter. Along with this, unwanted reductant breakthroughs are reduced and under some circumstances even completely suppressed. With the overall shorter regeneration period (residence time of the internal combustion engine in rich or stoichiometric operating mode) also reduces the fuel consumption.

With Specifying the fixed thresholds can be a simple but timely very rapid control of the regeneration process can be provided. Preferably, when the end time is reached, a combustion air ratio of Internal combustion engine raised.

Furthermore, it is preferred to release the method only when a predetermined desired signal quality is maintained until the end time. This is to avoid that signal outliers or strongly changing mixture conditions during regeneration lead to premature termination and thus only incomplete restoration of NO _x storage capacity. The release of the method may also be carried out in dependence on the combustion air ratio and / or an exhaust gas mass flow and / or an exhaust gas temperature and / or a catalyst temperature and / or a current lambda value before the NO _x storage catalytic converter. No release of the process occurs when a fuel cut is performed during the NO _x regeneration. By taking into account the mentioned criteria, it can be ensured that the NO _x storage capacity of the storage catalytic converter is always almost completely restored. If disturbances occur during regeneration, regeneration is continued under conventional conditions to achieve this primary goal.

at the internal combustion engine is a gasoline engine, in particular a directly injecting gasoline engine, or to a Diesel engine, in particular a direct injection diesel engine.

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

The The invention will be explained in more detail with reference to the accompanying drawings. It demonstrate:

1 a schematic diagram of an internal combustion engine with a NO _x storage catalytic converter in the exhaust system and

2 a profile of the lambda signal value and its slope downstream of the NO _x storage catalytic converter.

The 1 shows a schematic diagram of an internal combustion engine 10 with an emission control system 12 , The internal combustion engine 10 For example, a gasoline engine, in particular a directly injecting gasoline engine. In any case, it has an injection system (not shown here) with which injection parameters such as an ignition angle, an injection quantity, an injection duration and possibly post-injection parameters can be influenced. Furthermore, the air volume flows can be controlled or regulated, for example, by an exhaust gas recirculation device or a throttle valve, which is likewise not registered here. As a reference variable for a control, for example, in the exhaust system 14 serve detected lambda value. This is done with the help of an oxygen-sensitive measuring device 16 recorded and in an evaluation and control unit 18 - as an integral part of an engine control unit - read and evaluated. About the engine control unit are then the actuators, such as the injection system, the throttle and the exhaust gas recirculation device, for a desired operating mode of the internal combustion engine 10 specify required manipulated variables.

The emission control system 12 comprises a catalyst system which, as shown here, consists of a precatalyst 20 and a NO _x storage catalyst 22 consists. The emission control system 12 may include additional sensor technology that allows, an exhaust gas composition, a concentration of individual exhaust gas components or a temperature of the exhaust gas or individual components of the emission control system 12 capture. The measuring device 16 is in particular a lambda probe. But it can also be designed as a NO _x sensor, as this principle, in addition to its NO _x -sen tive function the lambda value downstream of the storage catalytic converter 22 can capture.

The NO _x storage catalytic converter 22 stores in lean operating phases of the internal combustion engine 10 NO _x , since in these operating phases a reductant mass flow is often too low to allow complete conversion. As the operating time increases, a NO _x storage capacity and, consequently, the NO _x storage capacity decrease. Therefore, the NO _x storage catalytic converter 22 be regenerated at regular intervals by forcing a change to stoichiometric or rich operation. The settings of the regeneration parameters and their adaptation to the possibly new conditions that exist in the emission control system 12 can be coordinated with the help of the engine control unit.

Of the 2 Among other things, a variation of the lambda value λ downstream of the NO _x storage catalytic converter can be achieved 22 in the form of a typical signal curve for a lambda probe (curve 24 ). Below the curve 24 is a curve of the slope of this lambda signal value as a curve 26 played. Immediately after detection of a need for regeneration of the NO _x storage catalytic converter 22 becomes a change to a stoichiometric or rich operating mode of the internal combustion engine 10 enforced (time t ₁ ). With a time delay caused by the dead volume, the signal subsequently rises in the sequence downstream of the NO _x storage catalytic converter 22 arranged measuring device 16 abruptly until there is a balance in the reduction processes in the NO _x storage catalyst 22 has set (time t ₂ ). During this phase of NO _x regeneration, the signal rises only slightly constant, since due to increasing depletion of oxygen-supplying gas components (NO _x , O ₂ ) in the catalyst, the conditions are changed only gradually. From a time t ₃ , the NO _x regeneration is substantially completed and as a result the atmosphere changes at the measuring device 16 drastically and the signal rises sharply. A corresponding course of the curve 26 Of course this results.

In contrast to the conventional methods, a regeneration end is not achieved by reaching a certain specification for the signal of the measuring device 16 displayed. Namely, this signal does not directly provide information about the actual progress of the NO _x regeneration. Rather, a threshold value S ₁ for the temporal signal increase dU / dt of the curve 26 predetermined, with the achievement of an imminent end of a complete NO _x regeneration is fictitious. This end time t _S is an indication of the actual conditions in the NO _x storage catalytic converter 22 , Towards the end of the regeneration, the reducing agents increasingly attack the stored oxygen, which in its chemical reaction is more inert than the stored NO _x . This results in an increase in the signal slope (curve 26 ). This increase is just used by specifying the threshold value S ₁ for the engine-side termination of the regeneration. As a result, the regeneration is terminated earlier and, if appropriate, the internal combustion engine 10 be operated stoichiometrically or lean again as in conventional process management again.

Alternatively or in addition to the specification of the threshold value S ₁ , it is conceivable to set a threshold value for a change over time of the signal gradient ΔU (= d ² U / dt ² ) of the curve 26 pretend. In this way, the safety of the prognosis on the progress of the NO _x regeneration can be increased and a premature termination of the NO _x regeneration can be avoided.

The latter requirement can also be taken into account by including further boundary conditions for releasing the method. Thus, in particular the presence of a predetermined desired signal quality can reduce an error of the prognosis due to signal outliers and the like up to the time t _S. It can also be provided that the method is only released when permissible ranges for a combustion air ratio of the internal combustion engine 10 , an exhaust gas mass flow, an exhaust gas temperature, a catalyst temperature and / or a lambda signal are maintained. Also, the method can be suppressed when disturbances on the NO _x regeneration, such as fuel cut, are detected. Overall, it can be ensured in this way that the NO _x storage catalytic converter 22 in any case, at least as far as possible, regenerated. If the method is enabled, ie if there are no disturbing influences and if the parameters are in the desired ranges, the end time t _S can be advanced significantly with the aid of the method, so that as a consequence a reduction agent breakthrough and also fuel consumption are reduced.

10

Internal combustion engine

12

emission control system

14

exhaust gas line

16

oxygen-sensitive measuring device

18

evaluation and control unit

20

precatalyzer

22

NO _x storage catalyst

24

Curve for the Waveform of a lambda broadband probe

26

Slope of the curve 24

λ

lambda value

t _1-3

timings

t _s

end time

S ₁

threshold

U

Voltage / waveform

you

signal slope

.DELTA.U

modification the signal slope

Claims (6)

Method for controlling a NO _x regeneration in the exhaust gas line ( 14 ) an internal combustion engine ( 10 ) arranged NO _x storage catalytic converter ( 22 ), in which the NO _x storage catalytic converter ( 22 ) during the NO _x regeneration with a rich exhaust gas (λ <1) is acted upon and by means of a downstream of the NO _x storage catalytic converter ( 22 ), oxygen-sensitive measuring device ( 16 ) detects a lambda value and in an evaluation and control unit ( 18 ), characterized in that an end time (t _S ) of the NO _x regeneration upon reaching a first threshold value (S ₁ ) for the time derivative of the lambda value (dU / dt) and / or when a second threshold value for a change the time derivative of the lambda value (d ² U / dt ² ) is determined. A method according to claim 1, characterized in that the end time (t _S ) with reaching the first and second threshold value (S ₁ , S ₂ ) is determined. A method according to claim 1 or 2, characterized in that upon reaching the end time (t _S ), a combustion air ratio of the internal combustion engine ( 10 ) is raised. Method according to claims 1 to 3, characterized in that a release of the method takes place when a predetermined desired signal quality is present until the end time (t _S ). Method according to claims 1 to 4, characterized in that a release of the method as a function of the combustion air ratio and / or an exhaust gas mass flow and / or an exhaust gas temperature and / or a catalyst temperature and / or a current lambda value before the NO _x storage catalytic converter ( 22 ) he follows. Method according to claims 1 to 5, characterized in that a release of the method is suppressed, if during the NO _x regeneration, a fuel cut occurs.