Device and method for controlling the nox regeneration of a nox storage catalyst

Abstract

The invention relates to a method and device for controlling the NOx regeneration of a NOx storage catalyst (14) disposed in the exhaust gas train (12) of an internal combustion engine (10) of a motor vehicle. The NOx regeneration is at least initiated when a threshold value is exceeded for a load state of a NOx storage catalyst (14) or a NOx emission downstream from the NOx storage catalyst (14). According to the invention, it is detected (a) whether the internal combustion engine (10) is idling and (b) alternately or in any possible combination, the threshold value for the load state or the NOx emission is increased; the NOx regeneration is only initiated after a specific amount of time has elapsed and a current NOx regeneration is interrupted when a shift occurs into an idling mode.

Description

BACKGROUND OF THE INVENTION

The invention relates to a method and device for controlling the NO_x regeneration of a NO_x storage catalyst located in the exhaust gas channel of an internal combustion engine of a motor vehicle and having the features recited in the preambles of the independent claims.

It is known to integrate an exhaust gas cleaning device in the exhaust gas channel for the purpose of cleaning the exhaust of internal combustion engines. The exhaust gas cleaning device typically includes components such as a particle filters or catalysts. If a raw emission of NO_x of the internal combustion engine is to be reduced, then these catalysts include a reduction catalyst. If the mass flows of reducing pollutants, such as carbon monoxide CO and incompletely combusted carbohydrides CH are sufficiently large in the region of the reduction catalyst, then the reducing agent NO_x promotes a conversion to nitrogen.

To minimize fuel consumption, it has proven to be advantageous to operate the internal combustion engine under the lean air conditions. However, the operation in the range optimized for fuel consumption is associated, on one hand, with increased NO_x emission and, on the other hand, with reduced mass flows of reducing agents. To prevent high NO_x emission levels, a NO_x storage device is associated with the catalyst which absorbs the NO_x in form of a nitrate. The NO_x storage device can be combined with the catalyst as a so-called NO_x storage catalyst.

The mass storage capacity of the NO_x storage catalyst is, of course, limited, so that a NO_x regeneration has be performed in regular intervals to prevent NO_x breakthroughs. During the NO_x regeneration, the operating mode changes to stoichiometric or rich. The NO_x that had been absorbed in the form of nitrate is thereby released. Typically, a NO_x regeneration is initiated when a threshold value for a load state of the NO_x storage catalyst or a NO_x emission detected downstream by a NO_x-sensitive measuring device (breakthrough emission) is exceeded. This has the disadvantage that identical criteria are applied for all operating phases of the motor vehicle to determine the need for regeneration. However, if the NO_x regeneration is initiated in the idle phase, significantly less exhaust gas flows and therefore the mass flows of reducing agents that are present are also smaller, so that the desorbed NO_x can only be incompletely reduced on the catalyst component. NO_x regeneration during the idle phase does not only result in an undesirably high NO_x emission, but also leads to increased fuel consumption as compared to other operating phases where the internal combustion engine runs under higher load. Disadvantageous, the NO_x regeneration in the idle phase is frequently also accompanied by generation of undesirable noise. Moreover, NO_x regeneration under idle conditions takes longer due to the smaller exhaust gas flows, and the operating conditions with unfavorable fuel consumption must be maintained for a longer time.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and a device which can obviate the aforedescribed disadvantages of the conventional technology. The obtained solution should be easily integratable with proven process control models.

The object is solved according to the invention by the device and method for controlling the NO_x regeneration of the NO_x storage catalyst having the characterizing features recited in the independent claims. For example, a lean phase in idle mode can be extended to the next absolutely essential NO_x regeneration or can be controlled with this method according to the predetermined time intervals by:

• • (a) determining if an internal combustion engine is switched into an idle mode, and
  • (b) alternatively or in any combination
    • increasing in the idle mode the threshold value for the load state or the NO_x emission,
    • initiating the NO_x regeneration after predetermined time intervals have passed, and
    • interrupting a current NO_x regeneration when changing into the idle mode.

The device according to the invention includes means for carrying out the aforedescribed method steps. Such means is preferable a control device in which a procedure is stored in digitized form which enables control of the NO_x regeneration in idle mode. The control device can be implemented as an independent control unit or can be integrated into an often already existing engine controller.

If a NO_x regeneration is performed during a change into the idle mode, then the NO_x regeneration is preferably completed, if the change into the idle mode occurs in a fuel-cutoff phase, if a rotation speed exceeds a predetermined threshold value or a motor vehicle speed also exceeds a predetermined limit speed. If the NO_x regeneration is interrupted, then a marker is set which causes the NO_x regeneration to continue in a subsequent acceleration phase. Of course, the marker is removed if a NO_x regeneration had to be already performed in the idle mode.

Preferably, the NO_x regeneration is performed by setting a lambda value in the range between 0.85 to 1.0. In any event, the NO_x regeneration should be performed under less rich conditions than is otherwise typical for NO_x regenerations. In this way, the noise level can be reduced in comparison to the “normal” NO_x regeneration at lambda values that are typically significantly less than 0.85. According to another preferred embodiment of the method, a NO_x regeneration in idle mode is always initiated if for any reason a change into a λ=1 operation is required. This can be the case, for example, when the pressure in a brake booster should be increased.

Generally, the aforedescribed procedures can reduce the number of NO_x regenerations in idle mode as compared to the other operating phases of the motor vehicle, so that fuel consumption, NO_x emission during the NO_x regeneration, as well as the noise generation are reduced.

Additional preferred embodiments of the invention are disclosed as additional features in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail with reference to an embodiment illustrated in the appended drawings. It is shown in:

FIG. 1 a schematic diagram of an internal combustion engine with a NO_x storage catalyst arranged in the exhaust gas channel, and

FIG. 2 a schematic block diagram for controlling a NO_x regeneration of the NO_x storage catalyst in idle mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an internal combustion engine 10 with a NO_x storage catalyst 14 arranged downstream in the exhaust channel 12. Associated with the exhaust gas channel 12 is a suitable sensor circuitry for measuring the air conditions in the exhaust or the fractions of specific pollutants. For example, a gas sensor 16 can be provided as a lambda probe or a gas sensor 18 as a NO_x-sensitive measuring device. The data measured by the sensor circuitry are supplied in a known manner in a motor controller 20. Models are stored in the motor controller 20 in digitized form for determining regulated values for the components associated with the internal combustion engine 10. The components enable control of an air-fuel ratio, an ignition angle or an injected fuel quantity in the combustion process. For example, regulated values can represent an opening angle of an exhaust return valve 22 or a position of a throttle 24. The device and the method for regulating the combustion process are sufficiently known and are therefore not described in detail at this place.

In addition, other status parameters, for example a flap position of a throttle or a gas pedal angle are inputted into the motor controller 20, which can be used to determine in a known manner if the motor vehicle is idling. The status of the motor vehicle is subsequently read into a control device 36 which is implemented in the motor controller 20.

If an excess of oxygen is present during the combustion process of an air-fuel mixture, a raw emission of NO_x of the internal combustion engine 10 increases, while at the same time the quantity of reducing agents carbon monoxide CO and incompletely combusted carbohydrides CH, which are required for converting NO_x, decreases. Since this operating range has proven to have a particularly advantageous fuel consumption, the NO_x has to be absorbed in a storage component of the NO_x storage catalyst 14 to prevent NO_x emission. If the operating mode changes into a stoichiometric or rich mode, then the NO_x, which is stored in form of nitrate, is desorbed again very quickly, at least immediately after the atmosphere in the NO_x storage catalyst 14 changes. If the mass flows of reducing agents are too low, then the reducing agents cannot be supplied in sufficient quantity to the catalyst component of the NO_x storage catalyst, which can cause undesirable NO_x emission.

With the method described hereinafter (see FIG. 2), the lean operating mode can be maintained longer under idle conditions which typically have lower exhaust gas flows, which reduces the number of NO_x regenerations in the idle mode in comparison to other operating phases. Moreover, noise generation caused by the NO_x regeneration can be suppressed.

First, it is determined in an initial query if the motor vehicle is idling (step S1). If this is not the case, then the NO_x regeneration of the NO_x storage catalyst 14 can be controlled by a conventional process. For example, a load state of the NO_x storage catalyst 14 or a NO_x emission downstream of the NO_x storage catalyst 14 can be monitored (step S2). If this quantity exceeds a threshold value, the NO_x regeneration is initiated by changing into the stoichiometric or rich operation.

If the motor vehicle is idling, then it is determined in a subsequent query (step S3), if the change into the idle mode occurs during an ongoing NO_x regeneration. If this is affirmative, then it is determined in step S4, if a fuel cutoff phase exists and/or if the motor vehicle still has a speed above a predetermined limit speed, and/or if a rotation speed exceeds a predetermined threshold value. If these boundary conditions are fulfilled, then the NO_x regeneration is first completed (step S5). Otherwise, the current NO_x regeneration is interrupted and a marker is set (step S6). Setting this marker ensures that the NO_x regeneration is resumed at the end of the idle phase, for example in a subsequent acceleration phase of the motor vehicle.

After the steps S5and S6or if the change into the idle mode does not occur during an ongoing NO_x regeneration (step S3), new threshold values for determining the need for regeneration are set (step S7). The threshold values which are used with conventional processes for the load state and/or the NO_x emission are then increased. It will be understood that the values have to be set in a manner that no significant NO_x breakthroughs can occur during in idle mode, which due to the low exhaust gas mass flows can be guaranteed even if the threshold values are higher than for the other operating phases of the internal combustion engine 10.

As an alternative to the latter approach, a fixed time interval can be set in step S7, wherein the NO_x regeneration has to be performed at the end of this time interval. Advantageously, in addition to the aforedescribed approach for controlling the NO_x regeneration in idle mode, the airfuel ratio during the NO_x regeneration can be set to a value in a range of λ=0.85 to 1.0, and at least less rich than is otherwise typical for NO_x regenerations, which reduces the noise generation.

LIST OF REFERENCE NUMERALS

• • 10 internal combustion engine
  • 12 exhaust gas channel
  • 14 NO_x storage catalyst
  • 16 gas sensor
  • 18 gas sensor
  • 20 engine controller
  • 22 exhaust gas return valve
  • 24 throttle flap
  • 36 control device

Claims

1. Method for controlling a NOx regeneration of a NOx storage catalyst (14) arranged in an exhaust gas channel (12) of an internal combustion engine (10) for motor vehicles, wherein the NOx regeneration is initiated at least when a threshold value for a load state of the NOx storage catalyst (14) or a threshold value for a NOx emission downstream of the NOx storage catalyst (14) is exceeded, the method comprising the steps of: (a) determining if the internal combustion engine (10) is operating in an idle mode, and (b) performing at least one of the following steps for extending a lean operation in the idle mode if the determination is that the engine is operating in the idle mode: increasing the threshold value for the load state of the NOx storage catalyst, increasing the threshold value for the NOx emission, initiating the NOx regeneration after a predetermined time interval has passed, and interrupting a current NOx regeneration.

2. Method according to claim 1, wherein a current NOx regeneration is not interrupted when changing into the idle mode, if a fuel cut-off phase occurs, if a motor vehicle speed exceeds a predetermined limit speed or if a rotation speed exceeds a predetermined threshold value.

3. Method according to claim 2, wherein if a current NOx regeneration is interrupted, a marker is set which causes the NOx regeneration to be continued in a subsequent acceleration phase of the motor vehicle.

4. Method according to claim 3, wherein the marker is removed if a NOx regeneration already had to be performed in idle mode.

5. Method according to claim 1, wherein the NOx regeneration is performed in idle mode by setting a lambda value in a range of 0.85 to 1.0, but in any event less rich than is otherwise typical for NOx regeneration.

6. Method according to claim 1, wherein the NOx regeneration is initiated during the idle mode when a change into a λ=1 operation is required.

7. Device for controlling a NOx regeneration of a NOx storage catalyst (14) arranged in an exhaust gas channel (12) of an internal combustion engine (10) for motor vehicles, wherein the NOx regeneration is initiated at least when a threshold value for a load state of the NOx storage catalyst (14) or a threshold value for a NOx emission downstream of the NOx storage catalyst (14) is exceeded, wherein the device comprises a means for determining if an internal combustion engine (10) is operating in an idle mode, and wherein, if the means determines that the engine is operating in the idle mode, the means performs at least one of the following steps for extending a lean engine operation in the idle mode: increasing the threshold value for the load state of the NOx storage catalyst, increasing the threshold value for the NOx emission, initiating the NOx regeneration after a predetermined time interval has passed, and interrupting a current NOx regeneration.

8. Device according to claim 7, wherein the means comprises a control device (36), in which control device a procedure is stored in digitized form for controlling the NOx regeneration of the NOx storage catalyst (14) in idle mode.

9. Device according to claim 8, wherein the control device (36) is integrated in an engine controller (20).