EP1403492B1 - Method for operating a direct injection engine - Google Patents

Abstract

Process for operating a direct injection I.C. engine containing a catalyst system with a nitrogen oxides storage catalyst comprises storing nitrogen oxides in the storage catalyst under a first operating condition, and releasing the nitrogen oxides from the catalyst under a second operating condition. A predetermined maximum time duration is assigned to each point in a load rotational speed mapping for each first operating condition which then switches to a second operating condition assigned to this point. The first operating condition comprises a lean operation, especially a shift operation and/or homogeneous lean operation. The lambda value for an air-fuel ratio in the first operating condition is more than 1.1. The second operating condition comprises a stoichiometric operation with a lambda value for an air-fuel ratio equal to 1. The predetermined maximum time duration is 0-20 seconds, preferably 0-5 seconds.

Description

The invention relates to a method for operating a direct-injection internal combustion engine, which has a catalyst system with a NOx storage catalytic converter, in various operating modes, with NOx being stored in the NOx storage catalytic converter in at least one first operating mode and NOx being removed from the NOx converter in at least one second operating mode. Storage catalyst is discharged, according to the preamble of claim 1.

In direct injection, stratified charge gasoline engines with catalyst system several modes are possible, each point in a load-speed map of the internal combustion engine is assigned a specific operating license: A stoichiometric homogeneous operation with lambda equal to 1.0 in the entire map area; a shift operation at low loads and speeds as well as a homogeneous lean operation with lambda up to 1.6 in the range of average loads and speeds. In principle, the aim is to select the operating point-dependent most fuel-efficient operating mode. For various reasons, however, this is not always possible: With a cold engine and catalyst system, only stoichiometric homogeneous operation is initially permitted until the catalyst system has reached or exceeded a certain minimum temperature. Additionally, for example, stratified operation and homogeneous lean operation must be periodically exited for NOx regeneration or desulfurization.

From the EP 0 894 962 A2 a catalyst arrangement is known in which a Gemischanfettung is allowed only if the internal combustion engine has reached a stable state after a certain period of time after switching the mode. DE 100 01 837 and US 2002128116 representing the prior art.

The invention has for its object to modify a method of the above type with respect to. The mode switch so that while maintaining a total of low-emission operation, a reduction in consumption is achieved.

This object is achieved by a method of o.g. Art solved with the features characterized in claim 1 or 2 features. Advantageous embodiments of the invention are specified in the dependent claims.

For this purpose, it is provided according to the invention that at least one point in a load-speed characteristic map of the internal combustion engine is assigned a predetermined, maximum period of time for each first operating mode, after which a switch is made to a second operating mode assigned to this point.

Alternatively, in a load-speed map of the internal combustion engine between a first map area in which the first mode is permitted, and a second map area in which the second mode is permitted, a third map area is provided in which the internal combustion engine only for a predetermined Duration is operated in the first mode and is switched after the expiration of the predetermined period of time in the second mode of the second map area.

This has the advantage that in a short-term change of the state of the internal combustion engine from a first operating state in which the internal combustion engine is operated in the first mode, in a second operating state in which the internal combustion engine is to be operated in the second mode, not immediately is switched to the second mode and possibly in a NOx regeneration, which would block an execution of a low-consumption first mode for a certain subsequent time, so that overall sets a consumption reduction with no significant deterioration of the emission characteristics.

For example, the first operating mode comprises a lean operation, in particular a stratified operation and / or a homogeneous lean operation, with a value lambda for an air-fuel ratio greater than 1.1 and the second operating mode a stoichiometrically homogeneous operation with a value lambda for an air Fuel ratio equal to 1.

Expediently, a third operating mode with a NOx regeneration of the NOx storage catalytic converter and a lambda value for an air-fuel ratio smaller than 1 is provided, which is switched depending on an operating state of the internal combustion engine and a loading state of the NOx storage catalytic converter to this third mode ,

For example, the predetermined maximum period of time is less than or equal to 80% of the time that elapses between two NO x regeneration in a continuous operation in the first mode associated with that point.

In a preferred embodiment, the predetermined, maximum period of time has a value between 0 and 20 seconds, in particular a value between 0 and 5 seconds.

Fig. 1

eine grafische Darstellung eines Last-Drehzahl-Kennfeldes gemäß dem Stand der Technik,

Fig. 2

eine grafische Darstellung von Fahrgeschwindigkeit und angefordertem Moment, welche über eine Zeit aufgetragen sind, gemäß dem Stand der Technik,

Fig. 3

eine grafische Darstellung eines Last-Drehzahl-Kennfeldes gemäß einer ersten bevorzugten Ausführungsform der Erfindung,

Fig. 4

eine grafische Darstellung von Fahrgeschwindigkeit und angefordertem Moment, welche über eine Zeit aufgetragen sind, gemäß der ersten bevorzugten Ausführungsform der Erfindung von Fig. 3,

Fig. 5

eine grafische Darstellung eines Last-Drehzahl-Kennfeldes gemäß einer zweiten bevorzugten Ausführungsform der Erfindung.

Further features, advantages and advantageous embodiments of the invention will become apparent from the dependent claims, and from the following description of the invention with reference to the accompanying drawings. These show in

Fig. 1

a graphical representation of a load-speed characteristic diagram according to the prior art,

Fig. 2

FIG. 4 is a graphical representation of vehicle speed and requested torque plotted over time according to the prior art. FIG.

Fig. 3

FIG. 2 is a graphical representation of a load-speed map according to a first preferred embodiment of the invention; FIG.

Fig. 4

a graphical representation of vehicle speed and requested torque, which are plotted over a time, according to the first preferred embodiment of the invention of Fig. 3 .

Fig. 5

a graphical representation of a load-speed map according to a second preferred embodiment of the invention.

Fig. 3 1 illustrates graphically a first preferred embodiment of a load-speed characteristic diagram of an internal combustion engine for a method according to the invention, wherein a rotational speed is plotted on the horizontal axis 10 and an engine torque (Nm) on the vertical axis 12. The load-speed map is divided into areas 14, 15, 16 and 18. A stoichiometrically homogeneous operation is in the region 14, in the region 16 a homogeneous lean operation and in the region 18 a shift operation is permitted. The modes stoichiometric homogeneous and homogeneous-lean are also in the respective underlying areas 15, 16 and 18 authorized. Fig. 4 graphically illustrates an associated progression of travel speed 20 and requested torque 22 plotted on vertical axis 24 versus time (sec) plotted on horizontal axis 26. There are in 3 and 4 special operating points 28, 30, 32, 34 and 36 marked.

The aforementioned modes of stoichiometric homogeneous, homogeneous-lean and stratified charge operation are subject to a certain hierarchical order. The shift operation represents the lowest hierarchy, followed by the homogeneous-lean operation. The highest hierarchical level is stoichiometrically homogeneous. This means that the mode with the lowest hierarchy, the shift operation, is allowed only in the correspondingly assigned operation window (area 18), while the higher order modes may also occur in the operation windows (areas 15, 16 and 18) of the lower order modes , Thus, at low loads and speeds (range 18), both the shift operation and, for example, the homogeneous-lean operation can be permitted, while at high loads and speeds (ranges 14, 15 and 16) the shift operation is by no means permitted.

The size and shape of the operating windows (areas 14, 15, 16 and 18) depend on different requirements. An important criterion is to favor the mode with the lowest consumption. As further criteria exhaust gas composition, smoothness, exhaust gas temperature, tendency to knock etc. are used. If, in dynamic operation, an operating mode range 16 or 18 is left in the direction of higher speeds and / or loads, the system switches over to a higher-priority operating mode, i. for example, from shift operation (area 18) to homogeneous lean operation (area 16) or stoichiometrically homogeneous operation (area 14) and from homogeneous lean operation (area 16) to stoichiometric homogeneous operation (area 14).

The operating mode limit of homogeneous-lean operation is largely defined by the exhaust gas composition. The NOx storage catalytic converter commonly used in lean-running gasoline engines requires to achieve optimum NOx storage efficiency an admission with the lowest possible NOx mass flow with the lowest possible exhaust gas mass flow and at the same time maintaining a working temperature window of about 250 ° C to a maximum of about 500 ° C. At higher loads (> approx. 5 bar effective mean pressure) the NOx mass flow increases significantly and can exceed values of 50 g / h. For a longer lasting operation the NOx storage catalyst is saturated very quickly at these NOx mass flows, so that either a very frequent NOx regeneration is required, which fuel consumption disadvantages and also emission disadvantages brings with it, with each NOx regeneration, a small pollutant breakthrough, or at higher acceptance NOx breakthroughs the required pollutant conversion is not achievable.

According to the invention in accordance with the load-speed map Fig. 3 In addition, the area 15 is provided in which only a temporary residence is provided in an operating mode in which NOx stores itself in the NOx storage catalytic converter. In the illustrated embodiment, around the region 16 in which the homogeneous-lean operation is permitted, a load belt 15 of 1 to 4 bar width and / or a speed belt of max. 1000 / min inserted into the operating window 14 for the stoichiometric homogeneous operation. In this region 15, the homogeneous-lean operation is allowed for a predetermined period of time, for example, 0.5 s to 20 s, in particular 2 s to 5 s. When this time window is exceeded, ie when the current operating state of the internal combustion engine is longer than the predetermined time period in the region 15, the homogeneous-lean operation is left in favor of the stoichiometric homogeneous operation. This belt 15 is essentially unsuitable for continuous homogeneous lean operation due to the high NOx raw emissions.

To set out the particular advantage of the invention, is in the 3 and 4 By way of example, a course of operation over operating points 28, 30, 32, 34 and 36 is shown, which corresponds to a corresponding course of operation in the case of a conventional load / speed characteristic map according to FIG Fig. 1 and 2 is compared. In Fig. 1 and 2 the same parts are marked with the same reference numerals as in the 3 and 4 , After prolonged lean operation (28 to 32), the load limit for homogeneous-lean operation is briefly exceeded (operating point 34) and then the operating state of the internal combustion engine is again in the operating window 18 for the shift operation (operating point 36).

As is clear from the Fig. 1 and 2 results in such a course of operation at the operating point 34 immediately stoichiometric homogeneous operation requested. Since the NOx storage catalytic converter has already stored a certain amount of NOx at this time, a NOx regeneration is requested immediately, since without this NOx regeneration in operation with lambda = 1, the previously stored nitrogen oxides (NOx) would be expelled largely unconverted again. The short load request at 34 thus causes longer operation in a NOx regeneration mode, such as indicated by a bold line. However, since the operating state of the internal combustion engine immediately after the short load request at 34 again falls into the operating window 18 for the shift operation and a NOx regeneration because of a not yet saturated NOx storage catalyst would not have been necessary, the forced NOx regeneration leads to emissions - and consumption-poorer operation, because of the engine parameters instead of the NOx regeneration mainly operation in lean or shift operation would have been possible.

In comparison with 3 and 4 shows that the invention offers significant advantages here. The power demand at 34 leads to a change of the operating state of the internal combustion engine in the transition region 15 between approval of homogeneous-lean operation (area 16) and stoichiometric homogeneous operation (area 14). As a result, the mode of operation is continued homogeneous-lean and not immediately requested the stoichiometric homogeneous operation, which also does not lead to a NOx regeneration. Since the load request remains at 34 with approximately 1.5 s in time under the predetermined time for the region 15 time, is not switched to the stoichiometric homogeneous operation. Rather, the operating state of the internal combustion engine at 36 is again in the range 18 for the shift operation and the internal combustion engine is accordingly operated in stratified operation, since the NOx storage catalyst is not yet completely saturated. Overall, an additional consumption for a stoichiometric homogeneous operation and the advanced NOx regeneration is eliminated. The emission behavior is also more favorable overall because, because of the very short residence time in the area 15, the increased NOx raw emissions do not yet lead to a supersaturation of the NOx storage catalytic converter and thus to NOx breakthroughs. In addition, the emission peaks occurring during NOx regeneration are eliminated.

Like the immediate comparison of Fig. 3 and 1 results, according to the invention in the load-speed map in addition to a transition from a first area (area 16) with a first mode with NOx storage in the NOx storage catalyst (homogeneous-lean operation) to a second area (area 14) In a second mode with NOx output from the NOx storage catalyst (stoichiometric homogeneous operation), a third region (belt 15) is provided in which the first mode (homogeneous lean operation) is allowed only for a predetermined period of time. Only when exceeding this predetermined period of time, ie, when the operating condition of the internal combustion engine is longer than the predetermined period in the third area (belt 15) is switched from the first mode (homogeneous-lean operation) in the second mode (stoichiometric homogeneous operation). The third area (belt 15) lies in a region of the load-speed characteristic map in which actually the first operating mode (homogeneous-lean operation) would not be permitted. Thus, the hard boundary between regions 14 and 16 is softened in favor of the first mode (homogeneous-lean operation).

An alternative embodiment of the invention is shown in FIG Fig. 5 shown. This again illustrates a load-speed characteristic map of an internal combustion engine (engine map), analogous to Fig. 3 on the horizontal axis 10, the speed and on the vertical axis 12, the engine torque (Nm) is plotted. For each operating point of this engine map according to the invention a maximum residence time in the first mode with NOx storage in the storage catalytic converter (stratified and / or homogeneous-lean operation) set before the emission-safe second mode (stoichiometric homogeneous operation) is requested when the internal combustion engine is longer than the dwell time at or near the operating point or in a corresponding region of the engine map. As a result, the indicated only with dotted lines hard boundaries between different areas of the engine map are softened with different mode approval. Thus, in the illustrated example, an extremely high value is entered at the operating point 38 for the stratified operation and the homogeneous lean operation, for example 100,000 seconds. At the operating point 40, the value 0 seconds is entered for the stratified operation and again 100,000 seconds for the homogeneous lean operation. The value "0 seconds" at an operating point for an operating mode essentially means that this mode is not permitted at the corresponding operating point. A high value for the residence time means that the corresponding operating mode is unrestrictedly permitted at this operating point. At the operating point 42, the value 0 seconds is again entered for the shift operation and, for example, the value 3 seconds for the homogeneous lean operation. A corresponding number of such operating points in the engine map generates an area in which the first operating mode is permitted only temporarily, ie for a maximum of the predetermined time period or dwell time. Expediently, the dashed lines define boundaries at which the predetermined periods of time for the first operating modes (lean operation or homogeneous lean operation) change. However, within these limits, changes in the predetermined time periods are possible. So is at the operating point 44 next the value 0 seconds for the shift operation a value deviating from the operating point 42 for the homogeneous-lean operation of, for example, 7 seconds entered. At operating point 46, the value 0 seconds is entered for stratified operation and homogeneous lean operation.

In both alternatives explained above, the predetermined period of time or residence time is, for example, less than or equal to 80% of the time interval which lies between two successive NOx regenerations during operation with the first operating mode (lean operation: stratified operation and / or homogeneous lean operation). In concrete numbers, this means a value for the predetermined period of time or residence time of, for example, in the range between 0.5 s to 20 s or in the range between 2 s to 5 s.

Claims (5)

1. Method for operating a direct-injection internal combustion engine, which has a catalytic converter system with a NO_x storage catalytic converter, in various operating modes, with NO_x being accumulated in the NO_x storage catalytic converter in a homogeneous lean mode, and with NO_x being discharged out of the NO_x storage catalytic converter in a stoichiometric homogeneous mode, characterized in that, in a load-speed characteristic map of the internal combustion engine, between a first characteristic map region in which the homogeneous lean mode is permitted and a second characteristic map region in which the stoichiometric homogeneous mode is permitted, a third characteristic map region is provided in which the internal combustion engine is operated only for a predetermined time period in the homogeneous lean mode and is switched into the stoichiometric homogeneous mode of the second characteristic map region after the predetermined time period has elapsed, with the third characteristic map region being inserted, as a load belt of 1 to 4 bar width and/or as a speed belt of a maximum of 1000 rpm, into the second characteristic map region around the first characteristic map region in which the homogeneous lean mode is permitted.
2. Method according to Claim 1, characterized in that, in the homogeneous lean mode, a value lambda for an air/fuel ratio is greater than 1.1.
3. Method according to at least one of the preceding claims, characterized in that a third operating mode is provided with a NO_x regeneration of the NO_x storage catalytic converter and a value lambda for an air/fuel ratio less than 1, with a switch being made to said third operating mode as a function of an operating state of the internal combustion engine and a loading state of the NO_x storage catalytic converter.
4. Method according to Claim 3, characterized in that the predetermined maximum time period is less than or equal to 80% of the time interval which elapses between two NO_x regenerations during uninterrupted operation in the first operating mode assigned to said point.
5. Method according to at least one of the preceding claims, characterized in that the predetermined maximum time period has a value of between 0 and 20 seconds, in particular a value of between 0 and 5 seconds.

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