DE10235592A1 - Method for controlling an internal combustion engine - Google Patents

Abstract

The invention relates to a method for controlling an internal combustion engine running at least occasionally in lean operation as a function of at least one operating parameter of a NO¶x¶ storage catalytic converter arranged in an exhaust gas duct and at least one exhaust gas parameter, using at least one measuring and control unit for determining the at least one operating parameter and of the at least one exhaust gas parameter. DOLLAR A It is provided that the internal combustion engine is controlled so that it has an at least two-dimensional lean operating field, which is defined by predefinable operating points, the predefinable minimum requirements for the at least one operating parameter of the NO¶x¶ storage catalytic converter in combination with the at least one exhaust gas parameter comprise, wherein the lean operating field is divided into at least two-dimensional segments for the at least one operating parameter of the NO¶x¶ storage catalytic converter and the at least one exhaust gas parameter and only the segments for lean operation are blocked, in which at least one of the operating points den in the spanned at least two-dimensional lean operating field specifiable minimum requirements are not sufficient.

Description

The invention relates to a method to control an internal combustion engine that can run at least occasionally lean using a measuring and control unit with the features the preamble of claim 1.

It is known to drive modern internal combustion engines, in particular those used to drive motor vehicles, over the largest possible load ranges in a fuel-efficient lean-burn operation. The lean operation is characterized by an excess of oxygen in an air-fuel ratio supplied to the internal combustion engine, that is to say by λ> 1. Special exhaust gas aftertreatment is required to meet the required exhaust emission limit values. Usually, therefore, the internal combustion engine is followed by at least one catalytic converter for storing and / or converting various exhaust gas components in an exhaust gas tract, in particular NO _x storage catalytic converters are used. Add this NO _x storage catalysts in the lean operating phases of NO _X in the form of nitrate and then reduce the NO _X to N _2, CO ₂ and water.

A common problem of practically all catalyst systems, in particular of NO _x storage catalysts, is the strong dependence of a storage and / or conversion activity of the catalysts on the catalyst temperature. Thus, NO _x storage catalysts have a usable working temperature window of approximately 200 to 550 ° C. in which there is sufficient catalyst activity. In the case of direct-injection gasoline engines and with strict legal exhaust gas values (e.g. the European exhaust gas standard EU IV), NO _x storage catalytic converters can even be used only in a temperature range of around 280 to 500 ° C, i.e. with a NO _x reduction rate of at least 90% become. Below this temperature window, the storage and regeneration processes take place with reaction speeds which are too low, while above the temperature window the thermally unstable nitrates decay, so that there is no sufficient storage rate.

In general, the lean operation of the internal combustion engine is therefore blocked as soon as the catalyst temperature lies outside a lean temperature range defined by the working temperature window of the catalyst. For example, in vehicles with direct-injection stratified gasoline engines to comply with the exhaust gas limits of the European emissions standard EU III outside the permissible lean temperature range of the NO _x storage catalytic converter, lean operation with λ> 1.2 is only permitted for a short time, for example for a maximum of 4 s. The only exceptions to this are in overrun fuel cut-off phases and / or during desulfurization of the NO _x storage catalytic converter. As long as the NO _x storage catalytic converter is not within the lean temperature range, the internal combustion engine must therefore be operated in the stoichiometric or rich operating mode, even if the current operating point of the engine would permit lean operation. This means that considerable consumption disadvantages in favor of lower exhaust gas emissions have to be accepted.

The behavior of the NO _x storage catalytic converter can be simulated by a mathematical model implemented in the engine control. As an important input parameters for modeling the NO _X -Speicherverhaltens are the NO _X -Rohemission to call the previous adsorption time, the exhaust gas mass flow and exhaust temperature. With the help of the catalytic converter model, the NO _x mass stored in the NO _x storage catalytic converter and the current efficiency of the NO _x storage can be determined. A regeneration can then be triggered, for example, when a maximum permissible NO _x load is exceeded or when the storage efficiency falls below a predetermined minimum.

In principle, however, this method is very imprecise, since, for example, the raw NO _x emissions can change significantly over the course of time, through series production and through external influences such as ambient temperature and air humidity. The catalyst behavior can also change due to sulfurization and thermal aging. An unacceptably high NO _x emission of NO _x storage catalyst can thus be reliably avoided only with large safety margins. Since an unfavorable case with high NO _x raw emission and low NO _x storage capacity of the catalytic converter is assumed here, a lean operating field that is limited compared to the state that can be achieved in a favorable manner, as well as unnecessarily frequent regeneration and thus increased fuel consumption result.

Alternatively, it is known to request a regeneration phase when a predetermined NO _x concentration is reached downstream of the NO _x storage catalytic converter. The. the threshold value for the NO _x concentration set for this purpose can be selected to be fixed or dependent on the operating point, that is to say depending on the speed, load and injection quantity of the internal combustion engine. This threshold value represents a maximum value for undesired slip on NO _x and thus for the degree of loading of the NO _x storage catalytic converter. Since the slip increases with the speed, the load or the injection quantity, the threshold value can be selected according to the operating point using this method. However, information about the storage capacity cannot be obtained with this method.

Meeting strict emission regulations requires very high average storage efficiencies, so that even at low _NOx slip a NO _X -Re generation must be triggered. If the storage times that can be achieved are very short, lean burn operation no longer achieves a consumption advantage, since the fuel expenditure for NO _X regeneration compensates for the consumption advantages in the lean area.

For this reason, the operating limits of the NO _x storage catalytic converter are adapted to its storage capacity. As already stated, it is known to couple the lean operation approval to the storage catalytic converter temperature. For temperature adaptation, it is checked, for example, whether the minimum time between two NO _X regenerations in lean operation is undershot. If this is the case, the upper temperature limit for lean operation approval is reduced by a predeterminable amount. If the predeterminable minimum time is exceeded at a catalyst temperature near this upper temperature limit, this upper temperature limit is raised by a predeterminable amount. With this measure, lean operation at high catalytic converter temperatures at which the NO _x storage catalytic converter declines in its storage effect can be restricted depending on the storage capacity of the catalytic converter.

It is also known to use the NO _x mass flow upstream of the NO _x storage catalytic converter as a criterion for blocking or permitting lean operation. A check is carried out to determine whether a given minimum time in lean operation between two NO _X regenerations is exceeded for a specific NO _X mass flow. If this is the case, the maximum permissible NO _x mass flow upstream of the catalytic converter is reduced by a predeterminable amount to enable lean operation. The permissible NO _x mass flow is increased in an analogous manner to the adaptation of the temperature limit. However, it is disadvantageous that a coupling between mass flow and temperature is not taken into account. For example, falling below the minimum storage time that lean operation at higher NO _X -Massenströmen leads in a constant operating point to upstream of the catalyst or at higher temperatures in the catalyst is no longer permitted.

The invention is therefore based on the object of providing a method which makes it possible to optimally utilize the storage capacity (capacity) of a NO _x catalytic converter. In particular, the operating limits should be better adapted to the actual NO _x storage capacity. The interplay of NO _x mass flow and temperature within the NO _x storage catalytic converter should be taken into particular account when determining the parameters for driving in advantageous lean operation.

According to the invention, this object is achieved by a Method with the features of claim 1 solved. Make the subclaims Preferred variants.

The method according to the invention is characterized in that the internal combustion engine is controlled in such a way that it has an at least two-dimensional lean operating field, which is defined by predefinable operating points, the predefinable minimum requirements for the at least one operating parameter of the NO _x storage catalytic converter (preferably a temperature within the NO _x Storage catalytic converter) in combination with the at least one exhaust gas parameter (preferably the NO _x mass flow upstream of the NO _x storage catalytic converter), the lean operating field being divided into at least two-dimensional segments for the at least one operating parameter of the NO _x storage catalytic converter and the at least one exhaust gas parameter and only those segments are blocked for lean operation in which at least one of the operating points in the spanned, at least two-dimensional lean operating field does not meet the minimum requirements that can be specified.

The storage and regeneration process in the NO _x storage catalytic converter can thus be optimally designed and the catalytic converter can be set such that it has a lean operating field for the input variables, preferably for a NO _x mass flow upstream of the NO _x storage catalytic converter, and has a catalytic converter temperature. whose threshold values can be set to achieve peak intercalation efficiency. This means that the specified lean operating field can be extended from the outset to its most favorable limits, which depend on the catalyst chemistry, for example in the case of barium storage catalysts to a temperature range of 230 - 570 ° C and a maximum of 50 mg NO _X / sec per liter of catalyst volume. Lean operation release regulations according to the prior art, on the other hand, only have windows in the range, for example, of 270-550 ° C. and, independently of the temperature, in the best case, 25 mg NO _x / sec per liter of catalytic volume.

The creation of a lean operating field in the form of a NO _x mass flow / storage catalytic converter temperature diagram with segments of defined step sizes and predefinable operating points with minimum requirements enables different design variants for the behavior of the NO _x storage catalytic converter.

In principle, the following results: At an operating point, the minimum storage requirements regarding temperature and NO _X mass flow are no longer met, i.e. the values for temperature and NO _X mass flow are either both outside the permitted upper limit or the temperature value is below the lower limit and the NO _X mass flow is above the permitted upper limit. In the present method, the lean operation is only for segments with too high NO _X -Mas flow and too high temperatures or with too low temperatures and too high NO _X mass flows are no longer permitted. This means that the segments are blocked for lean operation, for which the operating point in the spanned lean operating field no longer meets the minimum storage requirements for NO _x mass flow and temperature.

In a preferred embodiment, the method is controlled in such a way that the lean operation is also no longer permitted even for slightly lower NO _x mass flows and / or slightly lower temperatures than in the mentioned operating point, ie the segments in which these operating points are located, that no longer meet the minimum storage requirements for lean operation are selected to be slightly larger than the operating point actually requires. Such inclusion preferably applies to deviations of preferably around 0.5 mg NO _x / sec for the mass flow and 5 K for the temperature. Analogously, this process also applies to slightly lower NO _x mass flows and slightly higher temperatures in the area of the lower temperature limit.

An operating range lock using an algorithm that combines the criteria of NO _x mass flow and temperature is easy to control in terms of control technology. The blocking of segments with a fixed step size with respect to temperature and NO _x mass flow is preferred and requires little computing effort. Temperature windows between 5 ... 50 K step size, preferably 5 ... 25 K and NO _X mass flow windows between 0.25 ... 5.0 mg / sec, in particular 0.5 ... 2.1, are particularly preferred for this mg / sec. In addition to preferably rectangular segments, segments with a different shape, for example polygonal segments, triangular segments or the like, are also possible while accepting an increased computing effort.

As an alternative or in addition, the method can be controlled in such a way that the catalyst-specific most favorable operating range can be learned at any initialization values in vehicle use, the lean operating field being expanded beyond the respective upper or lower predefinable threshold value for the temperature and the NO _x mass flow in defined increments , Due to the sluggish reaction kinetics and the risk of pollutant breakthroughs during regeneration, an adaptation of an operating range at very low temperatures and NO _X mass flows is limited to a hard, predefinable minimum temperature threshold, preferably to 200 ... 250 ° C, particularly preferably to 230. ..240 ° C.

According to the invention, the lean operation lock at higher NO _x mass flows and higher catalyst temperatures when the minimum storage requirements are undershot at operating points located further inside in the lean operating field is also provided and useful.

Furthermore, it can be assumed that in an area of higher NO _X mass flows and lower temperatures as well as in an area of lower NO _X mass flows and higher temperatures an inadequate storage capacity is also to be expected, so that these segments are directly blocked without checking. This extension of the restricted area can also be extended into the low temperature range.

It is preferred in the process according to the invention a temperature threshold as the boundary between high and low temperature adaptation established. The method has proven to be particularly preferred to be controlled so that the maximum temperature is above the threshold is learned and below the threshold the minimum temperature learned up, preferably above a threshold in the range from 320 ... 400 ° C, especially in a range of 320 ... 340 ° C.

The division of the entire operating field into segments according to the invention enables an almost perfect adaptation to the storage capacity in a few adaptation steps. A method is provided that allows a flexible adaptation of temperature and NO _x mass flow taking into account the actual conditions in the NO _x storage catalytic converter.

In a further preferred embodiment, the method according to the invention is characterized in order to improve the storage capacity in addition to the NO _x mass flow and the temperature, if appropriate, also to include further parameters of the lean operation approval as criteria for an approval, for example the exhaust gas mass flow and catalyst fill level. Multi-dimensional lean operating fields (more than two-dimensional) can then be used for this.

Further preferred configurations the invention result from the remaining, mentioned in the subclaims Features.

The invention is hereinafter in embodiments based on the associated Drawings closer explained. Show it:

1 an arrangement of a NO _x storage catalyst in an exhaust duct of an internal combustion engine and

2 to 11 various lean farm fields.

The method according to the invention serves to control an internal combustion engine that can be operated at least temporarily with excess air, as schematically shown in FIG 1 is shown. The 1 shows an arrangement 10 with a NO _x storage catalyst 12 in an exhaust duct 14 an internal combustion engine 16 , Of course the arrangement is 10 just a very simplified version Example, and there can also be additional NO _x storage catalysts or pre-catalysts in the area of the exhaust duct 14 to be ordered. Such arrangements are known and will not be explained in more detail here.

In addition, sensors are arranged in the exhaust gas duct, which allow a conclusion to be drawn about a current catalytic converter state. In the arrangement 10 are, for example, a gas sensor arranged downstream of the NO _x storage catalytic converter 18 and a temperature sensor arranged upstream of the NO _x storage catalytic converter 20 shown. The sensors 18 . 20 deliver signals within an engine control unit 22 can be evaluated. Furthermore, the internal combustion engine 16 medium 24 assigned an at least temporary influence on the operating parameters of the internal combustion engine 16 enable. The lean operating or regeneration parameters can be set in a known manner by influencing the operating parameters of the internal combustion engine 16 respectively.

In the 2 to 11 are lean operating fields of the internal combustion engine 16 shown by the NO _x mass flow in front of the NO _x storage catalyst 12 and the catalyst temperature of the NO _x storage catalyst 12 are defined. The NO _X mass flow and the temperature can be modeled and the engine control unit 22 be transmitted for evaluation. Furthermore, the temperature in the NO _x storage catalytic converter 12 also based on the one with the sensor 20 measured temperature can be calculated. It is also conceivable to arrange a suitable gas sensor upstream of the NO _x storage catalytic converter 12 or a temperature sensor directly in the NO _x storage catalytic converter 12 ,

The NO _x mass flow in mg NO _x / sec and the temperature T in ° C are plotted here. Based on the representation in 2 it becomes clear that there is a lean operating field 30 results that is limited by a lower temperature threshold T ₁ and an upper temperature threshold T ₂ and a lower NO _X mass flow threshold NO _X1 and an upper NO _X mass flow threshold NO _X2 . At an operating point 32 are the minimum storage requirements for the NO _x storage catalyst 12 no longer met. This criterion is fulfilled, for example, when the gas sensor 18 measured NO _x concentration downstream of the NO _x storage catalytic converter one of that in the NO _x storage catalytic converter 12 stored NO _x mass-dependent limit value. This operating point 32 is characterized by a storage catalytic converter temperature T ₃₂ and a NO _X mass flow NO _X32 . Applied to the method according to the invention, this means that storage catalytic converter temperatures T> T ₃₂ and NO _X mass flows> NO _X32 lead to lean operation of internal combustion engine 16 not being permitted. This results in an operating window 34 in which lean operation is not permitted. As in 2 clarifies, the operating window in the direction of the lower threshold temperature T ₁ and the lower NO _X mass flow threshold value NO _{X 1 is} slightly larger, so that even at correspondingly lower NO _X mass flows and slightly lower temperatures T than at the operating point 32 lean operation is no longer permitted. This enlargement of the operating window 34 is, for example, T ₃₂ - 5 K or NO _X32 - 0.5 mg NO _X / sec.

Furthermore - how 3 clarifies - a lean operating window 36 be learned during vehicle use. This is based on an operating point 38 , by the minimum storage requirement on the NO _x storage catalyst 12 are met, the upper temperature threshold T _{2 raised} to the temperature threshold T ₃ . This additional operating window is defined 36 further by the NO _X mass flow NO _X3 .

How 4 illustrates, the inventive method for defining the lean operating field 30 can also be used at the lower temperature threshold T. Here, the lower temperature threshold T, due to the slower reaction kinetics and the risk of pollutant breakthroughs during regeneration at very low temperatures T and NO _X mass flows NO _{X, should be} limited to a hard minimum temperature threshold T _min that cannot be undercut. Taking this minimum temperature threshold T _min into account, a temperature threshold must be defined as the boundary between high-temperature adaptation and low-temperature adaptation. High-temperature adaptation relates to the exemplary embodiment according to 3 the embodiment according to during low-temperature adaptation 4 concerns. The limit can be approximately 320 to 400 ° C., in particular 320 to 340 ° C., the maximum temperature T _{3 being} learned down above this threshold value and the minimum temperature T _{min being} learned up below this threshold value.

5 shows a lean operating field 30 at which the operating point 32 , in which the minimum storage requirements for the NO _X storage catalytic converter 12 can no longer be met, further inside the lean operating field 30 lies. An operating window is also defined here 34 , in which lean operation of the internal combustion engine is not permitted. The operating window 34 is in this case correspondingly larger, that is to say over a larger temperature difference between T ₃₂ and T ₂ , and a larger NO _X mass flow difference, that is between NO _X32 and NO _X2 . This definition of the larger operating window 34 makes sense, it can be assumed as that in between T ₃₂ and T ₂ lying storage catalyst temperature T and the same NOx mass flow NO _XS2 or in between NO _X -Massenstrom NO _X32 and NO _X -Massenstrom NO _X2 lying NO _X -Massenstrom and the same temperature T ₃₂ also does not meet the minimum storage requirements.

In addition - how 6 clarifies - in operating windows 40 higher NO _X mass flows and lower catalyst temperatures T and lower NO _X mass flows NO _X and higher catalyst temperatures T with inadequate minimum storage requirements for the NOx storage catalytic converter 12 be calculated, so for these operating windows 40 also a lean operation of the internal combustion engine 16 is excluded.

This additional lock for lean operation can also be extended into the low temperature range, like an operating window 42 at the lower temperature threshold T, clarified.

Based on 7 to 11 becomes the segmentation of the lean operating field 30 clarified again. 7 shows that the lean operating field 30 into individual segments, here rectangular segments 44 , is divided. According to further exemplary embodiments, not shown, the shape of the segments can 44 also deviate from a rectangular shape, for example triangular, trapezoidal, polygonal or the like.

Below is rectangular segments 44 went out. Here are the segments 44 preferably the same size and have an edge length (temperature window) of 5 to 50 K, in particular of 15 to 25 K. The other edge length (NO _X mass flow window) is, for example, between 0.25 and 5 mg NO _X / sec, in particular between 0, 9 and 2.1 mg NO _X / sec.

By dividing the entire lean operating area 30 in such segments 44 can optimally adapt the lean operation of the internal combustion engine 10 the storage capacity of the NO _X storage catalytic converter 12 in a few adaptation steps. 8th clarifies here how, based on an operating point 32 , in which the minimum storage requirements were not met, as segments 44 as an operating window 34 from the lean operating field 30 were taken out.

In a next step, the in 9 is made clear, are also based on the operating points 38 in which the minimum storage requirement has been met, the additional operating windows 36 learned (see also explanation for 3 ).

In addition, based on operating points 38 ' at the upper threshold value NO _{X2 of} the NO _X mass flow, a learning of the lean operating field 30 take place, so that additional additional learned operating windows 36 ' for the lean operating field 30 be available. In this way an increase in emission security is achieved ( 10 ).

Finally, in a further step, how 11 clarifies, based on operating points at the lower temperature threshold 38 , in which the minimum storage requirements are met, at the lower temperature threshold T, adjacent additional operating windows 36 be learned, which are limited by the harsh minimum temperature threshold T _min in any case.

From the explanations it becomes clear that such an optimized lean operating field 30 to control the internal combustion engine 16 in lean operation, taking into account the minimum storage requirements of the NO _X storage catalytic converter 12 can be done.

10

Internal combustion engine

12

NO _x storage catalytic converter

14

exhaust duct

16

Internal combustion engine

18

sensor

20

sensor

22

control unit

24

control unit

30

Lean operation field

32

operating point

34

operating window

36 36 '

operating window

38 38 '

operating point

40

operating window

42

operating window

44

segments

T ₁

lower temperature threshold

T ₂

upper temperature threshold

T32

Storage catalytic converter temperature

T _min

Minimum temperature threshold

NO _X1

lower NO _X mass flow threshold

NO _X2

upper NO _X mass flow threshold

NO _X32

NO _X mass flow

bp Not

Operating point in which the minimum storage requirements are not met

bp Yes

Operating point in which the minimum storage requirements are met

bf out

cut out NO _X mass flow / temperature operating window

bf learned

additionally learned NO _X mass flow / temperature operating window

Claims (16)

A method for controlling an at least partly running in the lean operating internal combustion engine depending on at least one operating parameter of an object placed in an exhaust passage of NO _X storage catalytic converter and at least one exhaust parameter, using at least one measuring and control unit for determining the at least one operating parameter and the at least one exhaust parameter, characterized characterized in that the internal combustion engine is controlled so that it has an at least two-dimensional lean operating field, which is defined by predetermined operating points, the predetermined minimum requirements for the at least one operating parameter of the NO _x storage catalytic converter in combination with the at least one exhaust gas parameter sen, wherein the lean operating field is divided into at least two-dimensional segments for the at least one operating parameter of the NO _x storage catalytic converter and the at least one exhaust gas parameter and only the segments for lean operation are blocked in which at least one of the operating points in the spanned at least two-dimensional lean operating field meet the predefinable minimum requirements not enough. A method according to claim 1, characterized in that the exhaust parameter, the upstream of the NO _x storage catalytic NOx present mass flow and the operating parameter is a temperature in the NO _x storage catalyst ( 12 ) and that the lean operating field is created in the form of a NO _x mass flow / storage catalytic converter temperature diagram and is divided into segments for the NO _x mass flow and the catalytic converter temperature. Method according to claim 2, characterized in that the segments have defined step sizes for the temperature and / or for the NO _x mass flow A method according to claim 3, characterized in that the defined temperature step sizes 5 ... 50 K, preferably 5 ... 25 K amount Method according to claim 3, characterized in that the defined step sizes for the NO _x mass flow are 0.25 ... 5 mg NO _x / sec, preferably 0.5 ... 2.1 mg NOx / sec. Method according to one of the preceding claims, characterized characterized that in addition Segments for a lean operation be blocked, which is slightly from the operating point in which the Minimum requirements are given. A method according to claim 6, characterized in that segments are blocked, whose deviations are a maximum of 0.5 mg NO _X / sec and / or a maximum of 5 K. Method according to one of the preceding claims, characterized in that the catalyst-specifically most favorable operating range with predeterminable initialization values in vehicle operation for operating points which meet the predefinable minimum requirements is adapted via a predefinable algorithm, the lean operating field above the respective upper or lower predefinable threshold value for the Temperature and the NO _X mass flow is extended in defined increments. A method according to claim 8, characterized in that a Predefinable temperature threshold as the boundary between high and low temperature adaptation is set, preferably in the range of 320 to 400 ° C. Method according to claim 8 or 9, characterized in that in the event that the storage temperature is high and close to the upper threshold value in the permitted operating point and the NO _x mass flow is several orders of magnitude (defined segments) below the predefinable threshold value, an adaptation of the lean operating field for the temperature limit is above the upper threshold Method according to claim 8 or 9, characterized in that in the event that the NO _x mass flow value is high and close to the upper threshold value and the temperature is several orders of magnitude (defined segments) below the predeterminable threshold value, an adaptation of the lean operating field for the NO _X mass flow limit is above the upper threshold. A method according to claim 8 or 9, characterized in that in the event that the temperature in the permitted operating point is low and close to the lower threshold and the NO _x mass flow is also low, an adaptation of the lean operating field for the temperature above the lower threshold takes place, but only up to a clearly defined, predeterminable minimum temperature threshold, preferably also 200 to 250 ° C. Method according to one of claims 8 to 12, characterized in that that the scope of the adaptation depends on the operating points that meet the minimum requirements, is specified. Method according to one of the preceding claims, characterized characterized that the parameters for the catalyst state for Approval of lean operation continuously or after expiry of a a predetermined period of time determined again determined and / or calculated. Method according to one of the preceding claims, characterized in that in order to optimize the storage capacity of a NO _x storage catalytic converter, in addition to the NO _x mass flow and the temperature, further parameters of the lean operation approval are also included. Method according to one of the preceding claims, characterized in that the predefinable threshold values for the lean operating field, that is the maximum permissible values for the input variables NO _x mass flow and catalyst temperature, depending on the material used Catalyst coating can be selected.