DE10257172B4 - Method for operating an internal combustion engine with a flexible adaptation of the component protection design - Google Patents

Abstract

A method for operating an internal combustion engine (10) which has an exhaust system (12) with an exhaust gas cleaning system with at least one catalytic converter (16, 18), in which an engine lambda value as a function of a modeled or measured exhaust gas or component temperature (TEMPKRIT) is at least a critical point of the exhaust system (12) is set to a temperature-dependent engine lambda value deviating from normal operation in such a way that an exhaust gas temperature is lowered when the determined exhaust gas or component temperature (TEMPKRIT) at the at least one critical point of the exhaust system (12) is a predetermined one Exceeds the temperature value, characterized in that the predetermined temperature value is determined as a function of the duration and / or level of a temperature load which shortens the service life of the exhaust system (12).

Description

The invention relates to a method for operating an internal combustion engine having the features mentioned in the preamble of claim 1.

For the aftertreatment of exhaust gases from internal combustion engines, it is common practice to purify the exhaust gas catalytically. For this purpose, the exhaust gas is passed over at least one catalyst, which performs a conversion of one or more pollutant components of the exhaust gas. Different types of catalysts are known. Oxidation catalysts promote the oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO), while reduction catalysts promote the reduction of nitrogen oxides (NO _x ) of the exhaust gas. Furthermore, 3-way catalysts are used to simultaneously catalyze the conversion of the three aforementioned components (HC, CO, NO _x ).

Catalysts of internal combustion engines, which are used for example in motor vehicles, age by exposure to high temperatures. As a result, the light-off temperature, that is, the temperature at which 50% pollutant conversion is achieved, is shifted in the direction of higher temperatures, and the peak conversion rate decreases compared to that in the case of an undamaged catalyst. The peak conversion rate for 3-way catalysts in homogeneous stoichiometric operation is almost 100%. With increasing temperature, the catalyst deactivation increases disproportionately, whereas the exposure time leads to a decreasing catalyst aging.

To reduce the catalyst aging, it is known to monitor the maximum permissible temperature at various points of the exhaust system and limit by setting the engine operating parameters, in particular by adjusting the air-fuel ratio to specifiable temperature thresholds, which may be exceeded at most short-term depending on the method.

Out DE 43 44 137 A1 is a method for protecting catalysts from overheating known, in which the actual catalyst temperature is determined and when exceeding a maximum allowable catalyst temperature, a measure for the catalyst cooling is taken. In this case, the maximum permissible catalyst temperature is predetermined as a function of the operating state of the internal combustion engine and / or of ambient conditions and / or of a change in the actual catalyst temperature.

A disadvantage of these methods, however, is that the design of this component protection function or the determination of the temperature thresholds is independent of the driving profile during the vehicle life and thus must be interpreted as a worst-case vote for high proportions particularly catalyst damaging driving. This results in an unnecessary additional consumption if the vehicle is operated only occasionally in or in only weakly catalyst-damaging driving.

The invention is therefore based on the object, a generic method for operating an internal combustion engine, which has an exhaust system with emission control system to optimize such that only occasional or only slightly pronounced catalyst damaging operation leads to no unnecessary extra consumption.

This object is achieved by a method having the features mentioned in claim 1.

For this purpose, a method for operating an internal combustion engine is provided according to the invention, comprising an exhaust system with exhaust gas purification system having at least one catalyst, in which an engine lambda value depending on a modeled or measured exhaust gas or component temperature at least one critical point of the exhaust system deviates from normal operation set to a temperature-dependent engine lambda value, preferably lowered, that an exhaust gas temperature is lowered when the determined exhaust gas or component temperature at the at least one point of the exhaust system exceeds a predetermined temperature value, wherein the predetermined temperature value as a function of duration and / or height an already on the exhaust system exerted service life shortening temperature load is set. This results in a flexible adaptation of the component protection design with respect to the maximum permissible exhaust gas and catalyst temperatures, so that no additional consumption results from a catalyst-friendly driving behavior.

Preferably, in the method at several points of the exhaust system, primarily in the range of one or more catalysts, the temperature load is determined, since the damage is to prevent or at least slow down. In this case, the predetermined temperature values for different locations in the exhaust system may differ from one another.

For this purpose, an exhaust gas or component temperature TEMPKRIT is measured at at least one point of the exhaust system or modeled, wherein the Limiting the computational effort and for the correct detection of the temperature load preferably a comparatively slow time grid is used to determine the exhaust gas or component temperature TEMPKRIT. Thus, measuring intervals of 500-5000 ms and preferably of 1000-2000 ms are provided.

With this exhaust gas or component temperature TEMPKRIT, first a non-dimensional parameter KG is correlated, which ideally increases disproportionately with increasing exhaust gas or component temperature TEMPKRIT. Instead of this sub-step of the method, a formula-based approach for determining the parameter KG can be selected to limit the computational effort. For this purpose, a temperature threshold TMXOS is first defined or determined experimentally, in which just no or at most slight damage to the catalyst system occurs. For example, in the case of NO _x storage catalysts, this temperature threshold is 740 ° C., measured in the NO _x storage catalytic converter. The parameter KG then results from the formula (I) KG = (TEMPKRIT - TMXOS) ^N (I), wherein N> 1, preferably 1.4 to 7 and particularly preferably 1.5 to 2.2. If TEMPKRIT <TMXOS, then KG = 0 is set.

The parameter KG determined in the previous step is then converted into a weighted average KGF. This is preferably done by means of a digital low-pass filter which has the formula (II) KGFnew = KGFold + m · (KG - KGFold) (II) is working. KGFnew represents the output size of the current work step, KGFold the output size of the previous work step and m represents a weighting constant.

In order to be able to detect the component load optimally, it is preferably provided to provide a faster upshifting of the weighted average value KGF. For this purpose, the weighting constant m is made variable. The weighting constant m is assigned a higher value if the size KG is greater than or equal to the weighted average KGF, and a lower value if the size KG is smaller than the weighted average KGF. This is done in each case in comparison to the method with the weighting constant m as a constant size.

If exhaust gas or component temperatures TEMPKRIT are to be taken into account at several points of the exhaust system, the method steps described above are carried out several times in parallel.

Subsequently, the filtered characteristic KGF is correlated with a maximum component temperature TMAXBT, which represents the predetermined temperature value. The maximum component temperature TMAXBT decreases as the weighted average KGF increases. This step may also be carried out in parallel for all critical points in the exhaust system.

Finally, in a known manner, the maximum component temperature TMAXBT is correlated with an engine lambda input or the engine lambda input is controlled by a difference TEMPKRIT - TMAXBT. In the case of several critical points in the exhaust system, a minimum selection of the maximum component temperature TMAXBT or of the engine lambda input is made in a known manner in order to remain safely below the critical temperature at all critical points.

According to a preferred embodiment of the method according to the invention, the maximum component temperature TMAXBT is limited to a variable maximum and / or minimum value, wherein preferably the maximum and minimum values are correlated with a directly measured or indirectly predicted catalyst damage.

For correlation of maximum and minimum value with a catalyst damage, a measurement of the oxygen storage capacity of the catalyst system or at least one catalyst is carried out in a known manner. Alternatively, as known in the use of NO _x storage catalysts, the lean NO _x storage capability may be determined as compared to an undamaged catalyst system. Also, in a known manner, a measurement of the catalytic activity-induced increase in temperature over the catalyst compared to an uncoated monolith or an undamaged catalyst possible. In the case of NO _x storage catalysts, the minimum and / or maximum permissible lean operating temperature and / or the maximum permissible NO _x mass flow can be used as a further criterion.

In principle, as the measured or predicted catalyst damage increases, the maximum and minimum values of TMAXBT are lowered, whereby the maximum value is more limited than the minimum value to ensure optimum emission safety. Preferably, the minimum value of TMAXBT represents a fixed value.

In the previously described embodiments of the method according to the invention are only after the occurrence of a catalyst damage measures taken. In order to advantageously limit the maximum load even before the occurrence of catalyst damage, a second critical temperature threshold TKHART is defined for one or more locations of the exhaust system according to a particularly preferred embodiment of the method according to the invention, which can deviate up and down from the first temperature threshold TMXOS. In a non-volatile totalizer SUMTK, the difference (TEMPKRIT - TKHART) ^{P is added} to the already stored value for each calculation loop, if TEMPKRIT> TEMPHART. Instead, the product exhaust gas mass flow · (TEMPKRIT - TKHART) ^P can be added. P is preferably between 0.8 and 7, ideally between 1 and 4, optimally between 1.2 and 2.

The maximum and / or minimum value of the maximum component temperature TMAXBT is now correlated with the value from the sum counter SUMTK, wherein at low values of the sum counter SUMTK the maximum and minimum values of the maximum component temperature TMAXBT initially preferably remain approximately constant or at best fall slightly, with increasing Values of the sum counter SUMTK initially preferably progressively and in the further course preferably fall degressive and run counter to very hard limits at very high values of the sum counter SUMTK.

Preferably, the value of the sum counter SUMTK is reduced or reset to zero by ECU reading or writing devices, such as when changing one or more catalysts.

This embodiment variant of the method according to the invention can, alternatively or additionally to the measured catalyst damage, be taken into account when determining the maximum component temperature TMAXBT. In the case of several critical points in the exhaust system, a minimum selection is again to be made for the maximum component temperature TMAXBT.

Further preferred embodiments of the invention will become apparent from the remaining, mentioned in the dependent claims characteristics.

The invention will be explained in more detail in embodiments with reference to the accompanying drawings. Show it:

1 in a schematic diagram of an internal combustion engine having an exhaust system with emission control system;

2 in a diagram, the inventive method for operating an internal combustion engine with a flexible adaptation of the component protection design;

3 in a diagram, the sub-method according to the invention for determining the characteristic KG;

4 in a diagram a minimum selection in the use of multiple critical temperatures in the exhaust system of an internal combustion engine;

5 in a diagram, the sub-method according to the invention for the limitation of TMAXBT and

6 in a diagram, a variant of the sub-method according to the invention for the limitation of TMAXBT.

The in 1 shown internal combustion engine 10 is an exhaust system 12 downstream. The exhaust system 12 has an exhaust duct 14 in which a pre-catalyst arranged close to the engine 16 and a main catalyst 18 are located. In addition to the pre-catalyst 16 and the main catalyst 18 has the exhaust duct 14 Usually different, but not shown here gas and / or temperature sensors for controlling the internal combustion engine 10 on.

Shown in 1 merely exemplary two temperature sensors 20 . 22 that are at the precatalyst 16 or at the main catalyst 18 are arranged and each provide a signal for a component temperature TEMPKRIT. The component temperature signals TEMPKRIT are sent to an engine control unit 24 in which this for controlling the internal combustion engine 10 be used. In the engine control unit 24 is also a control unit 26 , are stored in the thresholds, models, algorithms and the like for engine control integrated. By means of the engine control unit 24 and the control unit 26 becomes at least one operating parameter of the internal combustion engine 10 , in particular an air-fuel mixture to be supplied (combustion lambda), influenced.

At the in 2 schematically illustrated method for controlling an internal combustion engine 10 Initially, a measured component temperature TEMPKRIT is assumed. A repetition of the query of the component temperature TEMPKRIT takes place to reduce the computational effort in an interval of preferably 1000-2000 ms. By correlation of the measured component temperature TEMPKRIT becomes a dimensionless Characteristic KG received. The characteristic KG increases disproportionately in the course of repeated queries with increasing component temperature TEMPKRIT.

Alternatively, according to 3 the parameter KG can also be determined by calculation. For this purpose, a temperature threshold TMXOS is first defined or determined experimentally, with just no or at most slight damage to the precatalyst 16 or the main catalyst 18 occurs. The parameter KG then results from the formula KG = (TEMPKRIT - TMXOS) ^N , where N> 1. If the component temperature TEMPKRIT is smaller than the temperature threshold TMXOS, then the parameter is set equal to zero. The determined parameter KG is then determined by means of a digital low-pass according to the formula (II) KGFnew = KGFold + m · (KG - KGFold) (II) converted into a weighted average KGF. The weighted mean value KGFnew represents the output quantity of the current working step, the weighted mean value KGFold the output size of the preceding working step and m a weighting constant. The weighting constant m is designed to be variable, ie the temperature load of the primary catalytic converter 16 and the main catalyst 18 to capture optimally. The weighting constant m is assigned a higher value if the size KG is greater than or equal to the weighted average KGF, and a lower value if the size KG is smaller than the weighted average KGF. This is done in each case in comparison to the method according to the invention, in which the weighting constant m is a constant size, which is not shown separately here. The weighted average KGF is then correlated, as known from the prior art, with a maximum component temperature TMAXBT. The maximum component temperature TMAXBT decreases as the weighted average KGF increases. Finally, but not shown here, in a known manner, the maximum component temperature TMAXBT correlated with a combustion chamber lambda input or the combustion chamber lambda input is controlled by a difference of the component temperature and the maximum component temperature TEMPKRIT - TMAXBT. At several critical points where a temperature sensor 20 . 22 in the exhaust system 12 is arranged, the individual process steps are carried out in parallel and in a known manner and as in 4 indicated a minimum selection of the component temperature TMAXBT made.

According to a preferred embodiment of the method according to the invention, the maximum component temperature TMAXBT, as in 5 shown limited to a variable maximum and / or minimum value, wherein in each case the maximum and minimum values are correlated with a directly measured or indirectly predicted catalyst damage. The rule here is that as the measured or predicted catalyst damage increases, the maximum and minimum values of the maximum component temperature TMAXBT are lowered. To ensure optimum emission safety, the maximum value is more limited than the minimum value of the maximum component temperature TMAXBT, which can also represent a fixed value.

According to a particularly preferred embodiment of the method according to the invention 6 becomes for the pre-catalyst 16 and the main catalyst 18 defines a second critical temperature threshold TKHART, which may deviate up and down from the first temperature threshold TMXOS. In a non-volatile sum counter SUMKT the difference (TEMPKRIT - TKHART) ^{P is added} to the already stored value of the sum counter SUMTK for each calculation loop, if the component temperature TEMPKRIT is greater than the second critical temperature threshold TEMPHART. P is preferably between 0.8 and 7, ideally between 1 and 4, optimally between 1.2 and 2. The maximum and / or minimum value of TMAXBT are now correlated with the value of the sum counter SUMTK, and at low values of the sum counter SUMTK the maximum and minimum values of the maximum component temperature TMAXBT initially preferably remain approximately constant or at best slightly fall, preferably progressively with increasing values of the sum counter SUMTK and preferably degressive in the further course and run counter to hard lower limits at very high values of the sum counter SUMTK. The value of the sum counter SUMTK is by unillustrated ECU read or write devices such as when changing one or more catalysts 16 . 18 diminished or reset to zero. At several critical points in the exhaust system 12 again is a minimum choice for TMAXBT according to 4 hold true.

LIST OF REFERENCE NUMBERS

10

Internal combustion engine

12

exhaust system

14

exhaust duct

16

precatalyzer

18

main catalyst

20, 22

temperature sensor

24

Engine control unit

26

control unit

KG

parameter

KGF

weighted average

KGFnew

weighted average of a current work step

KGFold

weighted average of a previous work step

m

Evaluation constant

SUMTK

non-volatile totalizer

TEMPKRIT

Exhaust and component temperature

TKHART

second critical temperature threshold

TMAXBT

maximum component temperature

TMXOS

temperature threshold

Claims (23)

Method for operating an internal combustion engine ( 10 ), which is an exhaust system ( 12 ) with exhaust gas purification system having at least one catalyst ( 16 . 18 ), in which an engine lambda value in dependence on a modeled or measured exhaust gas or component temperature (TEMPKRIT) at at least one critical point of the exhaust system ( 12 ) is set to a temperature-dependent engine lambda value deviating from normal operation such that an exhaust gas temperature is lowered when the determined exhaust gas or component temperature (TEMPKRIT) at the at least one critical point of the exhaust system ( 12 ) exceeds a predetermined temperature value, characterized in that the predetermined temperature value as a function of the duration and / or height of the exhaust system ( 12 ) applied life shortening temperature load is set. A method according to claim 1, characterized in that the engine lambda value is lowered compared to the normal operation. Method according to one of the preceding claims, characterized in that the at least one critical point of the exhaust system ( 12 ) in the region of the at least one catalyst ( 16 . 18 ) lies. Method according to one of the preceding claims, characterized in that the exhaust gas or component temperature (TEMPKRIT) at at least two points in the exhaust system ( 12 ) and deviating predetermined temperature values for the at least two locations of the exhaust system ( 12 ) are predetermined. Method according to one of the preceding claims, characterized in that a) the at least one modeled or measured exhaust gas or component temperature (TEMPKRIT) correlates with a parameter (KG) or the parameter (KG) by means of the formula (I) KG = (TEMPKRIT - TMXOS) ^N (I) where N> 1 and TMXOS is the temperature threshold at which no damage or only slight damage of a catalyst of the exhaust gas purification system occurs, and in the case where TEMPKRIT <TMXOS, the parameter KG = 0 is set, b) the parameter (KG) is then converted into a weighted average (KGF) of the parameter (KG), c) subsequently the weighted average (KGF) of the parameter (KG) is correlated with a maximum component temperature (TMAXBT) and d) finally the maximum Component temperature (TMAXBT) is correlated with an engine lambda value or the engine lambda value is determined by the difference between the exhaust gas or component temperature (TEMPKRIT) and the maximum component temperature (TMAXBT). A method according to claim 5, characterized in that the parameter (KG) increases disproportionately with increasing exhaust gas or component temperature (TEMPKRIT). A method according to claim 5 or 6, characterized in that the current weighted average (KGF) is determined by the formula (II): KGFnew = KGFold + m · (KG - KGFold), (II), wherein the new weighted average (KGFnew) is the output of a current operation, the old weighted average (KGFold) is the output of a previous operation, and (m) is a weighting constant. A method according to claim 7, characterized in that the weighting constant (m) is variable, wherein the weighting constant (m) is assigned a higher value if the parameter (KG) is greater than or equal to the weighted average (KGF), and a lower value if the parameter (KG) is smaller than the weighted average (KGF), than when using a constant for (m). Method according to one of claims 5 to 8, characterized in that for determining the at least one exhaust gas or component temperature (TEMPKRIT) a time grid of 500-5000 ms is used. A method according to claim 9, characterized in that for determining the at least one exhaust gas or component temperature (TEMPKRIT), a time grid of 1000-2000 ms is used. Method according to one of claims 5 to 10, characterized in that (N) is between 1.4 and 7. A method according to claim 11, characterized in that (N) is between 1.5 and 2.2. Method according to one of claims 5 to 12, characterized in that when using multiple critical points of the exhaust system ( 12 ) the process steps are carried out in parallel. Method according to one of claims 5 to 13, characterized in that when using multiple critical points of the exhaust system ( 12 ) a minimum selection of the engine lambda preset is made. Method according to one of claims 5 or 14, characterized in that the maximum component temperature (TMAXBT) is limited to a variable maximum and / or minimum value. A method according to claim 15, characterized in that the variable maximum and / or minimum value of the maximum component temperature (TMAXBT) is correlated with a directly measured or indirectly predicted catalyst damage. A method according to claim 16, characterized in that - a catalyst damage by measuring the oxygen storage capacity of the at least one catalyst is determined or - when using NO _x storage catalysts, the lean NO _x storage capacity is determined in comparison to an undamaged catalyst system or - a measurement an increase in temperature over the catalyst caused by catalytic activity in comparison to an uncoated monolith or an undamaged catalyst or, when using NO _x storage catalysts as a further criterion, the minimum and / or maximum permissible lean operating temperature and / or the maximum permissible NO _x mass flow can be used. A method according to claim 16 or 17, characterized in that as the measured or predicted catalyst damage increases, the maximum and minimum values of the maximum component temperature (TMAXBT) are lowered, whereby the maximum value can be more restricted than the minimum value. A method according to claim 18, characterized in that the minimum value of the maximum component temperature (TMAXBT) represents a fixed value. Method according to one of claims 1 to 19, characterized in that for at least one critical point of the exhaust system ( 12 ) defines a second temperature threshold (TKHART) that can deviate up and down from the first temperature threshold (TMXOS), and that in a non-volatile totalizer (SUMTK) the difference (TEMPKRIT - TKHART) ^P , where P is between 0.8 and 7, is added to an already stored value, if the exhaust gas or component temperature (TEMPKRIT) is greater than the second temperature threshold (TEMPHART), or that the product Exhaust mass flow × (TEMPKRIT - TKHART) ^P and then that the maximum and / or minimum value of the maximum component temperature (TMAXBT) is correlated with the value of the sum counter (SUMTK), wherein at low values of the totalizer (SUMTK) the maximum and minimum values of the maximum component temperature (TMAXBT ) initially remain constant or approximately constant or at best fall slightly, with increasing maximum component temperature (TMAXBT) initially progressive and degressive in the further course and run counter at very high values of the sum counter (SUMTK) hard lower limits. A method according to claim 20, characterized in that the value of the sum counter (SUMTK) can be reduced manually or automatically or set to zero. A method according to claim 20 or 21, characterized in that P is between 1 and 4. A method according to claim 20 or 21, characterized in that P is between 1.2 and 2.

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