DE10001310A1 - Device and method for controlling a NOx regeneration of a NOx storage catalytic converter - Google Patents

Abstract

The invention relates to a method for controlling an NOX regeneration of an NOX storage catalyst that is situated in the exhaust gas system of an internal combustion engine of a vehicle, comprising the following steps: (a) in each lean operating phase of the internal combustion engine, at least one status parameter of the NOX storage catalyst is determined using a measured or calculated NOX crude emission (mroh) of the internal combustion engine and an NOX breakdown emission (m) detected downstream in the exhaust gas by an NOX-sensitive measuring device; (b) a deviation of the status parameter of a current lean operating phase from the status parameters of a preceding lean operating phase is determined and (c) in the event that regeneration is necessary, an NOX regeneration is carried out n times according to the deviation (multiple NOX regeneration). The invention also relates to a device which has means for carrying out the steps of the inventive method.

Description

The invention relates to a device and a method for controlling a NO _x regeneration of a NO _x storage catalytic converter arranged in the exhaust line of an internal combustion engine for motor vehicles with the features mentioned in the independent claims.

During the combustion process of an air-fuel mixture in the internal combustion engine, pollutants such as carbon monoxide CO, unburned hydrocarbons HC or nitrogen oxides NO _x are produced to varying degrees. To purify the exhaust gas, it is known to pass the exhaust gas through catalysts arranged in the exhaust line. To this end, CO and HC are usually converted to carbon dioxide and water using oxygen on so-called oxidation catalysts. In contrast, NO _x is reduced by the reducing agents CO and HC on the NO _x storage catalytic converter. In lean operation, in particular in the consumption-optimized area of gasoline engines, with an air ratio of λ about 1.1, the reducing agent mass flows are no longer sufficient to ensure complete conversion of the NO _x . A so-called NO _x storage device is therefore assigned to the catalytic converter, which absorbs the NO _x under these conditions (combined with the catalyst component to form the NO _x storage catalytic converter).

A storage capacity of the NO _x storage catalytic converter is naturally limited, so that NO _x regeneration must be initiated at regular intervals by changing to a rich atmosphere. For this purpose, a rich target value is usually specified until a lambda probe arranged downstream of the NO _x storage catalytic converter falls below a predetermined rich threshold value. Then normal operation of the internal combustion engine is resumed.

After the change to a rich atmosphere, as described, the stored NO _{x is desorbed} and, at the same time, the same is reduced with the reducing agents now present on the catalyst component of the NO _x storage catalytic converter. At the beginning of the NO _x regeneration, this process takes place in a kinetically controlled manner, since NO _{x is} sufficiently present at an interface between the exhaust gas and the NO _x storage catalytic converter. However, with increasing duration of the fat phase, diffusion processes take place to an ever greater extent, which are significantly slower. Such diffusion inhibition increases with increasing NO _x loading and with increasing layer thickness of the NO _x storage. At the end of the fat phase, the NO _x stored in the form of nitrate must first diffuse from deeper layers of the NO _x storage in the direction of the interface. The diffusion inhibition is not taken into account in the conventional methods for controlling the NO _x regeneration, because it is assumed here that complete NO _x regeneration is already given after the rich threshold value has been reached. In particular at high loading rates, however, the NO _x regeneration is actually only incomplete, so that a subsequent lean phase is shortened and the fuel consumption is usually increased as a result.

The object of the present invention is therefore to provide a device and a method with which the NO _x regeneration can be controlled in such a way that a complete emptying of the NO _x store is ensured.

The described disadvantages of the prior art can be overcome by the method according to the invention and the device according to the invention for controlling the NO _x regeneration of the NO _x storage catalytic converter with the features mentioned in the independent claims. According to the procedure

• a) in each lean operating phase of the internal combustion engine, at least one state parameter of the NO _x storage catalytic converter on the basis of a measured or calculated _NOx -Rohemission the internal combustion engine and a downstream by an NO _x -sensitive measuring device detected NO _x - breakthrough emission is determined in the exhaust gas,
• b) a deviation of the state parameters of a current lean operating phase from the state parameters of a previous lean operating phase determined and
• c) if there is a need for regeneration depending on the deviation, the NO _x regeneration is carried out n times (multiple NO _x regeneration).

Storage catalytic converter, a multiple-NO _x in certain operating situations - - thus selected in compliance with state parameters of the NO _x regeneration is initiated that cause the complete emptying of the _NOx catalyst.

For this purpose, the device has means with which the aforementioned method steps can be carried out. These means can include a control unit in which a procedure for controlling the multiple NO _x regeneration is stored in digitized form. The control unit can be integrated into an already frequently existing engine control unit.

In a preferred embodiment of the method are used as state parameters

• a NO _x absorption capacity of the NO _x storage catalytic converter for a given raw NO _x emission and / or
• a NO _x breakthrough emission for a given NO _x raw emission and / or
• a time interval, starting with a complete regeneration (NO _x or SO _x regeneration) until reaching a predetermined NO _x breakthrough emission and / or
• - A raw NO _x emission with a given NO _x absorption capacity

determined.

The quantities mentioned allow a particularly reliable determination of the need to initiate the multiple NO _x regeneration. In this way, an unnecessary multiple NO _x regeneration and thus an unnecessary additional consumption of fuel can be avoided.

In a preferred manner, the multiple NO _x regeneration is realized in that a NO _x storage catalytic converter is initially charged n times with an exhaust gas in accordance with a rich target specification (rich phases). The rich phase ends when a lambda value in the range of λ = 0.999 to 0.95 (rich threshold value) is reached downstream of the NO _x storage catalytic converter. At the end of the rich phases, the NO _x storage catalytic converter is (n - 1) times to n times exposed to an exhaust gas in accordance with a lean target specification, and until a lambda value in the range of λ = 1.001 to downstream of the NO _x storage catalytic converter 1.2 (lean threshold) is reached. The lean target value is preferably in a range from λ = 1.05 to 1.5, that is to say significantly below the air conditions which prevail in normal operation in a consumption-optimized lean phase, so that only very small additional NO _x emissions can be expected . This short lean phase is already terminated when the lambda value has reached the lean threshold value, that is to say that oxygen loading of the NO _x storage catalytic converter has also largely been completed. As an alternative to this, the lean target specification can also be specified as a function of the value of the rich target specification. Such a lean target specification is preferably to be selected at least 0.04 units larger than the bold target specification.

The number (n) of fat phases can in principle be predefined. It has however, this has been shown to be advantageous depending on a level of deviation to determine the state parameters. For example, is the absorbency of the current lean phase compared to a previous lean phase strong dropped, the number is consequently increased. This can preferably be done Number to prevent growth against infinity by specifying one Limit the maximum value.

As an alternative or in combination with the aforementioned procedure when determining the number of fat phases, the success of previous multiple NO _x regenerations can also be taken into account. This can be done, for example, by comparing the state parameters before and after the previous multiple NO _x regeneration. If the multiple NO _x regeneration has not led to an improvement in the storage properties of the NO _x storage catalytic converter to the desired extent, the number of rich phases for the next multiple NO _x regeneration can be increased. Such a determination can be implemented in a particularly preferred and simple manner by specifying threshold values for the success of the previous multiple NO _x regeneration. If these threshold values are undershot, the number (n) is then increased. A reduction in the number (n) of the fat phases can of course take place in the same manner per se, in that the number is reduced again in the case of previous “successful multiple NO _x regeneration”, of course only up to a minimum number of two fat phases.

Further preferred refinements of the invention result from the remaining ones in the features mentioned in the subclaims.

The invention is described below in exemplary embodiments based on the associated Drawings explained in more detail. Show it:

Figure 1 is a schematic diagram of an internal combustion engine with an exhaust gas purification system that includes a NO _x storage catalyst.

Figure 2 is a graph showing a absorption capacity in dependence upon a NO _x -Rohemission.

Figure 3 is a graph illustrating a NO _x -Durchbruchsemission in function of time.

Fig. 4 shows the course of the air conditions downstream and upstream of the NO _x storage catalyst during a multiple NO _x regeneration and

5 is a flow chart for controlling the multiple NO _x -. Regeneration.

Fig. 1 shows schematically an internal combustion engine 10 having disposed in the exhaust line 12 exhaust gas purification plant 14. The exhaust gas purification system 14 comprises a pre-catalytic converter 16 , for example in the form of a three-way catalytic converter, and a NO _x storage catalytic converter 18 arranged downstream thereof. Furthermore, the exhaust line 12 is equipped with a sensor system that allows the local air conditions, gas compositions and temperatures to be recorded. For this purpose, on the one hand up to two temperature sensors 20 , 22 and also two gas sensors 24 , 26 are integrated in the exhaust line 12 . The gas sensor 24 is, for example, a lambda probe, which then provides information about an air ratio immediately after the internal combustion engine 10 . The gas sensor 26 carries a NO _x sensitive measuring device so that NO _x emissions downstream of the NO _x storage catalytic converter 18 (NO _x breakthrough emissions) can be detected. Such a NO _x sensor usually also enables a simultaneous determination of the air conditions, that is to say provides a lambda value downstream of the NO _x storage catalytic converter 18 .

The signals detected by the sensors are usually passed on to an engine control unit 28 . Procedures can be stored in the engine control unit 28 , among other things, in digital form, which allow control of a combustion process in the internal combustion engine 10 . By specifying suitable manipulated variables, for example, an exhaust gas recirculation rate of an exhaust gas recirculation device 30 , an intake volume in an intake manifold 32 with a throttle valve 34 and an injection system (not shown here) can be influenced in such a way that either rich, stoichiometric or lean mixtures are obtained. Such a control is known and will therefore not be explained in more detail here.

There is also a control unit 36 - here integrated in the engine control unit 28 - in which the signals detected by the sensor system are also provided. A procedure for controlling a multiple NO _x regeneration of the NO _x storage catalytic converter 18 is stored in the control unit 36 in digitized form.

During operation of the internal combustion engine 10 under lean conditions, in particular from the point of view of optimized consumption, a _crude NO _x emission m mostly is increased. The reducing agents carbon monoxide CO and unburned hydrocarbons HC required to convert the NO _x are not sufficiently available under such conditions. To remedy this, the catalytic converter has a NO _x accumulator, which binds NO _x as nitrate. Through diffusion, the nitrate also migrates into deeper layers of the NO _x storage as the lean phase increases. For complete NO _x regeneration, the nitrate bound in the deeper layers must therefore first diffuse in the direction of an interface between the exhaust gas and the NO _x storage catalytic converter 18 . Since this process is much slower than the reaction with the reducing agents CO and HC, there is an inhibition of diffusion at the end of the NO _x regeneration. In order to ensure that the NO _x storage is largely completely emptied, the following must be carried out:

First, in each lean operating phase of the internal combustion engine 10, at least one state parameter of the NO _x storage catalytic converter 18 is determined on the basis of the measured or calculated _raw NO _x emission m _raw and the NO _x breakthrough emission m detected by the NO _x - sensitive measuring device. Furthermore, a deviation of the state parameters of a current lean operating phase and the state parameters in a previous lean operating phase is determined and, if there is a need for regeneration of the NO _x storage catalytic converter 18, the NO _x regeneration is carried out n times as a function of the deviation (multiple NO _x -Regeneration).

A NO _x absorption capacity A of the NO _x storage catalytic converter 18 for a given _raw NO _x emission m _{raw, A} is suitable as the state parameter, for example. The determination of a deviation ΔA ₁ can be seen in FIG. 2. The NO _x absorption capacity A decreases with increasing raw emissions m _raw . At the beginning, ie after the end of a previous regeneration, the NO _x absorption capacity A can ideally reach a value of 1, that is to say the entire raw emission is bound in the NO _x storage catalytic converter 18 and no breakthrough emission m can be detected at the gas sensor 26 . With increasing loading, the absorbency A decreases, since the nitrate bound at the interface must first diffuse into deeper layers of the NO _x store. If a total NO _x storage capacity is already reduced by sulfurization or incomplete NO _x regeneration, the NO _x absorption capacity A drops more. Three curves 40 , 42 , 44 are entered as examples. Curve 40 shows a curve of the absorption capacity A for a fresh and unloaded NO _x storage catalytic converter 18 . The curve 42 shows the course of the absorption capacity A in the case of a NO _x storage catalytic converter 18 , the storage capacity of which is already reduced compared to the fresh state, for example due to thermal damage or sulfur loading. The curve 44 has been recorded in time following the curve 42 , and here the absorption capacity is reduced even further. At the intersections 46 , 48 , 50 with the given raw emission m _{raw, A} the values A ₀ , A 'and A "are thus available for the absorption capacity. By forming the difference between the current size A" and the size A' from the previous lean phase there is a deviation ΔA ₁ .

In the same way, a time interval t can serve as the state parameter. Fig. 3 shows, the emission temporal profiles of the breakdown m of a fresh storage catalyst (curve 52), a profile in a current lean phase (curve 56) and a course in a precedent thereto lean phase (curve 54). The time interval t begins with a complete regeneration (NO _x or SO _x regeneration) and ends when a predetermined breakthrough emission m _{t is reached} . The time intervals t ₀ , t 'and t "can therefore be determined from the intersections 58 , 60 , 62 with the curves 52 , 54 , 56. By forming the difference, a deviation Δt ₁ between the time intervals t' and t" is obtained.

In addition to the aforementioned state parameters, it should also be pointed out here that other parameters characterizing the storage capacity of the NO _x storage catalytic converter 18 can of course also be used. For example, it is conceivable to determine a breakthrough emission m for a given raw emission m _{raw, m} or a raw emission m _raw for a given absorption capacity A _{m, raw} . In order to increase the accuracy, the deviations of the state parameters that occur can be combined with one another. For a better comparison, the detected condition parameters can be standardized with the condition parameters of a fresh storage catalytic converter.

Depending on the previously determined deviations ΔA ₁ or Δt _{1, it} can now be determined whether the conditions for initiating the multiple NO _x regeneration are met. For this purpose, the deviations ΔA ₁ , Δt ₁ can be read into a map, the output variable of which is a number n of the fat phases of the multiple NO _x regeneration. With very small deviations ΔA ₁ , Δt ₁ , n is set to the value 1, so that the NO _x regeneration is carried out in a manner known per se.

FIG. 4 shows a course of the lambda values upstream and downstream of the NO _x storage catalytic converter 18 during a multiple NO _x regeneration with two fat phases (n = 2). The solid bold line stands for the lambda value in front of the NO _x storage catalytic converter 18 , and the dashed line shows the course of the lambda value downstream thereof.

Initially, the internal combustion engine 10 is operated under lean conditions, the state parameters being continuously ascertained in the aforementioned manner. After a need for regeneration has been determined, a change to a first rich phase takes place from a time T ₁ in that the internal combustion engine 10 is adjusted to a rich target specification SV _f . With a time delay caused by a dead volume, the lambda value downstream of the NO _x storage catalytic converter 18 drops to λ = 1. During this time, the absorbed NO _{x takes place} in the area of the interface and the interface of nearby layers of the NO _x storage device. From a time T ₂ λ continues to drop. The CO and HC reducing agents formed to an increased extent by the change in a rich atmosphere are therefore no longer used in full to reduce NO _x .

After falling below a rich threshold value SW _f (time T ₃ ), a lean exhaust gas is again provided on the engine side, namely in accordance with a lean target specification SV _{m, 1} . The bold threshold lies in particular in the range from λ = 0.999 to 0.95 and the lean target specification SV _{m, 1} in a range from λ = 1.05 to 1.5. Under the latter conditions, the _raw NO _x emission m _raw emitted by the internal combustion engine 10 is relatively low or can be largely compensated for by the reducing agents CO, HC still present, so that renewed absorption in the NO _x store takes place only to a very small extent.

The lambda value downstream of the NO _x storage catalytic converter 18 also changes in accordance with the change in atmospheric conditions. It initially rises to a value of λ = 1 and remains there until oxygen loading of the NO _x store is complete (time T ₄ ). If the lambda value exceeds a predetermined lean threshold value SW _{m, 1} at a time T ₅ , the air / fuel mixture is again set on the engine side in accordance with the rich target specification SV _f . The lean threshold value SW _{m, 1} is in a range from λ = 1.001 to 1.2. If, at a time T _6, the lambda value downstream of the NO _x storage catalytic converter 18 again falls below the threshold value SW _f , normal operation of the internal combustion engine 10 is released again. Under certain circumstances, it may make sense to first apply the storage catalytic converter 18 in accordance with the lean target specification SV _{m, 1} instead of a direct transition to normal operation and only from a point in time when the lean threshold value SW _{m, 1 is} exceeded Resume normal operation. The number of lean and rich target specifications SV _{m, 1} , SV _f is then the same.

In addition to the previously described preferred range for the value of the lean target specification SV _{m, 1} , it is also conceivable to determine the value depending on the position of the rich target specification SV _f . A lean target specification SV _{m, 2} exceeds the rich target specification SV _f by at least 0.04, so that SV _{m, 2 may also} still be in the rich range. Of course, a new threshold value SW _{m, 2} must also be defined for the latter case, which lies between the two target values SV _{m, 2} , SV _f .

A number n of the fat phases during the multiple NO _x regeneration can preferably be determined in such a way that a value is output using the characteristic map as a function of a level of the deviations (for example ΔA ₁ , Δt ₁ ). Alternatively or in combination, the number n can be determined depending on the state parameters A, m, t, m _raw before and after a previous multiple NO _x regeneration. FIG. 5 is shown in a corresponding flow chart. First, in a step S1, in the manner already described, the extent to which the state parameters of a current lean phase differ from a previous lean phase is determined. If there is only a slight deviation, the number n is set to 1 (step S2) and NO _x regeneration is carried out in a conventional manner. In the case of larger deviations, the number n is set to a value greater than 1 (step S3).

A subsequent query checks whether a previous multiple NO _x regeneration has been carried out successfully (step S4). For this purpose, the state parameters before the multiple NO _x regeneration and after the same are compared, in principle with increasing positive deviation an increased success, that is to say a greater NO _x storage capacity is available for the next lean phase. Such a query can be carried out in such a way that threshold values SW _{M, A} , SW _{M, m} , SW _{M, t} and SW _{M, m, are} given in _{raw form} , and if they are undershot, the number n is increased to n = n + 1.

If the deviation lies above the last-mentioned threshold values, a query S6 checks whether the number n can be reduced by 1 without the value becoming less than 2. If the answer is affirmative, the number n is set to n = n − 1 (step S8). Otherwise, the number n is set to n = 2 (step S7). If necessary, the determined value is still limited to a maximum value n _max (step S9).

Claims (13)

1. Experience to control a NO _x regeneration of a NO _x storage catalytic converter arranged in the exhaust line of an internal combustion engine for motor vehicles, in which

• a) in each lean operating phase of the internal combustion engine at least one state parameter of the NO _x storage catalytic converter based on a measured or calculated _raw NO _x emission (m _raw ) of the internal combustion engine and a NO _x breakthrough emission (m) detected downstream by a NO _x -sensitive measuring device Exhaust gas is determined
• b) a deviation of the state parameters of a current lean operating phase from the state parameters of a previous lean operating phase is determined and
• c) if there is a need for regeneration depending on the deviation, the NO _x regeneration is carried out n times (multiple NO _x regeneration).

2. The method according to claim 1, characterized in that as a state parameter

• - A NO _x absorption capacity (A) of the NO _x storage catalytic converter for a given NO _x raw emission (m _{raw, A} ) and / or
• - a NO _x breakthrough emission (m) for a given NOr raw emission (m _{raw, m} ) and / or
• a time interval (t) starting with a complete regeneration (NO _x or SO _x regeneration) until reaching a predetermined NO _x breakthrough emission (m _t ) and / or
• - a _raw NO _x emission (m _raw ) with a given NO _x absorption capacity (A _{m, raw} )

be determined. 3. The method according to claim 2, characterized in that the determined state parameters (A, m, t, m _raw ) are normalized to the state parameters of a fresh NO _x storage catalyst (A ₀ , m ₀ , t ₀ , m _{raw, 0} ). 4. The method according to any one of the preceding claims, characterized in that during the multiple NO _x regeneration of the NO _x - storage catalyst

• - An exhaust gas is applied n times in accordance with a rich target specification (SV _f ) (rich phases) until a lambda value in the range of λ = 0.999 to 0.95 (rich threshold value SW _f ) is reached downstream of the NO _x storage catalytic converter and
• - Starting with the end of the rich phases, the NO _x storage catalytic converter (n - 1) times to n times is subjected to an exhaust gas according to a lean target specification (SV _{m, 1} , SV _{m, 2} ) until downstream of the NO _x - Storage catalytic converter a lambda value in the range of λ = 1.001 to 1.2 (lean threshold SW _{m, 1} , SW _{m, 2} ) is reached.

5. The method according to claim 4, characterized in that the lean target specification (SV _{m, 1} ) is in the range of λ = 1.05 to 1.5. 6. The method according to claim 4, characterized in that the lean target specification (SV _{m, 2} ) has a lambda value which is at least 0.04 above the rich target specification (SV _f ). 7. The method according to any one of the preceding claims, characterized in that the number (n) of the fat phases is determined as a function of a height of the deviation of the state parameters (A, m, t, m _raw ). 8. The method according to claim 7, characterized in that the number (n) is limited by a predetermined maximum value (n _max ). 9. The method according to any one of the preceding claims, characterized in that the number (n) of the fat phases depending on the state parameters (A, m, t, m _raw ) is determined before and after a previous multiple NO _x regeneration. 10. The method according to claim 9, characterized in that the number (n) is increased if a deviation of the state parameters (A, m, t, m _raw ) before the multiple NO _x regeneration and after the same a predetermined threshold value (SW _{M, A} , SW _{M, m} , SW _{M, t} , SW _{M, m, raw} ). 11. Device for controlling a NO _x regeneration of a NO _x storage catalytic converter arranged in the exhaust line of an internal combustion engine for motor vehicles, in which means are available with which

• a) in each lean operating phase of the internal combustion engine, a state parameter of the NO _x storage catalytic converter based on a measured or calculated _raw NO _x emission (m _raw ) of the internal combustion engine and a NO _x breakthrough emission (m) detected downstream by a NO _x sensitive measuring device in the exhaust gas can be determined,
• b) a deviation of the state parameters of a current lean operating phase from the state parameters of a previous lean operating phase can be determined and
• c) if there is a need for regeneration depending on the deviation, the NO _x regeneration can be carried out n times (multiple NO _x regeneration).

12. The apparatus according to claim 11, characterized in that these means comprise a control device ( 36 ) in which a procedure for controlling the multiple NO _x regeneration is stored in digitized form. 13. The apparatus according to claim 12, characterized in that the control device ( 36 ) is part of an engine control unit ( 28 ).