DE102015119249B4 - Method for the targeted generation of NH3 during the regeneration process of a NOx storage catalyst - Google Patents

Abstract

Method for the specific generation of NH during the regeneration process of an in an exhaust system of an internal combustion engine (1) arranged NO storage catalytic converter (LNT), wherein the regeneration by setting a reducing exhaust gas atmosphere before the NO storage catalyst (LNT) is triggered and the time course of at least during the regeneration phase, lambda values (λ, λ) determined upstream and downstream of the NO storage catalytic converter (LNT) in the exhaust gas are determined upstream and downstream of the NO storage catalytic converter (LNT), characterized in that the following steps are carried out: a) locating a crossover time (K), at which the determined lambda values (λ, λ) have the same value upstream and downstream of the NO storage catalytic converter (LNT) b) Determining one from the crossing time (K) to a reference time (T) through the time profiles the lambda values (λ, λ) upstream and downstream c) determining an amount of NH in the exhaust gas downstream of the NO storage catalyst (LNT) on the basis of the differential area (D); d) terminating the regeneration process if the determined NH3 Quantity in the exhaust gas downstream of the NO storage catalyst (LNT) exceeds a defined limit.

Description

The invention relates to a method for the targeted generation of NH ₃ during the regeneration process of an arranged in an exhaust system of an internal combustion engine NO _x storage catalytic converter, wherein the regeneration is triggered by setting a reducing exhaust gas atmosphere in front of the NO _x storage catalytic converter and the time course of at least during the regeneration phase upstream and downstream of the NO _x storage catalytic converter determined lambda values in the exhaust gas upstream and downstream of the NO _x storage catalyst is determined.

For the fulfillment of current and future emission limit values, there are various possibilities in the operation of the initially mentioned internal combustion engines. A NO _x storage catalyst, which is also known as LNT ( "Lean-NO _x -Trap"), may be combined with an SCR catalyst for its operation is necessary NH _3, which can be formed by prolonged rich operation in the LNT. This internal NH ₃ generation during prolonged LNT regeneration in combination with a downstream SCR is referred to as a passive SCR. Alternatively or additionally, an active NH ₃ dosage can be provided from the outside.

Typically, the end of the regeneration of a NO _x storage catalytic converter, wecher as LNT (Lean NOx Trap) is known, recognized on the basis of a characteristic decrease of the lambda value of an arranged downstream of the NO _x storage catalytic converter lambda probe, see WO 00/19075 A1 , Alternatively, the point of intersection (lambda crossing) of the temporal courses of the lambda values upstream and downstream of the NO _x storage catalytic converter is used to detect the end of the regeneration ( DE 10 2010 001 202 A1 ). Furthermore, it is from the DE 10 355 037 B4 It is known to use a characteristic increase and subsequent decrease in an NO _x value of an NO _x sensor disposed downstream of the NO _x storage catalyst.

However, known methods have the disadvantage that they are relatively inaccurate and / or that for the measurement additional and expensive measuring sensors (eg at least one NO _x probe, currently costing about three times the value of simple lambda probes) is required.

It is the object of the invention to avoid these disadvantages and to propose a method which requires no additional expensive sensors and enables high-quality statements.

1. a) Aufsuchen eines Kreuzungszeitpunktes, an welchem die ermittelten Lambdawerte stromaufwärts und stromabwärts des NO_x-Speicherkatalysators den gleichen Wert aufweisen,
2. b) Ermitteln einer ab dem Kreuzungszeitpunkt bis zu einem Bezugszeitpunkt durch die zeitlichen Verläufe der Lambdawerte stromaufwärts und stromabwärts des NO_x-Speicherkatalysators aufgespannten Differenzfläche;
3. c) Ermitteln einer NH₃-Menge im Abgas stromabwärts des NO_x-Speicherkatalysators auf der Basis der Differenzfläche;
4. d) Beendigen des Regenerationsvorganges, wenn die ermittelte NH₃-Menge im Abgas stromabwärts des NO_x-Speicherkatalysators einen definierten Grenzwert überschreitet.

According to the invention, this is achieved by carrying out the following steps:

1. a) looking for a crossover point at which the determined lambda values have the same value upstream and downstream of the NO _x storage catalytic converter,
2. b) determining a differential area spanned from the time of intersection to a reference time by the temporal profiles of the lambda values upstream and downstream of the NO _x storage catalytic converter;
3. c) determining an amount of NH ₃ in the exhaust gas downstream of the NO _x storage catalyst based on the differential area;
4. d) terminating the regeneration process when the determined amount of NH ₃ in the exhaust gas downstream of the NO _x storage catalyst exceeds a defined limit.

The inventive method allows a deliberate NH ₃ generation by defined extension of the regeneration process. The NH ₃ can then be stored in the exhaust aftertreatment system, for example a downstream SCR catalyst. The determination of the lambda values can be implemented with any probes that can measure lambda values - for example, dedicated lambda probes or probes that measure O ₂ values, but also probes that determine the lambda value as an intermediate value. For NH ₃ determination, therefore, no expensive NO _x probes with the corresponding NH ₃ cross-sensitivity are necessary.

The crossing time in a) is the time from or after which the lambda value downstream of the NO _x storage catalytic converter is lower than the upstream lambda value. Before this crossing time, the downstream lambda value is greater than upstream.

The reference time in b) is the time of the end of regeneration, where the downstream lambda value increases again, in particular over the value at the time of crossing or above. This reference time is also characterized by the change of the upstream lambda value back to the normal operating value outside the regeneration mode.

The limit in d) is defined in particular by the amount of NH ₃ required in the downstream exhaust system, in particular to be stored in a downstream SCR catalyst.

In a first variant of the invention, the NH ₃ amount determined ₃ amount in c) with reference to an empirically determined relationship between the difference of surface and NH. The relationship is in a known manner by an empirical formula or a corresponding map produced. Among others, the following parameters can be considered: temperature of the exhaust gas and the catalyst (or LNT); Exhaust gas mass flow; Engine operating points such as engine speed and load. The respective differences of the upstream and downstream lambda values are weighted with empirically determined factors and the weighted differences are integrated into the differential area. Thus, at any time by means of lambda difference value on the empirically determined relationship and the exhaust gas mass flow to an NH ₃ amount inferred, which is integrated into a total NH ₃ amount. In step d), the regeneration is stopped when a certain amount of NH _{3 is} reached.

Thus, a higher-level SCR coordination ensures that sufficient NH _{3 is present} in the exhaust system or stored in an SCR catalytic converter in order to ensure a desired NO _x conversion.

According to a second variant, a particularly simple determination of the amount of NH ₃ in the exhaust gas results in c) if an H ₂ concentration in the exhaust gas downstream of the NO _x storage catalyst is determined on the basis of the differential area and the NH ₃ amount in the exhaust downstream of the NO _x storage catalyst is determined on the basis of the determined H ₂ concentration. After the crossing point, stored O _{2 is} consumed and H ₂ formation occurs, which can be detected accordingly. This means that lambda values should be the same upstream and downstream when all O _{2 is} consumed. The formation of H ₂ takes place, for example, by splitting up water molecules in H ₂ and ½ O ₂ in the SCR catalyst. Because of the greater cross sensitivity of the sensor, are determined with the lambda values - especially for example a lambda probe - for H ₂ as for O ₂ , the ratio of concentrations of H ₂ to O ₂ in the probe is greater than in the exhaust gas in the catalyst and therefore the lambda value smaller ,

The NH ₃ quantity determined can be fed as input variable to a NH ₃ -Beladungsmodell for a downstream of the NO _x storage catalyst by the exhaust gas flows SCR catalyst (SCR = Selective Catalytic Reduction).

According to the invention, the NH _{3 is} balanced on the basis of the, for the period after regeneration end, spanned surface of the lambda values before and after the NO _x storage catalyst and used in a NH ₃ loading model for a subsequent SCR catalyst.

The invention is based on the property of the "empty" NO _x storage catalytic converter to produce ammonia (NH 3) with continued under-engine operation (rich operation, lambda value <1). At a NO _x storage catalyst, H ₂ from CO is initially produced predominantly by the so-called "water gas shift" reaction but also by "steam reforming" from HC. This H ₂ is used for the reduction of of the storage locations of the NO _x storage catalyst desorbed NO _x. As soon as NO _x more is stored, or in some cases before, NH ₂ and NO _{3 are} formed.

The invention is based on the fact that after the end of the regeneration of the NO _x storage catalytic converter - which is detected by means of the so-called lambda crossing - the lambda probe downstream of the NO _x storage quasi used as H ₂ sensor by on the basis of the spanned differential area of the Lambda values or O ₂ signals of the lambda probes upstream and downstream of the NO _x storage catalyst, the H ₂ concentration is calculated.

Among other things, the invention makes use of the observation that the H ₂ concentration is proportional to the differential area D spanned by the temporal courses of the lambda values upstream and downstream of the NO _x storage catalytic converter: $$\text{D}=\text{<figure-callout id="1" label="K" filenames="DE102015119249B4\_0007.png" state="{{state}}">K</figure-callout>}1\cdot {\text{<figure-callout id="2" label="H" filenames="DE102015119249B4\_0007.png" state="{{state}}">H</figure-callout>}}_2,$$

The amount of NH ₃ formed is in turn proportional to the amount of H ₂ present: $${\text{H}}_2=\text{K2}\cdot {\text{NH}}_3,$$

K1 and K2 are each proportionality factors. In addition, it is considered that the O ₂ signals of the lambda probes have a higher sensitivity with respect to H ₂ than to CO and HC and O ₂ .

• 1 eine schematische Anordnung einer Brennkraftmaschine zur Durchführung des erfindungsgemäßen Verfahrens;
• 2 die zeitlichen Verläufe des Lambdawertes, der H₂-Konzentration und der NH₃-Menge im Abgas stromabwärts des NO_x-Speicherkatalysators.

The invention will be explained in more detail with reference to a non-limiting embodiment in the figures. Show it:

• 1 a schematic arrangement of an internal combustion engine for carrying out the method according to the invention;
• 2 the time courses of the lambda value, the H ₂ concentration and the amount of NH ₃ in the exhaust gas downstream of the NO _x storage catalytic converter.

1 schematically shows an internal combustion engine 1 with an exhaust system 2 in which a NO _x storage catalytic converter LNT (LNN = Lean NO _x Trap), a particulate filter cDPF and an SCR Catalyst SCR are arranged. In the exemplary embodiment, the NOX storage catalytic converter and the particulate filter cDPF are designed as a structural unit. With reference number 3 is the inlet line and with reference numerals 4 a between the particulate filter cDPF and the SCR catalyst from the exhaust system 2 branching and into the intake system 3 opening exhaust gas recirculation system called. However, the invention can be applied not only in the illustrated low-pressure exhaust gas recirculation (removal of the exhaust gas upstream of an exhaust gas turbocharger), but also in high-pressure exhaust gas recirculation (removal of the exhaust gas upstream of an exhaust gas turbocharger).

Upstream of the NO _x storage LNT a first lambda sensor L1 and downstream of the NO _x storage LNT a second lambda sensor L2 is arranged. Basically, any sensors can be used with which a lambda value can be determined - lambda probes according to the described embodiment are one of several possibilities. The lambda sensors (lambda sensors) L1, L2 measure the residual oxygen content O ₂ in the exhaust gas, which results after combustion of all previously unburned fuel components (eg HC, CO), in order to be able to set the ratio (lambda value) A from combustion air to fuel. If there is too little oxygen for this oxidation, it is pumped out of the exhaust gas in the reverse direction via an oxygen pump integrated in the lambda probe. This negative oxygen pumping current is calculated as negative oxygen concentration and subsequently as lambda smaller than 1. The lambda value serves as a control sensor for a lambda control circuit (not shown).

The NO _x storage catalyst LNT is used for temporary storage of nitrogen oxides contained in the exhaust gas (NO _x ). For intermediate storage of the nitrogen oxides, the NO _x storage catalyst LNT on suitable carriers on a noble metal catalyst such as platinum and a NO _x storage component, for example, barium on. In the lean, that is oxygen-rich, atmosphere, the nitrogen oxides are oxidized under the action of the noble metal catalyst, absorbed to form nitrates such as barium nitrate in the catalyst and thus removed from the exhaust gas stream. By a regular brief "enrichment" of the exhaust gas mixture, ie lowering the lambda value λ- run these reactions in the opposite direction, whereby the NO _x molecules are released back into the exhaust stream and by existing in the rich atmosphere reducing components such as C _m H _n -Incompletely burned hydrocarbons - and / or CO are further reduced.

If the absorption capacity of the NO _x storage catalytic converter LNT is exhausted, the engine electronics temporarily set a rich, reducing exhaust gas mixture for a few seconds (about two to ten seconds). In this short, fat cycle, the nitrogen oxides NO _x temporarily stored in the catalyst are reduced to nitrogen N ₂ and thus the NO _x storage catalyst is prepared for the next storage cycle. In order to keep the engine operation as short as possible in the rich cycle, it is essential in classic LNT operation that the reduction end be recognized as accurately as possible.

In the downstream SCR catalytic converter, with passive SCR, NO _x molecules are reduced by NH ₃ to N ₂ for further nitrogen oxide reduction. An SCR catalyst model is used to determine the amount of NH _{3 to be} supplied.

2 shows the curves of the lambda value λ ₁ upstream of the NO _x storage LNT, the lambda value λ ₂ downstream of the NO _x storage LNT, the course of the shares of nitrogen oxide NO _x downstream of the NO _x storage LNT, the course of the proportions of hydrogen H. ₂ downstream of the NO _x storage, the course of the proportions of ammonia NH ₃ downstream of the NO _x storage LNT, the course of the proportions of carbon monoxide CO downstream of the NO _x storage LNT and the course of the shares of unburned hydrocarbons HC downstream of the NO _x storage catalyst LNT, in particular during a regeneration phase REG of the NO _x storage LNT over the time t applied.

During the regeneration phase REG, the internal combustion engine 1 operated stoichiometrically, as can be seen from the operating mode indicative line M. Here, "1" stands for rich operation and "2" for lean operation. In the point marked with reference number R1, the change takes place from lean to rich engine operation.

The regeneration phase REG is divided into two time periods. The first time period A is characterized in that the lambda value λ ₁ upstream of the NO _x storage catalytic converter LNT is smaller than the lambda value λ ₂ downstream of the NO _x storage catalytic converter LNT. In the second period of time, however, the lambda value λ ₁ upstream of the NO _x storage catalytic converter LNT is greater than the lambda value λ ₂ downstream of the NO _x storage catalytic converter LNT. At the crossing time K of the curves of the two lambda values λ ₁ , λ ₂ , the lambda values λ ₁ , λ ₂ determined by means of the lambda sensors L1, L2 have the same value upstream and downstream of the NO _x catalyst.

Surprisingly, it was found that there is a relationship between the differential area D between the curves of the lambda values λ ₁ , λ ₂ , starting with the crossing point K and ending with the reference time point T _B , and the generated during the regeneration process NH ₃ . In a first variant of the invention, therefore, an empirically determined relationship is used, which is produced by an empirical formula or a corresponding characteristic field. Among other things, the following parameters can be taken into account: temperature of exhaust gas and / or catalyst (eg LNT); Exhaust gas mass flow; Engine operating parameters such as engine speed and load.

The lambda differences are weighted with empirically determined factors and the weighted differences are integrated into the differential area D. After reaching a desired amount of NH ₃ downstream, the regeneration is stopped.

A further variant of the invention is as follows: As soon as all cached nitrogen oxides NO _x reduced to nitrogen N ₂ and the NO _x storage LNT is thus "empty", H _{2 is} produced in this hydrogen, indicated by reference numeral R2. The proportion of hydrogen H ₂ is usually more than ten times as large as the proportion of carbon monoxide CO, which is again more than zehmals as large as the proportion of unburned hydrogen HC (Comparison R3 and R4). Upstream of the NO _x storage catalyst LNT hardly H _{2 is} observed (or is oxidized with the residual oxygen or the O ₂ stored in the LNT immediately). Investigations have shown that the hydrogen concentration H ₂ generated in the second period B of the regeneration phase REG is proportional to the differential area D between the profiles of the two lambda values λ ₁ , λ ₂ - starting at the point of intersection K and ending with the reference point T _B.

Since the differences of the lambda values are thus determined primarily by the hydrogen H ₂ , the lambda sensor L1, L2 can be used as a hydrogen sensor. In addition, different sensitivities of the lambda probe to carbon monoxide CO and hydrocarbons HC are also known.

wobei K1 einen ersten Proportionalitätsfaktor darstellt.The concentration of hydrogen H ₂ can thus be set as proportional to the differential area D: $$\text{D}=\text{<figure-callout id="1" label="K" filenames="DE102015119249B4\_0007.png" state="{{state}}">K</figure-callout>}1\cdot {\text{<figure-callout id="2" label="H" filenames="DE102015119249B4\_0007.png" state="{{state}}">H</figure-callout>}}_2.$$

where K1 represents a first proportionality factor.

The production of hydrogen is mainly due to the so-called "water gas shift" reaction H ₂ from CO H ₂ O + CO ↔ CO ₂ + H ₂ but also due to "steam reforming" from HC: 3H ₂ O + C ₃ H ₆ ↔ 3CO + 6H ₂ .

This H ₂ is used for the reduction of of the storage locations of the NO _x storage catalyst desorbed NO _x. As soon as NO _x more is stored, or in some cases before, NH ₂ and NO _{3 are} formed: 8H ₂ + Ba (NO ₃ ) ₂ → 5H ₂ O + BaO + 2NH ₃ .

wobei K2 einen zweiten Proportionalitätsfaktor darstellt.The amount of NH ₃ formed is in turn proportional to the amount of H ₂ present: $${\text{<figure-callout id="2" label="H" filenames="DE102015119249B4\_0007.png" state="{{state}}">H</figure-callout>}}_2=\text{K2}\cdot {\text{NH}}_3.$$

where K2 represents a second proportionality factor.

The amount of NH ₃ determined in this way can be supplied as an input variable to a known NH ₃ charge model for a SCR catalytic converter SCR through which exhaust gas flows downstream of the NO _x storage catalytic converter.

Claims (6)

A method for the targeted generation of NH ₃ during the regeneration process of a in an exhaust system of an internal combustion engine (1) arranged NO _x storage catalyst (LNT), wherein the regeneration is triggered by setting a reducing exhaust gas atmosphere in front of the NO _x storage catalytic converter (LNT) and the Timing of at least during the regeneration phase upstream and downstream of the NO _x storage catalytic converter (LNT) determined lambda values (λ ₁ , λ ₂ ) in the exhaust upstream and downstream of the NO _x storage catalyst (LNT) is determined, characterized in that the following steps performing: a) finding a crossing time point (K) at which the determined lambda values (λ ₁ , λ ₂ ) have the same value upstream and downstream of the NO _x storage catalytic converter (LNT); b) determining a differential area (D) spanned by the temporal courses of the lambda values (λ ₁ , λ ₂ ) from the point of intersection (K) up to a reference time (T _B ) upstream and downstream of the NO _x storage catalytic converter (LNT); c) determining an amount of NH ₃ in the exhaust gas downstream of the NO _x storage catalyst (LNT) based on the differential area (D); d) termination of the regeneration process, when the determined NH ₃ amount in the exhaust downstream of the NO _x storage catalyst (LNT) exceeds a defined limit. Method according to Claim 1 , characterized in that in step c) the NH ₃ amount is determined on the basis of an empirically determined relationship between the differential area (D) and the NH ₃ amount. Method according to Claim 1 characterized in that in step c) an H ₂ concentration in the exhaust downstream of the NO _x storage catalyst (LNT) based on the differential area (D) determined and the NH ₃ amount in the exhaust downstream of the NO _x storage catalyst (LNT ) is determined on the basis of the determined H ₂ concentration. Method according to Claim 3 , characterized in that the H ₂ concentration in the exhaust gas downstream of the NO _x storage catalyst (LNT) is assumed to be proportional to the differential area (D): $$\text{D}=\text{K}1\cdot {\text{H}}_2.$$

where K1 is a first proportionality factor. Method according to Claim 3 or 4 , characterized in that the amount of NH ₃ in the exhaust gas downstream of the NO _x storage catalyst (LNT) is assumed to be proportional to the hydrogen concentration H ₂ : $${\text{H}}_2=\text{K2}\cdot {\text{NH}}_3.$$

where K2 is a second proportionality factor. Method according to one of Claims 1 to 5 Characterized in that the NH ₃ determined -Beladungsmodell of the NO _x storage catalyst for a downstream (LNT) flowed through by the exhaust gas SCR catalyst (SCR) is fed as input variable to a ₃ amount NH.

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