DE102007045263A1 - A method of controlling reductant delivery to an exhaust aftertreatment system with an SCR catalyst - Google Patents

Abstract

The invention relates to a method for controlling the supply of reducing agent in an exhaust aftertreatment system of an internal combustion engine, at least containing an SCR catalyst for removing nitrogen oxides from the exhaust gas, and a metering device for reducing agent and a control unit, wherein a control signal is passed from the control unit to the metering device, the under Consideration of at least the measured variables temperature, storage level of the SCR catalyst to reducing agent and nitrogen oxide raw emission in a model-based function of formal-kinetic approaches in the control unit is calculated. The invention further relates to a controller which is designed to carry out the aforementioned method.

Description

The The invention relates to a method for controlling the supply of reducing agent in an exhaust aftertreatment system with an SCR catalyst for removal of nitrogen oxides from the exhaust gas of an internal combustion engine according to Preamble of claim 1 and a controller for execution this procedure.

State of the art

to Reduction of pollutants, in particular for the reduction of nitrogen oxides, Various processes have established themselves in which reducing Fluids (gases or liquids) in the exhaust system of a Internal combustion engine to be initiated.

to Reduction of nitrogen oxides has become especially the SCR technology proven in the oxygen-rich exhaust gas contained Nitrogen oxides (NOx) with the help of ammonia or a corresponding to Ammonia convertible precursor substance selectively to nitrogen and water are reduced. Preference is given here to aqueous Urea solutions are used. The urea solution is by means of hydrolysis catalysts or directly on the SCR catalyst hydrolyzed to ammonia and carbon dioxide. This is the urea solution by means of special dosing systems upstream of the hydrolysis catalyst or the SCR catalyst injected into the exhaust stream. emission control systems with an SCR catalyst are characterized by a high degree of conversion of nitrogen oxide emissions.

Around a maximum conversion rate of nitrogen oxides (NOx) and thus To achieve an efficient removal of nitrogen oxides from the exhaust gases generally an exact and needs-based dosage of the used Reducing agent necessary. An underdose leads to an undesirable nitrogen oxide emission. An overdose of reducing agent, on the other hand, an undesirable emission, a so-called reductant breakthrough result. Such a reductant breakthrough, especially in toxic Substances like ammonia must be avoided at all costs. In the SCR catalyst can various reduction processes, depending from the input concentrations of the exhaust gas components and the reducing agents, each leading to different conversion rates can. In addition, the temperature is also in SCR catalyst a significant factor in terms of nitric oxide reduction.

additionally At high temperatures in the exhaust system, the NOx conversion may be due to undesirable side effects, such as isomerization of the reducing agent decrease. These influences are therefore present a limitation of the maximum possible turnover rate.

A Method in SCR technology for reducing agent breakthroughs To avoid seeing a conservative Bedussung the injection and a permanent underdose of reducing agent. A maximum possible conversion rate of nitrogen oxides is at this Method not considered but considered constant. The nitrogen oxide removal can not be done with this procedure the various influencing factors in the exhaust system are adapted. The As a result, nitric oxide removal can not match the maximum possible Efficiency and only with a significantly reduced efficiency.

The DE 103 08 287 B4 discloses a method for removing nitrogen oxides from the lean exhaust gas of an internal combustion engine in which the nitrogen oxides contained in the exhaust gas are first adsorbed after a cold start or idle of an internal combustion engine on a nitrogen oxide storage catalyst and desorbed at elevated exhaust gas temperature. The nitrogen oxides are then reacted with reducing agent at a downstream SCR catalytic converter in the exhaust gas line, with a compound decomposable to ammonia being fed in between the nitrogen oxide storage catalytic converter and the SCR catalytic converter. In order to avoid an ammonia breakthrough in the event of an overdose, an additional oxidation catalyst downstream of the SCR catalyst is used as ammonia blocking catalyst in this system. This is for a very complex apparatus and disadvantageous in terms of the reducing agent consumption and the efficiency in the removal of nitrogen oxides.

In the DE 101 00 420 A1 a method for controlling an exhaust aftertreatment system for an internal combustion engine is described, in which the amount of supplied reducing agent is adapted from measures that characterize the effect of the exhaust aftertreatment system. The measured variables are supplied, for example, by sensors for nitrogen oxide or ammonia emission. The sensors are used in the exhaust tract after the exhaust aftertreatment system. The measured values are then compared with expected values and, based on the comparison, the amount of reducing agent is corrected. A disadvantage of this method is that the adaptation of the amount of reducing agent can be made either from the exhaust gas only after the occurrence of a reductant breakthrough indicated by the sensors or only when an unsatisfactory result of the removal of nitrogen oxides has occurred. An adjustment therefore always takes place only in response to an already occurring mismatch of nitrogen oxide emission and reducing agent. This system requires additional To set an optimal amount of reducing agent for nitrogen oxide removal and to safely avoid Reduktionsmitteldurchbrüchen several adaptation cycles and is slow and not very efficient.

task

task The present invention is therefore a method of control the reducing agent supply in an exhaust aftertreatment system with a To provide SCR catalyst, which is a reliable and highly effective nitrogen oxide removal from the exhaust gases of an internal combustion engine ensures and optimization of the amount of reducing agent supplied regardless of only after the reduction reaction allows determinable measured variables.

This is according to the invention with a method of control the reducing agent supply in an exhaust aftertreatment system according to the Patent claim 1 and a controller for the execution of Method according to claim 5 achieved. In the dependent Claims are each preferred developments of Invention specified.

According to the invention a method for controlling the supply of reducing agent in an exhaust aftertreatment system an internal combustion engine at least containing an SCR catalyst for removing nitrogen oxides from the exhaust gas and a metering device proposed for reducing agent and a control unit, in which a control signal from the control unit to the metering device passed that takes into account at least the measured quantities Temperature, storage level of the SCR catalyst to reducing agent and raw nitrogen oxide emissions in a model-based formal-kinetic function Approaches in the control unit is calculable.

advantageously, is inventively provided that the control signal is calculable in the control unit. This is understood according to the invention, that in the control unit for the various possible Operating conditions and parameters of the internal combustion engine and / or the exhaust aftertreatment system the respectively required Data are stored and a control signal for a customized and passed optimized reducing agent supply to the metering device can be. The control signal is in a model-based Function from formal-kinetic approaches under consideration at least the measured variables temperature, storage level of the SCR catalyst to reducing agent and nitrogen oxide raw emission calculated.

Under Temperature is inventively exhaust and or the SCR catalyst temperature understood. These have direct Influence on whether and what quantity produced by the internal combustion engine Nitrogen oxides can be implemented on the SCR catalyst. For example, during a cold start of an internal combustion engine an SCR catalyst usually does not yet have its light-off temperature achieved, so that no or only below 50% of the nitrogen oxides from the catalyst can be implemented. Accordingly low can therefore directly the the exhaust aftertreatment system amount of reducing agent be dosed and fed.

Under Nitrogen oxide emission is understood as the emission behind the Exhaust gas outlet of the internal combustion engine, for example by a corresponding NOx sensor, measured and transmitted to the control unit can be.

Under storage level of the SCR catalyst to reducing agent is mainly the surface coverage Θ _NH3 understood, the values between 0, which corresponds to no surface coverage, and 1, which corresponds to a complete coverage of the catalyst area with reducing agent may assume. This size can be determined from the level of the reducing agent storage model.

advantageously, become the measurands on the basis of which Calculation of the control signal to the metering device for the supply of the reducing agent takes place before and / or in the exhaust aftertreatment system by appropriate sensors he averages. Therefore, by the method according to the invention for different operating conditions and conditions the internal combustion engine and / or the exhaust aftertreatment system the need-based amount of reducing agent in advance, that is before the catalytic reduction reaction, determined and fed to eliminate as much as possible the nitrogen oxides to ensure from the exhaust. At the same time, overdose of the reducing agent and exceeding the maximum Storage level of the SCR catalyst and thus a reducing agent breakthrough safely avoided. The efficiency of exhaust aftertreatment systems with SCR catalytic converter for internal combustion engines can by significantly improved the inventive method become.

• 1.) 4 NO + 4NH₃ + O₂ → N₂ + 6H₂O mit einem Verhältnis NO:NH₃ = 1:1,
• 2.) NO + NO₂ + 2NH₃ → 2N₂ + 3H₂O mit einem Verhältnis NO_x:NH₃ = 1:1, und
• 3.) 6NO₂ + 8NH₃ → 7N₂ + 12H₂O mit einem Verhältnis NO_x:NH₃ = 3:4.

In an SCR catalyst system, when ammonia is used as the reducing agent, various reactions take place whose stoichiometry depends on the nature of the reactants. The main reactions are the following:

• 1.) 4 NO + 4NH ₃ + O ₂ → N ₂ + 6H ₂ O with a ratio NO: NH ₃ = 1: 1,
• 2.) NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O with a ratio NO _x : NH ₃ = 1: 1, and
• 3.) 6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O with a ratio NO _x : NH ₃ = 3: 4.

It could be determined that the reaction 2.) is particularly fast compared to the other two. However, this presupposes the presence of the same amounts of NO and NO ₂ in the raw emission, which is not the case in general operation. Conversely, the reaction shows 3.) an unfavorable ratio of reducing agent to nitrogen oxide, which would lead to an additional consumption of about 25 to reducing agent.

Therefore, it is already advantageous from the consideration of the main reactions to achieve a ratio of the nitrogen oxides in the raw emission of NO to NO ₂ near 1, but to have no excess of NO ₂ in the raw emission. The positive influence of the ratio of the nitrogen oxides is achieved inter alia with the preferred use of an oxidation catalyst before the actual SCR catalyst, the above-mentioned requirements are sought.

In A preferred embodiment of the method can be described as model-based function is a function for the maximum possible Conversion rate of nitrogen oxides are used. Under the maximum possible conversion rate is the proportion of nitrogen oxides understood in the exhaust stream, through the exhaust aftertreatment system, in particular by the SCR catalyst and a preferably upstream Oxidation catalyst, in any detectable operating condition maximum can be implemented. The maximum possible conversion rate is limited the amount of reducing agent necessary for NOx reduction. surplus supplied reducing agent can then for example for Increase of the stored amount in the SCR catalytic converter serve.

The model-based function can be purely theoretical from formal-kinetic Approaches are set up. Alternative or cumulative can model-based function from formal-kinetic approaches be established together with empirically determined characteristics. For example, the empirical characteristics on a external engine test bench are determined.

In a further preferred variant of the method can therefore with the inventive method also targeted an excess or deficiency of reducing agent used and thus an increase or reducing the amount of reducing agent stored in the catalyst be brought about. Equally, too a constant storage level kept controlled become. In addition, it can be determined what proportion of the existing and / or supplied reducing agent for nitrogen oxide reduction is used and what proportion to a change in the SCR memory level can serve.

In a further embodiment of the invention Method may be by the control signal of the controller the storage level of the SCR catalyst varies or kept constant. Thus, depending on the operation as needed, an increase, reduction or a constant Value of the amount of reducing agent stored in the SCR catalyst be brought about.

For example is the storage capacity of an SCR catalyst for Reducing agent is temperature dependent and may increase with increasing Decrease temperature. Likewise, the catalytic activity an SCR catalyst dependent on temperature. So it is the optimal storage level of an SCR catalyst with Reducing agent dependent on the temperature in the SCR catalyst.

The different measurands can with appropriate known measuring devices, for example sensors or probes, be determined. For example, temperature and Nitrogen sensors are used. Depending on the position of the sensor and / or the probe may also be model-based theoretically dependent the known changes in the location a value for another position can be determined. So can, for example, based on a measured temperature value before the catalyst system, a corresponding value for a Position within the catalyst system model-based extrapolated become.

A preferred process variant provides that as a reducing agent Ammonia or a precursor compound of ammonia used become. As a reducing agent for nitrogen oxides can but also the reducing agents present in the fuel, such as hydrocarbons or carbon monoxide can be used. Advantageously, with the method according to the invention an overdose of the reducing agent, which in the case of hydrocarbons can lead to increased fuel consumption.

mNOx

= Stickoxid-Massenstrom [mg/s]

Vkat

= Katalysatorvolumen [l]

wobei die Reaktionsgeschwindigkeit der möglichen Konzentrationsänderung an NOx pro Zeiteinheit und pro Liter Katalysatorvolumen entspricht und sich nach der Formel II berechnet

wobei

Θ_NH3

den Oberflächenbedeckungsgrad des SCR-Katalysators

R

die allgemeine Gaskonstante,

k

die Geschwindigkeitskonstante,

E_A

die Aktivierungsenergie,

c_NOx

die Gesamt-Konzentration an NOx und

T

die Temperatur innerhalb des SCR-Katalysators bedeuten.

The maximum possible conversion rate of the reaction of nitrogen oxides with ammonia can be calculated according to the following formula I.

mNOx

= Nitrogen oxide mass flow [mg / s]

Vcat

= Catalyst volume [l]

the rate of reaction of the possible Concentration of NO x per unit of time and per liter of catalyst volume corresponds and calculated according to the formula II

in which

Θ _NH3

the surface coverage of the SCR catalyst

R

the general gas constant,

k

the rate constant,

E _A

the activation energy,

c _NOx

the total concentration of NOx and

T

mean the temperature within the SCR catalyst.

The Nitrogen oxide mass flows can for example be calculated from the volume flow of the exhaust gas, which is made Air mass flow and injected fuel quantity and the density of respective molecular compound n dependence of the temperature results.

In According to another preferred embodiment, the calculation the conversion rate and the determination of the target supply quantity Reduction agent done in an electronic control unit. In this case, the control unit for the operation of modern internal combustion engines be provided anyway, higher-level engine control, in which the implementation of the invention Process necessary metrics processed become. The control unit may determine the determined target supply quantity transmit to reducing agent as a control signal to the corresponding metering device and so the exhaust aftertreatment system the optimal amount to supply reducing agent. For example, is the current one Conversion rate below the certain maximum possible Conversion rate, can be an underdose of the reducing agent present in the SCR catalyst storage, the following by a increased supply of reducing agent can be optimized. Even a defect in the exhaust aftertreatment system can in this way quickly detected and, for example, to an on-board diagnostic unit (OBD) are transmitted and displayed.

The The invention therefore further relates to a control device for an exhaust aftertreatment system used to carry out the Process is designed.

In In a particularly preferred embodiment, the controller Be part of the higher-level engine control.

According to the invention In addition, a computer program provided that program code means has, in order to carry out the method according to the invention, if the computer program on a computer, especially in one Control unit, for an exhaust aftertreatment system a motor vehicle is running. So in this case the invention by a stored in the control unit Program realized so that this program provided with the control unit in the same way represents the invention as the method, to whose Execution the program is suitable.

The Invention further relates to a computer program product with program code means, which are stored on a computer-readable medium, in order to carry out the method according to the invention, if the program product is on a computer, especially on a computer Control unit for an exhaust aftertreatment system a motor vehicle is running. In this case will the invention realized in such a way by a data carrier, that the inventive method carried out can be if the program product or the disk in a control device for an internal combustion engine in particular a motor vehicle is integrated. As a disk can in particular an electrical storage medium, for example a Read-only memory (ROM), an EPROM or even a permanent electrical memory For example, a CD-ROM or DVD, are used.

The The invention will be described below by way of example with reference to the drawing explained. However, the invention is not limited to the one shown Embodiment limited.

In this shows:

1 a schematic flow diagram of a method according to the invention.

1 shows a schematic flow diagram of a method according to the invention, wherein as a reducing agent for the nitrogen oxides NO / NO ₂ ammonia, or the precursor substance according to DIN 70070 / AUS32 (Adblue) is used. The actual measured quantities are based on the raw NOx emission and the temperature in the SCR catalytic converter. In addition, in this process variant, the surface coverage of the catalyst Θ _{NH₃ flows} into the calculation of the maximum possible conversion rate.

in this connection can also reduce the reaction rates of side reactions be taken into account. Side reactions can for example, unwanted isomerization of the used Urea solution, which will then influence the available has standing amount of ammonia as a reducing agent.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 10308287 B4 [0007]
• - DE 10100420 A1 [0008]

Cited non-patent literature

• - DIN 70070 / AUS32 [0037]

Claims (8)

A method for controlling the supply of reducing agent in an exhaust aftertreatment system of an internal combustion engine at least comprising an SCR catalyst for removing nitrogen oxides from the exhaust gas and a metering device for reducing agent and a control device, characterized in that a control signal is transferred from the control unit to the metering device, taking into account at least the measured variables Temperature, storage level of the SCR catalyst to reducing agent and nitrogen oxide raw emissions can be calculated in a model-based function of formal-kinetic approaches in the control unit. Method according to claim 1, characterized that as a model-based function a function for the maximum possible conversion rate of nitrogen oxides is used. Method according to one of claims 1 to 2, characterized in that as a reducing agent ammonia or a precursor compound of ammonia is used. Method according to one of claims 1 to 3, characterized in that by the control signal of the control unit the storage level of the SCR catalyst varies or is kept constant. Control unit for an exhaust aftertreatment system, characterized in that the control device for carrying out the method according to one of claims 1 to 4 configured is. Control unit for an exhaust aftertreatment system according to claim 5, characterized in that it is part of the parent Motor control is. Computer program with program code means to the To carry out the method of the preceding claims 1 to 4, if the program is on a computer, especially a control device for an exhaust aftertreatment system is performed. Computer program product with program code means, stored on a computer-readable medium are to the method according to one of the preceding claims 1 to 4, if the program product on a Computer, in particular a control device for a Exhaust after-treatment system is executed.