DE102007060221B4 - Process for the provision of reducing agents in an exhaust gas aftertreatment system - Google Patents

Abstract

Method for providing reducing agent in an exhaust gas aftertreatment system (1) of an internal combustion engine (10) with an exhaust gas duct (20) in which an SCR catalytic converter (30) is provided, whereby gaseous ammonia is used as the reducing agent, which is obtained by heating a reservoir (41) with an ammonia -Storage medium is generated in a reducing agent storage system (40), is introduced by means of a metering device (50) into the exhaust gas duct (20) in the flow direction of the exhaust gas upstream of the SCR catalytic converter (30), the heating of the memory (41) with regard to a The target release rate for the gaseous ammonia is regulated and pre-controlled by means of a control unit (100) and a mass flow of reducing agents through a reducing agent feed (45) in the direction of the metering device (50), a pressure increase gradient in the storage (41) of the reducing agent as the control variable (44) Storage system (40) and / or a change in a property that can be correlated with the degree of storage can be used in the storage medium in the memory (41).

Description

State of the art

The invention relates to a method for providing reducing agent in an exhaust gas aftertreatment system of an internal combustion engine with an exhaust gas duct in which an SCR catalyst is provided, with gaseous ammonia as reducing agent, which is generated by heating a store with an ammonia storage medium in a reducing agent storage system by means of a metering device is introduced into the exhaust gas duct in the flow direction of the exhaust gas upstream of the SCR catalytic converter.

Appropriate exhaust gas aftertreatment is required in connection with future legal requirements with regard to nitrogen oxide emissions from motor vehicles. The selective catalytic reduction (SCR) can be used to reduce the NO _x emissions (denitrification) of internal combustion engines, in particular of diesel engines, with predominantly lean, ie oxygen-rich, exhaust gas. Here, a defined amount of a selectively acting reducing agent is added to the exhaust gas. This can be in the form of ammonia, for example, which is metered in directly in gaseous form, or it can also be obtained from a precursor substance in the form of urea or from a urea-water solution (HWL). Such HWL-SCR systems have been used for the first time in the commercial vehicle segment.

A method for providing reducing agent of the type mentioned is in WO 2006/012 903 A2 specified. The heating of the storage tank to release the ammonia is regulated by means of a control unit.

Such a method is also shown by the one not previously published EP 1 977 817 B1 , whereby there is also a feedforward control.

Further processes for the supply of reducing agents are in the DE 10 2006 008 400 A1 and the US 6 399 034 B1 shown.

In the DE 101 39 142 A1 describes an exhaust gas purification system of an internal combustion engine in which _{an SCR catalytic converter is used to reduce NO x} emissions, which uses the reagent ammonia to reduce the nitrogen oxides contained in the exhaust gas to nitrogen. The ammonia is obtained from the urea-water solution (HWL) in a hydrolysis catalytic converter arranged upstream of the SCR catalytic converter. The hydrolysis catalyst converts the urea contained in the HWL into ammonia and carbon dioxide. In a second step, the ammonia reduces the nitrogen oxides to nitrogen, with water being produced as a by-product. The exact process has been adequately described in the specialist literature (cf. WEISSWELLER in CIT (72), pages 441-449, 2000 ). The HWL is provided in a reagent tank.

The disadvantage of this method is that HWL is consumed during operation of the internal combustion engine. The consumption is around 4% of the fuel consumption. The supply of urea-water solution would have to be ensured over a correspondingly large area, for example at petrol stations. Another disadvantage of the process is the required operating temperature range. The hydrolysis reaction of the urea-water solution only takes place quantitatively from temperatures of 200 ° C on the hydrolysis catalytic converter with the release of ammonia. In the case of diesel engines, for example, these temperatures in the exhaust gas are only reached after a long period of operation. At temperatures below 200 ° C., deposits can lead to blockages in the metering unit, which at least impede the supply of the urea-water solution to the exhaust tract. Furthermore, metering in the urea-water solution at temperatures below 200 ° C. can lead to an inhibition of the necessary catalytic properties on the hydrolysis catalytic converter or on the SCR catalytic converter due to polymerization.

In order to avoid having to carry another operating material with you, a plasma process for on-board generation of reducing agents has been proposed in an as yet unpublished document by the applicant. The ammonia required to reduce nitrogen oxides is produced from non-toxic substances in the vehicle as required and then fed into the SCR process.

In the DE 19 728 343 C1 describes a method for selective catalytic NO _x reduction in oxygen-containing exhaust gases using ammonia and a reduction catalyst, in which the gaseous ammonia is made available by heating a solid storage medium which is placed in a container. Ammonia is bound to the storage medium by adsorption and / or in the form of a chemical complex. Such ammonia storage substances from which ammonia is released by thermal desorption or thermolysis, ie as a result of the effect of temperature, are, for example, salts, in particular chlorides and / or sulfates of one or more alkaline earth elements, such as MgCl ₂ , CaCl ₂ , and / or one or more 3d- Subgroup elements such as manganese, iron, cobalt, nickel, copper and / or zinc. Such salts are described, for example, in WO 2006/012 903 as ammonia storage media. Furthermore, organic adsorbers and ammonium salts, such as for example ammonium carbamate, suitable ammonia storage substances.

In processes for selective catalytic NO _x reduction (SCR process), in which gaseous ammonia is provided by heating such storage media, the system dynamics depend on how quickly the release can be adapted to the current demand.

It is therefore the object of the invention to provide a method in which the provision of ammonia as a reducing agent is optimally adapted to the current requirement with a view to optimal exhaust gas cleaning and while avoiding or minimizing ammonia slip.

Disclosure of the invention

Advantages of the invention

The object of the invention is achieved in that the heating of the store is regulated and precontrolled by means of a control unit with regard to a target release rate for the gaseous ammonia. By pre-controlling the heating output, the reducing agent can be made available more quickly than with pure regulation. Changes in driving conditions can be recognized at an early stage and exhaust gas cleaning can thus be optimized so that compliance with exhaust gas limit values can be ensured even in dynamic operation. In addition, the consumption of reducing agents and the risk of a possible ammonia slip as a result of a possible overdose can be reduced.

According to the invention, a mass flow of reducing agent through a reducing agent feed in the direction of the metering device, a pressure (increase gradient) in the memory of the reducing agent storage system and / or a change in a property of the storage medium that can be correlated with the storage level are used as a control variable for monitoring, regulating and adapting the reducing agent release , e.g. a change in the electrical conductivity, the thermal conductivity or the dielectric constant, is used in the memory.

It is advantageous if, in the regulation or pre-control of the supply of reducing agent, a correlation between the manipulated variable of heat input and the target variable of reducing agent release is stored in terms of amount and time behavior. This means that complex physical or chemical relationships can also be mapped.

Monitoring can take place when an NO _x content of an exhaust gas probe and / or a temperature signal from a temperature sensor in the flow direction of the exhaust gas upstream of the SCR catalytic converter and / or a NO _x / NH ₃ content of a further exhaust gas probe downstream of the SCR catalytic converter in the Regulation or precontrol of the reducing agent supply are taken into account. The dosage of the reducing agent can be optimally adjusted as a function of the NO _{x content upstream of the SCR catalytic converter.} Monitoring the NO _x / NH ₃ content after the SCR catalytic converter can be used to _{monitor efficiency or to detect a possible NH 3} slip.

It has proven to be advantageous if the storage state of the storage of the reducing agent storage system is taken into account in the model. A very precise prognosis for the reducing agent release to be expected at the moment can be made with the introduction of a certain thermal energy, which has a positive effect on the control behavior.

In particular, if the reducing agent release rate is monitored by evaluating control variables as state parameters of the reducing agent storage system, optimal regulation can take place. In this context, locally resolved information about the reducing agent storage system can also be of interest.

With regard to a simple evaluation of the control variables mentioned and deriving an optimal heating output, it has been found to be advantageous if the model of the storage status of the memory of the reducing agent storage system and the input-side control variables are stored in one or more characteristic maps.

If a stationary pre-control value is deliberately overridden when dynamically changing between two target release rates, the rise time can also be shortened.

The current operating state of the internal combustion engine can be optimally taken into account when metering the reducing agent if the target release rate of the reducing agent corresponds to the current metering quantity and the metering quantity is derived from a model-based algorithm, taking into account operating state parameters such as engine speed or torque, an engine control of the internal combustion engine and a loading strategy for the SCR catalytic converter is determined.

It can also be advantageous if the target release rate is also influenced by a predicted driving state as part of the pre-control. For example, constant travel, acceleration or deceleration during the Feedforward control must be taken into account. If location data from a GPS receiver are used for the predicted driving condition, the future route profile, possibly also the height profile of the driving route, can be predicted and the prognosis can thus be adjusted.

With regard to a further optimization of the pre-control of the reducing agent release, data on the traffic situation and / or data on the distance to vehicles traveling ahead can be used in a variant of the method for the predicted driving state. Such data can be obtained from TMC data, for example. The Traffic Message Channel (TMC) is a digital radio data service that is used to transmit traffic disruptions to a suitable receiving device.

A preferred application of the method with the features described above provides for use in reducing agent storage systems in which ammonia is released from salts and / or organic adsorbers by desorption and / or thermolysis. In particular, the sometimes quite complex chemical and physical relationships during desorption or thermolysis can be predicted particularly well and the ammonia release can thus be precisely controlled or regulated.

Figure list

• 1 eine schematische Darstellung einer Abgasnachbehandlungsanlage einer Brennkraftmaschine mit einem Reduktionsmittel-Speichersystem.

The invention is explained in more detail below with reference to the embodiment shown in the figure. It shows:

• 1 a schematic representation of an exhaust gas aftertreatment system of an internal combustion engine with a reducing agent storage system.

Embodiments of the invention

1 shows schematically the technical environment in which the method according to the invention can be used.

An exhaust gas aftertreatment system 1 for an internal combustion engine is shown 10 , their exhaust gases via an exhaust system 20th are performed, in the flow direction of the exhaust gas in the example shown initially an exhaust gas probe 70 to determine a NO _x content 71 in the exhaust gas, a temperature sensor 80 , which is a temperature signal 81 supplies and an SCR catalyst 30th are provided. For the reduction of nitrogen oxides is in front of the SCR catalytic converter 30th from a reducing agent storage system 40 via a reducing agent feed 45 and a metering device 50 Ammonia can be supplied as a reducing agent. SCR catalysts 30th work according to the principle of selective catalytic reduction, in which the reducing agent ammonia in oxygen-containing exhaust gases is used to reduce nitrogen oxides to nitrogen and water.

The internal combustion engine 10 is with a motor control 60 linked in the way that operating status parameters 61 , such as speed, torque, air volume, temperature, etc., from sensors in the engine block or in the intake system or in the fuel metering system of the internal combustion engine 10 can be transmitted and recorded, processed and made available for further evaluation within the engine control system. There is also another exhaust gas probe in the example shown 90 in the direction of flow of the exhaust gas behind the SCR catalytic converter 30th arranged with a NO _x / NH ₃ content 91 can be determined.

The reducing agent storage system 40 includes a memory as essential components 41 for ammonia storage substances of the type mentioned at the outset, in which ammonia is released from salts and / or organic adsorbers by desorption and / or thermolysis, and a heating device 42 , which has a power regulator 43 , which can be a switching relay in the simplest case, controlled in terms of power and by means of a vehicle battery 120 can be supplied with energy.

The memory 41 can be designed as a monolithic block, segmented in a container or in several cartridges. The heat is introduced either uniformly in the entire storage volume or locally limited in individual sections with a change in the place of introduction in or on the storage medium depending on the degree of storage. The energy to increase the temperature can be introduced electrically, by means of heating conductors, as electromagnetic radiation of a suitable wavelength, for example microwave, or electromagnetically, by doping the storage medium with metal. The introduction of mechanical energy is also conceivable, for example through friction or through pressure oscillations. Another heating method that is particularly effective in vehicles is heating with a gas, oil and / or petrol-fired burner. Also a chemical reaction on the container wall or in recesses in the memory or phase transitions of media between the memory and a waste heat source circulate, can be used for heating.

For controlling or regulating the reducing agent storage system 40 is a control unit 100 provided, in which the inventive method for providing reducing agent is implemented. Part of the control unit 100 are essentially four functional units, one pilot control unit 101 , a control unit 103 , an SCR catalytic converter model 102 and a correlation unit 104 with which a dosing specification 105 for the dosing device 50 determined or the target release rate for the gaseous ammonia by acting on the power regulator 43 of the reducing agent storage system 40 is regulated and piloted.

Inside the pilot control unit 101 it is provided that the target release rate corresponds to the current dosing amount and the dosing amount from a model-based algorithm, taking operating state parameters into account 61 the engine control 60 the internal combustion engine 10 as well as a loading strategy for the SCR catalytic converter 30th is determined. In addition, in the example shown, the target release rate can be used as part of the pre-control of a predicted driving condition 110 to be influenced.

The supply of reducing agent is regulated within the regulating unit 103 by taking into account the NO _x content 71 the exhaust probe 70 and / or the temperature signal 81 of the temperature sensor 80 in front of the SCR catalytic converter 30th and / or the NO _x / NH ₃ content 91 the exhaust probe 90 behind the SCR catalytic converter 30th . In the SCR catalytic converter model 102 include the NO _x content after the SCR catalytic converter 30th , an ammonia level and an ammonia slip are stored as models.

Furthermore, it can be provided that in the model the memory status of the memory 41 of the reducing agent storage system 40 is taken into account. In addition, the reducing agent release rate can be determined by evaluating controlled variables 44 as state parameters of the reducing agent storage system 40 be monitored. As a controlled variable 44 can be a mass flow rate of reducing agents through the reducing agent feed 45 in the direction of the metering device 50 , a pressure increase gradient in the accumulator 41 of the reducing agent storage system 40 and / or a change in a property of the storage medium in the memory that can be correlated with the degree of storage 41 be used. Ideally, the model is the memory state of the memory 41 of the reducing agent storage system 40 as well as the input-side controlled variables 44 stored in one or more maps.

When regulating or pre-controlling the supply of reducing agent, it is provided according to the invention that within the correlation unit 104 a correlation between the manipulated variable of heat input (or the electrical heating power P _el ) and the target variable of reducing agent release (n _ammonia ) is stored in a model according to amount and time behavior.

The functional units within the control unit 100 can be implemented as software and / or hardware and, as in the example shown, can be stored in a separate unit or at least be part of a higher-level engine control system.

Claims (12)

Method for providing reducing agent in an exhaust gas aftertreatment system (1) of an internal combustion engine (10) with an exhaust gas duct (20) in which an SCR catalytic converter (30) is provided, whereby gaseous ammonia is used as the reducing agent, which is obtained by heating a reservoir (41) with an ammonia -Storage medium is generated in a reducing agent storage system (40), is introduced by means of a metering device (50) into the exhaust gas duct (20) in the flow direction of the exhaust gas upstream of the SCR catalytic converter (30), the heating of the memory (41) with regard to a The target release rate for the gaseous ammonia is regulated and pre-controlled by means of a control unit (100) and a mass flow of reducing agents through a reducing agent feed (45) in the direction of the metering device (50), a pressure increase gradient in the storage (41) of the reducing agent as the control variable (44) Storage system (40) and / or a change in a property that can be correlated with the degree of storage can be used in the storage medium in the memory (41). Procedure according to Claim 1 , characterized in that during the regulation or pre-control of the supply of reducing agent, a correlation between the manipulated variable of heat input and the target variable of reducing agent release is stored in a model according to amount and time behavior. Procedure according to Claim 1 or 2 , characterized in that a NO _x content (71) of an exhaust gas probe (70) and / or a temperature signal (81) of a temperature sensor (80) in the flow direction of the exhaust gas upstream of the SCR catalytic converter (30) and / or a NO _x / NH ₃ content (91) of a further exhaust gas probe (90) downstream of the SCR catalytic converter (30) can be taken into account in the regulation or pre-control of the supply of reducing agent. Method according to one of the Claims 1 until 3 , characterized in that the storage state of the store (41) of the reducing agent storage system (40) is taken into account in the model. Method according to one of the Claims 1 until 4th , characterized in that the reducing agent release rate is monitored by evaluating control variables (44) as state parameters of the reducing agent storage system (40). Method according to one of the Claims 1 until 5 , characterized in that the model of the storage state of the store (41) of the reducing agent storage system (40) and the input-side controlled variables (44) are stored in one or more characteristic diagrams. Method according to one of the Claims 1 until 6th , characterized in that a stationary pilot control value is deliberately overridden when dynamically changing between two target release rates. Method according to one of the Claims 1 until 7th , characterized in that the target release rate corresponds to the current dosing amount and the dosing amount is determined from a model-based algorithm taking into account operating state parameters (61) of an engine control (60) of the internal combustion engine (10) and a loading strategy for the SCR catalytic converter (30). Method according to one of the Claims 1 until 8th , characterized in that the target release rate is also influenced by a predicted driving state (110) in the context of the pre-control. Procedure according to Claim 9 , characterized in that location data from a GPS receiver are used for the predicted driving condition (110). Procedure according to Claim 9 , characterized in that data on the traffic situation and / or data on the distance to vehicles traveling ahead are used for the predicted driving condition (110). Application of the method according to one of the Claims 1 until 11th in reducing agent storage systems (40) in which ammonia is released from salts and / or organic adsorbers by desorption and / or thermolysis.

Images