Method for heating a reducing agent metering valve in an SCR system for exhaust aftertreatment of an internal combustion engine

Abstract

Method for operating an electromagnetically controlled reducing agent metering valve (24) arranged in the exhaust system (12) of an internal combustion engine (10) and controlled for metering reducing agent (26) with a first current profile (46), characterized in that a measure of the temperature (T(24)) of the reducing agent metering valve (24) is determined and compared with a threshold value (T_S) and that, if the determined temperature (T(24)) is lower than the threshold value (T_S), the reducing agent metering valve (24) is controlled with a second current profile (48) which differs from the first current profile (46), wherein the second current profile (48) is generated by applying a DC voltage to a solenoid coil of the actuator of the reducing agent metering valve (24) and the DC voltage is predetermined in such a way thatthat the reducing agent metering valve (24) has not yet been opened and the procedure is carried out independently of whether the reducing agent metering valve (24) and its supply line (30) are filled with reducing agent (26).

Description

Prior ArtThe invention relates to a method according to the preamble of claim 1 and to a control device according to the preamble of claim 4. electromagnetically controllable reducing agent metering valves have a magnetic coil, the magnetic field of which lifts a nozzle needle from a sealing seat in the case of a sufficiently large coil current and thus opens the reducing agent metering valve.A first current profile of the coil current serves to open and / or keep open the reducing agent metering valve in order to control a flow rate of reducing agent. Such a method and such a control device are known for use in motor vehicles such as passenger cars and trucks from the publication Diesel Engine Management, 4th edition, Friedrich Vieweg and Sohn Verlag, ISBN 3-528-23873-9, there page 338. The publication DE 10 2005 003 583 A1 discloses a method for heating parts of an exhaust gas purification system which carry reducing agent. Parts which carry reducing agents are referred to as a nozzle, a pump, a storage container, an intake line and a high-pressure line. When reducing agent is frozen in the pump, the coil of the pump serving for the pump drive is supplied with a continuous current or a pulsating current of high frequency. The heat of resistance produced is intended to quickly heat the pump. With regard to the likewise required heating of lines and storage containers, electric heating wires are mentioned which are connected to parts carrying reducing agent.DE 197 21 549 A1 shows an electromagnetically actuatable actuator, in particular for slip-controlled motor vehicle brake systems. At low temperatures, at which the viscosity of a liquid located in the actuator increases and can lead to switching delays, the current coil serving for electromagnetic actuation of the actuator is to be controlled for heating purposes.U.S. Pat. No. 5,201,341 A shows an electromagnetically actuatable valve with which a liquid to be metered is heated by variation of a magnetic field strength. This is intended to improve vaporization of fuel for a cold start of an internal combustion engine. The electromagnetically controllable valve is arranged in an intake pipe of an internal combustion engine. The heating takes place in a time-controlled manner after a cold start.The selective reduction of nitrogen oxides (SCR=selective catalytic reduction) is based on the fact that selected reducing agents reduce nitrogen oxides (NOx) even in the presence of oxygen. Selective means here that the oxidation of the reducing agent preferably takes place (selectively) with the oxygen of the nitrogen oxides and not with the molecular oxygen present in the exhaust gas in a substantially more abundant manner. Ammonia (NH 3) has proven its worth as a reducing agent with the highest selectivity. In the motor vehicle, ammonia is not carried along in pure form, but rather is metered to the exhaust gas from a carried urea-water solution. Urea (NH 2)2 CO has very good solubility in water and can therefore be easily metered into the exhaust gas. When a reducing agent is mentioned in this application, this term is also intended to refer to intermediates, carrier substances and media such as water in which a carrier substance or the reducing agent is contained in dissolved form. Therefore, the urea-water solution is also referred to below as reducing agent to be metered in.A urea-water solution known under the trade name AdBlue, having a mass concentration of 32.5% urea, has a freezing point at -11° C. A eutectic forms there, as a result of which demixing of the solution in the case of freezing is ruled out.Even if undesirable segregation does not occur with this composition, freezing of the reducing agent metering valve and other components of the system, for example freezing of lines, must be prevented as far as possible. In the case of a frozen system, the reducing agent could no longer be metered, which would result in increased nitrogen oxide emissions of the motor vehicle. If the system should nevertheless freeze under unfavourable environmental conditions, it must be able to be thawed again during operation of the motor vehicle. This applies in particular to the reducing agent metering valve, which generally consists of various metals and plastics. The reducing agent metering valve is arranged directly on the exhaust system. Therefore, there is a risk that it will overheat when the exhaust system is hot and the exhaust gas is hot. In order to avoid such thermal damage, the reducing agent metering valve is generally provided with a cooling body which allows large amounts of heat to be conducted away to the environment. Conversely, in the case of low temperatures, this cooling body increases the risk of freezing of the reducing agent metering valve and makes it more difficult for a frozen reducing agent metering valve to thaw.Disclosure of the InventionAgainst this background, the object of the invention is to prevent freezing of a reducing agent metering valve and / or to enable thawing of such a reducing agent metering valve with the simplest possible means and the lowest possible costs and the highest possible operating reliability.This object is achieved in each case by the features of the independent claims. Determining a measure for the temperature of the reducing agent metering valve and comparing the measure with a threshold value allows a detection of situations in which there is a risk of the reducing agent metering valve freezing.According to the invention, the reducing agent metering valve is actuated in such a situation with a second flow profile which differs from the first flow profile. While the first current profile serves to control the flow through the reducing agent metering valve, the output of the second metering valve takes place with the aim of releasing heat in the ohmic resistance of the coil, which heat the reducing agent metering valve from the inside out.Due to this multiple use of the magnet coil of the electromagnetically controllable reducing agent metering valve for controlling the flow cross section and as a heating winding, a separate heating device for the reducing agent metering valve can be dispensed with. As a result, the structure of the reducing agent metering valve is simplified. Furthermore, its space requirement and its production costs are reduced, while its operational reliability is simultaneously increased.Further advantages are evident from the dependent claims, the description and the attached figures.It is understood that the features mentioned above and those still to be explained below can be used not only in the respectively specified combination, but also in other combinations or alone, without departing from the scope of the present invention.Brief Description of the DrawingsExemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. They show, in each case in schematic form: FIG. 1 shows the technical environment of the invention; FIG. 2 shows embodiments of a first and a second current profile; FIG. 3 shows an equivalent circuit diagram of the reducing agent metering valve together with an end stage of an embodiment of the control unit; FIG. 4 shows a further embodiment in the form of an equivalent circuit diagram of the reducing agent metering valve together with an end stage; and FIG. 5 shows an embodiment of a method according to the invention.FIG. 1 shows an internal combustion engine 10 with an exhaust system 12 and a control unit 14. the control unit 14 is preferably the control unit which controls the internal combustion engine 10 and for this purpose receives signals from a sensor system 16 about operating parameters of the internal combustion engine 10 and processes them to control variables for control elements 18 of the internal combustion engine 10. The signals of the sensor system 16 typically allow the control unit to determine the air mass drawn in by the internal combustion engine 10, the rotational angle position of a crankshaft of the internal combustion engine 10, a temperature of the internal combustion engine 10, etc. From these signals, the control unit 14 typically forms control variables for metering fuel into combustion chambers of the internal combustion engine 10, for setting a charge pressure of an exhaust gas turbocharger, an exhaust gas recirculation rate, etc. Alternatively, the control unit 14 is a separate control unit which communicates with the control unit of the internal combustion engine 10 via a bus system.The exhaust system 12 includes an oxidation catalyst 20 and an SCR catalyst 22. Between the oxidation catalytic converter 20 and the SCR catalytic converter 22, a reducing agent metering valve 24 is arranged, via which reducing agent 26 is metered from a storage container 28 to the exhaust gas. The reducing agent metering valve 24 is actuated electromagnetically and for this purpose is actuated by the control unit 14 with a control current I, which flows through a magnetic coil of the reducing agent metering valve 24. In this case, the reducing agent 26 is supplied to the reducing agent metering valve 24 via a feed line 30, which is fed with the reducing agent 26 by a pump 32. The pump 32 is preferably designed as a controllable suction and pressure pump, which generates the injection pressure necessary for metering reducing agent 26 into the exhaust system 12 in the pressure mode and which allows emptying of the supply line 30 of reducing agent 26 in the suction mode. For this purpose, the pump 32 is likewise controlled by the control unit 14. Such emptying takes place, for example, between two driving cycles, or at the end of a driving cycle, in order to avoid intermediate freezing of the reducing agent 26 in the feed line 30, which is usually designed as a hose line, and in the reducing agent metering valve 24.In order to avoid freezing of the reducing agent 26, the feed line 30 is furthermore equipped with a hose heater 33, which is likewise controlled by the control unit 14. Alternatively or additionally, a further heater 34 is arranged in the storage container 28, which is also controlled by the control unit 14.In order to control the selective catalytic reduction of nitrogen oxides by metering reducing agent 26 to the exhaust system 12 of the internal combustion engine 10, various sensors 36, 38, 40, 42 and 44 are also provided, which detect operating parameters of the exhaust system 12 and supply corresponding measured values to the control unit 14. In one embodiment, the sensors 36 and 40 are temperature sensors, while the sensor 38 is preferably used for detecting the NOx concentration in the exhaust gas upstream of the SCR catalytic converter 22. Another NOx sensor 42 is disposed behind the SCR catalyst 22. The sensor 44 detects an ammonia concentration in the exhaust gas downstream of the SCR catalytic converter 22 and thus allows the determination of an overdosage of reducing agent 26.FIG. 1 thus shows in particular the technical environment in which the invention is used. It is understood that the invention is not limited to the configuration of internal combustion engine 10 with an exhaust system 12 and all the illustrated sensors 16, 36, 38, 40, 42, 44, 45 and 46 and actuators 18, 24, 32, 33, 34 shown in FIG. 1.With regard to method aspects of the invention, it is essential that the reducing agent metering valve 24 is arranged in the exhaust system 12 of the internal combustion engine 10 and is actuated for metering reducing agent 26 with a first flow profile, that a measure for the temperature of the reducing agent metering valve 24 is determined and compared with a threshold value, and that, if the determined temperature is lower than the threshold value, the reducing agent metering valve 24 is actuated with a second flow profile which differs from the first flow profile.With regard to the control unit 14 configured to control the reducing agent metering valve 24, it is essential that the control unit 14 is not only configured to actuate the reducing agent metering valve 24 with a first flow profile for metering reducing agent, but is also configured to determine a measure for the temperature of the reducing agent metering valve, to compare it with a threshold value, and then, if the determined temperature is lower than the threshold value, to actuate the reducing agent metering valve 24 with a second flow profile which differs from the first flow profile. Embodiments of the control unit 14 are configured to control a sequence of a method according to one of the dependent method claims.The threshold value is preferably predetermined such that it separates temperature ranges with and without the risk of freezing from one another. The temperature of the reductant dosing valve is measured and / or modeled. In one embodiment, a measurement is carried out by a special temperature sensor, which is not shown in FIG. 1. In a further embodiment, the temperature can be determined by measuring the current through the solenoid of the reducing agent metering valve 24, by using Ohmic Law, by using its resistance, and by using the resistance, by using the temperature of the solenoid as a measure of the temperature of the reducing agent metering valve 24. In internal combustion engines 10 where the ambient temperature, for example, the intake air temperature, is detected, a measure of the temperature of reductant dosing valve 24 may be modeled based on the detected ambient temperature and a temperature measured in exhaust system 12 or modeled for exhaust system 12. Modeling is understood to mean a computational replication of the temperature as a function of relationships stored in control unit 14, with additional consideration of the temperatures mentioned and / or other operating parameters of internal combustion engine 10 or exhaust system 12.FIG. 2 shows in its part a) a configuration of a first flow profile 46 and in its part b) a configuration of a second flow profile 48, wherein the horizontal dashed line 50 both in FIG. 2 a) and in FIG. 2 b) marks in each case a flow level which is required for opening and keeping open the reducing agent metering valve 24. The first current profile 46 has a first part in which the current I through the solenoid of the reducing agent metering valve 24 is set to a first, comparatively high value I 1 in order to open the reducing agent metering valve 24 quickly. Following the first part, the first current profile 46 has a second part in which a lower current I 2 is set. However, the lower current I2is still above the dashed line 50 marking a holding current level. As a result, the reductant dosing valve 24 with the first flow profile 46 is opened and maintained open between times t_ 0 and t_ 1.In contrast, the second flow profile 48 has an average flow level E 3 which is below the holding flow level 50 necessary for opening and keeping the reducing agent metering valve 24 open, such that the reducing agent metering valve 24 is not opened by the second flow profile 48 generated between the times t_ 2 and t_ 3. FIG. 2 b) thus shows in particular an embodiment of a second flow profile 48 which differs from a first flow profile 46 with which the reducing agent metering valve 24 is controlled (opened) for metering reducing agent 26.FIG. 3 shows an equivalent circuit diagram of the reducing agent metering valve 24 together with an output stage 52 of an embodiment of the control unit 14. the output stage 52 has a DC voltage source 54 and a switch 56, which is actuated by a further component 58 of the control unit 14. Block 58 summarizes the hardware aspects of processing input signals by control unit 14, i.e., in particular, an input signal processing and processing with the aid of a program stored in a memory of control unit 14. In the equivalent circuit diagram of the reducing agent metering valve 24, the magnetic coil is shown as a series circuit comprising a pure inductance 58 and an ohmic resistor 60. One connection each of the DC voltage source 54 and the solenoid of the reducing agent metering valve 24 is connected to a reference potential 62, for example a control unit ground. The respective complementary connections of the magnetic coil and the DC voltage source 54 are connected to one another or disconnected from one another via the switch 56.The current profiles 46 and 48 from FIG. 2 are generated by the configuration of FIG. 52 from FIG. 3 by a corresponding control of the switch 56. The current level I1 is obtained, for example, by the switch 56 being closed until the induction voltage of the inductance 58 has decayed to such an extent that the full or nearly full DC voltage of the DC voltage source 54 is established via the magnet coil. The DC voltage provided by the DC voltage source 54 is preferably greater than a threshold voltage at which a non-frozen reducing agent metering valve 24 opens. The current levels I2 and I3, on the other hand, result from the fact that the switch 56 is alternately closed (current rise) and opened (current fall) in the case of induction voltages which have not yet decayed.Each current flow through the solenoid of the reducing agent metering valve 24 is associated with a release of Joule heat because of the ohmic resistor 60. If the reducing agent metering valve 24 is controlled with the first flow profile 46 in order to meter reducing agent 26, this heat release may be disruptive. This applies in particular when the metering takes place with hot exhaust system 12 and hot exhaust gas, because there is then the risk of thermal damage to reducing agent metering valve 24. The reduction of the current intensity from the value I1 to the value I2, which is still sufficient to keep the reducing agent metering valve 24 open, reduces the release of heat, which is disruptive in this case, by the ohmic resistor 60 of the magnet coil.At low temperature of the exhaust system 12 and / or the exhaust gas of the internal combustion engine 10, the release of Joule heat in the ohmic resistor 60 is used for a desired heating of the reducing agent metering valve 24. The second flow profile 48 in FIG. 2 represents in this context an embodiment of a flow profile in which an average flow I 3 for heating the reducing agent metering valve 24 is generated, but which is still so small that an opening of the reducing agent metering valve 24 and thus a metering of reducing agent 24 to the exhaust system 12 does not yet take place. In this embodiment, the second current profile 48 is generated by applying a clocked DC voltage to a solenoid of the actuator system of the reducing agent metering valve 24.The generation of different current profiles 46 and 48 thus allows decoupling of the metering effect and the heating effect of current I by the solenoid of reducing agent metering valve 24 insofar as heating of reducing agent metering valve 24 is possible without simultaneous metering of reducing agent. This applies regardless of whether the reducing agent metering valve 24 and the feed line 30 are filled with reducing agent 26 or not.However, it is understood that a heating effect can also be generated by a second flow profile 48 in a metering operation. In one embodiment, this is done in that the second current profile 48 is generated by applying the full DC voltage to the solenoid of the actuator system of the reducing agent metering valve 24, without clocked switching on and off of the voltage source 54. It is also understood that the second flow profile can have all conceivable mixed forms of the flow profiles 46 and 48. Thus, the base current level I 3 of the second current profile 48 may be maintained over a longer period of time to achieve a continuous heating effect. If a metered addition of reducing agent 26 is then to be carried out, the difference from the first current profile 46, the first current profile 46 or the holding current I 2 of the first current profile 46 is superimposed on the basic current level I 3 in order to generate a second current profile. In this case, the reducing agent metering valve 24 is temporarily opened, wherein the heating effect is maintained even when the reducing agent metering valve 24 is not opened.In order to achieve an increased heating effect without metering reducing agent 26, a further embodiment provides that the control unit 14 controls the pump 32 such that the pump 32 draws reducing agent from the feed line 30 and the reducing agent metering valve 24 back into the storage container 28. In this case, the now pressureless reducing agent metering valve can be heated with high currents and thus high heating effects, without an undesired metering of reducing agent 26 to the exhaust system 12 taking place.FIG. 4 shows a further embodiment in which the control unit 14 is configured to generate a second current profile by applying an alternating voltage to a magnet coil of the actuator system of the reducing agent metering valve. In this case, the output stage 52 according to FIG. 4 has an AC voltage source 64 which replaces or supplements the DC voltage source 54 from FIG. 3. With the necessary changes, all the embodiments explained in connection with FIG. 3 can also be achieved with the AC voltage source 64 according to FIG. 4. Thus, an AC voltage source 64, for example an AC voltage source with a controllable frequency, can be operated at such a high frequency that the reducing agent metering valve 24 cannot follow this frequency because of its mechanical inertia.The alternating current which nevertheless flows through the ohmic resistor 60 then heats the reducing agent metering valve 24. This configuration can also be carried out independently of a filling of the reducing agent metering valve 24 and its feed line 30 with reducing agent 26.If opening of the reducing agent metering valve 24 is permitted or desired, a further configuration provides operation with a frequency of the alternating voltage which is so low that a non-frozen reducing agent metering valve 24 opens and closes with the frequency of the alternating voltage. The changed frequency can be effected either by alternately connecting a further alternating voltage source which provides an alternating current with a lower frequency, or by a controlled change in the frequency of the alternating voltage source 64.As a further alternative, the magnet coil can also be connected alternatively or additionally via separate switches to a DC voltage source 54 and an AC voltage source 64, so that the magnet coil is fed only with the current from one of the two voltage sources 54, 64 or with the sum of the currents.Here too, it applies that opening of the reducing agent metering valve 24 is permitted if, before opening, a suck back of reducing agent from the feed line 30 and the reducing agent metering valve 24 has taken place.To further improve the heating effect, a further embodiment provides that the reducing agent metering valve 24 and its supply line 30 is alternately filled with reducing agent 26 and emptied in order to convey heat to non-electrically heated parts of the system using warm reducing agent 26. The reducing agent 26 is heated by the heater 33 and / or 34 and pumped back and forth in the system by the pump 32.FIG. 5 shows an embodiment of a method for operating the electromagnetically controllable reducing agent metering valve 24, as is controlled by the control unit 14. In a step 64, the temperature T(24) of the reducing agent metering valve 24 is determined. As explained further above, this can be done by a measurement and / or a modeling. Subsequently, the temperature T(24) thus determined is compared in a step 66 with a predetermined threshold value T_S, which separates temperature ranges with and without the risk of freezing from one another. If T( 24) is greater than the threshold T_S, the program branches to step 68 where the reductant dosing valve is operated with the first current profile 46 without special heating actions. If, on the other hand, the temperature T( 24) is less than the threshold value T_S, operation of the reducing agent metering valve 24 with the second flow profile 48 takes place in step 70.

Claims

Method for operating an electromagnetically controllable reducing agent metering valve (24), which is arranged in the exhaust system (12) of an internal combustion engine (10) and which is actuated with a first current profile (46) for metering reducing agent (26), characterized in that a measure for the temperature (T(24)) of the reducing agent metering valve (24) is determined and compared with a threshold value (T_S), and in that, if the determined temperature (T(24)) is lower than the threshold value (T_S), the reducing agent metering valve (24) is actuated with a second current profile (48), which differs from the first current profile (46), wherein the second current profile (48) is generated by applying a direct voltage to a magnet coil of the actuator system of the reducing agent metering valve (24), and the direct voltage is predetermined in such a way, that no opening of the reducing agent metering valve (24) takes place yet and wherein the method is carried out independently of a filling of the reducing agent metering valve (24) and its feed line (30) with reducing agent (26).Method according to Claim 1, characterized in that before its execution, reducing agent (26) is sucked back from the reducing agent metering valve (24) and its feed line (30).Method according to one of the preceding claims, characterized in that the reducing agent metering valve (24) and its feed line (30) are filled and emptied alternately with reducing agent (26) in order to convey heat to non-electrically heated parts of the system using warm reducing agent.A control device (14) which is configured to control an electromagnetically controllable reducing agent metering valve (24) arranged in the exhaust system (12) of an internal combustion engine (10) and is configured to actuate the reducing agent metering valve (24) with a first flow profile (46) for metering reducing agent (26), characterized in that the control device (14) is configured to determine a measure for the temperature (T(24)) of the reducing agent metering valve (24) and to compare it with a threshold value (T_S) and, if the determined temperature (T(24)) is lower than the threshold value (T_S), to actuate the reducing agent metering valve (24) with a second flow profile (48) which differs from the first flow profile (46), wherein the control device is configured to actuate the reducing agent metering valve (24) with a second flow profile (48) which differs from the first flow profile (46), the second current profile (48) is generated by applying a direct voltage to a magnet coil of the actuator system of the reducing agent metering valve (24), and the direct voltage is predetermined such that the reducing agent metering valve (24) is not yet opened, and wherein the control device is configured to carry out the method independently of filling the reducing agent metering valve (24) and its feed line (30) with reducing agent (26).Control device (14) according to Claim 4, characterized in that it is designed to control a sequence of a method according to one of the dependent method claims.

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