Device for providing a liquid additive and method for heating the additive

Abstract

Device (1) for providing a liquid additive into an exhaust line (2), comprising a tank (3), a delivery line (4), a heating element (5) made of a PTC thermistor for heating the additive in the tank (3) and / or the delivery line (4) and a metering device (6) for metering the liquid additive with a coil (7) and a movable component (8), wherein the movable component (8) is movable by means of the coil (7) and the coil (7) is electronically connectable in series with the heating element (5) by means of a first switching element (9).

Description

The present invention relates to an apparatus for supplying a liquid additive to an exhaust line and to a method for heating the additive in the apparatus. The invention is used in particular in a motor vehicle which is provided with a tank for storing the liquid additive.It is to be regarded as known that methods for exhaust gas after-treatment are used in which liquid additive is added to the exhaust gas. This is intended in particular to ensure that the pollutants contained in the exhaust gas are converted as completely and efficiently as possible.For example, it is known to supply an oxidizing agent (for example fuel or hydrocarbon) to the exhaust gas. This additive can be used to react directly with constituents of the exhaust gas and thus to react pollutants. However, it is also possible for this oxidizing agent to be used to convert ambient conditions in the exhaust system into a desired state suitable for the conversion of pollutants. Thus, fuel or hydrocarbon can be supplied to an oxidation catalyst, wherein the temperature of the exhaust gas in the exhaust system can be significantly increased due to an exothermic reaction in the oxidation catalyst. This serves in particular for regenerating a diesel particle filter.It is also known to supply a reducing agent to the exhaust gas. Although it is possible in principle to introduce solid or gaseous reducing agent into the exhaust system, conveying systems and metering systems have proven particularly suitable, which bring about a liquid supply of the reducing agent. For example, the method of selective catalytic reduction (SCR: selective catalytic reduction) is known. In this case, nitrogen oxide compounds are reacted in the exhaust gas with the aid of the reducing agent. For this purpose, ammonia or ammonia-forming substances can be used in particular. A reducing agent which is already widely used for this purpose is urea-water solution. A 32.5% urea-water solution is known under the trade name AdBlue ®. This liquid additive is then first added to the exhaust gas and then passed over a suitable catalyst in which the desired chemical conversion then takes place (inter alia).Since, for example, liquid additive, such as urea-water solution, already freezes at temperatures around -11 ° C., heating systems have been proposed which are intended to achieve rapid thawing of the liquid additive in the tank and / or a connected conveying line. The desired exhaust gas purification method should thus be available already quickly after engine start of a motor vehicle and / or damage to the components required for storing, delivering and / or adding the liquid additive should be avoided.As heating devices, liquid heaters (e.g., heat exchangers that cooperate with the engine cooling system), electric heaters, radiant heaters, and combinations thereof have already been proposed in this technical field. However, the quickest possible and possibly even possible heating in the container could not yet be achieved satisfactorily. In particular, the proposed systems for heating the container are technically complicated, cost-intensive and / or difficult to regulate or control. In these heating devices, there is the risk that the additive is heated above a critical temperature. Thus, in AdBlue, ® components thereof can crystallize above 65° C., which increases the risk of undesired deposits and blocking.It is also known that the heating device has a single electrically operable heating element. An electrically operable heating element can likewise be activated and deactivated at predefined times. This is effected, for example, on the basis of ohmic resistance heating. A known electrically operable heating element, which is particularly preferably used, is a so-called PTC thermistor or PTC heating element (PTC: positive temperature coefficient). A PTC thermistor or PTC heating element contains a material whose electrical resistance increases as the temperature rises. Conveying devices with PTC heating elements are known from DE 10 2010 038 361 A1 and US 2010 / 0 107 615 A1.In such devices with a PTC thermistor as the heating element, a current spike occurs after switching on by applying a constant voltage because of the initially low resistance. The current through the PTC thermistor therefore rises sharply when it is switched on. During further operation, the PTC thermistor heats up due to Joule heat and the electrical resistance of the PTC thermistor thus also changes. In a temperature range in which the electrical resistance of the PTC thermistor changes greatly even with small temperature changes, the current through the PTC thermistor then sets to a constant level. After a certain time, when a characteristic temperature is reached, a nearly constant current flows (at a constant voltage) through the PTC thermistor, wherein the value of the characteristic temperature depends on the properties of the PTC thermistor. All current-conducting elements and in particular also all connection points of the current-conducting elements must be designed for the maximum current intensity of the current peak. In addition, the current spike can result in a transient electrical transient ("electrical transient") disturbance which could influence the electronics of the motor vehicle. It is therefore desirable to keep the current peak as low as possible.It is therefore the object of the present invention to at least partially alleviate the problems described with reference to the prior art. In particular, a device for providing a liquid additive into an exhaust line is to be specified, in which the inrush current is limited in a cost-effective manner by a heating element made of a PTC thermistor. In addition, a method for heating the additive in the device is to be specified, in which only an inrush current of limited magnitude flows.These objects are achieved with an apparatus and a method according to the features of the independent claims. Advantageous further developments of the device and of the method are specified in the dependent patent claims. The developments are more precisely described in the description.These objects are achieved in particular by a device for supplying a liquid additive into an exhaust line, comprising a tank, a delivery line, a heating element made of a PTC thermistor for heating the additive in the tank and / or the delivery line, and a metering device for metering the liquid additive, having a coil and a movable component, wherein the movable component is movable by means of the coil and the coil is electronically switchable in series with the heating element by a first switching element.The liquid additive is in particular a reducing agent, preferably a urea-water solution.The tank is in particular a tank for storing a reducing agent, in particular urea-water solution. However, this could also be used for oxidizing agents and other liquid additives. The tank can be made of plastic. In any case, the tank should be stable and designed for permanent storage of the liquid additive. The additive is stored in the tank and conveyed to the exhaust line via the conveying line.The heating element is formed in particular in and / or on the tank. Alternatively or cumulatively, the heating element is formed in and / or on the conveying line. If the additive has solidified due to low temperatures or if it threatens to solidify due to low temperatures, the temperature of the additive in the tank and / or in the delivery line can be increased by means of the heating element, so that it reaches the liquid state or remains in the liquid state.A PTC (positive temperature coefficient) element is a material whose electrical resistance increases as the temperature rises. The PTC thermistor thus has a positive temperature coefficient. The PTC thermistor operates in a self-regulating manner about a setpoint temperature when a constant voltage is applied. For example, ceramic materials such as barium titanate ceramics and / or doped polymers are used as PTC thermistors.The heating element is preferably designed such that, when a constant voltage is applied at the level of 12, 24 or 48 V, a constant current is established, with which the heating element adjusts to a temperature of at least 10° C., preferably at least 50° C.The term "metering device for metering the liquid additive" is intended to mean all elements of the device which convey the additive, such as a pump, for example, and which can limit the flow of the additive in the conveying line, such as a valve, for example. The metering devices contain a coil or an inductive element which exert an electromagnetic force on the movable component on account of a current flow generated therein and the magnetic field generated thereby. The movable component is in particular a magnetic element which is set in motion on account of the electromagnetic force generated by the coil. In a pump, this may be a membrane moving rod, for example. In a valve, this is preferably the valve needle itself.The present invention now proposes that, in particular during the switch-on process of the heating element, the coil can be electronically connected in series with the heating element by a first switching element. In particular, all known switching elements such as transistors, preferably MOSFETs, are used as switching elements. When the heating element is switched on, the current thus flows both through the coil and through the heating element. Due to the inductive effect of the coil, the current intensity of the current peak is greatly reduced when it is switched on. Thus, all further components through which the current flows, in particular electrical contacts and / or lines, of the device can also be designed for a lower current intensity. Furthermore, electrical transient interference of further electronic components is reduced.Coils as inductive components for filtering high frequencies or for limiting rapid current fluctuations are already known, but these components have hitherto been used as passive components. Coils are thus generally provided as chokes as separate components in circuits. The present invention teaches for the first time the use of an active coil to limit the inrush current of a heating element in a device for providing a liquid additive.According to an advantageous development of the device, it comprises a control device having the first switching element which is electronically connected in series with the coil and the heating element, and a second switching element which is connected in parallel with the first switching element and the coil and in series with the heating element. The control device is preferably formed spatially separated from the dosing device.The first switching element and the coil can thus be bypassed with the second switching element, such that the heating element can be operated independently of the switching position of the first switching element. This is particularly useful when the heating element has reached its operating temperature and the heating element is to be operated continuously. In this case, the coil and thus the dosing device can thus be operated independently of the heating element.In particular, if the second switching element is closed at the same time, the coil can be energized via a third switching element. Thus, after the switching-on process of the heating element, the coil and thus the dosing device can be operated independently of the heating element by opening and closing the third switching element.According to a further aspect of the invention, a method for heating an additive in a device according to the invention is proposed, comprising the following steps:determining a temperature of the additive,comparing the determined temperature with a first predefinable value,switching on the heating element when the determined temperature is lower than the first predefinable value, in that the coil is electronically connected in series with the heating element by a first switching element.To determine the temperature of the additive, the temperature can be measured directly by means of a temperature sensor. The temperature of the additive can, however, also be determined by measuring a temperature at another point of the device or of the motor vehicle, it being possible thereby to draw conclusions about the temperature of the additive. The first predefinable value is, in particular, the freezing temperature of the additive or a value that is at least 2° C., preferably at least 5° C., greater than the freezing temperature of the additive. Thus, in the case of freezing or already frozen additive, the heating element is switched on. The heating element is switched on by the heating element being electronically connected in series with the coil of the dosing device. The inductance of the coil limits the current intensity of the current peak in its height.It is advantageous if the heating element is directly connected to a voltage source by connecting the coil in parallel with the heating element by a second switching element and a third switching element.This parallel connection is preferably effected after the heating element has been switched on. Thus, the heating element and the coil can be operated independently of each other when the heating element has reached its operating temperature.According to a further aspect of the invention, a motor vehicle having an internal combustion engine and an exhaust line is proposed, which comprises a device according to the invention and is configured and designed to carry out the method according to the invention.The embodiments made with respect to the device can be transferred to the method and applied and vice versa.The invention and the technical field are explained in more detail below with reference to the figures. It should be noted that the embodiments illustrated in the figures are not intended to limit the invention. In particular, the features shown and explained together in the figures can also be considered separately from one another and / or be combined with other (individual) features of other figures, provided this is not technically possible or, below, the association of features is not explicitly pointed out as obligatory. Accordingly, a large number of possible combinations of the invention illustrated schematically and by way of example in the figures will be apparent to the person skilled in the art. They show schematically: FIG. 1 : a motor vehicle having a device for providing a liquid additive, FIG. 2 : shows an apparatus for providing a liquid additive, FIG. 3 is a circuit diagram for an embodiment of a device according to the invention.FIG. 1 schematically shows a motor vehicle 14 having an internal combustion engine 15 to which an exhaust line 2 is connected. A catalyst 16 is formed in the exhaust pipe 2. Upstream of the catalytic converter 16, a liquid additive can be introduced into the exhaust gas by means of a device 1. The apparatus 1 comprises a tank 3 which is connected in terms of flow via a conveying line 4 to a metering device 6 for metering the additive. In the exemplary embodiment shown, the dosing device 6 is a valve which comprises a coil 7 and a movable component 8, here a valve needle. During operation of the motor vehicle 14, liquid additive can be conveyed into the exhaust line 2 using the device 1.FIG. 2 schematically shows a device 1 with a tank 3. A container 18 is formed in the tank 3, in which container a metering device 6 for metering liquid additive is again arranged. The dosing device 6 is realized here in the form of a pump. The dosing device 6 (pump) comprises a coil 7 and a movable component 8 for actuating the pump. The metering device 6 is connected to a conveying line 4, through which the additive is conveyed from the tank 3 via the metering device 6 to an exhaust line 2, not shown. A heating element 5 made of a PTC thermistor is furthermore arranged in the tank 3. The heating element 5 can additionally or alternatively also be arranged in the wall of the tank 3. The dosing device 6 and the heating element 5 are connected to a control device 10, which in turn is connected to a voltage source 13. By means of the heating element 5, the additive can be held in a liquid state or can be converted into a liquid state in the tank 3.FIG. 3 shows a circuit diagram of a device 1. The circuit comprises a voltage source 13, a first switching element 9, a second switching element 11, a third switching element 12 and a fourth switching element 17 as well as a coil 7 of a dosing device 6 and a heating element 5. the heating element 5 can be connected in series with the coil 7 or can be connected in parallel thereto depending on the position of the switching elements 9, 11, 12, 17.When the fourth switching element 17 is closed, the coil 7 can be connected in series with the heating element 5 by closing the first switching element 9. A current thus flows both through the coil 7 and through the heating element 5, which is advantageous in particular when the heating element 5 is switched on, since the peak current is thus limited in its magnitude.If the heating element 5 is at operating temperature, the heating element 5 can be operated parallel to the coil 7 by closing the second switching element 11 and opening the first switching element 9. In this case, the coil 7 and the heating element 5 can be switched on or off independently of one another via the third switching element 12 and the fourth switching element 17.The third switching element 12 and the fourth switching element 17 can be used to operate the coil 7 with pulse width modulation of the current.The height of the inrush current through a heating element 5 made of a PTC thermistor material can be limited in a cost-effective manner by the present inventions.List of reference characters1 Device 2 Exhaust line 3 Tank 4 Delivery line 5 Heating element 6 Metering device 7 Coil 8 Movable component 9 First switching element 10 Control device 11 Second switching element 12 Third switching element 13 Voltage source 14 Motor vehicle 15 Internal combustion engine 16 Catalytic converter 17 Fourth switching element 18 Container

Claims

Device (1) for providing a liquid additive into an exhaust gas line (2), comprising a tank (3), a feed line (4), a heating element (5) made of a PTC thermistor for heating the additive in the tank (3) and / or the feed line (4) and a metering device (6) for metering the liquid additive having a coil (7) and a movable component (8), wherein the movable component (8) is movable by means of the coil (7) and the coil (7) is electronically switchable in series with the heating element (5) by a first switching element (9).The device (1) according to claim 1, comprising a control device (10) having the first switching element (9) electronically connected in series with the coil (7) and the heating element (5), and a second switching element (11) connected in parallel with the first switching element (9) and the coil (7) and in series with the heating element (5).Device (1) according to claim 2, wherein the control device comprises a third switching element (12), via which the coil (7) can be energized when the first switching element (9) is open.Method for heating an additive in a device (1) according to one of Claims 1 to 3, comprising the following steps: - determining a temperature of the additive, - comparing the determined temperature with a first predefinable value, - switching on the heating element (5) if the determined temperature is less than the first predefinable value, in that the coil (7) is electronically connected in series with the heating element (5) by a first switching element (9).Method according to claim 4, wherein the heating element (5) is directly connected to a voltage source (13) by connecting the coil (7) in parallel with the heating element (5) through a second switching element (11) and a third switching element (12).

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