Methods for exhaust aftertreatment of an internal combustion engine and exhaust aftertreatment system

Abstract

Method for exhaust aftertreatment of an internal combustion engine (10) of a motor vehicle, wherein at least one reducing agent is used to reduce NOx in the exhaust gas, wherein the reducing agent is stored in at least one storage container (12), wherein the reducing agent is conveyed in a circuit (14) at least from the storage container (12) and into the storage container (12), and wherein the storage container (12) can be heated by means of a heating device (16), characterized in that the reducing agent is conveyed into the storage container (12) after the internal combustion engine (10) is switched off, and that the heating device (16) is heated by means of waste heat from at least one element of the motor vehicle.

Description

The invention relates to a method for the exhaust gas after-treatment of an internal combustion engine of a motor vehicle, wherein at least one reducing agent is used for reducing NO x in the exhaust gas, wherein the reducing agent is stored in at least one storage container, wherein the reducing agent is conveyed in a circuit at least out of the storage container and into the storage container, and wherein the storage container can be heated by means of a heating device.In addition, the invention relates to an exhaust gas aftertreatment system for an internal combustion engine, having at least one storage container for storing a reducing agent for reducing NO x in the exhaust gas of the internal combustion engine, having at least one injection system, having at least one circulation pump having at least one venting device, having at least one circuit for conveying the reducing agent at least from the storage container and into the storage container and having at least one heating device for heating the storage container.Due to the development of exhaust gas legislation, vehicle manufacturers are faced with the challenge of reducing the crude engine emissions and purifying the exhaust gas still arising by means of corresponding exhaust gas post-treatments. The legislation requires, especially in diesel vehicles, a reduction of the nitrogen oxide emission to a certain extent. The reduction of nitrogen oxides is often achieved with selective catalytic reduction technology (SCR). In this process, NO x is reduced to nitrogen and water vapor. As reducing agents, ammonia (NH3), ammonia in aqueous solution or urea in aqueous solution are generally used. The reducing agent is injected into the exhaust system as part of the exhaust gas aftertreatment system by means of an injection system in order to reduce the nitrogen oxides there in the catalytic converter.In modern diesel vehicles, the so-called ad blue liquid has become established as reducing agent for reducing the NO x concentration in the exhaust gas. The Ad-Blue liquid has the disadvantage that it freezes at -11 degrees Celsius. Since the Ad-Blue liquid has a proportion of >60% of water, there is the risk of volume expansion in the event of frost. In order to prevent damage due to this volume expansion, the liquid must be heated by the vehicle electrical system battery in the event of frost, as is customary in the prior art. The heating duration is limited in time and depends on the battery charge.DE 10 2005 002 318 A1 attempts to bypass this problem by conveying the reducing agent in a circuit, even if the metering is already concluded, so that freezing of the reducing agent is prevented.DE 10 2008 061 471 A1 discloses a method for melting a reducing agent in an SCR exhaust gas aftertreatment system. Reducing agent, which can freeze cold temperatures in feed lines, is melted by means of a heating device. In order to prevent renewed freezing, the reducing agent is circulated and further heated by the heating device.DE 10 2004 026 866 A1 shows a method for heating a reducing agent carried along in a container of a motor vehicle for exhaust gas after-treatment. For this purpose, the document is based on the assumption that the amount of heat required for the thawing process is supplied to the reducing agent outside the storage container. It is provided that the reducing agent is removed from the storage container at a removal point and heated and is returned again at a return pointDE 10 2010 061 753 A1 relates to an exhaust gas aftertreatment system and an operating method for NO x- reduction in the exhaust gas of an internal combustion engine, having a storage tank, a delivery module and a metering module for introducing a reducing agent into an exhaust tract of the internal combustion engine. A latent heat storage device is connected to a heat exchanger of the storage tank via a circulating heat transport medium. The latent heat storage device is either designed as a heat exchanger in an external arrangement or designed as a casing of the storage tank or arranged as a central heating unit in the storage tank.DE 10 2014 226 749 A1 discloses a method and a device for checking a temperature sensor and an electrical heating device in an exhaust gas aftertreatment system of an internal combustion engine, wherein the exhaust gas aftertreatment system comprises at least one reduction catalyst, a reducing agent storage container for storing liquid reducing agent, a fill level sensor for determining the amount of reducing agent in the reducing agent container, a reducing agent pump for conveying the reducing agent, and a metering device for introducing the reducing agent into an exhaust line of the internal combustion engine. The temperature sensor and the electrical heating device are arranged within the reducing agent storage container. In an operating state of the internal combustion engine, in which frozen reducing agent is present in the reducing agent storage container due to the prevailing temperatures, the heating device is switched on and checked whether the signal of the temperature sensor rises starting from a starting temperature at the switch-on time of the heating device and during the heating process, the temperature of the reducing agent remains constant for a predetermined period of time when the freezing point of the reducing agent is reached. The temperature sensor and the heating device are recognized as fault-free if the temperature remains constant during the predetermined time period and subsequently rises within a predetermined time period up to a predetermined setpoint value.The object of the invention is to specify a method for the exhaust gas aftertreatment of an internal combustion engine and an exhaust gas aftertreatment system in which the reducing agent can be maintained in the liquid state for a longer time and in an energy-optimum manner or can be stored in the storage container without damage without electrical heating.This object is achieved in the present invention by the features of the characterizing part of claim 1 first in that the reducing agent is conveyed into the storage container after the internal combustion engine has been shut down and in that the heating device is heated by means of waste heat of at least one element of the motor vehicle.Ammonia (NH3), ammonia in aqueous solution and / or urea in aqueous solution can be used as reducing agent. Preferably, the reducing agent is an Ad-Blue liquid. Ad-blue liquids are known from the prior art. Ad-Blue consists of a mixture of demineralized water and about 32% urea.The storage container is a container which is suitable for receiving the reducing agent. The storage container is dimensioned in such a way that all the reducing agent in the circuit can be stored therein. The circuit is understood to mean a fluidic connection, in particular pipelines, through which the reducing agent can be conveyed out of the storage container. In the circuit, the reducing agent can be conveyed to further units or elements of the vehicle. In particular, the reducing agent is conveyed via an injection system to the catalyst for reducing NO x before it is conveyed at least partially back into the storage container.The storage container can be heated by means of a heating device. The heating device is not, as is customary in the prior art, an electric heater, but in particular a heat exchanger which is designed to absorb waste heat from at least one element of the motor vehicle. An element of the motor vehicle is understood to mean any device of the vehicle that warms up during the course of vehicle operation and can advantageously be used to transfer the heat to the storage container. Preferably, the elements that are located in the vicinity of the storage container are, for example, elements inside or in the vicinity of the engine compartment.This thermal integration has the effect that the storage container and above all the reducing agent are heated to a certain extent and thus freezing can be prevented. According to the invention, it is provided that the circuit, i.e. the pipelines and further fluidic connections, is completely flushed, so that no reducing agent is present any longer in the circuit. The reducing agent is then completely located in the storage container. In this way, the risk of reducing agent freezing within the circuit is no longer present. The flushing can be understood to mean, for example, a ventilation of the circuit, in which the reducing agent present in the circuit is conveyed into the storage container by means of air.The storage container is preferably insulated, so that the heat loss inside the storage container to the environment is as low as possible.Further preferred embodiments of the invention result from the other features mentioned in the dependent claims.In a first embodiment of the method according to the invention, it is provided that the reducing agent is firstly pumped in the circuit after the internal combustion engine has been shut down until the temperature of the reducing agent is in equilibrium with the engine temperature or with the temperature of the engine compartment. The reductant is thus conveyed further past the warmer portions of the engine compartment so that heat transfer may take place. With this additionally introduced thermal energy, the reducing agent can be present in the isolated storage container in the liquid state for a longer time.In order to be able to heat the reducing agent to a certain extent within the storage container, it is provided in a further embodiment of the method according to the invention that the heating device is heated by means of waste heat from an engine cooling system of the motor vehicle. The heating device, in the form of a heat exchanger for example, can use the waste heat of the engine cooling system in order to heat the storage container of the reducing agent or to maintain it at a specific temperature level. In this case, it can be provided that a heating heat exchanger, as an element of the engine cooling system, is supplemented by an additional plate pack including a continuous spiral. Engine heat then reaches the storage container via a line. The storage container is correspondingly extended by a water-conducting heating spiral which can be immersed in the storage container. A circulation pump can ensure the on-demand circulation.Additionally or alternatively, in a further embodiment of the method according to the invention, it is provided that the heating device is heated by means of waste heat from an oil cooler and / or from a transmission oil cooler of the motor vehicle. Analogously to the above embodiment, the waste heat of the oil cooler and / or of the transmission oil cooler can also be used. In this case, it can be provided that the oil cooler and / or the transmission oil cooler of the internal combustion engine are supplemented by an additional plate pack including a continuous spiral. The waste heat then reaches the storage container via a line, wherein the storage container is supplemented by a water-conducting heating spiral, as part of the heating device, wherein the heating spiral is immersed in the tank. Only heat is transferred in this case. There is no material transport between the line and the storage container. A circulation pump ensures the circulation in need.In a further alternative or a further supplement, it is provided that the heating device is heated by means of waste heat from an exhaust system of the motor vehicle. Analogously to the aforementioned possibilities, the heat is taken from the exhaust system of the internal combustion engine. In this case, the exhaust pots can be widened by a water-conducting spiral in the interior of the pot. Exhaust heat then reaches the storage container via a line. The storage container is correspondingly extended by a water-conducting heating spiral which can be immersed in the storage container. A circulation pump ensures the circulation in need. This variant can be combined with the run-on time of the vehicle within the scope of a regeneration of the prescribed diesel particulate filter. Here, the thermal energy, which has to be reduced when the motor vehicle is shut down, shortly after regeneration of the soot particle filter, can be emitted directly via the circuit to the storage container.The aforementioned object is also achieved by an aforementioned exhaust gas aftertreatment system for an internal combustion engine, having at least one storage container for storing a reducing agent for reducing NO x in the exhaust gas of the internal combustion engine, having at least one injection system, having at least one circulation pump, having at least one venting device, having at least one circuit for conveying the reducing agent at least from the storage container and into the storage container and having at least one heating device for heating the storage container. It is provided that the exhaust gas aftertreatment system is configured to execute a method according to one of Claims 1 to 5. The above explanations relating to the method according to the invention also apply accordingly to the exhaust gas aftertreatment system according to the invention.The injection system is configured to inject the reducing agent into the exhaust system as part of the exhaust gas aftertreatment system in order to reduce nitrogen oxides formed in the catalyst there. The circulation pump serves to pump the reducing agent in the above-mentioned circuit. The venting device is designed to convey the reducing agent completely into the storage container, wherein the lines of the circuit are vented through the venting device, for example by means of air.In a first embodiment of the exhaust gas aftertreatment system according to the invention, it is provided that the heating device is arranged within the storage container and that the heating device comprises a heat exchanger for absorbing waste heat of at least one element of the motor vehicle. The heating device can be designed, for example, as a liquid-conducting heating spiral which can be immersed in the storage container. Only heat is transferred by the heating device. There is no material transport between the heat exchanger and the storage container.In order to speed up a possible thawing of the reducing agent, it is provided in a further embodiment of the exhaust gas aftertreatment system according to the invention that the storage container comprises a first storage container and a second storage container and that the first storage container has a smaller volume than the second storage container. Depending on the volume of the storage container, a possible thawing process of the reducing agent can take more or less time. The thawing time can be significantly reduced by using a first storage container and a second storage container. The first reservoir has a smaller volume than the second reservoir, so that there is a main tank and a small auxiliary tank. The first, smaller reservoir serves to quickly thaw the reducing agent required immediately upon engine start-up. The first storage container can also be heated with an electric heating device, since the amount of energy required is comparatively small. In the larger second storage container, the thawing process takes somewhat longer. The second storage container does not rely on an electric heating device, but can be heated with the available engine waste heat.In order to further improve the flexibility of the exhaust gas aftertreatment system, it is provided in a further embodiment of the exhaust gas aftertreatment system according to the invention that the storage container is designed to be elastic, so that a volume expansion is made possible. The reservoir is thus configured to be stretchable. Preferably, the reservoir can withstand an expansion of the volume >10%, in order to be able to compensate for the expansion of a possibly frozen reducing agent. This solution has the advantage that after the reducing agent is conveyed into the storage container there is no risk of frost or damage and no electrical energy is necessary to heat the reducing agent.In addition, in a further embodiment of the invention, it can be provided that the storage container is designed to be spherical. The spherical configuration minimizes the surface of the storage container, so that the heat loss via the surface of the storage container is likewise minimized.The various embodiments of the invention mentioned in this application can be combined with one another with advantage unless stated otherwise in the individual case.The invention is explained below in exemplary embodiments with reference to the associated drawings. The following are shown: FIG. 1 shows a schematic illustration of an exhaust gas aftertreatment system with the use of the waste heat of an engine cooling system, FIG. 2 shows a schematic illustration of an exhaust gas aftertreatment system with the use of the waste heat of an oil cooler, FIG. 3 shows a schematic illustration of an exhaust gas aftertreatment system with the use of the waste heat of a transmission oil cooler, and FIG. 4 shows a schematic illustration of an exhaust gas aftertreatment system with the use of the waste heat of an exhaust system.FIG. 1 shows the execution of a method for the exhaust gas aftertreatment of an internal combustion engine 10 of a motor vehicle, not shown here. A reducing agent is used for reducing NO x in the exhaust gas. The reducing agent is stored in a storage container 12 and can be conveyed out of the storage container 12 and into the storage container 12 via a circuit 14. The storage container can be heated by means of a heating device 16. In the present exemplary embodiment, the heating device operates with the waste heat of an engine cooling system 18. In addition to the engine cooling system 18, in the region of the internal combustion engine 10, in particular oil cooling systems, such as an oil cooler 20 and a transmission oil cooler 22, but also the exhaust system 24 itself, are to be mentioned as heat sources.An injection system 28 is also required for an exhaust gas aftertreatment system 26, which injection system is arranged in the circuit 14 of the reducing agent. The reducing agent is injected into the exhaust system 24 by means of the injection system 26 in order to reduce the nitrogen oxides there in the catalytic converter. The exhaust gas aftertreatment system further includes a circulation pump 30 and a venting system 32.In the exemplary embodiment shown, after the internal combustion engine 10 has been shut down, the reducing agent is flushed in the existing circuit 14, so that the liquid reducing agent is heated by the residual heat of the environment. The thermal energy is stored in the reducing agent. The flushing is carried out in such a way that the reducing agent circulates in the circuit 14, wherein the circulation is driven by the circulation pump 30 or the venting system 32. The circulation is continued until the temperature of the reducing agent is in equilibrium with the engine temperature and in equilibrium with the environment of the engine region, respectively.Once equilibrium is reached, the circuit 14 is vented, that is, the circulation pump 30 delivers all the reducing agent to the reservoir 12 and purges the lines of the circuit 14 with, for example, air. The conduits and the injection system 28 are thus frost-proof, as they are free of liquid reductant. If, for example, Ad-Blue is used as reducing agent, the freezing point is approximately -11 degrees Celsius.Only the storage container contains liquid reducing agent after the flushing. The storage container 12 is thermally insulated in order to preserve the thermal energy for as long as possible. Moreover, the heating device 16 is arranged inside the storage container 12. The heating device 16 can be controlled by the on-board power supply system in a demand-oriented manner. The comparatively higher residual heat increases the initial temperature of the reducing agent, so that, for example, service lives overnight do not represent a risk for the freezing of the reducing agent.The heating device 16 is heated by means of waste heat from the engine cooling system 18 of the motor vehicle. The heating device 16 comprises a heat exchanger 34, which can use the waste heat of the engine cooling system 18 in order to heat the storage container 12 of the reducing agent or to maintain it at a specific temperature level. In this case, a heating heat exchanger, not shown here, is supplemented as an element of the engine cooling system 18 by an additional plate pack including a continuous spiral. The storage container 12 is correspondingly extended by a water-conducting heating spiral, as part of the heat exchanger 34, which is immersed in the storage container 12.FIG. 2 shows the exemplary embodiment analogous to FIG. 1, wherein the waste heat for heating the storage container 12 is not taken from the engine cooling system 18 but from the oil cooler 20.FIG. 3 shows the exemplary embodiment analogously to FIGS. 1 and 2, wherein the waste heat for heating up the storage container 12 is not taken from the engine cooling system 18 or the oil cooler 20, but rather from the transmission oil cooler 22.FIG. 4 shows the exemplary embodiment analogously to FIGS. 1, 2 and 3, wherein the waste heat for heating up the storage container 12 is taken from the exhaust system 24. Moreover, the storage container 12 is divided into a first storage container 36 and a second storage container 38. The first storage container 36 has a smaller volume than the second storage container 38; in this way, possible thawing of the reducing agent can be accelerated. Depending on the volume of the storage container 12, a possible thawing process of the reducing agent can take more or less time. The thawing time is reduced by dividing the storage container 12 into two storage containers 36, 38. The first reservoir 36 has a smaller volume than the second reservoir 38, so that there is a main tank and a small auxiliary tank. The first, smaller storage container 36 serves to quickly thaw the reducing agent required immediately upon engine start-up. The first storage container 36 can also be heated by an electric heating device, since the amount of energy required is comparatively small. In the larger, second storage container 38, the thawing process takes somewhat longer. The second storage container 38 does not rely on an electric heating device, but can be heated with the available waste heat of the exhaust system 24 by the heating device 16.In this exemplary embodiment, the first storage container 36 is designed to be elastic. In this way, a volume expansion of the first storage container 36 is possible. The first reservoir 36 is thus configured to be stretchable. The first storage container 36 can withstand an expansion of the volume >10%, in order to be able to compensate for the expansion of a possibly frozen reducing agent by volume. This solution has the advantage that after the reducing agent has been conveyed into the first storage container 36, there is no risk of frost or damage and no permanent electrical energy is necessary to heat the reducing agent.In addition, the first storage container 36 is designed to be spherical. The spherical configuration minimizes the surface of the first storage container 36, such that the heat loss via the surface of the first storage container 36 is likewise minimized.List of reference characters10 Internal combustion engine 12 Storage container 14 Circuit 16 Heating device 18 Engine cooling system 20 Oil cooler 22 Transmission oil cooler 24 Exhaust gas system 26 Exhaust gas aftertreatment system 28 Injection system 30 Circulation pump 32 Ventilation system 34 Heat exchanger 36 First storage container 38 Second storage container

Claims

Method for the exhaust gas after-treatment of an internal combustion engine (10) of a motor vehicle, wherein at least one reducing agent is used for reducing NOx in the exhaust gas, wherein the reducing agent is stored in at least one storage container (12), wherein the reducing agent is conveyed in a circuit (14) at least out of the storage container (12) and into the storage container (12), and wherein the storage container (12) can be heated by means of a heating device (16), characterized in that the reducing agent is conveyed into the storage container (12) after the internal combustion engine (10) has been shut down, and in that the heating device (16) is heated by means of waste heat of at least one element of the motor vehicle.Method according to Claim 1, characterized in that, after the internal combustion engine (10) has been shut down, the reducing agent is firstly conveyed in the circuit (14) until the temperature of the reducing agent is in equilibrium with the engine temperature.Method according to Claim 1 or 2, characterized in that the heating device (16) is heated by means of waste heat from an engine cooling system (18) of the motor vehicle.Method according to one of Claims 1 to 3, characterized in that the heating device (16) is heated by means of waste heat from an oil cooler (20) and / or from a transmission oil cooler (22) of the motor vehicle.Method according to one of Claims 1 to 4, characterized in that the heating device (16) is heated by means of waste heat from an exhaust system (24) of the motor vehicle.Exhaust gas aftertreatment system (26) for an internal combustion engine (10), having at least one storage container (12) for storing a reducing agent for reducing NOx in the exhaust gas of the internal combustion engine (10), having at least one injection system (28), having at least one circulation pump (30) having at least one venting device (32), having at least one circuit (14) for conveying the reducing agent at least from the storage container (12) and into the storage container (12) and having at least one heating device (16) for heating the storage container (12), characterized in that the exhaust gas aftertreatment system (26) is designed to carry out a method according to one of Claims 1 to 5.Exhaust gas aftertreatment system (26) according to Claim 6, characterized in that the heating device (16) is arranged within the storage container (12), and in that the heating device (16) comprises a heat exchanger (34) for absorbing waste heat of at least one element of the motor vehicle.Exhaust gas aftertreatment system (26) according to Claim 6 or 7, characterized in that the storage container (12) comprises a first storage container (36) and a second storage container (38), and in that the first storage container (36) has a smaller volume than the second storage container (38).Exhaust gas aftertreatment system (26) according to one of Claims 6 to 8, characterized in that the storage container (12) is of elastic configuration, with the result that volume expansion is made possible.Exhaust gas aftertreatment system (26) according to one of Claims 6 to 9, characterized in that the storage container (12) is of spherical configuration.

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