Patent Publication Number: US-2015086455-A1

Title: Method for operating an exhaust gas system, method for operating a motor vehicle and motor vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation, under 35 U.S.C. §120, of copending International Application No. PCT/EP2013/060305, filed May 17, 2013, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2012 104 669.7, filed May 30, 2012; the prior applications are herewith incorporated by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method for operating an exhaust system of an internal combustion engine, with the intention of reducing the pollutant emissions of the internal combustion engine, in particular, after a cold start. The invention also relates to a method for operating a motor vehicle and a motor vehicle. 
     Exhaust-gas regulations for internal combustion engines have become ever more stringent in recent years. That also applies, in particular, to the cold-start behavior of internal combustion engines, that is to say the time period immediately after restarting of the internal combustion engine of a motor vehicle. In that combustion process in particular, increased quantities of hydrocarbons are discharged, unburned, through the exhaust line into the environment. In the case of catalytic converters being used in the exhaust line, the conversion of the unburned hydrocarbons in the catalytic converter only takes place above a particular temperature. The so-called light-off temperature is, however, only reached when the catalytic converter has been correspondingly heated by exhaust gas or by using other heating devices. 
     Furthermore, systems and configurations are known with which the cold-start behavior of catalytic converters is improved by virtue of electrically heatable structures being placed in contact with the catalytic converter and/or the exhaust gas in order to accelerate the heating-up process up to the light-off temperature and thus rapidly initiate effective catalytic conversion. Operation with adaptation for that heating process is desirable, specifically in relation to energy consumption, reactivity of the exhaust-gas pollutants and/or ambient conditions. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method for operating an exhaust gas system, a method for operating a motor vehicle and a motor vehicle, which overcome the hereinafore-mentioned disadvantages and at least partially solve the highlighted problems of the heretofore-known methods and vehicles of this general type. In particular, it is sought to specify a method for operating an exhaust system, which ensures that the lowest possible quantity of unburned hydrocarbons flows through the exhaust line into the environment. Furthermore, it should be possible for a system of this type to be set in a particularly effective manner and operated efficiently, with regard to pollutant values, for the entire service life or operating duration in a motor vehicle. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a method for operating an exhaust system of an internal combustion engine in which the exhaust system has at least one heating device and a catalytic converter for the conversion of hydrocarbons. The method comprises operating the heating device with adaptation as a function of the aging state of the catalytic converter. In particular, this method is suitable for the operation of an exhaust system of a diesel combustion engine. In exhaust systems of diesel engines, use is made of diesel oxidation catalytic converters (DOC) which, by oxidation, convert particles, hydrocarbons and carbon monoxide contained in the exhaust gas. In this case, in the new state of the catalytic converter, the light-off temperatures are approximately 130 to 180° C. These, however, shift toward higher temperatures as the catalytic converter ages. 
     A catalytic converter for the conversion of hydrocarbons refers, in particular, to a so-called oxidation catalytic converter or diesel oxidation catalytic converter (DOC), that is to say a substrate body (for example in the form of a ceramic and/or metallic honeycomb body) with a coating with oxidizing action. The coating with oxidizing action may include a carrier substance (for example a so-called washcoat) that is doped with high-grade metals and/or rare earth elements. In this case, platinum, palladium and the like are used in particular. 
     The at least one heating device or heater includes, in particular, an electrical, regulable heating element. In this case, the heating device is preferably constructed in such a way that the exhaust gas can flow through it (for example by being formed with a multiplicity of throughflow openings or ducts) and it is suitable for increasing the temperature of the exhaust gas as it flows through. It is very particularly preferable for the heating device to include a honeycomb body, in particular a honeycomb body constructed with metallic sheets and/or foils. 
     The heating device is preferably disposed upstream of the catalytic converter, in such a way that the exhaust gas flows firstly through the heating device and then through the catalytic converter. In particular, the heating device is at least partially integrated in the catalytic converter and/or disposed directly adjacent thereto. In this case, it is particularly preferable for the heating device and the catalytic converter to form a structural unit in which a corresponding catalytic coating for the conversion of hydrocarbons is applied directly to the heatable heating device, in such a way that as a result of the activation of the heating device, the catalytic coating is directly heated to correspondingly desired temperatures. 
     For the operation of the at least one heating device, the latter is generally connected to a corresponding current supply/voltage supply and to a controller. In this way, it is possible for an activation or deactivation of the at least one heating device to be triggered by using the controller at predefined times and/or in the presence of predefined parameters. The way in which this regulation or this operation is performed is the subject matter of the present invention. 
     Accordingly, it is the intention for the heating device to be operated with adaptation as a function of the aging state of the catalytic converter. In other words, this also means that the heating power that is provided is approximated, adjusted and/or set up for the present aging state of the catalytic converter. This means in particular that, during the operation of the heating device, the present conversion capability of the catalytic converter is determined and taken into consideration. This encompasses, in particular, operating processes which give rise to operation that varies over the service life of the exhaust system (exclusively) as a function of the aging state of the catalytic converter. In this case, the aging state of the catalytic converter may be characterized, in particular, by the following factors:
         Thermal Aging:   In this case, the thermal loading of the catalytic converter causes a reduction in size of the catalytically active surface over the service life, in particular as a result of sintering processes at temperatures above 800° C. and/or melting.   Chemical Contamination:   Chemical reactions with foreign substances (fuel, oil additives, etc.) can likewise destroy the catalytically active coating.   Mechanical Contamination:   Over the service life of the catalytic converter, the catalytically active coating may also become covered to an increasing extent, for example by lead, sulfur, phosphorus, manganese, etc., or by fuel and/or oil.       

     The aging state of the catalytic converter may be determined, in particular, on the basis of the distance traveled with the catalytic converter or by the motor vehicle. Furthermore, the aging state may be determined on the basis of the time period since the catalytic converter was manufactured and/or since it was installed in an exhaust system. Further parameters that may play a role in the determination of the aging state are: type of catalytic loading, coating type, coating thickness, number of regeneration cycles during operation, maximum temperatures reached, averaged temperature (or temperature histogram), amount of exhaust gas that has flowed through, exhaust-gas type, fuel type, possible combination of different fuel types (additives of bioethanol etc.), oil consumption, etc. A (differently weighted) combination of the parameters for the determination and/or weighting of the aging state is also possible. Furthermore, it is also possible for the conversion performance of the catalytic converter to be detected by a sensor device, for example using the lambda sensors, which are disposed in and/or downstream of the catalytic converter and which determine the storage capacity of the catalytic converter for oxygen. It is also possible to determine the conversion performance of the catalytic converter from the measurement values. 
     In accordance with another advantageous mode of the method of the invention, the heating device is operated at least as a function of a predefined threshold temperature of the catalytic converter, wherein the predefined threshold temperature varies as a function of a present age of the catalytic converter. 
     The light-off temperature is lower in the case of a new catalytic converter than in the case of an already aged catalytic converter. This may have the effect that the light-off temperature of a new catalytic converter lies in a temperature range so low that the catalytic converter is already ready for use when only an insignificant quantity of unburned hydrocarbons has passed through the catalytic converter. In this case, for example, in the case of this configuration, heating of the exhaust system is not imperatively necessary. Due to the progressive aging of the catalytic converter, however, the light-off temperature and/or the conversion effectiveness of the catalytic converter is shifted toward higher temperature values, in such a way that over the course of the numerous operating hours of the system and thus over the service life of the system, an inadmissible quantity of unburned hydrocarbons passes through the catalytic converter before the catalytic converter disposed downstream even reaches the light-off temperature. It is then the intention, in accordance with this present situation, for a supply of heating power to be implemented. 
     For this reason, the operation of the heating device with adaptation to a predefined threshold temperature of the catalytic converter is proposed herein. The threshold temperature in this case is not constant over the service life or operating duration of the exhaust system, but is variable. The threshold temperature is affected by the present age or aging state of the catalytic converter. For the (preferred) situation in which the threshold temperature (approximately) represents the light-off temperature of the catalytic converter, the predefined threshold temperature of the catalytic converter consequently generally increases in this time period. This may take place continuously and/or in stepped fashion. If appropriate, the threshold temperature may be lowered again after a cleaning or decontamination of the catalytic converter. 
     In this regard, it is also advantageous for the age-dependent threshold temperature at which the catalytic converter reaches the light-off temperature to be known at all times during the operation of the internal combustion engine. The threshold temperature correspondingly varies as a function of the present age or aging state of the catalytic converter. Consequently, with progressive aging of the catalytic converter, the heating device is activated correspondingly earlier, or for longer, in order for the catalytic converter to be brought, preferably (precisely) then, to a temperature above or equal to the threshold temperature. In this case, it must be taken into consideration, in particular, that not the entire surface area of the catalytic converter disposed downstream has to be heated to the threshold temperature by the electric heating device; rather, it is in particular sufficient for only a small sub-region to be heated in such a way that the threshold temperature is reached locally. As a result of the then locally occurring catalytic conversion of the unburned hydrocarbons, sufficient energy is released from the exothermic reaction of the combustion of the hydrocarbons in such a way that very rapid heating of the entire catalytic converter occurs, and correspondingly, a large quantity of (possibly also temporarily stored) hydrocarbons can be converted. 
     In accordance with a further preferred mode of the invention, the method has at least the following steps:
         determining a conversion temperature of the catalytic converter,   comparing the conversion temperature with a predefined threshold temperature, and   operating the heating device if the conversion temperature is lower than the predefined threshold temperature.       

     The conversion temperature refers, in particular, to the temperature of the catalytic converter at the time at which a certain (still admissible) quantity of unburned hydrocarbons has already flowed through the catalytic converter without being converted by the (active) catalytic converter. In particular, the determination of the conversion temperature of the catalytic converter is performed only when it is to be assumed, due to the aging state of the catalytic converter, that the threshold temperature is already higher than the conversion temperature. Cyclic and/or continuous interrogation of the conversion temperature of the catalytic converter is also possible. For reliable conversion of the unburned hydrocarbons, the conversion temperature preferably corresponds to the temperature that the catalytic converter must be at at the time when a number of unburned hydrocarbons reaching the catalytic converter would exceed the admissible quantity of unburned hydrocarbons that may be discharged to the environment. The admissible quantity is, in particular, predefined by emissions regulations. In particular, the conversion temperature does not need to be measured at the catalytic converter itself and/or correspond to temperatures measured at other locations in the exhaust line. Rather, the conversion temperature may also be determined mathematically on the basis of the exhaust-gas temperature. Accordingly, the heating device for heating the catalytic converter is, in particular, operated only when the conversion temperature falls below the present threshold temperature, that is to say for example when the light-off temperature of the catalytic converter has not yet been reached. 
     In accordance with an added mode of the method of the invention, an adaptation of a heating power of the heating device is realized by operating a different number of heating segments or by implementing a different activation duration of the heating device. The measures may self-evidently be implemented in combination with one another. By operating individual heating segments and/or by heating sub-regions of the catalytic converter (separately), it is possible to realize rapid local heating of the catalytic converter, with little electrical power being used. As a result of the light-off temperature being reached locally, the conversion of the unburned hydrocarbons takes place there first. As a result of the exothermic reaction, however, adjacent regions of the catalytic converter heat up so rapidly that further electrical heating can be dispensed with. 
     With the objects of the invention in view, there is also provided a method for operating a motor vehicle, which comprises implementing the method not in a first operating phase of the motor vehicle but only when the predefined threshold temperature reaches a particular value. This means in particular that, until a particular threshold temperature is reached, or until a certain aging state of the catalytic converter is reached, it can be assumed that heating of the catalytic converter is not required, because the catalytic converter generally reaches the light-off temperature before an inadmissible quantity of unburned hydrocarbons has passed through the catalytic converter. Accordingly, an unnecessary determination of the conversion temperature of the catalytic converter is eliminated, and thus the controller of the motor vehicle is not burdened by such unnecessary interrogations. In this case, the first operating phase may, in particular, be characterized by at least one of the following parameters:
         kilometerage or mileage of the motor vehicle, in particular 20,000 km;   operating duration of the motor vehicle, in particular 100 hours.       

     With the objects of the invention in view, there is concomitantly provided a motor vehicle, comprising an internal combustion engine, an exhaust system and a controller, wherein the exhaust system has at least one heating device and a catalytic converter disposed downstream of the heating device. In this case, the controller is constructed or programmed for carrying out the method according to the invention. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims, noting that the features specified individually in the claims may be combined with one another in any desired technologically expedient manner and form further embodiments of the invention. 
     Although the invention is illustrated and described herein as embodied in a method for operating an exhaust gas system, a method for operating a motor vehicle and a motor vehicle, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. The description, in particular in conjunction with the figures, explains the invention further and specifies supplementary exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a block diagram of a motor vehicle having an exhaust system; 
         FIG. 2  is a diagrammatic, cross-sectional view of a heating device; 
         FIG. 3  is a diagram showing a possible variation of a threshold temperature of a catalytic converter as a function of its age; and 
         FIG. 4  is a diagram showing an operating state of an internal combustion engine. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now in detail to the figures of the drawing for explaining the invention and the technical field in more detail by showing particularly preferred structural variants to which the invention is not restricted and in which the same reference numerals are used to denote identical objects, and first, particularly, to  FIG. 1  thereof, there are seen components that are preferably provided for the method according to the invention. In this case, a motor vehicle  6  has an internal combustion engine  5  with an exhaust system  1 , in which a heating device or heater  4  and a catalytic converter  3  are provided in the exhaust system  1 . The catalytic converter  3  is assigned sensors  13  which are suitable, for example, for determining the respectively prevailing temperature of the catalytic converter  3 . The sensors  13  are connected by control lines  14  to a controller  7 , so that it is possible firstly for the determined sensor values to be transmitted and also secondly for electric heating of the heating device  4  to be realized over the control lines  14 . Furthermore, the controller  7  is connected to the internal combustion engine  5  through control lines  14 , in such a way that the controller  7  can receive information regarding operating states of the internal combustion engine  5 . The controller may additionally be formed with at least one memory element or have access to such a memory element in which, for example, characteristic maps, empirical values, parameter threshold values and the like can be accessed and/or adapted. 
       FIG. 2  shows, in a cross section, a structural variant of a heating device  4  in the form of a honeycomb body wherein, in this case, various heating segments  10  are additionally disposed annularly around one another. The heating device  4  has smooth metallic foils  17  and structured metallic foils  16  which are stacked and/or wound and/or coiled on one another to form the heating device  4 . For this purpose, the structured foils  16  and/or smooth foils  17  are connected to one or more non-illustrated electrodes, in such a way that the heating device  4  can be operated, for example, as a resistance heater. 
       FIG. 3  shows a potential profile or course of a threshold temperature  8  over the operating duration or service life of the catalytic converter in the exhaust system as a function of the age  11  of the catalytic converter. The temperature  9  is plotted on the vertical axis, and the age  11  of the catalytic converter is plotted on the horizontal axis.  FIG. 3  shows that, with progressive aging  11  of the catalytic converter, the threshold temperature  8  is also shifted toward higher temperature values. Accordingly, at a particular time  12 , an aging state of the catalytic converter is reached at which a particular threshold temperature  8  can be determined. 
       FIG. 4  shows an operating state of the internal combustion engine  5 , wherein the time  15  is plotted on the horizontal axis and the temperature  9  is plotted on the vertical axis. In this case, the temperature  9  is, in particular, the respectively present temperature detected by using a temperature sensor  13 . At a particular time  12 , the determined temperature  9  of the catalytic converter corresponds to the conversion temperature  2  at which the catalytic conversion of hydrocarbons by the catalytic converter must take place because a quantity of unburned hydrocarbons that is still just admissible has already passed through the catalytic converter. At this time  12 , corresponding to  FIG. 4 , a threshold temperature  8  prevails which lies above the presently determined temperature  9  or the conversion temperature  2 . This means that heating by using the heating device is necessary in order to heat the catalytic converter to a corresponding threshold temperature  8  in such a way that the unburned hydrocarbons can be converted in the catalytic converter. 
     The present invention, with all of its variants, solves the problems highlighted with regard to the prior art. In particular, a method for operating an exhaust system is specified, which ensures that the lowest possible quantity of unburned hydrocarbons flows through the exhaust line into the environment. Furthermore, the proposed system can be set in a particularly effective manner and operated efficiently, with regard to pollutant values, for the entire service life or operating duration in a motor vehicle. In particular, this yields cost advantages, and permits practically complete conversion of the hydrocarbons over the entire operating duration of the exhaust system, with little expenditure of energy.