Abstract:
The invention relates to an exhaust gas after-treatment device and a method, having a storage container and a buffer reservoir, both of which can be heated. In this manner it is possible to significantly reduce the energy requirement for providing gaseous ammonia for an exhaust gas after-treatment device of an internal combustion engine.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/PCT/EP 2009/054926 filed on Apr. 24, 2009. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an exhaust gas after-treatment device and a method for exhaust gas after-treatment. 
     Description of the Prior Art 
     The nitrogen oxide or NOx emissions limit values, which are becoming evermore stringent for diesel- or lean-engine-operated vehicles, require exhaust gas posttreatment that reduces the nitrogen oxides, beyond a certain vehicle weight. A very effective exhaust gas posttreatment known from the prior art is known as the selective catalytic reaction (SCR). In it, a reducing agent, namely ammonia, is injected as needed into the exhaust gas posttreatment device of the internal combustion engine and reacts in a special catalytic converter together with the nitrogen oxides of the exhaust gases to form the harmless compounds of nitrogen and water. One example of this kind of SCR exhaust posttreatment with gaseous reducing agent is known from International Patent Disclosure WO 99/012105. 
     In the exhaust gas posttreatment device known from the prior art, an ammonia storage substance, or a mixture of various ammonia storage substances, is present in the storage container, and from these substances, ammonia is released by thermal desorption or thermolysis, or in other words by the effect of temperature. Suitable storage substances can for instance be salts, in particular chlorides or sulfates or one or more alkaline earth elements, such as MgCl 2  or CaCl 2  and/or one or more 3d side group elements, such as manganese, iron, cobalt, nickel, copper, and/or zinc. 
     Organic absorbers and ammonium salts, such as ammonium carbamate, are also suitable ammonia storage substances that can be used in the exhaust gas posttreatment device of the invention and the method of the invention. It is definitive for all these substances that the decomposition process is completely reversible. This means nothing else than that after the reservoir has cooled down to the initial temperature level, the initial substances are present again in unchanged form. 
     In order for the driver of a vehicle equipped with this kind of exhaust gas posttreatment device not to have to refill the storage container himself, the storage capacity of the storage container is designed such that it does not need to be refilled except during scheduled maintenance, such as an inspection. 
     In practice, a storage volume of approximately 10 liters has proved suitable. A reservoir with a volume has a not inconsiderable thermal capacity. 
     Since, because of thermal conduction inside the storage container, a virtually constant temperature prevails in the entire storage container, every time the engine is started the storage container, which has meanwhile cooled down, must be reheated again to the operating temperature of 60° C. or 70° C., for example. The resultant energy consumption leads to an increase in fuel consumption of the engine. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is the object of the invention to furnish an exhaust gas posttreatment device and a method for operating an exhaust gas posttreatment device of this kind whose energy consumption is markedly reduced compared to the prior art. Moreover, the embodiment according to the invention should be economical and problem-free. 
     In an exhaust gas posttreatment device for an internal combustion engine, having an exhaust gas tube, having an SCR catalytic converter, having a metering valve for injecting a gaseous reducing agent into the exhaust gas tube, having a heatable storage container for the reducing agent, and having a buffer reservoir, this object is attained according to the invention in that the buffer reservoir is heatable. 
     Since the buffer reservoir has only a fraction of the storage volume of the storage container, the energy consumption for heating the buffer reservoir is very much less than in the exhaust gas posttreatment device of the prior art. The storage container of the exhaust gas posttreatment device of the invention, unlike in the prior art, is not heated every time the engine is put into operation but rather only whenever the buffer reservoir is nearly completely empty and has to be refilled with reducing agent from the heatable storage container. As a result, the storage container is heated only comparatively seldom, and thus the energy for heating the storage container in the exhaust gas posttreatment device of the invention can be reduced markedly compared to the prior art. 
     The exhaust gas posttreatment devices of the invention furthermore offer advantages in terms of energy management. For instance, the storage container and/or the buffer reservoir can be electrically heated. Alternatively, it is also possible for the storage container, in particular, to be heated with a liquid heat transfer medium, especially coolant or the motor oil, and/or the waste heat contained in the exhaust gases of the engine. As a result, the waste heat that occurs anyway during engine operation can be utilized. It is also possible, by a combination of electric heating and heating by means of a liquid heat transfer medium, to combine the advantages of the two types of heating and thereby also reduce the demand for electrical energy for heating the storage container and/or the buffer reservoir. 
     To ensure that the buffer reservoir will not empty itself into the storage container, a check valve or alternatively a switching valve is provided between the storage container and the buffer reservoir. 
     It is furthermore provided that a pressure sensor and/or overpressure valve is disposed between the buffer reservoir and the metering valve. 
     With the aid of the pressure sensor, it is possible, taking into account the temperature of the buffer reservoir and the storage container, to ascertain the charge state of the buffer reservoir and as a result to trip the refilling of the buffer reservoir with ammonia from the storage container. 
     In a further advantageous embodiment of the invention, it is provided that at least the storage container has a heat insulator. As a result, the energy losses are reduced, and the heat energy demand for the storage container and/or the buffer reservoir is reduced still further. 
     Since in modern motor vehicles there is often only little space for a storage container, it is provided in a further advantageous feature of the invention that the storage container is subdivided into a plurality of decentralized partial storage containers. As a result, the requisite storage volume can be distributed among various “niches” in the vehicle and thus the existing space can be optimally utilized. 
     If a plurality of partial storage containers is present, then a check valve or alternatively a switching valve is provided between the buffer reservoir and each partial storage container. 
     In a method for operating an exhaust gas posttreatment device for an internal combustion engine, having an exhaust gas tube, having an SCR catalytic converter, having a metering valve for injecting a gaseous reducing agent into the exhaust gas tube, having a heatable storage container for the reducing agent, and having a heatable buffer reservoir, the object stated at the outset is also attained in that the buffer reservoir is heated each time the engine is put into operation; and that the storage container is heated only for charging the buffer reservoir. 
     As a result, the advantages of the invention are attained, namely the economy in terms of heating energy for the storage container. 
     In a further advantageous feature of the method of the invention, to make it possible to ascertain fully automatically by means of the engine control unit whether the buffer reservoir needs to be recharged, a pressure sensor which detects the ammonia pressure prevailing in the buffer reservoir is provided. 
     Since there is a relationship between the charge state and the ammonia pressure in the buffer reservoir, and this relationship depends essentially only on the temperature of the buffer reservoir, the charge state of the buffer reservoir can be ascertained from the ammonia pressure prevailing in the buffer reservoir. In good time before complete emptying of the buffer reservoir occurs, the storage container is then heated. This is preferably done according to the invention when the engine is still in operation. The on-board electrical system then has enough power to heat the storage container. It is especially advantageously if the heater of the storage container is preferentially activated whenever the engine is in the overrunning mode, or in other words for instance during a braking event or when driving downhill. Then the mechanical energy required for driving the generator can in fact be recovered from the kinetic energy stored in the vehicle, without an additional expenditure of force. 
    
    
     
       Further advantages and advantageous features of the invention can be learned from the ensuing drawings, their description, and the claims. All the characteristics disclosed in the drawings, their description, and the claims can be essentially to the invention both individually and in arbitrary combination with one another. 
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in detail below in conjunction with the accompanying drawings, in which: 
         FIG. 1 , the schematic layout of an exhaust gas posttreatment device according to the invention; and 
         FIG. 2 , one exemplary embodiment of a method of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In  FIG. 1 , an internal combustion engine  1  with an exhaust gas posttreatment device  3  is shown, highly simplified and schematically. The exhaust gas posttreatment device  3  includes an exhaust gas tube  5 , an oxidation catalytic converter  7 , a particle filter  9 , and an SCR catalytic converter  11 . The flow direction of the exhaust gas through the exhaust gas tube  5  is indicated by arrows (without reference numerals). 
     For supplying the SCR catalytic converter  11  with reducing agent, a metering valve  13  for the reducing agent is disposed upstream of the SCR catalytic converter  11 , on the exhaust gas tube  5 . The metering valve  13  injects gaseous reducing agent as needed into the exhaust gas tube  5  upstream of the SCR catalytic converter  11 . 
     The metering valve  13  is opened as needed, via an engine control unit, not shown, so that gaseous ammonia can flow from a buffer reservoir  15  to the exhaust gas tube  5 . The buffer reservoir  15 , in the exemplary embodiment shown in  FIG. 1 , has an electric resistance heater  17 , which is activated each time the engine is turned on. It is understood that the electric resistance heater  17  may have a power regulator, not shown, for limiting the consumption of electrical energy to what is necessary. 
     A pressure sensor  21  is disposed in a connecting line  19  between the buffer reservoir  15  and the metering valve  13 . This pressure sensor  21  can be used to regulate the power of the electric heater  17 . Between the pressure in the buffer reservoir  15 , or in the connecting line  19 , that is filled with gaseous ammonia and the temperature of the buffer reservoir  15 , there is an unambiguous relationship, so that from the ammonia pressure that is detected by the pressure sensor  21 , the temperature of the buffer reservoir  15  can be ascertained. 
     It is furthermore possible, from the temperature and the pressure in the buffer reservoir  15 , to draw a conclusion about the charge state of the buffer reservoir  15 . From the temperature, the quantity of energy contained in the buffer reservoir  15  can be ascertained. Taking into account the pressure in the buffer reservoir  15 , the charge state of the buffer reservoir  15  can be ascertained from this quantity of energy. The quantity of energy contained in the buffer reservoir  15  can also be ascertained by means of an energy balance that takes into account the heating capacity introduced and the heat losses. 
     The pressure sensor  21 , like the metering valve  13 , is connected to the engine control unit via signal lines, not shown. 
     In the exhaust gas posttreatment device of the invention, a storage container  23  is also present, which is surrounded by a heat insulator  25 . The storage container  23  furthermore also has a heater, embodied here as an electric resistance heater  27 . Between the storage container  23  and the buffer reservoir  15 , a check valve  29  is provided, which ensures that only gaseous ammonia can flow out of the storage container  23  into the buffer reservoir, and the return path is closed. The storage volume of the buffer reservoir  15  is markedly smaller than the storage volume of the storage container  23 , since the latter has to suffice for the travel distance between two regular inspection intervals, which for example is 20,000 km to 30,000 km. 
     The storage volume of the buffer reservoir  15  is dimensioned such that the average distance traveled in a vehicle without shutting off the engine can be covered with the ammonia stored in the buffer reservoir  15 . The optimum is between 10 and 100 cycles, preferably between 30 and 60 cycles, before the buffer reservoir has to be recharged. The buffer reservoir  15  is in fact heated every time the engine is turned on, and it naturally cools down again from heat losses to the environment after the engine is shut off. It has proved advantageous if the ratio of the storage capacity of the storage container  23  and of the buffer reservoir  15  is approximately 66:1, corresponding for instance to a storage capacity of the storage container  23  of 10 kg and of the buffer reservoir  15  of 150 g. The storage container  23  is heated only whenever the buffer reservoir  15  is just about to become empty. Charging the buffer reservoir  15  with ammonia stored in the storage container  23  will be described in further detail below in conjunction with  FIG. 2 . 
     In  FIG. 2 , in a total of four graphs one above the other, the ammonia filling of the buffer reservoir  15  (see line  33 ), the ON time of the electric resistance heater  17  (see line  35 ) of the buffer reservoir  15 , the ammonia filling of the storage container  23  (see line  37 ), and the ON time of the electric heater  27  (see line  39 ) of the storage container  23  are plotted. 
     In a first cycle of the motor vehicle, that is, when the engine is started and the buffer reservoir  15  is full, corresponding to a filling at the standardized fill level  1 , when the engine is started the heater  17  of the buffer reservoir  15  is activated (see line  35 ) and is active, with one exception, as long as the vehicle is in operation. The heat demand decreases in the exhaust gas posttreatment device of the invention to less than 20% of a conventional system. As the topmost line in  FIG. 2  shows, the fill level in the buffer reservoir  15  decreases continuously during the first cycle. However, the charge state of the buffer reservoir  15  is still markedly higher than a limit filling  31  that is indicated by a dashed line  31  in  FIG. 2 . 
     It is therefore unnecessary during the first cycle for the buffer reservoir  15  to be charged with ammonia from the storage container  23 . Consequently, the heater  27  of the storage container  23  remains off in the first cycle, and the charge state of the storage container  23  also remains unchanged, at the outset value 1. 
     When the motor vehicle is shut off, and in other words a cycle is ended, the heater  17  is also shut off, and the buffer reservoir  15  cools down. 
     If now, in the n th  cycle, the buffer reservoir  15  has emptied enough that it reaches the limit charge  31 , recharging of the buffer reservoir  15  with ammonia from the storage container  23  is necessary. This is done in that when the limit charge  31  is reached, the heater  17  of the buffer reservoir is shut off. A short time before that, the heater  27  of the storage container  23  is activated, since the storage container  23  requires a certain amount of time until it is ready for operation. As a result of the shutoff of the heater  17  of the buffer reservoir  15  after the limit charge is reached, the buffer reservoir  15  cools down, while at the same time the temperature of the storage container  23  rises. The result is an overpressure of the ammonia in the storage container  23 , as a result of which gaseous ammonia flows from the storage container  23  through the check valve  29  into the buffer reservoir  15 . There, because of the reversibility of the storage operation and the decrease in temperature of the buffer reservoir  15 , it is stored in the storage material. This process can also continue after the shutoff of the vehicle, that is, after the end of the n th  cycle, until the buffer reservoir  15  has again reached the initial charge of 1.0. After that, in the n+1 th  cycle, the method already described in conjunction with the first cycle begins all over again. 
     Even from this simple description it will have become clear that the storage container  23  needs to be heated only once every n th  operating cycle, while in the method of the prior art it had to be heated in every travel cycle. As a result, there is a drastic reduction of energy needed for heating the storage container  23  and consequently an entirely relevant fuel economy. 
     The foregoing relates to the preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.