Abstract:
The invention relates to a plastic vehicle tank for an aqueous urea solution used for reducing the hydrogen oxide content in exhaust gases of an internal combustion engine. In the preferred embodiment, the inventive tank comprises a functional unit comprising at least one pump, at least one pressure control valve, at least one internal container provided with an internal electrical heating and at least one suction line. The functional unit is preferably mounted in the form of a lid on the container opening for closing it.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2006/066155 filed Sep. 8, 2006. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a plastic vehicle tank for an aqueous urea solution used for reducing the hydrogen oxide content in exhaust gases of an internal combustion engine. 
     2. Description of the Prior Art 
     In vehicles operated with diesel fuel, among other needs, the pollutant NOx will have to be reduced considerably, given the impending more-stringent laws on engine exhaust in the next few years. One method that is employed is known as SCR (selective catalytic reduction), in which the pollutant NOx (nitrogen oxides) is reduced to N 2  and H 2 O, with the aid of aqueous urea solution. To that end, the aqueous urea solution is injected upstream of the SCR catalytic converter into the exhaust tube via a metering valve. The aqueous urea solution evaporates in the hot exhaust gas and forms ammonia, which accumulates in the SCR catalytic converter. The ammonia accumulated in the catalytic converter converts the nitrogen oxides contained in the exhaust gas into elemental nitrogen and water vapor. The aqueous urea solution is stored in a tank. This reducing agent makes special demands of the tank. In utility vehicles, the SCR method is already in mass production. It employs tanks of special steel or aluminum. which are heated via the engine coolant. 
     SUMMARY AND ADVANTAGES OF THE INVENTION 
     Making a tank of aluminum or special steel is expensive and furthermore puts limitations on its shaping. For this reason, a plastic tank with various structural details is proposed. This plastic tank can be produced especially economically by blow molding. 
     Further advantages will become apparent from the characteristics described in the specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  shows a tank wall  2  of a reducing agent tank according to the invention; 
         FIG. 1   b  shows a reducing agent tank with its contents; 
         FIG. 2  shows a reducing agent tank with an insulation layer; 
         FIG. 3  shows a reducing agent tank with a heating bar; 
         FIG. 4  shows a reducing agent tank with a built-in cup; 
         FIG. 5  shows a reducing agent tank with one line; 
         FIG. 6  shows a reducing agent tank with one line and one cup line; 
         FIG. 7  shows a reducing agent tank with lines that are provided with throttle restrictions; 
         FIG. 8  shows a relatively flat aluminum body; 
         FIG. 9  shows a reducing agent tank with a cup and a heater; 
         FIG. 10  shows a cup of a reducing agent tank; 
         FIG. 11  shows a heater; 
         FIG. 12  shows a reducing agent tank with a cup and a tank; 
         FIG. 13  shows a pumping module; and 
         FIG. 14  is a schematic illustration of a tank with ventilation elements. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  in a schematic view (without showing a tank opening for filling the tank and without showing a connection to an exhaust gas posttreatment arrangement), in part b, shows a tank  1  with a tank wall  2 . The tank is made from plastic, such as PE (polyethylene), PA (polyamide), or PAA (polyarylamide). With “aqueous urea solution” as the reducing agent, the diffusion of ammonia through the tank wall to the outside must be avoided. This can be assured either by means of a suitably great wall thickness or a so-called Coex material. In the Coex material, the tank wall comprises a plurality of different layers of material, and one layer can for instance act as a barrier layer. Coex materials are typically made by means of coextrusion of a multilayer composite material.  FIG. 1   a  shows the construction of the tank wall  2  in the form of a Coex layer comprising three partial layers, which are an inner layer  11  oriented toward the tank contents, an outer layer  13 , and a barrier layer  15  located between the inner and outer layers. The aqueous urea solution  5  freezes below −11° C. Above the surface of the liquid, there is an air cushion. If a temperature lower than −11° C. surrounds this tank, then the tank first freezes in the region near the bottom and the walls. Since the air cushion  7  above the liquid acts as insulation, the liquid at the surface does not freeze until the end. The air cushion above the liquid thus acts as a compensation volume for the freezing aqueous urea solution. The intentional freezing at the bottom and the wall makes it impossible for the tank to burst, since the increase in volume is intentionally carried into the air-filled region of the tank. In  FIG. 1   b , the circled numerals  1  through  4  mark the order of the regions in which the tank contents freeze if the temperature goes below −11° C. The result is a hump  9  in the middle of the tank in the region of the air cushion  7 . 
       FIG. 2  shows a tank  21  with a tank wall  2  of plastic and with an additional insulation  23 , for instance in the form of a thermal insulation layer, mounted on the tank bottom, in the middle. At the phase transition at −11° C., the aqueous urea solution experiences an expansion of approximately 7%. As a result of this increase in volume, damage to the tank can occur. A locally controlled freezing of the tank contents is an additional aid or precaution in this respect; it is assumed that the tank is not completely full; that is, there is an air cushion above the surface of the liquid. By purposefully mounting the insulation material at certain points of the tank, such as an insulation  23  here, the freezing process in the tank can be steered in a locally purposeful way. This insulation can be selected such that the growth of the ice can be shifted to certain regions that are not critical to the tank, for instance to the surface, where an air cushion can be created. The insulation  23  on the bottom changes the three-dimensional freezing process in such a way that in the middle of the tank (see the circled numeral  5 ), a region with still unfrozen liquid can be maintained for a long time; on the one hand, this means that the mechanical forces of the ice on the tank are less, and on the other, that for a relatively long time, even at low temperatures, one region of the tank is still available that holds a liquid that can be removed by suction and used to operate an exhaust gas posttreatment arrangement. The circled numerals  1  through  5 , analogous to the view in  FIG. 1 , show the order of the regions in which the tank contents freeze when the temperature goes below −11° C. 
     If thawing occurs, which is done for instance via an electric heater or engine coolant circulating in lines that are in thermal contact with the tank, the heat for thawing can be concentrated on an intentional partial volume, by suitable disposition of the heating means. In  FIG. 3 , a tank with an electrical thawing mechanism, which is an electric heating bar  32  along with its electrical connections, is shown. Since there is only limited electrical power available, only a limited volume of ice  34 , which moreover depends on the fill level in the tank, can be thawed, yet this thawing can be accomplished in only a short time, so that a downstream exhaust gas posttreatment arrangement or the tank arrangement is already operationally ready extremely quickly. If the tank is nearly empty or the fill level is low, then the volume available for the thawing becomes quite small; in other words, after only a short travel distance, there is no longer any thawed reducing agent available, especially since the liquid around the heating bar is removed by suction, and thus the heat can no longer be dissipated to the frozen reducing agent by way of the air. 
     It is advantageous if a partial volume to be thawed is thermally insulated from the rest of the tank volume. Otherwise, the heat input distributes itself over the frozen total volume, and the thawing times increase. With the concentration on a small partial volume, the system is already operationally ready extremely quickly. Accordingly,  FIG. 4  shows a tank  41  with a plastic wall  2 ; in its interior, this tank has a cup  43  which demarcates a partial volume from the rest of the tank. A heating element not identified by reference numeral here, analogous to the heating element that is shown in  FIG. 3 , can be disposed in this partial volume or on the cup wall, in particular on the inside of the cup wall, in order to heat or thaw the partial volume in a purposeful way. The cup is preferably positioned in the middle of the tank. This is advantageous since the liquid freezes first in the region near the wall and only after that does it freeze in the middle. Since it takes a relatively long time until even the middle of the tank is also frozen, when a cup is positioned in the middle of the tank the heater needs to be switched on only rarely, even though the outdoor temperature is below −11° C. This kind of cup may also have an overflow, as described below for instance in  FIG. 5 . 
       FIG. 5  shows a tank arrangement or tank  51 , whose cup  43  disposed in it (in the middle of the tank) has an overflow  53 . The cup  43 , in the embodiment of  FIG. 5 , is intended to be completely fill at all times. This permanently full state of the cup is assured by providing that by a pump  57 , accommodated in a pumping module, the liquid from outside the cup, that is, from the “remaining” tank, is pumped via a line  55  to the downstream exhaust gas posttreatment arrangement, not shown. The mechanical pressure regulation of the system is effected via a mechanical pressure regulating valve  61 . This valve is accommodated in the pumping module, and the overflow quantity of this pressure regulator flows into the cup  43  via the return line  59 . As a result, the cup is always maximally filled, only an excess quantity leaves the cup  43  via the overflow  53  in the direction of the outer region of the tank. At low temperatures, the ice in the cup is now first thawed out, preferably completely, by means of an electrical heating element, not shown in detail, disposed in the region of the cup. Initially, the ice in the remainder of the tank is not thawed out. This cup  43  is likewise of plastic and thus forms an insulation layer between the cup that is to be thawed out and the frozen total volume. 
       FIG. 6  shows a tank arrangement  62  which additionally has a cup line  63  inside the cup  43 . Directly upstream of the pumping module, above the overflow  53 , this cup line communicates with the line  55  and can aspirate reducing agent from the cup and supply it to the downstream exhaust gas posttreatment arrangement. Thus when the line  55  that discharges into the tank is closed at its intake opening  67  by the frozen remainder of the tank, the thawed reducing agent is aspirated from the cup through the cup line  63  that is located in the cup and that has already thawed out. The tank  62  has an electric heater  65  disposed in the bottom region of the cup  43 . The term “pumping module” is understood to mean the sum of the following components: pump  57 , pressure regulating valve  61 , selectively additionally including the return line  59 , the line  55 , and the cup line  63 . 
     It is advantageous if the cup  43  is full at all times, so that shortly after the cup has thawed, a sufficient quantity of reducing agent that can be removed by suction is always available. The principle of the constantly full cup is satisfied if, in the thawed state (of the cup and remainder of the tank), aspiration is done simultaneously via the lines  55  and  63  from both the cup and the remainder of the tank. In the arrangement of  FIG. 7 , this is shown by way of suitable throttle restrictions in the lines  55  and  63 : The line  55  has a first throttle restriction  75 , and the cup line  63  has a second throttle restriction  77 . It should be taken into account that the liquid removed by suction from the remainder of the tank is always more than the quantity to be injected into the exhaust gas system. Via the return of the pressure regulator  61 , a permanently full cup  43  is thus assured. By way of the overflow  53  of the cup  43 , excess reducing agent can optionally be returned (return quantity  73 ) to the outer region of the tank. 
       FIG. 8  shows an embodiment of a heater  65  in the form of a relatively flat aluminum body  81 , in which one or more point-type heating elements  83 , such as PTC heating elements (PTC=positive temperature coefficient) that are self-regulating at high temperatures, are integrated. The point-type heating elements dissipate the heat in the region of the cup to the relatively flat aluminum body; the direction of the heat flow is marked here with arrows  85 . Because of its good thermal conductivity, aluminum is advantageous. The hole  82  in the middle of the aluminum body serves to secure the body to a mounting. 
       FIG. 9  shows one possible disposition of a heater  91 , in the form of the relatively flat aluminum body  81  that is insert-molded with plastic. By means of the barlike mounting  93  with which the power supply  93  is integrated, this relatively flat heating construction with integrated PTC heating elements is mounted near the bottom in the cup. The plastic insert molding serves to protect the PTC heating elements and the aluminum body from the aqueous urea solution. 
       FIG. 10  shows the disposition of a heater in a cup  43 ; here, the relatively flat aluminum body  81  insert-molded with plastic has peripheral convection bores  103 , which connect reducing agent regions located near the bottom, below the aluminum body, to regions located above the aluminum body. This heater is based on convection, and a convection flow or flow circulation  105  occurs around the convection bores. Because of how the heater is disposed spatially, that thawing occurs first around the heating body. Via an upward-oriented convection flow, the heat in the already thawed medium is continuously transported upward. On being cooled there, it drops down again toward the heater. Hence a flow circulation from the heater upward arises, particularly in the region of the convection bores mounted in the aluminum. After a certain length of time, the thawing flow of heat reaches the surface of the ice. Thus the partial volume in the cup is partially thawed (thawed region  107  and (still-) frozen region  109 ). If a connection is now made between the aspiration point of the cup line  63  and the air above the thawed liquid (up to the surface of the ice, or to the melted region  108  toward the air cushion), the system can now enter into operation as soon as the aspirating cup line  63  has also thawed. The volume of ice still present in the partial volume, which is preferentially at the surface, is now additionally hawed out by the warm medium  111  flowing back via the pressure regulating valve. This warm medium drops onto the bits of ice remaining at the surface and thus reinforces the thawing process. 
       FIG. 11  shows an example of a heater  113  that can be used inside a cup, having a mounting element which for instance is joined integrally to the relatively flat aluminum body and with which a cup line  163  for removal by suction of the reducing agent from the cup is integrated. The electric supply lines  115 , which supply one or more PTC heating elements  83  with electric current, extend parallel to the cup line  163 . The mounting element with the integrated cup line  63  with an intake point near the bottom is injection-molded in plastic (not shown in further detail) in the same way as the relatively flat aluminum body. The cup line  63  extends via the cup into the pumping module. The electric supply line (supply lines  115 ) to the PTC is embodied as a resistor wire. It extends in the immediate vicinity of the cup line  163 . Thus the cup line  163  can be thawed in good time by way of the heat generated in the resistor wire. Thus in an extremely short time, the quantity of liquid present in the cup is available to the system for its operation. Hence after an extremely short time, the operation of the system can be assured. With a thawed quantity present, it is thus possible for instance to drive a distance of approximately 500 to 1000 km. It is assumed that after that period of time, a further amount of aqueous urea solution in the overall tank will have thawed because of the surroundings and is then partially aspirated via the line  55 . 
       FIG. 12  shows a further tank arrangement  121 , in which a further tank  129  is disposed around the cup  143 . This tank has a sump  123  in its bottom region, or in other words a well, with a shield valve  125  in the lowermost region. This tank  129  has an overflow  127  to the tank. Via the hydrostatic pressure, the liquid passes through the shield valve  125  to reach the tank. From there, the liquid is aspirated into the cup via the line  55 . Hence the return quantity first fills the inner cup. By way of the cup overflow  53 , the liquid then reaches the tank  129 . Once that is full, the liquid flows back to the tank via the tank overflow  127 . The cup overflow  53  is located above the tank overflow  127 . Thus the tank overflow  127  causes the tank  129  to have a higher fill level than the remainder of the tank. Hence the line  55  that removes fluid by suction from the tank is always far below the surface of the liquid. This “cup in tank, tank in tank” construction makes the tank invulnerable to sloshing and ice impacts, the aspiration of air, and noise from sloshing and ice impacts. The “cup in tank” arrangement in the middle of the tank is furthermore advantageous in the event of sloshing motions and ice impacts. The liquid frozen in the tank forms a frozen ring, which is caught in the middle by the tank and thus has only a limited freedom of motion. 
       FIG. 13  shows an embodiment in which the pumping module  133  is placed on the cup  143  extending deeply into it. Thus without additional lines and connecting points, the pumping module and tank can communicate directly. The pumping module is furthermore positioned on the tank in a well, not shown in detail, of the tank wall. It is thus assured that along the surface of the tank, any slight leakage cannot reach the surroundings or not even reach the outside at all, since it is concealed or caught in the well. Hence the leakage does not get into the vehicle and can easily be removed from the well. The cup with the heater, the tank with the shield valve, and the pumping module form a function unit  131  that is virtually independent of the tank and that can be removed as a whole from the tank. This function unit can selectively be embodied as well with suitable sensors for the liquid level, temperature, or the quality of the reducing agent. The pumping module can be covered flush with the thank by a closure cap. The tank construction is embodied such that all the heat sources (tank heater, heater not shown in further detail of the pumping module, pump motor, etc.) are concentrated in the function unit and thus contribute jointly to thawing in a core region of the tank to assure that the warmup period is short. 
       FIG. 14  schematically shows a tank with a tank wall  2  without showing the other components, such as the cup, pumping module, etc., in further detail. Because of fluid withdrawal, temperature fluctuations, and decomposition of the aqueous urea solution, the tank contents can build up different pressures in the tank in the region of the air cushion  6 . By way of ventilation and venting valves  141  and  145 , respectively, to the surroundings, these pressure fluctuations are taken into account. Since upon venting the risk is that ammonia will escape to the ambient atmosphere and cause odors, the venting operation is performed only at high internal tank pressures; that is, the tank is advantageously constructed in such a way that it can absorb moderate internal pressures without changing its shape impermissibly. Optionally, an activated charcoal filter may be provided, by way of which the gas containing ammonia can be carried upon venting. 
     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.