CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 USC 119 to German Patent Application No. 10 2010 061 430.0, filed on Dec. 21, 2010, the entire disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a pressure tank system for a motor vehicle, with a fuel tank that is operated in a differential pressure range between a negative pressure and a positive pressure. The invention also relates to a method for operating a pressure tank system of this type and to a motor vehicle with a supporting structure and a pressure tank system of this type. 
         [0004]    2. Description of the Related Art 
         [0005]    DE 10 2008 062 243 A1 discloses a method for operational venting control on a plastics fuel tank. The method maintains the volume of the fuel tank within a predetermined differential pressure range relative to the ambient pressure by at least one operational venting valve that has a pressure maintaining function. 
         [0006]    It is an object of the invention to provide a simple cost effective pressure tank system for a motor vehicle, with a fuel tank that is operated in a differential pressure range between a negative pressure and a positive pressure. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention relates to a pressure tank system for a motor vehicle, with a fuel tank that is operated in a differential pressure range between a low pressure and a high pressure. The low pressure is limited to ambient pressure plus/minus 50 hectopascals. Limiting the negative pressure enables the advantageous elimination of the internal stiffeners that are required in conventional pressure tanks, thereby considerably simplifying the production of fuel tanks. 
         [0008]    The fuel tank may be equipped with a negative pressure limiting valve that limits the negative pressure in the fuel tank to the specified value. Thus, ambient air can flow into the fuel tank via the separate negative pressure limiting valve to prevent a greater negative pressure from occurring. 
         [0009]    The fuel tank may be equipped with a negative pressure protection valve device that prevents a negative pressure from occurring in the fuel tank. In this case, the negative pressure protection valve device is designed so that only positive pressure or ambient pressure occurs in the fuel tank during the operation of the pressure tank system. 
         [0010]    The pressure tank system may be characterized in that the pressure tank system comprises a conventional fuel tank made of plastic or sheet steel, thereby considerably reducing the production costs for the pressure tank system. 
         [0011]    The fuel tank preferably is surrounded, at least partially, by an outer reinforcing structure. Thus, the fuel tank is reinforced only with an outer reinforcing structure and does not include any inner reinforcing structure. 
         [0012]    The fuel tank preferably has a regenerating filter device. The regenerating filter device preferably contains an activated carbon filter material with which hydrocarbons from fuel vapors in the fuel tank can be bound. 
         [0013]    The pressure tank system preferably has a tank shutoff valve connected between the fuel tank and the regenerating filter device. The fuel tank is connected to the regenerating filter device via the tank shutoff valve as required. 
         [0014]    The invention also relates to a method for operating the above-described pressure tank system so that the negative pressure in the fuel tank is limited to ambient pressure plus/minus 50 hectopascals. More particularly, the pressure tank system is operated so that only positive pressure or ambient pressure, and possibly a slight negative pressure, is applied to the fuel tank during operation. 
         [0015]    The invention further relates to a motor vehicle with a supporting structure and the above-described pressure tank system that preferably is operated in accordance with the above-described method. The supporting structure defines an outer reinforcing structure of the fuel tank. The supporting structure of the fuel tank preferably defines a monocoque body formed, for example, from carbon fibers and aluminum. 
         [0016]    The motor vehicle preferably is a hybrid vehicle, in particular a plug-in hybrid vehicle that can be charged at a socket of a power supply system. 
         [0017]    Further features of the invention emerge from the description below, in which an exemplary embodiment is described in detail with reference to the drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a simplified illustration of a pressure tank system according to the invention with an internal combustion engine and a fuel tank. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]      FIG. 1  is a greatly simplified representation of a pressure tank system according to the invention with an internal combustion engine  1  and a fuel tank  4 . The fuel tank  4  has a filler neck  5  via which fuel is introduced into the fuel tank. 
         [0020]    The fuel tank  4  is assigned sensor devices  8 ,  9  and a tank cap release device  10 . The sensor devices  8 ,  9  detect the pressure and the temperature in the fuel tank  4 . The sensor devices  8 ,  9  are connected to a control device  14  via signal lines. 
         [0021]    The filler neck  5  can be closed by a tank cap that can be locked or released with the aid of the tank cap release device  10 . The tank cap release device  10  is connected in terms of control to the control device  14  via a control line. The tank cap preferably is released by the tank cap release device  10  only when ambient pressure prevails in the interior of the fuel tank  4 . 
         [0022]    The fuel tank  4  is connected to a regenerating filter device  18  via a venting line  16 . The filter device  18  comprises activated carbon filter material that functions to absorb or bind hydrocarbons from fuel vapors in the fuel tank. A tank shutoff valve  19  is connected into the venting line  16  between the fuel tank  4  and the filter device  18 . 
         [0023]    The filter device  18  can be connected to the surroundings via a line  21  to conduct ambient air through the filter device  18  for flushing the filter device  18 . The filter device  18  can be connected to the internal combustion engine  1  via a connecting line  24  for flushing. A tank venting valve  25  is arranged in the connecting line  24 . A flushing flow from the filter device  18  via the open tank venting valve  25  to the internal combustion engine  1  is indicated by an arrow  26 . 
         [0024]    The fuel tank  4  further has a negative pressure protection valve device  30 . The negative pressure protection valve device  30  comprises, for example, a negative pressure limiting valve  32  and functions to limit the operating pressure in the interior of the fuel tank  4  to no negative pressure at all or to a very slight negative pressure of, for example, 50 hectopascals. 
         [0025]    The negative pressure protection valve device  30  of the invention limits the operating pressure parameters of the fuel tank  4  during operation of the pressure tank system so that negative pressure is prevented from occurring in the fuel tank  4  and only a positive pressure or ambient pressure is permitted in the interior of the fuel tank  4 . 
         [0026]    Limiting the operating pressure parameters of the pressure tank system enables use conventional standard tanks made of sheet metal or of plastic produced, for example, by blow molding. 
         [0027]    No reinforcements or stiffening means are required in the interior of the fuel tank  4 . Thus, standard production methods can be used to produce the fuel tank  4  and the costs for producing the fuel tank can be reduced considerably. 
         [0028]    The fuel tank  4  illustrated in  FIG. 1  is provided only with outer supports  35 ,  36 . The outer supports  35 ,  36  constitute an outer reinforcing structure  40  to ensure that the fuel tank  4  withstands the positive pressure of, for example, 300 to 800 hectopascals, in particular 400 to 500 hectopascals, that occurs during the operation of the pressure tank system. 
         [0029]    The outer reinforcing structure  40  may comprise, for example, a steel framework and/or a carbon fiber structure. The outer reinforcing structure can be provided particularly advantageously by a monocoque motor vehicle body. The use of the monocoque motor vehicle body as an outer reinforcing structure  40  affords the advantage that no additional reinforcements are required.
A pressure tank system for a motor vehicle has a fuel tank ( 4) operated in a differential pressure range between a low pressure and a high pressure. To ensure a simple construction and can be produced cost-effective construction, the low pressure is between ambient pressure minus 50 hectopascals ambient pressure and plus 50 hectopascals.