Abstract:
A circuit for filtering a liquid, comprising a storage vessel ( 10 ), a pump ( 17 ), a consumer device ( 14 ), a filter element ( 23 ) and a controller ( 24 ). The circuit maybe used in systems where liquid, e.g. water, oil or fuel, is filtered and at least part thereof is returned to the storage vessel. Liquid is pumped in the circuit from the storage vessel by the pump through lines ( 11, 13, 15, 16 ), filtered by filter ( 23 ) and supplied to the consumer device. Excess liquid not required by the consumer device ( 14 ) is returned via return line ( 15 ) to the controller ( 24 ) which directs the liquid either to the filter or the storage vessel—depending on liquid temperature. The controller ( 24 ) and the filter are arranged in a common housing ( 18 ) having connectors for an unfiltered liquid inlet ( 19 ), a filtered liquid outlet ( 20 ), a liquid return ( 21 ) and a liquid outlet ( 22 ). The connectors ( 19–22 ) enable rapid assembly of the circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of international patent application no. PCT/EP01/06548, filed Jun. 9, 2001, designating the United States of America and published in German as WO 02/01061, the entire disclosure of which is incorporated herein by reference. Priority is claimed based on Federal Republic of Germany patent application no. DE 100 30 324.2, filed Jun. 27, 2000. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to a liquid circuit comprising a liquid storage vessel, a liquid pump, a device which consumes or utilizes the liquid, a filter element having an unfiltered liquid side and a filtered liquid side, and a controller for returning excess liquid not needed by the consumer device at least partially to the filter element. 
   German Patent No. DE 197 14 488 discloses a method and a fuel injection system for heating a fuel. In the method of heating fuel, the fuel is compressed by a fuel pump to an injection pressure necessary for injection. A part of the compressed fuel is supplied via a fuel filter to an internal combustion engine, where it is injected by fuel injectors. The fuel which is not injected is depressurized via a valve or a leakage line and at least partially supplied to the fuel pump again. The valve opens as soon as the fuel has a fuel temperature below a minimum temperature. 
   To carry out this method, the fuel injection system has a fuel supply line which is connected to the fuel tank, and a fuel pump is provided for delivering the fuel from the tank into the fuel line, in which a fuel filter for filtering the fuel is positioned. The fuel line is connected to a fuel injector of an internal combustion engine. A valve, which is connected via a return line to the fuel supply line, is provided on the fuel line. The valve opens as a function of predetermined operating states. For this purpose, the valve is constructed as a thermostat valve, which automatically connects the fuel return line to the fuel supply line when the fuel temperature is below a minimum temperature. The return line is connected to the fuel supply line in such a way that the heated fuel mixes with the cold fuel from the tank, whereby the mixing may occur in a mixing chamber. 
   However, the fuel injection system includes a large number of lines, connected by valves and thermostats to a central processing unit. In addition, numerous fuel lines must be connected to the components. In this case, during the installation of the fuel injection system on an internal combustion engine, there is a significant outlay for installation and there is also a significant danger that different lines may be confused with one another. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide an improved liquid circuit for a liquid filtering system. 
   Another object of the invention is to provide a liquid circuit which supplies a heated liquid at least partially to a filter element. 
   A further object of the invention is to provide a liquid circuit which is easy to understand and simple to install. 
   It is also an object of the invention to provide a liquid circuit which is particularly adapted for filtering water from diesel fuel supplied to the fuel injectors of a diesel, internal combustion engine. 
   These and other objects are achieved in accordance with the present invention by providing a liquid circuit comprising a liquid storage vessel, a liquid pump, a consumer device, a filter element having an unfiltered liquid side and a filtered liquid side, and a controller with a temperature detection unit for determining the temperature of a liquid in the liquid circuit; the controller comprising a valve, responsive to the temperature of the liquid, for returning excess liquid not needed by the consumer or user device at least partially to the filter element; wherein the filter element and the controller are integrated into a common liquid filter system comprising a housing having an unfiltered liquid inlet communicating with the unfiltered liquid side of the filter element, a filtered liquid outlet communicating with the filtered liquid side of the filter element, a liquid return which communicates with the controller, and a liquid outlet which communicates with the controller; the liquid storage vessel being connected to the unfiltered liquid inlet by an unfiltered liquid line; the filtered liquid outlet being connected to the consumer device by a filtered liquid line; the consumer device being connected to the liquid return by a liquid return line, and the liquid outlet being connected to the storage vessel by a liquid drain line; wherein the controller is connected by a connection line to the unfiltered side of the filter element, and the temperature detection unit is arranged between the liquid storage vessel and the consumer device. 
   For this purpose, the liquid circuit according to the present invention comprises a liquid storage vessel, a liquid pump, a filter element, a device which consumes or uses the liquid, and a controller, the filter element and the controller being integrated into a liquid filter system formed by a housing. If desired, multiple parts, such as a filter unit having a filter element and a controller, may be assembled into a module in the housing and thus form the liquid filter system. The liquid storage vessel is connected via an unfiltered liquid line to an unfiltered liquid inlet fixed on the housing. The liquid line pump is positioned in the liquid circuit in such a way that the liquid may circulate in the liquid circuit. The filter element has an unfiltered liquid side and a filtered liquid side, the unfiltered liquid inlet communicating with the unfiltered side of the filter element. The filtered side of the filter element communicates with a filtered liquid outlet, which is also fixed on the housing. A filtered liquid line, which conducts a liquid, for example, water, emulsions, oil, or fuel, to the user device, is attached to the filtered liquid outlet. 
   The liquid consuming device accepts, at most, only a part of the liquid pumped through the circuit. The remaining liquid, or all of the pumped liquid if the user device is not accepting any liquid, is supplied to the controller via a liquid return line which is connected to a liquid return fixed on the housing. The liquid may be heated by the user device or by other surrounding components, which are optionally part of the liquid circuit. As a result of this heating effect, the liquid temperature of the returned liquid may be significantly above the liquid temperature in the liquid storage vessel. 
   The controller comprises a temperature detection unit for determining a liquid temperature existing in the liquid, and a valve, which is controllable in accordance with the liquid temperature. The liquid temperature may be detected before or after the filter element. As soon as the liquid temperature is above a defined temperature, the controller conducts the liquid back into the liquid storage vessel via a liquid drain line which is connected to a liquid outlet fixed on the housing. If the liquid temperature is below the defined temperature, the liquid not accepted by the user device is returned from the controller to the filter element via a connection line. In this way, the unfiltered liquid is mixed with the returned liquid, which has a higher liquid temperature, through which the viscosity of the liquid to be filtered is reduced. Consequently, the resistance to flow produced by the filter element may be decreased. 
   If only a part of the liquid which is not accepted by the user device is to be supplied to the filter element, a return line valve is provided on the connection line, through which a part of the liquid may be supplied to the filter element and a further part of the liquid may be supplied to the liquid storage vessel. 
   The liquid consumer or user device may be, for example, a fuel injection system of an internal combustion engine, which only consumes a part of the liquid, for example, diesel fuel, and supplies the unused fuel to the controller via a liquid return line. In this case, the excess pumped fuel is supplied via the liquid return line to the controller, which supplies the fuel to the filter element and/or to the liquid storage vessel depending on the fuel temperature. If the liquid temperature, which is to be detected after the filter element, is below a defined temperature, for example, 20° C., the returned fuel is supplied via the connection line to the filter element, whereby the cold fuel from the liquid storage vessel mixes with the heated fuel from the connection line. In this way, paraffins, which are contained in the fuel and clog the filter element at low fuel temperatures, may be dissolved. 
   Another possible liquid consuming device be, for example, the transmission of an internal combustion engine, which is connected to an oil circuit. The oil circuit is a closed system, in which the oil is used to reduce the material wear. The colder the oil is, the more viscous it is. This is particularly disadvantageous during cold starting of an engine, since in this case the lubricant film may be interrupted and therefore significant wear may occur. By mixing the cold oil from the liquid tank with the heated returned oil, the viscosity of the oil may be reduced and the lubrication in the cold state may be improved. 
   One advantageous embodiment of the present invention includes a two-part housing, which comprises a cover and a container, which may be either detachably connected or permanently bonded to one another. Detachable connections of the two housing parts may be produced, for example, through screwing or snapping. If screwing is used, a thread may be molded onto the housing parts or self-tapping screws may be used, which are inserted through one housing part and screwed into the other part. Of course, the screws may also be fixed using nuts. Permanent bonds of the housing parts may be produced, for example, through gluing, welding, or soldering. 
   In this embodiment, the cover and the container may be made of, e.g., metal or plastic; whereby the two parts can be made of the same or of different materials. That is to say, it is conceivable within the scope of the invention to have a container made of metal and a cover made of plastic or a container made of plastic and a cover made of metal. 
   According to a further embodiment of the present invention, the temperature detection unit is mechanically connected to the valve. In this case, the temperature detection unit is designed in such a way that the liquid temperature triggers a direct action, in particular a change in volume in a temperature-sensitive material, in the temperature detection unit. The temperature detection unit may, for example, be formed by an expansion/contraction element. As soon as the temperature detection unit comes into contact with a cold liquid, the temperature-sensitive material contracts, and the valve, which is mechanically coupled to the temperature detection unit, opens the connection line through which the returned liquid is supplied to the filter element. A part of the returned liquid may, however, be conducted into the liquid storage vessel, whereby defined pressure ratios can be produced and/or maintained in the liquid circuit. 
   It is advantageous to position a ventilation device in the liquid filter system, through which gases may be separated. This ventilation device may be designed, for example, as a leakage line or as a pressure-dependent valve. If the ventilation device is constructed as a valve, it may open automatically above a predetermined pressure and allow the gas to escape. The ventilation device is preferably positioned before the filter element, since gases accumulate at this point. 
   In another embodiment of the ventilation device, the valve of the controller is constructed as a piston, with the dimensions and tolerances of the piston being designed in such a way that a defined piston play exists, via which the ventilation of the liquid filter system may occur. 
   A further variant of the present invention includes a device in the liquid filter system for removing a separated second liquid, in particular a water drain screw. The separated second liquid may be, for example, water, which is separated, for example, from a lubricating oil or a fuel oil. 
   It is advantageous to integrate a liquid heater into the liquid filter system, through which the liquid in the liquid circuit may be heated. This liquid heater may be positioned at any desired points. It is particularly advantageous to position a heater before the filter element in the flow direction. In this case, one may select between two embodiments. The first embodiment exclusively heats the liquid which is pumped out of the liquid storage vessel, and the heated liquid from the liquid storage vessel subsequently is mixed with the returned liquid from the filter element. In the second embodiment, the cold liquid from the liquid storage vessel is first mixed with the returned liquid, and the resulting liquid mixture is subsequently heated by the liquid heater. In this embodiment as well, the liquid is heated before the filter element. 
   A further embodiment of the present invention includes a non-return valve or check valve, positioned downstream from the controller, which may be formed, for example, by a shield valve. This check valve prevents uncontrolled emptying of the liquid filter system and is preferably positioned between the controller and the liquid drain line. In other check valve configurations, the valve may be integrated in the liquid drain line, as a result of which the liquid is not able to drain into the liquid storage vessel. 
   In accordance with a further embodiment of the present invention, a pressure relief or overpressure valve, which is preferably positioned downstream from the controller, is integrated into the liquid filter system. The overpressure valve is designed in such a way that in the event of excessive liquid pressure in the liquid circuit, at least a part of the liquid may drain into the liquid storage vessel, so that damage to the components located in the liquid circuit may be prevented. This overpressure valve may be positioned between the controller and the liquid drain line. In an alternative embodiment, the overpressure valve may be integrated into the liquid outlet. Furthermore, the overpressure valve and the check valve may be combined into a single component, so that assembly is made easier and installation space is saved. 
   It is advantageous for the cover to be attached to the container in a liquid-tight manner using screws. In this case, these housing parts may be made of different materials. To facilitate liquid-tight attachment, a seal may be provided between the cover and the container. Such a seal may be fixed to either of the two housing parts or may be inserted loosely and clamped between them. 
   According to a further embodiment of the present invention, the liquid filter system is installed in a fuel injection system of a diesel-operated internal combustion engine. In this case, the liquid will be diesel fuel, which is pumped by a fuel pump out of a fuel tank and through a fuel filter to injection nozzles. The fuel pump delivers more fuel than is necessary for the operation of the internal combustion engine. The excess pumped fuel is conducted to the controller, from which, depending on the temperature, it is supplied to either the fuel filter or the fuel tank. 
   These and other features of preferred embodiments of the invention, in addition to being set forth in the claims, are also disclosed in the specification and/or the drawings, and the individual features each may be implemented in embodiments of the invention either alone or in the form of subcombinations of two or more features and can be applied to other fields of use and may constitute advantageous, separately protectable constructions for which protection is also claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be described in further detail hereinafter with reference to illustrative preferred embodiments shown in the accompanying drawings, in which: 
       FIG. 1  shows a liquid circuit according to the invention; 
       FIG. 2  is a sectional view of a controller in the installed state; 
       FIG. 3  is a sectional view showing a modified controller; 
       FIG. 4  is a perspective view of a liquid filter system, and 
       FIG. 5  is a sectional view of the liquid filter system of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   A liquid circuit is schematically illustrated in  FIG. 1 . The liquid circuit is formed by a liquid storage vessel  10 , an unfiltered liquid line  11 , extending from the liquid storage vessel  10 , a liquid filter system  12  attached to the unfiltered liquid line  11 , a filtered liquid line  13  which connects a device  14  which consumes or uses the liquid to the liquid filter system  12 , a liquid return line  15 , and a liquid drain line  16 . In order that a liquid may circulate in the liquid circuit, a liquid pump  17  is provided, which generates a liquid pressure in the liquid circuit and has a larger delivery volume than the maximum consumption of the user device  14 . 
   In this illustrative embodiment, the liquid pump  17  is positioned in the unfiltered liquid line. However, the positioning of the liquid pump  17  within the liquid circuit is arbitrary. Consequently, if desired, pump  17  could be positioned, for example, in the liquid storage vessel, in the liquid filter system, or in other liquid-conducting lines. 
   The liquid filter system  12  comprises a housing  18 , which has an unfiltered liquid inlet  19 , a filtered liquid outlet  20 , a liquid return  21 , and a liquid outlet  22 , all fixed on the housing. A filter element  23  and a controller  24  are integrated into the housing  18 . The filter element  23  has an unfiltered liquid side  25  and a filtered liquid side  26 , with the two sides being separated from one another by a seal around the filter element  23 . The unfiltered liquid inlet  19  communicates with the unfiltered side  25  of the filter element  23  and is connected to the liquid storage vessel  10  by the unfiltered liquid line  11 . The filtered liquid outlet  20  communicates with the filtered side  26  of the filter element  23 , the filtered liquid line  13  being connected at one end to the filtered liquid outlet  20  and at the other end to the user device  14 , both connections being sealed. In this way, filtered liquid from the liquid storage vessel  10  is supplied to the user device  14 . 
   The excess liquid delivered by the liquid pump  17  is conducted back to the liquid filter system  12  via a liquid return line  15 , which is connected to the liquid return  21 , and is directed further by the controller  24  as a function of a liquid temperature existing in the liquid. In this embodiment, the controller  24  is provided with a temperature detection unit  27  for detecting the liquid temperature which senses the liquid temperature on the filtered side  26  of the filter element  23 . The detection of the liquid temperature may, however, be performed at any desired points in the liquid circuit, such as in the liquid storage vessel  10 , in other liquid lines  11  or  13 , or on the unfiltered side  25  of the filter element  23 . 
   Furthermore, the controller  24  includes an associated valve  28 , which opens or closes a connection line  29 , which communicates on one side with the liquid return  21  and on the other side with the unfiltered side  25  of the filter element  23 , depending on the liquid temperature. At low liquid temperatures, particularly below 20° C., the valve  28  is opened, so that the excess liquid delivered by the liquid pump  17  is at least partially supplied via the connection line  29  to the unfiltered side  25  of the filter element  23 . The excess liquid delivered by the liquid pump  17  has a somewhat higher temperature than the liquid in the liquid storage vessel. The increase in temperature is caused by the liquid pump  17  as it generates the liquid pressure. Furthermore, an increase in temperature may also be caused by a heat source, such as the liquid consumer or user device or other components positioned around the liquid circuit. The liquid delivered from the liquid storage vessel  10  mixes with the warmed recycle liquid supplied through the connection line  29  before they flow through the filter element  23 . 
   As soon as the liquid temperature lies above the predetermined temperature, the excess pumped liquid is conducted back to the liquid storage vessel  10 . If desired, a liquid cooler may be arranged on the liquid drain line  16 , so that the liquid in the liquid storage vessel  10  is not heated above a critical liquid temperature. 
   In order to prevent the filtered liquid line  13  from running empty and a consequent insufficient supply of liquid to the user device  14 , a check valve  8  may be positioned in the filtered liquid line  13 . Furthermore, in another embodiment, an overpressure valve  9  may be provided, which connects the filtered liquid line  13  to the connection line  29  or the liquid drain line  16 . The overpressure valve  9  serves to relieve excess liquid pressure, so that the user device  14  will not be damaged by excessive pressures. 
     FIG. 2  is a sectional view of a controller  24  in the installed state. In this illustrative embodiment, the temperature detection unit  27  is mechanically connected to the valve  28 . The temperature detection unit  27  is formed by a sleeve  31 , partially filled with a temperature-sensitive material  30 , the sleeve  31  being made of a material which has good thermal conductivity. The sleeve  31  is in contact with the liquid in filtered side  26  so that the liquid temperature is relayed to the temperature-sensitive material  30 . If the liquid temperature is cold, the temperature-sensitive material  30  contracts. At a high liquid temperature, the temperature-sensitive material  30  expands. 
   The valve  28  is formed by a piston rod  32 , a piston  33 , and a spring guide  34 . The piston rod  32  preferably has a cylindrical configuration; however, piston rods  32  constructed in other shapes, such as parallelepiped, are also conceivable. The piston rod  32  is inserted into the sleeve  31 , which is filled with the temperature-sensitive material  30 , care being taken that the piston rod  32  is smooth-running, but that the temperature-sensitive material  30  may not escape between the piston rod  32  and the sleeve  31  as it expands. 
   Upon expansion of the temperature-sensitive material  30 , the piston rod  32  must be at least partially pushed out of the space  31 . It is advantageous if a part of the piston rod  32  remains in the sleeve, due to which the piston rod  32  is provided with a guide region  35 , which prevents canting of the piston rod  32  during its movements within the sleeve  31 . 
   Upon contraction of the temperature-sensitive material  30 , the piston rod  32  is urged into the sleeve  31  by a spring  36 , which is clamped between the piston  33  and an end cap  37 . The spring  36  is arranged under a slight pre-stress or bias in a valve chamber  38 , which, together with the end cap  37 , forms the outer contour of the controller  24 , so that the temperature-sensitive material  30  may still displace the piston rod  32 . The end cap  37  may be detachably connected or permanently bonded to the valve chamber  38 . 
   The piston  33  has a cylindrical configuration and includes a lateral surface  39  which is guided in the valve chamber  38 . The piston  33  has a tight tolerance with the valve chamber  38  such that no and/or only a slight leakage flow may escape. 
   In this illustrative embodiment, the liquid flows through the controller  24 . The liquid reaches the controller  24  from the liquid return  21 . If the liquid temperature lies above a defined temperature, the temperature-sensitive material  30  is expanded and the piston rod  32  projects far out of the sleeve  31 , so that the piston  33  is located in a first end position shown in solid lines in which the connection line  29  is blocked off and the liquid is conducted into the liquid outlet  22 . 
   At a liquid temperature below a defined temperature, the temperature-sensitive material  30  contracts and the piston rod  32  projects only slightly out of the sleeve  31 . In this operating position (shown in broken lines), the piston  33  is in a second end position, in which it opens the connection line  29  so that the liquid may flow through the connection line  29 , as well as through the liquid outlet  22 . 
   If desired, flow regulating valves (not shown) may be provided on the liquid outlet  22  and/or on the connection line  29 , which control the flow rate of the liquid depending on where more liquid is to flow. The liquid return  21  and the liquid outlet  22  may be positioned in such a way that they lie opposite one another or are positioned at an angle to one another. The liquid return  21  may, however, also be positioned in a plane offset relative to the liquid outlet  22 . The connection line  29  is positioned such that the piston  33  blocks off the connection line at higher liquid temperatures and opens it at lower liquid temperatures. 
     FIG. 3  is a sectional view of a controller  24 . This controller  24  essentially corresponds to  FIG. 2 , except that the valve  28  and the positions of the liquid return  21  and of the liquid outlet  22  are modified. The position of liquid return  21  in relation to the liquid outlet  22  is selected in such a way that a distance D lies between them. In this embodiment, the valve  28  comprises a stopper piston  40 , which is positioned at a distance A from the piston  33 . The distance A and the distance D are tailored to one another in such a way that the valve  28  opens the liquid return  21  and the liquid outlet  22  in a first end position and closes the connection line  29  using the piston  33 , or opens the liquid return  21  and the connection line  29 , but closes the liquid outlet  22  using the stopper piston  40 , in a second end position (shown in broken lines). 
   In order to assure that an excessive liquid pressure, which is so high, for example, that it might damage the filter element, does not act on the filter element  23  in the second end position (shown in broken lines), a pressure relief or overpressure valve  54  may be provided. The overpressure valve  54  is positioned in an overpressure line  55 , which connects the connection line  29  to the liquid outlet  22 . As soon as the liquid pressure in the connection line  29  exceeds a defined liquid pressure, in particular 1 bar, the overpressure valve  54  opens the overpressure line  55 , through which the liquid may drain into the liquid outlet  22 . Once the liquid pressure in line  29  falls below the defined liquid pressure, the overpressure valve  54  closes again and the liquid is only conducted through the connection line  29 . 
     FIG. 4  is a perspective view of a liquid filter system  12 , which is particularly adapted for use in a fuel injection system of a V-engine. The liquid filter system  12  is essentially formed by a housing  18  and a mounting bracket  43 . The housing  18  comprises a container  41  and a cover  42 , the cover  42  and the container  41  each having molded attachment regions  44 , which are secured to one another using screws  45 . In this illustrative embodiment, the unfiltered liquid inlet  19  and the liquid outlet  22  are positioned on the container  41 , and are plugged into the container  41  as plug-in parts using a press fit and glued to form a seal. The unfiltered liquid inlet  19  opens into a liquid pipe  46 , in which a liquid heater  47  is positioned, which may heat the entering liquid if it has a liquid temperature below a defined value. The liquid outlet  22  may be connected to the liquid storage vessel  10  using a liquid drain line  16  as shown in  FIG. 1 . 
   This illustrative embodiment has two filter liquid outlets  20  and two liquid returns  21 , which are integrated into the cover  42  and are glued to the cover  42  as plug-in parts to form a seal. In this way, two user or consumer devices  14  (see  FIG. 1 ) may be connected in parallel. In order to connect multiple consumer devices  14  to the liquid filter system  12 , a corresponding number of filtered liquid outlets  20  and liquid returns  21  should be provided, and the liquid pump  17  (see  FIG. 1 ) should be designed or selected so that it has a sufficient output capacity. Furthermore, the controller  24  (see  FIGS. 1–3 ) comprises the valve  28  and the temperature detection unit  27  and is integrated into the cover  42  and secured therein by the end cap  37 . 
   This liquid filter system  12  is provided with a water drain screw  48 , through which water or another separated liquid, which has settled in the container  41 , may be withdrawn from the liquid filter system  12 . Preferably, a sensor, which senses the separated liquid and emits a signal which initiates manual or automatic discharge of the separated liquid, is positioned in the bottom region of the liquid filter system  12 . This sensor may be constructed in such a way that it also detects a liquid pressure existing in the liquid filter system  12 . 
   The bracket  43  is shown attached to the liquid pipe  46 . However, the bracket  43  may also be attached at other points of the housing  18 . The bracket  43  has two holes  49  through which attachment screws may be inserted and screwed together in order to mount the liquid filter system  12  to other elements provided for this purpose. If desired, damping components (not shown), such as rubber elements, may be included in the mounting structure. Furthermore, additional attachment points may be provided on the container  41 , which may be inserted into corresponding receptacles, if desired, with interposition of suitable damping components. 
     FIG. 5  is a sectional view of the liquid filter system  12  of  FIG. 4  taken along section line A—A. This section A—A runs centrally through the filtered liquid outlets  20 , the liquid returns  21 , and the controller  24 , as a result of which the top of cover  42  is missing from the drawing figure. The controller  24  is integrated into the cover  42 , which is constructed in such a way that it has walls  51  which conduct the liquid in desired paths. The temperature detection unit  27  is in contact with the liquid on the filtered liquid side  26  of the filter. The piston  33  is arranged in a recess or chamber  50  which is formed by walls  51  of the cover, and is depicted in its end position in the cold state, so that the connection line  29  is open. Therefore, the unused liquid, which is returned through the liquid return  21 , may reach the connection line  29  and thus pass to the unfiltered liquid side  25  of the filter element  23  (see  FIG. 1 ). A further part of the liquid is supplied to the liquid outlet  22  via an outlet connection line  52 , since this outlet is not closed off. 
   In this illustrative embodiment, the filter element  23  is formed by two cylindrical filter cartridges, which are positioned below two outlets  53  in the container  41  (see  FIG. 4 ). 
   If the liquid temperature rises above a defined temperature, the piston  33  is moved to its end position in the warm state (shown in broken lines). In this position, the piston  33  blocks off the connection line  29 , so that the returned liquid is supplied through outlet connection line  52  to the liquid outlet  22 . 
   The foregoing description and examples have been set forth merely to illustrate the invention and are not intended to be limiting. Since modifications of the described embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed broadly to include all variations falling within the scope of the appended claims and equivalents thereof.