PATENT ABSTRACT
An air filter has a filter housing and a filter element inserted in the interior of the filter housing. The filter element has an inflow surface and an outflow surface and a filter medium that extends between inflow and outflow surfaces. The filter element has a circumferentially extending seal arranged in the area of the inflow surface or outflow surface and interacting in the mounting position with the sealing surface of the filter housing to separate inlet and outlet of the air filter. A support device clamps the filter element transversely to the insertion direction in the filter housing by a transverse clamping action in a support area that is spaced from the seal. The transverse clamping action results by an insertion movement of the filter element into the filter housing by interaction of the filter element with inner sides of the filter housing.

PATENT DESCRIPTION
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of international application No. PCT/EP2013/056003 having an international filing date of 22 Mar. 2013 and designating the United States, the International application claiming a priority date of 23 Mar. 2012, based on prior filed German patent application No. 10 2012 005 731.8, the entire contents of the aforesaid international application and the aforesaid German patent application being incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention concerns an air filter, in particular of an internal combustion engine, in particular of a motor vehicle, comprising a filter housing provided with an interior and comprising a filter element that is inserted in an insertion direction in a mounting position in the interior of the filter housing. Moreover, the invention concerns a filter element for such an air filter. 
     Flat filter elements for air filters are known on the market; they can be exchangeably inserted into a filter housing. In an area of an inflow surface or side of the flat filter element which forms the raw air side, or an outflow surface or side which forms the clean air side, the flat filter element comprises a sealing section for a seal for sealing the flat filter element relative to the filter housing. The seal is attached to the sealing section and is inserted together with the flat filter element into the filter housing. The flat filter element is arranged in such a way in the filter housing that it separates an inlet of the filter housing from an outlet for the air. The filter housing has a mounting opening through which the flat filter element can be inserted into the filter housing. 
     The invention has the object to design an air filter of the aforementioned kind or a filter element of such an air filter with which a simple, stable, and reliable mounting of the filter element in the filter housing is possible. 
     SUMMARY OF THE INVENTION 
     This object is solved according to the invention in that a support device is provided by means of which the inserted filter element, at least in a support area that is spaced apart from the seal in the insertion direction, is clamped at least transverse to the insertion direction in the filter housing. The transverse clamping action results in a simple way with interaction of the inner sides of the filter housing and of the filter element by the insertion movement of the filter element. 
     Especially, the invention concerns a filter element whose filter medium is open, i.e., is not formed to a closed circumferential wall that surrounds the interior. In case of a folded filter medium, this means that the end face folds as well as the end face edges are not connected to each other. 
     Especially, a flat filter element is concerned. Flat filter elements can be planar but can also have bends in various directions. In contrast to a flat filter element, in a round filter element the filter medium is closed, i.e., its end faces, in case of a folded filter element its end face folds, are connected to each other. 
     In case of a preferred embodiment, the filter housing comprises a mounting opening through which the filter element, with the end leading that is remote from the circumferentially extending seal, is inserted into the interior wherein the mounting opening is surrounded by the housing-associated sealing surface. This results in a particularly functionally reliable sealing action between the filter element and the filter housing that is exposed to loads as minimal as possible during assembly. 
     A robust transverse clamping action of the filter element results in that in a preferred inventive variant on at least one inner side of the filter housing at least one projection is arranged by means of which the filter element is clamped in the at least one support area transverse to the insertion direction. 
     This advantage is achieved to an even greater degree when at least two projections are arranged on inner sides of the filter housing that are oppositely positioned transverse to the insertion direction and between which the filter element is clamped in the at least one support area transverse to the insertion direction. 
     A particularly disruption-free mounting of the filter element results in an inventive variant in which the projection or the projections comprises or comprise a forward guide ramp in the insertion direction. Canting of the filter element during the insertion movement is counteracted in this way. 
     With respect to manufacturing technology and for reasons of stability, an inventive variant is preferred in which the projection or the projections is or are embodied in the form of a recess of the filter housing. 
     In the case of a preferred embodiment, the cross-section of the interior of the filter housing in the support area is tapering at least in two directions that are transversely extending relative to each other. A particularly stable support, holding or fixation of the filter element results. In this way, a simple and robust configuration is provided when the cross-sectional tapering section is realized by means of one or several projections. 
     A preferred variant is characterized by a comfortable and disruption-free mounting of the filter element wherein on at least one inner side of the filter housing at least one guide web for guiding the filter element during insertion movement is arranged. Preferably, in the insertion direction of the filter element, the guide web and its top side which is facing the filter element extend first away from the corresponding inner side into the interior in front of the at least one projection in the insertion direction. Due to the guide web, the filter element is lifted off the corresponding inner side and therefore pre-positioned for insertion into the cross-sectional tapering section formed by the projection. 
     In order to prevent upon insertion of the filter element into the cross-sectional tapering section an undesirable canting or pressing onto the guide web, the guide web preferably descends in front of the at least one projection toward the inner side. The filter element thus crosses the section of the guide web that is projecting farthest into the interior before it is clamped at the projection or projections transverse to the insertion direction. 
     A particularly stable holding or support of the filter element results in that the at least one support area is arranged in the area of the outflow or inflow surface that is remote from the circumferentially extending seal. This is in particular important in case of particularly long filter elements, i.e., filter elements in which the spacing between the inflow and outflow surface is multiple times greater than the extension (thickness) of the filter element in at least one transverse direction. 
     An undesirable noise development and damage upon operation of the air filter, for example, by vibrations of the air filter, is counteracted in case of a particularly preferred inventive variant in which the transverse clamping action of the filter element in at least one support area is the result of an elastic deformation of the filter element, or of at least one support element section on the filter element, transverse to the insertion direction. 
     In case of a preferred variant of the invention, for the purpose of a further increased stabilization of the support action of the filter element, a transverse clamping action of the filter element is provided by means of the support device in a further support area which, in the insertion direction, is arranged between the first support area and the circumferentially extending seal. 
     The technical object is furthermore solved by a filter element of an air filter described above and in the following which is designed such that the filter element, at least in a support area which is spaced apart from the circumferentially extending seal in the insertion direction, can be clamped at least transversely to the insertion direction in the filter housing, wherein the transverse clamping action may result with interaction of inner sides of the filter housing and of the filter element by the insertion movement of the filter element. 
     In case of a particularly preferred embodiment of the invention, the filter element in the at least one support area comprises at least one support element section as a part of the support device. A simple configuration results in that the at least one support element section projects transverse to the insertion direction relative to at least one circumferential side of the filter element. Preferably, relative to the filter medium of the filter element, it is elastically movable and/or deformable at least transverse to the insertion direction. For a particularly stable support action, the at least one support element section is arranged in the area of the inflow or outflow surface that is remote from the circumferentially extending seal. 
     Preferably, at least one support element section is thus provided which, in addition to the seal, can support the filter element on the inner side of the filter housing. In this way, the filter element can be supported on at least two locations, i.e., at the location of the seal and at the location of the at least one support element section, and held in the interior of the filter housing. The at least one support element section projects past the filter medium in at least one direction. In this way, it is arranged between the filter medium and the inner side of the filter housing and secures the filter medium at a spacing to the filter housing. 
     It is also possible to provide more than one support element section so that the filter medium can be supported at several locations on the inner side of the filter housing. In this way, positioning of the filter element in the filter housing can be further improved. The at least one support element section can be designed such that it is able to compensate component-related and/or mounting-related tolerances between the filter element and the filter housing. In addition or as an alternative, the at least one support element section can be designed such that it can guide the filter element upon insertion into the filter housing along a guide path associated with the housing, in particular a guide groove or a guide rail. It can therefore act additionally as a guide element. 
     In addition or as an alternative, the support element section can be designed such that it effects a damping action in particular of vibrations between the filter housing and the filter medium. In this way, the filter housing can be vibrationally decoupled from the filter element. The mechanical load of the filter medium can thus be reduced. A susceptibility to failure can thus be reduced. In this way, also the service life of the filter element can be extended. 
     On the filter element a seal can be arranged. The seal can advantageously be connected by foaming of polyurethane foam to the sealing section. As an alternative, the seal can be pre-manufactured as a shaped part, in particular of rubber, and can be mounted on the sealing section, in particular pushed on or inserted. Alternatively, or additionally, a seal can also be mounted on the filter housing, preferably fastened. In this case, it is not required that the filter element itself is provided with its own seal. 
     Also, more than one support element section can be provided. At least two of the support element sections can be arranged on a common support element. Between the support element sections support gaps can be provided so that no continuous support on the inner side is realized. This differentiates the support action of the support element sections according to the invention from the support action of a seal which is continuous. In this way, the installation of the filter element can be simplified. As a whole, it is not required that the at least one support element section fulfills a sealing function. The at least one support element section can be optimized with regard to its effect for supporting and/or positioning and/or damping and/or holding. Advantageously, the at least one support element section can be made of plastic material. It can thus be manufactured easily. The at least one support element section can thus be provided with minimal weight. Also, complex shapes can be realized with plastic material. The flat filter element can advantageously be exchangeably arranged in the filter housing. 
     In an advantageous embodiment, at least two support element sections can be supported at least in opposite directions at corresponding support areas on inner sides of the filter housing. In this way, the filter medium can be supported in opposite directions and be held stably in this way. The filter medium can thus be clamped between the support areas of the filter housing. Advantageously, the at least two support element sections can project past opposite circumferential sides of the filter medium. The at least two support element sections can project also past the same circumferential side of the filter medium. The support direction can then extend parallel to the circumferential side wherein the support directions of the two support element sections can be oppositely oriented. In particular, the support element sections can be located on opposite sides of a single support element which can be arranged on the circumferential side of the filter medium. At least one of the support element sections can be realized also such that it can absorb, in addition to force components in opposite directions, also force components transverse to these directions. 
     Advantageously, between the at least two support element sections a support gap may exist. In the support gap no support of the filter element relative to the inner side of the filter housing is realized. In this way, the size of the support element sections can be reduced. Accordingly, the space requirement of the support element sections can be reduced. Moreover, the material demand for the support element sections can be reduced. Also, the freedom in shaping the filter housing, in particular the inner sides of the filter housing, and/or the support element sections can be increased. It is not required that the shapes of the inner side of the filter housing and the shapes of the support element sections are matched continuously relative to each other. The support element sections can thus be arranged in such a way on the circumferential side of the filter element that they can develop an optimal supporting action. Moreover, in case of support element sections that are separate from each other, an insertion process of the filter element into the filter housing can be simplified. A force expenditure upon insertion of the filter element can be reduced. Advantageously, at least one guide groove or guide rail can be arranged on the inner side of the filter housing with which the at least one support element section can be guided upon insertion of the filter element. 
     In a further advantageous embodiment, the at least one support element section can be attached to a carrier element and the carrier element can be attached to at least one side of the filter medium. Advantageously, the carrier element can be a carrier plate with which an areal connection to the filter medium is possible. In this way, the force transmission between the carrier plate and the filter medium can be improved. The carrier element and the at least one support element section can thus be pre-manufactured in a simple way. They can be connected as a common component to the filter medium. The carrier element and the at least one support element section can be realized as a modular component. The carrier element and the at least one support element section can be realized advantageously as a two component part. Advantageously, the carrier element can be made of a hard stable material. In this way, force transmission between the at least one support element section and the filter medium can be improved. Moreover, the filter medium can be reinforced with a stable carrier element. Deformations of the filter medium, in particular by vibrations, can thus be reduced. This has a beneficial effect, in particular for a folded filter medium, in regard to shape stability of the filter element. 
     Advantageously, at least two support element sections can be attached to a common carrier element. In this way, the carrier element can form a force connection between the at least two support element sections. In case that the two support elements are supported in opposite directions on the filter housing, a force action onto the filter medium as a result of the supporting action on the filter housing can thus be avoided. The carrier element can be fastened immediately on the filter medium. Alternatively, the carrier element can also be connected indirectly, in particular by means of a connecting element, preferably a lateral strip or a frame, to the filter medium. The carrier element can be connected with the filter medium by means of an adhesive connection, a weld connection or a mechanical connection, in particular a clip-on connection, a clamping connection, a snap-on connection, a plug-in connection or a screw connection. The carrier element can be designed such that it can additionally seal the side of the filter medium. The carrier element can also be free of any sealing function. In this way, the carrier element can be optimized with regard to its mechanical properties, in particular its carrying function and stabilizing function. Advantageously, the carrier element can extend across the entire side of the filter medium. In the this way, the stability of the filter element can be improved. Alternatively, the carrier element may also cover only a part of the side of the filter medium. In this way, material and weight can be saved. The carrier element can advantageously be made of plastic material. It can thus be manufactured in a simple way and complexly shaped. By use of plastic material, the weight of the carrier element can be reduced. The carrier element can also have the shape of a frame which at least partially can frame the filter medium. 
     In a further advantageous embodiment, the at least one support element section, optionally the carrier element, can be attached by means of a lateral strip to at least one side of the filter medium. By means of the lateral strip a tolerance between the at least one support element section or the carrier element and the filter medium can be simply compensated. By means of the lateral strip, unevenness of the filter medium, in particular of a folded filter medium, can be compensated easily. The lateral strip can be advantageously flexible. The lateral strip can additionally act as a damping element between the filter medium and the at least one support element section or the carrier element. In this way, transmission of vibrations from the filter housing onto the filter medium can be further reduced. The lateral strip can be cut to length in a simple way, in particular by cutting, in order to match it to the dimensions of the filter medium. The lateral strip can project past the side of the filter medium at least at one edge. It can be folded or bent easily at the at least one edge of the side of the filter medium. The lateral strip can be extended close to the sealing section for the seal and/or can be extended into it. It can overlap the sealing section and thus also the seal. 
     By use of the lateral strip for attachment of the carrier element the sealing function and the holding function can be separated from each other. The lateral strip can be optimized with respect to the sealing function for sealing the side of the filter medium. The at least one support element section, optionally together with the carrier element, can be optimized with respect to the holding function. The lateral strip can be optimized with respect to the sealing function. It can be realized with a thickness that, relative to its areal extension, is relatively minimal and can therefore be arranged in a space-saving way between the filter medium and the at least one support element section, respectively, the carrier element. 
     Advantageously, the lateral strip can be a nonwoven. Preferably, the lateral strip can be made of a nonwoven. A nonwoven can combine a great tearing resistance with a high flexibility. A nonwoven can also be stretchable. A nonwoven moreover can be seal-tight for the fluid to be filtered by the flat filter element so that the side of the filter medium can be effectively sealed with it. A nonwoven can moreover be adjusted simply in regard to its shape, in particular cut, in order to match it to the dimensions of the side of the filter medium. 
     Advantageously, the at least one support element section, relative to the filter medium, can be elastically movable and/or deformable at least transverse to an insertion direction of the filter element into the filter housing. Due to the elasticity, the damping function can be improved. Moreover, a tolerance compensation between the filter element and the filter housing can be simply realized in this way. The at least one support element section upon insertion of the filter element into the filter housing can elastically yield so that the filter element can also pass constrictions and/or projections within the filter housing. In the correct mounting position of the filter element, the at least one support element section can support the filter element elastically with a mechanical tension relative to the inner side of the filter housing. The at least one support element section can be of an elastic material. Advantageously, the at least one support element section can be of a soft component, in particular a rubber-like plastic material. Alternatively or additionally, the at least one support element section can be connected by a connecting means indirectly, in particular by means of a carrier element and/or a lateral strip, or immediately elastically with the filter medium. The connecting means can be a spring element. However, the connecting means can also be elastically changeable and/or deformable in other ways. 
     In a further advantageous embodiment, the filter medium can be connected at one side with the seal for sealing the filter element relative to the filter housing and the at least one support element section can be arranged on a side of the filter medium which is facing away from the seal. In this way, the filter element can be supported and held in the correct mounting position within the filter housing at locations that are spaced apart from each other. In this way, a leverage action on the seal by the weight of the filter medium can be reduced. This is advantageous when the filter element is arranged horizontally in the operational position and is attached by means of the seal, viewed in horizontal direction, at one side on the filter housing. The filter element, viewed in horizontal direction, can then be additionally supported in the housing on the side opposite the seal. Advantageously, for the horizontal arrangement of the filter element, the at least one support element section can provide support spatially in downward direction relative to the filter housing. 
     Advantageously, the filter medium can be folded in a zigzag shape. With a filter medium that is folded in a zigzag shape, a great ratio between effective filter surface and required mounting space can be realized. 
     In case of a filter medium that is folded in a zigzag shape, the at least one support element section is preferably at one of the end face edges of the folded filter medium, i.e., at one of the two circumferential sides where the zigzag-shaped course of the folds shows. 
     In case of a preferred variant of the air filter, the filter housing is thus provided with a mechanical counter bearing in the form of a projection on which, in the correct mounting position of the filter element, the at least one support element section can be supported. The projection enables therefore an improvement of the support action. In this way, a damping action of the support element section can be further improved. Moreover, with the projection an improved point of attack for a force transmission onto the support element section can be realized. Advantageously, in an area of the filter housing in front of the at least one projection in the insertion direction of the filter element, a housing-associated guide means, in particular a guide groove or a guide rail, may be realized along which the at least one support element section can be guided during insertion. Upon reaching the projection, a holding force between the projection and the support element section can be realized which contributes to holding, in particular clamping, of the flat filter element in the filter housing in the correct mounting position. The mechanical tension may be caused in this context by an elastic deformation and/or displacement of the at least one support element section. Advantageously, the mounting opening for the flat filter element can at least partially form an inlet opening for the air to be purified. Alternatively, the mounting opening can at least partially form an outlet opening for the purified air. A separate mounting opening is thus obsolete. 
     In an advantageous embodiment, at least two projections can be arranged on opposite sides of the interior between which two corresponding support element sections can be supported at least in opposite directions. Accordingly, the filter element can be clamped between the projections on opposite sides. The filter element can thus be held and supported by means of the support element sections and the oppositely positioned projections optimally between the inner sides in the interior of the filter housing. The filter element can thus be positioned and held on the two opposite sides at a spacing to the inner sides of the filter housing, respectively. 
     Advantageously, on at least one inner side of the filter housing at least one guide web can be arranged which can extend in the insertion direction of the filter element and whose top side that is facing the filter element can extend, in the insertion direction, first away from the corresponding inner side and, after a bend, in front of the at least one projection, can extend toward the inner side. Advantageously, the filter medium upon insertion can be resting on the guide web. Along the curved top side of the guide web the filter element can be moved on a curved path upon insertion into the filter housing. In this way, positioning of the at least one support element section on the corresponding projection can be simplified. The at least one guide web can be positioned advantageously spatially at the bottom for a horizontal insertion direction. In this way, the filter element can rest on the guide web upon insertion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages, features, and details of the invention result from the following description in which an embodiment of the invention will be explained in more detail with the aid of the drawing. A person of skill in the art will consider the features disclosed in combination in the drawing, the description, and the claims also individually and combine them to other meaningful further combinations. 
         FIG. 1  shows an isometric illustration of an air filter of an internal combustion engine of a motor vehicle, comprising an air filter housing in which a flat filter element is arranged. 
         FIG. 2  shows the air filter of  FIG. 1  wherein a housing top part of the air filter housing is removed. 
         FIG. 3  shows the open air filter of  FIG. 2  in another perspective view. 
         FIG. 4  is a vertical longitudinal section of the air filter of  FIGS. 1 to 3 . 
         FIG. 5  is a horizontal longitudinal section of the air filter of  FIGS. 1 to 4  through a section plane perpendicular to the section plane of the vertical longitudinal section of  FIG. 4 . 
         FIG. 6  shows a first vertical cross-section of the air filter of  FIGS. 1 to 5 . 
         FIG. 7  is a second vertical cross-section of the air filter of  FIGS. 1 through 6  in a section plane that is parallel to the section plane of the cross-section of  FIG. 6 . 
         FIG. 8  shows a third vertical cross-section of the air filter of  FIGS. 1 to 7  in a section plane that is parallel to the section planes of the vertical cross-sections of  FIGS. 6 and 7 . 
         FIG. 9  is an isometric illustration of the flat filter element with a housing cover of the air filter housing of the air filter of  FIGS. 1 to 8 . 
         FIG. 10  is an isometric illustration of the flat filter element of the air filter of  FIGS. 1 to 9  without housing cover. 
         FIG. 11  is a plan view of the flat filter element of the air filter of  FIGS. 1 to 10 . 
         FIG. 12  is a longitudinal side view of the flat filter element of the air filter of  FIGS. 1 to 11 . 
     
    
    
     In the Figures, same components are identified with same reference characters. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In  FIGS. 1 to 8 , an air filter  10  of an internal combustion engine, not shown otherwise, of a motor vehicle with an exchangeable flat filter element  12  is illustrated in various perspective views and section views. The flat filter element  12  is illustrated in  FIGS. 9 to 12  in detail in various perspective views. 
     The air filter  10  is arranged in an intake manifold, not shown otherwise, of the internal combustion engine. It serves for purifying intake air. The air filter  10  comprises a filter housing  14  which is assembled of a housing bottom part  16  and a housing top part  18  as well as a housing cover  20 . The housing bottom part  16 , the housing top part  18 , and the housing cover  20  are each made of a hard plastic material. In the normal mounted state of the air filter  10  illustrated in  FIGS. 1 to 8 , in which the flat filter element  12  is horizontally arranged, the housing bottom part  16  is arranged spatially at the bottom and the housing part  18  at the top. The designations “horizontal” and “vertical” relate to the filter  10  in its normal mounted state in space. Upon appropriate rotation and/or tilting of the filter  10 , the orientations “horizontal” and “vertical” will change accordingly. 
     The housing bottom part  16  comprises an approximately parallelepipedal tub which is slanted at a closed end face and is provided with a mounting opening  22  at the opposite end face. The mounting opening  22  is illustrated in  FIGS. 4 and 5 . The mounting opening  22  is surrounded by a mounting frame  24  which is part of the housing bottom part  16 . The mounting frame  24  is closed circumferentially. A free edge of the mounting frame  24  forms a housing-associated sealing surface in the form of a sealing edge  26  on which, when the flat filter element  12  is installed, an approximately circumferentially closed rectangular seal  28  is resting. The seal  28  has an approximately U-shaped profile. The sealing edge  26  and the seal  28  are shown inter alia in  FIGS. 4 and 5 . The mounting opening  22  has an approximately rectangular cross-section. 
     A threaded bushing  30  is formed monolithically on the radial outer circumferential side of the mounting frame  24  at its corners, respectively. The threaded bushing  30  serves for receiving screws  32  with which the housing cover  20  can be connected detachably to the housing bottom part  16 . 
     A total of four angled fastening members  34  are monolithically attached to the exterior of the housing bottom part  16  on the bottom side which is facing away from the housing top part  18 . The angled fastening members  34  serve for attachment of the air filter  10 , for example, on a frame of the internal combustion engine. 
     An interior  36  of the filter housing  14  is divided into an element space section  38  and a deflection space section  40 . In the element space section  38  a filter medium  42  of the flat filter element  12  is arranged. The element space section  38  adjoins directly the mounting opening  22 . In the deflection space section  40  there are two curved deflection walls  44  which are attached monolithically to a bottom  45  of the housing bottom part  16 . The deflection walls  44  extend perpendicularly to the bottom  45  of the housing bottom part  16 . The deflection walls  44  serve for deflecting an air stream from the outflow side of the filter medium  42 , which forms its clean air side  46 , by almost 90 degrees in the direction to a sidewall  48  of the housing bottom part  16 . 
     A circumferential rim  50  on the free side of the housing bottom part  16  which is facing the housing top part  18  is bent in radial direction outwardly. At the side of the sidewall  48  in the area of the element space section  38 , the rim  50  extends, when viewed from the mounting frame  24 , at a slant toward the bottom  45  of the housing bottom part  16 . In the area of the transition of the element space section  38  to the deflection space section  40 , the rim  50  has a slanted step toward the bottom  45  of the housing bottom part  16 . The height of the sidewall  48  thus decreases significantly here. The rim  50  extends approximately at the same height about the deflection space section  40  and along a sidewall  52  opposite the sidewall  48  to a location close to the mounting frame  24 . In the area of the mounting frame  24 , the rim  50  has a slanted step away from the bottom  45  and passes into an upper rim section of the mounting frame  24  which is facing away from the bottom  45 . 
     In the area of the mounting frame  24  the sidewalls  48  and  52  are each recessed toward the element space section  38  so that here a front lateral projection  54  into the interior  36  is provided, respectively, as a part of a support device. The front lateral projections  54  each extend across the entire extension of the sidewalls  48  and  52  from the bottom  45  of the housing bottom part  16  to the rim  50 . The rim  50  is not provided with such recesses. The front lateral projections  54  extend in the normal mounting position of the air filter  10  in vertical direction. The front lateral projections  54  are positioned opposite each other. 
     Within the element space section  38  in the area of the transition into the deflection space section  40 , the sidewalls  48  and  52  are recessed toward the interior  36  to corresponding rear lateral projections  56 . The two rear projections  56  are positioned also opposite each other. 
     The bottom  45  of the housing bottom part  16  in the area of the mounting frame  24  is recessed inwardly toward the interior  36  so that here a front lower projection  58  is provided. The front lower projection  58  passes at its ends into one of the front lateral projections  54 , respectively. 
     The upper rim section of the mounting frame  24  which is facing away from the bottom  45  has a front upper projection  60  in the form of an inward recess. The front upper projection  60  is arranged opposite to the front lower projection  58 . 
     In addition to the rear lateral projections  56 , the bottom  45  of the housing bottom part  16  is recessed toward the interior  36  to a rear lower projection  62 , respectively. 
     Moreover, at the bottom  45  of the housing bottom part  16  two guide webs  64  are integrally formed. The guide webs  64  project into the interior  36 . They extend in the element space section  38  parallel to each other from the mounting frame  24  up to the deflection space section  40 . The top sides  66  of the guide webs  64  which are facing the housing top part  18  are located at the same height. Viewed from the mounting frame  24  in an installation direction  116  of the flat filter element  12  into the filter housing  14 , the top sides  66  rise first away from the bottom  45  toward the housing top part  18  in the normal mounted state of the air filter  10 . In front of the rear lateral projections  56  and the rear lower projections  62 , the top sides  66  descend, adjacent to a bend, toward the bottom  45 . The guide webs  64  are illustrated in the  FIGS. 6 and 7 . The guide webs  64  serve as guide ramps for the flat filter element  12  upon insertion into the filter housing  14 . 
     The bottom  45  of the housing bottom part  16  is provided with profiles  65  for stabilization which extend parallel to each other and parallel to the guide webs  64  through the interior  36  away from the mounting frame  24 . 
     In the area of the element space section  38 , the housing top part  18  has a lower height than the housing bottom part  16 . At the side of the sidewall  48  the vertical extension of the housing bottom part  18  is significantly less compared to the sidewall  48 . At the side of the sidewall  52 , the vertical extension of the sidewall  69  of the housing top part  18  is approximately as large as the vertical extension of the sidewall  52 . 
     A rim  68  of the housing top part  18  is resting circumferentially closed on the rim  50  of the housing bottom part  16 . The rim  68  of the housing top part  18  is welded seal-tightly to the rim  50  of the housing bottom part  16 . 
     The sidewall  69  of the housing top part  18  comprises a rear lateral projection  70  into the interior  36 , shown in  FIG. 7 , which, when the filter housing  14  is assembled, is aligned with the rear lateral projection  56  of the sidewall  52  of the housing bottom part  16 . 
     A cover wall  72  of the housing top part  18  which is positioned opposite the bottom  45  of the housing bottom part  16  rises from the sidewall  69  toward the opposite side. The element space section  38  widens from the sidewalls  52  and  69  toward the sidewall  48  in vertical direction. At the end of the element space section  38  at the level of the rear lower projection  62  of the housing bottom part  16 , the cover wall  72  is recessed toward the rear upper projections  74  into the interior  36 . The rear upper projections  74  are located in the normal mounting position of the air filter  10 , viewed in horizontal direction, on opposite sides adjacent to the rim  68 . The rear upper projections  74  are positioned, viewed in vertical direction, opposite one of the rear lower projections  62  of the housing bottom part  16 , respectively. 
     In the area of the deflection space section  40 , the cover wall  72  passes into an outlet flange section  76 . The outlet flange section  76  forms a dome-like expansion for the deflection space section  40 . The deflection walls  44  extend up to the inner side of the outlet flange section  76  which is facing the bottom  45  of the housing bottom part  16 . On its side which is facing the sidewall  48  of the housing bottom part  16 , the outlet flange section  76  has a connecting flange  80  with an outlet opening  78  for purified air. 
     The housing cover  20  is a frame whose circumferential wall is stepped multiple times. The housing cover  20  is open at the side which is facing the mounting frame  24  and at the opposite side. The housing cover  20  partially forms an inlet  84  of the air filter  10  for air to be purified. The openings and the interior of the housing cover  20  and the mounting opening  22  form an inlet opening of the inlet  84 . A sealing rim  82  of the housing cover  20  which is facing the mounting frame  24  fits the sealing edge  26  of the mounting frame  24 . The sealing rim  82  of the housing cover  20  has at the radial inner circumferential side a step in outward direction in which the seal  28  is arranged when the air filter  10  is assembled. An inner cross-section of the housing cover  20  tapers from the sealing rim  82  to a free rim facing away from the sealing rim  82 . 
     In the following, the flat filter element  12  will be explained with the aid of  FIGS. 10 to 12 . The flat filter element  12  comprises a filter bellows  86  of filter medium  42  folded in a zigzag shape. An imaginary envelope of the filter bellows  86  is approximately of parallelepipedal shape. The filter bellows  86  has an inflow surface or raw air side  90 , in  FIGS. 9 and 10  to the left and the rear and in  FIGS. 11 and 12  to the left; an outflow surface or clean air side  46 , in  FIGS. 9 and 10  to the right and at the front and in  FIGS. 11 and 12  to the right; two end faces  92 , in  FIGS. 9, 10, and 12  at the top and at the bottom; and two end face edges  94 , in  FIGS. 9 and 10  at the front to the left, respectively, at the rear to the right, and in  FIG. 11  at the top and at the bottom. The filter bellows  86  is open at its end face folds  96  and end face edges  94 , meaning that neither the end face folds  96  nor the end face edges  94  are connected to each other. The filter bellows  86  can therefore be referred to as “flat”. The end face folds  96  are the two outer folds at the end faces  92  of the filter bellows  86 . They extend parallel to the fold edges  98  along which the filter medium  42  is folded. The end face edges  94  have each the zigzag-shaped course of folds  104  of the filter bellows  86 . 
     The flow-through direction  100  of the air to be filtered through the flat filter element  12  from the raw air side  90  to the clean air side  46  is indicated in  FIGS. 2 to 8  by an arrow, respectively. The fold edges  98  extend perpendicularly to the flow-through direction  100 . Moreover, the fold edges  98  are positioned perpendicular to the end face edges  94  and thus to the planes that are defined by the end face edges  94 . The planes that are defined by the end face edges  94 , respectively, extend parallel to each other and to the flow-through direction  100 . They are perpendicular to the end faces  92 . In the normal mounted state of the filter  10 , the fold edges  98 , the flow-through direction  100 , and the end faces  92  extend horizontally; the planes that are defined by the end face edges  94  are extending vertical. The seal  28  is in a vertical orientation. 
     At the raw air side  90  the filter bellows  86  is framed in the rectangular seal  28  in a frame-like manner. The seal  28  projects past the filter bellows  86  at its end faces  92  and end face edges  94 . The seal  28  is foamed of polyurethane foam onto the filter bellows  86  in a seal-tight manner. The opening of the U-shape profile of the seal  28  points toward the clean air side  46 . 
     On the clean air side  46  of the filter medium  42 , six support glue beads  102  extend perpendicular to the fold edges  98 , respectively. The support glue beads  102  connect neighboring folds  104 , respectively, and stabilize in this way the filter bellows  86 . 
     A rectangular lateral strip  106  of nonwoven material is glued onto the end face edges  94 , respectively. The lateral strips  106  are flexible. The lateral strips  106  close seal-tightly the sides of the filter bellows  86  at the end face edges  94 . The lateral strips  106  project past the filter bellows  86  at the end faces  92  and at the fold edges  98 . At the raw air side  90  the lateral strips  106  are enclosed by the seal  28 . A section of the filter bellows  86  with the lateral strips  106  that is enclosed by the seal  28  forms a sealing section  108  of the flat filter element  12 . 
     A rectangular carrier plate  110  with two elongate support elements  112  is glued, respectively, onto the exterior side of the lateral strips  106  that are facing away from the filter medium  42 . The lateral strips  106  serves in this context as a flexible tolerance compensation between the carrier plates  110  and the filter bellows  86 . The carrier plates  110  are made of a hard plastic material. The respective lateral strip  106  projects past the carrier plates  110  on all sides. The support elements  112  are located in the vicinity of the rim of the carrier plate  110  that is facing the raw air side  90  and of the rim that is facing the clean air side  46  on the exterior side that is facing away from the lateral strip  106 , respectively. The support elements  112  are made of soft elastic plastic material. The carrier plates  110  are realized with the support elements  112  as two component parts. 
     The support elements  112  extend parallel to the imaginary planes that are defined by the end face edges  94  and perpendicular to an installation direction  116  of the flat filter element  12 , vertical in the normal mounted state of the filter  12 . The installation direction  116  extends in the present embodiment in the flow-through direction  100  of the flat filter element  12 , defining an axial direction (indicated by axis  130 ,  FIG. 5 ) extending through the filter element  12  along the axis defined by installation direction  116  and flow-through direction  100 . The support elements  112  project past the carrier plates  110  in the direction of the fold edges  98  in outward direction, respectively. Moreover, the support element sections  114  of the support elements  112  project past the carrier plates  110  on both sides at the end faces  92  of the filter bellows  86 , respectively. The support element sections  114  of a support element  112 , viewed perpendicular to the installation direction  116 , are positioned opposite each other. Also, each support element section  114  of one of the support elements  112  on an end face edge side is positioned opposite a support element section  114  of a support element  112  on the opposite end face edge side. Longitudinal axes of the support elements  112  on opposite end face edge sides are located on an imaginary plane perpendicular to the installation direction  116 , respectively. Each support element section  114  has three support webs  117  which on the side opposite the filter element  42  project outwardly in the direction of the fold edges  98 . The areas of the support elements  112  between the support webs  117  of the two support element sections  114  act as support gaps  119  where no support relative to the filter housing  14  is realized when the flat filter element  12  is installed. 
     Viewed in the installation direction  116 , a spacing between the support element sections  114 , respectively, the support elements  112  on the same end face edge side of the filter bellows  86  corresponds to a spacing between the front lateral projections  54  and the rear lateral projections  56  and  70 , the front lower projection  58  and the rear lower projections  62 , and the front upper projection  60  and the rear upper projections  74 . In the correct final mounting position of the flat filter element  12  in the filter housing  14 , one of the support element sections  114  is supported on each one of the projections  54 ,  56 ,  58 ,  60 ,  62 ,  70 , and  74 . 
     For installation of the flat filter element  12  in the filter housing  14 , the filter bellows  86 , with the mounted seal  28  leading, is inserted into the sealing rim  82  of the housing cover  20 . The flat filter element  12  with the mounted housing cover  20  is shown in  FIG. 9 . Subsequently, the flat filter element  12 , with the clean air side  46  leading, is inserted in the installation direction  116  through the mounting opening  22  of the mounting frame  24  of the housing bottom part  16  into the interior  36 . The lower end face  92  of the filter bellows  86  which is facing the housing bottom part  16  is positioned in this context on the top sides  66  of the guide webs  64  and is first tilted upwardly and then tilted downwardly in the element space section  38  along a curved path upon further insertion. In this context, the guide webs  64  assist in positioning the clean air side-associated support element sections  114  on the respective rear projection  56 ,  62 ,  70 , and  74 . 
     Upon insertion of the flat filter element  12 , the elastic support element sections  114  yields and adjusts to the contours of the inner sides of the filter housing  14 . In the correct mounting position of the flat filter element  12 , which is shown in  FIGS. 1 to 8 , the support element sections  114  are resting on the appropriate projections  56 ,  58 ,  60 ,  62 ,  70 , and  74  with elastic mechanical tension. In this context, oppositely positioned support element sections  114  of the raw air side-associated support elements  112  are clamped between the front upper projection  60  and the front lower projection  58 . Oppositely positioned support element sections  114  of the clean air side-associated support elements  112  are clamped between the respective rear lower projection  62  and the corresponding rear upper projection  74 . The forces that are produced between the support element sections  114  are transmitted by the stiff carrier plates  110 . Also, the two front support elements  112 , in particular the support webs  117 , are clamped between the two front lateral projections  54 . The two rear support elements  112 , in particular the support webs  117 , are clamped between the two rear lateral projections  56  and  70 , respectively. 
     In the correct mounting position, the sealing rim  82  of the housing cover  20  is resting seal-tightly on the sealing rim  26  of the mounting frame  24  with interposition of the seal  28 . 
     For fixation of the flat filter element  12 , the screws  32  are inserted through corresponding bushings  118  of the housing cover  20  and screwed into the threaded bushings  30  of the mounting frame  24 . 
     Upon installation of the flat filter element  12  into the filter housing  14 , a tolerance compensation is achieved with the elastic support element sections  114 . Upon operation of the air filter  10 , the elastic support element sections  114  act furthermore as damping elements and protect in this way the filter bellows  86  from vibrations. By clamping of the support element sections  114  between the respective opposite projections  54 ,  56 ,  58 ,  60 ,  62 ,  70 , and  74 , the flat filter element  12  is secured in the element space section  38  of the interior  36 . With the clean air side-associated lower support element sections  114  the flat filter element  12  is supported on the rear lower projections  62 . In this way, mechanical loading of the seal  28  by a leverage action as a result of the weight of the filter bellows  86  is prevented. 
     In the described embodiments, the support elements  112  and the support element sections  114  have no sealing function. A support action of the flat filter element  112   12  on the inner side of the filter housing  14  is achieved by means of the support element sections  114  at locations that are separate from each other. 
     The following modifications are possible inter alia in the afore described embodiment of an air filter  10 , of a flat filter element  12 , and of a filter housing  14 . 
     The invention is also not limited to flat filter elements of air filters of internal combustion engines. It can also be employed for different kinds of flat filter elements for fluids of internal combustion engines, for example, oil, fuel or water. Also, it can be employed outside of the field of internal combustion engines, for example, in building technology or water treatment. It can also be employed outside of automotive technology in other kinds of internal combustion engines, for example, in industrial motors. 
     The air filter  10  can also be arranged in a different way in space wherein the orientation of the components relative to each other does not change. 
     The filter housing  14  can also be comprised of more or fewer than three housing parts. The housing bottom part  16  and the housing top part  18  can also be realized as a monolithic housing part. 
     Folding of the filter bellows  86  can also be designed differently. Instead of extending horizontally, the fold edges  98  can also be extending vertically or at a slant. 
     Instead of the filter medium  42  folded in a zigzag shape, a different kind of filter medium can be provided also. For example, a solid parallelepipedal filter medium, for example, a foam-like block can be used also. Instead of the rectangular filter bellows  86 , a filter bellows may be used also whose sides extend at a slant relative to each other. It is also possible to provide a filter bellows in which at least one of the sides is not planar but has at least one bend. 
     Instead of the seal  28  of polyurethane foam, a different kind of seal, for example, a shaped seal, for example of rubber, can be also connected seal-tightly to the folded filter bellows  86 . 
     The seal  28 , instead of being connected fixedly to the filter bellows  86 , can also be detachably pushed onto it. The seal  28  can also be arranged on the housing. It can be, for example, connected fixedly to the filter housing  14 . It can also be fixedly connected to the housing cover  20  or to the housing bottom part  16 . 
     The seal  28  can also have a different profile instead of the U-shape profile. 
     The rim  82  of the housing part  18 , instead of being connected by means of welding, can also be connected by means of a different type of connection, for example, an adhesive connection, a snap-on connection, a clamping connection or another type of screw connection, seal-tightly to the rim  50  of the housing bottom part  16 . 
     It is also possible to have fewer or more than two support elements  112  on an end face edge side of the filter bellows  86 . 
     Instead of the elastic deformable support elements  112  also different kinds, for example, also rigid, support elements  112  and support element sections  114  can be provided which, for example, by means of an elastic connection with the filter bellows  86 , can be moved elastically relative to the latter. 
     The support element sections  114  can also be designed such that they project only past the end face edge sides or only past the end faces  92  of the filter bellows  86 . Also, support element sections  114  can be provided which project past the filter bellows  86  at the fold edges  98 . 
     The support elements  112  and the carrier plate  110 , instead of being provided as a two component part, can also be connected to each other in a different way. For example, they can be glued together, welded, clipped on or connected to each other by another type of clamping connection. 
     The support element sections  114  of a support element  112  can also be arranged obliquely opposite each other. In this case, also the corresponding oppositely positioned upper and lower projections  58  and  60  or  74  and  62  are arranged obliquely opposite to each other. 
     The support element sections  114 , instead of being arranged in pairs contiguously on a support element  112 , can also be arranged separate from each other on separate support elements  112 . 
     The carrier plates  110  can also be attached directly to the fold edges  98 . In this case, the lateral strips  106  are obsolete. 
     The carrier plates  110 , instead of being made of hard plastic material, can also be made of another, preferably hard, material. 
     Instead of the respective continuous carrier plates  110 , two or more separate carrier plates  110  can be arranged also on the end face edge sides of the filter bellows  86 . Each of the support elements  112  can then be arranged on its own carrier plate. 
     The support elements  112 , instead of being arranged on the carrier plate  110 , can also be attached to the respective lateral strip  106 . The carrier plates  110  are then obsolete. 
     It is also possible to provide more or fewer than six support glue beads  102 . The support glue beads  102 , instead of being on the clean air side  46 , can also be arranged on the raw air side  90 . It is also possible to provide support glue beads on the clean air side  46  as well as on the raw air side  90 . It may be possible to eliminate the support glue beads in certain circumstances. 
     While specific embodiments of the invention have been shown and described in detail to illustrate the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.