Abstract:
A filter ( 1 ) for filtering air, in particular an air filtering element ( 30 ), is provided for releasably fitting into a housing ( 10 ) which has a housing upper part ( 11 ) and in particular a central tube ( 20 ) fastened to the housing upper part ( 11 ), and also the housing for the filtering element ( 30 ) is provided. The filtering element has a seal ( 33, 34 ) for holding the filtering element ( 1 ) on the housing upper part ( 11 ) and the central tube ( 20 ) and for sealing the space ( 111 ) between the filtering element ( 30 ) and housing upper part ( 11 ) when the filtering element ( 30 ) is releasably fitted in the housing ( 10 ). The seal ( 33, 34 ) is connectable in a form-fitting manner to a sealing receptacle ( 111 ) of the housing upper part ( 11 ) and/or of the central tube ( 20 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of international application No. PCT/EP2012/061354 having an international filing date of Jun. 14, 2012 and designating the United States, the international application claiming a priority date of Jun. 15, 2011, based on prior filed German patent application No. 10 2011 106 502.8, the entire contents of the aforesaid international application and the aforesaid German patent application being incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a filter element which can be used for filtering air as well as to a housing for the filter element. The invention relates in particular to an air filter element, the housing for the air filter element and a filter assembly. 
     Filter elements are used for the filtration of fluid flows or gaseous media. They are, for example, used for the filtration of an air flow which is directed into a passenger compartment of a motor vehicle. In this connection, an air condition installed in the motor vehicle is in particular used to clean the outside air with regard to harmful substances, odors, etc. contained therein. Appropriate filters or cabin filters are in this context for example particle filters, odor filters or a combination thereof. 
     Furthermore, it is necessary to clean the air, which is directed to an internal combustion engine, from suspended particles. Such air filters, for example for trucks, construction machines, agricultural machines or even marine engines have to be designed in a reliable and robust manner, for they can be exposed to extreme mechanical loads during operation. 
     Normally, filters have a housing with a housing upper part, wherein the housing can be installed in an exchangeable filter element. Normally, the filter element has a tubular design and is frictionally engaged in the housing by means of a seal. 
     BACKGROUND OF THE INVENTION 
     DE 20 2007 004 475 U1 discloses a filter assembly in which a filter holder features at least one through-hole which serves to sealingly accommodate a filter element with an adapter collar. 
     For example foam made of polyurethanes (PU), also called PU foam, may be used as seal between housing or housing cover and filter element. Such foam made of polyurethanes, however, tends to relax after heat storage or due to exposure to high temperatures or temperature change. 
     In other words, the foam made of polyurethanes is force-free after heat storage, having lost elasticity. With filter elements frictionally engaged in the housing this is often the reason why greater critical movements between the individual parts occur with vibration excitation of the assembly from the parts housing with housing upper part, seal, and filter element. Such a vibration excitation can, for example, may occur during the operation of a motor vehicle into which the filter is installed. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to develop an improved air filter system, an improved air filter element, and an improved method for exchanging an air filter element. 
     Accordingly, the air filter system serves in particular for the intake air of internal combustion engines and comprises a housing and at least one air filter element for filtering air. The housing comprises a housing upper part with a clean air connection for discharging clean air cleaned in the air filter element from the housing and a sealing receptacle for a form-fitting connection with a cylindrical, in particular radially acting seal of the air filter element and for holding the air filter element. The sealing receptacle features a cylindrical, from the housing upper part into the interior space of the housing extending sealing surface which encloses the clean air connection and to which the seal of the air filter element can be applied, wherein at the sealing surface an annular projection protruding radially over the sealing surface is disposed which can be enclosed by the seal of the air filter element in a form-fitting manner. 
     The design of the sealing receptacle and of the seal guarantees in particular that the seal between housing and filter element does not become force-free during heat storage. 
     Advantageously due to the form fit, the filter element is securely held at the sealing receptacle without axial tension and cannot drop out of the receptacle. The seal and/or the housing shape or the sealing receptacle is, for example, designed in such a way that the form fit firmly and tightly holds the weight of the filter element loaded with particles. 
     The cylindrical shape of the sealing surface can preferably be designed as circular cylinder, elliptic cylinder or oval cylinder. The shape preferably forms the outer circumference of the air filter element. However, a polygonal, in particular rectangular shape can be chosen. 
     The air filter element is preferably designed as round filter element which comprises a zigzag folded, closed annular filter bellows that encloses a central interior space. The filter bellows is sealed on its axial ends by end disks, wherein one end disk features a central annular inlet or outlet opening at which a seal for separating raw from clean side is disposed. However, the air filter element can also feature two zigzag folded, closed annular filter bellows which are arranged one inside the other and connected with each other in such a way that one filter bellows can be radially flowed through from the outside to the inside and the other filter bellows can be radially flowed through from the inside to the outside. An air filter element with channels extending in flow direction is also conceivable which are formed by the fact that one smooth filter media layer and one undulated filter media layer are alternately placed on top of each other and that the channels formed therebetween are alternately closed. 
     As filter medium, a flat, porous filter medium can be used in each case, for example individual layers or combinations of cellulose media, glass fiber media, fleeces made of melt-blown or spun synthetic fibers. 
     In one embodiment, a through-flowable central tube is disposed in the housing in particular coaxially to and in continuation of the clean air connection for radially supporting the air filter element, wherein the central tube is either detachably installable into the housing or made in one piece with the housing upper part of the housing. Due to the arrangement of the central tube at the housing, a reliable support of the filter bellows in operation can be ensured without providing a means for support in the exchangeable filter element. This makes it possible to create a lighter and less expensive design of the filter element. Furthermore, a possibly existing secondary element disposed in the central tube is protected by the central tube against mechanical damages when the main filter element is installed or removed. 
     It is advantageous if the central tube is designed in such a way that a seal of the filter element is detachably attachable by interlocking at the central tube and/or that the central tube is detachably attachable by interlocking at the housing upper part. 
     In one embodiment, the annular projection protruding radially over the sealing surface is formed by an annular extending central tube collar which is firmly connected with the central tube, in particular made in one piece and/or in particular abutting the sealing surface. In this way, the undercut formed by the projection protruding over the sealing surface is easy to be manufactured. Furthermore, with an unchanged housing, different variations of the air filter element and central tube are possible, for example, by varying the length of central tube, air filter element and housing lower part mountable on the housing upper part different air filter systems with different filter surfaces can be realized with unchanged housing upper part. 
     In one embodiment, the sealing surface is formed radially outside or inside at a cylindrical collar projecting from the housing upper part into the interior space of the housing. 
     In one embodiment, the sealing receptacle is formed by an annular recess or groove in the housing enclosing the clean air connection, in particular in the housing upper part. 
     In one embodiment, the sealing surface is formed by a cylindrical, radial inner or outer wall of the recess or groove. 
     In one embodiment, the central tube is designed in such a way that a seal of the air filter element is detachably attachable to the central tube by interlocking. This can, for example, be realized by the fact that the seal of the filter element features an undercut in the shape of a radial annular groove which corresponds to the shape of the projection of the housing upper part or the central tube. For example, the undercut can be designed in such a way that the surface of the undercut is spaced apart from the projection when the filter element is installed in the housing. However, the undercut can, for example, also be designed in such a way that the surface of the undercut abuts the projection in a force-free manner or with a slight tensioning when the filter element is installed in the housing. 
     Preferably when installing the seal of the filter element, the seal is pushed over the projection and thereby radially tensioned and elastically deformed. Once the sealing surface of the seal exceeds the projection, it radially abuts the corresponding sealing surface of the housing. If an undercut is provided in the seal of the filter element, the projection engages into the undercut and thus realizes a form fit in an insertion direction between housing and filter element. However, the form fit can also be realized without the undercut, the projection at the housing permanently deforming the seal of the filter element when installed in such a way that a corresponding undercut is formed in the seal. 
     The central tube can also be detachably attachable by interlocking to the housing upper part. 
     The annular projection can form a closed ring or be interrupted regularly or irregularly. 
     In one embodiment, this comprises a main filter element and a secondary filter element, which is disposed downstream of the main filter element in flow direction of the air, wherein the main filter element is attachable on the outside of the central tube and the secondary filter element on the inside of the central tube when the air filter elements are detachably installed in the housing. 
     In one embodiment, the main filter element features a seal for holding the main filter element at the housing upper part and/or at the central tube as well as for sealing the space between main element and housing upper part when the main filter element is detachably installed in the housing, and the secondary filter element features a seal for holding the secondary filter element at the housing upper part and/or at the central tube as well as for sealing the space between secondary element and housing upper part when the secondary filter element is detachably installed in the housing. Preferably, both seals and their corresponding sealing surfaces and sealing receptacles feature a configuration according to the invention. 
     Preferably, a radially outwardly facing sealing surface, in particular for the main filter element, with a radially outwardly facing projection protruding over the sealing surface is provided at the housing. 
     Preferably, a radially inwardly facing sealing surface, in particular for the secondary filter element, with a radially inwardly facing projection protruding over the sealing surface is provided at the housing. 
     The invention relates furthermore to an air filter element for a detachable installation into a housing of an air filter system in particular according to the invention, wherein the air filter element comprises a seal for holding the air filter element at the housing and for sealing a space between the air filter element and the housing upper part when the air filter element is detachably installed in the housing, wherein the seal can be connected with the sealing receptacle of the housing upper part in a form-fitting manner. 
     The form fit is preferably realized in radial direction, specifically one part of the one component, preferably of the housing, engages radially into the other part, preferably the air filter element. A form-fitting connection is thereby realized which substantially resists axial movement of the filter element out of the seal seat, making removal more difficult. 
     The filter element is preferably an air filter element, for example for filtering combustion air of an internal combustion engine. However, the invention can also be used for all liquid filters, for example oil filters or fuel filters. 
     The proposed filter element is of easy construction and can be manufactured at low costs. At the same time; the design of the seal ensures that the seal between housing and filter element does not become force-free even with heat storage. 
     Due to the form fit, the filter element furthermore remains attached to the sealing receptacle without tensioning and cannot drop out. The seal and/or the housing shape or the sealing receptacle is, for example, designed in such a way that the form fit holds a weight of the filter element loaded with particles. 
     In embodiments of the filter element, the same is exclusively insertably connectable with a housing or the sealing receptacle. This simplifies installation and/or removal and avoids time-consuming bayonet or screw connections. 
     Preferably, the seal is designed in such a way that the filter element is detachably attachable to the central tube by interlocking. Thus, a particularly stable and fluid-tight connection can be realized even with oscillations or vibrations of the filter element and/or the housing. 
     In one embodiment, the seal forms an undercut into which protrudes the annular projection radially when the air filter element is detachably installed in the housing. 
     In one embodiment, the seal is designed in such a way that the filter element is detachably attachable to the housing upper part and/or to the central tube by interlocking, wherein in particular the interlocking of the filter element at the housing upper part and/or at the central tube creates a haptic perception during the detachable installation of the filter element. The ability to handle the filter element is thereby improved, for it becomes perceivable whether the filter element has been correctly and sealingly mounted. 
     In one embodiment, the seal of the filter element features an undercut in the shape of a radial annular groove which corresponds in particular to the shape of the projection of the housing upper part or the central tube. For example, the undercut can be designed in such a way that the surface of the undercut is spaced apart from the projection when the filter element is installed in the housing. However, the undercut can, for example, also be designed in such a way that the surface of the undercut abuts the projection in a force-free manner or with a slight tensioning when the filter element is installed in the housing. 
     The seal is preferably made at least partially of a casting compound, for example of a foamed PUR material. In particular the seal and/or the materials of the filter element or the housing are designed for operating temperatures between −30° C. and 90° C. The seal is preferably made of the same material in one piece with an end disk. Even furthermore, the end disk is preferably made of the casting compound and is sealingly connected with the filter bellows during casting. 
     Furthermore, the seal can feature at least one stiffening element for stiffening the seal. 
     In one embodiment, the seal has an inside sealing line on the interior side of the main filter element and a sealing web exterior side. Preferably, the seal has on the sealing web exterior side a plurality of notches which are disposed side by side along the circumference of the sealing web. 
     With the seals according to the invention, the exterior side of the sealing web is meant to generate a contact pressure to the in particular radially inside disposed sealing surface when the filter element is being mounted or is mounted in the housing. The exterior side of the sealing web is thereby preferably radially supported at, for example, an annular surface of the sealing receptacle. If such notches are provided, they reduce the pressure or mechanical resistance which is perceivable when mounting the filter element. 
     The invention relates furthermore to a method for exchanging an air filter element in an air filter system according to the invention in which the housing is opened, in particular by loosening a housing lower part from the housing upper part, subsequently an air filter element in particular according to the invention is removed from the housing and further subsequently an air filter element in particular according to the invention is mounted into the housing in such a way that the seal is connected with the sealing receptacle of the housing upper part in a form-fitting manner. 
     When installing the seal of the filter element it will preferably first of all be pushed over the projection and thereby radially tensioned and elastically deformed. Once the sealing surface of the seal has exceeded the projection, it then abuts radially the corresponding sealing surface of the housing. If an undercut is provided in the seal of the filter element, the projection engages into the undercut and thus realizes a form fit in insertion direction between housing and filter element. However, the form fit can also be realized without undercut if the projection at the housing permanently deforms the seal of the installed filter element in such a way that a corresponding undercut is formed in the seal. 
     With the described filter element, a form fit in the seal with respect to the housing is realized which has the advantage that the seal is kept in position even after aging or after high temperature impact or temperature change. Therefore, even at high oscillation loads which can, for example, occur during operation of a motor vehicle, critical movements between the individual components housing, seal and filter element are avoided. 
     The described interlocking of seal and central tube is realized by a special design of seal and central tube and generates therefore the form fit in the seal. The advantage of form fit is that the user receives a haptic feedback when mounting the filter element into the housing which is generated by snapping in of the form fits. 
     Another advantage of the described form fit in the seal is that the filter element cannot drop out of the housing, for example by the gravitational force, even if the housing has been opened for dismounting the filter element. 
     Further possible implementations of the invention comprise also not explicitly mentioned combinations of characteristics described previously or in the following with respect to the examples of an embodiment. In this context, the person of skill in the art will add also individual aspects as improvements or complements to the respective basic form of the invention. 
     Further embodiments of the invention are subject of the subclaims as well as of the examples of an embodiment of the invention described in the following. Furthermore, the invention is explained in detail based on examples of an embodiment with reference to the attached figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying Figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention. 
       Features of the present invention, which are believed to be novel, are set forth in the drawings and more particularly in the appended claims. The invention, together with the further objects and advantages thereof, may be best understood with reference to the following description, taken in conjunction with the accompanying drawings. The drawings show a form of the invention that is presently preferred; however, the invention is not limited to the precise arrangement shown in the drawings. 
         FIG. 1  is a three-dimensional view of an air filter according to one of the first examples of an embodiment; 
         FIG. 2  is a sectional view of the air filter in  FIG. 1  into which a filter element is detachably installed according to the first example of an embodiment; 
         FIG. 3  is a three-dimensional detailed view of the filter element according to the first example of an embodiment; 
         FIG. 4  is a detailed view of the filter element according to a second example of an embodiment; 
         FIG. 5  is a detailed view of a filter element according to a third example of an embodiment which is installed in the filter in  FIG. 1 ; 
         FIG. 6  is a three-dimensional view of a main filter element of the filter element according to a fourth example of an embodiment; 
         FIG. 7  is a three-dimensional detailed view of the main filter element of the filter element in  FIG. 6 ; 
         FIG. 8  is a detailed view of an air filter system according to a fifth example of an embodiment; and 
         FIG. 9  is a detailed view of an air filter system according to a sixth example of an embodiment. 
     
    
    
     In the Figures, the same reference numerals denote identical or similar components, unless otherwise stated Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
     DETAILED DESCRIPTION 
     In the following, the invention is described by means of a filter for the combustion air of an internal combustion engine such as, for example, a motor vehicle. Instead of filtering air, suitable filter elements can be designed, for example, for filtering another gaseous or liquid medium such as, for example, oil. 
     As shown in  FIG. 1 , the filter  1  comprises according to the first example of an embodiment a housing with a housing upper part  11  which is connected with a housing lower part  13  via a housing casing  12 . The housing  10  in  FIG. 1  is also a tripartite housing  10 . The housing lower part  13  is held by means of clamps  14  at the housing casing  12 . At the housing upper part  11  is disposed a raw air connection  15  with which raw air, unfiltered air, can be supplied into the housing  10  in the flow direction shown by the vertical block arrow. At the housing upper part  11  is also disposed a clean air connection  16  with which clean air, air cleaned from impurities, can be discharged from the housing  10  in the flow direction shown by the white block arrow. At the housing casing  12  or the housing lower part  13  is disposed a discharge valve  17  with which particles, that were contained in the raw air, can be discharged from the housing  10 . A cyclone filtration is, for example, disposed upstream. The housing  10  has several ribs  18  at the housing upper part  11 , the housing casing  12  and the housing lower part  13  which are, for example, meant to stiffen the housing  10  or the housing upper part  11 , the housing casing  12  or the housing lower part  13 . To provide a clear overview, not all ribs  18  in  FIG. 1  are provided with a reference numeral. 
       FIG. 2  shows a section through the housing  10  when a filter element  30  for filtering air is installed therein. The filter element  30  comprises a main element  31  and a secondary element  32  which can be mounted in the housing  10  independent of each other. The main element  31  is meant to filter first predetermined impurities from the air such as, for example, coarse dust. The secondary element  32  is meant to filter second predetermined impurities from the air such as, for example, fine dust. The secondary element  32  is disposed downstream of the main element  31  in flow direction of the air, as can be seen by the block arrows in  FIG. 1  and  FIG. 2 . 
     A clamped bracket  19  of a clamp  14  can be recognized in  FIG. 2  in order to connect the housing parts  12 ,  13  with each other. The housing  10  has furthermore a first central tube  20  at which the tubular main element  31  of the filter element  30  is disposed. The housing  10  has moreover a second central tube  21  at which the annular secondary element  32  of the filter element is disposed. The second central tube  21  has a grid-shaped pipe wall and is fluid-permeable. The first central tube  20  has also a grid-shaped pipe wall and is fluid-permeable, even if this cannot be clearly seen in  FIG. 2 . The first central tube is attached to the housing  10 , more precisely to its housing upper part  11 . This attachment of the first central tube  20  is realized by interlocking the first central tube  20  to the housing  10 , more precisely to its housing upper part  11 . The second central tube  21  can also be attached to the housing  10 , more precisely to its housing upper part  11 . The attachment of the central tubes  20 ,  21  as well as of the filter element  30  is described later in detail with regard to  FIG. 3  to  FIG. 5  which depict each in detail an area in  FIG. 2  identified by an arrow A. 
       FIG. 1  and  FIG. 2  show the case in which the housing lower part  13  is attached to the housing casing  12  by means of clamps  14  and brackets  19 . To install the filter element  30  or its main element  31  and/or its secondary element  32 , the clamps  14  can be declamped from the housing casing  12  so that the housing lower part  13  can be removed from the housing casing  12 . In this state, the filter element  30  or its main element  31  and/or its secondary element  32  can be pushed along the first central tube  20  and/or the second central tube  21  into the housing  10  opened from below. The housing lower part  13  can then be attached again to the housing casing  12  by means of the clamps  14 . 
       FIG. 3  shows the main element  31  disposed on the exterior side of the first central tube  20  as well as the secondary element  32  disposed on the interior side of the first central tube  20  and the second central tube  21 . The main element  31  has an annular seal  33  with an approximately T-shaped cross section. The seal  33  can also be called main element seal  33 . The seal  33  can seal the space between main element  31  and housing upper part  11 . The seal realizes a radial form fit. The material used for the seal  33  is foam made of polyurethanes (PU) or PUR foam. The seal  33  is disposed at one end of the annular main element  31 . More precisely, the seal  33  is disposed with the crossbeam  331  of the T-shaped seal at the end of the annular main element  31 . The line which is vertical with respect to the crossbeam, in other words the foot  332 , of the T projects into a recess  111  of the housing upper part  11 . The foot  332  of the T forms a projection and can also be called sealing web  332  of the seal  33 . Preferably, the seal  33  is attached to the main element  31 , for example by gluing. The seal  33  has support surfaces for radially supporting the main element  31  at the housing upper part  11 , wherein this means radial in the direction of the tube radius of the annular main element  31 . Thus, a radial seal is realized by means of the seal  33 . The recess  111  is one part of the space between main element  31  and housing upper part  11  which is to be sealed. 
     The seal  33  itself has furthermore a recess or undercut  333  into which projects one part of the first central tube  20 , a central tube collar  201 . The central tube collar  201  is a projection of the central tube  20 . The undercut  333  is disposed in  FIG. 3  at the transition from foot  332  to crossbeam  331  of the seal  33 . The undercut  333  of the seal  33  is adapted to the shape of one end of the central tube collar  201  facing away from the first central tube  20 . The undercut  333  and the central tube collar  201  realize, therefore, a form fit. Moreover, the recess  111  of the housing upper part  11  is disposed in such a way that the end of the central tube collar  201  facing away from the first central tube  20  engages into the undercut  333  of the seal  33  when the main element  31  of the filter element  30  is installed in the housing  10  at the first central tube  20 . As a result, the first central tube  20  and the seal  33  engage with each other. To be even more precise, the central tube collar  201  of the first central tube  20  and the undercut  333  of the seal  33  engage with each other. If during maintenance of the cabin filter  1  an old filter element  30  is dismounted from the housing  10  by loosening the interlocking between the first central tube  20  and undercut  333 , a new filter element  30  can then be detachably installed. The engagement of the central tube collar  201  into the undercut  333  or its interlocking can be haptically perceived as a snapping sound by the maintenance staff. As a result, the maintenance staff receives a feedback that the main element  31  of the filter element  30  is installed in the correct and therefore safe position in the housing  10 . 
     As can also be seen in  FIG. 3 , the central tube  20  has moreover at its central tube end  202  facing the housing upper part  11  a protrusion  203  which projects to the interior side of the first central tube  20  and to the exterior side of the first central tube  20 . The exterior side of the first central tube  20  faces the seal  33 . The interior side of the first central tube  20  faces the secondary element  32  and the second central tube  21 . On the one hand, the protrusion  203  projects into a corresponding undercut  112  of the housing upper part  11  which exists in a housing upper part web  113  of the housing  10 . On the other hand, the protrusion  203  projects into a corresponding undercut  341  of a seal  34  of the secondary element  32 . The shape of the protrusion  203  is adapted to the shape of the undercut  112 . Thus, the protrusion  203  and the undercut  112  can realize a form fit when the first central tube  20  is installed in the housing  10 , as shown in  FIG. 3 . The shape of the protrusion  203  is also adapted to the shape of the undercut  341 . Thus, the protrusion  203  and the undercut  341  can also realize a form fit when the first central tube  20  is installed in the housing  10 , as shown in  FIG. 3 . As a result, the first central tube  20  is interlocked with the housing upper part  11  as well as with the seal  34  of the secondary element  32 . As described above, this interlocking is detachable. 
     The seal  34  can seal the space between secondary element  32  and housing upper part  11 . The seal  34  can also be called secondary element seal  34 . Foam made of polyurethanes (PU) or PUR foam can be used as material for the seal  34 , the same as for seal  33 . The seal  34  is disposed at one end of the annular secondary element  31  and abuts a housing upper part projection  114  which is disposed on the interior side of the clean air connection  16 . The housing upper part projection  114  forms an axial stop for the seal  34  or the secondary element  32  connected therewith, for example by gluing. Axial means in the direction of the tube axis of the annular secondary element  32 . 
       FIG. 4  shows a similar detailed view, as in  FIG. 3 , to explain a second example of an embodiment of the filter element  30 . The filter  1  and the filter element  30  of the second example of an embodiment are largely identical with the filter  1  and the filter element  30  of the first example of an embodiment. Therefore, only the differences between the first and the second example of an embodiment are described in the following. For the rest, reference is made to the description of the first example of an embodiment. 
     In contrast to the first example of an embodiment, the filter element of the second example of an embodiment has at least one stiffening ring  334  in the seal  33 . As an example,  FIG. 4  shows three stiffening rings  334  which are disposed side by side in the seal  33 . The ring of the stiffening rings  334  in  FIG. 4  is disposed essentially parallel in relation to the crossbeam  331  of the seal  33 . The stiffening rings  334  can be made of a harder material than that of the seal  33  such as, for example, hard plastics or metal. 
     Thanks to one or more stiffening rings, the seal  33  has a higher stiffness against bending. 
       FIG. 5  shows a similar detailed view, as in  FIG. 3 , to explain a third example of an embodiment of the filter element  30 . The filter  1  and the filter element  30  of the third example of an embodiment are largely identical with the cabin filter  1  and the filter element  30  of the first example of an embodiment. Therefore, only the differences between the first and the third example of an embodiment are described in the following. For the rest, reference is made to the description of the first example of an embodiment. 
     In contrast to the first example of an embodiment, the seal  33  of the filter element  30  of the third example of an embodiment is a two-component seal. Moreover, the filter element of the third example of an embodiment has two foot prong projections  335  at the foot  332  or the sealing web  332  of the seal  33  at the side facing the first central tube  20  or the secondary element  32 . Furthermore, at the foot  332  of the seal  33  is realized a duckfoot bend projection  336  on the side facing the first central tube  20  or the secondary element  32 . The dimensions of the complete foot  332  are adapted to the recess  111  of the housing upper part  11 , as shown in  FIG. 5 . As a result, the foot  332  of the third example of an embodiment shown in  FIG. 5  can also realize a form fit with the recess  111  of the housing upper part  11 , even if the form fit is in this case not as distinctive as it is with the form fits of the first and second examples of an embodiment. The foot  332  can interlock with the recess  111  of the housing upper part  11  which causes a haptic perception when installing the filter element  30  in the housing  10 . 
       FIG. 6  shows a main element  31  of the filter element  30  according to a fourth example of an embodiment in a three-dimensional overall view. The filter  1  and the filter element  30  of the fourth example of an embodiment are largely identical with the filter  1  and the filter element  30  of the first, second or third example of an embodiment. Therefore, only the differences between the fourth and the other examples of an embodiment are described in the following. For the rest, reference is made to the description of the first to the third example of an embodiment. 
     The main element  31  in  FIG. 6  is a hollow-cylindrical body which features at its external circumference several rings  312  which, for the sake of clarity, do not all have a reference numeral in  FIG. 6 . At the bottom in  FIG. 6 , the main element  31  is completed by a base ring  313 . The seal  33  is disposed at the end of the main element  31  opposing the base ring  313 . The seal  33  has an inside sealing line  337  on the interior side of the main element  31  and a sealing web exterior side  338  of the foot  332  or the sealing web  332 . Up to this point, the main element  31  is not different from the main element  31  of the first, second or third example of an embodiment. 
     In this example of an embodiment, however, the seal  33  has in contrast to the first, second or third example of an embodiment a plurality of notches  339  which, for the sake of clarity, do not all have a reference numeral in  FIG. 6 . The notches  339  are disposed side by side along the circumference of the sealing web  332 .  FIG. 7  shows the shape of the seal  33  with the notches  339  in enlarged detail. 
     The sealing exterior side  338  of the seal  33  is meant to generate a contact pressure to the sealing surface when the main element  31  is being installed or installed in the housing  10 . The notches  339  reduce the pressure or the mechanical resistance which can be perceived during the installation of the main element  31 . Because of the notches  339 , the seal  33  is in fact non-confined or sandwiched in the recess  111  of the housing upper part. Instead, the sealing web  332  of the seal  33  of the main element  31  can escape into the free spaces formed by the notches  339  or fill them by deforming. As a result, a sufficient sealing effect can, however, be realized by means of the seal  33 . However, when mounting the main element  31 , less mounting force has to be applied than without notches  339 . The reason for this is that less material is to be compressed radially. 
     In  FIGS. 8 and 9  are shown further examples of an embodiment similar to the examples of an embodiment according to  FIGS. 3 and 4 . The embodiments according to  FIGS. 8 and 9  are different from the examples of an embodiment according to  FIGS. 3 and 4  insofar that the first central tube  20  is firmly and undetachably connected with the housing upper part  11 . This can, for example, be realized by a one-piece design in plastic injection molding or by a welded joint. A radially inwardly extending bulge  203  each which protrudes to the interior side of the first central tube  20  is disposed at the central tube  20 . The interior side of the first central tube  20  faces the secondary element  32  and the second central tube  21 . The bulge  203  engages into a corresponding undercut  341  of a seal  34  of the secondary element  32  which is realized in the shown examples of an embodiment by an elastic deformation of the seal  34 . However, the undercut  341  can also be provided as radial groove in the seal  34 , as described above. 
     The seal  34  can seal the space between secondary element  32  and housing upper part  11 . The seal  34  can also be called secondary element seal  34 . Foam made of polyurethanes (PU) or PUR foam can be used as material for the seal  34 , the same as for seal  33 . The seal  34  is disposed at one end of the annular secondary element  32  and abuts a housing upper part projection  114  which is disposed on the interior side of the clean air connection  16 . The housing upper part projection  114  forms an axial stop for the seal  34  or the secondary element  32  connected therewith, for example by gluing. Axial means in the direction of the tube axis of the annular secondary element  32 . Radially outside of the central tube  20  is disposed an annular web  350  with a circular cylindrical sealing surface  351  at the housing upper part  11  which features at its end extending into the interior space of the housing  10  a radially outwardly extending bulge  203  which engages into an undercut  333  of the seal  33 .  FIG. 8  shows two variants of the undercut, wherein the cross section of the smaller undercut corresponds to the cross section of the bulge  203  and thus tightly encloses the surface of the undercut  333  when the main filter element  31  is installed. The dashed representation of the variant of the undercut  333  features in inner surface which is spaced from the surface of the undercut  333  when the main filter element  31  is installed in such a way that a gap is formed between bulge  203  and undercut  333 . In the variant shown in  FIG. 9 , the undercut  333  is formed by an elastic deformation of the seal  33  (dotted representation). 
     All examples of an embodiment of the filter  1 , the filter element  30  and the housing  10  described above can be used individually or in all possible combinations. In this connection, the following characteristics are conceivable. 
     Other geometries and dimensions for the filter element  30  as those depicted and described can be chosen insofar as the conditions at the installation site of the filter element  30  have to be taken into account and a detachable, haptically perceivable installation of the filter element  30  in the housing  10  is possible. 
     Instead of an undercut in the seals  33 ,  34 , the seal  33  and/or the seal  34  can also feature a projection or a bulge which engages into an undercut of the housing upper part  11  or of the first central tube  20 . However, the variant of the form fit described in the examples of an embodiment between the seals  33 ,  34  and housing  10  is preferred for stability reasons. 
     Foil can also be used as material for the seal  33 . Likewise, foil can also be used as material for the seal  34 . 
     The number of notches  339  at the seal  33  can be chosen as appropriate. Only one notch  339  can also be provided. As an alternative, more notches  339  than shown in  FIG. 6  can also be provided. 
     In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.