Patent Publication Number: US-2021172409-A1

Title: Filter and Filter Cartridge

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/427,177, filed 30 May 2019, which is a continuation application of U.S. patent application Ser. No. 15/382,700, filed 18 Dec. 2016, now U.S. Pat. No. 10,377,471, which is a continuation of international application No. PCT/EP2015/063537 having an international filing date of 17 Jun. 2015 and designating the United States, the international application claiming a priority date of 18 Jun. 2014, based on two prior filed German patent applications Nos. 20 2014 004 897.5 and 20 2014 004 894.0, and a priority date of 11 Nov. 2014, based on prior filed German patent application No. 20 2014 008 899.3, the entire contents of the aforesaid international application and the aforesaid three German patent applications being incorporated herein by reference to the fullest extent permitted by the law. 
     This application is a continuation of U.S. patent application Ser. No. 16/439,847, filed 13 Jun. 2019, which is a continuation application of U.S. patent application Ser. No. 15/382,701, filed 18 Dec. 2016, which is a continuation of international application No. PCT/EP2015/063542 having an international filing date of 17 Jun. 2015 and designating the United States, the international application claiming a priority date of 18 Jun. 2014, based on two prior filed German patent applications Nos. 20 2014 004 897.5 and 20 2014 004 894.0, and a priority date of 11 Nov. 2014, based on prior filed German patent application No. 20 2014 008 899.3, the entire contents of the aforesaid international application and the aforesaid three German patent applications being incorporated herein by reference to the fullest extent permitted by the law. 
     This application is a continuation of U.S. patent application Ser. No. 16/429,903, filed 3 Jun. 2019, which is a continuation application of U.S. patent application Ser. No. 15/382,702, filed 18 Dec. 2016, now U.S. Pat. No. 10,337,472, which is a continuation of international application No. PCT/EP2015/063592 having an international filing date of 17 Jun. 2015 and designating the United States, the international application claiming a priority date of 18 Jun. 2014, based on two prior filed German patent applications Nos. 20 2014 004 897.5 and 20 2014 004 894.0, and a priority date of 11 Nov. 2014, based on prior filed German patent application No. 20 2014 008 899.3, the entire contents of the aforesaid international application and the aforesaid three German patent applications being incorporated herein by reference to the fullest extent permitted by the law. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention concerns a filter, in particular for internal combustion engines, for filtering a fluid, in particular air, as well as a filter cartridge, in particular for such a filter. 
     In particular in construction and agricultural machines, engine air filtration is becoming more and more important. On the one hand, more and more efficient air filters are used because higher engine performances and stricter emissions standards require an increased air throughput through the engine. On the other hand, the number of component groups that are installed by default, such as air-conditioning devices, increases. This reduces the available installation space in the vehicle. Finally, there is the desire to design vehicles smaller and more lightweight which also is done at the expense of the available installation space. 
     When used in construction or agricultural machines, vibrations are transmitted onto the filter in different operating states. It is therefore a primary goal to arrange the main filter cartridge in the filter so as to be vibration-resistant. But also for a secondary filter cartridge arranged downstream a secure seat is important during operation. 
     It is an object of the invention to provide a filter, in particular for internal combustion engines, for filtering a fluid, in particular air, that even under extreme operating conditions exhibits high reliability. 
     In the aforementioned application situations, it is necessary to design the overall construction of a filter cartridge as robust as possible. Protection of the filter medium in operation is a primary concern. In the configuration of such filter cartridges, also possible handling preferences upon change of the filter cartridge or for servicing are be taken into consideration. 
     It is thus a further object of the invention to provide a filter cartridge that even for extreme operating conditions and handling preferences ensures a reliable filtering function. 
     For use in vehicles that provide little installation space for filtration purposes, in general so-called compact air filters or Z-filters with alternatingly closed channels that can be flowed though inline and that are formed by corrugated and flat layers of a filter medium positioned on top of each other are employed. This means that the inflow direction and the outflow direction extend parallel and substantially are aligned with each other. Often, the air which is flowing out of the air filter, depending on the application situation, must be deflected after having flowed through the filter. For this purpose, for example, a pipe elbow can be connected downstream to the filter housing. However, such downstream components increase again the required installation space of the entire arrangement. 
     It is an object of the invention to provide a filter, in particular for internal combustion engines, for filtering a fluid, in particular air, that requires less installation space for identical filter performance. 
     The aforementioned object is solved respectively by different aspects of the embodiments of a filter to be described in the following, in particular for internal combustion engines, for filtering a fluid, in particular air, as well as a filter cartridge, in particular for such a filter. As can be seen in the embodiments, the different aspects can be provided advantageously individually or combined in embodiments wherein, in case of combination, individual aspects enhance the advantages of other aspects and a synergistic interaction leads to an advantageous product. 
     SUMMARY OF THE INVENTION 
     This object is solved by a filter cartridge for a filter, in particular for internal combustion engines, for filtration of a fluid, in particular air, comprising a prismatic basic shape, wherein the parallel arranged base face and top face have a polygonal basic shape, comprising a first lateral face and a second lateral face neighboring the first lateral face, wherein the first lateral face is an inflow surface and is substantially positioned perpendicular to the neighboring second lateral face, and wherein a third lateral face that is an outflow surface and is positioned at an angle relative to the first lateral face that is greater than 10° and smaller than 80°. 
     This object is further solved by a filter comprising a filter cartridge according to the invention, wherein the main inflow direction of the filter and the main outflow direction of the filter are positioned at an angle of more than 30°, in particular more than 45°, preferably more than 60°, especially preferred 90°, relative to each other. 
     This object is further solved by a filter comprising a filter cartridge according to the invention, wherein the main inflow direction of the filter is parallel and displaced relative to the main outflow direction of the filter. 
     This object is further solved by a filter comprising a main filter cartridge and a secondary filter cartridge, wherein the main filter cartridge and/or the secondary filter cartridge are/is embodied in accordance with the invention. 
     Further embodiments of the invention are disclosed in the dependent claims. 
     The filter cartridge according to the invention comprises a prismatic basic shape wherein the parallel arranged base face and top face have a polygonal basic shape. The filter cartridge comprises in accordance with the polygonal basic shape of the base and top faces at least three, preferably four lateral faces. A first lateral face neighbors a second lateral face. The first lateral face as well as optionally also the second lateral face are inflow surfaces. Alternatively, the first lateral face and preferably additionally the second lateral face are outflow surfaces. The first lateral face and the second lateral face are positioned substantially perpendicular to each other. Substantially encompasses an angle range of 80° to 100°. A third lateral face is preferably positioned substantially opposite the first lateral face and is spaced apart therefrom in the flow direction and, in contrast to the first and the second lateral faces, is an outflow surface when the first and the second lateral faces are inflow surfaces or the third lateral face is an inflow surface when the first and preferably the second lateral faces are outflow surfaces. The third lateral face is positioned relative to the first lateral face at an angle that is smaller than 80° and greater than 10° and is in particular between 70° and 20°. Particularly preferred are angles such as 45° or 60°. 
     In the following, the aforementioned first alternative is described in which the first and optionally also the second lateral faces are inflow surfaces and the third lateral face an outflow surface. The explanations apply analogously to the reverse situation of the second alternative in which the first and the second lateral faces are outflow surfaces and the third lateral face an inflow surface. For the described filter cartridge geometry, in general one lateral face provides an inflow surface where the main fluid flow impacts first. The second lateral face that is optionally additionally used as an inflow surface assists entry of the fluid into the filter cartridge and increases the inflow surface. The fluid flows out through the third lateral face. Since the third lateral face is slantedly positioned relative to the first lateral face, for a filter with such a filter cartridge there is the possibility to determine the outflow direction already within the filter housing in the desired way. An external deflection outside of the filter housing, for example, by means of a curved outflow socket, is no longer necessary. Thus, two advantages are achieved. On the one hand, a significantly increased inflow surface results which possibly provides for better utilization of the filter capacity. At the same time, the filter geometry according to the invention permits a deflection of the main flow direction within the filter cartridge so that the installation space which is required by the filter is optimized, i.e., can be reduced. 
     The main flow direction is understood as the average direction of the flow at one position of the path through the filter system and/or filter element. For example, the main flow direction within a cyclone block comprised of a plurality of inline cyclone cells is defined by the axial extension of the cyclone cells. When impacting on the inflow surface of a filter element, the main flow direction is substantially perpendicular to this inflow surface and in other respects is defined by the path of least resistance which, passing through a filter element, is defined in case of a folded filter body by the course of the fold pockets between raw side and clean side and in case of a filter body with alternatingly closed channels by the extension of the channels. 
     The secondary filter cartridge according to the invention is designed to be inserted downstream of a main filter cartridge into a filter. It comprises an inflow surface, an outflow surface, and an outflow direction. Moreover, the secondary filter cartridge comprises a filter body which can be flowed through along the outflow direction and a filter cartridge frame supporting the filter body. The filter cartridge frame comprises a seal for separating a filter interior of a filter in clean side and raw side as well as an adhesive connection between the filter body and the filter cartridge frame. The separate configuration of seal and adhesive connection enables a substantially more stable configuration of the adhesive connection. 
     In a preferred embodiment of the invention, the seal is designed to be acting radially to the main flow direction and is extending circumferentially about the secondary filter cartridge in particular substantially perpendicular to the main flow direction. The seal can comprise a cellular rubber. 
     According to one embodiment, the polygonal basic shape is a triangle, preferably a quadrangle or a pentagon. The resulting prismatic basic shape, depending on the application situation, can also have slightly curved lateral faces. Also, inflow or outflow can be realized through the base face and/or top face of the prism. 
     Preferably, the polygonal basic shape is a quadrangle wherein, further preferred, a fourth lateral face is provided which is positioned preferably opposite and parallel to the second lateral face. In this context, it is particularly preferred that the second and fourth lateral faces as well as the base and top faces are oriented perpendicular to the first lateral face. Further preferred, base face and top face are oriented perpendicular to the third lateral face. 
     The first lateral face is preferably the only inflow surface. The third lateral face is preferably the only outflow surface. 
     A preferred further embodiment of the invention provides that the filter cartridge comprises a filter body with a zigzag-shaped folded filter medium. Such a folded filter medium comprises a large surface area and can be brought relatively inexpensively into the desired basic shape. 
     One embodiment provides that the folds have outer fold edges, which are positioned on the first lateral face or define the latter, and inner fold edges (i.e., positioned at the clean side) that are positioned opposite the outer fold edges and positioned on the third lateral face or define the latter. Accordingly, inflow into the filter cartridge is substantially realized perpendicular to the fold edges. The same holds true for the outflow wherein the third lateral face is positioned slantedly relative to the first lateral face. 
     In the described zigzag-shaped folding of the filter medium, the folds have at least one, in general two, end face(s) which are formed by the zigzag-shape extension of the edges of the folded filter medium and either are located on the second (and fourth) lateral face or on the base face and top face of the prismatic filter body. Accordingly, inflow of the filter medium is realized through the fold edge and optionally also through the end face of the fold. The end faces in this case must be designed such that no direct inflow to the clean side of the filter cartridge can take place, for example, by one-sided adhesive connection at the clean side. 
     A particularly preferred embodiment of the invention provides that the depth of neighboring fold edges or folds differs and in particular every other fold has the same depth. The different depths of neighboring fold edges enables a slanted configuration of the filter cartridge and, in this way, the embodiment of the desired geometry. 
     A particularly preferred further embodiment provides in this context that the depth of neighboring folds is different, in particular such that the fold heights at the second and fourth lateral faces are different wherein the fold height between second and fourth lateral faces changes continuously, in particular linearly. 
     A particularly preferred embodiment of the invention provides that the folds each have two end faces. One end face of the folds is positioned on the second lateral face. The other end face of the folds is positioned on a fourth lateral face. The second lateral face is larger than the fourth lateral face. The fourth lateral face comprises more fold edges than the second lateral face, in particular twice as many. By doubling the fold edges on the fourth lateral face, it is possible to produce or adjust the angle between the third lateral face and the first lateral face. 
     An alternatively preferred embodiment of the invention provides that the folds each have two end faces. One end face of the folds is positioned on the base face and another end face on the top face. Base face and top face are preferably congruent. In this context, the second and fourth lateral faces are preferably each formed by the first or last fold of a fold bellows. Preferably, the fold heights on the second and fourth lateral faces are different wherein the fold height between second and fourth lateral faces changes continuously, in particular linearly. In this way, it is possible to produce and adjust the angle between the third lateral face and the first lateral face. Preferably, a right angle is formed, respectively, between the first lateral face, which is further preferred the inflow surface, and the second as well as the fourth lateral faces. As an alternative to the described fold arrangement, a layered configuration of alternating flat and corrugated layers can be also used and between them channels are formed that can be alternatingly closed off by means of adhesive. The angle which is formed between first and third lateral faces can be realized in this context by layering of layers of different lengths wherein the layer length between second and fourth lateral faces changes (dis-)continuously, in particular linearly. 
     The concept of the invention is also realized in a filter with a filter cartridge according to the invention. In such a filter it can be provided that the main inflow direction of the filter and the main outflow direction of the filter are positioned relative to each other at an angle of more than 30°, in particular more than 45°, preferably more than 60°, and especially 90°. The main outflow direction is understood as the average direction of the flow between outflow surface of the main filter element and inflow surface of the secondary filter element, if present, or the outflow opening out of the filter housing. 
     Alternatively, it can be provided that the main inflow direction of the filter is parallel and displaced relative to the main outflow direction of the filter. As a result of the slanted position, realized by the filter cartridge, of the outflow surface of the filter cartridge relative to the inflow surface of the filter cartridge, within the filter a deflection of the main inflow direction toward the main outflow direction can be realized such that the main outflow direction is positioned at an angle relative to the main inflow direction or is displaced by a certain length. This can be realized simply and inexpensively by appropriate outflow openings at the filter. 
     Another also advantageous further development of the invention provides a filter with a primary filter element according to the invention and a secondary filter element according to the invention. By configuring the primary and secondary filter cartridges with a prismatic basic shape, a particularly great freedom of deflection of the fluid flows in such a filter is created. 
     In a preferred embodiment, the filter cartridge, in the area of the outflow surface, comprises a seal for separating a filter interior in clean side and raw side and a spacer structure for determining a spacing between the filter cartridge frame and a filter cartridge that can be positioned downstream, in particular a secondary filter cartridge. By means of the spacer structure, a prior determined spacing between filter cartridge, that can be, for example, a main filter cartridge, and a downstream further filter cartridge, that is, for example, a secondary filter cartridge, can be maintained even under extreme conditions of use, for example, vibrations. This ensures that the further filter cartridge cannot move out of its designated position and thereby lose its sealing function and securing function. 
     In one embodiment of the invention it is provided that the spacer structure is arranged within the seal, in particular at several locations along the seal, and in particular at the circumference of the outflow surface. For such a configuration it can be provided that the outer circumference perpendicular to the main flow direction of the further filter cartridge follows in shape and size the seal and therefore provides a support surface for the spacer structure. 
     An advantageous embodiment of the invention provides that the normals of the outflow surface or/and of a sealing surface of the seal are positioned relative to the main flow direction, in particular the main flow direction within the filter cartridge, at an angle between 5° and 45°, in particular an angle of 24°±10° and in particular an angle of 24°±5°. In the regularly occurring situation of an inflow surface that is positioned perpendicular to the main flow direction (or first lateral face) this means that the outflow surface (or third lateral face) is slanted by the aforementioned angle relative to the inflow surface. An outflow surface which is angled in such a way to the main flow direction has a larger surface area compared to an outflow surface that is not angled. Generally, the angled positioning of the outflow surface or/and of the sealing surface provides the possibility of effecting a deflection of the main flow direction already within the filter cartridge or directly downstream of the filter cartridge. This saves installation space and improves the pressure loss within the filter as a result of shorter flow distances. 
     Advantageously, the spacer structure is formed integrally with the seal, in particular is molded with the seal. This enables, for example, the use of the same material and manufacture of the spacer structure in the same processing step used for the seal. 
     A particularly preferred embodiment provides that the spacer structure is designed to exert in the installed state of the filter cartridge a clamping or holding force on a further filter cartridge which can be arranged downstream in the main flow direction, in particular a secondary filter cartridge, wherein the force presses the further filter cartridge into its installation position. This can be achieved, for example, in that the spacer structure is made of an elastic material. When the filter cartridge is then inserted after the further filter cartridge, already the insertion can exert a force on the further filter cartridge by suitable selection of an appropriate geometry. This improves, in turn, the reliability of the filter because the further filter cartridge is permanently exposed to a force which keeps it in its designated position. 
     In a preferred embodiment of the invention, the seal acts axially in the direction of the main flow direction. This means that the force which is to be applied for the sealing action is extending parallel to the outflow direction. 
     In an embodiment of the invention, the filter body is embodied as a filter bellows, in particular with variable fold height. By means of the variable fold height, for example, an outflow surface that is extending at a slant to the main flow direction can be realized. A configuration of a zigzag-shaped folded filter body is understood as variable fold height when the fold heights across a length of the folded bellows defined by the sequence of fold edges is not constant but changes continuously or discontinuously or differs in neighboring sections. Preferably, the height of the folds changes continuously, and further preferred linearly, across the length of the bellows. 
     In an advantageous embodiment, a secondary filter cartridge can be arranged downstream of the main filter cartridge. The filter cartridge frame of the main filter cartridge or the main filter cartridge has preferably in the area of the outflow surface a seal for separating the filter housing in clean side and raw side as well as a spacer structure for determining a spacing between the filter cartridge frame and the secondary filter cartridge. 
     In an advantageous embodiment of the filter, it is provided that the filter housing has an insertion direction which is substantially perpendicular to the main flow direction, in particular the main flow direction within the main filter cartridge. This enables after insertion of the secondary filter cartridge an insertion of the main filter cartridge in such a way that the secondary filter cartridge is blocked against moving out of its designated seat. 
     In a preferred embodiment, it is provided that the secondary filter cartridge comprises a seal for contacting the filter housing wherein the seal is in particular designed to be radially acting relative to the flow direction of the secondary filter cartridge. It is thus not required to provide a sealing surface on the filter housing in axial extension of the main flow direction of the secondary filter cartridge. Instead, already upon insertion into the designated seat, the seal is effective perpendicular to the main flow direction against the filter housing. 
     According to an embodiment of the invention, the secondary filter cartridge comprises a secondary filter body and a secondary filter cartridge frame supporting the secondary filter body. In this context, it can be provided that, in the inserted state of the main filter cartridge and of the secondary filter cartridge, the spacer structure of the main filter cartridge contacts the seal or the secondary filter cartridge frame of the secondary filter cartridge. Accordingly, vibrations that occur in situations of use cannot cause the secondary filter cartridge to move out because the spacer structure of the main filter cartridge blocks the space through which the secondary filter cartridge has been inserted. 
     In a further embodiment which is also an invention on its own, the filter cartridge according to the invention for a filter for filtering a fluid, in particular air, in particular for an internal combustion engine, comprises an inflow surface, an outflow surface, a main flow direction, a filter body as well as a filter cartridge frame supporting the filter body. The filter cartridge frame comprises in the area of the outflow surface a seal for separating a filter interior of a filter in clean side and raw side and, in the area of the inflow surface, comprises an edge protection that is surrounding circumferentially the filter frame. The edge protection connects the filter body and the filter cartridge frame to each other. According to the invention, this provides thus a synergistic interaction of edge protection and of the connection of the filter body with the filter cartridge frame. This is particularly advantageous in the usual practice in the field of construction machines where filter cartridges are cleaned by blowing out or tapping. 
     In particular, in particularly preferred embodiment it is provided that the edge protection forms a seal-tight connection for the filtered fluid between filter body and filter cartridge frame. Accordingly, the connection of filter body and filter cartridge frame forms at the same time the sealing action of the filter body relative to the filter cartridge frame. 
     The filter cartridge according to the invention comprises preferably in the area of the inflow surface an edge protection that is extending externally about the filter frame and/or about the inflow surface and/or the first lateral face. The edge protection connects the filter body and the filter cartridge frame with each other or the edge protection is resting inwardly on the filter cartridge frame, in particular with form fit. According to the invention, this provides thus a synergistic interaction of edge protection and of the connection of the filter body with the filter cartridge frame. This is particularly advantageous in the usual practice in field of construction machines where filter cartridges are cleaned by blowing out or tapping. 
     In particular, in particularly preferred embodiments it is provided that the edge protection forms a seal-tight connection for the filtered fluid between filter body and filter cartridge frame, in particular, by form-fit embedding of filter body and filter cartridge frame by means of conventional potting compounds used in the field of filtration, such as polyurethane. Accordingly, the connection of filter body and filter cartridge frame forms at the same time the sealing action of the filter body relative to the filter cartridge frame. 
     In an inventive further embodiment of the invention, it is provided that the filter cartridge frame in the area of the edge protection has cutouts that are filled out by the edge protection. This enables a particularly good mechanical connection between the edge protection and the filter cartridge frame. 
     Particularly preferred, the cutouts extend perpendicular to the main flow direction. It is thus not required to provide in the filter cartridge frame an angled member for the edge protection. Instead, the edge protection can be placed completely about the inflow-side edges of the filter cartridge frame and, at the same time, can penetrate the filter cartridge frame through the cutouts and, in this way, provide a particularly stable mechanical connection. 
     In a further embodiment of the invention, the filter cartridge frame surrounds the filter body. In addition, the filter cartridge frame determines the main flow direction of the filter cartridge. 
     A particularly preferred embodiment of the invention provides that the edge protection partially penetrates the filter body. In this way, after hardening of the edge protection, a particularly stable connection is provided between the edge protection and the filter cartridge frame, on the one hand, and the filter cartridge frame and the filter body, on the other hand. 
     A further embodiment of the invention provides that the filter body is embodied as a filter bellows, in particular with variable fold height. The variable fold height enables an angled position between outflow surface and main flow direction. 
     According to the invention, it can be provided that the outflow surface or/and a sealing surface of the seal is positioned relative to the main flow direction at an angle between 5° and 45°, in particular an angle of 24°±10° and in particular an angle of 24°±5°. The angle between main flow direction and outflow surface or sealing surface enables a particularly compact configuration because a deflection of the main orientation can be realized already within the filter housing that receives the filter cartridge. 
     A particularly preferred embodiment of the invention provides that the connection between filter body and filter cartridge frame is an adhesive connection which is formed as one part together with the edge protection. In particular, the adhesive connection and the edge protection can be formed of the same material, in particular of a polyurethane. This enables technical advantages in particular during manufacture because the adhesive connection between the filter cartridge frame and the filter body, the sealing action between filter body and filter cartridge frame, and the edge protection can be produced in one processing step and a particularly robust embodiment is created at the same time. 
     According to the invention, the edge protection is designed such that, when tapping the filter cartridge, the resulting forces can be at least partially absorbed by the edge protection and in particular damage of the filter cartridge can be prevented. 
     The filter cartridge according to the invention comprises a prismatic basic shape wherein the parallel arranged base face and top face have a polygonal basic shape. The filter cartridge comprises in accordance with the polygonal basic shape of the base and top faces at least three, preferably four lateral faces. A first lateral face neighbors a second lateral face. The first lateral face as well as preferably also the second lateral face are inflow surfaces. Alternatively, the first lateral face and preferably additionally the second lateral face are outflow surfaces. The first lateral face and the second lateral face are positioned substantially perpendicular to each other. Substantially encompasses an angle range of 80° to 100°. A third lateral face is preferably positioned substantially opposite the first lateral face and is spaced apart therefrom in the flow direction and, in contrast to the first and the second lateral faces, is an outflow surface when the first and the second lateral faces are inflow surfaces or the third lateral face is an inflow surface when the first and preferably the second lateral faces are outflow surfaces. The third lateral face is positioned relative to the first lateral face at an angle that is smaller than 80° and greater than 10° and is in particular between 70° and 20°. Particularly preferred are angles such as 45° or 60°. 
     In the following, the aforementioned first alternative is described in which the first and preferably also the second lateral faces are inflow surfaces and the third lateral face an outflow surface. The explanations apply analogously to the reverse situation of the second alternative in which the first and the second lateral faces are outflow surfaces and the third lateral face an inflow surface. For the described filter cartridge geometry, in general one lateral face provides an inflow surface where the main fluid flow impacts first. The second lateral face assists entry of the fluid into the filter cartridge and increases the inflow surface. The fluid flows out through the third lateral face. Since the third lateral face is slantedly positioned relative to the first lateral face, for a filter with such a filter cartridge there is the possibility to determine the outflow direction already within the filter housing in the desired way. An external deflection outside of the filter housing, for example, by means of a curved outflow socket, is no longer necessary. Thus, two advantages are achieved. On the one hand, a significantly increased inflow surface results which possibly provides for better utilization of the filter capacity. At the same time, the filter geometry according to the invention permits a deflection of the main flow direction within the filter cartridge so that the installation space which is required by the filter is optimized, i.e., can be reduced. 
     The main flow direction is understood as the average direction of the flow at one position of the path through the filter system and/or filter element. For example, the main flow direction within a cyclone block comprised of a plurality of inline cyclone cells is defined by the axial extension of the cyclone cells. When impacting on the inflow surface of a filter element, the main flow direction is substantially perpendicular to this inflow surface and in other respects is defined by the path of least resistance. 
     According to one embodiment, the polygonal basic shape is a triangle, preferably a quadrangle or a pentagon. The resulting prismatic basic shape, depending on the application situation, can also have slightly curved lateral faces. Also, inflow or outflow can be realized through the base face and/or top face of the prism. 
     A preferred further embodiment of the invention provides that the filter cartridge comprises a filter body with a zigzag-shaped folded filter medium. Such a folded filter medium comprises a large surface area and can be brought relatively inexpensively into the desired basic shape. 
     One embodiment provides that the folds have outer fold edges, which are positioned on the first lateral face and inner fold edges that are positioned opposite the outer fold edges and are positioned on the third lateral face. Accordingly, inflow into the filter cartridge is substantially realized perpendicular to the fold edges. The same holds true for the outflow wherein the third lateral face is positioned slantedly relative to the first lateral face. 
     In the described zigzag-shaped folding of the filter medium, the folds have at least one end face which is located on the second lateral face. Accordingly, inflow of the filter medium is realized through the fold edge as well as through the end face of the fold. The end faces in this case must be designed such that no direct inflow to the clean side of the filter cartridge can take place. 
     A particularly preferred embodiment of the invention provides that the depth of neighboring fold edges differs and in particular every other fold has the same depth. The different depths of neighboring fold edges enable a slanted configuration of the filter cartridge and, in this way, the embodiment of the desired geometry. 
     A particularly preferred embodiment of the invention provides that the folds each have two end faces. One end face of the folds is positioned on the second lateral face. The other end face of the folds is positioned on a fourth lateral face. The second lateral face is larger than the fourth lateral face. The fourth lateral face comprises more fold edges than the second lateral face, in particular twice as many. By doubling the fold edges on the fourth lateral face, it is possible to produce or adjust the angle between the third lateral face and the first lateral face. 
     The concept of the invention is also realized in a filter with a filter cartridge according to the invention. In such a filter it can be provided that the main inflow direction of the filter and the main outflow direction of the filter are positioned relative to each other at an angle of more than 30°, in particular more than 45°, preferably more than 60°, and especially 90°. 
     Alternatively, it can be provided that the main inflow direction of the filter is parallel and displaced relative to the main outflow direction of the filter. As a result of the slanted position, realized by the filter cartridge, of the outflow surface of the filter cartridge relative to the inflow surface of the filter cartridge, within the filter a deflection of the main inflow direction toward the main outflow direction can be realized such that the main outflow direction is positioned at an angle relative to the main inflow direction or is displaced by a certain length. This can be realized simply and inexpensively by appropriate outflow openings at the filter. 
     Another also advantageous further development of the invention provides a filter with a primary filter element according to the invention and a secondary filter element according to the invention. By configuring the primary and secondary filter cartridges with a prismatic basic shape, a particularly great freedom of deflection of the fluid flows in such a filter is created. 
     In one embodiment, the filter cartridge and/or the filter body comprises an outer shape embodied as a truncated wedge. 
     A further embodiment of the invention provides that the filter body is embodied as a filter bellows, in particular with variable fold height. The variable fold height enables an angled positioning between the outflow surface and the main flow direction. 
     According to the invention, the edge protection is designed such that, when tapping the filter cartridge, the resulting forces can be at least partially absorbed by the edge protection and in particular damage of the filter cartridge can be prevented. 
     The filter according to the invention, in particular for internal combustion engines, for filtering a fluid, in particular air, comprises a filter housing with a raw-side area and a clean-side area. A main filter element is insertable into the filter housing and comprises a main filter element inflow surface, a main filter element flow direction, a main filter element outflow surface as well as a seal arranged on a sealing surface. For example, the main filter element can have a prismatic basic shape. In particular, a flow through the lateral faces can be realized. The seal serves for fluid-tight separation of the raw-side area of the filter housing from the clean-side area of the filter housing. Moreover, the filter optionally comprises a secondary filter element arranged downstream of the main filter element and provided with a secondary filter element inflow surface, a secondary filter element flow direction, and a secondary filter element outflow surface. The sealing surface is positioned at a slant to the main flow direction of the main filter element. The slanted sealing surface makes available within the filter housing downstream of the main filter element a space in which, on the one hand, the secondary filter element can be arranged. At the same time, due to the sealing surface positioned at a slant relative to the main flow direction, a deflection of the main flow direction is already realized in the filter housing. Accordingly, with a suitable arrangement of an outflow opening on the filter housing, a deflection of the fluid flow in the desired direction can be realized. It is thus possible to eliminate the otherwise required pipe elbow or the like downstream of the housing. 
     The sealing surface and the main flow direction can be positioned in particular at an angle that is between 85° and 10°. Within this angle range, a noticeable deflection of the fluid flow occurs. 
     One embodiment of the invention provides that the sealing surface is curved and in particular is positioned on a lateral cylinder surface. By means of a concave configuration of the sealing surface, viewed from the main filter element, the installation space available for the main filter element can be optimized and provides sufficient space for a secondary element with a suitable selection of the radius of curvature. At the same time, due to the curvature of the sealing surface, the outflow direction of the filter housing, depending on the position of the outflow opening, can be determined in a particularly simple way. Preferably, the axis of the cylinder of the lateral cylinder surface is perpendicular to the main filter element flow direction and perpendicular to the secondary filter element flow direction. 
     Alternatively, the sealing surface can be positioned in a plane. 
     A preferred embodiment of the invention provides that the sealing surface and the main filter element outflow surface extend parallel. In this way, a clearly defined separation plane between the raw side and the clean side of the filter housing is provided. At the same time, for a curved embodiment of the sealing surface, an installation space-optimized configuration of the filter interior and thus of the entire filter is provided. 
     Preferably, it is provided that the secondary element inflow surface is extending parallel and spaced-apart relative to the sealing surface. In this way, it is ensured that upon change of the secondary filter element the clean side of the filter housing does not become soiled. In a curved configuration of the sealing surface, a secondary element inflow surface is provided that is also curved. 
     In a preferred embodiment of the invention, the basic shape of the secondary filter element is a parallelepiped. This enables a simple construction of the secondary element, for example, from a flat element with straight folds in which the fold edges at the inflow side and outflow side each form planes that are preferably positioned parallel to each other at a spacing defined by the fold height. 
     Alternatively, the basic shape of the secondary element can be a prism with one or several curved lateral faces such as, for example, a hollow lateral cylinder sector. This can be realized, for example, by a curved flat bellows. This shape provides the possibility of adapting the secondary filter element to a curved main filter element outflow surface and to thereby further optimize the installation space. 
     In a preferred embodiment of the invention, the basic shape of the main filter element is a prism. In particular, the base face and the top face of the prism can be a quadrangle or a pentagon. The quadrangle or the pentagon can have two or three right angles, an acute and an obtuse angle. A preferred embodiment of the invention provides that a lateral face of the prism, viewed from the exterior, is convexly curved so that a main filter element outflow surface results which is embodied as a lateral cylinder surface, for example. 
     An advantageous embodiment of the invention provides that the main filter element is a folded bellows with at least two different fold depths. By means of two different fold depths, the main filter element outflow surface that is slanted relative to the main flow direction of the main filter element can be realized at the fold end faces. Alternatively, by a continuously increasing fold height the main filter element outflow surface that is slanted relative to the outflow direction of the main filter element can be realized at the fold edges. The terms fold depth and fold height are synonymously used herein. 
     An advantageous embodiment of the invention provides that the filter housing has an inflow direction, an outflow direction, as well as an outflow area with an outflow opening. An outflow socket can be attached to the outflow area wherein the outflow area comprises a fastening surface for the outflow socket and the fastening surface is positioned at an angle of 45° relative to the main filter element flow direction. The fastening surface which is slanted by 45° relative to the main flow direction is located particularly preferred in an arrangement on the filter housing in such a way that it is positioned, viewed in the main filter element flow direction, within the filter housing, i.e., it does not project past it. At the same time, this fastening surface, viewed in a direction perpendicular to the main flow direction of the main filter element, i.e., for example, in an insertion direction of the main filter element, is located below the main filter element. In summarizing, an arrangement results in which the main filter element and/or the secondary element in the main flow direction, in particular in main flow direction of the filter element, project past the outflow opening of the filter housing at least partially, preferably completely. An outflow socket which is mounted on this fastening surface can now distribute the outflowing filtered fluid comparatively easily in any directions. 
     A particularly preferred embodiment of the invention provides that the outflow socket is formed such that within the outflow socket a deflection of the flow direction by 45° results. This means that the outflow socket preferably is designed as a 45° elbow. In this way, only the orientation of the outflow socket on the outflow area, in particular on the fastening surface, is relevant with respect to the final outflow direction. This enables in a particularly advantageous way the determination of the outflow direction of the filtered fluid with a single component, i.e., the outflow socket. The final outflow direction of the filter housing results by suitable orientation of the outflow socket on the fastening surface. 
     A preferred further embodiment of the invention provides that the outflow socket comprises a rotation-symmetrically embodied fastening area for attachment on the fastening surface of the filter housing. Accordingly, by rotation of the outflow socket the final determination of the outflow direction of the entire filter housing can be determined. In this way, an angle range between 0° and 90° results between the inflow direction of the filter housing and the outflow direction of the filter housing with one and the same components. 
     In the same way, it may be determined that the outflow direction out of the outflow socket and the fastening surface are positioned at an angle of 45° relative to each other. 
     An alternative further embodiment of the concept of the invention provides that the fastening surface is positioned on a lateral cylinder surface. The axis of the cylinder is positioned in this context preferably perpendicular to the main flow direction of the main filter element. Such a curved fastening surface is preferably combined with a correspondingly curved fastening area of an outflow socket. The position of the outflow socket at the outflow area then determines the outflow direction of the filter housing. 
     Preferably, the fastening surface and the secondary element outflow surface extend parallel. This enables an extremely high integration and thus installation space optimization. 
     According to the invention, in an embodiment it can be provided that the main filter element is insertable and removable along an insertion axis into the filter housing wherein the insertion axis is positioned relative to the main flow direction at an angle between 90° and the angle at which the sealing surface and the main flow direction are positioned relative to each other, preferably however 90°. According to this embodiment, the filter housing has a cover that is designed such that, in the state in which it closes off the filter housing, it exerts a force on the main filter element in the direction of the sealing surface. This can be realized, for example, by blades that project from the cover, laterally relative to the main filter element, into the filter housing and that in particular are wedge-shaped and that have on their narrow side a support surface, respectively. The blades are supported preferably with a first support surface on the filter housing and are positioned with a second support surface that is opposite the first support surface on a contact surface of the main filter element or of its support frame and force in this way the seal of the main filter element against the sealing surface of the housing. Preferably, the support surfaces of the blades are positioned relative to each other at an angle which corresponds to the angle between inflow surface and outflow or sealing surface (or between first and second lateral faces). Due to the slanted position of the sealing surface, the force which is exerted on the cover is at least partially converted into an axial force in the sealing direction, i.e., into a force which is partially acting in the direction of the main flow direction and preferably perpendicular to the sealing surface and/or outflow surface. This provides a force which presses the main filter element with its seal mounted on the sealing surface against a main filter element seat on the filter housing. 
     The secondary filter cartridge is designed to be inserted downstream of the main filter cartridge into a filter. It has an inflow side, an outflow side, and an outflow direction. Moreover, the secondary filter cartridge comprises a filter body which can be flowed through along the outflow direction and preferably a filter cartridge frame supporting the filter body. The filter cartridge frame further comprises preferably a seal for separating a filter interior of the filter into clean side and raw side as well as, further preferred, an adhesive connection between the filter body and the filter cartridge frame. The separate configuration of seal and adhesive connection enables a substantially more stable configuration of the adhesive connection and is of independent inventive importance. 
     In a preferred embodiment of the invention, the seal is designed to be acting radially to the main flow direction and is extending in particular substantially perpendicular to the main flow direction about the secondary filter cartridge. The seal can comprise a cellular rubber. 
     Particularly preferred, it is provided that a support section of the filter cartridge frame is extending circumferentially about the filter body and the support section comprises on the inner side an inner edge, in particular a groove, for receiving an adhesive material of the adhesive connection. This enables a mechanically particularly stable connection between the filter body and the filter cartridge frame. In particular, it can be provided that the depth of the groove extends in the direction of the main flow direction. 
     In a further embodiment of the invention, a seal receiving section of the filter cartridge frame is extending circumferentially externally about the filter body. In particular when removing and installing the secondary filter cartridge, shearing forces acting on the seal are absorbed by the seal receiving section. 
     One embodiment provides that the adhesive connection comprises a polyurethane and in particular is foamed. In particular, the adhesive connection can penetrate the filter body partially and, in this way, can ensure a particularly strong adhesive connection between filter body and filter cartridge frame. 
     In a further embodiment of the invention, it can be provided that the filter cartridge frame comprises at the outflow side a grate that is covering the outflow surface. This increases the collapse resistance of the secondary filter cartridge at high differential pressures. In particular, the grate can be formed as one part together with the filter cartridge frame. 
     The secondary filter cartridge has preferably an inflow-side grip area. The grip area in this context is designed preferably such that an inflow into the filter body via the grip area and a manual removal of the filter cartridge are possible. This is of independent inventive importance. In this way, the area is enlarged by means of which inflow into the filter body is realized. Since in general there is anyway a possibility for handling the filter cartridge, in particular for installation and removal of the filter cartridge, the pressure drop or pressure loss caused by the filter cartridge is significantly reduced by designing the grip area so as to be flowed through by the fluid to be filtered. 
     In a preferred further embodiment of the invention, it is proposed that laterally to the main inflow surface an auxiliary inflow surface is provided. This provides for the generally improved inflow situation of the filter cartridge with a further inflow surface of the filter cartridge and reduces thus further the pressure loss caused by the filter cartridge. In this context, it can be particularly provided that the grip area enables inflow into the filter body laterally adjacent to the main inflow surface. 
     Preferably, the grip area is designed as a grip depression and the grip depression is facing toward the filter body. On the one hand, the grip depression provides a particularly simple possibility for removing the filter cartridge from its inserted position. On the other hand, the shape of a depression enables a particularly turbulence-free inflow of the fluid to the filter body, in particular to the main inflow surface and the auxiliary inflow surface. 
     In one embodiment of the invention, it is provided that in main flow direction the grip area is substantially not projecting past the inflow surface. Also, the filter body can be substantially of a parallelepipedal shape. 
     In a further configuration, the filter body is configured as zigzag-shaped folded filter bellows and in particular one of the end faces of the filter bellows folds (i.e., the side faces which are formed by the zigzag-shaped course of the folded filter medium) face in the direction of the grip area. Due to the configuration according to the invention of the grip area that is open toward this end face, the flowing fluid is guided to the end faces of the filter bellows folds. In case of a one-sided sealing action of the end faces only at the clean side of the folded filter medium, the end face partially remains open at the raw-side unglued gaps between the folds. In this way, the fluid can flow at the end face into the interior of the filter bellows without bypassing the filter body. In this way, in addition to the regular main inflow surface which is formed by the fold edges, the end face of the filter bellows can serve as an auxiliary inflow surface and can thus minimize the pressure loss. 
     In a particularly preferred embodiment of the invention, the filter cartridge is designed as a secondary filter cartridge. Since in particular the installation space which is downstream of the main filter cartridge is particularly tight not always allows for an inflow surface at the secondary filter cartridge to be realized to be equivalent to the total outflow surface of the main filter cartridge, an additional auxiliary inflow surface is of particularly great importance for optimizing the entire filter system. 
     In a further inventive embodiment of the invention, it is provided that the filter cartridge frame at the outflow side is provided with a grate covering the outflow surface. In particular at high pressure differentials or when changing the main filter element in operation, a particularly high collapse pressure of a secondary filter cartridge is important. This is decisively improved by providing an outflow-side grate. 
     The grate can be preferably formed as one part together with the filter cartridge frame. 
     In one embodiment of the filter cartridge, in particular secondary filter cartridge, which represents also its own invention, it is provided that the filter cartridge comprises a main inflow surface, an outflow surface, and a main flow direction. Moreover, the filter cartridge comprises a filter body which can be flowed through along the main flow direction, as well as a filter cartridge frame supporting the filter body. The filter cartridge frame comprises preferably a seal for separating a filter interior of a filter in clean side and raw side as well as preferably a grip area at the inflow side. The grip area is designed further preferred such that an inflow into the filter body via the grip area and a manual removal of the filter cartridge are possible. In this way, the area is enlarged by means of which inflow into the filter body is realized. Since in general there must be provided anyway a possibility for manual handling the filter cartridge, in particular for installation and removal of the filter cartridge, the pressure drop or pressure loss caused by the filter cartridge is significantly reduced by designing the grip area so as to be flowed through by the fluid to be filtered. 
     In a preferred further embodiment of the invention, it is provided that laterally to the main inflow surface an auxiliary inflow surface is provided. This provides for the generally improved inflow situation of the filter cartridge with a further inflow surface of the filter cartridge and reduces thus the pressure loss caused by the filter cartridge further. In this context, it can be particularly provided that the grip area enables inflow into the filter body laterally adjacent to the main inflow surface. 
     As an alternative to the one-part embodiment of the afore described filter cartridge, it is also possible that the filter cartridge frame is separable from the (main) filter cartridge and thus can be reused upon exchange of the filter cartridge. For this purpose, preferably a reusable filter cartridge frame is provided which can receive the main filter cartridge. This is referred to in the following as a two-part solution. 
     In this solution, it is preferably provided that the seal is arranged on the filter cartridge and is connected thereto in a non-detachable way. The seal has in this context preferably substantially a pointed shape in cross-section which ends in the preferred flattened sealing surface which can be axially pressed seal-tightly against a sealing contact surface of the filter housing. Further, the seal and the sealing surface are preferably located in this context outside of, in particular radially outside of, the filter body envelope which is defined in particular by the base and top faces as well as the second and the fourth lateral faces. 
     In a preferred further embodiment of the two-part solution, for reinforcement and better mechanical support of the seal on the filter frame on the end of the filter cartridge frame which is facing the seal, a seal holder is provided which comprises preferably an I-shaped or L-shaped cross-section, wherein the first web is projecting away from the filter cartridge frame outwardly and provides a seal support surface that is extending parallel to the sealing surface and on which the seal with its support surface, positioned opposite the sealing surface and surrounding the filter body circumferentially at least partially, can be supported. 
     The seal support surface represents in this context the I-shaped cross-section or the first leg of the L-shaped cross section. Optionally, for configuring the second leg of the L, a second web is provided which is extending away from the filter cartridge frame and surrounds the seal externally 
     In all embodiments, the seal can contact the filter body or preferably penetrate it so that a fluid-tight, in particular form-fit, sealing action between filter body and the seal is realized. 
     The main filter cartridge of the two-part solution comprises at its inflow side preferably an edge protection that is extending externally about the filter body. The edge protection is preferably designed such that upon tapping of the main filter cartridge, for example, for cleaning it, impacts against the filter cartridge can be absorbed and at least partially compensated. Accordingly, damage of the filter cartridge, for example, of the filter body, can be avoided. The edge protection extends, further preferred, about the inflow-side edge of the filter element. 
     On the inflow-side edge of the edge protection of the two-part solution, on the longitudinal sides (the edges toward the base and top face) at least one cutout (preferably two) is provided, respectively, in which support webs of the filter cartridge frame can engage particularly by form fit, in particular for stabilization of the filter cartridge frame and/or for a positional securing action of the filter cartridge. The edge protection of the two-part solution has moreover, in particular positioned in the installation direction (Z direction) between the cutouts, on the longitudinal sides, a laterally outwardly projecting projection which project past the exterior surface of the filter body, in particular the base and top faces. The projections upon insertion of the filter cartridge into the filter cartridge frame of the two-part solution can be guided in a groove which is extending in the insertion direction. The groove moreover comprises preferably, viewed in the insertion direction, shortly before the inflow-side end, i.e., spaced a few millimeter or centimeters away from the inflow-side edge of the filter cartridge frame, an elevation which is reducing the depth of the groove and is in particular so high that in the area of the elevation the groove completely or partially vanishes and toward the inflow-side end of the filter cartridge frame forms again an inflow-side end area. The projections and/or the elevation are preferably designed such that, when inserting of the filter cartridge into the filter cartridge frame, the elevation must be overcome, in particular with elastic deformation of the filter cartridge, in particular of the projections, before the projections, further preferred, come to rest in the inflow-side end area of the groove. Accordingly, a detachable, form-fit connection of the filter cartridge with the filter cartridge frame can be produced. Alternatively, it is also conceivable to position the elevation at the end of the insertion path so that the filter cartridge by means of the clamped projections is secured in its end position by friction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be explained in more detail with reference to the drawings. The terms filter cartridge and filter element are synonymously used. 
         FIG. 1  is a perspective section illustration of an embodiment of a filter according to the invention with inserted main filter cartridge and secondary filter cartridge. 
         FIG. 2  shows the filter of  FIG. 1  without inserted secondary filter cartridge. 
         FIG. 3  shows the filter of  FIG. 1  without inserted main filter cartridge. 
         FIG. 4  shows the filter of  FIG. 1  in a cross-sectional view. 
         FIG. 5  is a perspective external view of the filter of  FIG. 1  according to the invention. 
         FIG. 6  is a perspective view of a secondary filter cartridge according to the invention with filter body. 
         FIG. 6 a    is a perspective view of an alternative secondary filter cartridge filter body according to the invention. 
         FIG. 7  is a perspective view of the secondary filter cartridge of  FIG. 6  without filter body. 
         FIG. 8  is another perspective view of the secondary filter cartridge of  FIG. 6  without filter body. 
         FIG. 9  is a perspective section view of the filter of  FIG. 6 . 
         FIG. 10  is a section view of the filter of  FIG. 7 . 
         FIG. 11  is a perspective front view of a main filter cartridge according to the invention. 
         FIG. 11 a    is a perspective front view of a second embodiment of a main filter cartridge according to the invention. 
         FIG. 12  is a perspective rear view of the main filter cartridge of  FIG. 11 . 
         FIG. 12 a    is a perspective rear view of the main filter cartridge of  FIG. 11 a    in a support frame. 
         FIG. 12 b    is a perspective rear view of the main filter cartridge of  FIG. 11 a    without support frame. 
         FIG. 13  is a perspective section view of the main filter cartridge of  FIG. 11 . 
         FIG. 13 a    is a perspective section view of the main filter cartridge of  FIG. 11 a   . 
         FIG. 13 b    is a perspective section view of the support frame of the main filter cartridge of  FIG. 11 a   . 
         FIG. 13 c    is a section view of the main filter cartridge of  FIG. 11 a    in a support frame, wherein the section is extending centrally through the main filter cartridge, parallel to the main flow direction X, perpendicular to the inflow surface, and perpendicular to the installation direction Z. 
         FIG. 13 d    is a section view of the main filter cartridge of  FIG. 11 a    in a support frame, wherein the section is extending centrally through the main filter cartridge, parallel to the main flow direction X, perpendicular to the inflow surface, and parallel to the installation direction Z. 
         FIG. 14  is a perspective rear view of the main filter cartridge of  FIG. 11  without edge protection. 
         FIG. 15  is perspective external view of the filter of  FIG. 5  showing the outlet socket in a first position. 
         FIG. 16  is perspective external view of the filter of  FIG. 5  showing the outlet socket in a second position. 
         FIG. 17  is a side view of the filter of  FIG. 5 . 
         FIG. 18  is a side view of the filter of  FIG. 15 . 
         FIG. 19  is a side view of the filter of  FIG. 16 . 
       The embodiments described in connection with the above Figures represent detailed embodiments and further developments are described with the aid of  FIGS. 20 to 36  wherein the features are not mandatorily required but are envisioned to be added individually or in combination to an embodiment according to the following illustrated Figures. 
         FIG. 20  is a perspective front view of the filter. 
         FIG. 21  is a perspective rear view of the filter of  FIG. 20 . 
         FIG. 22  is an exploded view of the filter of  FIG. 20 . 
         FIG. 23  is a section illustration of the filter according to the invention. 
         FIG. 24  is a front view of the main filter element according to the invention. 
         FIG. 25  is a rear view of the main filter element according to the invention. 
         FIG. 26  is a first folding type as a schematic. 
         FIG. 27  shows an alternative folding type as a schematic. 
         FIG. 28  is a front view of the secondary filter element according to the invention. 
         FIG. 29  is a rear view of the secondary filter element according to the invention. 
         FIG. 30  shows the filter according to the invention of  FIG. 20  in a second configuration. 
         FIG. 31  shows the filter according to the invention of  FIG. 20  in a third configuration. 
         FIG. 32  is a front view of an alternative embodiment of a filter according to the invention. 
         FIG. 33  is a rear view of the filter of  FIG. 32 . 
         FIG. 34  is an exploded view of the filter of  FIG. 32   
         FIG. 35  is a section illustration of the filter of  FIG. 32 . 
         FIG. 36  is a section view of an alternative main filter element and secondary filter element design of the filter of  FIG. 32 . 
     
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     With reference to  FIGS. 1 through 5 , an embodiment of a filter  10  according to the invention will now be described. Such a filter  10  can be used, for example, in an air intake manifold of a construction or agricultural machine, a compressor or another device with an internal combustion engine, for filtering a fluid, in particular air. The filter  10  comprises a filter housing  12  which can be divided roughly into a raw-side area  14  and a clean-side area  16 . 
     The filter  100  is flowed through along a main inflow direction X. On an inflow side  16  the fluid to be filtered impacts on a coarse or a pre-separation module  18  which is designed in the present case as a cyclone block. In the cyclone block  18 , a plurality of individual pre-separation cells  20  are connected in parallel in a so-called multi-cyclone block. The dust and/or water that has been pre-separated in the cyclone block  18  is removed through a discharge socket  22  from the filter housing  12 . 
     Downstream of the cyclone block  18 , the fluid to be filtered flows into the main filter cartridge  100 . The main filter cartridge  100  is embodied in the present case as a prism. An inflow surface  110  of the main filter cartridge  100  is not positioned parallel to an outflow surface  112  of the main filter cartridge  100 . Instead, the inflow surface  110  and the outflow surface  112  are positioned at an angle to each. In the present case, the inflow surface  110  of the main filter cartridge  100  is smaller with regard to the desired surface area than the outflow surface  112  of the main filter cartridge  100 . At the outflow side of the main filter cartridge  100 , a secondary filter cartridge  200  is provided in the filter housing  12 . A main inflow surface  210  of the secondary filter element  200  is oriented toward the outflow surface  112  of the main filter cartridge  100  and in particular is arranged parallel thereto. An outflow surface  212  is oriented in this embodiment parallel to the main inflow surface  210  of the secondary filter cartridge  200 . Due to the slanted position of the outflow surface  112  of the main filter cartridge  100 , already upon inflow of the fluid from the main filter cartridge  100 , but also upon inflow from the secondary filter cartridge  200 , a deflection of the main flow direction X is occurring. Due to the outflow geometry of the filter housing  12  in the outflow area  24 , the flowing fluid is deflected to the outflow direction Y and guided toward an outflow socket  26 . In the present case, the main outflow direction Y is substantially perpendicular to the main inflow direction X. 
     However, other outflow directions are conceivable also. This will be explained in more detail particularly in connection with  FIGS. 15 to 19 . 
     The main filter cartridge  100  comprises a main filter cartridge inflow surface  110 , a main filter cartridge flow direction X 1 , a main filter cartridge outflow surface  112  as well as a seal  116  arranged on a sealing surface  114  for fluid-tight separation of the raw-side area  14  and of the clean-side area  16  of the filter housing  12 . A secondary filter cartridge  200  with a secondary filter cartridge inflow surface  210 , a secondary filter cartridge flow direction Y 1 , and a secondary filter cartridge outflow surface  212  is arranged downstream of the main filter cartridge  100 . The sealing surface  114  of the main filter cartridge  100  is slantedly positioned relative to the main flow direction Y 1  of the main filter cartridge  100 . In particular, the sealing surface  114  is positioned at an angle α that is preferably between 5° and 45° (see  FIG. 4 ), in particular the angle amounts to 24°±10° and 24°±5°. In the present embodiment, the angle α is 24° (see  FIG. 4 ). 
     The secondary filter cartridge inflow surface  210  extends substantially parallel and at a spacing to the sealing surface  114  of the main filter cartridge  100 . The spacing is less than 2 cm; in the present embodiment, the spacing is 1 cm. 
       FIGS. 11 to 14  show the main filter cartridge  100 . The main filter cartridge  100  comprises an inflow surface  110  (first lateral face) and an outflow surface  112  (third lateral face). The arrow for the insertion direction Z points in  FIG. 11  to the second lateral face which is oppositely positioned to the smaller fourth lateral face. The top face can be recognized in  FIGS. 11 and 12  as the surface on which the marking for the main flow direction X 1  is arranged. It is positioned parallel and congruent to the base face. Base face and top face as well as the second and fourth lateral faces are perpendicularly oriented relative to the first lateral face (inflow surface  110 ). Inflow to the main filter cartridge  100  is occurring along a main inflow direction X and flow through it occurs along a main flow direction X 1 . The main filter cartridge  100  comprises a filter cartridge frame  118  that receives a filter body  120 . The filter body  120  is in the present case designed as a folded bellows. Inflow-side fold edges  122  are positioned opposite outflow-side fold edges  124 . Inflow-side fold edges  122  and outflow-side fold edges  124  are positioned parallel, substantially perpendicular to the main flow direction X 1 , and substantially horizontally in  FIGS. 11 to 14 . This orientation of the fold edges  122 ,  124  enables a variation of the fold depth in the direction of an insertion direction Z. Along the insertion direction Z, the main filter cartridge  100  is insertable into the filter housing  12  of the filter  10 . In the present embodiment, the fold height is reduced along the insertion direction Z. This effects a tilting of the inflow surface  110  relative to the outflow surface  112 . 
     In the area of the outflow surface  110 , the filter frame  118  has a sealing surface  114  along which a circumferentially extending seal  116  is provided. The seal serves for separating the raw-side area  14  from the clean-side area  16  in the filter housing  12  of the filter  10  when the main filter cartridge  100  is inserted into the filter  10 . The seal  116  comprises substantially a U-shape in cross-section. 
     For reinforcement and better mechanical connection of the seal  116  to the filter frame  118 , a web  126  is provided which engages the U-shape of the seal  116 . At the same time, the seal  116  can contact the filter body  120  or penetrate it so that an adhesive connection of the filter body  120  with the filter cartridge frame  118  and a fluid-tight sealing action between filter body  120  and filter cartridge frame  118  is generated at the same time. 
     Moreover, the seal  116  comprises a spacer structure which is embodied in the present case as support knobs  128 . The support knobs  128  are components of the sealing material of the seal  116 . As can be seen in the cross-sectional view of  FIG. 4 , the support knobs  128  contact the secondary filter cartridge  200 , in particular a secondary filter cartridge frame, when main filter cartridge  100  and secondary filter cartridge  200  are inserted in the filter housing  12  of the filter  10 . In this state, the secondary filter cartridge  200 , even for vibration excitation that can be transmitted, for example, through the filter housing  12 , cannot move out of a seal seat in the filter housing  12 . Also, by means of the support knobs  128  it is ensured that the secondary filter cartridge  200  after installation of the main filter cartridge  100  and closure of the cover  13  is seated in the correct position in the filter housing  12 . 
     A plurality of the support knobs  128  are positioned along the seal  116  on the side which is facing the outflow surface  112 . They can be produced, for example, integrally with the seal  116  when producing the seal  116 . 
     The seal  116  is located at the outflow surface  112  of the main filter cartridge  100  and acts in a direction that is perpendicular to the outflow surface  112 , i.e., substantially axially along the main flow direction X 1 . 
     The main filter cartridge  100  comprises at its inflow side  110  an edge protection  130  which is extending externally circumferentially about the filter frame  118 . The edge protection  130  is designed such that upon tapping of the main filter cartridge  100 , for example, for cleaning, impacts against the filter frame  118  can be absorbed and at least partially compensated. In this way, breakage of the filter frame  118  or other damages of the filter cartridge  100 , for example of the filter body  120 , can be avoided. The edge protection  130  extends circumferentially about the inflow-side edge of the filter element  100 . In this context, individual interruptions, for example, the notches  134  can be provided. The notches  134  are generated when producing the edge protection  130 . In this context, the filter frame  118  together with filter body  120  is positioned in a casting mold. Webs secure the spacing between the casting mold bottom and the filter body  118  and cause the formation of the notches  134  during the casting process. 
     Cutouts  136  are provided at the inflow-side edge  132 . The cutouts  136  penetrate the sidewalls of the filter frame  118  and extend thus perpendicular to the main flow direction X 1 . During the already mentioned casting process, the casting material for the edge protection  130  penetrates the cutouts  136 , contacts the inner walls of the filter cartridge frame  118  and in particular the filter body  120 . In this way, a fluid-tight sealing action between the filter body  120  and the filter cartridge frame  118  and at the same time an adhesive connection of the two components is produced. Accordingly, the edge protection  130  is monolithically produced with the adhesive connection between the filter body  120  and the filter cartridge frame  118  and an also required sealing action between the two components. The edge protection  130  can be produced, for example, of a foamable polyurethane. However, also silicone-based material systems are conceivable. 
     The filter cartridge  100  comprises a grip  138 . The grip  138  interacts with the cover  13  of the filter  10  and ensures reliable seating of the main filter cartridge  100  in the filter housing  12  and, at the same time, exerts pressure directed axially in the direction of the main flow direction X 1  on the seal  116  and ensures in this way a fixed seal seat of the main filter cartridge  100  in the filter housing  12 . 
       FIGS. 11 a , 12 a , 12 b , and 13 a  to 13 d    show a variant  2100  of the main filter cartridge  100  in which the filter cartridge frame  2118  is detachably and reusably connected with the filter cartridge  2100 . The outer shape, recognizable as a truncated wedge, corresponds to the shape of the one-part embodiment of  FIGS. 11 to 14 . The significant difference to the one-part embodiment is that the filter cartridge frame  2118  can be separated from the (main) filter cartridge  2100  and thus is reusable upon exchange of the filter cartridge  2100 . The main filter cartridge  2100  comprises an inflow surface  2110  and an outflow surface  2112 . Inflow into the main filter cartridge  2100  occurs at inflow surface  2110  and flow through it along the main flow direction X 1 . The main filter cartridge  2100  comprises a filter cartridge frame  2118  which receives the main filter cartridge  2100 . The filter body  2120  is embodied as a folded bellows in the present case. The inflow-side fold edges  2122  are positioned opposite outflow-side fold edges  2124 . Inflow-side fold edges  2122  and outflow-side fold edges  2124  are positioned parallel, substantially perpendicular to the main flow direction X 1 , and substantially horizontally in  FIGS. 11 a , 12 a , 12 b , and 13 a  to 13 d   . This orientation of the fold edges  2122 ,  2124  enables a variation of the fold depth in the direction of an insertion direction Z. Along the insertion direction Z, the main filter cartridge  2100 , as an alternative to the main filter cartridge  100 , is insertable into the filter housing  12  of the filter  10  (see  FIGS. 1 through 5 ) in the same way. In the present embodiment, the fold height decreases also along the insertion direction Z. This effects tilting of the inflow surface  2110  relative to the outflow surface  2112  about angle α ( FIG. 13 d   ). 
     In the area of the outflow surface  2110 , the filter cartridge  2100  has a sealing surface  114  which is provided along a circumferential seal  2116 . The seal serves for separating the raw-side area  14  from the clean-side area  16  in the filter housing  12  of the filter  10  when the main filter cartridge  2100  is inserted into the filter  10 . The seal  2116  substantially comprises in cross-section a pointed shape which ends in the preferred flattened sealing surface  2114  which can be pressed so as to seal axially against a seal contact surface of the filter housing  12 . The seal  2116  and the sealing surface  2114  are located outside of, in particular radially outside of, the filter body envelope  2115  which is defined by the base and top faces as well as the second and fourth lateral faces. In this way, it is ensured that even for flow deflection downstream of the main filter cartridge  2100  no impairment of the flow by the seal  2116  occurs. 
     For reinforcing and better mechanical support of the seal  2116  on the filter frame  2118 , a seal holder is provided on the end of the filter cartridge frame  2118  which is facing the seal  2116  and which has preferably an L-shaped cross-section, wherein the first web projects away from the filter cartridge frame  2118  outwardly and provides a seal support surface  2113  extending parallel to the sealing surface  2114  on which the seal  2116  with its support surface  2115  that is opposite the sealing surface  2114  and at least partially surrounds the filter body  2120  can be supported. The seal support surface  2113  represents the first leg of the L-shaped cross-section. Moreover, for configuring the second leg of the L, a second web  2126  is provided which extends away from the filter cartridge frame  2118  and which surrounds the seal  2116  externally. At the same time, the seal  2116  can contact the filter body  2120  or penetrate it so that a fluid-tight sealing between filter body  2120  and the seal  2116  is generated. 
     Moreover, in analogy to the first embodiment, the seal  2116  also has a spacer structure embodied as support knobs  2128  in the present case. The support knobs  2128  are component of the sealing material of the seal  2116 . In analogy to the cross-section view of  FIG. 4 , the support knobs  2128  also contact the secondary filter cartridge  200 , in particular a secondary filter cartridge frame, when the main filter cartridge  2100  and secondary filter cartridge  200  are inserted into the filter housing  12  of the filter  10 . In this state, the secondary filter cartridge  200 , even in case of vibration excitation that can be transmitted, for example, by the filter housing  12 , cannot move out of its seal seat in the filter housing  12 . Also, by means of the support knobs  2128  it is ensured that the secondary filter cartridge  200  after the installation of the main filter cartridge  100  and closing of the cover  13  is seated in the correct position within the filter housing  12 . 
     A plurality of support knobs  2128  are positioned along the seal  2116  on the side which is facing the outflow surface  2112 . They can be produced, for example, integrally with the seal  2116  when producing the seal  2116 . 
     The seal  2116  is located on the outflow surface  2112  of the main filter cartridge  2100  and acts in a direction that is perpendicular to the outflow surface  2112 , i.e., substantially axially along the main flow direction X 1  and/or in accordance with the angle position of the outflow surface. 
     The main filter cartridge  2100  comprises at its inflow side  2110  an edge protection  2130  which is surrounding externally the filter body  2120 . The edge protection  2130  is designed such that, when tapping the main filter cartridge  2100 , for example, for cleaning, impacts against the filter cartridge  2100  can be absorbed and at least partially compensated. Accordingly, damage of the filter cartridge  2100 , for example, of the filter body  2120 , can be avoided. The edge protection  2130  extends circumferentially about the inflow side edge of the filter element  2100 . In this context, in analogy to the preceding embodiment, individual interruptions, for example, the notches  134  can be provided (not shown here). The notches  134  are produced when manufacturing the edge protection  2130 . In this context, the filter body  2120  is positioned in a casting mold. Webs secure in this context the spacing between the casting mold bottom and the filter body  2120  and result in the notches  134  being produced during the casting process. 
     At the inflow side edge  2132  of the edge protection, at least one cutout  2131  (preferably two, as shown) are provided, respectively, on the longitudinal sides (the edges relative to the base and top face) and engaged in particular by form fit by support webs  2133  of the filter cartridge frame  2118 , in particular for stabilization of the filter cartridge frame and/or for positional securing of the filter cartridge  2100 . The edge protection  2130  comprises moreover, in particular positioned in the installation direction (Z direction) between the cutouts  2131 , on the longitudinal sides a laterally outwardly projecting projection  2135 , respectively, which is projecting past the exterior surface of the filter body  2120 , in particular the base and top faces. The projections can be guided upon insertion of the filter cartridge  2100  into the filter cartridge frame  2118  in a groove  2137  extending in the insertion direction, i.e., opposite to the flow direction X. The groove  2137  comprises, viewed in the insertion direction, shortly before the inflow-side end, i.e., a few millimeters or centimeters spaced away from the inflow-side edge of the filter cartridge frame, an elevation  2117  that reduces the depth of the groove  2137  and is in particular so high that, in the area of the elevation  2117 , the groove  2137  completely or partially vanishes and forms again toward the inflow-side end of the filter cartridge frame in an inflow-side end area  2119 . Upon insertion of the filter cartridge  2100  into the filter cartridge frame, this elevation  2117  must be overcome with elastic deformation of the filter cartridge  2100 , in particular of the projections  2135 , before the projections  2135  come to rest at the inflow-side end area  2119  of the groove  2137 . In this way, a detachable form-fit connection of the filter cartridge  2100  with the filter cartridge frame  2118  can be produced. 
     In analogy to the first embodiment, the filter cartridge frame  2118  has also a grip  2138 . The grip  2138  interacts preferably with the cover  13  of the filter  10  and ensures a reliable seat of the main filter cartridge  100  in the filter housing  12  and exerts at the same time a pressure on the seal  116  directed axially in the direction of the main flow direction X 1  and ensures in this way the fixed seal seat of the main filter cartridge  100  in the filter housing  12 . 
     However, in both embodiments it is preferred that blades that extend from the cover  13 , laterally of the main filter element  2100 ,  100 , into the filter housing  12  and are preferably wedge-shaped, exert a force in the direction of the sealing surface  114 ,  2114  on the main filter element  2100 ,  1200  in the closed state. This can be realized preferably in that the blades at their narrow sides each have a support surface. The blades are supported in this context preferably with a first support surface on the filter housing and are resting with a second support surface opposite the first one on a contact surface  2121  of the main filter element  100  or of the support frame  2118  and press thereby the seal  116 ,  2116  of the main filter element  100 ,  2100  against the sealing surface of the housing  12 . In case of the first embodiment, the force is transmitted in this context by the support frame  118  directly to the seal  116  as a result of the form fit connection. In case of the second embodiment, the support force is transmitted from the blade against the contact surface  2121  and is in this way introduced into the support frame  2118  which, in turn, is supported on the seal support surface  2113  on the seal  2116 , in particular its support surface  2115 , and in this way introduces the force into the seal  2116  where then the compression of the sealing surface  2114  with the corresponding contact surface of the housing  12  is realized. 
     The  FIGS. 6 to 10  show an embodiment of a secondary filter cartridge  200 . The secondary filter cartridge  200  comprises a main inflow surface  210 , an outflow surface  212  as well as a main flow direction Y 1 . Moreover, the secondary filter cartridge  200  comprises a filter body  214  which is supported by a filter cartridge frame  216 . At the inflow side, the filter cartridge frame  216  with a frame area  218  circumferentially surrounding the filter body  214  is substantially flush with the filter body  214 . The frame area  218  can serve, for example, as an abutment for the support knobs  128  of the main filter cartridge  100  in the inserted state of both filter cartridges  100 ,  200 . 
     In the present embodiment, the filter body  214  is substantially parallelepipedal, however, other basic shapes like, for example, a prism are conceivable. At the outflow side, i.e., in the area of the outflow surface  212 , the filter cartridge frame  216  is provided with a grate structure  220 . The grate structure  220  covers the outflow surface  212  at least partially. In case of a high differential pressure between the inflow side  210  and the outflow side  212 , the grate structure  220  prevents an undesirable bending or even falling out of the filter body  214 . 
     At a narrow side of the parallelepipedal filter body  214 , the filter cartridge frame  216  is provided with a grip depression  222 . In order to provide for comfortable gripping in the grip depression  222  by the hand of a person who wants to exchange the secondary filter cartridge  200 , the frame area  218  in the area of the grip depression  222  is widened to a grip stay  224 . The width of the grip stay  224  is selected such in this context that a direct inflow of a fluid exiting from the main filter cartridge outflow side  112  toward the filter body  214  is possible in particular at the side which is facing the grip depression  222 . This is easily apparent in particular also in the cross-sectional view of  FIG. 4 . Also, from the uppermost edge  113  of the main filter element outflow surface  112 , exiting fluid can flow directly to the filter body  214  of the secondary filter cartridge  200 . In this context, the fluid can enter in particular through the auxiliary inflow surface  211  into the filter body  214 . 
     In this embodiment, the filter body  214  is designed as a filter bellows. The fold edges extend in this context parallel to the longitudinal axis of the secondary filter cartridge  200  so that the end faces of the folds form the auxiliary inflow surface  211 . The fold edges of the folds form the main inflow surface  210  and the outflow surface  212 . By the combination of grip depression  222  and filter bellows  214  with inflow laterally via the auxiliary inflow surface  211 , the pressure losses at the secondary filter cartridge  200  can be reduced because the secondary filter cartridge  200  is matched significantly better to the flow guidance from the main filter cartridge  100  to the outflow socket in the filter housing  12 . At the same time, the grate structure  220  at the outflow side  212  improves collapse resistance of the secondary filter cartridge  200 . Moreover, by means of the integrated grip at the grip depression  222 , an easy removal of the filter cartridge  200  is possible. 
     The secondary filter cartridge  200  comprises a filter frame  216  which provides a groove  226  extending circumferentially about the outflow-side rim of the filter body  214 . At the same time, a web  228  is provided on the side of the filter cartridge frame  216  which is facing the grip depression  222 . The groove  226  serves as a casting mold for a circumferentially extending adhesive connection and sealing action of the filter body  214  with the filter cartridge frame  216 . The sealing action and adhesive connection are effected by a sealing material  230  (see  FIG. 9 ). The sealing material  230  can be, for example, a foaming polyurethane. However, also silicone-based material systems are conceivable. 
     The groove  226  and additionally the web  228  ensure good mechanical coupling of the sealing material  230  on the filter cartridge frame  216 . This configuration has moreover the advantage that after introduction of the sealing material  230  into the groove  226  and insertion of the filter body  214  into the filter cartridge frame  216  and a subsequent foaming and hardening, no further processing steps such as, for example, cutting to size of the sealing material  230 , are required. Excess material can be absorbed partially by the filter body  214  or can reach the intermediate area between filter body  214  and filter cartridge frame  216  without this being disadvantageous. 
     The depth of the groove  226  extends substantially along the main flow direction Y 1  of the secondary filter cartridge  200 . The outflow-side grate structure  222  can be formed as one part together with the filter cartridge frame  216 . 
     At its inflow-side circumference, the filter cartridge frame  216  has a seal receiving groove  232 . In this seal receiving groove  232 , a seal  234  can be inserted that is, for example, manufactured of cellular rubber. The seal  234  is thus acting radially, i.e., perpendicular to the main flow direction Y 1  of the secondary filter cartridge  200 . 
       FIGS. 15 to 19  show the filter  10  with different orientations of the outflow socket  26 . The filter housing  12  of the filter  10  comprises in the outflow area  24  a fastening area  25 . The fastening area  25  is positioned relative to the main inflow direction X of the filter housing  12  at an angle of approximately 45°. An outflow socket  26  can be attached to the fastening area  25 . The outflow socket  26  is shaped such that a fluid flowing through the outflow socket  26  is subjected to a deflection of 45°. The outflow socket  46 , prior to final attachment on the fastening surface  25 , is rotatably attachable. Accordingly, at a very late point in time in the manufacture of the filter  10  the final deflection direction or outflow direction Y of the filter  10  can then be determined. In the geometry shown in this embodiment, an inline flow ( FIG. 15 ,  FIG. 18 ), a deflection by 90° ( FIG. 17 ) as well as intermediate angle ranges are possible. In the angle ranges that are between the extreme angles, an additional a lateral deflection occurs. 
     The embodiments that have been described above in connection with  FIGS. 1 to 19  represent detail embodiments and further developments of the embodiments which are described in the following with the aid of  FIGS. 20 to 36  wherein the features which have been added by the further developments are advantageous but not mandatorily required and also can be added individually to an embodiment according to the following illustrated Figures. 
     With reference to  FIGS. 20 to 23 , a first embodiment of a filter  1010  according to the invention will be described. The filter can be used in particular in the air intake manifold of construction or agricultural machines, compressors or other devices with internal combustion engines for filtering a fluid, in particular air. The filter  1010  comprises a filter housing  1011  that can be divided roughly into a raw-side area  1012  and a clean side area  1013 . In the clean-side area  1012  a main filter element  1020  is insertable into the filter housing  1011 . The main filter element  1020  comprises a main filter element inflow surface  1021  as well as a main filter element outflow surface  1022  and is flowed through along the main filter element flow direction X 1 . The main filter element flow direction X 1  coincides in the here described embodiment substantially with the inflow direction X 0  of the filter housing  1011 . However, also other constellations are conceivable in which the main filter element flow direction X 1  and the inflow direction X 0  of the filter housing  1010  are positioned at an angle to each other. Upstream of the main filter element  1020 , a coarse or pre-separation module is provided that is embodied in the present case as a cyclone block  1040 . In the cyclone block  1040 , a plurality of individual pre-separation cells  1041  are connected in parallel in a so-called multi-cyclone block. In the cyclone block  1040  the pre-separated dust and/or water is discharged through dust discharge socket  1024  from the housing. 
     Downstream of the main filter element  1020  a secondary filter element  1030  is arranged. The secondary element inflow surface  1031  of the secondary filter element  1030  is facing the main filter element outflow surface  1022 , the outflow surface  1032  of the secondary filter element  1030  is oriented in the direction of an outflow opening  1017  of the filter housing  1011 . The secondary filter element  1030  is arranged in the area  1013  of the filter housing  1011  which, relative to the main filter element  1020 , is a clean side and provides protection from penetration of contaminants into the intake system downstream of the filter, in particular in case of exchange of the main filter element  1020 . 
     The main filter element  1020 , shown in  FIG. 24  as a separate component, comprises at its outflow surface  1022  a sealing surface  1026  on which a seal  1024  for fluid-tight separation of the raw-side area  1012  and the clean side area  1013  of the filter housing  1011  is provided. The sealing surface  1026  is positioned at a slant to the main flow direction X 1  of the main filter element  1020 . The selected angle α is 60° in this embodiment. The angle value represents only an exemplary embodiment. According to the invention, the angle α can be varied between 10 and 80°. Particularly preferred, the angle range is between 70 and 30°. 
     The seal  1024  of the sealing surface  1026  is resting against an appropriate housing structure  1028  and is forced by a cover  1040  of the housing  1011  against the seal seat which is formed by the housing structure  1028 . The cover  1014  that closes off the housing can be removed perpendicularly to the main flow direction X 1  of the main filter element  1020  from the housing and has a blade-shaped pressing structure  1050 ,  1051  arranged in pairs on the cover  1014 . It is matched in its shape to the slant of the sealing surface  1026  and, in the closed state, exerts a force on the main filter element  1020  which is substantially perpendicular to the main flow direction X 1 . Due to the slanted position of the sealing surface  1026 , the force which is extending perpendicular to the main flow direction X 1  is deflected at least partially into a force that is extending in the main flow direction X 1  and leads thus to a reliable compression of the main filter element  1020  with the housing  1011 . In particular, in this way an axial pressing force, i.e., extending in the main flow direction X 1 , is generated that provides a particularly great sealing effect. At the same time, a particularly good contact of the seal  1026  at the corresponding housing structure is achieved because upon compression by the housing cover the seal glides somewhat and in this way unevenness is molded well into the sealing compound. 
     The main filter element  1020  may comprise a folded bellows as a basic construction. For obtaining the desired sealing surface slant, the fold height or fold depth along a direction perpendicular to the main flow axis X 1  can be reduced stepwise. With such a configuration, the fold edges are the inflow side. The end faces of the folds must be glued at least partially for such a configuration, either across the entire surface area or every other end face, so that raw side and clean side remain separated. In the section illustration of  FIG. 23 , in the described embodiment the fold edges extend perpendicular to the drawing plane, the end face of the folds is positioned in the drawing plane. This means that the main flow direction X 1  extends parallel to the planes that are formed by the end faces of the fold edges and perpendicular to the each fold edge. 
     Alternatively, in a folded bellows according to the invention, the orientation of the individual folds can also be such that the end face of the folds is extending substantially perpendicular to the drawing plane of  FIG. 23 , i.e., the cover  1014  extends parallel to the end face of the folds of the main filter element  1020 . The main filter element inflow surface  1021  is then formed by fold edges that are extending perpendicular to the main flow direction X 1 . The fold edges extend in  FIG. 20  perpendicular from top to bottom. The main filter element outflow surface  1022  is also formed by fold edges which extend along the slant according to the angle α. For such a folding, the fold height varies over the course of the fold edge. 
       FIGS. 24 and 25  represent the main filter element  1020  as a removed element. The  FIGS. 26 and 27  illustrate the aforementioned alternative folding types as schematics. In the schematics, an end face of the folds is identified with reference numeral  1023  and a fold edge with the reference numeral  1025 , respectively. 
     The secondary element  1030  which is illustrated in  FIGS. 28 and 29  as a removed element can be designed as a straight folded flat elements in the present embodiment. 
     With reference to  FIGS. 23, 30, and 31 , the filter  1010  will now be described with regard to the outflow situation in more detail. The filter housing  1011  comprises in the clean-side area  1013  an outflow area  1015 . The outflow area  1015  is provided with the fastening surface  1016 . The fastening surface  1016  is positioned relative to the inflow direction X 0  of the filter housing as well as relative to the main flow direction X 1  of the main filter element  1020  at an angle of 45°. An outflow socket  1018  is fastened to the fastening surface  1016 . The geometry of the outflow socket  1018  is selected such that the fastening area  1019  provided for attachment on the fastening surface  1016  is also slantedly positioned by 45° relative to the outflow direction determined by the outflow socket  1018 . Also, the fastening area  1019  of the outflow socket  1018  is designed to be of rotational symmetry, i.e., it forms a circular disk in which a penetration for the outflow opening  1017  of the housing  1011  is provided. As a result of the rotational symmetry, the outflow socket  1018  can be rotated prior to final attachment of the fastening area  1019  on the fastening surface  1016  of the outflow area  1015  and, in this way, the outflow direction Y 0  of the filter housing  1011  can be determined. Depending on the orientation of the outflow socket  1018 , the angle between the inflow direction X 0  and the outflow direction Y 0  can be varied between 0° (inline flow) and 90° (right angle outflow). In  FIGS. 20 and 23  a deflection of the inflow direction X 0  by 90° to the outflow direction Y 0  is shown. In  FIG. 30 , an inline flow direction is shown, i.e., the inflow direction X 0  and the outflow direction Y 0  are parallel to each other.  FIG. 31  shows an intermediate angle of approximately 45° between inflow direction X 0  and outflow direction Y 0 . 
     In  FIGS. 32 to 36 , an alternative embodiment of the filter  1010  with regard to the fastening surface  1016  of the outflow area  1015  is shown. In  FIG. 36 , additionally the geometry of main filter element and secondary filter element is changed in this alternative embodiment. 
     In  FIGS. 32 to 35 , the filter element  1010  is however changed such that a fastening surface  1116  is provided which is located on a lateral cylinder surface. The axis of such a lateral cylinder surface is positioned perpendicular to the drawing plane, i.e., also perpendicular to the main flow direction X 1  of the main filter element  1020 . In accordance with the fastening surface  1116  which, viewed from the exterior, is convex, an outflow socket  1118  is provided which has a corresponding concavely shaped fastening area  1119 . Depending on the fastening location of the outflow socket  1118  on the convex fastening surface  1116 , the outflow direction Y 0  of the filter housing  1011  can be varied. 
       FIG. 36  shows an embodiment in which the filter housing  1011  is substantially comparable to the embodiment of  FIGS. 32 to 35  but the filter element geometry is changed. For avoiding repetitions, the description of features which have been identified in the Figures with same reference characters is omitted. 
     The filter  1110  of  FIG. 36  comprises a main filter element  1120  that has a substantially planar inflow surface  1121  and an outflow surface  1122  that is convexly shaped when viewed from the exterior of the filter element. The outflow surface  1122  in the present invention is substantially a lateral cylinder surface but can also be provided, depending on the application situation, with other curvatures, also with non-uniform curvatures. In accordance with the curvature of the outflow surface  1122 , a secondary element  1130  is provided that comprises an also curved inflow surface  1131  and a curved outflow surface  1132 . The secondary filter element  1130  can have two differently curved inflow and outflow surfaces. In the present embodiment, the outflow surface  1132  is matched to the curvature of the fastening area  1116 . This embodiment represents an embodiment that is particularly optimized with regard to installation space. 
     The attachment of the outflow socket  1018 ,  1118  can be realized, for example, by fusing. Depending on the application situation, the outflow opening  1017  in the outflow area  1015  can be introduced prior to attachment of the outflow socket  1016 ,  1116  in a flexible way, for example, by stamping. 
     The secondary filter element  1130  can be realized, for example, as a curved flat bellows. In manufacture, a conventional flat element could be used that during assembly is then introduced into a correspondingly curved plastic frame and shaped. 
     In each of the illustrated variants, the secondary filter element can comprise an integrated grip for removal of the secondary filter element.