Patent Description:
It is known to use air filters in order to control combustion intake air for internal combustion engines in passenger vehicles, commercial vehicles, work machines, agricultural vehicles, but also stationary generators and the like. Such air filters generally comprise a housing, an air inlet, and an air outlet, with a removable and replaceable main or primary filter element disposed within the housing. The housing often includes a maintenance cover for access to the filter element inside the housing during maintenance. For this purpose, the filter element can be removed and either replaced by a new filter element, overtaken and reused, or replaced by a previously used, but overtaken filter element.

The filter elements of air filters are usually replaced after a certain operating time. Depending on the dust load on the filter element during usage, the service life of a filter can be several days, for example when used in construction machines, up to several months in a less dusty environment.

In particular in the case of frequent exchanges of filter elements, reliable and process-proof sealing of the filter element in a housing is important. The sealing should be temperature resistant and vibration tested. The sealing of the filter element must also be ensured on systems or devices which are exposed to strong vibrations. At the same time, however, the filter element itself should preferably not have any metallic elements, so that it can be disposed of thermally without problems. In order to protect the clean side of an air cleaner system of an internal combustion engine against penetrating dirt particles when replacing the main filter element, a so-called "secondary filter element" is used, which remains in the filter housing during replacement of the primary filter element. The secondary filter element is usually located on the clean side of the filter element, for example inside the primary filter element, and is connected to the housing of the filter system. The secondary filter element itself also has a filter medium which keeps residual dirt particles away from the filter outlet in the air stream.

Optionally, the secondary filter element itself can be exchanged as it can be loaded with dust particles. The lifetime of the secondary filter element, however, is much longer than the lifetime of the primary filter element.

<CIT> and <CIT> disclose filter systems comprising a housing, a cover and an exchangeable filter element. <CIT> discloses an air filter system in which a secondary filter element is disclosed which consists of a filter medium configured as a hollow cylinder which is closed at one side by a closed end cap and is open at the other end. With the open end it can be slipped over a support tube connected to the filter housing, thus protecting the outlet of the filter system against dirt particles. The primary filter element rests on the free end of the supporting tube and thus on the end cap of the secondary filter element. The primary filter element is mounted on the secondary filter element.

It is an object of the invention to provide a filter system for accommodating a primary filter element which ensures a defined flow characteristic of the fluid so that a mass flow sensor in the filter system has reliable operating conditions.

It is a further object of the invention to provide the use of a primary filter element for such a filter system.

The aforementioned object is achieved according to one aspect of the invention by a filter system according to claim <NUM>, comprising a housing, a fluid inlet and a fluid outlet each formed in a housing wall a primary filter element and a secondary filter element, the primary filter element being arranged upstream the secondary filter element, the secondary filter element being arranged on a stand pipe having a longitudinal axis, the stand pipe being rigidly connected to one of the housing walls, the inside of the stand pipe being in fluid connection with the fluid outlet, wherein the primary filter element comprises a first, preferably closed end cap at its top region and a second, preferably open end cap at its bottom region, and wherein the primary filter element and the one of the housing walls accommodating the stand pipe are configured with mutual self-positioning elements to arrange the primary filter element on the one of the housing walls in a defined rotational position with respect to one or more struts of the stand pipe, as well, according to another aspect of the invention by a primary filter element for such a filter system and, according to another aspect of the invention, by a housing wall for an inventive filter system wherein a self-positioning element is arranged around a stand pipe intended to cooperate with a corresponding self-positioning element arranged at a primary filter element.

Advantageous embodiments and advantages of the invention are described in the further claims, the description and the drawings.

According to a first aspect of the invention, a filter system is proposed comprising a housing, a fluid inlet formed in a housing wall, a fluid outlet formed in a housing wall, a primary filter element and a secondary filter element, both being accommodated in the housing. The primary filter element is arranged upstream the secondary filter element, the secondary filter element being arranged on a stand pipe having a longitudinal axis, the stand pipe being rigidly connected to one of the housing walls, the inside of the stand pipe being in fluid connection with the fluid outlet. The primary filter element comprises a first, preferably closed end cap at its top region and a second, preferably open end cap at its bottom region. The primary filter element and the one of the housing walls accommodating the stand pipe are configured with mutual self-positioning elements to arrange the primary filter element on the one of the housing walls accommodating the stand pipe in a defined rotational position with respect to one or more struts of the stand pipe.

According to one preferred embodiment, a first axial end of the stand pipe is connected to the housing wall and the opposite, second axial end of the stand pope defines the top region of the stand pipe. Preferably, the connection elements of the stand pipe are positioned at the top region of the stand pipe. It is further preferred that the one or more connection elements of the secondary filter element are provided on the secondary filter element end cap such that secondary filter element and stand pipe can be mutually connected by connecting the connection elements provided on the end cap and on the stand pipe.

Advantageously, by connecting the stand pipe rigidly with the bottom wall of the housing, the positions of the longitudinal struts of the stand pipe are known. By positioning the primary filter element in a distinct rotational orientation with respect to one or more struts of the stand pipe, disturbances of the flow characteristic due to one or more longitudinal seams of the filter medium of the primary filter element and/or secondary filter element can be reduced or eliminated. When the primary filter element is replaced by a fresh one, a defined rotational position of the primary filter element can be reproduced. Additionally a reproducible position of the seam or seams with respect to a mass flow sensor can be achieved in case the primary filter element is exchanged.

It is of advantage to position the secondary filter element on the stand pipe in a similar defined rotational position with respect to one or more struts of the stand pipe. As a result, the position of the seam or seams, for example, of the secondary filter element on the stand pipe is known, too and can be reproduced when the secondary filter element is exchanged. Preferably, the secondary filter element may have a longitudinal seam. The seam can be positioned radially in front of one particular strut or, alternatively, between two longitudinal struts. In a preferred air filter system for a combustion engine this allows to position the longitudinal struts of the stand pipe and the primary filter element with respect to a mass flow sensor in a way that the flow characteristics of the fluid, i.e. air, is known in the region of the mass flow sensor and the measurements of the mass flow sensor are accurate. The mutual self-positioning elements of the primary filter element and the stand pipe enable a defined position of the longitudinal seam of the filter medium of the primary filter element. As result a flow-disturbing influence of the longitudinal seam on the flow characteristics can be reduced or even eliminated. Additionally a reproducible position of the seam or seams with respect to a mass flow sensor can be achieved in case the secondary filter element is exchanged.

Moreover, a further advantage of such a filter system is the safe and stable assembly of the primary filter element and, optionally, of the secondary filter element, as well as a very economical interchangeability of the primary filter element and, if appropriate, of the secondary filter element, in the event of maintenance.

According to the invention, the mutual self-positioning elements comprise a radially extending protrusion and a guiding surface, the radially extending protrusion being configured to glide along the guiding surface on a relative rotational movement of the primary filter element and the one of the housing walls about a common rotational axis. Favorably, the common rotational axis may be the longitudinal axis. The primary filter element may be guided into its end position by a movement of the radially extending protrusion on the guiding surface. This allows for a reproducible mounting of a primary filter element in the housing. The guiding surface may be arranged at the housing wall or at the primary filter element. The radially extending protrusion may be arranged at the primary filter element or at the housing wall correspondingly.

According to a favorable embodiment, the radially extending protrusion may be arranged on the outside of the second end cap of the primary filter element and the guiding surface may be arranged on the inside of the one of the housing walls. In particular, the guiding surface may be arranged surrounding the stand pipe. This arrangement can be manufactured easily without altering the body of the filter medium. The radially extending protrusion may be integrated in the second end cap or a ring element embedded in the second end cap.

According to a favorable embodiment, the guiding surface may include a notch for accommodating the radially extending protrusion in its locked position. The radially extending protrusion can be locked firmly in the notch in a well-defined rotational position.

According to a favorable embodiment, a ring may be embedded in the second end cap, wherein the ring comprises one of the self-positioning elements. Advantageously, the ring can easily be embedded in the end cap during manufacturing the end cap.

According to a favorable embodiment, the secondary filter element and the stand pipe may be configured with mutual self-positioning elements to arrange the secondary filter element on the stand pipe in a defined rotational position with respect to one or more struts of the stand pipe. The mutual self-positioning elements may comprise a guiding surface and a projection element, wherein the guiding surface is intended to guide the projection element from an initial position to a final position where the secondary filter element is in its defined rotational position with respect to the one or more struts of the stand pipe. Appropriate pairs of mutual self-positioning elements can be chosen. Additionally or alternatively, the mutual self-positioning elements may comprise a contour arranged at an exterior surface of one of the stand pipe and the secondary filter element and a counter contour at an interior surface of the other one of the stand pipe and the secondary filter element. Favorably, a polygonal contour on the stand pipe can be used for mounting the stand pipe to the bottom wall of the housing in a position oriented manner. Dual use can be made of such a contour for aligning the secondary filter element on the stand pipe, too, when an appropriate counter contour is provided on the secondary filter element.

According to a further aspect of the invention the use of a primary filter element for the inventive filter system is proposed, according to claim <NUM>, the primary filter element comprising a filter medium, along a longitudinal axis and a first end cap at its top region and a second end cap at its bottom region, the second end cap having a feedthrough for a stand pipe. The second end cap is provided with a self-positioning element to arrange the primary filter element on one of the housing walls of the filter system in a defined rotational position with respect to one or more struts of a stand pipe arranged on the one of the housing walls of the filter system. The primary filter element allows for a reliable and reproducible mounting of the primary filter element into a housing.

According to a favorable embodiment of the primary filter element, a ring may be embedded in the second end cap, wherein the ring comprises one of the self-positioning elements. The self-positioning element can be easily manufactured without altering the primary filter element as such.

According to a favorable embodiment of the primary filter element, the self-positioning element may extend to the exterior of the primary filter medium. The self-positioning element can be easily manufactured without altering the primary filter element as such.

According to a favorable embodiment of the primary filter element, the first end cap may be provided with a protrusion element extending in axial direction. The filter body of the primary filter element as such does not need to be adapted to the self-positioning element. In particular, the protrusion element may be arranged eccentrically on the first end cap with respect to a centre axis of the first end cap. The centre axis may be the longitudinal axis of the primary filter element and/or the secondary filter element. The axial protrusion allows for facilitating the positioning of the primary filter element and the secondary filter element. Advantageously, the secondary filter element is configured with a corresponding recess at that end cap facing the axial protrusion of the primary filter element.

Advantageously, the inventive filter system and primary filter element can be used as an air filter, in particular as an air filter of an internal combustion engine. The safe operation of internal combustion engines is also based on safe and favorable filtering of the intake air for combustion operation, in particular with an undisturbed flow characteristic of the air at the mass flow sensor. The described filter system represents an economic and reliable possibility for this purpose. The use of the filter system as a particle filter, in particular as a particle filter of an internal combustion engine, is also advantageous. Again, the secure assembly and economic interchangeability of the described filter elements is of advantage. Conveniently, the secondary filter element may remain in the housing when the primary filter element is changed. This ensures that the clean side of the filter system is also effectively protected against penetrating dirt particles during maintenance of the filter system.

According to a favorable embodiment of the housing wall of the inventive filter system a self-positioning element is arranged around the stand pipe intended to cooperate with a corresponding self-positioning element arranged at the primary filter element.

According to a favorable embodiment of the housing wall, the self-positioning element may be arranged to accommodate the primary filter element in a defined rotational position with respect to one or more struts of the stand pipe. When the primary filter element has to be mounted to the housing, a defined rotational position of the primary filter element can be reproduced. Additionally a reproducible position of the seam or seams with respect to a mass flow sensor can be achieved in case the primary filter element is exchanged.

According to a favorable embodiment of the housing wall, the self-positioning element may be configured as a guiding surface surrounding the stand pipe. This arrangement can be manufactured easily without altering the body of the filter medium of the primary filter element or the secondary filter element. A corresponding radially extending protrusion may be integrated in the second end cap or a ring element embedded in the second end cap.

According to a favorable embodiment of the housing wall, the guiding surface has a maximum region and a minimum region arranged diametrically vis-à-vis the maximum region. Favorably, the minimum region may comprise a notch where a corresponding self-positioning element may rest in its final position.

Further advantages result from the following drawing description. Embodiments of the invention are shown in the drawings. The drawings, the description, and the claims contain numerous features in combination. The person skilled in the art will expediently also consider the features individually and combine them into sensible further combinations. For example,.

The figures show merely examples and are not intended to be limiting. Similar or equal elements are referred to with same reference numerals in the Figures.

<FIG> shows a longitudinal cut view of a filter system <NUM> according to an embodiment of the invention. <FIG> shows the filter system <NUM> of <FIG> in an exploded view.

The filter system <NUM> comprises a housing <NUM>, a fluid inlet <NUM> formed in a housing wall <NUM>, a fluid outlet <NUM> formed in a bottom housing wall <NUM>. In this embodiment, the housing <NUM> may consist of three segments, the bottom housing wall <NUM>, the intermediate ring-shaped housing wall <NUM> with the fluid inlet <NUM> and a cover part <NUM>. The segments are connected to each other by, e.g., clamps, screws or the like. A hollow cylindrical primary filter element <NUM> is accommodated in the housing <NUM>. For removing the primary filter element <NUM> for maintenance or exchange the housing walls <NUM>, <NUM> can be removed from the bottom housing wall <NUM> together in one piece or only the cover part <NUM> is removed.

The body of the primary filter element <NUM> is made of a filter medium <NUM> which may be pleated, for instance. The primary filter element <NUM> is covered at both ends by ring shaped first and second end caps <NUM>, <NUM>, which are made, for example, from polyurethane which is well known in the prior art. Sealing structures and supporting ribs are arranged at the exterior sides of the first end cap <NUM> and the second end cap <NUM>. The primary filter element <NUM> is clamped between the bottom housing wall <NUM> and the cover housing wall <NUM> in a sealing tight manner so that a fluid has to pass through the primary filter element <NUM> in a radial direction, which is indicated by bold arrows in <FIG>. The second end cap <NUM> of the primary filter element <NUM> is configured to have a feedthrough for the stand pipe <NUM> and is accommodated in the bottom housing wall <NUM> which is provided with a groove <NUM>. For radial fixation of the primary filter element <NUM>, a ring shaped projection <NUM> is arranged in the groove <NUM>.

In its front face <NUM>, the cover part <NUM> is provided with a recess <NUM> which extends into the interior of the housing <NUM>.

Inside the primary filter element <NUM>, a secondary filter element <NUM> is arranged. The primary and secondary filter elements <NUM>, <NUM> are arranged concentrically about an axis extending in a longitudinal direction L, referred to as longitudinal axis L. The secondary filter element <NUM> is arranged downstream the primary filter element <NUM> so that fluid has to pass through the secondary filter element <NUM> on its way to the fluid outlet <NUM>. The secondary filter element <NUM> is arranged on a stand pipe <NUM> which is rigidly connected to the bottom housing wall <NUM>. The lower part of the stand pipe <NUM> merges with the fluid outlet <NUM> of the bottom housing wall <NUM>. The secondary filter element <NUM> comprises a filter medium <NUM> forming a body <NUM> with at least one weld seam <NUM> along the longitudinal direction L.

Optionally, the secondary filter element <NUM> comprises a closed end cap <NUM> at its top region <NUM>, wherein the secondary filter element <NUM> and the stand pipe <NUM> are mutually connected at their top regions <NUM>, <NUM> by connection elements. The open ended side of the secondary filter element <NUM> is accommodated in a circular groove <NUM> in the bottom wall <NUM>.

The first end cap <NUM> at the top region of the primary filter element <NUM> is provided with a protrusion element <NUM> extending in axial direction towards the top region <NUM> of the secondary filter element <NUM>.

The bottom region <NUM> of the primary filter element <NUM> and the housing wall <NUM> accommodating the stand pipe <NUM> are configured with mutual self-positioning elements <NUM>, <NUM> to arrange the primary filter element <NUM> on the housing wall <NUM> in a defined rotational position with respect to one or more struts <NUM> of the stand pipe <NUM>. The struts <NUM> are arranged about the longitudinal axis L and are pointing to the interior of the stand pipe <NUM>.

In particular, a radially extending protrusion <NUM> is arranged at the bottom part <NUM> of the primary filter element <NUM> as self-positioning element <NUM>. A guiding surface <NUM> is arranged around the stand pipe <NUM> as self-positioning element <NUM>. The guiding surface <NUM> is formed as a ramp which has a maximum region with a maximum height in relation to the longitudinal axis L and a minimum region with a minimum height in relation to the longitudinal axis L at the opposite side of the stand pipe <NUM>. The radially extending protrusion <NUM> can glide on the guiding surface <NUM> from the maximum region to the minimum region when the primary filter element <NUM> and/or the bottom housing wall <NUM> are rotated about the longitudinal axis L. A notch <NUM> is arranged in the minimum region in which the radially extending protrusion <NUM> of the primary filter element <NUM> can snap into place.

<FIG> shows a first embodiment of a secondary filter element <NUM> with an end cap <NUM> at its top region <NUM> having a recess <NUM> at its exterior side. <FIG> shows a bottom wall <NUM> of a housing <NUM> of a filter system <NUM> with a stand pipe <NUM> rigidly attached to the bottom wall <NUM> to which the secondary filter element <NUM> of <FIG> can be attached.

<FIG> shows a first embodiment of a secondary filter element <NUM> with an end cap <NUM> having a recess <NUM> at its exterior side. The secondary filter element <NUM> has a body <NUM> consisting of a filter medium <NUM>. The filter medium <NUM> can be a nonwoven material, paper, cellulose or a mixed fiber of plastic and cellulose. The filter medium <NUM> can be designed endlessly on the circumference of the secondary filter element <NUM> with at least one longitudinal weld seam <NUM>. The filter body <NUM> formed thereof has a conical shape with a large diameter at a bottom region <NUM> and a smaller diameter at the top region <NUM> of the body <NUM> where the end cap <NUM> is mounted. Preferably the filter medium <NUM> is connected to the end cap <NUM> by way of a circumferential weld seam <NUM>. The recess <NUM> in the end cap <NUM> extends into the interior of the body <NUM>.

<FIG> shows a bottom wall <NUM> of a housing <NUM> of a filter system <NUM> with a stand pipe <NUM> attached to the bottom wall <NUM>. The lattice-like body of the stand pipe <NUM> is conically shaped and composed of longitudinal struts <NUM> and circumferential struts <NUM>, only two of each are referred to with reference numerals and reference lines for clarity reasons.

The stand pipe <NUM> is provided as a carrier of the secondary filter element <NUM> as shown in <FIG>. The secondary filter element <NUM> and the stand pipe <NUM> each comprise complementary connecting means with which they can be connected to one another at one of their end faces, shown on top of the secondary filter element <NUM> and the stand pipe <NUM> in the <FIG>. This allows for a safe and stable assembly of both the primary filter element <NUM> (<FIG>, <FIG>) and the secondary filter element <NUM>, as well as a very economical interchangeability of the primary filter element <NUM> and, if appropriate, of the secondary filter element <NUM> in the event of maintenance. The secondary filter element <NUM> rests firmly on the stand pipe <NUM>. Due to the closed end cap <NUM> of the secondary filter element <NUM>, even with dismounted primary filter element <NUM> the clean side of the filter system <NUM> is protected against particulate matter even if fluid is still sucked through the secondary filter element <NUM>.

The stand pipe <NUM> in this embodiment comprises a receptacle <NUM> at its top region <NUM>. The contour of the receptacle <NUM> corresponds to the exterior contour of the endcap <NUM> of the secondary filter element <NUM>. The receptacle <NUM> has an open ended bottom which ends in a connection element <NUM> for the connection elements (not shown) of the secondary filter element <NUM>. For instance, snap beams can be hooked on the bottom of the receptacle <NUM> as locking element and establish a snap fit connection between the stand pipe <NUM> and the secondary filter element <NUM>. The receptacle <NUM> is funnel shaped in its bottom region so that introducing the snap beams is facilitated.

<FIG> and <FIG> illustrate a bottom housing wall <NUM> according to an embodiment of the invention. <FIG> shows a side view of the housing wall <NUM> with a rigidly attached stand pipe <NUM>. <FIG> shows a cut view of the housing wall <NUM> according to <FIG> rotated by <NUM>° about a longitudinal axis L. The interior of the stand pipe <NUM> is in fluid connection with the fluid outlet <NUM>.

The stand pipe <NUM> is surrounded by a self-positioning element <NUM> being configured as guiding surface <NUM> having the shape of a ring ramp. The guiding surface <NUM> is formed as a ramp which has a maximum region with a maximum height in relation to the longitudinal axis L and a minimum region with a minimum height in relation to the longitudinal axis L at the opposite side of the stand pipe <NUM>. A radially extending protrusion <NUM> (<FIG> and <FIG>) can glide on the guiding surface <NUM> from the maximum region to the minimum region when the primary filter element <NUM> and/or the bottom housing wall <NUM> are rotated about the longitudinal axis L. A notch <NUM> is arranged in the minimum region in which the radially extending protrusion <NUM> of the primary filter element <NUM> can snap into place.

<FIG> shows a primary filter element <NUM> according to an embodiment of the invention comprising a radially extending protrusion <NUM> at the outer circumference of the second end cap <NUM>. <FIG> shows a ring <NUM> for accommodating the filter medium <NUM> of the primary filter element <NUM> comprising a radially extending protrusion <NUM> at the outer circumference of its second end cap <NUM>. The primary filter element <NUM> has a filter body consisting of a filter medium <NUM> which extends between a first end cap <NUM> and a second end cap <NUM> which has a feedthrough for the stand pipe <NUM> (<FIG> and <FIG>). The filter medium <NUM> is attached to the outer surface of a support structure <NUM> arranged in the interior of the primary filter element <NUM>. A self-positioning element <NUM> configured as a radially extending protrusion <NUM> is arranged at the second end cap <NUM>. The radially extending protrusion <NUM> is part of a ring <NUM> which is embedded in the material of the second end cap <NUM>. The ring <NUM> is shown in <FIG>. The ring <NUM> accommodates the filter body consisting of the filter medium <NUM> in its interior. The ring <NUM> has an outer rim which overlaps the bottom part of the filter body in axial direction.

<FIG> and <FIG> show side views of a primary filter element <NUM> mounted to a housing wall <NUM> in two rotational positions distinct by <NUM>° relative to the longitudinal axis L. In <FIG>, the guiding surface <NUM> is inclined from the maximum region on the right side of the primary filter element <NUM> to the minimum region on the left side of the primary filter element <NUM>. In <FIG>, the minimum region with the notch <NUM> is seen as front view. The radially extending protrusion <NUM> is snapped into the notch <NUM> and secures the rotational position of the primary filter element <NUM> in the bottom housing wall <NUM>.

<FIG> and <FIG> illustrate in cut views how the primary filter element <NUM> according to <FIG> is introduced in the bottom housing wall <NUM>. <FIG> shows a cut view of the primary filter element <NUM> in a state where the primary filter element <NUM> is mounted to the housing wall <NUM> accommodating the stand pipe <NUM>. The radially extending protrusion <NUM> is seen on the left side of the drawing where it contacts the maximum region of the guiding surface <NUM>.

<FIG> shows a cut view of the primary filter element <NUM> according to <FIG> where the primary filter element <NUM> is arranged at the housing wall <NUM> in its final, rotationally defined position with respect to struts <NUM> of the stand pipe <NUM>. The primary filter element <NUM> has been turned by <NUM>° about the longitudinal axis L and the radially extending protrusion <NUM> rests in the notch <NUM> of the guiding surface <NUM>.

<FIG> shows an embodiment of a secondary filter element <NUM> which is mounted to a stand pipe <NUM> before the secondary filter element <NUM> is in its final position. To mount the secondary filter element <NUM> onto the stand pipe <NUM>, the body <NUM> of the secondary filter element <NUM> is put over and moved along the stand pipe <NUM> until the end cap <NUM> comes close the receptacle <NUM> of the stand pipe <NUM>.

The end cap <NUM> enters the receptacle <NUM> towards the funnel-shaped bottom region of the receptacle <NUM> with the snap beams <NUM> first. The end cap <NUM> can be introduced into the receptacle <NUM> until the projection element <NUM> hits the guiding surface <NUM>. The longitudinal extension <NUM> of the projection element <NUM> is smaller than the depth <NUM> of the top segment <NUM> of the receptacle <NUM>. Thus, the upper segment <NUM> of the end cap <NUM> can be immersed partly into the top segment <NUM> so that the step <NUM> is safely inside the receptacle <NUM>. As a result, the end cap <NUM> can be guided further into the receptacle <NUM> safely without tilting.

The guiding surface <NUM> has one maximum point <NUM> in the bottom segment <NUM> of the receptacle <NUM> and is inclined on both sides of the maximum point <NUM>. The maximum point <NUM> of the guiding surface <NUM> is at the interface between the top segment <NUM> and the bottom segment <NUM> of the receptacle <NUM>. By turning the end cap <NUM> in either direction about the longitudinal axis L the end cap <NUM> moves further into the receptacle <NUM> because the projection element <NUM> is guided on the guiding surface <NUM> until the projection element <NUM> reaches the pocket <NUM> in the guiding surface <NUM>.

The end cap <NUM> now is moved axially until the projection element <NUM> is accommodated in the pocket <NUM>. As the snap beams <NUM> move axially downward, too, they can snap over the rim of the funnel shaped bottom segment <NUM>. The rim is the locking element <NUM> of the stand pipe <NUM>. The snap beams <NUM> lock the end cap <NUM> safely to the stand pipe <NUM>.

The end cap <NUM> may be removed from the stand pipe <NUM> by an opposite sequence of movement by pulling and turning the end cap <NUM>.

For turning the end cap <NUM>, a tool such as a handle or the like can be applied to the recess <NUM> of the end cap <NUM>.

With the projection element <NUM> accommodated in the pocket <NUM>, the secondary filter element <NUM> is positioned accurately in a well-defined rotational position with respect to the stand pipe <NUM>. Hence, the longitudinal weld seam (not shown) is in a distinct position with respect to the longitudinal struts <NUM> of the stand pipe <NUM> and, consequently, to a mass flow sensor arranged at a fixed position close to the filter elements. Preferably, the hooks at the snap beams <NUM> are provided with inclined surfaces so that these can come loose when some force is applied to the end cap <NUM>.

Claim 1:
A filter system (<NUM>) comprising a housing (<NUM>), a fluid inlet (<NUM>) formed in a housing wall (<NUM>), a fluid outlet (<NUM>) formed in a housing wall (<NUM>), a primary filter element (<NUM>) and a secondary filter element (<NUM>), both being accommodated in the housing (<NUM>),
wherein the primary filter element (<NUM>) is arranged upstream the secondary filter element (<NUM>), the secondary filter element (<NUM>) being arranged on a stand pipe (<NUM>) having a longitudinal axis (L), the stand pipe (<NUM>) being rigidly connected to one of the housing walls (<NUM>), the inside of the stand pipe (<NUM>) being in fluid connection with the fluid outlet (<NUM>),
wherein the primary filter element (<NUM>) comprises a closed first end cap (<NUM>) at a top region (<NUM>) of the primary filter element (<NUM>) and an open second end cap (<NUM>) at a bottom region (<NUM>) of the primary filter element (<NUM>), and
wherein the primary filter element (<NUM>) and the one of the housing walls (<NUM>) accommodating the stand pipe (<NUM>) and the bottom region (<NUM>) of the primary filter element (<NUM>) are configured with mutual self-positioning elements (<NUM>, <NUM>) to arrange the primary filter element (<NUM>) on the one of the housing walls (<NUM>) accommodating the stand pipe (<NUM>) in a defined rotational position with respect to one or more struts (<NUM>) of the stand pipe (<NUM>, characterized in that the mutual self-positioning elements (<NUM>, <NUM>) comprise a radially extending protrusion (<NUM>) and a guiding surface (<NUM>), the radially extending protrusion (<NUM>) being configured to glide along the guiding surface (<NUM>) on a relative rotational movement of the primary filter element (<NUM>) and the one of the housing walls (<NUM>) about a common rotational axis.