The invention concerns a filter cartridge and the relative filter group for filtering fluids, particularly in the field of motor engines, for filtering fuel such as diesel fuel or petrol, or other more or less viscous fluids, such as lubricating oil or the like.

As is known, the filtration of a fluid in the field of motor engines, whether the fluid is a type of fuel, a lubricating oil or other type of fluid, is generally obtained by means of a filtering group which comprises an external casing provided with a substantially beaker-shaped body, the open end of which is closed by a cover provided with an inlet for the fluid to be filtered and an outlet for the filtered fluid. The casing internally contains at least a filter cartridge which is designed to divide the internal volume of the casing into two distinct chambers, a first chamber of which communicating with the inlet and a second chamber of which communicating with the outlet. In this way, the fluid flows from the inlet towards the outlet of the filter group and is forced to pass through the filter cartridge, which retains any impurities that might be present in the fluid. Sometimes a filter cartridge can comprise a first and a second filter wall, which are designed such that the fluid to be filtered passes through the walls in series, thus performing a coarse pre-filtration and a fine filtration on the fluid passing through.

In some embodiments, the first and second filter walls are tubular in shape, coaxially arranged and reciprocally superposed in the direction of the common axis. The axial ends of each of the filter walls are constrained to an upper support plate and a lower support plate respectively. The upper support plate of the first filter wall is generally provided with a central opening and with at least an offset opening, both of which openings lead to the internal cavity of the first filter wall. The upper support plate of the second filter wall is provided with a hollow cylindrical shank, which communicates with the internal cavity of the second filter wall and is inserted in the internal cavity of the first filter wall, such as to subdivide the internal cavity into two separate chambers, a chamber of which communicates with the central opening and another chamber of which communicates with the offset opening, which openings are afforded in the upper plate of the first filter wall. In this way, the filter cartridge overall defines a forced trajectory for the fluid, the fluid thus being forced to pass through both the first and the second filter wall.

In filter cartridges of this type, the first and the second filter walls are connected to one another by snap-fastening means, which means comprise a portion associated to the upper plate of the second filter wall, which portion can snap-fasten to a corresponding portion of the lower plate of the first filter wall. The hermetic seal of this connection is usually guaranteed by an annular, gasket, which is axially interposed and compressed between the upper plate of the second filter wall and the lower plate of the first filter wall, such that the hollow shank is surrounded.

A drawback of this solution is due to the fact that the first and the second filter walls are exposed to greatly differing operating pressures. In certain cases, the difference between operating pressures is subject to very sharp and large variations, which tend to distance the second filter wall axially from the first. In such cases, the snap-fastening connection between the support plate of the second filter wall and the lower plate of the first filter wall may not be sufficiently rigid and stable to keep the axial seal gasket sufficiently compressed, thus allowing the gasket to leak small amounts of fluid, and compromising the correct functioning of the filter cartridge.

An aim of an embodiment of the invention is therefore to provide a more stable and rigid system than systems in the prior art for constraining the first filter wall to the second filter wall. A further aim is to achieve the above- mentioned objective within the ambit of a simple, rational and relatively low-cost solution.

These and other aims are achieved by the characteristics of the invention claimed herein below in independent claim 1. The dependent claims delineate preferred and/or preferably advantageous aspects of the invention.

In detail, an embodiment of the invention provides a filter cartridge comprising at least:

- a first and a second tubular-shaped filter wall, which are coaxial and reciprocally superposed in the direction of the common axis ,

- a first and a second intermediate support plate, which are singly constrained to an end of the first and the second filter walls respectively, in such a way that the support plates are axially interposed between the first and the second filter walls,

- a first and a second external support plate, which are singly constrained to the opposite end of the first and the second filter walls respectively, the first external support plate being provided with at least a central opening and at least an offset opening which is off-axis with respect to the central opening, both of which openings lead to the internal cavity of the first filter wall, and a hollow cylindrical shank associated to the second intermediate support plate, which shank communicates with the internal cavity of the second filter wall and is inserted in the internal cavity of the first filter wall via a through-hole in the first intermediate support plate, such that the internal volume of the first filter wall is divided into two separate chambers, of which a chamber communicating with the central opening and another chamber communicating with the offset opening of the first external support plate, wherein the first and the second filter walls are joined together by at least a fixing screw which is coupled with the first external support plate and with the cylindrical shank.

Thanks to this solution, the connection between the first and second filter walls is constructionally extremely simple, since it does not require the realisation of snap- fastening means in the intermediate plates of the two filter walls. Further, the fixing screw realises a much more stable and rigid connection than the traditional snap-fastening means.

Another advantage of this solution is that should it effectively be necessary to separate the two filter walls (for example in order to replace an individual filter wall), unscrewing the fixing screw completely releases the bond between the two filter walls, and the walls can therefore be separated simply by sliding one out from the other, in a very rapid way and without having to tap or shake the walls. In an aspect of the invention, the first and the second filter walls are preferably joined together by a plurality of fixing screws (for example three) which are singly coupled with the first external support plate and with the cylindrical shank.

In this way, the resulting connection between the filter walls is even more rigid and stable.

In an embodiment of the invention, the fixing screws can be distributed offset around the axis of the cylindrical shank. Further, the fixing screws can be reciprocally arranged at different annular distances, i.e. not all be angularly equidistant with respect to the central axis of the cylindrical shank.

This solution has the advantage of allowing the two filter walls to be joined only in a predetermined reciprocal orientation with respect to the central axis of the cylindrical shank, there being no need to provide any other system of angular constraint, such as for example the realisation of ribbings and the relative receiving cavities. In many applications it is in fact necessary for the filter walls of the cartridge to exhibit a predetermined reciprocal angular position, especially when other functional elements are provided which must be associated to both filter walls, such as for example a degassing conduit.

In another aspect of the invention, each fixing screw can be made of plastic or metal, for example of a polymer material or stainless steel respectively.

In more detail, each fixing screw can be inserted in a through-hole afforded in the first external support plate and can be screwed into the cylindrical shank.

For example, the fixing screw can be screwed into a tapped insert, made of plastic or metal, which is preliminarily inserted and stably anchored within the cylindrical shank.

Alternatively, the fixing screw could be a self-tapping screw, screwed directly into the cylindrical shank, possibly in a small pilot hole afforded in the shank.

In a further aspect of the invention, the cartridge comprises a radial gasket which is coaxially interposed between the cylindrical shank and the through-hole of the first intermediate support plate.

Thanks to this solution, the coupling between the two filter walls is no longer guaranteed by an axial gasket, i.e. a gasket which is axially compressed between the intermediate support plates, as is the case in the prior art, but by a gasket which acts radially, i.e. which is radially compressed between the cylindrical shank and the hole of the plate in which the shank is inserted. Therefore if the two filter walls should distance themselves slightly in an axial direction despite the presence of the fixing screws, this displacement would have no effect on the compression of the gasket and therefore no effect on the seal of the coupling between the two filter walls.

Lastly, another embodiment of the invention provides a filter group comprising an external casing which is provided with at least an inlet for the fluid to be filtered and at least an outlet for the filtered fluid, and the filter cartridge which is delineated herein above and contained within the external casing, such that the first and the second filter walls can be passed through in series by the fluid flowing from the inlet to the outlet.

This embodiment of the invention substantially achieves the above-mentioned advantages, including in particular the advantage of improving the hermetic seal between the chamber communicating with the inlet of the fluid to be filtered and the chamber communicating with the outlet of the filtered fluid, in a simple and relatively-economical way.

Further characteristics and advantages of the invention will emerge, with the aid of the appended drawings, from the description herein below, which is provided as a non- limiting example.

Figure 1 is a section view along an axial vertical plane of a filter group in a first embodiment of the present invention. Figure 2 is an enlarged detail of a filter cartridge belonging to the group which is shown in figure 1.

Figure 3 is a plan view of the filter cartridge of figure 2. Figure 4 is a side view of the filter cartridge which is partially sectioned along plane IV-IV of figure 3.

The above-mentioned figures show, in its entirety, a filter group 100 for filtering diesel fuel in a Diesel engine of a motor vehicle. In other embodiments, the filter group 100 could however be used for filtering other types of fuel, for example petrol, or other types of fluid, such as for example lubricating oil and the like.

The filter group 100 comprises an external casing, indicated in its entirety by reference numeral 105, which casing in turn comprises a beaker-shaped body 110, and an upper cover 115, able to close the beaker-shaped body 110. At least an inlet conduit 120 for the diesel fuel to be filtered, an outlet conduit 125 for the filtered diesel fuel, and a discharge mouth 130 for the water which can separate from the diesel fuel during the filtering process, are arranged on the upper cover 115. The outlet conduit 125 is arranged coaxially with the beaker-shaped body 110, while the inlet conduit 120 and the discharge mouth 130 are arranged in an offset position.

A filter cartridge, denoted in its entirety by reference numeral 200, is received inside the casing 105, the filter cartridge 200 substantially comprising two distinct, reciprocally superposed filter units, an upper filter unit 205 and a lower filter unit 210.

The upper filter unit 205 comprises a first filter wall 215 which is tubular in shape, in this specific case substantially cylindrically-shaped, which delimits an internal through-cavity. The first filter wall 215 can be realised as a depth . wall, for example via a melt-blown process. The first filter wall 215 is generally a coarse wall, i.e. exhibiting a somewhat high average porosity, the wall being prevalently designed to trap impurities of relatively large dimensions.

The upper filter unit 205 further comprises two support plates, an upper support plate 220 and a lower support plate 225 of discoid shape and generally made of a plastic material, which are coaxially constrained to opposite ends of the first filter wall 215, such as to close the internal cavity of the first filter wall 215.

As shown in figures 2 and 3, the upper support plate 220 comprises a cylindrical cover 230, which is coaxial with the first filter wall 215 and axially projects from the opposite side with respect to the internal cavity. At the bottom of the cylindrical cover 230, the upper support plate 220 coaxially comprises a central through-hole 235 which through-hole 235 is designed to place the internal cavity of the first filter wall 215 in communication with the external environment.

The upper support plate 220 further comprises a plurality of offset slots 240, each of which is radially positioned between the cylindrical cover 230 and the internal surface of the first filter wall 215, such as to be separate from, and offset with respect to, the central hole 235. The offset slots 240 pass through the thickness of the upper support plate 220, thus placing the internal cavity of the first filter wall 215 in communication with the external environment .

Returning to figure 1, the lower support plate 225 also exhibits a central through-hole 245 which is coaxial with the first filter wall 215, which hole is able to place the internal cavity of the first filter wall 215 in communication with the external environment. In detail, the central hole 245 is preferably delimited by a cylindrical wall, realised as a single body with the lower support plate 225, the cylindrical wall also extending for a brief tract inside the internal cavity of the first filter wall 215.

The lower filter unit 210 comprises a second filter wall 250, in this particular case of a tubular, cylindrical shape, which delimits an internal through-cavity. The second filter wall 250 can be realised as a thin wall, which for example is pleated and star-shaped. The second filter wall 250 is generally a fine filter, i.e. exhibiting a higher average porosity than that of the first filter wall 215, thus being capable of retaining impurities of smaller dimensions. The second filter wall 250 can be internally supported by a support core 255, which is inserted in the internal cavity and is provided with lateral openings 260 for the passage of the diesel fuel.

The lower filter unit 210 further comprises two disc-shaped support plates, an upper support plate 265 and a lower support plate 270, which are generally realised in plastic and are coaxially constrained to opposite ends of the second filter wall 250, such as to close the central cavity of the second filter wall 250.

In detail, the lower support plate 270 is continuous and completely closes the lower end of the second filter wall 250, while the upper support plate 265 comprises a cylindrical shank 275, arranged coaxially with the second filter wall 250, communicates with the internal cavity of the second filter wall 250 and axially projects externally. In the shown example, the cylindrical shank 275 is realised as a single body with the upper support body 265, while the support core 255 is realised as a separate body and is snugly constrained between the upper plate 265 and the lower plate 270.

In other embodiments, the cylindrical shank 275 could be realised as a single body with respect to the upper support plate 265 and/or realised as a single body with the core 255.

The cylindrical shank 275 is coaxially inserted and substantially snugly constrained in the central hole 245 of the lower support plate 225 of the upper filter unit 205. In this way, the first and the second filter walls 215 and 250 are coaxial and are reciprocally superposed in the direction of the common axis X, the support plates 225 and 265 facing each other and being axially interposed between the filter walls 215 and 250, and the support plates 220 and 270 positioned at the external ends.

In particular, the cylindrical shank 275 exhibits substantially the same lengthwise extension as the first filter wall 215 and the end of the cylindrical shank 275 coaxially engages inside the cylindrical cover 230 of the upper support plate 220, thus substantially abutting against the bottom wall and therefore surrounding the central hole 235. In this way, the cylindrical shank 275 divides the internal cavity of the first filter wall 215 into two separate chambers, a chamber 280 which is defined inside the cylindrical shank 275 and communicates with the central hole 235 and with the internal cavity of the second filter wall 250, and a chamber 285, which is defined between the cylindrical shank 275 and inner surface of the first filter wall 215, and communicates with the offset slots 240.

The hermetic separation between the two chambers 280 and 285 is guaranteed by an annular gasket 290, for example an CD- ring, which is radially and coaxially interposed between the outer surface of the cylindrical shank 275 and the inner surface of the central hole 245 of the support plate 225. In the illustrated example, the annular gasket 290 is axially retained inside a circumferential recess fashioned in the cylindrical shank 275, but it could be alternatively or further be received also in a circumferential recess fashioned in the central hole 245.

As shown in figures 2 and 3, the filter units 205 and 210 are joined together only by a plurality of fixing screws 295, made of a plastic or metal material, which are directly coupled with the upper support plate 220 and at the end of the cylindrical shank 275, such as to be stably constrained to one another. In more detail, each fixing screw 295 is oriented parallel to the X axis of the filter walls, is freely inserted in a through-hole afforded in the lower support plate 220, in this specific case in the bottom wall of the cylindrical cover 230, and is frontally screwed to the end of the cylindrical shank 275 which is inserted in the cylindrical cover 230, in this specific case at a thicker portion of the end of the cylindrical shank 275.

In the illustrated example, the fixing screws 295 are self- tapping screws, each screw being screwed into a respective pilot hole 300 which is previously realised at the end of the cylindrical shank 275. In other embodiments, the pilot holes 300 could be absent, or the self-tapping screws 295 could be replaced by common screws, each screw screwing into a corresponding tapped hole which is associated to the cylindrical shank 275. In more detail, the tapped hole could be realised by a tapped insert of plastic or metallic material and incorporated at the end of the cylindrical shank 275 during the manufacturing of the cylindrical shank 275.

As can be seen in figure 3, the fixing screws 295 are distributed in offset positions around the central axis X of the cylindrical shank 275, such that they are substantially aligned along an imaginary circumference which centres on the axis X. The fixing screws 295 are not angularly equidistant from each other along this circumference, being arranged instead at differing angular distances, i.e. the angular distance separating a consecutive pair of fixing screws 195 differs from the angular distance separating at least another pair of consecutive fixing screws 295. In this way, the two filter units 205 and 210 can be connected to each other only in a predetermined reciprocal orientation with respect to the central axis X of the cylindrical shank 275, there being no need to provide any other angular constraint system.

The fact that the two filter units 205 and 210 can be connected only in a predetermined reciprocal orientation is advantageous for various reasons, including that of facilitating the fitting of the degassing conduit 301 which is shown in figure 4.

The degassing conduit 301 comprises a pipe 305 exhibiting at least an access mouth 310 located externally of the first filter wall 215 at the level of the upper support plate 220 and fits in a suitable seating 221 afforded in the upper support plate 220. The pipe 305 is substantially L-shaped and exhibits a vertical portion, which descends from the upper support plate 220 towards the lower support plate 270, and a horizontal portion, the free end of which defines an outlet mouth 315 for the pipe 305. The outlet mouth 315 is hermetically inserted in a hollow connecting element 320 which is afforded as a single body in the lower support plate 270, and which is designed to put the pipe 305 in communication with the internal volume of the second filter wall 250. In practice, the connecting element 320 defines an elbow-shaped tubular tract exhibiting a horizontal portion which extends radially from the peripheral end of the lower support plate 270 towards the central zone thereof, and a vertical portion which terminates at the surface of the lower support plate 270 facing the internal cavity of the second filter wall 250.

The degassing conduit 301 further comprises a choke 325, the internal diameter of which is dimensioned such that that the drop in pressure that takes place when the diesel fuel passes upstream and downstream of the choke 325 is greater than the drop in pressure of the diesel fuel upstream and downstream of the second filter wall 250. In this way, the choke 325 ensures a preferential passage for gases through the degassing conduit 301 and, at the same time, ensures a preferential passage for diesel fuel through the second filter wall 250, under all operating conditions of the filter group 100.

In the illustrated example, the choke 325 is defined by a cylindrical insert which is sealedly inserted in the connecting element 320 downstream of the outlet mouth 315 of the pipe 305, the cylindrical insert exhibiting an inner diameter which narrows in the direction of passage of the fluid.

Returning to figure 1, the filter cartridge 200 comprises an evacuation conduit 330 for the water which can separate from the diesel fuel during filtration. The evacuation conduit 330 is also defined by a pipe 335, the outlet mouth 340 of which is sealedly inserted in a hollow connecting element 345 afforded as a single body with the upper support plate 220. The connecting element 345 is substantially dogleg- shaped and terminates with a cylindrical shank 350 that projects axially from the upper support plate 220 on the opposite side with respect to the first filter wall 215. Starting from the connecting element 345, the pipe 335 is substantially L-shaped and exhibits a vertical portion, which descends from the upper support plate 220, while the free end of the pipe 335 fits in a suitable seating 271 which is provided in the lower support plate 270, and ends with an access mouth 355 situated at a lower position.

To guarantee a determined reciprocal orientation between the two filter units 205 and 210, the connecting element 320 must be aligned with the joint seating 221 of the degassing conduit 301 fashioned in the upper support plate 220, and the connecting element 345 must be aligned with the snap-in seating 271 of the evacuation conduit 330 provided in the lower support plate 270.

In use, the filter cartridge 200 is coaxially inserted in the external casing 105, such that the perimeter edge of. the upper support plate 220, which is slightly raised, is firmly constrained between the beaker-shaped body 110 and the cover 115. To ensure that the external casing 105 is hermetically sealed, an annular gasket 360 is tightly coupled with the perimeter edge of the upper support plate 220, the annular gasket 360 being received and compressed within a seating which is defined between the cover 115 and the edge of the beaker-shaped body 110.

The cover 115 is positioned such that the water discharge mouth 130 inserts on the cylindrical shank 350 of the upper support plate 220, preferably with the interposing of a seal gasket. A portion of the outlet conduit 125 projecting inside the casing 105 is coaxially inserted in the central hole 235 of the upper support plate 220 and from there internally of the cylindrical shank 275. An annular seal 365 is interposed between the outlet conduit 125 and the cylindrical shank 275, which annular seal 365 is designed to ensure hermetic closing of the chamber 280 defined inside the cylindrical shank 275. The inlet conduit 120 is set in direct communication with the offset slots 240 and subsequently with the chamber 285 defined between the cylindrical shank 275 and the first filter wall 215.

In this configuration, the access mouth 355 of the water evacuation conduit 330 is positioned in proximity of the bottom of the casing 105, while the access mouth 310 of the degassing conduit 301 is positioned in the upper zone of the casing 105.

Thanks to this arrangement, the first and the second filter walls 215 and 250 are positioned in series with respect to the flow of the diesel fuel to be filtered. In practice, the diesel fuel to be filtered arriving from the inlet conduit 120 is conveyed firstly into the chamber 285 between the cylindrical shank 275 and the first filter wall 215. From the chamber 285, the diesel fuel is forced to pass radially through the first filter wall 215 from the inside towards the outside, then reaching an intermediate chamber 135 which is defined between the filter cartridge 200 and the casing 105. In this way the diesel fuel undergoes a first filtering stage, which is generally referred-to as the pre-filtration stage, in which the fuel is separated from the coarsest impurities. The degree of filtration of the first filter wall 215 (i.e. the dimensions of the particulate for which the filter wall ensures a filtering efficacy of 99%) can be between 5 and 100 μπι and is preferably between 10 and 40 μπι. In addition, during this prefiltration stage almost all the water which may be present in the diesel fuel is separated, the water collecting under gravity at the bottom of the casing 105 from where it can be periodically evacuated via the evacuation conduit 330.

From the intermediate chamber 135, the diesel fuel is forced to pass radially through the second filter wall 250 from outside towards the inside, thus reaching the chamber 280, which is defined inside the cylindrical shank 275, and from there the outlet conduit 125. In this way, the diesel fuel undergoes a second filtration stage, which is generally known as the fine filtration stage, in which the fuel is separated from the impurities exhibiting smaller dimensions, which are not retained by the first filter wall 215, and thus achieving the desired level of purity. To perform this action, the second filter wall 250 generally has a higher degree of filtration than the first filter wall 215, for example between 0.5 and 20 μπι, and preferably between 1 and 10 μπι.

The gases that can separate from the diesel fuel during the filtration tend to accumulate in the upper zone of the casing 105, where the access mouth 310 of the degassing conduit 301 is situated. In this way, the degassing conduit 301 ensures the evacuation of the air and any other gases which accumulate (substantially under pressure) in the upper portion of the casing 105 during both the starting stage and the normal operating conditions of the filter group 100.

In practice, the outflow of gases along the degassing conduit 301 is ensured by the fact that the pressure in the chamber 280 is lower than the pressure in the intermediate chamber 135, where the access mouth 310 of the degassing conduit is located. In this way, the gases tend to flow into the degassing conduit 301 downwardly from above and thence flow into the chamber 280, while the choke 325 prevents the passage of the diesel fuel.

Obviously a person skilled in the art could introduce numerous technical and applicational modifications to the filter group 100 without thereby forsaking the ambit of the invention as delineated in the following claims.