WO2005065801A1

WO2005065801A1 - Ultra fine high efficiency filter

Info

Publication number: WO2005065801A1
Application number: PCT/IB2003/006191
Authority: WO
Inventors: Wilhelmus Petrus Maria Schaerlaeckens
Original assignee: Ntz International Holding Bv
Priority date: 2003-12-28
Filing date: 2003-12-28
Publication date: 2005-07-21
Also published as: AU2003292464A1

Abstract

Filter device (20) comprising a set of coaxially filter elements (1, 3) within a filter devise housing having an oil inlet port, and an oil outlet port (18), a first, upstream filter (1) being a full flow filter through which all of the inlet port oil passes, and second, downstream filter (3) being an in-depth micro-filter through which only part of the oil passed through the first filter (1) passes to the outlet port (18), the remainder of the oil filtered by the first filter (1) by-passing the second filter (3) towards said oil outlet port (18). The by-pass (11) is realised by an pressure dependent by-pass valve (8, 8A, 10, 12; 12A, 12B; 13) , opening during normal operation of the filter device (20) at a pre-set oil pressure level (dPr) within the oil compartment (2) between the said two filter elements (1, 3).

Description

ULTRA FINE HIGH EFFICIENCY FILTER The present invention relates to a combined set of filters as defined in the preamble of claim 1. Such filters are generally known, e.g. from US patent publication 3.262.564. In this early example of combining filters, the main purpose was safety in aeronautical applications. It comprises a fine, filt material based filter incorporated as a main filter and a conventional filter, located coaxially interior, at least downstream of the fine filter, so as to catch any loose comings form the main filter. Each filter is provided with a safety valve so that should any filter inadvertently get clogged, oil may pass either through the back up, relatively coarse filter, or may pass through unfiltered. The main filter is incorporated in a conventional, pleated manner so as to enlarge filtering surface. Automotive applications are known from US patent 5078877, showing coaxially incorporated filters, having a relatively coarse filter to the exterior, and a relatively fine filter located interior, at least downstream of the main filter. The filters have a common fluid input, however a different fluid output path, in that the main filter feeds the engine while the second filter feeds the crankcase. Both filters are in conventional pleated paper form. A relatively more advanced embodiment having a high efficiency, i.e. in-depth micro-filter combined with a conventional full flow filter is known from US patent publication 647 8958-2002, where both filters are fully in line operable, i.e. with a single oil input and a single oil output. To this end a venturi device is incorporated in the filter device, so that at least part of the oil is drawn through the in-depth filter, which requires a considerably more high pressure difference than a convention full flow filter, commonly with filtering material incorporated in pleated shape. The remainder of the oil filtered by the full flow filter passes through the interior space of the in-depth filter to the oil outlet port of the filter device. The advantage of the latter configuration is that a high efficiency micro-filter may be used in an in-line configuration, virtually independent from the available pressure at the supply side of the filter. It may thus even be used in applications where line pressure, e.g. return line pressure is low, e.g. only about 1 bar. Disadvantages however, may be a relatively high flow rate that is required in this type of filtering device, and the relatively small range of flows for which a particular venturi filter may be designed. At low flow volumes a required under pressure will not be created, while at high flow rates the under pressure will become so high, that in combination with a required counter pressure at the discharge side of the filter, the energy losses will become much too high. In fact the venturi-solution is suited mainly for stationary flow situations. Micro-filters in the form of high efficiency filters are known per se, e.g. from the PCT-patent publication WO-0107142 in the name of Applicant. They are very often applied in by-pas configurations, due to the relatively high flow resistance that comes with the high efficiency and the ultra fine-grade micro filtration feature that comes with such in-depth filters. These phenomena are explained in and known from SAE paper 2001-01-0867 "Automatic transmission hydraulic system cleanliness - the effects of operating conditions, measurement techniques and high efficiency filters", which document is hereby regarded included. The present invention aims at improving filtering technology in general, and to increase the possibilities of applying such known high efficiency ultra-fine micro-filters in particular. One way of realising such is by applying the features of the characterising portion of claim 1. With an arrangement having the features of the present claim, both the filtering quality and the filtering capacity of a conventional system are improved in a manner that is easily replaceable, and in a manner which does not require additional space and additional leads for creating a separate passage towards a by-pass filter. Moreover, the filter device is suitable for coping with a wide range of oil flows, up to around 80 litres per minute. By choosing the filtering grades of each the full flow filter and the by-pass filter appropriately, the filtering capacity is increased to above the level of what each filter is capable of filtering separately since the overall surface for superficial filtering has significantly increased, and since the main filter is less burdened by fine particles, while the micro filter is at it's surface, i.e. the entry location of the oil , significantly less burdened by larger particles. The advantages of the novel filter device are in particular made possible by the use of a so-called by pass valve, known per se in the form of a safety valve, however, now set into operation in a totally different manner, in that it is combined with, i.e. provides access around a micro-filter and in that the valve is required to open continuously, i.e. while the filter device is in normal operation. By doing so, it is attained that the by-pass filter is during operation continuously provided with a pressure difference of a chosen level, e.g. of about 1 Bar. Further advantage of this type of filter device is that it may excellently be used in diesel engines, where soot particles are taken away from engine oil. The invention will now by way of example be elucidated further along a drawing in which: Figure 1 is an axial cross section of the inner part of a filter device according to the invention, depicted without a housing; Figure 2 is a graph illustrating solely the carrier structure for the filter of Figure 1 in a slightly differently shaped embodiment; Figure 3 graphically illustrates a typical pressure difference Pd required for a certain desired flow volume F through the by-pass filter at different oil temperatures; Figure 4 graphically illustrates a typical flow volume F delivered by a filter device according to the invention against an engine speed Re, at various temperatures of the oil; Figure 5 graphically represents typical oil characteristics for Viscosity V and Density D against temperature; Figure 6 graphically illustrates the typical fluctuation in oil pressure in a system into which the filter device may be incorporated, which fluctuation is due to the characteristic of the oil pump typically used in many automotive applications. In the figures, identical reference numbers relate to identical or at least equivalent technical features. Figure 1 shows the filtering unit 20 of a filter device having a housing not depicted, in which the filtering unit 20 is included for normal operation. The housing comprises an inlet opening allowing oil to flow around and through a first filter element 1 , however not to an oil outlet opening of the device which is made a unit with an oil outlet opening 18 of the filtering unit 20, in casu via a seal 19, here embodied by an O- ring. The oil filtering unit 20 comprises of a set of co-axially combined filter elements 1 and 3, the outer element 1 being a conventional type of filter element functioning on the basis of a sieving principle, while the inner filter 3 is a so called in-depth micro filter, which operates both on the basis of sieving and on the basis of electrostatic binding of miniscule particles that may be present within the oil. The full flow filter 1 is normally shaped pleated so as to optimise surface area. The elements 1 and 3 are at their respective axial ends closed for passage of oil. In casu there is at one side, in the picture the lower, i.e. oil outlet port 18 side a common closure element 6. In case of the conventional filter 1 , in line with common practice, a further closure element may be present, extending essentially solely over the projected area of the full flow filter element 1. O-rings may be provided for preventing leakage between any such closure element and the common closure element 6. The common closure element 6 is centrally provided with the opening 18, and allows outlet of oil from both the filter unit 20 and the filter device as a whole. In the projected area of contact with the in-depth filter 3, which contact is non-attached, coaxially provided projections are provided on the closure element 6. They protrude into the filter material and have an end part that is essentially triangular in cross section. The protrusions, also present on closure element 8, force any passing oil and/or accumulated dirt through the material of the micro-filter 3. The oil that has passed the full flow filter 1 enters a room 2 between said two filter elements 1 and 3, from where it may pass either trough the in-depth filter 3, or via a passage 11, directly towards the outlet opening 18 of the filter device. In this way it bypasses the in-depth filter 3. To this end, the in depth filter element 3 is at one, axial end, opposite the end contacting the common closure element 6, provided with a closure element 8 that is associated with, i.e. forms part of, is attached to, or comprises, a pressure relieve valve unit, here denoted by-pass valve. The by-pass valve unit comprises a valve 12 that allows oil to pass through opening 11 as soon as a required minimum operating pressure difference value dPr for the in-depth filter element 3 is reached. In the present case the in-depth filter 3 is designed for a minimum or operating pressure difference dPr of 1 Bar. In practice this means that only part of the oil that passes through the filter device, and thus through the full flow filter element 1 , passes though the in-depth filter 3. Due to the high efficiency nature of the latter, it will within short period of operation of the mechanical device into which the filter device is to be incorporated, have realised the desired level of cleanliness of the oil, determined by the in-depth filter material applied. In a typical Otto-engine or in a transmission application, the in depth filter 3 would e.g. be designed to filter down to 5 microns, while the full flow filter 1 filters particles down to 20 micrometer. For example in one embodiment realised, at a flow rate of 10 Liters per minute (l/m), which may be the amount passing through at stationary operation of a very large engine vehicle, 4.5 l/m would go through the in-depth filter, while 5.5 l/m would pass via the opening 11 of the by-pass valve unit. Alternative applications, e.g. for filtering soot particles in diesel engines, following the same principle of the invention may however be embodied with even finer filters, e.g. with 10 and 2 micrometer filtering grade filters respectively. Although the fine grade of filters applied renders the filter unit relatively expensive and may have to be exchanged within a relatively shorter period, it is effective against soot that enters the oil, and thereby enhances the life time of both the engine oil and any soot filter present in the exhaust system, which requirements are both more expensive than the filter device. Moreover, due to the cleanliness of the engine oil, the present filter also enhances lifetime of the engine itself. In this kind of application however, the full flow filter primarily acts as a safety device for the by-pass filter, rather than as a combination of differing types of filters in which the filtering capacity is enhanced by the combination itself. The by-pass valve unit as depicted here and realised in a fully functional filtering device according to the invention, is known perse. The by-pass valve according to the invention structurally forms a whole with a closure element 8 for the closing of an axial end face of an in-depth filter element 3. In operational sense it differs from known relieve valve applications in that it is designed to open for operational use of the filter, i.e. intentionally should open during normal operation of the filter device. Rather than being used for safety of the filtering device as in the known applications, the pressure relieve valve is according to the invention thus used to create a pressure difference that allows the in-depth filter 3 to operate. The by-pass valve unit as preferred is composed of structures known per se and is commonly provided with a cylindrical shape, however, alternative shapes may as well be used, e.g. rectangular in cross section. It may e.g. be located in the interior chamber 5 rather than on top of the micro filter 3. It comprises a cylindrical exterior part 8A, and a fitting interior part 12, alternatively denoted piston 12. The piston 12 closes off one or more openings 11 in the part 8A of the by pass valve unit when it is located in a lowermost position. The interior, piston 12 is kept in this lowermost position by means of an elastically deformable biasing means 13, in casu a helical spring means, urging the piston 12 against a stop 15. Here, the stop 15 is embodied by a step-wise thickening at the interior side of the cylindrical part 8A. The biasing means 13 is kept in position by a lid-part 10 of the by-pass valve unit. In casu the lid- part 10 is at least essentially closed for passage of oil that has passed the full flow filter 1. The valve part 8 forms an end closure for the in-depth filter element 3, and is provided with a central opening for passage of oil towards an inner chamber 5 of the in-depth filter 3. The valve unit and thereby the end closure part 8 is biased against the relevant axial end face of the in-depth filter 3 by means of a second elastically deformable biasing means 9, in casu embodied by a helical spring, positioned between the by-pass valve unit and a lid part 7 of the filtering unit 20. It is in this embodiment further positioned by a cross-sectionally correspondingly shaped, fitting part of the lid 10, extending axially within the helical spring 9. It is further located by protrusions 7A of the lid part 7. The latter protrusions 7A, in combination with further axial protrusions 7B, also receive the full flow filter element 1. The present design supports easy assembly of the filtering unit 20. For smallest radial dimensioning of the entire filter device, a position of the bypass valve unit on top of an axial end face of the micro-filter 3, is preferred, since such allows a largest possible flow of oil, by using the circumference of the wall 8A to a maximum for the openings 11. Thus, at a particular axial level, the wall 8A, may be opened by openings 11 for at least the major part of the circumference, preferably also with the openings 11 having an edge which is straight for the major part of the circumferential length of the wall 8A over which openings 11 are provided. In this manner a maximum response in oil flow may be provided, which response may also be provided instantaneously due to the width of the opening according to the invention at only slight displacement of the piston 12. Radial width may further be enhanced, or flow capacity of the by-pass filter 3 be increased by locating the by-pass unit on top of the main filter. In this embodiment, not depicted, the axial length is increased and the by-pass valve unit extends in radial direction over the main filter. In this respect the chamber 2 is provided with two at least predominantly square hook shapes, at which it is ensured that the minimal width of the chamber is maintained at all sections thereof. In this embodiment the structures 7A and 7B remain present. In a current embodiment they are connected to a cylindrical, downward extending outer wall section of the closure part 7. The by-pass valve unit opens at an oil pressure existing or created in chamber 3, which connects with opening 11 as soon as the oil pressure in chamber 2 reaches a pre-set pressure difference dPr determined by the according to desire chosen characteristic of the biasing means 13. The oil pressure dP may counteract the force exerted on the piston part 12 by the biasing means 13, i.e. the pressure difference dPr, via small edges 14, 15 created in an outer circumference part of the piston 12. The edges 14, 15 communicate with an opening 11 via a reduction in thickness at the outer side of the piston part 12B, in casu the lower end part. This piston part 12B is of slightly smaller cross sectional size, in casu of a smaller diameter which, by the transition to the part 12A with largest cross sectional size, creates at least a first plane 14 having a component transverse to the axial direction of the filtering device 20 and thereby to the axis of the by-pass valve device. A second, kind like plane 15 is relatively easy created by extending the smaller cross sectional part 12B to an axial end of the piston. Here a bevel 15 in the transition to the axial end face 12C of the piston provides for a second plane 15, having a directional component transverse to the axial direction of the filter device and the piston. Preferably the piston 12 is provided with at least one, preferably three circumferential grooves 18 in the largest cross sectional part 12A of the piston. More preferably the at least one, in casu the central groove 18 is fitted with an O-ring. The piston 12 is further provided cylindrically open at both axial ends. It is internally provided with the rim 12C acting as a stop to the biasing means 13. The pressure dPr exerted by the biasing means 13 is determined by the desired and/or minimum pressure difference at which the by-pass filter element 3 will operate. The remainder of the oil coming in upon the filtering device and through the full flow filter element 1, will pass through opening 11, which is opened at said determined pressure difference dPr. The actual amount of passage of oil through said opening 8 depends on the amount of axial displacement of piston 12 under the pressure building up in chamber 3. The biasing element 13 is designed to have only slight increase of resistance when it is counteracted - e.g. maximum 20% increase - and will therefore quickly open fully to allow large volumes of flow to pass. In one embodiment the preset pressure difference is set to 1 Bar, at which as from stationary operation 4,5 L/min flows through the filter element 1, while 5,5 l/min flows through the openings 11. The latter may at maximum operating conditions increase up to e.g. 70,5 l/min. The filter unit according to the invention is further provided with a relative to passage 11 , ultimately small bore or opening 17, in this example situated in the lid part 10 of the filter unit. This feature provides both a functional advantage at cold starts and a safety function, while due to its nature only minimally effects the operation of the filter unit. As to the latter, it causes a constant flow of oil to escape, i.e. to pass through the device at any given pressure. For instance at the opening pressure of the relieve valve, e.g. 1 Bar, about 2 l/m (Litre per minute) of oil will escape through the bore. In the current example filter, at an overall flow of 10 L/m, this causes the relieve valve to open some what later, in casu at 7 l/min rather than at 5,5 L/m of flow. As will be shown further in this description, this circumstance does not affect the performance in the major part of the operating range of flows, which go up to 76 L/m. Rather, when the oil to be filtered may still be cold, and it is because of a high viscosity relatively difficult for the oil to pass through the filters 1 and 3, it is particularly at low volumes of flow advantageous that a minimum amount of flow passes through the filter unit unhampered: for the small bore 17, the volume that passes through is dependent on the mass, i.e. density D of the medium, not on the viscosity thereof. The safety feature of the bore is such that when the by-pass filter 3 might get clogged, or should the relieve valve unit get stuck, a high pressure build up could occur, devastating the full flow filter 1. After all, any safety valve provided conventionally for the full flow filter 1 only operates in case of pressure differences e.g. a pressure value chosen within the range of 1 to 3 Bar, whereas the absolute value of the pressure in the filter unit may become very high, e.g. 30 Bar, and would be exerted on the full flow filter 3. By the presence of the small bore 17, any such build up of pressure would be fed back by the bore, thus allowing the conventional safety valve to open timely. While not depicted in the drawing, it is further remarked that the safety valve that conventionally could have been included outside the filter unit, is in the present embodiment incorporated within the lid part of the filter unit housing not depicted. Where the safety valve for the full flow filter would open at 1 Bar, and the pressure relieve valve for operating the by pass filter would also open at 1 Bar, the entire pressure difference required for the operation of the filter device according to the invention would amount 2 Bar. Figure 2 in fact depicts a carrier structure, equivalent to that of the filter unit in figure 1 , however in a slightly different embodiment. It shows the structure to be made in two separate main parts, with the common closure element 6 being loose from the perforated core 4 for carrying the material of the by-pas filter 3. It is remarked that while said core 4 forms a whole with the closure element 8 and the cylinder part 8A, it is urged against said common closure 6 by the biasing means 9 acting on cylindrical part 8A. To this end, the closure member 6 is provided with a positioning rim 21 , fitting the circumference of the core 4 and depressed in the closure member 6. For ease of assembly, both the core 4 and the positioning rim 21 are provided bevelled. Figure 3 illustrates, for a typical oil as used in engines, transmissions and mechanical devices requiring active lubrication in general, the pressure difference dPr, required for operation of the by-pass filter element 3. The flow F through the filter 3 is illustrated against pressure difference dP at three different oil temperatures. In this respect line T20 represents the flow characteristic of the filter at an oil temperature of 20 degrees Celsius, line T50 at 50 degrees Celsius and line T100 at 100 degrees. The graph illustrates that at start up temperatures the oil flow through the filter element is strongly limited, particularly when taking into account the modest operating pressure difference dPr of 1 Bar. In figure 4 it will be shown that the latter, in typical automotive applications, may moreover be influenced by the revolutions rate Re of the engine. Figure 4 for a typical automotive application illustrates both the flow F and the pressure difference dPr, again for different oil temperatures, as these may occur in dependence of the revolutions Re of the engine. Centrally, i.e. diagonal in the graph there is plotted a lihe Fm that represents the main flow, i.e. the flow through the full flow filter 1. Here the uppermost line T100 provides a maximum flow of 10 L/M through the by-pass filter 3, at a high operating temperature of 100 degrees Celsius. The lower most line correspondingly indicates a flow of about 1 L/M of the oil at a low operating temperature of 20 degrees Celsius. In the graph, the lines that initially follow the line Fm represent the lines for flow through the by-pass filter 3 in dependence of the engine speed Re. The lines that start to the left hand side of the main flow line Fm represent the pressure difference dP over the by-pass filter, which will not exceed the pressure set for the by-pass valve by the biasing means 13, in casu 1 Bar. It is remarked however that due to the nature of the present biasing means 13, the latter pressure might increase slightly with the amount of biasing, in casu depression of the helical spring 13, however normally not more than about 10%, with 20% to a maximum at very large amplitudes. The graph illustrates that at low oil ternperatures the pressure in the filter increases ultimately rapidly with engine speed Re, which circumstance is responsible for rapid opening of the by-pass valve. It is for these conditions that, apart from the safety function that it has for the filter device, the small bore 17 in the lid 10 is of significant value for the operation of the filter system. It may also be evident from this graph, that at subsequent operating conditions with increasing levels of flow F, the minor flow F through the lid 10 is not of significance, i.e. does not affect the operation of the by-pass filter negatively. Figure 5 illustrates the manner in which the small bore 17 is set into practice for the device according to the present invention. It illustrates that viscosity Vsc of oil drops significantly with the temperature T thereof, by a factor value of about 20. The density D of the oil however almost remains constant with the increase of temperature, i.e. decreases merely to a slight extend, i.e. less than 10%. Thus, while oil temperature is low and the flow through the by-pass filter is severely restricted through the viscosity of the oil, the flow through the bore 17 in the lid 10, which is only density dependent, is not affected, and remains in tact. Finally, by figure 6, a further, advantageous, dirt explosion related feature of the filter according to the invention is illustrated. Along arbitrary chosen, however indicative values of oil pressure P in a system in which the device according to the invention may be applied, the amount and amplitude of oscillation thereof is indicated at two different operational speeds of the engine, at least of the oil pump of the system. Since the oscillations are indicative the time factor set out along the X-axis is not relevant, however seconds could be appropriate. The graph illustrates that at low engine speeds, e.g. 750 rpm (rotations per minute), the oscillations have significantly larger amplitudes than at high speeds of the engine, at least of the pump, e.g. at 3000 rpm, which latter speed is more frequently operated than the lower speeds. Although the oscillations are here represented in regular pattern, corresponding to the pumping nature of many pumps, due to operational condition in the mechanical system in which the pump and filter are incorporated, significant irregularities in amplitude may occur. Thus it may occur that at low operational speeds, probably by the amplitude of the oil pressure P, so called dirt explosions occur at use of a conventional filter 1. Although the exact cause of this phenomenon has not yet been established in common, the phenomenon is according to one hypothesis due to, or initiated by particles that are only slightly smaller than the sieving rate of the conventional filter, under the constantly applied pressure, slowly creep through the filter material. At hefty amplitude changes such particles shock-wise come loose from the filter, thereby taking many particles up to then held back by the filter with them. It is now of advantage that particularly at the low engine speeds where such so called dirt explosions occur, the filter device according to the invention passes a significant part of the oil flow, at least at normal operating temperatures through the by-pass filter. The by-pass filter due to its in-depth filtering nature is not susceptible to dirt explosion and traps part of the exploded dirt in the oil part that passes through the by-pass filter. Thus the filter device according to the invention, apart from the main advantages described, is advantageous in that it diminishes a disadvantageous feature of the conventional filter, i.e. diminishes the effect of dirt explosions. This advantageous feature of the device according to the invention is also becomes significant when it is considered that the dirt once trapped in the by-pass filter 1 , will no longer be released within the system. The invention, apart from the following claims, also relates to the preceding description and all details and aspects in the drawing which are directly and unambiguously derivable there from, at least by a man skilled in the art.

Claims

1. Filter device (20) comprising a set of coaxially filter elements (1 , 3) within a filter devise housing having an oil inlet port, and an oil outlet port (18), a first, upstream filter (1) being a full flow filter through which all of the inlet port oil passes, and second, downstream filter (3) being an in-depth micro-filter through which only part of the oil passed through the first filter (1) passes to the outlet port (18), the remainder of the oil filtered by the first filter (1) by-passing the second filter (3) towards said oil outlet port (18), characterised in that the by-pass (11) is realised by a pressure dependent by- pass valve (8, 8A, 10, 12; 12A, 12B; 13) , opening during normal operation of the filter device (20) at a pre-set oil pressure level (dPr) within the oil compartment (2) between the said two filter elements (1 , 3).

2. Filter device according to claim 1 characterised in that the by-pass valve (8, 8A, 10, 12; 12A, 12B; 13) is of a type having inlet openings (11) for radial flow of inlet, opening by transverse, in particular axial movement of a valve (12) closing of the openings (11).

3. Filter device according to claim 1 or 2, characterised in that the by-pass valve is incorporated at an axial end face of the by-pass filter (3).

4. Filter device according to claim 1, 2 or 3 characterised in that the filter elements (1 , 3) are received in a common carrier (6).

5. Filter device according to the preceding claim, characterised in that the carrier (6) forms a closure element to at least the axial end face of the filter material of the bypass filter (3).

6. Filter device according to any of the two preceding claims, characterised in that the common carrier (6) is provided with a central aperture (18) connecting to the interior chamber (5) of the by-pass filter (3).

7. Filter device according to any of the preceding claims, characterised in that a closure member (8, 12), in particular a lid part (10) thereof, is provided with a bore (17) of relative small passage, providing a free passage of oil from the oil chamber (2) intermediate between the full flow filter (1) and the by-pass filter (1) to the interior chamber of the by-pass filter (3).

8. Filter device according to the preceding claim, characterised in that the maximum oil flow through said bore is at least 20 times smaller than the maximum possible flow through the filter device, in particular 20 times smaller than the flow through the by-pass valve opening (11).

9. Filter device according to any of the preceding claims, characterised in that the circumferential width of the openings (11) at one particular axial level of the valve part (8A) takes up the major part of the circumference of said part, in particular with the openings (11) having a straight part that takes up the major part of the circumferential part covered by the openings (11).

10. Filter device according to any of the preceding claims, characterised in that a second carrier (7) is provided, positioning and/or closing an axial end face of the full flow filter (1 ) and axially closing off the intermediate oil chamber (2).

11. Filter device according to any of the preceding claims, characterised in that the valve means (8A; 12) comprise a piston (12) which is provided with a O-ring at the circumferential side facing a valve housing (8A) in which the piston (12) may move axially.

12. Filter device according to any of the preceding claims, characterised in that an elastically biasing means (9) is provided between the second carrier (7) and a lid part (10) of an axial end closure element (8) for the by-pass filter (3), urging the element (8) to the axial end face of the by-pass filter element (3).

13. Filter device according to any of the preceding claims, characterised in that outer connection means are provided, e.g. in the forms of one or more frames, clips or brackets, maintaining the assembly of filters (1 , 3) and positioning parts (6, 7) together.

14. Filter device according to any of the preceding claims, characterised in that the device is provided with a full flow filter (1) filtering grade of particles down to 20 μm and a by-pass filter (3) of a filtering grade down to 5 μM.

15. Filter device according to any of the preceding claims, characterised in that the filtering device is intended for incorporation in a diesel engine, and the full flow filter (1) is of a filtering grade of 10 μm and the by-pass filter (3) of a grade down to 2 μm.

16. Filter device according to any of the preceding claims, characterised in that the perforated core (4) of the by-pass filter (2) and the axial end closure (8) comprising said by-pass valve are provided mutually releasably connected, in particular by a snap connection.

17. Filter unit (20) for use, in particular for replacement, in a filtering device, comprising a set of coaxially filter elements (1 , 3) and being provided with one or more of the characterising features as described in the preceding claims, at least with a bypass valve as described in claim 1.

18. Method of filtering oil comprising a first, e.g. relatively coarse, filter element (1) through which all of the oil to be filtered passes, and a second, e.g. relatively finer grade, filter element (3), through which only part of the oil passes, and in between which filters (1 , 3) pressure is build up to a minimal operating pressure (dPr) by means of a relieve valve unit provided with a relieve opening (11) that by-passes the second filter (3).

19. Filter device, in particular according any of the preceding claims, comprising at least two functionally serially incorporated filter devices, in which an end closure element (10) of a subsequent filter (3) is provided with a permanently opened bore, allowing part of the oil filtered by a preceding filter element (1) to by pass the filter element (3) with said bored end closure (10), preferably the part being a fraction, more preferably at least 20 times smaller than the maximum flow through the filter device.

20. System, in particular a mechanical device requiring lubrication oil during operation, provided with a filter device or a filtering method according to any of the preceding claims.