In conventional petrol and diesel engines, the moving parts are lubricated by oil which flows around the engine in a circuit. The oil also serves to cool the engine, and form seal between components. As the engines runs, the oil starts to become contaminated by impurities; in this way it also cleans the engine. These impurities may come from many sources, including water leaking into or condensing in the lubrication system, small particles worn from moving parts, and dust. As the oil becomes more contaminated, the viscosity increases, making the oil a less effective coolant and increasing the oil temperature. In the presence of water acids are formed, and with iron and copper particles acting as catalysts, the oil can become corrosive. These reactions increase with the temperature, and more particulate matter is formed; thus the deterioration of the oil accelerates if left unchecked.

In order to remove some of these impurities, it is conventional to place a filter in the oil circuit, so that the particulate matter is trapped by the filter whilst the oil flows through the filter. Such filters are usually capable of removing particles greater than 10 to 50 microns in size. The majority of oil contaminants are less than this size.

The general object of the present invention is to reduce the level of contamination of fluids such as oil.

According to the present invention there is provided a filter system for a fluid comprising a magnetic filter, capable of attracting and retaining ferrous particles and the like, and, located in proximity to the magnetic filter, a porous filter, capable of retaining particulate matter. Preferably the porous filter is of the radial flow type. Preferably fluid flows first from through the magnetic filter and then through the porous filter. The fluid may be oil.

Preferably, if the fluid flows in a main circuit, and the flow of fluid through the filtering system is parallel to that main circuit.

According to another aspect of the present invention, there is provided a magnetic filter for removing ferrous particles and the like from a fluid, including one or more magnets completely hermetically enclosed in a sealing element. The sealing element may be of a material which has a low susceptibility, and preferably has a smooth surface.

According to another aspect of the present invention, there is provided a magnetic filter for removing ferrous particles and the like from a fluid, including a plurality of magnetic raised elements forming in a spaced relationship which cause ferrous particles and the like through magnetic attraction to settle between or around the raised elements. Preferably, the raised elements are removable, and are magnetic. Preferably the raised elements are of varying polarity, and are of such a shape that the distance between neighbouring elements varies such that the strength and throw of the magnetic field varies. Preferably the raised elements are pole extensions, and extend substantially radially around a cylindrical magnet filter Preferably the raised elements form a plurality of open partitioned regions.

The raised elements are preferably formed from annular discs, and they may be deformed from a planar shape to form the plurality of open partitioned regions when arranged upon the cylindrical magnetic filter.

According to another aspect of the present invention, there is provided a magnetic filter for removing ferrous particles and the like from a fluid, the magnetic filter being situated in a flow path, the flux from the magnetic filter being such that only ferrous particles and the like from a selected volume of the flow path are substantially influenced by the magnetic field, such that after the deposition of ferrous particles has obscured a particular cross section of the flow path, the flow path will not be further obscured by further deposition.

A filter system embodying the invention will now be described, by way of example, with reference to the drawings, of which; Figure 1 shows a section of the filter system Figure 2 shows a section of the magnetic filter assembly.

Referring to figure 1, the bypass filter comprises a magnetic filter assembly 10 and a radial flow filter assembly 20 connected in series by associated tubing, and further tubing and mounting means for attaching the bypass filter in a fluid circuit to the oil system of an engine (not shown).

The inlet 12 and outlet 22 of the bypass filter both have similar threaded inner surface of some standard size and pitch, such as the British Standard pipe thread. The inlet of the bypass filter is connected to some

convenient point in the engine's oil circuit, such as at the oil pressure switch sensor. The outlet is similarly connected to another convenient point in the oil circuit (the oil pressure between the inlet and outlet must be adequate to urge the oil through the filter), such as a level plug or a drain plug adaptor to the engine sump.

The flow rate through the bypass filter is low compared to the flow rate of the oil circuit as a whole. The bypass filter operates by tapping off a small proportion of the oil and filtering it in parallel to the main circuit.

Typically, the bypass filter could accept 5-10 % of the total flow. A conventional oil filter, enmeshing particles of 15 microns and over, may still be included in the oil circuit in its conventional location.

The magnetic filter 30 is substantially cylindrical in form. The magnetic filter is housed in an extruded metal tubular housing 32, the magnetic filter being held coaxially by a support 34,35 fitting each end of the magnetic filter each support having four nodes for spacing the filter, so that an annulus exists between the outer surface of the magnetic filter 30 and the inner diameter of the tubular housing 32. Each end of the magnetic filter includes a conical end section 36,37 which guides the flow of oil over the sides of the magnetic filter in a smooth and non-turbulent manner.

Upper and lower end plates 40,41 are fitted to either end of the tubular housing, the thread between the end plates and the housing being sealed using o-rings. The inlet 12 and a connecting stub 14 are incorporated into the lower and upper end plates respectively.

Referring to figure 2, the core of the magnetic filter comprises a series of short cylindrical rare earth magnets 45, having opposite poles on the circular faces. These magnets alternately arranged with short

cylindrical (or disc like) pole extension members 46. The orientation of the magnets also alternates, so that the north and south poles of any magnet faces the north and south poles respectively of its two neighbouring magnets. This core is hermetically sealed in a close fitting smooth tubular body 48, closed at both ends.

A field intensifier matrix 59 is installed upon the magnetic filter body. The field intensifier matrix comprises a series of flat wire compression washers, that is, flattened so as to form an annular ring. Each washer has been deformed so that rather than occupying a plane, two points lie above such a plane and two points lie below the plane, in a smoothly curving manner. These crests are then aligned in alternating senses as the washers are threaded upon the magnetic filter, so that as each crest abuts the oppositely orientated crest of a neighbouring washer (this is more clearly seen in figure 1), the washers delineate a regular arrangement of walled cells, which resemble a honeycomb structure.

The pole extension members 46 direct the flux from each magnet 45 so that generally speaking it leaves the magnetic filter body perpendicular to the filters axis, before curving round and re-entering the magnetic filter body perpendicularly to complete a magnetic circuit to the magnet's south pole. Each washer 60 is substantially located over one of the internal pole extension members (but of course will also extend over both neighbouring magnets). The material of the washers is sufficiently permeable for the washers to act as pole extension members. Alternate washers then have alternate polarities.

The magnetic field is strongest closest at the point close to where two washers 60 abut, and weakens as the as one moves along one of the

washers to a minimum at a midpoint (that is, a point midway between two such crests), the field thereafter rising once more. Where the field is strongest against the washer however, the field strength falls rapidly with increasing radial distance, the magnetic flux extending a comparatively short distance, whereas the flux from the parts of the washer which have a weaker field at the surface of the washer, extends further. This distance from the magnet or pole extension is often referred to as the'throw'.

As magnetic particles (by magnetic particles is meant ferrous particles, or other such particles as are attractable by a magnetic field such as paramagnetic particles) in the oil flow past the magnetic filter 10, the field from the washers'midpoints attracts the particle towards the magnetic filter, and as the magnetic particle draws closer to the magnetic filter, it is drawn towards the crests of the washers. Magnetic particles gather then in the cells of the magnetic filter, congregating particularly around the washers'crests where even small particles are held by the strong local field generated.

As the amount of magnetic debris attracted to the magnetic filter increases, it may start to exceed the height of the walls formed by the washers. The magnetic filter is usually chosen so that its capacity is sufficient for this not to occur. Moreover, the flux from the magnetic filter, even from the midpoints of the washer from whence it extends furthest, is negligible at a radius from the magnetic filter which is less than the inner diameter of the housing 32. Since there exists this annulus which the magnetic filter has no influence over, there will always exist a flow path through the magnetic filter and it will never block itself by amassing too much magnetic debris.

In this manner (provided that the capacity of the magnetic filter has not been exceeded) magnetic particles as small as one micron, or smaller still, may be removed from the oil. The oil thereupon flows out of the magnetic filter assembly and into the radial filter assembly via a connecting stub.

The radial filter 25 comprises a stiff cardboard tube 27 with cellulose fibre sheeting 28 radially wrapped around so as to form a roll, the cardboard being supported by a carbon steel compression spring 42 situated in the cardboard tube's inner bore. Upper and lower end plates 47,49 are installed at opposite ends of the radial filter, both these plates including spigots and o-rings for this purpose. The radial filter 25 and the plates are contained in an extruded tubular radial filter housing 52. In a similar manner to the magnetic filter, the radial filter 25 is coaxially contained in the radial filter housing, with an annulus existing between the outer diameter of the radial filter and the inner diameter of the housing.

Oil enters from the connecting stub through a calibrated aperture 61 in the upper radial filter end plate 47 (which screws onto the housing to create a seal), and is directed into the annulus by an isolator plate 48, which features radial channels upon its surface. This redirection of the oil flow increases the life and performance of the radial filter 25, since a direct jet of oil could form channels through the cellulose. The oil is forced by a pressure gradient to flow through the cellulose sheeting and through the cardboard tube (the cardboard having perforations to allow this passage), into the inner bore of the radial filter, where the oil then flows through a further calibrated aperture 62 in the lower end plate 49, and through the outlet 22 of the filter system. These calibrated apertures ensure the optimum flow and pressure for the radial filter.

The cellulose stops any particle smaller than 0.5 microns from passing through it. It also absorbs any water which may be present in the oil.

Working in combination in this way, the life of the filter is greatly prolonged, since small ferrous particles are very hard and may succeed in tunnelling through the cellulose, this tunnel allow subsequent particles through and reducing the effectiveness of the radial filter.

The filter system may be dismounted in order to provide necessary maintenance. After regular intervals the radial filter should be changed as its pores become blocked. The lower end plate 49 may be unscrewed so that the radial filter 25, isolator plate and internal spring may be removed.

The radial filter may then be simply disposed of, and replaced by a new radial filter.

The magnetic filter may be removed by unscrewing the lower end plate 41. The magnetic filter 30 is then slid out, and may be cleaned of magnetic debris by removing each washer 60 in turn, which pulls most of the entrapped magnetic debris off with it. Out of the influence of the magnetic field, the magnetic debris may be easily washed off of the washers. Any remaining debris upon the magnetic filter body may be removed with a cloth, wiping the debris along the body until it reaches the body's flat circular ends, which emits no flux and therefore does not retain the debris. To avoid the user damaging the magnets, or getting magnetic debris upon them which cannot then be removed, they are not accessible to the user, but instead sealed in their body for life. This also ensures that the magnets themselves do not become chipped, since particles of magnet will

be extremely hard, and may not necessarily be attracted and retained by the magnets. The magnetic filter is reinstalled simply by re-threading the washers and reversing the removal process.

The size and type of the components will naturally vary depending upon the particular system the filter is being designed for. Both the magnetic and radial filter housings feature cooling fins upon their periphery, typically the oil temperature may be 90 °C, although the filter system may be designed for hotter systems if necessary.

The system may also usefully be applied to cleaning hydraulic fluid, which suffers from similar contamination problems. In a hydraulic system application, the inlet of the filter system could be tapped into the hydraulics return line system, and the outlet tapped into the hydraulic reservoir.

The system could of course be applied to any system where both and magnetic and non-magnetic particles are to be removed from a fluid, with necessary adaptations to the filter system. Although suited for a bypass arrangement, in a system where such a pressure drop and low flow rate were acceptable, the filter system could be fitted in series with the fluid circuit. A magnetic filter as described could be fitted on its own or in conjunction with some other type of filtering system. On its own, the magnetic filter would operate effectively in a high flow environment.