Filters for engine oil, hydraulic oil and diesel

Engine lubrication is a prerequisite for proper operation and good performance. Cleaning the lubricating oil and eliminating impurities are ensured by the filtration systems, which are increasingly effective and sophisticated also for agricultural motors. Today, the industry offers various types of filters that meet the specific needs of the different engines, and innovative materials that can improve the system's functionality

An essential condition for the good operation of an internal combustion engine is that it is constantly lubricated to preserve the smooth running of all those moving parts that need a thin layer of oil to reduce friction, reduce operating temperatures, and remove impurities that can be generated in the engines. Obviously, the oil must always be kept as clean as possible, since a high rate of particles suspended in it will lead to premature engine wear.

The engine oil filter is designed to continually clean the oil, eliminating the impurities in it, such as particles that are generated as a result of continuous abrasion of the moving parts, any dust resulting from environmental contamination, as well as carbonaceous combustion residues. Filtering the fluids used in modern agricultural machines is carried out by devices with significant technological content, although they have kept the same and simple appearance for more than fifty years.

The task they are expected to perform is challenging and tough. They must ensure that the oils and the diesel in circulation are completely free of foreign particles, and therefore the filter must be able to stop even the smallest particles, and at the same time it is necessary to consider that the filtering must take place while causing the minimum possible resistance to the fluid’s flow, with the lowest possible pressure losses. As one can imagine, this is not an easy task, because for a given filter surface area, the denser the mesh, the more back pressure will be generated, and the greater the orifice clogging by the suspended particles being stopped, the more the back pressure will rise even further. For tractors, excluding any solid particles coming from outside the engine or hydraulic system that can enter them when replacing the oil or doing some repair work, most of the time the particles form internally originating from metals. For example, these particles can result from the wear of the components as a result of continuous rubbing. They may also result from oxidation processes or chemical reactions of materials such as those in contact with combustion products (sludges, lacquers, etc.), as well as micro-lesions or small mechanical and/or thermal stress fractures. In any case, the particle diameters that absolutely need to be stopped are generally between 5 and 60 microns. In diesel cycle engines, one must consider that the combustion chambers also produce lampblack micro-particles, resulting from the incomplete combustion of diesel fuel. Once the particles pass the elastic bands, they are intercepted by the engine oil, where they tend to come together to form macro-aggregates of essentially the same size as the metal particles mentioned earlier. As for the hydraulic system, the issues are similar, even though there are no combustion residues inside the hydraulic circuit. An ineffective filtration would cause premature wear in the sealing rings made of materials that need to be rather soft. Any micro-lesions in the metal parts would be no less damaging, given the high pressures normally used in hydraulic circuits. On tractors, the maximum operating pressure of the hydraulic system of lifters and distributors reaches 200 bar, while the hydrostatic component of a modern continuously variable mechanical-hydraulic split-power transmission easily reaches more than 600 bar. The issue is slightly different for the fuel, whose absolute cleanliness is essential for the smooth operation of the injection system, i.e. pumps and injectors. Most notably on modern engines equipped with Common Rail systems, the injectors come equipped with flow orifices that are smaller and smaller, and therefore more and more susceptible to harmful occlusions. It is therefore clear that clean diesel fuel free of any trace of water saves on engine maintenance by avoiding costly interventions on the fuel system and improving exhaust emissions. For this reason, in some cases diesel filters present small differences compared to those used for oils. A pan is created in its lower part, where any water in suspension gets deposited.

Filter types. The most common type of filter used on tractors is without a doubt the mechanical one, although there are other types of filters such as magnetic, sediment and centrifugal filters, although these systems have been abandoned or are only used on large stationary motors. In general, mechanical filters clean the oil by means of a filter element that stops and retains all impurities and metal residues present in the fluid passing through it. Mechanical filters can be of two types; the simpler “immersion” filters, usually made of a simple filter cartridge, and “spin-on” filters, which are made with a series of components enclosed within a metallic casing. After many years of predominance of the composite filters, commonly referred to as “spin-on”, i.e. to be screwed, today we are witnessing the massive return of immersion engine oil filters, which are inserted in special spigot joint housings on the baseplate or, more and more often, in a module that also contains the water-oil heat exchanger. Immersion filters are simpler and more economical, they include just the filtering part which is supported by a plastic and/or sheet metal structure and are inserted into a special spigot joint housing on the engine block. This filter is usually sold with an o-ring that needs to be replaced with the filter. In this case, the bypass valve is installed in a place directly connected to the cartridge housing and is never replaced.

The “spin-on” filters, on the other hand, consist of metal enclosures inside which the filtering element and the bypass valve are placed. These are secured to the baseplate by simple screwing and are not removable. When they need to be replaced, the filtering element and the bypass valve are changed at the same time. In many cases the position of the filter is such that when removing the filter, all the oil on it would come out due to the force of gravity. In order to prevent this from occurring, a special non-return valve with anti-leakage function is normally used.     

In both cases, however, the pump channels and pushes the oil toward the filter, where it passes through the appropriate small holes laid out in a circle, entering the container that houses the filtering element that captures the impurities, allowing the cleaned oil (or diesel) to exit passing through a central hole.

The more the filter works, the more the filtering element inevitably gets clogged, trapping all of the above-mentioned particles and generating an increasing back pressure. To prevent it from reaching detrimental levels in the engine oil and lubricant oil filters, there are bypass valves which, if the pressure becomes too high, link the delivery with the return, thus letting the oil flow without passing through the cartridge. It is obvious that if this system is activated, the fluid flowing through the filter ceases to be cleaned.

The filtering materials. Speaking of filters, it is easy to think of classic “nets” with very fine meshes working like a sieve to capture the particles with a system with a larger diameter than its meshes. Actually, the situation is much more complex.  For filters used in agricultural machinery, the situation is somewhat different. There are no actual “meshes”, and the filtering material is made up of cellulose fibres arranged in a random way to form a highly porous structure, which is often impregnated with particular resins to make it more durable. Lately, cellulose fibres are increasingly replaced by synthetic fibres, which are characterized by a smaller average diameter of the gaps, and differentiated density multilayer structures are also being created more and more often. Filters that use “mixed” filtering materials are also very common today, consisting of a combination of cellulose and synthetic fibres, with the latter greatly contributing to their increased durability, chemical resistance to contaminants and mechanical strength.

The “accordion”. The particular accordion pleats of the material used to make the filter element helps provide a large filtration surface and small outer dimensions, ensuring a reduced resistance to the oil flow and a high useful life of the filter, due to the increased capacity for foreign material that can be embedded in the filter element before the flow resistance becomes excessive.