Throughout this site we use many technical terms, and given the breadth of readership our site enjoys, sometimes we are remiss and incorrectly assume everyone knows what we are referring to. For those that do not, here are some explanations of the technical terms use. |
During circulation, engine oil is unavoidably contaminated by the by-products of combustion. These products consist of various lacquers, carbons, acids and some water and unburnt fuel. Certain abrasive particles sometimes enter the engine via the crankcase breather or through the induction system. Small particles of rust tend to accumulate inside the engine and even during normal wear small fragments of metal from the engine find their way into the oil. The smallest of these contaminating particles are not dangerous as modern engine oils are designed to hold this material in suspension. But the larger particles must be removed. If, for instance, a small speck of rust were to find its way into the lubricant, it would circulate with the oil and penetrate the film of oil, sometimes only a few molecules thick, that separates the moving parts. This would interrupt the lubrication process and cause serious wear. The oil must therefore be filtered.
Generally two filters are provided, one in the sump and one in the high pressure part of the lubrication system, immediately after the oil pump. The small filter in the sump is normally referred to as a strainer because it uses a comparatively coarse mesh to minimize the risk of strangling the suction effect. The strainer can only intercept larger particles and it is mainly the high pressure oil filter which keeps the oil clean. The problem with filters is to accommodate in a small space a large area of filtering material that will pick up fine particles without impeding the oil flow. No form of metal mesh is suitable, and the modern oil filter uses specially plasticized and processed paper. The paper element is pleated to give a large filtering surface in a compact volume. In fact the filtering surface area of the element can be as much as 6,500 square centimetres (about 1,000 square inches). The pores in the element are minute and the oil filter is designed to remove all particles above 1/2000 mm in diameter. Particles below this size are not considered to be harmful to the engine.
The oil filter element is cylindrical, and is contained in a metal canister bolted to the side of the engine block. A rod and spring within the bowl locate the filter element firmly against the filter head. Oil is fed from the oil pump to the outside of the element, through the element, and into the centre of the filter. The filtered oil is then pumped into the engine's lubrication system. Oil filters have to be carefully assembled to prevent leakage. Normally a sealing plate is provided at the bottom of the filter bowl to stop oil leaking out of the bolt-hole, and a rubber ring between the bowl and the filter head prevents leakage from the top of the bowl. On average the oil filter has a life of about 10,000 km, during which time it picks up a large amount of contaminating material. If the element is not changed the filtration efficiency declines and the oil flow to the engine is reduced. Assuming that the filter and oil pump are in good condition, an average engine circulates all the oil in the sump about three times per minute at an engine speed of 2000 rpm. |
