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Just like the air breathed in by an engine needs to be filtered to remove particles of abrasive grit and dust, the oil that circulates through the engine and the fuel that is supplied to the carburettor or fuel injectors also need to be filtered. That is the job of the oil filter and fuel filter respectively.
Modern engines have a full-flow oil filter, in which all the oil coming out of the oil pump is filtered before it flows to the engine's working parts. In the early years of motoring, engines had no oil filters. Oil change intervals were short enough and any accumulated dirt was removed before it could do serious damage. But the development of pressure lubrication brought about the need for some sort of filtration to protect the oil pump from rapid wear. In 1946 the first modern, full-flow oil filter appeared on a mass-produced engine.
The early oil filters had a replaceable element inside a permanent housing, but in the mid-50s the spin-on oil filter was introduced, where housing and element form a self-contained unit which can be unscrewed from its mount and discarded when it has to be replaced. This made filter changes faster and more convenient. It remains so to this day, although there has been a move back to the replaceable-cartridge type lately among some companies because it generates less waste at each filter change.
In a spin-on oil filter the "dirty" oil enters the filter under pressure through the holes around the perimeter of the base plate. It then passes through the filter element where it is cleaned, and emerges in the central tube from where it flows into the engine through the hollow, threaded mounting stud.
The rubber gasket on the base of a spin-on filter is a vitally important component. Not only does it provide a seal to prevent oil under pressure from escaping, but the friction created when the gasket is compressed during installation is the only thing that prevents the filter being unscrewed by the vibration of the engine. The installation instructions for a filter should therefore be carefully followed. Put a smear of clean oil right around the base gasket, and tighten the filter by hand only, usually about three-quarters of a turn after the base gasket first contacts the mounting surface. Fuel filters on petrol engines used to be cheap, easily replaceable in-line filters in the good old days of carburettors.
On modern fuel injected engines, which have an electric fuel pump immersed in the tank, the situation is different. The fuel filter is now a more substantial component, often installed near the front right hand corner of the petrol tank, just downstream of the pump. When replacing it, the fuel system has to be depressurised before you disconnect any fuel lines.
Diesel filters have always been substantial contraptions. They are often located at the rear of the engine compartment and sometimes incorporate a water-drain plug at the bottom of the filter housing. - Motoring Reporter