Oil Filters and Filtration

It's all very well changing your oil often, but it's not just the oil that helps prevent engine wear. The oil filter does its part too. Dirt is the prime cause of engine wear. Not big dirt, like you'd see in a yard, but minute particles of dirt. It's dirt nevertheless, and it's abrasive. These contaminants vary from road dust (which are razor-like flakes from an engine's perspective) that doesn't get filtered out by the air filter, up to actual metal particles - the byproducts of the casting scarf from the original engine manufacture, and basic engine wear. All this nastiness is carried around by the oil into the minute parts of your engine, being rammed into the precision clearances between bearings and other moving parts.

Once in, they don't come out easy, but tend to stay there, wearing grooves, grinding and generally messing up your engine. Other debris that causes problems are a by-product of the mere way an engine works - sooty particles from the combustion process can be forced past the piston rings and transported around in the oil too. This is definitely A Bad Thing - the soot acts like a sponge and soaks up other oil additives reducing the oil's anti-wear properties, and messing up it's viscosity. All this dirt is why oil goes black when it's used. That lovely syrup-like yellow that it is when you put it in is pure oil. The black stuff that comes out at an oil change is the same oil full of contaminants and by-products from wear and tear.

That's where the oil filter comes in. It's job is to catch all this crap floating around in the oil, and to stop it from recirculating. Most oil filters that you or I will ever see are the spin-on type. They're shaped like an aluminium can and spin on to a threaded oil feeder poking out of the side of the engine somewhere. They're called 'full-flow' oil filters because they sit in the normal flow of the oil through the engine. Sort of like an electrical component in series with all the other electrical component. Because it sits in-line, it has to be designed not to restrict the flow of oil around the circuit, and thus can only really be effective at stopping the larger particles. Large, in this case, is around the 20micron size. So here's the catch. The smallest contaminants are in the 10-20micron size range. Not only is that "extremely small", but it means that they pass right through the oil filter and back out into circulation. This is why regular oil changes are a necessity, because these tiny little things can be the most damaging.
This is an exploded view of a typical spin-on oil filter used in automotive applications. I've sliced the filter element (the brownish-yellow part) so you can see the internal structure of the filter). Typically the engine oil enters through the ring of 5 or 6 holes in the base and into the main cannister. From there it is forced inwards through the filter element, through the drain holes in the central core and out through the central, threaded hole in the base.

There is another alternative, but it's only really used in heavy applications or for racing. That alternative is to fit a secondary bypass oil filter. This is sort of like a filter in parallel with the primary one. It doesn't restrict the flow of oil in the main circuit, but the oil that passes through it is filtered down to the 5 micron range, thus removing even the smallest contaminants. The newest filters claim to work down to 1 micron, though I can't confirm nor deny those claims. The upside is that by cleaning the oil so completely, bypass oil filters increase not only engine life, but also the life of the oil itself. This means longer service intervals.

Source : Carbibles