According to the filtration principle, the air filter can be divided into filter type, centrifugal type, oil bath type and compound type. The commonly used air filters in the engine mainly include inertia oil bath air filter, paper dry air filter, polyurethane filter element air filter and so on. Inertia oil bath type air filter has gone through inertia type filter, oil bath type filter, filter type filter three filtration, the last two kinds of air filter mainly through the filter element filter type filter. Inertia oil bath type air filter has the advantages of low intake resistance, can adapt to the dusty and sandy working environment, long service life, etc., previously used in a variety of models of cars, tractor engines. However, this kind of air filter has low filtration efficiency, large weight, high cost and inconvenient maintenance, and has been gradually eliminated in the automobile engine. The filter element of paper dry air filter is made of microporous filter paper treated by resin. The filter paper is porous, loose, folded, has certain mechanical strength and water resistance, and has the advantages of high filtration efficiency, simple structure, light weight, low cost, convenient maintenance, etc. It is the most widely used automobile air filter at present. The filter element of the air filter is made of soft, porous and spongy polyurethane, which has strong adsorption capacity. This air filter has the advantages of paper dry air filter, but it has low mechanical strength and is widely used in car engines. The disadvantages of the latter two air filters are shorter service life and unreliable operation under harsh environmental conditions.
