Air filters can be divided into filter type, centrifugal type, oil bath type, compound type. At present, the commonly used air filters in the engine mainly include inertia oil bath air filters, paper dry air filters, polyurethane air filters and other air filters. Inertial oil-bath air filter has gone through three levels of inertial filtration, oil-bath filtration and filtration filtration successively, and the latter two kinds of air filters are mainly filtered through filter element filtration. Inertial oil-bath air filter has the advantages of low air intake resistance, dust and sand environment, long service life, etc., which has been used in the engine of many types of cars and tractors. But this kind of air filter filters the efficiency is lower, the weight is big, the cost is high, the maintenance is inconvenient, has been eliminated gradually in the automobile engine. The filter core of paper dry air filter is made of microporous filter paper treated with resin. The filter paper is porous, loose and folded. It has certain mechanical strength and water resistance. Polyurethane filter core air filter core is made of soft, porous, spongy polyurethane, with strong adsorption capacity. This air filter has the advantages of paper dry air filter, but low mechanical strength, widely used in car engines. The disadvantage of the latter two air filters is that their service life is short and they are not reliable in harsh environmental conditions.