Air filter can be divided into filtration type, centrifugal type, oil bath type and composite type. At present, the air filters commonly used in the engine mainly include inertia oil bath air filter, paper dry air filter, polyurethane filter element air filter and so on. The inertia oil bath air filter has been filtered by inertia filter, oil bath filter and filter filter, and the latter two air filters are mainly filtered by filter element. Inertia oil bath air filter has the advantages of small air inlet resistance, being able to adapt to dusty and sandy working environment and long service life. It has been used in many types of automobile and tractor engines before. But this kind of air filter is low efficiency, heavy weight, high cost and inconvenient maintenance. The filter element 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. It has the advantages of high filtration efficiency, simple structure, light weight, low cost and convenient maintenance. It is the most widely used air filter for automobiles. The filter element of polyurethane filter element is made of soft, porous and spongy polyurethane with strong adsorption capacity. This air filter has the advantages of paper dry air filter, but its mechanical strength is low, so it is widely used in car engine. The disadvantages of the latter two kinds of air filters are short service life and unreliable operation under harsh environmental conditions.