According to the filtration principle, the air filter can be divided into several types: filter type, centrifugal type, oil bath type and composite type. The air filters commonly used in engines mainly include inertial oil bath air filters, paper dry air filters, and polyurethane filter air filters. The inertial oil bath air filter is filtered by inertial filter, oil bath filter and filter filter. The latter two air filters are mainly filtered through the filter. Inertial oil bath air filter has the advantages of low air intake resistance, can adapt to dusty and sandy working environment, and has long service life. It has been used in various types of automobile and tractor engines. However, such an air filter has a low filtration efficiency, a large weight, a high cost, and inconvenient maintenance, and has been gradually eliminated in automobile engines. The filter element of the paper dry air filter is made of resin-treated microporous filter paper. The filter paper is porous, loose and folded, has certain mechanical strength and water resistance, and has high filtration efficiency, simple structure, light weight and cost. Low and easy to maintain, it is the most widely used automotive air filter. The filter element of the polyurethane filter air filter is made of soft, porous and sponge-like polyurethane. It has strong adsorption capacity. This air filter has the advantages of paper dry air filter, but the mechanical strength is low. It is widely used. The latter two air filters have the disadvantage of a short service life and unreliable operation under harsh environmental conditions.
Various air filters have their own advantages and disadvantages, but inevitably there is a contradiction between the amount of intake air and the efficiency of filtration. With the in-depth study of air filters, the requirements for air filters are also increasing. Some new air filters have emerged, such as fiber filter air filters, duplex filter material air filters, muffle air filters, and constant temperature air filters to meet the needs of engine operation.