According to the principle of filtration, air filter can be divided into filter type, centrifugal type, oil bath type, compound several. The main air filters used in the engine are inertial oil bath air filter, paper dry air filter, polyurethane filter air filter and so on. Inertial Oil bath air filter has undergone inertial filtration, oil bath filtration, filter filtration three level filtration, the latter two air filters mainly through the filter filter filtration. Inertial Oil Bath air filter has the advantages of small intake resistance, can adapt to the multi-dust and sandy working environment, long service life and so on, previously used in a variety of models of automobiles, tractor engines. However, this air filter filtration efficiency is low, heavy weight, high cost, inconvenient maintenance, in the car engine has been gradually eliminated. Paper Dry air filter Filter cartridges are made of resin-treated microporous filter paper, with porous, loose, folding, a certain degree of mechanical strength and water resistance, with high filtration efficiency, simple structure, light weight, low cost, easy maintenance and other advantages, is the most widely used automotive air filter. Polyurethane Filter air filter filter is made of soft, porous, spongy polyurethane, adsorption capacity, this air filter has the advantages of paper dry air filter.