There are two main types of air filters, namely dry type and wet type.
1. The dry air filter is a filter that separates the impurities in the air through a dry filter element (such as a paper filter element). The paper dry air filter element is made of resin treated microporous filter paper. 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, and convenient maintenance. The air filter used in light vehicles, such as home cars, is generally single-stage. Its shape is oblate, elliptical and flat. The filter material is filter paper or nonwoven. The end cover of the filter element is metal or polyurethane. The shell material is metal or plastic. Under the rated air volume flow, the original filter efficiency of the filter element should not be less than 99.5%. Due to the harsh working environment of the engine of heavy vehicles or construction machinery, its air filter must be majority. The first stage is a cyclone pre-filter (such as vane ring, cyclone tube, etc.), which is used to filter coarse particles. Due to the guidance of the cyclone blade, there is a strong rotation inside. Under the action of centrifugal force, large particles of solid impurities mixed in the air are thrown onto the inner surface of the filter shell, and under the action of gravity, they settle to the bottom along the wall, with a filtering efficiency of more than 80%. The second stage of fine filtration is a microporous paper filter element (generally referred to as the main filter element), with a filtration efficiency of more than 99.5%. There is a safety filter element behind the main filter element, which is used to prevent dust from entering the engine when installing and replacing the main filter element or when the main filter element is accidentally damaged. The material of the safety core is mostly nonwoven, and filter paper is also used.

2. Wet air filter includes oil-immersed type and oil-bath type. The oil-immersed type is to separate the impurities in the air through an oil-immersed filter element. The filter element material includes metal wire fabric and foam material. The oil bath type is to introduce the inhaled dust-containing air into the oil pool to remove most of the dust, and then conduct further filtration when the air with oil mist flows upward through a filter element made of metal wire, and the oil droplets and the blocked dust return to the oil pool together. The oil-bath air filter is now generally used for agricultural institutions and marine power.
The filtration accuracy of the air filter represents the size value of the smallest particles that can filter dust, including 2mm, 5mm, 25mm, etc., and the standard filtration accuracy is 5mm. Imported diesel engine requires air filtration accuracy of 5 μm. Domestic diesel engine shall also be less than 20 μm。 The filtration accuracy of high-efficiency filter paper is 2 μm. 30 for ordinary imported filter paper μ m. The domestic filter paper is only 80 μm。 The filtration accuracy is directly related to the size of the vent hole of the filter element. The larger the aperture, the lower the filtration accuracy, but the lower the resistance loss.

3. Main performance indicators of dry air filter
Dry air filter mainly refers to microporous fiber filter paper filter, which has a series of advantages such as light weight, small size, low cost, and convenient manufacturing. Its main performance indicators include: rated flow, coarse filtration efficiency, original filtration efficiency, original resistance, laboratory test life, dust storage capacity test and cumulative efficiency.
Rated flow:
The rated flow of the air filter is specified by the main engine manufacturer, and the technician marks it on the product drawing of the air filter. The air flow through the air filter outlet at standard atmospheric pressure shall match the air supply required by the engine at rated power. Unit: m3/h. The rated air flow of the engine can be determined according to its working volume, rated speed and inflation coefficient.
Coarse filtration efficiency:
The primary filter is a device that uses inertia or centrifugal method to remove part of dust in front of the filter element. The types of coarse filter include cap type coarse filter, basin type coarse filter, vane ring type and tangential inlet type coarse filter, and cyclone tube type coarse filter.
Filtration efficiency:
Filtration efficiency is the most important technical index of air filter. It refers to the ability of the test piece to remove specific test dust, expressed as a percentage.
Original resistance:
Original resistance of the assembly: when passing through the assembly with a new filter element under the rated air flow, the static pressure measured at the air outlet of the assembly is expressed in pascal (mm water column).
Original resistance of filter element: the difference between the static pressure measured under the rated air flow rate and the static pressure measured after replacing the filter element with an ideal nozzle is expressed in pascal (mm water column).
Resistance at the end of blocking: refers to the resistance of the assembly at the end of the life test in the assembly laboratory, expressed in pascals (mm water column). When the resistance at the end of the blockage is reached, the internal combustion engine will produce problems such as black smoke and insufficient power due to insufficient air supply.
Laboratory test life:
Under the rated air flow rate, add dust into the assembly continuously and evenly at the specified dust concentration. When the assembly resistance reaches the end of blocking resistance, the cumulative time of adding dust into the assembly is expressed in hours. Laboratory test life of single-stage assembly: ≧ 8h; Laboratory test life of multi-stage assembly without cyclone tube coarse filter: ≧ 10h; Test life of multi-stage assembly with cyclone tube coarse filter in laboratory: ≧ 16h.
Dust storage capacity test and cumulative efficiency:
Too much dust and dust in the filter element causes blockage and increase of air inlet resistance. When the engine power is reduced by 5% or the fuel consumption is increased by 5%, the intake resistance is a limit value. When this value is reached, the filter element must be cleaned or replaced. During the test, the dust storage capacity of the filter element is the weight of accumulated dust when the inlet resistance or pressure drop reaches 7-46kPa, and the filtration efficiency during the test is the cumulative efficiency.