There are three kinds of filters in the engine: air filter, oil filter, fuel filter, generally called "three filters", plus air conditioning filter, commonly known as four filters. It is responsible for the filtration of the medium in the lubrication system, the combustion system, the engine intake system and the carriage air circulation system.

Air filter is mainly used in pneumatic machinery, internal combustion machinery and other fields, the role is to provide clean air for these mechanical equipment, to prevent these mechanical equipment in the work of inhaling air with impurity particles and increase the probability of abrasion and damage. The main components of the air filter are the filter element and the casing, of which the filter element is the main filtration part, which undertakes the filtration work of the gas, and the casing is the external structure that provides the necessary protection for the filter element. The working requirements of the air filter are to be able to undertake high-efficiency air filtration work, not to increase too much resistance to air flow, and to work continuously for a long time.

The oil filter is located in the engine lubrication system. Its upstream is the oil pump, and the downstream is the parts that need to be lubricated in the engine. Its function is to filter out harmful impurities in the oil from the oil pan, and supply the crankshaft connecting rod, camshaft, supercharger, piston and other movement pairs with clean oil, play the role of lubrication, cooling, cleaning, so as to extend the life of these parts, the oil filter is divided into exchangable, rotary, centrifugal according to the structure; According to the arrangement in the system can be divided into full-flow, shunt type. The filter materials used in the oil filter are filter paper, felt, metal mesh, non-manufactured cloth and so on.

Fuel filters include diesel filters, gasoline filters and natural gas filters. Its function is to filter out harmful particles and water in the engine fuel gas system to protect the oil pump nozzle, cylinder liner, piston ring, etc., to reduce wear and avoid clogging.

The structure of the diesel filter is roughly the same as that of the oil filter, and there are two kinds of interchangeable and rotary. However, the working pressure and oil temperature resistance requirements are much lower than that of the oil filter, and the filtration efficiency requirements are much higher than that of the oil filter. The filter element of the diesel filter is mostly filter paper, and there are also felt or polymer materials. In addition to filtering the mechanical impurities in diesel, there is also an important function of the diesel filter is to filter water. The existence of water is very harmful to the diesel fuel supply system, rust, wear, stuck and even worsen the combustion process of diesel. The water removal method of diesel oil filtration system is mainly precipitation. Either a sedimentation chamber is arranged in the lower part of the filter, or a special precipitator is used. Whether it is the settling chamber under the filter or the special settling device, the water discharge valve is provided, and the valve is opened when the water accumulates to a certain amount.

Gasoline filters are divided into carburetor type and electric injection type. The gasoline engine using the carburetor, the gasoline filter is located on the inlet side of the oil pump, the working pressure is small, and the nylon shell is generally used. The gasoline filter of the EFI engine is located on the outlet side of the oil pump, the working pressure is higher, and the metal shell is usually used. The filter element of the gasoline filter is mostly filter paper, and nylon cloth and polymer materials are also used.