What is a filter, where is it used, and how many parts does it consist of?

The air filter is located in the engine's intake system. It is a assembly composed of one or several filter components for cleaning the air. Its main function is to filter out harmful impurities in the air that is about to enter the cylinder, so as to reduce the early wear of the cylinder, piston, piston ring, valve and valve seat. There are two types of air filters, namely dry and wet.
A dry air filter is a type of filter that separates impurities from the air through a dry filter element, such as a paper filter element. The air filters used in light vehicles (including sedans and mini vehicles) are generally single-stage. Its shapes can be flat, round, oval or flat. The filter material is filter paper or nonwoven fabric. The end cover of the filter element is made of metal or polyurethane, and the shell material is metal or plastic. At the rated air volume flow rate, the original filtration efficiency of the filter element should not be less than 99.5%. Due to the harsh working environment, heavy-duty vehicles must have multiple air filters. The first stage is a cyclone pre-filter (such as blade rings, cyclone tubes, etc.), which is used to filter out coarse particle impurities, with a filtration efficiency of over 80%. The second stage fine filter is a microporous paper filter element (generally referred to as the main filter element), with a filtration efficiency of over 99.5%. After the main filter element, there is also a safety filter element, whose function is to prevent dust from entering the engine during the installation and replacement of the main filter element or in case of accidental damage to the main filter element. The material of the safety core is mostly nonwoven fabric, and some use filter paper.
Wet air filters include two types: oil-immersed and oil bath. Oil-immersed type separates impurities in the air through an oil-immersed filter element. The filter element materials can be metal wire fabrics or foamed materials. The oil bath type is to introduce the dust-containing air sucked in into the oil pool to remove most of the dust. Then, when the air with oil mist flows upward through a filter element made of metal wire, further filtration is carried out. The oil droplets and the trapped dust return to the oil pool together. Oil bath air filters are now generally used in agricultural institutions and Marine power.

The oil filter is located in the engine lubrication system. Its upstream is the oil pump, and its downstream are the various components in the engine that need lubrication. Its function is to filter out harmful impurities in the engine oil from the oil pan, and supply clean engine oil to the crankshaft, connecting rod, camshaft, turbocharger, piston rings and other moving pairs, playing a role in lubrication, cooling and cleaning, thereby extending the service life of these components. Engine oil filters are classified by structure into replaceable type, spin-on type and centrifugal type. According to the layout in the system, it can be divided into full-flow type and split type. The filter materials used in oil filters include filter paper, felt, metal mesh, nonwoven fabric, etc.
Before the 1980s, the oil filters used in domestic engines were mostly replaceable. This type of filter structure places the filter element and other components, such as springs and sealing rings, inside a metal casing, and connects and fixes the casing, filter element, etc. to a metal filter base through a pull rod. Its advantage is that the usage cost is low, and only regular maintenance and replacement of the filter element are required. The drawback lies in the excessive number of sealing points. During maintenance and replacement of the filter element, some parts may be missed, which can easily lead to leaks and is also troublesome to replace.
Since the early 1980s, when Bengbu Filter General Factory was the first in China to introduce a spin-on filter production line from Italy, spin-on filters have gradually been recognized and adopted by domestic Oems. The feature of this type of filter is that it is equipped with a check valve and a bypass valve inside, and there is only one sealing point. It can be replaced as a whole, which greatly improves the sealing performance and is easy to replace. The filter element material is mostly imported filter paper, so it has a high filtration efficiency, low flow resistance and long service life. At present, all domestic sedans adopt this type of oil filter structure. The vast majority of mini cars, large, medium and small-sized buses, light and medium-sized trucks, as well as some heavy-duty trucks and agricultural vehicles all use spin-on oil filters.
A centrifugal oil filter has a rotor sleeved on a shaft and two nozzles with opposite spray directions. When oil enters the rotor and exits from the nozzles, the rotor rotates rapidly, cleaning the oil inside the rotor. Impurities in the oil are centrifugally thrown onto the inner wall of the rotor, and the oil coming out of the nozzles flows back to the oil pan. The centrifugal oil filter is characterized by stable performance and a flexible structure. There is no filter element that needs to be replaced. As long as the rotor is disassembled regularly and the dirt deposited on the rotor wall is cleaned, it can be reused. Its lifespan can be equivalent to that of an engine. Its shortcomings lie in its complex structure, high price, heaviness, etc., and it has high technical requirements for the users.
Full-flow oil filters, such as the replaceable type, spin-on type, and split centrifugal type mentioned earlier, filter all the engine oil entering the system. The split-type filter only filters 5% to 10% of the engine oil supplied by the oil pump. The split-type oil filters are all fine filters and are generally used in conjunction with the full-flow type. Most low-power engines only use full-flow filters, while large-power diesel engines mostly adopt full-flow combined with split-flow filtration devices.

There are three types of fuel filters: diesel filters, gasoline filters and natural gas filters. Its function is to filter out harmful particles and moisture in the engine's fuel and gas system to protect the fuel pump, nozzle, cylinder liner, piston rings, etc., reduce wear and avoid blockage.
The structure of a diesel filter is roughly the same as that of an oil filter, and it is available in two types: replaceable and spin-on. However, the working pressure it can withstand and the oil temperature resistance requirements are much lower than those of the oil filter, while the filtration efficiency requirements are much higher than those of the oil filter. The filter elements of diesel filters are mostly made of filter paper, and some are made of felt or polymer materials. In addition to filtering mechanical impurities from diesel, a diesel filter also has an important function of filtering water. The presence of water poses a great threat to the fuel supply system of diesel engines, causing rust, wear, jamming and even deteriorating the combustion process of diesel. Diesel