The structure of the diesel filter is roughly the same as that of the oil filter, and there are two types: replaceable and spin-on. However, its working pressure and oil temperature resistance requirements are much lower than those of oil filters, while its filtering efficiency requirements are much higher than those of oil filters. The filter element of the diesel filter mostly uses filter paper, and some also use felt or polymer materials. In addition to filtering mechanical impurities in diesel, diesel filters also have an important function of filtering water. The presence of water is extremely harmful to the fuel supply system of diesel engines. Corrosion, wear, damage to the piston rings of the cylinder will even cause the phenomenon of cylinder pulling, which can worsen the combustion process of diesel. The diesel filtration system uses the fluid flow control technology to separate the water according to the density of the liquid to achieve the effect of filtering the water.
Oil-water separator is an instrument that removes oil and moisture.

The mechanism is mainly divided into oil-in-water separator and water-in-oil separator. In terms of use, it is mainly divided into industrial-grade oil-water separator, commercial oil-water separator and household oil-water. There are several types of separators; from the principle of separation, there are membrane-filtered oil-water separators, oil-water separators that use lipophilic materials, unpowered oil-water separators with different specific gravity layers, pharmacological demulsified oil-water separators, and oil-water separators. Application in petrochemical industry, automobile industry, sewage treatment industry, etc.
Automobile oil-water separator is a kind of fuel filter, the main function of which is to remove the moisture in the diesel oil, so as to reduce the fuel injection nozzle failure and extend the service life of the engine. The principle is based on the difference between the density of water and fuel oil, and the separator that removes impurities and moisture using the principle of gravity sedimentation. There are also separation elements such as diffusion cones and filters. The oil-water separator also has other functions, such as pre-heating the fuel to prevent wax formation and filtering impurities.