1. Paper filter element 
　　It is mainly composed of three parts: filter element, housing and filter seat. The fuel is sent to the fuel filter by the fuel delivery pump, passes through the paper filter element to remove impurities in the fuel, enters the inner cavity of the oil filter cartridge, and then passes through the oil collection cavity on the filter seat to the fuel injection pump. The filter seat is equipped with an oil return joint and a built-in overflow valve. When the fuel pressure in the fuel filter exceeds 0.08MPa, the excess fuel is returned to the fuel tank through the oil return joint. The low-pressure fuel pipeline should be connected in the direction of the arrow on the seat and cannot be connected wrongly. The sealing washer at the bottom of the filter element is installed in the spring seat, and the spring closes the sealing washer to the bottom surface of the nut for sealing. The filter seat and the shell are connected by a tie rod and sealed with a rubber ring. The upper end of the filter seat has a bleed screw, which can be loosened to clear the air in the fuel filter during use.

　　2. High-pressure gap filter
　　Before the fuel enters the fuel injector, many diesel engines are also equipped with a high-pressure slit filter for the last filtration. This filter consists of a ground steel filter element rod inserted into the sleeve with a small gap (about 0.020.04mm). There are 840 oil grooves milled on the filter rod, half of which is for oil inlet and the other half for oil outlet, and the two are staggered. Fuel enters from one end, passes through the gap, and flows out from the other end, leaving some tiny impurities in the tank to achieve the purpose of filtering. And can extend the use time of the fuel injection nozzle, but when the fuel quality is good, this filter can work for 400-500 hours before cleaning.

　　3. felt filter
　　The felt filter is mainly composed of a filter, a filter element and a filter cover. The filter element is installed in the filter body, which is composed of a metal mesh, a silk cloth cover and a felt filter. The filter consists of 7 thicker and eight thinner, which are alternately mounted on the filter net. A pressure plate is welded on the upper end of the filter, a base is welded on the lower end, and a nut is screwed on the base, and the felt filter is pressed tightly on the upper pressure plate through the lower tray. The filter cover is screwed with inlet and outlet pipe joints, and a cock for the vent pipe, and a vent nozzle is installed on the cock. The filter body and the filter cover are connected by a center rod, one end is screwed into the screw plug at the bottom of the filter body, and the other end is screwed with a nut to connect the two. The filter element is pressed tightly on the filter cover by the spring installed on the screw plug. The fuel enters the inner cavity between the filter element and the housing from the oil inlet pipe 7, and then flows into the inner cavity of the metal mesh through the felt filter and the silk cloth sleeve and flows from here along the oil pipe to the oil outlet pipe. The purpose of installing the fuel pipe is to suck clean and airless fuel, and the air is collected in the upper part of the filter. The bleed cock is used to release the air from the fuel cavity to filter the air in the fuel. The air must be vented before the diesel engine starts.