The function of the oil filter is to filter out metal wear debris, mechanical impurities and oil oxides in the oil. If these impurities enter the lubrication system with the oil, they will aggravate the wear of engine parts and may also block the oil pipe or oil passage.

Two types of oil filters are installed in front of and behind the oil pump. The thing installed before the oil pump is called oil filter. After the oil pump is installed, there are two types according to its layout: full flow type and split flow type.

The filter is generally of the filter screen type, which is installed in front of the oil pump to prevent large-sized impurities from entering the oil pump. There are two types of filters: floating filter and fixed filter.

The floating filter can absorb the cleaner oil on the oil surface, but the foam on the oil surface is easy to be absorbed, reducing the oil pressure and making the lubrication unreliable.

Full-flow oil filter
The full flow oil filter is connected in series between the oil pump and the main oil passage, so all oil is filtered through it. At present, full-flow oil filters are widely used in automobile engines.
There are many kinds of filter elements of full-flow oil filter. At present, most automotive engines use paper filter elements. The oil filter of the paper filter element has two structural forms, one is removable, and can be replaced only by replacing the paper filter element, as shown in the figure below. The other is integral type, which needs to be completely replaced.
The full flow oil filter has a certain service life and should be replaced when it expires. When the filter element of the oil filter is seriously blocked by impurities within its service life, the oil pressure at the oil inlet of the oil filter will increase. When the pressure reaches the specified value, the bypass valve in the oil filter will open and the oil will not enter the main oil passage directly through the filter element. At this time, although the oil is not filtered and delivered to all lubricating surfaces, it is much better than no lubricating oil.

Shunt oil filter
Especially for heavy truck engines, the full flow split oil filter is generally used, as shown in the figure below. Full-flow filter, also known as coarse filter, is used to filter impurities with particle size of more than 0.05 mm in engine oil. The shunt filter, also known as fine filter, is used to remove fine impurities with particle size of more than 0.001 mm, which is only 5%~10% of the oil supply of the oil pump.
There are two types of shunt fine filter: filtration type and centrifugal type. At present, centrifugal oil fine filter is widely used. There is a rotor in the filter, which is supported on the shaft by the rolling bearing, as shown in the figure below. There are two nozzles with opposite injection directions in the rotor. It uses the pressure energy of the lubrication system itself. When oil enters the rotor and sprays out from the nozzle, it will produce reaction torque, thus driving the rotor to rotate rapidly. Under the action of centrifugal force, the oil in the rotor separates solid impurities and accumulates on the inner wall of the rotor. The oil in the center of the rotor becomes clean and flows back to the oil pan from the nozzle.