In-line fuel filters are applied in fuel systems, especially in petrol carburettor engines and fuel injection engines, installed on the fuel pipe using quick connect fittings or band clips. These filters have a filtering cartridge with paper medium, which is pleated or warped. Warped cartridges are made of special, thin filtering material. It has a highly filtration efficiency but limited absorptivity; hence the filtering medium surface for these filters must be larger than for filters with pleated medium.
Fuel filters in plastic casings are a large group of in-line filters installed on fuel pipes using metal band clips on the casing ferrules. These filters are used as coarse filters in multistage fuel filtration systems of large engines. They are also installed as fine filters in one stage petrol filtration in small carburettor engines, and fuel injection engines, as well as in motorbikes and scooters. Filtering cartridge installed inside, may, according to requirements, have a net filter medium (coarse filters for large size engines) or standards filtering paper formed by pleating (filters for one stage petrol filtration). The plastic casing characteristic for these filters is usually transparent, which allows monitoring of the filtering medium contamination.
Characteristic construction used in cars with diesel engines are in-line filters equipped with water separator. Such a specific design "connects" can-type filter and in-line filter. In these filters, the cover with thread, which is tightened to the filter head, has been replaced with ferrules sticking out of the cover, which make the installation similar to that characteristic for in-line filters. Installing both the ferrules (fuel inlet and outlet) in one cover make the inner construction of the filter similar to spin-on filters. Such a combination resulted from installing a water separator in the filter, which is required for diesel oil filtration in modern engines. For a better performance, it is required to open the drain plug to let the water out to prevent serious problems with starting and exploitation of the engine, especially in winter.