Air filter casings in trucks are usually placed outside the engine compartments due to its large volume. Sometimes there are two filtering cartridges. In this case the inner cartridge works as a safety device stopping contaminants from flowing into the engine during cartridge replacement or protecting from mechanical damage of main cartridge medium surface during exploitation. Due to the large size of air filter casings for trucks, they are made of steel (traditional constructions) or reinforced plastic. Placing the air inlet tangent to the outer diameter makes the casing become the filter itself using the centrifugal force resulting from the air rotational motion around the filtering cartridge.