The engine of a self-propelled combine is mounted in an enclosure having an inlet through which cooling air is drawn by an engine-driven fan. The air is filtered by a rotating disk-like screen mounted over the air inlet before passing through a radiator adjacent the inlet. A duct extends across the face of the disk from its center to periphery, changes direction and terminates with an open end adjacent the inlet side of the fan by-passing the radiator. The duct is also open adjacent the face of the screen so that the fan draws air through the duct, (some of the air being necessarily drawn from inside the screen as it passes the duct opening) so that foreign material filtered from the mass of cooling air being drawn through the unobstructed portion of the screen and held on the screen exterior surface is removed and carried through the duct. A trash knife rigidly attached to the periphery of the screen extends outwards parallel to the screen's axis of rotation. The knife is shielded by and travels within an annular channel-like shroud as the disk rotates. The shroud is connected with and intersects the duct so that as the screen rotates the knife passes through the duct, closely adjacent a shear bar supported there. The knife cuts or dislodges any foreign material lodged in the duct where it changes direction so that it may continue its passage carried by the air flow through the duct. A relatively flexible material with a high percentage open area is used for the filter element of the screen and is held taut by a compression spring pressing outwards on a thrust washer at the center of the screen to form it into a shallow cone.