Lycoming IO-540 is a family of six-cylinder, horizontally opposed, air-cooled aircraft piston engines with direct drive. Its fuel-injection system meters fuel in proportion to airflow and delivers it to the intake ports. Lycoming's manual distinguishes this arrangement from the carbureted O-540, despite their shared basic six-cylinder architecture. Civil installations include the Cessna 182S and 182T, with the approved engine suffix defining the particular installation.