Compressors are used to compress air. They compress incoming air and send it back out upon demand as compressed air. Depending on the model, compressors are lubricated with oil or oil-free. They are divided into to groups according to functionality: turbo compressors and displacement compressors. Turbo compressors can accept large intake volumes, while displacement compressors achieve great compression with smaller amounts of air. Air compressors are normally reciprocating compressors. They work on the displacement principle and are often use in industry as drive units for compressed-air tools.