A jet engine is a reaction engine which uses ambient air as the working fluid, and converts it to a hot, high-pressure gas which is expanded through one or more nozzles. Two types of jet engine, the turbojet and turbofan, employ axial-flow or centrifugal compressors to raise the pressure before combustion, and turbines to drive the compression. Ramjets operate only at high flight speeds because they omit the compressors and turbines, depending instead on the dynamic pressure generated by the high speed (known as ram compression).