In turbojet engines, air is taken in from the front of the engine and compressed to many times its original pressure. Fuel is then added to the compressed air and ignited inside a combustion chamber to raise the fluid's temperature. The hot air that is produced then passes through a turbine, which powers the compressor. Excess pressure produced by the turbine is sent to the nozzle, or rear, of the turbojet engine, where it is expelled as a gas stream. This stream provides the thrust needed to power an aircraft. Aside from being used for aircrafts, turbojets are also found in missiles.
Turbofans consist of a large fan that draws in air from the front. Much like a turbojet, a turbofan engine relies on a compressor, combustion chamber and turbine to create thrust. Only a small percentage of air actually passes through the combustion chamber; the rest passes through a low-pressure compressor and is mixed with the gas-generated exhaust to form a "hot" jet. This system is designed to increase thrust without sacrificing fuel. Most modern commercial and military aircrafts use the turbofan engine.
The turboprop engine is found attached to an airplane's propellor. A turbine at its rear is powered by hot gas that powers a propellor and causes it to rotate at a rapid rate. Because the turboprop achieves more efficient propulsion at speeds below 500 miles per hour (805 kmh), these type of engines are typically found on transport aircrafts and smaller airliners.
Another variation of gas-turbine engine, the turboshaft engine is very similar to the turboprop. Instead of powering an external propellor, turboshaft engines provide power to a helicopter's rotor.
Considered the most basic aircraft engine, the ramjet is unique in that it contains no rotating or moving parts. A ramjet's ability to provide thrust is dependent on the speed of the aircraft to which it is attached. Since it develops very little thrust below the speed of sound, aircrafts using ramjet technology require some type of assisted takeoff, usually another aircraft. Space vehicles usually rely on the use of ramjets. Like the turbojet engine, ramjets are also used in guided missile systems.
