A jet engine is any type of engine that works by burning a compressed
air/
fuel mixture and shooting it out of the back for
propulsion.
Turbojet
The turbojet works by sucking in air with many rows of axial turbines. These are alternating stator and rotor turbines. The rotors of course turn to suck in air. The stators break up the centrifugal air flow. They also slow down the air, inducing compression.
After passing though the compressor, the air pressure is several atmospheres and is moving at a high rate of speed. Some of the air is funneled into the combustion chamber, while other air is bypassed. Here, it is mixed with the fuel, usually JP-5 kerosene, and burned. This creates immense heat and pressure. This mixture is cooled down just past the combustion chamber by ducting additional cool bypassed air.
At this point, the exhaust travels though the turbines. These axial blades gently let the pressure out of the exhaust and give power via a shaft to the compressor in front.
Turbofan
This type of engine provides power to all airliners and many military aircraft. It is very fuel efficient and is quiet. A high bypass turbofan works exactly the same way as a turbojet except it has a huge low pressure fan in front. Most of this air bypasses the rest of the engine and provides most of the thrust. In a turbofan, the bypassed air provides most of the thrust, while in a turbojet, it is solely the exhaust. A turbofan is capable of flying to about Mach 1.5.
Ramjet
This is the simplest of all jet engines, so simple it can be built at home. Ramjets are capable of flying to Mach 6, far faster than other forms of jet engines. They have seen use in high supersonic cruise missiles. The Bomarc cruise missile used a ramjet engine. Once it reached Mach 5.5.
A ramjet has to be moving at transonic speeds before it will work well. At high speeds, air is shoved into the intake without the use of turbines. It is then passed through the combustion chamber, where it is mixed with kerosene, hydrogen, or methane, and burned. It then exits the engine.
Supersonic ramjets (ramjets that fly over Mach 1.5) work exactly the same way except that they have an additional diffuser in their front to slow down the air to subsonic speeds. Diffusers are usually conical, as seen in the SR-71. They slow down the air by compressing it.
Ramjets cannot start unless they are moving fairly fast already. Therefore, a ramjet aircraft must start with another power source, such as a turbojet or rocket.
A variation on the ramjet is the SCRAMjet. This is the Supersonic Combustion Ramjet. It works exactly the same way as a ramjet, except it burns fuel in a supersonic, as opposed to subsonic, flow. SCRAMjets are currently under development. They will be able to achieve speeds of Mach 25, which would allow them to fly to orbit with no rocket power.
Pulsejets
Pulsejets are noisy, loud, inneficient, but simple. They saw service in the V-1 Cruise Missile during World War II.
A pulsejet works exactly the same way as a ramjet except it pulses. At the intake, there are one-way reed valves. Fuel is injected in pulses, and exploded by a spark plug. The valves close when the fuel and air explodes, directing the thrust toward the rear. A pulsejet sounds exactly the same as a two-stroke gasoline engine, except many, many, many times louder.
Motorjets
This was in early form of jet. This type of jet was actually employed on an aircraft in 1910. It crashed. Later on, a motorjet was utilized in the Italian Caproni-Campini fighter in 1940.
A motorjet is very simple. It has not rotors in the back. It uses another engine for compression. The Caproni-Campini used a 700 horsepower gasoline engine to drive the compressors. The motorjet was doomed to failure because it was inneficient and underpowered.