PROPULSION | Turbojet, Turboprop, and Turbofan | Air Turbine (Rotating Blades) Engines
The three most common types of jet engines are the turbojet, turboprop, and turbofan. Air entering a turbojet engine is compressed and passed into a combustion chamber to be oxidized. Energy produced by the burning fuel spins the turbine that drives the compressor, creating an effective power cycle. Turboprop engines are driven almost entirely by a propeller mounted in front of the engine, deriving only 10 percent of their thrust from the exhaust jet. Turbofans combine the hot air jet with bypassed air from a fan, also driven by the turbine. The use of bypass air creates a quieter engine with greater boost at low speeds, making it a popular choice for commercial airplanes.
Airplanes use either piston or turbine (rotating blades) engines to provide propulsion. In smaller airplanes, a conventional gas-powered piston engine turns a propeller, which either pulls or pushes an airplane through the air. In larger airplanes, a turbine engine either turns a propeller through a gearbox, or uses its jet thrust directly to move an airplane through the air. In either case, the engine must provide enough power to move the weight of the airplane forward through the airstream. (PROPULSION, Turbojet, Turboprop, and Turbofan, Air Turbine - Rotating Blades Engines)
The earliest powered airplanes relied on crude steam or gas engines. These piston engines are examples of internal-combustion engines. Aircraft designers throughout the 20th century pushed their engineering colleagues constantly for engines with more power, lighter weight, and greater reliability. Piston engines, however, are still relatively complicated pieces of machinery, with many precision-machined parts moving through large ranges and in complex motions. Although enormously improved over the past 90 years of flight and still suitable for many smaller general aviation aircraft, they fall short of the higher performance possible with modern jet propulsion and required for commercial and military aviation. (PROPULSION, Turbojet, Turboprop, and Turbofan, Air Turbine - Rotating Blades Engines)
The turbine or jet engine operates on the principle of Newton’s third law of motion, which states that for every action, there is an opposite but equal reaction. A jet sucks air into the front, squeezes the air by pulling it through a series of spinning compressors, mixes it with fuel and ignites the mixture, which then explodes with great force rearward through the exhaust nozzle. The rearward force is balanced with an equal force that pushes forward the jet engine and the airplane attached to it. A rocket engine operates on the same principle, except that, in order to operate in the airless vacuum of space, the rocket must carry along its own air, in the form of solid propellant or liquid oxidizer, for combustion. (PROPULSION, Turbojet, Turboprop, and Turbofan, Air Turbine - Rotating Blades Engines)
There are several different types of jet engines. The simplest is the ramjet, which takes advantage of high speed to ram or force the air into the engine, eliminating the need for the spinning compressor section. This elegant simplicity is offset by the need to boost a ramjet to several hundred miles an hour before ram-air compression is sufficient to operate the engine. (PROPULSION, Turbojet, Turboprop, and Turbofan, Air Turbine - Rotating Blades Engines)
The turbojet is based on the jet-propulsion system of the ramjet, but with the addition of a compressor section, a combustion chamber, a turbine to take some power out of the exhaust and spin the compressor, and an exhaust nozzle. In a turbojet, all of the air taken into the compressor at the front of the engine is sent through the core of the engine, burned, and released. Thrust from the engine is derived purely from the acceleration of the released exhaust gases out the rear. (PROPULSION, Turbojet, Turboprop, and Turbofan, Air Turbine - Rotating Blades Engines)
A modern derivative known as the turbofan, or fan-jet, adds a large fan in front of the compressor section. This fan pulls an enormous amount of air into the engine case, only a relatively small fraction of which is sent through the core for combustion. The rest runs along the outside of the core case and inside the engine casing. This fan flow is mixed with the hot jet exhaust at the rear of the engine, where it cools and quiets the exhaust noise. In addition, this high-volume mass of air, accelerated rearward by the fan, produces a great deal of thrust by itself, even though it is never burned, acting much like a propeller. (PROPULSION, Turbojet, Turboprop, and Turbofan, Air Turbine - Rotating Blades Engines)
In fact, some smaller jet engines are used to turn propellers. Known as turboprops, these engines produce most of their thrust through the propeller, which is usually driven by the jet engine through a set of gears. As a power source for a propeller, a turbine engine is extremely efficient, and many smaller airliners in the 19- to 70-passenger-capacity range use turboprops. They are particularly efficient at lower altitudes and medium speeds up to 640 km/h (400 mph).