A jet engine is a heat engine that is propelled in a forward direction as the result of the escape of hot gases from the rear of the engine. In an airbreathing jet engine, air entering the front of the engine is used to burn a fuel within the engine, producing the hot gases needed for propulsion (forward movement). Jet engines are used for the fastest commercial and military aircraft now available.
The scientific principle on which the jet engine operates was first stated in scientific terms by English physicist and mathematician Isaac Newton (1642–1727) in 1687. According to Newton's third law of motion, for every action there is an equal and opposite reaction. That principle can be illustrated by the behavior of a balloon filled with air. If the neck of the balloon is untied, gases begin to escape from the balloon. The escape of gases from the balloon is, in Newton's terms, an "action." The equal and opposite reaction resulting from the escape of gases is the movement of the balloon in a direction opposite to that of the movement of the gases. That is, as the air moves to the rear, the balloon moves forward.
Afterburner: A device added at the rear of a jet engine that adds additional fuel to the exhaust gases, increasing the efficiency of the engine's combustion.
Combustion: The process of burning; a chemical reaction, especially a rapid combination with oxygen, accompanied by heat and light.
Compress: To make more compact by using pressure.
Ramjet: A simple type of air-breathing jet engine in which incoming air is compressed and used to burn a jet fuel such as kerosene.
Turbojet: A type of air-breathing jet engine in which some of the exhaust gases produced in the engine are used to operate a compressor by which incoming air is reduced in volume and increased in pressure.
Turboprop: An engine in which an air-breathing jet engine is used to power a conventional propeller-driven aircraft.
Ramjets. The simplest of all jet engines is the ramjet. The ramjet consists of a long cylindrical metal tube open at both ends. The tube bulges in the middle and tapers off at both ends. As the engine moves forward at high speeds, the air entering it is automatically compressed (made more compact under pressure). The compressed air is then used to burn a fuel, usually a kerosene-like material. The hot gases produced during combustion within the engine are then expelled out the back of the engine. As the gases leave the back of the jet engine (the nozzle exit), they propel the engine—and the wing and airplane to which it is attached—in a forward direction.
A typical ramjet engine today has a length of about 13 feet (4 meters), a diameter of about 39 inches (1 meter), and a weight of about 1,000 pounds (450 kilograms). A ramjet engine of this design is capable of giving a maximum velocity of about Mach 4 (Mach 1 is equal to the speed of sound: 740 miles [1190 kilometers] per hour).
Turbojets. A turbojet differs from a ramjet in that it contains a compressor attached to a turbine. The compressor consists of several rows of metal blades attached to a central shaft. The shaft, in turn, is attached to a turbine at the rear of the compressor. When air enters the inlet of a turbojet engine, some of it is directed to the core of the engine where the compressor is located. The compressor reduces the volume of the air and sends it into the combustion chamber under high pressure.
The exhaust gases formed in the combustion chamber have two functions. They exit the rear of the chamber, as in a ramjet, providing the engine with a forward thrust. At the same time, the gases pass over the blades of the turbine, causing it to spin on its axis. The spinning turbine operates the compressor at the front of the engine, making possible the continued compression of new incoming air. Unlike a ramjet engine, which only operates after a high speed has been attained, the turbojet engine operates continuously.
Turboprop engines. In a turboprop engine, a conventional propeller is attached to the turbine in a turbojet engine. As the turbine is turned by the series of reactions described above, it turns the airplane's propeller. Much greater propeller speeds can be attained by this combination than are possible with simple piston-driven propeller planes. However, propellers cannot operate at high air speeds. The maximum efficient speed at which turboprop airplanes can operate is less than 450 miles (724 kilometers) per hour.
Afterburners. No more than about one-quarter of all the oxygen entering the front of the jet engine is actually used to burn fuel within the engine. To make the process more efficient, some jet engines are also equipped with an afterburner. The afterburner is located directly behind the turbine in the jet engine. It consists of tubes out of which fuel is sprayed into the hot exhaust gases exiting the tubing. Combustion takes place in the afterburner, as it does in the combustion chamber, providing the engine with additional thrust.