Jet Engine
Jet Engine
Background
The jet engine is the power plant of today's jet aircraft, producing not only the thrust that propels the aircraft but also the power that fuels many of the aircraft's other systems.
Jet engines operate according to Newton's third law of motion, which states that every force acting on a body produces an equal and opposite force. The jet engine works by drawing in some of the air through which the aircraft is moving, compressing it, combining it with fuel and heating it, and finally ejecting the ensuing gas with such force that the plane is propelled forward. The power produced by such engines is expressed in terms of pounds of thrust, a term that refers to the number of pounds the engine can move.
The jet engine, like many technological innovations, took a long time to progress from concept to design to execution. The first attempts to transcend the traditional piston engine were actually modifications of that engine, both heavy and complex. The turbine design was introduced in 1921, and it and the other basic components of the modern jet engine were present in a design for which a Royal Air Force lieutenant named Frank Whittle received an English patent in 1930. Although testing on Whittle's engine began in 1937, it did not fly successfully until 1941. Across the English Channel in a Germany rushing to arm itself for World War II, similar but entirely separate work had begun with a 1935 jet engine patent issued to Hans von Ohain. Four years later, a team of German engineers led by Dr. Max Hahn achieved success, conducting the first entirely jet-powered flight in history. Upon achieving success with the Whittle engine in 1941, the British promptly shipped a prototype to their allies in the United States, where General Electric immediately began producing copies. The first American jet engine, produced by G.E., took flight in a plane constructed by Bell Aircraft late in 1942. Although use of jets was somewhat limited during World War II, by the end of the war all three countries had begun to utilize elite squadrons of jet-powered fighter planes.
Today's commercial engines, up to eleven feet in diameter and twelve feet long, can weigh more than 10,000 pounds and produce more than 100,000 pounds of thrust.
Design
A jet engine is contained within a cowling, an extermal casing that opens outward, somewhat like a rounded automobile hood, to permit inspection and repair of the interior components. Attached to each engine (a typical 747 uses four) is a pylon, a metal arm that joins the engine to the wing of the plane. Through pumps and feed tubes in the pylons, fuel is relayed from wing tanks to the engine, and the electrical and hydraulic power generated by the engine is then routed back to the aircraft through wires and pipes also contained in the pylons.
At the very front of the engine, a fan helps to increase the flow of air into the engine's first compartment, the compressor. As the fan drives air into it, the compressor—a metal cylinder that gradually widens from front to rear—subjects the incoming air to increasing pressure. To accelerate the progress of the air through the engine, the compressor is fitted with blades that rotate like simple household fans. In the incredibly brief time it takes air to reach the inner end of a typical compressor, it has been squeezed into a space 20 times smaller than the intake aperture.
Expanding as it leaves the high-pressure compressor, the air enters the combustor, an interior engine cylinder in which the air will be mixed with fuel and burned. The combustion chamber is actually a ring, shaped something like a car's air filter. The air that passes through this ring as it exits the compressor is ignited, while another, larger stream of air merely passes through the center of the ring without being bumed. A third stream of air being released from the compressor is passed outside the combustion chamber to cool it.
As the air from the compressor mixes with fuel and ignites in the combustor to produce an incredibly hot volume of gas, some of that gas leaves the engine through the exhaust system, while another, smaller portion is routed into the engine's turbine. The turbine is a set of fans that extend from the same shaft which, further forward in the jet engine, rotates the compressor blades. Its job is to extract enough energy from the hot gases leaving the combustor to power the compressor shaft. In some models, the turbine is also used to generate power for other components of the plane. Because the turbine is subjected to intense heat, each blade has labyrinthine airways cut into it. Cool air from the compressor is routed through these passages, enabling the turbine to function in gas streams whose temperature is higher than the melting point of the alloy from which it is made.
The bulk of the gas that leaves the combustor, however, does so through the exhaust system, which must be shaped very carefully to insure proper engine performance. Planes flying beneath the speed of sound are equipped with exhaust systems that taper toward their ends; those capable of supersonic travel require exhaust systems that flare at the end but that can also be narrowed to permit the slower speeds desirable for landing. The exhaust system consists of an outer duct, which transmits the cooling air that has been passed along the outside of the combustor, and a narrower inner duct, which carries the burning gases that have been pumped through the combustor. Between these two ducts is a thrust reverser, the mechanism that can close off the outer duct to prevent the unheated air from leaving the engine through the exhaust system. Pilots engage reverse thrust when they wish to slow the aircraft.
Raw Materials
Strong, lightweight, corrosion-resistant, thermally stable components are essential to the viability of any aircraft design, and certain materials have been developed to provide these and other desirable traits. Titanium, first created in sufficiently pure form for commercial use during the 1950s, is utilized in the most critical engine components. While it is very difficult to shape, its extreme hardness renders it strong when subjected to intense heat. To improve its malleability titanium is often alloyed with other metals such as nickel and aluminum. All three metals are prized by the aerospace industry because of their relatively high strength/weight ratio.
The intake fan at the front of the engine must be extremely strong so that it doesn't fracture when large birds and other debris are sucked into its blades; it is thus made of a titanium alloy. The intermediate compressor is made from aluminum, while the high pressure section nearer the intense heat of the combustor is made of nickel and titanium alloys better able to withstand extreme temperatures. The combustion chamber is also made of nickel and titanium alloys, and the turbine blades, which must endure the most intense heat of the engine, consist of nickel-titanium-aluminum alloys. Often, both the combustion chamber and the turbine receive special ceramic coatings that better enable them to resist heat. The inner duct of the exhaust system is crafted from titanium, while the outer exhaust duct is made from composites—synthetic fibers held together with resins. Although fiberglass was used for years, it is now being supplanted by Kevlar, which is even lighter and stronger. The thrust reverser consists of titanium alloy.
The Manufacturing
Process
Building and assembling the components of a jet engine takes about two years, after a design and testing period that can take up to five years for each model. The research and development phase is so protracted because the engines are so complex: a standard Boeing 747 engine, for example, contains almost 25,000 parts.
Building components—fan blade
- 1 In jet engine manufacture, the various parts are made individually as part of subassemblies; the subassemblies then come together to form the whole engine. One such part is the fan blade, situated at the front of the engine. Each fan blade consists of two blade skins produced by shaping molten titanium in a hot press. When removed, each blade skin is welded to a mate, with a hollow cavity in the center. To increase the strength of the final product, this cavity is filled with a titanium honeycomb.
Compressor disc
- 2 The disc, the solid core to which the blades of the compressor are attached, resembles a big, notched wheel. It must be extremely strong and free of even minute imperfections, as these could easily develop into fractures under the tremendous stress of engine operation. For a long time, the most popular way to manufacture the disc entailed machine-cutting a metal blank into a rough approximation of the desired shape, then heating and stamping it to precise specifications (in addition to rendering the metal malleable, heat also helps to fuse hairline cracks). Today, however, a more sophisticated method of producing discs is being used by more and more manufacturers. Called powder metallurgy, it consists of pouring molten metal onto a rapidly rotating turntable that breaks the metal into millions of microscopic droplets that are flung back up almost immediately due to the table's spinning. As they leave the table, the droplets' temperature suddenly plummets (by roughly 2,120 degrees Fahrenheit—1,000 degrees Celsius—in half a second), causing them to solidify and form a fine-grained metal powder. The resulting powder is very pure because it solidifies too quickly to pick up contaminants.
- 3 In the next step, the powder is packed into a forming case and put into a vacuum. Vibrated, the powder sifts down until it is tightly packed at the bottom of the case; the vacuum guarantees that no air pockets develop. The case is then sealed and heated under high pressure (about 25,000 pounds per square inch). This combination of heat and pressure fuses the metal particles into a disc. The disc is then shaped on a large cutting machine and bolted to the fan blades.
Compressor blades
- 4 Casting, an extremely old method, is still used to form the compressor blades. In this process, the alloy from which the blades will be formed is poured into a ceramic mold, heated in a furnace, and cooled. When the mold is broken off, the blades are machined to their final shape.
Combustion chamber
- 5 Combustion chambers must blend air and fuel in a small space and work for prolonged periods in extreme heat. To accomplish this, titanium is alloyed to increase its ductility—its ability to formed into shapes. It is then heated before being poured into several discrete, and very complex, segment molds. The sections are removed from their molds, allowed to cool, and welded together before being mounted on the engine.
Turbine disc and blades
- 6 The turbine disc is formed by the same powder metallurgy process used to create the compressor disc. Turbine blades, however, are made by a somewhat different method than that used to form compressor blades, because they are subjected to even greater stress due to the intense heat of the combustor that lies just in front of them. First, copies of the blades are formed by pouring wax into metal molds. Once each wax shape has set, it is removed from the mold and immersed in a ceramic slurry bath, forming a ceramic coating about .25-inch (.63-centimeter) thick. Each cluster is then heated to harden the ceramic and melt the wax. Molten metal is now poured into the hollow left by the melted wax. The internal air cooling passages within each blade are also formed during this stage of production.
- 7 The metal grains in the blade are now aligned parallel to the blade by a process called directional solidifying. The grain direction is important because the turbine blades are subjected to so much stress; if the grains are aligned correctly, the blade is much less likely to fracture. The solidifying process takes place in computer-controlled ovens in which the blades are carefully heated according to precise specifications. The metal grains assume the correct configuration as they cool following their removal from the ovens.
- 8 The next and final stages in preparing turbine blades are machine-shaping and either laser drilling or spark erosion. First, the blade is honed to the final, desired shape through a machining process. Next, parallel lines of tiny holes are formed in each blade as a supplement to the interior cooling passageways. The holes are formed by either a small laser beam or by spark erosion, in which carefully controlled sparks are permitted to eat holes in the blade.
Exhaust system
- 9 The inner duct and the afterburners of the exhaust system are molded from titanium, while the outer duct and the nacelle (the engine casing) are formed from Kevlar. After these three components have been welded into a subassembly, the entire engine is ready to be put together.
Final assembly
- 10 Engines are constructed by manually combining the various subassemblies and accessories. An engine is typically built in a vertical position from the aft end forward, on a fixture that will allow the operator to manipulate the engine easily during build up. Assembly begins with bolting the high pressure turbine (that closest to the combustor) to the low-pressure turbine (that furthest from the cumbustor). Next, the combustion chamber is fastened to the turbines. One process that is used to build a balanced turbine assembly utilizes a CNC (Computer Numerically Controlled) robot capable of selecting, analyzing, and joining a turbine blade to its hub. This robot can determine the weight of a blade and place it appropriately for a balanced assembly.
- 11 Once the turbines and combustion chamber have been assembled, the high and low pressure compressors are attached. The fan and its frame comprise the forward most subassembly, and they are connected next. The main drive shaft connecting the low pressure turbine to the low pressure compressor and fan is then installed, thus completing the engine core.
- 12 After the final subassembly, the exhaust system, has been attached, the engine is ready to be shipped to the aircraft manufacturer, where the plumbing, wiring, accessories, and aerodynamic shell of the plane will be integrated.
Quality Control
As production begins on a newly designed engine, the first one built is designated a test engine, and numerous experiments are run to test its response to the various situations the engine model will encounter during its service life. These include extreme weather conditions, airborne debris (such as birds), lengthy flights, and repeated starts. The first engine built is always dedicated to quality testing; it will never fly commercially.
Throughout the entire process of building an engine, components and assemblies are inspected for dimensional accuracy, responsible workmanship, and material integrity. Dimensional inspections are undertaken in many different ways. One common method is CNC inspection. A coordinate measuring machine (CMM) will inspect key features of a part and compare them to the designed dimensions. Parts are also inspected for material flaws. One method is to apply a fluorescent liquid over the entire surface of a part. After the liquid has migrated into any cracks or marks, the excess is removed. Under an ultraviolet light any surface imperfections that could cause premature engine failure will illuminate.
All rotating assemblies must be precisely balanced to insure safe extended operation. Prior to final assembly, all rotating subassemblies are dynamically balanced. The balancing process is much like spin-balancing the tire on your car. The rotating subassemblies and the completed engine core are computer "spun" and adjusted to insure that they rotate concentrically.
Functional testing of a finished engine takes place in three stages: static tests, stationary operating tests, and flight tests. A static test checks the systems (such as electrical and cooling) without the engine running. Stationary operating tests are conducted with the engine mounted on a stand and running. Flight testing entails a comprehensive exam of all the systems, previously tested or not, in a variety of different conditions and environments. Each engine will continue to be monitored throughout its service life.
Where To Learn More
Books
Moxon, Julian. How Jet Engines Are Made. Threshold Books, 1985.
Ott, James. Jets: Airliners of the Golden Age. Pyramid Media Group, 1990.
Peace, P. Jet Engine Manual. State Mutual Book & Periodical Service, 1989.
EPeriodicals
Brown, David A. "Norwegians Expect to Develop Family of Radial Inflow Turbine Engines." Aviation Week & Space Technology. November 10, 1986, p. 63.
Kandebo, Stanley W. "Engine Makers, Customers to Discuss Powerplants for 130-seat Transports." Aviation Week & Space Technology. June 17, 1991, p. 162.
Kandebo, Stanley W. "NASA-Industry Propulsion Team Addressing HSCT Environmental Issues." Aviation Week & Space Technology. November 25, 1991, p. 58.
Proctor, Paul. "Advanced Fuel Systems Crucial to High-Speed Transport Progress." Aviation Week & Space Technology. February 9, 1987, p. 45.
"Going with the Flow in Jet Engines." Science News. July 30, 1988, p. 73.
—David Harris
Jet Engine
Jet Engine
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. Two general types of jet engines exist: the air-breathing jet engine and the rocket. In an air-breathing 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. In a rocket, air is not needed for propulsion. Instead,
some type of chemical, nuclear, or electrical reaction takes place within the rocket engine. Hot gases formed because of that reaction exit the engine from the rear, providing the engine with its thrust, or forward momentum. Some authorities reserve the term jet engine for the first of these two types, the air-breathing jet engine. Air-breathing jet engines are used for the fastest commercial and military aircraft now available.
Royal Air Force officer and inventor Sir Frank Whittle (1907–1996) and German-born American scientist and inventor Hans Joachim Pabst von Ohain (1911–1998) are considered the inventors of the jet engine. Working independently of each other, Ohain is generally considered the designer of the first operational jet engine, while Whittle was the first (in 1930) to patent a turbojet engine, with Ohain patenting his in 1936. However, Ohain’s jet design first flew in 1939, while Whittle’s design flew later in 1941.
Scientific principle
The scientific principle on which the jet engine operates was first stated in scientific terms by English physicist and mathematician Sir Isaac Newton (1642–1727) in 1687. According to Newton’s third law, for every action, there is an equal and opposite reaction. That principle can be illustrated by the behavior of a balloon filled with air. As long as the neck of the balloon is tied, gases (air) within the balloon push against all sides of the balloon equally, and no motion occurs. If the neck of the balloon is untied, however, 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 outward in one direction, the balloon moves outward in the opposite direction.
Rockets
Rockets can be broadly classified into one of two categories: those that use a chemical reaction as their energy source, and those that use some other kind of energy source. An example of the former are rockets that are powered by the chemical reaction between liquid oxygen and liquid hydrogen. When these two chemicals react with each other, they produce very hot steam (water vapor). The escape of steam from the back of the rocket provides the propulsive force that drives the jet engine forward.
Chemical rockets make use of either liquid fuels, such as the rocket described above, or of solid fuels. An example of the latter are the solid rockets used to lift a space shuttle into orbit. These rockets contain a fuel that consists of a mixture of aluminummetal (the fuel), ammonium perchlorate (the oxidizer), and a plastic resin (the binder).
Nuclear and electric rockets are examples of jet engines that make use of a non-chemical source of propulsion. In a nuclear rocket, for example, a source of nuclear energy, such as a fission or fusion reactor, is used to heat a working fluid, such as liquid hydrogen. The hot gases formed in this process are then released from the rear of the rocket, providing its forward thrust.
Various kinds of electrical rockets have been designed. In one type, a fluid within the engine is first ionized. The ions thus formed are attracted and/or repelled by strong electrostatic fields created within the engine. The escape of the ionized fluid provides the rocket with its forward thrust.
Ramjets
The simplest of all jet engines is the ramjet. The ramjet consists of a long cylindrical tube made of metal, open at both ends. The tube bulges in the middle and tapers off at both ends. This shape causes air entering the front of the engine to expand and develop a higher pressure in the center of the engine. Within the engine, the compressed air is 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. These exiting gases can be compared to the air escaping from a rubber balloon. As the gases leave the back of the jet engine (the nozzle exit), they propel the engine itself in a forward direction.
When the ramjet engine is at rest, no air enters the front of the engine, and the engine provides no thrust. Once the engine is moving through the air, however, it begins to operate more efficiently. For this reason, the use of ramjet engines is usually reserved for aircraft that travel at very high speeds.
A typical ramjet engine today has a length of about 13 ft (4 m), a diameter of about 39 in (1 m), and a weight of about 1,000 lb (450 kg). A ramjet engine of this design is capable of producing a thrust of 9,000 Newtons (N) (about 2,000 lb), giving a maximum velocity of about Mach 4 at higher altitudes.
Turbojets
One might guess that one way to improve the efficiency of a jet engine would be to increase the speed at which exhaust gases are expelled from the engine. In fact, that turns out not to be the case. Aeronautical engineers have discovered that a larger mass of gas moving at a lower velocity produces greater thrust in the engine. The modification that was developed to produce this effect is called a turbojet. The major difference between a turbojet and a ramjet is that the former 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. In the first place, 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 piston-powered engine, which has power strokes and exhaust strokes, the turbojet engine described here operates continuously. It is not subject, therefore, to the kind of vibrations experienced with a piston-powered propeller airplane.
Turbofan jets
A jet engine can be made more efficient by the addition of a large fan surrounded by a metal cowling at the front of the engine. The fan is somewhat similar to a propeller except that it has many more blades than a simple propeller. The fan is attached to a shaft that is also powered by the turbine at the rear of the engine. When exhaust gases from the compression chamber cause the turbine to spin, rotational energy is transmitted not only to the compressor, as described above, but also to the fan at the front of the engine.
The spinning fan draws more air into the engine, where some of it follows the pathway described above. Some of the air, however, bypasses the compressor and flows directly to the back of the engine. There, it joins with the exhaust gases from the combustion chamber to add to the engine’s total thrust.
The turbofan jet engine has the advantage of operating more efficiently and more quietly than turbojet engines. However, they are heavier and more expensive than are turbojets. As a consequence, turbofan engines are usually found only on larger commercial and military aircraft (such as bombers), while turbojets are the preferred engine of choice on smaller planes, such as smaller commercial aircraft and military fighters.
Afterburners
Combustion within any type of air-breathing jet engine is quite inefficient. Of all the oxygen entering the front of the engine, no more than about one-fourth is actually used to burn fuel within the engine. To make the process more efficient, then, 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. In a typical jet engine of moderate size, an afterburner can increase the takeoff thrust from about 50,000 N (11,000 lb) to about 70,000 N (15,500 lb).
Turboprop engines
When jet engines were first introduced in the 1940s, they were not very efficient. In fact, the cost of
KEY TERMS
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.
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.
Turbofan engine —A type of air-breathing jet that contains a large fan at the front of the engine operated by the turbine at the rear of the engine.
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.
operating a jet airplane was so great that only military uses could be justified. At the time, commercial airline companies decided to compromise between the well-tested piston engines they were then using and the more powerful, but more expensive, jet engines. The result was the turboprop engine. 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 that are possible with simple piston-driven propeller planes. The problem is that at high rotational speeds, propellers begin to develop such serious eddying problems that they actually begin to slow the plane down. Thus, the maximum efficient speed at which turboprop airplanes can operate is less than 450 mph (724 km/h).
Resources
BOOKS
Cumpsty, Nicholas A. Jet Propulsion: A Simple Guide to the Aerodynamic and Thermodynamic Design and Performance of Jet Engines. Cambridge: Cambridge University Press, 2000.
Flack, Ronald D. Fundamentals of Jet Propulsion with Applications. New York: Cambridge University Press, 2005.
Meyers, Robert A., ed. “Jet Propulsion.” In Encyclopedia of Physical Science and Technology San Diego, CA: Academic Press, 2002.
OTHER
Bucknell University. “The History of the Jet Engine.” <http://www.facstaff.bucknell.edu/mvigeant/therm_1/je2/history.htm> (accessed October 14, 2006).
Glenn Research Center, National Aeronautics and Space Administration. “Gas Turbine Propulsion.” March 16, 2006. <http://www.grc.nasa.gov/WWW/K-12/airplane/turbine.html> (accessed October 14, 2006).
HowStuffWorks Inc. “How Gas Turbine Engines Work.” <http://www.howstuffworks.com/turbine.htm> (accessed October 14, 2006).
David E. Newton
Jet Engine
Jet engine
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. Two general types of jet engines exist: the air-breathing jet engine and the rocket. In an air-breathing 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. In a rocket, air is not needed for propulsion. Instead, some type of chemical, nuclear, or electrical reaction takes place within the rocket engine. Hot gases formed as a result of that reaction exit the engine from the rear, providing the engine with its thrust, or forward momentum . Some authorities reserve the term jet engine for the first of these two types, the air-breathing jet engine. Air-breathing jet engines are used for the fastest commercial and military aircraft now available.
Scientific principle
The scientific principle on which the jet engine operates was first stated in scientific terms by Sir Isaac Newton in 1687. According to Newton's third law, for every action, there is an equal and opposite reaction. That principle can be illustrated by the behavior of a balloon filled with air. As long as the neck of the balloon is tied, gases (air) within the balloon push against all sides of the balloon equally, and no motion occurs. If the neck of the balloon is untied, however, 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 outward in one direction, the balloon moves outward in the opposite direction.
Rockets
Rockets can be broadly classified into one of two categories: those that use a chemical reaction as their energy source, and those that use some other kind of energy source. An example of the former are rockets that are powered by the chemical reaction between liquid oxygen and liquid hydrogen . When these two chemicals react with each other, they produce very hot steam (water vapor). The escape of steam from the back of the rocket provides the propulsive force that drives the jet engine forward.
Chemical rockets make use of either liquid fuels, such as the rocket described above, or of solid fuels. An example of the latter are the solid rockets used to lift a space shuttle into orbit . These rockets contain a fuel that consists of a mixture of aluminum metal (the fuel), ammonium perchlorate (the oxidizer), and a plastic resin (the binder).
Nuclear and electric rockets are examples of jet engines that make use of a nonchemical source of propulsion. In a nuclear rocket, for example, a source of nuclear energy, such as a fission or fusion reactor, is used to heat a working fluid, such as liquid hydrogen. The hot gases formed in this process are then released from the rear of the rocket, providing its forward thrust.
Various kinds of electrical rockets have been designed. In one type, a fluid within the engine is first ionized. The ions thus formed are then attracted and/or repelled by strong electrostatic fields created within the engine. The escape of the ionized fluid provides the rocket with its forward thrust.
Ramjets
The simplest of all jet engines is the ramjet. The ramjet consists of a long cylindrical tube made of metal, open at both ends. The tube bulges in the middle and tapers off at both ends. This shape causes air entering the front of the engine to expand and develop a higher pressure in the center of the engine. Within the engine, the compressed air is 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. These exiting gases can be compared to the air escaping from a rubber balloon. As the gases leave the back of the jet engine (the nozzle exit), they propel the engine itself in a forward direction.
When the ramjet engine is at rest, no air enters the front of the engine, and the engine provides no thrust. Once the engine is moving through the air, however, it begins to operate more efficiently. For this reason, the use of ramjet engines is usually reserved for aircraft that travel at very high speeds.
A typical ramjet engine today has a length of about 13 ft (4 m), a diameter of about 39 in (1 m), and a weight of about 1,000 lb (450 kg). A ramjet engine of this design is capable of producing a thrust of 9,000 Newtons (N) (about 2,000 lb), giving a maximum velocity of about Mach 4 at higher altitudes.
Turbojets
One might guess that one way to improve the efficiency of a jet engine would be to increase the speed at which exhaust gases are expelled from the engine. In fact, that turns out not to be the case. Aeronautical engineers have discovered that a larger mass of gas moving at a lower velocity produces greater thrust in the engine. The modification that was developed to produce this effect is called a turbojet. The major difference between a turbojet and a ramjet is that the former 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. In the first place, 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 piston-powered engine, which has power strokes and exhaust strokes, the turbojet engine described here operates continuously. It is not subject, therefore, to the kind of vibrations experienced with a piston-powered propeller airplane.
Turbofan jets
A jet engine can be made more efficient by the addition of a large fan surrounded by a metal cowling at the front of the engine. The fan is somewhat similar to a propeller except that it has many more blades than a simple propeller. The fan is attached to a shaft that is also powered by the turbine at the rear of the engine. When exhaust gases from the compression chamber cause the turbine to spin, rotational energy is transmitted not only to the compressor, as described above, but also to the fan at the front of the engine.
The spinning fan draws more air into the engine, where some of it follows the pathway described above. Some of the air, however, bypasses the compressor and flows directly to the back of the engine. There, it joins with the exhaust gases from the combustion chamber to add to the engine's total thrust.
The turbofan jet engine has the advantage of operating more efficiently and more quietly than turbojet engines. However, they are heavier and more expensive than are turbojets. As a consequence, turbofan engines are usually found only on larger commercial and military aircraft (such as bombers), while turbojets are the preferred engine of choice on smaller planes, such as smaller commercial aircraft and military fighters.
Afterburners
Combustion within any type of air-breathing jet engine is quite inefficient. Of all the oxygen entering the front of the engine, no more than about a quarter is actually used to burn fuel within the engine. To make the process more efficient, then, 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. In a typical jet engine of moderate size, an afterburner can increase the takeoff thrust from about 50,000 N (11,000 lb) to about 70,000 N (15,500 lb).
Turboprop engines
When jet engines were first introduced in the 1940s, they were not very efficient. In fact, the cost of operating a jet airplane was so great that only military uses could be justified. At the time, commercial airline companies decided to compromise between the well-tested piston engines they were then using and the more powerful, but more expensive, jet engines. The result was the turboprop engine. 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 that are possible with simple piston-driven propeller planes. The problem is that at high rotational speeds, propellers begin to develop such serious eddying problems that they actually begin to slow the plane down. Thus, the maximum efficient speed at which turboprop airplanes can operate is less than 450 MPH (724 km/h).
Resources
books
Boyne, Walter, Terry Gwynn-Jones, and Valerie Moolman. HowThings Work: Flight. Alexandria, VA: Time-Life Books, 1990.
Cumpsty, Nicholas A. Jet Propulsion: A Simple Guide to theAerodynamic and Thermodynamic Design and Performance of Jet Engines. Cambridge: Cambridge University Press, 1998.
Gunston, Bill. The Development of Jet and Turbine Aero Engines. 2nd ed. New York: Haynes Publishing, 1998.
"Jet Engine." In The Rand McNally Encyclopedia of Transportation. Chicago: Rand McNally, 1976.
"Jet Engines." In How It Works. New York: Simon and Schuster, 1971.
Karagozian, A. R. "Jet Propulsion." In Encyclopedia of Physical Science and Technology, edited by Robert A. Meyers. Orlando Academic Press, 1987.
Shaw, John M. "Jet Engines." In Magill's Survey of Science:Applied Science Series, edited by Frank N. McGill. Pasadena, CA: Salem Press, 1993.
David E. Newton
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- 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.
- 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.
- Turbofan engine
—A type of air-breathing jet that contains a large fan at the front of the engine operated by the turbine at the rear of the engine.
- 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.
Jet Engine
Jet engine
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.
Scientific principle behind jet engines
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.
Words to Know
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.
Types of jet engines
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.
jet engine
jet engine
jet en·gine • n. an engine using jet propulsion for forward thrust, mainly used for aircraft.