Jet Engine Technology

Summary

Jet engines are machines that add energy to a fluid stream and generate thrust from the increase of momentum and pressure of the fluid. Jet engines, which usually include turbomachines to raise the pressure, vary greatly in size. Applications include microelectromechanical gas turbines for insect-sized devices; mid-sized engines for helicopters, ships, and cruise missiles; giant turbofans for airliners; and scramjets for hypersonic vehicles. Jet engine development pushes technology frontiers in materials, chemical kinetics, measurement techniques, and control systems. Born in the desperation of World War II, jet engines have come to power most modern aircraft.

Definition and Basic Principles

The term “jet engine” is typically used to denote an engine in which the working fluid is mostly atmospheric air, so that the only propellant carried on the vehicle is the fuel used to release heat. Typically, the mass of fuel used is only about 2 to 4 percent of the mass of air that is accelerated by the vehicle.

Background and History

British engineer Frank Whittle and German engineer Hans Pabst von Ohain independently invented the jet engine, earning patents in Britain in 1932 and in Germany in 1936, respectively. Whittle's engine used a centrifugal compressor and turbine. The gas came in near the axis, was flung out to the periphery, and returned to the axis through ducts. Ohain's engine used a combination of centrifugal and axial-flow turbomachines. The gas direction inside the engine was mostly aligned with the axis of the engine, but it underwent small changes as it passed through the compressor and turbine. The axial-flow engine had higher thrust per unit weight of the engine and smaller frontal area than the centrifugal machine. Initially, the axial-flow machine, which had a large number of stages and blades, was much more prone to failure than the centrifugal machine, which had sturdier blades and fewer moving parts. However, these problems were resolved, and most modern jet engines use purely axial flow.

Several early experiments with jet engines ended in explosions. Ohain's engine was successfully used to power a Heinkel He 178 aircraft on August 27, 1939. Ohain's engine led to the Junkers Jumo 004 axial turbojet engine, which was mass-produced to power the Messerschmitt Me 262 fighter aircraft in 1944. The British Gloster E.28/39, which contained one of Whittle's engines, flew in 1941. In September, 1942, a General Electric engine powered the Bell XP-59 Airacomet, the first American jet fighter aircraft.

The de Havilland Comet jet airliner service began in 1952, powered by the Rolls-Royce Avon. In 1958, a de Havilland Comet jet operated by British Overseas Airways Corporation flew from London to New York, initiating transatlantic passenger jet service. In the mid-1950's, the Rolls-Royce Conway became the first turbofan in airliner service when it was used for the Boeing 707. The Rolls-Royce Olympus afterburning turbofan powered the supersonic Concorde in 1969. The majority of aircraft have come to be powered by jet engines, mostly turbofans, the largest, as of 2010, being the General Electric GE90–115, which produces nearly 128,000 pounds (rated at 115,000 pounds, or 570,000 Newtons) of thrust.

From 1903 to the 1940s, the power per unit weight of engines increased from 0.05 to about 0.8 horsepower per pound (hp/lb), with the largest engines producing 4,000 hp. With jet engines, the power per unit weight has risen to more than 20 hp/lb, with the largest engines producing more than 100,000 hp. The compressor pressure ratio has risen from 3:1 for initial engines to more than 40:1 for modern engines.

How It Works

Jet engines operate by creating thrust through the Brayton thermodynamic cycle. In a process called isentropic compression, a mixture of working gases is compressed, with no losses in stagnation pressure. Heat is chemically released or externally added to the fluid, ideally at constant pressure. A turbine extracts work from the expanding gases. This work runs the compressor and other components such as a fan or propeller, depending on the engine type and application. The gas leaving the turbine expands further through a nozzle, exiting at a high speed.

Jet engine developers try to maximize the pressure and temperature that the engine can tolerate. The thermal efficiency of the Brayton cycle increases with the overall pressure ratio; therefore, designers try to get the highest possible pressure at the end of the compression. However, the temperature rise accompanying the pressure rise limits the amount of heat that can be added before the temperature limit of the engine is reached. Thrust is highest when the greatest net momentum increase is added to the flow. The propulsive efficiency of the engine is highest when the speed of the jet exhaust is close to the flight speed of the vehicle. These considerations drive jet engine design in different directions depending on the application. For very-high-speed applications (typically military engines), engine mass and frontal area must be kept low, so a smaller amount of air is ingested and accelerated through a large speed difference. For engines such as those used on airliners, a large amount of air is ingested using a large diameter intake and accelerated through a small speed difference for best propulsive efficiency. The major components of a jet engine are the inlet, diffuser, compressor, fan, propeller, combustor, turbine, afterburner, nozzles, and gearbox.

Inlet and Diffuser. The engine inlet is designed to capture the required airflow without causing flow separation. An aircraft flying at supersonic speeds has a supersonic inlet in which a series of shocks slows down the flow to subsonic speeds with minimal losses in pressure. Once the flow is subsonic, it is slowed further to the Mach number of about 0.4 needed at the face of the compressor or fan. A supersonic inlet and diffuser may lose 5 to 10 percent of the stagnation pressure of the incoming flow when used with aircraft flying in excess of Mach 2.

Compressor, Fan, and Propeller. In 1908, René Lorin patented a jet engine in which a reciprocating piston engine compressed the fluid. In 1913, he patented a supersonic ramjet engine in which enough compression would occur simply by slowing the flow down to subsonic speeds. In 1921, Maxime Guillaume patented a jet engine with a rotating axial-flow compressor and turbine. Most modern engines use the turbomachine in some form. The compressor is built in several stages, with the pressure ratio of each stage limited to prevent flow separation and stall. A centrifugal compressor stage consists of a rotor that imparts a strong radial velocity to the flow, and the flow is flung out at the periphery of the blades with added kinetic energy. The flow is then turned and brought back near the axis by a diffuser stage in which the static pressure rises and the flow speed decreases. In an axial compressor, work is added to the flow in several stages, as many as fifteen in some engines. In each stage, a spinning rotor wheel with many blades that act as lifting wings, imparts a swirl velocity to the flow. This added energy is then converted to a pressure rise in stator blade passages, bringing the flow back to being axial, but with increased pressure. Some newer compressors have counter-rotating wheels in each stage instead of a rotor and a stator. Shock-in-rotor stages use blades moving at supersonic speed relative to the flow to create large pressure rise because of shocks. Supersonic through-flow compressor designs are being developed for future high-speed engines.

The fans of most engines are extensions of the first, or low-pressure, compressor stages. Fans have only one or two stages, and fewer, larger blades than the compressor stages.

Propellers are used in turboprop engines to produce a portion of the engine's thrust.

Combustor. The combustor is designed to mix and react the fuel with the air rapidly and to contain the reaction zone within an envelope of cooling air. At the exit of the combustor, these flows are mixed to ensure the most uniform temperature distribution across the gases entering the turbine. Older combustors were either several cans connected by tubes, arranged around the turbomachine shaft, or an annular passage. Some modern combustors are arranged in a reverse-flow geometry to enable better mixing and reaction. Ideally, a jet engine combustor must add all the heat that can be released from the fuel, at a constant pressure, with minimal pressure losses because of flow separation and turbulence. Heat addition must also be done at the lowest flow Mach number possible.

Turbine. The critical limiting temperature in a jet engine is the temperature at the inlet to the first turbine stage. This is usually tied to the strength of the blade material at high temperatures. A turbine has only a few stages, typically four or fewer. The highest mass flow rate through the engine is limited to the flow rate at which the passages at the final turbine stage are choked, in other words, when the Mach number reaches 1 at these passages. Turbine stage disks and blades are integrated into blisks and can be made as a single piece, with cooling passages inside the blades, using powder metallurgy.

The turbine is directly attached to the compressor through a shaft. To enable starting the compressor and better matching the requirements of the different stages, a twin-spool or three-spool design is used, where the outer (low-pressure, lower rotation speed) stages of the compressor, fan, and turbine are connected through an inner shaft, and the inner (high-pressure) stages are connected through a concentric, outer shaft.

Afterburner. Older military engines use an afterburner (also known as reheat) duct attached downstream of the turbine, where more fuel is added and burned, with temperatures possibly exceeding the turbine inlet temperature. Afterburners are highly inefficient but produce a large increment of thrust for short durations. Therefore, afterburners are used at takeoff, in supersonic dashes, or in combat situations.

Nozzles. For jet engines without afterburners, the exit velocity is at most sonic and the nozzle is just a converging duct. If the exhaust is expanded to supersonic speeds, the nozzle has a convergent-divergent contour. Thrust-vectoring nozzles either rotate the whole nozzle, as in the case of the Harrier or the F-22, or use paddles in the exhaust, as in the case of the Sukhoi-30.

Gearbox. When the engine must drive a rotor, propeller, or counter-rotating fan, a gearbox is used to reduce the speed or change the direction of rotation. This usually adds considerable weight to the engine.

Applications and Products

Ramjet and Scramjet.Ramjets are used to power vehicles at speeds from about Mach 0.8 to 4. The diffuser slows the flow down to subsonic speeds, increasing the pressure so much that thrust can be generated without a mechanical compressor or turbine. Beyond Mach 4, the pressure loss in slowing down the flow to below Mach 1 is greater than the loss due to adding heat to a supersonic flow. In addition, if such a flow were decelerated to subsonic conditions, the pressure and temperature rise would be too high, either exceeding engine strength or leaving too little room for heat addition. In this regime, the supersonic combustion ramjet, or scramjet, becomes a better solution.

Turbojet. The turbojet is the purest jet engine, with a compressor and turbine added to the components of the ramjet. The turbojet can start from rest, which the pure ramjet cannot. However, since the turbojet converts all its net work into the kinetic energy of the jet exhaust, the exhaust speed is high. High propulsive efficiency requires a high flight speed, making the turbojet most suitable near Mach 2 to 3. Because jet noise scales as the fifth or sixth power of jet speed, the turbojet engine was unable to meet the noise regulations near airports in the 1970's and was rapidly superseded by the turbofan for airliner applications.

Turbofan. The turbine of the turbofan engine extracts more work than that required to run the compressor. The remaining work is used to drive a fan, which accelerates a large volume of air, albeit through a small pressure ratio. The air that goes through the fan may exit the engine through a separate fan nozzle or mix with the core exhaust that goes through the turbine before exiting. Because the overall exhaust speed of the turbofan is much lower than that of the turbojet, the propulsive efficiency is high in the transonic speed range where airliner flight is most efficient, yet airport noise levels are far lower than with a turbojet. Turbofan engines are used for most civilian airliner applications and even for fighter and business jet engines.

Turboprop. In the turboprop engine, a separate power turbine extracts work to run a propeller instead of a fan. The propeller typically has a larger diameter than a fan for an engine of comparable thrust. However, the rotating speed of a propeller, constrained by the Mach number at the tip, is only on the order of 3,000 to 5,000 revolutions per minute, as opposed to turbomachine speeds, which may be three to ten times higher. Therefore, a gearbox is required.

Turboshaft. Instead of a propeller, a helicopter rotor or other device may be driven by the power turbine. Automobile turbochargers, turbopumps for rocket propellants, and gas turbine electric power generators are all turboshaft engines.

Propfan. Propfans are turbofans in which the fan has no cowling, so that it resembles a propeller and has a larger capture area, but the blades are highly swept and wider than propeller blades.

Air Liquefaction. The high pressures encountered in high-speed flight make it possible to liquefy some of the captured and compressed air at lower altitudes, using heat transfer to cryogenic fuels such as hydrogen. The oxygen from this liquid can be separated out and stored for use as the vehicle reaches the edge of the atmosphere and beyond. Turboramjet engines using this technology could enable routine travel to and from space, with fully reusable, single-stage vehicles.

Careers and Course Work

Jet engine development and manufacturing are parts of a highly specialized industry. Those interested in jet engine technology must have a very good basic understanding of thermodynamics and dynamics and can specialize in combustion, turbomachine aerodynamics, gas dynamics, or materials engineering. Airlines operate engine test cells and employ many technical workers to diagnose and repair problems and ensure proper maintenance and operating procedures. The National Aeronautics and Space Administration (NASA) and the U.S. Department of Defense offer many opportunities in all aspects of jet engine technology. Engine developers include the very large corporations that supply airliner engines and the smaller companies that develop engines for business jets, cruise missiles, and other applications. Companies doing research, development, and manufacture of jet engines are in the United States, Europe, Russia, Japan, and China. Many other nations also produce jet engines under license.

Social Context and Future Prospects

Jet engines have developed rapidly since the 1940s and have come to dominate the propulsion market for atmospheric flight vehicles. Because the air that makes up more than 95 percent of the working fluid is free and does not have to be carried up from the ground, air-breathing propulsion offers a huge increase in specific impulse over rocket engines for flight in the atmosphere. As technology advances to enable rotating machinery to tolerate higher temperatures, pressures, and stresses, jet engines can become substantially lighter and more efficient per unit of thrust. Hydrogen-fueled engines can operate much more efficiently than hydrocarbon-fueled engines. Turbo-ramjet engines may one day enable swift and inexpensive access to space. Helicopter engines and the lift fans developed for vertical-landing fighter planes bring personal air vehicles closer to reality. Supersonic airline travel using hydrogen fuel is much closer to becoming routine. At the other extreme, micro jet engines are finding use in devices to power actuators for many applications, including surgical tools and devices to control stall on wings and larger engines. A major area of jet engine research in the twenty-first century is the search for a quieter, more fuel-efficient method of jet propulsion; many companies are looking into electric-powered turbofans rather than the traditional gas-powered ones, for example.

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