Jet engines Summary

  • Last updated on November 10, 2022

An internal combustion engine that converts the chemical energy of fuel into mechanical energy in the form of thrust by the high-speed exhaust gases leaving the engine nozzle.

Description

The jet engine consists of several components: a compressor, a combustion chamber, a turbine, and an exhaust system. At the front of the jet engine is the compressor, driven by a shaft connected to the turbine. The compressor takes in air from the atmosphere and compresses it to produce high-pressure air. The air then enters the combustion chamber, where jet fuel is injected in fine droplets. Combustion occurs with ignition, and the hot gases exit the combustion chamber and enter the turbine, downstream of the combustion chamber. The hot gases leave the turbine through the exhaust system, exiting at high speed from the jet engine nozzle and propelling forward both the jet engine and the aircraft attached to it. The principle behind this propulsion is described by Newton’s third law of motion, which states that for every action there is a reaction equal in magnitude and opposite in direction. Jet propulsion is the movement of a small mass of gas at a very high velocity, whereas in a propeller plane, the propeller moves a large mass of air at low velocity.

History

The first patent for the modern gas turbine was granted in 1930 in England to Sir Frank Whittle, whose design led to the W-l turbojet engine with a centrifugal compressor. Simultaneously yet independently, German engineer Hans P. von Ohain also obtained a patent for a turbojet engine less than five years after Whittle had received his patent. Von Ohain’s engine also had a centrifugal compressor, whereas another German design, by Ernst Heinkel, had an axial compressor. A plane with von Ohain’s He-S3b engine made its test flight on August 27, 1939. Two years later, on April 12, 1941, a plane with Whittle’s turbojet engine was tested.

By the 1940’s, German turbojet engine prototypes had adopted the axial compressor, whereas British models all used the centrifugal compressor. By 1943, the two main turbojet engines were Germany’s Junkers Jumo 004 and Britain’s Rolls-Royce Welland. In the United States, General Electric Company engineers modified the Whittle engine and produced an American version called the I engine. In October, 1942, the I engine had its first test flight in the Bell P-59A.

During World War II, scientists from both Allied and Axis countries worked feverishly to design and test the jet engine. By 1946, several countries had successfully developed turbojet engines. In the United States, General Electric built the I-16 and the I-40. In England, Rolls-Royce built the Welland I, the Derwent I, and the Nene. In Germany, Junkers manufactured the Jumo 004-4.

By the 1950’s, the turbojet had been applied to civilian aviation. Early passenger jets included the De Havilland Comet I, which first flew in 1952 but was withdrawn from service two years later because of fatal accidents. By 1954, the United States had successfully tested its Boeing 707 passenger jet, with regular flights commencing four years later. After adopting the jet engine, commercial aviation quickly developed into an international business, with most countries operating their own national airlines. International jet aircraft industries manufacture many types of planes: wide-body models that can carry hundreds of passengers; supersonic planes that can fly at Mach 2; aircraft that are capable of vertical takeoffs and landings (VTOL); and military jet aircraft that can take off and land on the deck of an aircraft carrier.

The gas-turbine engine that powers all jet aircraft is, however, basically the same engine that was designed by Sir Frank Whittle in 1930. It consists of a compressor, combustion chamber, turbine, and exhaust system. There are four major manufacturers of jet engines: Société Nationale de Construction de Moteurs Aeronautiques (SNECMA) in France, Rolls-Royce in the United Kingdom, and Pratt & Whitney and General Electric in the United States.

ComponentsCompressor

The purpose of the compressor is to increase the pressure of the gas. In the compressor, atmospheric air is pressurized to typically ten to forty times the inlet pressure, and consequently the temperature of the air rises to between 200 and 550 degrees Celsius. The ideal gas law states the proportionality of the pressure and temperature of gases. The two basic types of compressors are the centrifugal-flow compressor and the axial-flow compressor.

The centrifugal-flow compressor, preferred for smaller engines, is a simpler device that uses an impeller, or rotor, to accelerate the intake air and a diffuser to raise the pressure of the air. The axial-flow compressor is favored for most engine designs, because it is capable of increasing the overall pressure ratio. The axial-flow compressor uses rotors fitted to many differently sized discs to accelerate the intake air and stationary blades, known as stators, to diffuse the air until its pressure rises to the correct value.

The type of compressor used in an engine affects the engine’s exterior appearance: An engine with a centrifugal compressor usually has a larger front area than an engine with an axial compressor. An engine with an axial compressor is longer and has a smaller diameter than an engine with a centrifugal compressor.

Combustion Chamber

In the combustion chamber, jet fuel, typically kerosene, is injected in fine droplets to allow for fast evaporation and subsequent mixing with the hot, compressed air. The compressed air is used for combustion, which occurs with ignition; the hot pressurized gases then reach temperatures of 1,800 to 2,000 degrees Celsius. To protect the combustion chamber walls from these high temperatures, some of the intake air, routed from the compressor, is used to cool the combustion chamber walls.

The three types of combustion chambers are the multiple chamber, annular chamber, and can-annular chamber. The multiple chamber, with individual chambers, or flame tubes, arranged radially, is used on engines with centrifugal compressors and early axial-flow compressor engines. The annular chamber has one annular flame tube with an inner and outer casing. The can-annular chamber combines characteristics of the multiple chamber and the annular chamber and has several flame tubes in one casing.

Turbine

In an aircraft engine, the sole function of the turbine, which is downstream of the combustion chamber, is to power the compressor. Similar to the compressor, the turbine has several large discs, though typically not as many as the compressor, fitted with many blades. Gases at temperatures between 850 and 17,000 Celsius exit the combustion chamber and enter the turbine. The hot gases impact the turbine blades, causing the discs carrying them to rotate at high speeds, averaging 10,000 revolutions per minute. The discs are mounted on a shaft that is connected at the other end to the compressor discs. The turbine blades are usually made of nickel alloys, because these materials are both strong and able to withstand the high temperatures within the turbine. The blades are fitted with many small holes through which cool air is forced to prevent the blades from melting.

Exhaust System

The jet engine’s exhaust system is configured so as to maximize the thrust of the engine. The exhaust system consists of a nozzle and may also include a thrust reverser and an afterburner.

In a basic exhaust system, the hot gases leaving the turbine are discharged through a propelling nozzle at a velocity that provides thrust. In VTOL aircraft, the nozzle swivels vertically so the aircraft can move up and down.

The thrust reverser enables the aircraft to slow down and stop more quickly upon landing, allowing the aircraft to land on shorter runways without relying solely on braking devices. Thrust reversal quite simply reverses the direction of exhaust gases to decelerate the aircraft. The two main thrust reversal methods use either clamshell-type deflector doors or bucket-type deflector doors on a retractable ejector.

Afterburner

An afterburner is used in some aircraft, such as supersonic jets (SSTs), including the Concorde and military aircraft, that need to reach high speeds in a short time. Unburned oxygen from the jet engine’s exhaust system flows into an afterburner, where more fuel is injected into the hot gases to augment the thrust of the engine. The temperature of the exhaust gases increases, thereby increasing the gas velocity and the thrust of the engine. This additional thrust allows for acceleration to supersonic speeds or for faster takeoffs to accommodate combat situations or the shorter runways of aircraft carriers.

Types of Jet Engines

The basic types of jet engines are the turbojet, turbofan, turboprop, and turboshaft. Turbojet and turbofan engines are called reaction engines, because they derive their power from the reaction to the momentum of the exhaust gases. The turboprop and turboshaft engines, however, utilize the momentum of the exhaust gases to drive a power turbine that, in turn, drives either a propeller or an output shaft.

Turbojet

The turbojet was the first jet engine type to be invented and flown. In a turbojet, all of the intake air passes through the compressor and is burned in the combustion chamber. The hot gases pass through the turbine and are then expelled through the exhaust nozzle to provide the thrust required to propel the engine and the aircraft attached to it forward. Examples of the turbojet appear in both civilian and military aircraft, including the Olympus 593 in the Concorde SST.

Turbofan

By the end of the twentieth century, the turbofan had become the most popular choice for aircraft propulsion in both civilian and military aircraft. In a turbofan engine, a large fan is placed at the front of the compressor of the jet engine. The amount of intake air is increased up to ten times. Most of this cool intake air either bypasses the compressor, combustion chamber, and turbine and exits the fan nozzle separately, as in separate-flow turbofans, or gets mixed with the turbine exhaust and exits through a common nozzle, as in the mixed-flow turbofan.

Afterburners in turbofan engines are equipped with a mixer to mix the cooler bypass air with the hot exhaust gases, thus allowing an easier burning of the bypassed air. Turbofan engines are characterized by their bypass ratio, which is the mass flow rate, in pounds per second, of air going through the fan divided by the mass flow rate of air going through the compressor. Low-bypass engines have ratios of up to two; medium-bypass engines have ratios from two to four, and high-bypass engines have ratios from five to eight. Ultrahigh-bypass engines have bypass ratios from nine to fifteen or higher. The highest bypass ratios, although providing high propulsion efficiency, likewise involve large, heavy components.

The advantage of the turbofan is its greater thrust on the same amount of fuel, which results in more efficient propulsion, lower noise levels, and an improved fuel consumption. Turbofan jet engines power all modern commercial aircraft, such as the Boeing 747; business jets, such as the Gulfstream IV; and most military airplanes, such as the F-18. Future turbofans may combine various bypass features. For example, the variable-cycle engine (VCE) would have both high-bypass and low-bypass features. Such an engine would be designed for planes that travel at subsonic and supersonic speeds. The VCE would operate by a valve that would control the bypass stream, either increasing it for subsonic speeds or decreasing it for supersonic speeds.

Turboprop

A turboprop engine is a turbojet engine with an extra turbine, called a power turbine, that drives a propeller. In the turboprop engine, the jet exhaust has little or no thrust. Planes powered by turboprop engines typically fly at lower altitudes and reach speeds up to 400 miles per hour (640 kilometers per hour). An example of the turboprop engine is the Rolls-Royce DART in the British Aerospace 748 and the Fokker F-27.

Turboshaft

A turboshaft is a turboprop engine without the propeller. The power turbine is instead attached to a gearbox or to a shaft. One or more turboshaft engines are used on helicopters to power the rotors. The turboshaft engine has industrial applications, such as in power stations, and marine applications, such as in hovercrafts.

Jet Engine Pollution

Because it is an internal combustion engine whose exhaust gases flow directly into the environment, a jet engine is a serious source of air pollution. Because of its high level of noise, its also causes noise pollution.

Air Pollution

Air pollution results from the combustion process of the gas-turbine engine. Jet-engine emissions, including carbon dioxide, carbon monoxide, hydrocarbons, and nitrogen oxide gas, contribute to both the greenhouse effect and atmospheric ozone depletion. They also endanger the health of people especially near airports.

Some regard aircraft transportation as more polluting than any other type of transportation, including the automobile. Generally, older aircraft are greater polluters than newer aircraft. The turbofan and bypass turbofan engines in particular use less fuel and therefore pollute less. A new MD-90 is about 50 percent more economical than a DC-9 or a DC-10, because the newer plane uses less fuel. Nevertheless, studies show that per passenger, an airplane uses twice as much fuel per passenger than does a car with three passengers, when the car drives the distance a jet travels in one hour (770 kilometers).

Airplane fuel consumption could be improved by eliminating various classes of cabins in the aircraft. Business- and first-class cabins seat fewer passengers, thereby reducing the overall fuel efficiency of the aircraft. If a reduction in carbon dioxide aviation emissions is to be realized, older aircraft must be replaced with newer ones that have more fuel-efficient engines. The most environmentally friendly aircraft include the B-777 and B-767. Carbon monoxide is contained in the combustion exhaust fumes. Both carbon monoxide and hydrocarbon emissions occur at the highest rates when airplanes idle their engines on runways, where often twenty planes are lined up waiting for takeoff. Airplanes pollute hundreds of times more when idling than when flying.

Nitrogen dioxide emissions contribute to acid-rain formation. The emission of hydrocarbons, especially radical ones, contribute to ozone formation. In terms of these emissions, the new high-bypass turbofan jet engines pollute much less than older turbofan and turbojet engines. Sulfur dioxide emissions also contribute to acid-rain formation. Nitrogen oxides have a possible role in ozone depletion, and its reduction can only be effected by less air traffic in general.

Noise Pollution

Noise is measured on a logarithmic scale in decibels, a unit of audio power. The decibel range is from zero decibels to about 160 decibels. A normal conversation takes place at about 40 decibels, and a noise level of 90 decibels would make it impossible to hear a normal conversation. The noise from a nearby jet takeoff is about 110 decibels. The main source of jet-engine noise is the propulsion system and the resultant noises generated by both internal and external processes.

In early turbojet engines, the noise occurred behind the exhaust nozzles when the hot exhaust gases mixed with the cool atmospheric gas. The high-bypass turbofan engines alleviated this noise problem.

Nevertheless, noise issues continue with the fan noise and core noise in high-bypass turbofan engines. Fan noise can be either broadband, discrete tone, or multiple tone, depending on whether the tip speed of the fan rotor blades is subsonic or supersonic. Core noise includes the noise from the rotation of the compressor, the noise from the turbulence generated in the combustion chamber, and the noise from the turbine.

Aircraft noise is regulated by federal rules that become increasingly stringent with time. Aircraft are classified as either stage one, for very noisy, 1960’s-era jetliners; stage two, for moderately noisy, 1970’s-era jetliners; or stage three, for more quiet, modern aircraft. Beginning in the year 2000, only stage-three aircraft may operate in the United States and Europe. Supersonic commercial aircraft, such as the Concorde, operate under different regulations and are only allowed to take off and land at certain airports because of the noise they make during takeoff.

To reduce external noise, the exhaust stream velocity may be decreased by flying jets that have a turbofan engine with a bypass ratio of five or higher. Such engines reduce exhaust noise considerably. With lower levels of external noise, internal noises are more audible.

To reduce internal noise, the fan tip speed can be decreased, although this would result in the necessity of more compressor stages, therefore resulting in a heavier engine. More spacing between the rotor and stator would also lessen the noise, but the larger spaces would require a larger engine.

Bibliography
  • Bathie, William W. Fundamentals of Gas Turbines. New York: John Wiley & Sons, 1984. A thorough history of the gas turbine, with sections on thermodynamics, fluid mechanics, combustion, component matching, and environmental impacts and a detailed section on jet engine noise.
  • Cohen, H., G. F. C. Rogers, and H. I. H. Saravanamuttoo. Gas Turbine Theory. Essex, England: Addison-Wesley Longman, 1996. A detailed explanation of how the different components of a gas turbine work, with graphs and equations and prediction of performance of a gas turbine.
  • Kerrebrock, Jack L. Aircraft Engines and Gas Turbines. Cambridge, Mass.: MIT Press, 1992. An explanation of the thermodynamic cycles of the different types of jet engines, component design, component matching, and aircraft engine noise.
  • Rolls-Royce. The Jet Engine. 5th ed. Derby, England: Author, 1996. An excellent overview of the subject, with emphasis on the turbojet engine and featuring easy-to-understand charts and diagrams.

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