This article describes the components and systems found in jet engines. It uses two example engines; the type most familiar to the general public, the modern airliner engine, and the military afterburning engine. The components and systems make up what is known as a bare engine.Systems of Commercial Turbofan Engines, An Introduction to Systems Functions, Andreas Linke-Diesinger, , Section 10-2
The article also has a section on inlets. Although the inlet is not part of the engine, the engine relies on it to help prevent compressor surging (by reducing inlet distortion), and to give a pressure boost to the engine which reduces its fuel consumption (by converting the relative speed of the approaching air into pressure).
The article also mentions the nacelle because the outside of an airliner engine has to be streamlined so that as little of its thrust as possible is lost to the drag of the air flowing past the engine.
Overview of components in a modern airliner engine
Engines for airliners are enclosed in a streamlined pod called a nacelle which hangs under the wing or, on smaller aircraft, on the side of the aircraft behind the wing.
The most fundamental part of the engine is the gas generator ( also known as the core) because every gas turbine engine needs one (it has 3 parts, compressor, combustion chamber and turbine).JET PROPULSION A Simple Guide to the Aerodynamic and Thermodynamic Design and Performance of Jet Engines, Nicholas Cumpsty 2003,, Fig.4.1 Early jet engines (turbojets) were just a gas generator until additional parts were added to reduce fuel consumption. In front was added a fan and, at the back, another turbine, both connected together by a shaft going through the middle of the gas generator, (and known as a fanjet, bypass engine, or turbofan).
For an airline passenger the gas generator is out-of-sight in the middle of the engine and all that can be seen of the engine itself are the fan at the front and the turbine for the fan at the back inside the core nozzle.
Major components
The major components of an airliner engine are shown on the 'Schematic of a high bypass turbofan'. The engine is identified as 2-spool. Spool is a name given to a complete rotor consisting of compressor, turbine and connecting shaft. Intake- A short length of ducting in front of the fan is not part of the engine ( it comes with the nacelle) but is required by the fan to make sure the air approaching it is reasonably smooth (not too much distortion). A rounded lip at the entry to the intake ensures this. Fan- The first part of the engine is the fan. It is a compressor and raises the pressure of the air which flows into the engine by a small amount (about 1/2 an atmosphere). Most of the air passing through the fan leaves the engine at the fan nozzle. A smaller amount goes into the LP compressor and then the gas generator ( which consists of HP compressor, combustion chamber and HP turbine).
Fan nozzle- Is the area required to back-pressure the fan to make it run properly as a compressor.JET PROPULSION A Simple Guide to the Aerodynamic and Thermodynamic Design and Performance of Jet Engines, Nicholas Cumpsty 2003,, p.120 At the same time the nozzle accelerates the fan airflow as it leaves the bypass duct.JET PROPULSION A Simple Guide to the Aerodynamic and Thermodynamic Design and Performance of Jet Engines, Nicholas Cumpsty 2003,, xiiThe Aerothermodynamics Of Aircraft Gas Turbine Engines, Oates, Air Force Aero Propulsion Laboratory, Wright-Patterson AFB, https://apps.dtic.mil/sti/citations/ADA059784, page 26-19 LP compressor- The air that passes through the fan close to the fan hub (with a small pressure rise) passes through the LP compressor with a further rise in pressure. This air is known as the core air and gives a pressure boost to the gas generator. Gas generator- In between the fan and its turbine is a gas generator where the fuel is burned and its energy transferred to the air passing through the combustion chamber. The gas generator consists of an HP compressor, a combustion chamber (fuel is burned continuously after initially being ignited during the engine start) and an HP turbine. The compressor is needed because fuel has to be burned in high pressure air to get the most energy out of itOn the thermodynamic spectrum of airbreathing propulsion, Carl Builder, Published Online:16 Aug 2012https://doi.org/10.2514/6.1964-243, p.2 (the LP booster has already helped this). The turbine is needed to drive the compressor. LP turbine- This is the turbine which drives the fan and LP compressor. It can be seen looking in the back of the engine. Core nozzle- Is the area required to back-pressure the gas generator compressor to make it run properly as a compressor.JET PROPULSION A Simple Guide to the Aerodynamic and Thermodynamic Design and Performance of Jet Engines, Nicholas Cumpsty 2003,, p.164 It also accelerates the core flow as it leaves the engine.The Aerothermodynamics Of Aircraft Gas Turbine Engines, Oates, Air Force Aero Propulsion Laboratory, Wright-Patterson AFB, https://apps.dtic.mil/sti/citations/ADA059784, page 26-19 Nacelle- The streamlined pod around the engine. It also incorporates a fan-air thrust reverser which has blocker doors and a set of turning vanes to direct the air forwards to help slow the aircraft on landing.
Two significant extra components are also used on modern airliner engines compared to the 2-spool arrangement shown above. Fan Drive Gear System- The FDGS is a planetary reduction gearbox. The power from the LP turbine goes through the gears and the fan turns at only 1/3 the turbine speed, when used on the Pratt & Whitney PW1000G. This type of engine is known as a geared turbofan. The 2-spool engine as described above is known as a direct drive turbofan. Intermediate Pressure spool- A third spool, known as the IP spool, is used on the Rolls-Royce Trent.
Overview of components in a modern supersonic engine
Most of the same components are needed but some differ noticeably in appearance, as does the bare engine itself. The fan is much smaller and has more than one set of blades, for example. There is no thrust reverser but there is an afterburner. One or two engines are installed in the body of the aircraft behind the cockpit.
Intake- Compared to the airliner intake, the one on a combat aircraft is a curved passage because the engine located behind the cockpit is offset from the entrance to the inlet. The relatively slow-moving air on the aircraft skin (boundary layer) has to be prevented from entering the intake. The air approaching the intake at supersonic speed has to be slowed to subsonic speed at the entrance, keeping as much of its energy as possible as a rise in pressure for the engine. Fan- The fan is much smaller because less air goes past the core than through it (For the airliner engine more than eight times as much bypasses the core). When much less bypass air is needed the engine gives its best performance if the bypass air is compressed more, so 3 stages are needed (only one on an airliner engine).
Afterburner- The cold fan air and the hot LP turbine exhaust come together in the afterburner. When the afterburner is selected fuel is sprayed in and ignited with igniters (the turbine exhaust is not hot enough to do this). The flame is held in position in the flowing gas using flame holders which can be seen by looking into the afterburner. Nozzle- The nozzle has an adjustable area which is increased when fuel is being burned in the afterburner (more fuel, bigger area). Increasing the area in proportion with the burning keeps the engine operating the same as when the afterburner is not lit (because the airflow through the engine hasn't changed).
How the components and systems work Compressors
The job of the compressor is to raise the pressure of ambient air as high as possible with the level of technology existing at the time (the higher the pressure the greater the proportion of energy in the fuel that is transferred to the air in the combustor, which means less wasted fuel).On the thermodynamic spectrum of airbreathing propulsion, Carl Builder, Published Online:16 Aug 2012https://doi.org/10.2514/6.1964-243, p.2 For the illustrated J79 engine it was 12 atmospheres (General Electric compressor knowledge in 1952).
The air is squeezed by very fast moving blades in a close-fitting tube. The exit from the tube is smaller than the entry as the squeezed air (higher density/smaller volume) takes up less space. The high pressure is attained at high speed and the small exit area is only correct for the high-speed density (small volume). At low engine speeds the slower moving blades don't compress the air so much and the exit area is too small to pass the low-speed (bigger) volume. This affects the angle the blades meet their approaching air (see following explanation).
The speed of the air moving through the compressor and the speed of the blades combine to give the blade angle of attack (explained using velocity trianglesIdeas and Methods of Turbomachinery Aerodynamics: A Historical View, Cumpsty et al., p.16, https://arc.aiaa.org/doi/abs/10.2514/1.9176?journalCode=jpp) and if too much, similar to an aircraft wing, the compressor air flow stalls. This condition happens at low compressor speeds and stops the compressor getting to its high-speed/works-well condition. Three additions to a compressor enable it to run without blade stall at low speeds (see following).
The J79 illustration shows the use of variable angle blades in the first half of the compressor. The J57 illustration shows a (combined 12 atmosphere) compressor split into two separately-spinning compressors, both of which work well because, individually, they only compress their own bit of airflow 3 or 4 times (3x4=12). The Avon illustration shows where air escapes at low engine speeds (three bleed valves).
These examples are all early jet engines. More recent engines use all three solutions together, as illustrated here on a model of the V2500 airliner engine
Sources:Powering the World's Airliners: Engine Developments from the Propeller to the Jet Age, Decher,, pp.143-146Flow Matching Of The Stages Of Axial Compressors, Wilde, Technical Series No.4,, pp.53 and 67
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Combustors
Why is a combustor required? The volume of air determines the amount of work which has to be put into compressing or which can be got out during expansion. So more work can be obtained after heating compressed air (due to the increased volume of the working fluid) by expanding it (in a turbine) than was needed to compress it. The combustor heats the air and with the turbine driving the compressor there is a surplus available to form a propelling jet (or drive an additional turbine).Gas Turbines And Their Problems, Hayne Constant 1948, Todd Reference Library, Todd Publishing Group Ltd., London W.1, p.11
The air leaving the compressor has to be slowed for combustion Slowing the air reduces the pressure loss in the combustor (less pressure loss, lower fuel consumption). Also, less pressure loss means combustion takes place at a higher pressure which increases combustion efficiency (lower fuel consumption).On the thermodynamic spectrum of airbreathing propulsion, Carl Builder, Published Online:16 Aug 2012https://doi.org/10.2514/6.1964-243, p.2 The air leaving the compressor is initially slowed when it enters a bigger annulus (see accompanying image of Adour engine).