Components of jet engines


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.
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, and to give a pressure boost to the engine which reduces its fuel consumption.
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 because every gas turbine engine needs one. Early jet engines 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,.
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 but is required by the fan to make sure the air approaching it is reasonably smooth. 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. 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.

  • Fan nozzle- Is the area required to back-pressure the fan to make it run properly as a compressor. At the same time the nozzle accelerates the fan airflow as it leaves the bypass duct.
  • LP compressor- The air that passes through the fan close to the fan hub 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 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 it. 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. It also accelerates the core flow as it leaves the engine.
  • 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 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. 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.
  • 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 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. Increasing the area in proportion with the burning keeps the engine operating the same as when the afterburner is not lit.

    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. For the illustrated J79 engine it was 12 atmospheres.
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 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. 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 volume. This affects the angle the blades meet the their approaching air.
The speed of the air moving through the compressor and the speed of the blades combine to give the blade angle of attack 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.
The J79 illustration shows the use of variable angle blades in the first half of the compressor. The J57 illustration shows a 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. The Avon illustration shows where air escapes at low engine speeds.
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
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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 by expanding it 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.

The air leaving the compressor has to be slowed for combustion

Slowing the air reduces the pressure loss in the combustor. Also, less pressure loss means combustion takes place at a higher pressure which increases combustion efficiency. The air leaving the compressor is initially slowed when it enters a bigger annulus.
Air going into the combustor with the fuel spray is still going too fast for a flame to exist. The flame front doesn't move fast enough to remain inside the combustor against the incoming air flow. A sheltered combustion zone with a reverse flow region has to be established. This enables the flame to remain in a stable location near the fuel nozzles and is a continuous ignition source for incoming air and fuel.

Cooling the combustor

After the air required for combustion has entered the front of the combustor further air enters through many holes in the wall of the combustor to provide wall-cooling with a film of cooler air to insulate the metal surfaces which have a protective thermal barrier.

Cooling the combustion gasses for the turbine

Since the turbine cannot withstand the stoichiometric temperatures in the combustion zone, the compressor air remaining after supplying the primary zone and wall-cooling film, and known as dilution air, is used to reduce the gas temperature at entry to the turbine to an acceptable level.

Turbines

Because a turbine expands from high to low pressure, there is no such thing as turbine surge or stall. The turbine needs fewer stages than the compressor. The blades have more curvature and the gas stream velocities are higher.
Designers must, however, prevent the turbine blades and vanes from melting in a very high temperature and stress environment. Consequently, bleed air extracted from the compression system is often used to cool the turbine blades/vanes internally. Other solutions are improved materials and/or special insulating coatings. The discs must be specially shaped to withstand the high stresses imposed by the rotating blades. Improved materials help to keep disc weight down.