An aircraft engine produces thrust by pushing air (or exhaust gas) backwards: the force on the aircraft is the rate at which the engine increases the momentum of the flow passing through it. This page explains the main families of engines, how a modern jet engine is built, and what the architecture fields of our database mean. Examples link to the database.
For a jet engine, thrust is the mass flow of air times the increase of its velocity, plus a pressure term when the nozzle does not expand the gas down to ambient pressure:
F = ṁ · (Vjet − Vflight) + (pexit − pambient) · Aexit
The same thrust can be obtained by accelerating a little air a lot (a turbojet) or a lot of air a little (a turbofan or a propeller). The second is far more efficient, because the kinetic energy left in the jet is wasted: the propulsive efficiency is approximately ηp = 2 / (1 + Vjet / Vflight). That is why airliner engines have grown huge fans with high bypass ratios, and why propellers remain the best choice at low speed.
The thermal efficiency (how much of the fuel energy becomes kinetic energy of the flow) depends mostly on the overall
pressure ratio and on the turbine inlet temperature. For an ideal cycle it is ηth = 1 − OPR−(γ−1)/γ,
so pressure ratios have risen from about 5 on the first jet engines to about 60 on the
GE9X. The product of both efficiencies is the overall efficiency, which sets the fuel burn
(the database stores it as SpecificFuelConsumption and, for the emissions certification, as FuelFlowTakeOff and the other fuel flows).
Every turbine engine (turbojet, turbofan, turboprop, turboshaft) runs the same Brayton cycle in a continuous flow:
A spool is a set of compressors and turbines on the same shaft. Splitting the machine into two or three spools lets each part turn at its best speed: the large fan must turn slowly (its blade tips must stay near the speed of sound) while the small high-pressure compressor turns fast. In the database, the number of spools follows from which turbines have stages: high-pressure and low-pressure turbines make a two-spool engine; an intermediate-pressure turbine makes it three-spool.
The cross-section below follows a CFM LEAP-1A as recorded in the database: one fan stage with 18 blades, a 3-stage booster, a 10-stage high-pressure compressor, a lean-burn combustor, a 2-stage high-pressure turbine and a 7-stage low-pressure turbine. It powers the Airbus A320neo. The upper half is mirrored below the axis.
About eleven times more air goes around the core, through the bypass duct, than through it (the bypass ratio). The fan alone produces most of the thrust; the core is the gas generator that drives it. The LP shaft (blue) links the fan and the booster to the low-pressure turbine; it runs inside the hollow HP shaft (red), which links the high-pressure compressor to the high-pressure turbine.
Engineers number the planes along the flow path (SAE AS755). The temperature fields of the database follow this numbering:
| Station | Position | Database fields |
|---|---|---|
| 0 | Free stream, far ahead of the engine | |
| 2 | Fan face (engine inlet) | |
| 13 | Fan exit, bypass side (18–19: bypass nozzle) | FanPressureRatio = P13 / P2 |
| 25 | High-pressure compressor inlet (booster or IP compressor exit) | |
| 3 | High-pressure compressor exit, combustor inlet | T3TakeOff, T3MaxContinuous… ; OverallPressureRatio = P3 / P2 |
| 4 | Combustor exit, high-pressure turbine inlet (the hottest point) | T4TakeOff, T4MaxContinuous… |
| 45 | High-pressure turbine exit, next turbine inlet (often where the exhaust gas temperature is measured) | T45TakeOff, T45MaxContinuous… |
| 5 | Low-pressure turbine exit | T5TakeOff… |
| 9 | Core nozzle exit | T9TakeOff… |
These fields of the jet engines describe the whole architecture, module by module from front to back, and its main cycle parameters.
| Field | Meaning | Examples |
|---|---|---|
FanStages | Number of fan rotor stages. One on airliner turbofans; two or three on military low-bypass fans. | 1 on the LEAP-1A; 3 on the F119 |
FanBlades | Number of blades of the fan (first stage). Fewer, wider blades are lighter and more efficient. | 36 on the CFM56-5B, 18 on the LEAP-1A, 16 on the GE9X |
FanTechnology | How the fan blades are made: solid or hollow titanium, carbon composite (woven, resin transfer moulding), metallic leading edge, blisk. | Solid titanium (CFM56), hollow titanium (Trent), 3D-woven carbon (LEAP, GE9X) |
GearedFan | Yes when a reduction gearbox sits between the low-pressure shaft and the fan. | Yes on the PW1100G |
Gear | Type of that gearbox: star epicyclic (fixed planet carrier, the fan turns the other way than the shaft) or planetary (the carrier drives the fan). | Star on the PW1100G; planetary on the Rolls-Royce UltraFan |
ReductionRatio | Low-pressure shaft speed divided by the fan speed. | About 3.06 on the PW1100G |
LPCompressorStages, LPCompressorTechnology | Low-pressure compressor (the "booster" of two-spool turbofans), on the same shaft as the fan. | 3 stages on the LEAP-1A, 4 on the CFM56-5B |
IPCompressorStages, IPCompressorTechnology | Intermediate-pressure compressor of three-spool engines, on its own shaft. Empty on two-spool engines. | 8 stages on the Trent XWB |
HPCompressorStages, HPCompressorTechnology | High-pressure compressor. Technology: axial or centrifugal stages, blisks, variable stator vanes… | 10 on the LEAP-1A, 6 on the Trent XWB, 8 on the PW1100G |
CombustionChamber | Type of combustor: annular, can-annular or cans; lean-burn (TAPS), rich-quench-lean (TALON), double annular. | TAPS II on the LEAP-1A |
HPTurbineStages, HPTurbineTechnology | High-pressure turbine, driving the HP compressor. Technology: cooled single-crystal blades, thermal barrier coatings, ceramic matrix composites. | 2 on the LEAP-1A, 1 on the CFM56 and the Trent XWB |
IPTurbineStages, IPTurbineTechnology | Intermediate-pressure turbine of three-spool engines, driving the IP compressor. | 2 on the Trent XWB |
LPTurbineStages, LPTurbineTechnology | Low-pressure turbine, driving the fan (and booster). Geared fans need fewer, faster stages. Technology: titanium aluminide blades, counter-rotation… | 7 on the LEAP-1A, 3 on the PW1100G |
Afterburner | Yes when fuel can be burnt again behind the turbine (reheat) for extra thrust. | M88, Olympus 593 |
Streams | Number of separate flows: 1 for a turbojet, 2 for a turbofan, 3 for adaptive-cycle engines with a third, variable bypass stream. | 2 on the LEAP-1A |
BypassRatio | Mass flow going around the core divided by the mass flow through the core. | 0 for a turbojet, about 0.3 for a fighter engine, 5–6 for the CFM56, 11 for the LEAP-1A, 12 for the PW1100G |
OverallPressureRatio | Pressure at the high-pressure compressor exit divided by the pressure at the fan face (P3 / P2). | About 33 for the LEAP-1A26, about 60 for the GE9X |
FanPressureRatio | Pressure ratio of the fan on the bypass side (P13 / P2). Low values (1.3–1.5) go with high bypass ratios. | |
BPCompressorPressureRatio | Pressure ratio of the low-pressure compressor ("BP": basse pression, the same as LP). | |
IPCompressorPressureRatio, HPCompressorPressureRatio | Pressure ratios of the intermediate- and high-pressure compressors. The product of the fan (core side), LP, IP and HP ratios gives the overall pressure ratio. |
All the air goes through the core and leaves as a fast, hot jet. Simple and compact, but noisy and thirsty below supersonic speed, so it has disappeared from new airliners. Some have an afterburner in the jet pipe.
Examples: Junkers Jumo 004, General Electric J79, Rolls-Royce/Snecma Olympus 593 of the Concorde.
The engine of combat aircraft: a multi-stage fan sends a small part of the air around the core (bypass ratio below 1). Both flows mix before the afterburner, and a variable nozzle follows.
Examples: Snecma M88, Eurojet EJ200, Pratt & Whitney F119, Pratt & Whitney F135.
See the cross-section above. A large single-stage fan and a booster on the LP shaft, the high-pressure core on the HP shaft. The usual architecture of airliner engines from General Electric and CFM International.
Examples: CFM56-5B (A320), CFM56-7B, LEAP-1A (A320neo), GE90-115B (777-300ER), GE9X (777-9), GP7270 (A380).
The fan has its own LP turbine, and an intermediate-pressure spool sits between it and the HP spool. Each spool turns closer to its best speed, with fewer stages and a shorter, stiffer engine. The Rolls-Royce architecture.
Examples: RB211-524G, Trent 900 (A380), Trent XWB (A350-900).
A reduction gearbox lets the fan turn slowly while the LP spool turns fast, so the LP compressor and turbine need fewer stages and the fan can be larger (higher bypass ratio).
The fan has no casing: its blades can be much larger, giving a very high bypass ratio and a lower fuel burn, at the cost of noise and installation constraints. Contra-rotating rotors, or a rotor followed by fixed vanes, recover the swirl.
Examples: Progress D-27, CFM International RISE.
The turbine drives a propeller through a reduction gearbox; the exhaust gives only a little thrust. Most efficient below about 650 km/h. Many have a free power turbine, on its own shaft, separate from the gas generator.
The same machine as a turboprop, but all the power goes to an output shaft: helicopter rotors, auxiliary power units, ships and generators.
No rotating parts: at high speed the inlet itself compresses the air. A ramjet gives no thrust at rest and works from about Mach 2 to Mach 5 (the air is slowed to subsonic speed before combustion); a scramjet burns in a supersonic flow for higher speeds.
Examples: Bristol Thor, Marquardt RJ43.
Combustion is intermittent: flap valves let air in, the mixture burns, the pressure closes the valves and pushes the gas out of the tail pipe, and the cycle repeats tens of times per second. Very simple, very noisy.
Example: Argus As 014 of the V-1.
Carries both its fuel and its oxidiser, so it works without air and in space. Pumps feed the propellants to the combustion chamber; the gas expands through a convergent-divergent nozzle to very high velocity.
Examples: Walter HWK 109-509, Rocketdyne F-1, RS-25.
Like a car engine: an intermittent cycle in cylinders turns a crankshaft that drives the propeller. Still the engine of most light aircraft.
Examples: Lycoming O-360, Rotax 912.
A battery (or a fuel cell, or a generator in a hybrid system) feeds the motor through an inverter. Simple and quiet, but batteries store far less energy per kilogram than fuel, which limits range.
Examples: Siemens SP260D, Emrax 268.