All turbine engines have a compressor whose job is to increase the pressure of the incoming air before it reaches the combustion section. Compressors fall into two broad categories - axial and centrifugal - both built from stages that pair a moving rotor with a stationary stator. An axial compressor lines up alternating stator and rotor blades from inlet to exit, while a centrifugal or annular compressor flings air outward through an impeller-diffuser pair. Some engines add a low-pressure compressor to receive intake air and then pass the flow to a high-pressure compressor for an even greater stage of compression, creating the twin-spool arrangement familiar in modern jets.
What is a compressor in an aircraft?
An aircraft compressor is a mechanical device that increases pressure by compressing air. And all turbine engines have a compressor, and high-compression jet engines use axial compressors. Axial compressors have advantages including greater pressure ratio and higher flow rate. The HP compressor is driven by the HP turbine, and compressor performance influences total engine performance.
A compressor in an aircraft is a machine used to compress gases to a higher pressure level. It increases the pressure of the gas by decreasing its volume, converting high-velocity air into slower, higher-pressure flow. In a twin-spool jet engine the high-pressure compressor (HPC) is the section that supplies the final stage of compression for the air ingested by the engine, while the low-pressure compressor receives the intake air first. Greater pressure ratio across the successive stages yields higher thrust and refines fuel efficiency.
What is the function of the compressor in an aircraft?
The compressor's primary function is to compress incoming air to higher pressure and temperature before it reaches the burners. By raising the pressure at which fuel is burned, the compressor increases the thermodynamic efficiency of the Brayton cycle, directly refining thrust while reducing fuel consumption. Compressors deliver air in the quantity and at the pressures required by the combustion burners, guaranteeing consistent combustion and reliable rotation of the power turbine. The compressor supplies bleed air for many systems throughout the engine and aircraft, aiding environmental, anti-ice, and hydraulic functions. Acting as the first step in the turbine's energy-production cycle, the compressor conditions air, making it suitable for ignition and assuring that exhaust exits only at the rear of the engine, thereby maximizing propulsive force.
What are the types of aircraft compressors?
The types of aircraft compressors are listed below.
- Axial compressors
- Centrifugal compressors
- Mixed flow compressors
Gas-turbine aircraft engines are generally served by two main compressor families: positive-displacement and dynamic. Within the dynamic branch the two principal devices are the axial compressor and the centrifugal compressor, while a third, less common, member is the mixed-flow compressor. Very small turbofans at the low-thrust end of the market sometimes add a mixed-flow stage, but most engines rely on axial and centrifugal elements alone. An axial compressor resembles an aircraft engine because its flow travels parallel to the axis of rotation, the air being squeezed by alternating rows of rotor and stator blades that are stacked into twenty or more stages. Axial stages are best for engines that demand a high overall pressure ratio, because they are made multi-staged, weigh less per unit pressure rise, swallow more airflow than a centrifugal design of the same diameter, and achieve peak efficiency at the high flight speeds typical of large turbojets, turbofans, turboprops and turboshafts. Each rotor blade exerts torque on the air, and successive variable stator vanes, or a twin-spool and occasionally triple-spool arrangement, maintain surge-free, high-efficiency compression.
The other widely used device is the centrifugal compressor. Easy to fix and maintain, it comes as a Plug-&-Play unit delivered from the factory, runs at lower speeds and is made either single-entry or double-entry. Whereas axial flow moves straight through the machine, centrifugal flow moves radially outward from the axis of rotation, accelerating in the impeller and collecting in the diffuser. Because a single radial stage seldom matches the pressure ratio of several axial stages, centrifugal compressors are generally reserved for small turbojets, turboshaft engines, auxiliary power units, ground-support equipment and process plants that accept a lower overall pressure ratio. They are combined with an axial front end, where they sit as the final compressor stage, or placed separately in engines whose size and simplicity matter more than ultimate efficiency and thrust-to-weight ratio. Choosing which compressor type is best therefore depends on pressure ratio, size, weight, speed and mission, but for the best combination of high compression and high efficiency, modern high-bypass turbofans continue to favour multi-staged axial compressors arranged in split-spool configurations.
What are the parts of a compressor in aircraft?
The parts of a compressor in aircraft are listed below.
- Impeller
- Diffuser
- Compressor manifold
- Ports
- Compressor case
- Stator vanes
- Fan rotor
- Interconnecting shafts
A turbine engine compressor includes the impeller, a rotor that gathers and accelerates incoming air outward through centrifugal force, converting it into kinetic energy. Following each impeller row, a diffuser (stator) is an integral part of the compressor manifold; this stationary row of stator blades converts that kinetic energy into pressure. The compressor manifold itself houses these stages, and in many cases the compressor case is horizontally divided into halves for maintenance access. The fan rotor, the first and very large rotor in the compressor, is clearly visible at the front of high-bypass turbofan engines. It provides the main thrust by directing part of the incoming air into the engine core and the rest into bypass ducts. Cold-section shafts transmit power from downstream turbine discs back to successive compressor blades, enabling the compressor to draw in air continuously.
How does an aircraft compressor work?
An aircraft compressor is engine-driven and starts working the moment air is drawn in through the inlet. The fan accelerates large volumes of this incoming air and, in a turbofan, directs part of it as bypass air for extra thrust and lower noise. The remaining air passes into the core compressor where the flow travels parallel to the axis of rotation. Rotor blades spin and accelerate the fluid, adding kinetic energy. The moving part called the rotor is linked to the turbine by a concentric shaft. After each rotor stage the flow meets the stator, the stationary part that diffuses fluid and slows the flow, turning velocity into pressure. Pressure increase occurs across both rotor and stator, and the pressure rise is multiplied row by row until the overall compressor pressure ratio reaches up to 40:1. Because air heats up rapidly as it is compressed, a heat exchanger cools the compressed air before it reaches the combustor. Throughout this process the compressor map shows the operating line, a safe corridor that keeps the unit a small distance below the surge line, the limit beyond which flow breaks down and becomes unstable.
Compressor stability is watched closely. Rotating stall is a local disruption of airflow within the compressor that continues to provide compressed air with reduced effectiveness, and when the disturbance grows, it triggers compressor surge, a violent reversal of flow that propagates through the entire compressor in less than a second and is accompanied by an increase in rotor speed. To avoid these events the engine design keeps the compressor operating on the operating line below the surge pressure ratio. The high-energy gases produced in the combustion chamber expand through the turbine. The turbine extracts energy from the hot gases and, via the shaft, drives the compressor, fan, and booster stages. This Brayton-cycle process repeats continuously, assuring that the compressor supplies high-pressure air to the combustor and that the engine generates sustained thrust.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
