Module 15 Gas Turbine Engine key terms

The ideal thermodynamic cycle for a gas turbine: air is compressed, heat is added at constant pressure, the gas expands through a turbine, and heat is rejected. Its efficiency rises with pressure ratio and turbine inlet temperature.

A standard scheme labeling engine cross-sections by number, such as inlet, compressor face, compressor exit, combustor exit, turbine exit and exhaust, so performance and temperatures can be referenced consistently.

The net force produced by an engine, equal to the mass flow times the velocity increase of the exhaust plus any pressure thrust from the nozzle, minus ram drag from incoming air.

The useful propulsive force remaining after subtracting inlet momentum drag from gross thrust. It is what actually accelerates the aircraft and is the basis for rating thrust values.

The momentum loss caused by slowing incoming air to engine speed, equal to inlet mass flow times flight velocity. It grows with airspeed and reduces net thrust at high speed.

Fuel burned per unit of thrust or power per unit time. Lower values mean better efficiency; it is used to compare engines and to track deterioration over the engine's life.

The mass flow of air passing around the core divided by the mass flow through the core. High bypass ratios improve propulsive efficiency and reduce noise, typical of modern turbofans.

Deliberately limiting an engine's certified thrust below its maximum capability at low ambient temperatures, so thrust stays constant over a temperature range and engine life is protected.

Operating an engine below its maximum rated thrust or temperature to reduce stress and extend component life. Takeoff, climb and cruise derates are common fuel-saving and maintenance practices.

A compressor in which air flows parallel to the shaft through alternating rotor and stator rows. Each stage raises pressure modestly; many stages give high pressure ratio with good efficiency.

A compressor that throws air outward from a rotating impeller into a diffuser, raising pressure in a single stage. It is compact and robust but has lower flow capacity than axial designs.

The ratio of compressor discharge pressure to inlet pressure. Higher ratios improve thermal efficiency but raise discharge temperature and demand more turbine work to drive the compressor.

A local breakdown of smooth airflow over compressor blades, causing reduced pressure rise, vibration and possible damage. It may affect one stage or spread into a full surge if not corrected.

A severe, often cyclic reversal of flow through the compressor when demand exceeds capability. It produces loud bangs, rapid EGT rise and thrust loss, and requires immediate corrective action.

Movable inlet guide vanes and stator vanes whose angle is scheduled with engine speed. They keep blade incidence correct across the operating range, widening the stall margin.

A valve that releases compressor air overboard or to another stage during acceleration and starting. It prevents stall and surge by matching airflow to what the downstream stages can accept.

A passage that slows air and converts velocity into static pressure. In the combustor inlet it prepares air for stable burning; in a centrifugal compressor it recovers pressure from the impeller exit flow.

The section where fuel is injected and burned with compressed air to raise gas temperature before the turbine. Designs include can, annular and can-annular arrangements.

A device that meters and atomizes fuel into the combustion zone so it mixes readily with air. Types include simplex, duplex and air-blast nozzles, chosen for flow range and atomization quality.

A single continuous ring-shaped combustion chamber surrounding the shaft. It gives uniform exit temperature, compact length and low pressure loss, and is the most common layout in modern engines.

Stationary vanes that turn and accelerate hot gas onto the turbine rotor blades at the correct angle. They are among the most highly cooled parts in the engine.

A rotating airfoil that extracts energy from hot gas to drive the compressor and accessories. Blades endure high centrifugal stress and temperature and often use internal cooling passages and thermal barrier coatings.

Using compressor bleed air through internal passages, film holes or impingement to keep blade metal below its limit. Cooling allows gas temperatures well above the metal melting point.

The small gap between a rotor blade tip and its surrounding casing or shroud. Excess clearance leaks gas and wastes efficiency, so active clearance control is used on some engines.

A system that sprays cooling air onto the turbine casing to shrink it and reduce tip clearance at cruise, then relaxes it for takeoff and transient conditions to avoid rubbing.

The gas temperature entering the turbine, a key limit on thrust and efficiency. Because it exceeds metal capability, it is managed with cooling and inferred from exhaust gas temperature.

The temperature measured at the turbine exit or exhaust, used as a proxy for turbine inlet temperature. It is a primary limit and health indicator, and rising EGT usually signals deterioration.

The difference between the observed exhaust gas temperature and the certified limit at a given power setting. Shrinking margin indicates wear or fouling and drives maintenance decisions.

A device that redirects fan or exhaust flow forward to produce braking force after landing. Common types are cascade, bucket-target and pivoting-door designs.

A thrust reverser using blocker doors to divert fan air through fixed cascade vanes that turn the flow forward. It is common on high-bypass turbofans and adds little weight.

A bearing using balls or rollers between inner and outer races to carry shaft radial and axial loads with low friction. It is the dominant type in turbine engine rotor support.

A thin oil film between a bearing outer race and its housing that absorbs vibration and reduces rotor dynamic loads. It improves stability and bearing life at high speeds.

A non-contacting seal with a series of knife-edge fins running close to a rotating surface. Each fin adds a pressure drop, limiting leakage between regions of different pressure.

The component that delivers a precisely controlled fuel flow to the nozzles in response to pilot demand and engine parameters. Modern units are commanded electronically by the FADEC.

Full Authority Digital Engine Control: a computer system that schedules fuel and geometry with no mechanical backup, providing precise control, protection limits and fault reporting.

Compressor air extracted for cabin pressurization, anti-ice, cooling and starting. Bleed extraction costs engine power, so it is scheduled and limited by the control system.

A system that heats engine inlet lips and other surfaces with hot bleed air to prevent ice buildup that would distort airflow and could shed into the engine.

A high-energy unit that converts low voltage into pulses for the igniter plugs, producing the spark that lights the fuel-air mixture during starting and as a relight safeguard.

The coordinated steps to bring an engine to self-sustaining speed: motoring by starter or air, ignition, fuel introduction, light-off, acceleration and stabilization at idle.

Measuring rotor vibration amplitude and frequency to detect imbalance, bearing wear or blade damage. Trends and signatures help isolate faults before they become failures.

Recording and plotting parameters such as EGT, fuel flow, speed and vibration over time to detect deterioration and plan maintenance before limits are reached.