Jet Engine Glossary

Afterburner (reheat): A second burner in the jet pipe behind the turbine. It sprays fuel into the hot exhaust and burns it, while the nozzle opens wider, for a large boost in thrust at a very high fuel cost.
Bleed air: Compressed air taken from the compressor for cabin pressure, anti-icing or cooling. Every kilogram bled off is air that does not make thrust.
Buzz: The violent, repeating in-and-out movement of the shock system at a supersonic inlet after an unstart. It shakes the inlet and can break it.
Bypass ratio: In a turbofan, the mass of air that flows around the core for each kilogram that flows through it. Fighter engines are below 1; airliner engines are about 5 to 12. A turbojet has a bypass ratio of 0.
Choked nozzle: A nozzle in which the gas reaches the speed of sound at its narrowest point. Past that point the flow through it cannot increase any further for the same upstream conditions, and leftover pressure at the exit adds pressure thrust.
Combustor: The chamber between compressor and turbine where fuel burns in the compressed air at nearly constant pressure. Most of its air is used for cooling and mixing rather than burning.
Compressor: Rows of spinning and fixed blades that raise the pressure of the incoming air before it reaches the combustor. Driven by the turbine on the same shaft.
Compressor surge (stall): A breakdown of the flow through the compressor when it is asked for more pressure rise than it can give, for example during a fast acceleration or a hot start. The flow can reverse with a loud bang and a flash of flame from the inlet.
Dyno (thrust stand): A frame that holds the engine while a load cell measures its thrust. The game's dyno runs the same test on every engine: sea level, standard day, FADEC on, with fuel use read at 70 % power.
EGT (exhaust gas temperature): The temperature of the gas leaving the turbine. It shows how hot the engine is running: it peaks during a start, rises with power, and has a red line that protects the turbine.
FADEC: Full Authority Digital Engine Control: the computer that turns the throttle lever into fuel flow while keeping speed and temperature inside the limits, and that manages starts. In the game you can switch it off and fly the fuel by hand.
Flameout: The flame in the combustor going out while the engine is running, for example from too little fuel, too much fuel, or (in a ramjet) too little airspeed. The engine then has to be relit.
Hot start: A start in which the exhaust temperature climbs too high, usually because fuel arrived before the compressor was moving enough air. It can damage the turbine within seconds.
Hung start: A start in which the engine lights but stops accelerating below idle, while the temperature keeps climbing.
Idle: The lowest stable running speed with the throttle closed. The engine keeps itself turning but makes little thrust.
Isolator: In a scramjet, a short duct between the inlet and the combustor that holds back the pressure rise from combustion so it cannot reach the inlet and unstart it.
Light-off: The moment the fuel ignites during a start, seen as a quick jump in exhaust gas temperature.
Mach number: Speed divided by the local speed of sound. Mach 1 is the speed of sound; ramjets work from roughly Mach 2 to 5, scramjets above about Mach 5.
N1 (rotor speed): The speed of the low-pressure spool as a percentage of its rated speed. In a single-spool engine, like the game's, it is simply the rotor speed. Two- and three-spool engines also show N2 (and N3) for the faster inner spools.
Nozzle: The exit duct that turns the remaining pressure and heat of the gas into speed. Afterburning engines have a variable nozzle that opens when the afterburner is lit.
Pressure ratio: Compressor exit pressure divided by inlet pressure. A higher pressure ratio generally means a more efficient engine: about 5 to 10 in early jets, 40 to 50 or more in modern airliner engines.
Propeller pitch: The angle of a propeller's blades. Fine pitch takes small bites for take-off, coarse pitch big bites for cruise, and reverse pitch pushes air forwards to brake.
Propulsive efficiency: The share of the jet's kinetic energy that actually pushes the aircraft, 2·V₀ / (V₀ + V_jet). It is highest when the jet is only a little faster than the aircraft, which is why fans and propellers save fuel.
Ram drag: The momentum the incoming air already has in flight, which is subtracted from the engine's gross thrust. It is zero on a test stand, which is why engines show more thrust on the ground.
Ramjet: A jet engine with no compressor or turbine: the inlet compresses the air using the aircraft's own speed, so it only works once it is already flying fast.
Scramjet: A supersonic-combustion ramjet. The air stays supersonic all the way through the engine and the fuel, usually hydrogen, has about a millisecond to burn.
Spool: A compressor and the turbine that drives it, joined by one shaft. Many real engines have two or three spools turning at different speeds; the game's turbine engines have one.
Spool-up: How long the engine takes to accelerate after the throttle is opened. The rotor's inertia and the risk of surge stop it from responding instantly. The game's dyno times it from idle to 95 % of full thrust.
Thrust: The force the engine produces, measured in newtons (N), kilonewtons (kN) or pounds-force (lbf). 1 kN is about 225 lbf.
Thrust-to-weight ratio: Thrust divided by the engine's own weight. Turbine engines are typically between about 3 and 10; ramjets can be far higher because they have no rotating parts.
TSFC (thrust specific fuel consumption): Fuel burned per unit of thrust per unit of time, for example mg/(N·s) or lb/(lbf·h). Lower is better. 1 lb/(lbf·h) equals about 28.3 mg/(N·s).
Turbine: Blades behind the combustor that take energy from the hot gas to drive the compressor (and in a turbofan or turboprop, the fan or propeller). It is the hottest and most stressed part of the engine.
Turbine inlet temperature: The temperature of the gas entering the turbine, the hottest point in the cycle. Higher means more power from each kilogram of air, but it is limited by blade materials and cooling. In the game, the hot-section material sets the limit.
Unstart: In a supersonic inlet, the shock system being pushed out in front of the inlet, often by too much fuel or too little airspeed. Airflow and pressure collapse, thrust drops, and the inlet may buzz.