The short version
A jet engine pulls air in, squeezes it, heats it by burning fuel, and lets it rush out of the back much faster than it came in. Pushing that air backwards pushes the engine forwards. Engineers number the places along the way, called stations, so they can talk about the pressure and temperature at each one.
1. Inlet (stations 0 to 2)
The inlet guides air smoothly to the compressor face. At low speed it simply has to avoid disturbing the flow; at supersonic speed it slows the air down through shock waves first, which is how a ramjet works with no compressor at all.
2. Compressor (stations 2 to 3)
Rows of spinning blades (rotors) and fixed blades (stators) each add a little pressure. The total rise is the pressure ratio: around 5 to 10 in early jets, 40 to 50 or more in modern airliner engines. A higher pressure ratio makes the engine more efficient (the Brayton cycle calculator shows how much), but each stage can only push so hard. Ask too much of it and the flow breaks down in a compressor surge, a bang and a burst of flame out of the front.
3. Combustor (stations 3 to 4)
Fuel is sprayed into the compressed air and burned at nearly constant pressure. The flame itself is far hotter than any metal could survive, so most of the air is used to cool the liner and dilute the gas before it reaches the turbine. The temperature leaving the combustor is the turbine inlet temperature, the hottest point in the engine.
4. Turbine (stations 4 to 5)
The hot gas spins the turbine, which is on the same shaft as the compressor and drives it. Only part of the gas's energy is needed for that; the rest stays in the gas to make thrust. Turbine blades sit in gas hotter than their own melting point, which is why their materials (nickel superalloys, single-crystal blades, ceramic composites) and internal cooling decide how hot, and so how powerful, an engine can be.
5. Nozzle (stations 5 to 9)
The nozzle turns the gas's remaining pressure and heat into speed. In a simple convergent nozzle the flow reaches the speed of sound at the exit (the nozzle is choked), and any pressure left over pushes on the nozzle as extra pressure thrust. An afterburner sits just before the nozzle and opens it wider when lit.
Where the thrust comes from
This is exactly how the game computes thrust every frame: the gas flow and velocity at the nozzle exit, plus pressure thrust, minus the momentum of the incoming air.
Starting the engine
A jet engine cannot start itself from rest, because the compressor must already be moving air before fuel can burn. A starter spins the shaft; at roughly a sixth of full speed the igniters fire and fuel is added; the flame lights (light-off) and the exhaust temperature jumps; then the turbine takes over and the engine accelerates to idle. The turbojet simulator walks through each step, and the game lets you try it by hand with the engine computer switched off.
Reading the gauges
- N1: rotor speed as a percentage of its rated speed. Thrust rises steeply with it near the top.
- EGT: exhaust gas temperature. It shows how hot the turbine is running; it peaks during a start and rises with power. The red line protects the turbine.
- Fuel flow: kilograms of fuel per second. Divide by thrust for the TSFC.
- Thrust: measured in kilonewtons by the load cell on the thrust stand.
Modern engines are run by a FADEC, a computer that turns the throttle lever into fuel flow while keeping speed and temperature within limits. The game's FADEC does the same, and you can switch it off.