Gas Turbine, the Running Engine

Air enters at the intake, which acts as a diffuser: it slows the air and raises its pressure before the compressor. The fan and compressor then raise the pressure further, and because compressing a gas heats it, the temperature climbs steadily through the compressor even though no fuel has yet been burned. Pressure peaks at the compressor delivery, which is the highest-pressure point in the engine.

In the combustion chamber fuel is burned at essentially constant pressure, so temperature peaks at the turbine inlet while pressure falls very slightly. The turbine then extracts work from the hot gas to drive the compressor and fan, so both pressure and temperature drop across it, while velocity rises. Whatever energy remains is converted to velocity in the propelling nozzle, and it is that momentum change that produces thrust.

A two-spool engine has an N1 shaft carrying the fan and low-pressure turbine, and an independent N2 shaft carrying the high-pressure compressor and turbine, so each can run at its own optimum speed. Raising the bypass ratio moves thrust production from a small fast core jet to a large slow fan flow, which is more propulsively efficient and much quieter. Turbine inlet temperature is the limiting design parameter: raising it improves performance but is capped by what the turbine materials and their cooling can survive.

• Try this: Watch the graph as you raise the pressure ratio. At which station does pressure peak, and at which does temperature peak, and why are they not the same place?
• Try this: Increase the bypass ratio at constant thrust lever. What happens to the core jet velocity, and why does that make the engine more efficient?
• Try this: Raise the turbine inlet temperature toward the limit. What does the EGT do, and what physically constrains how far you can go?