Starting a jet engine is a carefully controlled sequence that turns a stationary turbofan into a stable source of thrust. The goal is to spin the engine core fast enough for compression, then add fuel and ignition at the right moment. This matters because a mistimed start can overheat the engine, fail to light, or damage expensive components.
Pilots and computers monitor rotor speed, fuel flow, and exhaust gas temperature throughout the start.
Key Facts
- Starter torque spins the core before fuel is introduced.
- Compressor pressure ratio = compressor exit pressure / compressor inlet pressure.
- Light-off occurs when fuel ignites and exhaust gas temperature begins rising.
- Power = torque x angular speed, or P = tau omega.
- EGT must stay below the engine start limit to prevent hot-section damage.
- At stable idle, compressor speed, fuel flow, and turbine power are balanced.
Vocabulary
- Turbofan
- A jet engine that uses a large fan plus a gas turbine core to produce thrust efficiently.
- Starter
- A device that turns the engine core during start until the turbine can keep the engine running.
- Compressor
- A rotating section that raises the pressure of incoming air before it enters the combustor.
- Light-off
- The moment when the fuel-air mixture ignites in the combustor and the engine begins producing its own hot gas flow.
- Exhaust Gas Temperature
- The temperature of gases leaving the turbine, commonly monitored to protect the engine during start and operation.
Common Mistakes to Avoid
- Adding fuel before enough core speed is reached is wrong because the compressor may not supply enough airflow for safe combustion.
- Thinking the ignition alone spins the engine is wrong because the starter provides the initial rotation while igniters only create sparks.
- Ignoring a rapid EGT rise is wrong because excessive temperature during start can damage turbine blades and combustor parts.
- Assuming idle means no thrust is produced is wrong because a running jet engine still accelerates air and produces some thrust at idle.
Practice Questions
- 1 A starter brings the engine core from 0 rpm to 3600 rpm in 20 s. What is the average angular acceleration in rpm/s?
- 2 During start, EGT rises from 120 degrees C to 540 degrees C in 14 s. What is the average temperature rise rate in degrees C per second?
- 3 Explain why the start sequence requires airflow through the compressor before fuel is sprayed into the combustor.