The GATE Grind

GATE 2021 AE – Question 15

Propulsion · Basics of thermodynamics · 1 mark · Multiple choice

The ideal combustion process in a turboshaft engine is

  1. isentropic
  2. isobaric
  3. isochoric
  4. isothermal

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Show answer and explanation

Correct answer: (B) isobaric

Explanation

The gas turbine engine (turboshaft, turbojet, turbofan) is modelled by the **Brayton cycle**:

1. Compression in the compressor: isentropic.
2. Heat addition in the combustor: **at constant pressure** (isobaric).
3. Expansion in the turbine and nozzle: isentropic.
4. Heat rejection: at constant pressure.

In an ideal engine, the pressure losses in the combustor are neglected, so the combustion is **isobaric** (option B). Constant-volume (isochoric) heat addition belongs to the Otto cycle and the pulse-jet.