The GATE Grind

GATE 2026 AE – Question 18

Propulsion · Aerothermodynamics of aircraft engines: thrust, efficiency, range and the Brayton cycle · 1 mark · Multiple choice

The figure below depicts two ideal gas turbine cycles, cycle 1-2-3-4-1 and cycle 1-2-3'-4'-1, on a $T$-$s$ diagram. Which one the following statements is FALSE?

A T-s diagram. The compression 1-2 is common. Point 3 lies on the constant pressure line from 2, and point 3' lies further along the same line at a higher temperature. Expansion 3-4 and 3'-4' are vertical lines ending on the same constant pressure line through 1 and 4.
  1. The thermal efficiency of the two cycles is the same
  2. The specific work of the two cycles is the same
  3. The processes 2-3 and 2-3' are isobaric
  4. The amount of heat added in the combustion process is greater for the cycle 1-2-3'-4'-1

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Correct answer: (B) The specific work of the two cycles is the same

Explanation

Both cycles have the same pressure ratio, so the ideal Brayton efficiency $1 - \frac{T_1}{T_2}$ is the same (A true), and 2-3 and 2-3' are on the same isobar (C true). Cycle 1-2-3'-4'-1 reaches a higher turbine inlet temperature, so it takes more heat in (D true) and also has a larger enclosed area, so its specific work is greater. The statement that the specific work is the same (B) is false.