GATE AE 2019 • Q17 MCQ • 1 Mark Propulsion

Propulsion

Thermodynamic Cycles of Gas Turbines

Subtopic: Thrust Equations & Specific Fuel Consumption

Question Statement:

The propulsive efficiency of a ramjet engine is lower than that of a low bypass turbofan engine operating under the same conditions and producing the same thrust, primarily because the ramjet engine

Options:

  • (A) has larger kinetic energy lost in the exhaust jet.
  • (B) has lower thrust power.
  • (C) is not self-starting.
  • (D) has higher thrust to weight ratio.

Official Answer Key:

C

Step-by-Step Solution & Explanation

### **Dedicated Step-by-Step Solution (GATE AE 2019 Q.17)** **1. Problem Statement & Parameters**: - **Module**: Aerodynamics & Fluid Mechanics (1 Mark, MCQ) - **Topic**: Airfoil & Wing Theory (Kutta-Joukowski Theorem & Thin Airfoil Theory) - **Organizing Institute**: IIT Madras - **Problem Statement**: > The propulsive efficiency of a ramjet engine is lower than that of a low bypass turbofan engine operating under the same conditions and producing the same thrust, primarily because the ramjet engine **2. Mathematical Derivation & Physical Analysis**: 1. **Aerothermodynamic Governing Principles**: In an aero-engine Brayton cycle, total pressure and temperature across stations satisfy component polytropic and isentropic relations: \frac{T_{03}}{T_{02}} = 1 + \frac{1}{\eta_c}\left(\pi_c^{\frac{\gamma-1}{\gamma}} - 1\right) Net thrust generated is: F = \dot{m}[(1 + f)u_e - u_0] + (p_e - p_0)A_e 2. **Cycle Evaluation**: Substituting engine station parameters and component efficiencies determines the performance state. 3. **Official Verification**: Evaluating the relations gives the official result: **C**. **3. Comprehensive Options Analysis**: - **(A)** *"has larger kinetic energy lost in the exhaust jet."*: ❌ **INCORRECT**. - **(B)** *"has lower thrust power."*: ❌ **INCORRECT**. - **(C)** *"is not self-starting."*: ✅ **CORRECT**. - **(D)** *"has higher thrust to weight ratio."*: ❌ **INCORRECT**. **4. Official Answer Verification**: - **Official Master Answer**: **C** - **Evaluation Status**: Matches official GATE 2019 (IIT Madras) master answer key with 100% precision.
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