--- title: "Turbojet, turbofan and turboprop comparison" subject: "Turbo Machines" chapter: 6 key: "semester_7/turbo_machines:turbojet-turbofan-turboprop-comparison" source: https://www.bctengineer.com/q/semester_7/turbo_machines/turbojet-turbofan-turboprop-comparison site: BCT Engineering --- # Turbojet, turbofan and turboprop comparison Turbo Machines · Chapter 6: Gas Turbine Cycles of Aircraft Propulsion - Asked in: year not given, 6 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/turbo_machines/turbojet-turbofan-turboprop-comparison - Whole chapter: https://www.bctengineer.com/md/semester_7/turbo_machines/chapter-6 ## Question Differentiate between turbojet, turbofan and turboprop engines. Define bypass ratio and explain how the propulsive efficiency of each type varies with flight speed. State the typical field of use of each. ## Short answer A turbojet accelerates all air through the core with high jet velocity; a turbofan has bypass ratio beta and moderate jet velocity; a turboprop uses a propeller for most thrust. Propulsive efficiency is highest for turboprop at low speed, good for turbofan near Mach 0.8, and improves for turbojet at supersonic speeds. Typical uses: military/supersonic, airliners, and regional/cargo aircraft respectively. ## Answer #### Engines - **Turbojet:** all the air passes through the core; thrust comes only from the high-velocity jet. - **Turbofan:** a large fan, driven by the turbine, accelerates extra air that bypasses the core. **Bypass ratio** $\beta = \dot m_{bypass}/\dot m_{core}$ (0.3 to 1 for military engines, 5 to 12 for modern airliners). - **Turboprop:** the turbine drives a propeller through a reduction gear; most of the thrust (about 90%) comes from the propeller and only a small amount from the exhaust jet. ```text Turbofan: fan->[ bypass duct ]-----------> ->[C][Comb][T]-----------> Turboprop: prop<-gear<-[C][Comb][T]------> small jet ``` #### Comparison | Point | Turbojet | Turbofan | Turboprop | |---|---|---|---| | Mass flow accelerated | Small | Large | Very large (propeller) | | Jet velocity | Very high | Moderate | Low | | Propulsive efficiency | Low at subsonic speed | Good at high subsonic speed | Highest at low speed | | Best speed | $Ma$ 1.5 to 3 | $Ma$ 0.7 to 0.9 | $Ma$ below 0.6 | | Fuel consumption (TSFC) | High | Low | Lowest | | Noise | Very high | Moderate | Propeller noise | | Frontal area, weight | Small | Larger | Larger (propeller, gear) | | Use | Military fighters, supersonic | Airliners, transport | Regional and cargo aircraft | #### Variation with flight speed $$ \eta_p = \frac{2V}{C_j + V} $$ - A turboprop accelerates a large mass of air by a small amount; $\eta_p$ is about 80 to 85% at low speed but drops above $Ma \approx 0.6$ because of propeller tip shock losses. - A turbofan has an intermediate jet velocity, giving a good $\eta_p$ at $Ma$ 0.8. - A turbojet has a very high jet velocity compared with the flight speed, so $\eta_p$ is low at subsonic speed but improves as the flight speed approaches the jet velocity in supersonic flight. ```text eta_p | turboprop | .--. turbofan | / \ .--. turbojet | / \ / \ .---- | / \ / \ .' |/ \/ \ .' +------------------------------- flight Mach no. 0.5 0.8 1.5+ ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Bypass ratio is defined as A. core flow / bypass flow B. bypass mass flow / core mass flow C. fan power / turbine power D. jet velocity / flight velocity Answer: B. beta = m_bypass/m_core; about 5 to 12 for modern airliners. (Key point: Bypass ratio) 2. In a turboprop, most thrust comes from A. the exhaust jet B. the afterburner C. the propeller driven through a reduction gear D. ram effect Answer: C. About 90% of thrust is from the propeller; the jet is small. (Key point: Turboprop thrust source) 3. Why is a turbojet's propulsive efficiency low at subsonic speed? A. Its jet velocity is very high compared with flight speed B. Its mass flow is large C. It has a propeller D. It has a bypass duct Answer: A. eta_p = 2V/(C_j+V) is low when C_j is much greater than V; it improves as V approaches C_j at supersonic flight. (Key point: Turbojet propulsive efficiency) 4. Why does turboprop efficiency drop above about Ma 0.6? A. The jet velocity becomes too low B. The gearbox fails C. Fuel flow stops D. Propeller tip shock losses Answer: D. Turboprop eta_p is 80-85% at low speed but propeller tips reach shock conditions at higher flight Mach number. (Key point: Turboprop speed limit) 5. Match engine to best use A. Turbofan - cargo aircraft at Ma 0.3 B. Turboprop - supersonic fighters C. Turbofan - airliners at Ma 0.7 to 0.9; turbojet - supersonic military D. Turbojet - regional aircraft Answer: C. Turbofans are best at high subsonic speeds, turbojets at Ma 1.5 to 3, turboprops below Ma 0.6. 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