Turbojet, turbofan and turboprop comparison

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.

 Turbofan:   fan->[ bypass duct ]----------->
                 ->[C][Comb][T]----------->
 Turboprop:  prop<-gear<-[C][Comb][T]------> small jet

Comparison

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.

 eta_p
   |    turboprop
   |    .--.        turbofan
   |   /    \      .--.         turbojet
   |  /      \    /    \       .----
   | /        \  /      \    .'
   |/          \/        \ .'
   +------------------------------- flight Mach no.
        0.5         0.8        1.5+