7.2 Turbofan, Turbojet, and Turboprop Engines

Jet engines come in various types, each with unique characteristics. Turbojets generate thrust through high-velocity exhaust, turbofans use a large fan to accelerate air, and turboprops employ a propeller driven by a gas turbine. These designs cater to different flight needs.

Engine selection depends on factors like speed, altitude, and efficiency requirements. Turbofans, with their bypass ratios, offer a balance of thrust and fuel efficiency. Turbojets excel in high-speed applications, while turboprops are efficient at lower speeds and altitudes.

Turbofan, Turbojet, and Turboprop Engines

Types of jet engines

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• Turbojet engines compress air, burn fuel in a combustion chamber, and extract energy through a turbine to generate high-velocity exhaust gases for thrust
• Turbofan engines use a large fan to accelerate a high volume of air, splitting it into bypass and core streams, generating thrust from both streams (bypass provides most thrust in high-bypass designs)
• Turboprop engines employ a gas turbine to drive a propeller, which generates most of the thrust, with exhaust gases providing a small additional thrust component

Bypass ratio in turbofans

• Bypass ratio quantifies the ratio of bypass air mass flow rate to core air mass flow rate
• Higher bypass ratios improve propulsive efficiency by accelerating a larger mass of air to a lower velocity, reducing specific fuel consumption and noise levels (ideal for subsonic airliners like Boeing 747)
• Lower bypass ratios offer higher specific thrust, beneficial for high-speed applications, but with reduced fuel efficiency (used in fighter jets like F-16)

Engine selection factors

• Aircraft speed and altitude requirements dictate engine choice (turbojets for high-speed, high-altitude; turboprops for lower speeds and altitudes)
• Fuel efficiency and range considerations favor high-bypass turbofans for long-range aircraft (Airbus A350), while low-bypass and turbojets suit high-speed, shorter-range applications
• Noise restrictions and environmental concerns make high-bypass turbofans and turboprops attractive for noise-sensitive areas (airports near residential zones)
• Thrust requirements and aircraft size influence engine selection (larger aircraft need high-thrust engines like high-bypass turbofans; smaller aircraft can use turboprops or low-bypass turbofans)

Jet engine performance comparison

• Turbofan engines offer high thrust and fuel efficiency, especially with high-bypass designs, covering a wide range of applications from business jets (Cessna Citation) to large airliners (Boeing 787)
• Turbojet engines provide high specific thrust but lower fuel efficiency, suitable for high-speed applications like military fighters (Lockheed Martin F-22) and supersonic aircraft (Concorde)
• Turboprop engines excel in propulsive efficiency at lower speeds and altitudes, with lower fuel consumption than turbojets and low-bypass turbofans, commonly used in regional airliners (ATR 72), cargo planes (C-130 Hercules), and military transports, limited to subsonic speeds due to propeller efficiency constraints