Turbofan Engine

What Is a Turbofan Engine?

A turbofan engine is the dominant propulsion system in modern commercial aviation. Unlike a pure turbojet, a turbofan routes a large volume of air around the engine core via a massive front-mounted fan, generating the majority of thrust without burning additional fuel. This makes turbofans significantly more fuel-efficient and quieter than their turbojet predecessors.

How It Works

Incoming air is split at the fan stage into two streams:

• Bypass stream: The larger stream flows around the engine core through a duct. This cold air produces most of the thrust at lower cost.
• Core stream: A smaller amount enters the compressor, where it is pressurized and mixed with fuel. Combustion expands the gases through turbine stages that drive both the compressor and the fan. The hot exhaust exits at high speed, contributing additional thrust.

The proportion of bypass to core airflow is described by the Bypass Ratio. Modern high-bypass turbofans (BPR 10–13:1) are dramatically more efficient than early low-bypass designs used in military aircraft.

Performance Specifications

• Thrust range: 15,000–115,000 lbf for commercial engines
• Bypass ratio: 4:1 (older CFM56) to 13:1 (GE9X on 777X)
• Overall pressure ratio (OPR): up to 60:1 on latest generation engines
• Turbine inlet temperature: up to 1,700 °C using advanced ceramic coatings
• Noise reduction: 20–30 dB quieter than equivalent turbojets

Aircraft Examples

• Boeing 737 MAX: CFM International LEAP-1B (28,000–33,000 lbf, BPR 9:1)
• Airbus A320neo: CFM LEAP-1A or Pratt & Whitney PW1100G (24,000–33,000 lbf)
• Boeing 777X: GE Aviation GE9X (105,000 lbf, BPR 10:1, world's largest turbofan)
• Airbus A350: Rolls-Royce Trent XWB (74,000–97,000 lbf, BPR ~9.6:1)

The Nacelle encases the engine, while FADEC manages fuel delivery and performance automatically. Understanding Thrust output is central to aircraft performance planning.