高バイパス比ターボファン (HBT: High-Bypass Turbofan)

What Is a High-Bypass Turbofan?

A high-bypass turbofan is a jet engine where the large front fan moves far more air around the engine core than through it. The bypass ratio — the ratio of bypassed air to core air — exceeds 5:1, with modern commercial engines reaching 12:1 or higher. This configuration makes high-bypass turbofans the dominant propulsion system for commercial airliners and widebody freighters worldwide.

How It Works

Incoming air enters through the large-diameter fan, typically 2.8–3.5 m (110–140 in) across on widebody engines. A small fraction — perhaps 10–20% — passes through the engine core, where it is compressed by the compressor stages, mixed with fuel in the combustion chamber, and expelled through the turbine and exhaust nozzle. The remaining 80–90% bypasses the core entirely, accelerated only modestly by the fan and discharged as a large, slow-moving jet.

This bypass stream generates the majority of thrust while consuming far less fuel than accelerating a small mass of air to high velocity. The principle follows Newton's second law: a large mass flow at low velocity is thermodynamically more efficient than a small mass flow at high velocity, particularly below Mach 0.9.

Performance Specifications

• Bypass ratio: 5:1 to 13:1 (GE9X reaches 10:1; CFM LEAP-1B approximately 9:1)
• Thrust range: 100 kN to 500 kN (22,500 lbf to 112,000 lbf) for commercial variants
• Specific fuel consumption (SFC): approximately 0.50–0.55 lb/lbf/hr at cruise, 15–20% better than low-bypass predecessors
• Noise reduction: 20–30 EPNdB quieter than 1960s-era turbojets
• Cruise speed compatibility: Mach 0.78–0.90

Aircraft Examples

• GE90-115B on the Boeing 777-300ER — rated at 513 kN (115,300 lbf), the most powerful certified commercial turbofan
• GEnx-1B on the Boeing 787-9 — bypass ratio approximately 9.3:1
• Trent XWB-97 on the Airbus A350-1000 — rated at 430 kN (97,000 lbf)
• CFM56-7B on the Boeing 737-800 — bypass ratio 5.1:1, older but widely deployed

The high-bypass architecture's efficiency gains have enabled non-stop ultra-long-haul routes exceeding 17,000 km (10,500 mi), such as Singapore–New York operated by Airbus A350-900ULR.