Bypass Ratio (BPR)

What Is Bypass Ratio?

Bypass Ratio (BPR) is the ratio of the mass airflow bypassing the engine core to the mass airflow passing through the core in a Turbofan Engine. A BPR of 10:1 means ten kilograms of air flow around the core for every one kilogram that passes through it. Higher bypass ratios generally translate to greater fuel efficiency and lower noise.

How It Works

The fan at the front of a turbofan accelerates both bypass and core airstreams. The bypass air, expelled at moderate velocity through the fan duct, provides the bulk of thrust efficiently. The core air undergoes combustion and exits at very high velocity, contributing the remainder. The efficiency gain comes from the physics of momentum:

• Low BPR (1–3:1): High exhaust velocity, high noise, lower efficiency. Typical of older jets and military aircraft.
• Medium BPR (4–7:1): CFM56 family, older 737s and A320s. Good efficiency balance.
• High BPR (8–13:1): LEAP, GE9X, Trent XWB. Maximum commercial efficiency and minimum noise.

Performance Specifications

• CFM56-7B (737 NG): BPR ~5.1:1
• CFM LEAP-1B (737 MAX): BPR ~9:1 — 15% fuel saving vs CFM56
• PW1100G (A320neo): BPR ~12:1 — geared turbofan architecture
• GE9X (777X): BPR ~10:1 with 134-inch fan diameter
• GE90-115B (777-300ER): BPR ~8.7:1, previously world's most powerful engine

Aircraft Examples

• Airbus A220: Pratt & Whitney PW1500G, BPR ~12:1 — quiet regional operations
• Boeing 787 Dreamliner: GEnx-1B, BPR ~9.6:1, or Rolls-Royce Trent 1000, BPR ~10:1
• Concorde (retired): Olympus 593 turbojet, BPR 0:1 — pure core thrust for supersonic flight

BPR directly affects Specific Fuel Consumption (SFC). Higher BPR reduces SFC, extending aircraft range. Engine Fan Blades grow larger as BPR increases, requiring wider Nacelles.