比燃料消費率（SFC） (SFC: Specific Fuel Consumption)

比燃料消費率とは？

Specific Fuel Consumption (SFC) — also called Thrust Specific Fuel Consumption (TSFC) for jet engines — quantifies how efficiently an engine converts fuel energy into useful thrust. It expresses the mass of fuel burned per unit of thrust per unit of time. A lower SFC means a more fuel-efficient engine: less fuel is consumed to produce the same thrust, directly extending range and reducing operating costs.

仕組み

SFC is calculated as:

• SFC = (fuel flow rate) / (net thrust produced)
• In imperial units: lb of fuel per hour per lbf of thrust → lb/(lbf·h)
• In SI units: kg per hour per kN → kg/(kN·h) or equivalently mg/(N·s)

Key factors that influence SFC:

• Bypass Ratio: Higher Bypass Ratio dramatically improves SFC by accelerating more air at lower velocity. Doubling BPR from 5:1 to 10:1 can improve SFC by 10–15%.
• Overall Pressure Ratio (OPR): Higher compression improves thermodynamic efficiency. Modern engines achieve OPR of 45–60:1 vs. 15–20:1 in 1960s jets.
• Turbine Inlet Temperature (TIT): Higher TIT improves Carnot efficiency. Enabled by ceramic thermal barrier coatings and film cooling.
• Altitude: SFC improves at cruise altitude because cooler air increases the density ratio and reduces intake ram drag penalties.
• Airspeed: SFC increases at higher Mach numbers due to ram drag — a key economic penalty for supersonic flight.

性能仕様

• CFM56-7B (737 NG, circa 1997): ~0.545 lb/(lbf·h) at cruise
• CFM LEAP-1B (737 MAX, 2017): ~0.490 lb/(lbf·h) — approximately 15% improvement
• Pratt & Whitney PW1100G (A320neo, 2016): ~0.460–0.480 lb/(lbf·h) — best in class for single-aisle
• Rolls-Royce Trent XWB (A350, 2015): ~0.478 lb/(lbf·h) — best SFC of any large turbofan at launch
• Concorde Olympus 593 (supersonic): ~1.195 lb/(lbf·h) — over twice modern turbofans due to low BPR and supersonic penalties

航空機の事例

• Boeing 787-9 (GEnx-1B): SFC ~0.486 lb/(lbf·h); combined with composite airframe yields 25% better fuel burn per seat than 767
• Airbus A320neo (PW1100G): 20% better fuel efficiency than A320ceo, driven largely by SFC improvement plus airframe drag reduction
• ATR 72-600 (PW127M turboprop): Turboprops use shaft-horsepower SFC (lb/SHP·h): ~0.48 — far better than jets for short sectors below 400 nm
• Future Open Fan (CFM RISE program): Targeting 20% SFC improvement over LEAP by ~2035 using unducted fan architecture with BPR >20:1

SFC is the core metric linking engine efficiency to aircraft range. Engineers use the Breguet range equation, in which SFC appears as a direct denominator — halving SFC doubles range for a given fuel load.