Specific Fuel Consumption and Overall Efficiency

The engine performance may be described in several ways. One of the useful parameters is specific fuel consumption, or s.f.c. For turbojets and fans, the s.f.c. is usually expressed as the thrust specific fuel consumption or t.s.f.c.. It is defined as the weight of the fuel burned per unit time, per unit thrust. In English units, t.s.f.c. is usually quoted in lbs of fuel per hour per lb of thrust or just lb/hr/lb or 1/hr. (In SI units the t.s.f.c. is sometime expressed in kg/hr/kN.)

For turboprop or piston engines, the s.f.c. is often expressed as a power specific fuel consumption, i.e. weight of fuel per unit time per unit power delivered to the propeller. This quantity is often denoted b.s.f.c. (for brake-power s.f.c.) and has units of 1/length. It is expressed in the unwieldy, but familiar English units of lb / hr / h.p..

The overall efficiency of the propulsion system is given by:

h = Power Available to Aircraft / Rate of Energy Consumption = T V / w h

where T = thrust, V= aircraft speed, w = rate of fuel consumption (weight/unit time), and h = specific energy of the fuel (energy / unit weight).

In terms of the s.f.c.: h = V / tsfc h.

One must be careful to use consistent units in this expression.

Overall efficiency of several engines vs. Mach number.


Overall efficiency vs. bypass ratio for large commercial turbine engines. (From Dennis Berry, Boeing)


Trends in advanced engine efficiency.

Subsonic Engine Efficiencies: 
(At about min sfc throttle setting 80% at typical cruise conditions)
GE90     .361
PW4000   .348
PW2037    .351 (M.87 40K)
PW2037    .335 (M.80 35K)
CFM56-2   .305
TFE731-2  .234