Thrust-specific fuel consumption (TSFC) is the
fuel efficiency
Fuel efficiency (or fuel economy) is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical energy, chemical potential energy contained in a carrier (fuel) into kinetic energy or Mechanical work, w ...
of an
engine
An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy.
Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power ge ...
design with respect to
thrust
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...
output. TSFC may also be thought of as fuel consumption (grams/second) per unit of thrust (newtons, or N), hence ''thrust-specific''. This figure is inversely proportional to
specific impulse
Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine, such as a rocket engine, rocket using propellant or a jet engine using fuel, generates thrust. In general, this is a ratio of the ''Impulse (physics), ...
, which is the amount of thrust produced per unit fuel consumed.
TSFC or SFC for
thrust engines (e.g.
turbojet
The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and ...
s,
turbofan
A turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a combination of references to the preceding generation engine technology of the turbojet and the add ...
s,
ramjet
A ramjet is a form of airbreathing jet engine that requires forward motion of the engine to provide air for combustion. Ramjets work most efficiently at supersonic speeds around and can operate up to .
Ramjets can be particularly appropriat ...
s,
rocket
A rocket (from , and so named for its shape) is a vehicle that uses jet propulsion to accelerate without using any surrounding air. A rocket engine produces thrust by reaction to exhaust expelled at high speed. Rocket engines work entirely ...
s, etc.) is the mass of
fuel
A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work (physics), work. The concept was originally applied solely to those materials capable of releasing chem ...
needed to provide the net thrust for a given period e.g. lb/(h·lbf) (pounds of fuel per hour-pound of thrust) or g/(s·kN) (grams of fuel per second-kilonewton). Mass of fuel is used, rather than volume (gallons or litres) for the fuel measure, since it is independent of temperature.
Specific fuel consumption of air-breathing jet engines at their maximum efficiency is more or less proportional to exhaust speed. The fuel consumption ''per mile'' or ''per kilometre'' is a more appropriate comparison for aircraft that travel at very different speeds. There also exists
power-specific fuel consumption, which equals the thrust-specific fuel consumption divided by speed. It can have units of pounds per hour per horsepower.
Significance of SFC
SFC is dependent on engine design, but differences in the SFC between different engines using the same underlying technology tend to be quite small. Increasing
overall pressure ratio
In aeronautical engineering, overall pressure ratio, or overall compression ratio, is the amount of times the pressure increases due to ram compression and the work done by the compressor stages.
The compressor pressure ratio is the ratio of the ...
on jet engines tends to decrease SFC.
In practical applications, other factors are usually highly significant in determining the fuel efficiency of a particular engine design in that particular application. For instance, in aircraft, turbine (jet and turboprop) engines are typically much smaller and lighter than equivalently powerful piston engine designs, both properties reducing the levels of
drag on the plane and reducing the amount of power needed to move the aircraft. Therefore, turbines are more efficient for aircraft propulsion than might be indicated by a simplistic look at the table below.
SFC varies with throttle setting, altitude, climate. For jet engines, air flight speed is an important factor too.
Air flight speed counteracts the jet's exhaust speed. (In an artificial and extreme case with the aircraft flying exactly at the exhaust speed, one can easily imagine why the jet's net thrust should be near zero.) Moreover, since work is force (''i.e''., thrust) times distance, mechanical power is force times speed. Thus, although the nominal SFC is a useful measure of fuel efficiency, it should be divided by speed when comparing engines at different speeds.
For example,
Concorde
Concorde () is a retired Anglo-French supersonic airliner jointly developed and manufactured by Sud Aviation and the British Aircraft Corporation (BAC).
Studies started in 1954, and France and the United Kingdom signed a treaty establishin ...
cruised at 1354 mph, or 7.15 million feet per hour, with its engines giving an SFC of 1.195 lb/(lbf·h) (see below); this means the engines transferred 5.98 million
foot pound
The foot-pound force (symbol: ft⋅lbf, ft⋅lbf, or ft⋅lb ) is a unit of work or energy in the engineering and gravitational systems in United States customary and imperial units of measure. It is the energy transferred upon applying a ...
s per pound of fuel (17.9 MJ/kg), equivalent to an SFC of 0.50 lb/(lbf·h) for a subsonic aircraft flying at 570 mph, which would be better than even modern engines; the
Olympus 593 used in the Concorde was the world's most efficient jet engine.
The turbofan engine
", page 5. '' SRM Institute of Science and Technology, Department of aerospace engineering'' However, Concorde ultimately has a heavier airframe and, due to being supersonic, is less aerodynamically efficient, i.e., the lift to drag ratio is far lower. In general, the total fuel burn of a complete aircraft is of far more importance to the customer.
Units
Typical values of SFC for thrust engines
The following table gives the efficiency for several engines when running at 80% throttle, which is approximately what is used in cruising, giving a minimum SFC. The efficiency is the amount of power propelling the plane divided by the rate of energy consumption
Energy consumption is the amount of energy used.
Biology
In the body, energy consumption is part of energy homeostasis. It derived from food energy. Energy consumption in the body is a product of the basal metabolic rate and the physical acti ...
. Since the power equals thrust times speed, the efficiency is given by
:
where V is speed and h is the energy content per unit mass of fuel (the higher heating value
The heating value (or energy value or calorific value) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it.
The ''calorific value'' is the total energy release ...
is used here, and at higher speeds the kinetic energy of the fuel or propellant becomes substantial and must be included).
See also
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Notes
References
External links
GE CF6 website
NASA Cruise SFC vs. Year
{{DEFAULTSORT:Thrust Specific Fuel Consumption
Engine technology
Power (physics)