Specific fuel consumption
Encyclopedia
Thrust specific fuel consumption (TSFC) or sometimes simply specific
fuel consumption, SFC, is an engineering
term that is used to describe the fuel efficiency
of an engine
design with respect to thrust
output. It allows the efficiency of different sized engines to be directly compared.
TSFC may also be thought of as fuel consumption (grams/second) per unit of thrust (kilonewtons, or kN). It is thus thrust-specific
, meaning that the fuel consumption is divided by the thrust.
TSFC or SFC for thrust engine
s (e.g. turbojets, turbofan
s, ramjet
s, rocket engine
s, etc.) is the mass of fuel
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 vary more or less inversely with speed, which in turn means that the fuel consumption per mile or km can be a more appropriate comparison metric for aircraft that travel at very different speeds.
This figure is inversely proportional to specific impulse
.
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 and climate. For jet engines, flight speed also has a significant effect upon SFC; SFC is roughly proportional to air speed (actually exhaust velocity), but speed along the ground is also proportional to air speed. Since work done is force 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 to get a way to compare engines that fly at different speeds.
For example, the Concorde
cruised at Mach 2.05 with its engines giving an SFC of 1.195 lb/(lbf·h) (see below); this is equivalent to an SFC of 0.51 lb/(lbf·h) for an aircraft flying at Mach 0.85, which would be better than even modern engines, it was the world's most efficient jet engine. However, Concorde ultimately has a less aerodynamically efficient and heavier airframe- due to being supersonic 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.
Specific
Specific may refer to:* Specificity* Specific, a cure or therapy for a specific illnessLaw:* Specific deterrence, focussed on an individual* Specific finding, intermediate verdict used by a jury in determining the final verdict...
fuel consumption, SFC, is an engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
term that is used to describe the fuel efficiency
Fuel efficiency
Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...
of an engine
Engine
An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...
design with respect to thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
output. It allows the efficiency of different sized engines to be directly compared.
TSFC may also be thought of as fuel consumption (grams/second) per unit of thrust (kilonewtons, or kN). It is thus thrust-specific
Specific
Specific may refer to:* Specificity* Specific, a cure or therapy for a specific illnessLaw:* Specific deterrence, focussed on an individual* Specific finding, intermediate verdict used by a jury in determining the final verdict...
, meaning that the fuel consumption is divided by the thrust.
TSFC or SFC for thrust engine
Reaction engine
A reaction engine is an engine or motor which provides propulsion by expelling reaction mass, in accordance with Newton's third law of motion...
s (e.g. turbojets, turbofan
Turbofan
The turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...
s, ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
s, rocket engine
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
s, etc.) is the mass of fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
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 vary more or less inversely with speed, which in turn means that the fuel consumption per mile or km can be a more appropriate comparison metric for aircraft that travel at very different speeds.
This figure is inversely proportional to specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
.
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 ratioOverall pressure ratio
In aeronautical engineering, the term overall pressure ratio is defined as the ratio of the stagnation pressure as measured at the front and rear of the compressor of a gas turbine engine...
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
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
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 and climate. For jet engines, flight speed also has a significant effect upon SFC; SFC is roughly proportional to air speed (actually exhaust velocity), but speed along the ground is also proportional to air speed. Since work done is force 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 to get a way to compare engines that fly at different speeds.
For example, the Concorde
Concorde
Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...
cruised at Mach 2.05 with its engines giving an SFC of 1.195 lb/(lbf·h) (see below); this is equivalent to an SFC of 0.51 lb/(lbf·h) for an aircraft flying at Mach 0.85, which would be better than even modern engines, it was the world's most efficient jet engine. However, Concorde ultimately has a less aerodynamically efficient and heavier airframe- due to being supersonic 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
| Specific Impulse (by weight) |
Specific Impulse (by mass) |
Effective exhaust velocity |
Specific Fuel Consumption |
|
|---|---|---|---|---|
| SI | =X seconds | =9.8066 X N·s/kg | =9.8066 X m/s | =(101972/X) g/kN·s |
| Imperial units | =X seconds | =X lbf·s/lb | =32.16 X ft/s | =(3600/X) lb/lbf·h |
See also
- Brake specific fuel consumptionBrake specific fuel consumptionBrake Specific Fuel Consumption is a measure of fuel efficiency within a shaft reciprocating engine.It is the rate of fuel consumption divided by the power produced. It may also be thought of as power-specific fuel consumption, for this reason...
- Energies per unit mass
- Specific impulseSpecific impulseSpecific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
- Vehicle metricsVehicle metricsThere are a broad range of metrics that denote the relative capabilities of various vehicles. Most of them apply to all vehicles while others are type-specific....