To move an airplane through the air, a
propulsion system is used to generate
thrust.
The amount of thrust an engine generates is important. But the
amount of fuel used to generate that thrust is sometimes more
important, because the airplane has to lift and carry the fuel
throughout the flight. Engineers use an efficiency factor, called
thrust specific fuel consumption, to characterize an engine's
fuel efficiency. "Thrust specific fuel consumption" is quite a
mouthful, so engineers usually just call it the engine's TSFC.
What does TSFC mean?
The fuel consumption of TSFC is "how
much fuel the engine burns each hour." The specific
of TSFC is a scientific term meaning "divided by mass or weight." In
this case, specific means "per pound (Newton) of thrust." The
thrust of TSFC is included to indicate that we are talking
about gas turbine engines. There is a corresponding brake specific
fuel consumption (BSFC) for engines that produce shaft
power. Gathering all the terms together, TSFC is the mass of fuel
burned by an engine in one hour divided by the thrust that the
engine produces. The units of this efficiency factor are mass per
time divided by force (in English units, pounds mass per hour per
pound; in metric units, kilograms per hour per Newton).
Mathematically, TSFC is a
ratio
of the engine fuel mass flow rate mdot f
to the amount of thrust F produced by burning the fuel:
TSFC = mdot f / F
If we divide both numerator
and denominator by the engine airflow mdot 0, we obtain another form of the
equation in terms of the fuel to air ratio f, and the
specific thrust Fs.
TSFC = f / Fs
Engineers use the TSFC factor several different ways. If we
compare the TSFC for two engines, the engine with the lower TSFC is
the more fuel efficient engine. Let's consider two examples:
 Suppose we had two Engines, A and B, that produced the same
amount of thrust. And suppose that Engine A uses only half the
fuel per hour that Engine B uses. We would then say that Engine A
is more fuel efficient than Engine B. If we compute the TSFC for
Engines A and B, the TSFC of Engine A is one half the value of
Engine B.
 Looking at it another way, suppose we had two Engines, C and
D, and we fed the same amount of fuel per hour to each of them.
Suppose Engine C produces twice the thrust of Engine D. Then we
are getting more thrust from Engine C for the same amount of fuel,
and we would say that Engine C is more fuel efficient. Again, if
we compute the TSFC for Engines C and D, the TSFC of Engine C is
one half the value of Engine D.
Let's look at the second example with some numerical values.
In this case we are comparing a turbojet
engine and a turbofan engine. The
engines are being fed from a fuel tank that delivers 2000 pounds mass
per hour to each engine. The turbojet produces 2000 pounds of thrust,
while the turbofan produces 4000 pounds of thrust. Computing the TSFC
for each engine shows that the TSFC of the turbojet is equal to 1.0
(pounds mass/hour/pound) while the TSFC of the turbofan is 0.5
(pounds mass/ hour/pound). The turbofan, with a lower TSFC, is more
fuel efficient. The values of 1.0 for a turbojet and 0.5 for a
turbofan are typical sea level static values. The value of TSFC for a
given engine will vary with speed and altitude, because the
efficiency of the engine changes with atmospheric
conditions.
TSFC provides important information about the performance of a
given engine. A turbojet with an afterburner
produces more thrust than a plain turbojet. If the TSFC were the same
(1.0) for the two engines, in order to increase thrust, we would have
to increase the fuel flow rate by an equivalent amount. For
example,
Initial thrust = 2000 pounds
Thrust with afterburner = 3000 pounds
TSFC = 1.0
Fuel flow rate = 3000 pounds per hour.
But a turbojet with an afterburner has a typical TSFC value of
1.5. This says that adding the afterburner, although it produces more
thrust, costs much more fuel for each pound of thrust added. For
example,
Initial thrust = 2000 pounds
Thrust with afterburner = 3000 pounds
TSFC = 1.5
Fuel flow rate = 4500 pounds per hour.
Engineers use the TSFC for a given engine to figure out how much
fuel is required for an aircraft to perform
a given mission. If the TSFC = 0.5, and we
need 5000 pounds of thrust for two hours, we can easily compute the
amount of fuel required. For example,
5000 pounds x 0.5 pound mass/hour/pound x 2 hours = 5000 pound
mass of fuel.
The interactive Java applet
EngineSim
is now available. You can
study the effects of any engine component's performance on fuel
consumption and compare the efficiency of various types of turbine
engines.
Activities:
Guided Tours

EngineSim  Engine Simulator:

Calculating Fuel Flow Rate:
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