A turbofan engine is the
most modern variation of the basic gas turbine engine. As with
other gas turbines, there is a core engine. In the turbofan engine, the core engine is
surrounded by a fan in the front and an additional turbine at
the rear. The fan and fan turbine are composed of many blades,
like the core
compressor and core
turbine, and are connected to an additional shaft.
As with the core compressor and turbine, some of the fan
blades turn with the shaft and some blades remain stationary.
The fan shaft passes through the core shaft for mechanical
reasons. This type of arrangement is called a two spool
engine (one "spool" for the fan, one "spool" for the core.)
Some advanced engines have additional spools for even higher
How does a turbofan engine
work? The incoming air is captured by the engine
inlet. Some of the incoming air
passes through the fan and continues on into the core
compressor and then the
where it is mixed with fuel and combustion occurs. The hot
exhaust passes through the core and fan turbines and then out
nozzle, as in a
basic turbojet. The rest of the incoming air passes through
the fan and bypasses, or goes around the engine, just
like the air through a propeller. The air that goes through
the fan has a velocity that is slightly increased from free
stream. So a turbofan gets some of its thrust from the core
and some of its thrust from the fan. The ratio of the air that
goes around the engine to the air that goes through the core
is called the bypass ratio.
Because the fuel flow rate
for the core is changed only a small amount by the addition of
the fan, a turbofan generates more thrust for nearly the same
amount of fuel used by the core. This means that a turbofan is
very fuel efficient. In fact, high bypass ratio turbofans are
nearly as fuel efficient as turboprops. Because the fan is
enclosed by the inlet and is composed of many blades, it can
operate efficiently at higher speeds than a simple propeller.
That is why turbofans are found on high speed transports and
propellers are used on low speed transports. Low bypass ratio
turbofans are still more fuel efficient than basic turbojets.
Many modern fighter planes actually use low bypass ratio
turbofans equipped with afterburners. They can then cruise
efficiently but still have high thrust when dog-fighting. Even
though the fighter plane can fly much faster than the speed of
sound, the air going into the engine must travel less than the
speed of sound for high efficiency. Therefore, the airplane
inlet slows the air down from supersonic speeds.