>> How Things Work >> How Gas Turbine Engines Work
go to an airport and see the commercial jets there, you can't
help but notice the huge engines that power them. Most commercial
jets are powered by turbofan engines, and turbofans are one
example of a general class of engines called gas turbine engines.
have never heard of gas turbine engines, but they are used
in all kinds of unexpected places. For example, many of the
helicopters you see, a lot of smaller power plants and even
the M-1 Tank use gas turbines. In this article, we will look
at gas turbine engines to see what makes them tick!
are many different kinds of turbines:
have probably heard of a steam turbine. Most power plants
use coal, natural gas, oil or a nuclear reactor to create
steam. The steam runs through a huge and very carefully
designed multi-stage turbine to spin an output shaft that
drives the plant's generator.
dams use water turbines in the same way to generate power.
The turbines used in a hydroelectric plant look completely
different from a steam turbine because water is so much
denser (and slower moving) than steam, but it is the same
turbines, also known as wind mills, use the wind as their
motive force. A wind turbine looks nothing like a steam
turbine or a water turbine because wind is slow moving and
very light, but again, the principle is the same.
turbine is an extension of the same concept. In a gas turbine,
a pressurized gas spins the turbine. In all modern gas turbine
engines, the engine produces its own pressurized gas, and
it does this by burning something like propane, natural gas,
kerosene or jet fuel. The heat that comes from burning the
fuel expands air, and the high-speed rush of this hot air
spins the turbine.
does the M-1 tank use a 1,500 horsepower gas turbine engine
instead of a diesel engine? It turns out that there are two
big advantages of the turbine over the diesel:
turbine engines have a great power-to-weight ratio compared
to reciprocating engines. That is, the amount of power you
get out of the engine compared to the weight of the engine
itself is very good.
turbine engines are smaller than their reciprocating counterparts
of the same power.
disadvantage of gas turbines is that, compared to a reciprocating
engine of the same size, they are expensive. Because they
spin at such high speeds and because of the high operating
temperatures, designing and manufacturing gas turbines is
a tough problem from both the engineering and materials standpoint.
Gas turbines also tend to use more fuel when they are idling,
and they prefer a constant rather than a fluctuating load.
That makes gas turbines great for things like transcontinental
jet aircraft and power plants, but explains why you don't
have one under the hood of your car.
Gas Turbine Process
engines are, theoretically, extremely simple. They have three
- Compresses the incoming air to high pressure
area - Burns the fuel and produces high-pressure, high-velocity
- Extracts the energy from the high-pressure, high-velocity
gas flowing from the combustion chamber.
is sucked in from the right by the compressor. The compressor
is basically a cone-shaped cylinder with small fan blades
attached in rows (eight rows of blades are represented here).
Assuming the light blue represents air at normal air pressure,
then as the air is forced through the compression stage its
pressure rises significantly. In some engines, the pressure
of the air can rise by a factor of 30. The high-pressure air
produced by the compressor is shown in dark blue.
air then enters the combustion area, where a ring of fuel
injectors injects a steady stream of fuel. The fuel is generally
kerosene, jet fuel, propane or natural gas. If you think about
how easy it is to blow a candle out, then you can see the
design problem in the combustion area -- entering this area
is high-pressure air moving at hundreds of miles per hour.
You want to keep a flame burning continuously in that environment.
The piece that solves this problem is called a "flame
holder," or sometimes a "can." The can is a
hollow, perforated piece of heavy metal.
are at the right. Compressed air enters through the perforations.
Exhaust gases exit at the left. You can see in the previous
figure that a second set of cylinders wraps around the inside
and the outside of this perforated can, guiding the compressed
intake air into the perforations.
left of the engine is the turbine section. The turbines, the
shaft and the compressor all turn as a single unit.
far left is a final turbine stage, shown here with a single
set of vanes. It drives the output shaft. This final turbine
stage and the output shaft are a completely stand-alone, freewheeling
unit. They spin freely without any connection to the rest
of the engine. And that is the amazing part about a gas turbine
engine -- there is enough energy in the hot gases blowing
through the blades of that final output turbine to generate
1,500 horsepower and drive a 63-ton M-1 Tank! A gas turbine
engine really is that simple.
case of the turbine used in a tank or a power plant, there
really is nothing to do with the exhaust gases but vent them
through an exhaust pipe, as shown. Sometimes the exhaust will
run through some sort of heat exchanger either to extract
the heat for some other purpose or to preheat air before it
enters the combustion chamber.
here is obviously simplified a bit. For example, we have not
discussed the areas of bearings, oiling systems, internal
support structures of the engine, stator vanes and so on.
All of these areas become major engineering problems because
of the tremendous temperatures, pressures and spin rates inside
the engine. But the basic principles described here govern
all gas turbine engines and help you to understand the basic
layout and operation of the engine.
jetliners use what are known as turbofan engines, which are
nothing more than gas turbines combined with a large fan at
the front of the engine.
of a turbofan is a normal gas turbine engine like the one
described in the previous section. The difference is that
the final turbine stage drives a shaft that makes its way
back to the front of the engine to power the fan. This multiple
concentric shaft approach, by the way, is extremely common
in gas turbines. In many larger turbofans, in fact, there
may be two completely separate compression stages driven by
separate turbines, along with the fan turbine as shown above.
All three shafts ride within one another concentrically.
of the fan is to dramatically increase the amount of air moving
through the engine, and therefore increase the engine's thrust.
When you look into the engine of a commercial jet at the airport,
what you see is this fan at the front of the engine. It is
huge -- on the order of 10 feet (3 m) in diameter on big jets,
so it can move a lot of air. The air that the fan moves is
called "bypass air" because it bypasses the turbine
portion of the engine and moves straight through to the back
of the nacelle at high speed to provide thrust.
engine is similar to a turbofan, but instead of a fan there
is a conventional propeller at the front of the engine. The
output shaft connects to a gearbox to reduce the speed, and
the output of the gearbox turns the propeller.
of a turbofan engine is to produce thrust to drive the airplane
forward. Thrust is generally measured in pounds in the United
States (the metric system uses Newtons, where 4.45 Newtons
equals 1 pound of thrust). A "pound of thrust" is
equal to a force able to accelerate 1 pound of material 32
feet per second per second (32 feet per second per second
happens to be equivalent to the acceleration provided by gravity).
Therefore, if you have a jet engine capable of producing 1
pound of thrust, it could hold 1 pound of material suspended
in the air if the jet were pointed straight down. Likewise,
a jet engine producing 5,000 pounds of thrust could hold 5,000
pounds of material suspended in the air. And if a rocket engine
produced 5,000 pounds of thrust applied to a 5,000-pound object
floating in space, the 5,000-pound object would accelerate
at a rate of 32 feet per second per second.
is generated under Newton's principle that "every action
has an equal and opposite reaction." For example, imagine
that you are floating in space and you weigh 100 pounds on
Earth. In your hand you have a baseball that weighs 1 pound
on Earth. If you throw the baseball away from you at a speed
of 32 feet per second (21 mph / 34 kph), your body will move
in the opposite direction (it will react) at a speed of 0.32
feet per second. If you were to continuously throw baseballs
in that way at a rate of one per second, your baseballs would
be generating 1 pound of continuous thrust. Keep in mind that
to generate that 1 pound of thrust for an hour you would need
to be holding 3,600 pounds of baseballs at the beginning of
the hour. If you wanted to do better, the thing to do is to
throw the baseballs harder. By "throwing" them (with
of a gun, say) at 3,200 feet per second, you would generate
100 pounds of thrust.
In a turbofan
engine, the baseballs that the engine is throwing out are
air molecules. The air molecules are already there, so the
airplane does not have to carry them around at least. An individual
air molecule does not weigh very much, but the engine is throwing
a lot of them and it is throwing them at very high speed.
Thrust is coming from two components in the turbofan:
gas turbine itself - Generally a nozzle is formed at the
exhaust end of the gas turbine (not shown in this figure)
to generate a high-speed jet of exhaust gas. A typical speed
for air molecules exiting the engine is 1,300 mph (2,092
bypass air generated by the fan - This bypass air moves
at a slower speed than the exhaust from the turbine, but
the fan moves a lot of air.
can see, gas turbine engines are quite common. They are also
quite complicated, and they stretch the limits of both fluid
dynamics and materials sciences. If you want to learn more,
one worthwhile place to go would be the library of a university
with a good engineering department. Books on the subject tend
to be expensive, but two well-known texts include "Aircraft
Gas Turbine Engine Technology" and "Elements of
Gas Turbine Propulsion."
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