When you 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.
You may 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 edition of HowStuffWorks,
we will look at gas turbine engines to see what makes them
tick!
A Little Background There are many different
kinds of turbines:
You 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.
Hydroelectric
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 principle.
Wind 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.
A gas 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.
Advantages and Disadvantages So why 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:
Gas 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.
Gas turbine engines are smaller than their
reciprocating counterparts of the same power.
The
main 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.
The Gas Turbine Process Gas turbine engines
are, theoretically, extremely simple. They have three parts:
Compressor - Compresses the incoming air to high
pressure
Combustion area - Burns the fuel and produces
high-pressure, high-velocity gas
Turbine - Extracts the energy from the
high-pressure, high-velocity gas flowing from the combustion
chamber
The following figure shows the general
layout of an axial-flow gas turbine -- the sort of
engine you would find driving the rotor of a helicopter,
for example:
In this engine, air 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.
This high-pressure 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. Half of the can in
cross-section is shown below:
The injectors 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.
At the left of the engine is the turbine section. In
this figure there are two sets of turbines. The first set
directly drives the compressor. The turbines, the shaft and
the compressor all turn as a single unit:
At the 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.
In the 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.
The discussion 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.
Other Variations Large 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. Here's the basic (highly simplified) layout of a
turbofan engine:
You can see that the core 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 (shown in red in this picture). 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.
The purpose 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" (shown in purple above) 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.
A turboprop 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.
Jet Engine Thrust The goal 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.
Thrust 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:
The 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 kph).
The 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.
As you 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."
The links on the next page will help you learn more on the
Web. There is a surprising amount of activity in the
home-built gas-turbine arena, and you can find other people
interested in the same topic by participating in newsgroups
or mailing lists on the subject.
