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Jet Engines
How a Jet
Engine Works
By Interesting
Engineering
We're going to explain the processes involved so that
anyone could get a good understanding of the underlying principles behind jet
engines.
You may have wondered how a
jet engine works but gave up on the idea that you would be able to understand
rocket science.
But it's actually a simple
concept to grasp and one that will impress the person by your side on your next
flight.
So, we're going to explain
the processes involved so that anyone could get a good understanding of the
underlying principles behind jet engines.
Jet engines, more
commonly used for aircrafts, are a type of gas turbine engine.
Now you may know steam turbines,
where a fuel is burnt to produce high-temperature flowing steam which drives a
turbine, subsequently turning a shaft, before being exhausted from the system.
The turning of this shaft is
output power and it is this rotation which drives a rotating object.
A gas turbine resembles the
same underlying principles, however, a pressurized gas is responsible for
driving the turbine.
In jet engines, the
high-temperature pressurized gas powers the rotation of the compressor at the
front, but more importantly, what's exhausted from the system flies out at the
rear at high speeds, producing what's known as thrust.
Simply put, jet engines have a core which is
split into three main sections:
·
Compressor - at
the front of the engine are fan blades, some rotating (rotors) and some static
(stators), which draw air into the engine. There are many rows of blades and as
the air passes by each row it becomes more pressurized and the temperature
increases.
·
Combustion
chamber - this pressurized air is then sprayed with fuel (most commonly Jet A
or Jet A-1 which are of kerosene-type) and then an electric spark ignites the
fuel and air mixture in the chamber. This causes the air/fuel mixture to burn,
massively increasing pressure and temperature.
·
Turbines - the
hot pressurized gas is drawn out of the engine by a turbine at the rear which
takes energy out of the gas and causes a drop in pressure and temperature. As
the pressure decreases, the gas flows faster (think about letting go of an inflated
balloon). The energy from the gas that drives the turbine at the rear is what
powers the rotation of the compressor which draws in air at the front.
The high-velocity gases being released through
the nozzle at the rear is what causes the thrust.
To understand this we refer to Newton's Third
Law of Motion: for every action, there is an equal and opposite reaction.
As the gas rushes out of the back an equal and
opposite force is exerted forward.
Think about when you push the wall of a
swimming pool to glide off in the opposite direction; even though the force of
your pushing is directed towards the wall, an equal and opposite reaction force
causes you to travel in the opposite direction.
At approximately 400 mph, one
pound of thrust equals one horsepower, but at higher speeds, this ratio
increases and a pound of thrust is greater than one horsepower.
At speeds of less than 400
mph, this ratio decreases. This force allows large planes like the 747 to fly
at speeds up to 600mph.
There are different
types of the jet engine too such as the Turboprop.
You'll know if it's a
Turboprop type by the large extruding propellers at the front, which is
responsible for the thrust as most of the energy from the gas is transferred to
the compressor by the rear turbines, so the exerted gas is not responsible for
the thrust.
Turboshaft is the type found
in helicopter rotors, power plants, and even the M1 tank.
The process is similar to the
turboprop, however, instead of driving propellors, the rotating shaft can power
a variety of devices such as pumps, generators, wheels and generally anything
that spins.
Modern large aircrafts
use a High-Bypass Turbofan which is similar to the standard turbojet engine
except that a large fan at the front draws in more air to the engine.
However, not all of the air
goes through the compressor and turbines, with most of the air actually
bypassing the core and traveling through ducts on the outside of the core (on
average 5 times more air is bypassed than actually traveling through the core).
These are more efficient
especially at subsonic speeds (ie. below the speed of sound, 768 mph) and
are also a lot quieter whilst still having the ability to accelerate a vehicle
heavier than a locomotive from 0 to 200 mph in less than 60 seconds.
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