How Do Jet Engine's Work? Name:________
A jet engine works on the principle of Sir Isaac Newton's third
law of physics, i.e. for every action there is an equal and opposite
re-action. The action of forcing gases out from the rear of the jet
engine results in a re-active force in the opposite direction, and is
commonly referred to as 'thrust'. This thrust is measured in pounds
force (lbf ), kilograms force (kgf ), or Newtons (N). Engines of this
type are often referred to as 'Reaction Engines', a rocket engine
being another example. Newton's third law and the action of a jet
can be demonstrated in simple terms by inflating a balloon and
releasing it, the escaping air propels the balloon in the opposite
direction.
Creating thrust takes energy. The energy required is obtained
from burning fuels, whether it be in gas or liquid form such as
propane, kerosine, diesel or even vegetable oils! This fuel is
normally combined with pressurised air to increase the efficiency
and power output for a given engine size. This fuel/air mixture is
burned in some form of combustion chamber where the resulting
hot gases expand creating an increase in pressure inside the
combustion chamber. The expanding gases are then used to do
useful work. One example of this process is what happens inside
the cylinder of a car engine. Air and fuel are drawn into the cylinder
by the downward movement of the piston, the piston then moves up
and squeezes this mixture which is then ignited. The fuel burns
creating a sudden sharp rise in pressure inside the cylinder. This
pressure then forces the piston back down producing mechanical
work. The piston then moves back up the cylinder to eject the burnt
fuel ready for another cycle. This process is commonly referred to
as the 'Suck, Squeeze, Bang, Blow' cycle! (SSBB).
Comparison of the Operation of a Typical Jet Engine
and a Four Stroke Internal Combustion Engine
The way a basic Turbojet engine burns it's fuel is exactly the
same as in a car engine, but instead of burning the fuel in discrete
packets, the jet engine continuously sucks, squeezes, bangs and
blows all at the same time! Also, instead of using the expanding
gases to push on a piston, they are released through the turbine
blades which takes some of the energy to drive the compressor, the
rest being released to the atmosphere which results in 'Newtons'
thrust described above. In a basic turbo jet, the air enters the front
intake (suck) and is compressed by the compressor (squeeze), then
forced into combustion chambers where fuel is sprayed into them
and the mixture is ignited (bang). The gases which form expand
rapidly, and are exhausted through the rear of the combustion
chambers and out through the nozzle (blow) providing the forward
thrust. Just before the gases enter the engine nozzle, they pass
through a fan-like set of turbine blades which rotates the engine
shaft. This shaft, in turn, rotates the compressor, thereby bringing
in a fresh supply of air through the intake. All of these processes
are happening at the same time. Engine thrust may be increased by
the addition of an afterburner section into which extra fuel is
sprayed into the exhausting gases ( which contains surplus hot
oxygen ) to give the added thrust.
1.
Which of Sir Isaac Newton’s principles is the jey engine based on?
2.
What fuel do jet engines use?
3.
What does “SSBB” mean?
4.
Describe how “SSBB” applies to 4-stroke engines.
5.
Describe how “SSBB” applies to jet engines.
6.
Why is the turbine attached to the compressor (via the engine shaft)in a jet engine?
7.
What does an afterburner do?
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