How Do Jet Engines Work?
Jet engines put Isaac Newton's third law of motion into use.
The third law states "Every action has an equal and opposite
reaction." Jet engines shoot a high velocity stream of fuel
out the back of the engine through the nozzle, creating a
force called thrust that pushes the aircraft forwards.
Stronger engines create a stronger stream and therefore
generate more thrust. Today's jet engines usually generate
around 44 kN of thrust, that's 9,900 lbs! There are two ways
to generate a large thrust. The first theory is to take a
large amount of air and increase its velocity by any amount.
The second theory is to take a smaller amount of air and
increase its velocity by a very large amount. These theories
are governed by the thrust equation.
The Thrust Equation
The thrust is a force generated by the acceleration of a mass
of gas out of the back of the engine. A force is a change in
momentum over time. The thrust is dependent upon how much gas
is propelled out of the back and how much it is accelerated
(F=Ma). To keep track of the mass of the gas, we use the mass
flow rate (MFR), the amount of mass that flows through a given
plane per unit time. The momentum is described as the mass
flow rate multiplied by the velocity. If the exit pressure is
the same as the free stream pressure outside of the engine
then the thrust equation is simply (Thrust= (MFR*V)exit -
(MFR*V)free stream). However when the pressures differ then we
must account for this in the equation by adding the difference
between the free stream and exit pressure multiplied by the
area of the plane that the gas exits from (Ae). This is
summarized in the figure below.
How Does the Engine Accelerate the Air?
This is a nice video by CFM International that describes how
air travels through the core of the engine and the bypass air.
If the embedded video below does not work the video can be
found at:
https://youtu.be/eA699AKxT7s