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