Aeronautics, Energy, Mechanics, Temperature, Thermodynamics

Turbofan jet engine: how it works?

This post’s subject is the turbofan jet engine. It’s used in every commercial airplane due to high thrust and for being the most efficient and silent type.

Operation principle of turbofan jet engine

Air entrance

The turbofan has this name due to the big propeller which sucks air. Each blade has an aerodynamic shape, when wind arrives at the blade, a lift force is induced, like in airplane’s wing. This force on blades is the one that contributes more to plane’s thrust with turbofan engines.


When higher the propeller, higher the capacity to suck air to the motor.

turbofan fan
Source: OpenPR.

Air compression

To execute combustion in the engine, air must have high pressure and temperature. Right behind the fan, there is a low pressure compressor, and then, another compressor of high pressure, that spins faster than the previous one.

turbofan compressor
A compressor is formed by many blade stages. Source: Boldmethod.

Are used 2 types of compressors: axial and centrifugal. In axial type, the blades have airfoil shape and the air flow parallel to the engine’s shaft.

axial compressor
Source: AerMech.

An axial compressor has 1 or more stages of fixed blades, called stators. The stator’s function is to convert rotational energy in pressure and directs the flux to the next stage in rotation, called rotor.

centrifugal compressor
The centrifugal compressor has different shapes of blades, to change air flux’s direction. Source: Leeham News and Analysis.


After passing through compressors, air pressure and temperature become much higher. The next stage is the combustion chamber. 

combustion chamber
The combustion chamber has many holes for air entrance and fuel injection. Must be made of a material that resists a temperature of approximately 2000 ºC. Source: Jet Engine Trader.

The fuel goes from tank to the chamber by valves and a tube, aided by pumps. Many fuel injectors put fuel on the chamber like a spray, to mix with high pressure and high temperature air. An electric spark by igniter initiates the mixture’s combustion.

operation of combustion chamber on turbofan
Representative schematics of the combustion chamber. Source: Mechanical Boost.
This is the swirler, a component that induces turbulence inside the chamber to improve the mixture and flame stabilization. Source: AM.

The first part of “All fuels” show the fuels used in aviation.

All fuels (Part 1)Click here


The gases coming out of combustion chamber go to high-pressure and low-pressure turbines respectively. These turbines use kinetic energy of gases to spin the compressors and the fan, because they are all linked to the same axis.

turbofan turbines
The turbine’s blades have an airfoil shape. Source: Mechanical Boost.

Low pressure blades are longer, a lower speed reduce the centrifugal stress on blade’s base, increasing it’s lifespan.


After passing through turbines, the gas mixture is expelled by a nozzle. It must be narrow to increase airplane’s thrust. It’s an application of Newton’s third law, when the hot air is expelled through the nozzle at high speed, a force is produced. Air generates a reaction force which produces thrust to the plane.

The equation of turbofan thrust force F, in Newtons (N).

F=\dot{m}_{f}\cdot (V_{f}-V_{o})+\dot{m_{c}}\cdot (V_{e}-V_{o})

  • \dot{m}_{f}, the bypass air flux rate.
  • V_{f}, speed of bypass flux.
  • V_{o}, air speed before entering the engine.
  • \dot{m_{c}} is the air flux rate that passes inside the engine.
  • V_{e} is the air speed expelled by the nozzle.

Turbofan bypass rate

Most of air sucked by the big fan doesn’t go to combustion chamber, but to the side of engine. This is called bypass flux.

turbofan diagram
The cold air (pink) is expelled together with hot air (red). This is the reason for turbofan being the most silent and economical of jet engines. Source: Century of Flight.

Equation of bypass rate.


The 2 turbofan categories are: low-bypass and high-bypass. The high-bypass has a big air volume that pass by, in comparison to volume that enters in the engine. On low-bypass, rate between the fluxes is smaller.

turbofan engines
The majority of commercial airplanes use high-bypass engines. While many military planes use the low-bypass because it’s better to reach hypersonic speeds. Source: David Darling.
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