If you browse Jane's or look up your favorite planes on Wikipedia, you may have noticed that some planes claim to have turbojet engines and some turbofans.
You may be wondering (and with good reason) what the differences between each are.
In this article we will give you a brief understanding of what each mechanism is, how it works and explain its complexity.
We will also talk about the different stages of a turbojet engine as well as a brief history of its origins.
In conclusion, we come to the meat and potatoes of the article:
What is the difference between a turbojet and a turbofan engine?
Read on to find out more!
What is a turbo jet?
A turbojet is a type of gas generator, or more specifically a gas turbine, which is essentially a continuous internal combustion engine.
They work according to the Brayton cycle, a thermodynamic cycle that works at constant pressure and extracts energy from the heat supplied. (Reference:1)
In general, there are three different stages in a gas turbine:
1.compression stage– This is the stage where the incoming air is compressed by alternating the moving and stationary compressor blades. one
2.combustion stage– The stage where fuel is added and continuously burned.
3.turbine section– This is the step that extracts the energy released from the combustion step.
The compressor and turbine are connected via a shaft that can rotate between 10,000 and 25,000 rpm.
The diagram below shows the whole process:
Origins of the turbojet engine
French engineer Maxime Guillaume patented the first turbojet engine in 1921, but his project was not pursued because the state of metallurgy was not sufficiently advanced.
Materials that can withstand the high temperatures required in the turbine section had not yet been developed.
The first turbojet prototype was tested in 1937 by Frank Whittle of Great Britain. (ref:2)
Although the test was successful, Whittle failed to impress the military at the time and development was slowed.
Around the same time as the Whittle test, German engineer Hans von Ohain patented and developed a similar design.
Von Ohain managed to impress the aircraft designer Ernst Heinkel, who built an airplane based on von Ohain's engine.
This gave rise to the first existing jet aircraft, the HE-178, which made its maiden flight in 1939, just days before the outbreak of World War II in Europe.
World War II was the catalyst for the development of jet engines. Although the Nazi leadership seemed unimpressed at first, Ohain's project continued, resulting in the first operational combat aircraft, the now-famous ME-262.
In Britain, Whittle was chosen to further refine his design, resulting in the Gloster Meteor, which made its combat debut in 1944, a few months after the ME-262's introduction.
Related article–History of the Jet Age: Breaking the Sound Barrier and more
Types of turbojets
There are two main types of turbojets: centrifugal and axial.
Many early turbojets (like early Rolls-Royce engines) were centrifugal designs, similar to a car turbocharger.
The performance of the compressor stage was generally perpendicular to the axis of rotation of the impeller.
The main disadvantage of centrifugal turbojets is that they are too big and wide for their thrust capabilities.
The first jet planes were subsonic, so drag wasn't that important.
However, as airplanes got faster and new methods were developed to squeeze every ounce of performance, minimizing cross-sectional area was crucial, leading to the move to axial-flow turbojets.
Axial flow turbojets compress air parallel to the axis of rotation of the shaft.
Although axial-flow engines are generally longer than their centrifugal counterparts, this allows for smaller cross-sectional areas, which minimizes drag.
Technically, it also allows for more compressor stages, increasing thrust and engine performance.
What is a turbo fan?
Turbofan engines are an evolutionary development of the turbojet.
They still work on the same 3 principles and have the same 3 sections, compression, combustion and turbine.
However, the shaft not only connects to a compressor section, but also to a large fan at the front surrounded by a duct.
Only part of the air drawn into the fan is directed to the compressor stage of the engine, the rest of the air is expelled through the duct, which also acts as a nozzle, and around the main body of the engine.
The main reason for this development is better efficiency. A large fan area increases engine mass flow relative to fuel combustion.
The increase in efficiency depends on whether a turbofan is high-bypass or low-bypass.
A reduction in noise is another reason for using turbofans.
High deviation rate
A high bypass turbofan is generally defined as anything over a 2:1 ratio.
This means that two parts of the air are drawn into the duct part (the "fan" part of the engine), while one part is drawn into the compressor stage of the classic turbojet part.
For an example of a high-bypass turbofan, just take a look at the next plane.
Almost all aircraft in service today use a high-bypass turbofan for reasons of efficiency and noise.
Low Deviation Rate
Low-bypass turbofans have a bypass ratio of 2:1 or less.
Today, many military jet engines use these designs because they combine the benefits of a pure turbojet with the added efficiency of a turbofan.
Since the bypass ratio dictates the fan size, a low bypass ratio means a smaller fan, which reduces the motor diameter.
Turbojato versus Turbofan
While the turbofan is just an extension of the turbojet, there are different reasons for using the different designs.
A large bypass rate is desirable for noise and efficiency reasons.
However, a large fan increases drag and lowers the engine's maximum operating speed by limiting exhaust velocity.
Switching to a low-bypass design minimizes these limitations, resulting in an increase in engine speed and exhaust velocity while retaining the added efficiency of a turbofan.
For this reason, most modern military aircraft use low-drift designs. It's a compromise between performance and efficiency.
For pure speed applications, the turbojet is still unsurpassed.
The fastest aircraft in existence, the SR-71, MiG-25 and Concorde all used standard turbojets.
The lack of drag from using a fan and the resulting smaller cross-sectional area meant it was much easier to optimize aerodynamics for supersonic applications.
However, as with everything, compromises were made.
These legendary planes were limited by poor fuel efficiency and, in the case of the Concorde, noise.
Flying over the Atlantic at Mach 2+ doesn't bother many people, but climbing at full speed over Manhattan has angered people, and the noise of his Olympus 593 turbojets was a big factor in his resignation.
Still, the two speed demons mentioned above, the SR-71 and the MiG-25, used turbojets, and in military applications noise is not a factor.
Both were capable of reaching Mach 3 (although the MiG-25's engine was limited to Mach 2.8) and both were successfully used in reconnaissance roles, capable of flying fast and high.
Turbofans lose thrust with altitude and the air gets thinner, making high altitude the domain of the turbojet.
The turbofan is an evolution of the classic turbojet, and in modern times when efficiency in subsonic flight is desired, the turbofan is the choice of aircraft designers.
Modern military aircraft that require high performance and long flight times use low-bypass turbofans for efficiency, high exhaust velocities, and compact size.
For ultra-fast, ultra-high flights, the turbojet remains the king of the skies because it can compress thin air from the stratosphere better than a turbofan.
The choice between a turbojet, a low-bypass turbofan, and a high-bypass turbofan is an application problem.
Turbofan versus Turbojato
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Learn more about turbofan vs. turbojet, including how they work and the pros and cons of each.
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