Jet Engine Evolution - From Turbojets to Turbofans | EngineDIY

 In our last video on jet engines, we have learned that just like piston engines jet engines do intake, compression, combustion, and exhaust but the big difference is that in piston engines these events occur one after the other in every cylinder whereas in a jet engine, these events occur continuously, all the time and they occur simultaneously with each other.

In this video, we will explore how jet engines have evolved to become much more powerful and much more efficient. Now this engine is called a turbojet and by modern standards, this is very much obsolete. This right here is a turbofan, or more specifically a low by-pass turbofan, and an engine like this is nowadays most commonly found on fighter jets and other military aircraft. As you can see, even upon first glance, the engine is pretty different from our turbojet.

Now the first, and most important difference is that in a turbojet, all the thrust generated by the engine comes from the exhaust stream, or the jet of expanding gasses coming out of the back. In other words, all the air that comes through the front of the engine ends up inside the core which houses all the key mechanical components of the engine.

But in a turbofan, this is not the case. Not all the air ends up in the core, some of the air is bypassed around the core and never contacts the internal parts of the engine. So why would we bypass some of the air around the engine?

Well to understand that we must remember that jet engines are also called reaction engines. Essentially they move incredible masses of air. This movement creates a force. And as we know for every force there is a reaction force in the opposite direction. This reaction force moves the engine and because the engine is attached to the aircraft the entire aircraft moves.

This tells us that to travel faster and/or to move a larger heavier aircraft we must move greater masses of air. To move a greater mass of air we can either move more air or we can move the air faster.

A turbofan engine exploits the first concept and that is to move more air. Now we have two kinds of turbofans, a high bypass and a low by-pass turbofan. When a civilian like you or me flies in a commercial aircraft we are propelled through the sky by a high by-pass turbofan. A high bypass turbofan takes the concept of moving more air to the extreme because at the very front of the engine, we will find a giant fan. This is where the name comes from, turbofan. We have a giant fan and gas turbines at the back which harness the energy of the combustion and thus power the fan. Now because the fan is so large it is capable of moving absolutely incredible amounts of air and about 80% of the thrust of the engine actually comes from the fan and only around 20% comes from the exhaust jet coming out of the back of the engine.

Because most of the thrust comes from the fan it means that we don’t have to burn ridiculous amounts of fuel to move the aircraft. Modern fans are designed to be extremely efficient at cruising speeds and altitudes of commercial aircraft. The added benefit of the is that the bypassed air creates a sheath of air around the exhaust jet and this greatly reduces the noise pollution created by modern commercial aircraft.

But unfortunately moving more air has its limits. You can’t make infinitely large fans because the greater the size of the fan the greater the difference in speed between the blade root and the blade tip, because the tip covers a much greater distance than the root. In other words, an overly large fan will inevitably achieve supersonic speeds at the blade tips and this leads to inadequate and inefficient operation.

This is where low bypass turbofan engines like this one come in. Their bypass ratio is around 0.5 to 1 compared to the bypass ratio of commercial turbofans which is usually 9:1 and above. A bypass ratio of 9:1 tells us that for every kilogram of air going through the engine core 9 kilograms of air go around it. 

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