Jet Engine Evolution - From Turbojets to Turbofans

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Published 2024-02-25
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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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#d4a #jetengine

00:00 Moving More Air or Moving it Faster
02:36 High Bypass vs Low Bypass
07:56 More Shafts More Efficiency
11:14 Gyros and Ducatis
All Comments (21)
  • @aladecombatevirtualcuba4405
    As an aeronautical engineer I affirm that this material is of good quality. super recommended for all audiences.
  • @staygray6212
    As an aviation guy who watches your channel because cars are cool, I love to see you tackling stuff about aircraft. You have an incredible talent for explaining things my dogg.
  • @topquark6242
    I worked for Rolls-Royce for 32 years and have experience on a whole range of engines from Speys (A 'leaky-turbojet' more than a turbofan) to BR-700's. I think you did a marvelous job explaining the basics. Obviously it is far more complicated when it comes to practical applications!
  • @bowieinc
    Every video of yours where I “know” the topic, I come away realizing there’s so much more to learn.
  • @jtocher685
    As you said, turbofan engines of higher and higher bypass ratios have been developed to improve efficiency and reduce noise. To add a bit of engineering and physics to the discussion I offer the following: Noise - A significant source of jet engine noise is the shearing/mixing of high speed flow from the core with the lower speed flow surrounding it. In a turbojet engine, all the high speed core flow is interacting with the low speed flow around the outside of the engine, creating one very strong shear/mixing zone. With a turbofan engine the high speed core flow is mixing with the slower fan flow (still much faster than the overall flow over the engine) and the fan flow is mixing with the slower overall flow creating two less intense shear zones and therefore spreading out the mixing zone resulting in a gentler energy exchange and less noise. Fuel Efficiency - The job of the engine is to produce thrust. When thrust exceeds drag the aircraft accelerates. At cruise, thrust = drag. Thrust comes from increasing the momentum (mass flow rate X change in velocity) of the air flow. Fuel requirements are driven by energy considerations however, not momentum. Jet fuel is a storage medium for energy and is about 43Mj/Kg. Kinetic energy of the flow is (1/2 X mass flow X velocity squared). So, if we want an engine that produces say 1000 units of thrust, we can chose a low air flow rate/high airflow speed change solution (turbojet) or a high airflow rate/low airflow speed change solution (turbofan). But in either case, the fuel flow will be proportional to the kinetic energy change of the flow. So, suppose we want 1,000 units of thrust. We could choose a turbojet engine that processes 250 units of air flow and accelerates it 4 units of speed. Multiplying those parameters for momentum change/thrust we get 250 x 4 = 1,000 units of thrust and for kinetic energy change we get 2,000 units (0.5 X 250 X 4 X 4). Now we choose a turbofan engine that can process 500 units of air flow and accelerate it 2 units of speed. Multiplying again for momentum/thrust we have 500 X 2 = 1,000 units of thrust. However, the kinetic energy change required is now 0.5 X 500 X 2 X 2 = 1,000 units of kinetic energy...half of what the turbojet would require for the same thrust. In practice we can't get all that improvement and the resulting turbofan engine will be more complex, heavier and more expensive, but you can easily see why engineers pursue them!
  • @aphasic6707
    Now i know edactly what i should swap in my Vw Golf
  • @mattj65816
    This guy is amazing. From maybe three videos on turbine engines, he's (re)taught me 90% of the layman's understanding of turbines that it took me a decade to glean from AgentJayZ videos. No knock on AgentJayZ whatsoever, he just goes into way, way, way more depth. It takes more time to process. The high level view presented here condenses the core principles wonderfully.
  • @wonko3fc
    Your ability to explain complex ideas is unsurpassed sir. Better yet, you also add a little humor but not so much it becomes a distraction. You are an inspiration. Never change.
  • @kerrywsmyth
    I used to be a Harrier mechanic. I’m impressed that you know this about our Rolls Royce engine! Additionally, the Harrier uses 4 nozzles with a single engine. The two “cold” nozzles come from the Low pressure compressor and the “hot” nozzles from the high pressure compressor, combustion chamber and all the turbines. You can walk up to a running Harrier and put your hand in front of the LP cold nozzle and the air was only slightly warmer than ambient. But even at idle it is a LOT of volume of air at a pretty high speed. You wouldn’t want to put your face in front of it!
  • @samrobinson2640
    Absolutely fantastic, I work for an aircraft engine manufacturer and this has a far better explanation than any of the courses or literature the company has to offer. Thank you
  • @pnwchris2000
    I’m 24 and have worked on automobiles for about 6.5 years now, restoring my own collection of older trucks. I’ve always figured airplanes and their technology were out of my reach so I never gave them much thought. Until about mid December (‘23) I watched one recommended YT video of a C17 Globemaster III accidentally lane at the wrong airport that happened to be much smaller and it reversed back and still took off to the shock of the crew behind the camera. Ever since, I’ve done so much research of what kind of aircraft the US military and others have around the world and their capabilities and support roles. Now I’m going thru the process of enlisting in the Air Force and by watching this one video I’ve realized that their engines aren’t too complicated and that I can pretty easily comprehend how they function, just like how I learns how automotive engines work. I will forever help my future children in understanding that they can achieve anything in this world and that nothing is out of their reach as long as they have some kind of interest in it. Passion can really be incredible.
  • @joesantamaria5874
    As someone who knows nothing about engineering, it’s still striking that jet engines seem to have far fewer parts than their piston predecessors. No wonder they are so much more reliable. It’s a remarkable difference, and a seemingly much simpler machine. Amazing!
  • @LG-ct8tw
    I just can't get enough of your videos! Looking back at my days in school, wishing I had teacher like you.
  • @retiredtom1654
    In the mid 1960s I was a jet engine mechanic in the Navy. Your video was well done.
  • @phayzyre1052
    I’m a 30+ year aircraft technician, this is a very good video! One guy I knew who worked on aircraft from the 1950s to the 1990s told me those old turbojets were “designed to do two things: eat gas and haul ass!” He added fuel efficiency was just something that was not thought of back then.