Jet Engine Evolution - From Turbojets to Turbofans

In our previous video on jet engines we have  learned that, just like reciprocating piston engines, jet engines do intake, compression,  combustion and exhaust. The difference is that in every cylinder of a piston engine these events  occur one after the other, whereas in a jet engine these events occur all the time, continuously  and they occur simultaneously. In today's video we are going to learn how jet engines have  evolved during the past 50 or more years to become much more efficient and

much more powerful.  Now this type of engine that you can see right now is called a turbojet and by modern standards  this is very much obsolete. But something like this is not, because this is a turbofan or more  specifically a low bypass turbofan, and nowadays you most commonly find something like this in  fighter jets or similar military aircraft and as you can see, even upon first glance, this engine  is very different from our turbojet. Now, the first and most important difference is that i

n a turbojet, all the thrust generated by the engine comes from the exhaust stream or the jet of expanding  gases coming out at the back of the engine In other words, all the air that comes in at the  front ends up inside the core which houses all the key mechanical components. But in a turbofan this is not the case. Not all the air ends up inside 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 a

round the engine? What's the benefit of this? Well, to understand that we must remember  that jet engines are also called reaction engines Essentially they move very large masses of air.  Any movement of any mass creates a force and as we know for every force there is a reaction  force in the opposite direction. So, the engine pushes air out the back and the reaction moves the  engine forward. This reaction force also moves the entire aircraft because the engine is attached  to the aircraft, and

this tells us that to move a greater heavier aircraft or to travel faster  we have to move a greater mass of air within the same time period. And to do that we can either  move more air or we can move the air faster, at a greater rate Now a turbofan exploits the concept  of moving more air and we have two kinds of turbofans A high bypass turbofan and a low bypass  turbofan such as this one. But when a civilian like you or me travels in a commercial aircraft we  are actually propelled through th

e sky by a high bypass 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 We have a giant fan and a gas turbine at the back,  so turbo fan. The turbine harnesses the energy of the combustion and powers 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 f

rom the fan and only around 20% comes from the exhaust jet  at the back of the engine. Modern turbofans on commercial airliners can easily move upwards  of 1,200 kg of air per second Because most of the thrust comes from the fan and not  from the core we do not have to burn ridiculous amounts of fuel to travel through the air. On top  of this modern fans are designed to be extremely efficient at cruising speeds and altitudes of  modern commercial aircraft and they're also really well balanced an

d they offer almost zero  resistance to movement. In fact, the giant fans on modern jet engines of commercial jet engines  they're more than 3 metersin diameter but you can initiate their movement with just one finger. The  added benefit is that the large amount of bypassed air creates a sort of protective thick sheath  around the exhaust jet coming from the core of the engine and this helps to greatly reduce engine  noise. But unfortunately there are limits to moving more air. The first limit

is that we obviously  can't make the fan at the front infinitely large some modern aircraft do have fan diameters which  are greater than their fuselage and it is possible to attach very large turbo fans to aircraft but  there is still a limit to this because at some point we get issues with ground clearance. But  even if issues with ground clearance magically disappeared you still can't make infinitely large  fans because the greater the size of the fan the greater the difference in speed betwe

en 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 turbofans like this one come in.  Their bypass ratio is around 0.5 to1 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 goin

g  through the engine core, 9 kg of air go around it Conversely, for every kilogram of air going through  the core a bypass ratio of 0.5:1 gets only half a kilogram of air around the core. A low bypass  ratio is employed in order to improve the cruising efficiency of the engine and increase the range of a fighter jet. But at the same time the majority of the thrust comes by increasing the exhaust velocity. In other words instead of moving more air a low bypass turbofan moves the air faster. This

means that we are not limited by the size of the fan as with a high bypass turbofan. Here we  have much more thrust on tap. All we have to do is burn more fuel to increase the heat of the exhaust  jet because the hotter the gas the more it expands, the more it expands the greater its velocity, the  greater its velocity the greater the rate at which we move air through the engine, the greater this  rate the more thrust we generate. But unfortunately, here too there are limits and this time the l

imit  is temperature. At some point the gases in the combustion chamber will become so hot that they  will start to melt the turbine blades. In order to prevent this a lot of cooling air is added to the  combusted stream before it gets to the turbines Cooling air is also introduced through the turbine  blades themselves and it envelopes them creating a sort of protective film against the heat. These  large amounts of cooling air mean that jet engines contain large amounts of unburned air. In oth

er  words, they run pretty lean. To prevent turbine meltdown fighter jet engines make use of this fact  in order to generate incredible amounts of thrust and achieve supersonic speeds at the expense of  fuel efficiency. A reheater, more commonly known as an afterburner can be installed behind the  turbine wheels where it can add great amounts of fuel to the hot exhaust stream. The heat of the  exhaust stream is sufficient to combust the fuel and the unburned air in the exhaust stream means  that

we have all the preconditions to achieve near stochiometric air fuel ratios and incredibly high  temperatures of the exhaust stream We don't have to worry about turbine meltdown because the  afterburner is located after, behind the turbine The crazy temperatures achieved by the afterburner lead to dramatic gas expansion and insane exhaust gas velocity which leads to very very  high thrust. Unfortunately due to the very high fuel requirements afterburners can be employed  only for a short period

of time. The other key difference that you can spot between our ancient  turbojet and our modern low bypass turbofan is the increased number of compressor and turbine wheels  in the more modern engine. Our Turbojet only has a single compressor section and a single turbine  wheel to harness the energy of the expanding gases from the combustion chamber. The turbine wheel and  the compressor wheels are all mounted together on the same shaft so an increase in turbine speed  will lead to an equal in

crease in compressor speed Our modern turbofan does things differently.  It contains two shafts running concentrically One shaft runs inside the other. On the first shaft we  have the low pressure turbine wheels which spin the low pressure compressors. And on the second shaft we  have the high-pressure turbine wheel, a single wheel which spins the high pressure compressors. Such a  two shaft arrangement increases efficiency. Why? Because it allows us to run different wheels at  different speeds.

Now each compressor and turbine wheel achieves peak efficiency at a certain speed  or RPM. High pressure and low pressure wheels have different tasks, they off operate in different  environments and therefore they achieve their peak efficiency at different speeds, different  RPM. By having two shafts we can run each wheel set nearest to its peak efficiency RPM. On top of  this, it is always a good idea to have more wheels and wheel sets because this allows us to harness  the energy and increase

the pressure in a more incremental fashion, which in general improves  efficiency, however it also "improves" complexity, engine size and weight. Something else that  you might have noticed is that the shafts, the two shafts and the wheels on them, they spin in  different directions. Now this is not very common but it does exist on engines and when it is done,  again it's an attempt to increase efficiency A turbine or a compressor section consists of rotor  and stator blades. The rotors rotate,

the stators are stationary. The rotor blades sort of bite into  the air with the leading edge and push it back as they rotate, they accelerate the air. The actual  compression of the air and the pressure increase occurs in the stator, whose shape diverges. This  slows down the air. In simple terms, the aerodynamic shape of the stator converts the increased speed  of the air into increased pressure. The problem is that when the airflow leaves the stator it is not  traveling in an ideal direction

, so outlet guide vanes or guide nozzles are employed at the end  of a compressor or turbine section to rectify the air and direct it in a desirable direction  for the next set of compressor or turbine wheels Unfortunately these outlet guide vanes introduce  certain aerodynamic efficiency losses But if the next set of wheels spins in the opposite direction  then the need for these outlet guide vanes is greatly reduced or even completely eliminated. This  reduces aerodynamic losses and it also he

lps make the engine shorter and lighter. But there  is another reason why counter-rotating shafts like these are employed and that is to cancel out  the gyro effects of the engine. As you know if we take a wheel or a similar disc-shaped object and  spin it at very high speed it will tend to resist forces which try to change its position. As we  know, compressor and turbine wheels, they are wheels and they spin at very high speeds in a  jet engine. So the engine as a whole generates a gyroscopic

effect which means that it  makes it harder for the airplane to change its position Well, airplanes actually travel at very  high speeds pretty much all the time and at these high speeds the aero surfaces of the aircraft,  meaning the wing flaps and the rudders, they have a very high mechanical advantage, so they easily  overcome the gyro effect of the spinning wheels inside the engine and pilots really don't feel any  resistance or any issues with controlling the aircraft However, problems aris

e at low speeds,  when these aero surfaces have greatly reduced mechanical advantage, and this is for example why  the Rolls-Royce Pegasus engine employed in the Harrier jump jet, the hovering jet, this engine,  the Pegasus engine employed counter-rotating shafts The two shafts spinning in opposite  direction would cancel the gyro effect of each other leading to no net gyro effect on the  engine and no issues with the low speed hovering and maneuvering of this aircraft. This is also  important f

or example in motorcycles, Ducati is a nice example where they have an engine which  rotates in the opposite direction to the wheels So the engine cancels out some of the gyro effect  of the wheels and this makes it easier to change the direction of the motorcycle at very high  speeds. So that's pretty much it, there you have it The evolution of jet engines through multi  shaft arrangements and bypass ratios. As always, thanks a lot for watching, I'll be seeing you soon  with more fun and useful

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