Thank you for coming back to this course in aerospace engineering and aircraft fundamentals and advanced.
Today we will start at two topic on engine propulsion and specifically we will be talking about the
main properties, the fundamental ideas regarding jet engines.
So the most fundamental question that we have to ask ourselves when we are designing a jet engine is.
What are the key variables?
What are the key ideas that we need to satisfy?
So to produce an amount thrust and so that the aircraft can actually move as a consequence of a specific
engine.
And so, first of all, as we've discussed previously, right, pressure and velocity seem to be the
keys to fluid dynamics.
But also, temperature has a key role.
Okay.
And as we discussed through the Bernoulli's equation, which for you you might remember from previous
classes, pressure and velocity are linked and they are actually inversely linked.
Okay.
And we will see that the main operation of Jet Engine is really linked to pressure and velocity.
However, I just wanted to mention previously that all of the materials, all of the components for
these types of engines must be designed to withstand the incredibly hot temperatures that we have in
the interior of the engine, particularly, of course, in the combustion chamber and also on the turbine.
So how does a jet engine work and how can we explain it in a nutshell?
Right.
So as you can imagine, the aircraft is moving throughout air at a certain velocity.
So relative to the engine, we can also see that the air would be moving towards the engine.
Right.
And because this these air speed of the fluid moving to the inside of the engine, what we see is this
distribution.
Okay.
First the air goes through the compressor, then through the combustion chamber and through the turbine
and the nozzle.
This is the main idea, of course.
Now, how do we actually interact with this air and how do we produce a thrust as a consequence of this
air?
So the first thing we do is to compress the air.
Okay.
So just to make a clear explanation here, like the air intake and other intake, a given velocity,
this velocity is given by the speed at which the aircraft is moving with respect to the air.
And then this air is compressed.
Through the compressor, of course.
Compressing the air.
What does that mean?
It means that the pressure is increased.
So because of the compressed air, we are able to increase the pressure of the air.
Right.
So the pressure over here will be lower than the pressure at this stage.
So when the air goes through the compressor and it's literally compressed, right then this air is mixed
with fuel and this mixture takes place inside the combustion chamber.
What does this imply?
It implies a higher temperature, right?
This mixture basically increases the energy of the flow because it's combines with fuel.
It's not air only now.
Right.
This is a combination of air and also fuel.
And so as a consequence of that, this whole flow of air now becomes more energetic.
And it has a higher temperature.
Okay.
Therefore, the design of the combustion chamber, as you can imagine, really has to take into account
that increase in temperature.
Once this across the flow, which now contains fuel and also air right.
Is then expanded.
Through the turbine expanded means that it is reduced in pressure.
The pressure is decreased.
And as you may remember from the new lease law, if the pressure is increased, then the velocity is
increased.
So as you can see, we are already tricking the flow for it to have higher velocity.
Right.
And at the end, the nozzle, which is a clever geometry.
Right.
Allows us to further increase a bit more the velocity by also expanding this mixture.
Okay.
The question is.
How does this actually work?
Right.
I can see the pressure and velocity.
There's this different compress that allows us to increase the pressure.
Right.
The turbine decreases the pressure.
But why isn't this balanced?
Why is and how does this actually result in an increase of thrust?
So the.
My answer here would be that the combustion chamber, because we are also combining the air with fuel,
we are able to increase the energy of the flow.
So the flow that we had at the beginning, which could be called a cold flow right, becomes a hot flow
at the end.
As a consequence of this mixture with fuel.
And the second part is that this is actually true, right?
There is a balance between the pressure which is increased in the compressor and the pressure which
is decreased and the turbine.
This is not to say that the magnitude of the decrease and increase are the same, but rather the effectively
they act.
Conversely, right, the compressor increases the pressure and the turbine decreases the pressure.
And so the trick of Turbojet engines is actually an interesting one.
Right.
And the idea is that the turbine, when the flow is mixed.
Right.
This flow will actually act on the turbine.
This flow as I'm just going to repeat this okay acts on the turbine and the turbine as a consequence
of that will rotate.
And so you will ask.
Well, right.
But the compressor also rotates.
Yeah, it's true.
The compressor does also rotate, but it rotates because it's connected to the turbine through the shaft.
So.
The turbine is the one it's acted upon, right?
The air acts upon the turbine.
But because the turbine is forced to rotate and it's connected to the compressor, the compressor therefore
rotates too.
And so you can see that it's balanced.
Okay.
The compressor and the turbine will rotate it together following the movement of the shaft.
So this would be the main idea of how a jet engine operates.
Okay.
Now the components as mentioned the inlet, the compressor, the combustion chamber, the turbine and
the nozzle.
And you will find all of these components for any type of jet engine.
Okay.
Now the shaft is the geometry on the inside, which allows which connects the turbine to the compressor,
as mentioned.
And so it can be said that the compressor moves solidary so the turbine and not in the inverse way.
Right.
It's the turbine which causes the movements of the compressor.
So this is the main equation for us to calculate the thrust as a consequence of this.
Okay.
The thrust is generated by a jet engine and is mainly related to the exhaust velocity and the total
mass flow propelled by air plus fuel.
In other words.
The mass propelled per unit of time, which is what this point over here means, right times the velocity,
the effective velocity with respect to the velocity of the aircraft.
In other words, the thrust is just the amount of mixture which is going downstream, which is moving
down.
Okay.
As compared sorry, multiplied by the increase of velocity with respect to the velocity that the aircraft
already had.
Right.
So it's pretty trivial to some extent amount of flow, which means amount of mass amount of particles
multiplied by the velocity of a section as compared or relative to the velocity of the aircraft.
So there are plenty of types of jet engines throughout history that have been developed.
But fundamentally, fundamentally, the ones that you will see today are Turbo Jets.
So both fans and Turboprops, the Turbojet is the one that we already explained.
It's to some extent the most fundamental right.
The TURBOFAN combines an interesting way of it's actually an interesting way of improving the efficiency
of Turbojet.
Basically by putting more air to move not to the inside of the engine itself.
Right.
But rather through the outside of what would be called, you know, it's like the exterior, but inside
of the engine itself.
All of this engine is covered.
But it's two floors.
And by having these two floors, it can be proven that it's actually more efficient.
And this is what you'll see in commercial aircraft, typically.
So budgets are mainly used for high performing aircraft or you will find them also historically.
Okay, but it is not that common to have them.
I mean, it's not common at all to have them commercial aircraft, commercial aircraft people to see
turbofan, which are more fuel efficient.
Right.
And finally, there's the turboprops, which are more or less a category of their own, in the sense
that even if the geometry of the inside of the engine is similar to the turbojet and to the Turbofan,
the idea here is that the thrust is not actually generated as a consequence of the.
The increase in velocity of the flow multiplied by the total amount of flow moving down or moving back.
Right, but rather the capacity of the shaft to actually move a propeller.
Right.
And as a consequence of these movements of the propeller, this there's a lift, right?
This propellers capable of generating a certain lift.
And as a consequence of that, it will start moving and it will push the aircraft back.
Okay.
And and so this is the main idea.
So objects they create this difference in velocity with respect to the velocity of the aircraft multiplied
by the amount of fuel.
And under the mixture, it will it will be able to, you know, to provide the thrust to the engine.
So the Jets I'm sorry, Turbofan are similar, but they also have this exterior amount of air moving
on the surroundings of the interior of the engine, which provides higher fuel efficiency and turboprops.
They operate by providing lift through the propellers.
Okay.
And so I just wanted to put this as a bit of an idea of how to imagine this.
This is climatic of the temperature that you will find on a turbojet.
As you can see, the air which is just surrounding the aircraft at the beginning.
Comes inside the atmosphere, the engine at relatively low temperatures.
Okay.
But then as it is compressed, it will increase somewhat in temperature, but also on the combustion
chamber and through the turbine.
Okay.
We cables see increases on temperature.
And as you can see on the hand, Turbofan, this is a bit different is what I mentioned before.
Right?
That's if this is some of what we commercially see, a lot of the times we see the cover, right.
The the nozzle on the outside, the nozzle.
But what we can see is that this is interior of the engine itself, which looks similar to the turbojet.
But there's these conducts on the outside, right?
This ring on the outside, which is just all of the air moving and which is not it's its energy is not
increased as much as the fuel and air which is going through the inside.
Okay.
And so this is just for you to have a better picture of turbochargers and turbo fans, which will be
the most the engines that we will focus more on this course.
Okay.
So I hope that this is clear and now we can move to the next class.