This has really got me confused lately. I’ve read online that it increases efficiency, the potential energy of the air and helps with combustion, but my question is:
If the same amount of air is passing through the engine regardless, why pressurise it? Wouldn’t the gain in PE of the air just be equal to the loss of KE of the aircraft due to the air hitting/dragging the aircraft back as it’s compressed? I’m almost certain I’m wrong here, but any explanation which could clear this up for me would be great thanks.
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Air is thinner the higher you go, planes fly at different heights but want consistent supply
They only good part of the air is the oxygen in it, which is a relatively small proportion but is what aids with combustion. The more pressurised the air, the more oxygen is concentrated so the more it can contribute to the combustion.
The entire premise of a jet engine is that is has to eject mass out the back (due to Newton’s third law). Most of that with modern turbofans is just moving the air through it (it’s literally a giant freaking fan). But to spin that turbine, you need to move sufficient mass of exhaust gases (from air and fuel) out of the combustion chamber and through a smaller turbine. Since air and fuel need to be at a correct ratio to burn, you gotta squeeze a bunch of air into the combustion chamber to burn the required amount of fuel.
There is a relationship between the amount of air to the amount of fuel that can burn using the oxygen it brings. Squeeze more air in and you can burn more fuel, increasing the amount of energy from the reaction you liberate per second.
In a piston engine the work done is through expansion of the gasses in the cylinders. In a turbine engine the work is ALSO done through gas expansion, but its captured by turbine blades in the hot combustion part of the engine – those spin with the hot explody gas and drive the compressor.
Note that in a piston cylinder there is still compression going on – the piston itself does that during its cycle and just before the spark plug ignites it. This allows a small amount of fuel and a BIG amount of air to combine in just the right ratio to get the most bang for the buck.
So you still need some compression. If you didn’t compress your air at all you’d get a very weak little _pfumpf_ instead of a bang (or a continuous roar in a turbine).. unless you dumped a LOT more fuel in there – which would make the engine very fuel inefficient.
So if you throw some compression turbines on there – driven by the hot exhaust turbines, you can obtain the same air to fuel ratios that make the biggest bang for the buck.
Also there’s the problem that without an incoming cold air stream that is of equal or higher air pressure to the hot explody exhaust air, the explody exhaust would want to go back out the wrong way or hangout in the combustion chamber if the airflow dropped below a certain speed. So having a compressor on there keeps the airflow going the right way – doesn’t entirely solve the problem, you can still get an engine fire under some circumstances, but it minimizes the occurrence. In a pinch you can restart an engine through using bleed air or even a motor or something to spin the compressor up to speed and getting that airflow going again. If you didn’t have the compressor up there you’d have to dive and get the air flowing thru the engine.
Think about what you are trying to do, move air molecules from the front of the engine to the back. That is the premise, in order to do that, you want to spin a huge fan at the front which pushes air around bypass ducts, you do this in about an 8-1 ratio. For every cubic meter of air that goes through the combustion chamber, 8 goes through the ducts.
The compression stages get a bunch of air molecules close together, when they combust they flow over a turbine that turns the big fan in the front that we need to power to move the plane. The more compression you can get, the less fuel you need to use to get the same amount of power. It is like compressing air using a turbocharger, you do use some more fuel but the power you get from getting more air into it more than makes up for the fuel use. In essence, the more oxygen you can pack into that thing, the more power you get. The more power to the turbine means you can swallow more air to push around the bypass ducts. The more air you push around the bypass ducts, the more powerful the engine.
All heat engines (so turbines, piston engines, rockets, etc.) are governed by a rule called Carnot’s Law, which basically states that the maximum efficiency of any heat engine cycle is determined by;
eta = 1 – (Tc/Th)
where Tc and Th are the temperatures of the cold and hot side of the reaction. The cold side is the ambient environment, and the hot side is essentially a stand-in for the pressure and temperature of the chamber where combustion is happening. Thus, raising the pressure of the combustion chamber *before* combustion actually starts will increase your overall efficiency, because it’ll push the combustion temperatures higher as well.
As for why Carnot’s Law holds; it’s a consequence of entropy and the 2nd Law of Thermodynamics. Heat always seeks to flow from high energy to low energy (i.e. from hot to cold). The “forcefulness” of this flow (for lack of a better word) is tied directly to the difference in energy between the hot and cold sides of the reaction, thus the bigger the difference is, the more energy you can extract as work.
You HAVE TO compress it, it’s fundamentally how jet engines work. No compression, no jet engine.
It’s been a while since I’ve done this, but I’ll do my best to explain it. Jet engines (and power plants) work by abusing physics. As it turns out, air has different properties at different pressures. So by pressurizing the air, we change the properties as it burns. This allows us to get way more energy out than it takes to run the compressor. If you didn’t compress it, you’d only be able to only get the amount of energy it takes to run the compressor out (in theory, in reality it’s way less).
Look into thermodynamic cycles, specifically the Brayton cycle (what jet engines use). If you’d like, I can look this up in my textbook and give a more detailed answer if you’d like.
Because we add fuel to it at that stage, and the engine is created to have a lower pressure after the burners so the air expands in that direction gaining a lot of speed, driving a few turbines to help the engine keep going, then being ejected at high velocity out of the engine.
In the turbine engine first we have a fan, collecting air (it all sits on a single shaft, just to make it easy.
Then we have compressor stages, slowing the air down. But now we have more air in the same volume being injected into the burners where its mixed with fuel and ignited.
The air heats up so it wants to expand, or rather that is what hot air does, the molecules have a higher velocity. So they go to where the low pressure is which is after the burners where they bump into some fans driving the rest of the engine.
(the compressor stage may consist of up to 9 turbo fans, the driving stage usually just 3-4 powering the fan and the compressor stages) This is because the air is moving so much faster after the fuel has been burnt and everything is hot.
So we need the compressor stage to burn enough fuel to make all that happen.
The simplest way to explain it is that by compressing the air, adding fuel to it, and igniting the mixture makes it expand a great deal more when released. This provides more thrust from the engine than using uncompressed air. This does two things. Primarily, it provides the thrust that moves the plane forward. But it also passes through another set of turbines that then power the compressor stages of the engine. Thus making the system self sustaining. Once it gets started, the only way to stop it is to stop providing fuel or air. (or catastrophic destruction)
In regards to your question about the PE and KE causing drag: The compressor stage is the front third to half of the length of the engine. It’s a dozen or two alternating rows of turbines and such that pull air from the front of the engine and compress it a very great deal before it enters the combustion chamber. Jet engines have an enormous amount of suction and you’ll never want to be within 50-100 ft of the intake of one that’s running. So in essence you could say that the engine is partly using thrust to propel the plane forward while also using suction to aid the forward momentum.
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