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I am trying to understand the importance of having compressed air in the combustion chamber of a gas turbine engine. As far as I understand it, the reasons are twofold.

Firstly, the compressor is necessary for the engine to produce static thrust, as the pressure gradient gives the engine a "preferred" direction of airflow.

Secondly, the compressed air in the combustion chamber results in more molecules of oxygen resulting in more efficient combustion - the same reason why superchargers and turbochargers exist for ICEs.

Is there another reason - potentially thermodynamic - that I am missing? Like how an ICE requires a compression stroke for the power stroke to be able to occur, is there any fundamental reason why the jet engine requires the compressor? Would a jet engine need the compressor (ignoring air density) when the aircraft is at speed?

Thanks.

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    $\begingroup$ Please put your answers in a real answer, not in the comments. $\endgroup$
    – DeltaLima ♦
    Nov 19 at 16:25
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    $\begingroup$ Would en.wikipedia.org/wiki/Ramjet be an answer to your question, or are you looking for an possible engine where no compression occurs at all (regardless if powered by a shaft or air inertia)? $\endgroup$
    – Martin
    Nov 19 at 16:39
  • $\begingroup$ The ramjet has no compressor, only ram air is used, see Is the compressor required on jet engines? Can air be rammed into the turbine?. This is possible only after Mach 2 or 3, at this speed the required quantity of air can be rammed, mixed with fuel at the correct air/fuel ratio, and can deliver the required energy/thrust. So yes the first reason of the compressor is to feed the combustion chamber. $\endgroup$
    – mins
    Nov 19 at 17:08
  • $\begingroup$ @Martin , yeah I should have clarified, excluding ramjets. The idea essentially was why a continuous pulsejet (a contradiction, I know) doesn't exist. $\endgroup$
    – bchen
    Nov 19 at 17:52
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    $\begingroup$ @bchen the m in mv. Water molecules are 18, N2 is 28, O2 is 32. But adding liquid water to make steam increases volume greatly. It has been used to boost take-off thrust. $\endgroup$
    – Robert DiGiovanni
    Nov 23 at 22:26

6 Answers 6

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The reason is thermodynamics: without the compressor, the power output of a gas turbine would be exactly zero. It's not about being "more efficient" but about fundamentally being able to produce any power at all.

All heat engines work on the same principle: raise the pressure of a working fluid (gas) to a higher pressure and, and then lower the pressure again. In itself that's a futile exercise, unless you can somehow get more work out of lowering the pressure than you put in raising the pressure. That's why we do the combustion in the middle: you keep the pressure the same* but you just get "more" gas volume (it expands because it's hot) so you can get more work out of it than you put in initially.

If you didn't raise the pressure initially, you'd just have more, hot gas at ambient pressure, but it'd be useless unless you had a hot air balloon to fill.

*A combustor in a gas turbine cycle (Brayton cycle) is necessarily approximately isobaric (constant pressure) because it is open-ended on both sides. Other types of engines have different conditions during combustion; e.g., an ideal Otto cycle (petrol engine) is isochoric (constant volume) during combustion, resulting in an increase in pressure. This means that in theory, an Otto cycle doesn't need the compression stroke, leading to a degraded efficiency Lenoir cycle of a pulsejet.

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    $\begingroup$ Thanks for the answer. I'm still left wondering why raising the pressure is a necessary preliminary step. Is the pressure gradient analogous to the hot air balloon, providing a framework in which the expansion of the gas does useful work? Again, in an ICE it's easy to pass compression off as a requirement for efficient combustion, but it is clear that complete combustion is not the only effect at play here. $\endgroup$
    – bchen
    Nov 19 at 22:16
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    $\begingroup$ You are right: thermodynamically you need a "raise the pressure" step but it doesn't have to be a compressor. Like how a rocket uses the energy of burning fuel directly to raise the pressure and propel gas out the back. But a rocket has a closed front whereas a gas turbine needs to draw new air in to combust. And since a combustor with two open ends is largely isobaric you need the compressor. $\endgroup$
    – Sanchises
    Nov 20 at 5:29
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    $\begingroup$ An interesting case for comparison, intermediate between a turbine and a rocket is the ramjet in which the compressor is essentially just a nozzle, using the forward motion of the aircraft, missile etc. to compress the incoming air, and thus unable to run without forwad movement $\endgroup$
    – Chris H
    Nov 20 at 16:13
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    $\begingroup$ I don't understand your answer at all. The energy - the pressure raise - comes from burning fuel. An ICE does not need a compressor, why does a turbine engine? $\endgroup$
    – Violet Giraffe
    Nov 21 at 17:08
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    $\begingroup$ @VioletGiraffe Well, in order for a jet engine to work, air has to flow into the front of it. If you use combustion to create a high-pressure region, and you don't have a compressor, then air will stop flowing into that region (unless you're going fast enough for the dynamic pressure to overcome that static pressure), and your engine will stop working. $\endgroup$
    – Tanner Swett
    Nov 22 at 18:03
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You don't. A (sc)ramjet doesn't have a compressor, as the forward velocity of the jet provides the necessary pressure.

Most thermal engines do work by expanding a gas. In many cases, this gas expands against a piston, and the piston converts the heat energy into mechanical work. A turbine does not contain a piston to expand against, and the walls of the turbine offer neither movement as required by a piston, nor thrust even if they could move. Thus, the combusting fuel must expand out the front and the back of the engine. Obviously, expanding out the front is undesirable, as that provides negative thrust. Nor is it feasible to place a piston there for the gas to expand against. The solution is to create a virtual piston out of air, by squeezing incoming air so that when the combusting fuel expands, its only choice is to expand out the rear of the engine. The fact that this also improves oxidizer flow is a virtuous benefit.

When the fuel combusts, it increases the pressure at the site of combustion. This pressure wave expands spherically, because it doesn't "know" which direction it is "supposed" to go. The walls of the turbine confine the pressure wave in the radial direction, so all of the pressure must travel axially, both forwards and backwards. If the pressure in the front of the turbine is lower than the pressure created by combustion, then some of the exhaust will flow forwards through the engine, which I think we can all agree is not the desired behavior. The only way to prevent this forward flow is to ensure that the pressure in front of the combustion chamber is at least as high as the pressure created by combustion itself. And that establishes the minimum pressure you must generate just to make the combustion products flow strictly out the back of the engine.

So yes, the compressor is essential to guarantee that the exhaust only goes out the back of the engine, and not the front. And yes, the compressor helps improve oxidation of the fuel, just like a super/turbocharger in a piston engine (and why the original jet engines were called "turbojets"). And yes, forward movement of the engine is sufficient to provide compression, provided adequate airspeed.

Although a ramjet can theoretically operate at speeds as low as Mach 0.5, they are typically considered optimal between Mach 3 and Mach 6.

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    $\begingroup$ The compressor of a ramjet is the combination of speed and intake - remove one or both and it ceases to produce thrust. The compression step is necessary for all heat engines. Naming the part that covers this step "compressor" in only one type of them does not mean that all the others can live without the compression step. $\endgroup$
    – Peter Kämpf
    Nov 22 at 4:12
  • $\begingroup$ I'm confused. The first sentence of this answer is "You don't [need a compressor]." Then three paragraphs later it says, "So yes, the compressor is essential..." Which is it? $\endgroup$
    – Todd Wilcox
    Nov 22 at 17:40
  • $\begingroup$ @ToddWilcox Is it not clear that only ramjets can function without a physical compressor? $\endgroup$
    – Lawnmower Man
    Nov 22 at 20:34
  • $\begingroup$ It is not clear in the answer, no. Also, it's not clear what you're saying is the etymology of "turbojet" is. My understanding is that the "turbo" prefix refers to the use of a turbine. So a turbocharger is a turbine driven supercharger (forced induction device/air compressor). A turbojet engine is a jet with a turbine driven compression stage. Etc. $\endgroup$
    – Todd Wilcox
    Nov 22 at 21:14
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The compressor raises the pressure and temperature at which the fuel is burned, thereby increasing the thermodynamic efficiency of the cycle (Brayton) on which the jet operates.

Second, without the compressor stage in place, the combustion gases would be free to blow out the front of the engine as well as the exhaust, thereby generating no thrust.

The majority of the power output of the engine is used to operate the compressor. The remainder is used for propulsion.

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  • $\begingroup$ Thanks Niels, this answers the original question. It is curious that the majority of the power output operates the compressor. I assume that the power comes "extracting" the kinetic energy of the jet. Are there any measures in place to ensure that the momentum of the exhaust jet is relatively unaffected? That is to say - is the turbine designed such that the thrust produced is similar? $\endgroup$
    – bchen
    Nov 19 at 17:46
  • $\begingroup$ The turbine is designed to extract just enough energy from the exhaust to spin the compressor, or the compressor plus bypass fan. This sets the number of turbine discs, their diameter, blade pitch, and number of blades per disc. The remaining thrust is set by the mass flow rate times the pressure drop across the turbine discs. $\endgroup$
    – niels nielsen
    Nov 19 at 20:38
  • $\begingroup$ I am intrigued by the last sentence: I would have thought thrust would have related to the pressure drop between the free stream and the exhaust jet, as T = mdot * v? $\endgroup$
    – bchen
    Nov 19 at 22:18
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    $\begingroup$ Whilst it is true that the compressor consumes a lot of power, I don't think it is correct to contrast it with the power for propulsion. The compressor is a major contributor to the propulsive force, and thus to the propulsive power. $\endgroup$
    – DeltaLima ♦
    Nov 19 at 22:35
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    $\begingroup$ I thought the majority of a turbine engine's power is used to drive a propeller or a bypass fan? $\endgroup$
    – Nayuki
    Nov 20 at 0:38
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In order for a jet to create thrust, pressure must be created so the difference in pressure from the inside of the engine to the outside air will create a flow of a certain mass at a certain velocity.

This mv, or momentum, will drive the engine (and aircraft) in the opposite direction. We've all done this with a balloon, blowing it up and letting it go.

a "jet engine" is only one way of doing this

The aeolipile used heat to raise steam pressure. Rockets pump liquid oxygen and fuel into their combustion chambers, creating pressure by both phase change and temperature increase.

the name of the game is constant mass flow

As Lawnmower Man eloquently put it, we are creating a virtual piston, and the compressor is a virtual intake valve, continuously feeding air into the combustion chamber as heated air is exhausted.

because the incoming air is heated, the exhaust port will be larger than the inlet.

A bigger hole in back than in front. Going back to rockets, they must carry their own oxidizer. Breathing air with a compressor saves carrying all that extra weight.

Out of that bigger hole in the back comes thrust. The heat energy added into the system is also converted into mechanical energy to drive the turbine (which drives the compressor).

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  • $\begingroup$ Interesting observation that the compressor is a virtual intake valve. To take the analogy further, I assume that the combustion chamber acts as the compression stroke, with some blurring of the lines between the power and exhaust strokes shared between the turbine and exhaust nozzle? $\endgroup$
    – bchen
    Nov 20 at 11:01
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    $\begingroup$ @bchen exactly. The jet is continuous. $\endgroup$
    – Robert DiGiovanni
    Nov 20 at 12:08
  • $\begingroup$ Breathing air with a compressor saves carrying all that extra weight. - And even more importantly, the atmosphere gives you something to push on that you weren't carrying with you. Momentum and thus thrust scale linearly with exhaust velocity, but kinetic energy scales with v^2. So it's much more efficient to push a lot of air through an engine so the same amount of energy can create a lot more momentum from lower exhaust speeds. (Turbofans take this a step farther and take energy from the jet to move more air with a big fan.) $\endgroup$
    – Peter Cordes
    Nov 22 at 4:32
  • $\begingroup$ Energy density of jet fuel vs. jet fuel + LO2 is what, maybe a factor of 2 or 3, that kind of ballpark? But air-breathing turbojet or turbofan vs. pure rocket is what, something like a 20x difference in impulse (newton-seconds, which would be delta-v if there wasn't drag) per amount of fuel? sciencedirect.com/topics/earth-and-planetary-sciences/… "the specific impulse of an air breather can be an order of magnitude greater than rockets at the lower Mach numbers" (not hypersonic) $\endgroup$
    – Peter Cordes
    Nov 22 at 4:37
  • $\begingroup$ @PeterCordes actually not too surprised about the specific impulse comments. The GE-90 - an introduction and some nasa data states that the mass flow through the ge90 is "only" half that of the F1 engines on the saturn v first stage. Consider then that the rocket carries its own oxidiser and the v high jet velocities and it is understandable that rocket efficiency is so low in low and slow conditions. $\endgroup$
    – bchen
    Nov 22 at 23:23
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Would a jet engine need the compressor?

All heat engines require a compression step, only in turbomachinery the part involved is called compressor. But other engines also compress a medium as part of the cycle:

  • In steam engines it is the feed-water pump that must overcome the boiler pressure to add more water to the boiler. By doing the compression work on a liquid medium, it does so very efficiently. You could call it also a compressor - the task is quite similar.
  • In piston engines it is the cylinder-piston combination during the upstroke that compresses the fuel-air mixture before it is burnt.
  • Liquid-fuel rockets use turbopumps to feed fuel and oxydizer into the rocket's combustion chamber. Given the mass flow of rockets, their energy density is impressive. They could as well be called compressors.
  • Even pulsejets use a compression step similar to a piston engine, only here the piston is replaced by an oscillating pressure wave.
  • and, lastly, ramjets use the combination of intake flow speed and intake geometry to compress their working medium. Remove one of them and they cease to work.

That only jet engines give the compressor its proper name is no reason to believe that its contribution is not essential to the work of heat engines in general. The why is excellently explained by both Sanchises and sophit, so I do not repeat it here again.

What leaves to be said: The compression ratio determines engine efficiency and mass-specific thrust. The Jumo 004B had a compression ratio of just 3.2:1 and had twice the thrust-specific fuel consumption of the equally sized EJ200 (without afterburner) with a compression ratio of 26:1, but the modern engine has seven times more thrust at approximately the same mass as the old Jumo engine. Higher compression is also the reason for the better fuel efficiency of diesel engines.

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I think that both a practical and a thermodynamic answer can be given.

As far as I understand it, the reasons are twofold.

Correct

From a practical point of view, compressing the air before the combustion chamber allows:

  • the air to reach a temperature high enough to make the mixture air-gasoline self ignite; this is similar to any diesel engine that indeed does not need any spark plug;
  • the airflow to move from the compressor towards the turbine; the combustion chamber is opened at both ends and therefore the combustion products would naturally come out from both ends; having on one side of the chamber (the one towards the compressor) a much higher pressure than the other one (toward the turbine), forces the combustion products to flow towards the turbine; in a piston engine this job is done by the cylinder head.

Is there another reason - potentially thermodynamic - that I am missing?

From a thermodynamic point of view, the compression stage as well as the turbine stage are the two steps needed to extract work from the cycle:

Bryton cycle

In this picture (Wikipedia), the points:

  • 1 to 2 represent the compressor stage; the compressor raises pressure and temperature and diminishes volume;
  • 2 to 3 is the combustion chamber; being the combustion chamber open, pressure remains more or less constant and only volume and temperature of the combustion products increase;
  • 3 to 4 is the turbine; it reduces pressure and temperature and the combustion products expand;
  • 4 to 1 is the closing part of the cycle representing the fact that the exhaust gases return to their initial state.

The p-V diagram is quite useful since the area enclosed in the loop represents the work extracted from the cycle.

Now, what would happen if the compressor stage weren't there? Line 2-3 and 4-1 would overlap bringing the inside area to zero: no net work could be extracted from the engine which would simply behave like a furnace.

why can't the increase of pressure represented by 1-2 occur from combustion?

It definitely can occur in the combustion chamber and this is actually what happens in any Otto or Diesel engine: the combustion takes place in a fixed volume and therefore increases the pressure that pushes the piston down. But the combustion chamber of a jet engine cannot be confined, it must be open in order to continuously ingest fresh air from one end and expel the combustion products from the other end. The combustion happen at constant pressure (since the combustion chamber is open) but with increasing volume.

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    $\begingroup$ Really appreciated answer,and the PV diagram is very helpful in the understanding. Point of confusion which stems from the original question: why can't the increase of pressure represented by 1-2 occur from combustion? Since the majority of the work done by the engine originates from the fuel's chemical energy, why is the external pressure increase required? Thanks! $\endgroup$
    – bchen
    Nov 20 at 19:57
  • $\begingroup$ @bchen: simply because of all the practical reasons I've just listed. For the combustion to happen you need 1) a lot of air 2) at high temperature and 3) flowing from the inlet to the outlet of the engine. Compressing it is the way to get all of these 3 points. Plus (thermodynamic requirement) if the pressure at 2 and 3 would be like 1 then the turbine couldn't work either. $\endgroup$
    – sophit
    Nov 20 at 21:00
  • $\begingroup$ @bchen: is it a bit clearer now? $\endgroup$
    – sophit
    Nov 21 at 11:42
  • $\begingroup$ yes - it did take me a minute. I understand it as the thermodynamic pressure requirement is a "byproduct" of the practical considerations for the combustion to occur in the right way. Thank you very much! $\endgroup$
    – bchen
    Nov 21 at 18:33
  • $\begingroup$ @bchen: For me, understanding dawned when I considered a counterexample: extract work from combustion without compressing the input air before burning fuel mixed with it. The combustion itself can raise the pressure if you confine it (like a rocket nozzle), which avoids the problem of an empty area on the P-v diagram. But we wanted to design a continuous-combustion engine, not a pulse-jet, so we need to feed new air into the combustion chamber. Which has to be at higher pressure than the free stream to generate thrust, so air won't move into it without some kind of pump, aka compressor. $\endgroup$
    – Peter Cordes
    Nov 22 at 5:19

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