Why do Jet Engines Have a Compressor?

[Saladsamurai]

So like it says. It may be a silly question, but I am doing some reading on jet engines (on Wiki) and I am having trouble finding an explanation as to why we want to increase the pressure of the air before it goes into combustion. Does this just another way of increasing the thrust?

Thanks,
Casey

 

[jaap de vries]

That is actually a very good question. In the following link an attempt is made in answering this very basic question.

http://www.aerospaceweb.org/question/propulsion/q0033.shtml

The main reason given in the link above is quote: "If uncompressed, the air-fuel mixture won't burn and the engine can't generate any thrust". As a combustion engineer I don't like this explanation (the German V1 rocket did not have a compressor I believe). I see no reason why a fuel air mixture uncompressed would not burn.
The whole idea behind a jet engine is that you accelerate the fluid going through it with as driving force a pressure gradient. The pressure in a jet engine is the highest right after the last stage of compression (for axial compressors). The combustion then takes place with minimum pressure loss and the the expansion begins through the turbines and finally through the nozzle . The high pressure created by the compressor forces the fluid to go out through the back. In other words, it creates the favorable pressure gradient used to accelerate the flow and produce thrust. By drawing out a Brayton cycle you can also show the effect this has on the efficiency.

 

[Danger]

Good post. The V1, though, used a valved pulse-jet. Compression wasn't needed because the valves closed off the front of the combustion chamber upon firing. That made it essentially a rocket.

 

[Phrak]

For the same reason you get better power out of an internal combustion engine with a higher compression ratio. This is really a question in thermodynamics.

Danger, the buzz bomb used a tuned exhaust. After the exhaust had pulsed out the back, the low pressure returned a portion of it to toward the combustion chamber. This compressed the air fuel mixture that had entered during the low pressure phase.

 

[Averagesupernova]

I would say the main reason for the compressor is power density in any engine.

 

[Danger]

Danger, the buzz bomb used a tuned exhaust. After the exhaust had pulsed out the back, the low pressure returned a portion of it to toward the combustion chamber. This compressed the air fuel mixture that had entered during the low pressure phase.

From everything that I read when researching pulse-jets (which, admittedly, was more than 35 years ago), the original exhausts weren't tuned at all. Nobody expected the 'vacuum' intake effect. Something like 80% of the fresh intake charge got sucked in through the tailpipe, but that wasn't part of the design plan.
Subsequent designs capitalized upon that feature, and eventually the valve box was eliminated when people figured out how to properly tune the system so the pulses effectively closed the front of the chamber through pressurization.
The reason that the V1 was called the 'buzz bomb' was because of the noise made by the intake valves flapping.

 

[turbo]

Casey, a compressor on a jet engine prevents back-flow and it acts like a supercharger on a car's IC engine, delivering an increased load of oxygen so that more fuel can be burned more efficiently.

 

[Astronuc]

So like it says. It may be a silly question, but I am doing some reading on jet engines (on Wiki) and I am having trouble finding an explanation as to why we want to increase the pressure of the air before it goes into combustion. Does this just another way of increasing the thrust?

Thanks,
Casey

Ramjets don't require a dynamic (turbo) compressor, and instead use the forward motion of the engine to compress the air in the inlet.

Thrust comes from the mass flowrate and velocity.

The compressor increases the pressure of the atmosphere and momentum of the air flow. The air (oxidizer) combines with the fuel, which burns and provides energy to the flow. The combusion does two things - the combutsion process breaks fuel molecules to make more molecules and provides heat which increases the temperature which decreases the density which causes an increase in velocity of the flow.

The compressor 'pushes' the flow into the combusion chamber, and the flow passes out the exhaust with a much high velocity than it entered - and that combined with the mass increase from the fuel - causes the thrust.

Think of the continuity (mass flow) equation. Mass flow in = mass flow out, otherwise the mass in the control volume (jet engine) would increase or decrease.

The momentum equation: momentum out > momentum in, and the change in momentum gives the thrust (force).

Energy (in flow) equation: Work-Energy out = work in + energy (thermal) produced

 

[Andre]

In addition to all those excellent answers, there is also the air density problem. The higher the flight, the less dense the atmosphere, the less drag and the least fuel to be used. But at those densities there is not a lot of oxygen to burn and we really need high compression rates to sustain a nice little flame in that engine.

 

[jaap de vries]

Good post. The V1, though, used a valved pulse-jet. Compression wasn't needed because the valves closed off the front of the combustion chamber upon firing. That made it essentially a rocket.

Fair enough, my point was that the argument that uncompressed fuel air does not burn makes no sense to me.

 

[FredGarvin]

Thrust comes from the mass flowrate and velocity.

DING DING DING! We have a winner. Well said Astro. Concise and right on.

To initiate flow, there has to be a favorable pressure graidient in the engine. The compressor's job isn't to make sure it keeps flow moving in a certian direction, although most of the time it does that as a by product of increasing the density (although in surge it definitely stops doing that). Also, as Jaap pointed out, the idea that an air-fuel mixture wouldn't burn uncompressed is just silly.

The whole point is that thrust is directly proportional to the mass flow through the engine. The more mass flow, the more thrust.

 

[Phrak]

From everything that I read when researching pulse-jets (which, admittedly, was more than 35 years ago), the original exhausts weren't tuned at all. Nobody expected the 'vacuum' intake effect. Something like 80% of the fresh intake charge got sucked in through the tailpipe, but that wasn't part of the design plan.
Subsequent designs capitalized upon that feature, and eventually the valve box was eliminated when people figured out how to properly tune the system so the pulses effectively closed the front of the chamber through pressurization.
The reason that the V1 was called the 'buzz bomb' was because of the noise made by the intake valves flapping.

Interesting. I wonder if it would have worked at all without the accidental compression. I have part of a pulse jest--minus the tail pipe. It's amazingly simple: a set of reeds, a block of aluminum with holes for the intake, a fuel venturi, a starter spark plug and combustion chamber/exhaust tube.

 

[Danger]

I have part of a pulse jest--minus the tail pipe.

Neat! Original, or a reproduction?

 

[Phrak]

Like I said, this is question in thermodynamics.

In a jet turbine engine, a 4 cycle gasoline engine, diesel, or coal dust turbine an element of combustable fluid goes through a process of compression, ignition and expansion to obtain work.

I could find a believable answer that gave power efficiency as a function of pressures. What's the power efficiency ratio of a compressed vs. uncompressed working fluid? Is it 4 to 1, 5 to 1, twenty to 1?

 

[FredGarvin]

From a thermo point of view, the cycle efficiency is directly tied to the compressor's pressure ratio. The propulsive efficiency drops off as pressure ratio increases but the thermal efficiency increases with pressure ratio. This makes the overall efficiency increase with pressure ratio. If you look through some literature like Hill and Peterson, you can see plots of this very topic that illustrate this point.

 

[Borek]

my point was that the argument that uncompressed fuel air does not burn makes no sense to me.

As Andre wrote - when density of the mixture is too low, there is not enough energy produced by burning fuel to sustain the fire.

Simple thought experiment - we have a mixtures of oxygen and fuel of varying densities. On the one end we have mixture that we know will burn, on the other end we have vacuum (wih only traces of oxygen and fuel) - same mixture, but it will not burn. Somewhere in the middle must be the point where we move from "burning" to "not burning".

 

[Phrak]

From a thermo point of view, the cycle efficiency is directly tied to the compressor's pressure ratio. The propulsive efficiency drops off as pressure ratio increases but the thermal efficiency increases with pressure ratio. This makes the overall efficiency increase with pressure ratio. If you look through some literature like Hill and Peterson, you can see plots of this very topic that illustrate this point.

What is propulsion efficiency?

The efficiency of an Otto cycle, eta is given as

\eta = 1 - \left( \frac{V_{top}}{V_{bottom}} \right) ^{0.27}

as given in this link, http://courses.washington.edu/me341/oct22v2.htm that I can't generate a hyperlink for.

A volumetric compression ratio of 12 spits out 49% efficiency.

A ratio of one gives zero, which makes no sense, or course.

 

[FredGarvin]

For a basic definition, the propulsive efficiency is

\eta_p=\frac{T*u}{m\left[(1+f)(u_e^2/2)-u^2 /2\right]}

Where:

T = thrust
u= vehicle velocity
ue = exit velocity
f = fuel air ratio

It is also considered to be

\eta_p = \frac{\eta_{overall}}{\eta_{thermal}}

Note: the term T*u is often referred to as the thrust power.

Here's another reference on this with a reduced form:
http://en.wikipedia.org/wiki/Propulsive_efficiency
http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node81.html

 

[Saladsamurai]

Hmmm.. haven't logged in in awhile. Had no idea this thread was generating so many replies! Great stuff guys! This is all great. I am taking a thermodynamics class and so far we are not too far into the text. So the problems we have had have merely told us that there is a compressor before the combustion chamber, but not WHY.

Thanks!

 

[Phrak]

A ratio of one gives zero, which makes no sense, or course.

Of course it makes sense, Prank. If the volume doesn't change it does no work.

 

[Phrak]

Nice graphs, Fred.

This equation has power in the numerator and energy in the denominator--it should be unitless.

\eta_p=\frac{T*u}{m\left[(1+f)(u_e^2/2)-u^2 /2\right]}

 

[zee24]

did you know the formula PV=nRT, which n,R is constant and V(volume) we can say that constant too. so the temperature(T) based on pressure(P) and P=F/A, if we want to increase the thrust(F) we just have to decrease the area(A) or increase the pressure(P).
in ramjet case we just need incerase the volume of air and burn that to increase the thrust. that's why ramjet only effective in supersonic speed.
am i right?

 

[redargon]

not sure exactly how to answer your question, "am i right?", but I could point some things out.

1. PV=nRT is the ideal gas law and requires ideal gasses for it to be accurate, I think you can substitute in some conversion factors to apply it to certain gasses under certain conditions, but as far as I know, a fuel air mixture is not an ideal gas and shouldn't be modeled as such.
2. P=F/A, ie. F=PA, therefore to increase F you can either increase the A or increase the P or both, but I'm not quite sure how this relates to jet engines.
3. I'm not sure how you come to the ramjet conclusion. Ramjets can use the shockwave generated by supersonic flow to create the compression required for efficient combustion instead of using a mechanical compressor, but I'm not 100% sure you need to be going supersonic to use one. You could design it to have subsonic flow at the inlet and then cause it to go supersonic by using applicable geometries, but then choking could affect you mass flow rate, and as astronuc said earlier, having a large mass flow rate is part of the thrust generation.

Hope that helps.

 

[deepthishan]

if you think about it, if you compress the air entering, more volume can be burnt in the combustion chamber every minute

 

[prost22]

Isn't the answer as simple as this?
The compressor is needed to get you off the ground (ramjets are essentially turbojets with no compressor; you can't take off from the ground with a ramjet, without a booster of some sort). A jet engine is essentially a controlled explosion--if you had no compressor, and you were sitting in the ground, all you'd have is a fire in the combustor (the combustor injects fuel into the air stream, which starts on fire, heats up, goes through the turbine and out the back, through the nozzle); you'd get some 'thrust,' as the really hot air went through the turbine and through the back, but not enough to get you off the ground. Now, compress that air 20 times its original pressure, inject some fuel, and light it--voila! explosion! Air rushes out the back of the combustor through the turbine, turning the turbine, (which gives you enough energy to compress the air), the air blows out back through the nozzle.

deepthishan; since the combustion chamber has constant volume, I think you might have meant 'more mass, m' or 'more mass flow, m-dot' ("m" with a small dot on top of it, signifying dm/dt).

 

[rbeale98]

Isn't the answer as simple as this?
The compressor is needed to get you off the ground (ramjets are essentially turbojets with no compressor; you can't take off from the ground with a ramjet, without a booster of some sort). A jet engine is essentially a controlled explosion--if you had no compressor, and you were sitting in the ground, all you'd have is a fire in the combustor (the combustor injects fuel into the air stream, which starts on fire, heats up, goes through the turbine and out the back, through the nozzle); you'd get some 'thrust,' as the really hot air went through the turbine and through the back, but not enough to get you off the ground. Now, compress that air 20 times its original pressure, inject some fuel, and light it--voila! explosion! Air rushes out the back of the combustor through the turbine, turning the turbine, (which gives you enough energy to compress the air), the air blows out back through the nozzle.

deepthishan; since the combustion chamber has constant volume, I think you might have meant 'more mass, m' or 'more mass flow, m-dot' ("m" with a small dot on top of it, signifying dm/dt).

in the case of sitting on the ground with no compressor, the burning fuel/air would have no preferred direction to flow. I agree with your general point however that the basic purpose of the compressor is to make the air more explosive. The amount of energy spent driving the compressor is well worth the energy you get by blowing up a stream of compressed air mixed with fuel.

Another important point is that the compressor slows down the incoming flow as it compresses. this makes it easier to hold the flame. an earlier poster said there's no reason why uncompressed air would not burn. true, but uncompressed air would be fast enough to blow the flame out the back.

 

[Cyrus]

in the case of sitting on the ground with no compressor, the burning fuel/air would have no preferred direction to flow. I agree with your general point however that the basic purpose of the compressor is to make the air more explosive. The amount of energy spent driving the compressor is well worth the energy you get by blowing up a stream of compressed air mixed with fuel.

Another important point is that the compressor slows down the incoming flow as it compresses. this makes it easier to hold the flame. an earlier poster said there's no reason why uncompressed air would not burn. true, but uncompressed air would be fast enough to blow the flame out the back.

It also makes more drag than the entire airframe at top speeds.