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Why do turbine engines require a compressor section

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Why do turbine engines require a compressor section

28th Nov 2011, 23:11
#81 (permalink)

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Thermal to Kinetic Energy

All, a fun thread, but all of you are missing an important piece of information, the direct relationship between thermal energy and mechanical kinetic energy.

I'll skip the reason for a compressor, and go straight to why higher compression yields higher efficiency. The Brayton and Otto cycles are similar, if you look at the volume, pressure and heat curves. The important question is HOW does heat energy get converted into mechanical energy inside either a piston or a turbine engine? Remember that thermal energy in a gas is directly related to the kinetic energy of the individual molecules within that gas. The temperature of a gas is the average temperature of all the molecules within the volume being measured, as some will be hotter and some cooler. The temperature of an individual molecule is the amount of kinetic motion that molecule is experiencing. The hotter it is, the faster that molecule is vibrating or moving.

The bottom of a piston and the end of the exhaust nozzle of a turbine engine are both exposed to the ambient outside pressure. When an air/fuel mixture is compressed and ignited, the resulting heat energy is directly transferred to mechanical motion, when individual molecules transfer their heat (kinetic energy) to the metal molecules of the piston or turbine blade (think Newton's cradle). In other words, the transfer of many gas molecule's kinetic energy to the metal of the piston or turbine blade, causes the metal parts to move. This happens because the heat and kinetic energy of the gas molecules are used to create pressure within the engine by design, which causes movement of the piston or turbine blade. When that movement occurs, the gas molecules give up some of their kinetic heat energy to the moving parts, and exchange it for mechanical motion or work, which is just another form of kinetic energy (again think Newton's cradle). This lowers the temperature of the gas, since it has now given up some of its heat energy in exchange for mechanical motion.

When higher compression (piston) is used, the pressure difference after ignition between the top of the piston and the ambient outside pressure at the bottom of the piston is greater, and therefore more kinetic energy is transferred to the piston, exchanging more heat from the gas to the piston's movement. If you look at the Otto cycle curves, most of the work and most of the energy transfer takes place in the top half of the power stroke, and this is why ignition timing is so important for efficiency. Bad ignition timing means heat is released by the fuel at the wrong time, and doesn't transfer as much heat to mechanical work, thus the EGT is higher when the timing is off.

For the turbine, a greater pressure ratio means a higher pressure is created in the combustion chamber, thus the pressure differential between the combustion chamber and the exhaust nozzle (across the turbines) is greater, thus more exchange of the heat (kinetic) energy of the gas into mechanical motion. In a turbofan, this is really helpful as turning the fan produces much more thrust then the residual pressure out of the exhaust nozzle (which you still use).

To summarize, the whole point of greater compression or pressure ratios, is to create greater pressure differentials across the moving parts, so more heat within the gas can be exchanged for mechanical work. This is what creates greater efficiencies.

Last edited by Flight Safety; 28th Nov 2011 at 23:29.
29th Nov 2011, 05:05
#82 (permalink)
Thread Starter

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Hi Everyone,

First of all thanks for taking the time to reply to my original post. As is often the case with PPRuNe I do not really feel that I have a clear concise answer to take away from the discussion. However I believe compromise one of the universal tenets of science and indeed aviation and I certainly appreciate that there is no single factor that controls the efficiency of an engine nor the need for/benefit of a compressor section.

HOWEVER... I was really looking for a simple thermodynamic explanation that would provide the basic model on which I could add any number of exceptions and limitations. In this sense I think that slippery pete's explanation helped a lot. I have been trying to find an answer independent of this forum and have enjoyed mixed success.

From what I can summarise, and I'm happy to be corrected:

1/ Why do turbine engines require a compressor section?

There are obvious and practical reasons for having a compressor section such as were mentioned in the early replies such as the need to avoid a "bonfire" etc. but fundamentally the reason seems to be to increase the overall efficiency of the engine.

2/ Why does increasing the compression ratio (piston) or pressure ratio (turbine) improve efficiency?

I think that the general thermodynamic reasoning is that, compression when viewed as an ideal process is reversible. Meaning that if I expend a certain amount of work compressing air inside a cylinder then the same amount of work can be expelled by the piston in returning to it's original state. As it is compressed air (whether inside a piston or compressor) will increase in temperature.

The efficiency of any system is:

what you get out / what you put in

Carnot's Theorem tells us that in reality even with an idealised engine (no friction losses etc) efficiency is determined by the difference between the temperature at which heat enters the engine and the ambient temperature of the surrounding environment. Since we cannot really hope to change the ambient temperature of the environment the best we can do is improve the temperature at which the heat enters the engine. In aircraft this is done by increasing the temperature at which the fuel air mixture is ignited.

One must also consider of course the effect of practical real world factors such as material temperature limitations, volumetric efficiency effects, flame propagation, points of maximum pressure and so on so forth.

Any thoughts,

J
29th Nov 2011, 09:48
#83 (permalink)

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Originally Posted by QJB
but fundamentally the reason seems to be to increase the overall efficiency of the engine.
No.
Fundamentally, the reason is that you need a pressure differential for extracting work from heat. And since there is no pressure increase in the combustion chamber of a turbine (turbine = open flow engine), the pressure increase must be created by a compressor.

Without a compressor, there no work that can be extracted !

Luc
29th Nov 2011, 10:12
#84 (permalink)

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Without a compressor, there no work that can be extracted !
Or, in other words, the efficiency of the engine drops to 0.

Golf-Sierra
29th Nov 2011, 10:31
#85 (permalink)

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Other related tidbits:

Volumetric Efficiency - Primarily related to power production. The amount of air and oxygen pumped through a given engine's physically limited size, determines the amount of fuel that can be burned, thus peak power production. However any engine design, piston or turbine, has a sweet spot (or range) of power production that has the highest thermal efficiency (converting heat to work), thus the lowest specific fuel consumption for work produced in that power range.

Turbochargers or Superchargers - Increases the amount of air and oxygen pumped through an engine, and the amount of fuel that can be burned. Superchargers NEVER add thermal efficiency only power, however a turbocharger can add thermal efficiency in 2 ways. One, it allows a smaller engine to produce the same power as a larger engine, making it possible to operate a smaller engine closer to its optimum thermal efficiency power range in a given application. Two, the recovered waste heat from the exhaust is used by the turbo to pump air through the engine, thus taking over the engine's air pumping duties and reducing or eliminating the engine's normal pumping losses.

Atkinson Cycle - A piston engine design where the power stroke is longer than the compression stroke. The purpose is to increase thermal efficiency by extracting more work from the heat of the burned air/fuel charge, at the expense of power density. True Atkinson engines used various mechanical linkages to create the dissimilar length of compression and power strokes, whereas modern Atkinson cycle engines (such as those used in hybrid autos) use clever valve timing to achieve the same result.

Lean Burning - Can increase thermal efficiency. Stoichiometric fuel burning is designed to burn just enough fuel to consume all the oxygen in an air/fuel charge, and still produce complete combustion of the hydrogen and carbon in the HC fuel, with no oxygen left over. However lean burning burns less fuel than the available oxygen can support. Less heat is added relative to the amount of air compressed in either the Otto or Brayton cycle, making it possible to convert more of a smaller heat product into mechanical work with less residual wasted heat. It can also be argued that a more open throttle for a given power output while lean burning, reduces pumping losses. Like the Atkinson cycle, lean burning produces more thermal efficiency at the expense of overall power. However clever engine design allows lean burn to be a selectable mode rather than a constant condition, to preserve an engine's high power production when needed.

Last edited by Flight Safety; 29th Nov 2011 at 10:53.
29th Nov 2011, 17:06
#86 (permalink)

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It seems to me that this is all getting overcomplicated - AND back-to-front, as it were.

The simplest answer to "Why a compressor?" is - to provide "artificial airspeed" through the engine so that it will run even when sitting still. Actually, of course, it is REAL airspeed - but localized to within the confines of the engine.

Without that flow - you get, as previously mentioned - a bonfire. Google "turbine hot start."

Pure internal-combustion jets (e.g. ramjet) require substantial forward speed (= airflow through the engine) to function at all. At least 100 mph (160 kph) for minimal function. "Best" operational speed is at Mach 0.5 or higher.

Very hard to taxi (or pull up to the gate) while maintaining Mach .5

So - there needs to be a way to keep the engine up to minimal self-sustaining spin, even with little or no external inflow of air from the speed of the aircraft/vehicle itself. Thus the compressor.

A ramjet works with neither compressor NOR power turbine! The main function of the power section in a turbojet, at least at lower aircraft speeds, is - to drive the compressor to sustain ignition! Ideally subtracting as little power from the exhaust thrust as possible.

In a turboprop/turboshaft engine, that is the main function of the power turbine - period. A turbine helicopter can hover while the compressor keeps the airflow/speed through the engine up in the mach numbers.

If an aircraft reaches a high enough speed, in theory one could fold the compressor blades out of the way, and just let the ram air do its thing unimpeded. Hard to build a hinge strong enough to take the stresses, though, and in fact, development went the other direction (fanjets and turboprops) where the compressor (or at least a part of it) becomes the primary source of thrust.

Now, there is lots of room for theory and engineering to make the compressor (and the turbine driving it) work as effectively and efficiently as possible, which is the reason for all the theory and measurements and charts.

But that answers not "Why a compressor" - but "HOW a compressor?"

@ FS (below) - yes, OK, I was referring to compressor as the physical bit of machinery. A ramjet does need compression - from ram airspeed, augmented by venturi compression (usually). Just not the spinning fan.

Last edited by pattern_is_full; 29th Nov 2011 at 17:46.
29th Nov 2011, 17:13
#87 (permalink)

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Actually, both ramjets and scramjets still require the "compressor" function, as they are both still heat engines using external oxygen for combustion. It's just that the "compressor" is external to the engine, using a ram effect from high speed airflow, instead of rotating blades.
29th Nov 2011, 18:02
#88 (permalink)

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A ramjet works with neither compressor NOR power turbine!
Well, almost. The ramjet needs a high-pressure fuel pump, and it's generally driven by an air turbine, by tapping some inlet air from the diffuser.
29th Nov 2011, 18:08
#89 (permalink)

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If an aircraft reaches a high enough speed, in theory one could fold the compressor blades out of the way, and just let the ram air do its thing unimpeded.
This effect can be achieved by valving, gradually opening a bypass duct while closing down airflow to the gas turbine. Several schemes have been tested, going back to P&W's entry in the SST race of the late 60s.
29th Nov 2011, 18:13
#90 (permalink)

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Barit1, I believe the J58 of the Blackbird worked the way you described. Accord to Ben Rich, 80% of the power produced by the J58 at Mach 3.2 came from the resulting ram effect.
29th Nov 2011, 19:11
#91 (permalink)

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The Blackbird used a hybrid engine before hybrids were cool. (IIRC, the engine begins as a turbojet and as it increases in speed, the flow more closely approximates a ramjet by use of valves and moving the nose cone backwards. This picture from a wiki article seems to tell that tale as well).

29th Nov 2011, 20:39
#92 (permalink)

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Without the pressure differential the heated air would still do work, it's just that it wouldn't be much use. Gas flows from a high pressure region to a low pressure region. No compressor and the heated gas, which is now at higher pressure (pv=nkt) sees lower and equal pressure at either end of the tube. Compressor (or ram effect, or reflected shock as in those exhaust pipe trick engines) gives the right gradient to send the gas to the designated exhaust where it expands, cools, and does work. The higher the initial pressure of the incoming gas, the higher its density and so it can support a richer mix, which increases temperature,etc.
29th Nov 2011, 22:19
#93 (permalink)

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Originally Posted by J
Why do turbine engines require a compressor section?

Hi guys,

Can anyone give me an answer on this? Also why is it that both the piston engine and the turbine engine can have their efficiencies increased by increasing the pressure ratio (compression ratio for piston)? Is there some sort of simple thermodynamic explanation for this?

Cheers,

J
First off, I'm just a pilot. My bachelors is in professional aviation. I normally don't do math in public.

I think the answer to the "why" question is simple. It's because they are intended to produce thrust in a specific speed range. (my remarks apply to airplanes only, but could likely be used in other discussions). What is thrust? "Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system."

You can't have an equal and opposite reaction without something to push against. When the exhaust gasses move, they have to push against something in order for there to be an opposite reaction. In a piston type reciprocating engine, the expanding gases push against the piston, which is connected to a crankshaft which converts the linear force to rotating force. In a turbine engine, the expanding gases ultimately have to have something to push against and absorb the energy produced. The compressor blades absorb that push, transfer it to the bearings on their shaft which are connected to the engine cases which are connected to the engine pylon which is connected to the airframe and the entire airframe moves opposite of the exhaust.

I realize that ram/scram jets fly, but they are highly specialized and also work in a specific velocity range. I'm out of my league here, but I think that ramjets also have "something to push against" but that "thing" isn't a structure, it is a pressure wave inside the combustion chamber. Hitler's "buzz bomb" used a pulse jet which was a ramjet with a flapper door at the entrance to the combustion chamber. That flapper provided the expanding exhaust gas with something to push against.
30th Nov 2011, 15:07
#94 (permalink)

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Originally Posted by Mr Optimistic
No compressor and the heated gas, which is now at higher pressure (pv=nkt) sees lower and equal pressure at either end of the tube
No.
This is a very common misunderstanding of turbine engine.
The gas heating is done at constant pressure not at constant volume !
The temperature increase is exactly compensated by a density decrease.
Without compressor, the heated air would just remain at atmospheric pressure and expand in both forward and backward direction and wouldn't produce any work (no pressure differential).

Luc

Last edited by Luc Lion; 30th Nov 2011 at 18:07.
30th Nov 2011, 22:53
#95 (permalink)

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Why compress air using a compressor in jet engines ?

Mc2

Increase the mass (using compression i.e.take a mass of

air and compress into a small space) to increase power output.

In a jet engine airmass accelleration is caused by heating the air in

combustion chambers.This action (accelleration) causes an equal reaction

(thrust).

To use only ram air,which I think the questioner is thinking of, there'd be

a relatively low mass, hence low thrust. A hypersonic engine, on the other

hand, relies on ram air, but requires very high velocities to compress

before heating.

This is the way I remember and understand how it works.
30th Nov 2011, 23:34
#96 (permalink)

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TTex600:
You can't have an equal and opposite reaction without something to push against.
Absolutely NOT true, and the obvious example is a rocket engine in a vacuum. The thrust is the result of the mass flow of gas and its exit velocity, and does not require "something to push against".

So the example of the piston in a cylinder has no relevance to the propulsive utility of a jet or rocket. The piston does not propel the aircraft; its mechanical linkage (connecting rod, crankshaft, perhaps a gearset) is to a propeller, which does.
30th Nov 2011, 23:40
#97 (permalink)

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Does anyone remember the pulsejet engine? The German V-1 used it.
1st Dec 2011, 00:11
#98 (permalink)

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barit1

Add a second nozzle pointing in the opposite direction from the one you started with, on the other side of the reaction chamber.

The Third law is honored, and movement (velocity) is nil.
1st Dec 2011, 01:16
#99 (permalink)

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Hi QJB.

Glad you came back and posted again.

The efficiency of any system is:

what you get out / what you put in

Carnot's Theorem tells us that in reality even with an idealised engine (no friction losses etc) efficiency is determined by the difference between the temperature at which heat enters the engine and the ambient temperature of the surrounding environment. Since we cannot really hope to change the ambient temperature of the environment the best we can do is improve the temperature at which the heat enters the engine. In aircraft this is done by increasing the temperature at which the fuel air mixture is ignited.
Yup, exactly what I've been saying from the start - this is the thermodynamic reason why higher compression engines are more efficient. Glad it helped you answer the question from your OP.

Obviously how the engine is designed, the operating RPM range, type of fuel etc. etc. are all things which are important for designing an engine for a certain application.
1st Dec 2011, 01:31
#100 (permalink)

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barit1

In your rocket in a vacuum, is the rocket motor open on both ends?

Originally Posted by barit1
Does anyone remember the pulsejet engine? The German V-1 used it.
You bust on me, and you didn't even read my post. It doesn't lend you much credibility.

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