Why do turbine engines require a compressor section
Yes, I know but that wasn't the point I was trying to make.
Second Law
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A ram jet has sufficient compression without the need for a fan assembly.
Go sub routine velocity dependence!
Why do we compress - to release more energy to do useful work per unit time within the engine (be it piston or gas turbine).
CW
Go sub routine velocity dependence!
Why do we compress - to release more energy to do useful work per unit time within the engine (be it piston or gas turbine).
CW
Why do we compress - to release more energy to do useful work per unit time within the engine (be it piston or gas turbine).
Burning a certain amount of fuel produces a certain amount of energy, let's call it X. You can't change the amount of energy released by burning a fuel by burning it faster or slower or under more pressure. Burn it as fast or slow or under as much pressure as you like, the amount of energy stored in that fuel is constant.
The energy produced by X is divided into waste heat, useful energy and sound by an engine. The sound is so negligible against the power output of the engine, it can be ignored.
So energy from the fuel = useful work + waste heat
The ONLY way to get more energy out of the same amount of fuel is to make less waste heat. Higher compression results in less waste heat transferred to the air by the end of the cycle.
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Any aircraft engine supplies a propulsive force by capturing a mass of air and then accelerating that air. It may be a propeller doing the job, or it may be a gas turbine (we call it a jet engine). In any case, a pressure increase is needed to force (accelerate) the air mass aft. Thus, the need for a compressor. The prop is a compressor; the fan in a turbofan is a compressor; and so is the compressor (doh!) in a straight jet or a turboprop. 
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slippery....
If you confine yourself to framing everything in terms of thermal efficiency you will never be able to explain how or why an engine works.
The topic of this thread is simply "why do turbine engines require a compressor section". That is the question that was being answered by CW. His answer emphasised the requirement for power.
Nobody wants an aircraft that can taxi out on a thimble of fuel before failing miserably to accelerate to flying speed.
The ONLY way to get more energy out of the same amount of fuel is to make less waste heat. Higher compression results in less waste heat transferred to the air by the end of the cycle.
The topic of this thread is simply "why do turbine engines require a compressor section". That is the question that was being answered by CW. His answer emphasised the requirement for power.
Nobody wants an aircraft that can taxi out on a thimble of fuel before failing miserably to accelerate to flying speed.
Last edited by oggers; 22nd Nov 2011 at 14:17.
Second Law
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Power v heat waste losses
Slippery Pete, sir,
Saying that "the main reason for thermodynamic efficiency increase in high compression is reduced heat waste to the fluid" is, in my view, wrong. It is the "main reason" aspect of the statement that I quarrel with.
The primary reason for compressing the air is the enhanced power per unit time the engine can then produce; heat transference losses are also reduced it is true for the reasons you give but I submit that that it is a secondary (albeit welcome) effect of the compression.
With compressed air you have more oxygen molecules in the combustion chamber and you can burn more fuel per unit time. You release more energy to do useful work per unit time. The time dependence is critical.
The main reason is power but you are right in that, under high compression, the loss of heat this way is a smaller % of the process i.e. it helps but it's not the key driver.
If we really want to get obscure the parallel with Le Chateliers Principle when justifying running big pressures on systems that are making fewer moles of gas in production processes is a good one - you "notice" the drop in entropy less than at low pressures.
Yes yes there's a rate of reaction factor at elevated pressures too but let's not go there either!
I suspect we shall have to agree to differ short of doing a lot of sums in public.
CW
Saying that "the main reason for thermodynamic efficiency increase in high compression is reduced heat waste to the fluid" is, in my view, wrong. It is the "main reason" aspect of the statement that I quarrel with.
The primary reason for compressing the air is the enhanced power per unit time the engine can then produce; heat transference losses are also reduced it is true for the reasons you give but I submit that that it is a secondary (albeit welcome) effect of the compression.
With compressed air you have more oxygen molecules in the combustion chamber and you can burn more fuel per unit time. You release more energy to do useful work per unit time. The time dependence is critical.
The main reason is power but you are right in that, under high compression, the loss of heat this way is a smaller % of the process i.e. it helps but it's not the key driver.
If we really want to get obscure the parallel with Le Chateliers Principle when justifying running big pressures on systems that are making fewer moles of gas in production processes is a good one - you "notice" the drop in entropy less than at low pressures.
Yes yes there's a rate of reaction factor at elevated pressures too but let's not go there either!
I suspect we shall have to agree to differ short of doing a lot of sums in public.
CW
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Methinks you are all missing the point. Suppose I build an aircraft that is electrically powered (motor driving a prop), or hydraulically powered, or compressed air, or human powered. In all cases the prop is a compressor - a device which captures a mass of air and accelerates it aft.
Now if we put a simple turbojet in the plane, it still has to - ahem - capture a mass of air and accelerate it aft. But we have to somehow drive the compressor, and so we have a burner and turbine to recover some of the energy in the airflow to create a continuous Brayton cycle (named after American engineer George Brayton (1830–1892)).
Now if we put a simple turbojet in the plane, it still has to - ahem - capture a mass of air and accelerate it aft. But we have to somehow drive the compressor, and so we have a burner and turbine to recover some of the energy in the airflow to create a continuous Brayton cycle (named after American engineer George Brayton (1830–1892)).
Luc/MathFox/Dan and many others -- thank you for a fun discussion.
Note as to reasons why we use a compressor, per the original question.
Yes.
Not quite.
That may be an outcome of the process of burning oxygen to release energy in the fluid to get something to turn (shaft or turbine) but it isn't the reason to do it.
Other point made earlier: without a delta P, (Pressure differential between inside the turbine engine and the outside air) you get no work out of the turbine, be it in turbo shaft, turbo prop, turbofan, or turbojet.
You build turbine engines to get work out of them.
My degree isn't important, but I did take some thermogoddamics and a few courses on compressible flow and turbomachinery. Otto, Brayton, Carnot ... all familiar names.
Put energy in, get directed energy out to make something turn.
That is why you have an engine.
Note as to reasons why we use a compressor, per the original question.
Chris: Why do we compress - to release more energy to do useful work per unit time within the engine (be it piston or gas turbine).
Slippery_Pete :It's so that less heat is added to the air over the entire cycle.
That may be an outcome of the process of burning oxygen to release energy in the fluid to get something to turn (shaft or turbine) but it isn't the reason to do it.
Other point made earlier: without a delta P, (Pressure differential between inside the turbine engine and the outside air) you get no work out of the turbine, be it in turbo shaft, turbo prop, turbofan, or turbojet.
You build turbine engines to get work out of them.
My degree isn't important, but I did take some thermogoddamics and a few courses on compressible flow and turbomachinery. Otto, Brayton, Carnot ... all familiar names.
Put energy in, get directed energy out to make something turn.
That is why you have an engine.
Hi Chris Weston, respectfully I disagree.
More oxygen molecules in the combustion chamber? Only in a turbine. In a piston engine, an 8:1 engine has the same volume of air in the combustion chamber as a 12:1 compression ratio engine. If your theory of more air was important, it would not effect a piston engine (which it does).
Burning more fuel per unit time has nothing to do with it. The entire CONCEPT of the original post is about thermodynamic efficiency (ie being able to get more useable work out of the same amount of fuel). Anyone can just increase the fuel flow and get more power out of an engine ... has nothing to do with thermodynamic efficiency.
Let's make it absolutely clear... we are talking about two identical engines, same size, same fuel flow, same RPM, same EVERYTHING - except one has a higher compression ratio - why is the one with higher compression getting more energy out of the same fuel? That's what we are talking about here.
With compressed air you have more oxygen molecules in the combustion chamber
and you can burn more fuel per unit time
Let's make it absolutely clear... we are talking about two identical engines, same size, same fuel flow, same RPM, same EVERYTHING - except one has a higher compression ratio - why is the one with higher compression getting more energy out of the same fuel? That's what we are talking about here.
Hi Blackhand.
Look, I'm sure we all know that a piston engine with the same physicals can't have a different compression ratio, and in order to change it without changing the shape of the cylinder you would end up changing the volumetric capacity very slightly.
So, for the purposes of answering the original post, yes, if the capacity of the engines is the same, the compression ratio will not change the amount of air molecules. Lets say 2x 350 cubic inch engines - you will theoretically have the same amount of air in the cylinder at TOPD if it has a 8:1 or 15:1 compression ratio.
I've talked about this before, and the fact that it relates to limitations of how a piston engine is operated (ie at inefficient high RPM or across a wide RPM band, low throttle settings etc). Not related to the OP, which was about how compression, and compression alone makes a difference in thermodynamic efficiency.
Nope, no sciolist. Just someone with a basic knowledge of physics. As an engine tuner, I'm sure you know more about me on how to tune an engine on a dyno for best power and driveability over a large RPM range... interesting, but not related to the OP, nor the thermodynamic efficiency of an engine. It really is simple physics.
Are you sure about this?
So, for the purposes of answering the original post, yes, if the capacity of the engines is the same, the compression ratio will not change the amount of air molecules. Lets say 2x 350 cubic inch engines - you will theoretically have the same amount of air in the cylinder at TOPD if it has a 8:1 or 15:1 compression ratio.
From work on performance engines, it is the compression pressures and not really the comp ratio that is important. Compression pressures are related to the comp ratio but are more influenced by valve and ignition timing. Of course the dynamics of the intake and combustion chambers has a direct affect.
I work on the practical side, so is interesting talking to a theorist - unless of course you are just a sciolist.
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In a piston engine, an 8:1 engine has the same volume of air in the combustion chamber as a 12:1 compression ratio engine.
we are talking about two identical engines, same size, same fuel flow, same RPM, same EVERYTHING - except one has a higher compression ratio -
From work on performance engines, it is the compression pressures and not really the comp ratio that is important. Compression pressures are related to the comp ratio but are more influenced by valve and ignition timing. Of course the dynamics of the intake and combustion chambers has a direct affect.
I work on the practical side, so is interesting talking to a theorist - unless of course you are just a sciolist.
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1 Q 10 A
So typical of PPrune...
I love the technical questions, and really would like to learn the answer....but with so many answers I often end up more confused..
and it is almost Friday...
What about a mod filter, after say 1 week to concatinate all the answers, and make 1 complete response......
I must go to Friday Jokes Forum....
glf
I love the technical questions, and really would like to learn the answer....but with so many answers I often end up more confused..
and it is almost Friday...
What about a mod filter, after say 1 week to concatinate all the answers, and make 1 complete response......
I must go to Friday Jokes Forum....
glf
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Taking the brayton cycle to the next level
Supercritical carbon dioxide Brayton Cycle turbines promise giant leap
Supercritical carbon dioxide Brayton Cycle turbines promise giant leap
Second Law
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Slippery
I suspect trolling here so more fool me but I will have one final attempt and share some simple physics with you.
pV = nRT
so
p = n/V x RT
Putting that V R and T are constant we have that
p is directly proportionate to n
The greater pressure you have the more moles of gas you will have in a fixed volume.
CW
I suspect trolling here so more fool me but I will have one final attempt and share some simple physics with you.
pV = nRT
so
p = n/V x RT
Putting that V R and T are constant we have that
p is directly proportionate to n
The greater pressure you have the more moles of gas you will have in a fixed volume.
CW
Chris,
I'll say it again REALLY SLOWLY so you can understand.
Your theory of more compression = more air molecules only applies in a turbine with a fixed volume combustion chamber. It doesn't apply in a fixed volume piston engine (because the swept volume determines the number of molecules you can get in there). And yet a higher compression piston engine is also more efficienct. If it was (as you argue) all about the number of oxygen molecules, a high compression piston engine would be no more thermodynamically efficienct than a low compression one.
As they say on mythbusters, myth BUSTED.
In fact, thermodynamic efficiency has nothing to do with how many oxygen molecules are there - the only time that becomes important is if there are not enough oxygen molecules to combust all the fuel.
When will all the lies and myths end? Every new page on this thread brings more people with more false arguments. So far we've had volumetric efficiency (consequence of operating a piston engine inefficiently), faster burning fuel, incomplete combustion, valve timing, ignition timing... it goes on and on and on.
These are all important things for tuning or selecting and engine for an application, because they make it practical and useable over a large RPM range (rather than super-efficienct at one particular RPM), and because we have to manage inadequacies of the fuel, and the temperature of the components.
But the fact remains, the only thermodynamic reason a higher compression or pressure ratio engine is more efficient (THE QUESTION IN THE OP) - is because by the end of the cycle, less waste heat has been transferred to the air. IT'S SIMLPE FN PHYSICS.
You put in an amount of energy (X) into an engine in the form of fuel, you want to get some of that energy out in the form of useful work (Y), but unfortunately you get a lot of waste heat (W). Maximum thermodynamic efficiency is the concept of ... for a given amount of fuel energy (X), you get the maximum useful work (Y). High compression or pressure ratio engines do this better because less of the original fuel energy (X) is converted to waste heat (W).
Conservation of energy - I would expect most high school students to know this.
Anyway, let the next round of myths begin on thermodynamic efficiency at higher compression. I wonder what we'll get next...
I'll say it again REALLY SLOWLY so you can understand.
Your theory of more compression = more air molecules only applies in a turbine with a fixed volume combustion chamber. It doesn't apply in a fixed volume piston engine (because the swept volume determines the number of molecules you can get in there). And yet a higher compression piston engine is also more efficienct. If it was (as you argue) all about the number of oxygen molecules, a high compression piston engine would be no more thermodynamically efficienct than a low compression one.
As they say on mythbusters, myth BUSTED.
In fact, thermodynamic efficiency has nothing to do with how many oxygen molecules are there - the only time that becomes important is if there are not enough oxygen molecules to combust all the fuel.
When will all the lies and myths end? Every new page on this thread brings more people with more false arguments. So far we've had volumetric efficiency (consequence of operating a piston engine inefficiently), faster burning fuel, incomplete combustion, valve timing, ignition timing... it goes on and on and on.
These are all important things for tuning or selecting and engine for an application, because they make it practical and useable over a large RPM range (rather than super-efficienct at one particular RPM), and because we have to manage inadequacies of the fuel, and the temperature of the components.
But the fact remains, the only thermodynamic reason a higher compression or pressure ratio engine is more efficient (THE QUESTION IN THE OP) - is because by the end of the cycle, less waste heat has been transferred to the air. IT'S SIMLPE FN PHYSICS.
You put in an amount of energy (X) into an engine in the form of fuel, you want to get some of that energy out in the form of useful work (Y), but unfortunately you get a lot of waste heat (W). Maximum thermodynamic efficiency is the concept of ... for a given amount of fuel energy (X), you get the maximum useful work (Y). High compression or pressure ratio engines do this better because less of the original fuel energy (X) is converted to waste heat (W).
Conservation of energy - I would expect most high school students to know this.
Anyway, let the next round of myths begin on thermodynamic efficiency at higher compression. I wonder what we'll get next...
Last edited by Slippery_Pete; 26th Nov 2011 at 01:59. Reason: Typo
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Taking the brayton cycle to the next level
Supercritical carbon dioxide Brayton Cycle turbines promise giant leap
Supercritical carbon dioxide Brayton Cycle turbines promise giant leap
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the propeller on a turboprop is in such a point of view also a "compressor" but its main work is to create thrust, it has very little effect on pre compressing the air which enters the inlet for combustion, regardless if its a multishaft free turbine ( eg.pt6a) or a singleshaft fixed turbine ( rr dart, tpe331 e.g ) .
when it comes again to ( 4 stroke) pistons the amount of air in the cylinder is determined on the suction cycle and the suction, as we all know it ends with the piston at bottom dead centre. the amount of air being able to suck in is describes in volumentric efficiency. - at natural aspirated engines typically below 1 , at turbocharged engines above 1 - here the engine not just sucks the air, it is forced into the cylinder. the the piston moves up again and compresses the amount that was sucked in.
so when the amount of sucked air in suction cycle stays the same a low compression engine will have the same amount of air as the high compression engine at end of the compression cycle. at the higher compression engine this amount of air is just squeezed more .
its the same when you take an airballon , put an amount of air in it and then squeeze it. sqeezing it more will rise the compression inside the ballon, not the amount of air in it ( since no further air is forced inside when you squeeze it)
cheers !
when it comes again to ( 4 stroke) pistons the amount of air in the cylinder is determined on the suction cycle and the suction, as we all know it ends with the piston at bottom dead centre. the amount of air being able to suck in is describes in volumentric efficiency. - at natural aspirated engines typically below 1 , at turbocharged engines above 1 - here the engine not just sucks the air, it is forced into the cylinder. the the piston moves up again and compresses the amount that was sucked in.
so when the amount of sucked air in suction cycle stays the same a low compression engine will have the same amount of air as the high compression engine at end of the compression cycle. at the higher compression engine this amount of air is just squeezed more .
its the same when you take an airballon , put an amount of air in it and then squeeze it. sqeezing it more will rise the compression inside the ballon, not the amount of air in it ( since no further air is forced inside when you squeeze it)
cheers !
The prop is a compressor; the fan in a turbofan is a compressor; and so is the compressor (doh!) in a straight jet or a turboprop
Does a prop actually compress? If so what is it's actual practical effect on the overall compression cycle?
The gas turbine section of a turboprop can be (but isn't always) de-linked from the prop, a free power turbine drives the prop. I would have to see the numbers but I'm not sure what contribution to the overall compression cycle the prop will actually give. Especially in some of the more complex reverse flow turbine engines.
In a directly linked turboprop such as the RR Dart, would there be any compression effect from the prop? I can't see how there would be much flow increase from the root of the blade into the intake. Discuss.
The fan in a turbofan can be referred to as the LP compressor (EG RR RB211) and I can see how that would work.
In an aft-fan config of course it would have no effect on the compression at all. Maybe that's one of the reasons the idea was only used briefly. Convair 880 is the only on I can think of, no idea what make/model the engine is.)
No axe to grind just trying to understand what the OP is after.
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Slippery_pete:
Wrong again. The "unswept volume" is the combustion chamber (or at least part of it depending how semantic you want to be). It is the swept volume x the volumetric efficiency that determines the number of molecules you get in - not the "unswept volume".
It doesn't apply in a fixed volume piston engine (because the unswept volume determines the number of molecules you can get in there [there being the combustion chamber])
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TURIN:
Well, a prop blade is an airfoil, and Bernoulli tells us all about that. Static pressure is increased on the aft face of the blade. This may have little or no effect on the ENGINE cycle, but it's this pressure rise that integrates to propeller thrust i.e. the motive force for the airplane.
Granted the pressure rise is not huge, but the prop disc area is relatively large, and F=p x A. The prop moves a large mass of air at relatively low pressure.
Now if the prop has many blades and is enclosed in a shroud (i.e. a ducted fan), then the pressure rise is greater although the area may be less. This higher pressure acts to supercharge the downstream core engine.
Erm, I have to query that.
Does a prop actually compress? If so what is it's actual practical effect on the overall compression cycle?
Does a prop actually compress? If so what is it's actual practical effect on the overall compression cycle?
Granted the pressure rise is not huge, but the prop disc area is relatively large, and F=p x A. The prop moves a large mass of air at relatively low pressure.
Now if the prop has many blades and is enclosed in a shroud (i.e. a ducted fan), then the pressure rise is greater although the area may be less. This higher pressure acts to supercharge the downstream core engine.