Glad you've come over to the dark side, aerobat! :ok:

All the arguments Oggers posted relating to small throttle settings (VE), or flame front speed, or burning fuel faster, or ignition timing blah blah blah - don't relate to the original post about thermodynamic cycle efficiency of low vs. high compression. :ugh:

In fact, these problems disappear in a constant pressure heat engine like a turbine, where the concept is the same:

Less heat transferred to the fluid = more useable energy available
More useable energy available from same energy input = higher thermodynamic efficiency :ok:

I'd really like to hear Oggers explain how ignition timing, or valve timing, or mixtures, or flame fronts, or pumping losses, or any other of his false arguments explain why a high compression turbine engine is more efficient than a low compression turbine. :D

Now that is very clever, change the parameters of your argument.
You were talking about the Otto cycle, not the Brayton cycle:D

gents, are you talking turbines or pistons ? valve timing in a turbine ? when it comes to turbine engines we also discuss pressure ratio, not compression. maybe its just a misunderstanding since different things are mixed up.

the original question is well answered but of course a little bit like asking why a piston engine needs pistons . also the ram-jet explanation directs a wrong way since a ram jet is not a turbine engine. the job of a turbine disc is ( beyond driving the propeller via a gearbox at a turboprop) to drive the compressors , and when there are no compressors before the combustion chamber , for what does the turbine disc spin ?

maybe the thread beginner meant a jet engine, since the question for what a compressor in a turbine engine is self answering.

Sorry Blackhand, you really have missed the boat here.

I originally chose to talk about the Otto cycle because I thought it was easier to explain why thermodynamic efficiency goes up with higher compression of the fluid. The fact is that in both the Otto and Brayton cycles, more compression before the combustion results in less heat exchange to the fluid - and a thermodynamically more efficient engine.

I obviously made a mistake, because I didn't count on people like yourself and Oggers being unable to understand such a simple concept. Oggers then went off on several tangents, and used a myriad of false arguments about ignition timing, and valve timing, and mixing, and flame fronts, and all sorts of other things to try and squeeze his way out of the fact that nothing of those things have to do with the original question about the thermodynamic efficiency of higher compression.

Hi Aerobat. Yes, these things don't occur in a turbine - this was the point I was trying to make. Oggers used these arguments (valve timing, fuel mixing, incomplete combustion, VE at low throttle) as his reasoning for the difference between the efficiencies of high and low compression engines.

My switch over to turbine was to show that his arguments don't apply on a turbine, where in fact the same thermodynamic principle does (higher compression before combustion results in less energy wasted heating the fluid = more available work).

Slippery_Pete,

you stated several time the idea
I may be misunderstanding the point you intend to make, but I think that the statement is either wrong or more or less correct but very misleading.

Your sentence suggests that we add energy to the fluid and that this energy is split between useful work and heat. And so, the less heat the more work.
That's misleading.
Actually, the sequence of events is:
- we take a gas that has an internal energy U1
- optionally, we increase its energy to U2
(this is the compression phase. Mandatory for turbines. For steam machines, just assume U2=U1)
- we pour in some heat Q and get the internal energy U3 = U2 + Q
- we extract a percentage of the internal energy U3 into work through an expansion of the gas (usually adiabatic expansion).
- the resulting energy U4 = U3 - W
The percentage of extracted energy depends on the pressure ratio.
Actually it depends on the temperature ratio T4/T3 and is 1 - T4/T3 .
For adiabatic expansions, it can be shown that 1 - T4/T3 = 1 - (p4/p3)^((γ-1)/γ)

You can see that, for instance if the extraction efficiency is 40%, we extract 40% of U3 which is 40% of U2 plus 40% of Q. The more heat is added to the fluid, the more work is extracted.

This picture is, of course, a simplification. I looked at the efficiency of just one phase of the cycle rather than looking at the efficiency of the whole cycle. However it shows that:
- heat is the object of the energy extraction efficiency and is not an adjustement variable of the efficiency
- theoritical efficiency is just linked to the temperature ratio during expansion phases or to the pressure ratio which can be considered as a surrogate for temperature ratio

Note that the 40% U2 extracted is not magic energy ; it is only if the gas has been compressed initially from U1 to U2 with U1 being 60% of U2 that the final pressure ratio allows a 40% reduction. So the 40% extracted of U2 is just some invested energy that is reclaimed.
If there is no initial compression (steam engine) the increase of pressure obtained by adding Q in a fixed volume results in a pressure increase that allows a very poor extraction efficiency, much less than Q/U1.

Note also that the final state U4 is equivalent to the initial state U1 except that we extracted only a part of Q. So grossly, U4 = U1 + 60%.Q
And the final temperature of the gas is not a variable that can be acted upon ; it is dictated by be heat that could not be extracted.

Luc

Slippery_pete: please don't do this to yourself. You know full well that all of those things were being discussed in the context of the piston engine. Anyone who reviewed our exchange could see that - the clue is in the liberal use of the terms 'crank', 'piston', and 'cylinder', including in your first post where this started :rolleyes:

aerobat77: thanks for the link to that paper. There won't be any surprises in there for those who've been schooled in piston engine theory and it's a useful case study for those that might be interested. :8

Specifically on the point of exhaust temperature there is no surprise that it dropped as compression rose: this is not in dispute.

Folks when you get all done with this piston engine stuff (no compressors or turbines), could you do a google-idiot style translation into turbine engines for the casual thread reader.

Maybe a distinction could then be made between TIT (Turbine Inlet Temperature), EGT (Turbine Exhaust Gas temperature) out the tailpipe and pressure out of the compressor,

I doubt that timing enters into this for a gas turbine since the flowrate is constant and so is the fuel and ignition.

The casual reader would appreciate simple comparisons with everyday effects e.g. welding torches, bunsen burners, baloons, home oil heaters etc.

Turbine inlet temperature - Not indicated on turbine engines I work on.
Usually ITT, interstage turbine temperature between compressor turbine and power turbine sections is indicated
EGT is idicated on piston engines as an aid to leaning the mixture, not so much on turbine engines.
In any case will be lower than ITT and TIT.
Pressure out of the compressor is a function of the compressor stages in axial flow and compressor size/effenciency in centrifugal compressor; and is expressed as a ration over inlet pressure.
Which leads us to bleed valves and interstage bleeds to decrease stall margins etc.
BH

Hi Oggers.

I've already explained why I used the words crank, piston and cylinder in my first post - because I thought it would be easier for people like you to understand the thermodynamic concept ask by the OP if I talked about the Otto Cycle. I used those words to try and explain the concept to most readers on this forum, but I never said anything other than the fact that the main reason for thermodynamic efficiency increase in high compression is reduced heat waste to the fluid. All of the false arguments were introduced by you.

I'm listening.

FOR THE FOURTH TIME, I'm still listening. You really are choosing to avoid this.

Luc,

Why don't you read all my posts, rather than glaze over the last one?

The point regarding adding heat to the fluid is the total heat added to the fluid over the entire cycle (ie, once the fluid is returned to atmospheric pressure).

I'll say it again, for either a piston or turbine pressure, then higher the compression ratio or pressure ratio, the less overall energy will have been absorbed by the fluid by the time it has returned to atmospheric pressure.

Higher compression/pressure ratio means for a given fuel flow, less heat exchange has occurred to the exhaust gas by the end of the cycle. It allows more energy to be extracted as useful work.

ok, when we come to turbines i will give here some impressions on the turboprop GA aircraft i currently fly- a cheyenne III with the pt6a-41 engines. the pt6a-41 is a turboprop two shaft engine which incorporates a three stage axial compressor followed by a 1 stage centrifugal compressor driven by a 1 stage compressor turbine. after this we have a 2 stage power turbine which drives the prop via a gearbox. the both shafts are not mechanically coupled and the low pressure turbine drives only the prop - so we call the pt6a is a free turbine.

the engine is like common nowaday design flat rated - so the rated power output is a mechanical limit and the engine is able to keep rated power above ISA or keep rated power in thinner air when you climb until its thermodynamical limit ( ITT or compressor speed) is reached. in other words- the engine could develop on ground more power that the gearbox is approved for.

basicly on this engine you give with the power levers an input to the FCU ( fuel control unit ) to set a target compressor speed. in regard to air density and outside temperature a given amount of fuel is needed to keep this speed. this will result in a given force to the power turbine and a given torque - so power output.

at take off you are mostly torque ( so power output) limited and the turbine is at its mechanical limit . the ITT and compressor speed are below its limit. when you climb out and do not touch the power levers the compressor speed stays the same. the ITT also but torque and fuel flow decreases. this is due the fact the FCU ( fuel control unit) keeps like said a target compressor speed . in a climb out the air gets thinner and the "resistance" on the compressor stages also. so the compressors try to spin faster and the FCU has to decrease fuel flow to keep the same speed. due to less fuel and gas driving the power turbine the torque ans power output also decreases.

when you want to keep the same power output in a thinner air in a climb you will have to push the power levers more and more forward. this will result in a faster and faster spinning compressors and a higher and higher ITT until at a given altitude you match the maximum ITT or compressor speed. here the turbine reaches its thermodynamical limit.

sooo... when the air gets thinner and the compressors deal with a lower pressure ratio ( in pistons compression) the ITT rises .thats a fact. i found and attached a pic at our top of climb in FL 280 with a cheyenne III with pt6a-41 engines so you can have a look what the torque, ITT, compressor speed amd fuel flow is here. at this altitude the engine is at its thermodynamical limit - so the compressor speed is at company limits resulting in a given ITT and torque far below its redline ( mechanical limit)

now we can talk why it is so a a turbine engine.

Aerobat77:

Turbine engines utilise air for cooling of the combustion chamber and the turbines themselves. ITT will rise due to the reduced mass flow rate of air going to the combustion chamber as you climb.

Less mass of air going into combustion chamber, but same mass required for combustion itself = less left over for cooling. I'm not saying this is the only cause (I have a mere pilot's understanding of turbines) but it is the only cause I was taught at the Royal Naval Flying Training School. It is important to helicopter pilots because you can become power limited in the hover due to turbine temp before you reach the service ceiliing - a factor in so called 'hot and high' operations.

...if you didn't have a compressor it would be a ram jet. Anyone mentioned stoichiometric yet ? As someone said, need to have the pressure gradient to avoid a bonfire - imagine starting a ram jet at zero speed.

Thermodynamics
 

Gentlemen, as a mere physical chemist and at the risk of causing more confusion, may I suggest we try to stick to what is to me the fundamental process here?

The more we compress the air the more oxygen molecules we supply per unit time to the combustion chambers so we can burn more mostly hydrocarbon fuel per unit time and release more energy to do useful work per unit time.

That's the simple central point here.

Let's dig a little deeper.

The Second Law of Thermodynamics can be expressed as

ΔG = ΔH -TΔS

The combustion products are hotter than the fuel or the air reactants so the ΔS term becomes more positive as their entropy is now larger, the negative sign above then makes the TΔS term negative.

T is large and positive again making the TΔS term negative

The combustion process (yes yes ideally stoichiometrically , we'll have that discussion if you wish but not here for the moment please) is an exothermic reaction that mostly releases energy to the surroundings in the form of heat so ΔH has a negative sign on this side of the pond.

So ΔG (the free energy) is large and negative and we have lots of free energy available to do useful work - such as provide thrust.

Finally, to go back to the original question of "why compress?", we can now say "so as to release more energy per unit time from the fuel"

CW

oggers:Well, this may or may not be true for a given engine type, depending on the characteristics of the control system (whether governing to a constant core speed, constant corrected speed, constant torque, constant CDP, etc etc.)

If an electronic control, it's all done with ones and zeroes; if hydromechanical, then it's bellows and cams and flyweights.

Blimey - PPRuNe at it's best. A simple question asked by someone seeking enlightenment and they get a thesis and a sh!t fight!

I would like to answer the original queries by by answering the second question first. In simple terms, the power output of a reciprocating engine can be expressed as bore x stroke x rpm x compression ratio. Based on this, if you force more of a charge into the engine by using a turbo or superchager, the compression ratio will increase and the power output will go up.

As for the first part, obviously you understand the OTTO cycle an the principle of induction, compression, conmbustion and exhaust, or suck, squeeze bang and blow. The turbojet engine does the same. The suck is the flow into the engine, the squeeze comes from the compressor, the bang occurs in the combustion chamber and the blow is the thurst. If you look at it like this, if you didn't have the compressor, the engine wouldn't work and as someone else said, all you would have is a bonfire!

Barit:

No doubt you're quite right. However, I was responding to aerobat's specific scenario regarding his specific type in which he observes an ITT rise.

If you follow the thread back you will find this comment by aerobat which gives context:

My point is that if you observe such a rise in those circumstances there is a good alternative explanation; namely less cooling air available.

Pedant head on.

Internal combustion engine (Otto) cycle - suck, Squeeze, bang, fart.

Gas turbine (I refuse to call it Brayton as Mr Whittle formulated it years before) cycle - Suck, squeeze, burn, blow.

Just sayin'. :\




Anyway, to answer the original question...

If the turbine engine didn't have a compressor, then the turbine is just a spinning disc getting in the way. It has to drive something.

I suppose in theory you could have a turbo-prop with no compressor and rely on a very efficient ram effect for compression but as no-one has done it yet I suspect it won't work, for all the very clever and frankly complex answers given on the last 3 pages. :ok:

actually it won't spin unless there is a pressure drop across it. The turbine needs a forced movement of air through it.

Erm, I don't quite understand your point. :confused:

EG. A ram jet with a turbine stuck up it's arris. :\ Pointless but you get my drift?

yea, but how do you get it up to speed to ram it? Difficult to start one on the ground.

Yes, I know but that wasn't the point I was trying to make.

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

It's so that less heat is added to the air over the entire cycle.

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.

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. :)

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.

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

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)).

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.

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.

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?
I can see that you are talking theoretical engine, as some parameter of the engine has to be changed to alter compression ratio; combustion bowl size, head height, or piston stroke.
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.

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

Taking the brayton cycle to the next level
Supercritical carbon dioxide Brayton Cycle turbines promise giant leap

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

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...

That seems to apply to stationary power generation, as a replacement for steam turbines. I don't see it replace turbo jet engines as these CO₂ turbines require an external heat source.

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 !

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?

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. :ok:

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".

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.