Oggers, I'm reposting your Otto cycle diagram from your interesting post #118.

http://upload.wikimedia.org/wikipedi...f/P-V_otto.png

Work accomplished can be seen in the diagram. Energy added by burning the air/fuel charge and the rapid pressure rise is represented by line 2-3. Residual heat remaining when the exhaust valve opens at point 4, is represented by line 4-1. Notice carefully that line 2-3 is longer than line 4-1. The relationship between heat and pressure in a gas is well known, therefore heat has been converted into mechanical work between points 3 and 4, to yield a shorter pressure line 4-1 than pressure line 2-3.

Efficiency could therefore be expressed as the ratio between the heat added to the process to raise the pressure from point 2 to point 3 (a larger pressure change), and the residual heating remaining in the pressure drop from point 4 to point 1.

Why then does higher compression yield more efficiency? Work is done by the pressure difference between the top and bottom of the piston. Higher compression increases the pressure difference, and therefore converts more heat in the pressurized gas into mechanical work, yielding a residual heat value that is lower, and an efficiency ratio that is higher. Notice too that most of the work is done in the top half of the expansion (or power) stroke, where most of the benefit from higher compression is located and is most useful.

In the turbine and rocket engine, higher pressures yield higher velocities imparted to the reaction mass, and thrust as we know is a product of the velocity and mass. The higher the velocity imparted to the same mass, the higher the thrust. However is a turbine, lower velocities imparted to a larger air mass (the turbofan) is more efficient that adding higher velocities to a lower air mass (pure turbojet). With either turbine type, higher compression (or pressure ratios) yields higher velocities.

Oggers...

Regarding my agreement with Chris Weston, my posts have ALWAYS related to the thermodynamic efficiency of higher combustion ratios, and in fact not the question which was in the subject line of the OP (about the fundamental purpose of a compressor) until now.

I will happily admit that my explanation in my first post was not ideal, but then it wasn't meant to be. It was, infact, meant to be an easy way for someone with little or no knowledge of basic physics and conservation of energy to grasp the concept of minimising waste heat. It certainly wasn't supposed to be taken so literally, and after you chipped me on it, I clarified what I meant in subsequent posts (temperature change over the entire cycle).

See above. It's obvious why this is not going to work (or will work very poorly), but allows people to conceptualise what is going on and what is trying to be achieved.

The fact remains that EVERY FN TIME you refuse to answer my question about two different compression turbines with the same fuel flow and the difference in their thermodynamic efficiencies. How many times have you avoided this now? I've asked AT LEAST 5 TIMES now how your "better mixing, flame front speeds" BS applies in this situation, but you simply refuse to answer.

YOU JUST DON'T KNOW seems to be the only explanation.

I'm still waiting.

Slippers...

:suspect: In post #63 CW answered that very question: "why compress - to release more energy to do useful work per unit time within the engine (be it piston or turbine)." But in the very next post you quoted him and replied: "It's so that less heat is added to the air over the entire cycle" and then went on about efficiency again. And so on, throughout the thread. It's clear to me you have shifted position, however, I'm not interested in a subjective exchange so I'll move along.

I wouldn't say "not ideal". I would say wrong.

[BTW, small point but temp change over the entire cycle is zero in both cases. Has to be if the engine is in a steady state because internal energy is a state variable. I only mention that because you said "argue all you like, but I have a physics degree and the principles of thermodynamics have been unchallenged for a few hundred years" ;)]

I assume this was your clarification from the post after I first "chipped you" on it:
Yes, we know the exhaust is cooler. That's a given if we consider increased efficiency in an idealised cycle. The question is why is it cooler? So back to what you wrote originally:

Three questions for you:

1. If that wasn't an attempt to explain why the exhaust ended up cooler, where in that first post is it?

2. Where in any of your posts do you clarify that what you wrote above isn't meant to suggest that 'a hot charge absorbs less heat during the combustion process and vice versa'?

3. If that's not what you were getting at, why write this:

...and by post #31 you were still reiterating the point:

?

What I'm getting at is - despite what you may say now - you were definitely writing that by increasing CR, the working fluid would absorb less heat during the heat addition phase.

Finally:


I'll leave the finer points of combustion in a turbine for someone else. I'll just say this: you chose the piston engine as your original example and no matter how these factors relate to a turbine it doesn't change the fact they are critical in a piston engine, unless you limit your knowledge to an idealised approximation of the cycle. In the real world no such ideal engine exists.

Just speed read this... strewth, talk about handbags at dawn!!

I'm surprised no-one has yet mentioned expansion ratio, rather than compression ratio (at least I don't think it was mentioned). The work output comes from the expansion of the gases, not the compression. A higher CR provides more expansion of the gases after combustion, which is an explanation for the lower EGT of a high CR engine.

The disadvantage of the use of a very high CR is the instability of the fuel/air mix as the peak cylinder temperature increases beyond a critical level, depending on which fuel is being burned. Detonation can occur if this is too high, which results in a "kick" to the piston, rather than a controlled push.

Talking of which, Slippery Pete wrote: Yes, this is incorrect. It would have been more accurate to say that peak cylinder pressure should occur just after TDC, about 17 degrees ATDC in fact. This is for geometric reasons of the conrod pushing round the crank in the most efficient way.

Isn't the expansion limited by the atmosphereric pressure and thus has only small variations?

It would seem that the imput pressure is what varries the most, so in what way does the term of using the expansion ratio differ from the compression ratio in a gas turbine?

We really do need to corral this discusion around gas velocities in a turbine, other wise we wouldn't have a compressor and a means of expansion across a turbine stage or an exhaust jet pipe

I think the point is that the more compression, the higher the pressure in the combustion chamber, and the more the gas expands in the turbine section. To me the real question is why is the energy gained from the extra expansion is greater than the energy used to compress the gas further in the first place.

The only thing I can figure out is that there's effectively less gas in the compressor section (because it hasn't yet been heated in the combustion chamber), and compression therefore take less energy than you get back from expansion.

Imagine climbing a mountain with a 10 pound weight, then letting it go to roll back down. If you had a magic weight that increased to 20 pounds when you let it go, the weight would release more energy rolling down the mountain than you put into it climbing up. And the higher you climbed before you released it, the greater the energy difference would be.

Heating the fuel-air mixture in the combustion chamber is a little like increasing the weight from 10 pounds to 20, except it's due to the heat energy from combustion, not magic. But it still means you get back more energy from the turbine than you put in with the compressor.

At least that's my (doubtless somewhat confused) story.

Chu Chu,

Under high compression you can burn more fuel in a given amount of time as high compression crams in more oxygen molecules et al and you simply release more energy per unit time from the increased levels of combustion.

Specifically you release enough extra energy available to do useful work than you need to use in the compression step. The relationship between compression and power out is exponential and not linear. I will play hunt the graphs.

turbo graphs - Google Search

Increased expansion comes from now having generated more combustion product molecules CO2/H2O/NOx and friends in a given space or volume (can/cylinder etc.)

Gas turbine and piston engines are both fully open systems thermodynamically, exchanging energy and matter with their surroundings.

CW

Oggers,

Thought so.

Dear oggers & Slippery_Pete,

You have both been slinging handbags since Nov 11, and have even developed affectionate names for one another e.g. "Slippers.."

Please could you exchange your love letters via PMs?

In a piston engine, higher compression ration permits faster combustion (especially useful in high revving engines), thus it is possible to get the maximum mean pressure to do useful work (hence more efficient) .

A gas turbine needs high compression to force the air into the combustion chamber against the combustion pressure.
It is introduced through a much smaller surface area than the exhaust gas exit area.

My wife is starting to become suspicious :ok:, so that's a great idea!

I actually went down that path about a month ago, but Oggers refused to answer my PM :D... for probably the same reason he won't answer my question in the public forum either.

Well, not really. The combustor does not create pressure, unlike a piston engine. In fact there's a small pressure drop as air flows through the burner. The main restriction to flow is the turbine nozzle vanes (guide vanes) just downstream of the burner; this restriction, coupled with the pumping flow rate of the compressor, determines the pressure ratio of the machine.

It's very much like a garden hose; the spigot controls the flow into the hose, but if the nozzle is wide open, there's not much pressure in the hose. But close down the nozzle, and the pressure in the hose increases.

:)

Hi Bart1,
Well if doesn't create dynamic pressure - what accelerates the gas?

The static pressure may drop through any venturi - but when you measure EPR surely the exhaust pressure exceeds inlet pressure by the indicated ratio.

Turbine Combustors
 

The combustion system in a turbine engine receives engine airflow that is highly compressed from the compressor, adds heat energy to this airflow and delivers the hot gases to the turbine. The combustor must deliver uniformly mixed hot gases to the turbine just slightly below stoichiometric fuel-air mixture combustion temperatures. At stoichiometric conditions, the maximum amount of heat is released and all the available fuel and oxygen is consumed. Over stoichiometric, excess fuel acts as a heat sink reducing the heat released.

There are generally two regions in a combustor system that are divided into about equal volumes but perform different functions. The upstream region is the primary combustion zone where nearly stoichiometric burning takes place with the correct fraction of air flow. The downstream region is the secondary or dilution zone where the excess air is mixed with the hot combustion products to provide the desired turbine inlet temperature. The average flow velocities in typical combustors range from 60 to 100 Ft/sec. All in all, it is a very complex system.

Combustor efficiency is a measure of the ratio of actual to theoretical heat release and must be as high as possible over the entire operating range of the engine.

The total pressure loss of the combustion system is defined as the difference between the averaged stream total pressure at the compressor exit station and the turbine inlet station. In general, higher pressure losses result in better combustor performance, but of course the engine cycle performance is reduced. A balance must be found between these opposing factors.

TD

The temperature rise in the burner expands the gas - thus its velocity increases smartly. But it's still subsonic flow, in an environment where the velocity of M1.0 is greatly increased compared to ambient. The flow becomes choked at the first stage turbine nozzle, and this sonic flow is what drives the turbine rotor.

It is all about thermodynamic efficiency
 

My vote is with SP. Assuming that the OP's question could be rephrased as "why is compression desirable" or more accurately "why does engine efficiency increase with compression ratio (or pressure ratio for turbines)". The answer is that compression causes an increase in the thermodynamic efficiency of the engine. Compression causes a temperature increase of the working fluid (gas). This results in the heat charge (combustion) being introduced at a higher temperature. The result is an increase in thermodynamic efficiency.

So the answer to the OPs question is that compression results in higher thermodynamic efficiency. Everything else is fluff...

Crabman: No. You have not answered the question, merely repeated it. We know there is an increase in efficiency - half the question was predicated upon that very observation.

The question is why? If you read through the thread properly you will find it has been answered along with the other question posed in the OP. However, the answer slippery gave on page 1 is incorrect.

Crabman 137


Crabman;

I think Oggers and Turbine D et al are right here, I suggest that Slippery P is wrong.

It's true that compression will elevate the temperature of gas phase molecules and that that takes us in the right direction, but it's a fairly trivial contribution relative to the enhanced combustion processes so nicely described by Turbine D in post 135.

Let's stay with the notion of temperature change in gas turbines.

Do the Maths; remember these are fully open systems as thermodynamically they exchange matter and energy with the surroundings.

ΔT1 and the energy from the compression of the gases (courtesy of Van der Waals + Dipole-Dipole et al bond formation) will be, at very best, 100s of degrees C or kJ if you want to calculate the energy released and (unless we're in ram jet territory which I guess we're not) is far less than we require for the energy needed for the compression process.

ΔT2 and the energy released to the surroundings from the enhanced ΔHc (combustion processes see post 128) will be orders of magnitude greater than that needed for the compression and therefore available to do useful work - such as provide thrust.

CW

oggers: I think that I did answer where the increase in efficiency comes from (I certainly was trying to):

"Compression causes a temperature increase of the working fluid (gas).This results in the heat charge (combustion) being introduced at a higher temperature. The result is an increase in thermodynamic efficiency."

CW: I'm not saying that the delta T (not good at typing Greek on my ancient keyboard) of compression is somehow solely the cause of the efficiency increase. Without combustion, it would all be for naught. What I am saying is that delta T of compression results in the heat of combustion (delta Q - from which the work is derived) occurring at a higher temperature. The maths are fairly straightforward and have everything to do with the pressure ratio of the turbine (and its effect on the delta T of compression and the resulting delta T of exhaust, after combustion).

Maybe I'm not expressing it clearly (and perhaps we are even in violent agreement). I will say it simply one more time. Thermodynamic efficiency is a function of Compression Ratio (or a slightly different different function of Pressure Ratio in a turbine). Why? Because of its effect on the temperature changes of the working fluid. What is this effect? The heat of combustion is introduced into the system at a higher temperature.

CW: Your reference to Mr. Van der Waals and the compression of gases left me somewhat puzzled (No, don't bother explaining it - I'm not a chemist - I only work with ideal gases), but started me thinking ... Wouldn't it be interesting if, sometime, we could all have a discussion about something everyone surely agrees upon, such as what causes lift. (and drop names like Newton and Bernoull - and maybe even Navier and Stokes).

Or, perhaps, planes on conveyor belts...