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Why do turbine engines require a compressor section

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Why do turbine engines require a compressor section

Old 19th Dec 2011, 14:19
  #101 (permalink)  
 
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QJB

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

...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.
You will not find the answer to your question in the Carnot cycle where, for two given heat reservoirs, you can achieve the same efficiency regardless of compression ratio used. With your statement above you are effectively hoping to use heat from the hot reservoir to increase the temperature difference between the two reservoirs. A hopeless endeavour.

If you want to consider the effect of increased CR in the Otto cycle with a notional perfect constant volume combustion process occuring just after TDC then you are simply left with a longer expansion path, a correspondingly longer compression path and therefore the difference between the two is also maintained over a greater path ie its more; more net work.

But you cannot increase the theoretical efficiency of the Otto cycle by taking energy out the crank and feeding it back to increase the temperature of the fuel/air charge prior to combustion. This is where slippery_pete has given you a bum steer. If you look beyond the cycle itself to the combustion process then the picture changes because you can increase the speed of combustion by increasing the temperature and pressure in the combustion chamber.

Last edited by oggers; 20th Dec 2011 at 10:22.
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Old 19th Dec 2011, 18:29
  #102 (permalink)  
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Spot on Oggers

At high compression the rate of the combustion reaction increases as the effective concentration of the fuel/air mix increases.

This happens because there is now a greater statistical probability of collisions at above or equal to the Energy of Activation for the reaction

Crucially we're back to time dependence. A more rapid reaction means more bangs per unit time and more power per unit time.

Why do we compress? To make the engine more powerful.

Carnot is predicated on the most efficient cycle for converting a given amount of thermal energy into work

Carnot cycle - Wikipedia, the free encyclopedia

CW

Last edited by chris weston; 20th Dec 2011 at 11:42.
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Old 19th Dec 2011, 19:29
  #103 (permalink)  
 
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To further Chris's post, compressorless atmospheric jet engines are actually quite common. There's one in most DIY's tool kit, it's his blow torch.

The blow torch makes pretty minimal thrust for the fuel used. The combustion chamber pressure is limited to about atmospheric, and air is induced only by venturi effect from the high speed fuel jet supplying the burner. Without combustion chamber pressure, there is no exhaust jet thrust. I've measured the thrust from a small torch (bored at work one day). Used a torch lighter. It managed to make about 1/10gram of thrust. Nothing.

Compressorless jet engines also exist in the non-air-breathing set. Oxy-fuel torches all the way to liquid fuel rocket engines. These ARE compressorless jet engines. In liquid fuelled rockets, the combustion chamber pressures are limited only by material limitations and the pressure your fuel/oxy pumps can deliver. Thus, the chamber pressure can be high, and so can the thrust.

Without a method to adequately concentrate the oxidizer, and supply it to the burner at a pressure above chamber pressures, jet engines cannot produce meaningful thrust on a continuous basis. It is possible intermittently, as in the Argus pulsejet (or indeed any pulsejet engine) on the V1 (pulsejets aren't truely compressorless either, actually, as they use tuned pipe resonance to achieve compression acoustically).

Food for thought.

J
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Old 19th Dec 2011, 19:51
  #104 (permalink)  
 
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In liquid fuelled rockets, the combustion chamber pressures are limited only by material limitations and the pressure your fuel/oxy pumps can deliver. Thus, the chamber pressure can be high, and so can the thrust.
Exactly, and the oxidizer pump performs exactly the same function as a gas turbine's compressor. Without pressure in the rocket's chamber, there is no thrust. And without a fuel pump supplying a like pressure, fuel would never flow into the pressurized rocket chamber.
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Old 19th Dec 2011, 20:00
  #105 (permalink)  
 
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TTex600:
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.
Sorry, I had missed your point. You are of course correct.

But I was addressing the common misconception of the escaping exhaust of a jet or rocket "reacting" against the atmosphere behind the vehicle. I used to have to shoot down that "logic" in the classes I taught.

You are exactly right, the gases in the rocket chamber force against the forward wall, and that is where the reaction is felt.
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Old 24th Dec 2011, 06:28
  #106 (permalink)  
 
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Hello all, and Season's Greetings to you.

Why do we compress? To make the engine more powerful.
I'd agree with that in principle. It allows useful work to be extracted over a wide range of conditions.

If you look beyond the cycle itself to the combustion process then the picture changes because you can increase the speed of combustion by increasing the temperature and pressure in the combustion chamber.
This is misleading. You can't increase the speed of the combustion reaction to extract more energy - we've been over this before. Please see post #70. I'll repost the question I posed for you below so you don't ignore it again, and you can consider them turbine engines (this will prevent you bringing in your multiple false arguments again.

Consider two identical TURBINE 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?

Last edited by Slippery_Pete; 25th Dec 2011 at 23:17.
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Old 24th Dec 2011, 14:01
  #107 (permalink)  
 
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Old 27th Dec 2011, 12:47
  #108 (permalink)  
 
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Slippery

"My explanation from page 1 until page 5 has remained unchanged, and until the laws of physics change - will remain so."
It certainly isn't the laws of physics at issue, so let's have another look at your first post:

"If you consider two cups of water - 1x 50 degrees celsius, 1x 100 degrees celsius... if you put them over a flame of 200 degrees for exactly one second, the cooler cup of water will absorb more heat (because the temperature split between the two is larger)... The same applies in an engine cylinder."
No. In a cylinder, when you burn a given quantity of fuel 'Q', you get:

ΔU = Q - W

The idealised cycle has a constant volume combustion process and looks like this:



The time may vary by a millisecond or two but ALL the heat of combustion goes into the working fluid because W = 0 during this idealised process [#2 to #3 on the diagram].

Bearing that in mind it is clear that your first post is ill-founded:

"When the ignition occurs, a lower compression ratio engine will have a cooler air/fuel charge in the cylinder - and so it will absorb more energy (which is wasted as exhaust gas heat).

A high compression ratio engine will ignite a hotter air/fuel charge which will absorb less heat. Less energy wasted as heat = more energy transferred to the crank."
To be clear, the energy absorbed during this idealised heat addition process will NOT vary with CR in the way you suggest. No useful work can be done - in theory or in practice - with any of the heat until it HAS been absorbed. The clue is in the phrase 'heat addition process'.

"Of course, it follows then that if you were to have two almost identical piston engines (one low/one high compression) burning exactly the same amount of fuel, the exhaust gases from the higher compression engine would be slightly cooler than the low compression engine."
The exhaust gas temperature would vary with the change in PdV work during the power stroke. Not because "A high compression ratio engine will ignite a hotter air/fuel charge which will absorb less heat".
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Old 27th Dec 2011, 12:58
  #109 (permalink)  
 
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http://web.me.unr.edu/me372/Spring20...on%20Cycle.pdf
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Old 28th Dec 2011, 22:37
  #110 (permalink)  
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The problem here is a lot of what is being said is true. There is not one answer to the original question, which in turn is rather artificial.

It is true you need a compressor otherwise you haven't got a jet engine; all you've got is an oil fire. Also higher compression tends to lead to more fuel being consumed. However the point of compressors is to raise pressure. The thermodynamic efficiency of internal combustion engines increases with pressure and temperature. The explanations given above may be true; it doesn't really matter. All you need to know is you want to get the pressure as high as you can without suffering from side effects like spontaneous combustion/explosion.

Note that pressure is not the same as compression ratio. That's why superchargers and turbochargers raise pressure - they help stop brake mean effective pressure falling as rotational speed increases or air thins.
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Old 29th Dec 2011, 12:03
  #111 (permalink)  
 
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Slippery

The edit to your last post is welcome. Crucially you now agree with what CW has been saying in principle, so well done, we are halfway there.

As for the other change:

This is misleading. You can't increase the speed of the combustion reaction to extract more energy - we've been over this before.
I'm not going to rehash the whole thread to prove that you are taking my comments out of context. I will just paste your reply from the time:

"the ignition timing is simply adjusted to ensure the maximum pressure in the cylinder is occuring at TDC."
Which isn't a good concept to be promoting.

Last edited by oggers; 29th Dec 2011 at 15:16.
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Old 29th Dec 2011, 12:52
  #112 (permalink)  
 
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Oggers, I'm reposting your Otto cycle diagram from your interesting post #118.



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.

Last edited by Flight Safety; 29th Dec 2011 at 21:46.
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Old 31st Dec 2011, 09:53
  #113 (permalink)  
 
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Oggers...

Crucially you now agree with what CW has been saying in principle, so well done, we are halfway there.
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).

the ignition timing is simply adjusted to ensure the maximum pressure in the cylinder is occuring at TDC
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.
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Old 5th Jan 2012, 13:36
  #114 (permalink)  
 
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Slippers...

"Regarding my agreement with Chris Weston, my posts have ALWAYS related to the thermodynamic efficiency of higher combustion ratios, and not the question which was in the subject line of the OP until now."
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 will happily admit that my explanation in my first post was not ideal, but then it wasn't meant to be."
I wouldn't say "not ideal". I would say wrong.

"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)."
[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:
"A higher compression ratio adds the heat to a hotter air charge, so once the engine reaches BDC the higher compression engine "fluid" will be cooler. By "absorb less heat", I meant at the end of the cycle the fluid has absorbed less total heat during the cycle (not saying it's cooler at the point of ignition - it is, in fact, hotter as you said)."
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:

"A high compression ratio engine will ignite a hotter air/fuel charge which will absorb less heat. Less energy wasted as heat = more energy transferred to the crank. When the ignition occurs, a lower compression ratio engine will have a cooler air/fuel charge in the cylinder - and so it will absorb more energy (which is wasted as exhaust gas heat)."
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:

"consider two cups of water - 1x 50 degrees celsius, 1x 100 degrees celsius... if you put them over a flame of 200 degrees for exactly one second, the cooler cup of water will absorb more heat (because the temperature split between the two is larger)."
...and by post #31 you were still reiterating the point:

"The DIFFERENCE between the fluid temperature and the burning temperature of the fuel at the point of ignition is LOWER in a high compression engine."
?

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:

"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."

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.
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Old 5th Jan 2012, 15:41
  #115 (permalink)  

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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:
the ignition timing is simply adjusted to ensure the maximum pressure in the cylinder is occuring at TDC.
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.
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Old 5th Jan 2012, 18:55
  #116 (permalink)  
 
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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
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
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Old 6th Jan 2012, 01:00
  #117 (permalink)  
 
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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.
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Old 6th Jan 2012, 20:28
  #118 (permalink)  
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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

Last edited by chris weston; 9th Jan 2012 at 11:23.
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Old 7th Jan 2012, 06:46
  #119 (permalink)  
 
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Oggers,

YOU JUST DON'T KNOW seems to be the only explanation.
Thought so.
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Old 7th Jan 2012, 11:48
  #120 (permalink)  
 
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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?
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