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...

Crabman,

Thank you, nice idea.

But .....don't forget the importance of the little guys in chemical bonding. The 787 is in trouble if we do.

CW

Hi Crabman.

You are on the right track.

What you have to understand here, is that the concepts which others have discussed (flame front speeds, timing and better burning) relate to design parameters of a particular engine using the Otto cycle - not the theoretical thermodynamics of the engine.

Oggers and CW say that faster “flame front speeds” make engine X more efficient at higher compression. I say you can make engine X just as efficient by operating it at low RPM. In fact, this is what the world's most efficient piston engines do - they operate at extremely low RPM.

Oggers and CW say that “better burning” make engine Y more efficient. If this were true, consider what would happen if you increased the compression ratio of an engine at constant fuel flow. Oggers and CW say that you will more completely combust much more of the fuel. This should mean that the exhaust gas temperature should increase. We saw from the linked document in post 41 that in fact, completely the opposite occurs – EGT reduces. Ask them to explain why “better burning” (which obviously they are implying means less unburnt fuel being wasted) causes the EGT to reduce.

You can easisly take their piston engine arguments out of the equation by running a large diesel engine at extremely low RPM (where flame front speeds mean jack !!!!! and where the fuel is essentially completely combusted). But why does compression ratio still affect the thermodynamic efficiency?

I can tell you why - because you are adding less energy to the fluid.

As for the practicalities in a car or aircraft of a large, extremely low RPM diesel - I obviously understand this and a multitude of other reasons why for practical purposes there other considerations in making piston engines adaptable and useable over a large range for a particular application. But these are inconsequential to the FUNDAMENTAL concept of the OP.

Not only will Oggers and CWs arguments not apply to a large, efficiently designed and operated piston engine, they also fail to apply to a turbine. This is why they have both failed to answer my question about this so many times – because their reasoning simply can't explain it.

Hi Slippery Pete,
Er... I'd say that if the fuel was burned completely by the correct moment during the cycle, then more useful work can be extracted from the energy supplied, therefore you would need less fuel compared to the inefficient cycle, therefore the EGT would reduce.

Your engine X may be very efficient at low RPM, because the combustion is completed by the time the piston is at position Z. However it is not producing as much power as a faster revving engine.

As the revs increase, we need faster burning (hence higher compression ratio) so that combustion is complete by the time the piston is at position Z.

Slippery Pete,

At no stage have I referred to flame front speed or indeed to "better" burning in this thread, I have never referred solely to piston engines.

I have carefully not considered the kinetics of the hydrocarbon combustion processes as they follow and do not lead in this discussion on "why do we compress?" We can go into the kinetics if you wish presumably to bring some order into this discussion if you wish, the pun is intended.

I do however agree that rate (or flame front speed) will show a form of direct proportionality to gas pressure during combustion. It follows from simple rearrangement of the ideal Gas Equation pV = nRT. Pressure can be equated as concentration on rearrangement etc.

What I have stressed is that compression allows more combustion per unit time thereby releasing more energy to do useful work per unit time.

We compress primarily to make the engine more powerful. You can be as efficient as you like in your isolated system engine but if you lack the power to reach Vr what's the point! I believe Oggers said something similar some time ago albeit in a different way.

Your efficiency argument is predicated on thermodynamically closed systems.

Remember that piston or gas turbine engines are more or less fully open systems thermodynamically.

The heating effect due to increased combustion per unit time is orders of magnitude greater than that relating to heating from compression alone.

Beyond a defined threshold, the need to add less combustion energy to an already compressed fluid is relatively trivial in an open system.

Hope this helps!

CW

Compression
 

This is a little off topic but valid.. 3 'planes take off to the north but the route is to the south. (A) takes off, leaves the flaps down, turns south then cleans up and accelerates. (B) takes off, cleans up the flap, remains at 0 flap speed, turns south then accelerates. (C) cleans up, accelerates to 250 Kts IAS, then turns south. Which has the best performance and burns the least fuel?
It's "C". Why? At 250 Kts IAS there is more ram air and therefore more power. "C" was further south and higher on less fuel than "A" & "B". More compression (ram) means more power. This was done in the late '70s.

No doubt you climb faster, but since TAS & GS are greater, your rate of turn (assuming same bank angle) is less, time & distance traveled is greater, and total trip distance is greater.

So the "more power" aspect is fighting the physical facts of more time and distance. And you don't get "more power" except by burning more fuel. Perhaps "C" is correct, but not by much.

(Although - I certainly can sympathize with a Canadian hurrying to fly south...) :p

I read all the above and reckon there’s room for another angle on this so I’m going to resurrect this thread and wade in here – this is one of those questions which keeps me awake at night – something that’s generally accepted (e.g. because a computer model or a graph tells you) but there are rarely any good/satisfying answers to be found …

The question was put in terms of gas turbines and also reciprocating engines. I’ll answer in terms of the former because that’s what I know about. I’ll assume that the same principle can be read across to reciprocating engines. That exercise is left to the student...

So why do we compress? I distinctly remember answering this with ‘because it packs more wallop’ at a university interview nearly 30 years ago - not a bad answer really, I got a degree in the end….

Let’s put a few things to bed:

1. It is nothing to do with rate of combustion.

I can show (below) that you can get more thrust (and/or better thermal efficiency) from a turbojet with a 14:1 OPR (overall pressure ratio) than from a turbojet with a 12:1 OPR both designed for the same inlet airflow and operating at the the same fuel flow.


2. It is nothing to do with keeping the air going in the right direction.

Mr Whittle figured he’d need a compressor for cycle efficiency reasons and was probably jolly glad that this also kept things going aftwards – so that was something he didn’t have to worry about. I can also show that an engine with hardly any OPR (1.1:1) will develop some thrust but will have a shocking thermal efficiency. You don’t need much pressure ratio to keep things going rearwards – a slightly inclined tube with a fire in it does the job pretty well - think of the Mt Blanc tunnel fire of 1999…

Essentially, the answer is that for the higher OPR engine, the nozzle pressure ratio (NPR) is higher so you get more pressure thrust. Momentum thrust is the same (in our example).

For a convergent nozzle: Gross Thrust = W8.v8 + (Ps8-Pambient).A8

... where 8 is the nozzle throat plane.
W is flow
Ps is static pressure
v is velocity
A is area

Momentum thrust (W8v8) is the same because W8 is the same (we fixed that at the start in the way we set up the comparison for both engines). Exhaust jet velocity v8 is the same because T8 is the same and the nozzle is choked (Mach 1).

But why is T8 the same for the second engine, and indeed, and why is NPR higher i.e. why is nozzle entry pressure (P7) higher?

Furthermore why, for the higher compressor PR, doesn’t the turbine operate at a similarly higher pressure ratio to make the NPR the same as before?

Burning questions indeed ....

Let’s look at two design points as described above. I ran these on a proprietary Gas Turbine simulation program (GasTurb – you can download a free reduced functionality version from the website). This saved me doing the calcs myself. The first engine had 12:1 OPR and the second 14:1 at the same airflow and fuel flow (for this we need a smaller turbine throat area and final nozzle area – these areas fall out of the thermo calcs).

Sure enough – we get a higher NPR in the 14:1 case at much the the same T8 because the extra T we’ve added at the compressor is taken off by the turbine, with about the same combustor heat addition in each case. Thrust is higher (due to increased NPR) and so is thermal efficiency (obviously, because we are getting more thrust at the same fuel flow).

Furthermore, Gasturb allows you to throttle back the second engine (i.e. run slightly slower at fixed geometry using nominal component maps) to get the same thrust as the first engine. Lo and behold, the thermal effy was slightly higher in the second engine case (throttled back – so no longer operating at 14:1).

The 1.1 OPR example gave quite a bit of thrust by virtue of the momentum term (v was significantly increased due to the temp rise) but the cycle was at ~1% thermal effy!
Compare this with Engine 1 at 40% and Engine 2 at 42%.

I haven’t answered those questions yet though - all the GasTurb run showed me is what we know happens, though it does illustrate very clearly that it is not a rate-of-combustion thing...

Essentially the answer is in the relationship:

Pressure Ratio = Temp Ratio ^ (gamma/gamma-1)

Because the turbine has to develop a deltaT (starting at a higher T due to the combustion process) to drive the compressor, the turbine temp ratio is smaller than the compressor temp ratio and consequently the pressure ratio is smaller. For an increase in OPR, the turbine therefore satisfies the extra power requirement with some pressure ‘to spare’ thus NPR increases.

You can do these calcs by hand, just using the equation above – making them very simple by assuming 100% component efficiency and assuming an identical combustor temp rise for each engine (not far from the truth in these two examples – remember we set the airflow and fuel flow to be the same). You can also assume constant gas properties (gamma and Cp) – realities such as varying gamma and Cp just make the sums trickier but don’t change the basic principle.

I realise that I haven’t really explained things all that satisfactorily as my ‘explanation’ still relies on an unchallenged thermodynamic equation (I’m happy to accept it!). But it does illustrate where the benefit comes from, and possibly more clearly than ‘explaining’ things in terms of Temperature-Entropy charts where the pressure lines ‘conveniently’ diverge! (for the same reason of course).

So, a slightly different, and perhaps long-winded - angle on the subject to what’s gone before. And hopefully useful !

Really :confused:

But the subject did mention Gas Turbine

From a standing start how do you expect a rotational compressor to turn if the gas doesn't move in the direction of a turbine somewhere downstream of the combustor or are we getting into a related subject like a ram-jet ?

I like Chris Weston's comment about compression. Isn't it the reason computers, geting smaller, also get faster? The Physics is similar? Faster is better. And faster implies proximity, yes?

Starting is an entirely separate issue. Claarly, the stable flow/burning process needs to be initiated by persuading everything to flow in the right dierction. However what this thread is all about (I believe - correct me if I'm wrong), is why an increase in compression ratio results in an increase in thermal efficiency.

The thread is whatever the responders make it

This is so often true in the technical section where the thread starter drops a one-liner subject header and disappears into the nether leaving us to expound all sorts of intelligence that falls on deaf ears.

but it's fun anyway :)

Keeping the gas going in the "right direction" is everything to do with propulsion.

The starting cycle is a simple way of understanding the need for compression. Without compression, there is no work. How can compression become irrelevant to the process at any time?

Starting is compression, it contains gases in a dynamic flow by directing its passage from a large to small annulus. Without the pipe, there is no power, and without the fan, there is only multidirectional expansion.

Water injection, Open Iris Afterburner, Hypergolic starting, Start carts, Compression is our friend. The difference between early centrifugal compressors and axial flow instructs as to the OP post. PUSH, PACK, POP, off you go.

Lomapaseo – quite right, we do (and should) pick up stuff around the edge of the discussion, however I was addressing what I considered to be the main question. And yes – it is fun.

Lyman, you said “Keeping the gas going in the "right direction" is everything to do with propulsion”.


I’m not disagreeing with you – I was suggesting that the reason the compressor is there is for fundamental cycle effy reasons and not primarily to set the flow direction (it’s perhaps more of a philosophical point I am making here).

You also said : The starting cycle is a simple way of understanding the need for compression. Without compression, there is no work. How can compression become irrelevant to the process at any time?

I’m certainly not saying compression is irrelevant to the process. I’ve established that it is fundamental to getting work out of a machine at a good thermal efficiency. Work is required in the start phase just as in other operational phases (steady-state, accels and decels alike)

But starting is a means to an end, it is all about getting the engine to a stable, self sustaining operation to deliver the required thrust. Starting needs compression, sure, and it needs external cranking power (or windmilling). And yes, compression sets the flow direction, though you have to be especially careful to control the fuel flow (and variable geometry if you have any) otherwise the compressor might get upset (stall). Same comment applies for ‘normal’ (engine-started) operation where reverse flow may occur under some circumstances.

‘Compression is our friend’ – certainly, it keeps me in a job. Compressors however are a nuisance!

Hi jh5speed,

Our local National Trust house has a very old gas turbine installed in the kitchen.

It consists of an open coal fire (at ambient atmospheric pressure) and a very long vertical jet pipe called a chimney. A turbine (set of fan blades) is placed above the fire and its rotation is converted through a suitable system of bell cranks and levers to turn the spit and roast the meat.

It doesn't seem to be very efficient - would a compressor help?

Without a doubt, though probably the extra cost of control-system improvements would outweigh the efficiency gain (however calculated - I guess it would need a labour cost element as well as fuel cost to account for those poor servants who, at present, have to waste valuable time checking that the meat is browning evenly, and fetching in the extra odd bucket of coal). But then, if they weren't doing that, what would they be getting up to?

Lyman will advise you on getting the fire lit without smoking the place out.

Once lit, I reckon if smoke isn't coming back into the room and the meat is turning, you'll probably be better off leaving well alone.

Oh - and it might be a good idea to send a child up the chimney with a tub of goose fat to grease all those bellcranks and pulleys (thus increasing mechanical efficiency) ... While he (or she) is up there a decoke of the turbine blades might be a good idea.

Hi jh5speed,

Thanks for the tips and advice which I've passed on to the N.T. kitchen turbine department.
Apparently they have tried using a compressor (mechanical bellows of soft leather nailed between flat wooden plates) which increases the kinetics and raised the combustion temperature. The more energetic exhaust flue gas turned the turbine more quickly.

It seems to be a win win situation, but the bellows operator is demanding extra pay because of the increase in thermal efficiency. They are now working on a mechanically driven set of bellows from the extra power from their turbine.
They want to know if it is worth adding a second turbine, downstream of the first to work the bellows?

They are grateful for the advise on the turbine wash and suitable gearbox lubrication.

jh5speed, rudderrat

Hmmm. Save the coal for your children at Christmas, seal the bellows at the top of the flue and have your valet pump the bellows into the chimney in reverse flow. Change the pitch on your fan, and use the gear to drive your abacus.

Do your part for the Planet :ok:

Why do turbine engines require a compressor section
 

Apologies if anyone has already mentioned the real reason . . .

...............................http://upload.wikimedia.org/wikipedi...lovmeerkat.jpg

. . which is, of course, to absorb the power developed by the turbine and prevent it from overspeeding - simples!

more thoughts..if I dare
 

I think this is a new angle and hope it is not in the mind-numbing category (not derogatory but complimentary in this context) of most of the previous posts. It is not in answer to any outstanding question, be it either still to be answered or of great merit.

As already stated the RJ needs (and has) a compressor just as does a jet engine. I suspect the reason for the initial observation that it doesn't have one is the belief that they are fundamentally different machines. The RJ and TF are just different breeds of the same with very similar cycle requirements but obtained from very different looking parts. I don't think there's any great stumbling block to be made of the fact that a conventional RJ doesn't work at zero speed but the TF can.

At zero airspeed the TF does no more than an RJ. It can sit on a test bed running at TO from Monday to Sunday (with slave oil supply) and it has actually done less than nothing. It has moved nothing, except the thrust cradle a few thou, but has cost a weeks worth of fuel/oil/cell occupancy/creep life/etc. Once it starts down the runway though, it has taken the first step to being a ramjet and the turbom/c compressor has taken the first step to being redundant. eg you can't use the SLS 43:1 PR of a big fan in an F-22.

The B777 at cruise has the same subsonic piece of compressor hardware as the pre-SCRJ, a piece of ducting. the only difference is that the rest of the compressor is downstream on the TF but upstream on the RJ.

I find it satisfying to look for underlying similarities rather than thinking there are fundamental differences.

To get the job done both the RJ compressor, with its attendant shock compression, and the TFC, with its supersonic regimes, treat the air with the utmost violence on the one hand, and then gently on the other, with the touchy subsonic diffusion in the duct or rotating blade rows and fixed stator passages.

The degree of brutality which the RJC metes out has always been foisted upon it by the missile cruise requirements. The turbom/c, on the other hand, has increased in brutality from the gentle subsonic compressor of a J79, for example, where the inevitable low stage PR required 17 stages to get about 13:1. You have to thrash a lot more energy into the air if you want a compact HPC where 10 stages give about 20:1. The road to this level began with turbocompressors entering the RJ compressor regime by using supersonic blade relative MN.

Therein lies a similarity. It's high relative MN between air and pieces of metal that give you the makings of a compressor. You don't necessarily need relative motion between the metal of the compressor and its 'mother'.

But the conventional RJ still can't get off the ground!
Bear in mind that the TF only exists to cruise just like the RJ. It needs to be a 'different' machine to get there, in as much as, in its money making regime, it has different ratings, ECS bleeds, turbine clearance bleeds, etc. compared to TO and CL. A B777 won't economically get to 35000ft in its cruise 'config'. The TF is a hybrid.

So what if an RJ also needs a bit of hybridization to get to cruise.

Barit 1 A car with a turbo charger (or supercharger) produces more power due to the compressed air being forced into the intake but does not burn more fuel. Hence compressing the intake air (more volume) makes a smaller engine produce the same power as a non turbo charged larger engine. Why do jet engines require a compressor? Because they wouldn't work otherwise. Simple. Don't make a mountain out of a mole hill.