english jet engine

8/4/2019 English Jet Engine

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Turbine Engine Technical English Course

Unit 1

FIRST PRINCIPLES

Dialogue

Welcome to Air-Alpha, the aero-engine school which runs courses in jet propulsion for

students from all over the world. A dozen men have gathered in the school's main

lecture hall for their first session with Mr. North, the Chief Instructor. Some are

examining the engines on display around the room, while others are talking quietly

together near the instructor's desk. Then Mr. North enters, and the session begins.

Mr. North: Good morning. We have a lot to do, so I suggest we get straight down

to business.1 I shall be working fast, but please stop me If there's

anything you don't understand. The more questions you ask, the more

you will all learn.

Chris: May I ask the first one? As a pilot, I'm particularly interested in

*controls and *instrumentation. Will we be dealing with these during

the course?

Mr. North: Yes. In fact I hope to cover the whole fleld2 of jet propulsion from the

first principles, which you must master before you begin to specialise,

right up to the latest developments in *supersonic flight.

Ben: Im a technician, so I know how a piston engine works. Is that

knowledge going to help me here?

Mr. North: Yes, indeed. The two principles are very similar.Chris Surely not! A piston engine works on a four-stroke cycle: induction,

compression, combustion and exhaust. But air flows straight through a

jet engine.

Mr. North: True. Yet a jet engine has the same four stages. Look at the diagram of

a *ram jet.

As you see, it is basically a *tapered tube, open at both ends, into which

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*fuel can be injected.

Ben: If that's all there is to jet engines, I needn't stay!

Chris: I suppose air flows into the tube from the left. Is that the induction

stage ?

Mr. North: It is. Now, once inside, the air slows down because the tube widens,and the *kinetic energy released by this loss of velocity is converted

into pressure energy and heat.

Ben: Why? I don't quite get that.3

Mr. North: Well, energy can't be lost; it can only change its form.

Chris: That happens in a car. If you slow down, the lost speed reappears as

heat in the brakes.

Ben: I think I see4 now. And in a jet engine this energy reappears both as

heat and as an increase in pressure?

Mr. North: Exactly. Pressure, after all, is just another form of energy.

Chris: This presumably corresponds to the compression stroke in the piston

engine.Ben: Does the compressed air then flow into the *combustion chamber?

Mr. North: Yes. Then fuel is injected and ignited.

Chris: That must cause a big rise in temperature, as it does in a piston engine.

Ben: And a big increase in pressure, too.

Mr. North: No, Ben. The air becomes very hot, but it's free to escape through the

rear of the tube, so there's very little rise in pressure.

Chris: Both combustion and exhaust stages occur simultaneously, then?

Mr. North: Yes. In fact all four stages are going on at the same time, but in

different parts of the engine. In a piston engine, they all occur in one

place, the *cylinder, but at different times.

Ben: If there is so little rise in pressure, surely the combustion chamber can

be made much lighter?

Mr. North: Quite right. And this lightness is a great advantage where aircraft are

concerned.

Chris: Couldn't the compression be raised by putting some sort of

supercharger near the intake?

Mr. North: A good question, Chris. But I'm afraid we can't go into it now;5 let me

deal with it next time, when we meet the other members of the jet

family. They're all related to the ram jet we've been studying today.

Conversational expressions

1. get . . . down to business: start work

2. cover the whole field: discuss the whole subject

3. I dont . . . get that: I dont understand that

4. see: (here) understand

5. we cant go into it now: we cant talk about it now.

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Vocabulary

Controls.- The various switches and levers by which the operations of the engine can

be regulated.

Instrumentation.- The system of sensors and instruments that enable the pilot to know

how the engines are performing.Supersonic.- Faster than the speed of sound.

Ram jet.- A type of jet engine in which fuel is burned in a duct using air compressed by

the forward speed of the aircraft.

Tapered tube.- A tube progressively decreasing in diameter.

Kinetic energy.- The energy of a body in motion.

Cylinder.- A tube in which a piston can slide.

Exercise 1: Comprehension questions

1. What should a student of jet propulsion master before he begins to specialise?

2. How would you describe the shape of the ram jet engine shown on page 1?

3. Why is there a rise in pressure energy when air enters the front of the engine?

4. What happens after the compressed air flows into the combustion chamber?

5. Why can the combustion chamber of a jet engine be made of light material?

6. Where are lightweight engines particularly useful?

Exercise 2: structural practice

Notice this structure from the conversation:

If that's all there is to jet engines, I neednt stay!

Use this structure to make sentences from the following:

Example:the design / study it

Response: If that's all there is to the design, I needn't study it.

Now, you do it.

1. the design / study it2. this course / work very hard

3. the diagram / look at it

4. the problem / bother about it

5. piston engines / spend any more time on them

6. this job / worry about it

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Exercise 3: progressive substitution drill

Statement: Ihope to cover the whole field of jet propulsion.

Prompt: We

Response: We hope to cover the whole field of jet propulsion.

Now you do it.

Statement: I hope to cover the whole field of jet propulsion.

Prompts:

1. We

2. He hopes

3. intends

4. expects

5. to deal with

6. the whole subject of

7. aircraft engines

8. engine design

Exercise4: further structural practiceNotice this structure:

We'll meet the other members of the jet family next time.

Use this structure to respond to the following questions:

Example: Have you met the other members of the jet family?

Prompt: next time

Response: No, we'll meet the other members of the jet family next time.

Now you do it.

1. Have you met the other members of thejet family? next time

2. Have you had your first session at Air-Alpha? soon

3. Have you looked at the diagram? in a few minutes

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Now do these.

4. Has he dealt with controls and instrumentation ? in the next few

weeks

5. Has he answered your question? when he's got

time

6. Has he covered the whole field of jet propulsion? during the course

Further Reading

To learn how a jet engine works, we must remember Sir Isaac Newton's third law of

motion: "For every force acting upon a body, there is an equal and opposite reaction".In the present case, the "body" is the volume of air that is passing through the engine.

The force that accelerates this air towards the rear of the engine also exerts an equal

force in the opposite direction, thus driving the engine and the plane itself forwards. lt is

important to understand that this forward *thrust occurs inside the engine itself; it is not

caused by the high-pressure exhaust gases acting directly on the outside atmosphere, as

many people believe. This point is well illustrated by rocket engines which, like

aeroplane engines, work on the principle of *internal reaction. We all know that these

are capable of forward motion even in outer space, where there is no surrounding

atmosphere for their exhaust gases to "push against".

Exercise 5: comprehension questions

1. What is Sir Isaac Newton's third law of motion?

2. What does the force that accelerates the air towards the rear of the engine also do?

3. Where does the forward thrust occur?

4. Do rocket engines work on the principle of internal reaction?

5. What is the density of the atmosphere in space?

Exercise 6

Use the following words in sentences of your own to show that you understand their

meaning and use:

1. tube 7. velocity

2. controls 8. tapered

3. exerts 9. instrumentation

4. cycle 10. stage

5. pressure 11. caused by

6. flow 12. supersonic

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Unit 2

THE JET FAMILY

Dialogue

Mr. North: Good morning. At our last session we saw how a ram jet works, and

today I want to introduce you to the other members of the jet family.

Ben: Before you do that, can I ask one more question about ram jets? What

forces the air in at the front end?

Chris: The forward motion of the plane, of course.

Mr. North: Thats correct.

Ben: Im not with you.1 What if the plane is stationary on the ground?

Mr. North: Chris is still right. A ram jet can only work at high velocityalready

moving forward at high speed. Thats why it is not used in

conventional aeroplanes.

Chris: What about pulse jets? They can run while nearly stationary, cant

they?

Ben: What on earth are they ? Ive never heard of them.

Mr. North: Pulse jets are similar to ram jets, but they have *flap valves at the front

end. These open to let air in, but close again when back-pressure is

created by the fuel burning in the combustion chamber. As the burnt

gases escape to the rear, so the pressure at the front end drops to a point

below that of the outside atmosphere. At this point, the flap valves re-

open, and more air rushes in.

Chris:Does a pulse jet work by internal reaction, too?Mr. North: Yes; ram jets, pulse jets, *rockets and even propeller-driven aircraft are

all practical applications of Sir Isaac Newtons third law of motion.

Ben: Which jet engines are most used in aeroplanes today?

Mr. North: Undoubtedly the turbo-jet. This is rather like the ram jet, but it has a

*compressor at the front to force air in.

Chris: I suggested that idea at the end of the last lesson.

Mr. North: You did. And we shall be studying compressors in detail next time.

Ben: What drives the compressors? The energy must come from somewhere.

Mr. North: Think, Ben. Have a shot at2 answering that one yourself.

Ben: Well, theres plenty of energy at the back, where the hot gases leave the

combustion chamber. If you put a *turbine there and connected it to thecompressor by a *shaft, that would do the trick,3 wouldnt it?

Mr. North: Thats a pretty fair4 description of a turbo-jet. Well done, Ben.

Chris: What about the other members of the jet family?

Mr. North: Theyre all variations on the turbo-jet principle. Theres the turbo-prop,

for example, where the turbines rotate both the compressor and an

ordinary *propeller.

Chris: Whats the advantage of that?

Mr. North: Well, at lower flying speeds, propellers used to be a more efficient form

of propulsion than pure jets. But by-pass engines were later developed

Ben: How do they work?

Mr. North: Will you have a go at5 explaining that, Chris?

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Chris: Il1 try. In by-pass engines only some of the air goes straight through;

the remainder is passed through *ducts, round the combustion chamber

and turbines, but rejoins the main stream at the rear. This makes the

engine quieter and more efficient.

Mr. North: Good. Fan engines are an extension of this principle. In their. case, the

first set of rotor *blades of the compressor are enlarged to form, a sortof multi-blade propeller, known as a fan.

Chris: Is this air from the fan ducted to the rear of the engine ?

Mr. North: In some cases, yes; but not always.

Ben: Is that the last of the jet engine family? I'm beginning to get thoroughly

confused.

Mr. North: Yes, Ben, thats the lot.6 As you see, there are ram and pulse jet engines

that have no compressors or turbines, and which youre hardly likely to

meet; then there are the various types of turbo-jet, many of which have

by-pass systems to make them more efficient. Some work by jet

reaction alone, while others use much of their power output to drive

either a conventional propeller or a fan.


1. Im not with you: I dont understand you2. Have a shot at . . . : Try to . . .3. that would do the trick: that would give the desired result4. pretty fair: reasonably satisfactory5. have a go: try to6. thats the lot: thats all

Vocabulary

Flap valve.- A simple type of valve, used particularly in the pulse jet engine.

Rocket.- A jet engine that produces its own propelling fluid by the combustion of liquid

or chemically decomposed fuel with oxygen, which it carries, thus enabling it to operate

outside the earths atmosphere.

Compressor.- The part of the engine that raises the pressure of the air from the intake

before feeding it to the combustion chamber.

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Turbine.- A disk fitted with blades which turns when placed in a moving stream of air

or gases.

Shaft.- A rotating member for the transmission of power.

Propeller.- Revolving blades set at an angle that drive a ship or plane forwards.

Duct.- A passage or tube through which air or gases can pass.

Blades.- Metal aerofoils forming part of a propeller, compressor or turbine.


1. What does a pulse jet have which a ram jet does not have?

2. What drives the compressor of a turbo-jet?

3. What is the function of the compressor?

4. When is a propeller more efficient as a means of propulsion than a pure jet?

5. What happens to the air in a by-pass engine after it has gone through thecompressor?



As the burnt gases escape, so the pressure drops.

Now use this structure to join the following sentences:

Example The speed of the aircraft increases.The efficiency of its propeller decreases.

Response: As the speed of the aircraft increases, so the efficiency of its propeller

decreases.

Now you do it.

1. The speed of the aircraft increases.

The efficiency of its propeller decreases.

2. The fuel burns.

The air expands.

3. The pressure goes up.The valves begin to close.

4. The velocity drops.

The pressure increases.

5. The plane moves forward.

Air is forced into the engine.

6. The flap valves re-open.

More air rushes in at the front end.

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Statement: I suggested that idea at the end of the last lesson.

Prompt: He

Response: He suggested that idea at the end of the last lesson.

Now you do it.

Statement: I suggested that idea at the end of the last lesson.

Prompts:

1. He

2. explained

3. this means of propulsion

4.

Newtons law5. pulse jets6. at the start of

7. your

8. beginning of last week

Exercise 4: further structural practice

Complete the following sentences with their appropriate question tags.

Example: A ram jet can only work at high velocity.Response: A ram jet can only work at high velocity, cant it?

1. A ram jet can only work at high velocity.

2. A pulse jet hasnt got a turbine.

3. The first jet engines were very inefficient.

4. The first true turbo-jet plane flew in 1939.

5. Jet reaction is not an external phenomenon.

6. You had to study the principles of jet propulsion.

German flying bomb V1 in World War II was powered by a pulsejet

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Further Reading

Every member of the jet family has its uses. Ram jets, for example, are suitable for

propelling certain *missiles that are launched from parent aircraft or by rockets. Their

inability to work unless travelling forward at high speed then ceases to be a

disadvantage, and they are both cheap and easy to manufacture. Pulse jets are not seentoday because of their high fuel consumption, but they were used successfully in the

Second World War to power Germanys V1 flying bombs.

Most modern aeroplanes have turbine/compressor engines. At supersonic speeds,

turbo-jets are highly efficient, so they are ideal for fighter aircraft. But, at slower

speeds, an engine that slightly accelerates a large mass of air is more economical on fuel

than one which produces the same thrust by imparting considerable acceleration to a

small mass of air. So by-pass engines, with their larger *airflows at lower velocities,

are more suitable for civil aircraft, where low running costs are more important than

mere speed.

The last member of the jet family is the rocket. Because it carries its own supply of

oxygen, this is the only one suitable for use in outer space.

Vocabulary

Missiles.- A military rocket.

Airflow.- The movement of air.


1. What are ram jets suitable for?

2. Why are pulse jets no longer used?

3. What type of engine do most modern aeroplanes have?

4. Why are by-pass engines more suitable for civil aircraft?

5. Why is the rocket the only member of the jet family suitable for use in outer space?

Exercise 6

Use the following words and phrases in sentences of your own to show

that you understand their meaning and use.1. energy 7. atmosphere

2. turbine blade 8. compressor

3. propulsion 9. low running costs

4. inability 10. duct

5. supersonic 11. efficient

6. forward motion 12. stationary

Saturn 5

Launch vehicle for Apollo XIII


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Unit 3

THE COMPRESSOR

DialogueBen: You promised to tell us about compressors today, Mr. North.Mr. North: Yes, Ben, so I did. There are, in fact, two sorts: centrifugal compressors

and *axial-flow compressors. The latter are more efficient and are used in

most jet engines today, but I think you ought to know a little about theformer, too.

Chris: How does a centrifugal compressor work, then?Mr. North: Like this. A vaned disc, called the *impeller, is attached to the turbine

shaft near the *air intake. As the disc rotates, it spins the air in contact with

it. And centrifugal force flings the air outwards at high velocity.

The impeller

Ben: But I though we needed to increase the pressure of the air, not its velocity.Mr. North: Hold on,1 Ben, I havent finished yet! Do you remember how pressure was

increased in the ram jet engine?

Chris: Yes. The air entered a widening, or divergent, tube.Mr. North: Exactly. And in this case, the air is flung off the impeller into divergent

passages formed by diffuser vanes attached to the outer *casing.Ben: Oh, I see. When the air enters these passages, it slows down, and the lost

kinetic energy reappears as pressure energy. Is that right?Mr. North: Absolutely right. From there, of course, the compressed air is ducted to the

combustion chamber of the engine.Chris: It sounds simple enough in theory.Mr. North: Yes. And in practice this type of compressor is both robust and fairly easy

to manufacture.

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Ben: But you said its not so efficient as the other kind.Mr. North: Yes, I did. So lets move on to2 axial-flow compressors, which youre

more likely to meet, anyway.Chris: Are these compressors *mounted on the *turbine shaft, too ?Mr. North: Yes. But instead of driving the air outwards by centrifugal force, they

force the air straight backwards by means of a series of multi-blade fansmounted one behind the other.Ben: Well, that sounds a more efficient arrangement, for a start!Mr. North: It is. Between each set of rotating blades, or *"rotor blades", as they are

called, are similar sets of stationary blades, attached to the outer casing.

These *"stator blades" re-direct the air to the correct angle for the next setof rotor blades. Each set of rotor blades with its stator blades is known as a

*"stage".Chris: I suppose that in a multi-stage axial-flow compressor the air must reach

very high velocities.

Mr. North: Not at all. Each stage accelerates the air only a little. But as the air passes

through the spaces between the blades, there is a drop in velocity ...Ben: . . . and the lost kinetic energy reappears as pressure energy! This is where

I came in,3 Ithink!Mr. North: Yes, I think it is. But at least you can see why I insisted on you learning

the first principles.Chris: Yes. For a while I thought you were wasting our time with all that talk

about ram jets. I know better now!Ben: Theres one thing that still bothers me. You say that the velocity of the air

remains almost constant throughout the compressor. Yet, at the same time,

I can see that theres a progressive rise in pressure.Mr. North: Yes, thats quite true.

Ben: But isnt that a contradiction? I mean, surely you cant alter one without

changing the other, can you?

A typical axial-flow compressor

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Mr. North: A very good point, Ben. But times up4 for today, so I mustleave you to try to answer it yourself. Study the construction of a typical

axial-flow compressor. Heres a diagram to help you. If you look

carefully, you may find the solution to your problem.Chris: And if we dont find it?

Mr. North: Im sure you will. But Ill give you the answer next time, when we look ataxial-flow compressors in a little more detail.


1. Hold on: Wait a moment2. lets move on to: lets talk about. . . now3. This is where I came in: Ive heard about this before, this is what we were discussing

earlier4. Times up: we have no more time left

Vocabulary

Axial-flow compressor.- A compressor in which the air flows from the front to the rearparallel to the engine shaft.

Impeller.- A vaned disc of a centrifugal compressor.

Air intake.- The front part of the engine, which collects the air and directs it into decompressors.

Casing.- The outer part of an engine.

Mount.- Attach.

Turbine.- A disc fitted with blades which turn when placed in a moving stream of air orgases.

Rotor blades.- The rotating blades in a compressor or turbine.

Stator blades.- The fixed blades in a compressor or turbine.

Stage.- One set of stator and rotor blades in a compressor or turbine.


1. What is the difference between the airflows in centrifugal and in axial-flow

compressors?

2. What are the main advantages of centrifugal compressors?3. What is the most common sort of compressor in use today?

4. What happens as the air passes between the rotor and stator blades ?5. Why does an axial-flow compressor require many stages?

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Combine these sentences in the following way:

Statement: The impeller rotates. The air spins.

Response: If the impeller rotated, then the air would spin.

1. The impeller rotates. The air spins.

2. The air enters a divergent passage. Its velocity drops.

3. You add more stages. You create new problems.4. You alter one factor. You alter all the others.

5. You look at the diagram. You find the answer.

5. You fail to find the solution. I have to give it to you myself.

Further Reading

There is a constant relationship between the volume, the temperature and the pressure of the air as

it passes through a jet engine. Briefly, it can be said that the absolute temperature of the air at any

point is a product of the pressure and the volume of that air. When the volume of the air is being

reduced in either a centrifugal or an axial-flow compressor, there is an increase in both pressure and

temperature. During combustion, when fuel is added and ignited, there is a rise in both the

temperature and the volume of the air, but the pressure remains almost constant because the

combustion chamber is not sealed. As energy is taken from the stream of exhaust gases by the

turbine, both the temperature and pressure decrease, while the volume increases.

The more efficient the design of a compressor, the higher will be the pressure generated for a

given rise in temperature. Similarly, the better the design of the turbine, the more power it will

produce. for a given drop in the temperature of the exhaust gases.


1. Is there a constant relationship between the volume, temperature and pressure of the air

as it passes through a jet engine ?2. What happens when the volume of the air is being reduced in a centrifugal

compressor?

3. What happens to the air during combustion?4. Is the combustion chamber sealed?

6. What decreases when energy is taken from the stream of exhaust gases by the turbine?

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Exercise 6

Use the following words and phrases in sentences of your own to show that you understand

their meaning and use:

1. centrifugal force 5. relationship 9. in theory2. axial-flow 6. generated 10. in practice3. impeller 7. the former 11. progressive

4. casing 8. the latter 12. robust


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Unit 4

ROTORS AND STATORS

Dialogue

Chris: You gave us a problem to solve last time, Mr. North.Mr. North: Thats right. And did you find the solution?

Ben: Well, we think so. If you remember, you asked us how the design of an

axial-flow compressor ensures that the air passing through remains at moreor less a constant velocity even though there is a progressive rise in

pressure.Mr. North: Well?Chris: Well, we think the velocity can be controlled by gradually restricting the

size of the passage through which the air has to pass.Mr. North: And how would you restrict the size of this air *annulus?Ben: By fitting a tapered *drum round the turbine shaft, and then attaching the

blades to it.Mr. North: I see you studied that diagram carefully!Chris: Yes, we did. And we thought of another way of getting round1 the

problem: by tapering the casing round the compressor.

Mr. North: Yes, that method is used, too. But can you tell me what effect this has on

the blades themselves ?Ben: Yes. The ones at the front can be quite large in diameter, but they must be

made shorter and shorter the farther back they are placed.Mr. North: Thats right.Chris: Great centrifugal *stresses must be put on the rotor blades when the

compressor is turning fast. How are they attached to the rotor drum?

Mr. North: There are a number of methods. Sometimes they arent attached to a drumat all, but to discs that are *splined to the rotor shaft. This helps todistribute the centrifugal and axial stresses that are involved.

Ben: What about the shape of the blades, Mr. North?Mr. North: That would be getting us into really deep water,2 Ben The design of

compressor and turbine blades is a very complicated subject indeed, and

one that we cant possibly examine in detail at this stage. Tell me, Chris,

how would you make a compressor more efficient?Chris: By careful design of the rotor and stator blades, I suppose.

Mr. North: Ah, but even the best-designed blade will only work at maximum

efficiency within a limited range of conditions. Outside these conditions,

the blade may *stall, or the whole compressor *surge, with a consequentloss of power. This can be overcome partially by fitting adjustable *inletguide-vanes to the intake, and by *"bleeding off" air at certain points.

Ben: Im afraid thats a bit beyond me.3 Cant you make a compressor more

powerful simply by sticking on4 more stages?Mr. North: Yes, of course. But that can cause headaches, too. At low speeds, the

turbine may not have enough power to turn a high-compression

compressor.

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Chris: Why not have two compressors: a low-pressure one, and a high-pressureone?

Ben: That would be complicated.Chris: Not if you had two turbines.Mr. North: How would the turbines be connected to the compressors?

Ben: By two shafts, of course. Thats dead easy.

5

Chris: Yes, a large hollow shaft, and a smaller one revolving inside it.Mr. North: Well, I really must congratulate you. Youve just described what is known

as a twin-spool system, one of the most common designs today. This type

is invariably used in by-pass engines, with the L.P. compressor at the front

supplying enough air for both the H.P. compressor and the by-pass system.Ben: What about fan engines?

Mr. North: In their case, the fan is attached to the front of the L.P. compressor. Some

of the air then passes into the H.P. compressor, while the remainder passesback, often through ducts, to the atmosphere.

Chris: What happens to the air after it leaves the H.P. compressor? Does it go to

the combustion system?Mr. North: It does. And that, gentlemen, will be the subject of our next session!

The twin spool system


1. a way of getting round: a solution to2. That would be getting us into really deep water: that would lead to a discussion of

very complicated things3. a bit beyond me: too difficult for me to understand4. sticking on: adding (slang)5. dead easy: very easy

Vocabulary

Annulus.- A double ring formed by one cylinder fitted inside another.

Drum.- The cylindrical or tapering part of a compressor, to which the rotor blades areattached.

Stress.- Intense force acting on a part, and tending to deform it.

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Splined.- Of a shaft and wheel: made to turn together by having projections on the shaftand matching grooves in the hub of the wheel.

Stall.- The condition that occurs when a few stages in a compressor are affected by

turbulence; the engine vibrates and the turbine gas temperature rises rapidly.

Surge.- The condition that occurs when all stages of a compressor are affected by

turbulence; the engine vibrates and the temperature of the turbine gases rises rapidly.Inlet guide vanes.- Fixed blades at the front of an engine which guide the air onto the first

set of rotor blades.

Bleed off.- Remove a fluid from the main stream.


1. How is the velocity of the air kept nearly constant in an axial-flow compressor?

2. What are the stator blades attached to?

3. Briefly describe a twin-spool system.

4. Where does the air go after it leaves the compressor?6. Where is the fan usually attached in a fan engine?



The velocity can be controlled by restricting the size of the passage.


Example: How can you control the velocity?

Prompt:restrict the size of the passage

Response: The velocity can be controlled by restricting the size of the passage.

Now you do it.

1. How can you control the velocity? restrict the size of the passage

2. How can you distribute the stresses? attach the rotor blades to discs3. How can you improve the efficiency? design the rotor bladescarefully

4. How can you overcome this difficulty ? fit adjustable guide vanes

5. How can you restrict the size of the passage? taper the casing6. How can you solve this problem?fit two compressors

7. How can you connect the turbines? use two shafts

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Statement: I see you studied the diagram carefully.

Prompt: know

Response: I know you studied the diagram carefully.

Now you do it.

Statement: I see you studied the diagram carefully.

Prompts:

1. know

2. hope

3. He hopes

4. looked at

5. examined

6. the problem

7. thoroughly

8. the design

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How would the turbines be connected?

Use this structure to respond to the following statements:

Example: Wed connect the turbines.Response: How would the turbines be connected?

Now you do it.

1. Wed connect the turbines.2. Wed distribute the stresses.

3. Wed duct the air back.

4. Wed design the engine.

5. Wed make it more efficient.7. Wed attach the blades.

Further Reading

The efficiency of an axial-flow compressor depends principally upon the design of itsrotor and stator blades. Rotor blades are of *aerofoil section, and are made of steel,

aluminium alloy or titanium. However skilfully they may be made, they can only work

with maximum efficiency within a limited range of operating conditions. Outside theseconditions, the smooth flow of air past the blades is upset by unwanted *turbulence. When

one stage "stalls" in this way, the compressor may "cough" and start to vibrate. If all thestages stall, and there is a complete breakdown of the airflow, then the compressor is said to"surge". A bang is heard from the engine, and there is a rapid rise in the temperature of the

exhaust gases near the turbine.

Stator blades are also of aerofoil section. They may be attached directly to the casing ofthe compressor, or to retaining rings which in turn are fitted to the casing. Whereas rotor

blades are usually mounted individually on their drum or disc, stator blades for the front

stages of a compressor can be fitted in *packs, with connecting shrouding at their tips to

give them greater rigidity.

Vocabulary

Aerofoil.- Shaped like the wing of an aeroplane.

Turbulence.- Disruption of the smooth flow of air or gas.

Packs.- Several stator blades made in one piece.

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22


1. What does the efficiency of an axial-flow compressor principally depend upon?

2. What are rotor blades made of?3. Can rotor blades work with maximum efficiency under all conditions ?

4. What happens to the compressor if there is a complete breakdown of the airflow?5. What gives stator blades greater rigidity?

Exercise 6

Use the following words in sentences of your own to show that you understand their

meaning and use:

1. problem 5. annulus 9. maximum

2. solutions 6. stresses 10. distribute

3. constant 7. design 11. hollow4. restrict 8. breakdown 12. power


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Unit 5

THE COMBUSTION CHAMBER

Dialogue

Mr. North: Today we reach the heart of the jet engine-the combustion chamber, wherefuel is mixed with the air from the compressors and ignited.

Ben: Will this be even rougher going1 than last time? Things were getting a bit

tough,2 you know.Mr. North: No. Funnily enough,3 the combustion chamber is relatively simple. Think

of it first as two tubes, one inside the other. The inside one, known as the

*flame tube, is fitted at the front with a perforated *flare behind an entry*snout. In the centre of the flare there are a number of swirl vanes.

Chris: Theres a diagram here which shows that.

Combustion chamber

Ben: So there is. And I can see bigger holes at various places along the flame

tube.

Chris: That must be the *burner nozzle, right in the centre of the flare.Ben: I can see how its made, but how does it work?

Mr. North: Well, the air comes from the compressors at high velocity-as much as four

or five hundred feet per second....

Ben: Thats quite a speed!4

Chris: So fast, in fact, it would surely blow out the flame, wouldnt it ?Mr. North: Precisely. And, besides, there is more air than is needed for combustion.

*Kerosene, you see, burns best at a ratio of about 15 to 1; but if all the airwere used, the ratio could be as high as 120 to 1.

Ben: So thats why there are two tubes: only the quantity of air needed for

combustion goes into the inner tube, the rest passes outside it.Chris: The flare presumably helps to decrease the velocity of the air entering the

flame tube....

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Ben: And to raise its temperature and pressure. Dont forget that!Mr. North: Yes, youre both right. Inside, the air is still travelling at about 75 feet per

second. Even this is too fast, but the swirl vanes induce a reverse flow of

air in the region of the burner nozzle, which gives us the low velocity werequire.

Ben: How much air goes into the flame tube proper?Mr. North: About 18 % passes in directly, while a further 10 to 15 % enters the

combustion zone by the other holes you can see at the front. Theremaining air is introduced progressively farther back along the flame tube

to mix with the exhaust gases which have already been burnt.

Chris: Forgive me if I seem a bit stupid, but I dont see why the compressorsshould be made to deliver so much air when only about 30% of it is needed

for combustion.

Ben: I thought I was the stupid one! Isnt it used for cooling the outside of theflame tube, Mr. North?

Chris: Yes, but does it need so much?

Mr. North: Youre forgetting that the temperature of the gases leaving the combustionzone may be as high as 2,000 centigrade. And thats much too hot to feed

into the turbine. In fact, about half the excess air is needed to dilute andcool these exhaust gases.

Ben: How does the *fuel nozzle work?

Mr. North: Fuel is fed to the nozzle by pumps, and lit by a spark from an *igniter plug.

Once started, the flame is selfsustaining.Chris: Arent there different types of nozzle?

Mr. North: Yes. Fuel is usually injected into the combustion zone as *atomized spray,

but there is another possible method....Ben: Whats that?

Mr. North: Hang on, Ben; Im coming to it!5 In the second method, the swirl vanes and

flare are replaced by a *baffle plate that supports fuel sprayers. The air passes through holes in the baffle; then, laden with fuel, it flows into a

number of U-shaped tubes located behind the baffle plate in the

combustion zone. Here the fuel vaporizes before being swept forward andignited.

Chris: Is that the only difference?

Mr. North: Yes. In both cases, the burnt gases are cooled by the rest of the air, and

passed back to the turbines. And turbines, gentlemen, are the subject of ournext session.


1. rougher going: more difficult2. a bit tough: rather difficult3. funnily enough: strangely4. Thats quite a speed!: Thats very fast5. Hang on, Im coming to it: Wait, Im just going to talk about it

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EXERCISE 2: STRUCTURAL PRACTICE


The ratio could be as high as 120 to 1.


Statement: How high is the ratio?Prompt: 120 to 1

Response: It could be as high as 120 to 1.

Now you do it.

1. How high is the ratio? 120 to 1

2. How fast is the speed of the air? 75 feet per second

3. How low is the best ratio? 15 to 14. How large is the quantity of air that's taken in? 18%

5. How high is the temperatura of the gases? 2,000 centigrade

6. How much is the velocity? 500 feet per second

EXERCISE 3: PROGRESSIVE SUBSTITUTION DRILL

Statement: The air is travelling at 75 feet per second.

Prompt: moving

Response: The air is movingat 75 feet per second.

Now you do it.

Statement: The air is travelling at 75 feet per second.

Prompts:

1. moving

2. 400

3. 500

4. high velocity

5. more slowly

6. very fast

7. lt's

8. lt would be

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EXERCISE 4: FURTHER STRUCTURAL PRACTICE

Put the following sentences into the active form:

Statement: The fuel can be re-ignited by an electric plug.

Response: An electric plug can re-ignite the fuel.

1. The fuel can be re-ignited by an electric plug.2. The air is driven into the combustion chamber by the compressor.

3. The velocity of the air is reduced by the flare.

4. A reverse flow of air is created by swirl vanes.5. Fuel is fed to the nozzle by pumps.

6. The fuel is vaporized in the U-tubes by the heat of the combustion zone.

Further Reading

Although all combustion chambers work on the principles already describes, they may be

*installed in the engine in a number of different ways. The multiple combustion chamberlayout is often used with engines having centrifugal compressors. Here, a number of flame

tubes, each with its own outer casing, are disposed radially round the engine. Annular and

tubo-annular combustion chambers are more often

seen, however. The latter have flame tubes groupedround the engine, as in the multiple layout, but instead

of each having a separate outer casing, they are all

disposed in a common annular casing, shaped like two broad rings, one inside the other. With annular

combustion chambers, the flame tube itself is in the

form of a double ring which in turn is fitted into anannular casing of two more rings.

We have seen that multiple combustion chambers

may be the most suitable form for use with centrifugalcompressors. Tubo-annular chambers are easier to manufacture and overhaul, while

annular chambers, besides possessing these advantages, are also more compact. In

addition, annular chambers are so efficient from the point of view of combustion that they

considerably reduce many of the problems of air pollution.Combustion chambers are subject to many stresses; they must be capable of withstanding

not only very high temperatures and changes of temperature, but also the corrosive effects

produced by the products of combustion. In recent years, great progress has been made indeveloping materials capable of meeting these requirements.

Vocabulary

Install.- To fit or put in its correct place.

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EXERCISE 5: COMPREHENSION QUESTIONS

1. When is the multiple combustion chamber often used?2. How are the flame tubes arranged in a multiple combustion chamber?

3. Which of the combustion chambers described is the most compact?4. Why do annular combustion chambers help to reduce air pollution?

5. What do combustion chambers have to withstand?

EXERCISE 6

Use the following words in sentences of your own to show that you understand theirmeaning and use:

1. combustion 5. overhaul 9. decrease2. ignite 6. Vane 10. extinguish

3. perforated 7. Nozzle 11. deliver

4. multiple 8. Ratio 12. dilute

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Unit 6

TURBINES

Dialogue

Ben: Good morning, Mr. North. What have you thought up for us1 today?Chris: Really, Ben! You know he's going to tell us about turbines. He said so

last time.

Mr. North: That's right. The basic principles of turbine design are relatively easy tounderstand, but their practical application is very complicated, and we

shall only be able to scratch the surface2 of the problem. First, what does

the turbine do?

Ben: It drives the compressor, of course.

Mr. North: Anything else?

Chris: Yes. It drives the propeller shaft in turbo-prop aircraft, and the rotor shaftin helicopters, both through reduction *gears.

Ben: I suppose it drives various *accessories, too. Fuel pumps, and things likethat.

Mr. North: Very good. Now, let's see how a turbine works. The hot exhaust gases

from the combustion chamber first pass fixed nozzle guide vanes.

Chris: They look like the stator blades in a compressor, don't they?

Mr. North: Yes, in a way. They're aerofoil-shaped blades that are often made hollowinside. Do you know why?

Ben: No, I don't!

Mr. North: Could you guess if I added that cool air supplied by the compressor maybe made to flow through them?

Ben: Ah, I'm with you now. The exhaust gases make them so hot that they haveto be cooled.

Mr. North: Exactly. Immediately behind the guide vanes are the rotating blades of the

turbine. These, too, are of basic aerofoil shape. They have to withstand

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enormous temperatures and strains, and great attention must be paid to the

way in which they are attached to the *turbine disc.

Chris: One way is shown here in a diagram. It's called the "Fir tree root" method.That's quite descriptive, isn't it?

Ben: Yes, I agree. What sort of stresses are involved here, Mr. North?

Mr. North: Well, a blade may weigh only a couple of ounces, but when the turbine isrotating at speed, it can exert a load of two tons or more. Do you notice

anything else about the blade?

Ben: Yes. Theres a sort of segment on the outer end. I suppose that, when allthe blades are in place, these segments form a kind of outer ring. Is that to

prevent leaks?

Mr.North: Thats right. Now lets see the principles upon which the turbine works.What you have learnt about compressors will be useful here.

Ben: I know! In a divergent passage.Chris: Hold on, Ben, youve got it wrong. As far as I know, the nozzle guide

vanes form convergentpassages.

Mr.North: Exactly. But its not only the impulse of the gases striking the blades thatworks the turbine; a reaction force is also generated between the blades

themselves. In most engines about half the power is produced by impulse,

and the other half by reaction.

Chris: The turbine blades seem to be twisted.

Ben: Youre dead right;3 they are.

Mr. North: Yes, their *stagger angle is greater at the tip than at the *root. This ensures

that the gases do equal work throughout the length of the blade, and thatthey leave the turbine at a uniform axial velocity.

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Ben: We saw that there may be more than one turbine; one to drive the L. P.compressor, and one for the H. P. compressor. Are they mounted one

behind the other?

Mr. North: Yes, they are. A high ratio by-pass fan engine may even have three

turbines, and three concentric shafts, running independently. This forms

what is known as a triple-spool system. Such engines are not only veryefficient, they are also extremely quiet.

Christ: What happens to the gases when they have left the turbines, Mr. North?

Mr.North: We must stop now, Christ, but Ill tell you all about that the next time we

meet.

Conversational expressions1. What have you thought up for us? : What have you prepared for us?2. scratch the surface: Study superficially3. dead right: absolutely right


1. What purpose does the turbine serve?2. Why are the nozzle guide vanes often hollow?3. Why must the turbine blades be very firmly attached to the turbine disk?4. What do the segments on the blades prevent?5. What are the two forces that drive the turbine round?



The gases are so hot that they have to be cooled.


Example: Are the gases hot?

Prompt: cool

Response: Yes, theyre so hot that they have to be cooled.

Now you do it.

1. Are the gases hot? cool

2. Is the velocity high? decrease

3. Is the movement of the air fast? slow down

4. Is the pressure low? raise

5. Are combustion engines subject to many stresses? strengthen6. Is that engine design complicated? simplify

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7. Is the volume of air small increase8. Is the temperature high? Lower


Statement: They have to withstand enormous temperatures

Prompt: The inlet guide vanes

Response: The inlet guide vanes have to withstand enormous temperatures.

Now you do it.

Statement: They have to withstand enormous temperatures.

Prompts:

1. The inlet guide vanes

2. The turbine blades

3. are required to

4. resist

5. very destructive

6. stresses

7. strains

8. heat conditions



These engines are not only efficient, they are also quiet.

Use this structure to complete the following sentences:

1. the air / hot / under pressure

2. the blade / very light / very strong

3. this type of compressor / robust / easy to manufacture

4. the blades / hollow / twisted

5. this principle / simple / very useful

6. this combustion chamber / more efficient / lighter

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Further Reading

The performance of a jet engine depends very largely upon the efficiency of its

turbines, and enormous technological progress in their design has been made in recent

years. Very high temperatures and stresses are involved, for air issuing from the

combustion chamber may be travelling at over 2,000 feet per second, and may exceed1,000C in temperature. In addition, a turbine is the more efficient the faster it rotates,

but limits are imposed by the fact that stresses increase as the square of speed.1 The

problem of turbine design can be appreciated when it is realized that the blades, while

glowing red hot, may be moving at more than 1,300 feet per second at their tips. Only

very special materials can withstand such conditions without soon suffering from

*creep, corrosion, *fatigue or *thermal shock. At the same time, the blades must be

made of materials that can be formed with accuracy, and machined by current

manufacturing methods.

It is in the turbine that much can be done to reduce engine noise, a problem that is

causing world-wide concern as air traffic increases. Triple-spool systems that are

efficient yet quiet are an important development in the war against noise pollution.

1The square of speed: briefly, a stress level at 1,000 r.p.m. will increase by a factor of four at 2,000 r.m.p., and by a factor of nine at3,000 r.p.m.


1. What speed does the air from the combustion chamber travel at?

2. Does the temperature exceed 1,500C?

3. How do stresses increase?4. If inferior materials are used for the turbine blades, what will they suffer from?

5. What kind of materials must be used for making turbine blades?

EXERCISE 6

Use the following words and phrases in sentences of your own to show that you

understand their meaning and use:

1. practical application 5. in addition 9. descriptive2. complicated 6. withstand 10. exert

3. accessories 7. accuracy 11. impulse

4. stuff 8. exceed 12. uniform velocity


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Unit 7

REHEAT (AFTERBURNING)

Dialogue

Mr. North: Airline operators require engines that are powerful, yet economical andquiet. But sometimes performance is the major consideration. When do

you think that would be?

Chris: In fighter aircraft during combat, for instance; or when a plane needs to

take off and climb very rapidly.

Mr.North: Yes, thats right. How can one best increase performance for short periodslike that?

Ben: Put in a bigger engine!

Chris: No, thats just not on.1 It would increase the weight and lead to higher

overall fuel consumption.

Ben: I know! Why not use the oxygen that wasnt burnt in the combustionchamber? You could add more fuel and ignite it.

Chris: You would wreck the turbine if you raised the temperature of the exhaustgases.

Ben: Not if you did it after the turbine stage, in the *jet pipe.

Mr. North: You are being bright2

today, Ben. Thats precisely what can be done with*afterburning, or reheat, as its also called.

Chris: Do most planes have it?

Mr. North: No, because the afterburner makes an engine less efficient when its not in

use. The jet pipe must be heavier, and the extra equipment somewhat

restricts the gas flow, reducing thrust.

Ben: What does it consist of, Mr. North?

Mr.North: Well, youve studied combustion chamber design. You tell me.

Ben: There must be fuel nozzles, for a start.

Mr.North: Yes, These are often in the form of a circular *manifold, with holes at therear which allow the fuel to escape.

Chris: How does the fuel get into the manifold?Mr.North: Its pumped there through *feed pipes in the manifold support *struts.

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Ben: Hang on! You asked me to have a go at3 explaining it.

There must also be an igniter plug, of course.

Chris: Why? I would have thought the temperature of the exhaust gases would be

high enough to cause *ignition.

Mr. North: Surprisingly enough, an igniter is required. Kerosene needs a temperature

of at least 800 C for proper combustion, and the exhaust gases may becooler than that at high altitudes.

Chris: What about the problem of the gases blowing out the flame? The velocity

of the air in the combustion chamber had to be reduced because of this

possibility.

Mr. North: Yes, and the same problem occurs with afterburning. An annular ring, V-

shaped in section, can be fitted downstream from the manifold. This

creates turbulence and the necessary area of low-velocity gas flow.

Ben: Is all the oxygen burned?

Mr. North: No. The jet pipe becomes extremely hot, so its fitted with an inner heatshield, and some air is made to pass between this and the outer casing for

cooling purposes.Chris: Can afterburning be fitted to any sort of engine? What about by-pass

engines?

Mr. North: In their case, two methods are possible: either the by-pass and turbine

streams are mixed before the reheat stage; or fuel is added to both streams

separately, and ignited.

Ben: Well, even I can get the hang of that!4

Mr. North Wait, Ben, theres more to come! At the rear end of the jet pipe there is the*propelling, or exit, nozzle that controls the correct balance of pressure,

temperature and thrust in the engine. With a small nozzle, these values

increase, while with too large a nozzle they may become too low.

Chris: Ah, I think I see what youre getting at.5 If the exit nozzle is the correct

size when the afterburner is notbeing used, it will be too small when it isin use.

Ben: So the temperatures and pressures in the engine could rise dangerously

high.Mr. North: Precisely. Tell me, how can we get round

6 that one?

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Ben: Thats easy. Fit a *variable nozzle.

Mr. North: Very good, Ben. Youre catching on fast7 today! Well, we have to stopnow, but next time we shall look at the exhaust system in more detail.

Conversational expressions1. thats . . . not on: thats not possible2. bright: intelligent3. have a go at: try to4. get the hang of that: understand that5. I see what youre getting at: I understand what youre trying to see6. get round: solve7. catching on fast: understanding things quickly


1. How can the unburnt oxygen remaining in the exhaust gases be used to increasethe power of an engine?

2. Is an afterburner economical on fuel?3. Is the temperature of the gases high enough to cause ignition?4. What is the purpose of the annular V- shaped ring fitted downstream from the

fuel manifold?

5. Must the area of the propelling nozzle be increased or decreased when theafterburner is operating?



The velocity Had to be reduced because ofthis possibility.


Example: Why did the velocity have to be reduced?

Prompt: this possibilityResponse: The velocity had to be reduced because of this possibility.

Now you do it.

1. Why did the velocity have to be reduced? this possibility

2. Why did the blades have to be strengthened? the stresses3. Why did the igniter have to be used? the low temperature

4. Why did the performance have to be improved? these requirement

5. Why did an annular ring have to be fitted? the velocity of the exhaust gases6. Why did the power have to be reduced? the danger of overheating

7. Why did the engine have to be replaced? structural damage8. Why did the noise have to be reduced? complaints from the public

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Statement: The temperature could rise dangerously high.

Prompt: extremely

Response: The temperature could rise extremely high.

Now you do it.

Statement: The temperature could rise extremely high.

Prompts:

1. extremely

2. very

3. be

4. become

5. pressure

6. low

7. will

8. mightEXERCISE 4: FURTHER STRUCTURAL PRACTICE

Notice this structure:

You would wreck the turbine if you raised the temperature.

Use this structure to complete the following sentences:

1. increase the weight / put in that engine

2. improve the performance / use an afterburner

3. soon get the hang of it / listen to the instructor4. solve the problem / fit a variable nozzle

5. see what Im getting at / look at the diagram

6. blow out the flame / increase the velocity of the air

7. protect the jet pipe / install a heat shield

8. create the necessary turbulence / fit an annular ring

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Further Reading

Great care must be taken in the design of the propelling nozzle at the rear of the

engine. If the flow of exhaust gases is impeded by too small an exit, temperatures and

pressures will build up inside the engine, while too large an exit will make them fall,

and create a loss of thrust.When afterburning is in operation, the area of the propelling nozzle can be increased

by opening two eyelids that partially obstruct the nozzle aperture when closed.

Alternatively, a ring of interlocking *flaps hinged to the outer casing can serve the same

purpose. The pilot actuates these eyelids, or flaps, through *pneumatic rams, which in

turn are linked to the fuel supply system. As they open the supply of fuel is increased,

and as they close the supply is reduced again. In addition to this, a pressure ratio control

unit is fitted to monitor temperature and pressure conditions around the turbine. Should

these rise excessively, the control unit automatically actuates the rams, the eyelids or

flaps open, and normal conditions are quickly restored.

Planes fitted with by-pass engines that burn relatively little of the oxygen available in

their combustion chambers can benefit spectacularly from the use of afterburning.Thrust con be increased by 70 % or more for short periods of time. This enables them to

reach an economical cruising height far more quickly than planes not fitted with

afterburners, a fact that partially compensates for their basically higher fuel

consumption.


1. What happens if the flow of gases is impeded by too small an exit?

2. What may serve the same purpose as the eyelids?

3. What is fitted to monitor temperature and pressure conditions around the turbine?4. Which type of engine can benefit spectacularly from the use of afterburning?

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38

5. What is the possible increase in thrust?

EXERCISE 6

Use the following words and phrases in sentences of your own to show that youunderstand their meaning and use:

1. economical 5. consist of 9. cruising height

2. performance 6. manifold 10. restricts

3. interlocking 7. rear 11. struts

4. thrust 8. reduce 12. variable


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Unit 8

THE EXHAUST SYSTEM

Dialogue

Ben: We were talking about the propelling nozzle last time, Mr. North, but we

havent really studied the exhaust system in detail yet. Do you think we

could go over it 1 with you today?

Mr. North: Thats just what I had in mind, 2 Ben. Lets start where the exhaust gases

leave the turbine. Can you hazard a guess at their temperature at this

point?

Chris: I think you said it could be less than 800 C. Thats why an afterburner

requires its own igniter.Mr.North: Ah, Chris, youve remembered that. Good. To be more exact, the

temperatures there may vary between 550 and 850 C. Naturally by-pass

engines are cooler than non-by-pass engines, for the latter send all the air

through their combustion chambers.

Ben: Even these lower temperatures are pretty hot, to my way of thinking.3

Mr. North: Yes, they are. So a protective *cone is usually fitted to the rear of the

turbine disc to prevent these hot gases from blowing across it.

Chris: Why is it cone shaped?

Mr. North: Somehow, I think Ben would like to answer that one.

Ben: Would I? Thats news to me!4

Mr. North: Just think, Ben. How must the cone affect the area of the jet pipe at thatpoint?

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Ben: It makes it progressively larger, like a divergent passage. Oh, I see what

youre driving at! 5 In a divergent passage, theres a drop in velocity. . . .

Mr.North: Precisely! But why should we want that, Chris? Before you answer,

remember that the gases leaving the turbine may be travelling at more

than 1.000 feet per second.

Chris: At that speed, they must produce high friction losses. So if you decreasedthe velocity, youd make the engine more efficient.

Mr.North: Thats right, youve got it.6 Further losses can be caused by residual

*whirl velocity in the gas stream produced by the turbine; but the support

*fairings in the exhaust system can be designed to straighten out the gas

flow to some extent, so reducing these losses.

Ben: What happens exactly when the exhaust gases reach the propelling

nozzle?

Mr.North: Thats not easy to explain in a few words, but Ill try to keep it short.7

The propelling, or exit, nozzle forms a convergent duct. When the exhaust

gases passing through it reach the speed of sound, the nozzle becomes

*choked and no further increase in exhaust gas velocity is possible.Ben: What is the speed of sound?

Chris: That depends on the temperature, doesnt it? The higher the temperature,

the higher the speed of sound.

Ben: So, if you make the exhaust gases hotter, you can achieve a higher exit

velocity, I suppose?

Mr. North: True, but if you raise the temperature too much, youll destroy the turbine!

Incidentally, when the nozzle becomes *choked, the static pressure of

the gas at exit rises above that of the outside atmosphere. This produces

what is known as pressure thrust. This is small, of course, compared with

the main thrust of engine which is produced by the momentum change of

the gas stream.

Chris:I rather thought that the propelling nozzle was convergent-divergent inshape.

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Mr. North Some are, especially on high pressure ratio engines. When the exhaust

gases enter the convergent part their velocity rises. . . .

Ben: And when they pass on into the divergent part, theres an increase in

pressure.

Mr.North: Yes. And this pressure acting on the walls produces yet more thrust.

Chris: Let me get this quite straight.8

Thrust is produced by the acceleration ofthe gas stream through the engine. More thrust is produced across the face

of the nozzle. Is that right?

Mr.North: Yes. That is known as pressure thrust.

Chris: And yet more thrust is obtained from the pressure of the gases on the

walls of the divergent part of the nozzle.

Ben: Bravo, Chris. I couldnt have explained that better myself. And dont ask

me to try, Mr. North, as I can see that times up for today!

Conversational expressions1. go over it: discuss it2. had in mind: had intended to do3. to my way of thinking: in my opinion4. Thats news to me! I didnt know that!5. driving at: trying to explain6. got it: understood it7. keep it short: explain briefly8. get this quite straight: understand this properly


1. Are the exhaust gases of a by-pass engine cooler than those of a turbo-jet?2. Why is there a cone downstream of the turbine disc?3. What might be the velocity of the gases as they leave the turbine?4. What is the speed of sound?5. How is the most thrust obtained in a jet engine?



If you decreased the velocity, youd make the engine more efficient.

Use this structure to make complete sentences:

Example: decrease the velocity / make it more efficient

Response: If you decreased the velocity, youd make it more efficient.

Now you do it.

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1. decrease the velocity / make it more efficient

2. fit an adjustable nozzle / be able to control the temperature3. fit a protective cone / get more protection

4. make the exhaust gases hotter / increase the exit velocity

5. add this modification / improve the design6. change the design / straighten out the gas flow

7. add an afterburner / make it more powerful8. use a triple spool system / have a quieter engine


Statement: Lets start at the point where the gases leave the turbine.

Prompt: Shall we

Response: Shall we start at the point where the gases leave the turbine?

Now you do it.

Statement: Lets start at yhe point where the gases leave the turbine.

Prompts:

1. Shall we

2. begin

3. at the stage

4. the exhaust gases

5. I suggest we

6. the combustion chamber


Reply to the following sentences in the negative:

Example: Is the exhaust flow of a by-pass engine hotter than that of a turbo-jet?

Response: No, it isnt so hot.

Now you do it.

1. Is the exhaust of a by-pass engine hotter than that of a turbo-jet?

2. Do piston engines turn faster than jet engines?

3. Would this engine have worked better with a different fuel?

4. Are flying conditions worse in summer than in winter?5. Do jet engines work better at low altitudes than at high ones?

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43

Further Reading

Although the basic principles of the engine are widely understood, doubt often remains

in peoples minds as to the exact position of the thrust forces involved. In fact, these are

distributed throughout the engine. Some are in a forward direction, while others are

towards the rear, and it is the amount by which the sum of the former exceeds the sumof the latter that determines the efficiency, or rated thrust, of an engine. As a rule, it can

be said that forward thrust is created wherever there are divergent passages which

convert velocity, or kinetic energy, into pressure energy. Conversely, rearward thrust

occurs in convergent passages, where pressure drops but velocity rises.

Following the air as it passes through the engine; we see that considerable forward

thrust occurs in the compressors because of the rise in pressure there. The air then

passes to the combustion chambers, where even greater forward pressure is exerted on

the walls as the heated air expands. The expanding gases next meet the turbine guide

vanes, where they are accelerated and deflected onto the turbine blades. Both vanes and

blades are subjected to large rearward forces, often known as *drag. Slight forward

thrust occurs on the turbine cone, while the convergent shape of the propelling nozzleproduces yet more drag.


1. Where are the thrust forces?

2. What is another word for velocity energy?

3. Where does rearward thrust occur?

4. Why does considerable forward thrust occur in the compressor?

5. What happens when the expanding gases meet the turbine guide vanes?

EXERCISE 6

Use the following words and phrases in sentences of your own to show that you


1. system 5. deflected 9. whirl

2. requires 6. friction 10 basic principles

3. protective 7. forward direction 11. difference

4. cone 8. residual 12 acceleration


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Unit 9

NOISE SUPPRESSION

DialogueChris: You mentioned some time ago, Mr, North, that airline operators require

engines that are not only powerful and economical, but quiet, too. How

can quietness best be achieved?

Mr. North: Its a difficult problem, Chris, and one to which theres no perfect

solution. Lets face it, 1 aeroplanes are naturally noisy things.

Ben: Youre telling me! 2 I once lived near an airport, and life was pure hell at

times.

Chris: You can talk, 3 Ben! That old motor-bike of yours is worse than any jetplane.

Mr. North: Yes, but were concerned with aero-engines. There are, in fact, three main

sources of noise in a jet engine.Can you name them for me?

Ben: Well, the obvious one is the exhaust gas stream coming out at the back.Then I suppose the turbine itself must be quite noisy.

Chris: Yes, and so are the compressor. But which is the noisiest?

Mr. North: That depends on the type of engine. With pure jets, the most noise iscreated by the external mixing of the exhaust gas stream with the

atmosphere.

Ben: You cant stop that happening.

Mr. North: No, of course not. But by careful design of the exhaust nozzle, you can

make the gas flow merge more efficiently with the atmosphere. This

creates less turbulence, and therefore less noise.

Chris: How are such nozzles made, then, Mr. North?

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Mr. North: There are two main types of noise suppressor. One consists of metal*corrugations inside the nozzle, with parts open to the atmosphere. Air

flows into these corrugations and mixes with the jet stream, reducing the

turbulence.

Ben: Theres a diagram to illustrate that. And another one to show the second

method.

Mr. North: Yes, The other noise suppressor works on a similar principle, but in this

case the exhaust gases are split to flow through a number of * lobes,

where again they mix efficiently with the air.

Chris: Dont these suppressors reduce the efficiency of the engine?

Mr. North: Yes, they do. A nozzle with deep corrugations or large lobes must bebigger in diameter than an unsuppressed nozzle if it is to pass the same

volume of gases, and the increased size inevitably means greater weight

and more drag.

Ben: I dont think pure jets can ever be made really quiet. But what about by-

pass engines; they are less noisy, arent they?

Mr. North: Even so, they have their problems. Their exhaust jet may be quieter, but

their compressor and turbines are by no means silent. In fact the fan of a

high by-pass ratio engine is sometimes as noisy as a pure jet.

Chris: What exactly creates all this noise?

Mr. North: There are two main sources. Discrete tones and Harmonics are produced

as the rotor blades pass through the wakes set up in the air by the preceding stationary blades.4 And background noise is produced by the

reaction of each blade to the air passing over its surface.

Ben: Then weve had it! 5 Theres no solution unless you take out all the blades

in the compressors and turbines; and you cant do that!

Chris: No, of course not. But a lot of noise is created by the guide vanes andnone are needed in front of a large single-stage fan. Such a quiet fan

could be driven by a high by-pass ratio engine which is fairly quiet, too.

Mr. North: Chris is quite right. And if the speed of the fan can be reduced without

loss of total thrust, then the engine will be quieter still.

Ben: That would be possible with a triple-spool engine fitted with a variable

area exhaust nozzle. By reducing the area of the nozzle, you could reduce

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the speed of the fan and its turbine, so cutting down the amount of noise

they make.

Chris: But that would increase the velocity of the exhaust gases, and they wouldmake more noise!

Mr. North: Exactly, so a compromise must be reached. The exhaust nozzle must be

reduced only to the point where the noise levels from the fan, turbines andexhaust stream are the same. That will produce the quietest engine of all

with the least loss of efficiency.

Conversational expressions1. Lets face it: admit the facts2. Youre telling me!: I know that only too well!3. You can talk!: You are guilty too, so you should not criticize other people4. Discrete tones: noises separated by fixed intervals of pitch; harmonics: secondary

vibrating noise. Wakes are areas of air disturbed by the stator blades.

5. weve had it!: theres nothing we can do


1. What are the three main sources of noise in a jet engine?2. What creates the most noise in a pure jet engine?3. How can you make the gas flow merge more efficiently with the atmosphere?4. Do the suppressors reduce the efficiency of an engine?5. What happens if you reduce the speed of the fan and its turbine?



The fan of a high by-pass ratio engine is sometimes as noisy as a pure jet.


Example: Is the fan noisy?

Prompt: sometimes / a pure jetResponse: Itssometimes as noisy as a pure jet.

Now you do it.

1. Is the fan noisy? sometimes / a pure jet

2. Is the exhaust stage important? just / the combustion stage3. Are centrifugal compressors efficient? almost / axial-flow compressors

4. Is this design good? quite / that design5. Are metal corrugations effective? just / lobes

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6. Is the turbine noisy? nearly / the exhaust gas flow


Statement: Isuppose the turbine must be quite noisy.

Prompt: the compressors

Response: I suppose the compressors must be quite noisy

Now you do it.

Statement: I suppose the turbine must be quite noisy

Prompts:

1. the compressors2. the exhaust gas stream

3. is

4. makes a lot of noise

5. most of the

6. should think


Complete the following sentences with their appropriate question tags:

Example: Airline operators require engines that are quiet.

Response: Airline operators require engines that are quiet, dont they?

1. Airline operators require engines that are quiet.

2. We are only concerned here with jet engines.

3. You can stop that happening.

4. There was a diagram on page fifty.

5. Jet engines cant be made completely silent.

6. They would have made quieter engines if it had been possible.

Further Reading

Many people feel that the day man first landed on moon was a critical point in our

attitude towards technology. Before that day, there seemed something frighteningly

inevitable about the march of scientific progress. If a project were feasible, it hadto be

attempted, at whatever cost to the environment. Only when Neil Armstrong took thathistoric step did mankind gain sufficient self-confidence to be able to say no to the

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Unit 10

THRUST REVERSAL

Dialogue

Ben: Weve been talking about aircraft and the effect they have on theenvironment. But its not just a matter of the horrible noise they make.

Chris: No. Airports too create problems. As planes become heavier and faster,they need longer runways that swallow up more and more precious land.

Mr.North: Designers are aware of this. Thats why so much research is going into

the development of vertical take-off and S. T. O. L. aircraft.

Ben: S. T. O. L.? What does that stand for?1

Mr.North: The letters are for Short Take Off and Landing.

Chris: Thats all very fine, but 2 arent there less revolutionary ways of slowing

down a plane?

Ben: You bet there are! 3 Didnt you know that the disc brakes you find on somany cars today were first developed for aeroplanes?

Mr.North: Thats true. But even disc brakes are inadequate on runways that are wetor covered by snow and ice.

Chris: Some planes deploy small parachutes on landing, but personally Ive

never thought much of that idea.

Ben: No, it does seem a bit primitive. With propeller-driven planes, of course,

the problem has already been solved simply by reversing the *pitch ofthe blades.

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Mr.North: And a very efficient method it is, too. Some of these planes can even taxibackwards!

Chris: Yes, I once saw a military transport plane do that. I could hardly believemy eyes!

Mr.North: Im sure you couldnt, but lets keep to the point.4 Most modern planes

dont have propellers; so how do you stop a jet?Ben: Turn the engine round and make it blow the other way!

Chris: Oh, dont be silly, Ben

Mr.North: Hes not quite as silly as you might think, Chris. Perhaps it isnt practical

to turn the whole engine round, but there is nothing to stop you

deflecting the flow of the exhaust gases forwards. In fact its a method

thats used both when the plane is in flight, to make a more rapid rate of

descent possible, and for braking purposes as it lands.

Ben: you see, Chris, Im not as stupid as I look. But how is the airflow

reversed, Mr. North?

Mr.North: Well, you tell me.

Ben: Let me see. If you blocked off the exhaust exit. . . .Chris: Ive got it! Look at this diagram. *Bucket doors or clamshell doors are

fitted near the rear of the engine. In normal flight they lie *flush with the

sides of the engine, allowing the gases to pass without hindrance.

Ben: In that position they also *seal off the forward-facing *thrust reverser

ducts that lead to the outside atmosphere.

Chris: Then, when the plane lands, the pilot operates a lever and the doors close,

blocking off the normal exit for the exhaust gases.Ben: As they close, they uncover the thrust reverser ducts. . .

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Mr.North: . . . and the gases are deflected forwards into the outside atmosphere.

Chris: It seems such an obvious method, now we know. But does it really

work?

Mr.North: It does indeed. The doors are operated by pneumatically or hydraulically

operated rams, and are of course built to withstand the high temperature

and pressures of the exhaust gases.Ben: In a ducted fan engine you could reverse just the flow of the cold by-pass

air. Then heat wouldnt be such a problem.

Mr.North: That method is used. Folding *blocker doors, or flaps, are fitted so that

in normal flight they allow the cold airstream to pass.

Chris: They also block off the thrust reverser ducts. . .

Ben: . . . and a cowl covers the outside of the duct to give a cleaner line to theengine, and so reduce drag.

Mr.North: Good. Now, when the doors fold back, they slide the cowl open and re-

direct the cold airstream through the thrust reverser ducts. At the same

time, other doors swing back to *spoil the flow of the hot exhaust gases.

Chris: It all sound quite simple!Mr.North: I know. But in fact theres a lot for the pilot to remember. For one thing,

he has to be very careful with his operational technique, and ensure that

thrust reversal is cancelled before the plane speed drops below a certain

point. Otherwise, there can be severe ingestion of dust and grit from the

runway into the engine.

Well, I think thats all for today, then. Next time well talk about fuel

system.

Conversational expressions1. stand for: represent2. Thats all very fine, but. . .: Thats a good idea as far as it goes, but. . .3. You bet there are!: There certainly are!4. lets keep to the point: lets just talk about the subject we meant to discuss


1. What do the letters S.T.O.L. stand for?2. When are disc brakes least effective?3. How may propeller-driven planes be braked?4. How can the exhaust gas flow be deflected forwards in a jet engine?5. How are the doors actuated?



The doors are operated by pneumatic or hydraulic rams.


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Example: How do you operate the doors?

Prompt: pneumatic or hydraulic ramsResponse: The doors are operated bypneumatic or hydraulic rams.

Now you do it.

1. How do you operate the doors? pneumatic or hydraulic rams2. How do you slow down the plane? deflecting the flow of gases3. How do you reverse the airflow? the use of clamshell doors

4. How do you reduce drag? fitting a cowl5. How do you drive the compressor? the turbine6. How do you alter the velocity? restricting the size of the air annulus7. How do you feed fuel to the nozzle? pumps8. How do you light the fuel? a spark


Statement: You could reverse the flow of the cold by-pass air.

Prompt: the gases

Response: You could reverse the flow ofthe gases.

Now you do it.

Statement: You could reverse the flow of the cold by-pass air.

Prompts:

1. the gases

2. airflow

3. deflect

4. fit clamshell doors

5. bucket

6. folding blocker

7. They

8. Theyve fitted

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EXERCISE 6

Use the following words and the phrase in sentences of your own to show that you


1. environment2. remote areas3. pitch4. reverse5. flush6. compromise7. airstream8. automatically9. spoil10.fold11.solution12.deflect

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Unit 11

FUEL SYSTEM

DialogueBen: How does the pilot control the speed of his plane, Mr. North? Does he

vary the flow of fuel to the combustion chamber, as you do with a car?

Mr.North: Yes, in a way. But things are rather more complicated with a jet engine.You see, an engines thrust depends upon the weight of air passing

through it, and this varies according to the aircrafts speed, its altitude

and the temperature of the air itself.

Chris: You mean that if any of these alter, then the speed of the plane will alter

too, even though the pilot hasnt touched the *throttle controls?

Mr.North: Precisely.

Ben: That could be dangerous.

Mr.North: Yes, but in fact every engine is fitted with an automatic control unit thatsenses these variables and applies the necessary corrections.

Chris: How does this unit work, Mr. North?

Mr.North: It would take up too much time if I went into details, for the units are

rather complicated; but they all give the pilot precise control of his speed,

whatever the conditions.

Ben: How does the fuel get from the *tanks to the combustion chambers?

Chris: May I butt in1 here? Pumps are used, arent they? One type consists of anumber of spring-loaded pistons or *plungers fitted to a root *assembly.

The outer ends of the plungers *bear against a non-rotating *camplate.

As the plungers revolve, they are forced down by the camplate, then

released again. This produces the required pumping action.

Ben: Good heavens, Chris, where on earth did you learn that?

Chris: Elementary, my dear Watson!2 Ive been studying the diagram.

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Mr. North: You might have added, Chri, that a *servo piston is linked to thecamplate so that the angle of its inclination can be varied.

Ben: Oh, I see. By altering the angle of the camplate, you can alter the *strokeof the plungers and therefore the quantity of the fuel being pumped. And

the servo piston itself is regulated by the automatic control unit we were

talking about just now.Chris: Is it true that such a pump can deliver up to 2,000 gallons of fuel per

hour?

Mr.North: It is, and a pressure of up to 2,000 lbs per square inch. Mind you, 3 it

takes a lot of power from the engine to work it, 50 or more horse-power

in some cases.

Ben: Isnt there another sort of pump that works on the principle of meshinggears?

Mr.North: Yes,

english jet engine

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