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INTERNAL COMBUSTION ENGINES (IncludingAir Compressors and Gas T\¡rbines and Jct Propulsion)

By

R.K. RAúIPUT M.E. (Heat Power Engg.).Élons.-Gold Medallist ;Gtad' (Mech' Eryg' M.l.E. (Indb) ; M'S.E.S.I. ; M.I.S.T.E. ; C.E. (Iúb)

&

Elect' Erqg')

Princlpal (Formcrlfl, Puaiob College of Infonnotion

Teehtplql

PATIAIA (Puajob)

rN(Ml BANGALORE. JALANDHAR O

PUBLICATIONS (P) tTD CHENNAI KOLKATA

o GOCHIN o LUCKNOW NEW DELHI

¡..

o GUWAHATI o MUMBAI

o o

HYDERABAD RANCHI

PREFACE TO THE SECOND EDITION I am pleased to presenü the Second edition ofthis book. The warm reception, which the previous edition ofttre book has enjoyed all over India, has been orgt"át ;atisfaction to -att"r me. " l*

It

t

-ii',Í#f,8rrtb er rro 1lB, Golden

r.qlo{r

House, Daryasani.

New Delhi_il000i phonc :011_4A bg 25 00 .Far : 011-4Ít 5g 2E 28

The book has been thoroug_ hly rwised, besides adding a new chapter (No. 22) on..short the itudents to prepare more effectively forpro ctical Viua-uüe E xamhtatia ns and I nter v iew s. Any suggestions for improvement of this bbok will be thankfully acknowledged and

Answer Quections'to enable

incorporated in the next ediüon.

{

-Author

www.Ian ipublicatio¡e.com

. info@l¡nipublicaüions.com Compiled @

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of this publilxrtfun

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io oo, fotm or by aryr means, "lrteii, ;;h";ü;;;*;;;,*, rcañ.it* "t""t*"í., or otherwise without the pnor urrtten penni,ssion of the publishen Price:fu.

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EIC.O65O.395.INTERNAI COMBUSTION ENGINES Ilpeeet at : Goswami Associatee, Delhi.

U

c_t74üilosto4 Printed

at: l¡jitfuintars, Delhi.

{ i..

PREFACE TO THE FIRST EDITION gos turbínes) on olnternal Combustion Engineco (Induditg

CONTENTS

contains

This treahise lucid and direct language' It eirvelops comprehensive treatment ofÁ-e su¡ject matter in a_simple, typical worked examples from a large number of solved p*ff"-J n""nerly graited including view. of point examination

.$ T,

I

Ir I

Thebookcomprises2lchapters.Allchaptersa¡esaturatedwithmuchneededtext, "upport"J órí¡:.ti""

¡y ,i.pf"."rr¿ *fi"*pi"".tow-figurás-.At the end of eactr chapter-Highlights' have been Eximples ivp.'qo."u"i", it .i.tiá {uestiors and unsolved Objective Tlrye Questions

Bank'

containing

"Adütional

"queJion Sofii""-Comments)",'Theoretical Questions withAnswers" and (with Answer" "oa ..Addiüional Typical p*"ipl"" (Includ,ing l¡niuersities and Competitiue Exomination rnake the bóok a comprehensive and a complete unit in all

added ; besides tfris

A;;J¡;;;;

t

"r,"

a

i;

-t

I.

BASIC CONCEPTS OF TEERMODYNAMICS

1.1. 1.2.

L""o io"to¿"Tto

respects. preparing for engineering undergraduThe book will prove to be a boon to the students examinations' competitive other and U'P'S'C' graduate, post e.ü.i.O., ut",

Theaut,hor,sthanksaredueüohiswifeRameshRajputforextendingallcotiperation

during preparation ofthe manuscript and proofreading' his graütude üo Shri R.K. Gupta, Chairman, In the end t;.e author wishes üo expresspu¡ucauons hrt. Ltd., New Delhi for taking a sh. saurabh Gupta, Managi"g Di;.**, l,axmi in a short span oftime' good presentation very with book out"th; lot of pains in bringrng

Althougheverycarehasbeentakentomakeühebookfreeoferrorsbothintextasrvellas enors present are brought to ltis i¡r solved examples, v.t trr" u"irro, shall feel obliged if any received' warmly be will book ,roti.". Corr"trrr.tive criticism of the

Pages

Chapter

1.3. 1.4. 1.5. 1.6. 1.7. 1.8. 1.9.

Deffnition of ThermodYuamics Thermoclynamic SYstems 1.2.1. System, bobndary and surroundings 1.2.2. Closed sYstem 1.2.3. OPen sYstem

L.2.4. L.2.5. L.2.6. L.2.7.

Isolated sYetem Adiabatic sYstem Homogeneous sYstem Hetemgeneous sYstem

Pure Subgtance Thermodynanic Equilibrium Properties of SYstems State Process

Cycle

Point Function

1-32 1

2 2

2 2 a o

ó

3 3 3

4 4 4 D

1.10. Path Function

5

1.11. TemPerature

D

!.12.

7'erclh Law of Thermodynamics

1.13,

PresEure

b

1.13.1. Definition of Pressure

-Author 1.14. 1.15.

1.13.2. Unit of Pressure 1.13.3. \rpes of pressure meaaurement devices R¿versible and Irreversible Process€s EnergY, Work and Heat 1.15.1. EnergY 1.15.2. Work and heat

1.16. First Law of Thermod¡namics 1.17. The Perfect

6

Gas

1.17.1. The characterietic equation of state

1.17.2. SPecific heats 1.17.3. Joule'e law

1.17,4. R¿lationship between two specific heats 1.17.5. EnthalPY i 1.1?.6. Ratio of sPecific heats

b 7

8

I I

9 10 11

1l t2 13 13

t4 15

(vni)

(ir)

Chapter

... ^. ... ... ... ... ... ... ...

1.lg.l.

Enerry relatio¡s for flow process 1.19. Limitations of First f., of fn"._-Jyotio

1.20. performance of Heat Eagine and n"r1,"""¿ lleat 1.21. Statement¡ of.Seconil f¿-* of fn"._Jilaurics

Engine

1.21.1. Clausius statement 1.21.2. Kelvin-planck statement

L.22. Entropy

1.22.1. Introduction L.22.2. Tempetature.euhopy diagram 1.22.8. Characteristics of entrop| 1.28. The Third Law ofThermodynariics Hightights

Objectiue fupe euestions Th¿oretical euestions

IMRODUCTION

2.I. ?.2. 2.3. 2.4. ?.5. 2.6. 2.7. Z.B. 2.9.

I1O

TNIEnNAL CoMBUsfioN ENGINES

Heat Engines Developmeat of I.C. Engines Claseification of I.C. Engines Appücation of I.C. Eagil-es Engine Cycte-Eou"gy B"l*""

Basic ldea of t.C. Engineg Different parts of I.C. Engines Terms Connected with I.C: Oi6ne" Working Cycles

2.10. Indicator Diagra-

2-.Il 12 ? 2.I3.

Four Stroke Cycle Engines TVo Srroke Cycle Engines

Intake for Compression Igaition Engines 2.L4. Comparison of Four Strokl ."a f*iit-t" Cycle Engines 2.15. Comparison of Spark lgn¡tion fs.i.l anJáoirop."".ioo Ignition (C.I.) 2.16. Comparison between a petml Engine and a Diesel Engine 2.L7. Hott to Tell a T$o Shoke Cy"l" ñ;;;'-f; a Four Stroke Cycle Engine

!

Highl,ighta O bje ct iv e Ilpe e ue stians Theoretical euestions

3. AIR STANDARD CYCI,ES 3.1. Deñnition of a Cycle 3.2. Air Standard Efficiencv 3.3. The Carnot Cycle 3.4. Constant Volume or Otto Cycle

L7 18 18 19 19 20 20 20

20

2l 2L 22

... ... ... ... ... ... ...

...

Atkinson Cycle

3.10. 3.11. 3.12. 3.13.

Stirling Cycle

110

t20 136 136 13? L37

138

Ericsson Cycló Brayton Cycle

741

t42

Mille" Cycle

153

) :..

Lenoir Cycle

155

156 IDó 158 159 160

i

bj ectiu e Type Q ues tions Theoreticol Questinns O

32

Unsolued Enmples

/4.

FT,'EI-AIR AND ACTUAL CYCI,ES

4.L.

33 95 35 38 39 40 41 66 68 69 69

Fuel-air Cycles

4.I.7.

4.1.2.

4.1.3.

4.7.4. 4.1.5. 4.1.6. 4.1.7. 4.1.8. 4.1.9.

73

77

4.2.

Introduction Factorg considered for fuel-air cycle calculations Aesumptions oade for fuel-air cycle analysis Importance of fuel_air cycle Variable specific heats Effect ofvariation of epesific heats Dissociation Thermal efficienc¡r and fi¡el consumption Efect of @r¡¡mon engine variables

4.1.10. Charact¿ristics of co¡stant volume fuel-air cvcle 4.1.11. Combustion charts 4.1.12. Gas tables Actual Cycles

4.2.L. Introduction 4.2.2. Causes of der¡iation of actual cycles hom fuel-air cycles 4.2.3. Real fuel-air engine cycles 4.2,4. Difference between real cycle and fuel-air cycle 4.2.5. Comparison of operations and working media for ,air cycle,,

79 81 81

,actual cycle' of S.I.-engrnes _-. - -. _fuel-air cycle' and Highli.ghts Objectiue Type euestians Theoretical Questions Unsolved Exarnples

82 83

u

8L161 ... 85 ... ...

S.8. 3,9.

Highlights

78

... ... ... '.. ...

Constant Pressure or Dieeel Cycle Dual Combustion Cycle Cornparison of Otto, Diesel and Dual Combustion G¡rcles A.1.L. Efficiency versus compression ratio 3.7.2. For the sane coopression ratio anil üe same heat input 3.7,5, For congtant maximum pressu¡e anil heat supplied

23 25

33_&t

... ... ...

Pages

3.5. 3.6. 3.7.

16

... ... ... ... ...

1.24. Available and Unavailabt" irr""gy-

2.

Chapter

Pages

1.18. Steady Flow Eaergr Equation (S.F.E.E.)

l

162_200

... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ..

COMBUSTION IN S.T. ENGINES

6.

86 86

5.1.

Introduction

5.1.1. 5.7.2.

94

{,..

i

Definition of combustion Ignition limits

163 163 163

170

L7t 172 173 178 178 178 181 181 181 181 182

...

... ... ...

762 762 162

róo r97 197 198 199

20t-226

... ... ...

201 201 201

(¡)

(¡i

Chapter Pages

5.2. 3 I5.4.

5.5. 5.6.

J.I. D.ó.

5.9.

Combustion phenomenon 5.2.L. Normal combustion 5,2.2. Abnormal conbustion Effect of Engine Variables on Ignition Lag Spark Advance and Factors Afiecting lgni-tion Timing

205

Detonation

m3

Pre-ignition

5.6.1. Introduction 5.6.2. Process of detonation or knockiug 5.6.3. Theories of detonation 5.6.4. Efects of detonation 6,6.5. Factors affecting detonation/k¡ocks Performance Nuober (pN) Higheat Usefi:l Compression Ratio (HUCR) 99Tbuslion Ch¡-ber Desigrr-'S.I. Engines 5.9.1. Induction swirl

5.9.2. Squish and tumble 5.9.3. Quench area 5.9,4. Turbulence 5.9.5. Flarne propagation 5.9.6. Swirl ratio 5.9.7. Surface-to-volume ratio 6.9.8. Stroke-to-bore ratio 5.9.9. Compression ratio (C.R.)

5.10. Some Tlpes of Combustion 5.10.1.

Chambe¡e Divided combustion chambers

Highlights

Objective Type euestians Theoretical euestbns 6.

COMBUSTION IN C.I. ENGINES

6.1. 6.2. 6.3. 9^_ 6.5. 6.6.

6.7.

in Diesel Engines Delay period (or Ignition Lag) in C.I. Engines Diesel Knock C.I. Engine Combustion Chambers 6.6.f . P¡ima¡y considemtions in the desiga of combustion chambers for C.I. engines 6.6.2. Basic methods of generating air swirl in C.I. engines

6.6.3.

combu.stion chambers

Types of combustion chambers Cold Starting of C.I. Engines

Highlights Objective Type euestíons Theoretical euestíons

Pages

Chopter

202 202 204

24&-268

AIR CAPACITÍ OF IIOT'R SIROI{B ENGINES

7.t. 7.2. 7.3. 7,4. 7.5.

248 249

208

Introduction Ideal Air Capacity Volumetric EfEciengy Effect of Various Factors o¡ Volu.metric EfEciency Inlet Valve Mach Index Highlights

208

Objectiue Type Questions

265

2r0

Theoretial Questions

210

unsolued

m4

208

Emmples

249 250 253

264

)

2r0

zLl

8.

ztl

TWO SIROIiE:EF{GINES

8.1.

2Ll 212 213 213 214 215

275 215 216 276

8.2. 8.3. 8.4.

218

220 223 224 225

6.D.

8.6. 8.7.

226-247

Objective Type Qrestbns Theoretical Questiorc

226 227

9'

230

9.1.

237 237 238 l

Chemical Thermodynmics

9.1.1. General aspects 9.1.2. Ba¡ic chemi¡try 9.1.3. Fuels 9.1.4. Combuetion equations 9.1.5. Theoretical air and excess air 9.1.6. Stoichiometric air-fuel (A/F) ratio 9.1.7. Air-fuel ratio from analysis of products 9.1.8. Analysis of exhaust and flue gas 9.1.9. Internal energy and enthalpy of reaction 9.1.10. Enthalpy of formation (AlI.) 9.1.11. Heating values of fuels

{. r.i

265 266

297-28r 267 267

269 269 270 270

270 271 274 275 277

278 280 280

281

CI{N}trCAL TIIERMODYNAMICS AND FTJELS (CONI¡ENTIONAL AND ALTERNATIVE)

234 236

210 241 245 246 246

General Aspects 8.1.1. Construstion and working 8.f.2. Comparison between two-stmke cycle and four-stroke cycle engine 8.1.3. Disadvantagee oftwo-stroke S.I. engine comtared to twoshlke C.I. engine 8.1.4. R¿ason8 for use oftwo-e¿roke C.L engines for marine propulsion 8.1.5. Reasons for the use of two-stroke S.I. engines for low horse power two wheelers Intake for Two stroke Cycle Engines Scavenging hoces Scavenging Parmeteru Scavenging Systems Crankcase Scavenging Scavenging Pumps and Blowers

HishliAhts

Introduction Combustion phenomenon in C.I. Engines Fundamentale of the Conbustion pr-ocegs

)

282-356 282 282

282 283

284 286 286 287

287 289 293 294

(xii)

I

(

Chapter Pqg¿"

9.1.12. Adiabatic flame tenpe¡atur€

Pog""

9.1.13. Chemical equilibriui

9.2.

296

9.1.14. Actual combuetion

2W

Conventional Iuels (For 9.2.L Introduction

298 238

_¿""i" lClffi""¡--

9.2.2. Desirable-propertiee of good I.C.cngioes 9.2.9. Gaseous fuels 9.2.4. Liquid fuels 9.2.8. Structure ofpetrole¡n 9.2.6, petrolelo aod coinpoeition of crude oil 9.2.7. Fuels for "p""f_ig"iUoo 9.2.9. Knor ""gi""l-

s,,Jil:ffi"jj*;H;",T:*ne 9.2.10. s2

9.A.

Dieeet fuel

298 2gfl 300 300 303 305 310 314 314 316 316

tue,s

General aspecto and dieadvanrases of using alrer¡ative fuele

l.i:i. ifl:i:i.*""

9.9.4. Alcohol-gasoline fuel ble¡ds 9.8.5. Hydrogen 9.8.6. Natural gas (Eetha¡e) 9.9.2. LpG and LNG 9.8.8. Biogas Hisht@hb Objective Type euestions Theoretical euestions

F.UEr./ArR MrxTuRE REQUIREMEIYTS

10.1. Introduction 10.2. FueUAir Mixture

Bequirements for Steady ' --' vwqqr IRunning Optimum FueUAir RaUo" 10.4. Idling and Low Load 10.5. Normal power Range or Cruise Ranse 10.6. Maximum power RLge -- -'v '.*¡Es 10.7. Transient Mixture Requirements 10.?.1. Starting and warming up hi¡tu¡.e requirements 10.2.2. Mixture fol ;;;n"" 10.8. Effects of operarine "eqrrire-ent variables ;; üj;;;"-t"quiremenk .. 10.9. Mixture Requirements f"" Di"""l-;;;;;;

l0'S

Highlights

--- -'¡6¡¡¡ee

Objectiue Type euestions

Theoretical euestinns

rT. CARBT.IRETION A¡ID CANBUR¡TTORS 11,1. Introduction 11.2. Induction System

11.9.1. Eseential featu¡es o,fgood commercial carburettor for automotive engines Injection

11.10.1. 11.10.2. 11. 10.3. 11. 10.4. I l. 10.S.

3r7 318 323

Drawbacks of modern carbu¡ettors Introduction to fuel iqiection Direct injection Indirect injection Injection considerations

11'10'6' comparison ofpetror idection and carburetted fuel supply

368 369 370

37L 372 373

381 381 382 383

391 391 391 391 391 393

systeme

325 325

11. 10.2.

393 394 395

Theoretical euestions Unsolued, Eramples

411

Electrouic fuel injecti,on 11.11. Theory of Simple Carburetüor Highlights Objectíue Type etnstiow

327 327 351 354 355

11.9.2. $pes of carburettors 11.9.3. Description of some important maLes of carburettors

^ Petrol 11.10. -_ _

350

Unsolued Examples

ro.

11.3. Factors Influencing Ca¡brretion 11.4. Mixture Requirenents 11.5. Distribution 11.6. Transient Mixtu¡e Requirenents f1.7. A Sinple or Elementary Ca¡burettor 11.8. Complete Carburettor 11.9. Carburettors

29E

fuels

Alternative Fuels for I.C, Engines

9.4.1.

úii)

Chapter

12. FUEL INJECIION

357-366

72.r. L2.2. 12.3. 12.4.

357 357 360 361 362

L2.5.

362 35:t 363

363 364 364

It a

366

10 a

366 366

t2.8.

367 367

.l i-..

i

412 413

SYSTEMS FOR C.I. ENGINES

Introduction Functional Requiremeats of an $ection System Fun¡tions of a Fuel Injection S¡rstem Fuel Injection Systens 12.4.1. Air injection 12.4.2. Solid or ai¡less injection Pumn and Fuel Injector (Atomiser) l^ue_l 12.5.1. Fuel pump 12.5.2. Fuel atomiser or injector 12.5.3. Faults, causes and remediee of injectors T}pes of Nozzles üd Fuel Spray pattems 12.6.-1. Main reqrri¡s6s¡ts of an injector nozzle 12.6-2. Classification and description of nozzles Engine Starting Systems Fuel,Injection Computation in C.I. Engines Highlights O bje c t iu e Type eue stians Theoretical Questions Unsolued Etamples

367_414

410

416-440 415 415

tro ato 416 417

420

420 423 425

426 426 426 429 430 438 439 439 439

.

Chapter

Pqgss

13. IGMTTON

SYSTEMS (S.r. ENGINES)

...

Objectiue Type Questions

Theoretical euestbns

14. ENGINE

FB,ICTTON

¡ND LI]BRICATION

14.1. Introduction 14.2. Total Engine Friction 14.3. Effect of Engine Parameters on Engine Friction 14.4. Determination of Engine Friction 14.5. Lubrication 14.5.1. 14.5.2. 14.5.3. 14.5.4.

Definition and objects Behaviour of a journal in its bearing Properties of lubricantg Types of lubricants 14.6. Lubúcation Systems 14.6.1. Introduction 14.6.2. Wet sump lubrication s¡rstem 14.6.3. Dry sump lubrication systen 14.6.4. Mist lubrication system 14.6.5. Lubrication of different engine parts 14.6.6. Lubrication of ball and roller bearings . L4.6.7. Oil filters 14.7. Crankcase Ventilation Hishlights Objectiue Type Questions

?heoretical Questians

15.2.

15.4.1. Heat transfer 15.4.2. Temperature distribution 15.4.3. Temperature profiles

4{9 452 452

and Temperature profiles

16. SUPERCEARGING OF I,C. ENGINES

468

16.1.

459

16.2. Supercharging of S.I. Engines 16.2.1. Natually aspirated cycle of operation

,t60

...

... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ,..

46:|

1U

16.3.

,164

465 466 469

16.4.

r6.5. 16.6. 16.7.

177 171 472

engine pressure_volume diagrams Boost pressure and pressure ratio The effect of presaure ratio on air charge r€mperature Thermodynanic cycle and superchargiag power Supercharging limits of S.I. enginee Supercharging of C.I. Engines 16.3.1. Supercharging limits of C.I. engines Modification of an Engine for Supercharging Supercbargers Supercharging Arrangements Turbochargers

16.7.1. Introduction 16.7.2. Altitude compensation 16.?.3. Turbocharging-Buchi system 16.7.4. Methods of turbcharging 16.7.5. Limitations of turbocharging Highlights

174 175 475 477 477

485 485 487 488

Purpose of Superchaiging

511 513 513 513 5L4 514 515 516 518

16.2.4. 16.2.5. 16.2.6. 16.2.7.

4&

482 484 484

) 511-536

16.2.2. Supercharged cycle of operation 16.2.3. Comparison of actual natwally aspirated and supercharged

re r 461

478 478 478 480

49r 493 493 496 503 508 508 509

Objectiue Type Questions Theoretical Questions

4153

455 456

490

15.7.1. Ai¡-cooling system 15.?.2. WaterÁiquid cooling system 15.8. Components of üIater Cooling System Hishlights

412 112

482-510

Necessity of Engine Cooling Areas of Heat Flow in Engines Gas Temperature Variation

15.3. 15.4. Heat Transfer,'Temperature Distribution

,l,ll

461--{81

15. ENGINE COOLING 15.1.

Pages

15.5. Effects of Operaüng Vriables on Engiue Heat Tlansfer 15.6. Cooling Air and Water Requirements 15.7. Cooling Systeqs

tul

... ... ... ... ... ... ... ... ... ... ... ...

13.10. Electronic Ignition Systems Hightights

'

Chapter

441-460

13.1. Introduction 13.2. Requirements of an Ignition System 13.3. Basic Igaition Systems 13.4. Battery (or Coil) Ignition System 13.5. Magneto Ignition System 13.6. Firing Order 13.7. Ignition Timing 13.8. Spark Plugs 13.9. Limitations of Cbnveútional Ignition

(p)

bj ec tiu e Typ e Quc stions Theoretical Questinns Unsolued Etamples O

L7.

TESTING AND PERÍ'ORIITANCE OF I.C. ENGINES 17.1. 77.2. 1a e

L7.4. L7.5.

r/.b-

Introducüion Performance Parameters Bagic Measurements Engine Performance Curoes Comparison of Petrol and Diesel Engines_Fuel ConsumDtron Inad Outputs and Exhauet Composilion Governing of I.C. Engine Noise Abatement

Highlights

aló

,

519 520 520 520

52r 521 Kta

524 526 . ¿a ññ cor'+. ír rr ^ t1o

2.5o,

"H,

Air-Fuel Ratio and Analysis of producüs of Combustion acetylenz

5¿v

Solution. The Stoichiometric air equation (Example g.1) is w¡itten as :

rt;reiJvi*#; ;""t"*;;;1",n"rfilffi;."*T" t-'"';,",1iJ';Tilio1H,1xXT""¡"1;*;ü{,*ff,f;"'" r;", p,,n,;;;;*-.'i¡,n t¡ut

,fii]3:" (iir) when biogas is used as a "ogiou tuel, N4

CHEMICAL THERMODYNAMICS AND FUELS

tz.s A/F

,n" ,,.11irL""?retical

air-fuel ratio on

.

tz.s

(fi)

c{r,r.

The

w,

+tz.s(!9)

=

= b9.b mol airlmol fuel

, ,'u.. bui,

---\?ll

is found by

introducing the molecular weight of

330

ENGINES

JJ I

CÍIEMICAL THERMODYNAMICS AND FUELS

-t I

I

M

59.5 (28.97)

=G

f¿

Example "9.4.

* ¡. le)

One hg of octane plete combustion determine ;

_

fuel.

= 15.08 kg air/kg

@dIr)

By balancing co rbon ato¡ns on both the sides, we get

(Ans,)

uith

is burned.

By balancing orygen atoms on both the sides, we get

air. Assuming com-

2001o theoretical

(ii)' we get Substituting the value of ó 1= Z - a) from eqn' (i) in eqn'

For

200o/o

(#)

Nr-----+

8co,

+

ego

+

(0.9) (3'5) (2) = tut + 2

and

C,II. + (0.9) (3.5) O,

o,

+ (2) (12.5)

(,'e)

(¿')

Air-fuel ratio

[t

#)

1.e.,

air

Mass of

of

Mole fraction of

tlO

=

fr

+

Volumetric analysis of dry products of combuttion is as follows

p++a's

1q

= 3o'2s

fuel ¡a

ñ7

"O (z)(1r.5)

[;1)

= 128.5 moles/mole fuel

The saturation temperature corresponding to this pressure is 39.7'C which is also the dew-

t¡p=39.7'C. (Ans.)

Noúe. T?re water conden¡ed from the products of combustion usually contains some dissolved gases md therefore may be quite corrosiue. For this reason the products of combustion are often Éepl c boue the ilew point until discharged to the otmosplere.

_ Example 9.5. Onc hg of eth.ane (C2II) is burned with glEo of theoretical aír. Assuming complete cornbustíop of hydrogen in the fuel detennine the vol,urnetric analysis of the dry prod,ucts of combustion. \

Solution. The complete combustión equation for CrHu is written as CrI\ + 3.5 Or---+ 2CO, + SH,O

[aJ

N,

a cor+ó

9.6 . Metlwr¿e

=

¡fr6

x 100 = 6.06%. (Ans.)

tbe prod'ucts QH ) is burrcd with otmosplwrh air' Tlw arwlysis of

CO2= 10'0Mo, 02=2'37%' CO = 0'539o' Nz= 87'1Mo' the oir'fii'el ratia; G\ Deterrníne the comiustion equatinn; Gi) Calculate

(íii) Percent th'eoretical air' Solution. (i) Combuetion equaüion

:

be wútten, keeping in mind From the analysis of the products, the following equation can that this analYsis is on a dry óosrc. r CHn iy O, + z Nr---+ 10'0 CO2 + 0'53 CO + 2'37 O,+ o H'O + 87'1 N2 for each of the elements' To determine all the unláown áefñcients let us find balance

Nitrogen bolance : z = 87'L Since all the nitrogen comes frorr the air,

t

t

:

Co + BHro + (o.e) (3.5)

(;?)

-,

:

basís is as follows :

:

The combustion equation for CrHu for 90% theo¡eticalqir is written as

(#) O--

sExample on a'dry'

= O.OtrS

point temperature

crHu + (0.g) (3.5) o, + (0.e) (3.5)

/29\

x 100 = 9.3M. (Ans')

co, = ,fi6

Partial pressure ofHrO = 1ü) x 0.0728 = 2.28 kPa

Hence

Nr

r7e)

=

r2.5 +

-_

(0.e) (3.5) = 1.3 + 0.7 + [ 21J of fuel = 1.3 + 0.? + 11.85 = 13.85 moles/mole

A/F = 3095. (Ans.) (ri) Dew point of the products, tun : Total number of mole.s of products =8+e

[1J

-----+ 1'3 CO, + 0.? CO + sHrO + (0'9) (3'5)

28.92 = s448.8 kg/mole of fuel

= Mass

/7e)

Total number of moles of dry products of combustion

:

Air-fuel ratio. A/T

:

+ (0.9) (e.s)

*,

Mass of fuel = (1) (8 x 12 + 1 x 18) = 114 kg/mole

-

I

b=2-ai-2-l'g=O'7

Q2.$[#)

"

Mass of air = (2) (rz.s)

I

Thus the combustion equation becomes

-----+ 8CO, + eHrO + (1) (12.5)

I

a=1.3

rz.s(lf) n,

-79,n 2L "

=

--9f'1-'= QslzD

23.16

Carbonbalance: r= 10.00+0'63 = 10'53 Ilydrogen

i

a+3

6.3=o+5

theoretical cir the combustion equation would be

CrH,, + (2') (12.il Oz + Q)

-

I I

...(ü)

(0.9)(35)(2) =tut+b+3

(í) Air-fuel ratia (ii) Dew point of the prducts ot a total pressure 100 kpa. Solution, The equation for the combustion of CrH* w.ith theoretical air is

crn,, + t2.5or+tr r

...(r)

2=a+b

bolonce: a'=2t=2

x 10'53 = 21'06

55¿ INIERNAL COMBUSTTON ENG¡NES

provides a check o" tn" ,".r*""??,fíli:#ffi'f#:iHtl-j.::,"":i"'itTl:::-o:"1sorved u..*á.ylffi";:'T:1":TT balance

r

9{9

= 1o.oo +

+ 2.sz

*

3lS

Substituting these values for '*, y, z and o, we have,

=

\

CO' = '

is

rheair.tuer"^r,",::X1u;,:'i"-.[ffi LO.47

The rheoretical

Hr=

x28.97

(#)N,

____+

(12;

co, + zl"o +rzl (,,eJ rv,

üD-

air&g

tueL

percent theoretical

Example ash. catcutatle

.

of

G) The stoichiometric

NF-ratin;

Solution. (j) The etoichiomeüric

1 kg ofcoal contains 0.g2 kg C and

... 1 kg ofcoal

Le*h

."n",,i

11?

(ii) The -'"- anolysis of

¡¡f ratio: O.l0l;;,

,ont"irr, 0.82

e oxygen

coat is gioen as g2%

^

c, la{o H" and

the products by uolume.

.lt c_* 0.82

0.10

t

+

x

CO"

+8,26* N, ___+ o Co, + ó IIrO + 8.76¡ N.,

Y =" ... a =0.06gmoles balance : z, ff = zt .., ó = 0.05 moles

Orygenbalance:2x=ya6

.,= (::Ueiel)

=o.oes_or.,

x r0o = lo.74o. (Ans.)

air-fiul ratia for

30 per cent excess

(Ane.)

the combusti.on of a sample

of

Hydrogen (Hr) = 4 per cent Nitrogen (N) = 1 per ceni

= 3.5 Per cent (S) = 0.5 per cent

Mass per kg coal

Then,Carbonbalanee : Hydrogen

g7o

.ilff :l:ñ-"::: ::,",

U,

L4.664o. (Ans.)

Ash=SDercent

air is supplied. deternúne

:

(i) Air-fuel ratio (íi) Wet dr1 analysís of the products of combustion by volume. Solution, Stoichi brake I.C. engine efiiciency for S.I. and C.I. engines are of the o¡der 357o a¡d 407o respectively. Fig. 1?.7. Specifrc fuel consumptionC.I. engine. The flat curve of Fig. 17.7 illustrates brake mem effective pressure curve for the C.I. engine. that atpart load the compression ignition engine is more economi¡ol than the spork ignition er4Éne. This is the benefit of quality control rather than quantity control of power. 1

Soeed ------->

7.5. COMPARISON OF PETROL AND DIESEL ENGINES_FUEL CONSUMPTION LOAI) OLTTPUTS AND EXIIAUST COMPOSITION

Fig.1?.5. Power-speed and torque-sped curyes for the I.C. engine'

Specific fuel consumption relations : S.I. Engines : Refer. Fig. 1?.6. The cuwes are plotted for constant throttle opening, constant speed. and constont ígnition setting, The only uariable is the air-fuel ra¿io. The eflect of

))l

TESTINC AND PERFORMANCE OF I,C. ENGINES

L Fuel Consumption: Fig. 1?.8 shows fuel consumption loops, for both petrol and diesel engines, plotted on a base of brake mean effective pressure (b'm.e.p). o = Excessively rich mixture gives slow and unstable

B¡ch

combustion.

Petrol engino

!

= Muimum b.m.e.p. with something like 1G-207o rich mixture. c = Conect stoichiometric mixture of 14.7 : 1 by weight d = Maximum thermal efficiency with something like IO-2|Vo weak mixture (approaches ideal constant á

{

=o

+

E I

E

c

'$*,

o

\s4 'b& -\¡-ñ------i't

f

Sto¡chiometric

.9

bmep

bmep

------)

-----f

=oo

(b)

(a)

0

\

o

25

volume combustion)' = Excessively weak mixtue gives slow buming and popping back through aü intake. /= Maimum b.m.e.p. with satisfactory clear exhaust requi¡es mixture strengü of about 18 : 1 by weight. g-h = Muimum thermal efliciency, minimu speciñc fuel consumption ranges between 5O-85% of muimum b.m.e.p. i = No-load Oow speed idle) requires mixtue strength e

50

75

100

10G-75:1 byweight.

Fig. 17.8. Comparision offuel consumption loops for petrol md diesel engines on a base of engine load (b.m.e.P.).

In case of a diesel engine, Ioad. and, speed output is controlled entirely by uarying the quantity of fuel injected. into the cylinder wíthout misft'ring occurring, that is, from

Wsak

Sto¡ch¡ometr¡c

Rich

(c)

17.6. Specific fuel consumption-brake mean effective pressure cunes fo¡ the S.I. engine.

0-700Vo of the maximum b.m.e.p- developed. engines, however, if there was no throttle (full th¡ottle position) the effects ofvarying the mixture strength from the richest position (o) to the rveakest position

with the petrol

INTERNAL COMBUSTION ENGINES

variations ofb.m..e.p. Qoad) on only 259 that is, from 75-L00% o ¡' -' p. ?r''i;"i?.!TlñT"HtT o u tp u t c o n t r o t o n n ot u! achieued olone "' bv throttting th" " 'oryl,s *" ^¡t ii"-ti""áí

^*'u.'

i}"rli

^i*trru "oi

with rnixture strengths greater than 207o rich, that is 18 monoxide (CO) is present in the exhaust (FiC. 17.10).

lff ;t

on

II. Load Ouúputs:

o

I 15

re¡nain reasonably constant to no load.wherer. rú orlh^9fÍrnping from full load. is re.duced due to the.^ c,mprng lossesfor apetrol erwine progress.iuely rtr",* tiJl.^,".p, losses

I

g o a

c)

aE'

9:

120

Diesel

engine

-,_

I

(ú-

60

o

40

9¡e Ev

J

Exhaust

-F-}l

emmission

E

d)

I

20

1

0 I

lt

lim¡l

for

dies'et

engines

2o;Eo

,"

-

hishe;

o,i

100

\ \

i0

\

l.o ;o

iv, i.,

l\

ico,' ! \,'

i\

i.i',

il'

19 17 15

13

I

i i i

t9 17 15 13

m'a;i^1{i.::::! fiXtri'Ji,ljy;

1Cl

___>

*i!¡'"1*í"s have a 15 to e"á.1e ll:j:;"' o¡esel engine (Fig r7'9)'

" ul)r.ll,,l!,Íiffiil","f,Tü:':: ::{;ff:i;!i:,l":##1:í:f::1s for pe,ot and dieset engines orca¡bon monoxide (co) re¡¡¡s¿ ¿5 ¿h. ff'.iüT*:3'i":iTr:"l:',Hii}:,nlllr 10:lrhisb.i"s1;1!,^13'";;ñ;,sr":iJ-;t;rt,:J.n-.t:'r::llHlilXffi and full power ónditions. In contrast-th" diur"t

The carbon dioxide (COr) emission produced by the diesel engine relative to the petrol engine is always much lower, particularly as the engine load is reduced, whereas the petrol engine in the stoichimet¡ic (14.7 : 1) band operates with the highesi level of COr.

17.6. GOVERNING OF I.C. ENGINE

Fig' 17'g' comparisou ofload (b.rn'e.p.) fo¡ petrol and diesel engine on base ofair-fuer ratio. A petror engíne can effectivery under steady .op-erate conditions ransins from 20 : I.ti t0 : oyer a nixture strength L;;;;;;"'";;;iJi i"h cannormauy otnly utilize 80vo "ngir".*t "J';:tir;:Yf;írith o '"o'ánobiv "t,,J,l'iá',',,, ,iu operate from r8 : ri that is 2avo weah

Irr. Exhaust composition

\

Air/fuel rato by we¡ght

a

Airl tuel ral¡o by weight

o

io

Fig. 17.10. Comparison ofcomposition ofexhaust gases for petrol md diesel engines on a base ofair-fuel ratio.

o e o o

oc

co.'

¡O ttr.

'i.

jilrr"* t

100 80

f?'¡

21i

.t:..;

pehol engine

i

i\r/

l"z

0

4

weight, so that no carbon

Petrol engine

12

-".i,u,,üi"ñ.i";;ffi :".ffi1":t'i1*1;*::::J1l*ll"l;:o'"i."iilá'iJi"i" :,:fi.1J.',?:311"':j#,i!:'1.:'"r" oi"'""1u-"iti;::Tl"T;T"'*::n:Tilf,i:,".7

;* ljn Hidf *ffift,Tr

i

ai*.

"ij" "iii"

::lff nff ú: jl;J";:;xs;ff

Diesel engine

18

al

fn case ofadi¿sel ensine

i

: 1 by

Áá'í**p*

:i::t:ix,x:f:i:#ir:{'"i!'frf ;ne í!:";;:""";::::;;;;"".'::*,,"" listed the right 17.8. consumption.loops are

553

TESTÍNG AND PERFORMANCE OF I.C. ENGINES

(e) p¡oduces a

":iiil

j

under fufl loail n-ever ope¡ates "nginn

The function ofa governor is to keep the speed of engine constant irrespective of the changes jn load on the engine. The governor is usually ofcentrifugal type. In petrol engine, the control is exercised by means ofa throttle valve which is placed in intake manifold. The quantity ofmixture entering the cylinder depends on the amount of opening ofthrottle valve. The position of throttle valve is controlled by the governor (centrifugal type). In diesel engines, the flow offuel is controlled by centrifugal governor which actuates link rods which in tr.rrn operate some device on the fuel pump and consequently portion of the fuel by passes. The governor in plunger type injection pump alters the relative angular position of the plunger.

Following are the methods ofgoverning I.C. engines

:

(i) Hit and miss method

(jj) Qualitygoverning

(iii) Quantity governing (i) Hii, and miss method. Refer Fig. 17.11. When the speed increases the permissible value the governor sleeve S gets lifted up, as a ¡esult of whiclt'he leverÁ lifts the distant piece B, so that the peckerK misses it. Thus the gas inJg¡fal*lf¿l---oo not open and the usual charge does not eni,er the cylinder. This continues un-ti}-thlspeed is reduced andB occupies its initial position. Explosions are thus missed intermittently but every charge is of normal strength. This method is commanly used, in gas engines,

T-__ 554

INTERNAL COMBUSTION ENG¡NES

555

OF I'C' ENCINES TESTINO AND PEI(FORMANCE

ensine vibrations for foequencies to give the flywheel the proper stiffness to absorb fluid flows to other locations, flywheel the J"J"."tion.occurs wit"" condition. at that it more absorbent to the making and flywheel of tne stiffness changing the overalt new vibration frequencY' connecting the engine-to the auto' Some automobiles have lrydrou lic engine mounts danpen engíne uibrations and' and. b absorb aJts mttunts ¡n thes'e bod.y. Fluicl mob¿te '¡"áí"ir1n"Á compartment' Engine mounts using' electrorheo'

fr"* Á;";;;;;;;"r better víbration dompening at ;"ii"l i"¿¿ á* uni, a"uaol^ent which wi-ll altow by as much as a all frequencies. T}te vis""sity of these fluids can be changed voltage. Engine noise (vibration) factor of 50 :

1

with the application of an extern-al

engine manageo"""tnri'ty¿iíi, which feed this information into the is applied voltage proper and is-analysed frequency lUe ment system ervrsl. ri"t" of the order is time Response frequéncy' thut to the engine noun; tol;;tdtmp"tt

i, .""J¡y

of 0.005 second.

'an!inois,e-:1.:?:::L-":t-,""*t"" .- Active noise abatemen is accomplish edbv generating analyztng the lrequency This is done by sensl ng the noise-with a receiver' but out^of.phase,with the origi"*ftu"tf""ise. the noise, ona tnro g"ii.lo]iiii naís"e of equal freq-uency' of bú 180' out ofpñase' the wave fronts nol noise.If the noises "";;td" tutu i""'q"""tyThis method works well with constant is elimináted' noise tt¡e and concel eoch other equipment (receiver' f¡eelectronic aaditiottal fi *lttir"s speed automobil" Fig. 17.11. Hit ard miss gweming.

(li) Quality governing. ln this method ofgoverning, the mixture strength

is

altered.In gas

¡

it is effected by reducing the amount of gas supplied to the engine. This is accomplished by varying the lift of the gas valve. In oil engines, quality goveming is carded out by varying the engine

quality of fuel oil entering the cylinder per cycle, it is done by changing the angular position of the helical groove ofthe pump plunger. In this type of gouerning, the ignitian is not alwa.ys satisfa4tory and thermal efficiency is

EMS computers' ".,gi""t' quency analyzer' transmitter) than that used with normal mounted under seats in the transmitters ancl receivers have automobiles Some engine noise abatement system' Similar syspassenger ao-putt-"ttt át u" ""tit" pipe, a major source ofengine-related noise' tail ofthe tems are us"d near the enil are now equipped with a Noise reduction has been so successful that some automobiles that the safety switch is quiet so is engine the idle speed, Ál ,tu,io' safety switch on trr" engine when it is already running' required to keep drivers from trying to start the WORJ(EDEXAMPLES

reduced.

(iii) Quantity governing. Hete mi.rture strength remaíning tlrc

sam.e, tlte quantíty of míxture enteríng the cylinder is altered. When the speed is too high a lesser amount of charge is admitted into the cylinder. The compression ratio and air standard efficiency remain unchanged. The pressure after compression and during working stroke is lower, the less work thus obtained during the cycle reduces the speed. This method is preferred, for large engines.

17.7. NOISE

Exampleil1.!.Atwo.strokecycleinternalcombustíonenginehasameanffictivepressure piston and' stroke are 170 mm bar. The speed of the engine i tOOl ''p'^' If the d'inmeter of power dcueloped' ittd'icated the and 140 mtn respectíuely, fi,nd' Solution. Mean effective pressure (indicated)' Pn¿ = 6 bar

of 6

Engine speed, Diameter of the

ABATEMENT

A lot of research and developement is in progress towards reducing engine and erhause noise. This can be accomplished by the following three ways: 1. Passive 2. Semi-active 3. Active o Noise re{uction is accomplished passively by correct design dnd the use of proper

components.

t In semi-active

noíse abatement systems,

hydraulics are often

Indicaüed Power develo¡red,

and

P:

,.".= s'1#ll9 **

Indicated power,

Here, ¿ = No. of cYlinders = 1' ft = 1 ,.,.. for 2-stroke cycle engine. 1

used.

Some engines are equipped with flywheels which have hydraulic passages through which fluid flows. At idle and other constant-speed operation, the system is designed

1t/ = 1000 r.p.m. D = 110 mrr = 0'11 m

L=140mm=0.14m

Stroke length,

materials.' The use of ríbs and stiffners, cotnposite núterials, and sand,wích construction is now routítu. This type ofcoostruction reduces noise uibrations in the uarious engine

piston,

... r.p.=

x 6 x 0-14 x

4

x(0trf

#-=13.3kw.

x1000x

1

x 10

(Ans')

ExqDple

NTERNAL coMBusrtoN ENctNEs

A 4-nli.¿.¡ r^,.-

TESTING AND PERFORMANCE OF I.C. ENGINES

-. petrol er4 mean effectiue pressure deuelops 14.7 htil at is u.!':P":P*:strohe í bar. calculate rh;i;";;;;fi'e 1oo0 r.p.m. The Ls r.o ttmes the strohe of the enginc, bore. if the length of stroke Solution. Number of cylinder, n 4 r7.2.

Lengthofstroke, For four stmke

k=

+ x s.s

I.p, =

x t.so

14.7 =

+

60 r

!L. Wk rto kw

x(I x n2)x

t7.5.

f

A singte-cylind.er, tot'r'strohe fo_ur-stron" ]".^XTjr: Xt1r" tr"k;;;;;;;T'7:I"rtuer' *heet

cvs!¿ o;;f oiil ¿¡s¿ae is fi2:d with a rope brake. The fitted "yrt" "n" roDe d,iantot.-,--]!'_l: the brake tne engine runs at?u.y: 4s0 r.p.m., ,h;;;iu "" be the brahe

ís 600 mm and, the

!1yit71 í

is 200 N and !I,!:,!':tu tk" power oower of the engine enginz ??

ii; r";;;;;"í::iff;,Ji ' -'4 ev tv' tr "o,i";;;;;;;;":#;;i:?;;;::::i7y: :"fl?!,:::j::,i;ü';:;:::::,K;;il,^,;;;:::;i:::::::4:i";;#;;i::i Solution. Diameter of the

nop"

b

aia-ut"",;;# Tl;;T",,D¿

= 6oo mm

=0.6m

Dead load on the brake, W= 200 ñ spring balance readin¡¡, S = g0 N Engine speed, l/ = 450 r.p.m.

Brake power, B.p.

_

torq^ue

.Example 17,4.4 four -. ,.

of rco

, -

im;;;;;;:í "t':der, ",ii, ^á,I#::;:: j

brahe

(GATE-1992)

T,

=

1Éi0

Nm,

N = 3000 r.p.m.,pa = 960 kPa = 960 x 10s N/m2 = 9.6 bar; D = |

x 0.13822 x 0.1387

d.e.

=50kw = 150 kw

(iu) Network d,one by turbíne =40kW If.the brahe mean effect_iue pressure is 0.6 MPa, d.etermine the bore and. strohe ol. the engile tahing the ratio of bore to strohe ai 1 and engine ,p"rá o" fi00 r.p,m. (GATE_lggg) 6 bar

;

o7

=

t

t

*

=1ooo r.p.m

L: =

-

(50 + 1S0 + 40) = b80 kW

nx p* IANhxtO 6

580 =

l:;::;;:,,;:,X;;i:::lXif:;r:.*:";yii,i;::

i

(A¡rs.)

(ü Work done during compression and, expansíon g2O kW =

6x6xDxID2

rodo-...---

tu¡S:irr" being 4-stroke cycle),

= 0 1387 m or 138'7 urm'

m3. (Ans.)

(ii) Worh done during intake and, ethaust (iii) Rubbing fríctínn ín the engine

B.p

B0)n(0.6 + 0.026)

x10

A turbocharged. sít-cylind.er díesel engine has the fottowing performance

Net wo¡k available = 820

x 450 60 = 2'5 kW. (Ans.) " four-stroke, spark ignition engine d.euetsps s mat¿mum

errectiue pressu,,

Solution. Giuen : n 4 = ;h_

-

tuits

D,

=," -iJ:,130.r," o* (200

;

"

= 0.002095 17.5,

)

;U3

= 7Dz x L =

Solution. Gíuen : p^u= 0.6 Mp, -

:

Brake power is given by, B.p.

t -^^^-

(,#ffi] n-a

Displacement

Example

j

6

L=D=138.7mm.

*,,

xB00ox

50.265 = 18849.6 ¡'3

j

noox x ro '-........-.....".-

r.si ¡ xl6oo- xld. = 0.0006806 = or 8Z.O mm. (A¡s..r -0._08?9 I'3 = 1.6 x 87.9 tgl.g mm. (Ans.) =

and

=

D=

s.s x

,


Moving blades

Fixed

A¡r

2.

delivery

(id)

Aluminium

(tu) Steel

Botor:

o For rotor sñofüs and disc ..........'súe¿1." o Aircraft engines may use titanium d.t the front stages and' "níckel

S = Stalor (Fixed) blades R = Rolor (Moving) blades

alloy" in the rest-

3. Stator blod'ings :

o

Same materials as that of rotor but st¿¿l is the most cornnlon, 4. Co*tings. These may be of c¿sú magnes¿um, eluminíum, steel or iron or fobricated frorn titanium or steel.

blades Fig. 20.61. Axial flow comprsor. The annular a¡ea is usually reduced from inlet to outlet ofthe compressor. This is to heep the flow uelocity c.onsto,nt throughout the compressor t"ngtn.'tÁ-lne diverging passages of the moving blades, there is rise in temperature due io diffusion. The absolute velocity is also increased due to work input. The "fixed. blades,' ser¡e the following two purposes : (t Convert a part of_the K.E, of the fluid into pressure energ!. This conuersion -- is achieued

r

,

715

COMPRESSORS

NC (Numerically controlled) machines make dies aDd the blades are m¿rnufactured by precision forging. Blades a¡e also machined by CNC copying machines. 20.4.3.3.2. Velocity Diagrams and \fork Done of a stage of Axial Flow compre6sors. Fig. 20.62 shows the velocity triangles for one stage of an axial flow compressor. All angles are measured from the axial direction and the blade velocity C" is taken to be samz at blade entry and exist. This is because thb air enters and leaves the blades at almost equal ra'd'ii.

-

by diffusion process carried, out in thi d¿rárs" tioiríassoges. (ii) Guide and' red'irect the fluíd flow so that entry to the neú stage is without shoch.

Working

tli

Basically, the compression is performed in a similar manner to that ofthe centrifugal type. The work input to the rotorshaft_is iransfer¡ed by the iovingblailes to the air, thus accelerating it. The blades are so a¡ranged_that the spares Letueen the blad,es ¡or^ aiffisrl passages, and. hence,th.e uela.íty of the aii rerotiue to thi brades ii-d"rr"or"¿ as the air posses through them, and' there is a rise in pressure. The air is then further díffused in the stotor ilades, which are also arranged to form diffuser passages' In the fixed stator úlades the cir is turned tir.ough an angle so tltat its directiort is such that ítian be.allowed, to pass to a second. row of movíng rotor blades. It is usual to haue a relatiuely large number of stageli and. to maintain a cónstunt iork input per stage (e.9.,

:.1

I

from 5 to 14 stages have been useá).

-

The necessary réduction in vorume may be allowed by flaring the stator or by flaring the rotor. It is more common to use a flared rotor, andihis typle is dhown rliagrammati_

cally in Fig. 20.61. It is usually arranged to have an equ ar temperature rise in the mouing and, the btades, and.'to heep the axiat uerocity ;;";;;"; ilÁíin""r'iir"i,o'^ir""ror. fired, rnu" "f similar with r"goíd to of.,th.e compression is exactiy otr r"ii'ity and blad' ?::!,"r"c,9 tnlet and outlet angles, A diffusing flow is ress stabre than a converging flow, and for this reason the bratrr shape and profile is much more impbrtant for L compressor than for turbine. ,]:_!?:r1: of compressor blades is based on aerodjrnamic theorya reaction and an aerofoil s¿o¿€ ts used_

k- C*.-rl+-

uli

c* ------¡l

,,.Ar

t;.,

lii'i

"t

-

Fig. 20.62. Velocity diagrans for axial flow compressor.

o Air approaches

the rotor blade with absolute velocity C, and at an angle or. The relative velocity C"r, obtained by the vectorial addition of absolute velocity Ct and blade velocity Co,, has the inclination pt with the axial direction.

t

I

ii .

¡

INTERNAL COMBUSTION ENGINES

Due üo diffusion in the diverging passages formed b mtor blades, sure rise. This is at ttr" ofrelative velocity and so the there is some p¡escreases from c,, * q".'^9*oense ""p?.rr"u'o?";iT,:::::E relative velocity de-

J"'T::iü: J:1["rr"ffii"::";il;;

wour¿ urti¡natlü ñ.i;, The air then enters tn"

rh;;;ñ;,

ot:9.:

rno the diverging passase ""t1.to: r3r: ¡1"¿".. ni"rrü'iill

;;#',;;ü::"J¡,"

uelocity

c^,

4=tancr+tanB,

'I rl

I

Assume ,l kg of flow

=

F2

"^

+

From 'Iangential force per kg C,z_ = Work absorbed by the stage p.,

I

f,:::".",,

and

c,rare

I

c*,"f

L-:r-i

is missing in axial flow compresso¡s. Due to tbis reason the pressure

raüo per stege in axíal flow cotrLpressor is much less than that of centrifugal compressor, The stage temperature dse;dgardless of efficiency of compression, will be given by . the equation

(^T),"t=

-9

-

\rr"^g,

Pt *Ct2

t! of"i., = co(r o2 -

r or)*,

^=-!:'9,: ^) "".o"i*iu,"i;;*ft:,;,?líf,-l.,,lTi,

and

exit;":?:;iJl

*^-,

tnut

y:fflr::fff:t ,vu¡¡r uu¡¡rponenr ar the jifl*:'J,Hi becausee or of rne thó tact fact that ""#,lii\i:!-iie,entrance til;i;;--"." -

From eqns. (20.85) and (20.8'6), we have

.(20.88)

p2p2 -

Pz

ran F,)

(20.91)

( c,' c"'\ -Pl-.'-;-l

\¿)

= (Pz)¡,,n Since

*Cz2

the static isentropic pressure rise may'be expressed in te¡ms.of the inlet dynamic (Ap),*n = (p2)¡,n

orthe ur úne compressor compress or iis notzero ai¡ flows to-ws atially axially aná and not not rad.íally.) TL^ ^_--- for work donr The expression "";;"i;;"^"r¡u' be put in terms of flodaxial velocity and air angles. (t^^ d w^. = Cu c,, C, a- (tan ¡ og +-tan ar) -',mar

!"

curt -

Pressure rise in isentropic flow through a cascade : ....Consider the incompressible isentropic and steady flow through a cascade from uniform condition 1 to uniform condition 2. From Bérnoulli's equation, we have

stage

C,t Y:

the whirr

77'7

The second term represents the increment of K.E, in rotating cascade that has to be converted into pressure euergy in stationary cascade. compaiing this equation to the work input to centrifugal compressor, ." fird that the term'cent¡ifusal action

...(20.85.)

tan a"

o/a# through t,,e compressor --"'Ftwoou Newton's s.econd law of molior,,

-

"i.

From the velocity triangles, we have

?

COMPRESSORS

and decereration takes prace rn

rt. reaves_tbe staror brades with "l 'itr'jly¡"i!T,Hf."t?::,:l;:;:"::":,:,_:::':;""ros,, ceo";il, i,t.-Ii",."¿ __-._é ,,,o wu.pr.essor stage equals the r Ct approach

and'

AIR

- tl(rr'

+ c,r2)

-

(c¡22 + c,22)l

Cn= C¡2= C¡ (4p)¡,.o=

2 ...(20,',2) t ".r, - C,r\ = cf &anzcr- tan2 cr) C,r= t'.' Crtan a, and C,"= Cltan o"zl

20.4.3.3.3. Degree of Reaction

since Fu¡ther

Wn

C¿^ = Cw, C¿¡ , =

By use orverocir,

=

C6¡"

Crz

C,z- c,,t =.r1, _

-

Cbt,

j

Degree of reaction (.R ) is defined as the ratío of pressure rise in the compressor stage. _ hessure rise in the rotor blades

-o=@

Crt _

tiu-, = ",*> is modified the above equation ",*))as,

,.i"n*rli|] Í;Ííl;r;Í*], w"'=

c,*, _ c,*,

= cu, ac,, = c,

9'it'i* c'":c"

Pressure rise in the compressor stage gu,

ili:L',i#;i:fi3ffi:**'"'which

;;";J;;;";"

wórk input per stage and is

= Pressure rise in the rotor blades is at the expense ofK.E. and is

(ar)*,

c,,t -cr"t 2

...(2o.eo)

teh) c, = ,)'""tt"'t' "": The _ term first on the rieit siáe of.the. above equation introduces the part of the work is pressure aue to dirrusion action (second

eq,,,"'"ls

(C,, -Crr)

:'

R,

"

C"' -C"' = 2Cil (C*, c.,) -

Reler inlet and outlet velocity triangles

:

C*, = Cu, - Crtan P, Cr, = Qur- Crtan Bt

...(20.93)

AIR COMPRESSORS INTERNAL COMBUSTION ENCINES

Cr"

-

Cr, = C¡(tan

Similarly fiom velocity triangles,

-

tan pr)

c,,2 = (C)' + (c, tan

pr)2

(Crbn

Fz)2

C""2 = (C)" +

crr'

9¡

- c"r' = cfz

(tan2

-

.(20.94)

tan2 gr¡

o*ffi=;;i ##+1ft*=!

So

F1

cr

Let us conside¡ the compression process ofa multistage compressor on ?-s plot ofFig. 20.63.

IrT-

...(20,95)

(tan p, + tan Br)

AT^

Degree of reaction is usually kept as 0.5,

IC¡ ub= -'C"

Kr,/o^,., (stage)

'.'---tm/

I

lmacnine) |

(*0r+tanPr)

C^,

6 =r*

F1+tanP,

C^t

But

C, = tan

..

9w

C,

q1 + tan Fr = tan o9 + tan B,

(from velocity triangles)

= tan pr + tan 9z = tan crl + tan Fl =

ta., g'r+ tan p,

From this

so with uo*

'"""ÍJiff;fr,!;",**rressors haue svmmetrícar brades and. wíth thís ";'" ii)"ír_"á. and. fluid. friction losses are mini¡nutn. !: "l"oron"" **t""rt':;ü;lTJ;*" work input to a cornpressor, with usual

type of set up losses in flow path In symtnetrical bl"e., ,¡O

20'4'3'8'4'

Fig. 20.63. Concept ofpolytmpic efficiency. pop

notarions,

(T' ¡ \ =c^(Tno,-r^,rr(Tor-4\ P w¿ (4"-4T)=""1;4)xQ*'-r')

A/n \isenVntc) =

"''

=

tl "01,""or,,(X--. 4,fl&n)".l(pr) \.or J= u- n*,

rl I

Stagnation isentropic efliciency fo¡ a

¿eriveredto it from tire preceding stage.

""",""[rlillTflling Polytropic ,'rl;"":tt'

stages, rhe concept

rise

A?o can be represented as

M,_

or

^o"hio"

\isen (ml

or

'lie¡ (n/c)

tT^ = " l¡*(o¡ ' Equating expressions (i) and (ii), we get )

c\

\¡en(m/c\

_

I

:

(AT"')*"ti* E(dro')w

(LTo',

ofpolytropic

efficiency ,:^r!:.:::r:or:,efficienqr of one stdge of d multístage compre..sor. stage efficiencv is constant f"; "t;';;;;;;" q a cotnpressor with inlinite number of

s¿a€,e, (sú)

dq

The total actual ternperature

;H,'j""fffi:i;"J*:".*Tf':'"";rili;il.;,,1iñi'*","u"" orthe overar peJrormance orthe efficienq,;ii;";;";;;:":"#f;.y;::#:#n::;#r,i:!:;::i:;;"s,i"et"tio. ouerau diff"""nt

-

n. ..=dro' '14?4(3,

...(2o.s7)

Eqn' (20'97) indicates that for the same isentropic efficiency rl¡"n and pressure ratio po2 , the work input is proportíonal. to ihe initiat temperature. Thus in a compressor coosistirrf,olr several stages bf equal isentropi" u".-ñ"í""Ja"g stage wilr have io perform more work because it has to deal with a fl-"ia "m"i"oqr, ofitt"."rrL¿i"-;"."t"r"

the perform"n.u of.orrrp."J"o"rn"irh

The gas is being compressed from pressurepor hpo, in four stages ofequal pressure ratio. po and poo are the intermediate pressures. Now by definition : Overall isentropic e{Iiciency (stagnation) fot the m.achine (mlc),

z (dTo' 'lise¿

"'(t) ...(tr) I t

)

"bsc (rÉ)

-(ATo')mrchire

Iise¿(sr) L(dTo')"b8"

..(20.98) I

INTERNAL COMBUSTfON ENGINES

From Fig. 20.63, we have : (LTo')_*hí* 1i

on ?.s prot, ,h"

Thus, by the definition ofpolytropic or small stage efficienc¡ we get

= (l_/.) + (t,_m,) +(m,,-n,) +(n,_2,)

".:(fl",;ñ.,.1::tirJ.::_y);::r::":f l_m, > l,-m"

;r,-'

m,,_¡r, ^ say that E(dTo)ou" > (LTo,)*"u.n" ;:.:"":1 rro:t eqn 20'98, r¿,.n("r) ) r¡on (-r") l:tussmall stage fr", effiaency \¡",1,¡¡ uhich is consto _,^__ and is nt for all stages is called polytropic ciency desígnited"by \r. effi"Pllvjropic efficienóy' in terms .--_ of entrv s-nd deli.very pressure tures and ttr. r"tio .i and temperaheats .

"iecifrc Refer Fig. 20.63. The actual lu¡¡¡P¡cssron path l-2 is-ineuersíble, bu, the end 2 are íno equílilrium;";';i"*"": equítibrium and. tie on states onrr::1:'::liT":?in^1,?t:,:'*":rs.íbte,-but the same polytropic path ,n"r""tnrX"J'tl*, p"uor = constant,

Let the corresponding reveÁiilu

. b ¡rreversible ¡or

path, we may write

1-2,

r

get d,po=

Substituting the value of

we

get dp"=

From eqns. 20.99 and 20.100, we

a""= =

r'" . = Lrf .dqn. podTo

with eud states r and 2, cha¡acterised

n^

o¡ '|)r

-#"

=

Actual stage temperaturo, dT^ =

sjmir¡r t¡earment

the stage isentropic

ro

le

*",r1","

dp^

-

n,=

RT"

"u![ut*¡tu]=

...(20.100)

"

.r.

*[L,ufir"]

rór./

t?'*"

of specific heat

.,(2o:o4)

lnll!¿l [4rl of

pressure

"r_ Cot

.

stage is defined as.

Co,

= C,,t + C*¡ = Cn (tan B, + tan cr)

c", ,' c¡1(tan Bl + ran crl) C¡1= C¡2= C¡

'

Also,

pooo"r=

rol)

7, woukl give

(2r, 102)

pol

y.

0-=

(20

ratio @ , ,emperaturc

20'4'3'3'5' Flow coefficient, Head or Work coefficient, Deflection coefficient and Pressure Coefficient l' Flow coefficient (Q/ The flow coefficient ofaxial frow compressor Since

dp^ll.-1) e

/^, ,\ ,-'15 \ Y )P'

-#= l"l*¿l

c",

-#

following tltelaw

,^lrr)tl

The eqn (20'704) gives the polytropic efficiency in terms

. T^^ ratb and the ratio ,* r01

ndp.-dp"= dp"(n-t)

dT; = dpal

\

^,"'t_t".t-l \

"l!"k"_¿"!e.]

"",nor!.I "l::r", t"-fo"utu." a4, expressed as

/

TLr.fpor')

p^

J

:ss

(m \

..(20.99)

..

Po

haive

the,o"rr

1 dP, =I -

dTo

3l=r-r¡nfPozl '" \Tot) y -"[ro,i

;l+*";J:llZ1 'o

dpo= n.dpo

:

-t;tpo Integrating between the two end states 1 and 2, we get

or

t="" on differentiarion,

O"

from eqn. (20.101) into eqn. (20.103) we have

tt^

n"lnl

.",

-u ,Il

and

zz

zrnpon-t.d,p.= n

Now, the characteristic gas equation

,

...(2o.roa) Eqn 20.103 gives the value ofpolytropic efficiency in te¡ms of exponent z and the adiabatic exponent y.

P6 = Z1Po"

On differentiation, we

:

¿ ') - _lr-r)1, y ",=l Jl"-r)

or

zl

i.u"t"opi.;:th

= conatant

Pour

v- r'\ r ¿T, dP.lt;I )P" l: rl-=*'o \ ' d?. = -,^("-I\\ '-\ n )po f

)

,n_h,

.

AIR COMPRESSORS

"'

Cr' O,= 'r- c¡lt^"í;;;ü

- rr;¡;;;n

-=

al

...(20.105)

I

. '(20.106) p, + ta" o, 2. Head or work coefficient (Q¡). It is d.efined "rn as the ratio of actual worh d.one to the hinetic energyr correspond,ing to the meon peripheral uelocity. .Ihus.

r, -

2+ "bI l.

cd =?9+eL.

=z

,r [!els,:: ", ] \ tan p2 + Lan (J2.)

.(20.707 )

3. Deflection coefficient (0¿i).

It is

The overall pressu¡e ratio is given by

defined as,

oo",=4&#=C'tC" c¡t crf "'

or Q¡=2Q¿"r

4. Pressure coef;ticient (0,). It ¿s defined as tlu ratio of isentropic worh done eilergy corresponding to the peripheral ueloclfy. Thus, c- LT.--q,=VlE

of

20.4.3.3.6. Pressure Increase

=r¡.o

,, = [t.oL,*"]"

..,(20.113)

...(20.108)

to kinetic

,o=,ln ll(?)

...(20.114)

, [ro)"6""1

where

...(20.109)

0¡

Súagee

The pressure ratio is expressed as

T,-TrT"'-' (sei Tr Pt=f,*. LI

P,

...(20.110)

Ict ?, and ?, denote the temperature ofthe working fluid at inlet and outlet of rotdting blades. Hence the temperature increase is

...

100

+

190

...(20.r11)

I I

6

blod¿s is given by,

áao

c"'-c"'

r3- ^ tr=-1"",

,r5=

c @

¡ . _ n/(1-1) pl=lr+n-.951

...

g'

g

70

U'

Hence pressure

"r"

l,í ro! ,r";;""'4J

60L

pz= pzf{r. n" +}"1-t

The pressure increase in a stage is

&"'= &*+

and

brades is

0.5

_

4p" =ps

LT"r=

Lt

tz)

_

poz

=l

,t

n

'

l 1.

LP"

r. ¡

or"T"' ' n¡oF¿¿TIJ

If the pressure ratio per

(r) '

=#_" _

...(20.112)

1.7

Profile lossee on the ¡urface of the blodee

:

cascade By profile losses, we rrean the total pressure loss of two dimensional rectilinear ;t"i;; f; tnÉ "n" friction on túe surface and due to the mixing of flow particles after the blades. These losses are usually determined experimentally'

3. Secondary flow lossée :

prod,uced' by an axial flow compressor blade channels, certairi secondary flows are combíned' effects of curuature and boundary layer'

r In

Poz-Pos*Po(tv+r)

por poz

1.5

-------|

2" Skin frlction loss on the annulus walls : on the annulus r The wall f¡iction total pressure losses arising from the skin friction layer growth on walls and the secondary losses a¡e dfficult io anolyse as bound'ary phenomena' three'd'ímensíonal a complex ís walls these e Empirical relations (by Howéll, Haller) are available for calculating drag coeflicient'

stage bé the same, then

P'stdsc

1.3

1.1

Fig. 20.64. Losses in @mpressor stege.

If the work done per stage is assumed to be the same, then the number of stages (N) is given

*

0.9

Flow coaffclent

L!*1 ¡7"

Pot L-

o.7

rl

The stagnation pressure ratio is given by

by,

to l.2'

shown in Fig. 20.64.

ZXcp

Pr L -'41 in stationary

l.l2

varies from

C't! =C'22

-

P2=fr*n" 4"n|'tt-tt

The temperature rise

(ro)rros"

20.4.3.3.7. Losses in Axisl Flow Compressor Stage In actual practice, various losses occur while the fluid flows through a compressor $tage. The total pressure loss arises in three ways: 1. Profile losses on the surface ofthe blades' 2. Skin friction on the annulus walls' 3. SecondarY flow losses' coeflicient is The various losses represented on graph between stage efliciency and flow

in a Súage of en Axial Flow Compressor and Number

aTP- T'- T' '

783

COMPRESSORS

¡NTERNAL COMBUSTION ENGINES

poN

oSecond'aryflowispród,uceilwhenastreamwisécomponentsofvelocítyisdeveloped

*

INTERNAL COMBUSTION ENCINES

from the

deflecüon__of

AIR

an initially sheared

COMPRESSORS

flow. such secondary flow occurs when a developed pipe flow:-o!".-. u ¡""a, *hen .*"heared flow pr"r"" oo"" a¡ aerofoil offinite thickness or an aerofoil of finite liit boundary level meets an obstacle normar "ll¡1" aa wind it i" browing pasr a relegraph pole). ¡'t'lctr

the section lMof the curve, the flow is not stabre. A fa]l in mass flow rate wil' be accompanied by a fall in p¡essure ratio. In this situation any small disturbance causiirg a cbeck in rnass flow wilr cause a fall in pressure ratio and t},e flow muy reuerse dv some point. when the temporary disturbance is rernoved, the flow wilr pick up and it is found that small disturbances cause the flow to oscillate rapidly. The oscillations is noisy and can, ifallowed to continue, cause st¡uctural damage in the compressor. It is called 'surge' and the point M on the curve marks the rimit of ,riuf.,l operation of the compressor. If a compressor is runni.g normally at the point where-surge usually commences it is possible to induce surge rnerely by passing ihe hand across the inlet. It is found that compressor efficiency is highest at point adjacent to M and it is therefore advisable to able to operate as close to M u oossibk. 'stalling. "stallíng" ofa stage ofaxiar ftow is defined as the aerod,yna¡níc stall

fl;;j;; ffi

. f1,3:iltjil",j'l

of secondarv-no," *'o'"',,, uiat turbo"","':;::T:3;::;::"3":;i: orthemáchine,ou,aert,i"áL;;:#-;í;ilí:Íi:f:;r"tr:#i?,f*;"ohubwatis machinery

20'4'3'3'8' Surging, choking and stauiíg-compressor characteristics nsurging'is

:g': when

an uwtabte timit oropera. qi[#lT#,X X:;?:i;:i:#;Ti;n*:"' the-coip,i"'li"ii,-rí',r",í,'iiíi1l:""Ji1:'"-::d r11|sat of flow throush tt n

"oÁpi,,l, correspond.ins to *"'tÁi,,i,i!"',Í,,ir:l'."F"1ífi value, this surge can.reach iu"r, mechanical failures mav rl,sutL ".mrg,,ii;J"1"'##a""rr the compresso¡ and rn many cases "'rt"."r"tri; "ü'..". to which the rotor of the machine is

rk^:Jí"*ff":^""#Kxl^("'iiyi*gili:;

r¡"

ily""::,:":Ht5.:Httrf,

:'tril":fil{*if .i,ii{:#tr:;:r,":*:a;Hro,orbrading

Choking. When the Dressuro rnt;^ :- ,,-:.-.-,.-

nr"x:i;ii'"';xifi;"ü:¡í[í:J:*"i;t'í#.-"ii!;;:ilrrfi rr!.'!:::::;:!í ;::;:::":";"T:,""n j::":,:xil{;::::r:;i#"#r#:'É:',:;",:;i";!i::,:, .

Fig. 20.65, shows the compressor In the compressor wh*: tl,u flow is"horo"t"-rirti"".

rncorrect fluid angles retarive to

against

,i"1""".,,"u

gradient the incidence ross

due to ,*o::qJ.üi.".r*",r.r," (r,) falls the desis'n p"i"i. rm.'m., added to the rricrion ross which ,"1"T:'J"i#:::?::::lf"::f w'r ss llow rate, gives a pressure ratio-mass n"* i"t"."i"1.. as Fig. 20.65. shown in

t¡,. ¡1"a". i".uil;l:

^¿l)

¡ on

.

orthe-breakwayoftheflowfromsuctionsideofthebladeaerofoil.Itmaybe "o^pr"""o,

d.uetolesserflowrate

than designed value or dueto non'uniforrnity'in tn" itááe profite. Thus stalling is ohead plrcnornenon of surging,

A multi-stage compressor may operate stable in the unsurged region with one o¡ more ofthe , stages stalled and rest of the stages unstalled. In other word{ snrtng is a locar phenomenon whereas surging is a complete system phenomena. 20.4.3.3.9. Performance of Axial Flow Compressor o Fig' 20'66 (c) shows the relationship between pressure ratio, power and efficiency uerszs flow rate for yarious values of speeds such as Nr, N;,;".-A; i certoin efficiency increases as the flow rote increases and,eáchels' a marimum varue"pnea, after ylicn it d'ecreases. Accordingry as the flow rate rncreases the power consu¡ned, also tncreases. + I

.e i6

+

e

1

I I

I

*

E E l

.9

I

:

E

E

u o

o

3 L

Flow rate

-----.}

Volume flow rate

(s) Mass flow rate, ¡ir _______) Fig. 20.65. The compressor characteristie.

is cttohe! and is passrnsrrre maximum rnass flou rate. *fii"","11]"Í:i;::::íj:"t#?ji mass now."i" *irr*"".,,rt i"

: á:fi:':"*":#;T:1lT:i a rise in pressure

*u"

Fig. 20.66. perfomance cunes of axiul flo*

a

ao-pru."or.

------|

to'

Fig. 20.66 (ó) shows the performance and constant efliciency curves, such a plot does not take into account the varying inlet temperaturc and pressure. In addition to this, these plots cannot show the coriparison of performance for similar compressors of diffe¡ent sizes. To, account for alr tirese, the performan""

or" plotted' with'dimensionless parameters'. These dimensionless parameters are "urii"' : pressure '

2¡¡e -42 ; speed paramete.,

Pt

l/' Jt

"u¿

flow parametcr

toJrL., p1

nu¡.. Fig.2o.67(¿ and

ó

r.

786

INTERNAL COMBUSTION ENCINES

AIR

COMPRESSORS

787 I

Ailaptability to Iow

Adoptability

+

4

6

e

1.0

)

I

E U.ó co ^^ ? u.b 6

1 3

É o

u-o

1.0

/N\

o.¿

12.

Operoting attention

More

I¿ss

13.

Míxing of worhing fluid uith lubricating oil

AJways a chance

No chance

74.

Suitability

For low, medium and high pressures md low and medium gas

sus

6

volmes.

0

0.4

COMPRESSORS

mlot S.No.

Por

(ó)

l.

Fig.20.67

20.5. COMPARISON BETWEEN RECIPROCATING AND CEIVTRIFUGAL COMPRESSOR,S

Reciprocating 6.

parts the machine is poorly bal. mced)

I¡wer

Mechanical effrciznc1

(due to the presence of several sliding or bearing rnembers)

Installed. first-cost

Higher

Asp€cts

Reciprocating air mmpressors

Rotary air compressors

Suitability

Sütable for low discharge ofair at high p¡essure

Suitable for handling large volmes of ai¡ at low pressures. Usuallyhigh

Operotional speed

I¡w

Air supply

hüating

Continuous

Balancing Lubricdting slsten

Cyclic vibrations occur

I¡sqvibrations

QuaLity of air dcliuered

Generally contaminated with oil

Genually

omplieted

Generally simple lubrication systems are required

cmpresaors G¡eat¿r übration problems (due to the presence ofrtriprocating

For low and medium presmdlarge gas volumes.

20.6. COMPARISON BETWEEN RECIPROCATING AND ROTARY AIR

por

(a)

'i

0.8

mat.,

Vibration probkms

Adaptability to high speed, low maintenance cost drivers such as tu¡bines

speed drive

Less vibrational problems since the machine dos not have reciprccating parts.

is relatively more

clem. 7. 8. 9.

I{8her ompuatively (due to the absence of numerous sliüng or bearing membere)

1U.

Air atmpressor size Free airhondhd

Iange for the given discharge

Small for same discharge

250-300ms/min

Delivery pressure Usual standard of compression

Hi&

200G-3000m¡/min Low Isentropic compression

Isothemal compression

.{

1ü

il 'l

are

favourable. Pressure ratio per stage

About 5 to 8

About 3 to 4.5.

Type of flow

Axial (Parallel to the direction ofaxis ofthe machine)

Rádial

Capability to dzliuer pressure

.High presue @y nultistaging, high deüverypressue upto 5000 atm. may be achieved),

Medim pressme (By multistaging, the delivery pres-

Pressure ratio per stage

High, about 4.5 : J. Thus unit is conpdct

Inw, about

su¡e upto 400 atm, may be

In

supérsonic compressors, -the pressue ratio is about 10

achiéved). C apabilíty

of 7.

of dzliue r ing uol u me

airl gal,

F lexibility in sure ranSe

npacity

and

p re

s

-

uit

Small (By using multicyünders, the volume nta¡r be increased,).

G¡eater (per space).

Greate¡

No flexibility in capac'ty and

of

but at the cost ofefficiency. Op. eration is noú so difficult and

building

risky.

ó.

Maintenonce etpenses

Higher

Lower

Continuiti of

Lesser

G¡eater

10

Conpression efficienqr

Higher, at compression ratio

Ffigher, at compression ratio less than 2.

seruice

above 2.

ri

1.2 : l This is due to absence ofcentrifugal action. To achieve the pressure ratio equal to that per stage in centrifugal compressor 10

llilil

to 20 stages are required.

lll

Thus the unit is

le ss

id lg tfl

conpoct

and,Iess rugged.

pressu¡e mnge.

9.

rl i$

20.7. COMPARISON BETWEEN CENTRIFUGAL AND AXIAL FLOW COMPRESSORS

I¡wer (where pr*sue md

volume conditions

Ai¡ delivered

il

Isothermol efficiercy

About 80 to 827o

About 86 to 887o (rvith modern aerofoil blades)

it

Frontal area

Inrger

Smalle¡ (This makes the

jt

axial flow compressors more suítable for jet engínes due

ti

il I ,I I

I I

t

I

INTERNAL COMBUST¡ON ENCTNES

:

--;: (dye to adjustable to.r:

1

whirl o.

Part load.performance

7.

Effect of deposit foimation on

8.

Star ting S

uitab

to rq

ilitl

fo

w

üffuservmes)

Better Pe rfo

the surface of impeller rotor

o

and

pre_

Poo¡

r ma n ce not ad.uerseb

Petformne aduerselt

affectcd

req uire d,

affected

Low.

r m ultis tdging

Hish

Slightlydilñotr

.

More suitable

for

multistaging Delive ry pres s u re possible

Applications

Upto

bar

40O

upto 20 bar

Us.ed in blowing cngines in

stel

mUs,.low prssure refrigeration-

org central air conditioninc

ptants, fe¡tiliser and

indutryl

superchargilg I.C. engines,

ilI?,i:

rli rij I

t"

Previously it was ued -engrnes

Effubncy

vs. speed, curve

g;

Iong disrance pipe in

ArR

COMPRESSORa

^n (li) The work done per kg of air, W : W = Co, Cr(tan c, _ tan og) 240 x 190 = -;Oa-ttun 45" - tan 14") = 34.29 krtr. (Ans.) Example 20'43. An axiar flow cotnpressor hauing eight stages and, wíth s,vo reacti.on design cornpresses air in the pressure ratio of 4 : 1. The air enters the compressor at 20"c and, flows through it with a constatut speed, of gT'mrs. The rotating blad,es of cotnpressor rotate with a mean speed of 180 ml s. Isentropic efficiency of thz, compresslr *oy b" ioninZ," áb%. cot."ulote : (i) Worh done by the machine (ii) BIaO", onrr"r. Assume I = 1.4 and c, 1.005 tal / hg K. = .

Mostly used in jet ensines (due to higher efficienci and smalle¡ frontal a¡ea). Also preferred in power plant eas tu¡bines and steel mills.

Solution, Also

jet

More flat (Fig. 24.68 )

lVork required./kg

= co(Tr-

Now, work done/kg

= Numbe¡ of stages

Cmlritugal

Ft.or 50Vo

tan

í."., rhe

(i) The pressure rise

.

(ii¡ 7¡" worh

190 m/s

d,one

; al = 45" i

az

per kg

gcfftan2 c,

of air.

= 74",p = I kelma

- tan2 q)

(tgot"' = 1" t(tun 4S.)2 _ (tan 2^

,-.--*J

l's-

...lEqn. (20.s2)l 14")2J

= 0.169 bar.

{fiffi

= 174.47

kJ/ke.

(Ans.)

-

tan crr) [Refer Fig. 20.621

= r.rau

tancr+tanp,= I = # =, -Lf9U tan p, - tan al = 1.g46 tanpr+tancr=2

1

The pressure rise through a ring of rotating blades, Ae =

o, =

2gB)

...(r) ...Qi)

From (j) and (ji), we get

fouowing axiar ftow compressor: Cut=240tn/s, Cr= ""r"::":::: "n lS0 mls, % =45"; o= r0., p= i hglm' Calculate.

Solution. Giuen : Co, 240 m/s = ; C, = (i) The pressure rise, Ap :

c,-tan

-

* Ctt(C., - C*r)

reaction blading, a2 = g, and c,, = B, 1.346 = tan 91 - tan c,

Now, 2o.44.

= 1.005(466.6

L74.47 = 8 x Cu, Cr(tan %

"'

Exampre

Ir)

(Ans.)

2 tan

pl = 3.346 = 59'1' = crz. (Ans.) d. - l8.t'= R la-. \ Pr

and Exampre

2o.46. An an ouera, isentropic efficiency of BSVo ".Í;;rlllr;ru,Z;,,lfi)o compresses it in the presrur, ,otio of 4 : r. The mean blade speed. and. !:i::,:t::, ?!,Lan.d, f.tow ue¿oclty are constant throughout the. compressor. Assu*ing

blade uelocity as 180 nrls (i) FIou uelocity

iná

(ii) Number of stages Taht: a, = I2', Ft = 42".

worh

50Eo reaction Itl"á¡rig and

nprt'¡g"to, a, i,"i|, calculate

:

t.hing

INTERNAL COMBUSTION ENGINES

Solution. Given i

1¡".n

ÍPI

Pressure ratio,

= 85%, T1=

2O

lf

AIR COMPRESSORS

+ 273 = 293 K

Po Po(Fl)

=¿,C.=180m/s

Po(rH)

Work input factor

= O.82

t-l

L4-l

t2 ;ll =[ul' \&/

(4)-n-

= 1.486

K- C"r-d

'2 = 293 x 1.486 = 435.4K

Co¡

---il

Tz'-Tt

Now

4i*o = Tz 0.85 =

-Tt

-{i-rr3

293

435.4

?z = 460.5 K Theoretical work required per kg = co(T2Frorn velocity As (Fig. 20.62)

fl)

= 1.005(460 .5 _ 2gB) = 168.98 kJ

Cú------¡

K-

85 kWgndrtrh

0.36

PM < 85 kWg/kW}

0.61

PM

20N

1996

-

6.903

Reduceq HC and CO Reduces N0 Reduces aldehydes Use same design for all vehicles

Inng life

or HC +

Ne

norms

0.97

8.0t s&Wh - l.?03 (glt