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The Basic Design of Two Stroke Engines
Table of Contents
CHAPTER 1
INTRODUCTION TO THE TWO-STROKE ENGINE 1
1.0 Introduction to the two-stroke cycle engine 1
1.1 The fundamental method of operation of a simple two-stroke engine 6
1.2 Methods of scavenging the cylinder 9
1.2.1 Loop scavenging 9
1.2.2 Cross scavenging 10
1.2.3 Uniflow scavenging 12
1.2.4 Scavenging without employing the crankcase as an air pump 13
1.3 Valving and porting control of the exhaust, scavenge
and inlet processes 17
1.3.1 Poppet valves 18
1.3.2 Disc valves 18
1.3.3 Reed valves 19
1.3.4 Port timing events 20
1.4 Engine and porting geometry 21
1.4.0.1 Units used throughout the book 22
1.4.0.2 Computer programs presented throughout the book 22
1.4.1 Swept volume 23
1.4.2 Compression ratio 23
1.4.3 Piston position with respect to crankshaft angle 24
1.4.4 Computer program, Prog. 1.1, PISTON POSITION 25
1.4.5 Computer program, Prog. 1.2, LOOP ENGINE DRAW 25
1.4.6 Computer program, Prog.1.3, QUB CROSS ENGINE DRAW 27
1.5 Definitions of thermodynamic terms in connection with two-stroke
engines 28
1.5.1 Scavenge ratio and delivery ratio 28
1.5.2 Scavenging efficiency and purity 29
1.5.3 Trapping efficiency 30
1.5.4 Charging efficiency 30
1.5.5 Air-to-fuel ratio 31
1.5.6 Cylinder trapping conditions 32
1.5.7 Heat released during the burning process 32
1.5.8 The thermodynamic cycle for the two-stroke engine 32
1.5.9 The concept of mean effective pressure 34
1.5.10 Power and torque and fuel consumption 36
1.6 Laboratory Testing of two-stroke engines 37
1.6.1 Laboratory testing for performance characteristics 37
1.6.2 Laboratory testing for exhaust emissions from
two-stroke engines 40
CHAPTER 3
SCAVENGING THE TWO-STROKE ENGINE 115
3.0 Introduction 115
3.1 Fundamental theory 115
3.1.1 Perfect displacement scavenging 117
3.1.2 Perfect mixing scavenging 117
3.1.3 Combinations of perfect mixing and perfect
displacement scavenging 118
3.1.4 Inclusion of short-circuiting of scavenge air flow in
theoretical models 118
3.1.5 The application of simple theoretical scavenging models 119
3.2 Experimentation in scavenging flow 121
3.2.1 The Jante experimental method of scavenge flow assessment 122
3.2.2 Principles for successful experimental simulation of
scavenging flow 126
3.2.3 Absolute test methods for the determination of
scavenging efficiency 127
3.2.4 Comparison of loop, cross and uniflow scavenging 130
3.3 Comparison of experiment and theory of scavenging flow 135
3.3.1 Analysis of experiments on the QUB single-cycle gas
scavenging rig 135
3.3.2 A simple theoretical scavenging model to correlate
with experiments 138
3.4 Computational Fluid Dynamics (CFD) 143
3.5 Scavenge port design 149
3.5.1 Uniflow scavenging 150
3.5.2 Conventional cross scavenging 151
3.5.3 QUB type cross scavenging 155
3.5.4 Loop scavenging 157
3.5.4.1 The main transfer port 159
3.5.4.2 Rear ports and radial side ports 160
3.5.4.3 Side ports 160
3.5.4.4 The use of Prog.3.4, LOOP SCAVENGE DESIGN 160
NOTATION for CHAPTER 3 162
REFERENCES for CHAPTER 3 164
CHAPTER 4
COMBUSTION IN TWO-STROKE ENGINES 167
4.0 Introduction 167
4.1 The spark ignition process 168
4.1.1 Initiation of ignition 168
4.1.2 Air-fuel mixture limits for flammability 169
4.1.3 Effect of scavenging efficiency on flammability 171
4.1.4 Detonation or abnormal combustion 171
4.1.5 Homogeneous and stratified combustion 173
1.7 Potential power output of two-stroke engines 44
1.7.1 Influence of piston speed on the engine rate of rotation 45
1.7.2 Influence of engine type on power output 46
NOTATION for CHAPTER 1 47
REFERENCES for CHAPTER 1 48
CHAPTER 2
GAS FLOW THROUGH TWO-STROKE ENGINES 51
2.0 Introduction 51
2.1 Motion of pressure waves in a pipe 54
2.1.1 Nomenclature for pressure waves 54
2.1.2 Acoustic pressure waves and their propagation velocity 56
2.1.3 Finite amplitude waves 57
2.1.4 Propagation and particle velocities of finite amplitude
waves in air 59
2.1.4.1 The compression wave 59
2.1.4.2 The expansion wave 61
2.1.5 Distortion of the wave profile 62
2.2 Motion of oppositely moving pressure waves in a pipe 62
2.2.1 Superposition of oppositely moving waves 63
2.2.2 Reflection of pressure waves 66
2.3 Reflections of pressure waves in pipes 68
2.3.1 Reflection of a pressure wave at a closed end in a pipe 68
2.3.2 Reflection of a pressure wave at an open end in a pipe 69
2.3.2.1 Compression waves 69
2.3.2.2 Expansion waves 70
2.3.3 Reflection of pressure waves in pipes at a cylinder boundary 71
2.3.3.1 Outflow 74
2.3.3.2 Inflow 75
2.3.4 Reflection of pressure waves in a pipe at a sudden area change 76
2.3.5 Reflections of pressure waves at branches in a pipe 79
2.4 Computational methods for unsteady gas flow 81
2.4.1 Riemann variable calculation of flow in a pipe 81
2.4.1.2 d and 6 characteristics 81
2.4.1.3 The mesh layout in a pipe 84
2.4.1.4 Reflection of characteristics at the pipe ends 84
2.4.1.5 Interpolation of d and 6 values at each mesh point 85
2.4.2 The computation of cylinder state conditions at each time step 87
2.5 Illustration of unsteady gas flow into and out of a cylinder 91
2.5.1 Simulation of exhaust outflow with Prog.2.1, EXHAUST 94
2.5.2 Simulation of crankcase inflow with Prog.2.2, INDUCTION 103
2.6 Unsteady gas flow and the two-stroke engine 111
NOTATION for CHAPTER 2 111
REFERENCES for CHAPTER 2 112
5.3.1.2 The effect of the exhaust dynamics on charge
trapping 240
5.3.1.3 The effect of engine speed on expansion chamber
behavior 241
5.3.1.4 The accuracy of computer models of
high-performance engines 242
5.4 Single cylinder high specific output two-stroke engines 243
5.5 Computer modelling of multi-cylinder engines 247
NOTATION for CHAPTER 5 251
REFERENCES for CHAPTER 5 252
CHAPTER 6
EMPIRICAL ASSISTANCE FOR THE DESIGNER 253
6.0 Introduction 253
6.1 Design of engine porting to meet a given performance characteristic 254
6.1.1 Specific time areas of ports in two-stroke engines 255
6.1.2 The determination of specific time area of engine porting 263
6.2 The use of the empirical approach in the design process 264
6.2.1 The use of specific time area information 265
6.2.2 The acquisition of the basic engine dimensions 265
6.2.3 The width criteria for the porting 266
6.2.4 The port timing criteria for the engine 267
6.2.5 The selection of the exhaust system dimensions 269
6.2.5.1 The exhaust system for an untuned engine, as in
Prog.5.1 271
6.2.5.2 The exhaust system of the high-performance
engine, as in Prog.5.2 272
6.2.6 Concluding remarks on data selection 278
6.3 The design of alternative induction systems for the
two-stroke engine 279
6.3.1 The empirical design of reed valve induction systems 282
6.3.2 The use of specific time area information in reed
valve design 284
6.3.3 The design process programmed into a package, Prog.6.4 288
6.3.4 Concluding remarks on reed valve design 290
6.4 The empirical design of disc valves for two-stroke engines 291
6.4.1 Specific time area analysis of disc valve systems 291
6.4.2 A computer solution for disc valve design, Prog.6.5 294
6.5 Concluding remarks 295
REFERENCES for CHAPTER 6 2974.2 Heat released by spark ignition combustion 174
4.2.1 The combustion chamber 174
4.2.2 Heat release prediction from cylinder pressure diagram 174
4.2.3 Heat release from a two-stroke loop scavenged engine 177
4.2.4 Combustion efficiency 178
4.3 Modelling the combustion process theoretically 181
4.3.1 A heat release model of engine combustion 181
4.3.2 A one-dimensional model of flame propagation 183
4.3.3 Three-dimensional combustion model 185
4.4 Squish behavior in two-stroke engines 186
4.4.1 A simple theoretical analysis of squish velocity 187
4.4.2 Evaluation of squish velocity by computer 192
4.4.3 Design of combustion chambers to include squish effects 193
4.5 Design of combustion chambers with the required clearance volume 196
4.6 Some general views on combustion chambers for particular
applications 197
4.6.1 Stratified charge combustion 198
4.6.2 Homogeneous charge combustion 199
NOTATION for CHAPTER 4 200
REFERENCES for CHAPTER 4 201
CHAPTER 5
COMPUTER MODELLING OF ENGINES 205
5.0 Introduction 205
5.1 Structure of a computer model 206
5.1.1 Physical geometry required for an engine model 206
5.1.2 Geometry relating to unsteady gas flow within the
engine model 210
5.1.3 The open cycle model within the computer programs 212
5.1.4 Theclosedcycle model within the computer programs 216
5.1.5 The simulation of the scavenge process within the
engine model 217
5.1.6 Deducing the overall performance characteristics 221
5.2 Using Prog.5.1, ENGINE MODEL No. 1 221
5.2.1 The analysis of the data for the QUB 400 engine at
full throttle 222
5.2.2 The QUB 400 engine at quarter throttle 227
5.2.3 The chainsaw engine at full throttle 229
5.2.4 Concluding discussion on Prog.5.1, ENGINE MODEL NO.l 232
5.3 Using Prog.5.2, "ENGINE MODEL No.2" 235
5.3.1 Analysis of data for a Husqvama motorcycle engine
using Prog.5.2 237
5.3.1.1 The pressure wave action in the expansion
chamber at 6500 rpm 2378.4 Some fundamentals of silencer design 364
8.4.1 The theoretical work of Coates(8.3) 364
8.4.2 The experimental work of Coates(8.3) 365
8.4.3 Future work for the prediction of silencer behavior 373
8.5 Theory based on acoustics for silencer attenuation characteristics 373
8.5.1 The diffusing type of exhaust silencer 374
8.5.2 The side-resonant type of exhaust silencer 378
8.5.3 The absorption type of exhaust silencer 380
8.5.3.1 Positioning an absorption silencer segment 381
8.5.3.2 A possible absorption silencer segment for a
two-stroke engine 382
8.5.4 Silencing the intake system 385
8.5.4.1 The acoustic design of the low-pass intake silencer 386
8.5.4.2 Shaping the intake port to reduce
high-frequency noise 388
8.6 Silencing the exhaust system of a two-stroke engine 388
8.6.1 The profile of the exhaust port timing edge 390
8.6.2 Silencing the tuned exhaust system 392
8.6.2.1 A design example for a silenced expansion
chamber exhaust system 393
8.6.3 Silencing the untuned exhaust system 396
8.7 Concluding remarks on noise reduction 397
NOTATION for CHAPTER 8 398
REFERENCES for CHAPTER 8 398
POSTSCRIPT 401
COMPUTER PROGRAM APPENDIX 405
ProgList 1.0 407
ProgList 1.1, PISTON POSITION 407
ProgList 1.2, LOOP ENGINE DRAW 408
ProgList 1.3, QUB CROSS ENGINE DRAW 421
ProgList 1.4, EXHAUST GAS ANALYSIS 434
ProgList 2.0 437
ProgList 2.1, EXHAUST 442
ProgList 2.2, INDUCTION 447
ProgList 2.3, CYLINDER-PIPE FLOW 461
ProgList 3.0 465
ProgList 3.1, BENSON-BRANDHAM MODEL 465
ProgList 3.2, BLAIR SCAVENGING MODEL 466
ProgList 3.3, QUB CROSS PORTS 470
ProgList 3.4, LOOP ENGINE DESIGNCBA PTER 7
•RED! UCTION OF FUEL CONSUMPTION AND EXHAUST EMISSIONS 299
?.(Q Introduction 299
7.1.1 Some fundamentals regarding combustion and emissions 301
7.1.2 Homogeneous and stratified combustion and charging 303
121 The simple two-stroke engine 306
7.2.1 Typical performance characteristics of simple engines 309
7.2.1.1 Measured performance data from a
QUB 400 research engine 309
7.2.1.2 Typical performance maps for simple two-stroke
engines 313
£? •> Optimizing the emissions and fuel economy of the simple
two-stroke engine 316
7.3.1 The effect of scavenging behavior 317
7.3.2 The effect of air-fuel ratio 320
7.3.3 The effect of exhaust port timing and area 322
7.3.3.1 The butterfly exhaust valve 324
7.3.3.2 The exhaust timing edge control valve 324
7.3.4 Conclusions regarding the simple two-stroke engine 327
It ! The more complex two-stroke engine 328
7.4.1 The stratified charging and homogeneous combustion engine 332
7.4.1.1 The QUB stratified charging engine 332
7.4.1.2 The Piaggio stratified charging engine 333
7.4.1.3 An alternative mechanical option for stratified
charging 339
7.4.1.4 The stratified charging engine by Institut
Francais du Petrole 339
7.4.2 The stratified charging and stratified combustion engine 342
7.4.3 Direct in-cylinder fuel injection 350
7.4.3.1 Air-blast injection of fuel into the cylinder 352
E; 5 Concluding comments 354
S 3FERENCES for CHAPTER 7 354
Ǥ\PTER 8
m: UCTION OF NOISE EMISSION FROM TWO-STROKE ENGINES 357
W ) Introduction 357
ft : Noise 357
8.1.1 Transmission of sound 358
8.1.2 Intensity and loudness of sound 358
8.1.3 Loudness when there are several sources of sound 359
8.1.4 Measurement of noise and the noise-frequency spectrum 361
tt 2 Noise sources in a simple two-stroke engine 362
E i Silencing the exhaust and inlet system of the two-stroke engine 3638.4 Some fundamentals of silencer design 364
8.4.1 The theoretical work of Coates(8.3) 364
8.4.2 The experimental work of Coates(8.3) 365
8.4.3 Future work for the prediction of silencer behavior 373
8.5 Theory based on acoustics for silencer attenuation characteristics 373
8.5.1 The diffusing type of exhaust silencer 374
8.5.2 The side-resonant type of exhaust silencer 378
8.5.3 The absorption type of exhaust silencer 380
8.5.3.1 Positioning an absorption silencer segment 381
8.5.3.2 A possible absorption silencer segment for a
two-stroke engine 382
8.5.4 Silencing the intake system 385
8.5.4.1 The acoustic design of the low-pass intake silencer 386
8.5.4.2 Shaping the intake port to reduce
high-frequency noise 388
8.6 Silencing the exhaust system of a two-stroke engine 388
8.6.1 The profile of the exhaust port timing edge 390
8.6.2 Silencing the tuned exhaust system 392
8.6.2.1 A design example for a silenced expansion
chamber exhaust system 393
8.6.3 Silencing the untuned exhaust system 396
8.7 Concluding remarks on noise reduction 397
NOTATION for CHAPTER 8 398
REFERENCES for CHAPTER 8 398
POSTSCRIPT 401
COMPUTER PROGRAM APPENDIX 405
ProgList 1.0 407
ProgList 1.1, PISTON POSITION 407
ProgList 1.2, LOOP ENGINE DRAW 408
ProgList 1.3, QUB CROSS ENGINE DRAW 421
ProgList 1.4, EXHAUST GAS ANALYSIS 434
ProgList 2.0 437
ProgList 2.1, EXHAUST 442
ProgList 2.2, INDUCTION 447
ProgList 2.3, CYLINDER-PIPE FLOW 461
ProgList 3.0 465
ProgList 3.1, BENSON-BRANDHAM MODEL 465
ProgList 3.2, BLAIR SCAVENGING MODEL 466
ProgList 3.3, QUB CROSS PORTS 470
ProgList 3.4, LOOP ENGINE DESIGN 475
CHAPTER 7
REDUCTION OF FUEL CONSUMPTION AND EXHAUST EMISSIONS 299
7.0 Introduction 299
7.1.1 Some fundamentals regarding combustion and emissions 301
7.1.2 Homogeneous and stratified combustion and charging 303
7.2 The simple two-stroke engine 306
7.2.1 Typical performance characteristics of simple engines 309
7.2.1.1 Measured performance data from a
QUB 400 research engine 309
7.2.1.2 Typical performance maps for simple two-stroke
engines 313
7.3 Optimizing the emissions and fuel economy of the simple
two-stroke engine 316
7.3.1 The effect of scavenging behavior 317
7.3.2 The effect of air-fuel ratio 320
7.3.3 The effect of exhaust port timing and area 322
7.3.3.1 The butterfly exhaust valve 324
7.3.3.2 The exhaust timing edge control valve 324
7.3.4 Conclusions regarding the simple two-stroke engine 327
7.4 The more complex two-stroke engine 328
7.4.1 The stratified charging and homogeneous combustion engine 332
7.4.1.1 The QUB stratified charging engine 332
7.4.1.2 The Piaggio stratified charging engine 333
7.4.1.3 An alternative mechanical option for stratified
charging 339
7.4.1.4 The stratified charging engine by Institut
Frangais du Petrole 339
7.4.2 The stratified charging and stratified combustion engine 342
7.4.3 Direct in-cylinder fuel injection 350
7.4.3.1 Air-blast injection of fuel into the cylinder 352
7.5 Concluding comments 354
REFERENCES for CHAPTER 7 354
CHAPTER 8
REDUCTION OF NOISE EMISSION FROM TWO-STROKE ENGINES 357
8.0 Introduction 357
8.1 Noise 357
8.1.1 Transmission of sound 358
8.1.2 Intensity and loudness of sound 358
8.1.3 Loudness when there are several sources of sound 359
8.1.4 Measurement of noise and the noise-frequency spectrum 361
8.2 Noise sources in a simple two-stroke engine 362
8.3 Silencing the exhaust and inlet system of the two-stroke engine 363
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