THE EFFECT OF COMPRESSION RATIO ON THE PERFORMANCE OF AA Thesis submitted for ...

267 Pages·2015·7.12 MB·English

THE EFFECT OF COMPRESSION RATIO ON THE PERFORMANCE OF A
DIRECT INJECTION DIESEL ENGINE
A Thesis submitted for the degree of Doctor of Philosophy
by
Razmik Aivaz Balian
Department of Mechanical Engine~ring, Brunel University, Uxbridge
March 1990
Brunel University
Uxbridge
Department of Mechanical Engineering
Razmik Aivaz Balian
The Effect of Compression Ratio on the Performance of a
Direct Injection Diesel Engine
March 1990
ABSTRACT
This thesis considers the effect of compression ratio on the
performance of a direct injection diesel engine.
One aspect of engine performance is considered in great detail,
namely the combustion performance at increased clearance volume. This
aspect was of particular interest because variable compression ratio
(VCR) systems normally operate by varying the clearance volume.
The investigation relied upon results obtained both from
experimental and computer simulating models. The experimental tests were
carried out using a single-cylinder direct-injection diesel engine,
under simulated turbocharged conditions at a reduced compression ratio.
A number of one-dimensional computer models were developed; these
simulate the induction and compression strokes, and the fuel spray
trajectories in the presence of air swirl.
The major objectives of the investigation were: to assess the
benefits of VCR in terms of improvements in output power and fuel
economy; to assess the effects on combustion of increased clearance
volume, and investigate methods for ameliorating resulting problems;
develop computational models which could aid understanding of the
combustion process under varying clearance volume conditions.
It was concluded that at the reduced compression ratio of 12.9:1
(compared to the standard value of 17.4:1 for the naturally-aspirated
engine), brake mean effective pressure (BMEP) could be increased by more
than 50%, and the brake specific fuel consumption (BSFC) could be
reduced by more than 20%. These improvements were achieved without the
maximum cylinder pressure or engine temperatures exceeding the highest
values for the standard engine. Combustion performance deteriorated
markedly, but certain modifications to the injection system proved
successful in ameliorating the problems. These included: increase in the
number of injector nozzle holes from 3 to 4, increase in injection rate
by about 28%, advancing injection timing by about 6°CA. In addition,
operation with weaker air fuel ratio, ~n the range of 30 to 40:1 reduced
smoke emissions and improved BSFC.
Use of intercooling under VCR conditions provided only modest gains
in performance.
The NO emission was found to be insensitive to engine operating
x
conditions (fixed compression ratio of 12.9:1), as long as the peak
cylinder pressure was maintained constant.
Engine test results were used in order to assess the accuracy of
four published correlations for predicting ignition delay. The best
prediction of ignition delay with these correlations deviated by up to
SOt from the measured values.
The computer simulation models provided useful insights into the
fuel distribution within the engine cylinder. It also became possible to
quantify the interaction between the swirling air and the fuel sprays,
using two parameters: the crosswind and impingement velocities of the
fuel spray when it impinges on the piston-bowl walls. Tentative
trends were identified which showed that high crosswind velocity
coincided with lower smoke emissions and lower BSFC.
ACKNOWLEDGEMENTS
The author wishes to acknowledge Dr. Nicos Ladommatos for his
guidance, support, and patience during the supervision of this project.
He would also wish to thank Dr. Richard Stone for his ever constructive
criticisms and comments during the course of this study.
The author also wishes to thank Mr. Godfrey Reading, Mr. John
Langdon and all the laboratory and workshop technicians for their
technical advice and support.
Finally, I would like to specially thank my parents, Emik and Avo
Aivaz Balian for their immense help, and their endless moral and
financial support without which,my studies would have been impossible.
With this, I would like to dedicate this thesis to my parents, specially
my mother Emik, who died on 14th of January 1989 with her only wish
being the successful completion of this project and with it my studies.
NOMENCLATURE
A Area.
A Cylinder cross-sectional area.
c
Effective leakage area.
A Orifice area.
o
A Percentage squish area.
sq
ATDC After top dead centre.
B Cylinder bore. m
BDC Bottom dead centre.
BMEP Brake mean effective pressure. bar
BSAC Brake specific air consumption. kg/kWh
BSFC Brake specific fuel consumption. kg/kWh
BTDC Before top dead centre.
Discharge coefficient.
CI Compression ignition.
CR Compression ratio.
d Nozzle hole diameter. m
o Combustion bowl diameter. m
01 Direct injection.
F Constant defined by Eq 5.13.
f Skin friction coefficient for the
cyl
cylinder walls.
f Skin friction coefficient for the
pc
piston crown and cylinder head.
HSD! High speed direct injection.
IC internal combustion.
101 In-direct injection.
IVC Inlet valve closure.
1 Valve lift. m
L Characteristic length. m
LOV Laser doppler velocimeter.
m mass. kg
m Trapped air mass at inlet valve closure. kg
ivc
.m Maximum trapped air mass during induction. kg
.m ax
m mass flow rate. kg/s
m Average air mass flow rate during time
av
interval ot. kg/s
in Air mass flow rate at time t. kg/a
t
m Air mass flow rate at time t + ot. kg/a
tt
n Number of nozzle holes.
N Engine speed. rev/min
P Pressure. bar
Ambient pressure. bar
P Cylinder pressure. bar
c
p Manifold pressure. bar
o
Cylinder pressure at time t. bar
Cylinder pressure at time t + 0 t. bar
q Volume of fuel injected per engine cycle.
Momentum flux of the swirling air.
Momentum flux of the fuel spray.
r Radius. m
R Specific gas constant. kJ/kgK
s Distance between the piston crown and
cylinder head at any crank angle. m
s Tangential displacement of the fuel
spray tip. m
SI Spark ,ignition.
SMD Sauter mean diameter. m
T Temperature,
. or Torque. Nm
T Manifold temperature.
o
t Time t.
tt Time t + 0 t.
v Combustion bowl volume.
v Volume.
v Cylinder volume.
c
V Crosswind velocity. m/s
cw
Impingement velocity. m/s
V Mean injection velocity. m/s
lnj
Squish velocity. m/s
W Turbine power. kW
t
X Fuel spray tip penetration in
t quiescent air. m/s
X Fuel spray tip penetration in
tw. a swirling air environment. m/s
Ratio of the specific heats of air.
Small time interval between time t and tt. s
Pressure drop across the injector nozzle. bar
p
Density.
Density of the compressed air within
the cylinder.
Density of air at atmospheric conditions. kg/m3
~tlfl
Density of the standard diesel fuel.
.~
A Fuel density.
P Absolute viscousity of air.
O
.Measured injection period. degree
T Shear stress.
w angular velocity. rad/s
Angular velocity of the swirling air
during compression. rad/s
Average angular velocity of the swirling
ot.
air during compression over interval rad/s
CONTENTS
Abstract
Acknowledgements
Nomenclature
Chapter 1 - Introduction
1.1 Background 1
1.2 Objectives of the study 6
1.3 Outline of the thesis 6
Chapter 2 - Literature Survey
2.1 Fundamentals of Internal Combustion Engines 8
2.1.1 Diesel Engine Types 9
2.2 The Combustion Process in 01 Diesel Engines 12
2 .. 3 Bulk Flow Within the Cylinder 16
2.3.1 Swirl 16
2.3.2 Squish 24
2.3.3 Turbulence 25
2.4 Effect of Swirl, Squish, and Turbulence on
Air Fuel Mixing 28
2.5 Effect of Clearance Height on Swirl and Squish 31
2.6 Matching of Injection System With In-Cylinder
Air Activity 33
2.6.1 Fuel Spray Structure and its Effect
on combustion 33
2.6.2 Optimisation of a Fuel Injection System 35
2.7 Exhaust Pollutant Emissions 38
2.8 Conclusions 41
Chapter 3 - Experimental System and Commissioning Tests
3.1 Description of the Experimental System 44
3.1.1 Outline 44
3.1.2 Control of Engine Operation 47
3.1.3 Measured Parameters and Instrumentation Systems 50
3.1.4 Cyclic Data Acquisition System 58
3.2 Baseline Tests 60
3.2.1 Purpose of Baseline tests 60
3.2.2 Experimental Procedure for Baseline Tests 61
3.2.3 Discussion of Baseline Tests Results 62
3.3 Steady Flow Rig 67
3.4 Injector Nozzle Tests 69
Chapter 4 Experimental Results at Lower Compression Ratio
4.1 Outline of Tests 71
4.2 Effect of Injection Timing on Engine Performance 76
4.2.1 Relation Between Ignition Delay and Maximum
Rate of Pressure Rise 76
4.2.2 Effect of Injection Timing on Ignition Delay
and Maximum Rate of Pressure Rise 76
4.2.3 Effect of Injection Timing on Boost Pressure 77
4.2.4 Effect of Injection Timing on BMEP 77
4.2.5 Effect of Injection Timing on BSFC 78
4.2.6 Effect of Injection Timing on Smoke Emission 78
4.2.7 Tradeoffs Between BMEP, BSFC, and Smoke with
Varying Injection Timing 79
4.3 Effect of Injection Rate on Engine Performance 80
4.3.1 Effect of Injection Rate on Fuel Spray
Characteristics 80
4.3.2 Effect of Injection Rate on Ignition Delay 81
4.3.3 Effect of Injection Rate on the Maximum Rate
of Pressure Rise, Boost Pressure, and BMEP 81
4.3.4 Effect of Injection Rate on BSFC and
Smoke Emission 82
4.3.5 Tradeoffs between BSFC, BMEP and Smoke
Emissions with Higher Injection Rate 83
4.4 Effect of Air Fuel Ratio on Engine Performance 84
4.4.1 Effect of Air Fuel Ratio on BMEP, BSFC and Smoke 84
4.4.2 Tradeoffs Between BSFC, BMEP, and Smoke
Emissions With Varying Air Fuel Ratio 85
4.5 Attempts to Eliminate After Injections 86
4.6 Effect of Intercooling on Engine Performance 86
4.7 Metal Temperature Around the Combustion Chamber 87
4.8 Noise 88
4.9 Oxides of Nitrogen 88
4.10 Optimisation of other engine parameters 89
4.11 Engine Performance at the Standard Compression
Ratio with the Four-Hole Nozzle and the High
Injection Rate Pump 91
4.12 A Correlation for the Prediction of Ignition Delay 94
4.12.1 Introduction 94
4.11.2 Ignition Delay Correlations 94
4.13 Summary and Conclusions 101
Chapter 5 Computer Simulation of In-Cylinder Air Flow and Fuel
Spray Trajectories at Low Compression Ratio
5.1 Introductory Overview of the Computer Model 103
5.2 Mathematical Model 104
5.2.1 Prediction of Mass Flow Rate Through the
Inlet Valve During Induction 104
5.2.2 Prediction of Total Angular Momentum at IVC 106
5.2.3 Prediction of Swirl During Compression Stroke 108
5.2.4 Prediction of Squish During Compression Stroke 111
5.2.5 Prediction of Crosswind Velocity 111
5.3 Computer Simulation Program 118
5.3.1 Simulation of the Induction Process -
Program "ITERAM" 118
5.3.2 Simulation of Total Moment of Momentum at IVC -
Program "SUMMOM" 119
5.3.3 Estimation of the Swirl and Squish Velocities
During Compression Stroke - Program "COMPRESS· 120
5.3.4 Estimation of Crosswind Velocity at Impingement -
Program "CWVEL" 120
5.4 Demonstration and Validation of the Computer Model 122
5.4.1 Prediction of Instantaneous Mass Flow Rate During
Induction, and Total Mass Trapped at IVC -
Program "ITERAM" 122
5.4.2 Prediction of Moment of Momentum at IVC -
Program "SUMMOM" 122
5.4.3 Prediction of Swirl and Squish During
Compression Stroke - Program "COMPRESS" 123
5.4.4 Prediction of Crosswind Velocity and Spray Tip
Trajectory - Program "CWVEL" 126
5.5 Injection System Optimisation for High Crosswind
Velocity 129
5.6 Discussion of Experimental Results and Computer Data 132
5.7 Conclusion 134
Chapter 6 -Conclusion. and recommenda~ion. for fu~ure work
6.1 Conclusions 135
6.2 Future work 138
Appendix A 139
Appendix B 145
Appendix C 190