residual gas mixing in engines
TRANSCRIPT
Residual Gas Mixing in Engines
by
Andrew G. Bright
A thesis submitted in partial fulfillment
of the requirements for a degree of
Master of Science
(Mechanical Engineering)
at the
University of Wisconsin – Madison
2004
ii
i
Abstract
The mixing of fresh charge with residual gases was studied in a spark-ignition engine
using planar laser-induced fluorescence (PLIF) of a homogenous air/fuel/tracer mixture. An
adjustable, dual-overhead cam cylinder head and throttled operation provided a range of
elevated residual gas fractions. The bulk residual fraction was measured with a sampling
valve and exhaust emissions were recorded for 15 experimental conditions covering two
engine speeds and five valve overlap strategies.
Residual gas fractions ranged from 24% to 40% at 600 RPM and 21% to 45% at 1200
RPM. Indicated mean effective pressure ranged from 146 kPa to 271 kPa across all
conditions, with variability levels consistently below 6%. Calculated heat release confirmed
the high dilution levels with universally slow burning rates.
A non-intensified CCD camera was used to capture the PLIF signal and operated with
a peak signal-to-noise ratio of 21:1. The negative-PLIF imaging technique was verified with
a quantitative measure of intake charge homogeneity, and a fuel-cutoff experiment that
isolated unwanted fluorescence signal from residuals. Data images were analyzed with first
and second statistical moments of pixel intensity, as well as an ensemble PDF curve.
All fired conditions showed a clear increase in spatial variation from the
homogeneous condition, a trend that was qualitatively verified visually in the corrected data
images. Inhomogeneity in the compressed charge increased rapidly above 35% residual gas
fraction, independent of engine speed or overlap strategy. The intake cam advance valve
overlap strategy was found to provide reduced spatial variation over equivalent symmetric
valve overlaps and exhaust cam retard overlaps.
ii
Acknowledgements
First to thank for the completion of this project is my advisor, Professor Jaal B.
Ghandhi for giving me the opportunity to pursue my graduate work at the Engine Research
Center. Prof. Ghandhi has been an exceptional point of reference for the myriad challenges
that have presented themselves over the past two years.
The support staff at the ERC also have to be thanked, particularly Sally Radecke and
Susan Strzelec in the office, for tolerating my approach to procedure and paperwork. Also,
Ralph Braun has provided the supplies and access to shop facilities essential to completing
this project.
Very little would have been accomplished without the help of fellow students here,
past and present. Matt Wiles got me started in the engine lab and familiarized me with all
aspects of the laser imaging procedure. Randy Herold has been an invaluable aid throughout
the project with the optical system and emissions analyzers. Lonny Peet provided his time in
completing the accumulator fuel system, which has been a major improvement in the lab.
Brian Albert, Dennis Ward, Bob Iverson, Tongwoo Kim, Soochan Park, Jared Cromas, Nate
Haugle, Karen Bevan, Daniel Rodriguez and Anton Kozlovsky have all given substantial
help along the way. Cheers to all.
The Wisconsin Small Engine Consortium generously assumed funding support mid-
way through this project. The representatives of Briggs & Stratton, Fleetguard/Nelson,
Harley-Davidson, Kohler, Mercury Marine, MotoTron and the Wisconsin Department of
Commerce are to be thanked. Preliminary funding came through a grant from the National
Science Foundation, to which I am equally grateful.
iii
Table of Contents
ABSTRACT..............................................................................................................................I
ACKNOWLEDGEMENTS .................................................................................................. II
TABLE OF CONTENTS .....................................................................................................III
LIST OF FIGURES ..............................................................................................................VI
LIST OF TABLES ................................................................................................................. X
1. INTRODUCTION........................................................................................................... 1 1.1. MOTIVATIONS FOR RESIDUAL GAS STUDY ................................................................ 1
1.1.1. Small Engines Issues............................................................................................. 2 1.1.2. High-Dilution Automotive Engines....................................................................... 3 1.1.3. Homogeneous-Charge Compression-Ignition ...................................................... 4
1.2. PROJECT OBJECTIVES................................................................................................. 6 1.3. OUTLINE .................................................................................................................... 6
2. BACKGROUND ............................................................................................................. 8 2.1. RESIDUAL GAS EFFECTS ON COMBUSTION ................................................................ 8
2.1.1. Combustion Thermodynamics............................................................................... 8 2.1.2. Flame Speed Effects.............................................................................................. 9 2.1.3. Oxides of Nitrogen Formation............................................................................ 11 2.1.4. Cycle-to-Cycle Variations................................................................................... 12
2.2. BULK RESIDUAL GAS FRACTION MEASUREMENT.................................................... 13 2.2.1. Measurement Principle....................................................................................... 13 2.2.2. Sampling Valves.................................................................................................. 14 2.2.3. Sampling Valve Operation.................................................................................. 15
2.3. ONE-DIMENSIONAL STUDIES OF RESIDUAL GAS...................................................... 17 2.3.1. Early Work .......................................................................................................... 17 2.3.2. Recent Work ........................................................................................................ 19
2.4. PLANAR LASER-INDUCED FLUORESCENCE .............................................................. 22 2.4.1. Laser Source ....................................................................................................... 23 2.4.2. Tracer Chemical Selection.................................................................................. 23 2.4.3. Camera................................................................................................................ 25
2.5. PLIF MEASUREMENTS IN ENGINES.......................................................................... 26 2.5.1. 2-d Quantification of SI Engine Flow Inhomogeneity ........................................ 27 2.5.2. Direct Visualization of Residual Gas.................................................................. 30 2.5.3. Negative Visualization of Residual Gas.............................................................. 33
3. EXPERIMENTAL SETUP.......................................................................................... 36 3.1. SINGLE-CYLINDER RESEARCH ENGINE.................................................................... 36
iv3.1.1. Base Engine ........................................................................................................ 36 3.1.2. Optical Access..................................................................................................... 37 3.1.3. Cylinder Head and Combustion Chamber.......................................................... 38 3.1.4. Valvetrain Timing System ................................................................................... 40 3.1.5. Dynamometer...................................................................................................... 43 3.1.6. Engine Fluid Systems.......................................................................................... 43 3.1.7. Engine Aspiration Systems.................................................................................. 44 3.1.8. Fuel Delivery System .......................................................................................... 45 3.1.9. Engine Control System........................................................................................ 48
3.2. COMBUSTION DATA ACQUISITION ........................................................................... 49 3.2.1. Cylinder Pressure Measurement......................................................................... 49 3.2.2. Sampling Valve ................................................................................................... 51 3.2.3. Emissions Bench ................................................................................................. 53
3.3. OPTICAL MEASUREMENT SYSTEM ........................................................................... 55 3.3.1. Laser Source ....................................................................................................... 55 3.3.2. Laser Optics ........................................................................................................ 56 3.3.3. Camera................................................................................................................ 58 3.3.4. Optical Triggering .............................................................................................. 60
4. ENGINE OPERATING CONDITIONS..................................................................... 63 4.1. SELECTION CRITERIA............................................................................................... 63
4.1.1. Optical Engine Considerations........................................................................... 63 4.1.2. Establishing Engine Conditions.......................................................................... 64
4.2. COMBUSTION ANALYSIS .......................................................................................... 67 4.2.1. Cylinder Pressure Data ...................................................................................... 68 4.2.2. Heat Release Analysis......................................................................................... 69
4.3. EXHAUST GAS EMISSIONS MEASUREMENT.............................................................. 74 4.3.1. Emissions Measurement Procedure.................................................................... 75 4.3.2. Emissions Analysis.............................................................................................. 75 4.3.3. Emissions Measurements .................................................................................... 77
4.4. BULK RESIDUAL GAS FRACTION MEASUREMENT.................................................... 79 4.4.1. Sampling Valve Measurement Technique........................................................... 79 4.4.2. Residual Gas Fraction Calculations................................................................... 83 4.4.3. Residual Gas Fraction Measurements................................................................ 84
5. IMAGING SYSTEM DEVELOPMENT AND ANALYSIS ..................................... 86
5.1. PLIF IMAGE PROCESSING ........................................................................................ 86 5.1.1. Image Acquisition Procedure ............................................................................. 86 5.1.2. Image Correction Procedure .............................................................................. 89 5.1.3. Median Filtering ................................................................................................. 91 5.1.4. Image Statistics ................................................................................................... 92 5.1.5. Probability Distribution Function ...................................................................... 94 5.1.6. Image Presentation ............................................................................................. 95
5.2. IMAGING SYSTEM PERFORMANCE............................................................................ 96 5.2.1. Camera Selection................................................................................................ 96
v5.2.2. Region of Interest and Spatial Resolution .......................................................... 97 5.2.3. Signal-to-Noise Ratio.......................................................................................... 99 5.2.4. MicroMax Comparison with Intensified CCD.................................................. 101
5.3. ASSESSMENT OF INTAKE CHARGE HOMOGENEITY................................................. 102 5.3.1. First and Second Moments of Homogeneous Data........................................... 102 5.3.2. Homogeneous Image PDF................................................................................ 104
5.4. DIRECT-INJECTION TEST OF IMAGING TECHNIQUE ................................................ 105 5.4.1. Skip-Direct Injection Experiment ..................................................................... 106 5.4.2. Skip-DI Imaging and Results ............................................................................ 109
6. RESIDUAL GAS MIXING........................................................................................ 111 6.1. SAMPLE IMAGING DATA ........................................................................................ 111 6.2. CORRELATION OF SPATIAL-MEAN PIXEL INTENSITY WITH MEASURED RESIDUAL GAS FRACTION .......................................................................................................................... 113 6.3. CORRELATION OF RESIDUAL GAS FRACTION TO IMAGE INTENSITY VARIATION.... 115
6.3.1. Cycle-Averaged Image Intensity COV Correlation .......................................... 115 6.3.2. Lower Residual Fraction Case-to-Case Comparison....................................... 118 6.3.3. Higher Residual Fraction Case-to-Case Comparison...................................... 121
6.4. PRIOR-CYCLE EFFECT ON IMAGE INTENSITY VARIATION ...................................... 123 6.5. ENGINE OPERATING CONDITIONS EFFECT ON DATA IMAGE INTENSITY VARIATION 126
6.5.1. Symmetric Overlap Increase............................................................................. 128 6.5.2. Intake Cam Advance ......................................................................................... 129 6.5.3. Exhaust Cam Retard ......................................................................................... 133
7. SUMMARY AND CONCLUSIONS ......................................................................... 134 7.1. PROJECT SUMMARY ............................................................................................... 134 7.2. RESULTS SUMMARY............................................................................................... 135 7.3. CONCLUSIONS........................................................................................................ 138 7.4. RECOMMENDATIONS FOR FUTURE WORK .............................................................. 140
REFERENCES.................................................................................................................... 141
APPENDIX A – ENGINE OPERATING CONDITIONS .............................................. 144
APPENDIX B – IMAGE STATISTICS............................................................................ 149
vi
List of Figures FIGURE 1.1. STRATEGIES PURSUED FOR HCCI CONTROL IN CURRENT RESEARCH. REPRINTED
FROM [9]. ........................................................................................................................... 5 FIGURE 2.1. EXPERIMENTAL MEASUREMENTS OF GASOLINE LAMINAR FLAME SPEED IN
EXHAUST GAS-DILUTED MIXTURES RELATIVE TO UNDILUTED MIXTURES, SU(0), FOR A RANGE OF DILUENT FRACTIONS, EQUIVALENCE RATIOS AND INITIAL BOMB PRESSURES. REPRINTED FROM [3]. ...................................................................................................... 11
FIGURE 2.2. SAMPLE CYLINDER PRESSURE DATA FOR IN-CYLINDER SAMPLING IN A SMALL 2-STROKE ENGINE, WITH VALVE LIFT DURATION MEASURED BY AN INDUCTIVE PROXIMITY SENSOR SHOWN. REPRINTED FROM [12]. ......................................................................... 16
FIGURE 2.3. CORRELATION OF MEASURED [CO2] TO LOCAL N2 TEMPERATURE USING CARS. THE PLOT ON THE LEFT IS FOR DATA ACQUIRED AT 30° BTDC WITH A CORRELATION COEFFICIENT OF 0.486. THE PLOT ON THE RIGHT IS AT 5° BTDC WITH A CORRELATION OF 0.420. REPRINTED FROM [20].......................................................................................... 18
FIGURE 2.4. EXPERIMENTAL SETUP FOR RAMAN SCATTERING MEASUREMENTS IN A MODERN 4-VALVE PENT-ROOF COMBUSTION CHAMBER. REPRINTED FROM [8]. ................................ 19
FIGURE 2.5. RESIDUAL GAS MOLE FRACTION VS. CRANK ANGLE, BASED ON ENSEMBLE-AVERAGED CONCENTRATION MEASUREMENTS OF VARIOUS SPECIES. REPRINTED FROM [8].......................................................................................................................................... 21
FIGURE 2.6. LEVELS OF VARIANCE IN DATA FOR ENSEMBLE-AVERAGED MEAN RESIDUAL GAS MOLE FRACTION GIVEN IN FIGURE 2.5. REPRINTED FROM [8]. ......................................... 22
FIGURE 2.7. ABSORPTION AND EMISSION PROPERTIES OF 3-PENTANONE IN LIF APPLICATIONS [17]. ................................................................................................................................. 24
FIGURE 2.8. MEASURED TEMPERATURE DEPENDENCY OF LIF SIGNAL OF ACETONE AT ATMOSPHERIC PRESSURE, NORMALIZED TO ROOM TEMPERATURE CONDITION. REPRINTED FROM [18]. ....................................................................................................................... 25
FIGURE 2.9. MEAN H2O PLIF SIGNAL TREND WITH INTAKE MAP. REPRINTED FROM [22]..... 31 FIGURE 2.10. CYCLIC VARIATION IN H2O PLIF SIGNAL FOR INCREASING LOAD. REPRINTED
FROM [22]. ....................................................................................................................... 31 FIGURE 2.11. CORRELATION OF LOAD-NORMALIZED RESIDUAL GAS FLUCTUATION TO CCV OF
0-0.5% HEAT RELEASE DURATION USING H2O PLIF. REPRINTED FROM [22].................. 32 FIGURE 2.12. COMPARISON OF FLOWFIELD EFFECT ON RESIDUAL GAS DISTRIBUTION AS
MEASURED BY NEGATIVE-PLIF. BOTH CONDITIONS ARE 1200 RPM, ΗVOL = 0.6. REPRINTED FROM [23]. .................................................................................................... 34
FIGURE 2.13. MEAN RESIDUAL GAS DISTRIBUTION ACROSS COMBUSTION CHAMBER (DIRECTION ALONG PENT-ROOF AXIS) FOR TWO BULK FLOWFIELD CONDITIONS. IMAGE DATA TAKEN WITH NEGATIVE-PLIF AT SPARK TIMING (27° BTDC). 1200 RPM, ΗVOL = 0.6. REPRINTED FROM [23]. .................................................................................................... 35
FIGURE 3.1. VALVETRAIN TIMING LAYOUT FOR DOHC CYLINDER HEAD................................ 41
viiFIGURE 3.2. COMPARISON OF MEASURED CYLINDER PRESSURE TRACES AT WALL-MOUNT
LOCATION TO CONVENTIONAL ROOF-MOUNT. MOTORING ENGINE CONDITION WITH OHV HEAD, 1200 RPM............................................................................................................. 51
FIGURE 3.3. IN-CYLINDER SOLENOID-ACTUATED SAMPLING VALVE MOUNTED TO BLOCK-HEAD SPACER RING. TEFLON SAMPLED GAS LINE TRAVELS TO AN ADJACENT ICE BATH AND THEN TO THE ANALYZER. .......................................................................................................... 52
FIGURE 3.4. 266 NM LASER PULSE SEPARATION AND DELIVERY OPTICS (PLAN VIEW).............. 57 FIGURE 3.5. LASER SHEET-FORMING OPTICS SETUP FOR 266 NM PLIF IMAGING. .................... 57 FIGURE 3.6 MICROMAX CAMERA MANUAL SUMMARY OF DIF-MODE TIMING. IMAGE
EXPOSURE TIMES ARE SHOWN IN THE SECOND LINE. READY AND SCAN ARE OUTPUT SIGNALS FROM THE CAMERA CONTROLLER, EXT. SYNC IS THE INPUT TRIGGER TTL, LASER OUTPUT SHOWN IS FOR A DOUBLE-PULSE LASER, THIS EXPERIMENT ONLY USES THE FIRST PULSE. REPRINTED FROM [24]. ......................................................................................... 59
FIGURE 3.7 SCHEMATIC FOR TTL TIMING OF LASER PULSE AND CAMERA, SYNCHRONIZED WITH MOTOTRON SKIP-FIRING IGNITION BY A “ONE-AND-ONLY-ONE” CIRCUIT. ....................... 62
FIGURE 4.1 SUMMARY OF FOUR VALVE OVERLAP STRATEGIES. BASELINE CAM TIMING IS INDICATED BY THE DASHED LINE IN ALL PLOTS. ARROWS INDICATE CAM SHIFT FROM BASELINE. THE BASELINE OVERLAP DURATION IS 20°, THE 600 RPM EXTENDED OVERLAPS ARE 30° DURATION, AND THE 1200 RPM CONDITIONS ARE 60° OVERLAP DURATION. ....................................................................................................................... 66
FIGURE 4.2 HEAT RELEASE RATE AND CUMULATIVE HEAT RELEASE FOR ALL CAM STRATEGIES AT 600 RPM LOW LOAD.................................................................................................. 70
FIGURE 4.3 HEAT RELEASE RATE AND CUMULATIVE HEAT RELEASE FOR ALL CAM STRATEGIES AT 600 RPM MID LOAD. ................................................................................................. 71
FIGURE 4.4 HEAT RELEASE RATE AND CUMULATIVE HEAT RELEASE FOR ALL CAM STRATEGIES AT 1200 RPM MID LOAD. ............................................................................................... 72
FIGURE 4.5 SKIP-FIRING SEQUENCE EXAMPLE (1200 RPM BASELINE OVERLAP SHOWN). SAMPLING VALVE IS ACTUATED ON COMPRESSION STROKE OF SKIP-FIRED CYCLE (SEE TABLE 4.5)....................................................................................................................... 80
FIGURE 4.6 SAMPLE PRESSURE DATA FOR SKIP-FIRED CYCLE WITH SAMPLING VALVE ACTUATION. THE AVERAGE FIRED CYCLE PRESSURE TRACE AND THE SAMPLING VALVE LIFT TRANSDUCER SIGNAL FOR THAT SKIP-FIRED CYCLE (NO PHYSICAL UNITS) ARE OVERLAYED. 1200 RPM EXHAUST CAM RETARD CONDITION SHOWN.............................. 81
FIGURE 4.7 FREQUENCY HISTOGRAM OF PRIOR-CYCLE IMEP FOR SKIP-FIRING OPERATION AT 600 RPM LOW LOAD SYMMETRIC OVERLAP INCREASE CONDITION. DATA COMPILED FROM 100 CONSECUTIVE SAMPLED CYCLES. .............................................................................. 82
FIGURE 4.8 FREQUENCY HISTOGRAM OF PRIOR-CYCLE IMEP FOR SKIP-FIRING OPERATION AT 1200 RPM EXHAUST RETARD CONDITION. DATA COMPILED FROM 100 CONSECUTIVE SAMPLED CYCLES............................................................................................................. 83
FIGURE 5.1 SAMPLE 100-IMAGE MEAN BACKGROUND IMAGE. PIXEL INTENSITY SCALE IS ON RIGHT............................................................................................................................... 87
FIGURE 5.2 100-IMAGE MEAN FLATFIELD IMAGE, 30° BTDC 600 RPM MID LOAD EXHAUST RETARD CONDITION. FLATFIELD IMAGES HAVE BEEN BACKGROUND-SUBTRACTED. ....... 88
FIGURE 5.3 SAMPLE RAW DATA IMAGE (NO CORRECTIONS), 30° BTDC 1200 RPM EXHAUST RETARD CONDITION. ........................................................................................................ 89
viiiFIGURE 5.4 SAMPLE HOMOGENEOUS IMAGES ACQUIRED AT 30° BTDC FOR THE 1200 RPM,
ZERO OVERLAP CONDITION DEMONSTRATING VERTICAL BANDING IN THE CORRECTED IMAGES. SEE SECTION 5.1.6 FOR IMAGE PRESENTATION CONVENTION. ........................... 93
FIGURE 5.5 LOCATION OF ROI WITHIN COMBUSTION CHAMBER, DOHC CYLINDER HEAD. DISTANCE H IS BETWEEN LASER SHEET PLANE AND PISTON FACE, AND IS TABULATED FOR IMAGE TIMINGS IN TABLE 5.1........................................................................................... 97
FIGURE 5.6 CAMERA NOISE CHARACTERIZATION, AS A FUNCTION OF SIGNAL INTENSITY - MICROMAX FRAME-STRADDLING CCD. REPRINTED FROM [14]. .................................... 99
FIGURE 5.7 COMPARISON OF THEORETICAL SHOT NOISE INTENSITY VARIATIONTO MEASURED
HOMOGENOUS PIXEL INTENSITY VARIATION ( )y yσ µ
................................................ 103 FIGURE 5.8 PROBABILITY DISTRIBUTION FUNCTION FOR PIXEL INTENSITY IN HOMOGENEOUS
IMAGE SETS AT FOUR IMAGE TIMINGS FOR ALL THREE ENGINE SPEED/LOAD POINTS. BASELINE VALVE OVERLAP. EACH PDF CURVE CONTAINS INFORMATION ABOUT 100 CORRECTED HOMOGENOUS IMAGES. .............................................................................. 105
FIGURE 5.9 DIRECT-INJECTION EXPERIMENT CYLINDER PRESSURE TRACE COMPARISON WITH DOHC BASELINE VALVE OVERLAP. 600 RPM. ............................................................. 108
FIGURE 5.10 DIRECT-INJECTION EXPERIMENT CYLINDER PRESSURE TRACE COMPARISON WITH DOHC BASELINE VALVE OVERLAP. 1200 RPM. ........................................................... 108
FIGURE 6.1 SAMPLE HOMOGENEOUS IMAGE SEQUENCE, 60° BTDC. ..................................... 111 FIGURE 6.2 SAMPLE DATA IMAGE SEQUENCE, HIGH RESIDUAL FRACTION CONDITION, 60°
BTDC. ........................................................................................................................... 111 FIGURE 6.3 SAMPLE DATA IMAGE SEQUENCE, MID-RANGE RESIDUAL FRACTION, 60° BTDC. 112 FIGURE 6.4 SAMPLE DATA IMAGE SEQUENCE, LOW RESIDUAL FRACTION CONDITION, 60°
BTDC. ........................................................................................................................... 112 FIGURE 6.5 CORRELATION OF MEAN IMAGE INTENSITY RATIO TO MEASURED RESIDUAL
FRACTION FOR ALL 15 EXPERIMENT CONDITIONS. .......................................................... 114 FIGURE 6.6 PIXEL INTENSITY COV VS. RESIDUAL GAS FRACTION FOR ALL ENGINE CONDITIONS
AT 30° BTDC. SHOT NOISE-LIMITED MAXIMUM SNR WAS ~22:1 FOR THIS IMAGE TIMING........................................................................................................................................ 116
FIGURE 6.7 PIXEL INTENSITY COV VS. RESIDUAL GAS FRACTION FOR ALL ENGINE CONDITIONS AT 45° BTDC. SHOT NOISE-LIMITED MAXIMUM SNR WAS ~20:1 FOR THIS IMAGE TIMING........................................................................................................................................ 116
FIGURE 6.8 PIXEL INTENSITY COV VS. RESIDUAL GAS FRACTION FOR ALL ENGINE CONDITIONS AT 60° BTDC. SHOT NOISE-LIMITED MAXIMUM SNR WAS ~18:1 FOR THIS IMAGE TIMING........................................................................................................................................ 117
FIGURE 6.9 PIXEL INTENSITY COV VS. RESIDUAL GAS FRACTION FOR ALL ENGINE CONDITIONS AT 99° BTDC. SHOT NOISE-LIMITED MAXIMUM SNR WAS ~15:1 FOR THIS IMAGE TIMING........................................................................................................................................ 117
FIGURE 6.10 SAMPLE DATA IMAGES FOR 600 RPM, LOW-RESIDUAL CONDITION. ................. 119 FIGURE 6.11 SAMPLE DATA IMAGES FOR 1200 RPM, LOW-RESIDUAL CONDITION. ............... 119 FIGURE 6.12 100-IMAGE PIXEL INTENSITY PDF FOR 600 RPM LOW-RESIDUAL CONDITION. . 120 FIGURE 6.13 100-IMAGE PIXEL INTENSITY PDF FOR 1200 RPM LOW-RESIDUAL CONDITION.121 FIGURE 6.14 SAMPLE DATA IMAGES FOR 600 RPM, HIGH-RESIDUAL CONDITION. 45° BTDC.
....................................................................................................................................... 123
ixFIGURE 6.15 SAMPLE DATA IMAGES FOR 1200 RPM, LOW-RESIDUAL CONDITION. ............... 123 FIGURE 6.16 PRIOR-CYCLE IMEP VS. IMAGE INTENSITY COV. 600 RPM LOW LOAD, SYM.
INCREASE 60° BTDC. YR = 40.4%, IMEP=152 KPA, COVIMEP = 6.0%, ( )y yσ µ
=5.2%............................................................................................................................ 124 FIGURE 6.17 PRIOR-CYCLE IMEP VS. IMAGE INTENSITY COV. 1200 RPM, SYM. INCREASE 60°
BTDC. YR = 43.7%, IMEP=253 KPA, COVIMEP = 1.2%, ( )y y nσ µ
=7.3%............... 125 FIGURE 6.18 MEAN IMAGE INTENSITY VARIATION VS. CA AT 600 RPM LOW LOAD, ALL
OVERLAPS. ..................................................................................................................... 126 FIGURE 6.19 MEAN IMAGE INTENSITY VARIATION VS. CA AT 600 RPM MID LOAD, ALL
OVERLAPS. ..................................................................................................................... 127 FIGURE 6.20 MEAN IMAGE INTENSITY VARIATION VS. CA AT 1200 RPM, ALL OVERLAPS. ... 127 FIGURE 6.21 INTAKE ADVANCE DATA IMAGES AT 600 RPM MID LOAD. 45° BTDC. ........... 130 FIGURE 6.22 EXHAUST RETARD DATA IMAGES AT 600 RPM MID LOAD. 45° BTDC. ........... 130 FIGURE 6.23 INTAKE ADVANCE DATA IMAGES AT 1200 RPM. 45° BTDC. ........................... 130 FIGURE 6.24 EXHAUST RETARD DATA IMAGES AT 1200 RPM. 45° BTDC............................ 130 FIGURE 6.25 INTAKE ADVANCE 100-IMAGE PIXEL INTENSITY PDF AT 600 RPM MID LOAD, 45°
BTDC. ........................................................................................................................... 131 FIGURE 6.26 EXHAUST RETARD 100-IMAGE PIXEL INTENSITY PDF AT 600 RPM MID LOAD, 45°
BTDC. ........................................................................................................................... 131 FIGURE 6.27 INTAKE ADVANCE 100-IMAGE PIXEL INTENSITY PDF AT 1200 RPM 45° BTDC.
....................................................................................................................................... 132 FIGURE 6.28 EXHAUST RETARD 100-IMAGE PIXEL INTENSITY PDF AT 1200 RPM 45° BTDC.
....................................................................................................................................... 132
x
List of Tables TABLE 1.1. SAMPLE RESULTS FROM A HIGH-DILUTION STOICHIOMETRIC DISI ENGINE. CASE 1
REPRESENTS THE BASELINE ENGINE RUNNING THROTTLED WITH PORT FUEL INJECTION. CASE 2 IS A 70-CAD WIDENED VALVE OVERLAP WITH DIRECT INJECTION, SUPPLEMENTED WITH A SECONDARY AIR INJECTION AND A HIGH-ENERGY VARIABLE-GAP IGNITION SYSTEM. BOTH CONDITIONS ARE AT 1500 RPM AND 400 KPA BMEP. [5] ....................... 4
TABLE 3.1. FIXED INTERNAL DIMENSIONS OF GM-TRIPTANE ENGINE. VALVE TIMINGS ARE FOR INTERNAL SINGLE CAMSHAFT USED FOR OHV ENGINE OPERATION.................................. 37
TABLE 3.2. MAJOR COMBUSTION CHAMBER DIMENSIONS FOR GM-TRIPTANE ENGINE WITH DOHC ADJUSTABLE-CAM CYLINDER HEAD. .................................................................... 40
TABLE 3.3 FUEL PROPERTIES FOR PURE ISO-OCTANE AND THE 20% 3-PENTANONE TRACER BLEND USED FOR THIS EXPERIMENT. ................................................................................ 46
TABLE 3.4. HORIBA EXHAUST EMISSIONS ANALYZER BENCH SUMMARY. ............................... 54 TABLE 3.5 TRIGGER TIMING DELAYS FOR OPTICAL MEASUREMENT SYSTEM. DELAYS ARE
RELATIVE TO THE LEADING EDGE OF THE TRIGGER SIGNAL FROM THE CRANKSHAFT ENCODER.......................................................................................................................... 61
TABLE 4.1. AIR/FUEL ENGINE OPERATION PARAMETERS FOR THE THREE EXPERIMENTAL SPEED/LOAD POINTS. THESE VALUES WERE HELD CONSTANT FOR EACH CAM STRATEGY. 67
TABLE 4.2 MEAN EFFECTIVE PRESSURE DATA FOR 100-CYCLE AVERAGE PRESSURE DATA AT ALL EXPERIMENTAL CONDITIONS. PERCENTAGES SHOWN ARE CHANGES RELATIVE TO THE BASELINE OVERLAP CONDITION FOR THE INDIVIDUAL SPEED/LOAD POINTS AT EACH CAM STRATEGY. ....................................................................................................................... 68
TABLE 4.3 FLAME DEVELOPMENT ANGLES AND OVERALL BURNING ANGLES FOR DIFFERENT OVERLAP STRATEGIES, DETERMINED BY A SINGLE-ZONE HEAT RELEASE CODE. PERCENTAGES INDICATED ARE CHANGES RELATIVE TO THE BASELINE OVERLAP CONDITION AT EACH SPEED/LOAD POINT. ........................................................................................... 73
TABLE 4.4 SUMMARY OF EXHAUST EMISSIONS SPECIES MEASUREMENTS, CONCENTRATIONS SHOWN ARE CORRECTED TO A WET BASIS FROM THE RAW READINGS. AIR/FUEL RATIO AND COMBUSTION EFFICIENCY COEFFICIENT HAVE BEEN CALCULATED FROM THE CONCENTRATION DATA. ................................................................................................... 77
TABLE 4.5 SAMPLING VALVE OPERATION FOR ALL EXPERIMENTAL CONDITIONS. SAMPLING FREQUENCY IS LISTED AS THE NUMBER OF FIRED CYCLES BETWEEN SAMPLED CYCLES (SEE FIGURE 4.5). .................................................................................................................... 80
TABLE 4.6 SUMMARY OF BULK RESIDUAL GAS FRACTION MEASUREMENTS AT ALL EXPERIMENTAL CONDITIONS. PERCENTAGES SHOWN ARE CHANGES RELATIVE TO THE BASELINE OVERLAP CONDITION AT EACH INDIVIDUAL SPEED/LOAD POINT. ...................... 85
TABLE 5.1 DISTANCE FROM PISTON FACE TO LASER SHEET ROI FOR EXPERIMENT IMAGE TIMINGS. .......................................................................................................................... 98
xiTABLE 5.2 VALUES OF SPATIAL-MEAN DATA IMAGE INTENSITY AND RESULTING SHOT NOISE-
LIMITED MAXIMUM SNR FOR THREE SPEED/LOAD POINTS. EACH SET IS THE MEAN VALUE FOR THE FIVE VALVE OVERLAP STRATEGIES................................................................... 100
TABLE 5.3 DIRECT INJECTION EXPERIMENT ENGINE CONDITIONS AND UNBURNED HYDROCARBON EMISSIONS MEASUREMENTS. * INDICATES THE APPROXIMATE IGNITION TIMING. .......................................................................................................................... 106
TABLE 5.4 DIRECT INJECTION EXPERIMENT IMAGING RESULTS. 100-IMAGE MEAN SIGNAL LEVEL FOR FLATFIELD, SKIP-FIRED, AND MOTORED SKIP-DI PLIF DATA........................ 109
TABLE 6.1 COMPARISON OF LOWER-RESIDUAL CONDITIONS AT 600 AND 1200 RPM.
DEVELOPMENT OF IMAGE ( )y yσ µ
[%] WITH CRANK ANGLE..................................... 119 TABLE 6.2 COMPARISON OF HIGHER-RESIDUAL CONDITIONS AT 600 AND 1200 RPM.
DEVELOPMENT OF ( )y yσ µ
[%] WITH CRANK ANGLE. ............................................... 122
1
1. Introduction
1.1. Motivations for Residual Gas Study
Residual gas plays an important role in the combustion development process in four-
stroke cycle spark-ignition (SI) engines. This type of internal combustion has to this day
been the dominant prime-mover in automobiles and utility engine applications. Residual gas
is present in all engines and has important implications to the designer in terms of engine
stability and pollutant emissions.
Residual gas is especially significant in its role as a diluent species during
combustion. This property provides the major benefit to increased residual gas fractions –
reduction in NOx generation during combustion. NOx is a major pollutant species in internal
combustion engine exhaust.
The advent of variable valvetrain actuation (VVA) systems in recent years has
provided much more freedom to the spark ignition engine designer to utilize the exhaust
residual for pollutant reduction and load control, in addition to improvements in volumetric
efficiency across the engine speed and load range. VVA, commonly performed by
mechanical or electro-hydraulic phase-shifting of the camshaft, is becoming increasingly
common on new automotive engine designs.
More information about the participation of residual gas in engine flows preceding
combustion reactions will be critical to achieving the maximum potential (in terms of SI
engine emissions and efficiency) of this and other dilution-controlling technologies.
21.1.1. Small Engines Issues
Small engines can be defined as the category of internal combustion engines below
500 bhp used for non-automotive applications, principally in power equipment, motorcycles
and marine transportation. Despite sharing similar if not identical operation fundamentals,
small engines have unique engineering considerations to automotive SI engines. When faced
with new challenges related to emissions regulations, small engine manufacturers do not
have the luxury of simply adopting mature technologies from the automotive industry.
Of particular concern is NOx emissions, which have only been reduced to
environmentally acceptable levels in cars by universal use of three-way exhaust catalysts
(TWC). For many small engines, the unit cost of the automotive TWC exceeds that of the
entire engine, and as such this technology is not deemed practical in the category. Instead of
aftertreatment, focus is being placed on charge dilution strategies for NOx reduction, and the
simplest delivery mechanism is through internal recirculation via residual gas.
Since VVA systems also fall outside the cost-acceptable realm of most small engine
designs, elevated residual gas fractions will likely be provided by fixed camshaft profiles.
This presents a strong challenge to the combustion chamber designer, with the need to
accommodate high-dilution mixtures throughout the engine speed and load range without
negatively impacting performance felt by the user. More must be learned about charge
composition development at high dilution levels in small engines for this worthy goal to be
achieved.
31.1.2. High-Dilution Automotive Engines
New applications of high residual gas dilution occur in novel engine designs.
Olafsson et al. in [5] describe a high-dilution spark ignition engine designed at Saab to
reduce fuel consumption and NOx emissions. The engine has a similar objective as seen with
direct injection spark ignition (DISI) engines which typically operate without intake
throttling and thus enjoy large improvements in part-load fuel efficiency. The critical
drawback to DISI engines is that by using excess fresh air, the highly effective and durable
three-way catalyst cannot be used to control NOx, CO and HC emission. By utilizing the
exhaust gas residual instead of excess air, Olafsson et al. were able to operate at overall
stoichiometric conditions with a 10% reduction in part-load fuel consumption from the
conventional SI engine. This engine design requires complicated engine systems such as
continuously variable camshaft phasers to control residual dilution, air-assisted in-cylinder
fuel injection, and most notably, a variable spark plug gap to consistently ignite dilute
mixtures. Sample results from this project are presented in Table 1.1.
4 Case 1 Case 2 Change MAP [kPa] 50 93 --- BMEP [kPa] 400 400 --- PMEP [kPa] 54 11 --- COV of IMEP [%] 1.0 1.5 --- BSFC [g/kWh] 265 228 - 14 % BSNOx [g/kWh] 16 0.6 - 96 % BSHC [g/kWh] 6 9 + 50 % BSCO [g/kWh] 19 9 - 50 % Exhaust Temp [C] 560 450 --- 0-10% HR [CAD] 24 35 --- 10-90% HR [CAD] 20 22 --- IGN timing [bTDC] 25 41 ---
Table 1.1. Sample results from a high-dilution stoichiometric DISI engine. Case 1 represents the baseline engine running throttled with port fuel injection. Case 2 is a 70-CAD widened valve overlap with direct injection, supplemented with a secondary air injection and a high-energy variable-gap ignition system. Both conditions are at 1500 RPM and 400 kPa BMEP. [5]
1.1.3. Homogeneous-Charge Compression-Ignition
Homogeneous Charge Compression Ignition (HCCI) is a rapidly developing new
engine combustion strategy that could combine some of the best operating characteristics of
SI and diesel engines. In particular, HCCI can achieve the part-load fuel efficiency of diesel
engines with substantially reduced in-cylinder soot and NOx emissions on the level of SI
engines. Like knock in homogeneous charge SI engines, HCCI involves a controlled
autoignition that can be obtained with a variety of petroleum-based fuels. Controlling the
autoignition of a mixture is separated into 2 strategies: altering the fuel mixture reactivity
kinetics and altering the time-temperature history of the mixture. Cooled external EGR is
often explored for the former, given the usual need to delay the onset of compression
5ignition. The latter strategy commonly involves significant heating of the fuel/air charge
which can encourage the onset of autoignition in engines with lower compression ratios.
Figure 1.1. Strategies pursued for HCCI control in current research. Reprinted from [9].
This lower-compression ratio configuration would enable dual-mode operation with
part-load HCCI combustion transitioning to full-load spark ignition combustion. Intake air
heating, while convenient in a laboratory, is not deemed practical for mobile applications.
Instead, the focus is being placed on the use of VVA to deliver high residual fractions for
heating of the charge. High-dilution operation may be a likely application of HCCI for
improving the efficiency of gasoline automotive engines [9, 10]. For this and a variety of
other reasons, the mixing and chemical kinetics of the exhaust gas residual is a growing topic
of research.
6
1.2. Project Objectives
Four broad objectives have been identified for this research:
1. To provide high-quality, spatially and temporally resolved, two-dimensional
quantification of residual gas mixing with fresh homogenous air/fuel charge through a
range of positions in the SI engine cycle.
2. To supplement and correlate the mixing data with engine-out operating information
such as cylinder pressure data and exhaust emissions analysis for a range of residual
gas dilution levels.
3. To extract conclusions from the residual gas mixing measurements and engine
performance data that will be helpful to the field in designing high-dilution engines.
4. To aid in the development of Planar Laser-Induced Fluorescence as an invaluable
combustion diagnostic in SI engines.
1.3. Outline
This thesis will be divided into six subsequent chapters. Chapter 2 presents the
project background in the form of a literature review of residual-effected SI combustion,
sampling valve measurements, prior optical studies of residual gas and the use of PLIF in
engines. Chapter 3 contains a detailed, design-oriented discussion of the experimental
facility including the research engine, combustion diagnostic instrumentation, and the optical
system. Chapter 4 will present the engine operating conditions covered in the project,
including the basis for their selection and the measurements of bulk residual gas fraction at
7each condition. Chapter 5 will discuss the development of the imaging technique,
particularly the selection criteria for the hardware and processing steps and subsequent
performance of the data images. Chapter 6 will contain the residual gas mixing data derived
from the PLIF images, with discussion. Finally, chapter 7 contains project summary,
conclusions and recommendations.
8
2. Background
2.1. Residual Gas Effects on Combustion
Recycled exhaust gas has a substantial effect on combustion processes by acting as a
diluent, meaning that it does not participate in the oxidation of the fuel but is present and
absorbing the released energy in a quantity significant enough to reduce flame speed and gas
temperature [2]. Decreasing flame front speed inherently lengthens the time to reach 10, 50,
and 90% mass-fraction burned levels, extending combustion reactions further into the
expansion stroke. If the engine control system is not able to adjust other parameters
properly, residual gas dilution can slow the burning rate to a point where partial-burn and
misfire cycles emerge with severe penalties on emissions and performance. The
temperature-mitigating effect of residual gas is well-known as a strategy for reducing oxides
of nitrogen (NOx) production in internal combustion engines.
2.1.1. Combustion Thermodynamics
Residual gas in a spark-ignition engine running at a stoichiometric air/fuel ratio is
composed predominantly of N2, CO2, H2O and O2. Engines that operate fuel-rich of
stoichiometry, such as small air-cooled utility engines, will see significant CO and H2 and
very little remaining O2 in the residual gas. In most SI engines, pollutant species such as
NOx and unburned hydrocarbon compounds (HC) normally sum to 1% or less by volume [1].
9Based on this composition, it can be seen that when added to a mixture of vaporized fuel and
air, residual gas will lower the mass-specific heating value of the mixture. For constant-
volume combustion, the first law of thermodynamics can be expressed as
reactants products ad f( , ) = ( , )i iU T p U T p (2.1)
where Tad is called the adiabatic flame temperature and is easily calculated from a balanced
reaction equation by assuming adiabatic conditions, ideal gas behavior, and no dissociation
of reactants or products into minor species [4]. These assumptions make exact calculations
difficult but the trend of in-cylinder flame temperature vs. initial reactant composition
becomes clear. Residual gas species reduce the total enthalpy (formation plus sensible) of
the reactants, which is related to the initial internal energy by the universal gas constant, and
thus reduce the flame temperature from that of undiluted air/fuel mixtures.
2.1.2. Flame Speed Effects
The effect of reducing adiabatic flame temperature is observed in reduced burning
velocity. Combustion in an SI engine occurs via a turbulent, thin-sheet wrinkled flame
structure, which, despite being inherently complex is locally modeled closely by laminar
flame propagation rates. The laminar flame speed, SL has been measured [24], and for
conventional hydrocarbon fuels has been found to obey the power law equation:
10
,00 0
uL L
T pS ST p
α β
=
(2.2)
where the reference values are standard temperature and pressure and SL,0, α and β are
tabulated constants for particular combinations of fuel and equivalence ratio. The term Tu
represents the unburned gas temperature just ahead of the reaction zone in the flame front.
Rhodes and Keck [3] studied gasoline combustion with controlled residual concentration in a
constant-volume bomb experiment and quantified a laminar flame speed correction factor for
Equation (2.2) given the inclusion of a residual gas fraction in the reaction, based on the data
of figure 2.1:
0.77( ) ( 0)(1 2.06 )L r L r rS x S x x= = − (2.3)
Decreasing the flame temperature and velocity represents a significant challenge to
maintaining appropriate engine performance. If, for whatever reason, reactant preheating
temperatures fall below 1900 K, flame velocity will be at or near the partial-burn and misfire
lower limit [5]. This situation might typically arise if the exhaust valve opens prior to
completion of flame propagation, or if the flame is prematurely extinguished [1]. Partial
burn and misfire are extreme symptoms of cycle-to-cycle variation (CCV) in engine power
output. Besides contributing to unwanted engine roughness characteristics, the incomplete
combustion of the fuel charge represents a very significant emission of HC pollutants.
11
Figure 2.1. Experimental measurements of gasoline laminar flame speed in exhaust gas-diluted mixtures relative to undiluted mixtures, Su(0), for a range of diluent fractions, equivalence ratios and initial bomb pressures. Reprinted from [3].
2.1.3. Oxides of Nitrogen Formation
Another major consequence of the dilution effect of residual gas is reduced NOx
formation. NOx is a primary ingredient in photochemical smog found in the lower
atmosphere mainly above major cities. It also is known to contribute to acid rain. NOx is
also regrettably known for being somewhat inextricably linked with engine performance and
efficiency. Rate equations for the formation of NOx are non-linear functions of time,
elevated temperature and availability of nitrogen and oxygen molecules. Peak NOx
formation at optimal combustion phasing occurs close to stoichiometric air/fuel ratio, which
also represents the operating point for peak engine stability, power output and efficiency [4].
122.1.4. Cycle-to-Cycle Variations
Increased residual fractions are expected to locally affect small-scale mixture
homogeneity, which describes imperfect distribution of fuel vapor within the air and residual
charge. It is assumed that low to moderate spatial inhomogeneity will affect combustion
only during the earliest stages near the discharge of the spark plug and the formation of a
flame kernel. The scales of non-uniformity are larger or of the same order of the enflamed
volume during these critical early instants. As the flame front area grows much larger, the
effect of inhomogeneity is averaged out in a global sense [7, 8].
The variation of air/fuel ratio and residual dilution in the vicinity of the spark gap has
an important effect on cycle-to-cycle variations (CCV) in SI engines. Local mixtures outside
the ignition limit or too dilute to rapidly transition into a fully developed turbulent flame are
common causes of misfire and high CCV [1]. In their literature review of cyclic variation,
Ozdor et al. [6] summarized several studies of mixture inhomogeneity on flame development.
They point to a general uncertainty in applicable length scales of non-uniformities, but to a
demonstrated effect of controlled in-cylinder turbulence (particularly swirling motion) at
time of spark on reducing CCV. At the time of writing (1994), they point out that none of
the dozens of papers reviewed were able to quantify the impact of spatial inhomogeneity of
residual gas on CCV.
13
2.2. Bulk Residual Gas Fraction Measurement
In this project, residual gas mixing quantifications will be performed for varying
levels of residual gas fraction. This quantity, denoted yr, is defined as the mass of burned
exhaust gases carried over from the previous cycle’s combustion process relative to the total
cylinder mass. Like most other in-cylinder quantities, yr is subject to cycle-by-cycle
variation in magnitude. However, cycle-averaged values can be measured using in-cylinder
gas sampling as will be discussed in this section.
2.2.1. Measurement Principle
The exhaust gas emissions analyzer bench has become a standard engine test cell
instrument and typically provides concentration measurements of CO2, CO, O2, NO and HC
present in a stream of exhaust gas. Given this measurement capability, the most direct way
of quantifying total cylinder residual gas fraction is by the relation:
%( )%( )
CO2
CO2
compr
exh
xx
x= (2.5)
which defines a ratio of mole fractions of CO2 in the cylinder during the compression stroke
(after IVC) and the exhaust system downstream of the engine, typically after passing through
a mixing volume. It is important that this calculation be made on a “wet basis,” where the
absence of water vapor in NDIR CO2 analyzers is accounted for. Water is always condensed
14out of the exhaust sample lines since it is damaging to instruments. There are a few
techniques for correction and they typically involve knowledge of fuel chemistry, CO2 and
CO “dry basis” readings and intake air relative humidity [1].
2.2.2. Sampling Valves
Extracting an emissions analyzer sample during the compression stroke from the
closed cylinder is most directly performed with a category of hardware known as the fast-
acting sampling valve. Sampling valves have been employed as early as 1927 to aid the
study of chemical and physical processes in engine combustion.
Zhao and Ladommatos [14] document a more comprehensive summary of valve
designs employed in the engine literature. Most sampling valves covered were either of the
outward-opening poppet type or inward-opening needle type. Needle valves hold advantages
of smaller tip diameters, which can be advantageous in space-confined combustion chamber
surfaces, and also a lack of physical intrusion into the combustion chamber volume. Poppet
valves benefit from better sealing performance, aided by combustion pressures and potential
for smaller crevice volumes via flush-mount machining. It is proposed by the authors that
needle valve sampling volumes will be slightly larger in reach across the combustion
chamber.
Although mechanical and electro-hydraulic sampling valves have been used for
engine studies in the past, the most popular actuation mechanism is electromagnetic force.
Typically driven by a linear solenoid, this design must feature a high traction force to
counteract a strong return spring used for valve sealing and high armature acceleration for
15minimum lift duration [11]. Utilization of programmable research/calibration-type digital
engine control systems has greatly improved control of valve response. Additionally,
monitoring the valve stem lift with an inductive proximity sensor in the back side of the
valve body can provide necessary feedback for exact location of the valve window [12].
2.2.3. Sampling Valve Operation
For sampling of residual gas mixtures, the ignition system should be synchronized to
shut off during the cycle of valve actuation to prevent alteration of the residual concentration.
Monitoring the effect of skip-firing the engine is important in controlling the quality of the
analyzed residual gas mixture. It is expected that after the misfire of the sampled cycle, the
following cycle will be strong due to the residual gas being composed of additional unburned
fuel/air. It is necessary to ensure that the next sampled cycle follows a cycle that is
representative of the steady-state engine performance. One example from the literature is
that Hinze & Miles, in [7], found that the third cycle following the skip-fired cycle had an
average IMEP equal to the steady 100-cycle average for a 32 kPa MAP, 800 RPM condition.
For residual fraction measurement, sampling valve opening frequency must be optimized for
maximum sample gas flow rate and minimum deviation of sampled cycle characteristics
from steady-state conditions.
16
Figure 2.2. Sample cylinder pressure data for in-cylinder sampling in a small 2-stroke engine, with valve lift duration measured by an inductive proximity sensor shown. Reprinted from [12].
One other concern with global residual fraction measurements with fast-acting
sampling valves is that the volume of sampled gas must be representative of the total cylinder
charge. In designing the UW/ERC poppet-type sampling valve in [15], Foudray referenced
sources that indicated that a minimum of 10% to 25% of cylinder volume is adequate to
characterize cylinder composition, depending on degree of stratification. Although that
research was focused on 2-stroke cycle engine exhaust scavenging, the same criteria are
believed to hold for the 4-stroke cycle engine. Using a bellows flow meter, Foudray
estimated a sampling mass flow to be within a range of 33% to 66% of per-cycle cylinder
mass. Leakage was measured to be approximately 3% of the sample flow rate and neglected
in calculations.
17
2.3. One-Dimensional Studies of Residual Gas
Raman scattering has been used for many years to provide in-cylinder temporally-
resolved measurements in IC engines. Three papers are reviewed here where this one-
dimensional optical technique has been used to characterize residual gas participation in SI
engine flows.
Line spectroscopy studies hold advantages over two-dimensional imaging in the
reduced impact of optical access and the ability in many cases to track individual chemical
species without the use of tracers. They are inherently limited by their one-dimensional
nature and within that, a limited spatial resolution.
2.3.1. Early Work
Lebel and Cottereau in [20] performed an early study of residual gas effects on SI
combustion. They measured simultaneous CO2 concentration and N2 temperature using a
Coherent Anti-Stokes Raman Scattering (CARS) setup, with a fixed measurement region 1
cm long and 100 µm in diameter. CO2 was chosen to track residual gas, while charge
temperature was monitored to ensure that same-cycle burned gases in the firing engine were
not present in the measurement region. Laser beam intensity referencing was used to allow
comparison of single-shot measurements. Correlations were reported, at a single operating
condition, between [CO2] and temperature, cycle peak cylinder pressure (PP) and location of
peak pressure (LPP) at instants before and after ignition and two locations near and far from
the spark plug.
18Very poor correlation was found between [CO2] and PP/LPP in measurements taken 1
mm from the spark plug and 5° bTDC (considered end of ignition delay). Since this is
counter-intuitive, the authors conclude that, given their limited measurement region, it
indicates that the residual gas is not perfectly mixed at the end of the compression stroke.
The only meaningful correlation reported in this paper is between increasing [CO2] and
increasing T (figure 2.3), which is somewhat obvious given the charge heating property of
residual gas. As local temperature readings did not correlate with pressure data, this would
reinforce the statement that residual gases (and thus local charge temperatures) are stratified
late in the compression stroke. Direct correlations of [CO2] with PP/LPP yielded coefficients
from -0.2 to 0.2, limiting the authors to very basic conclusions for effects of local residual
gas concentrations on engine performance with this technique.
Figure 2.3. Correlation of measured [CO2] to local N2 temperature using CARS. The plot on the left is for data acquired at 30° bTDC with a correlation coefficient of 0.486. The plot on the right is at 5° bTDC with a correlation of 0.420. Reprinted from [20].
192.3.2. Recent Work
Hinze and Miles at Sandia National Laboratories performed two subsequent line-
imaging studies of residual gas mixing [7, 8], developing a detailed statistical quantification
for mean and fluctuating inhomogeneity components. Both studies utilized a laser
measurement volume in an axially centered position, in which CO2, H2O, N2, O2 and C3H8
concentrations were recorded. Binning on the CCD array divided the volume into individual
adjacent measurement points which established the spatial resolution. Data was presented in
15 CAD increments from start of intake to TDC compression. Homogenous propane/air
mixtures were supplied at stoichiometric conditions. Neither paper presents engine
performance data.
Figure 2.4. Experimental setup for Raman scattering measurements in a modern 4-valve pent-roof combustion chamber. Reprinted from [8].
20Ensemble-averaged measurements were taken to describe mean stratification of fresh
charge and residual gas, while 500-cycle single-shot images were analyzed to establish a
cycle-to-cycle fluctuating component. These data were used to generate spatial covariance
functions of species mole fractions (based on the adjacent measurement points), which were
broken down into fluctuation components coming from system noise, turbulence, and bulk
composition. These covariance functions, once developed, could be used to extract integral
length scales of local residual gas fraction fluctuation (the scale over which turbulent
fluctuations remain correlated.)
In their first paper [7], Miles and Hinze utilized a side-valve, side-spark optical
engine to test this technique at the same engine operating conditions in two bulk flowfields –
a semi-quiescent condition and a high-swirl condition. The measurement volume was 11 mm
long and 0.49 mm in diameter, divided into 12 measurement points. The quiescent flow was
shown to homogenize rapidly, with fluctuations in residual gas concentration nearly
eliminated by 150° bTDC. For the swirling flow, the measurement volume was radially
traversed away from the centerline to two additional measurement regions. Gradients were
observed throughout the cycle in the mean concentration data between these volumes which
suggested a bulk charge stratification which persisted throughout the compression stroke.
Rms fluctuations in the mixture composition at spark time were 5 times higher in the swirling
condition (5% vs. 1% for quiescent at -15 CAD.) Mixing length scales for both conditions
were found to vary from 2 to 5 mm.
In the second paper [8], Hinze and Miles moved to a more conventional pent-roof, 4-
valve cylinder head for their measurements and chose to focus on a single engine condition
representative of idle. Figure 2.5 shows the reported development of the ensemble-averaged
21residual gas fraction during the engine cycle. In this experiment, the measurement volume
was 14.5 mm long and 0.27 mm in diameter divided into 16 sub-regions, improving the
spatial resolution by nearly a factor of two. During the intake stroke, the authors were able to
track residual gas backflow into the intake and a later period where all the residual gas has
been re-inducted away from the measurement volume. The largest gradients in the
measurement volume occurred at BDC, as shown in Figure 2.6, with significant gradient
breakdown during compression similar to the first project. Length scales encountered at -180
CAD were on the order of 1 cm. Rms fluctuation (1%) and mixing length scale range (2-4
mm) at spark time were comparable to the previous experimental computations.
Figure 2.5. Residual gas mole fraction vs. crank angle, based on ensemble-averaged concentration measurements of various species. Reprinted from [8].
22
Figure 2.6. Levels of variance in data for ensemble-averaged mean residual gas mole fraction given in figure 2.5. Reprinted from [8].
2.4. Planar Laser-Induced Fluorescence
Planar laser-induced fluorescence (PLIF) is an increasingly popular advanced
combustion diagnostic. PLIF has the ability to provide quantitative two-dimensional
measurements in single-phase or multi-phase flows with exceptional spatial and temporal
resolution. A general summary of a PLIF measurement system is a high-energy, pulsed laser
sheet propagating through a flowfield containing a suitable fluorescent tracer species
resulting in absorption and subsequent emission of photons at a characteristic wavelength of
the tracer molecules. With a process time response on the order of nanoseconds, individual
laser shots can be captured by a CCD camera for correction and analysis.
23Detailed discussion of PLIF theory has been presented in the literature [13, 15] and
will not be repeated here. Instead, a summary of the important characteristics of the system
components used in this project are covered, including laser source, camera, and tracer
chemical.
2.4.1. Laser Source
The traditional laser source for PLIF work in engines is the Nd:YAG laser, which
offers high-power laser pulses at four harmonic wavelengths, 1064 nm, 532 nm, 354 nm and
266 nm. Laser pulses are delivered at an optimal repetition rate, most commonly 10 Hz.
Individual pulses are on the order of 8 ns duration with maximum energies exceeding 100
mJ. Nd:YAG lasers can operate with external triggering and can thus be synchronized with
engine events, although the low repetition rate typically precludes sequential measurements
in the engine cycle. Pulsed laser operation requires attention to shot-to-shot variation in laser
beam intensity and profile when making quantitative measurements.
2.4.2. Tracer Chemical Selection
Since neither air nor iso-octane fluoresce under the range of wavelengths supplied by
the Nd:YAG laser, a tracer chemical is doped into the intake charge at a controlled
concentration. Tracer addition can occur by either on-the-fly seeding of the intake air or by
pre-mixing in solution with the fuel, depending on the targeted measurement. Maximum
tracer concentration must yield maximum fluorescence signal without significant laser power
24attenuation or influence on combustion performance. The most popular class of tracers for
combustion PLIF is the di-ketone group, and the preferred match for iso-octane research is 3-
pentanone, based on its closely-related distillation curve. Tracer-matching is far more
important in multi-phase PLIF where evaporation rates must be matched than in pre-
vaporized homogenous charge studies.
1.0
0.8
0.6
0.4
0.2
0.0Rel
ativ
e A
bsor
ptio
n, F
luor
esce
nce
500450400350300250 λ (nm)
Absorption Fluorescence
Optical Properties of 3-Pentanone
Figure 2.7. Absorption and emission properties of 3-pentanone in LIF applications [17].
The excitation wavelengths for di-ketones fall in the ultraviolet, with an absorption
range of 225-320 nm [17]. Thurber et al. performed important studies on the temperature
[18] and pressure [19] dependence of acetone fluorescence at various excitation wavelengths.
It was shown that temperature dependence is practically eliminated on the range of 300-700
K using 289 nm. Likewise, an optimal wavelength for neglecting pressure effects is shown
25to be 308 nm. Making the extension of the acetone behavior to 3-pentanone, tuning the laser
wavelength to a value near 289 nm is highly beneficial in quantifying engine flows which are
at all temperature-stratified.
Figure 2.8. Measured temperature dependency of LIF signal of acetone at atmospheric pressure, normalized to room temperature condition. Reprinted from [18].
2.4.3. Camera
The di-ketone tracer group emits photons in a broadband range of 350-550 nm [17].
This visible light is best collected by a high-resolution scientific-grade CCD camera.
Charge-coupled devices contain a photo-sensitive pixel array, which when impacted by
photons, convert the photon energy to electron charge potentials with a quantum efficiency
26that is a property of the device. The individual pixel charges are read out sequentially into a
registry where they are amplified and digitized for computer processing [14].
There are four sources of noise important in making quantitative measurements with
CCD images: dark, read, pattern and shot noise. Dark noise arises from thermal generation
of electrons in the array and is limited with cooled (thermo-electric or cryogenic) CCD chips.
Read noise is a property of the array readout circuit and the programmed readout rate. Fixed
pattern noise can be traced from sources on either the CCD chip or the imaging subject, and
is unique in this discussion in that it can be eliminated with standard background and flatfield
image correction. Shot noise is typically the limiting noise element in high-fidelity CCD
imaging such as found in PLIF studies. Shot noise is completely independent of the CCD
type and arises from the probabilistic nature of photon impingement on the pixels. The shot-
noise limited signal-to-noise ratio is equal to the square root of the number of photons
incident per CCD pixel, based on Poisson statistics [13].
2.5. PLIF Measurements in Engines
As mentioned in the previous section, planar laser-induced fluorescence is a powerful
IC engine diagnostic tool due to its two-dimensional nature and superior spatial and temporal
resolution. Previous studies at the UW/ERC have achieved sufficient spatial resolution to
calculate scalar dissipation and used it to quantify the degree of mixedness in stratified DISI
flows [15, 16]. Additionally, using two high-shuttering speed intensified CCD cameras,
Rothamer [13] was able to simultaneously image unburned and burned mixtures to quantify
27flame-front equivalence ratio in a stratified-charge DISI engine. For the current study of
residual gas mixing in engines, it is important to first present basic techniques for quantifying
spatial charge inhomogeneity from PLIF intensity data and then introduce the limited
literature on residual gas studies using this technique.
2.5.1. 2-d Quantification of SI Engine Flow Inhomogeneity
Baritaud and Heinze conducted an early application of PLIF in an SI engine at the
Institut Français du Pétrole (IFP) in 1992 [21]. The subject of their experiment was
quantification of the development of fuel/air stratification in a PFI engine. A major portion
of this paper discusses the statistical means for describing charge inhomogeneity in PLIF
images.
The authors define a total standard deviation for a set of N single-shot images, based
on the idea that a single image’s inhomogeneity can be quantified by its standard deviation
about the spatial mean (σn). By ensemble-averaging this value after normalizing each by the
mean image intensity ( nI ), the influence of the pulse-to-pulse variation in laser intensity is
removed:
1
1 Nn
totnnN I
σσ=
= ∑ (2.6)
28The total standard deviation σtot is presented as a relative value, since absolute measures of
charge inhomogeneity cannot be correlated with individual engine cycles without bias error
from the pulse energy variations.
To extract the maximum potential information from the data images, the simple
standard deviation was broken down into fine-scale and large-scale contributions by
employing a basic spatial Fourier transform. First, a 3x3 smoothing procedure was twice
performed on the I x J pixel data image, with the resulting smooth field termed Φ(In(i,j)).
The large scale contribution to the inhomogeneity, arising from gradients in large-scale
structures in each data image n is:
( )( )( )2
n,lf,
1 , nni j
I i j IIJ
σ = Φ −∑ (2.7)
After ensemble averaging, the relative large scale variation is:
n,lf
1
1 N
LFniN I
σσ
=
= ∑ (2.8)
Likewise, small-scale fluctuations in each image can be tracked by examining the fluctuation
in the raw image intensities relative to the smoothed image:
( )( ) ( )( )2
n,hf,
1 , ,n ni j
I i j I i jIJ
σ = Φ −∑ (2.9)
29
This value is again ensemble averaged on a normalized basis:
n,hf
1
1 N
HFniN I
σσ
=
= ∑ (2.10)
If the ensemble-averaged pixel intensity field ( ),nI i j is used in place of the single-
image data in equation (2.9), a “hybrid” fluctuation arises which can describe the variation of
the large-scale inhomogeneities from cycle-to-cycle:
( )( ) ( )( )2
n,cyc,
1 , ,n ni j
I i j I i jIJ
σ = Φ −∑ (2.11)
Importantly, ( ),nI i j is biased by laser pulse variations, which limited its usefulness in this
initial study. Finally, this value can also be ensemble-averaged to a relative basis.
n,CCV
1
1 N
cycniN I
σσ
=
= ∑ (2.12)
The authors indicate that it is difficult using metrics such as σtot, σLF, σHF, and σcyc to
separate single-cycle inhomogeneity effects from cycle-to-cycle variations captured in the
data images.
302.5.2. Direct Visualization of Residual Gas
Direct visualization of combustion residual species such as H2O and NO2 is possible,
although challenging, with PLIF. In [22], Johansson et al. used water as a residual tracer,
which required use of strategy known as “2-photon” LIF, which is unique in its requirement
for an interaction of two photons at 248 nm to detect the water molecule. This approach
yields inherently lower signal levels than a single-photon LIF study like those done on fuel
tracers. Additionally, the authors were unable to provide a homogeneous distribution of
water molecules at a known concentration, which prevented signal calibration and therefore
quantification of the H2O intensity data.
The objective of this study was to observe the influence of residual gases on cycle-by-
cycle variations in engine power output. The optical access system required a vertical laser
sheet only 6 mm in height. The laser sheet centerline was passed 4.5 mm below the spark
plug and water concentration images were obtained for a range of engine loads (based on
intake MAP.) Cylinder pressure-derived heat release data were compiled to correlate
residual gas levels with initiation and propagation of SI combustion. The engine was
operated on homogeneous natural gas at 700 rpm, and the images were acquired 1° before
spark time. Imaging was performed with an intensified CCD gated to 100 ns exposure.
Resulting noise levels due to low signal strength and maximum intensifier gain were roughly
20%.
The conclusions made on ensemble-averaged water intensity data were fairly basic,
essentially confirming predicted trends in increasing residual gas concentration near the
spark plug with decreasing load. When normalized by the equivalence ratio of the data set,
31the duration of 0-0.5% heat release was shown to correlate well with the CCV of the water
concentration normalized by load point. This is thought to strengthen the argument that
fluctuation in residual gas near the spark plug is a major contributor to CCV in SI engines.
Unfortunately, quantitative values of the observed fluctuations were not available.
Figure 2.9. Mean H2O PLIF signal trend with intake MAP. Reprinted from [22].
Figure 2.10. Cyclic variation in H2O PLIF signal for increasing load. Reprinted from [22].
32Johansson et al. also attempted correlations with pressure and heat release data for the
single-cycle measurements. Although laser power intensity fluctuations were corrected in
this experiment by shot-resolved power meter readings, the poor SNR and small imaging
region created a large amount of scatter in these correlations. The correlation between
duration of 0-0.5% HR and [H2O] was optimized for radius of ROI within the image. At a
low-load condition, a peak 60% correlation was shown at a radius of 2.9 mm. This
correlation degraded with decreasing residual fraction, which was satisfactory since the
magnitude of the fluctuations relative to the image noise was expected to also decrease.
Figure 2.11. Correlation of load-normalized residual gas fluctuation to CCV of 0-0.5% heat release duration using H2O PLIF. Reprinted from [22].
332.5.3. Negative Visualization of Residual Gas
Residual gas can also be tracked with PLIF images by examining the negative of the
intensity field provided by a homogeneous air/fuel/tracer charge. Following up on the early
work described in Section 2.5.1, Deschamps and Baritaud at IFP [23] performed a negative-
PLIF visualization of burned gas distribution in an SI engine. Because this project sought to
observe separately the distributions provided by external EGR as well as internal residual
gas, the upstream intake air was chosen to be seeded with biacetyl. Air seeding via a
carburetor imparted more uncertainties and challenges than premixed fuel solutions. A 25-
mm wide horizontal laser sheet was passed 4 mm below the spark plug parallel to the ridge
of the cylinder head’s pent roof.
For the internal residual gas study, five engine effects were examined: fuel type, fuel
distribution, tumble level, spark plug location and volumetric efficiency. Mean image
intensity profiles in the direction of the sheet across the pent roof were examined, but only in
a qualitative manner.
The enhanced tumble experiment was conducted with propane to remove fuel
stratification effects. With enhanced tumble, mixing along the roof ridge direction was
observed to be more difficult during the intake stroke than during compression, where it is
assumed that the tumble motion normal to the laser sheet is broken down by turbulence.
However, by the end of compression, the enhanced tumble condition shows both a higher
concentration and flatter linear distribution than the standard case. The increased
concentration suggested that lower tumble levels leave a portion of the residual gas trapped
in the bottom of the combustion chamber. Increased charge motion then not only helps
34distribute the residual gas vertically in the combustion chamber, but laterally to create a more
homogenous mixture. Another property of enhanced tumble operation proposed by the
authors is improved SI combustion efficiency which often correlates with increased intake
MAP, reducing bulk residual fraction.
Figure 2.12. Comparison of flowfield effect on residual gas distribution as measured by negative-PLIF. Both conditions are 1200 RPM, ηvol = 0.6. Reprinted from [23].
35
Figure 2.13. Mean residual gas distribution across combustion chamber (direction along pent-roof axis) for two bulk flowfield conditions. Image data taken with negative-PLIF at spark timing (27° bTDC). 1200 RPM, ηvol = 0.6. Reprinted from [23].
With varying volumetric efficiencies, changes in the distribution of residual gas in the
data images taken at -30 CAD are explained primarily through assumed changes and
asymmetries in the intake port flows, imparting different bulk flowfields. The residual gas
concentration in the image ROI decreases with increasing volumetric efficiency as expected.
Deschamps and Baritaud conclude in this section of the paper that the interacting
parameters they studied were too complex for control of residual gas distribution in an
engine, and suggest choosing external EGR as a delivery mechanism instead. The remainder
of the paper discusses EGR effects in a similar manner, only with the addition of emissions
work.
36
3. Experimental Setup
3.1. Single-Cylinder Research Engine
This project was performed on a single-cylinder, optically-accessible research engine
mated to a regenerative AC dynamometer. For improved control of residual gas dilution, a
dual overhead cam cylinder head was integrated. Calibrated air flow was delivered from a
critical flow orifice rack and control of air-assisted fuel injection and spark timing was
provided by a commercial engine control and calibration system.
3.1.1. Base Engine
The base engine block for this project is the GM Research “Triptane Base 4”,
originally designed for alternative fuels research in the late 1950’s. It is of two-part
construction, with cast iron crankcase and cylinder barrel. The crankcase contains a
balancing shaft and a single fixed two-lobe camshaft for pushrod actuation of an overhead-
valve system. The cylinder barrel has been re-lined recently and contains a liquid coolant
jacket. The firedeck surface includes a groove for an o-ring seal with the cylinder head
spacer ring. The major fixed dimensions of the Triptane engine are provided in table 3.1.
37Bore [mm] 92.4
Stroke [mm] 76.2
Displacement [cc] 511
Connecting Rod Length [mm] 144.8
Exhaust Valve Open [CAD] 115
Exhaust Valve Close [CAD] 365
Intake Valve Open [CAD] 349
Intake Valve Close [CAD] -180
Table 3.1. Fixed internal dimensions of GM-Triptane engine. Valve timings are for internal single camshaft used for OHV engine operation.
3.1.2. Optical Access
The major feature of the Triptane engine is the Bowditch-type optical-access
piston/cylinder geometry. The extended-height cylinder barrel accommodates the aluminum
Bowditch piston and allows for mounting of the 45° mirror, which passes through the
cylinder barrel and allows for a periscope view of the combustion chamber via a transparent
piston cap.
The piston cap is fabricated of aluminum and is fastened to the Bowditch piston with
an internally threaded steel retaining ring. The cap contains an axially-centered 47 mm-
diameter 10 mm-thick sapphire window. The fit of the cap into the retaining ring is indexed
and the assembly locks with a small screw-fastened key. Cylinder sealing for the window-
cap and cap-retainer surfaces is performed with Viton O-rings.
38The piston rings used for this experiment are common to optical engine studies and
unique in that they operate without a lubricating oil film on the cylinder wall. Custom
manufactured by the C. Lee Cook Company based on dry gas compression technology, they
are composed of a spring-loaded oil control ring and a bronze-impregnated Nylon rider ring
below the mirror and an additional rider ring above the mirror. The single compression ring
is of a butt-cut design and is made of Vespel. The compression ring groove is located in the
steel retaining ring at a maximum height that does not cross the firedeck surface gap.
The final component of the optical access system is the steel spacer ring fastened
between the block and head, with an inside diameter matching the engine bore. The 25 mm-
tall ring contains four equally-sized window ports. Two ports contain 16.5 mm-thick quartz
windows for laser sheet propagation through the combustion chamber. The other two ports
are utilized for combustion diagnostics described in Section 3.2. Although the piston cap
crosses the plane of the windows near TDC, the compression ring stays below the spacer ring
throughout the cycle.
3.1.3. Cylinder Head and Combustion Chamber
In the interest of generating a range of residual gas fractions for this experiment, a
means of independent cam phasing was required. Since the base engine’s OHV camshaft is
of fixed geometry and difficult to access within the crankcase, a dual overhead camshaft
(DOHC) single-cylinder research cylinder head of near-identical bore was obtained from GM
Research Labs. Originally designed and used for gasoline direct- injection (GDI) studies, the
39cylinder head contains intake and exhaust cams that are independently phase adjustable via
taper-split drive pulleys.
As an additional lab improvement, the DOHC cylinder head provided a combustion
chamber geometry that is consistent with modern multi-valve SI engines. The pent-roof
cylinder head contains two intake valves and two exhaust valves with an axially-centered
M14 spark plug. The GDI injector bore (tangential, wall-guided orientation) was plugged in
this project. One intake port is cast in a helical approach for swirl generation, which can be
varied with a butterfly throttle on this port alone. This throttle was left full-open for this
project to generate maximum flowfield turbulence.
The major consideration in the integration of this cylinder head was its effect on
compression ratio. The DOHC head’s pent-roof occupies a 49.5 cc volume, whereas the
Triptane engine’s traditional OHV head is designed with a flat “pancake” roof. The
requirements of the optical access system prevented modification to either the spacer ring or
the Bowditch piston, so the highest-compression flat-top piston crown was used in this
project. With an OHV setup, this piston yields a CR of over 12:1 for compression ignition
studies, but with the DOHC head, we are able to obtain only 5.95:1. The upside of this
arrangement is that the engine free-spins without any valve-piston interference, allowing
infinite valve timing flexibility and reduced risk of catastrophic engine damage. During
initial testing, the low compression engine was demonstrated to operate stably at elevated
dilution conditions. The major combustion chamber dimensions for this project are
summarized in Table 3.2.
40Compression Ratio 5.95:1
Top-Ring Crevice Volume [cc] 4.13
Exhaust Valve Inner Seat Diameter [mm] 29.5
Intake Valve Inner Seat Diameter [mm] 34.4
Table 3.2. Major combustion chamber dimensions for GM-Triptane engine with DOHC adjustable-cam cylinder head.
Mating the 116 mm square bolt pattern of the DOHC head to the 117x86 mm
rectangular pattern of the Triptane firedeck required fabrication of four hardened steel mating
blocks for an offset 2-screw fastening method. The deck surface of the DOHC head was
lowered by 0.030” to accommodate a replaceable graphite head gasket to seal against the
spacer ring. As mentioned, the spacer ring sealed to the firedeck with a Viton O-ring.
Additional lab modifications for the DOHC setup are described in subsequent sections.
3.1.4. Valvetrain Timing System
The major feature of the DOHC cylinder head is its independent cam phasing
adjustment. This is accomplished with indexed taper-split pulleys on each camshaft. The
inner flange half is permanently fastened and keyed to the camshaft along with a backing
graduated degree wheel. The outer pulley half is fastened through radial slots against the
internal taper. The slots were machined so as to allow access to any practical cam phasing
arrangement.
41Prior to this project, an external, belt-driven half-speed shaft was added to the
laboratory to provide a timing signal for engine control software. For the DOHC
arrangement, this half-speed shaft was linked to the camshaft pulleys at a 1:1 ratio via a
Gates 1”-width 52”-length 3/8”-pitch trapezoidal tooth timing belt. An automotive OEM 1”
torsional belt tensioner was added to the half-speed shaft assembly for final belt tensioning.
Despite its apparent complexity, this dual-belt timing system eliminated the need for a very
long single timing belt and camshaft extensions, as well as providing required modularity
with the OHV engine setup.
Figure 3.1. Valvetrain timing layout for DOHC cylinder head.
42The degree wheels for each camshaft were first calibrated to valve open/close events.
The graduations on the degree wheels are to be read against markers bolted to the valve cover
with the engine rolled to TDC compression. This baseline point was used because it is
assumed that both cams will always be on the base circle at this time. Using a 0.006” valve
lift threshold of open/close timing and proper oil pressurization of the hydraulic valve lifters,
the engine is manually rolled over with a crankshaft degree wheel, watching a dial indicator
on the appropriate valve stem. This procedure does suffer from significant degree value
uncertainty of valve timing, due to the effect of engine rotational speed on hydraulic lifter
response. However, engine operation data have shown the timing system to be highly
repeatable in terms of engine-out performance.
A related procedure was developed for selecting valve timings during the experiment.
The engine was rolled to TDC compression, and the cam taper was broken with the timing
belt still taught. The pulley half is backed off slightly (not completely off) to allow relative
rotation between the two halves. The flange half contains a hex nut that can be used to turn
the camshaft to a different degree wheel position relative to the timing belt, which remains
locked to the crankshaft. The pulley was then re-fastened on the taper. It is important in this
procedure that the engine always is manually rotated in its proper counter-clockwise
direction to avoid the multi-degree backlash in the belt tensioner and that the timing belt
tension is preserved throughout the process.
433.1.5. Dynamometer
The crankshaft of the Triptane engine is connected, via a flywheel, to a three-phase
440 VAC General Electric dynamometer. The control system is a Reliance Electric Max Pak
Plus VS Drive box. The dyno system can provide motoring or generating operation up to
1500 RPM and 30 kW load. Manual selection of engine speed is performed with a rheostat
and feedback control loop, which is periodically optimized.
3.1.6. Engine Fluid Systems
The Triptane engine and DOHC cylinder head are liquid cooled in a conventional
block-thru-head loop, linked by external hoses. A 50/50 water-antifreeze mixture was used
for corrosion resistance. The external, motor-driven circulating pump is pressure-fed by a
standing column reservoir. An electric water heater is operated continuously during
experiments to bring the engine to operating temperature and a feedback-controlled solenoid
valve is used to meter cold building water through a copper counter-flow heat exchanger to
maintain a system set point during operation. Previous optical engine projects have
determined the optimum coolant temperature of 68° C.
Oil pressure and flow rate was also provided by an external pump, with a commercial
in-line filter. For this project, a Triptane internal post-main bearing oil gallery was selected
to provide an external high-pressure feed to the DOHC head cam journals and valve lifters.
A low-pressure drain line was also installed from the head to the reservoir in the crankcase.
Engine oil selected for optical studies is SAE 40, for its higher viscosity and resistance to
44infiltrating the combustion chamber. With low speeds, loads and temperatures, engine oil
grade selection is not considered critical to Triptane performance.
To provide further defense against oil fouling of the optical access system, vacuum
pumps are applied to both the crankcase and the cylinder head valve cover. Crankcase
vacuum cuts down on blow-by past the oil control ring, which can quickly foul the turning
mirror, camera lens and back surface of the piston window. Valve cover vacuum was
necessary in this project to reduce oil migration past the valve stem seals, which fouls all
combustion chamber windows.
3.1.7. Engine Aspiration Systems
Intake air is metered through a critical flow orifice rack, where the upstream pressure
was varied to obtain set mass flow rates. Three orifice diameters are used to provide an
adequate range of air flow at the common engine operating speeds of 600 and 1200 RPM:
0.125”, 0.100” and 0.050”. All three orifices are calibrated with a bellows flow meter for the
range of upstream pressures providing choked flow. Mass air flow rate is then determined
from a density correction. During the experiment, the supply air (separated, filtered and
humidity-controlled central compressed air) is monitored for consistent upstream properties
with the calibration condition.
For this project, only the smallest 0.050” orifice is used, since all experiment
conditions were “throttled” or sub-atmospheric intake manifold absolute pressure (MAP).
Also for this consideration, a new pressure-tested rigid copper intake runner was fabricated to
link the DOHC head with the existing 14 gal intake air surge tank. Intake MAP is monitored
45with a Wallace & Tiernan 0.1-psi resolution absolute pressure gauge at the surge tank. An
atmospheric intake vent is opened for all transient dyno speed selection periods.
For the exhaust, a new steel runner was fabricated. Approximately 10 cm
downstream of the cylinder, the runner contains an axially-located K-type thermocouple used
to confirm thermal steady-state firing operation. Engine exhaust emissions are sampled from
the near-exit centerline of a 12 gal mixing tank located 2.25 m downstream of the ports. This
surge tank is positioned to allow for modularity with the OHV head setup and contains a
perforated tube diffuser entrance for gas mixing. Exhaust back pressure is manually
controlled with a gate valve and monitored on an absolute pressure gauge. All conditions in
this project were set to 1.0 bar absolute back pressure.
3.1.8. Fuel Delivery System
The fuel used in all measurements of this experiment is 80% iso-octane, 20% 3-
pentanone, by volume. The tracer concentration was set as being the maximum level that did
not attenuate the laser sheet intensity across the combustion chamber. Since this experiment
involved a homogeneous, pre-vaporized mixture, the tracer was assumed to faithfully track
the fuel. More thorough discussions of the co-evaporation properties of iso-octane and 3-
penatnone are found in [14] and [15], where direct-injection spray studies mandated the extra
consideration. Relevant properties of the fuel mixture are given in Table 3.3.
46 Iso-octane 20% 3-pentanone / 80% iso-octane Molecular Weight 114.23 106.16 H:C Ratio 2.25 2.20 Stoich. Air/Fuel Ratio 15.1 14.25
Table 3.3 Fuel properties for pure iso-octane and the 20% 3-pentanone tracer blend used for this experiment.
This optical project required that the intake air and vaporized fuel/tracer solution be
thoroughly pre-mixed before entering the combustion chamber. To accomplish this, the fuel
injector is mounted far (1.3 m) upstream of the intake ports. Based on previous optical
engine studies, an Orbital air-assisted fuel injection system is used to provide the highest
level of initial fuel/tracer atomization. Degree of homogeneity is analyzed in Section 4.3.
The Orbital air-assist injector is primarily used in North America in the Mercury
Marine Optimax 2-stroke outboard engine line, where it operates in a GDI arrangement. In
this system, a fuel injector draws fuel from a pressurized fuel rail and fires into a mixing
volume on the entrance to an adjacent air injector, which is within a separate 80 psi
compressed air rail. A fixed delay of 4 ms occurs for initial air/fuel mixing before the air
injector fires the mixture into the intake runner for a fixed duration of 3 ms. The critical
operation parameter with this system is that the fuel supply rail be held at 10 psid above the
compressed air rail (= 90 psi). Fuel delivery rate is varied with the initial fuel injection pulse
width.
Due to the use of pure iso-octane fuel and 3-pentanone tracer, the OEM high pressure
pump and recirculating pressure regulator system are not used for the Orbital fuel rail.
Pressurized fuel/tracer and air are instead supplied by a clean accumulator system. Mixed
47fuel/tracer solution is drawn into the Tobul piston-type accumulator (Model 4.5A20-8-5763)
by a vacuum pump. The accumulator, which contains Teflon-encapsulated seals for
chemical resistance, is then pressurized with nitrogen. After passing through a safety shut-
off valve and a 0.5 micron filter, the fuel is regulated with a Go single-stage regulator (Model
00-HO2073) to the necessary 10 psi differential pressure. An Orange Research differential
pressure gauge (Model 1516D6) is used to directly monitor this value during the experiment.
Regulated compressed air for the injector is supplied by a medical-grade cylinder.
Fuel delivery rate was calibrated before the experiment using a gravimetric technique
for varying durations of fuel injector pulsewidth. The engine control system was operated
using its internal timing generator mode set at 600 RPM. Permanent injector characterization
settings along with fuel-air delay and air injector duration were fixed in software, with only
the fuel injector duration varied. The air-assist injector rail was mounted on an atmospheric-
pressure mixing volume and drained through a ¼” tube into a capped glass beaker on an
Ohaus Scout II digital scale. Overall mass flow rate (over a six minute duration after initial
flow equilibrium) was then converted to a mass per injection value. As expected, the mass
flow rate was linear across the delivery rate range of this project (10-18 mg/inj). Gain and
offset were then entered into the engine control software.
There is substantial uncertainty in the overall air/fuel ratio with this experimental
setup. Particular sources are from the just-described injector calibration, which is not
performed in a negative-pressure environment as found in the intake system, the well-known
chamber sealing inefficiencies of the optical engine piston ring pack, uncertainties in the
intake orifice calibration and small air leaks in the intake system.
483.1.9. Engine Control System
Control of fuel injection, ignition, sampling valve and camera triggering for this
project was provided by the MotoTron commercial engine control and calibration package.
Using triggers from an interpolated crankshaft encoder and a camshaft Hall-effect encoder,
the ECU software is able to generate output signals with 1/16-CAD resolution. A major
advantage of this particular package is its built-in support for the Orbital air-assist fuel
injection system, including characterization and calibration software inputs and integrated
electronic driver circuits. Furthermore, it provides up to six spark ignition TTL signals,
which can be given independent timings and pulse widths for triggering of external systems.
For this project, Mototron engineers generously provided the laboratory with a new
programmed feature for “skip-fired” operation. When enabled, this mode provides a user-
defined number of firing cycles to occur before cutting the ignition on a single cycle. During
that skip-fired cycle, a TTL signal of user-defined timing and duration is activated for
triggering of a sampling valve or camera.
The ignition coil used for this experiment is a Mercury Marine “DFI” model, with
maximum spark energy of 150 mJ. Due to the high residual dilution levels and low
compression ratio of this project, the maximum coil dwell was used at all times. Likewise,
the AC Delco spark plug (Model 41-954) was gapped to 2.1mm, a very large amount, but
one that was proven to consistently sustain spark propagation.
49
3.2. Combustion Data Acquisition
To add relevance to the optical studies of this project, multiple combustion
diagnostics were needed. Most important is cylinder pressure data, which in addition to
allowing for optimization of performance at each running condition, allows for analysis of
cylinder heat release rate and cycle-by-cycle power variations. To quantify bulk residual gas
fraction at each running conditions, a solenoid-actuated in-cylinder sampling valve is used in
conjunction with an exhaust gas emissions analyzer. This emissions bench is also used to
describe general running trends in pollutant formation.
3.2.1. Cylinder Pressure Measurement
Cylinder pressure was measured with an AVL model QC42D-E C109 water-cooled
piezoelectric transducer. The charge output was sent to a Kistler model 5010 amplifier,
operated with a medium time constant. The amplified signal was logged on a Hi-Techniques
A/D conversion PC running REVelation software. Pressure readings were recorded in sets of
100 cycles at 0.25 CAD resolution, based on a simultaneous signal from a high-resolution
BEI optical crankshaft encoder. The pressure transducer/amplifier were calibrated using a
hydraulic dead-weight tester with an excellent resulting linearity (R2 > 99.9%).
The relative pressure signal was pegged to the intake MAP at -180 CAD in software.
The REVelation program provided instant display of an averaged trace, IMEP and COV of
the recorded data in the laboratory, but additional statistics and cycle-resolved data were only
accessible using a binary data extraction program. Extracted pressure traces for each final
50running condition were entered into a single-zone heat release code which operates in the
Engineering Equation Solver (EES) environment, for analysis of cumulative and
instantaneous heat release rate.
An important consideration in this project was the location of the AVL pressure
transducer (M14 thread) within the combustion chamber. Space constraints in the 4-valve
DOHC head prevented traditional roof access. An unused window in the optical spacer ring
was selected and a special tapped window was machined. There were initial concerns about
dynamic effects on the pressure trace at this location from the piston, which passes the
window location near TDC, placing the transducer in the top ring crevice volume.
A test was performed with the OHV head, using two pressure transducers (Figure
3.2). One was mounted in that head’s roof location and the other in the window port. Both
were logged simultaneously and compared. The window transducer followed the shape of
the roof transducer exactly, except for a small (< 5%) deviation of peak pressure near TDC.
The behavior of the cylinder wall location was deemed acceptable for this project.
51
Pressure Trace Comparison
0
500
1000
1500
2000
2500
-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90
CAD
p [k
Pa]
Roof MountWall Mount
Figure 3.2. Comparison of measured cylinder pressure traces at wall-mount location to conventional roof-mount. Motoring engine condition with OHV head, 1200 RPM.
3.2.2. Sampling Valve
Residual gas fraction levels are measured by comparing the concentrations of CO2
trapped in-cylinder before ignition to that in the combustion products in the exhaust. To
obtain the in-cylinder CO2 concentration, a solenoid-actuated sampling valve is installed in
the head/block spacer ring in a modified steel window opposite the pressure transducer. The
design details of this sampling valve are covered in [12].
The sampling valve outlets into a 1 m long ¼” Teflon line connected to an ice bath.
The condensing ¼” stainless steel tube coil removes water from the stream and two external
52coalescing filters remove impurities before passing the sampled gas stream through a 10 m
long ¼” Teflon line to the emissions bench.
The sampling valve 48 VDC driver circuit is triggered by the skip-fire MotoTron
TTL signal. Skip-firing mode is used for cylinder sampling to ensure faithful measurement
of pre-ignition trapped charge composition. Sampling valve timing and duration are
optimized for the engine running condition to provide maximum flow rate to the emissions
bench. The target flow rate is 2.5 lpm, measured by a rotameter at the bench entrance.
Figure 3.3. In-cylinder solenoid-actuated sampling valve mounted to block-head spacer ring. Teflon sampled gas line travels to an adjacent ice bath and then to the analyzer.
Before the experiment, the leakage rate past the sampling valve seat was measured
during three continuous-firing baseline engine operating conditions without opening the
53valve. A Hewlett-Packard 1-10-100 ml soap bubble flow meter was used for this experiment.
At the 600 RPM 64 kPa MAP load condition, the leakage was 1.11 ml/sec. At 600 RPM 46
kPa MAP, the leakage was reduced to 0.57 ml/sec. Finally, at 1200 RPM 50 kPa MAP, the
leakage rate was 1.03 ml/sec. Given these values and a worst-case sample flow rate of 1.5
lpm, the highest possible leakage gas concentration was 4.4%. Since most conditions were
assumed to be below this value, the sealing performance of the sampling poppet valve was
deemed acceptable.
3.2.3. Emissions Bench
A five-gas Horiba emissions analyzer was used to measure steady-state exhaust
species concentration sampled from the engine exhaust mixing tank. After exiting the
mixing tank, the sample was transported to the emissions bench by an electrically heated line.
The line was temperature-controlled to 190° C to avoid hydrocarbon and water condensation.
Adequate flow is provided by a vacuum pump in the bench and a regulated manifold tree to
the individual analyzers. Before entering the infrared analyzers, water was condensed from
the stream in a 0° C refrigerant bath.
The five analyzers were CO, CO2, O2, HC and NOx, each paired to a signal
amplifier. The CO and CO2 analyzers were calibrated through a Stec gas divider to a
second-order polynomial fit of voltage vs. concentration. The remaining three analyzers
were linear in operation and required only two-point calibrations. Table 3.4 summarizes the
Horiba emissions bench hardware.
54Gas Span Level Horiba
Analyzer # Analyzer Type Horiba Amplifier
# CO2 10.1 % AIA-23 ND-Infrared OPE-135 CO 2.56 % AIA-23 ND-Infrared OPE-115 O2 1.01 % MPA-21A Paramagnetic OPE-335 NOx 101 ppm CLA-22A Chemiluminescent CLA-22A HC 6286 ppm
(C3H8) FIA-23A Flame Ionization Detector
(FID) FIA-23A
Table 3.4. Horiba exhaust emissions analyzer bench summary.
All amplifier output signals were passed through an A/D converter card and logged
on a PC using LabView 6.0. LabView was used to automatically perform the voltage
calibration and average multiple samples for final data reporting.
NOx measurement during this experiment was precluded by the inability of the
analyzer to achieve a steady-state during the short firing duration of the optical engine. The
engine could not be fired continuously for more than five minutes, where NOx readings were
still increasing for all conditions. Therefore, NOx will not be reported in the results section.
To rapidly switch from exhaust emissions measurement to the sampling valve stream
during the experiment, the front side calibration port of the CO2 analyzer was used to receive
the line from the ice bath. In this arrangement, the low-flow sampling valve stream was
supplied exclusively to the CO2 analyzer, which was the only measurement needed. Before
and after sampling valve measurement runs, the valve plumbing system was purged with
nitrogen.
55
3.3. Optical Measurement System
The mixing of the residual gas with the fresh homogeneous fuel/air charge in the
combustion chamber of the engine was performed using planar laser-induced fluorescence
(PLIF). The measurement system consisted of a laser source, beam-transport and sheet-
forming optics and a camera. The optical system was synchronized with the engine
crankshaft to capture PLIF images at several crank angles during the compression stroke.
3.3.1. Laser Source
The laser used in this project was the Spectra-Physics GCR-170 Nd:YAG. The
fundamental 1064 nm output was frequency quadrupled to 266 nm, providing a pulse
duration of 4-5 ns and a peak energy of 90 mJ/pulse. Pulse-to-pulse energy stability is listed
as <10%. The laser was externally triggered at its design repetition rate of 10 Hz, with an
optimized delay between flash lamp and q-switch trigger signals of 186 µs.
Laser power, which could only be measured safely before the last two optical
elements ~35 cm upstream of the engine entrance, was monitored with a ScienTech Mentor
MD-10 power meter with a UV-sensitive power head. Laser energy at this point was
adjusted to 30 mJ/pulse +/- 3 mJ, although this value is an integrated time average and can
not be resolved to a pulse-by-pulse basis. The Nd:YAG laser was able to supply more
power, but damage to the engine quartz windows near the focused laser sheet prevented
higher powers.
563.3.2. Laser Optics
The 266 nm output of the Nd:YAG laser was separated from the higher harmonics by
a Pellin-Broca prism located at the exit of the laser. The visible and infrared beams were
captured by beam dumps. The 266nm beam was again turned 90° by a dichroic mirror to
traverse the length of the laser table to a second Pellin-Broca Prism for final wavelength
separation.
A dichroic mirror directed the 266 nm beam into a 1 m focal length spherical lens,
designed to focus the laser at the center of the engine bore. After the spherical lens, the beam
was vertically traversed by two right-angle prisms from laser output height to engine window
height. The traverse distance is ~10 cm and was adjusted by a micrometer translation stage.
A 100 mm positive focal length cylindrical lens was located 60 cm downstream of the
spherical lens to develop the laser sheet. Finally, a 2 in diameter dichroic mirror was used to
direct the laser sheet into the engine’s through the quartz windows in the space ring.
The optical system is presented in Figures 3.4 and 3.5 for clarity.
57
Figure 3.4. 266 nm laser pulse separation and delivery optics (plan view).
Figure 3.5. Laser sheet-forming optics setup for 266 nm PLIF imaging.
583.3.3. Camera
The primary camera used for PLIF imaging in the Triptane engine was the Roper
Scientific MicroMax. This camera lies in a category of scientific CCD cameras known as the
“frame-straddling” type and is specified with a nominal quantum efficiency of 45% at the
peak fluorescence wavelength range from Figure 2.7. The MicroMax CCD is front-side
illuminated and cooled thermoelectrically to -20°C. A twin-blade fast mechanical shutter is
used to protect the device from combustion luminosity. The CCD array measures 1300x1030
pixels with a pixel size of 6.7 µm. In this experiment, the camera was binned on-chip 6-by-6
to increase PLIF signal and shorten read-out time. The binning selection is discussed in
Section 4.2.2. The readout rate for the device is 5 MHz, with 12-bit digitization. The
MicroMax camera is not intensified.
The principal design feature of the MicroMax is a “Double-Image Feature” (DIF)
mode designed for particle-image velocimetry (PIV), which allows two separate exposures to
be captured on the CCD array in rapid succession without mechanical shuttering. This is
accomplished by “interline transfer” on-chip, which is the reason why this camera was used
in the project. In DIF mode, the mechanical shutter is pre-opened once the camera is finished
reading the previous image and the chip is actively drained of charge before the exposure
trigger arrives. The CCD is divided into alternating columns of masked and unmasked pixels
in this mode and the unmasked pixels are charged during the first exposure for a time
programmed in software as short as 1 µs. The camera then performs the interline transfer of
the charged pixels over to the masked pixels, which requires 200 ns, or some longer
programmed duration. Then, a second exposure is taken on the unmasked pixel columns.
59The second exposure has no place to be shifted to, so it must be exposed until the mechanical
shutter is closed for read-out. A diagram of DIF-mode operation is shown in Figure 3.6. The
basis for selecting the MicroMax over other high-quality CCD cameras is discussed in
Section 5.2.
In this project, the exposure time was set to 10 µs, with the second image from each
engine cycle deleted from the analysis. When pixels were binned 6-by-6, the resulting read-
out time was 0.3 sec, easily fast enough to keep up with our engine skip-firing frequency.
MicroMax images were saved on a Pentium III Windows PC operating Roper Scientific
WinView/32 v. 2.4.8 as multiple-frame 12-bit grayscale TIFF files.
The lens selected for the primary camera was an 85 mm Nikkor f/1.4 model mounted
on a 20 mm Kenko extension tube in addition to a C-mount to F-mount adapter. The
selection criteria for this lens are detailed in [15].
Figure 3.6 MicroMax camera manual summary of DIF-mode timing. Image exposure times are shown in the second line. Ready and Scan are output signals from the camera controller, Ext. Sync is the input trigger TTL, Laser Output shown is for a double-pulse laser, this experiment only uses the first pulse. Reprinted from [24].
603.3.4. Optical Triggering
The requirements of the optical triggering system were threefold: 1) To supply the
Nd:YAG laser with an uninterrupted 2-pulse trigger sequence at 10 Hz; 2) to ensure that the
camera triggers were delivered on skip-fired cycles only; and 3) to gate both the primary and
reference camera with sub-microsecond resolution to capture the 5 ns laser pulse. To
perform this, the BEI crankshaft encoder and the MotoTron engine control were each utilized
with an interface at a special TTL logic circuit.
The “clock” signal for the laser and both cameras was provided by the shaft encoder,
via a TTL counter box supplied by Mercury Marine. The A-pulse (divided by 4 to one-per-
CAD frequency) and the Z-pulse (TDC) were input into the counter box. An advance or
delay value of 0-99 CAD before or after TDC could be selected with surface-mounted
control switches. One TTL pulse per engine revolution was output from the counter box at
the input advance/delay timing.
A Berkeley Nucleonics model 555 pulse/delay generator was used to supply the high-
precision trigger signals to the optical system. This device was operated in external gate
mode with an input clock signal from the counter box. At 600 RPM, the counter box
frequency (one per revolution) was already at 10 Hz, while at 1200 RPM, the pulse/delay
generator had to be operated in divide-by-2 mode, where synchronization with the
compression stroke had to be verified. The outputs of the pulse/delay generator were used to
provide the signals summarized in Table 3.5, with delays relative to the leading edge of the
counter box pulse:
61Channel Device Width Delay
A Flash Lamp 5.000 ms 0.000 µs
B Q-Switch 5.000 ms 186.0 µs
C MicroMax (primary camera) 5.000 ms 180.8 µs
D PI-Max (alternate camera) 5.000 ms 180.0 µs
Table 3.5 Trigger timing delays for optical measurement system. Delays are relative to the leading edge of the trigger signal from the crankshaft encoder.
The laser trigger pulses were delivered to the Nd:YAG directly to provide the
uninterrupted 10 Hz operation. The camera triggers were sent to two “one and only one”
TTL logic circuits (chip #4013), where they provided the “clock” input to the circuit diagram
shown in Figure 3.7. The MotoTron “skip-fire” TTL signal, previously used for activating
the sampling valve, was supplied as the “enable” input, at a timing of -180 CAD. On receipt
of the enabling signal, the circuit outputs the next clock signal and only that one pulse. This
allows the camera to capture the laser sheet that is fired on the compression stroke of the
skip-fired engine cycle and prevents the MicroMax camera from being triggered during a
combustion cycle. With the camera exposure set to 10 µs, the circuit’s insertion loss
(predicted to be ~10-20 ns) does not affect the capturing of the laser pulse.
62
Figure 3.7 Schematic for TTL timing of laser pulse and camera, synchronized with MotoTron skip-firing ignition by a “one-and-only-one” circuit.
63
4. Engine Operating Conditions
4.1. Selection Criteria
Utilizing the adjustable-camshaft feature of the cylinder head, conditions for varying
levels of residual gas dilution were established at our two operating speeds. For this project,
five categories of cam-phasing strategies were covered: a “baseline” valve overlap, an
enlarged valve overlap symmetric about TDC exhaust, an enlarged valve overlap with the
intake cam advanced from the baseline, an enlarged valve overlap with the exhaust cam
retarded from the baseline, and finally a zero-overlap (IVC=EVO) setting. At each of these
five categories, three engine conditions were established: a “low” load at 600 RPM, a “mid”
load at 600 RPM, and a “low” load at 1200 RPM. The result was a test matrix of 15 distinct
engine operating conditions.
4.1.1. Optical Engine Considerations
The objective of this project was to study conditions of high residual gas fraction.
Severe limitations in operating a fired optically accessible engine made this objective
challenging. The low compression ratio (5.95:1), despite being conducive to increased
trapped residual gas levels, impacts the ignition and combustion stability of the engine. To
address this, fuel was delivered at consistently rich conditions to aid the spark ignition (with
the added benefit of increased tracer density for optical measurement). Additionally, the
64spark plug was gapped to 2.1mm and the ignition coil was permanently set on maximum
dwell. The second major limitation was from thermal loading of the oil-less ring pack for the
Bowditch piston. Despite the relatively cold block temperature (§ 3.1.6), the Triptane engine
could not be fired continuously for more than four minutes at the 1200 RPM and 600 RPM
“mid” load conditions or six minutes for the 600 RPM “low” load conditions.
Mass loss through the non-metallic ring pack and the temporally evolving nature of
the mass loss under firing operation introduce uncertainty into cylinder pressure analyses (§
4.2). Secondly, time-limited firing operation effects steady-state exhaust gas emissions
measurement uncertainty (§ 4.3). A final mention before proceeding must be given to the
consideration of engine speed, bulk flowfield and manifold wave dynamics when extending
optical engine combustion data to conventional SI engines.
4.1.2. Establishing Engine Conditions
A baseline valve overlap duration of 20° was selected, based on a presumed-typical
value for a 4-valve engine of 510 cc displacement operating at 600-1200 RPM. For
simplicity, this overlap duration was positioned symmetrically about TDC exhaust. Intake
air mass flow rates were established at the baseline valve overlap for the three engine
speed/load combinations, and were held consistent for the other overlap cases. In
establishing the baseline air flow rates, fuel delivery and spark timing were both freely
adjusted to optimize IMEP and COV of IMEP.
The 600 RPM “low” load condition was set by varying the air mass flow rate to find a
comfortable minimum for stable combustion operation. Based on the measurement
65requirements of both the optical and sampling valve techniques, a combustion stability
criteria of <10% COV of IMEP was established. Since the objective was to increase dilution
with increased valve overlap from this baseline condition, an absolute minimum air delivery
rate was not chosen. The air mass flow rate chosen for the five 600 RPM “low” load
conditions, 144 mg/cycle, resulted in an intake MAP of approximately 50 kPa at the baseline
overlap. The 1200 RPM “low” load condition was established by adjusting the intake mass
flow rate to provide the same MAP as 600 RPM “low” load at the baseline overlap. The
result was 181 mg/cycle, although IMEP and exhaust temperatures were significantly higher
at the increased speed.
The 600 RPM “mid” load point was established by increasing the intake mass flow
rate to a comfortable upper limit for safe engine operation. At 208 mg/cycle, peak cylinder
pressure was at 15 bar and steady exhaust temperature was at 400 C, both acceptably close to
the upper limit. Intake MAP increased from the low load condition to 61 kPa. A “mid” load
point at 1200 RPM could not be established due the safety limits in combustion pressures and
temperatures. Likewise, full-load atmospheric-MAP firing operation could not be performed
at either speed, so no quantitative pressure-based reference for load points could be
established. Therefore, only intake air mass flow rate is used as a basis for engine load.
With air mass flow rates fixed, the advanced valve overlap conditions could be
established. Both fuel mass and spark timing were varied and 10% COV was used an upper
limit for combustion stability. Since the engine was less tolerant of increased overlap at 600
RPM compared to 1200 RPM, different overlap levels were tested for the two speeds. At all
600 RPM increased-overlap conditions (both loads), a total valve overlap duration of 30° was
set. This 10° increase from baseline was established from the maximum amount tolerable at
66the low-load condition at either intake-advance, exhaust-retard or symmetric-increase. At
1200 RPM, a 40° increase in overlap from baseline (60° total) was achieved for all three cam
strategies.
-50 0 500
2
4
6
8
10
12
valv
e lif
t [m
m]
Symmetric Overlap Increase
-50 0 500
2
4
6
8
10
12Intake Cam Advance
-50 0 500
2
4
6
8
10
12
valv
e lif
t [m
m]
CAD aTDC
Exhaust Cam Retard
-50 0 500
2
4
6
8
10
12
CAD aTDC
Zero Valve Overlap
Baseline 600 RPM 1200 RPM
Exh.
Exh.
Exh.
Exh.
Int.
Int. Int.
Int.
Figure 4.1 Summary of four valve overlap strategies. Baseline cam timing is indicated by the dashed line in all plots. Arrows indicate cam shift from baseline. The baseline overlap duration is 20°, the 600 RPM extended overlaps are 30° duration, and the 1200 RPM conditions are 60° overlap duration.
Fuel delivery rate was set by the highest injection mass required in the increased
overlap conditions for each of the three speed/load combinations. Fuel mass flow was then
fixed for all overlap conditions at the particular speed/load, to provide consistency in the
67optical measurements. A summary of the targeted air/fuel delivery rates for each of the
speed/load combinations is shown in Table 4.1. Spark timing was not fixed in this
experiment, but was optimized at each of the 15 test conditions to provide consistent
combustion phasing.
Mass Air Flow Fuel Injection Targeted AFR 600 RPM, Low Load 144 mg/cycle 14.5 mg/cycle 9.93:1 600 RPM, Mid Load 208 mg/cycle 18.0 mg/cycle 11.56:1
1200 RPM, Low Load 181 mg/cycle 18.0 mg/cycle 10.06:1
Table 4.1. Air/fuel engine operation parameters for the three experimental speed/load points. These values were held constant for each cam strategy.
A complete summary of engine operating conditions recorded in the laboratory can be
found in the master summary in Appendix A.1.
4.2. Combustion Analysis
Cycle-resolved cylinder pressure data were recorded throughout this experiment
using the system discussed previously. Ensemble-averaged indicated mean effective
pressure (IMEP) and pumping mean effective pressure (PMEP) were directly calculated by
the post-processing program along with COV of IMEP, which was used as a basis for
combustion stability. Averaged pressure traces were also used to compute heat release
information for each of the 15 test conditions.
684.2.1. Cylinder Pressure Data
Table 4.2 contains a summary of the IMEP and PMEP data for the test conditions.
The data are organized by valve overlap strategy and include a percent change relative to the
baseline overlap condition for the particular speed/load point. As previously discussed,
combustion phasing was kept consistent (location of peak pressure ~ 15° aTDC) for all
conditions.
Cam Strategy Speed [RPM] /Load
IMEP [kPa] (gross)
COV of IMEP [%] PMEP [kPa]
600 low 147.3 () 4.1 () 46.2 () 600 mid 260.9 () 2.05 () 35.3 () Baseline
Overlap 1200 low 228.2 () 1.32 () 51.1 () 600 low 151.7 (+ 3%) 6.03 (+ 47%) 45.0 (- 3%) 600 mid 270.7 (+ 4%) 2.07 (+ 1%) 30.3 (- 14%) Symmetric
Increase 1200 low 253.4 (+ 11%) 1.2 (- 10%) 35.3 (- 31%) 600 low 156.4 (+ 6%) 3.89 (- 5%) 45.4 (- 2%) 600 mid 269.8 (+ 3%) 2.46 (+ 17%) 31.1 (- 12%) Intake Advance 1200 low 260.6 (+ 14%) 1.84 (+ 40%) 32.2 (- 37%) 600 low 157.8 (+ 7%) 3.77 (- 8%) 43.5 (- 6%) 600 mid 271.5 (+ 4%) 2.64 (+ 20%) 29.1 (- 18%) Exhaust Retard 1200 low 240.7 (+ 6%) 3.64 (+ 176%) 35.1 (- 31%) 600 low 141.6 (- 4%) 2.99 (- 27%) 51.5 (+ 12%) 600 mid 255.1 (- 2%) 1.2 (- 41%) 39.2 (+ 11%) Zero Overlap 1200 low 229.3 (+ 0%) 0.87 (- 50%) 53.3 (+ 4%)
Table 4.2 Mean effective pressure data for 100-cycle average pressure data at all experimental conditions. Percentages shown are changes relative to the baseline overlap condition for the individual speed/load points at each cam strategy.
From the trends shown in Table 4.2, it can be seen that the baseline overlap selected
was not an ideal setting for any of the three speed/load points. The 600 RPM low load
condition was least sensitive to the cam-phasing changes, but nevertheless showed a
69consistent increase in IMEP with extended overlaps. The 600 RPM mid load points enjoyed
larger volumetric efficiency improvements (12 to 18 %) with the increased overlap durations
with minor increases in IMEP, indicating that the pumping improvements were nearly
overshadowed by losses due to charge dilution and reduced compression/expansion times.
The 1200 RPM points show the most significant reductions (> 30%) in pumping work, and
also the largest gains in IMEP, with the notable exception of the exhaust cam retard case.
The zero-overlap condition predictably reversed the trend due to penalties in pumping work.
4.2.2. Heat Release Analysis
A single-zone heat release code was used to process ensemble-averaged cylinder
pressure data. The code, which runs in the EES equation solver, uses an iterative
optimization scheme within the numerical integration to obtain wall heat transfer
characteristics. Combustion efficiency was calculated at each condition, using exhaust gas
measurements (§ 4.3.2), with a resulting range from 72% to 90%. The limits of integration
were optimized for each of the 15 engine conditions to obtain appropriate cumulative heat
release traces. The graphical results of the analysis are presented in Figures 4.2-4.4,
organized by speed/load point.
70
-30 0 30 60 90 120 1500
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
CAD aTDC
Hea
t Rel
ease
Rat
e [k
J/de
g]
BaselineSymmetric Incr.Zero OverlapIntake AdvanceExhaust Retard
600 RPMLow Load
-30 0 30 60 90 120 150 1800
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
CAD aTDC
Cum
ulat
ive
Hea
t Rel
ease
BaselineSymmetric Incr.Zero OverlapIntake AdvanceExhaust Retard
Figure 4.2 Heat release rate and cumulative heat release for all cam strategies at 600 RPM Low Load.
71
-30 0 30 60 90 120 1500
0.005
0.01
0.015
0.02
0.025
0.03
CAD aTDC
Hea
t Rel
ease
Rat
e [k
J/de
g]
BaselineSymmetric Incr.Zero OverlapIntake AdvanceExhaust Retard
600 RPMMid Load
-30 0 30 60 90 120 150 1800
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
CAD aTDC
Cum
ulat
ive
Hea
t Rel
ease
BaselineSymmetric Incr.Zero OverlapIntake AdvanceExhaust Retard
Figure 4.3 Heat release rate and cumulative heat release for all cam strategies at 600 RPM Mid Load.
72
-30 0 30 60 90 120 150 1800
0.005
0.01
0.015
0.02
0.025
CAD aTDC
Hea
t Rel
ease
Rat
e [k
J/de
g]
BaselineSymmetric Incr.Zero OverlapIntake AdvanceExhaust Retard
-30 0 30 60 90 120 150 1800
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
CAD aTDC
Cum
ulat
ive
Hea
t Rel
ease
BaselineSymmetric Incr.Zero OverlapIntake AdvanceExhaust Retard
Figure 4.4 Heat release rate and cumulative heat release for all cam strategies at 1200 RPM Mid Load.
73The most immediate trend from the plot sequence is the long tail of the heat release
curves for this engine, which extend far into the expansion stroke and approaches the EVO
timing. Even the baseline and zero overlap conditions demonstrate protracted burn durations,
demonstrating the influence of the low compression ratio and inherently high trapped
residual mass of the engine.
Table 4.3 summarizes the flame development duration and overall burning duration,
defined as crank angle 0-10% cumulative heat release and 10-90% heat release [1],
respectively. Once again, trends relative to the baseline 20° overlap are included at each
condition.
Cam Strategy Speed [RPM] /Load
Flame Development Angle
(0-10% HR)
Overall Burning Angle (10-90% HR)
600 low 53° () 59° () 600 mid 36° () 60° () Baseline Overlap 1200 low 36° () 68° () 600 low 61° (+ 15%) 79° (+ 34%) 600 mid 41° (+ 14%) 67° (+ 12%) Symmetric
Increase 1200 low 57° (+ 59%) 87° (+ 28%) 600 low 61° (+ 14%) 75° (+ 27%) 600 mid 43° (+ 20%) 67° (+ 12%) Intake Advance 1200 low 57° (+ 59%) 84° (+ 24%) 600 low 61° (+ 14%) 78° (+ 32%) 600 mid 42° (+ 17%) 70° (+ 17%) Exhaust Retard 1200 low 60° (+ 68%) 93° (+ 37%) 600 low 40° (- 25%) 60° (+ 2%) 600 mid 27° (- 27%) 59° (- 2%) Zero Overlap 1200 low 33° (- 7%) 72° (+ 6%)
Table 4.3 Flame development angles and overall burning angles for different overlap strategies, determined by a single-zone heat release code. Percentages indicated are changes relative to the baseline overlap condition at each speed/load point.
74The extended valve overlaps have a more pronounced effect on the heat release data
than the IMEP data of Table 4.2. At 600 RPM low load, the 10° increases in overlap
duration show up clearly in the 10-90% burn angle, with a consistent 25% increase, and a
smaller influence on the flame development angle. The overall increase in the burn duration
was lowest for the elevated load (600 mid) conditions. At 1200 RPM, where the largest
valve overlap extensions occurred and the highest residual fractions were anticipated, the
effect on early flame development was most severe. Since the baseline overlap at 1200 RPM
was predicted to be one of the lowest-residual conditions, this result was not surprising.
Higher turbulence levels at 1200 RPM help explain the lessened impact on 10-90% burn
duration at the high-overlap conditions. Once again, the 1200 RPM exhaust cam retard
condition is an outlier, showing a larger impact than either symmetric increase or intake
advance.
4.3. Exhaust Gas Emissions Measurement
Downstream exhaust gas emissions were recorded for the 15 established test
conditions as part of the residual fraction measurement described in Section 4.4. CO2
readings were critical for those measurements, but the additional CO, HC and O2 readings
are used here to better-quantify air/fuel ratio and combustion efficiency.
754.3.1. Emissions Measurement Procedure
Downstream emissions were recorded during continuously-fired operation with the
sample line feeding all five gas analyzers. Readings were acquired at 30 second intervals
during the time allowed for engine firing – four minutes at 600 RPM mid load and 1200
RPM, six minutes at 600 RPM low load. At that time, the engine had to be stopped (1200
RPM) or motored (600 RPM) for several minutes to avoid damage to the piston rings. The
readings were then graphically reviewed to determine the region of steady-state behavior,
and the corresponding measurements in that region were averaged to yield the reported
measurement value.
The NOx analyzer was not able to achieve steady-state in any of the measured
conditions. Rather than report suspect values, NOx measurements will not be included in the
project. With the highly fuel-rich operating conditions and low exhaust temperatures,
readings were predicted to be low (<< 1000 ppm) and should be negligible in combustion
calculations.
4.3.2. Emissions Analysis
Since all analyzers were recorded as “dry” measurements from the exhaust sample
line, a dry-to-wet correction factor had to be applied to raw the emissions bench data. This
value, Kexh, is defined as:
76
( )2 ,
exhexh
exh H O exh
nKn n
=+
(4.1)
where exhn and 2 ,H O exhn are derived from a carbon balance and the fuel’s combustion
stoichiometry (see §3.1.8). The concentration of H2 gas in the rich combustion products was
determined by the equation:
[ ]( )
[ ]2
:
4fuel
dry dry
H CH CO= × (4.2)
Knowledge of the incomplete combustion species, the water concentration and the
dry-to-wet correction factor allows for computation of the air/fuel ratio by way of a chemical
balance. Combustion efficiency was determined by the wet-basis mole-fraction equation
[25]:
( )[ ] [ ] [ ]{ }
[ ] [ ] [ ]
, 298 , 298 , 29822
, 298
2
% 100 (4.3)
100 (4.4)
CO T K H T K HC T K
cfuel T K
N CO h H h HC h
h
NHC CO CO
η= = =
=
+ += −
≡+ +
The molar enthalpy of combustion for the unburned HC is assumed to be equal to that of the
fuel. All concentration measurements are to be used on a “wet” basis.
774.3.3. Emissions Measurements
Steady-state downstream exhaust emissions measurements are summarized in Table
4.4. Analyzer readings shown are on a “wet” basis, after correction.
Cam Strategy
Speed [RPM] /Load [CO2] [CO] [O2] HC
[ppm C1] AFR ηc
600 low 8.1 6.5 0.4 10,629 10.7 82 % 600 mid 10.6 4.8 0.6 5769 12.1 90 % Baseline
Overlap 1200 low 10.1 5.9 0.6 6159 11.5 89 % 600 low 8.1 6.3 1.1 18,822 10.5 72 % 600 mid 10.7 4.6 0.7 5838 12.2 90 % Symmetric
Increase 1200 low 8.9 6.0 0.8 9015 11.3 85 % 600 low 8.4 6.3 0.9 14,463 10.8 77 % 600 mid 11.3 3.9 0.6 5697 12.5 90 % Intake
Advance 1200 low 10.3 5.0 0.7 6879 12.0 89 % 600 low 8.2 6.4 0.9 15,483 10.7 76 % 600 mid 11.1 4.3 0.5 5766 12.3 90 % Exhaust
Retard 1200 low 7.9 6.7 0.8 12,666 10.7 79 % 600 low 7.9 6.6 0.9 10,551 10.9 82 % 600 mid 10.5 4.8 0.7 5253 12.2 91 % Zero
Overlap 1200 low 8.4 6.4 0.8 6858 11.2 88 %
Table 4.4 Summary of exhaust emissions species measurements, concentrations shown are corrected to a wet basis from the raw readings. Air/fuel ratio and combustion efficiency coefficient have been calculated from the concentration data.
The combination of fuel-rich stoichiometry and the abnormally large crevice volume
of the Bowditch piston contributed to the very high hydrocarbon readings shown in Table
4.4. Pressure data summarized in Section 4.2.1 were taken simultaneous with these emission
measurements and did not indicate a single misfire (less than 1 bar IMEP) for any of the 15
78conditions. When the heat release data of section 4.2.2 are considered, it seems likely that
incomplete combustion at EVO was a major culprit in HC concentration, particularly at the
two low-load conditions.
It is notable that the intake cam advance strategy demonstrated the lowest CO
readings in the experiment and the lowest HC readings for extended overlap at all three
speed/load points. The 600 RPM mid load condition was least sensitive to overlap strategy
in terms of pollutant emissions. Combustion at the 600 RPM low load condition fared the
poorest at the symmetric overlap increase condition, with comparable results found at the two
asymmetric overlap extensions. At 1200 RPM, the worst-performing strategy was once
again the exhaust retard case, consistent with the pressure and heat release data.
The O2 readings were higher than would be anticipated at the equivalence ratio
indicated by our recorded CO concentrations. Since the highest O2 measurements were
consistently found at 600 RPM low load (~ 1%), where IMEP was lowest, it is proposed that
the high readings are a consequence of thermally-dependent sealing inefficiencies in the ring
pack during cylinder scavenging. The exhaust system was kept under positive back pressure
(above ambient) throughout the experiment and was newly constructed and leak-tested.
Furthermore, the emissions vacuum sampling line was tested with pure N2 gas with a
successful zero reading on the O2 analyzer.
Air/fuel ratio calculations were consistent across both of the 600 RPM load points. A
significant variation was encountered at the 1200 RPM condition, which is believed to be a
consequence of difficulty reading the time-limited measurement. 1200 RPM were the
hardest-running conditions in the experiment, with the high piston speeds and gas
temperatures. All analyzer-derived AFR readings were higher than the targeted delivery
79rates, indicating some error in the fuel injector and/or intake air orifice calibrations
(discussed in § 3.1.7-8).
4.4. Bulk Residual Gas Fraction Measurement
For this project, the primary basis for comparing optically measured residual gas
mixing phenomena is the bulk in-cylinder residual gas mass fraction (yr). To measure this
value, the fast-acting sampling valve was installed in the Triptane engine to measure in-
cylinder CO2 as described in Section 3.2.2.
4.4.1. Sampling Valve Measurement Technique
The sampling valve experiment was performed simultaneously with the exhaust
emissions measurements described in the previous section. In-cylinder CO2 readings were
taken under skip-fired operation at each condition immediately prior to switching to
continuous-firing for the downstream sampling and pressure data logging. The skip-fired
sequence is graphically presented in the pressure trace of Figure 4.5. The settings for the
sampling valve and the sampled gas flow rates are summarized in Table 4.5.
80
Figure 4.5 Skip-firing sequence example (1200 RPM baseline overlap shown). Sampling valve is actuated on compression stroke of skip-fired cycle (see Table 4.5).
600 RPM 1200 RPM Sampling Valve Open 56° bTDC 75° bTDC Valve Open Duration 17 ms 14 ms Sampling Valve Close 5° aTDC 25° aTDC Sampling Frequency 4 cycles 6 cycles
Table 4.5 Sampling valve operation for all experimental conditions. Sampling frequency is listed as the number of fired cycles between sampled cycles (see Figure 4.5).
Like the downstream emissions measurements, the skip-fired in-cylinder readings
were limited by the continuous-firing time limits of the engine. In a similar procedure, the
81steady-state reading of the CO2 analyzer was determined graphically. Measurements taken
at the two “low load” condition sets at the smaller baseline and zero overlaps demonstrated a
weaker flow to the bench, which required fully opening the flow control valve on the
analyzer entrance, resulting in flow pulsations to the instrument. All other conditions were
able to operate with a non-pulsating flow delivered to the bench at the analyzer’s optimal
flow rate of 2.5 lpm.
-100 0 100 200 300 400 5000
1
2
3
4
5
6
7
8
9
10
CAD aTDC
P [k
Pa]
Fired CycleSampled CycleValve Lift Transducer Signal
Figure 4.6 Sample pressure data for skip-fired cycle with sampling valve actuation. The average fired cycle pressure trace and the sampling valve lift transducer signal for that skip-fired cycle (no physical units) are overlayed. 1200 RPM exhaust cam retard condition shown.
82A concern with measuring in-cylinder composition with this technique is that the
mixture trapped in-cylinder on the skip-fired cycle be representative of the bulk cylinder
charge composition. Particularly concerning was the possibility of the influence of partial-
burn or misfire on the cycle prior to the sampling valve open event. Even at high dilution
conditions, this was not found to be a major problem, as can be seen for the histograms of
prior-cycle IMEP shown in Figures 4.7 and 4.8, which are the two conditions having the
slowest burning rate for the two engine speeds..
Figure 4.7 Frequency histogram of prior-cycle IMEP for skip-firing operation at 600 RPM low load symmetric overlap increase condition. Data compiled from 100 consecutive sampled cycles.
83
Figure 4.8 Frequency histogram of prior-cycle IMEP for skip-firing operation at 1200 RPM exhaust retard condition. Data compiled from 100 consecutive sampled cycles.
4.4.2. Residual Gas Fraction Calculations
The residual gas fraction was calculated on a molar basis by comparing the mole
fractions of CO2 in the compressed charge, xCO2,cc, with the downstream exhaust
measurement, xCO2,exh.
2
2
,
,
CO ccr
CO exh
xx
x= (4.5)
84The dry-to-wet factor for the exhaust, Kexh, was determined in Equation 4.1 and used to
convert the denominator term from its raw dry-basis analyzer reading. The dry-to-wet factor
is different in the compressed charge, and is defined in [26] to be:
11
exh rcc
r
K xKx
+=
+ (4.6)
Equations 4.1, 4.5, 4.6 and the raw CO2 analyzer readings can be used to iteratively solve for
xr. It is assumed that the molecular weights of the compressed charge mixture and the
exhaust gas mixture are equal, and that the mole fraction xr is then equal to the mass fraction
yr. Additionally, the small relative humidity in the dried intake air is neglected.
4.4.3. Residual Gas Fraction Measurements
The results of the bulk residual gas fraction study are presented in Table 4.6
organized by cam phasing strategy. As in previous sections, the percent change from the
baseline is provided for ry . Values for 2 ,CO ccx and ccK are in the master conditions summary
in Appendix A.1.
The intake cam advance strategy is shown to yield the smallest increases in residual
fraction. For the two 600 RPM loads, the symmetric overlap extension provided the largest
residual gas fraction increase. Since the baseline overlap condition at 600 RPM low load was
already at 37 % residual fraction, it is not surprising that the engine was not especially
tolerant of increased valve overlap durations there. The largest residual gas fraction was
85measured at 1200 RPM with the exhaust cam retarded, which is consistent with the analysis
of the pressure, heat release, and emissions measurements. In general, the range of residual
fractions covered (21.9% - 44.8%) is believed to be representative of those encountered in a
high-dilution engine design. It is not a major concern for this project that the lower end of
the measured ry range does not correlate well with typical values in conventional SI engines
(5% - 25%).
Cam Strategy Speed [RPM] /Load yr
600 low 0.377 () 600 mid 0.296 () Baseline Overlap 1200 low 0.273 () 600 low 0.404 (+ 7%) 600 mid 0.358 (+ 21%) Symmetric
Increase 1200 low 0.437 (+ 60%) 600 low 0.387 (+ 3%) 600 mid 0.325 (+ 10%) Intake Advance 1200 low 0.408 (+ 50%) 600 low 0.399 (+ 6%) 600 mid 0.327 (+ 10%) Exhaust Retard 1200 low 0.448 (+ 64%) 600 low 0.287 (- 24%) 600 mid 0.245 (- 17%) Zero Overlap 1200 low 0.219 (- 20%)
Table 4.6 Summary of bulk residual gas fraction measurements at all experimental conditions. Percentages shown are changes relative to the baseline overlap condition at each individual speed/load point.
86
5. Imaging System Development and Analysis
5.1. PLIF Image Processing
Before discussing the selection of the camera for the experiment, an overview of the
procedure for correcting planar laser images in engines will be covered. First the image
acquisition sequence at each measurement condition will be described, followed by the
numerical corrections applied to the images to extract a faithful representation of the tracer
molecules in the laser sheet. Finally, the statistical processes used to evaluate and quantify
the inhomogeneity of the fresh charge in the engine are discussed.
5.1.1. Image Acquisition Procedure
Each PLIF measurement consists of three series of TIFF-format grayscale intensity
images – a background image sequence, a flatfield image sequence, and a data image
sequence. These images are all acquired at the same crank angle timing (engine
motoring/firing), with the camera focused on the laser sheet plane and the room lights turned
off.
Fifty background images are acquired before the data measurement and 50 more are
acquired afterward. The background images are taken with the laser sheet firing through the
combustion chamber, but without tracer addition (i.e. no fuel injection). Background images
contain signal from blemishes in the turning mirror and piston window, as well as from
87scattered laser sheet light impinging on cylinder head roof surfaces. These images are
subtracted from all subsequent images to better isolate the laser sheet. A sample 100-image
mean background is shown in Figure 5.1, where all four valves, the spark plug and the
periphery of the piston window are all visible in the full-CCD image.
Figure 5.1 Sample 100-image mean background image. Pixel intensity scale is on right.
Following the first set of 50 background images, 100 flatfield images are acquired
with the fuel injector activated and the ignition disabled. With the far-upstream fuel injection
(§ 3.1.8), this mode is considered to provide a very nearly homogeneous in-cylinder mixture.
Laser-induced fluorescence images of the homogeneous tracer distribution are used to
perform a “flatfield correction” of the data images, whereby the laser sheet intensity profile is
normalized. In this project, the flatfield image sets are additionally useful, since they can be
used to define the homogeneous image condition, when the mean flatfield correction is
applied to the 100 individual flatfield images. Since the flatfield images are acquired while
motoring, the residual gas contains fuel/tracer and these images can be used for comparison
88with the fired data images which contain residual gases. Provided the intake charge is
thoroughly pre-mixed (§ 5.3), the corrected homogeneous images provide the statistical
definition for a “completely mixed” cylinder charge, limited by the detection system signal-
to-noise characteristics. Figure 5.2 contains a mean flatfield image acquired 30° bTDC, and
demonstrates the spatial variation in laser sheet intensity.
Figure 5.2 100-image mean flatfield image, 30° bTDC 600 RPM Mid Load Exhaust Retard condition. Flatfield images have been background-subtracted.
Data images were acquired with the engine in skip-fired operation. Data images
contain fresh air/fuel/tracer charge mixing with combustion residuals. Like the homogeneous
images, data images are background subtracted and flatfield corrected. A sample raw data
image is shown in Figure 5.3. The correction technique will be developed in the following
section.
89
Figure 5.3 Sample raw data image (no corrections), 30° bTDC 1200 RPM Exhaust Retard condition.
5.1.2. Image Correction Procedure
Image processing in this experiment was performed using the Matlab programming
environment. First, 100 background images are read in and converted from a 12-bit integer
value to double precision (to avoid truncation of low signal in subsequent computations). An
ensemble mean background image,,i j
B , is computed by the formula:
( )( ) ( )
100
, ,, 1
th, ,
1 (5.1)100
B intensity of pixel , of n background image for all , in CCD array
i j ni j n
i j n
B B
i j i j=
=
=
∑
The 100 flatfield images are then read in and converted to double precision. An
ensemble mean flatfield image is computed by Equation 5.2:
90
( )( ) ( )
100
, ,, ,1
th, ,
1 (5.2)100
F intensity of pixel , of n flatfield image for all , in CCD array
i j ni j i jn
i j n
F F B
i j i j=
= −
=
∑
The mean maximum pixel value from the 100 flatfield images, maxF , is stored and used as a
scaling factor for a normalized mean flatfield image used for data image correction, 0 1 ,i jF − .
This image will be divided into the individual data images. In order to preserve the full range
of the original pixel count scale after division, 0 1 ,i jF − is created as a normalized zero-to-one
double precision array.
( )
,0 1 ,
max
(5.3)
for all , in mean flatfield image
i ji j
FF
F
i j
− =
To avoid amplifying noise-level pixels in the flatfield correction, pixels in 0 1 ,i jF − below
25% of the maximum value are forced to zero. Zero-value pixels are not included in the
flatfield correction.
The homogeneous images and data images are corrected by the normalized flatfield
after background subtraction (Equations 5.4 and 5.5, respectively).
91
( )
, , ,, ,
0 1 ,
,
(5.4)
for each of n=100 flatfield images
i j n i ji j n
i j
i j
F BH
F
F
−
−=
( )
, , ,, ,
0 1 ,
,
(5.5)
for each of n=100 engine raw data images
i j n i ji j n
i j
i j
Q BD
F
Q
−
−=
In this project, the data and homogenous image corrections are performed on a region of
interest completely located within the laser sheet. The coordinates of the ROI within the
CCD array vary based on the exact alignment of the laser sheet at each condition. The ROI
properties are discussed in Section 5.3.1.
5.1.3. Median Filtering
Homogeneous and data image sets ( , , , , and i j n i j nH D ) are transformed with the 3-by-3
median spatial filter built in to Matlab’s Image Processing Toolbox. This operation was
performed after all corrections from the previous section and before any statistical
calculations or output presentation. Median filtering has shown to be a valuable technique
for noise removal in images while preserving gradients.
925.1.4. Image Statistics
The two-dimensional spatial mean pixel intensity and standard deviation were
computed on the homogenous and data image ROI using built-in array operators in the Image
Processing Toolbox. Since the fluorescence intensity of the 3-pentanone tracer was not
calibrated to an absolute scale (due to shot-to-shot laser power variation and thermal
gradients in-cylinder), a coefficient of variation had to be defined to quantify inhomogeneity
in each image n ( nCOV ), relative to the mean signal.
( )( )
( )
( )
(5.6)
where = standard deviation of pixel intensities on ROI of image n
and = spatial mean pixel intensity on ROI of image n
I nn
n
I n
n
COVI
I
σ
σ
=
During the experiment, a significant error in the flatfield correction occurred,
imposing vertical bands on the corrected images (Figure 5.4). This phenomenon has been a
common occurrence in prior PLIF studies [13, 15, 16] and can been attributed to slight
temporal variation in the laser sheet profile during the measurement. The non-physical
structures introduce error into the two-dimensional statistics, so a modified “column COV”
was used to calculate the relative standard deviation only in the direction of the vertical
banding. This quantity, although technically a coefficient of variation, will be referred to
symbolically as ( )y yσ µ indicating statistics performed in the vertical direction across an
entire image ROI and then ensemble-averaged.
93
( ) ( )1
th
1 (5.7)
where and are the mean pixel intensity and standard deviation
for each pixel column j in the n image ROI ( columns wide)
Nj
y y nj j
j j
N I
I
J
σσ µ
σ
=
= ∑
If the laser sheet is aligned with the camera’s pixel array, the column COV term better
indicates variation in pixel intensity due to engine flows only. As a convention in the
nomenclature, over-bars will used to indicate spatial-mean values within an image and angle
brackets < > will used to denote ensemble-mean values.
Yr = 21.87%, <(σy / µy)> = 1.74%
1200 RPM Homogeneous, 30° bTDC
220
240
260
280
300
320
340
360
380
0.9 × Imax
0.5 × Imax
Figure 5.4 Sample homogeneous images acquired at 30° bTDC for the 1200 RPM, zero overlap condition demonstrating vertical banding in the corrected images. See Section 5.1.6 for image presentation convention.
945.1.5. Probability Distribution Function
The term ( )y yσ µ was developed in the previous section to quantify intensity
variation in individual data images, and also the mean stratification for a particular set of
images. Since this ensemble mean encompasses a large amount of pixel values, the
probability density function (PDF) is a relevant tool for presenting graphically the measure of
central tendency for a set of 100 images.
Pixel intensities were gathered from the regions of interest in both corrected data and
homogeneous images. A two-dimensional count of these data would be biased by the
vertical intensity banding shown in Figure 5.4. Therefore, as in the ( )y yσ µ calculations, a
vertical column-based analysis was developed. For each column j in each image ROI, the
mean intensity, jI , was computed and used to generate a “pseudo-image”, ,i jD′ , of the ROI
where each pixel was normalized by it’s column mean.
( )
( )
, ,1.. 1..
1
, for corrected data image (5.8)
1 , for each column in the ROI (5.9)
i j i jj i M j N
M
ji
D i jD D
I
I D i j jM
= =
=
′ =
= ∑
The relative intensities of ,i jD′ on the (M x N) region of interest are decoupled from
the flatfield correction errors and can be processed by two-dimensional statistics. Each
image n in the 100-image data set is analyzed by the image histogram function in the Matlab
Image Processing Toolbox. Since this function only operates on unsigned integers, the
95double-precision values of ,i jD′ are first multiplied by 10,000 counts to avoid truncation
before conversion to16-bit integer class. The histogram bin widths are set to 4 counts (total =
16,384 bins) and the bin counts from each image are summed for the data set to form the full
histogram.
The full histogram is integrated and then normalized by the integral value to form the
PDF curve. The bin scale is normalized to the mean intensity from the ,i jD′ pseudo-images.
At each condition where the PDF is examined, the homogenous image set is overlaid on the
skip-fired data for comparison.
5.1.6. Image Presentation
Figure 5.4 provides an example of the PLIF data image presentation format for this
project. Regions of interest from 25 successive corrected images are compiled into a tiled
array. The images are in successive left-to-right, top-to-bottom order from the 100-frame
raw data file, taken at the same image timing at the engine cycle imaging frequency
(typically one every 6th cycle). Figure 5.4 also includes an intensity scale at right. As
labeled, the upper and lower limits of this scale are the 90% and 50% levels, respectively, of
the maximum intensity found in the ROI at that imaging condition. These cutoff levels were
used for visual representation help to better illustrate regions of high and low fresh charge
concentration in the fired data images. In all future images this same grayscale criteria has
been employed.
96
5.2. Imaging System Performance
An objective of this project was to apply the highest-fidelity image-based
measurement system to residual gas mixing processes. The evaluation criteria for the camera
are signal-to-noise ratio and spatial resolution. The MicroMax CCD camera, described in
Section 3.3.3, was selected after comparison with other alternatives. The lens selection
resulted in a small, centrally-positioned laser sheet region of interest in the combustion
chamber. A test target was used to determine the spatial resolution of the system. Finally,
image data were analyzed to determine a maximum range of SNR.
5.2.1. Camera Selection
The MicroMax camera is categorized as a frame-straddling CCD, and was analyzed
in a comparative study on PLIF detection systems by Rothamer and Ghandhi in [14]. The
highest-performing device in that study (assuming a strong PLIF signal) was from the slow-
scan category of cameras. The Apogee AP7 CCD, which had produced SNR greater than
80:1 in prior non-firing direct-injection mixing studies [15, 16], was not capable of shuttering
residual background luminosity during the skip-fired compression stroke. The 16-bit CCD
saturated at the lowest shutter duration time (20 ms), and was eliminated from consideration
in the project. The MicroMax, with the electronic interline transfer on-chip shuttering
feature described in Chapter 3, was able to easily reject this luminosity at an exposure of 10
µs.
975.2.2. Region of Interest (ROI) and Spatial Resolution
As demonstrated in the background image in Figure 5.1, the full CCD array captures
a region of the combustion chamber located below the spark plug and slightly off-center
toward the exhaust valve-side of the pent-roof axis. Figure 5.5 more exactly locates the ROI
relative to the cylinder head and bore diameter, while Table 5.1 contains the vertical distance
separating the ROI from the piston face at the four experimental image timings.
Figure 5.5 Location of ROI within combustion chamber, DOHC cylinder head. Distance h is between laser sheet plane and piston face, and is tabulated for image timings in Table 5.1.
98Image Timing [CAD aTDC] h [mm]
- 30 6.4 - 45 13.7 - 60 22.9 - 99 49.0
Table 5.1 Distance from piston face to laser sheet ROI for experiment image timings.
The small pixel size (6.7 µm) of the 1300-by-1030 pixel MicroMax CCD required
binning the pixels 6-by-6 to obtain an equivalent pixel size in the imaging plane on the order
of 185 µm. This value was measured by Wiles [15] in a bench experiment to optimize the
photonic flux on the CCD array given our use of a large-aperture lens and also the fixed
dimensions of the lab’s Bowditch piston (~45cm camera-ROI distance). Bin size in this
experiment was determined prior to test target measurement using a less-precise scale image.
The equivalent pixel size, and thus the physical size of the ROI, was computed using
an image taken of the USAF 1951 optical test target. The target was back-illuminated and
placed in the laser sheet plane above the piston with the cylinder head removed. Using the
standard 4% contrast criterion, the image’s spatial resolution was measured to be 4.0 line
pairs per mm (lpmm). The equivalent pixel size was measured on the target to be 174 µm in
the imaging plane. The binned pixel size is comparable to that of the focused laser sheet
thickness [15] and therefore does not reduce system spatial resolution.
All regions of interest in this project were 163 pixels long in the direction parallel to
the laser sheet propagation. Condition-to-condition variation in laser alignment resulted in a
range of ROI widths (across sheet profile) of 103-130 pixels. Therefore, the ROI sizes
99recorded in Chapter 6 range from 18-by-28 mm to 23-by-28 mm in real size within the
cylinder.
5.2.3. Signal-to-Noise Ratio
Rothamer and Ghandhi [14] demonstrated that, in the presence of a sufficient
threshold signal level, the frame-straddling camera will operate in the shot noise-limited
regime. This is important, as it reduces the influence of other CCD noise sources (§ 2.4.3)
from the calculation of the maximum possible SNR. This condition is verified graphically in
Figure 5.6, where a characteristic curve for the MicroMax camera is shown with a reference
line indicating the shot noise-limited slope of ½.
Figure 5.6 Camera noise characterization, as a function of signal intensity - MicroMax frame-straddling CCD. Reprinted from [14].
100Signal-to-noise evaluations of the data images were based on the two-dimensional
mean intensity level in the background-subtracted data images (not flatfield-corrected). This
value was calculated on the laser sheet ROI described in the previous section. Day-to-day
and also set-to-set variation in laser power caused a range of mean signal levels to occur.
Table 5.2 contains mean signal level and shot noise-limited SNR (Equation 5.10) for each of
the four experiment image acquisition times. Due to the increased number density from
compression, the later timings were expected to yield higher signal, and thus SNR.
Image Timing [aTDC]
Mean Signal Intensity [counts]
Shot Noise-Limited Maximum SNR
- 30 102.3 21.2 : 1 - 45 89.5 19.8 : 1 - 60 68.2 17.3 : 1
600 RPM Low Load
- 99 45.0 14.1 : 1 - 30 125.7 23.5 : 1 - 45 105.7 21.6 : 1 - 60 86.5 19.5 : 1
600 RPM Mid Load
- 99 58.4 16.0 : 1 - 30 106.4 21.6 : 1 - 45 90.1 19.9 : 1 - 60 70.8 17.6 : 1
1200 RPM Low Load
- 99 46.0 14.2 : 1
Table 5.2 Values of spatial-mean data image intensity and resulting shot noise-limited maximum SNR for three speed/load points. Each set is the mean value for the five valve overlap strategies.
The signal levels of Table 5.2, when located on the abscissa of Figure 5.6 provide a
similar range of SNR as published by Rothamer and Ghandhi. The equation used for shot
noise-limited maximum signal-to-noise ratio is:
101
max * (5.10)outSNR I G=
The output gain term outG is related to digitization and equal to 4.4 e-/ADU for the
MicroMax camera. On examination of the characteristic curve in Figure 5.6, it can be seen
that the camera is operating in the shot-noise limited region, although the lower-signal
images at 99° bTDC are very near the transition away from shot noise-limited behavior.
Flatfield images, and thus homogenous images, have higher signal levels since they do not
contain residual gases. Due to the marginal signal level at some data image conditions, there
was interest in exploring potential benefits to using an intensified slow-scan camera in place
of the MicroMax.
5.2.4. MicroMax Comparison with Intensified CCD
Intensified cameras are commonly used in combustion PLIF measurements since their
extremely short gating time capabilities allow maximum light rejection. The MicroMax
camera was compared with the Roper Scientific PI-Max intensified slow-scan camera. In a
moderate signal level environment, the two devices were found to provide very comparable
signal-to-noise ratios, and the MicroMax yielded a slightly lower measure of spatial variation
on the homogeneous image condition.
Given the moderate-signal conditions and the intensified camera’s fundamental
tendency to blur gradients, the MicroMax camera was selected to maximize spatial
performance of the detection system.
102
5.3. Assessment of Intake Charge Homogeneity
A major experimental assumption of this project is that the fresh charge contains only
air, fuel and tracer and that it is homogenously mixed before IVO. Far-upstream air-assisted
fuel injection of pure hydrocarbon fuels has previously been verified for similar laboratory
setups in [13, 15, 16]. Nevertheless, the homogenous PLIF images will be analyzed here to
quantify degree of homogeneity.
5.3.1. First and Second Moments of Homogeneous Data
The homogenous images were tested by comparing their intensity variation
( )y yσ µ with the theoretical level of shot noise for the mean flatfield signal level at the
four timings at all 15 engine conditions. The column statistics were performed on unfiltered
corrected flatfield images, since the non-linear filtering is not predicted by shot noise theory.
The mean signal level mξ was that of the 100 flatfield images after background subtraction.
From this value, the theoretical shot noise level can be defined in terms of a normalized
standard deviation as:
( ) (5.11)
1where
mshot
m
out
AD
AD G
ξσ µ
ξ=
=
103A plot of the homogeneous image spatial variation compared to the theoretical shot
noise flatfield is shown in Figure 5.7. A 1-to-1 line is shown indicating the shot noise floor
for the ( )y yσ µ metric. All homogeneous images lie correctly above the shot noise floor,
although the small offset, close grouping and similar slope of the data points indicate that the
variations measured in all homogeneous conditions are primarily influenced by shot noise on
the CCD. The small offset can be partially attributed to slight misalignment of the laser sheet
vertical banding to the pixel columns where ( )y yσ µ is calculated as well as the
contribution of read and dark noise.
0 1 2 3 4 5 6 7 8 9 100
1
2
3
4
5
6
7
8
9
10
<(σ / µ)>shot [%]
<(σ
y / µ y)>
hmg
[%
]
Figure 5.7 Comparison of theoretical shot noise intensity variation ( )shot
σ µ to measured
homogenous pixel intensity variation ( )y yσ µ.
104
5.3.2. Homogeneous Image PDF
As a second check of the homogeneous fresh charge and also a demonstration of the
pixel intensity PDF function described in Section 5.1.5, the 12 homogeneous image sets
taken at the baseline valve overlap are shown in Figure 5.8. This plot shows the strong
grouping about the mean pixel intensity in the corrected images. The 600 RPM Low Load
condition is the curve showing the lowest peak, and also the condition of highest ( )y yσ µ
and lowest SNR in the experiment. Additional PDF figures will be shown in the next
chapter, accompanying data images, which will indicate that most homogeneous image sets
fall near the taller curves in Figure 5.8 (peaks near 30) and also that the skip-fired residual
gas data PDF’s are substantially lower in profile.
105
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50
5
10
15
20
25
30
I / Imean
PD
F
Figure 5.8 Probability distribution function for pixel intensity in homogeneous image sets at four image timings for all three engine speed/load points. Baseline valve overlap. Each PDF curve contains information about 100 corrected homogenous images.
5.4. Direct-Injection Test of Imaging Technique
Residual gas mixing was measured in this experiment using what amounts to a
“negative PLIF” approach. The presence of residual gases in the image ROI is denoted by
regions of low fluorescence intensity. The fresh charge, in which the tracer molecules are
homogenously distributed, is assumed to provide the entire PLIF signal. If molecules
trapped or re-inducted with the residual gas fluoresce in the laser sheet, that signal would
then skew the identification of the residual in the image. A test was then performed to isolate
this signal from the predominant PLIF signal from the fresh air/fuel/tracer charge. Gasoline
direct-injection (GDI) hardware and the MotoTron skip-fire sequence were used to do this.
1065.4.1. Skip-Direct Injection Experiment
In order to isolate the fluorescence signal of the residual gases, fuel injection had to
be shut off on the image cycle. Direct fuel injection is capable of achieving this, so the
Triptane engine’s pushrod OHV cylinder head, which is outfitted with access for a high-
pressure Chrysler Pressure-Swirl injector, was used. The details of this combustion chamber
and fuel injection strategy are well documented in [13, 15, 16]. A summary of the engine
conditions for the GDI experiment are covered in Table 5.3
600 RPM 1200 RPM
CR 9.8 : 1
airm•
[mg/cycle] 212 172
fuelm•
[mg/cycle] 16.80 13.37
AFR 12.62 12.86
IMEP [kPa] 180 151
COV of IMEP [%] 13.2 7.7
iMAP [kPa] 55.84 46.53
EOI [° aTDC] -270 -270
IGN [° aTDC] -20* -20*
Texh [° C] 320 450
ry [%] 12.28 13.25
HC [ppm C1] 13,044 7706
Table 5.3 Direct injection experiment engine conditions and unburned hydrocarbon emissions measurements. * indicates the approximate ignition timing.
107To isolate the residual gas fluorescence signal on the image cycle, the direct injection
was disabled on the intake stroke preceding the image timing. The only means to accomplish
this was by using the one available skip-fire TTL output from MotoTron to perform both fuel
injection and ignition timing. The MotoTron sequence was set to trigger the SOI timing, via
a Berkeley Nucleonics Model 555 pulse/delay generator. The pulse generator provided the
fuel injection driver signal and, on a time delay, the ignition coil signal. This operation
disabled both fuel injection and ignition on the image cycle, but introduced an uncertainty
into the ignition timing, due to the time-based delay from the ECU signal. This resulted in
the high COV of IMEP shown for early-injection GDI operation.
The remainder of Table 5.3 shows that the OHV cylinder head provides a
significantly higher geometric compression ratio than the DOHC head (CR = 5.95:1). This is
reflected in the much lower residual gas mass fraction measured here (compare with Table
4.6). Based on the significant change in combustion chamber geometry, the experimental
conditions covered in Chapter 5 could not be matched. Instead, worst-case conditions for HC
emissions were set at 600 and 1200 RPM (within engine load limits). Hydrocarbon
emissions were considered indicative of the presence of unburned residual 3-pentanone (or
other fluorescent compounds) in the residual gas mixing images. By advancing the ignition
timing to phase location of peak pressure (LPP) near TDC, HC levels from crevice volume
outgassing were maximized. Measured HC was found to be comparable to the imaging
conditions in Table 4.4. Figures 5.9 and 5.10 present pressure data at both 600 RPM and
1200 RPM relative to the baseline overlap condition at each speed.
108
-180 -90 0 90 180 270 360 450 5400
200
400
600
800
1000
1200
1400
1600
1800
2000
CAD (aTDC)
p cyl [k
Pa]
Direct injection, OHV headBaseline overlap, DOHC head
Figure 5.9 Direct-injection experiment cylinder pressure trace comparison with DOHC baseline valve overlap. 600 RPM.
-180 -90 0 90 180 270 360 450 5400
200
400
600
800
1000
1200
1400
1600
CAD (aTDC)
p cyl [k
Pa]
Direct injection, OHV headBaseline overlap, DOHC head
Figure 5.10 Direct-injection experiment cylinder pressure trace comparison with DOHC baseline valve overlap. 1200 RPM.
1095.4.2. Skip-DI Imaging and Results
Two engine speeds were examined (Table 5.3); images were acquired at 10° bTDC
compression. First, the flatfield condition was imaged using upstream homogenous fuel
injection. The mean signal from the 100 flatfield images was compared to two direct-
injection conditions. First, the engine was operated with both skip-direct injection and skip-
firing enabled. This operation provided the “best case” for low concentration of unburned
fluorescent compounds in the combustion chamber, since the prior cycle involved complete
combustion. Second, the skip-fire ignition signal was disabled and the engine was motored
with the skip-direct injection. This condition provided the “worst case” for hydrocarbon
concentration in the residual gas, simulating a prior cycle with partial burn or misfire quality.
The results of the imaging experiment are shown in Table 5.4, with all mean intensity values
representing that of an ROI within the laser sheet after background subtraction.
600 RPM 1200 RPM
Motored Flatfield 165.5 119.0
Skip-Fire / Skip-DI 4.2 2.7
Skip DI, motoring 48.2 30.5
Table 5.4 Direct injection experiment imaging results. 100-image mean signal level for flatfield, skip-fired, and motored skip-DI PLIF data.
The data of Table 5.4 indicate a very low signal from the fired residual gas – less than
3% of the flatfield intensity for both engine speeds. Under non-fired operation, where
110misfire residual gas is simulated, the signal level rose to nearly 30% of the flatfield. The
engine operating conditions outlined in the next chapter were established to avoid high
cyclic-variability operation. Therefore, provided that the combustion is nearly complete, the
low fluorescence intensity levels shown by the skip-DI/skip-fire test indicate that the
technique is faithful in depicting residual gas mixing with homogenous fresh air/fuel charge.
111
6. Residual Gas Mixing
6.1. Sample Imaging Data
Before entering into statistical analysis of residual gas mixing, sample data images
representative of engine conditions with high, medium, and low residual gas fractions are
presented in Figures 6.2-6.4. A sample homogeneous image set from the same engine timing
is shown in Figure 6.1. All engine images are presented in sets of 25 consecutive frames, and
the grayscale assignment follows the procedure outlined in Section 5.1.6.
Yr = 0%, <(σy / µy)> = 1.94%
600 RPM Homogeneous Images, 60° bTDC
160
180
200
220
240
260
2800.9 × Imax
0.5 × Imax
Figure 6.1 Sample homogeneous image sequence, 60° bTDC.
Yr = 44.9%, <(σy / µy)> = 8.9%
1200 RPM Exh. Retard, 60° bTDC
90
100
110
120
130
140
1500.9 × Imax
0.5 × Imax
Figure 6.2 Sample data image sequence, high residual fraction condition, 60° bTDC.
112
Yr = 35.7%, <(σy / µy)> = 4.7%
600 RPM Sym. Increase, 60° bTDC
70
75
80
85
90
95
100
105
110
115
1200.9 × Imax
0.5 × Imax
Figure 6.3 Sample data image sequence, mid-range residual fraction, 60° bTDC.
Yr = 21.9%, <(σy / µy)> = 4.0%
1200 RPM Zero Overlap, 60° bTDC
100
110
120
130
140
150
160
170
1800.9 × Imax
0.5 × Imax
Figure 6.4 Sample data image sequence, low residual fraction condition, 60° bTDC.
All data image shown have been background-subtracted, flatfield-normalized and 3 x
3 median-filtered. All pixel variance data, ( )y yσ µ , are ensemble-mean values, computed
in the vertical direction (§5.1.4). This figure sequence serves to introduce the data produced
by the measurement system outlined in Chapters 3 and 5, and also visually demonstrates the
effect of elevated residual gas fractions on the homogeneous air/fuel/tracer mixture shown in
Figure 6.1. A more thorough discussion of the correlation between residual fraction and
( )y yσ µ will be covered in Section 6.3.
113
6.2. Correlation of Spatial-Mean Pixel Intensity with Measured Residual Gas Fraction
The ratio of the spatial-mean fluorescence signal of the skip-fired data image to that
of the motored flatfield image can be assumed to correlate with the bulk residual gas fraction
if important assumptions are made. The region of interest captured by the imaging system (§
4.3.1) is small relative to the combustion chamber and inherently two-dimensional, and
therefore extensions of the ROI image properties to entire cylinder volume are uncertain.
Ensemble-averaging of the maximum number of fired data images available can improve the
characterization, but nevertheless assumptions about the ROI have to be made. Secondly,
knowledge of the different in-cylinder conditions between skip-fired and motored operation
indicates an influence of the temperature-dependent fluorescence intensity on any
calculations. The flatfield condition, without any combustion products, contains a cooler
mixture, while the skip-fired data images contain tracer molecules that have been heated by
the residual gases shown to be mixing with them. Figure 2.8 demonstrates the di-ketone
group’s decreasing intensity yield with increasing temperature at 266-nm laser excitation,
and therefore a ratio of fired data to motored data will slightly over-predict our residual gas
fraction (Equation 6.1).
For these calculations, only background subtraction was applied to the raw CCD
image data, to preserve the detection system’s absolute scaling at each image timing and
engine condition. For each of the 15 engine speed/load/overlap conditions, the intensity ratio
was calculated as the mean of the four image acquisition timings by Equation 6.1:
114
30 45 60 99
114
data data data dataratio
ff ff ff ff
I I I III I I I
− − − −
= − + + +
(6.1)
The correlation between ratioI and ry is shown in Figure 6.5. The correlation can
be considered surprisingly good, given our assumptions about the ROI. The temperature
dependence of fluorescence appears in this plot as the offset between the slope of the data
point grouping, which is not far from parallel to the 1:1 line. The influence of residual gas
charge heating on PLIF uncertainty indicated by this correlation certainly has important
implications for the results of this project. A likely source of scatter in Figure 6.5 is slight
inconsistencies in mean laser sheet power between corresponding flatfield and skip-fire
imaging measurements.
0 10 20 30 40 50 60 70 800
10
20
30
40
50
60
70
80
Yr [%]
< I ra
tio >
[%
]
Figure 6.5 Correlation of mean image intensity ratio to measured residual fraction for all 15 experiment conditions.
115
6.3. Correlation of Residual Gas Fraction to Image Intensity Variation
The relationship between the degree of mean spatial variation in the corrected data
images and the bulk residual gas fraction was investigated across all 15 experiment
conditions. Four image timings (30°, 45°, 60°, and 99° bTDC) were studied at each
condition and a consistent trend of increasing charge stratification with increasing residual
fraction was found for all timings. This trend invited further exploration of our image data
based on residual fraction, independent of engine speed or load.
6.3.1. Cycle-Averaged Image Intensity COV Correlation
Figures 6.6 through 6.9 present the correlation observed between measured residual
gas fraction and the ensemble-mean pixel intensity variation, ( )y yσ µ , captured in the
skip-fired data images. Separate plots are prepared for each of the four image timings and
each plot contains the appropriate reading for each of the 15 experiment conditions.
Additionally, the spatial variations for the corresponding homogeneous image data are shown
as a relative measure of the absolute shift in stratification when measuring the fired engine
flow.
The sequence in Figures 6.6-6.9 includes the mean SNR values calculated from all
data points at the individual image timings (Table 5.2). This value clearly decreases with the
lower-charge density images at the advanced timings. This effect is also seen in the
incremental upward shift in homogeneous image variation level at the advanced timings.
116
0 5 10 15 20 25 30 35 40 45 500
2
4
6
8
10
12
30° bTDC
Yr [%]
<(σ
y / µ y)>
[%
]
Fired DataHomogeneous
Figure 6.6 Pixel intensity COV vs. residual gas fraction for all engine conditions at 30° bTDC. Shot noise-limited maximum SNR was ~22:1 for this image timing.
0 5 10 15 20 25 30 35 40 45 500
2
4
6
8
10
12
45° bTDC
Yr [%]
<(σ
y / µ y)>
[%
]
Fired DataHomogeneous
Figure 6.7 Pixel intensity COV vs. residual gas fraction for all engine conditions at 45° bTDC. Shot noise-limited maximum SNR was ~20:1 for this image timing.
117
0 5 10 15 20 25 30 35 40 45 500
2
4
6
8
10
12
60° bTDC
Yr [%]
<(σ
y / µ y)>
[%
]
Fired DataHomogeneous
Figure 6.8 Pixel intensity COV vs. residual gas fraction for all engine conditions at 60° bTDC. Shot noise-limited maximum SNR was ~18:1 for this image timing.
0 5 10 15 20 25 30 35 40 45 500
2
4
6
8
10
12
99° bTDC
Yr [%]
<(σ
y / µ y)>
[%
]
Fired DataHomogeneous
Figure 6.9 Pixel intensity COV vs. residual gas fraction for all engine conditions at 99° bTDC. Shot noise-limited maximum SNR was ~15:1 for this image timing.
118The most notable feature of Figures 6.6-6.9 is the quasi-exponential growth in
( )y yσ µ at the highest recorded residual gas fractions. This trend, particularly the
transition range of 35% to 40% residual fraction where both 600 RPM and 1200 RPM data
points are located, indicates that there is a residual dilution level at which mixture
composition inhomogeneity begins to rapidly increase, independent of speed or load. Since
the engine could only exceed 40 % residual fraction at 1200 RPM, the maximum ( )y yσ µ
points naturally occur at 1200 RPM only.
Figures 6.6 through 6.9 also demonstrate the absolute magnitude shift in the intensity
variation metric from the motored flatfield condition. Corrected homogeneous images fall
near 1-2% ( )y yσ µ , depending primarily on the shot noise encountered at the image
timing. The difference between the fired data points and the homogeneous points is a clear
and consistent sign of the presence of residual gas unmixedness.
6.3.2. Lower Residual Fraction Case-to-Case Comparison
At the lower end of the measured scale in Figures 6.6-6.9, the nearest match between
600 RPM and 1200 RPM conditions was for the baseline overlap 1200 RPM set ( ry =27.3%)
and the zero overlap 600 RPM low load set ( ry =28.7%). The ( )y yσ µ data for these
conditions are presented in Table 6.3. With the exception of the early 99° bTDC timing, the
1200 RPM data show consistently lower variation than 600 RPM. Both data sets
demonstrate an increasing image intensity variation at 30° bTDC.
119% ry 30° bTDC 45° bTDC 60° bTDC 99° bTDC
600 RPM, Zero Overlap 28.7 5.21 4.37 4.44 5.47 1200 RPM, Baseline OV 27.3 4.18 3.99 4.09 5.90
Table 6.1 Comparison of lower-residual conditions at 600 and 1200 RPM. Development of
image ( )y yσ µ [%] with crank angle.
The data of Table 6.3 would seem to suggest an engine speed influence on residual
gas mixing at this dilution level, with the higher engine speed doing a better job of mixing
the residual gas with the homogeneous fresh charge in the imaging plane. Sample image
data acquired for the two conditions at 45° bTDC are presented in Figures 6.10 and 6.11.
Yr = 28.7%, <(σy / µy)> = 4.37%
600 RPM Low Load, Zero OV, 45° bTDC
110
120
130
140
150
160
170
180
0.9 × Imax
0.5 × Imax
Figure 6.10 Sample data images for 600 RPM, low-residual condition.
Yr = 27.3%, <(σy / µy)> = 3.99%
1200 RPM, Baseline OV, 45° bTDC
120
130
140
150
160
170
180
190
0.9 × Imax
0.5 × Imax
Figure 6.11 Sample data images for 1200 RPM, low-residual condition.
120Visually comparing the 600 RPM data images in Figure 6.10 with the 1200 RPM set
in Figure 6.11 indicates the difficulty in qualitatively distinguishing two conditions. The
( )y yσ µ values for these are in fact similar, with the brighter and more numerous flow
structures shown at 600 RPM likely accounting for the difference. The PDF is a more
quantitative comparison between the two conditions, as shown in Figures 6.12-6.13.
Figures 6.12-6.13 are an introduction to the nature of the residual gas data image
series PDF, where a substantial deviation from the homogeneous mixture data is seen. At
600 RPM, where the mean intensity variation was lower and shows a slightly lower peak in
the distribution, although the homogenous data was also less tightly grouped.
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50
5
10
15
20
25
30
45° bTDC600 RPM Low LoadZero Overlap
I / Imean
PD
F
HomogeneousFired Data
Yr = 28.7 %<(σy / µy)> = 4.37 %
Figure 6.12 100-image pixel intensity PDF for 600 RPM low-residual condition.
121
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50
5
10
15
20
25
30
45° bTDC1200 RPMBaseline Overlap
I / Imean
PD
F
HomogeneousFired Data
Yr = 27.3 %<(σy / µy)> = 3.99 %
Figure 6.13 100-image pixel intensity PDF for 1200 RPM low-residual condition.
6.3.3. Higher Residual Fraction Case-to-Case Comparison
Since the engine was more tolerant of elevated residual fractions at 1200 RPM,
comparison of image data between engine speeds was not possible for the maximum dilution
levels. The closest match at the high-end occurred between the 600 RPM low load,
symmetric increased overlap ( ry = 40.4%) and the 1200 RPM intake cam advance condition
( ry = 40.8%). Again, the development in ( )y yσ µ is presented in Table 6.4.
122% ry 30° bTDC 45° bTDC 60° bTDC 99° bTDC
600 RPM, Sym. Incr. OV 40.4 5.93 5.19 5.15 6.68 1200 RPM, Intake Advance 40.8 5.92 7.09 7.78 8.02
Table 6.2 Comparison of higher-residual conditions at 600 and 1200 RPM. Development of ( )y yσ µ [%] with crank angle.
At the latest image timing, the variation was identical for the two conditions.
However, the earlier development of intensity variation was much different, with the 600
RPM condition reaching an early minimum at 60° bTDC and the 1200 RPM data decreasing
steadily from a high initial level of 8%. At this dilution level, the influence of engine speed
seems to be reversed, as the longer mixing time found at 600 RPM produced lower mixture
variation in the ROI. Given the conflicting conclusions on engine speed, it would then
appear that bulk residual gas fraction is a more applicable parameter in predicting the mixture
inhomogeneity. Sample data images for the two conditions of this section are shown in
Figures 6.14-6.15.
123
Yr = 40.4%, <(σy / µy)> = 5.19%
600 RPM Low Load, Sym. Incr., 45° bTDC
110
120
130
140
150
160
170
180
0.9 × Imax
0.5 × Imax
Figure 6.14 Sample data images for 600 RPM, high-residual condition. 45° bTDC.
Yr = 40.8%, <(σy / µy)> = 7.09%
1200 RPM Intake Advance, 45° bTDC
100
110
120
130
140
150
160
1700.9 × Imax
0.5 × Imax
Figure 6.15 Sample data images for 1200 RPM, low-residual condition.
6.4. Prior-Cycle Effect on Image Intensity Variation
Cylinder pressure was recorded for all engine cycles during the image acquisition
periods and single-cycle IMEP data could be extracted in software. Since the experiment
involved skip-firing, same-cycle pressure data was not relevant to the engine operating
condition. The best correlation possible to the imaged engine flow would have to come from
the previous engine cycle. It was proposed that strong and weak prior cycles would have
some degree of influence over the residual gas fraction on the skip-fired cycle. Sample
124
results of the extracted prior-cycle IMEP and corresponding data image COV ( )y y nσ µ are
shown in Figures 6.16 and 6.17, with the axes scaled relative to the data-set mean. These
figures display the prior-cycle IMEP data for all 100 images at 60° bTDC. Figure 6.16 is for
the 600 RPM low load, symmetric overlap condition and Figure 6.17 is the same overlap
strategy at 1200 RPM. These engine conditions were chosen based on their high residual
fraction and high pixel intensity variation.
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20.75
0.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1.2
1.25
(σy / µy)n / <(σy / µy)>
[IME
P n-1] /
[IM
EP m
ean]
Figure 6.16 Prior-cycle IMEP vs. image intensity COV. 600 RPM Low Load, Sym.
Increase 60° bTDC. Yr = 40.4%, IMEP=152 kPa, COVIMEP = 6.0%, ( )y yσ µ =5.2%.
125
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20.75
0.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1.2
1.25
(σy / µy)n / <(σy / µy)>
[IME
P n-1] /
[IM
EP m
ean]
Figure 6.17 Prior-cycle IMEP vs. image intensity COV. 1200 RPM, Sym. Increase 60°
bTDC. Yr = 43.7%, IMEP=253 kPa, COVIMEP = 1.2%, ( )y y nσ µ
=7.3%.
All engine conditions demonstrated a similar random data scatter and no correlation
was found between prior-cycle IMEP and ( )y y nσ µ . It was assumed that quantifying the
large-scale cylinder mixture influence of the prior cycle combustion performance with a
small ROI is poorly suited for the single-image analysis done here. Ensemble-averaged ROI
data, such as presented in Section 6.3, are more likely to yield a better correlation.
126
6.5. Engine Operating Conditions Effect on Data Image Intensity Variation
A major feature of this project was to establish engine conditions of varying residual
gas dilution by means of valve overlap strategies. For this experiment, a baseline 20° overlap
was established and three strategies for elevated residual fraction were explored – intake cam
advance, exhaust cam retard, and symmetric overlap increase. Finally, zero valve overlap
was studied to establish a minimum residual fraction. Trends of ( )y yσ µ for the various
overlap strategies are shown in Figures 6.18-6.20, organized by speed/load.
-120 -100 -80 -60 -40 -20 00
1
2
3
4
5
6
7
8
9
10
CAD aTDC
<(σ
y / µ y)>
[%
]
600 Low
Baseline OverlapSymmetric IncreaseIntake AdvanceExhaust RetardZero Overlap
Figure 6.18 Mean image intensity variation vs. CA at 600 RPM low load, all overlaps.
127
-120 -100 -80 -60 -40 -20 00
1
2
3
4
5
6
7
8
9
10
CAD aTDC
<(σ
y / µ y)>
[%
]
600 Mid
Baseline OverlapSymmetric IncreaseIntake AdvanceExhaust RetardZero Overlap
Figure 6.19 Mean image intensity variation vs. CA at 600 RPM mid load, all overlaps.
-120 -100 -80 -60 -40 -20 00
2
4
6
8
10
12
CAD aTDC
<(σ
y / µ y)>
[%
]
1200 Low
Baseline OverlapSymmetric IncreaseIntake AdvanceExhaust RetardZero Overlap
Figure 6.20 Mean image intensity variation vs. CA at 1200 RPM, all overlaps.
128The 600 RPM conditions in Figures 6.18 and 6.19 demonstrate a consistent “hook-
up” in ( )y yσ µ approaching 30° bTDC. This trend is presumed to be indicative of a bulk
in-cylinder flow component, mostly likely a tumble motion induced by the pent-roof
geometry, consistently delivering pockets of unmixed fluid to the imaging plane. At the
elevated load conditions in Figure 6.19, the intake cam advance set does not demonstrate the
hook-up trend, indicating that the change in phasing of the induction process may have
affected the timing of the flow at that intake manifold pressure.
The most prevalent trend in the 1200 RPM plot of Figure 6.20 is the large magnitude
shift in flow inhomogeneity from the low-residual baseline and zero overlap conditions to the
increased overlap strategies. As with much of the combustion data in Chapter 4, the exhaust
retard condition at 1200 RPM demonstrated the largest effect of residual dilution, with the
highest image intensity variation levels (and also image-to-image variation) recorded in the
experiment. In general, the 1200 RPM conditions are not as conclusive as to bulk flowfield
influence on residual gas transport as the 600 RPM data.
6.5.1. Symmetric Overlap Increase
At the 600 RPM low load condition, the symmetric 10° increase in valve overlap did
not significantly shift ( )y yσ µ from the baseline values at either 99° or 60° bTDC (6.8%
and 5.2%, respectively). At the later timings, image variation became more pronounced,
particularly at 45° bTDC, where a 12% increase in ( )y yσ µ from the baseline was
129calculated. In general, the level of variation was not distinguishable from the other overlap
strategies.
The mid load condition at 600 RPM demonstrated a flatter progression in Figure 6.19
through the compression stroke, roughly splitting the difference in ( )y yσ µ between the
other two increased-overlap strategies. Similar behavior is seen in the 60° total overlap 1200
RPM data in Figure 6.20, although the level of variation is noticeably increased with the
higher residual fraction (43.7%) at this condition.
6.5.2. Intake Cam Advance
In Figure 6.18, the intake cam advance data points are indistinguishable in ( )y yσ µ
from the exhaust retard condition and at the later timings also from the symmetric increased
overlap. This plot seems to indicate that at 600 RPM low load the effect of cam phasing
strategy is small, perhaps limited by the small increases in dilution level that were tolerable.
At the higher IMEP conditions in Figures 6.19 and 6.20, there are clearer distinctions
in the behavior of the three strategies. At both 600 RPM and 1200 RPM, the intake cam
advance strategy delivered the lowest intensity variation at high residual fraction. The 1200
RPM data set is the only one in the experiment to exhibit continuously decreasing ( )y yσ µ
throughout the four image timings. The improvements in image inhomogeneity can be
partially attributable to the lower measured residual gas fraction relative to the symmetric
increase and exhaust retard cases. Figures 6.21 through 6.24 compare intake advance and
exhaust retard imaging at both speeds, followed by the PDF’s for all 4 images.
130
Yr = 32.5%, <(σy / µy)> = 4.09%
600 RPM Mid Load, Int. Advance, 45° bTDC
110
120
130
140
150
160
170
180
190
0.9 × Imax
0.5 × Imax
Figure 6.21 Intake advance data images at 600 RPM Mid Load. 45° bTDC.
Yr = 32.7%, <(σy / µy)> = 4.92%
600 RPM Mid Load, Exh. Retard, 45° bTDC
130
140
150
160
170
180
190
200
210
220
0.9 × Imax
0.5 × Imax
Figure 6.22 Exhaust retard data images at 600 RPM Mid Load. 45° bTDC.
Yr = 40.8%, <(σy / µy)> = 7.09%
1200 RPM Intake Advance, 45° bTDC
100
110
120
130
140
150
160
1700.9 × Imax
0.5 × Imax
Figure 6.23 Intake advance data images at 1200 RPM. 45° bTDC.
Yr = 44.8%, <(σy / µy)> = 8.71%
1200 RPM Exhaust Retard, 45° bTDC
110
120
130
140
150
160
170
180
190
0.9 × Imax
0.5 × Imax
Figure 6.24 Exhaust retard data images at 1200 RPM. 45° bTDC.
131
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50
5
10
15
20
25
30
45° bTDC600 RPM Mid LoadIntake Cam Advance
I / Imean
PD
F
HomogeneousFired Data
Yr = 32.5%<(σy / µy)> = 4.09%
Figure 6.25 Intake advance 100-image pixel intensity PDF at 600 RPM Mid Load, 45° bTDC.
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50
5
10
15
20
25
30
45° bTDC600 RPM Mid LoadExhaust Cam Retard
I / Imean
PD
F
HomogeneousFired Data
Yr = 32.7 %<(σy / µy)> = 4.92%
Figure 6.26 Exhaust retard 100-image pixel intensity PDF at 600 RPM Mid Load, 45° bTDC.
132
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50
5
10
15
20
25
30
45° bTDC1200 RPMIntake Cam Advance
I / Imean
PD
F
HomogeneousFired Data
Yr = 40.8%<(σy / µy)> = 7.09%
Figure 6.27 Intake advance 100-image pixel intensity PDF at 1200 RPM 45° bTDC.
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50
5
10
15
20
25
30
45° bTDC1200 RPMExhaust Cam Retard
I / Imean
PD
F
HomogeneousFired Data
Yr = 44.8%<(σy / µy)> = 8.71%
Figure 6.28 Exhaust retard 100-image pixel intensity PDF at 1200 RPM 45° bTDC.
1336.5.3. Exhaust Cam Retard
As shown in the previous image sequence, the exhaust cam retard strategy
demonstrated unique qualitative image behavior relative to the intake advance conditions at
both 1200 RPM and 600 RPM mid load. Particularly at the lower residual gas fraction 600
RPM case in Figure 6.21, the intake advance data images show a distinctly more continuous
distribution of tracer in the images. Using the consistent 90%/50% scaling method,
structures in the intake advance ROI tend to be defined by “shades of gray”, as opposed to
“black and white” behavior in the exhaust retard images.
The exhaust cam retard strategy yielded the most consistently high ( )y yσ µ values
in the experiment. Even at the 600 RPM mid-load condition where the symmetric overlap
increase gave an 11% larger increase in residual fraction from the baseline (Table 4.6), the
exhaust retard data display a higher magnitude of intensity variation.
134
7. Summary and Conclusions
7.1. Project Summary
An experiment was performed to measure mixing processes between fresh intake
charge and residual gas in a spark-ignition engine. A single-cylinder, optically accessible
test engine was upgraded with an adjustable dual-overhead cam cylinder head for providing a
range of residual gas fraction conditions. A direct in-cylinder sampling valve was used to
measure bulk residual fraction, while exhaust emissions and cylinder pressure analyses
quantified engine operation at 15 distinct conditions.
Laser-induced fluorescence imaging was performed during the later portion of the
compression stroke during a programmed skip-fired engine operation cycle to allow use of a
fast-shuttering non-intensified CCD camera. The fluorescence was obtained with 3-
pentanone doped into the iso-octane fuel. The premixed fuel system provided a
homogeneously mixed intake charge and the residual gases were found to be largely devoid
of fluorescent components. Thus, the image inhomogeneity was indicative of fresh
charge/residual gas mixing.
An image correction algorithm was developed to extract engine flow information
from the raw fluorescence data. First and second statistical moments were calculated in a
manner which avoided error introduced by correction process imperfections. Normalized
spatial variation in fluorescence intensity, corresponding to compressed charge
135inhomogeneity, was quantified for the 15 experimental conditions at four unique crank angle
timings and compared with the combustion analyses.
7.2. Results Summary
The engine was considerably more tolerant to residual gas dilution at 1200 RPM than
at 600 RPM, particularly at the low load (MAP) condition. IMEP levels increased for all
extended overlap conditions, although change was lower at 600 RPM than 1200 RPM. This
improvement appeared to be largely a result of increasing volumetric efficiency as indicated
by PMEP, which dropped up to 37% from the baseline overlap at 1200 RPM. The zero valve
overlap condition reversed these trends from the baseline. Combustion stability, denoted by
COV of IMEP, was very good throughout the experiment (< 6%). Perhaps more importantly,
misfires, which could strongly skew cycle-resolved residual gas measurements, were not
recorded at any condition.
Heat release rates and cumulative burn curves showed a very prolonged burning
duration indicative of elevated charge dilution. Ignition delay and flame development, as
indicated by the 0-10% HR duration, were also quite long at all conditions. At the low load
600 RPM condition, the three extended overlap strategies generated comparable burn rate
data, with approximately a 30% increase in 10-90% HR duration. The mid-load condition at
600 RPM was similarly insensitive to overlap strategy, and the increases from the baseline
overlap values were smaller. Due to the larger increase in overlap at 1200 RPM, the
magnitude of the burn duration increase was more notable. Intake advance and symmetric
136increase conditions showed comparable heat release data, while the exhaust retard case was a
clear outlier, resulting in the largest flame development and overall burn angles in the
experiment. All zero overlap conditions showed a reduction in flame development time, with
little change in overall burn duration.
Exhaust emission measurements confirmed the highly fuel-rich operating conditions,
with measured equivalence ratios of Φ = 1.14 through 1.36. Not surprisingly, combustion
efficiency values were consistently below 90%, with high CO and unburned HC levels.
Hydrocarbon readings were known to be skewed by the large crevice volume of the optical
engine, although the extended heat release curves indicate substantial incomplete
combustion. The intake cam advance condition generated the lowest CO and HC levels of
the extended overlap strategies for all three speed/load points, resulting in the smallest
penalty in combustion efficiency from the baseline overlap. Emission measurements at zero
valve overlap were comparable to the baseline conditions.
Bulk residual gas fraction measurements ranged from 27% to 38% at the baseline
overlap, because of the low compression ratio and throttled operation. Given the high
residual fraction at the baseline condition, at 600 RPM the residual fractions did not increase
significantly with the 10° overlap increases, and at low-load, the growth was essentially
indifferent to cam strategy. The mid-load condition showed a slightly higher sensitivity to
overlap increase, with the symmetric increase case yielding the largest increase from the
baseline dilution level. The 1200 RPM conditions contained a more distinct indication of
cam phasing effect on residual gas fraction. With the 40° increase in valve overlap, all three
strategies were far above the baseline residual, with the intake cam advance giving the
smallest increase (+50% to ry =41%) and the exhaust retard case giving the largest residual
137fraction recorded (+64% from baseline to ry =45%). Zero overlap resulted in roughly 20%
decreases in residual gas fraction at all three speed/load points.
The measurement technique of visualizing residual gas mixing by fluorescing a
homogeneous fresh charge was verified using PLIF tests of compressed charge homogeneity
during non-firing operation, and of fluorescence intensity of unburned hydrocarbons
contained in the residual gas. Spatial variation in the corrected homogeneous images was
shown to correlate very close to the theoretical shot noise floor, indicating a sufficiently
homogeneous fresh charge. A fuel cutoff experiment performed with direct fuel injection
indicated that for non-misfire cycles, the fluorescence intensity of residual compounds was
less than 3% of the homogeneous charge flatfield signal.
The mean fluorescence signal of the skip-fired residual gas data images was
compared to that of the motored flatfield at the same image timing. Although this ratio was
subject to inherent spatial uncertainty, it over-predicted the measured residual gas fraction by
an average of 52%. This result was largely attributed to different in-cylinder temperatures
between the fired and motored cases, which affects the fluorescence yield of the tracer
molecules. The 15 measurement conditions did generally follow a 1:1 increasing slope with
the measured data.
The spatial variation of the images was shown to shift significantly from the
homogenous images’ low values, which were near the noise floor and varied from 1.5% to
4% at the lowest-residual conditions. The absolute shift in image behavior was also
quantified by probability density functions of pixel intensity across the 100-image sets.
At all image timings, except the low-SNR 99° bTDC points, data image sets taken at
conditions with ry < 35% tended to converge to ( )y yσ µ values between 4% and 5%, not
138considering engine speed/load or overlap strategy. Beyond 35% residual gas fraction and
particularly for the 1200 RPM data where ry > 40%, the mean spatial variation metric grows
rapidly with residual fraction to a peak level of 9%. Data acquired at 45° bTDC and 60°
bTDC show a close grouping of data points along this quasi-exponential curve. The latest
image timing, 30° bTDC, demonstrated slightly more condition-to-condition variation.
At the two 600 RPM load points, an increasing slope in the spatial variation term was
found at 30° bTDC, which was considered indicative of a bulk tumbling transport motion of
residual gases through the ROI during compression.
The intake cam advance and exhaust cam retard conditions were compared, as they
routinely provided the best and worst cases, respectively, for spatial variation levels at
elevated valve overlaps.
7.3. Conclusions
The most significant result of this project was the qualitative and quantitative
observation of inhomogeneity of the compressed charge near ignition timing at elevated
residual fractions in the range of 21-45%. This information is important for combustion
modeling in high-dilution engines. In particular, the notion of homogenously distributed
residual gases in the central regions of the combustion chamber has been brought under some
doubt in this project.
The rate of growth of the spatial variation of the charge with increasing residual
fraction at all four experiment image timings (Figures 6.6-6.9) indicate that a significant
139increase in charge stratification occurs at ry ≈ 35%. This correlation was observed across
the speed/load points and overlap strategies, which indicates that the bulk residual fraction
can be a dominant term in predicting the level of compressed charge inhomogeneity.
Of the three cam-phasing strategies employed to obtain the high residual fractions,
the intake cam advance case was shown to provide the lowest charge inhomogeneity levels
for a given overlap duration. This was partly explained by the fact that intake advance also
provided slightly lower residual fractions than exhaust retard or symmetric increase. But
additionally, intake advance introduces the fresh charge to the combustion chamber earliest,
thereby maximizing the mixing time with the residual gas. The exhaust retard cases had the
latest IVO timing of the extended overlaps, and at 1200 RPM demonstrated the highest
spatial variation measurements of the project. From a mixedness point of view, it can then be
argued that the intake cam advance strategy is preferable as it provides the largest amount of
time for charge mixing before ignition.
This project was successful in generating a range of residual fractions that can be
considered appropriate for a high-dilution design. Fuel-rich operation and a large spark gap
were demonstrated to be useful in obtaining stable running conditions at these dilution levels
in an optical engine.
The technique of tracking residual gases by negative-PLIF of a homogenous
air/fuel/tracer mixture was demonstrated to be sound. A fundamental issue of pre-ignition
in-cylinder luminosity was discovered, which has important implications in the use of high-
fidelity CCD cameras for these measurements. This project demonstrated the utility of the
frame-straddling camera in circumventing this problem.
140
7.4. Recommendations for Future Work
This project covered the high end of residual gas dilution levels in spark ignition
engine designs. The results of the correlation between data image spatial variation and bulk
residual fraction suggested a very slow rate of decrease in charge inhomogeneity below ry =
20%. It was shown in this project that the zero-residual fraction homogeneous condition
demonstrated a spatial variation near that of the shot noise floor. This means that this trend
of ( )y yσ µ vs. ry must at some point break from its leveled-off spatial variation toward
homogeneity in the low and moderate range of residual fractions not covered in this
experiment. With spatial variation still clearly present at 21% residual fraction, data below
this level will certainly be desirable, particularly from an experiment which could identify a
threshold residual gas fraction below which homogeneity can be safely assumed.
A more robust research engine could be used to explore the range of residual fractions
above 40% where the rapid increase in charge stratification was measured. Extensive heat
release data from a more extensive parametric study of engine conditions could better
identify this behavior as being a fundamental limitation to high-dilution engines, or a more
engine-specific phenomenon of insufficient in-cylinder mixing. This information would
likely be very valuable to HCCI research, where novel variable-cam re-breathing strategies,
and therefore very high levels of residual gas dilution, are being pursued.
141
References [1] Heywood, John B., 1988, Internal Combustion Engine Fundamentals, McGraw-Hill,
New York. [2] Fox, J.W., Cheng, W.K., and Heywood, J.B., 1993, “A Model for Predicting Residual
Gas Fraction in Spark Ignition Engines”, SAE 931025. [3] Rhodes, D.B. and Keck, J.C., 1985, “Laminar Burning Speed Measurements of
Indolene-Air-Diluent Mixtures at High Pressures and Temperatures,” SAE 850047. [4] Turns, Stephen R., 1996, An Introduction to Combustion: Concepts and Applications,
McGraw-Hill, New York. [5] Olofsson, E., Alvestig, P., Bergsten, L., Ekenberg, M., Gawell, A., Larsen, A., and
Riemann, R., 2001, “A High Dilution Stoichiometric Combustion Concept Using a Wide Variable Spark Gap and In-Cylinder Air Injection in Order to Meet Future CO2 Requirements and Worldwide Emissions Regulations,” SAE 2001-01-0246.
[6] Ozdor, N., Dulger, M. and Sher, E., 1994, “Cyclic Variability in Spark Ignition
Engines: A Literature Survey,” SAE 940987. [7] Hinze, P.C. and Miles, P.C., 1999, “Quantitative Measurements of Residual and
Fresh Charge Mixing in a Modern SI Engine Using Spontaneous Raman Scattering,” SAE 1999-01-1106.
[8] Miles, P.C. and Hinze, P.C., 1998, “Characterization of the Mixing of Fresh Charge
with Combustion Residuals using Laser Raman Scattering with Broadband Detection,” SAE 981428.
[9] Stanglmaier, R.H. and Roberts, C.E., 1999, “Homogeneous Charge Compression
Ignition (HCCI): Benefits, compromises and Future Engine Applications,” SAE 1999-01-3682.
[10] Epping, K., Aceves, S., Bechtold, R., 2002, “The Potential of HCCI Combustion for
High Efficiency and Low Emissions,” SAE 2002-01-1923. [11] Zhao, Hua and Ladommatos, Nicos, 2001, Engine Combustion Instrumentation and
Diagnostics, Chapter 4, SAE International, Warrendale, PA. [12] Foudray, H.Z., 2002, Scavenging Measurements in a Direct-Injection 2-Stroke
Engine, M.S. Thesis, Department of Mechanical Engineering, University of Wisconsin-Madison.
142 [13] Rothamer, D.A., 2002, Investigation of Flame-Front Equivalence Ratio during
Stratified Engine Combustion, M.S. Thesis, Department of Mechanical Engineering, University of Wisconsin-Madison.
[14] Rothamer, D.A. and Ghandhi, J.B., 2002, “On the Calibration of Single-shot Planar
Laser Imaging Techniques in Engines,” SAE 2002-01-0748. [15] Wiles, M.A., 2003, Characterization of Operating Parameters’ Authority on the
Flow-Field Mixedness of a DISI Engine, M.S. Thesis, Department of Mechanical Engineering, University of Wisconsin-Madison.
[16] Probst, D.M., 2001, Spray Mixing in Engines, M.S. Thesis, Department of
Mechanical Engineering, University of Wisconsin-Madison. [17] Hansen, D.A. and Lee, E.K.C., 1975, “Radiative and Nonradiative Transitions in the
First Excited Singlet State of Symmetrical Methyl-Substituted Acetones,” J. Chem. Physics., v.62 no.183.
[18] Thurber, M.C., Grisch, F., Kirby, B.J., Votsmeier, M., and Hanson, R.K., 1998,
“Measurements and Modeling of Acetone Laser-Induced Fluorescence with Implications for Temperature Imaging Diagnostics,” Applied Optics, v.37 no.21 pp4963-4978.
[19] Thurber, M.C. and Hanson, R.K., 1999, “Pressure and Composition Dependences of
Acetone Laser-Induced Fluorescence with Excitation at 248, 266 and 308 nm,” Applied Physics, v.69, pp.229-240.
[20] Lebel, M. and Cottereau, M.J., 1992, “Study of the Effect of the Residual Gas
Fraction on Combustion in a S.I. Engine Using Simultaneous CARS Measurements of Temperature and CO2 Concentration,” SAE 922388.
[21] Baritaud, T.A. and Heinze, T.A., 1992, “Gasoline Distribution Measurements with
PLIF in a SI Engine,” SAE 922355. [22] Johansson, B., Neij, H., Juhlin, G., and Alden, M., 1995, “Residual Gas Visualization
with Laser Induced Fluorescence,” SAE 952463. [23] Deschamps, B. and Baritaud, T.A., 1996, “Visualization of Gasoline and Exhaust
Gases Distribution in a 4-Valve SI Engine; Effects of Stratification on Combustion and Pollutants,” SAE 961928.
143[24] Roper Scientific, 2000, Princeton Instruments 5MHz MicroMax User Manual. [25] Stivender, D.L., 1971, “Development of a Fuel-Based Mass Emission Measurement
Procedure,” SAE 710604. [26] Albert, B.P., 2004, Residual Gas Effects on Combustion in an Air-Cooled Utility
Engine, M.S. Thesis, Department of Mechanical Engineering, University of Wisconsin-Madison.
144
Appendix A – Engine Operating Conditions
600 RPM Low Load 600 RPM Mid Load 1200 RPMRPM [RPM] 600 600 1200EVO [aTDC] 140 140 140EVC [aTDC] 370 370 370IVO [aTDC] 350 350 350IVC [aTDC] -135 -135 -135Exhaust MAP [kPa] 100.0 100.0 100.0Intake MAP [kPa] 49.6 61.3 51.7m_air [mg/cycle] 144 208 181m_fuel [mg/cycle] 14.5 18 18IGN [aTDC] -60 -40 -40T_exh [C] 347 400 477
IMEP [kPa] 147.25 260.91 228.15COV of IMEP [%] 4.1 2.05 1.32PMEP [kPa] 46.22 35.33 51.08Peak Pressure [kPa] 1021 1416 1280Location of PP [aTDC] 12 13.5 12
0-10% HR [CAD] 53 35.5 35.510-90% HR [CAD] 58.5 59.5 68
[CO2] (dry) [%] 8.12 10.64 9.21[CO] (dry) [%] 6.46 4.75 5.94[O2] (dry) [%] 0.42 0.59 0.56[HC] (dry) [ppm C1] 10629 5769 6159n_comb [ ] 0.821 0.899 0.891measured AFR [ ] 10.69 12.1 11.46K_exh [ ] 0.871 0.865 0.871
[CO2] SV [%] 2.759 2.807 2.253K_cc [ ] 0.964 0.968 0.972Y_r [%] 37.65 29.59 27.27
BASELINE OVERLAP
145
600 RPM Low Load 600 RPM Mid Load 1200 RPMRPM [RPM] 600 600 1200EVO [aTDC] 145 145 160EVC [aTDC] 375 375 390IVO [aTDC] 345 345 330IVC [aTDC] -140 -140 -155Exhaust MAP [kPa] 100.0 100.0 100.0Intake MAP [kPa] 54.8 67.2 68.2m_air [mg/cycle] 144 208 181m_fuel [mg/cycle] 14.5 18 18IGN [aTDC] -60 -45 -60T_exh [C] 340 389 462
IMEP [kPa] 151.65 270.68 253.40COV of IMEP [%] 6.03 2.07 1.20PMEP [kPa] 45.00 30.32 35.32Peak Pressure [kPa] 796 1437 1228Location of PP [aTDC] 17 14 15
0-10% HR [CAD] 61.0 40.5 56.510-90% HR [CAD] 78.5 67.0 86.5
[CO2] (dry) [%] 8.11 10.73 8.86[CO] (dry) [%] 6.33 4.55 6.04[O2] (dry) [%] 1.06 0.68 0.79[HC] (dry) [ppm C1] 18822 5838 9015n_comb [ ] 0.719 0.897 0.846measured AFR [ ] 10.54 12.22 11.29K_exh [ ] 0.858 0.865 0.869
[CO2] SV [%] 2.93 3.44 3.50K_cc [ ] 0.958 0.964 0.959Y_r [%] 40.39 35.75 43.65
SYMMETRIC OVERLAP INCREASE
146
600 RPM Low Load 600 RPM Mid Load 1200 RPMRPM [RPM] 600 600 1200EVO [aTDC] 140 140 140EVC [aTDC] 370 370 370IVO [aTDC] 340 340 310IVC [aTDC] -125 -125 -175Exhaust MAP [kPa] 100.0 100.0 100.0Intake MAP [kPa] 53.4 65.5 67.6m_air [mg/cycle] 144 208 181m_fuel [mg/cycle] 14.5 18 18IGN [aTDC] -60 -45 -60T_exh [C] 334 407 474
IMEP [kPa] 156.40 269.82 260.59COV of IMEP [%] 3.89 2.46 1.84PMEP [kPa] 45.37 31.11 32.21Peak Pressure [kPa] 896 1385 1256Location of PP [aTDC] 16 15 15
0-10% HR [CAD] 58.0 42.5 56.510-90% HR [CAD] 74.5 66.5 83.5
[CO2] (dry) [%] 8.39 11.32 10.32[CO] (dry) [%] 6.26 3.92 4.96[O2] (dry) [%] 0.88 0.61 0.67[HC] (dry) [ppm C1] 14463 5697 6879n_comb [ ] 0.771 0.899 0.891measured AFR [ ] 10.81 12.48 11.95K_exh [ ] 0.863 0.863 0.865
[CO2] SV [%] 2.91 3.29 3.79K_cc [ ] 0.961 0.966 0.960Y_r [%] 38.66 32.54 40.80
INTAKE CAM ADVANCE
147
600 RPM Low Load 600 RPM Mid Load 1200 RPMRPM [RPM] 600 600 1200EVO [aTDC] 150 150 180EVC [aTDC] 380 380 410IVO [aTDC] 350 350 350IVC [aTDC] -135 -135 -135Exhaust MAP [kPa] 100.0 100.0 100.0Intake MAP [kPa] 55.8 67.9 70.3m_air [mg/cycle] 144 208 181m_fuel [mg/cycle] 14.5 18 18IGN [aTDC] -65 -45 -65T_exh [C] 336 389 435
IMEP [kPa] 157.79 271.53 240.73COV of IMEP [%] 3.77 2.64 3.64PMEP [kPa] 43.54 29.07 35.07Peak Pressure [kPa] 929 1385 1218Location of PP [aTDC] 14 15 14
0-10% HR [CAD] 60.5 41.5 59.510-90% HR [CAD] 78.0 69.5 92.5
[CO2] (dry) [%] 8.16 11.07 7.89[CO] (dry) [%] 6.36 4.31 6.65[O2] (dry) [%] 0.90 0.54 0.76[HC] (dry) [ppm C1] 15483 5766 12666n_comb [ ] 0.758 0.899 0.793measured AFR [ ] 10.68 12.27 10.66K_exh [ ] 0.863 0.863 0.869
[CO2] SV [%] 2.92 3.23 3.20K_cc [ ] 0.960 0.965 0.958Y_r [%] 39.85 32.67 44.78
EXHAUST CAM RETARD
148
600 RPM Low Load 600 RPM Mid Load 1200 RPMRPM [RPM] 600 600 1200EVO [aTDC] 130 130 130EVC [aTDC] 360 360 360IVO [aTDC] 360 360 360IVC [aTDC] -125 -125 -125Exhaust MAP [kPa] 100.0 100.0 100.0Intake MAP [kPa] 44.8 58.6 50.7m_air [mg/cycle] 144 208 181m_fuel [mg/cycle] 14.5 18 18IGN [aTDC] -45 -25 -35T_exh [C] 363 418 490
IMEP [kPa] 141.57 255.14 229.25COV of IMEP [%] 2.99 1.20 0.87PMEP [kPa] 51.53 39.21 53.27Peak Pressure [kPa] 977 1306 1182Location of PP [aTDC] 13 17 15
0-10% HR [CAD] 40.0 26.5 33.010-90% HR [CAD] 60.0 58.5 72.0
[CO2] (dry) [%] 7.92 10.51 8.37[CO] (dry) [%] 6.56 4.75 6.36[O2] (dry) [%] 0.92 0.71 0.75[HC] (dry) [ppm C1] 10551 5253 6858n_comb [ ] 0.821 0.906 0.877measured AFR [ ] 10.93 12.19 11.24K_exh [ ] 0.872 0.867 0.876
[CO2] SV [%] 2.04 2.29 1.64K_cc [ ] 0.971 0.973 0.977Y_r [%] 28.70 24.47 21.87
ZERO OVERLAP
149
Appendix B – Image Statistics
Filtered Unfiltered Filtered Unfiltered Filtered Unfiltered
30 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 5.55 8.40 4.46 6.48 4.18 6.07Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 2.10 4.50 1.71 3.50 1.65 3.19
45 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 4.62 8.94 4.01 6.35 3.99 6.35Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 2.20 4.78 1.81 3.87 1.71 3.59
60 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 5.22 9.56 4.10 6.82 4.09 6.82Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 2.71 5.99 1.94 4.16 1.86 4.06
99 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 6.81 12.47 5.26 8.72 5.90 9.26Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 3.13 7.14 2.34 5.22 2.31 5.22
600 RPM Low Load 1200 RPM600 RPM Mid LoadBASELINE OVERLAP
142.3
67.8
49.5
70.817.65
25.02
17.27
23.82
20.11
128.9
91.9
12.03
14.76
113.822.38
196.829.43
126.023.55
241.432.59
75.420.27
169.227.29
93.4
110.322.03
221.131.19
137.124.56
289.135.67
18.21
112.022.20
16.25
20.21
32.9 62.6 60.0
65.3 114.2 92.822.42
16.60
16.95
150
Filtered Unfiltered Filtered Unfiltered Filtered Unfiltered
30 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 5.93 7.75 5.17 7.60 8.24 10.71Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 1.73 3.35 1.88 3.81 1.72 3.47
45 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 5.19 7.47 4.99 8.22 8.41 11.95Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 1.92 3.88 2.15 4.61 1.90 4.09
60 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 5.15 8.17 4.70 8.00 7.31 11.61Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 2.18 4.46 2.19 4.77 2.29 5.10
99 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 6.68 10.56 6.08 10.05 9.37 15.44Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 2.56 5.65 2.62 5.87 2.51 5.64
SYMMETRIC OVERLAP INCREASE600 RPM Low Load 600 RPM Mid Load 1200 RPM
110.9 97.4 70.722.09 20.70 17.64
256.7 212.5 245.333.61 30.58 32.85
93.8 73.0 50.120.32 17.92 14.85
192.6 196.8 172.629.11 148.00 27.56
71.2 67.0 42.817.70 17.17 13.72
151.0 127.8 116.825.78 23.71 22.67
47.3 47.9 28.614.43 14.52 11.22
96.2 89.8 100.520.57 19.88 21.03
151
Filtered Unfiltered Filtered Unfiltered Filtered Unfiltered
30 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 6.03 8.52 4.07 6.30 5.92 8.13Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 2.04 4.08 1.67 3.44 1.61 3.14
45 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 5.34 8.27 4.09 6.65 7.09 9.31Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 1.93 4.14 1.76 3.78 1.59 3.24
60 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 5.77 9.67 4.49 7.56 7.78 10.30Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 2.26 5.01 1.97 4.31 1.74 3.62
99 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 7.45 12.73 5.64 9.95 8.02 12.19Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 3.05 6.86 2.48 5.64 2.12 4.65
INTAKE CAM ADVANCE600 RPM Low Load 600 RPM Mid Load 1200 RPM
89.8 121.7 95.019.88 23.14 20.45
194.6 249.5 302.629.26 33.13 36.49
82.8 106.0 84.119.09 21.60 19.24
181.3 210.8 273.028.24 30.46 34.66
62.2 81.3 70.516.54 18.91 17.61
134.1 169.8 218.224.29 27.33 30.99
42.1 56.8 41.613.61 15.81 13.53
84.5 110.1 136.519.28 22.01 24.51
152
Filtered Unfiltered Filtered Unfiltered Filtered Unfiltered
30 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 5.85 7.71 5.46 9.02 8.74 10.74Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 1.79 3.54 1.74 5.13 1.60 3.17
45 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 4.96 7.16 4.92 7.16 8.71 10.88Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 1.83 3.70 1.81 3.73 1.62 3.44
60 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 5.92 8.46 4.95 7.47 8.90 11.60Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 2.04 4.19 1.98 4.10 1.75 3.78
99 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 7.44 10.94 6.64 9.72 10.77 15.41Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 2.48 5.42 2.38 5.11 2.19 4.88
EXHAUST CAM RETARD600 RPM Low Load 600 RPM Mid Load 1200 RPM
122.1 127.5 92.023.18 23.69 20.12
258.4 252.5 307.133.72 33.33 36.76
102.9 103.7 82.121.28 21.36 19.01
212.1 209.5 254.530.55 30.36 33.46
79.7 93.4 66.618.73 20.27 17.12
167.5 182.2 208.727.15 28.31 30.30
51.8 61.8 39.515.10 16.49 13.18
106.0 119.7 136.921.60 22.95 24.54
153
Filtered Unfiltered Filtered Unfiltered Filtered Unfiltered
30 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 5.21 7.09 4.46 6.02 4.34 6.18Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 1.84 3.67 1.68 3.34 1.74 3.50
45 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 4.37 6.84 3.82 5.82 3.95 6.08Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 1.92 3.98 1.72 3.62 1.65 3.46
60 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 4.44 7.50 4.03 6.63 4.14 6.64Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 2.18 4.58 1.94 4.18 1.84 4.00
99 bTDC Mean Data Image SignalMean Data SNRFired <(σ_y/µ_y)> 5.47 9.63 5.39 8.77 6.07 9.38Mean FF Image SignalMean FF SNRFlatfield <(σ_y/µ_y)> 2.75 6.03 2.51 5.63 2.46 5.54
ZERO OVERLAP600 RPM Low Load 600 RPM Mid Load 1200 RPM
118.0 156.0 137.222.79 26.20 24.57
219.7 261.1 238.731.09 33.89 32.41
100.3 132.0 123.921.01 24.10 23.35
187.7 218.8 236.628.74 31.03 32.27
78.5 97.3 98.618.58 20.69 20.83
145.1 167.2 178.625.27 27.12 28.03
50.8 62.7 60.214.95 16.61 16.28
91.6 100.4 104.620.08 21.02 21.45