Residual Gas Fraction (Bright)

8/21/2019 Residual Gas Fraction (Bright)

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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


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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.


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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.


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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 R ESIDUAL GAS STUDY ................................................................ 11.1.1. Small Engines Issues............................................................................................. 2

1.1.2. High-Dilution Automotive Engines....................................................................... 3 1.1.3. Homogeneous-Charge Compression-Ignition ...................................................... 4

1.2. PROJECT OBJECTIVES................................................................................................. 61.3. OUTLINE .................................................................................................................... 6

2. BACKGROUND............................................................................................................. 8

2.1. R ESIDUAL 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 R ESIDUAL 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. O NE-DIMENSIONAL STUDIES OF R ESIDUAL GAS...................................................... 17

2.3.1. Early Work.......................................................................................................... 17

2.3.2. Recent Work ........................................................................................................ 19 2.4. PLANAR LASER -I NDUCED 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 E NGINES .......................................................................... 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 R ESEARCH E NGINE .................................................................... 36


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3.1.1. 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 A NALYSIS .......................................................................................... 67

4.2.1. Cylinder Pressure Data ...................................................................................... 68 4.2.2. Heat Release Analysis......................................................................................... 69

4.3. EXHAUST GAS EMISSIONS MEASUREMENT.............................................................. 744.3.1. Emissions Measurement Procedure.................................................................... 75

4.3.2. Emissions Analysis.............................................................................................. 75

4.3.3. Emissions Measurements.................................................................................... 77 4.4. BULK R ESIDUAL 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


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5.2.2. 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 I NTAKE CHARGE HOMOGENEITY................................................. 102

5.3.1. First and Second Moments of Homogeneous Data........................................... 102 5.3.2. Homogeneous Image PDF................................................................................ 104

5.4. DIRECT-I NJECTION 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 I NTENSITY WITH MEASURED R ESIDUAL GASFRACTION .......................................................................................................................... 113

6.3. CORRELATION OF R ESIDUAL GAS FRACTION TO IMAGE I NTENSITY 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 I NTENSITY VARIATION ...................................... 123

6.5. E NGINE OPERATING CONDITIONS EFFECT ON DATA IMAGE I NTENSITY 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

............................................................................................... 1347.2. R ESULTS SUMMARY............................................................................................... 1357.3. CONCLUSIONS........................................................................................................ 138

7.4. R ECOMMENDATIONS FOR FUTURE WORK .............................................................. 140

REFERENCES.................................................................................................................... 141

APPENDIX A – ENGINE OPERATING CONDITIONS.............................................. 144

APPENDIX B – IMAGE STATISTICS............................................................................ 149


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List of Figures

FIGURE 1.1. STRATEGIES PURSUED FOR HCCI CONTROL IN CURRENT RESEARCH. R EPRINTED

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 ARANGE OF DILUENT FRACTIONS, EQUIVALENCE RATIOS AND INITIAL BOMB PRESSURES.

R EPRINTED 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. R EPRINTED 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. R EPRINTED FROM [20].......................................................................................... 18

FIGURE 2.4. EXPERIMENTAL SETUP FOR R AMAN SCATTERING MEASUREMENTS IN A MODERN 4-VALVE PENT-ROOF COMBUSTION CHAMBER . R EPRINTED FROM [8]. ................................ 19

FIGURE 2.5. R ESIDUAL GAS MOLE FRACTION VS. CRANK ANGLE, BASED ON ENSEMBLE-

AVERAGED CONCENTRATION MEASUREMENTS OF VARIOUS SPECIES. R EPRINTED FROM [8].......................................................................................................................................... 21

FIGURE 2.6. LEVELS OF VARIANCE IN DATA FOR ENSEMBLE-AVERAGED MEAN RESIDUAL GAS

MOLE FRACTION GIVEN IN FIGURE 2.5. R EPRINTED FROM [8]. ......................................... 22FIGURE 2.7. ABSORPTION AND EMISSION PROPERTIES OF 3-PENTANONE IN LIF APPLICATIONS

[17].................................................................................................................................. 24FIGURE 2.8. MEASURED TEMPERATURE DEPENDENCY OF LIF SIGNAL OF ACETONE AT

ATMOSPHERIC PRESSURE, NORMALIZED TO ROOM TEMPERATURE CONDITION. R EPRINTED

FROM [18]. ....................................................................................................................... 25FIGURE 2.9. MEAN H2O PLIF SIGNAL TREND WITH INTAKE MAP. R EPRINTED FROM [22]..... 31

FIGURE 2.10. CYCLIC VARIATION IN H2O PLIF SIGNAL FOR INCREASING LOAD. R EPRINTED

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. R EPRINTED 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. R EPRINTED 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 TAKENWITH NEGATIVE-PLIF AT SPARK TIMING (27° BTDC). 1200 RPM, ΗVOL = 0.6.

R EPRINTED FROM [23]. .................................................................................................... 35

FIGURE 3.1. VALVETRAIN TIMING LAYOUT FOR DOHC CYLINDER HEAD................................ 41


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FIGURE 3.2. COMPARISON OF MEASURED CYLINDER PRESSURE TRACES AT WALL-MOUNTLOCATION TO CONVENTIONAL ROOF-MOUNT. MOTORING ENGINE CONDITION WITH OHV

HEAD, 1200 RPM............................................................................................................. 51

FIGURE 3.3. I N-CYLINDER SOLENOID-ACTUATED SAMPLING VALVE MOUNTED TO BLOCK -HEAD

SPACER RING. TEFLON SAMPLED GAS LINE TRAVELS TO AN ADJACENT ICE BATH AND THENTO 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. .................... 57FIGURE 3.6 MICROMAX CAMERA MANUAL SUMMARY OF DIF-MODE TIMING. IMAGE

EXPOSURE TIMES ARE SHOWN IN THE SECOND LINE. R EADY AND SCAN ARE OUTPUT

SIGNALS FROM THE CAMERA CONTROLLER , EXT. SYNC IS THE INPUT TRIGGER TTL, LASEROUTPUT SHOWN IS FOR A DOUBLE-PULSE LASER , THIS EXPERIMENT ONLY USES THE FIRST

PULSE. R EPRINTED FROM [24].......................................................................................... 59

FIGURE 3.7 SCHEMATIC FOR TTL TIMING OF LASER PULSE AND CAMERA, SYNCHRONIZED WITHMOTOTRON SKIP-FIRING IGNITION BY A “ONE-AND-ONLY-ONE” CIRCUIT. ....................... 62

FIGURE 4.1 SUMMARY OF FOUR VALVE OVERLAP STRATEGIES. BASELINE CAM TIMING ISINDICATED BY THE DASHED LINE IN ALL PLOTS. ARROWS INDICATE CAM SHIFT FROMBASELINE. 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. ................................................................................................. 71FIGURE 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)....................................................................................................................... 80FIGURE 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) AREOVERLAYED. 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. .............................................................................. 82FIGURE 4.8 FREQUENCY HISTOGRAM OF PRIOR -CYCLE IMEP FOR SKIP-FIRING OPERATION AT

1200 RPM EXHAUST RETARD CONDITION. DATA COMPILED FROM 100 CONSECUTIVESAMPLED 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 EXHAUSTR ETARD CONDITION. FLATFIELD IMAGES HAVE BEEN BACKGROUND-SUBTRACTED. ....... 88

FIGURE 5.3 SAMPLE RAW DATA IMAGE ( NO CORRECTIONS), 30° BTDC 1200 RPM EXHAUST

R ETARD CONDITION. ........................................................................................................ 89


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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. ........................... 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 FORIMAGE TIMINGS IN TABLE 5.1........................................................................................... 97

FIGURE 5.6 CAMERA NOISE CHARACTERIZATION, AS A FUNCTION OF SIGNAL INTENSITY -

MICROMAX FRAME-STRADDLING CCD. R EPRINTED FROM [14]. .................................... 99FIGURE 5.7 COMPARISON OF THEORETICAL SHOT NOISE INTENSITY VARIATIONTO MEASURED

HOMOGENOUS PIXEL INTENSITY VARIATION( ) y yσ µ

................................................ 103FIGURE 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 WITHDOHC BASELINE VALVE OVERLAP. 600 RPM. ............................................................. 108

FIGURE 5.10 DIRECT-INJECTION EXPERIMENT CYLINDER PRESSURE TRACE COMPARISON WITH

DOHC BASELINE VALVE OVERLAP. 1200 RPM. ........................................................... 108FIGURE 6.1 SAMPLE HOMOGENEOUS IMAGE SEQUENCE, 60° BTDC. ..................................... 111

FIGURE 6.2 SAMPLE DATA IMAGE SEQUENCE, HIGH RESIDUAL FRACTION CONDITION, 60°

BTDC. ........................................................................................................................... 111FIGURE 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. .......................................................... 114FIGURE 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 99° BTDC. SHOT NOISE-LIMITED MAXIMUM SNR WAS ~15:1 FOR THIS IMAGE TIMING........................................................................................................................................ 117FIGURE 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.. 120FIGURE 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


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FIGURE 6.15 SAMPLE DATA IMAGES FOR 1200 RPM, LOW-RESIDUAL CONDITION. ............... 123FIGURE 6.16 PRIOR -CYCLE IMEP VS. IMAGE INTENSITY COV. 600 RPM LOW LOAD, SYM.

I NCREASE 60° BTDC. YR = 40.4%, IMEP=152 K PA, COVIMEP = 6.0%, ( ) y yσ µ

=5.2%............................................................................................................................ 124

FIGURE 6.17 PRIOR -CYCLE IMEP VS. IMAGE INTENSITY COV. 1200 RPM, SYM. I NCREASE 60°

BTDC. YR = 43.7%, IMEP=253 K PA, 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. ..................................................................................................................... 126FIGURE 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. ... 127FIGURE 6.21 I NTAKE 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 I NTAKE ADVANCE DATA IMAGES AT 1200 RPM. 45° BTDC. ........................... 130

FIGURE 6.24 EXHAUST RETARD DATA IMAGES AT 1200 RPM. 45° BTDC............................ 130FIGURE 6.25 I NTAKE 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 I NTAKE ADVANCE 100-IMAGE PIXEL INTENSITY PDF AT 1200 RPM 45° BTDC.

....................................................................................................................................... 132FIGURE 6.28 EXHAUST RETARD 100-IMAGE PIXEL INTENSITY PDF AT 1200 RPM 45° BTDC.

....................................................................................................................................... 132


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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 IGNITIONSYSTEM. BOTH CONDITIONS ARE AT 1500 RPM AND 400 K PA 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. ................................. 37TABLE 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 TRACERBLEND 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 ARERELATIVE 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. 67TABLE 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 CAMSTRATEGY. ....................................................................................................................... 68

TABLE 4.3 FLAME DEVELOPMENT ANGLES AND OVERALL BURNING ANGLES FOR DIFFERENTOVERLAP STRATEGIES, DETERMINED BY A SINGLE-ZONE HEAT RELEASE CODE.

PERCENTAGES INDICATED ARE CHANGES RELATIVE TO THE BASELINE OVERLAP CONDITION

AT EACH SPEED/LOAD POINT. ........................................................................................... 73TABLE 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. ................................................................................................... 77TABLE 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). .................................................................................................................... 80TABLE 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. ...................... 85TABLE 5.1 DISTANCE FROM PISTON FACE TO LASER SHEET ROI FOR EXPERIMENT IMAGE

TIMINGS. .......................................................................................................................... 98


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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. .................................................................. 100

TABLE 5.3 DIRECT INJECTION EXPERIMENT ENGINE CONDITIONS AND UNBURNED

HYDROCARBON EMISSIONS MEASUREMENTS. * INDICATES THE APPROXIMATE IGNITIONTIMING. .......................................................................................................................... 106

TABLE 5.4 DIRECT INJECTION EXPERIMENT IMAGING RESULTS. 100-IMAGE MEAN SIGNALLEVEL 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


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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.


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1.1.1. 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.


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1.1.2. 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.


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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-CADwidened valve overlap with direct injection, supplemented with a secondary air injection anda 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


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ignition. 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.


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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


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each 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.


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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].


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Based 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 T ad 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, S L has been measured [24], and for

conventional hydrocarbon fuels has been found to obey the power law equation:


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,0

0 0

u L L

T pS S

T p

α β

=

(2.2)

where the reference values are standard temperature and pressure and S L,0, α and β are

tabulated constants for particular combinations of fuel and equivalence ratio. The term T u

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 r S 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.


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Figure 2.1. Experimental measurements of gasoline laminar flame speed in exhaust gas-diluted mixtures relative to undiluted mixtures, S u(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].


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2.1.4. 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.


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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

comp

r

exh

x x

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


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out 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


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minimum 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.


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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. Reprintedfrom [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.


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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.


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Very 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 ploton 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].


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2.3.2. 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].


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Ensemble-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


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residual 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].


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Figure 2.6. Levels of variance in data for ensemble-averaged mean residual gas molefraction 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.


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Detailed 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


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attenuation 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

0.0 R e

l a t i v e

A b s o r p

t i o n ,

F l u o r e s c e n c e

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


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to 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


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that 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


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flame-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 ( n I ), the influence of the pulse-to-pulse variation in laser intensity is

removed:

1

1 N ntot

nn N I

σ

σ == ∑ (2.6)


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The 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 termedΦ(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, nn

i j

I i j I IJ

σ = Φ −∑ (2.7)

After ensemble averaging, the relative large scale variation is:

n,lf

1

1 N

LF ni N I

σ σ

=

= ∑ (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 n

i j

I i j I i j IJ

σ = Φ −∑ (2.9)


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This value is again ensemble averaged on a normalized basis:

n,hf

1

1 N

HF ni N I

σ σ

=

= ∑ (2.10)

If the ensemble-averaged pixel intensity field ( ),n I 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 n

i j

I i j I i j IJ

σ = Φ −∑ (2.11)

Importantly, ( ),n I 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

cycni N I

σ σ

=

= ∑ (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.


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2.5.2. 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,


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the 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].


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Johansson 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].


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2.5.3. 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


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distribute 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].


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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 atspark 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.


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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.


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Bore [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 internalsingle 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

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Residual Gas Fraction (Bright)

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