EXPERIMENTAL MEASUREMENT AND ANALYSIS OF IN-CYLINDER FUEL-AIR MIXING AND COMBUSTION USING AN OPTICAL DI DIESEL ENGINE UNDER REALISTIC OPERATING CONDITIONS
By
Cody Squibb
A DISSERTATION
Submitted to Michigan State University In partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
Mechanical Engineering
2012 ABSTRACT
EXPERIMENTAL MEASUREMENT AND ANALYSIS OF IN-CYLINDER FUEL-AIR MIXING AND COMBUSTION USING AN OPTICAL DI DIESEL ENGINE UNDER REALISTIC OPERATING CONDITIONS
By
Cody Squibb
In order to improve engine-out emissions and performance of Diesel combustion systems, continued insight into the complex combustion and fuel spray processes must be gained. An optically accessible Diesel engine and modern diagnostic imaging methods can contribute to the understanding of the fuel spray, combustion, and soot formation phenomena that occur during the four-stroke engine cycle. This work will discuss nonintrusive methods of data acquisition used to gather in-cylinder information using an International VT275-based, optical DI Diesel engine with an electro-mechanical engine control system implemented for consistency of engine boundary conditions. Infrared, visible, and intensified OH imaging techniques are used to characterize fuel spray, combustion, and soot formation processes over a range of different operating conditions, including conventional and low-temperature combustion schemes.
Probability maps of the in-cylinder occurrence of fuel, combustion, and soot volumes under low temperature combustion conditions are presented to analyze the effects that changes in engine load have on the process of combustion. The influence of the injector nozzle geometry on fuel spray characteristics and how these characteristics affect combustion is discussed. Comparisons between the combustion and soot production processes of oxygenated, bio-derived fuels and pump Diesel fuel are evaluated. General trends of in-cylinder soot formation are shown to compare well with engine-out filter smoke number emissions data across a range of loads, nozzle geometries, and fueling conditions. DEDICATION
To Amanda.
iii ACKNOWLEDGEMENTS
I would like to take this time to thank and acknowledge the many people who have made this dissertation possible. I would like to thank Dr. Harold Schock for his belief in, and support of, me through the many years of working with him. The opportunities that have been afforded through his guidance have been much beyond what I might have imagined possible upon entering graduate school. Much gratitude is due to Dr. George Zhu who greatly aided in the setup of the engine controller and general overall understanding of data acquisition. Without Dr. Zhu’s many hours of guidance, support, and patience, the bulk of this work would not have been able to be accomplished. To the other members of my guidance committee, Dr. Giles Brereton and Dr.
Dennis Miller, I owe many thanks for your support in this endeavor. Acknowledgement is also due to Dr. Tonghun Lee for his guidance and willingness to share equipment in support of this work.
To all of the staff of the Engine and Automotive Research Laboratory, thank you for providing an enjoyable and entertaining work environment. In particular, Thomas Stuecken and
Mulyanto Poort, should be recognized for their support. Without your great friendship and support, this work would not have been able to be completed. Tom was always available and willing to give support on the experimental setup; Muly was always willing to “turn knobs and click buttons” and provide guidance in image processing. Kyle Crayne and Andrew Nuttall should be recognized for their assistance. Much gratitude is owed to Zhen Ren, Xiaojian Yang, and Xuefei Chen for their help with implementing and programming of the engine controller. I would like to acknowledge the aid that Casey Allen provided in this work, as well. To Ed Timm and Andrew Huisjen, thank you for your great friendship through these past years. To Jeff Higel,
Jen Higel, and Gary Keeney, sincere gratitude for all of the assistance you have given.
iv To my family and friends, the support you have provided has been immense and unwavering, thank you. This work has been the culmination of a large amount of support from you. To Amanda, thank you, for everything.
v TABLE OF CONTENTS
LIST OF TABLES…………………………………………………………………………..….viii
LIST OF FIGURES…………………………………………………………………………...... ix
LIST OF SYMBOLS AND ABBREVIATIONS……………………………………………...xviii
CHAPTER 1 INTRODUCTION AND BACKGROUND…………………………………….……………...…1
CHAPTER 2 LITERATURE REVIEW...... 5 2.1 Soot Formation in Diesel Engines...... 5 2.2 Optical diagnostic Techniques...... 9 2.3 In-Cylinder Variability...... 12
CHAPTER 3 EXPERIMENTAL SETUP……………………………………………………..……...... …..13 3.1 Engine Specifications………………………………………………..……...... 13 3.2 Optical Piston Design…………………………………………………...... 16 3.3 Data Acquisition System………………………………………………....…...... 17 3.4 Dynamometer………………………………………………………….…...... 17 3.5 Fuel Delivery System …………………………………………………..…...... 17 3.6 Intake Charge System…………………………………………………..…...... 19 3.7 Coolant System………………………………………………………………...... 22 3.8 Engine Control System……………………………………………………...... 22 3.9 Testing of Engine Control System.…………………………………….…...... 27 3.10 Safety of Engine Test Cell...... 29 3.11 Cameras…………………………………………………………………...... 31 3.12 Camera Setup………………………………………………………….…...... 32 3.13 Smoke Meter...... 33 3.14 Source of Uncertainty and Error...... 37 3.15 Probability Map Study Experimental Setup...... 42 3.16 Injector Nozzle Comparison Study Experimental Setup…………………...... 43 3.17 Biodiesel Study Experimental Setup……………………………………...... 45
CHAPTER 4 IMAGE PROCESSING..…………………………………………………………...... …...... 49 4.1 Fuel Spray Image Processing...... 49 4.2 Combustion Image Processing...... 52 4.3 Probability Maps Image Processing...... 54
CHAPTER 5 PROBABILITY MAP STUDY RESULTS...... 56 5.1 Mass Fraction Burned...... 56
vi 5.2 Engine Speed Variations...... 57 5.3 Soot Deposition Effects...... 58 5.4 Sample Probability Maps...... 59 5.5 Projected Areas of Fuel Spray ...... 60 5.6 Projected Exothermic Areas from Combustion ...... 63 5.7 Projected Areas of Soot ...... 65 5.8 Fractional Areas of Projected Exothermic Areas from Combustion...... 67 5.9 Radial Distance of Projected Exothermic Areas from Combustion...... 73 5.10 Engine-Out Soot Emissions...... 79
CHAPTER 6 NOZZLE GEOMETRY STUDY RESULTS………………………...………..…….…...... 81 6.1 Nozzle Study Fuel Spray Results...... 81 6.2 Nozzle Study Combustion Results...... 99
CHAPTER 7 BIODIESEL STUDY RESULTS..………………………………………..….....…...... …...137 7.1 Composite Cycle Sample Images...... 137 7.2 Pressure and MFB...... 140 7.3 Liquid Fuel Penetration Lengths...... 148 7.4 Projected Area of Combustion Gases with IR Imaging...... 149 7.5 Average Pixel Intensity of Combustion Gases with IR Imaging...... 153 7.6 Projected Area of Soot with Visible Imaging...... 158 7.7 Projected Area of OH with Intensified Imaging...... 161 7.8 Average Pixel Intensity of OH...... 164 7.9 Combustion, Soot, and OH Lengths from Injector...... 167 7.10 Engine-Out Soot Measurements...... 178
CHAPTER 8 SUMMARY AND CONCLUSIONS………………...…………………………...... ……...182 6.1 Probability Map Summary/Conclusions...... 182 5.1 Shallow Bowl Summary/Conclusions……………………………...... ……...... 184 5.2 Biodiesel Summary and Conclusions…………………………………...... ….…186
CHAPTER 9 RECCOMENDATIONS………………...………………………...... …………………...... 189
BIBLIOGRAPHY……………………………………………………………...... ……...... 191
vii LIST OF TABLES
Table 1: Engine Parameters...... 13
Table 1: Probability Map Combustion Operating Conditions...... 39
Table 2: Nominal Engine Operating Parameters for Nozzle Study...... 41
Table 3: Properties of Fuels Used in Biodiesel Study...... 42
Table 4: Biodiesel Study Operating Conditions Including Optimized Timings and Pulse Width...... 44
viii LIST OF FIGURES
Figure 1: Example cross-section of a combusting fuel plume. For interpretation of the references to color in this and all other figures, the reader is referred to the electronic version of this dissertation...... 3
Figure 2: Images of engine setup (left) and “piston view” (right)...... 14
Figure 3: Images describing the “valve view”...... 14
Figure 4: Optical engine system flowchart...... 15
Figure 5: Section view of the optical piston assembly showing the sapphire window (purple) as well as piston cap, a section of the Bowditch piston, and gaskets (green)...... 16
Figure 6: View of the fuel cart showing the common rail, pressure sensor, and drive electronics...... 18
Figure 7: Electronically controlled valves with blue housings used in the intake charge engine subsystem...... 20
Figure 8: Bleed volume with oxygen sensor and pressure compensation transducer installed...... 20
Figure 9: View of the intake heaters wrapped in aluminum insulating tape installed upstream of the intake plenum (black)...... 21
Figure 10: Engine control system diagram showing the signal flow through the system...... 22
Figure 11: View of I/O Box (top, cream) and Opal-RT (bottom, black) hardware...... 23
Figure 12: Fuel delivery control subsystem diagram...... 24
Figure 13: Control diagram of intake charge subsystem...... 26
Figure 14: Coolant control subsystem diagram...... 27
ix Figure 15: Example pressure traces of consistency testing with traces from before control system (single condition)...... 28
Figure 16: Example pressure traces of consistency testing with traces from after control system implementation (four conditions)...... 28
Figure 17: Pressure (black) and injection (red) traces of engine malfunction test...... 29
Figure 18: Polycarbonate barrier with steel frame painted green surrounding test cell...... 30
Figure 19: Shear pin assembly to the left of the rubber coupling, along with the location of the stub shaft failure...... 31
Figure 20: Side view diagram of the camera setups for the different views relative to the engine...... 34
Figure 21: Top view diagram of the simultaneous imaging camera setup relative to the engine cylinder...... 35
Figure 22: Trigger timings of the cameras of separate experiments...... 36
Figure 23: Plot of various cycles of a sample test showing the difference in combustion as the experiment progresses...... 40
Figure 24: Sample camera-out signals of recording and exposure illustrating the delay from trigger until recording (left) and the unsynchronized exposure with the trigger (right)...... 41
Figure 25: Diagram of FSSC...... 46
Figure 26: Description of piston view fuel spray metrics...... 50
Figure 27: Example fuel spray valve view image...... 51
Figure 28: Raw, visible combustion image and a translated Cartesian contour plot, showing the nearest distances to the injector of soot...... 54
x Figure 29: Description of geometry for the probability maps...... 55
Figure 30: Mass fraction burned of the probability map study operating conditions...... 57
Figure 31: EPA3 MFB 50% point probability map of sets of10 consecutive cycles illustrating the effect of soot impingement at the outside of the bowl window...... 58
Figure 32: Exothermic (top), fuel spray (middle), and soot (bottom) probability maps for the EPA1 (6.75 bar IMEP) condition...... 60
Figure 33: Fuel spray probability maps for the EPA1-4 conditions...... 62
Figure 34: Exothermic probability maps for the studied conditions...... 64
Figure 35: Soot probability maps across the conditions studied...... 66
Figure 36: Fractional area error bar plots plotted through the engine cycle for the different conditions...... 68
Figure 37: Fractional area coefficient of variation plotted through the engine cycle for the different conditions…...... 69
Figure 38: Standard deviation of fractional area for the EPA1 operating condition...... 70
Figure 39: Standard deviation of fractional area for the EPA2 operating condition…...... 71
Figure 40: Standard deviation of fractional area for the EPA3 operating condition…...... 72
Figure 41: Standard deviation of fractional area for the EPA4 operating condition…...... 73
Figure 42: Combustion length error bar plots plotted through the engine cycle for the different conditions……...... 74
Figure 43: Combustion length coefficient of variation plotted through the engine cycle for the different conditions……...... 75
xi Figure 44: Standard deviation of combustion length for the EPA1 operating condition...... 76
Figure 45: Standard deviation of combustion length for the EPA2 operating condition...... 77
Figure 46: Standard deviation of combustion length for the EPA3 operating condition...... 78
Figure 47: Standard deviation of combustion length for the EPA4 operating condition...... 79
Figure 48: Average filter smoke number of the operating conditions…...... 80
Figure 49: Sample fuel spray images from the piston view and valve views for the 6x.100x160 nozzle, EPA1 condition…...... 82
Figure 50: Liquid (dashed) and vapor (solid) fuel plume lengths for the EPA1 condition…...84
Figure 51: Liquid (dashed) and vapor (solid) fuel plume lengths for the EPA2 condition…...85
Figure 52: Liquid (dashed) and vapor (solid) fuel plume lengths for the EPA3 condition…...86
Figure 53: Liquid (dashed) and vapor (solid) fuel plume lengths for the EPA4 condition…...87
Figure 54: Vapor fuel plume widths for the EPA1 condition…...... 88
Figure 55: Vapor fuel plume widths for the EPA2 condition…...... 89
Figure 56: Vapor fuel plume widths for the EPA3 condition…...... 90
Figure 57: Vapor fuel plume widths for the EPA4 condition…...... 91
Figure 58: Fuel plume lengths for the 6x.100x160 nozzle across all conditions…...... 93
Figure 59: Fuel plume widths for the 6x.100x160 nozzle across all conditions…...... 94
Figure 60: Fuel fractional areas in the valve view for the EPA1 condition…...... 96
xii Figure 61: Fuel average pixel intensities in the valve view for the EPA1 condition…...... 97
Figure 62: Fuel fractional areas in the valve view for the EPA2 condition…...... 98
Figure 63: Fuel average pixel intensities in the valve view for the EPA2 condition…...... 99
Figure 64: Pressure traces for the different nozzles for the EPA1 operating condition….....101
Figure 65: MFB traces for the different nozzles for the EPA1 operating condition...... 102
Figure 66: Pressure traces for the different nozzles for the EPA2 operating condition….....103
Figure 67: MFB traces for the different nozzles for the EPA2 operating condition...... 104
Figure 68: Pressure traces for the different nozzles for the EPA3 operating condition….....105
Figure 69: MFB traces for the different nozzles for the EPA3 operating condition...... 106
Figure 70: Pressure traces for the different nozzles for the EPA4 operating condition….....107
Figure 71: MFB traces for the different nozzles for the EPA4 operating condition...... 108
Figure 72: 6x.120x160 nozzle sample IR and visible images from the EPA1 condition...... 109
Figure 73: 6x.120x160 nozzle sample IR and visible images from the EPA4 condition...... 110
Figure 74: Fractional areas for the piston view of the EPA1 condition...... 112
Figure 75: Fractional areas for the valve view of the EPA1 condition...... 113
Figure 76: Fractional areas for the piston view of the EPA2 condition...... 114
Figure 77: Fractional areas for the valve view of the EPA2 condition...... 115
xiii Figure 78: Fractional areas for the piston view of the EPA3 condition...... 116
Figure 79: Fractional areas for the valve view of the EPA3 condition...... 117
Figure 80: Fractional areas for the piston view of the EPA4 condition...... 118
Figure 81: Fractional areas for the valve view of the EPA4 condition...... 119
Figure 82: Maximum values of combustion fractional area curves through the piston view...... 121
Figure 83: Maximum values of projected soot fractional area curves through the piston view...... 122
Figure 84: Maximum value of combustion fractional area curves in the valve view...... 123
Figure 85: Maximum value of soot fractional area curves in the valve view...... 124
Figure 86: Distances of soot and combustion for EPA1...... 125
Figure 87: Distances of soot and combustion for EPA2...... 126
Figure 88: Distances of soot and combustion for EPA3...... 127
Figure 89: Distances of soot and combustion for EPA4...... 128
Figure 90: Minimum soot and combustion distances for the different nozzles across the operating conditions...... 129
Figure 91: Minimum soot distances for the different nozzles across the operating conditions...... 130
Figure 92: Soot distance and pulse width correlation plot...... 131
Figure 93: Average FSN values for the nozzles under the operating conditions...... 132
xiv Figure 94: Average FSN and maximum projected area of soot for all nozzles and conditions...... 134
Figure 95: Maximum projected area of soot versus FSN for the nozzles and conditions...... 135
Figure 96: Integrated projected area of soot versus FSN for the nozzles and conditions...... 136
Figure 97: Sample images taken from the CC3 operating condition...... 139
Figure 98: Pressure data curves for the LTC1 operating condition...... 141
Figure 99: MFB curves for the LTC1 operating condition...... 142
Figure 100: Pressure data and MFB curves for the CC2 operating condition...... 143
Figure 101: Pressure data and MFB curves for the CC2 operating condition...... 144
Figure 102: Pressure data and MFB curves for the CC3 operating condition...... 145
Figure 103: Pressure data and MFB curves for the CC3 operating condition...... 146
Figure 104: Liquid fuel penetration curves for the three operating conditions...... 148
Figure 105: Projected areas of combustion gases for the LTC1 combustion condition...... 150
Figure 106: Projected areas of combustion gases for the CC2 combustion condition...... 151
Figure 107: Projected areas of combustion gases for the CC3 combustion condition...... 152
Figure 108: Average intensity values of the infrared images in the areas where combustion occurs for the LTC1 combustion condition...... 154
Figure 109: Average intensity values of the infrared images in combustion areas for the CC2 combustion condition...... 156
xv Figure 110: Average intensity values of the infrared images in the areas where combustion occurs for the CC3 combustion condition...... 157
Figure 111: Projected areas of soot for the LTC1 condition...... 159
Figure 112: Projected areas of soot for the CC2 condition...... 160
Figure 113: Projected areas of soot for the CC3 condition...... 161
Figure 114: Projected areas of OH for the LTC1 condition...... 162
Figure 115: Projected areas of OH for the CC2 condition...... 163
Figure 116: Projected areas of OH for the CC3 condition...... 164
Figure 117: Average intensity values of the OH images for the LTC1 condition...... 165
Figure 118: Average intensity values of the OH images for the CC2 condition...... 166
Figure 119: Average intensity values of the OH images for the CC3 condition...... 167
Figure 120: Radial combustion distances nearest to the injector for the LTC1 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip)...... 169
Figure 121: Radial soot distances nearest to the injector for the LTC1 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip)...... 170
Figure 122: Radial OH distances nearest to the injector for the LTC1 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip)...... 171
Figure 123: Radial combustion distances nearest to the injector for the CC2 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip)...... 172
Figure 124: Radial soot distances nearest to the injector for the CC2 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip)...... 173
xvi Figure 125: Radial OH distances nearest to the injector for the CC2 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip)...... 174
Figure 126: Radial combustion distances nearest to the injector for the CC3 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip)...... 175
Figure 127: Radial soot distances nearest to the injector for the CC3 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip)...... 176
Figure 128: Radial OH distances nearest to the injector for the CC3 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip)...... 177
Figure 129: Relative filter smoke number values for the different fuels and operating conditions...... 179
Figure 130: Maximum projected area of soot versus relative FSN for the fuels and conditions...... 180
Figure 131: Integrated projected area of soot versus relative FSN for the fuels and conditions...... 181
xvii LIST OF SYMBOLS AND ABBREVIATIONS
ATDC After Top Dead Center BTDC Before Top Dead Center CA 90 CAD of 90% Mass Fraction Burned CAD Crank Angle Degree CAIO CAD of 10% Mass Fraction Burned Can/DBS 60% by Volume Canola, 40% by Volume Dibutyl Succinate CAS Combustion Analysis System CC Conventional Combustion CC2 CC Biofuels Load Condition CC3 CC Biofuels Mid Load Condition Cv Coefficient of Variation DBS Dibutyl Succinate DI Direct Injection EGR Exhaust Gas Recirculation EPA1 EPA Nozzle Study Condition 1 EPA2 EPA Nozzle Study Condition 2 EPA3 EPA Nozzle Study Condition 3 EPA4 EPA Nozzle Study Condition 4 F Fahrenheit F/O Fuel to Oxidizer Ratio FAMES Fatty Acid Methyl Esters fps Frames Per Second FSN Filter Smoke Number FSSC Fuel Sample Separation Cylinder FUR Forward Looking Infrared GE-DC General Electric - Direct Current Hz Hertz I/O Input/Output IMEP Indicated Mean Effective Pressure IR Infrared LOL Lift-Off Length LTC Low Temperature Combustion LTC1 LTC Biofuels Condition MAP Manifold Absolute Pressure MFB Mass Fraction Burned mm Millimeter ms Millisecond nm Nanometer NOX Nitrous Oxides ns Nanosecond OH Hydroxyl pC Picocoulombs PWM Pulse Width Modulation
xviii RPM Revolution per Minute SI Spark Ignited TDC Top Dead Center TTL Transistor-Transistor Logic μs Microsecond UV Ultraviolet V Volts W Watt
xix CHAPTER 1
INTRODUCTION AND BACKGROUND
Diesel engines can have significant increases in efficiency, but may also have increased engine-out emissions and production costs compared to gasoline fuelled engines [1-3]. The lack of a throttle, increased compression ratios, and the ability to operate fuel-lean all contribute to an increase in efficiency over gasoline fuelled counterparts. However, an inherently very inhomogeneous distribution of fuel can lead to difficulties in meeting emissions goals. Should emission standards be able to be reached with these combustion systems, Diesel engines may play an important role in the future of propulsion, because of the additional efficiency afforded by these engines. It is therefore necessary to understand the complex in-cylinder fuel spray and combustion events to aid in the understanding of how certain emissions are created. This work presents an experimental investigation into direct injection (DI) Diesel engine events using an optically accessible Diesel engine based on an International VT 275 series engine. Three imaging techniques were implemented to study the in-cylinder events: infrared imaging, visible imaging, and imaging of hydroxyl (OH). Comparisons of in-cylinder events are drawn between different operating conditions, nozzle configurations, and fuels.
Much investigation has been completed into the fuel spray, combustion, and soot production processes of Diesel engines. Combustion inside a DI Diesel engine comes from injection of fuel directly into the combustion chamber at high pressure and temperature conditions near top dead center (TDC) after the compression stroke. As fuel is injected, droplets break up and evaporate, changing from liquid fuel to vapor. At the periphery of this jet of fuel vapor, edge vortices entrain the ambient in-cylinder charge of intake air and re-circulated exhaust
1 gases. When the proper amount of oxygen has entrained into the fuel jet to create a combustible mixture at the pressures and temperatures of the fuel plume, combustion begins locally in the fuel jets. The benefit to this type of combustion is control of engine power by fuel delivery rather than air restriction as is in spark-ignited (SI) engines, and the ability to achieve fuel-efficient, overall lean combustion. However, because fuel is directly injected in distinct plumes, Diesel combustion schemes lead to very inhomogeneous fuel concentrations and can cause undesired amounts of emissions, leading to the need for expensive aftertreatment systems in order to meet emissions standards.
Low-temperature combustion (LTC) schemes are used in order to reduce peak cylinder temperatures and reduce nitrous oxides (NOx) production. The general concept implemented for
LTC is to use increased levels of exhaust gas recirculation (EGR) to lengthen the ignition delay and restrict the rate of combustion. Under certain conditions, the longer ignition delays produced by this combustion scheme can also produce increased premixed portions of combustion [1].
Even though an increase in the premixed portion of Diesel combustion typically leads to a reduction in soot production, soot formation can still be affected negatively, owing to the lower oxygen in the cylinder, leading to larger amounts of soot-conducive, fuel-rich regions than might occur with higher in-cylinder oxygen concentrations. The reduced in-cylinder temperatures that occur for this combustion scheme might also reduce the amount of post-combustion soot oxidation, also negatively affecting engine-out emissions of particulates [1-2].
In order to understand the images produced of the events that transpire inside the combustion chamber of an optical engine, a general description of what a camera might observe must be understood. A sample combustion volume is presented in Figure 1 to illustrate a general case of a combusting fuel jet. This combusting volume is a simplified example of a cross-section
2 of a combusting fuel jet with a rich, soot producing center region surrounded by a leaner, non- soot producing region. The products of combustion in the figure are simplified to carbon dioxide, water, and soot, although many other combustion species would be expected throughout the combustion volume. Three different lines-of-sight are shown: Line-of-sight 1 where no soot is seen, Line-of-sight 2 where a significant amount of soot occurs, and Line-of-sight 3 where a small amount of soot might be observed. These lines-of-sight can be thought of as pixels of images, with the goal of the work presented in this document as describing the characteristics of combustion through the use of these pixels. More in-depth discussion about Diesel combustion processes is presented in the following chapter.
Figure 1: Example cross-section of a combusting fuel plume. For interpretation of the references to color in this and all other figures, the reader is referred to the electronic version of this dissertation.
This work will analyze fuel spray and combustion development under various operating conditions, nozzle geometries, and fuels. Infrared, visible, and intensified OH imaging
3 techniques will be used to produce images that are post-processed to characterize in-cylinder events. Probability maps will lend insight into the variations in combustion that occur under LTC conditions that was previously unknown. A study on soot formation for different nozzle geometries under a range of LTC conditions is also presented to illustrate how fuel spray, combustion, and soot are affected by the number and size of injector nozzle holes. Biologically- derived Diesel surrogate fuels (biodiesels) have been shown to decrease soot emissions, and this effect is investigated using certain biodiesels. The analyses provided in this paper will aid in the understanding of the processes that might be implemented to utilize the higher efficiencies of
Diesel engines while also meeting emissions goals.
4 CHAPTER 2
LITERATURE REVIEW
The purpose of this chapter is to provide an introduction to the Diesel combustion process and the current status of optical diagnostics of engine combustion research. Through the discussion presented here, the goal is to create an understanding of the fuel spray, combustion, and soot production processes that occur inside the combustion chamber of a Diesel engine.
Optical techniques are discussed in this section, along with what information these techniques provide. Through the reviewed previous works, it is clear that optical diagnostics implemented in optically accessible engines can provide significant insight into the complex in-cylinder processes that occur inside a Diesel engine.
2.1 FORMATION AND ENGINE-OUT EMISSIONS OF PARTICULATES IN
DIESEL ENGINES
Engine-out emissions of particulates, or soot, can be thought of as two competing processes: soot production during combustion and subsequent oxidation. Particulates formed that are not oxidized prior to the exhaust stroke will result in engine-out emissions of soot. In- cylinder soot imaging is typically accomplished using laser incandescence, two-color methods, or imaging of natural luminosity. However, the soot that is observed does not necessarily correlate with engine-out emissions, as the competing processes of production and oxidation may vary between conditions, loads, fuels, and other possible differences to boundary conditions.
Dec et al [4-5] have presented a Diesel jet combustion model based on optical studies involving laser sheet imaging. The model involves a Diesel spray jet entraining air as it moves
5 outward from the injector. As the jet entrains air and combustion begins, the model details where in the fuel jet certain emission species of interest, such as soot and NOx, form. Soot is shown to form in the fuel rich interior sections of the jet, whereas NOx formation areas are shown towards the outside of the combusting fuel plume. These papers also go on to discuss the soot formation process, describing small unburned hydrocarbons in the rich regions forming polycyclic aromatic hydrocarbons that are generally accepted as precursors of particulates.
Several other papers have used optical techniques to analyze soot formation under high
EGR, LTC conditions. Musculus [6] and Bobba et al. [7] utilized a heavy-duty optical Diesel engine to better understand the soot formation processes under these conditions. Musculus illustrated that the greatest soot production under LTC conditions comes from the head of the fuel jet, where the mixing of fuel and oxidizer is the lowest [6]. Bobba et al. also presented that soot formation under LTC conditions can form in numerous parts of the fuel jet, including inside the portion that reflects off the bowl wall as well as prior to contact with the bowl wall, depending on when energy release takes place [7].
Pressure vessel experiments are also a useful tool in analyzing combustion of single fuel spray plumes, because additional optical access can be afforded in these devices. Pickett and
Siebers [8] and Idicheria and Pickett [9] have used combustion pressure vessel experiments to analyze soot formation in single Diesel fuel spray jets. In these works, the authors observe combustion beginning when a certain amount of oxygen is entrained into the jet, regardless of the oxygen concentration in the environment. That is to say that the distance that combustion begins away from the injector varies depending on the amount of oxygen entrained, rather than at a constant length from the injector. The authors also present that soot formation begins inside the
6 fuel jet at an estimated equivalence ratio of approximately 2 based on a model of jet-ambient entrainment.
Other works have also discussed in-cylinder soot formation based on how fuel is delivered. The Internal Combustion Engine Handbook [1] discusses soot formation in fuel-rich regions of the cylinder, but also points out that post-combustion soot oxidation is a major factor in tailpipe emissions. The book Particulate Emissions from Vehicles by Peter Eastwood [10] and the 2002 SAE paper “Optical Investigation of the Effect of Fuel Jet Wall Impact Position on Soot
Emissions in a Single Cylinder Common Rail Direct Injection Diesel Engine” [11] are additional references on this subject. It is presented in these works that wall impingement of the fuel spray has a direct impact on the amount of soot produced. It is also presented in various other works that longer liquid phase penetration lengths can cause an increase in wall impingement of the fuel spray, leading to increased soot production [12-15]. Fuel droplet size has been shown to cause an impact on soot emissions, because larger droplets evaporate slower and have richer combustion regions around the droplets for longer times [1,16]. Increases in engine load are also shown to increase engine-out particulates, because of the increased fuel needed to achieve higher loads [17].
2.1.1 Soot Formation with Biodiesel Fuels
Plant oil-derived fatty acid methylesters (FAMES) fuels, or biodiesel, are well documented to have reduced engine-out soot emissions, with slightly increased or similar engine out NOx emissions and reductions in fuel economy [18-25]. The reduction in fuel economy is caused by the decrease in specific energy of the biofuels compared to typical Diesel fuel.
Biodiesel is typically produced from a transesterification reaction of a bio-derived feedstock
(such as Canola oils, as is used in the work presented later) and methanol. This reaction takes a
7 triglyceride molecule and strips off the fatty acid branches. The resulting fuel is similar to Diesel fuel and has shown promise in its ability to offset petroleum dependence and curb certain engine emissions.
Previous optical studies have given insight into the spray and combustion processes of biodiesel and similar oxygenated fuels. Issues with changes in engine calibration and fuel properties have been shown to negatively affect engine-out emissions when fuelling with biodiesel [25]. Other studies using optical engines have utilized optical diagnostics to demonstrate and discuss the decrease of in-cylinder soot production for oxygenated fuels [26-
28]. Similarly, optical pressure vessel experiments are a useful tool to evaluate the soot reduction of biofuels and other oxygenated fuels in single combusting plumes [29-31]. The reduction of soot production and emissions is shown, in these optical engine and pressure vessel studies, to be caused, in large part, by the included oxygen in the fuel, which reduces the local fuel-to-oxidizer
(F/O) ratio throughout the combusting plume, and leads to fewer areas of soot production. It is also necessary to note that it is believed that the carbon to oxygen single bond is more important in the oxygenated fuel than if the oxygen was double bonded, as the double bonded oxygen tends to form carbon dioxide directly, effectively wasting oxygen [32-33]. Other studies have discussed the aforementioned cause of the NOx increase, but this effect is beyond the scope of the work presented in this dissertation [e.g. 32].
2.2 OPTICAL DIAGNOSTIC TECHNIQUES
Optical techniques have been used extensively in combustion analysis [4-9,11-13,24-43].
Many different techniques are utilized when examining combustion, including natural luminosity imaging, infrared imaging, and intensified OH imaging, amongst others. “Natural luminosity”
8 refers to the natural emission of combustion in the visible light range. Infrared imaging utilizes detectors that are sensitive to emission in infrared wavelengths. OH is typically imaged through the use of an intensified camera to view emission in a specific ultraviolet (UV) range. The goal of the following sections is to provide a general understanding of how the techniques used in the work discussed later are implemented and what might information be obtained from the resulting images.
2.2.1 High-Speed, Visible Imaging Techniques
The advantages of using high-speed, visible imaging techniques lie in the ability to image liquid fuel spray and the natural luminosity of soot with a high frame rate. Liquid fuel visualization methods rely on Mie scattering of laser light by fuel droplets. Thus, as the droplets breakup and evaporate, the fuel becomes increasingly difficult to view. This effect allows for a measure of the length that liquid fuel penetrates outward from the injector. Visible light radiation emitted from combustion reactions can also be observed in the visible light range. Natural luminosity of combustion is composed of two major radiators: soot radiation and chemiluminescence [6,35]. Natural luminosity of soot in the visible wavelengths has been shown to be orders of magnitude higher than the chemiluminescence in Diesel combustion, allowing for this technique to be used to visualize in-cylinder soot formation processes [6,35]. Although the soot observed does not always yield a direct correlation with engine-out emissions [36], this imaging technique is still a useful tool when analyzing how and where soot forms in the combustion chamber.
9 2.2.2 Infrared Imaging Techniques
The use of infrared (IR) cameras to gather information about Diesel engine events has been shown to be a viable in-cylinder imaging technique in optical engines [42-43], as some substances that are not easily seen in the visible light range can be observed in the infrared wavelength range. A paper that utilizes infrared imaging in an optical engine is the paper titled
“Infrared Spectral Analysis of Engine Preflame Emission” by Marcis Jansons et al. [42]. In this work, a SI single cylinder optical engine was used with an infrared camera and a spectrometer to image through a window in a Bowditch piston. Squibb et al. gave an introduction into infrared imaging used in Diesel engines, illustrating the differences between infrared and visible imaging techniques [43]. These works offer insight into the benefits of utilizing infrared imaging when observing in-cylinder engine events. Few other previous works were found that utilized infrared imaging in optical engines.
A main benefit in using IR imaging techniques in viewing Diesel fuel sprays is that
Diesel fuel vapor is nearly transparent in the visible range, however, in the infrared region, the fuel spray is easily seen. The vapors are evident because oxygen and nitrogen in the cylinder do not emit significantly in the infrared region until higher temperatures, while fuel emits significantly, allowing for a distinction of the fuel spray plumes [44]. Combustion energy is also beneficial to view using IR imaging. In the images produced from infrared imaging techniques, the total area of hot gases of combustion may be better seen than in the visible range where soot radiation dominates what is observed.
A major disadvantage of current infrared technology is the speed at which image acquisition can be made. In engine applications, infrared cameras are limited to one usable frame
10 per cycle. The other imaging techniques used in this work are able to capture many more pictures per cycle. Because of the lower speed of the infrared camera, cycle-to-cycle variations appear in the composite cycles produce from this imaging technique.
2.2.3 Intensified OH Imaging Techniques
Imaging of OH has also been shown to be a beneficial imaging technique for combustion of Diesel fuel jets [4-5,29-31]. In a 2001 paper, Higgins and Siebers covered OH imaging with respect to combustion of Diesel fuel jets and illustrated that the 310 nm emission band was the best wavelength region for imaging this radical [37]. This paper also illustrated that when imaging to determine the lift-off length (LOL) of Diesel fuel spray jets, OH imaging was the best indicator. This imaging technique is widely used to determine the distance from the injector to the nearest point of combustion, or LOL, in the combusting fuel plume [31,37-38]. The LOL of
Diesel combustion is an important parameter in determining how much oxygen is entrained into the fuel plume prior to combustion. For the same fuel chemistry and jet boundary conditions, a longer LOL would yield an increased amount of oxygen entrained into the fuel plume. It is also believed that the regions in which OH exists are regions with proper chemical and energy conditions in which soot oxidation could occur [6,39-41]. As a general trend, an increase of in- cylinder OH occurrence could lead to an increase in post-combustion soot oxidation.
2.3 IN-CYLINDER VARIABILITY
Optical diagnostic techniques are widely used to study different fuel spray and combustion parameters, including in-cylinder combustion and fuel spray events as well as soot formation. However, optical engine experiments rarely investigate the variability of the engine events and how differences in combustion and fuel spray patterns contribute to the variations in
11 combustion and lead to differences in soot production processes. The flow fields inside an optical engine are typically studied using flow tracking techniques (MTV, PIV, etc) [45-48].
Studies have investigated the cyclic variability of both large scale turbulent in-cylinder motions and turbulent in-cylinder fuel spray processes. Although these studies present excellent flow field measurements, to what extent the variations in fuel spray or in-cylinder flow fields affect differences in combustion is not investigated. Bates discussed variations in the combustion of a
SI gasoline engine using visible imaging techniques, showing that cyclic variations could be studied using optical engines [49]. Few other optical studies were found to have discussed variability of the in-cylinder processes, especially with Diesel combustion as a focus.
12 CHAPTER 3
EXPERIMENTAL SETUP
3.1 ENGINE SPECIFICATIONS
A single-cylinder, Diesel engine with optical access available through a window in a
Bowditch piston and through a periscope window that replaces an exhaust valve was used in the experiments presented in this paper. The optical engine is a single-cylinder engine based from a
4.5 L V6 International VT 275 production Diesel engine. Parameters of the optical engine can be found in Table 1. The engine is fitted with a pressure transducer placed in lieu of the glow plug to record in-cylinder pressure data. A Bowditch style piston with a sapphire window is used for optical access into the combustion chamber along the bore centerline; this view is referred to as the “piston view”. The engine setup and an image of the piston view can be seen in Figure 2. The
Bowditch piston is an extended piston which gives a standoff from the piston of the crank-slider linkage that allows for a mirror to be placed below the piston to view the combustion chamber.
An additional view is available through a window that replaces an exhaust valve; this view is referred to as the “valve view”. Images describing this view are shown in Figure 3. A periscope- style tube is used to seal the window from the exhaust runner.
Table 1: Engine Parameters
Bore (mm) 95 Stroke (mm) 105 Connecting Rod (mm) 175 Geometric Compression Ratio 17.5:1 Piston Rings Graphite
13 Figure 2: Images of engine setup (left) and “piston view” (right).
Figure 3: Images describing the “valve view”.
14 A flow chart of the optical engine system is presented in Figure 4. Black arrows in this diagram illustrate the flow of gases through the system; the forest green arrows show the flow of fuel through the fuel system. The engine is fed from a combination of compressed air and nitrogen. The nitrogen acts as an inert gas that is used in lieu of exhaust gas recirculation (EGR).
Pressure regulators are used to reduce the pressure inside the intake lines such that valves can meter the correct amount of each component to achieve the correct intake manifold pressure and oxygen content.
Figure 4: Optical engine system flowchart.
15 The oil system on the optical engine is a dry sump system. A pump is used to supply the cam and crankshafts with oil from a reservoir that is separate from the engine block. The pressure of the supply was set to 30 psi gauge. A vacuum pump is used to pull the oil back to the reservoir from the cam and block. The vacuum pressure was set to 5 psi vacuum inside the reservoir.
3.2 OPTICAL PISTON DESIGN
The combustion bowl of the optical piston that was used in these experiments is a bowl design that is modified from the production piston to allow greater optical access with less optical distortion. Figure 5 shows a section view of the optical piston. A piston cap screws the sapphire to the Bowditch extension, creating the optical piston. Aluminum gaskets are used to seal the window to the extension and piston cap.
Figure 5: Section view of the optical piston assembly showing the sapphire window (purple) as well as piston cap, a section of the Bowditch piston, and gaskets (green).
16 3.3 DATA ACQUISITION SYSTEM
A Phoenix Combustion Analysis System (CAS) from A&D Technology was integrated with the engine to record data. The included software is able to convert the recorded data to a
Matlab format. This system has 12 channels of data acquisition, with 8 channels that can be used to record analog signals. This system uses in-cylinder pressure, crank angle, top dead center
(TDC) and cam signals to synchronize the data acquisition to the engine crank angle and engine events. For these experiments, the CAS was also set to record the manifold absolute pressure
(MAP), intake charge temperature in the intake plenum, coolant temperature, fuel temperature, intake oxygen percentage, exhaust oxygen percentage, fuel rail pressure, and the injection signal.
A resolution of 1 crank angle degree (CAD) was used for the experiments presented in this document.
3.4 DYNAMOMETER
The dynamometer used for the optical engine experiments is a General Electric Direct
Current (GE-DC) Dynamometer. The GE-DC dynamometer is a 180 volt, 140 amp, 38 horsepower dynamometer. A Galco electronic drive system is used to power and control the dynamometer.
3.5 FUEL DELIVERY SYSTEM
The fuel system used with this engine is a common rail fuel system. Only one port of the common rail is used to supply fuel to the injector; the others are blocked, with one port housing a pressure transducer. The pressure transducer is a Kistler model 6229A quartz high pressure sensor with a sensitivity of 2.5 pC/bar. A Kistler 5010 charge amplifier is used to output the rail pressure at 500 bar/V. Because this type of pressure transducer will drift over time, the
17 transducer baseline is reset before each set of experiments. The fuel pump is a prototype pump based on the production Ford fuel pump part number 9A543 from an F-250 HD. The pump utilizes two solenoids to determine the fuel pressure and volume flow rate to the common rail.
The pump is driven with an electronic drive that is set to operate at 40 Hz. A view of the fuel cart with fuel rail, pressure transducer and drive electronics can be seen in Figure 6, the pump and electric drive motor are on the bottom level of this cart.
Figure 6: View of the fuel cart showing the common rail, pressure sensor, and drive electronics.
The injector used with this engine is a Siemens piezo injector. The piezo stack is used to activate the nozzle pintle to allow flow through the nozzle. Different nozzles with various
18 geometries are able to be used with this injector by removing the cone nut and reassembling the injector with a different nozzle. The injector driver requires 100V and 12V power signals and is activated by a 5V TTL signal. The injector driver was found to require 1.8 ms to reenergize between injections.
3.6 INTAKE CHARGE SYSTEM
The intake MAP and oxygen concentration are controlled with two PV series electronic proportioning control valves from Omega Engineering that can vary the flow rates of the air and nitrogen components of the intake charge independently. These valves utilize a 0-5V control signal to determine the position of each valve, and, hence, the amount of each intake component allowed into the intake system. These valves are shown in Figure 7. In order to determine the manifold pressure a Kistler 4045A5 piezoresistive absolute pressure transducer with a 0-5 bar range was installed. This pressure transducer gives a 0-10V output signal at 0.5bar/V. The oxygen percentage is determined using an ECM Lambda 5220 which outputs a 0-10V signal based on the amount of oxygen in the intake charge. A bleed volume off of the intake plenum is used to reduce pressure effects on the sensor, even though this oxygen sensor model is pressure compensated. A highly repeatable pressure regulator was installed upstream of the bleed volume to ensure constant pressure inside the volume. Figure 8 shows the bleed volume with the oxygen sensor installed.
19 Figure 7: Electronically controlled valves with blue housings used in the intake charge engine subsystem.
Figure 8: Bleed volume with oxygen sensor and pressure compensation transducer installed.
20 Two 3400W intake heaters were installed in the intake system to allow control of the temperature of the intake charge. The heaters are 3174K62 replacement heat gun parts from
McMaster-Carr. These heaters are installed upstream of the plenum, but downstream of the component valves. The heaters were installed in this location both to promote uniformity of the thermal energy, but also to promote mixing of the gas components. The heaters and lines running to the intake plenum are insulated to reduce heat loss to the surroundings. A Crydom
MCPC2425C control relay utilizes a 0-10V input signal to control the heater load. A K-type thermocouple is installed at the top of the intake plenum and is used to read the temperature of the gases inside the intake plenum. Figure 9 shows a view of the intake heaters with the intake plenum and optical engine in the background.
Figure 9: View of the intake heaters wrapped in aluminum insulating tape installed upstream of the intake plenum (black).
21 3.7 COOLANT SYSTEM
The engine coolant system is a modified hot water heater that is plumbed to the cylinder and cylinder head and is used to preheat the engine components to over-the-road operating temperatures. The heating element is a 2400 W heater. The tank is plumbed to the engine via rubber lines. J-type thermocouples are used to measure the temperature inside the tank and at the outlet of the cylinder head. A Teledyne model SH24A25 25 amp solid-state relay is used to regulate the power status to the heating element.
3.8 ENGINE CONTROL SYSTEM
An Opal-RT based electronic control system was implemented for control of the boundary conditions to the optical engine. This system supplies the control signals for fuel rail pressure, fuel injection events, intake oxygen percentage, intake temperature, intake manifold pressure, and coolant temperature. The control system operates with a Simulink interface.
Through this interface, it is possible to set and modify the control parameters and desired values for control of the subsystems in real time. Figure 10 is a general diagram of the control system illustrating how the control signals move through the system.
Figure 10: Engine control system diagram showing the signal flow through the system.
22 An input/output (I/O) box was created to convert hardware signals to the Opal-RT electronic control system and vice versa. This box was designed with BNC connectors on the front of the box, such that signals could be easily probed. Connectors on the back side of the box were hard wired to the respective devices. Power supplies were installed inside the I/O box to power the injector driver as well as several other auxiliary systems, as needed; 13.5 V, 96 V, and
5V power are all available from the I/O box. Figure 11 shows an image of the front of the I/O box and Opal RT system.
Figure 11: View of I/O Box (top, cream) and Opal-RT (bottom, black) hardware.
3.8.1 Fuel Delivery Subsystem
The fuel delivery subsystem is divided into two parts: fuel injection and fuel pressure control. Fuel injection events are controlled with the Opal RT system by the number of injection pulses and when these pulses occur. The control system generates a 5V transistor-transistor logic
23 (TTL) signal for a specified duration in order to activate the injector driver. The system works by using the crank signals in order to determine when the injection should start and then provides a time delay until the injection ends. Fuel rail pressure is controlled with a 0-5V control signal to determine the load of the fuel rail and volume solenoids. This control signal is converted to a pulse width modulated (PWM) signal via the use of an OES PWM module. This board module has two inputs and outputs that are used to connect to the solenoids. The controller utilizes a feedforward/feedback control system. The feedforward portion of the controller is a lookup table with values for voltages of both solenoids and corresponding rail pressures. The feedback controller is used to adjust the solenoid voltages from the feedforward values to attain the desired rail pressure. Figure 12 shows a control diagram of the fuel delivery subsystem.
Figure 12: Fuel delivery control subsystem diagram.
3.8.1 Intake Charge Subsystem
A feedforward/feedback control system was implemented to control the electronic valves of the intake charge subsystem to create the desired oxygen concentration and intake manifold pressure conditions in the intake system. The feedforwad portion of the controller uses a lookup table to provide an approximate initial voltage to the valves based on the desired setpoint. The
24 feedback portion then adjusts the voltage based on the error between the desired value and the measured value, in order to achieve the desired conditions. The lookup table for the feedforward controller portion was created by motoring the engine at a constant 1500 rpm with constant voltages supplied to the valves and recording oxygen concentration and manifold pressure for these voltages. These results were then inverted to produce a table provides approximate supply voltages of the valves based on the desired inputs. The feedback controller parameters were tuned by running the engine and adjusting the parameters to give a reasonable system response with minimal settling time and small steady-state error. Heater control is implemented in a similar feedforward/feedback scheme. The lookup table for the heater control was produced by running the engine at a constant 1500 rpm and constant manifold pressure while varying the heater voltages. The engine was run until the temperature reached a steady value and the temperature was recorded for each voltage. It was found that different manifold pressures did not significantly affect the steady-state temperature that the heaters would produce for a given input voltage. A proportional-integrating controller was implemented and tuned to achieve the desired intake temperature with acceptable accuracy. The diagram presented in Figure 13 is a presentation of the controller scheme for the intake charge subsystem.
Figure 13: Control diagram of intake charge subsystem.
25 3.8.1 Coolant Subsystem
Control of the coolant system is accomplished using a simple hysteresis control scheme.
The temperature of both the water tank and just downstream of the engine head are read into the controller. The controller is set to limit either of these temperatures based on 2 second averages with ~+/-2°F of drift. The tank temperature can reach 200°F while the head is limited to 190°F.
The tank is set higher than the head to ensure that the head temperature is the limiting measurement when coolant is circulating. However, if the coolant is not circulating through the head, this scheme eliminates unwanted overheating of the coolant. The control diagram for this subsystem can be found in Figure 14.
Figure 14: Coolant control subsystem diagram.
3.9 TESTING OF THE CONTROL SYSTEM
Once the control system and hardware modifications were established and tested, it was necessary to confirm the accuracy of the engine control system to create repeatable, consistent boundary conditions to the optical engine. This examination was completed by acquiring in- cylinder pressure traces for the same operating and boundary conditions setting of the control
26 system. Through this testing it was found that a significant increase in the repeatability of in- cylinder pressure behavior of combustion occurred. An example of this result can be seen in
Figures 15 and 16 where a sample of an average of approximately 50 fired cycles of repeated test conditions with and without controller implemented can be seen. The new, with controller, case illustrates the repeatability of pressure data of repeated conditions. A large amount of confidence is found in the control system to create repeatable boundary conditions across a range of loads through this testing.
Figure 15: Example pressure traces of consistency testing with traces from before control system (single condition).
27 Figure 16: Example pressure traces of consistency testing with traces from after control system implementation (four conditions).
3.10 SAFETY OF ENGINE TEST CELL
During the consistency testing and validation of the engine control system, an engine malfunction caused significant damage to the experimental setup. The Opal-RT engine control system became overrun with data and injected fuel for more than a full cycle under a high load operating condition. This injection caused spontaneous ignition during the compression stroke of the cycle following the end of injection, creating approximately 140 bar in-cylinder pressure around 20 CAD BTDC. Figure 17 shows the pressure trace (black) and injection trace (red) of the trial on the left and the highest pressure cycle on the right. Note the cycle-long injection signal, followed by an increase in peak pressure of the next cycle. The pressure spike sheared the
1” diameter steel stub shaft between the dynamometer and the engine, which untimed the piston and cam. The piston and valves collided, causing a small deformation of the piston, and a large
28 deformation of the valvetrain pushrods. The sapphire piston window did not fail. Although this event was a large failure of the experimental engine, it spoke to the inherent robustness of the optical engine setup and design, as the engine was usable after relatively minor repairs.
However, additional safety measures were implemented to avoid a similar event in the future.
Figure 17: Pressure (black) and injection (red) traces of engine malfunction test.
After the malfunction, safety precautions were implemented to avoid a similar engine event and limit the consequences should one occur. A pulse width limitation box and new Opal-
RT software has been added to prevent the injection event from lasting longer than 2 ms. A 0.5” polycarbonate barrier with a steel frame was also added to encase the engine setup and protect those outside of the barrier from flying debris, should another malfunction of this type occur. A shear pin assembly was created and installed between the dynamometer and crankshaft timing pulley to keep the piston and valves timed properly. Should too much torque be applied, as was the case with the malfunction, the assembly will shear at the shear pin rather than between the crank and the timing pulley. Figure 18 shows the test cell with safety barrier installed and Figure
19 is a picture of the shear pin assembly.
29 Figure 18: Polycarbonate barrier with steel frame painted green surrounding test cell.
30 Figure 19: Shear pin assembly to the left of the rubber coupling, along with the location of the stub shaft failure.
3.11 CAMERAS
The infrared camera used in the experiments presented in this document is a FLIR
Phoenix MID infrared camera. This camera has a 3-5 m m viewable wavelength region. The camera is liquid-cooled and has the ability to be triggered by a 5V TTL pulse. The acquisition speed of this camera is limited to 60 frames per second (fps). This acquisition rate translates to one picture being taken every 57.6 CAD at 1500 rpm, thereby allowing for only one usable picture per combustion or fuel spray event. Two settings were used for the infrared camera: one for the in-cylinder fuel spray experiments and one for the in-cylinder combustion experiments. A
31 neutral density filter allowing 3.5% of energy through for the camera viewing wavelengths was used for the experiments where in-cylinder combustion was expected, but this filter was not used for the in-cylinder fuel spray experiments in the absence of combustion. The neutral density filter was used to restrict the energy reaching the camera such that the images taken from the in- cylinder combustion experiments would not be overexposed. Other camera settings were similar between experiments, in that an integration time of 0.02 ms was used along with the same Janos
Technology ASIO 100 mm MWIR lens for both fuel spray and combustion experiments. Gold, first-surface mirrors were used when imaging with this camera.
The high-speed visible camera used is a Photron Fastcam APX RS, which has the ability to acquire images in the visible wavelength range in color RGB format at 10,000 fps with a 512 x 512 pixel resolution. This speed translates to the ability to capture one picture every 0.9 CAD at 1500 rpm, thereby allowing many more pictures per engine event than the infrared camera can acquire. A shutter speed of 98 μs was used for the experiments. A Nikon AF Micro Nikor 105 mm lens was used. Gold, first-surface mirrors were used when imaging with this camera. Like the infrared camera, the high-speed camera also has the ability to be triggered with a 5V TTL pulse. The visible camera can be coupled with an Oxford 20W copper-vapor laser operated at maximum power. The laser is directed along the centerline of the bore for flood illumination of the engine cylinder and is used to view the fuel spray events using a Mie-scattered technique with this camera. The laser creates a beam at 510.6 nm and 578.2 nm, causing green hues in the high-speed pictures presented later, and does not interfere with IR camera wavelengths. The length of the laser pulse is 25 ns. The laser frequency is driven by the high-speed camera output at 10,000 pulses per second.
32 An Invisible Vision UVi model 1850-10 intensifier was used in conjunction with a
Photron Fastcam SA5 for OH imaging. This camera is able to record images at 10,000 fps with a resolution of 896x848 pixels in grayscale. The camera and intensifier were set to 98 μs integration times. The intensifier was set to a gain of 50% for the experiments in which this imaging technique was used. A Sodern Cerco UV 100 mm lens was used in front of the intensifier. An Asahi Spectra ZBPA310 narrow bandpass filter at 310 nm was placed in front of the lens to show the effect of OH without interference from soot, similar to other optical studies involving OH imaging (e.g. [39]). Protected aluminum mirrors were used with this imaging technique.
3.12 CAMERA SETUP
Simultaneous and non-simultaneous methods of image acquisition were used in the experiments presented in this paper. In non-simultaneous imaging, a single camera is placed in front of the mirror that reflects along the centerline of the cylinder. Non-simultaneous image acquisition utilized a single experiment for each imaging technique. It is important to note that some attenuation of the pictures can occur from the hardware setup. To flood illuminate the engine cylinder for the visible fuel spray experiments it was necessary to transmit the copper vapor laser beam through a fiber optic cable, which is mounted at the base of the mirror that reflects directly into the combustion chamber. The housing of this cable can cause interference as it can sit in front of the mirror and cause an artificial shadow in the images that required the use of the laser. Figure 20 contains camera setup diagrams for both views relative to the engine.
33 Figure 20: Side view diagram of the camera setups for the different views relative to the engine.
When using a simultaneous method of image acquisition, the visible and IR cameras were placed opposite each other to simultaneously record events. A diagram of this setup can be seen in Figure 21. The visible camera reflects off the front side of an infrared beamsplitter. The IR camera is placed such that the energy from the cylinder goes through the beamsplitter and reflects off a gold, first-surface mirror. When using this technique, the cameras can be simultaneously triggered such that comparisons can be drawn between the imaging techniques and information gained about specific engine events.
34 Figure 21: Top view diagram of the simultaneous imaging camera setup relative to the engine cylinder.
In both imaging methods, the cameras were triggered by 5V TTL pulses. Because the IR camera was limited to one frame per engine event, a computer program was used to trigger this camera at a set of delays in 100 m s increments, starting with a zero delay and adding 100 m s for each subsequent recorded image from the following cycle. Forty-one cycles of IR images were recorded. Unless otherwise noted, this image timing scheme was used for the experiments presented in this document. The visible and OH cameras acquired 20 cycles of images at 10,000 fps recording 100 frames per cycle. The injection pulse was used as the baseline trigger for both camera timings, capturing the entire engine event. Figure 22 depicts the timing of image
35 acquisition, showing the image timing for the IR at one image per cycle and the timings of the high speed images (visible and OH).
Figure 22: Trigger timings of the cameras of separate experiments.
3.13 SMOKE METER
An AVL 415s smoke meter was used to acquire engine-out soot emissions data from the optical engine. The 415s is a filter-type smoke meter that uses an optical meter to evaluate the amount of blackening (via the reflectivity of the paper) that occurs on the filter. The 415s has a heated sample line. The sampling line of the smoke meter was plumbed into the exhaust manifold of the optical engine. A separate set of experiments to the imaging experiments were
36 used to gather emissions data. Smoke emissions data were taken by running the optical engine for 50 firing cycles (~2 seconds at 1500 rpm). The smoke meter was set to acquire measurements for 6 seconds, effectively capturing the soot produced by all of the firing cycles. The acquisition process was as follows:
· The engine was motored to achieve proper steady boundary conditions.
· The smoke meter was triggered to acquire data.
· Within two seconds of triggering the smoke meter, the set of 50 injections was
activated, ensuring that the 6 second sampling time would be long enough to
capture the entire set of combustion cycles.
· Four measurements were taken at each condition.
In utilizing this process to acquire data, the engine-out soot emission data produced are values corresponding to a weighted filter smoke number (FSN) average over the entire set of combustion cycles and sampling time. That is, the values produced from this acquisition do not correspond to the steady-state FSN values that might be produced from metal engine experiments, but, instead, are meant as a relative comparison.
3.14 SOURCES OF UNCERTAINTY AND ERROR
Uncertainties in data and image acquisition can be caused by a variety of measurement and experimental setup errors. Besides the typical measurement uncertainties described by
Moffat [50], several other uncertainties and errors may occur that are specific to the experimental setup and data acquisition process used in the experiments presented in this document. The uncertainties specific to these experiments include cyclic variation, uncertainties in the start of image exposure, amongst others.
37 3.14.1 Engine Variations
Cycle-to-cycle variations inherent in engines are well-described (e.g. [2]). The many complex processes that occur during the engine cycle can lead to substantially different behavior from one cycle to the next. The differences in combustion behavior between cycles is almost certainly caused by a combination of the largely turbulent in-cylinder flows, variations in fuel concentrations and temperature gradients, and fluctuations or disturbances in boundary conditions, amongst many other possibilities. Because of these inherent variations in engines, in- cylinder data are usually presented as averages of several cycles with error bars to indicate the average behavior of the combustion events along with how consistent the combustion events were.
Although it has been shown that there are many differences between optical and all-metal engines that affect in-cylinder conditions [51], the experimental setup used in the experiments closely recreates the in-cylinder conditions of the all-metal engine at the times of the engine- cycle that are of interest. The differences between the optical and production engines occur largely due to blowby and heat transfer properties of the piston. In the optical engine used for the experiments in this paper, the bowl of the piston is replaced with a sapphire window and mounted on a Bowditch piston to allow optical access. The cylinder bowl will therefore have different heat transfer properties compared to the all-metal engine because of the differences in materials. The graphite piston rings used in this experimental setup also yield more blowby than in a traditional ring pack. Because of this increased blowby, it was necessary to modify the intake manifold pressure in order to achieve the desired in-cylinder pressures when the engine was motoring.
38 Besides the inherent engine cyclic variation, the method of optical engine operation in these experiments could also lead to differences in the combustion behavior between cycles. The operating procedure to acquire data in the optical engine was to motor the engine with the controller operating, such that proper in-cylinder pressure and engine boundary conditions could be achieved. One issue with this process of engine operation is the increase of in-cylinder temperature caused by combustion. When combustion occurs, the combustion chamber wall temperature will increase; the first cycle may have a lower wall temperature than a later cycle.
Another possible source of variation in this operation scheme is the increase of exhaust gases that is trapped internally; the first cycle will not have any internal exhaust gases, while a later cycle likely could have more trapped gases if these gases buildup. Figure 23 contains a plot of several combustion cycles throughout an engine test, showing the possible variation of in- cylinder pressure through a single experiment. Other problems might also exist, but the previously discussed issues are likely to be the largest sources of differences caused by the engine operation method.
39 Figure 23: Plot of various cycles of a sample test showing the difference in combustion as the experiment progresses.
3.14.2 Camera triggering
A significant issue was discovered toward the end of this work involving the triggering capabilities of the high-speed, visible camera. It was discovered that an overvoltage to an input caused damage to the processing board inside the camera. Because of this damage, the start of frame exposure would not reset to the rising edge of the trigger signal as it should. The delay to the beginning of recording was also observed to be around 170 m s, rather than the typical delay of less than 10 m s. Therefore, the timings of the visible images needed to be adjusted to correctly place the images in the cycle. The beginning of the exposure of the first image of a video would
40 not correspond directly to the trigger, but could vary, with whichever frame was exposing when the recording signal turned positive. The start of image exposure can vary by up to an entire frame and the image timings could be off by ½ of a frame from the given point, or 0.45 CAD at the typical engine speeds used in the experiments presented in this document. However, because the exposure timing is a random event, ensemble averaging will have the effect of somewhat neutralizing the errors caused by this uncertainty. The IR camera did not exhibit any of these triggering issues. An example of the triggering, recording, and exposure signals from the visible camera can be found in Figure 24.
Figure 24: Sample camera-out signals of recording and exposure illustrating the delay from trigger until recording (left) and the unsynchronized exposure with the trigger (right).
3.14.3 Data Acquisition
Uncertainties involved with data recording are typically related to the resolution of the acquisition system. The CAS that was used for data acquisition has a 15 bit resolution, which can produce a 5 digit precision in signal measurement. Because of the high precision of the system,
41 electronic noise of the signals can be a large source of errors. The data are filtered and/or averaged, as needed to reduce the effect of noise on the recorded signals.
3.15 PROBABILITY MAP EXPERIMENTAL SETUP
Four operating conditions were used to examine differences between operating conditions with a 6-hole nozzle with 0.120 mm holes and a 160 degree cone angle. The conditions chosen are LTC conditions, with variations in load from 6.75 bar to 12.75 bar goal indicated mean effective pressure (IMEP). The operating conditions for the combustion conditions can be found in Table 2. The operating conditions used for the fuel spray experiments were the same as those for the combustion experiments, except that the fuel spray experiments used only nitrogen supplied to the engine such that in-cylinder combustion would not occur. All operating conditions were run at 1500 rpm with an injection pressure of 2000 bar. For the two lower load combustion conditions, 10 experiments were completed with 5 images taken at steps of 200 m s after the injection signal, yielding a total of 50 images per delay step. Because of soot deposition on optical accesses, only 5 experiments were completed at the higher load combustion conditions in a similar manner, yielding 25 total images per delay step for these two conditions. The fuel spray experiments were completed in a single test, with 50 images recorded at each delay step.
Table 2: Probability map combustion operating conditions.
Intake Motoring Goal Case Inj PW Inj Timing Temp Pressure O2 % IMEP (CAD (bar @ (ms) BTDC) (deg C) TDC) (bar) EPA 1 0.62 -15 33 50 12.3 6.75 EPA 2 0.79 -13 37.5 62 12.3 8.5 EPA 3 1.04 -12 42.8 78 12 11 EPA 4 1.17 -12 51.2 91 12 12.75
42 3.16 INJECTOR NOZZLE COMPARISON STUDY EXPERIMENTAL SETUP
The injector nozzle comparison study tests were completed to gain an understanding of the differences in fuel spray and combustion between nozzle number and hole size, across a range of loads. Four nozzles were used in this experiment: 2 nozzles with 7 holes and 2 nozzles with 6 holes. Each pair of nozzles had one nozzle with .100 mm and one nozzle with .120 mm diameter holes. All nozzles had a cone angle of 160 degrees. Four load conditions were run with each of these nozzles, ranging from a goal IMEP of 6.75 bar up to 12.75 bar. The fuel spray experiments were run with only nitrogen boosting, such that no combustion would occur.
For the fuel spray imaging of the piston view, a non-simultaneous imaging technique was used. On-hundred cycles of IR images were recorded, starting at 200 ms in 100 ms steps to 1100 ms with 10 images per step. Visible imaging was completed with 30 recorded images per cycle, with 40 total recorded cycles. The copper-vapor laser was used to illuminate the fuel spray events when the visible camera was used.
Experiments with combustion used 41 consecutive combusting cycles and simultaneous image acquisition. The IR images were recorded in 100 m s delay steps with one image per cycle, starting with the injector trigger signal. The 41 cycles and stepped delays allowed for the IR camera to capture the entire combustion event. The visible images were 100 frames recorded at
10,000 fps, triggered with the injector signal for all 41 cycles.
When using the valve view, a non-simultaneous technique was used. For the combustion experiments with this view, 41 combustion cycles were used for the IR images with the same image timings as the piston view. The fuel spray experiments with this view only utilized IR imaging; visible imaging of fuel spray was not completed for this view, as no droplets would be expected to be observed under this view. Both sets of IR images were recorded at one frame per
43 cycle with 100 ms delay steps between cycles. One-hundred visible images per cycle were recorded at 10,000 fps of 20 cycles through this view. This method of data acquisition allows for analysis to be completed, despite lending to variations between experiments.
The operating conditions that were used in the nozzle study experiments can be found in
Table 3. In this table, the nominal values used for these experiments are presented. The four different conditions that were used were for 4 load conditions. The IMEP and injection timings were held constant for these cases, along with the other boundary conditions. Through the images produced under these conditions, the differences in the combustion and fuel spray behavior between nozzles can be compared.
Table 3: Nominal engine parameters for nozzle study.
Inj Intake Goal Peak Peak Inj PW Timing Temp MAP IMEP Press Press (CAD (bar (CAD Nozzle Case (ms) BTDC) (deg C) abs) O2 % (bar) (bar) ATDC) 7x.100x160 EPA 1 0.65 -15 33 1.31 12.3 6.75 72 5-7 7x.100x160 EPA 2 0.76 -13 37.5 1.63 12.3 8.5 98 5-7 7x.100x160 EPA 3 0.92 -12 42.8 2.12 12 11 114 5-7 7x.100x160 EPA 4 0.99 -12 51.2 2.47 12 12.75 125 5-7 7x.120x160 EPA 1 0.66 -15 33 1.31 12.3 6.75 72 5-7 7x.120x160 EPA 2 0.75 -13 37.5 1.63 12.3 8.5 98 5-7 7x.120x160 EPA 3 0.85 -12 42.8 2.12 12 11 114 5-7 7x.120x160 EPA 4 0.905 -12 51.2 2.47 12 12.75 125 5-7 6x.100x160 EPA 1 0.80 -15 33 1.31 12.3 6.75 72 5-7 6x.100x160 EPA 2 1.00 -13 37.5 1.63 12.3 8.5 98 5-7 6x.100x160 EPA 3 1.40 -12 42.8 2.12 12 11 114 5-7 6x.100x160 EPA 4 1.55 -12 51.2 2.47 12 12.75 125 5-7 6x.120x160 EPA 1 0.62 -15 33 1.31 12.3 6.75 72 5-7 6x.120x160 EPA 2 0.79 -13 37.5 1.63 12.3 8.5 98 5-7 6x.120x160 EPA 3 1.04 -12 42.8 2.12 12 11 114 5-7 6x.120x160 EPA 4 1.17 -12 51.2 2.47 12 12.75 125 5-7
44 3.17 BIODIESEL STUDY EXPERIMENTAL SETUP
The fuels that were used in the biodiesel study were a Canola-derived FAMES biodiesel
(Canola), a blend of the Canola biodiesel and DBS (Can/DBS), and a pump Diesel fuel as a baseline fuel. The blend of Canola and DBS fuel was in a 60% majority Canola and a 40% minority of DBS by volume. The Canola was distilled with methanol using a transesterification reaction. The Diesel fuel was a pump Diesel. Fuel properties can be found in Table 4. Note the additional available oxygen (single bond) for the DBS.
Table 4: Properties of fuels used in biodiesel study.
Fuel Appx. Density Flash C H O C-O C-O Cetane (kg/L) Point Single Double Number (K) Bonds Bonds Diesel 50 0.839 398 ~12 ~23 0 0 0 Canola 55 0.912 453 ~19 ~35 2 1 1 DBS 35 0.977 408 12 22 4 2 2
The biofuels were supplied to the injector using a Fuel Sample Separation Cylinder
(FSSC) which allowed the pressure of the common rail fuel system to be transferred through a piston to the sample fuels. In using this device, it was possible to use less fuel than would need to be used with the entire common rail fuel system. A diagram of this device can be seen in Figure
25. Diesel fuel was supplied directly to the injector from the common rail. Although differences in fuel temperature may be expected between fueling with the FSSC and the standard common rail, these effects were considered negligible, as previous studies have shown fuel temperature to have little effect on the in-cylinder combustion behavior, although some effect on engine-out emissions might be expected [31,52].
45 Figure 25: Diagram of FSSC.
Three operating conditions were tested and filmed during the biodiesel study: a low load low temperature combustion (LTC1), a low load conventional combustion condition (CC2), and a mid-load conventional combustion condition (CC3). The three conditions were chosen to examine the difference that combustion strategy would play in a lower load condition (between the CC2 and LTC1 conditions) as well as to examine combustion differences between a lower load and higher load conventional combustion case (between the CC2 and CC3 conditions). The operating conditions used in these experiments are tabulated in Table 4. Note the optimized operating conditions for the biofuels, which are the cases modified to match the in-cylinder combustion pressure data of the baseline Diesel conditions. In order to match the load and combustion timing between the fuels, the pulse widths and injection timings were adjusted to achieve the desired in-cylinder behavior. Injection timings were adjusted to match the point of
10% burn (CA10) within 1 CAD of the Diesel condition. Pulse widths were adjusted such that
IMEP values were within 10% of the baseline Diesel value. The pulse widths of the pilot injections were increased at approximately the same percent as the main injection pulse widths in an attempt to ensure similar energy releases between the fuelling conditions. The 10% variation in load was allowed because of the restrictions to finding these conditions in an optical engine and to reduce the amount needed to run the engine. These variations in load should be noted as
46 they may lead to slightly different results presented later. All operating conditions were run at
1500 rpm with a common rail injection pressure of 2000 bar.
Table 4: Biodiesel study operating conditions including optimized timings and pulse widths.
Fuel Operating Pilot Pilot Main Main Intake Motoring O2% Avg Condition Inj Inj Inj Inj Temp Pressure IMEP PW Timing PW Timing (°C) @ TDC (bar) (ms) (CAD (ms) (CAD (bar) ATDC) ATDC) Diesel LTC1 N/A N/A 0.38 -12 60 49 11.5 3.76 Diesel CC2 0.24 -20 0.3 0 90 49 14.5 3.07 Diesel CC3 0.255 -22 0.375 -2 50 56 14.5 5.43 Canola LTC1 N/A N/A 0.38 -12 60 49 11.5 3.23 Canola CC2 0.24 -20 0.3 0 90 49 14.5 2.46 Canola CC3 0.255 -22 0.375 -2 50 56 14.5 4.22 Can/DBS LTC1 N/A N/A 0.38 -12 60 49 11.5 2.89 Can/DBS CC2 0.24 -20 0.3 0 90 49 14.5 1.81 Can/DBS CC3 0.255 -22 0.375 -2 50 56 14.5 3.62 Canola LTC1 N/A N/A 0.4 -10 60 49 11.5 3.75 (Optimized) Canola CC2 0.24 -19 0.32 1 90 49 14.5 2.93 (Optimized) Canola CC3 0.26 -22 0.4 -2 50 56 14.5 4.91 (Optimized) Can/DBS LTC1 N/A N/A 0.42 -11 60 49 11.5 3.80 (Optimized) Can/DBS CC2 0.25 -20 0.33 0 90 49 14.5 2.89 (Optimized) Can/DBS CC3 0.27 -22 0.43 -2 50 56 14.5 5.33 (Optimized)
Five different experiments were conducted at each operating condition with each fuel, such that four imaging techniques and engine-out particulate measurements could be completed for each fuel at the operating conditions of interest. The first test used the IR camera for recording images. The second test was for visible, high-speed with laser illumination to study liquid fuel penetration of the fuels. The third tests were run without laser illumination of the piston view using the visible camera to study the natural luminosity of soot without the
47 background interference of the laser illumination. The fourth tests were run to visualize in- cylinder OH. The last tests were run to acquire relative engine-out soot measurements with the smoke meter.
48 CHAPTER 4
IMAGE PROCESSING
Once the imaging of in-cylinder fuel spray and combustion events was completed, it was necessary to analyze the images produced by these experiments. The images recorded in these experiments were post-processed in numerous ways, as is discussed in this section. The image analyses focused on examining fuel spray characteristics, how and when combustion occurred, and how these processes led to differences in soot formation. Projected areas of the in-cylinder volumes of combustion gases and soot are calculated, along with different radial distance metrics, in order to characterize the in-cylinder events for different boundary conditions, nozzle geometries, and fuels.
4.1 FUEL SPRAY IMAGE PROCESSING
Images of fuel spray in the absence of combustion were processed to examine differences in fuel spray behavior for different operating conditions, geometries, and fuels. Threshold values were used to determine whether fuel existed at each pixel. In studying these images, it was necessary to subtract the backgrounds of the images to leave only the effects of fuel spray. The backgrounds were taken as the first image of the series; no fuel spray would occur in this image.
Any excess glare that occurred in the images around the areas of interest was also removed. The infrared images were processed to provide information on both the liquid and vapor portions of the fuel plumes, while the visible images provided only the liquid fuel droplets via a Mie scattered technique, as discussed previously.
For the piston view images, the length and width of the fuel plumes were calculated. The fuel penetration into the cylinder has been shown to be a good determinant of oxidizer mixing into
49 the fuel plume [53]. The average spreading angles (widths) of the plumes were calculated for the infrared images to determine the effect of nozzle geometry on fuel plume spreading angle. Figure
26 is an illustration explaining the studied fuel spray characteristics in the piston view.
Thresholds for the infrared images were set at a point that corresponded to approximately 2000 counts, or about 15% of the sensor detector maximum. Because of differences in experimental setup of the laser, thresholds for the visible liquid fuel images varied between 4% and 15% of maximum pixel value, depending on laser light intensity, to best calculate the liquid fuel penetration.
Piston View Image Fuel Plume Angle
Vapor Length
Liquid Fuel Length
Figure 26: Description of piston view fuel spray metrics.
50 Through the valve view images, it was possible to examine the fuel that would enter the region directly below the valve window with the infrared images. Fuel in the squish region is thought to be a large source of flame quenching, and, hence, unburned hydrocarbons and carbon monoxide [54]. The images used for this data were first processed to remove the areas inside the combustion bowl. The image in Figure 27 gives an example of an IR fuel spray valve view image and indicates the combustion bowl area that was subtracted. The fraction of the area in which fuel vapors were observed was calculated. The average pixel intensity was also calculated for those pixels at which fuel existed. A lower average intensity could point to a lower concentration in the areas of fuel, with the assumption that the fuel vapors are at similar temperatures, yielding the difference in intensity largely due to line-of sight concentration effects. Valve View Image
Edge of Piston Bowl (Subtracted)
Figure 27: Example fuel spray valve view image.
51 4.2 COMBUSTION IMAGE PROCESSING
The combustion images through both views were analyzed to examine the projected areas of combustion or soot volumes that occur for the various experiments. The areas that occur in the images are the volumes of combustion, soot, or OH projected into the viewing plane along the line-of-sight of the camera. The visible images were processed to produce the in-cylinder areas where soot occurs, while the IR images provide information on the exothermic areas of hot combustion gases. The intensified OH images showed areas where this radical existed. The total number of pixels that fit the criteria was normalized by the total pixels corresponding to the area of interest in that view, yielding the fractional area of the view in which the parameters were observed.
Threshold values of pixels were used to determine whether combustion, soot or OH occurred for a given pixel. The threshold for the IR images was taken to correspond to approximately
2000 counts of the camera. For reference, the compression of fuel in the absence of combustion yielded maximum intensities of around 1500 counts for the camera settings used for the experiments in which combustion was expected. Unless otherwise noted, a floating threshold was used for determination of soot in the visible images in order to reduce the effect of soot luminosity reflection off of the cylinder head. The method to determine this threshold value began by analyzing the area of the image in an 80 pixel by 80 pixel area around the nozzle. The average pixel value in this area was then multiplied by 2 and added to 40 to give the threshold value used in the analysis of visible images. Upper and lower visible image pixel intensity threshold limits were set at 200 and 50, respectively. The OH images were observed to have relatively low background noise and reflection, and it was determined that a constant threshold of 2% of the maximum pixel value could be used.
52 Another metric analyzed in the images produced from the experiments in which combustion occurred is the radial distance away from the injector nozzle that combustion, soot, or OH occurred. The distances produced by this analysis are the point nearest to the injector that combustion, soot, or OH occurs for each individual combusting plume. This analysis was accomplished by dividing the image into sections corresponding to each fuel plume. The individual plume distance values were then averaged for each image to produce the average distance away from the injector for the plumes. If no soot, combustion, or OH was observed for a given combusting plume, that plume was not used in averaging. An image explaining the image processing technique is seen in Figure 28. In the left side of this figure, a raw image is marked with the different divisions of the cylinder, indicated with red lines, as well as the nearest points in each section, marked with green dots. The image on the right side of the figure is a translation of that image into a contour plot in Cartesian coordinates. As discussed in the literature review section, the distance from the tip of the nozzle to a combusting plume, the lift-off length (LOL), is an important combustion parameter, as, for the same chemistry and boundary conditions of the jet, a shorter LOL would typically mean less ambient in-cylinder gas entrainment, leading to increased soot production [31,37-38]. Typically the lift-off length is analyzed using OH imaging, with the minimum OH radial distances of most interest, as these values would indicate the nearest LOL and the point of least ambient gas entrainment into the fuel jet.
53 Figure 28: Raw, visible combustion image and a translated Cartesian contour plot, showing the nearest distances to the injector of soot.
4.3 PROBABILITY MAPS IMAGE PROCESSING
Probability maps were created to examine the likelihood that fuel spray, exothermic areas from combustion, or soot would occur at different points of the cylinder using threshold values.
The maps were created by normalizing the number of frames in which each parameter was observed along that in-cylinder line-of-sight with respect to the total number of images taken at a given CAD. A constant threshold was maintained for the visible images to determine if soot was present if the red values were greater than 200 and the green values greater than 50 for a given pixel. Figure 29 shows an example probability map of the exothermic areas produced by combustion with the positions of the intake valves, exhaust valve, and exhaust valve window.
The direction of swirl is in the counterclockwise direction, as noted in the figure. A reference jet centerline is also illustrated with the other jets observed around the nozzle in the center of the view. The dark red in these images means fuel spray or combustion was seen at that in-cylinder
54 position in every image. The dark blue means that neither combustion nor fuel spray was seen at that position in any image. It is important to note that the portion of increased probability of the nozzle and fuel jets in the middle of the view that may appear in the exothermic combustion area or soot probability maps is not from combustion or soot occurring here, but, instead, is an artifact caused by reflection of energy off the liquid fuel and injector nozzle. Likewise, in some images, reflection off the valves or window may also appear, even though background subtraction was used to reduce this effect.
Figure 29: Description of geometry for the probability maps.
55 CHAPTER 5
PROBABILITY MAP STUDY RESULTS
The probability maps study was completed with a single injector nozzle, under 4 different
LTC load conditions. The goal of these experiments was to characterize the variations inherent to the engine when operating under these conditions and analyze how general combustion behavior might vary under different loads. Pressure data as well as IR and visible images taken from these tests are used in the analysis to characterize how fuel spray, soot, and combustion processes change as load is increased.
5.1 Mass Fraction Burned
The mass fraction burned (MFB) curves of the operating conditions are calculated from the pressure data and presented in Figure 30. In analyzing these curves, it is clear that the two lower load conditions behave similarly to each other, while the two higher load conditions combust in a different manner. The combustion of the lower load conditions is much faster than that of the higher load conditions, indicating that the combustion of the higher load conditions must be restricted by the longer fuel delivery needed to achieve these loads. In examining the error bars of the curves, it is also clear that there are larger variations in the MFB curves of the lower load conditions, compared to the EPA3 and EPA4 conditions. In an attempt to explain the differences in the error bars, the images that were acquired from these experiments are investigated.
56 Figure 30: Mass fraction burned of the probability map study operating conditions.
5.2 Engine Speed Variations
When analyzing these images, it was determined that changes in engine speed during engine operation, coupled to constant image delay timings being used, caused variations of when in the cycle the images were actually acquired. The crank angles presented in this section for the experiments in which combustion occurred may vary with maximum standard deviations of 0.4
CAD. The engine speed of the fuel spray experiments did not vary during the test, as there was no increase in engine load due to combustion, hence the CAD of the images remained constant.
The method of determining the actual position in the cycle an image was taken was to use the
57 speed at the beginning of a particular cycle and calculate to what CAD a given frame would correspond for the delay from the trigger signal. The results presented for this study are the only figures in this dissertation that are corrected for engine speed variations; the other sections had similar engine speed variations within conditions such that even if the image timings vary from those presented, the points used to make comparisons will be similar.
5.3 Soot Deposition Effects
Soot deposition on the Bowditch piston window can be a source of significant imaging errors. The deposition on the window causes less energy to be acquired by the camera, and can cause errors during post-processing of the images. In order to quantify this effect, an experiment was performed in which 50 consecutive cycles were run under the EPA3 condition, a condition with significant soot production. Probability maps were created using the same threshold determination for sets of 10 consecutive cycles and are presented in Figure 31. From these maps, it is clear that little effect of soot deposition is observed until the set of the 21st-30th cycles. After these cycles, significant attenuation of the signal can be observed towards the outside of the bowl in locations where the fuel plume contacts the bowl just down-swirl of the jet centerline. Because of this result, only the first 25 cycles of a test are used to create the probability maps when significant soot deposition is expected (in the high load, EPA3 and EPA 4, conditions).
Figure 31: EPA3 MFB 50% point probability map of sets of10 consecutive cycles illustrating the effect of soot impingement at the outside of the bowl window.
58 5.4 Sample Probability Maps
As an introduction to the probability maps results, sample maps of fuel spray, exothermic areas from combustion, and soot occurrence are presented from the EPA1 condition in Figure 32.
Four timings of MFB are presented in the following figures, such that the approximate timings of ignition (1.5% MFB), the onset of combustion at 10% MFB, the midpoint of combustion (50%
MFB), and the end of combustion around 90% MFB could be examined. From the images in
Figure 32, there are a few notes to be made about the in-cylinder processes that hold true throughout all conditions. Combustion starts inside the fuel plume in the down-swirl direction from the jet centerline, with soot formation beginning at similar areas inside the fuel plume, only slightly later in the combustion event. Also note that exothermic areas from combustion at ignition are less likely to occur at the bottom of the view, which is the region of the exhaust valve window. The lack of exothermic combustion gases in this region indicates that cooling caused by the valve window may play a role in delaying the ignition of fuel in this part of the cylinder. However, as combustion progresses, a more homogeneous distribution of exothermic sites of combustion gases is observed around the combustion bowl, including the area of the valve window. It is also interesting to note that the areas of high probability of exothermic areas from combustion are larger than the fuel spray areas at the end of combustion, showing an effect on the spread of exothermic gases from expansion caused by combustion, compared to distributions of exothermic gases dictated solely by the areas of fuel. The analysis presented in the following sections will provide insight into the difference in variations between the low and high load conditions as well as how these differences relate to the soot production processes.
59 Figure 32: Exothermic (top), fuel spray (middle), and soot (bottom) probability maps for the EPA1 (6.75 bar IMEP) condition.
5.5 Projected Areas of Fuel Spray
Probability maps of fuel spray into nitrogen without combustion are presented in Figure
33 for the four operating conditions. Although these probability maps do not give information about the concentration of fuel, which might vary greatly within the fuel volume and between operating conditions, these maps yield information about where fuel exists in the combustion bowl and how these patterns vary between conditions. From these maps, it is observed that, at the beginning of combustion, more mixing from reflection off of the bowl wall would be
60 expected for the lower load cases. This increased mixing can be attributed to the longer ignition delays that occur for the lower load conditions, leading to increased time for mixing of the fuel.
When qualitatively analyzing the edges of the fuel spray plumes, similar gradients can be observed around the edges of the plumes between conditions, indicating that the combustion variations resulting from variations in the locations of fuel spray should be similar between conditions. Again, note that variations in concentration within these volumes are likely to occur and these variations may lead to combustion variations. Also note that through the maps of the
90% MFB points, the differences in the total amounts of fuel injected can be observed, as larger areas of fuel occur as load (and pulse width) increases.
61 Figure 33: Fuel spray probability maps for the EPA1-4 conditions.
62 5.6 Projected Exothermic Areas from Combustion
Figure 34 contains the probability maps of the exothermic sites produced by combustion.
The increased energy release as load increases can be evaluated as larger exothermic areas are observed at the same MFB point as load increases. As load decreases the variation observed in the probability maps increases, a trend especially evident in the probability maps at the 10%
MFB points and echoing the variations in the MFB curves. For the EPA1 and EPA2 conditions, the exothermic area probability maps at 10% MFB show a probability distribution that is more dispersed around the outside of the combustion bowl than the higher load conditions. This distribution pattern of the lower load conditions is indicative of large-scale, bulk mixing of the combustion volumes. In comparison, under the higher load conditions, the distributions of exothermic volumes produced by combustion are observed to be more dictated by fuel delivery and plume structure. These differences in combustion patterns, coupled with the differences in
MFB variation that occur between the lower and higher loads, reveals that the increased variations that occur for the lower load can be attributed more to the large-scale turbulent in- cylinder bulk mixing processes, rather than variations in fuel delivery.
63 Figure 34: Exothermic probability maps for the studied conditions.
64 5.7 Projected Areas of Soot
Figure 35 contains the soot probability maps for the four conditions, determined using visible natural luminosity as an indicator of soot. The probability maps of the EPA1 operating condition depict soot that maintains solitary pockets of soot that are related to the outward portion of the individual fuel plumes. The soot probability maps of the EPA2 condition show similar initial soot production to the EPA1 condition, though an increased area of soot is observed in a torus around the outside of the bowl by 50% MFB. From the soot probability maps presented for the EPA3 and EPA4 conditions, soot is still observed to be formed around the edge of the combustion bowl, as in the lower load conditions, but also forms further inwards along the fuel plumes. This result is especially obvious in the soot probability maps at 50% MFB, where soot is observed to protrude much further into the bowl for the higher load conditions.
Intuitively, as fuel is injected into areas that are already combusted (and oxygen used locally), rich regions that are more conducive to soot formation will be created further down the fuel plume, increasing the probability that soot would form closer to the injector. Therefore, the longer that an injection occurs, as with an increase in load, the more likely it would be to form soot further up the fuel plume. Indeed, the soot formation observed under the EPA4 condition shows the furthest inward protrusion, as the pulse width of this condition is the longest.
65 Figure 35: Soot probability maps across the conditions studied.
66 5.8 Fractional Areas of Projected Exothermic Areas from Combustion
Along with the probability maps, several other metrics of combustion were also studied using the images gathered from these experiments. In order to understand the variability of the fractional area of the exothermic areas of combustion, this metric was calculated for the different tests under the studied operating conditions. Figure 36 shows the average behavior from the tests plotted through the cycle with error bars of one standard deviation and Figure 37 contains the coefficient of variation (Cv) for the four operating conditions. From this figure, it is clear that, as the load increases, less variation of the fractional projected areas can be expected. This result is again in agreement with the trends determined by pressure data and relates to the results taken from the probability maps, in that the higher load conditions have been shown to be more dependent on the fuel plume structure and less variable than the more bulk-motion distributed combustion volumes of the lower load conditions. Note in this plot, the earlier onset in the cycle of combustion area increase as the load is increased. Also note the reduced final projected area for the lowest load, EPA1, condition, which is related to the reduced load and energy release of this condition. This decreased area is in comparison to nearly the entire view filled with exothermic combustion gases in the other three conditions.
67 Figure 36: Fractional area error bar plots plotted through the engine cycle for the different conditions.
68 Figure 37: Fractional area coefficient of variation plotted through the engine cycle for the different conditions.
The standard deviations of the fractional area results are also examined. The standard deviations for the different tests are plotted in Figures 38-41. Each point corresponds to the standard deviation within that particular test, whereas the solid line “All” refers to the deviation of all of the data points. From the presentation of these data, it can be determined that the variability of this metric may be attributed in a significant portion to the differences between tests (for all the data) as opposed to the cycle-to-cycle variations within a test. Indeed, very few individual tests saw deviations through the tests that were greater than the deviation between all of the data. This result indicates that slight differences in the engine boundary conditions
69 between tests might significantly contribute to experimental variation between tests, rather than solely caused by the inherent cycle-to-cycle variations.
Figure 38: Standard deviation of fractional area for the EPA1 operating condition.
70 Figure 39: Standard deviation of fractional area for the EPA2 operating condition.
71 Figure 40: Standard deviation of fractional area for the EPA3 operating condition.
72 Figure 41: Standard deviation of fractional area for the EPA4 operating condition.
5.9 Radial Distance of Projected Exothermic Areas from Combustion
Another metric that was studied is the length from the injector that combustion occurs.
Figure 42 shows the average distances with error bars and Figure 43 contains the coefficient of variation that were found from this analysis. As the load increases, combustion is observed further inward toward the nozzle earlier in the cycle. A much smaller Cv that is more uniform through the cycle is observed for this metric compared to the fractional area analysis. This result is likely caused by the mathematical definition of Cv, as, for the fractional area, the denominator is initially very small. However, the combustion distance analysis still shows smaller variations
73 with smaller error bars through the cycle than the area analysis. Note that this metric shows similar variation (error bars) between conditions, indicating less effect of combustion scheme on the variation of this metric. That is, the minimum radial distance to combustion volumes is less variable than the total area that combustion occurs. This result may be logical, as the radial distance to combustion should be more closely related to the physical processes of fuel delivery and ambient charge entrainment, which was shown to be less variable than the combustion processes.
Figure 42: Combustion length error bar plots plotted through the engine cycle for the different conditions.
74 Figure 43: Combustion length coefficient of variation plotted through the engine cycle for the different conditions.
The standard deviation of the combustion length is also analyzed and presented in
Figures 44-47. Similar to the fractional area results, the combustion length results show deviations between all of the data that are usually greater than within a single test. A larger percentage of data points are seen above the “All” deviation line for the lower load conditions than occur for the higher load conditions. Referencing the fuel plume reliance of the higher load conditions explains this trend, as the distances from the injector that exothermic gases of combustion occur for the lower load conditions has less to do with the plume structure as in the higher load conditions.
75 Figure 44: Standard deviation of combustion length for the EPA1 operating condition.
76 Figure 45: Standard deviation of combustion length for the EPA2 operating condition.
77 Figure 46: Standard deviation of combustion length for the EPA3 operating condition.
78 Figure 47: Standard deviation of combustion length for the EPA4 operating condition.
5.10 Engine-Out Soot Emissions
With the soot production processes of the various conditions characterized, it is useful to examine the engine-out soot emissions produced under these conditions to determine whether the trends found from the soot probability maps maintain validity in engine-out measurements.
Figure 48 shows the average FSN values for these conditions. Reasonable qualitative agreement can be observed between these data and the soot formation maps, in that as load is increased and an increased probability of soot formation is observed, increased engine-out emissions are observed. Note that significantly more engine-out soot occurs for the higher load conditions, with very little engine-out soot produced under the EPA1 condition.
79 Relative FSN of EPA Conditions 6
5
4
3 FSN 2
RelativeFSN 1
0 EPA 1 EPA 2 EPA 3 EPA 4 Operating Condition
Figure 48: Average filter smoke number of the operating conditions.
80 CHAPTER 6
NOZZLE STUDY RESULTS
As outlined in the experimental setup section, four different injector nozzle geometries were studied under 4 operating conditions to evaluate the effects of injector hole size and number on fuel spray, combustion, and soot production processes under various LTC load conditions.
Fuel spray and combustion events are analyzed and examined through both the piston and valve views. This section of this chapter is divided into two parts: one section that discusses fuel spray differences between nozzle geometries in the absence of combustion and a second section that discusses the combustion and soot production differences that occur between the different nozzle geometries.
6.1 Nozzle Study Fuel Spray Results
As previously stated, fuel spray experiments were run with only nitrogen supplied to the intake system, such that combustion would not occur in the cylinder. The metrics that were analyzed for this set of experiments include the length of the liquid and vapor portions of the fuel plumes through the piston view. Fractional areas and average pixel intensities of fuel in the squish region where fuel is seen were evaluated for the valve view. The goal of this analysis is to describe where fuel exists in the cylinder and how nozzle geometry affects certain fuel spray characteristics.
6.1.1 Sample Fuel Spray Images
A sample composite cycle of a fuel spray event can be found in Figure 49. A set of overlaid images from the piston view and a set of IR valve view images are presented in this
81 figure from the 6x.100x160 nozzle. The overlaid images are simultaneously acquired IR (red) and visible (green) fuel spray images. These images are provided to illustrate the general in- cylinder behavior of fuel spray. From these images, the swirl of the engine is seen to move the vapors (IR, red) off of the line of liquid fuel injection (visible, green) in a clockwise direction.
The valve view images show how the fuel is drawn into the region outside of the combustion bowl. In these valve view images, the bowl has been removed. Both sets of images are taken from the EPA1 fuel spray conditions. Note that the piston and valve view images are not at the same CAD. From these images, it is clear that general processes can be described but more in depth analysis is needed to fully characterize these events.
Figure 49: Sample fuel spray images from the piston view and valve views for the 6x.100x160 nozzle, EPA1 condition.
6.1.1 Piston View Fuel Spray Results
Figures 50-53 show the fuel penetration of liquid and vapor portions of the plumes. In these figures, the nozzle center is at 0mm and the bowl edge is at 32 mm. It was observed that the vapor penetration into the cylinder is similar between nozzles, although each operating condition showed the vapor portion of the 6x.100x160 nozzle progressed outwards slightly faster
82 than the others, while the 7x.120x160 nozzle progressed at the slowest rate. The width of the liquid fuel curves is related to the injection of liquid fuel: as fuel continues to inject, liquid fuel is observed in the cylinder; once the fuel injection has stopped, the curves return to the nozzle baseline. Note the lack of a substantial difference in quasi-steady liquid fuel penetration length between nozzles in these figures. The “quasi-steady” portion of the liquid penetration curves is the plateau at which the liquid penetration length reaches, where the energy entraining into the fuel jet is balanced by the energy needed to vaporize the liquid fuel, and is noted in Figure 50, along with the bowl edge and nozzle center. The quasi-steady liquid fuel penetration length increases slightly over time, especially in the higher load conditions, an effect attributed to in- cylinder charge cooling of the fuel injection. A substantial difference in quasi-steady liquid fuel penetration length between nozzles does not occur except for the two highest load conditions. It was unexpected that the 7x.120 and 6x.120 nozzles would cause the biggest difference in liquid fuel penetration lengths, but this difference may be related to a trade-off in droplet size and injection pulse width/momentum of the liquid fuel. The differences in liquid fuel penetration between nozzles may also be partially caused by difficulties in determining the actual maximum penetration length, as the signal/noise ratio for the visible technique is lower than that of the IR imaging of fuel spray. Differences in power and orientation of the laser between tests would also affect the ability to effectively determine the penetration of droplets into the combustion chamber. Aside from these difficulties, similar liquid fuel penetration trends are still observed within a condition.
83 Figure 50: Liquid (dashed) and vapor (solid) fuel plume lengths for the EPA1 condition.
84 Figure 51: Liquid (dashed) and vapor (solid) fuel plume lengths for the EPA2 condition.
85 Figure 52: Liquid (dashed) and vapor (solid) fuel plume lengths for the EPA3 condition.
86 Figure 53: Liquid (dashed) and vapor (solid) fuel plume lengths for the EPA4 condition.
Figures 54-57 show the calculated fuel plume widths of the vapor portions of the fuel spray for the nozzles under the four load conditions. The shape of these curves indicates that as the spray progresses, the width of the fuel plume increases. One should also note that a sharper increase in the width of the plumes occurs after the plume contacts the bowl wall, as noted in
Figure 54, indicating that the fuel plumes spread from the reflection caused by this bowl contact.
It is apparent from these figures that nozzle geometry has little effect on the average width of the fuel plume within a condition. Only in the higher load conditions is a difference in fuel plume spreading angle observed between the nozzles. It is interesting to note that the 0.100 hole sized nozzles show larger observed spreading angles of the fuel plumes for the 3 and 4 conditions. In
87 general, however, for similar engine boundary conditions, the shape of the fuel plumes seems to not be greatly affected by the nozzle geometries tested.
Figure 54: Vapor fuel plume widths for the EPA1 condition.
88 Figure 55: Vapor fuel plume widths for the EPA2 condition.
89 Figure 56: Vapor fuel plume widths for the EPA3 condition.
90 Figure 57: Vapor fuel plume widths for the EPA4 condition.
An example of the difference in fuel spray behavior between operating conditions can be seen in Figures 58 and 59, where the 4 operating conditions are plotted for the 6x.100x160 nozzle. Note that the abscissa in this figure is not in units of CAD, but, instead, is plotted as the number of image in the cycle (delay from SOIc). It is interesting to note that as the load increases
(and the in-cylinder pressure and temperature increase), both the liquid and vapor penetration lengths are delayed or decreased. The quasi-steady liquid penetration lengths become shorter as the in-cylinder pressure and temperature increase for increasing load. These results are intuitive, as the increased in-cylinder pressure and temperature will lead to more drag on the droplets and greater heat transfer into the droplets, both of which will lead to an increased vaporization of the
91 liquid fuel. The difference in liquid penetration seems to not have a significant effect on the vapor penetration, as the vapor portion of the fuel plume maintains similar behavior between the operating conditions. However, a trend can be seen that towards the outside of the bowl
(>~22.5mm), the plumes of the higher load conditions are slightly shorter for the same delay from injection. In comparison to the vapor penetration of the fuel plumes, the widths of the vapor portions of the fuel plumes show differences between the conditions. As load increases, the spreading angle of the fuel plumes also increases. When taken with the reduction in liquid fuel penetration, it may seem logical that swirl may affect the fuel vapor to a greater extent if the fuel is vaporizing more quickly. This is an important result, in that, although the penetration lengths of the vapor portions of the fuel plumes do not vary significantly, the increase of in-cylinder pressure and temperature causes wider plumes. That is, the so-called “envelope” of fuel existence is shown to change mostly from changes in the width of the fuel plumes across the studied operating conditions and is not greatly affected by nozzle geometry. Additionally, it must be noted that even though the envelope of fuel may not differ greatly between nozzles within an operating condition, the concentration of the fuel within the envelope almost certainly will vary.
Decoding effects of fuel concentration was beyond the scope of this study, however.
92 Figure 58: Fuel plume lengths for the 6x.100x160 nozzle across all conditions.
93 Figure 59: Fuel plume widths for the 6x.100x160 nozzle across all conditions.
6.1.1 Valve View Fuel Spray Results
When viewing the fractional areas of fuel spray and average pixel intensities in areas of fuel taken from the valve view, presented in Figures 60-63, it was noticed that, even though the piston view showed very similar behavior between nozzle geometries in the envelope of fuel, a reduced area of fuel is observed in the area under the valve view for the 7x.120 nozzle under the
EPA1 condition. The lower area of the 7x.120 nozzle also shows a lower average intensity, while the other nozzles are similar. The reduced fuel amount observed for this nozzle in this region is likely the result of the shorter injection pulse widths needed for this nozzle and, hence, a reduced momentum of the fuel. Less momentum of the fuel would provide less ability of the fuel to move
94 outwards in the combustion bowl and into the squish region. The other three nozzles show similar trends to each other in the squish region for this condition in both area and intensity.
When the pulse widths are increased as needed for the EPA2 condition, similar behavior occurs between all nozzles. The analysis for the EPA2 condition indicated behavior between the nozzles that was similar enough to not warrant further study for the higher load conditions.
95 Figure 60: Fuel fractional areas in the valve view for the EPA1 condition.
96 Figure 61: Fuel average pixel intensities in the valve view for the EPA1 condition.
97 Figure 62: Fuel fractional areas in the valve view for the EPA2 condition.
98 Figure 63: Fuel average pixel intensities in the valve view for the EPA2 condition.
99 6.2 Nozzle Study Combustion Results
6.2.1 Combustion Pressure Data
The firing pressure data and MFB curves of each of the nozzles under the different operating conditions are presented in Figures 64-71. Boundary conditions were maintained constant in the engine controller, although a slightly lower motoring pressure occurred for the 6- hole nozzle tests. The decreased motoring pressure was likely the result of a change of compression rings between the 7-hole and 6-hole nozzle tests. The main conclusions that can be drawn from the comparison of pressure data are the lower and later peak pressures for the 6-hole nozzles. When analyzing the MFB curves, it is apparent that combustion progresses faster for the
7-hole nozzles than the 6-hole nozzles, with the 6x.100x160 nozzle showing the longest ignition delay. The difference in combustion progression is almost certainly caused by the rate of fuel delivery, as the 7-hole nozzles deliver fuel more rapidly than the 6-hole nozzle because of the difference in nozzle flow area. That is, there is a greater area to flow fuel for the 7-hole nozzles, because there are more holes to deliver fuel through. To that end, the 6x.100x160 nozzle showed the latest peak pressure and longest MFB10-90 times for all cases, and also had the longest pulse widths. However, pulse width is not the only determinant in combustion progression, as the
7x.100 nozzle showed the shortest ignition delay of the nozzles, even though the 7x.120x160 nozzle had the shortest injection pulse widths. Also note the similarities in MFB consistency between conditions between this study and the previous probability maps study in that as load increases, the pressure and MFB curves are less variable for all nozzle geometries. The images of combustion produced by these experiments are analyzed to evaluate how combustion events differ for different nozzle geometries.
100 Figure 64: Pressure traces for the different nozzles for the EPA1 operating condition.
101 Figure 65: MFB traces for the different nozzles for the EPA1 operating condition.
102 Figure 66: Pressure traces for the different nozzles for the EPA2 operating condition.
103 Figure 67: MFB traces for the different nozzles for the EPA2 operating condition.
104 Figure 68: Pressure traces for the different nozzles for the EPA3 operating condition.
105 Figure 69: MFB traces for the different nozzles for the EPA3 operating condition.
106 Figure 70: Pressure traces for the different nozzles for the EPA4 operating condition.
107 Figure 71: MFB traces for the different nozzles for the EPA4 operating condition.
6.2.2 Combustion Sample Images
Sample combustion images from the nozzle study experiments are presented in Figures
72 and 73 for the 6x.120x160 nozzle under EPA1 and EPA4 operating conditions. Both infrared and visible images are given for the piston and valve views for the highest and lowest load conditions. The piston view images are simultaneously acquired. The bowl area has been removed for the valve view images to highlight the behavior of combustion in the squish region.
Note the increase in intensity and combusting area for the higher load IR images as well as the
108 significantly higher amount of soot in the visible images for the higher load case. Combustion parameters of the images are analyzed and discussed to better characterize the in-cylinder combustion and soot processes.
Figure 72: 6x.120x160 nozzle sample IR and visible images from the EPA1 condition.
109 Figure 73: 6x.120x160 nozzle sample IR and visible images from the EPA4 condition.
6.2.3 Nozzle Study Projected Fractional Areas of Combustion and Soot
The fractional areas of the piston view in which combustion and soot occur lend additional insight into the in-cylinder combustion events of different nozzle geometries. The plots of these areas through the cycle are presented in Figures 74-81. In these figures, the IR
(solid lines) curves can be taken to be the total fractional area of hot gases from combustion, while the soot fraction can be approximated by the visible areas (dashed lines). The “+” markers designate the soot fraction taken from the corresponding simultaneous images between the IR and visible imaging techniques; note that these points show little additional information about the soot behavior on single point basis. Although the projected areas are not the total in-cylinder
110 mass or volume of soot or combustion produced nor do these values contain information about concentrations or absolute temperature, this analysis still lends insight into how combustion and soot progress for the different nozzles. The 7x.120x160 nozzle shows a lower overall combusting area than the others, indicating that, for the same energy release, these areas may be more energy dense than the combusting areas of the other nozzles. Note the delay between the expansion of combustion area and the onset of soot production seen in the cylinder, caused by oxygen being locally used and leading to rich combustion regions. For the 6-hole nozzles, a longer time is seen between the onset of combustion and when projected areas of soot are seen. A lower sooting area trend is observed for the 6x.100x160 nozzle in the squish region, where significantly less soot is seen for all load cases. Despite similar areas of combusting gases occurring in this view for all nozzles, less projected area of soot is observed, indicating that soot may not be produced towards the outside of the combustion bowl for the 6x.100x160 nozzle as heavily as the other nozzles.
The maximums of these curves are discussed in more detail in the following analysis.
111 Figure 74: Fractional areas for the piston view of the EPA1 condition.
112 Figure 75: Fractional areas for the valve view of the EPA1 condition.
113 Figure 76: Fractional areas for the piston view of the EPA2 condition.
114 Figure 77: Fractional areas for the valve view of the EPA2 condition.
115 Figure 78: Fractional areas for the piston view of the EPA3 condition.
116 Figure 79: Fractional areas for the valve view of the EPA3 condition.
117 Figure 80: Fractional areas for the piston view of the EPA4 condition.
118 Figure 81: Fractional areas for the valve view of the EPA4 condition.
As a summary to the fractional area curves observed through the piston view, the maximum values of the projected areas of combustion gas are shown in Figure 82 and the maximum fractional area of soot volumes are presented in Figure 83. The main trend observed through the maximum areas of combustion gases is the increase in peak area as the load increases related to the greater energy release. It is immediately obvious in the soot maximums that larger areas of soot are observed as load increases, although a slight reduction in soot area exists between the EPA3 and EPA4 cases, even though EPA4 is a higher load case. It is speculated that, because this was the last operating condition tested before cleaning the window,
119 soot impingement on the window from the other operating conditions played a role in reducing the maximum soot areas observed by the camera, and may have led to lowered peak projected areas. The trends between the nozzles within a condition can still be examined, as all conditions showed similar amounts of soot impingement. Another trend that can be taken from this data is the increased projected soot area for increasing nozzle number. That is, the 7-hole nozzles show larger projected areas of soot than the 6-hole nozzles. To this extent, the highest areas of soot occur for the 7x.100x160 nozzle whereas the lowest areas are usually observed for the
6x.100x160 nozzle. Although the soot trends between the nozzles are likely related to the soot production of each individual combusting plume, and a higher concentration of soot could be possible inside the plume for the longer injections of the 6-hole nozzles, the end result is a larger maximum in-cylinder area in which soot is formed for the 7-hole nozzles. It must also be noted that, while larger areas may be observed for one nozzle compared to another, engine-out emissions may still differ if soot oxidation is different between the nozzles.
120 Figure 82: Maximum values of combustion fractional area curves through the piston view.
121 Figure 83: Maximum values of projected soot fractional area curves through the piston view.
Similar to the piston view summary, Figures 84 and 85 contain the maximum values of fractional area curves of combustion and soot observed through the valve view. In this view, similar areas of combustion gases are observed between nozzles, with the exception of the
7x.120x160 nozzle showing less combustion in the lowest load condition. This result is in agreement with the fuel spray results, where less fuel was observed for this nozzle under this condition. The most interesting trend in the squish region was the significantly less peak soot area seen in squish region for 6x.100x160 for all cases. This reduced area indicates that less soot would move outward from the bowl into the area under the valve view.
122 Figure 84: Maximum value of combustion fractional area curves in the valve view.
123 Figure 85: Maximum value of soot fractional area curves in the valve view.
6.2.4 Radial Distance of Projected Areas of Combustion and Soot
Another characteristic of the combustion behavior of the different nozzles that was studied is the radial distance from the injector that hot gases of combustion and soot were observed. The plots produced from this study are presented in Figures 86-89. Fuel is seen in the
IR images to combust toward the outside of the bowl for each of these nozzles and operating conditions and quickly moves back along the fuel plumes towards the nozzle. Shortly after ignition, at similar lengths as combustion began, soot starts to form. This location of soot formation is intuitive, as oxygen is locally used in the combustion volumes as combustion progresses, leading to areas of increased equivalence ratio that are conducive to soot production.
124 As combustion comes closer to the injector, soot does as well until the soot reaches a nearly constant length away. The hot gases of combustion nearly fill the cylinder later in the cycle, as shown in the previous fractional area of combustion section, coming almost back to the injector.
A minimum distance was set for these curves at 5 mm away from the nozzle in order to reduce the influence of reflection by the injector nozzle, causing the flat portions of the IR-indicated combustion curves at this distance in the two higher load conditions. As the reaction completes, the hot gases move outward, toward the bowl edge, especially for the 7x.120 nozzle. This outward motion can be attributed to the in-cylinder swirl causing centripetal acceleration of the gases as well as cooling of the in-cylinder charge causing less total area to be able to be detected.
Soot
Combustion
125 Figure 86: Radial distances from the injector of soot and combustion for EPA1.
Figure 87: Radial distances from the injector of soot and combustion for EPA2.
126 Figure 88: Radial distances from the injector of soot and combustion for EPA3.
127 Figure 89: Radial distances from the injector of soot and combustion for EPA4.
Similar to the previously presented maximums of the fractional area curves, the combustion and soot distance results are summarized with the values during the cycle that are nearest to the injector in Figures 90 and 91. For the 3 highest load conditions, the shortest soot distance is seen for the 6x.100x160 nozzle, likely related to the longer injection pulse widths needed with this nozzle. It was shown that this nozzle produces less overall area of soot and, therefore, the soot forming regions must exist in a thinner region inside the fuel plume in order to produce a lower overall projected soot area. The longer distance than the other nozzles for the lowest load case of the 6x.100x160 nozzle could be explained by the lower area of soot formed under this condition. In general, an inverse relationship occurs as shorter distances occur as
128 longer pulse widths are used. It is logical that the longer an injection occurs into a combusting region, more oxygen will be locally used, yielding richer, soot forming regions closer to the nozzle.
Figure 90: Minimum soot and combustion distances for the different nozzles across the operating conditions.
129 Figure 91: Minimum soot distances for the different nozzles across the operating conditions.
The trend of longer the pulse widths creating shorter the soot distances holds true, as the higher the load conditions (and, hence, longer pulse widths needed to inject increasing amounts of fuel), create shorter soot lengths. A correlation is shown in Figure 92, where the minimum soot lengths are plotted against the pulse width that produced that distance. A power series is shown on the plot to demonstrate a general correlation between the pulse width and soot distance. Pulse widths also play a role in the minimum IR combustion distance, as the longest combustion distance occurs for 7x.120x160 nozzle under the two lowest load conditions, correlated with the shortest injection pulse widths. At the higher load cases, the minimum IR- indicated combustion distance reaches the distance threshold for all nozzle geometries.
130 Soot Distance vs. PW
25
20
15 Soot Distances Power (Soot Distances) 10
Soot Distance Soot(mm) Distance
5
0 0 0.5 1 1.5 2 Pulse Width (ms)
Figure 92: Soot distance and pulse width correlation plot.
6.2.5 Nozzle Study Engine-out Soot Measurements
The soot that is produced during a combustion event may or may not be directly correlated to the engine-out soot emissions [36], as significant amounts of soot can be oxidized post-combustion [1-2]. In order to evaluate whether the trends observed of in-cylinder soot formation engine-out translate to emissions, FSN measurements were produced from the four
EPA conditions for comparison with the results found from the imaging portion of the nozzle study. The average relative FSN values for the different nozzles and operating conditions produced from these experiments can be found in Figure 93. The main trends in engine-out measurements indicate that slightly higher relative FSN values are created by the 6-hole nozzles
131 when operating under the higher load (EPA3 and EPA4) conditions. However, the 6x.120x160 nozzle produces the least amount of engine-out soot emissions for the lower load conditions.
Note that even though 6x.100x160 nozzle showed the lowest projected area of soot, this nozzle produced the highest engine-out emissions of particulates for the higher load conditions. It is interesting to note that there is not a large increase in relative FSN between the two higher load conditions. A nearly linear relationship between load and relative FSN can be observed between the first three conditions. However, the relative FSN for the highest condition is approximately equal to the previous condition for all the nozzles. This result might have been expected from the soot area results, where similar maximum projected areas of soot occurred for these conditions.
Average Relative FSN for EPA Conditions
7
6
5 7x100 4 7x120 6x100 3 6x120
Relative FSN Relative 2
1
0 EPA 1 EPA 2 EPA 3 EPA 4 Operating Condition
Figure 93: Average FSN values for the nozzles under the operating conditions.
132 The relationship between the soot observed and relative FSN engine-out measurements warrants further analysis. To this end, a bar graph is presented in Figure 94, with both the average relative FSN and the average maximum projected area of soot. From this figure, no distinct trends are immediately obvious between the maximum amount of soot observed and engine-out soot emissions. However, when relative FSN is plotted against maximum soot area, as in Figure 95, it becomes clearer that there is a correlation for a given nozzle. The 7-hole nozzles show a nearly linear relationship between the maximum projected soot areas and relative
FSN and values, with R2 values of nearly one. The 6-hole nozzles do not show as linear of a trend, but the correlations are still reasonable, with R2 values greater than 0.8. It is interesting to note that, if the linear trends are taken as true, for the same maximum projected soot area, the
7x.100 nozzle would lead to decreased engine-out particulates while the 6x.100 nozzle might lead to more. This result can be interpreted as increased post-combustion soot oxidation for the 7 x.100 nozzle and/or as an increased concentration of soot within the combustion areas of the
6x.100 nozzle.
133 Relative FSN and Max Projected Area of Soot
1
6 0.9
0.8 5
0.7 7x100 FSN 7x120 FSN 4 0.6 6x100 FSN 6x120 FSN 0.5 3 7x100 MS
Relative FSNRelative 0.4 7x120 MS
Max Soot Max Area 6x100 MS 2 0.3 6x120 MS
0.2 1 0.1
0 0 EPA 1 EPA 2 EPA 3 EPA 4 Operating Condition
Figure 94: Average FSN and maximum projected area of soot for all nozzles and conditions.
134 Relative FSN vs Maximum Projected Soot Area
1
0.9 2 2 R = 0.9918 R = 0.993 0.8 2 R = 0.8273 7x100 0.7 7x120 0.6 6x100 6x120 0.5 2 R = 0.8175 Linear (7x100) 0.4 Linear (7x120) Linear (6x120) 0.3 Linear (6x100) 0.2
Maximum Projected Soot Area
0.1
0 0 1 2 3 4 5 6 7 Relative FSN
Figure 95: Maximum projected area of soot versus FSN for the nozzles and conditions.
The integrated soot area was also investigated to determine whether a better correlation could be found between the observed in-cylinder soot and engine-out soot emissions. The integrated soot area is the integration of the total area of soot observed through the cycle. The results from this analysis are presented in Figure 96. It was determined that this prediction of engine-out soot was no better than using the maximum observed area. Indeed, worse correlations exist for this analysis than for the maximum area analysis.
135 Relative FSN vs Integrated Projected Soot Area
18
16 R2 = 0.937
2 14 R = 0.9681 7x100 R2 = 0.8249 12 7x120 6x100 10 6x120 2 Linear (7x100) 8 R = 0.6918 Linear (7x120) 6 Linear (6x120) Linear (6x100) 4
Integrated Projected Integrated Soot Area 2
0 0 1 2 3 4 5 6 7 Relative FSN
Figure 96: Integrated projected area of soot versus FSN for the nozzles and conditions.
136 CHAPTER 7
BIODIESEL STUDY RESULTS
The biodiesel experiments were completed using 3 different fuels under three operating conditions. The goal of these experiments was to understand differences in the physical in- cylinder combustion characteristics that occur and what effect these differences have on soot formation for different fuelling conditions. Similar to the previous section, with the nozzle geometry study, parameters of combustion and soot production were studied for the different fuels. Along with IR imaging of the exothermic areas of combustion and visible imaging of fuel spray and soot luminosity, intensified imaging of hydroxyl was also completed for this study.
7.1 Composite Cycle Sample Images
A sample of images from these experiments is given in Figure 97. For brevity, only the images from the CC3 operating condition are presented, and only for the optimized biofuel injection timings and amounts. The third cycle of the 41 recorded cycles of the OH and non- illuminated visible images are presented every 4.5 CAD in this figure. The IR images are of every fifth image in the composite cycle, again every 4.5 CAD. Through these sample images, it is clear that similar combustion events transpire inside the cylinder between the fuelling cases.
Fuel is injected and begins to react towards the outside of the combustion bowl in a similar manner between the fuels, as seen in the IR images. Note, also, in the IR images, the similar characteristics of combustion, in terms of area of hot gases and energy scale amongst the fuels.
The visible images show a reduced amount of natural luminosity of combustion for the 60/40
Can/DBS mixture, with only a small amount observed in the frames between 370 and 380 CAD.
Canola and Diesel fuels show similar amounts of natural luminosity to each other, indicating
137 similar areas of soot occurring in the cylinder. A necessary note to be made about the soot observed under this condition is the reflection of the visible light emitted from soot off the cylinder head. A sample of the reflection can be seen in the Diesel and Canola visible frames at
370.4 CAD. This operating condition, CC3, produced the most significant soot of the three conditions examined, and reflection of soot energy was only significant under this operating condition. Care was taken during processing by using increased threshold values, when reflection was present, to determine the sooting regions. The OH images in this figure also show fairly comparable in-cylinder trends of this radical production between the fuels. The earliest OH is present near the bowl edge and then moves inward along the combusting plume. As the reaction concludes, the OH moves outward and disperses amongst the combustion products in distinct pockets, in each fuel plume. It must be remembered that the images presented in this figure are only sample images and the average behavior may be somewhat different to that seen in these images, but these images still allow for a general introduction to the data. Additional analyses on the recorded images are completed in following sections of this paper.
138 Figure 97: Sample images taken from the CC3 operating condition.
139 7.2 Pressure and MFB
Average in-cylinder pressure data and MFB of these experiments is presented in Figures
98-103. These data are taken from a single test, approximately 50 fired cycles per test. From these data, a need to optimize the pulse widths and timings of the biofuels is evident, as the non- optimized combustion of the biofuels is significantly different from the baseline Diesel curves.
The MFB are shown here in order to show the agreement with MFB 10% points between the optimized biofuels conditions and the Diesel baseline. The slight initial decrease in some of the
MFB curves is attributed to blowby of the graphite piston rings. In all cases, it is seen that less than one CAD is seen between the fuels at the MFB10 point for the optimized fuel conditions with the baseline Diesel. As was discussed in the experimental setup section, the average IMEP of each test was calculated from this pressure data and matched between the fuels for the optimized conditions.
140 Figure 98: Pressure data curves for the LTC1 operating condition.
141 Figure 99: MFB curves for the LTC1 operating condition.
142 Figure 100: Pressure data and MFB curves for the CC2 operating condition.
143 Figure 101: Pressure data and MFB curves for the CC2 operating condition.
144 Figure 102: Pressure data and MFB curves for the CC3 operating condition.
145 Figure 103: Pressure data and MFB curves for the CC3 operating condition.
7.3 Liquid Fuel Penetration Lengths
Liquid fuel penetration lengths of the fuels were calculated to determine the length needed for liquid fuel to travel in order to vaporize. The length of the liquid fuel is an important parameter, as longer vaporization lengths could result in increased bowl impingement of the fuel plumes, leading to increased soot production [1,12]. Twenty cycles of the laser-illuminated, visible combustion images were used for this analysis. The engine conditions used were the baseline Diesel fuel conditions and non-optimized pulse widths and timings for the biofuels, such that the same pulse width and timing of the fuel injection were similar between experiments.
Only the liquid fuel penetration lengths of the main injections were of interest in this study. The
146 liquid fuel penetration length results under the three operating conditions are presented in Figure
104. The liquid fuel for all cases exits the nozzle at similar speeds, as seen by comparing the initial slopes of the curves. Note that the biofuels have longer maximum lengths for the operating conditions studied here, which is in agreement with literature [37,39,49-50]. The longer maximum lengths are to be expected, as the biofuels are less volatile than pump Diesel, as seen from the flash points presented previously in the experimental setup section in Table 4. Also note that the maximum liquid penetration lengths between the two biofuels are not significantly different within an operating condition. This result indicates that the higher volatility DBS has little effect on the vaporization of the lower volatility Canola fuel. This result is in agreement with literature, where it is presented that the less volatile compounds in fuel blends are the major determining factor in liquid fuel length [12]. A correlation with injection pulse width can also be seen in these plots, as longer liquid penetration lengths are observed for longer injections. That is, the 0.38 ms pulse width of the LTC1 condition yields maximum liquid fuel lengths between
18 and 20 mm, with 14.5-17 mm maximum lengths from the 0.3 ms pulse width of the CC2 condition. It is interesting to note that approximately a 2 mm maximum liquid fuel length difference is observed between the LTC1 and CC3 conditions, despite only a 0.05 ms difference in injection pulse width. This difference in maximum liquid fuel penetration lengths between these conditions can be attributed to the increase of in-cylinder temperature from the pilot injection combustion of the CC3 condition as well as the higher in-cylinder gas density for the
CC3 condition.
147 Figure 104: Liquid fuel penetration curves for the three operating conditions.
7.4 Projected Area of Combustion Gases with IR Imaging
The fractional projected areas of combustion gases were calculated from the IR images to evaluate in-cylinder combustion development. The curves of the projected area of IR-indicated combustion gases for the operating conditions can be found in Figures 105-107. Through all operating conditions studied, similar behavior is observed between the optimized conditions and the Diesel baseline. Note the lower area of combustion gases later in the cycle for the non- optimized biofuel cases, related to the decreased energy release of these conditions. However,
148 when the pulse widths and injection timings are adjusted to account for fuel differences, the combustion behavior is similar between the fuels. Cetane effects can also be evaluated through these plots, as the non-optimized Can/DBS blend shows the longest ignition delay of the fuels and the non-optimized Canola case shows the earliest ignition. For those cases which included a pilot injection (CC2 and CC3), differences can be seen in the combustion behavior of this injection. The smaller, initial increase in combustion energy from around 350 CAD lasting to about 365 CAD is the combustion of the pilot injection, as indicated with arrows in Figures 74 and 75. A relationship between the cetane of the fuel and the pilot combustion is seen, as the highest cetane fuel, Canola, shows the larger IR-indicated areas of combustion of the pilot injection and the lowest cetane Can/DBS shows the least. That is to say that the energy content of the fuel was observed to have a smaller role than the autoignition properties of the fuels in the combustion gas area of the pilot injection. The limitations of the injector driver are also evident in Figures 106 and 107, as the onset of the pilot injection of the Canola was unable to be retarded enough to match the pilot combustion rise of the baseline Diesel case. Another note to make about the curves is the decrease in combustion gas area for all fuels and conditions after the end of combustion, around 375 CAD, which can be attributed to cooling and mixing of in-cylinder gases as the piston moves downward.
149 Figure 105: Projected areas of combustion gases for the LTC1 combustion condition.
150 Figure 106: Projected areas of combustion gases for the CC2 combustion condition.
151 Figure 107: Projected areas of combustion gases for the CC3 combustion condition.
In summary of the results presented from the analysis of the IR-indicated projected areas of combustion volumes, it was observed that when the biofuels are not optimized for combustion timing and load, unwanted combustion behavior occurs. Shorter ignition delays were seen for the higher-cetane Canola fuel. When blending DBS with this fuel, the reduction in energy content and cetane led to significantly lower areas of combustion gases, with the increase in combustion gas area occurring later in the cycle for the non-optimized case. For those cases which had pilot injections, the combustion of the pilot injections were observed to be different between the fuels, even when optimized, owing to injector driver limitations. When adjusted for load and CA10, the
152 timing of the increase of the combustion gas area produced by the main injection and the final combustion area for the biofuels were similar to the baseline Diesel fuel, illustrating the need for proper in-cylinder control when using fuels which may have different properties than Diesel fuel.
7.5 Average Pixel Intensity of Combustion Gases with IR Imaging
As an additional investigation into the characteristics inside the combustion volumes of the different fuels, the average pixel intensities of the IR images in the areas of combustion are examined to determine whether the behavior within the combusting regions varies greatly between the fuels. The pixel intensity is a scaled radiation amount captured by the IR camera, and the average values were calculated only for the areas of combustion gases. Intensity values can be affected by both emissivity (soot/combustion concentrations and optical path length) and temperature; increases in both parameters yield higher pixel intensity values [55]. If differences in pixel intensities are observed, differences inside the volumes of combusting gases might also exist. Although decoding to what extent the temperature or emissivity affects the pixel intensities is beyond the scope of this study, general behavior can be examined to determine whether significant differences in the combustion gases occur between the fuels.
The LTC1 condition shows similar average pixel intensity behavior between the fuels under the optimized conditions, as seen in Figure 108. The average intensity increases at nearly the same rate, with similar values for the optimized biofuel and Diesel fuel conditions at 365
CAD. The non-optimized Can/DBS case shows a lower maximum average intensity, from the lower and later energy release of this fuel condition. The non-optimized Canola condition shows an earlier rise in average pixel intensity, caused by the higher cetane properties of this fuel and corresponding with the earlier combustion that was shown in the area analysis. Overall, the optimized conditions show very similar average intensity behavior to Diesel fuel under this
153 operating condition, indicating that similar combustion can be achieved with the biofuels, when adjusted for timing and load.
Figure 108: Average intensity values of the infrared images in the areas where combustion occurs for the LTC1 combustion condition.
Larger differences in the average pixel intensities in areas of combustion are observed for the CC2 condition between the fuels than were observed for the LTC1 condition. Figure 109 shows the CC2 average pixel intensity curves. The Canola shows earlier and slightly higher increase in energy emission of the pilot injection than the pump Diesel fuel. The Can/DBS blend
154 shows lower pixel intensities of the pilot injection combustion that occur later than the other fuels, even for the optimized conditions. The combustion of the pilot injection for both cases of the Can/DBS is also more inconsistent, seen by examining consecutive points in this pilot injection region. The variation in consecutive points of the pilot injection in the figure shows the lack of combustion consistency between cycles of the pilot injection for this fuel. Similar to the
LTC1 condition, these results mimic the projected combustion area results, where lower areas of combusting gases occurred for the combustion of the pilot injection of the Can/DBS than the other fuels. When the main injections react, the average intensity values for the optimized biofuel cases increase to values like those of the Diesel fuel, again showing the need for optimization of the injections to match combustion behavior between the fuels.
155 Figure 109: Average intensity values of the infrared images in combustion areas for the CC2 combustion condition.
The curves in Figure 110 show that the CC3 operating condition displayed similar average pixel intensity behavior in the onset of main combustion between the Diesel and optimized biofuel conditions. The differences between the fuels for this condition are the earlier pilot ignition for Canola and higher final average intensities caused by Diesel. Cetane effects are noticeable in the initial combustion of the pilot injection, as the Canola ignites earlier than the other fuels. The non-optimized Can/DBS condition again shows poor pilot combustion as well as lower final intensities. A higher final value of average intensity occurs for the Diesel case. It is unclear exactly why higher intensities occur for the Diesel fuelling condition, but this result
156 could be related to the increased in-cylinder soot formation shown later for this fuel and operating condition (emissivity) as well as a slightly higher load (temperature) for the Diesel fuel compared to the biofuels in this particular experiment.
Figure 110: Average intensity values of the infrared images in the areas where combustion occurs for the CC3 combustion condition.
In summary of the average pixel intensity of combustion, the major trend observed was the need to optimize the biofuels in order to match the combustion behavior of the baseline
Diesel, reinforcing the observations from in the previous section. Inconsistent combustion was observed for the non-optimized condition of the Can/DBS mixture and an earlier energy release
157 was also seen for the non-optimized Canola case. However, when optimized, the biofuels displayed similar combustion behavior to the Diesel fuel conditions, illustrating that similar in- cylinder behavior can be achieved in areas of heat release.
7.6 Projected Area of Soot with Visible Imaging
Through the analysis of pressure data and combustion gas areas indicated in the IR images, the need to modify the pulse widths and injection timings of the biofuels to match the combustion behavior to the baseline Diesel fuel was shown. An examination of the in-cylinder soot production under these optimized operating conditions is warranted to evaluate the fuel effects on the in-cylinder soot production processes. To this end, visible images were analyzed using a threshold analysis. In the figures presented in this section, only the optimized operating conditions are shown such that the soot production from similar combustion conditions can be studied.
The visible fractional area curves of the projected in-cylinder soot volumes are presented in Figures 111-113. Using natural luminosity of visible light as an indicator of soot, the main result found from this analysis is that very little soot is formed with the Can/DBS blend compared to the other fuels across these conditions. For Diesel fuel, a higher maximum area of soot occurs earlier in the cycle compared to that of the other fuels, for all the conditions studied.
A slight reduction in maximum soot area is observed for Canola compared to Diesel. Note in the
LTC1 condition that the volumes of soot exist longer into the cycle for Canola than the other fuels, which could suggest slower and/or less oxidation of the soot for this condition. A longer delay from the onset of combustion until soot production is also observed for the biofuels.
Longer delays of soot formation from biofuels were also seen in previous studies [29] and can be explained, intuitively, in that the oxygen included in the fuel allows for non-sooting, lower F/O
158 areas in the combusting plume for a longer time. When the CC2 operating condition is compared with the LTC1 condition, it can be seen that lower max areas of soot occur, which is likely the result of a lower load for the CC2 condition. Additionally, an increased area of soot production is observed under the CC3 condition, likely related to the higher load of this condition. Though the reduction of in-cylinder soot is demonstrated through this analysis, if the reduction in area relates to reduced engine-out emissions and whether the cause of this reduction is solely from additional oxygen in the fuel is unclear and is further evaluated in the following sections.
Figure 111: Projected areas of soot for the LTC1 condition.
159 Figure 112: Projected areas of soot for the CC2 condition.
160 Figure 113: Projected areas of soot for the CC3 condition.
7.7 Projected Area of OH with Intensified Imaging
To evaluate possible differences in soot oxidation abilities of the fuels, the OH fractional areas have been plotted. OH is thought to be an indicator of areas where soot oxidation could be possible; an increased area of OH detected by the camera could yield an increase in the capacity to oxidize soot [24-25, 28]. The curves of the projected areas of OH are presented in Figures
114-116. For the LTC1 and CC3 operating conditions, very little difference in OH area is seen between the fuels, in terms of fractional area and the timing of production. The behavior of the areas of OH is most different under the CC2 condition, where the biofuels show greater areas of
161 OH production for this operating condition. The analysis of the plots shown in this section indicates that a greater oxidation of the increased soot production observed in the Diesel fuelled cases would not be expected.
Figure 114: Projected areas of OH for the LTC1 condition.
162 Figure 115: Projected areas of OH for the CC2 condition.
163 Figure 116: Projected areas of OH for the CC3 condition.
7.8 Average Pixel Intensity of OH
An additional investigation was conducted into in-cylinder OH production. The average pixel intensity in the areas where OH was present was calculated to show information about the relative amount of in-cylinder OH in those areas. A major assumption behind this analysis is that the intensity of the OH is dominated by the amount along the line-of-sight (concentration and optical path length) rather than temperature or other influences. Increased amounts of OH along the line-of-sight of the camera should yield higher pixel intensities. Figures 117-119 contain the
164 average pixel intensity curves. The results indicate that, under the CC2 and CC3 conditions, more OH is present for the biofuels, while similar amounts were seen for all three fuels under the
LTC1 condition. Interestingly, when different, higher intensities are seen for the Canola compared to the Can/DBS blend, even though the Can/DBS blend includes more oxygen in the fuel. This result indicates that OH production is not directly coupled to the amount of oxygen in the fuel. These results are meant as a relative comparison between the fuels and, when coupled with the OH area results from the previous section, again indicate that a greater in-cylinder soot oxidation capacity should not be expected when fueling with Diesel fuel.
Figure 117: Average intensity values of the OH images for the LTC1 condition.
165 Figure 118: Average intensity values of the OH images for the CC2 condition.
166 Figure 119: Average intensity values of the OH images for the CC3 condition.
7.9 Combustion, Soot, and OH Lengths from Injector
The previous sections have shown a decreased area of soot production for the biofuels.
However, it is unclear whether the significant decrease in soot production from the Canola to the
Can/DBS is solely due to the additional oxygen in the Can/DBS. To evaluate the soot production processes further, an analysis of the radial distance from the nozzle that combustion, soot, and
OH occurred is also presented.
Through the distance curves found in Figures 120-122 from the LTC1 operating condition, the IR-indicated combustion, soot, and OH production can be well described. Ignition
167 is observed in the IR images to initiate toward the outside of the combustion bowl, as seen in
Figure 120. Combustion moves inward along the plumes, until the exothermic gases reach a nearly level value around 13 mm away from the nozzle, in the IR images. The OH LOLs in
Figure 122 follow similar paths as the IR combustion distances while the reaction is ongoing. For this condition, the Canola shows a minimum LOL that is 3 mm closer than the other fuels. The minimum LOLs for Diesel and the Can/DBS are within 1 mm under this operating condition. As the reaction concludes, the minimum OH distance moves outward toward the bowl wall from the momentum of the jet and the reduction in area. However, the IR-indicated hot gases of combustion still remain at the closer distances. Soot is formed toward the outside of the bowl slightly after combustion begins. Similar to the combustion gases, the soot stops at a nearly constant level, only further away than the hot gases produced by combustion, between 17 and 22 mm away depending on the fuel. As the soot disappears from oxidation and decreasing soot temperature, the areas of the natural luminosity of soot decrease, causing the distances to increase. Interestingly, the furthest minimum soot distance for this condition is seen from the
Diesel case, even though this fuel provided the largest areas of soot production. Towards the end of combustion, the soot and OH curves are somewhat noisy, even though these values are averages from 20 cycles. This variation is caused by the lowered areas of soot leading to increased variation as well as in-cylinder swirl drawing the plumes through different divisions of the cylinder, and was seen for each operating condition. It is possible that additional images would reduce this variation.
168 Figure 120: Radial combustion distances nearest to the injector for the LTC1 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip).
169 Figure 121: Radial soot distances nearest to the injector for the LTC1 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip).
170 Figure 122: Radial OH distances nearest to the injector for the LTC1 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip).
Figures 123-125 show the IR, visible, and OH imaging distances from the nozzle for the
CC2 condition. The combustion of the pilot injection, in Figure 123, of this condition shows IR- indicated combustion distances starting at different lengths, but reaching comparable values when the combustion of the main injection begins. Differences in the IR-indicated combustion distances of the main injection are largest under this condition. The IR images showed combustion that came 6 mm closer to the injector for the Canola than Diesel. LOLs in the OH images were also observed to be shorter for Canola than the other fuels, as seen in Figure 125.
The visible images showed that the Canola and Diesel fuels formed soot at nearly the same distance from the injector, with significantly longer soot lengths observed for the Can/DBS blend. These longer lengths occur, despite having an OH-indicated LOL that was between the
171 Canola and Diesel values under these conditions, a result of the increase in oxygenates of the fuel.
Figure 123: Radial combustion distances nearest to the injector for the CC2 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip).
172 Figure 124: Radial soot distances nearest to the injector for the CC2 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip).
173 Figure 125: Radial OH distances nearest to the injector for the CC2 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip).
The distance curves calculated from the images taken from the highest load, CC3, operating condition are presented in Figures 126-128. A closer initial IR-indicated combustion of the pilot injection of the Canola was observed in Figure 126, although, by the time the fuel from the main injection ignites, the other fuels show combustion distance values like those of the
Canola. OH occurred closer to the injector during the combustion of the pilot injection for the
Canola than for the other fuels, as well. As the main injection reacts, the combustion distances of the fuels move inward at similar rates. Again, Canola shows less distance for fuel mixing with air, as a 3.5 mm closer minimum OH-indicated LOL occurs for the Canola, in Figure 128. The
Diesel and Can/DBS fuels show OH-indicated LOLs that are again within 1 mm of each other at their minimums. The soot trends in Figure 127 for this operating condition are similar to those
174 observed in Figure 124 from the CC2 condition, with Diesel and Canola forming at similar lengths, but at a further distance from the nozzle for the Can/DBS.
Figure 126: Radial combustion distances nearest to the injector for the CC3 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip).
175 Figure 127: Radial soot distances nearest to the injector for the CC3 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip).
176 Figure 128: Radial OH distances nearest to the injector for the CC3 condition (30mm=outer edge of combustion bowl, 0=injector nozzle tip).
Analyses of the curves produced from the calculated distances from the nozzle to the IR- indicated combustion, visible-indicated soot, and OH lift-off lengths lend additional insight into the soot production of the different fuels. The combustion curves taken from the IR images show a very similar structure of the combusting plumes between the fuels under the same operating condition, in terms of the distance from the injector through the cycle. The OH distance results point to a closer LOL for the Canola compared to that of the other fuels. Similar minimum LOL values are observed for the Diesel and Can/DBS fuels for all three operating conditions, with a slightly closer value for Can/DBS under the CC2 condition. An important result is that the longer
LOLs that occur when adding DBS indicates that blending this fuel with the Canola has the effect of reducing the autoignition properties, allowing more air to be entrained into the fuel
177 plumes prior to the LOL, while also increasing the amount of oxygenates in the fuel. This result is echoed in the soot distances, where soot is formed not only in decreased projected areas, but also further away from the nozzle for the Can/DBS blend than the pure Canola biodiesel.
7.10 Engine-Out Soot Measurements
In order to evaluate whether the decreases in soot area observed when fuelling with the biofuels produce reductions in engine-out soot emissions, the smoke meter was used to record relative FSN values for each of the operating conditions and fuels. The average relative FSN values for the three fuels under the optimized conditions are presented in Figure 129. From the values presented in this figure, it is clear that the reduction in projected soot area observed for the
Can/DBS blended fuel did, in fact, produce a lower FSN value across the range of studied conditions. The pure Canola fuel showed a decrease in soot compared to the Diesel values for the
CC2 and CC3 conditions, but had higher soot emissions under the LTC1 condition. Recall in the soot area curves for the LTC1 condition, that when fuelling with Canola, although the maximum area of soot was reduced, soot volumes were observed later in the cycle than the Diesel case. The implication of this result, coupled with the engine-out measurements, is that the soot produced by the Canola fuelled case may not have been oxidized to the extent that the soot produced by the
Diesel fuel was under this condition. Another argument for increased soot oxidation for Diesel under this condition is that this condition was the only condition under which slightly higher OH values occurred for Diesel combustion. It is also interesting to note that the LTC1 condition had the longest liquid penetration lengths of the studied conditions, with the biofuels showing longer lengths than the Diesel. This increased liquid penetration for this case could have also played a larger role in fuel impingement.
178 Relative FSN - Biofuels
0.25
0.2
0.15
Diesel Canola Can/DBS
Relative FSNRelative 0.1
0.05
0 LTC1 CC2 CC3
Operating Condition
Figure 129: Relative filter smoke number values for the different fuels and operating conditions.
In order to further compare the relative filter soot measurements, the relative FSN were plotted against maximum projected area of soot and the integrated soot amount, similar to the results from the nozzle geometry study. The maximum values versus relative FSN are presented in Figure 130 and the integrated values versus relative FSN are presented in Figure 131. Unlike
179 the nozzle results, a better correlation between soot area values and smoke number is observed for the integrated area, rather than the max area. However, the maximum area correlation still shows reasonable agreement with engine-out soot amounts. The difference between these correlations with those in the nozzle study results may be attributed to the substantially different combustion schemes that were used in this study (LTC and CC) compared to the nozzle study
(all LTC conditions).
Relative FSN vs Maximum Projected Soot Area
0.6
0.5 2 R = 0.9154
0.4 Diesel Canola Can/DBS 0.3 2 R = 0.7862 Linear (Canola) Linear (Diesel) 0.2 Linear (Can/DBS)
2 0.1 R = 1
Maximum Projected Area of Soot
0 0 0.05 0.1 0.15 0.2 0.25 Relative FSN
Figure 130: Maximum projected area of soot versus relative FSN for the fuels and conditions.
180 R elative FS N vs Integrated P rojected S oot Area
8
7 2 R = 0 . 9 4 6 6 6
D i e s e l 5 C a n o l a C a n / D B S 4 Linear (Canola) 2 R = 0 . 9 4 8 Linear (Diesel) 3 Linear (Can/DBS)
2
1 2
Integrated Projected AreaR of = Soot 1
0 0 0 . 0 5 0 . 1 0 . 1 5 0 . 2 0 . 2 5 Relative FSN
Figure 131: Integrated projected area of soot versus relative FSN for the fuels and conditions.
181 CHAPTER 8
SUMMARY AND CONCLUSIONS
8.1 PROBABILITY MAP STUDY SUMMARY AND CONCLUSIONS
Experiments were completed with an optical Diesel engine in order to study the variability of in-cylinder events between different LTC load conditions. Mass fraction burned curves were calculated from pressure data. Probability maps of projected areas of fuel spray, exothermic areas produced by combustion, and soot occurrence were studied. From the results of these experiments, the following conclusions were made.
· The MFB curves illustrated differences in combustion behavior between the lower load
and higher load conditions. The combustion events of the lower load conditions were
observed to occur faster than the higher load conditions. Smaller variation of MFB was
observed for the two higher load conditions.
· The fuel spray probability maps showed that, as the load increased, less reflection off of
the combustion bowl would be expected at the beginning of combustion. This result is
attributed to the increased ignition delay of the lower load conditions allowing a longer
time for fuel mixing. Similar variations in the locations of fuel spray were observed,
which should cause similar resulting combustion variations caused by fuel spray events,
although variations in fuel concentration inside these volumes might lead to combustion
differences.
· It was concluded from the exothermic combustion probability maps that as the load
increases (and, hence, more fuel injected), the structure of the exothermic areas from
combustion becomes more dependent on fuel delivery and less on the in-cylinder bulk
182 mixing processes. While variations in fuel spray certainly play a role in the variations of
combustion for all conditions, the distribution of the lower load conditions indicative of
turbulent in-cylinder bulk flows, coupled with the increased variation seen in these
conditions, indicates that the bulk-mixing processes play a larger part in combustion
variations.
· An increase in the probability of observed projected soot area was observed as load
increased. Soot formed further down the fuel plume towards the injector as load and
injection amount is increased.
· A qualitative agreement was seen between the soot probability maps and engine-out soot
emissions; a higher probability of soot formation correlated to an increase in engine-out
soot emissions.
183 8.2 NOZZLE STUDY SUMMARY AND CONCLUSIONS
An optical Diesel engine was used to visualize combustion and fuel spray of different nozzles using an infrared and a visible camera through a window in a Bowditch piston and a periscope window replacing an exhaust valve. Several LTC load conditions were tested with different nozzle geometries, and the images produced from these experiments were post- processed to evaluate combustion and fuel spray parameters. Different metrics were calculated from these images and the following conclusions were observed:
· Within an operating condition, slight differences in fuel vapor behavior were observed
between the nozzles, in both penetration and spreading angle of the fuel. The 6x.100x160
nozzle showed the fastest penetration, while the 7x.120x160 nozzle showed the slowest
penetration. The spreading angles of the nozzles with 0.100 holes were larger than the
nozzles with 0.120 mm holes. Liquid fuel penetration lengths of the fuel spray showed
similar fuel spray behavior between the nozzles within a condition. Increasing the intake
pressure and temperature yielded shorter quasi-steady liquid fuel penetration lengths; fuel
vapor progression was similarly affected, albeit to a lesser extent. Between conditions,
changes in the envelope of fuel were mostly by the width of the fuel plume and not by
fuel penetration into the cylinder.
· Pressure data from the nozzle study combustion experiments showed later and lower
pressures for the 6-hole nozzles compared to the 7-hole nozzles. This result is attributed
to the decreased flow rate of fuel caused by the lessened flow area of the 6-hole nozzles
and restricting the rate at which fuel can be injected and combust.
· The projected fractional areas of exothermic gas volumes showed reduced areas for the
7x.120x160 nozzle for the lowest load condition, which was attributed to the increased
184 flow rate and shorter injection pulse for this nozzle, through both views studied. The
other nozzles had similar observed areas of exothermic combustion gases within an
operating condition. Larger projected areas of combustion were observed as load
increased, related to the greater energy release.
· Fractional areas of the piston view in which soot was observed showed larger projected
areas of in-cylinder soot for the 7-hole nozzles, with the lowest areas for the 6x.100x160
nozzle. In general, as load increased, larger areas of soot were observed. The valve
window showed a lower area of soot for the 6x.100x160 nozzle.
· Radial soot lengths away from the nozzle showed the shortest soot distances for the
6x.100x160 nozzle, despite having the lowest projected areas of soot volumes. An
inverse correlation was found between fuel injection pulse widths and minimum soot
distance: as pulse width increases, distances from the injector decreased.
· Little difference in engine-out particulate emissions occurred between nozzles. The 6-
hole nozzles produced more engine-out soot for the higher load conditions; despite
having the lowest observed soot area, the 6x.100x160 nozzle produced the most engine-
out particulates for the EPA3 and EPA4 conditions. It was found that the maximum
projected area of soot was a better predictor of engine-out relative FSN compared to the
integrated soot area for these nozzles and conditions. A nearly linear relationship exists
between the maximum area and relative FSN for the 7-hole nozzles, with the 6-hole
nozzles showing an agreement with an R2 value greater than 0.8.
185 8.3 BIODIESEL SUMMARY AND CONCLUSIONS
Analysis of fuel spray and combustion processes in an optical DI Diesel engine with different load and fuel conditions were presented. Three engine operating conditions were examined with respect to pump Diesel fuel, a Canola derived FAMES biodiesel, and a blend of this biodiesel with Di-butyl Succinate, an oxygenated, lower-cetane biofuel. Liquid fuel penetration lengths were calculated using a Mie-scattered imaging technique. Using IR radiation as a measure of combustion gases, natural luminosity as a measure of soot, and intensified chemiluminescence at
310 nm as a measure of OH production, combustion, soot, and OH fractional projected areas and distances from the injector were analyzed. Based on the results of these experiments, the following conclusions were made:
· Longer liquid fuel penetration lengths were observed for the biofuels compared to the
baseline Diesel fuel. Little difference in maximum liquid fuel penetration was observed
between the pure Canola biodiesel and the Can/DBS blend within operating conditions,
indicating little effect of DBS on the liquid fuel penetration lengths of the Canola. The
length of the liquid fuel in the fuel plume was also seen to correlate to the pulse width of
the fuel injection.
· Dissimilar IR-indicated combustion behavior was seen when a pilot injection occurs. The
pilot injections of pure Canola showed the least variation in combustion gas area between
consecutive cycles, the Can/DBS showed the most variation. Similar main injection
combustion behavior was seen when the fuels were optimized for load and timing. Use of
the biofuels indicated the requirement of an optimization of the combustion strategy, as
early or late combustion with less energy release was seen for the non-optimized biofuel
conditions.
186 · A longer time between the onset of IR-indicated combustion and soot production was
observed for the biofuels compared to Diesel fuel. This result can be attributed to the
additional oxygen in the biofuels.
· Similar projected areas of OH production were seen for the LTC1 and CC3 conditions for
all fuels. The CC2 condition showed increased projected areas of OH for both biofuels.
The average OH pixel intensity calculations of OH indicated an increased intensity for
the biofuels under the CC2 and CC3 conditions. The LTC1 average OH intensity values
were similar between the three fuels, although Diesel showed slightly higher values.
From these trends, it is concluded that it would be more likely for soot oxidation to occur
for the biofuels than Diesel fuel.
· The Can/DBS blend shows significantly less projected area of soot producing volume
than either of the other two fuels. The soot reducing effect of adding DBS to the Canola
is twofold. First, the DBS contains additional oxygen, reducing the local equivalence
ratio. Second, the low cetane DBS reduces the autoignition properties of the fuel,
increasing the LOL, allowing more oxygen to be entrained into the fuel jet. Canola
showed a slight decrease in soot producing area compared to the baseline Diesel fuel. The
soot reduction for the higher cetane Canola-derived biodiesel is likely due to the
oxygenated fuel, but was hampered by shorter LOLs.
· Relative engine-out FSN measurements indicated that the reduction in soot area observed
for the Can/DBS mixture produced less engine-out soot across all operating conditions.
When fuelling with Canola FAMES, reductions in engine-out soot were observed for the
conventional combustion conditions; a higher relative FSN value occurred for the low
temperature condition. The lack of reduction in engine-out soot measurements for the
187 LTC1 condition is attributed to increased soot oxidation for the Diesel under this operating condition. In opposition to the nozzle geometry study results, the relative FSN correlated better to the integrated soot area than the maximum soot area.
188 CHAPTER 9
RECOMMENDATIONS
1. The next step in this research should be to implement optical diagnostic techniques that
could yield more quantitative information about species existence inside the combustion
plumes. The simplest of these techniques would be an implementation of a two-color
thermography technique using the visible camera. Because the camera outputs images in
RGB format, it may be possible through filtering and calibration to apply such a
technique on a pixel-by-pixel basis, yielding temperature and soot concentration maps
throughout the view. Similarly, should the ability to insert a laser sheet into the
combustion chamber be achieved, laser diagnostics, such as PLIF and PLII, would be an
excellent source of quantitative information. Future work involving techniques such as
these should focus on describing the various combustion schemes with respect to how
and where certain emissions species are produced.
a. Specific to this work, quantitative information on the concentration of fuel and
mass of soot inside the fuel plumes would be extremely beneficial to future
discussions.
2. Larger changes in nozzle geometries could be analyzed to determine if the envelope of
fuel could be more largely affected than was observed for the nozzles used in this work.
3. Further investigation into what extent the autoignition and oxygenated properties of a fuel
affect soot production might be warranted. It would be useful to isolate the effects of
cetane and oxygen to determine how an optimum future bio-derived surrogate Diesel fuel
might behave during combustion.
189 4. A metal engine could be assembled with geometry that duplicates that of the optical
engine. Large-scale sweeps (injection timing and amount, EGR rates, etc) could be
accomplished more quickly with this engine than with the optical engine, with the goal of
relating the combustion and emissions results of certain operating points to the
information gained from the optical engine. More exotic combustion schemes (PCCI, etc)
might also be achieved in a metal engine, without the fear of damaging optical windows.
5. Emissions equipment could be upgraded to include the ability to measure NOx, CO2, CO,
and UHCs, amongst other species. Soot measurements could also be completed with
faster sampling equipment, such that individual cycles can be compared with the trends
observed in imaging.
6. The control system of the dynamometer could be adjusted such that variations in speed
occur to a lesser extent when combustion occurs. A skip-fired combustion method may
also be implemented to reduce the power needed to dissipate, more appropriately
maintaining engine speed.
7. In-depth validation of CFD codes could be completed with the images and analysis
presented in this work.
190 BIBLIOGRAPHY
191 BIBLIOGRAPHY
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