TRANSIT BUS LOAD-BASED MODAL EMISSION RATE MODEL DEVELOPMENT A Thesis Presented to The Academic Faculty by Chunxia Feng In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the School of Civil and Environmental Engineering Georgia Institute of Technology May 2007 TRANSIT BUS LOAD-BASED MODAL EMISSION RATE MODEL DEVELOPMENT Approved by: Dr. Randall Guensler, Advisor Dr. Michael Hunter School of Civil and Environmental School of Civil and Environmental Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Michael Rodgers Dr. Jennifer H. Ogle School of Civil and Environmental Civil Engineering Department Engineering Georgia Institute of Technology Clemson University Dr. Michael Meyer School of Civil and Environmental Engineering Georgia Institute of Technology Date Approved: March 06, 2007 ACKNOWLEDGEMENTS I would like to express the deepest gratitude to my advisor, Dr. Randall Guensler, for his full support, expert guidance, understanding and encouragement throughout my study and research. Without his incredible patience and timely wisdom and counsel, my dissertation would have been a frustrating and overwhelming pursuit. To the other members of my thesis committee, Dr. Michael Rodgers, Dr. Michael Meyer, Dr. Michael Hunter, and Dr. Jennifer Ogle, I want to say thank you for having served in my defense committee. Their thoughtful questions and comments were valued greatly. Particular appreciation is reserved for Michael Rodgers for numerous consultations. His voracious appetite for knowledge was intimidating, challenging, infectious, and admirable. I am grateful for financial support from many funding agencies and organizations, including the USEPA, the Federal Highways Administration, and Georgia Department of Transportation. Without their support I would not have been able to pursue a doctoral degree. Lastly, my heartfelt gratitude is to my husband, Daiheng, and sons, Andrew and Daniel. Their endless love, understanding and dedication are elements that have sustained me throughout this journey. iii TABLE OF CONTENTS Page ACKNOWLEDGEMENTS iii LIST OF TABLES viii LIST OF FIGURES xiii SUMMARY xxi CHAPTER 1 INTRODUCTION 1 1.1 Emissions from Heavy-Duty Diesel Vehicles 1 1.2 Current Heavy-Duty Vehicle Emissions Modeling Practices 2 1.3 Research Approaches and Objectives 3 1.4 Summary of Research Contributions 4 1.5 Dissertation Organization 5 2 HEAVY-DUTY DIESEL VEHICLE EMISSIONS 6 2.1 How Diesel Engine Works 6 2.2 Diesel Engine Emissions 10 2.3 Heavy-Duty Diesel Vehicle Emission Regulations 13 2.4 Heavy-Duty Diesel Vehicle Emission Modeling 16 3 HEAVY-DUTY DIESEL VEHICLE EMISSIONS MODELING 17 3.1 VMT-Based Vehicle Emission Models 17 3.2 Fuel-Based Vehicle Emission Models 25 3.3 Modal Emission Rate Models 26 4 EMISSION DATASET DESCRIPTION AND POST-PROCESSING PROCEDURE 41 iv 4.1 Transit Bus Dataset 41 4.2 Heavy-duty Vehicle Dataset 56 5 METHODOLOGICAL APPROACH 65 5.1 Modeling Goal and Objectives 65 5.2 Statistical Method 66 5.3 Modeling Approach 77 5.4 Model Validation 79 6 DATASET SELECTION AND ANALYSIS OF EXPLANATORY VARIABLES 81 6.1 Dataset Used for Model Development 81 6.2 Representative Ability of the Transit Bus Dataset 83 6.3 Variability in Emissions Data 86 6.4 Potential Explanatory Variables 99 6.5 Selection of Explanatory Variables 106 7 MODAL ACTIVITY DEFINITIONS DEVELOPMENT 112 7.1 Overview of Current Modal Activity Definitions 112 7.2 Proposed Modal Activity Definitions and Validation 115 7.3 Conclusions 123 8 IDLE MODE DEVELOPMENT 124 8.1 Critical Value for Speed in Idle Mode 124 8.2 Critical Value for Acceleration in Idle Mode 127 8.3 Emission Rate Distribution by Bus in Idle Mode 131 8.4 Discussions 137 8.5 Idle Emission Rates Estimation 140 8.6 Conclusions and Further Considerations 143 9 DECELERATION MODE DEVELOPMENT 145 v 9.1 Critical Value for Deceleration Rates in Deceleration Mode 145 9.2 Analysis of Deceleration Mode Data 149 9.3 The Deceleration Motoring Mode 157 9.4 Deceleration Emission Rates Estimations 160 9.5 Conclusions and Further Considerations 165 10 ACCELERATION MODE DEVELOPMENT 167 10.1 Critical Value for Acceleration in Acceleration Mode 167 10.2 Analysis of Acceleration Mode Data 172 10.3 Model Development and Refinement 179 10.4 Conclusions and Further Considerations 236 11 CRUISE MODE DEVELOPMENT 238 11.1 Analysis of Cruise Mode Data 238 11.2 Model Development and Refinement 245 11.3 Conclusions and Further Considerations 292 12 MODEL VERIFICATION 294 12.1 Engine Power vs. Surrogate Power Variables 294 12.2 Mean Emission Rates vs. Linear Regression Model 297 12.3 Mode-specific Load Based Modal Emission Rate Model vs. Emission Rate Models as a Function of Engine Load 300 12.4 Separation of Acceleration and Cruise Modes 304 12.5 MOBILE 6.2 vs. Load-Based Modal Emission Rate Model 306 12.6 Conclusions 307 13 CONCLUSIONS 309 13.1 Transit Bus Emission Rate Models 311 13.2 Model Limitations 313 13.3 Lessons Learned 315 vi 13.4 Contributions 316 13.5 Recommendation for Further Studies 316 REFERENCES 318 VITA 327 vii LIST OF TABLES Page Table 2-1 National Ambient Air Quality Standards 14 Table 2-2 Heavy-Duty Engine Emissions Standards 16 Table 3-1 Heavy-Duty Vehicle NOx Emission Rates in MOBILE6 21 Table 3-3 Heavy-Duty Vehicle HC Emission Rates in MOBILE6 21 Table 4-1 Buses Tested for USEPA 42 Table 4-2 Transit Bus Parameters Given by the USEPA 44 Table 4-3 List of Parameters Used in Explanatory Analysis for Transit Bus 54 Table 4-4 Summary of Transit Bus Database 55 Table 4-5 Onroad Tests Conducted with Pre-Rebuild Engine 57 Table 4-6 Onroad Tests Conducted with Post-Rebuild Engine 58 Table 4-7 List of Parameters Given in Heavy-duty Vehicle Dataset Provided by USEPA 59 Table 4-8 List of Parameters Used in Explanatory Analysis for HDDV 63 Table 4-9 Summary of Heavy-Duty Vehicle Data 64 Table 5-1 ANOVA Table for Single-Factor Study 73 Table 6-1 Basic Summary Statistics for Emissions Rate Data for Transit Bus 89 Table 6-2 Basic Summary Statistics for Truncated Emissions Rate Data 92 Table 6-3 CARB Emission Regime Definition 97 Table 6-4 Percent of High Emission Points by Bus 98 Table 6-5 Correlation Matrix for Transit Bus Dataset 107 Table 7-1 Comparison of Modal Activity Definition 114 Table 7-2 Four Different Mode Definitions and Modal Variables 116 viii Table 7-3 Results for Pairwise Comparison for Modal Average Estimates In Terms of P- value 119 Table 7-4 Sensitivity Test Results for Four Mode Definition 122 Table 8-1 Engine Power Distribution for Three Critical Values for Three Pollutants 127 Table 8-2 Percentage of Engine Power Distribution for Three Critical Values for Three Pollutants 127 Table 8-3 Engine Power Distribution for Four Options for Three Pollutants 130 Table 8-4 Percentage of Engine Power Distribution for Three Critical Values for Three Pollutants 131 Table 8-5 Median, and Mean of Three Pollutants in Idle Mode by Bus 134 Table 8-6 Engine Power Distribution in Idle Mode by Bus 135 Table 8-7 Idle Mode Statistical Analysis Results for NOx, CO, and HC 141 Table 8-8 Idle Emission Rates Estimation and 95% Confidence Intervals Based on Bootstrap 143 Table 9-1 Engine Power Distribution for Three Options for Three Pollutants 146 Table 9-2Percentage of Engine Power Distribution for Three Options for Three Pollutants 146 Table 9-3 Median, and Mean for NOx, CO, and HC in Deceleration Mode by Bus 152 Table 9-4 High HC Emissions Distribution by Bus and Trip for Deceleration Mode 154 Table 9-5 Engine Power Distributions in Deceleration Mode by Bus 155 Table 9-6 Comparison of Emission Distributions between Deceleration Mode and Two Sub-Modes (Deceleration Motoring Mode and Deceleration Non-Motoring Mode) 160 Table 9-7 Emission Rates Estimation and 95% Confidence Intervals Based on Bootstrap for Deceleration Mode 164 Table 10-1 Engine Power Distribution for Three Options for Three Pollutants 170 Table 10-2 Percentage of Engine Power Distribution for Three Options for Three Pollutants 170 Table 10-3 Engine Power Distribution for Acceleration Mode and Cruise Mode 171 Table 10-4 Median, and Mean of Three Pollutants in Acceleration Mode by Bus 174 ix Table 10-5 Engine Power Distribution in Acceleration Mode by Bus 177 Table 10-6 Original Untrimmed Regression Tree Results for Truncated Transformed NOx Emission Rate in Acceleration Mode 186 Table 10-7 Trimmed Regression Tree Results for Truncated Transformed NOx Emission Rate in Acceleration Mode 187 Table 10-8 Original Untrimmed Regression Tree Results for Truncated Transformed CO Emission Rate in Acceleration Mode 190 Table 10-9 Trimmed Regression Tree Results for Truncated Transformed CO Emission Rate in Acceleration Mode 191 Table 10-10 Original Untrimmed Regression Tree Results for Truncated Transformed HC Emission Rate in Acceleration Mode 193 Table 10-11 Trimmed Regression Tree Results for Truncated Transformed HC in Acceleration Mode 195 Table 10-12 Secondary Trimmed Regression Tree Results for Truncated Transformed HC Emission Rate in Acceleration Mode 197 Table 10-13 Final Regression Tree Results for Truncated Transformed HC and Engine Power in Acceleration Mode 198 Table 10-14 Regression Result for NOx Model 1.1 200 Table 10-15 Regression Result for NOx Model 1.2 202 Table 10-16 Regression Result for NOx Model 1.3 203 Table 10-17 Regression Result for NOx Model 1.4 205 Table 10-18 Regression Result for NOx Model 1.5 207 Table 10-19 Comparative Performance Evaluation of NOx Emission Rate Models 210 Table 10-20 Regression Result for CO Model 2.1 213 Table 10-21 Regression