Field Evaluation of Carbon Dioxide Emissions at High Speeds

Abstract

This study has two main objectives. First, the impact of vehicle cruise speed on its carbon dioxide (CO₂) emission rates at speeds up to 95 mph is evaluated with field data gathered under real-world driving conditions. Second, the cycle-based CO₂ emission rates for a broad range of average cycle speeds are investigated. A portable emission measurement system unit was used to measure tailpipe emissions of three light-duty gasoline vehicles and three Class 2b diesel vehicles. The vehicles were tested on a speed track 9 mi high in Pecos, Texas, while the driver followed predeveloped drive patterns. The drive patterns were used to ensure good coverage of various steady-state and transitional operational modes of the vehicles. The results show that the CO₂ emissions of gasoline vehicles follow a sharply increasing pattern between 40 mph and 70 mph. At speeds higher than 70 mph, the vehicle's fuel economy tends to decrease at a small rate or to stay flat. The Class 2b diesel vehicles, in contrast, show a monotonically increasing trend for speeds higher than 40 mph. The second-by-second emission data were used to develop a series of instantaneous emission models capable of providing accurate emissions at each speed and acceleration rate. The models were applied to a series of representative drive cycles to provide distance-based average CO₂ emission rates. The results indicate that the gasoline vehicles have a minimal CO₂ emission interval between 55 and 65 mph. Diesel vehicles, however, seem to reach their minimum CO₂ production at 55 mph and the trend increases on both sides of this optimal speed.