This study has two main objectives. First, the impact of vehicle cruise speed on its carbon dioxide (CO(2)) emission rates at speeds up to 95 mph is evaluated with field data gathered under real-world driving conditions. Second, the cycle-based CO(2) 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 CO(2) 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 CO(2) emission rates. The results indicate that the gasoline vehicles have a minimal CO(2) emission interval between 55 and 65 mph. Diesel vehicles, however, seem to reach their minimum CO(2) production at 55 mph and the trend increases on both sides of this optimal speed.
