Evaporative hydrocarbon emissions from gasoline-powered vehicles continue to be a major concern in areas where the national ambient air quality standard for ozone is violated. As a result, accurate estimates of real-world emissions from in-use motor vehicles are of vital importance in assessing the progress made in reducing emissions, as well as in determining the need for and required magnitude of additional emissions reductions.
In this study, real-world evaporative emissions testing was performed on 50 late-model vehicles (30 passenger cars and 20 light-duty trucks), ranging in age from the 1992 to 1997 model year. Six of the 50 vehicles were equipped with enhanced evaporative emission control systems. Forty-nine of the 50 vehicles were procured from an Arizona State Inspection and Maintenance (I/M) Program test lane located in Mesa, Arizona, and one vehicle was procured from an employee of the test facility. Hot soak, running loss, and real-time diurnal testing was performed using tank fuel that averaged 6.5 Dry Vapor Pressure Equivalent (DVPE). Hot soak and running loss testing was performed at about 95 °F and one- and three-day diurnal testing was done using a diurnal heating range of 72-96 °F.
The data collected indicate that the small percentage of the vehicle fleet with evaporative emission control system defects contributes disproportionately to the total evaporative emissions of the fleet. This observation, which has also been noted in previous studies examining real-world evaporative emissions from older vehicles, suggests that a few very high emitting vehicles produce the majority of the total fleet emissions. Identification and repair of vehicles with evaporative control system problems is required to fully achieve the intended reductions in real-world evaporative emissions.
Comparison of the data collected from late-model vehicles with data from older vehicles in previous related studies indicates that, after the few extremely vehicles (referred to as high emitters) are eliminated, evaporative emissions from newer vehicles are, as expected, lower than those from older vehicles.
Although only a limited number of vehicles with enhanced evaporative emission control systems were included in this test program, a comparison of data from those vehicles with the data from vehicles with the preceding generation of non-enhanced (basic) evaporative emission control systems indicates that the improvements in evaporative emission controls mirror what would be expected based on the differences in the evaporative emissions certification standards applicable to the two systems.
Comparisons of the data collected in this study with emission predictions from U.S. EPA's MOBILE5b and CARB's MVEI7G models showed mixed results. The biggest discrepancies identified were the overprediction of running loss emission rates from vehicles with basic evaporative emission control systems by both models; the overprediction of total diurnal and resting loss emissions for vehicles with basic evaporative systems, which was more pronounced with MOBILE5b than for MVEI7G; and the overprediction of day 2 and day 3 diurnal emission rates by MOBILE5b for vehicles with both types of evaporative control systems.