Illustrative Comparison between Rated and Real-World Fuel Economy and Emissions

摘要

New light duty vehicles sold in the U.S. market must demonstrate compliance with applicable exhaust emission standards of either the U.S. Environmental Protection Agency or state of California. Compliance is demonstrated via a series of dynamometer driving cycle measurements of selected pollutants for multiple driving cycles, including Federal Test Procedure (FTP), Highway Fuel Economy (HFET) test, US06, SC03 and Cold FTP cycles. These dynamometer test cycles are also used by US EPA to estimate the rated fuel economy for new vehicles. While the dynamometer cycles are based on observations of real-world driving, and collectively cover a wide range of engine load, they are not necessarily representative of actual on-road operation of a given vehicle. Since 2008, NC State University has conducted field measurements of 113 light duty gasoline vehicles, including passenger cars and passenger trucks (i.e. pickup trucks, sport utility vehicles, and minivans). The measurements were conducted using Portable Emission Measurement Systems (PEMS), on-board diagnostic (OBD) scantools and global position system (GPS) receivers with barometric altimeter, from which vehicle activity, energy-use and emissions were quantified. Emissions were measured for nitric oxide, carbon monoxide, hydrocarbons, and carbon dioxide. Vehicles were measured on a common set of four routes that include a range of road types and traffic conditions. For each vehicle, a modal fuel-use and emission rate model was developed using Vehicle Specific Power (VSP) as an indicator of engine power demand. The VSP modal rates can be weighted by the time spent in each VSP mode for any driving cycle to estimate cycle average rates. The purpose of this work is to conduct a vehicle-by-vehicle comparison of certification test emission rates and rated fuel economy reported by the U.S. EPA with the estimated cycle average emission rates and fuel economy for the dynamometer and real-world cycles based on real-world VSP-based modal emission rates. The procedure enables evaluation of similarities and differences in certification emission values and rated fuel economy versus those observed for an in-use vehicle of the same make and model. For an example vehicle, the comparison illustrates some similarities in trends across driving cycles for relative variability in cycle average emission rates and fuel consumption. The absolute values of the emission rates typically differ between the dynamometer-based versus real world based measurement methods.