Improved Binning of Operating Modes in EPA Simulator Program for Transit Buses Case Study in Beijing

The Motor Vehicle Emissions Simulator (MOVES) quantifies emissions as a function of the operating mode (opmode) and emissions rates. The opmode, the determinant parameter in estimating emissions, is defined by two critical parameters: speed and scaled tractive power (STP). Activity characteristics of transit buses are commonly recognized as being quite different from those of other vehicles, and this study found the values of the two parameters for transit buses to be much smaller than those for other vehicles. However, the MOVES program uses an identical opmode binning method for transit buses and other vehicles, a method that likely leads to errors in emissions estimations for transit buses. This paper developed a binning method based on massive field data collected in Beijing to improve the opmode binning for transit buses. To this end, STP fractions, vehicle kilometers traveled (VKT) fractions, and emissions contributions were first investigated. The STP values were grouped into nine bins on the basis of analysis of emissions rates and emissions contributions. Three speed bins were then determined with the hierarchical clustering method and the averaging of VKT fractions. As a result, 29 opmode bins were defined for transit buses. Finally, the proposed method was applied to real-world emissions data in Beijing. The results indicated that the proposed binning method could reduce errors in emissions estimation errors. On average, the relative errors in estimating carbon dioxide, carbon monoxide, nitrogen oxide, and hydrocarbon emissions by the proposed method were 2.0%, 5.9%, 1.6%, and 1.5% lower, respectively, than errors made by the MOVES method.