Modal Analysis of Vehicle Operation and Particulate Emissions from Connecticut Transit Buses

Abstract

Transit buses represent a significant source of particulate exhaust emissions, especially in urban areas, but few previous studies have quantified these emissions by using real-world, onboard sampling while the vehicles operate in the transportation network. In this study, real-world particle number emissions for hybrid diesel–electric (HDE) and conventional diesel (CD) buses are examined for various vehicle operating conditions and road types in the Hartford, Connecticut, region. The results presented in this paper are based on analysis of the unique second-by-second Connecticut Transit on-road transit bus emissions and operations data set collected between January and November 2004. The modal analysis results indicate that hybrid buses operate differently than do conventional diesel buses. Although the distributions of vehicle specific power (VSP) values (in kw) were similar for the two bus types, the distributions of engine operation parameters (load and speed) were different. More important, VSP alone cannot be used to distinguish between vehicle types for modeling engine operation of, and possibly emissions from hybrid and conventional vehicles. Furthermore, modal analysis of ultrafine particle emissions indicates that in some situations the HDE buses do not outperform the CDs and may even produce higher emission rates than do the CD buses tested. Thus, there are routes and conditions in which transit authorities should avoid the use of HDE buses similar to those tested here when particle emissions are of concern. The observed high particle emissions on steep uphill grades for hybrids indicate that careful route selection for hybrid buses is warranted to optimize the environmental benefits of the hybrid vehicles.