Comparative Emissions from Diesel and Biodiesel Fueled Buses from 2002 to 2008 Model Years 2010-01-1967
Fuel economy and regulated emissions were measured from eight forty-foot transit buses operated on petroleum diesel and a “B20” blend of 80% diesel fuel and 20% biodiesel by volume. Use of biodiesel is attractive to displace petroleum fuel and reduce an operation's carbon footprint. Usually it is assumed that biodiesel will also reduce particulate matter (PM) emissions relative to those of petroleum diesel. Model years of the vehicles evaluated were newer 2007-08 Gillig low-floor buses, 2005 Gillig Phantom buses, and a 2002 Gillig Phantom bus. Engine technology represented three different emissions standards, and included buses with OEM diesel particulate filters. Each bus was evaluated using two transient speed-time schedules, the Orange County Transit Authority (OCTA) driving schedule which represents moderate speed urban/suburban operation and the Urban Dynamometer Driving Schedule (UDDS) which represents a mix of suburban and higher speed on-highway operation. On road coast down runs confirmed that the road load was appropriate. Average fuel economy was 33% higher over the UDDS and average distance specific oxides of nitrogen emissions were 30% lower that observed over the OCTA. Distance specific particulate matter emissions were higher over the UDDS for five of the eight buses including both buses with and without aftertreatment. A comparison between tests performed with diesel fuel and those performed with B20 fuels showed no discernable difference in fuel economy, inconsistent but slightly higher oxides of nitrogen emissions. The use of B20 resulted in significant reductions in distance specific particulate matter emissions as compared to the use of conventional diesel fuel with a 26% reduction over the OCTA and a 32% reduction over the UDDS. Oxides of nitrogen emissions from the 2007-08 buses were the lowest of those examined, followed by the 2005 buses while the 2002 bus had the highest NOx emissions which was consistent with certification NOx levels for their engine families. Particulate matter emissions from the 2007-08 buses, which were equipped with diesel particulate filters (DPFs) were over 90% lower than those from the 2005 and 2002 model year buses. There were fuel economy effects associated with both bus size and bus model year.