In-Use Study on B20 in Transit Buses Finds Slight Decrease in Fuel Economy, Equivalent Reliability to ULSD Buses
|Average fuel economy of the B20 and ULSD buses. The data shows a continuous slight decline in fuel economy for both. Click to enlarge.|
A 12-month evaluation by the National Renewable Energy Laboratory (NREL) of the in-use performance of buses operating on B20 biodiesel (20% biodiesel, 80% petroleum diesel) found that the B20 buses exhibited 1.7% lower fuel economy than the ULSD (ultra low sulfur diesel) study group. Reliability, as measured by MBRC (miles between road calls), and maintenance costs between the two groups were comparable.
This evaluation of buses in the St. Louis (Missouri) Metro fleet was conducted under a Cooperative Research and Development Agreement (CRADA) between the National Renewable Energy Laboratory (NREL) and the National Biodiesel Board (NBB). The study is the first B20 in-use fleet study using exhaust gas recirculation (EGR) equipped buses, and is also the first study to compare the use of B20 to ultra-low sulfur diesel (ULSD).
NREL researchers examined fifteen 40-foot MY2002 transit buses manufactured by Gillig equipped with MY2002 (2004 emissions certification) Cummins ISM engines. Eight of these buses operated exclusively on B20 and the other seven operated exclusively on petroleum ULSD. The B20 and ULSD study groups operated from different depots at St. Louis Metro, but bus routes were matched for duty cycle parity.
Average fuel economy over the 12-month period for the ULSD buses was 3.58 mpg US; average fuel economy for the B20 buses was 3.52 mpg US.
Average MBRC values over the evaluation period were 2,375 and 2,627 for ULSD and B20 groups, respectively. MBRCs for the engine and fuel systems show that by the end of the 12-month evaluation, the B20 buses exhibited higher reliability, with engine and fuel system MBRC values of 6,924 and 8,211 for ULSD and B20 groups, respectively.
However, the B20 study group had a higher incidence of fuel filter and fuel injector replacements. Analysis of B100 and B20 samples did not indicate poor fuel quality. No fuel injectors were retained for tear-down analysis to determine failure mode and cause.
Based on the available data, the cause of the higher rate of fuel injector replacement for the B20 buses cannot be determined with certainty. On the one hand, exposure to B20 may have been the cause, but on the other hand, the higher mileage of the B20 buses might also have lead to a higher number of injector failures. This is not atypical for a 12-month evaluation, as a significantly longer time is generally required to fully understand fuel impacts on engine durability and maintenance. Note that the evaluation is being continued for a second year, and the additional data will hopefully clarify the situation.
Lube oil analysis indicated no harm, and some potential benefits, with B20 use; notably, soot and wear metals were lower with B20 use. Viscosity, total base number, and corrosive metals were generally more positive with ULSD use, but these qualities were still “in-grade” for the B20 buses throughout the oil drain interval.
R. Barnitt, R.L. McCormick, and M. Lammert (2008) St. Louis Metro Biodiesel (B20) Transit Bus Evaluation: 12-Month Final Report (NREL/TP-540-43486)