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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.




So B20 gets 1.7% less MPG's but reduces hydrocarbons by 10-12%, acid rain precusors by 18% and has 0 sulfer.

Oh, 1.7% is 6/100'ths of a gallon. I'll take that trade.


Good start, now consider stop/start or other hybridisation.

Did anyone read the paper? On page 7, we have this:

The B20 and ULSD study fleet fuel consumption and economy data are presented in Table 6. The calculated 12-month average fuel economy for the B20 buses is 1.7% lower than that of the ULSD buses. This difference is expected due to the approximately 2% lower energy content in a gallon of B20. The 12-month fuel economy for each bus was used to compare ULSD and B20 groups in a two-tailed, paired t-test. By conventional criteria, the difference between the two groups is not statistically significant with a high degree of confidence (P = 0.3).

Emphasis added. A quick check of table 6 on page 8 concurs.


Very nice reassuring figures for bio,
I wonder what bio rating Cummins place on the ISM engine?


Recently asked the Volks dealer about the 2009 TDI being introduced in Canada. He said they were contemplating a B5 certification. We suggested that B20 would both help the consumer demand more bio-d pumps, and further green the TDI image. Cool car.

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