1. NREL/CP-540-40128. Posted with permission.
Presented at the Powertrain and Fluid Systems 2006-01-3253
Conference and Exhibition, October 2006, Toronto, Canada
100,000-Mile Evaluation of Transit Buses Operated on
Biodiesel Blends (B20)
Kenneth Proc, Robb Barnitt, R. Robert Hayes, Matthew Ratcliff, and Robert L. McCormick
National Renewable Energy Laboratory—U.S. Department of Energy
Lou Ha
Regional Transportation District
Howard L. Fang
Cummins, Inc.
Copyright © 2006 SAE International
ABSTRACT INTRODUCTION
Nine identical 40-ft. transit buses were operated on B20 Biodiesel is a diesel fuel blending component made from
and diesel for a period of two years – five of the buses vegetable oil, waste cooking oil, or animal fat by reaction
operated exclusively on B20 (20% biodiesel blend) and with methanol to form methyl esters. Biodiesel blends
the other four on petroleum diesel. The buses were are used to reduce petroleum consumption as well as
model year 2000 Orion V equipped with Cummins ISM greenhouse gas and pollutant emissions, and to comply
engines, and all operated on the same bus route. Each with mandates for the use of alternative fuels.
bus accumulated about 100,000 miles over the course of Reductions of particulate matter (PM), carbon monoxide
the study. B20 buses were compared to the petroleum (CO), and total hydrocarbon emissions (THC) can be
diesel buses in terms of fuel economy, vehicle achieved with biodiesel use [1]. Based on life-cycle
maintenance cost, road calls, and emissions. There was analysis, the use of biodiesel produces real reductions in
no difference between the on-road average fuel petroleum consumption and carbon dioxide (CO2)
economy of the two groups (4.41 mpg) based on the in- emissions [2]. Energy content per gallon is slightly lower
use data, however laboratory testing revealed a nearly resulting in minor reductions in peak torque and fuel
2% reduction in fuel economy for the B20 vehicles. economy, but thermal efficiency is unchanged [3].
Engine and fuel system related maintenance costs were
nearly identical for the two groups until the final month of There are few quantitative studies of in-use operational
the study. Component replacements near the end of the performance. Bickel and Strebig [4] report a two-year
study on one B20 bus caused average maintenance field trial of B20 use in road maintenance vehicles in
costs to be higher for the B20 group ($0.07 vs. $0.05 per Minnesota. Nearly 25,000 gallons of B20 were consumed
mile). However, engine and fuel system maintenance over roughly 135,000 miles of operation. The B20
costs varied widely from bus-to-bus so the $0.02 per mile vehicles exhibited the same average fuel economy (in
average difference between the two groups is not miles per gallon) as the diesel control vehicles. Oil
statistically significant. There was no significant analysis conducted at 5,000-mile intervals indicated no
difference in miles between road calls. Analysis of B20 unusual engine wear or fuel dilution.
samples during the study period revealed early problems
with fuel blending. There also were occasional fuel filter Chase et al. [5] described operation of a heavy-duty, line-
plugging events for the B20-fueled buses that were likely haul truck on a blend of 50% biodiesel and 50% No. 2
caused by out of specification biodiesel, however the diesel (B50) for more than 200,000 miles. No operational
exact cause could not be conclusively determined. Oil problems were reported, inspection of the engine at the
analysis results indicate no additional wear metals from conclusion of the study showed no excessive wear, and
the use of B20, with similar rates of TBN and ZDDP tests indicated no injector degradation.
decay. Soot levels in the lubricant were significantly
lower for the B20 vehicles. Laboratory chassis emissions The BIOBUS project was conducted in Montreal from
tests comparing the in-use B20 and petroleum diesel on March 2002 to March 2003 [6]. Biodiesel from vegetable
the CSHVC cycle showed reductions in all measured oil, animal fat, and waste cooking oil was tested as B5
pollutants, including a reduction in nitrogen oxides. and B20 in over 150 buses. Engines included primarily
mechanically governed 2-stroke and 4-stroke engines

2. manufactured by Detroit Diesel Corporation and • Verification of the emissions benefits of biodiesel
Cummins, Inc., respectively. The study found no impact • Performance changes in buses operating on
of biodiesel on fuel economy or on the cost to maintain biodiesel
fuel pumps and fuel injectors. Problems with low
temperature fuel filter plugging were noted for a few • Fuel consumption changes with biodiesel
specific vehicles. Pollutant emission measurements • Effects of biodiesel on bus mechanical reliability and
revealed reductions in THC, CO, and PM with no effect service availability
or a small increase for NOx. • High altitude and cold weather performance and
stability of biodiesel
Fraer et al. [7] compared operation of cargo vans and
truck tractors on B20 and petroleum diesel and • Drivers and passengers acceptance of biodiesel.
performed teardown and analysis of engines and
B20 has typically cost more than No. 2 diesel, and at the
components. After four years of operation and 600,000
time of the evaluation, averaged $0.17 higher in the
miles accumulated on B20, no differences in wear were
Denver, Colorado, area. As of August 24, 2006, B20
noted. In comparing maintenance costs between the two
averaged $2.62 in the United States compared to $2.44
groups, only minor differences could be attributed to B20
for No. 2 diesel (taxes not included) [8]. RTD must weigh
use.
this additional cost against potential benefits.
In this study, nine identical 40-ft. transit buses were
operated on B20 and diesel for a period of two years – APPROACH
five of the buses operated exclusively on B20 and the
other four on petroleum diesel. The buses operated in VEHICLE SELECTION
the Regional Transportation District (RTD) fleet in
Boulder, Colorado. A quantitative comparison of mileage The vehicles chosen for this study are 40-ft Orion V
accumulation, fuel use, road calls, maintenance costs transit buses and seat 43 passengers. These buses
and events, fuel analysis, oil analysis, and pollutant were selected in part because they operate on a
emissions is reported. dedicated route. They operate in Boulder, Colorado, on
RTD’s “Skip” route, a 16.1-mile roundtrip route that
OBJECTIVE provides high-frequency bus service along a heavily
traveled corridor. The Skip route is served by the nine
buses, which have special exterior graphics showing
The objective of this study is to compare vehicles
they are dedicated for the route (Figure 1). Table 1
operating on B20 and conventional diesel in terms of
provides a summary description of the Skip buses.
engine performance, fuel economy, vehicle
maintenance, and emissions. The results will help
RTD—and other potential biodiesel users—understand
the costs and benefits of B20 use, and any changes to
maintenance and operating procedures that might be
required. The results also will help engine manufacturers
in exploring the effects of B20 on engine durability.
Additionally, RTD has specific objectives for its
participation in the project. Located at a mile above sea
level, RTD operates and maintains a fleet of over 1000
heavy-duty transit buses serving the transportation
needs of over 2.5 million people in the Denver
Metropolitan area. The high altitude plus the high desert
climate—very low humidity, hot in the summer, and cold
and snowy in the winter—create unique challenges for
RTD bus propulsion systems. Some of the common
Figure 1. RTD Skip Bus in Service
problems experienced at this area are low vehicle power,
incomplete combustion due to lack of oxygen, and
engine overheating due to reduced airflow through the VEHICLE FUELING
radiators.
During this study, five of the nine buses operated on
RTD is working to reduce exhaust emissions from its B20, and four operated on standard petroleum diesel as
buses. In evaluating biodiesel, RTD expects to obtain a control group. All buses were fueled daily at which time
information on the operation of biodiesel to help its the hubodometer reading, fuel amount, and amounts of
planning of a strategy for improving RTD fleet operating any other fluids added were logged. Diesel buses were
conditions and public image through vehicle emissions fueled at an indoor fueling island, whereas the B20
reduction, use of renewable fuel, and fleet reliability. This buses were fueled at a temporary fueling facility located
information includes: just outside the bus garage (Figure 2). The B20 fueling
station contained a 6,000-gallon, above ground storage
tank and a pedestal-mounted dispenser.

3. Table 1. RTD B20 Evaluation Transit Bus Description Summary quality and compare oil degradation. On-road fuel
Vehicle Information Evaluation Buses economy, maintenance, and road call calculations were
based upon records provided by RTD and were reviewed
(B20 and Diesel)
for accuracy by the National Renewable Energy
Number of Buses 5 B20, 4 Diesel Laboratory (NREL). Each individual event was examined
Chassis Manufacturer/Model Orion V for missing or incomplete information. For example, each
Chassis Model Year 2000 fuel record was first checked for data entered in all fields,
Engine Manufacturer/Model Cummins ISM then for an accurate hubodometer reading (in sequence
Engine Model Year 2000 for a given date), and finally for fuel economy (further
Engine Ratings examination if grossly out of range). Records that were
Max. Horsepower 280hp @ 2,100 rpm recorded or entered incorrectly, thereby casting doubt
Max. Torque 900 lb-ft @ 1,200 rpm upon their accuracy, were removed from the
Diesel Fuel System Capacity 125 gal calculations, thus improving the level of confidence in the
Transmission ZF Ecomat 5HP592 on-road data. Results are typically reported as a running
Manufacturer/Model or cumulative average, that is, the average results from
Curb Weight 28,800 lb the beginning of the study to any given point in the study.
Gross Vehicle Weight Rating 40,600 lb
(GVWR) CHASSIS DYNAMOMETER EMISSIONS TESTING
Chassis dynamometer testing allows emissions to be
accurately measured in g/mile while the vehicle is driven
over a reproducible duty cycle in the laboratory. The
dynamometer system simulates the vehicle payload as
well as aerodynamic drag and rolling resistance. Driving
cycles consist of a speed versus time schedule that is
followed by the vehicle driver. This study employed a
chassis dynamometer consisting of twin 40” rolls
connected via gears to a 380 hp DC dynamometer and
to 47” diameter flywheels. The base inertia of the
dynamometer system as configured for this testing was
approximately 32,000 lbs. Additional vehicle inertia, drag,
and rolling resistance were simulated using load applied
electrically by the dynamometer.
Figure 2. B20 Fueling Station
The emissions measurement system employs full scale
The buses were fueled using a dry-lock nozzle dilution with constant volume sampling for mass flow
manufactured by Emco-Wheaton, but to separate fuel measurement. Gaseous emissions—including CO2,
types, the B20 buses used a nonstandard connector. nitrogen oxide (NOx), THC, and CO—are measured
The fueling nozzle at the B20 station was identical in continuously. PM emissions are measured
appearance and operation to the standard diesel fuel gravimetrically for samples collected onto 47 mm Teflon
nozzle, but it used a four-pin instead of the standard filters and weighed on a microbalance in a clean room
three-pin connector. This different pin configuration environment. The emission measurement system meets
ensured that the test buses were only fueled with B20, the requirements of the current code of federal
and B20 was dispensed only into these buses. regulations for heavy-duty engine emissions certification
(40 CFR, part 86). In addition, direct mass flow fuel
PERIOD OF OPERATION consumption is measured in line with a high accuracy
(+/-0.5% of reading) fuel metering system, which
RTD has 303 Orion buses in its fleet that entered service comprises a volume flow meter and in-line density meter
beginning in late 2000. In the summer of 2001, nine of to measure fuel mass flow.
the buses were dedicated to the Skip route, and B20
fueling began in July 2004 with each of the buses at The Skip buses have a rated gross vehicle weight of
about 160,000 miles. By late July 2004, controlled fueling 40,600 lb. For chassis dynamometer testing, a vehicle
of the five B20 buses and continuous data collection inertia of 35,000 lb was employed with estimated rolling
were in place. The data collection period reported is from resistance and drag coefficients set at CRR=0.01 and
August 1, 2004 through July 31, 2006. CD=0.5. The test driving cycle selected was the City-
Suburban Heavy-Vehicle Cycle (CSHVC). This cycle was
DATA COLLECTION AND ANALYSIS chosen because certain parameters of the cycle are a
close match with the typical Skip bus route, as shown in
Operation and maintenance data were collected for 24 Table 2. The CSHVC speed-time trace is shown in
months during normal operation and analyzed to Figure 3.
evaluate performance. Periodic fuel and used oil
samples were collected and analyzed to verify fuel

4. Table 2. Comparison of Parameters for CSHVC and Skip Bus Route
Running Average Monthly Miles per Bus
CSHVC Skip Route Diesel Group B20 Group
Average Speed, mph 14.2 15.6
6,000
Maximum Speed, mph 44 40
Stops per Mile 0.75 0.78 5,000
4,000
Miles
3,000
2,000
1,000
-
Aug- Oct- Dec- Feb- Apr- Jun- Aug- Oct- Dec- Feb- Apr- Jun- Aug-
04 04 04 05 05 05 05 05 05 06 06 06 06
Figure 4. Running Average Monthly Miles
FUEL ECONOMY
Individual bus fuel economy over 24 months is shown in
Table 4. Fuel economy values are calculated by dividing
total miles driven by total gallons of fuel used. Buses are
grouped by fuel type (B20 or diesel), and fuel economy
results presented in Figure 5.
Figure 3. Speed-Time Trace for the CHSVC.
Although diesel bus 2205 exhibited a fuel economy
RESULTS consistently about 5% lower than the rest of the diesel
baseline group, there is no apparent basis for removing it
from this dataset as an outlier. As a result, there is no
MILEAGE ACCUMULATION AND FUEL USE
difference (4.41 mpg diesel vs. 4.41 mpg B20) between
the diesel and B20 study group fuel economies.
In the 24 months of data collection, about 100,000 miles
were driven by each of the study vehicles. For the B20
If diesel bus 2205 were removed as an outlier, the 24-
buses, more than 100,000 gallons were consumed in
month average fuel economy for the diesel group
total during the study. Table 3 provides mileage
becomes 4.46 mpg. In this case, the fuel economy for
accumulation details. The B20 buses had about the
the B20 buses is 1.2% lower than that of the diesel
same use as the diesel comparison buses. Accumulated
buses (p-value = 0.02). A small fuel economy reduction
mileage numbers are very similar, and both groups
is expected due to the lower energy content of B20 as
averaged over 4,000 miles per bus per month (Figure 4).
compared to diesel fuel.
Table 3. Accumulated Mileage Details
Table 4. Individual Bus Fuel Economy
Bus VIN Total Data
Bus Fuel Economy
Number Period
Number (mpg)
Mileage
Diesel Group
Diesel Group
2203 4.46
2203 1VH5H3H2XY6501249 105,499
2204 4.46
2204 1VH5H3H2XY6501250 106,788
2205 4.25
2205 1VH5H3H2XY6501251 110,133
2206 4.46
2206 1VH5H3H2XY6501252 105,981
Average 4.41
B20 Group
B20 Group
2207 1VH5H3H2XY6501256 102,614
2207 4.37
2208 1VH5H3H2XY6501258 100,484
2208 4.40
2209 1VH5H3H2XY6501259 95,358
2209 4.41
2210 1VH5H3H2XY6501260 101,815
2210 4.45
2211 1VH5H3H2XY6501261 100,962
2211 4.41
Average 4.41

5. Fuel Economy Comparison Running Total Maintenance Cost per Mile
Diesel Group B20 Group
Diesel Group B20 Group
6
5 0.80
4
0.60
MPG
3
$/mile
0.40
2
1 0.20
0 0.00
Aug- Oct- Dec- Feb- Apr- Jun- Aug- Oct- Dec- Feb- Apr- Jun- Aug- Aug- Oct- Dec- Feb- Apr- Jun- Aug- Oct- Dec- Feb- Apr- Jun- Aug-
04 04 04 05 05 05 05 05 05 06 06 06 06 04 04 04 05 05 05 05 05 05 06 06 06 06
Figure 5. Fuel Economy Comparison by Fuel Group Figure 6. Running Total Maintenance Cost Comparison by Fuel Group
MAINTENANCE RTD codes and categorizes labor events and parts
replacements according to vehicle subsystem or
This analysis examines both total maintenance costs, as maintenance activity. For example, maintenance
well as maintenance costs related to the engine and fuel performed on the engine, fuel system, or as part of a
system. Total maintenance costs include the costs of preventative maintenance program is coded differently.
parts and labor, but do not include warranty costs (the Using these codes, the maintenance and repair data
five-year extended warranty expired in 2005). The labor were analyzed in more detail to assess differences at the
rate for maintenance has been arbitrarily set at $50 per engine and fuel system level—the systems that B20 use
hour and is not intended to reflect RTD’s current labor might be expected to impact.
mechanic rate. Cost per mile is calculated as follows:
Bus maintenance costs over 24 months related to the
Cost per mile = ((labor hours * $50) + parts cost)/mileage engine and fuel system are presented in Table 6. The
running average maintenance costs for the diesel and
Bus maintenance costs over 24 months are presented in B20 groups are compared in Figure 7. The engine and
Table 5. The running average of maintenance costs for fuel system maintenance cost per mile for the B20 group
the diesel and B20 groups are compared in Figure 6. is 39% higher than for the diesel group (p-value = 0.16).
This running average or cumulative presentation of Engine and fuel system maintenance costs are very
maintenance costs shows the average of the costs up to similar for most of the test period. However, during the
a given month and smoothes occasional monthly spikes last 3 months of the study, average B20 maintenance
in maintenance. The total maintenance cost per mile for costs increased due to component replacements on Bus
the B20 group was 5.2% lower than for the diesel group 2211 in May and June of 2006 (details to follow). The
(p-value = 0.27). This difference will be explored in 5.2% lower total maintenance cost per mile exhibited by
further discussion regarding maintenance cost the B20 group is not attributable to fewer engine and fuel
breakdown by vehicle system. system repairs. In calculation of the total maintenance
costs, the higher engine and fuel system repair costs for
Table 5. Bus Maintenance Costs the B20 group were offset by higher maintenance costs
for transmission repairs in the diesel group (unrelated to
Bus Miles Labor Parts Total Cost fuel use).
Number Driven Hours Cost ($/mile)
Diesel Group Table 6. Bus Engine and Fuel System Maintenance Costs
2203 105,499 892 $11,965 $0.54
2204 106,788 835 $14,254 $0.52 Bus Miles Labor Parts Total Cost
2205 110,133 965 $14,178 $0.57 Number Driven Hours Cost ($/mile)
2206 105,981 852 $13,555 $0.53 Diesel Group
Totals 428,401 3,544 $53,951 $0.54 2203 105,499 48 $3,427 $0.06
B20 Group 2204 106,788 38 $3,227 $0.05
2207 102,614 770 $7,366 $0.45 2205 110,133 57 $3,205 $0.05
2208 100,484 888 $11,507 $0.56 2206 105,981 29 $3,234 $0.04
2209 95,358 844 $8,647 $0.53 Totals 428,401 171 $13,093 $0.05
2210 101,815 757 $11,957 $0.49 B20 Group
2211 100,962 800 $13,145 $0.53 2207 102,614 25 $2,910 $0.04
Totals 501,233 4,059 $52,622 $0.51 2208 100,484 81 $3,402 $0.07
2209 95,358 80 $3,474 $0.08
2210 101,815 27 $4,104 $0.05
2211 100,962 70 $7,118 $0.11
Totals 501,233 284 $21,008 $0.07

6. event for Bus 2211 caused the running average engine
Running Engine, Fuel System Maintenance Cost
and fuel system maintenance costs for the B20 group to
per Mile
spike during the last few months of the study. To mitigate
Diesel Group B20 Group the impact of the high variability in maintenance costs
0.10
between vehicles, future studies will need to assess a
larger fleet or a similar sized fleet for a significantly
0.08
longer period of time.
0.06
$/mile
0.04 Table 7. Replacement Fuel System Parts
0.02 Bus Part Date Quantity Total
0.00 Number Description Replaced Cost
Aug- Oct- Dec- Feb- Apr- Jun- Aug- Oct- Dec- Feb- Apr- Jun- Aug- Diesel Group
04 04 04 05 05 05 05 05 05 06 06 06 06
2203 Gasket Fuel
Pump 07/13/05 1 $4.81
Figure 7. Running Engine and Fuel System Maintenance Cost Gasket Fuel
2203
Comparison by Fuel Group Pump 07/14/05 1 $4.91
2203 Fuel Pump 07/14/05 1 $622.67
Looking specifically at fuel system parts that may be 2203 Tube Fuel
considered potentially susceptible to B20 use, Supply 01/24/05 1 $15.31
maintenance items found in the data included the Gasket Fuel
following: 2204 Pump 07/15/05 1 $4.91
2205 Fuel Injector 01/27/06 1 $548.49
• Fuel pump Gasket Fuel
• Fuel pump gasket 2206 Pump 10/21/04 1 $1.76
• Fuel injector 2206 Fuel Pump 10/21/04 1 $555.37
Gasket Fuel
• Fuel injector O-ring 2206 Pump 02/20/06 1 $4.94
• Fuel lines Total $1,763
• Fuel filter.
B20 Group
2208 Fuel Injector 07/08/05 1 $709.2
These parts are all categorized as fuel system parts, with
Fuel
the exception of the fuel filter. The fuel filter is grouped Solenoid
with a suite of preventative maintenance repair checks 2208 S/Off 07/08/05 1 $36.47
and part replacements. Preventative maintenance events 2208 Oring Injector 07/11/05 1 $1.59
are scheduled by RTD to occur every 6,000 miles of use. 2208 Oring Injector 07/11/05 1 $1.61
2208 Oring Injector 07/11/05 1 $1.78
The fuel system replacement parts installed during the Gasket Fuel
study are shown in Table 7 for both diesel and B20 2209 Pump 10/26/04 1 $1.76
buses. During the evaluation period, fuel pumps were 2209 Fuel Pump 10/26/04 1 $555.37
replaced for two diesel buses (2203 and 2206), and a Tube Fuel
fuel pump was replaced for a B20 bus (2209). A single 2209 Supply 10/13/05 1 $15.82
fuel injector was replaced for one diesel bus (2205) and 2211 Oring Injector 05/24/06 6 $10.32
one B20 bus (2208). However, B20 Bus 2211 had all six 2211 Fuel Injector 05/24/06 6 $2,479.14
injectors replaced due to a “no-start” condition in May 2211 Fuel Injector 06/14/06 6 $2,479.14
2006. In June 2006, Bus 2211 had a cylinder head Total $6,293
replacement due to a “burnt valve” and all six injectors
were again replaced as part of the rebuilt cylinder head
replacement, although it does not appear that this ROAD CALLS
second replacement of 6 valves was actually necessary.
The labor and parts costs associated with the May – A road call is defined as a call-in to dispatch reporting a
June 2006 fuel injector and cylinder head replacements mechanical problem. Depending on the nature of the
of Bus 2211 make up the difference in engine and fuel problem, dispatch may instruct operators to continue
maintenance costs between the B20 and diesel groups driving their routes. However, a road call may stem from
in this study. Further analysis of the replaced parts is an issue that requires the bus to stop driving, allowing for
ongoing to determine if B20 use is related to their failure. roadside mechanical repair or towing back to the
maintenance facility. Road calls and average miles
These results highlight certain challenges inherent in (driven) between road calls (MBRC) are an important
controlled fleet evaluations, and in particular the high reliability indicator for the transit industry. For the
variability in maintenance costs from vehicle to vehicle, purposes of this analysis, data received from RTD
independent of the fuel used. For this group of vehicles indicating the occurrence of a road call was recorded as
transmission repairs that were unrelated to fuel use such, regardless of its relative severity.
caused the total maintenance costs for the diesel group
to be higher. At the same time, a single maintenance

7. Figure 8 shows the cumulative MBRC for all road calls Fuel was removed from the vehicle fuel tanks and tested
for the B20 and diesel baseline groups. Average MBRC for several properties as shown in Table 8. None of the
values over 24 months are 3,197 and 3,632 for diesel fuel samples exhibited excessively high levels of
and B20 groups, respectively. The B20 buses are biodiesel or cold filter plugging point (CFPP, determined
apparently 14% higher, but the difference is not by ASTM D6371). Water determined by Karl Fischer
significant (p-value = 0.59) and there is no evidence in method (ASTM D6304) indicated higher levels than
the data to suggest this improvement is attributable to typical of a No. 2 diesel fuel but not excessively high.
fuel use. Differences during the first months of the study The Bug Alert™ ATP test for microbial growth does not
are related to the variability of MBRC month-to-month, indicate that microbial contamination is an issue.
with a few months required for the running average of
each group to settle. After 24 months of evaluation, there Table 8. Vehicle Fuel Testing Results (April 2005)
is no negative impact on MBRC from the use of B20.
Percent CFPP Water, Bug Alert™
Sample Biodiesel ºC ppm ATP
However, in April 2005 two buses reported road calls for
engine misfiring and stalling caused by plugged fuel 2207 18.4 -24 72 139 (med)
filters. The first incident happened with Bus 2210 on April 2208 16.9 -25 77 27 (low)
8, 2005. The plugged fuel filter was removed from the 2209 19.2 -25 88 57 (low)
vehicle and cut open for examination. A brown “grease- 2210 20.3 -25 97 1 (very low)
like” material was found in the filter pleats and was the 2211 15.0 -30 78 93 (low-med)
suspected cause of the filter plugging (Figure 9).
The dark brown gelatinous residue coating RTD fuel filter
Running Miles Between Road Calls (MBRC) No. 2210 was analyzed by gas chromatography mass
Diesel Group B20 Group spectrometry (GC-MS) (Agilent 6890 GC equipped with a
5890 MSD mass selective detector). The sample was
10,000 prepared by scraping 23 mg of residue from a filter pleat
8,000
and dissolving this in 1.0 mL of toluene. 1.0 µL of the
solution was injected into the GC-MS using a split
6,000
injection (100:1) onto a 30m x 0.25mm column, (0.50 μm
Miles
4,000 DB-5 film).
2,000
0
The resulting chromatographic data are shown as the
Aug- Oct- Dec- Feb- Apr- Jun- Aug- Oct- Dec- Feb- Apr- Jun- Aug- total ion current (TIC) signal from the MSD, as a function
04 04 04 05 05 05 05 05 05 06 06 06 06 of component elution time in Figure 10. The multiple
peaks in the 8-17 minute region are identified as diesel
Figure 8. Running Average MBRC Comparison by Fuel Group hydrocarbons. The larger peaks in the 18-23 minute
region are fatty acid methyl esters (FAME) from the
soybean derived biodiesel. The presence of these
components is due to the fact that no attempt was made
to extract them from the sampled filter residue.
FAME Sitosterol
Stigmast-4-en-3-one
Campesterol
Stigmasterol
Total Ion Current
Diesel Hydrocarbons
39 40 41 42 43 44 45
Figure 9. Bus 2210 Plugged Filter Examination Phytosterols
The second incident with Bus 2208 occurred three days
later on April 11, 2005. The plugged filter from this bus 0 10 20 30 40 50
also contained the brown material. The filters on the Retention Time, min
other three B20 buses were changed as a precautionary
measure, and inspection of these used filters also Figure 10. GC-MS results for brown material found on plugged fuel
filter from Bus 2210
revealed signs of the brown material, but not of the
quantity and consistency of the plugged filters.
Plant (or phyto) sterols were detected in the 39-45
minute region. The compounds were identified by
matching mass spectra of the peaks with library spectra.

8. Campesterol, stigmasterol, sitosterol and stigmast-4-en- Table 9. Extra Fuel Filter Maintenance Costs
3-one were the major species identified at retention Bus Extra Labor Parts Total
times 39.59, 40.06, 41.30 and 44.34 minutes,
Number Fuel Hours Cost Cost
respectively. The relative amounts are in rough
Filters
agreement with those reported in soybean oil [10]. While
this analysis is semi-quantitative, the total level of sterols B20 Group
is significantly higher than expected for B20. Thus, this 2207 3 1 $19.45 $69.45
analysis suggests that high levels of plant sterols might 2208 3 0.8 $18.84 $58.84
be responsible for the filter plugging. These sterols are 2209 4 1 $25.12 $75.12
much higher molecular weight (≥400 amu) and would 2210 2 0.5 $12.56 $37.56
have a higher freezing point than typical of FAME or 2211 4 1.5 $26.34 $101.34
diesel fuel components. However, based on this analysis Totals 16 4.8 $102.31 $342.31
alone we cannot rule out other potential causes.
FUEL TESTING
Two other filter plugging events occurred during the
remaining study period. One happened about two
Fuel delivery load samples were collected for analysis of
months later on June 14, 2005. Bus 2209 filled from the
biodiesel blend content. Fuel was delivered about twice a
B20 dispenser with a report of slow fueling by the
month with samples starting in September 2004.
dispenser operator. The fuel filter on Bus 2209 plugged
Biodiesel content was determined by infrared
shortly thereafter. It was later discovered that the B20
spectroscopy (FTIR). The fuel blender employed splash
tank was nearly empty. Bus 2209 required several fuel
blending to mix biodiesel with petroleum diesel to make
filter changes and vehicle tank drainage to correct the
the required B20 blend. Biodiesel from a heated storage
plugging problem. The remaining B20 was also drained
tank was loaded into the delivery truck and then driven to
from the storage tank, the tank cleaned, and then refilled
the fuel terminal where No. 2 diesel fuel (or both No. 2
for continued B20 service.
and No. 1 diesel in the winter months) was typically
bottom loaded to make the blend. From the terminal, the
A final plugging event happened during the last month of
delivery truck would drive to the RTD Boulder facility and
the study period. Two buses, 2207 and 2211, were road
offload all of the B20 into the outdoor storage tank
called for plugged fuel filters on July 7, 2006. The fuel
(typically 2,500 gallons per delivery). Load samples were
storage tank was again near empty in anticipation of
taken from the top of the delivery truck tank at the
completion of the study and removal of the tank.
terminal, prior to delivery.
Because the tank is drawn from the bottom, this implies
that a material less dense than biodiesel was floating on
In examining the delivery load samples for biodiesel
top and was pumped into the vehicles as the tank
content, blend levels were found to range from less than
became nearly empty. One well known quality issue with
1% to over 80% (Figure 11). After the discovery of erratic
biodiesel is the presence of soap, which in large enough
blend levels from the first group of samples tested in May
concentration will float on top of a fuel tank.
2005, the blender reported changing its blending
procedure to include recirculation of fuel within the
As mentioned previously, fuel filters are not included in
delivery truck tank prior to delivery. Later samples
the analysis of engine and fuel system labor and parts
seemed consistently B20 for a short period then again
costs because they are considered preventative
became erratic.
maintenance. Due to fuel filter plugging events fuel filters
were replaced on the B20 buses in excess of their
preventative maintenance schedule. Table 9 lists the Delivery Sample Biodiesel Content (Volume %)
number of extra fuel filters replaced, and indicates the
associated labor and parts cost. In addition, labor for 90
diagnosis and related work (draining and refilling fuel 80
tanks) amounted to $712.50 for a total cost of $1,054.81. 70
These additional fuel filter replacements were not 60
significant additions to the maintenance analysis, adding 50
an additional maintenance cost per mile to the B20 group 40
of only $0.002. However, disruptions in transit service 30
and related costs (bus substitution, affected ridership) 20
are not captured in the maintenance costs, and were 10
considered significant to RTD. 0
Aug-04 Nov-04 Feb-05 May-05 Sep-05 Dec-05 Mar-06 Jul-06
Figure 11. B100 Content of Delivery Load Samples
Inconsistent blend levels for B20 fuel are not uncommon.
A survey of 50 B20 samples taken across the United
States indicated varying blend levels and noted problems
with splash blending [9]. In the RTD study however, all

9. the fuel from each delivery truck was offloaded into the base number) decay, oxidation, fuel dilution, viscosity,
storage tank, and it appeared to have been completely soot loading, and wear metals).
blended in this process as evidenced by vehicle tank
ZDDP decay and lubricant oxidation were assessed
samples taken in April 2005 (Table 8) and March 2006 at
using FTIR. For ZDDP the strengths of the C-O-P
or near B20 (Table 10). -1
stretching band at 957 cm and of the P=S stretching
-1
band at 670 cm were measured. The decay of these
In addition to blend level, several other properties were
bands with mileage for all samples listed in Table 12 is
tested to examine fuel quality in March 2006. Acid value,
shown in Figure 12. This indicates exponential decay
peroxides, and aldehydes (or alkanals) were determined
with mileage:
using the Saftest™ method. Acid and peroxides are
consistently low in comparison to levels observed in the
C = C0exp(-αm) (1)
B20 quality survey [9]. Alkanals indicate some oxidative
degradation of the biodiesel but are not high.
ln (1 + (C-C0)/C0) = - αm (2)
Table 10. B20 Vehicle Sample Testing March 2006 Where m = mileage
Vehicle B100 Acid Peroxide Aldehyde C0 = initial intensity
Number Content Value Saftest™ Saftest™ C = intensity at mileage m
α= exponential decay constant
Volume % mgKOH/g ppm nmol/mL
2207 20.3 <0.1 -- 58.212
2208 18.4 <0.1 13.22 57.902 Table 12. Oil Samples Taken for Analysis, Miles Indicate Mileage
Since Last Oil Change
2209 17.4 <0.1 11.59 55.696
2210 18.7 <0.1 16.75 73.35 Vehicle No. Sample Date Miles
2211 19.7 <0.1 11.42 61.546 Diesel Group
2203 3/14/06 1295
Samples collected in March 2006 from both the diesel 2203 4/26/06 7770
and biodiesel vehicle fuel tanks were combined to 2203 6/8/06 5375
produce composite diesel and B20 samples. These were 2204 3/14/06 1392
subjected to more detailed analysis as shown in Table 2204 4/22/06 6850
11. These results show the reduction of fuel sulfur 2204 6/6/06 6035
content caused by blending in of biodiesel as well as the 2205 5/17/06 7463
2.4% reduction in energy content. The B20 blends exhibit 2206 6/1/06 5840
significantly higher cetane number. B20 Group
2207 3/14/06 2900
Table 11. Results of Testing of B20 and Diesel Composite samples 2207 6/6/06 7256
Obtained from Vehicles March 2006 2208 3/14/06 1834
2208 4/25/06 7314
B20 Diesel 2209 4/25/06 802
Composite Composite 2210 4/29/06 3634
Water & D2709 0.01 0.01 2211 5/24/06 8800
Sediment vol %
Cloud Point ºC D2500 -13 -14
Sulfur ppm D5453 324.1
Sulfur ppm D2622 272 0
Aromatics vol % D1319 25.6
Olefins vol % 1.3 -2
Saturates vol % 73.1
C mass% D5291 84.71 86.6
ZDDP decay
-4
H mass% 12.88 13.21
Derived Cetane D6890 51.0 47.9
Number -6
LHV BTU/lb D240 17859.7 18306.6
-8 957 cm-1
OIL TESTING 670 cm-1
-10
Oil was sampled from several of the test vehicles during 0 2000 4000 6000 8000 10000
oil drain intervals in March through June of 2006 and at Mileage
the mileage indicated in Table 12. These data provide a
snapshot of oil performance for the two fuels in terms of Figure 12. ZDDP Decay in Lubricants from All Vehicles. C-O-P
ZDDP (zinc dialkyldithiophosphate) decay, TBN (total Monitored at 957 cm-1 and P=S Monitored at 670 cm-1

10. Fitting of the data to equation 2 (Figure 13) yields TBN also decays exponentially with mileage, as shown in
exponential decay constants of 0.00007/mile for the C-O- Figure 15. The decay constant is consistent with
P stretch and 0.0001/mile for the P=S stretch, which are Cummins experience for conventional diesel fuel. TBN is
consistent with previous testing at Cummins and with a sensitive function of fuel sulfur content. As noted,
expectations for oil decay for this engine. Differences biodiesel dilutes the sulfur content of the diesel fuel.
between the fuels for ZDDP decay were small and Table 13 lists TBN values and other oil properties as a
probably not significant. The buildup of oxidation function of mileage. Examination of these values
products was monitored via the acid carbonyl IR band at suggests that TBN decay is occurring more slowly for the
-1
1700 cm . Oxidation products grow exponentially with B20 blends; however, this cannot be proven based on
mileage as shown in Figure 14. Lubricants contain over- this limited dataset.
based detergent, which neutralize acids by formation of
carboxylate. The formation of carboxylate was also Fuel dilution of the lubricant was examined by a gas
-1
monitored via the IR band at 1640 cm which also shows chromatography method for some samples. This method
exponential growth. As seen in Figure 14, both acid and yields the amount of petroleum diesel fuel in the lubricant
carboxylate growth has essentially the same slope or and results are shown in Table 13. Fuel dilution by
exponential growth constant. This indicates that the lubricant is low in all cases, and even lower for the B20
acids being formed are being adequately neutralized by blends. Additionally, lubricant dilution by methyl ester
the detergent. No difference was observed between from the biodiesel fuel blend was examined by FTIR. No
diesel and B20. ester was detected in the lubricant from the B20 buses
except for the 7314 mile sample from bus 2208, which
0.5 contained 0.6% methyl ester.
y = -7E-05x + 0.0124
R2 = 0.7142 0.8
0
Ln(ZDDP decay)
-0.5 y = -2E-05x + 0.6024
0.6 R2 = 0.5653
Ln(TBN decay)
-1
-1.5 y = -0.0001x - 0.0352 0.4
Ln(957)
R2 = 0.7035
Ln(670)
-2
0 2000 4000 6000 8000 10000
Mileage 0.2
0 2000 4000 6000 8000 10000
Mileage
Figure 13. Results of Fitting ZDDP Decay Data to Linearized
Exponential Law, Equation 2
Figure 15. Results of Fitting TBN Decay Data to Linearized
Exponential Law, Equation 2
3
Metals analysis was performed by ICP-AA (inductively
y = 0.0002x + 0.2317 coupled plasma/atomic absorption). Calcium, zinc, and
2
R2 = 0.8788 phosphorus increase in concentration as the lubricant is
consumed by evaporation, thus can be used to track oil
Ln(oxidation)
1 consumption. However for these oil samples the
concentrations of these metals do not exhibit any trend
with mileage and show no discernable difference for
0 diesel and B20. Iron, copper, and chromium are
indicators of engine wear. There is no discernable
different for oil samples from the diesel and B20 buses.
-1 y = 0.0003x - 0.5963 ln(fC=O) Finally, sodium levels are low in all cases indicating no
R2 = 0.9268
coolant leak or contamination with high soap content
Ln(carboxy)
-2 biodiesel.
0 2000 4000 6000 8000 10000
Mileage Soot in oil was determined by a thermogravimetric
analysis method, and these values are also shown in
Figure 14. Results of Fitting Acid Carbonyl and Carboxylate Growth Table 13. Soot levels are low in all cases but about 50%
Data to Linearized Exponential Law, Equation 2 lower on average in the B20 lubricant samples. This
lower soot loading for B20 is a significant potential
advantage that should be examined in more detail in

11. Table 13. Lubricant Properties at Various Mileage
Vehicle No. Miles TBN Fuel % Ca, Zn, P, Fe, Cu, Cr, Na, Soot,
by GC ppm ppm ppm ppm ppm ppm ppm wt%
Unused Oil 0 9.27 0 3261 1246 1215 2 0 0 0 0
Diesel Group
2203 1295 8.4 0.70 3265 1265 1177 5 0 0 1 --
2203 5375 4.21 -- 3213 1259 1111 24 1 1 3 3.83
2203 7770 5.86 -- 3421 1323 1173 21 1 2 4 3.09
2204 1392 7.57 0.70 3411 1312 1234 7 6 1 2 --
2204 6035 4.64 -- 3385 1299 1178 22 3 3 3 4.16
2204 6850 5.37 0.30 3731 1401 1255 23 10 3 4 3.42
2205 7463 4.7 -- 3399 1297 1168 20 1 1 4 3.9
2206 5840 4.73 -- 3293 1311 1171 25 1 2 4 4.27
B20 Group
2207 2900 7.75 0 3684 1286 1210 6 2 3 3 --
2207 7256 5.82 -- 3213 1268 1144 13 3 5 3 1.9
2208 1834 9.03 0.20 3508 1258 1208 3 0 0 1 --
2208 7314 6.95 0 3802 1413 1277 13 1 1 6 1.75
2209 802 7.47 -- 3405 1295 1237 4 0 0 1 0.32
2210 3634 5.93 -- 3367 1327 1179 20 2 2 4 3.72
2211 8800 5.93 -- 3330 1319 1181 20 1 1 4 2.53
lubricant performance tests. The lubricant samples were are reported in Table 15. The data show that for these
also tested for viscosity and viscosity index. These vehicles on this test cycle, operation on B20 reduced all
values did not decay significantly during the oil drain regulated pollutants, including NOx. Fuel economy on a
interval for lubricant from either group of vehicles. mpg basis was decreased by roughly 2%, in agreement
with the lower energy content of B20. In most cases
Overall the lubricant analysis indicates no negative p<0.05, indicating changes are significant with 95% or
impact from the use of B20 and suggests some potential better confidence.
benefits. To quantify these potential benefits will require
additional study. Table 14. Fuel Properties of Fuel Used for Emissions Testing
CHASSIS EMISSION TESTS B20 Diesel
B100 Content (Vol %) 21.1 NA
Emission testing was conducted on two vehicles: bus Distillation T90, ºF (D86) 644.4 617.4
numbers 2208 and 2211. The buses were tested using Flash Point, ºF (D93) 159 151
both the in-use No. 2 diesel and the in-use B20 fuels. Copper Corrosion (D130) 1a 1a
Properties of these fuels are listed in Table 14. At the Kinematic Viscosity, cSt@40ºC 2.726 2.438
time of this testing, the B20 was being blended with the (D445)
same diesel fuel used by the diesel buses. The B20 Ash, %Mass (D482) 0 0
shows significantly higher cetane number, in part
Carbon Residue, %mass (D524) <0.010 0.04
because of the relatively high cetane number of biodiesel
which is typically 54 [9], but this can only account for a Cetane Number (D613) 47 40
cetane number increase of about 3. The additional Cloud Point, ºF (D5773) 6 0
increase is caused by the multifunctional diesel additive Total Sulfur, ppm (D5453) 280 364
used by the biodiesel supplier, which includes a cetane Water & Sediment, %Vol 0 0
improving additive. The aromatic content of the B20 (D2709)
appears to be slightly higher than the aromatic content of Aromatics, %Vol (D1319) 28.5 27.1
the diesel fuel; however biodiesel is known to interfere in Heat of Combustion, BTU/gal 134,650 137,720
method D1319 [11] and may provide falsely high values (D240)
for B20 blends. Two drivers were used to perform the
test cycles. Testing was performed so that each driver Acid Number, mg KOH/gram 0.16 0.01
performed the same number of test runs on each bus, (D664)
per fuel. Peroxide Number, ppm (D3703) 8.1 0
Six to eight total repeated test cycle runs were driven for
each bus on each fuel, results for each test run are Clearly the oxygen content provided by the biodiesel is
reported in Appendix A; and summary results analyzed primarily responsible for the reductions in THC, CO, and
statistically for each bus and for both buses combined PM; in agreement with previous studies [1]. The situation
for NOx emissions is less clear. B20 exhibited a cetane

12. number of 47 versus 40 for the in-use diesel fuel. For systems were $0.05 and $0.07 per mile,
fuels that are otherwise identical, increasing cetane respectively. Because of high variability in
number from 40 to 47 could produce a NOx reduction of maintenance costs between vehicles, the engine and
3% [12]. However, in testing of a third mechanically fuel system maintenance costs for the two groups
identical transit bus where neither fuel contained are not significantly different with a high degree of
additives we have observed NOx reductions ranging from confidence (p~0.5). The increased engine and fuel
2 to 5% [13]. Thus it seems unlikely that additive effects system costs for the B20 group were due to fuel
are the main cause of the NOx reduction observed here. injector and cylinder head replacements on Bus
2211. Further study will be required to determine if
By testing two buses, with two drivers each, on both B20 fuel use caused these failures.
fuels, some information regarding variability between
vehicles and between drivers can also be acquired. This • Miles between road calls averaged 3,197 for the
analysis can be useful to evaluate whether the results diesel group and 3,632 for the B20 group. There is
can be interpreted to likely apply generally to the Skip no evidence in the data to suggest this difference is
bus fleet or if vehicle-to-vehicle variability prevents such related to fuel use.
a generalized conclusion.
• Fuel filter plugging on the B20 buses caused road
The data show good agreement between the two test calls, and required extra filter replacements in the
vehicles, with no statistically significant difference (at B20 group. Although the additional maintenance cost
alpha of 0.05), between the two buses for any of the was small, adding only $1,054.81 to the B20 group
regulated emissions. There was a statistically significant or $0.002 per mile, the events were significant to the
difference between the two buses for fuel economy with transit district because of resulting disruptions to
bus 2211 achieving approximately 3% better fuel normal bus service. Fuel filter plugging may have
economy than bus 2208, on average. been caused by the presence of high levels of plant
sterols in the biodiesel or other fuel quality issues.
There was no statistically significant difference between
drivers (alpha of 0.05) for NOx emissions, THC • Measurement of biodiesel blend level showed erratic
emissions, or fuel economy. There was a statistically biodiesel content for delivery load samples. Vehicle
significant difference between drivers for CO and PM samples, however, were consistently at or near B20
emissions, with these emissions from driver A averaging indicating complete blending had occurred during
about 22%-23% higher than driver B. delivery and offloading of the fuel.
Table 15. Summary Results for Bus Emission Testing • Oil analysis results indicate no additional wear
metals from the use of B20, with similar rates of TBN
Bus NOx THC CO PM FE and ZDDP decay, oxidation, fuel dilution, and
g/mile g/mile g/mile g/mile mpg viscosity. Soot levels in the lubricant were
2211 significantly lower for the B20 vehicles. Quantifying
Base 19.81 0.871 3.60 0.274 4.70 potential benefits will require additional study.
B20 18.65 0.625 2.63 0.226 4.59
%Δ -5.8 -28.2 -26.8 -17.3 -2.4 • Laboratory chassis testing on the CSHVC cycle
p-value <0.001 0.001 <0.001 0.042 0.032 using the in-use fuels found that B20 reduced
2208 emissions of all regulated pollutants and caused a
Base 19.44 0.807 3.44 0.2648 4.60 small fuel economy decrease.
B20 18.67 0.571 2.73 0.2150 4.45
%Δ -3.9 -28.0 -20.3 -19.9 -2.1 ACKNOWLEDGMENTS
p-value 0.039 0.002 0.071 0.153 0.018
The FreedomCAR and Vehicle Technologies Program,
Fuels Technologies Subprogram of the U.S. Department
CONCLUSIONS of Energy sponsored NREL’s participation in this project.
In the 100,000-mile evaluation of transit buses operated The authors thank the Technical Services group and the
on B20, the following operational differences were found Boulder maintenance staff of RTD. Special thanks are
related to vehicle fuel: extended to Dean Shaklee from Operations; Dave
Richardson and Dave Ober from Maintenance Reporting
• The fuel economy for both petroleum diesel and B20 Systems; and Hugh Willis and Ray Fernandez from
groups was 4.41 mpg based on in-use fleet data. An Boulder Maintenance. Additionally the authors wish to
approximately 2% reduction in fuel economy for B20 thank Dave Forrester of Power Service Products for B20
was measured in laboratory emission testing. testing during the fuel filter plugging incident; Dale Rains
of Gray Oil; and Sean Lafferty and Ryan Lafferty of Blue
• Total maintenance costs per mile were $0.54 for the Sun Biodiesel.
diesel group and $0.51 for the B20 group, and
maintenance costs specific to the engine and fuel

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EPA420-P-02-001, 2002.
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Milestone Report. National Renewable Energy
Laboratory, NREL/MP-540-40554, October 2006.

14. APPENDIX A
Table A1. Chassis Emissions Test Data
Bus 2211 Fuel
Fuel Run NOx THC CO PM Economy
g/mile g/mile g/mile g/mile mpg
Base 332 19.90 0.906 4.18 0.2380 4.68
Base 333 20.56 1.033 3.74 0.2185 4.70
Base 337 19.69 0.816 3.48 0.3055 nm
Base 338 19.81 0.854 3.08 0.2616 4.68
Base 339 19.33 0.789 3.78 0.3263 4.67
Base 340 19.54 0.829 3.32 0.2940 4.77
Average 19.80 0.871 3.60 0.2740 4.70
Standard deviation 0.42 0.09 0.39 0.04 0.04
Coefficient of variation 2.1% 10.2% 10.8% 15.2% 0.9%
B20 326 18.78 0.604 2.48 0.2421 nm
B20 328 18.54 0.598 2.94 0.2200 4.54
B20 329 18.83 0.581 2.69 0.2365 4.50
B20 345 18.36 0.485 2.97 0.2603 4.55
B20 346 18.83 0.729 2.35 0.1961 4.65
B20 347 18.57 0.754 2.36 0.2037 4.69
Average 18.65 0.625 2.63 0.2264 4.59
Standard deviation 0.19 0.10 0.28 0.02 0.08
Coefficient of variation 1.0% 16.0% 10.6% 10.8% 1.7%
Percent difference with base: -5.8% -28.3% -26.8% -17.4% -2.4%
Bus 2208 Fuel
Fuel Run NOx THC CO PM Economy
g/mile g/mile g/mile g/mile mpg
Base 364 19.81 0.806 2.84 0.2178 4.67
Base 365 19.86 0.818 2.77 0.2001 4.67
Base 366 19.24 0.724 3.64 0.2901 4.57
Base 367 19.08 0.938 3.60 0.2804 4.60
Base 372 18.68 0.747 4.35 0.3358 4.52
Average 19.34 0.807 3.44 0.2648 4.60
Standard deviation 0.50 0.08 0.65 0.06 0.06
Coefficient of variation 2.6% 10.3% 18.9% 21.0% 1.4%
B20 358 19.01 0.542 3.18 0.2583 4.24
B20 359 18.75 0.566 2.37 0.1933 4.33
B20 360 18.47 0.546 2.93 0.2439 4.37
B20 362 19.17 0.564 2.43 0.1581 4.44
B20 377 18.33 0.533 3.61 0.3261 4.45
B20 378 18.38 0.597 2.38 0.1868 4.59
B20 379 18.20 0.616 2.53 0.1821 4.62
B20 380 19.08 0.608 2.42 0.1714 4.55
Average 18.67 0.571 2.73 0.2150 4.45
Standard deviation 0.38 0.03 0.46 0.06 0.13
Coefficient of variation 2.0% 5.6% 16.9% 26.4% 3.0%
Percent difference with base: -4.11 -38.94 -25.54 -49.29 -2.08