NREL/CP-540-40128. Posted with permission. 

Presented at the Powertrain and Fluid Systems 

Conference and Exhibition, October 2006, Toronto, Canada 2006-01-3253 

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 

Nine identical 40-ft. transit buses were operated on B20 

and diesel for a period of two years – five of the buses 

operated exclusively on B20 (20% biodiesel blend) and 

the other four on petroleum diesel. The buses were 

model year 2000 Orion V equipped with Cummins ISM 

engines, and all operated on the same bus route. Each 

bus accumulated about 100,000 miles over the course of 

the study. B20 buses were compared to the petroleum 

diesel buses in terms of fuel economy, vehicle 

maintenance cost, road calls, and emissions. There was 

no difference between the on-road average fuel 

economy of the two groups (4.41 mpg) based on the inuse data, however laboratory testing revealed a nearly 

2% reduction in fuel economy for the B20 vehicles. 

Engine and fuel system related maintenance costs were 

nearly identical for the two groups until the final month of 

the study. Component replacements near the end of the 

study on one B20 bus caused average maintenance 

costs to be higher for the B20 group ($0.07 vs. $0.05 per 

mile). However, engine and fuel system maintenance 

costs varied widely from bus-to-bus so the $0.02 per mile 

average difference between the two groups is not 

statistically significant. There was no significant 

difference in miles between road calls. Analysis of B20 

samples during the study period revealed early problems 

with fuel blending. There also were occasional fuel filter 

plugging events for the B20-fueled buses that were likely 

caused by out of specification biodiesel, however the 

exact cause could not be conclusively determined. Oil 

analysis results indicate no additional wear metals from 

the use of B20, with similar rates of TBN and ZDDP 

decay. Soot levels in the lubricant were significantly 

lower for the B20 vehicles. Laboratory chassis emissions 

tests comparing the in-use B20 and petroleum diesel on 

the CSHVC cycle showed reductions in all measured 

pollutants, including a reduction in nitrogen oxides. 

INTRODUCTION 

Biodiesel is a diesel fuel blending component made from 

vegetable oil, waste cooking oil, or animal fat by reaction 

with methanol to form methyl esters. Biodiesel blends 

are used to reduce petroleum consumption as well as 

greenhouse gas and pollutant emissions, and to comply 

with mandates for the use of alternative fuels. 

Reductions of particulate matter (PM), carbon monoxide 

(CO), and total hydrocarbon emissions (THC) can be 

achieved with biodiesel use [1]. Based on life-cycle 

analysis, the use of biodiesel produces real reductions in 

petroleum consumption and carbon dioxide (CO2) 

emissions [2]. Energy content per gallon is slightly lower 

resulting in minor reductions in peak torque and fuel 

economy, but thermal efficiency is unchanged [3]. 

There are few quantitative studies of in-use operational 

performance. Bickel and Strebig [4] report a two-year 

field trial of B20 use in road maintenance vehicles in 

Minnesota. Nearly 25,000 gallons of B20 were consumed 

over roughly 135,000 miles of operation. The B20 

vehicles exhibited the same average fuel economy (in 

miles per gallon) as the diesel control vehicles. Oil 

analysis conducted at 5,000-mile intervals indicated no 

unusual engine wear or fuel dilution. 

Chase et al. [5] described operation of a heavy-duty, linehaul truck on a blend of 50% biodiesel and 50% No. 2 

diesel (B50) for more than 200,000 miles. No operational 

problems were reported, inspection of the engine at the 

conclusion of the study showed no excessive wear, and 

tests indicated no injector degradation. 

The BIOBUS project was conducted in Montreal from 

March 2002 to March 2003 [6]. Biodiesel from vegetable 

oil, animal fat, and waste cooking oil was tested as B5 

and B20 in over 150 buses. Engines included primarily 

mechanically governed 2-stroke and 4-stroke engines 
manufactured by Detroit Diesel Corporation and 

Cummins, Inc., respectively. The study found no impact 

of biodiesel on fuel economy or on the cost to maintain 

fuel pumps and fuel injectors. Problems with low 

temperature fuel filter plugging were noted for a few 

specific vehicles. Pollutant emission measurements 

revealed reductions in THC, CO, and PM with no effect 

or a small increase for NOx. 

Fraer et al. [7] compared operation of cargo vans and 

truck tractors on B20 and petroleum diesel and 

performed teardown and analysis of engines and 

components. After four years of operation and 600,000 

miles accumulated on B20, no differences in wear were 

noted. In comparing maintenance costs between the two 

groups, only minor differences could be attributed to B20 

use. 

In this study, nine identical 40-ft. transit buses were 

operated on B20 and diesel for a period of two years – 

five of the buses operated exclusively on B20 and the 

other four on petroleum diesel. The buses operated in 

the Regional Transportation District (RTD) fleet in 

Boulder, Colorado. A quantitative comparison of mileage 

accumulation, fuel use, road calls, maintenance costs 

and events, fuel analysis, oil analysis, and pollutant 

emissions is reported. 

OBJECTIVE 

The objective of this study is to compare vehicles 

operating on B20 and conventional diesel in terms of 

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 

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 

radiators. 

RTD is working to reduce exhaust emissions from its 

buses. In evaluating biodiesel, RTD expects to obtain 

information on the operation of biodiesel to help its 

planning of a strategy for improving RTD fleet operating 

conditions and public image through vehicle emissions 

reduction, use of renewable fuel, and fleet reliability. This 

information includes: 

• Verification of the emissions benefits of biodiesel 

• Performance changes in buses operating on 

biodiesel 

• Fuel consumption changes with biodiesel 

• Effects of biodiesel on bus mechanical reliability and 

service availability 

• High altitude and cold weather performance and 

stability of biodiesel 

• Drivers and passengers acceptance of biodiesel. 

B20 has typically cost more than No. 2 diesel, and at the 

time of the evaluation, averaged $0.17 higher in the 

Denver, Colorado, area. As of August 24, 2006, B20 

averaged $2.62 in the United States compared to $2.44 

for No. 2 diesel (taxes not included) [8]. RTD must weigh 

this additional cost against potential benefits. 

APPROACH 

VEHICLE SELECTION 

The vehicles chosen for this study are 40-ft Orion V 

transit buses and seat 43 passengers. These buses 

were selected in part because they operate on a 

dedicated route. They operate in Boulder, Colorado, on 

RTD’s “Skip” route, a 16.1-mile roundtrip route that 

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 

they are dedicated for the route (Figure 1). Table 1 

provides a summary description of the Skip buses. 

Figure 1. RTD Skip Bus in Service 

VEHICLE FUELING 

During this study, five of the nine buses operated on 

B20, and four operated on standard petroleum diesel as 

a control group. All buses were fueled daily at which time 

the hubodometer reading, fuel amount, and amounts of 

any other fluids added were logged. Diesel buses were 

fueled at an indoor fueling island, whereas the B20 

buses were fueled at a temporary fueling facility located 

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. 
Table 1. RTD B20 Evaluation Transit Bus Description Summary 

Vehicle Information Evaluation Buses 

(B20 and Diesel) 

Number of Buses 5 B20, 4 Diesel 

Chassis Manufacturer/Model Orion V 

Chassis Model Year 2000 

Engine Manufacturer/Model Cummins ISM 

Engine Model Year 2000 

Engine Ratings 

Max. Horsepower 

Max. Torque 

280hp @ 2,100 rpm 

900 lb-ft @ 1,200 rpm 

Diesel Fuel System Capacity 125 gal 

Transmission 

Manufacturer/Model 

ZF Ecomat 5HP592 

Curb Weight 28,800 lb 

Gross Vehicle Weight Rating 

(GVWR) 

40,600 lb 

Figure 2. B20 Fueling Station 

The buses were fueled using a dry-lock nozzle 

manufactured by Emco-Wheaton, but to separate fuel 

types, the B20 buses used a nonstandard connector. 

The fueling nozzle at the B20 station was identical in 

appearance and operation to the standard diesel fuel 

nozzle, but it used a four-pin instead of the standard 

three-pin connector. This different pin configuration 

ensured that the test buses were only fueled with B20, 

and B20 was dispensed only into these buses. 

PERIOD OF OPERATION 

RTD has 303 Orion buses in its fleet that entered service 

beginning in late 2000. In the summer of 2001, nine of 

the buses were dedicated to the Skip route, and B20 

fueling began in July 2004 with each of the buses at 

about 160,000 miles. By late July 2004, controlled fueling 

of the five B20 buses and continuous data collection 

were in place. The data collection period reported is from 

August 1, 2004 through July 31, 2006.

DATA COLLECTION AND ANALYSIS 

Operation and maintenance data were collected for 24 

months during normal operation and analyzed to 

evaluate performance. Periodic fuel and used oil 

samples were collected and analyzed to verify fuel 

quality and compare oil degradation. On-road fuel 

economy, maintenance, and road call calculations were 

based upon records provided by RTD and were reviewed 

for accuracy by the National Renewable Energy 

Laboratory (NREL). Each individual event was examined 

for missing or incomplete information. For example, each 

fuel record was first checked for data entered in all fields, 

then for an accurate hubodometer reading (in sequence 

for a given date), and finally for fuel economy (further 

examination if grossly out of range). Records that were 

recorded or entered incorrectly, thereby casting doubt 

upon their accuracy, were removed from the 

calculations, thus improving the level of confidence in the 

on-road data. Results are typically reported as a running 

or cumulative average, that is, the average results from 

the beginning of the study to any given point in the study. 

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. 

The emissions measurement system employs full scale 

dilution with constant volume sampling for mass flow 

measurement. Gaseous emissions—including CO2, 

nitrogen oxide (NOx), THC, and CO—are measured 

continuously. PM emissions are measured 

gravimetrically for samples collected onto 47 mm Teflon 

filters and weighed on a microbalance in a clean room 

environment. The emission measurement system meets 

the requirements of the current code of federal 

regulations for heavy-duty engine emissions certification 

(40 CFR, part 86). In addition, direct mass flow fuel 

consumption is measured in line with a high accuracy 

(+/-0.5% of reading) fuel metering system, which 

comprises a volume flow meter and in-line density meter 

to measure fuel mass flow. 

The Skip buses have a rated gross vehicle weight of 

40,600 lb. For chassis dynamometer testing, a vehicle 

inertia of 35,000 lb was employed with estimated rolling 

resistance and drag coefficients set at CRR=0.01 and 

CD=0.5. The test driving cycle selected was the CitySuburban Heavy-Vehicle Cycle (CSHVC). This cycle was 

chosen because certain parameters of the cycle are a 

close match with the typical Skip bus route, as shown in 

Table 2. The CSHVC speed-time trace is shown in 

Figure 3. 
Table 2. Comparison of Parameters for CSHVC and Skip Bus Route 

CSHVC Skip Route 

Average Speed, mph 14.2 15.6 

Maximum Speed, mph 44 40 

Stops per Mile 0.75 0.78 

Figure 3. Speed-Time Trace for the CHSVC. 

RESULTS 

MILEAGE ACCUMULATION AND FUEL USE 

In the 24 months of data collection, about 100,000 miles 

were driven by each of the study vehicles. For the B20 

buses, more than 100,000 gallons were consumed in 

total during the study. Table 3 provides mileage 

accumulation details. The B20 buses had about the 

same use as the diesel comparison buses. Accumulated 

mileage numbers are very similar, and both groups 

averaged over 4,000 miles per bus per month (Figure 4). 

Table 3. Accumulated Mileage Details 

Bus 

Number 

VIN Total Data 

Period 

Mileage 

Diesel Group 

2203 1VH5H3H2XY6501249 105,499 

2204 1VH5H3H2XY6501250 106,788 

2205 1VH5H3H2XY6501251 110,133 

2206 1VH5H3H2XY6501252 105,981 

B20 Group 

2207 1VH5H3H2XY6501256 102,614 

2208 1VH5H3H2XY6501258 100,484 

2209 1VH5H3H2XY6501259 95,358 

2210 1VH5H3H2XY6501260 101,815 

2211 1VH5H3H2XY6501261 100,962 

Running Average Monthly Miles per Bus

-

1,000

2,000

3,000

4,000

5,000

6,000

Aug04Oct04Dec04Feb05Apr05Jun05Aug05Oct05Dec05Feb06Apr06Jun06Aug06

Miles

Diesel Group B20 Group

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. 

Although diesel bus 2205 exhibited a fuel economy 

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 

difference (4.41 mpg diesel vs. 4.41 mpg B20) between 

the diesel and B20 study group fuel economies. 

If diesel bus 2205 were removed as an outlier, the 24-

month average fuel economy for the diesel group 

becomes 4.46 mpg. In this case, the fuel economy for 

the B20 buses is 1.2% lower than that of the diesel 

buses (p-value = 0.02). A small fuel economy reduction 

is expected due to the lower energy content of B20 as 

compared to diesel fuel. 

Table 4. Individual Bus Fuel Economy 

Bus 

Number 

Fuel Economy 

(mpg) 

Diesel Group 

2203 4.46 

2204 4.46 

2205 4.25 

2206 4.46 

Average 4.41 

B20 Group 

2207 4.37 

2208 4.40 

2209 4.41 

2210 4.45 

2211 4.41 

Average 4.41 
Fuel Economy Comparison

0

1

2

3

4

5

6

Aug04Oct04Dec04Feb05Apr05Jun05Aug05Oct05Dec05Feb06Apr06Jun06Aug06

MPG

Diesel Group B20 Group

Figure 5. Fuel Economy Comparison by Fuel Group 

MAINTENANCE 

This analysis examines both total maintenance costs, as 

well as maintenance costs related to the engine and fuel 

system. Total maintenance costs include the costs of 

parts and labor, but do not include warranty costs (the 

five-year extended warranty expired in 2005). The labor 

rate for maintenance has been arbitrarily set at $50 per 

hour and is not intended to reflect RTD’s current labor 

mechanic rate. Cost per mile is calculated as follows: 

Cost per mile = ((labor hours * $50) + parts cost)/mileage 

Bus maintenance costs over 24 months are presented in 

Table 5. The running average of maintenance costs for 

the diesel and B20 groups are compared in Figure 6. 

This running average or cumulative presentation of 

maintenance costs shows the average of the costs up to 

a given month and smoothes occasional monthly spikes 

in maintenance. The total maintenance cost per mile for 

the B20 group was 5.2% lower than for the diesel group 

(p-value = 0.27). This difference will be explored in 

further discussion regarding maintenance cost 

breakdown by vehicle system. 

Table 5. Bus Maintenance Costs 

Bus 

Number 

Miles 

Driven 

Labor 

Hours 

Parts 

Cost 

Total Cost 

($/mile) 

Diesel Group 

2203 105,499 892 $11,965 $0.54 

2204 106,788 835 $14,254 $0.52 

2205 110,133 965 $14,178 $0.57 

2206 105,981 852 $13,555 $0.53 

Totals 428,401 3,544 $53,951 $0.54 

B20 Group 

2207 102,614 770 $7,366 $0.45 

2208 100,484 888 $11,507 $0.56 

2209 95,358 844 $8,647 $0.53 

2210 101,815 757 $11,957 $0.49 

2211 100,962 800 $13,145 $0.53 

Totals 501,233 4,059 $52,622 $0.51 

Running Total Maintenance Cost per Mile

0.00

0.20

0.40

0.60

0.80

Aug04Oct04Dec04Feb05Apr05Jun05Aug05Oct05Dec05Feb06Apr06Jun06Aug06

$/mile

Diesel Group B20 Group

Figure 6. Running Total Maintenance Cost Comparison by Fuel Group 

RTD codes and categorizes labor events and parts 

replacements according to vehicle subsystem or 

maintenance activity. For example, maintenance 

performed on the engine, fuel system, or as part of a 

preventative maintenance program is coded differently. 

Using these codes, the maintenance and repair data 

were analyzed in more detail to assess differences at the 

engine and fuel system level—the systems that B20 use 

might be expected to impact. 

Bus maintenance costs over 24 months related to the 

engine and fuel system are presented in Table 6. The 

running average maintenance costs for the diesel and 

B20 groups are compared in Figure 7. The engine and 

fuel system maintenance cost per mile for the B20 group 

is 39% higher than for the diesel group (p-value = 0.16). 

Engine and fuel system maintenance costs are very 

similar for most of the test period. However, during the 

last 3 months of the study, average B20 maintenance 

costs increased due to component replacements on Bus 

2211 in May and June of 2006 (details to follow). The 

5.2% lower total maintenance cost per mile exhibited by 

the B20 group is not attributable to fewer engine and fuel 

system repairs. In calculation of the total maintenance 

costs, the higher engine and fuel system repair costs for 

the B20 group were offset by higher maintenance costs 

for transmission repairs in the diesel group (unrelated to 

fuel use). 

Table 6. Bus Engine and Fuel System Maintenance Costs 

Bus 

Number 

Miles 

Driven 

Labor 

Hours 

Parts 

Cost 

Total Cost 

($/mile) 

Diesel Group 

2203 105,499 48 $3,427 $0.06 

2204 106,788 38 $3,227 $0.05 

2205 110,133 57 $3,205 $0.05 

2206 105,981 29 $3,234 $0.04 

Totals 428,401 171 $13,093 $0.05 

B20 Group 

2207 102,614 25 $2,910 $0.04 

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 
Running Engine, Fuel System Maintenance Cost 

per Mile

0.00

0.02

0.04

0.06

0.08

0.10

Aug04Oct04Dec04Feb05Apr05Jun05Aug05Oct05Dec05Feb06Apr06Jun06Aug06

$/mile

Diesel Group B20 Group

Figure 7. Running Engine and Fuel System Maintenance Cost 

Comparison by Fuel Group 

Looking specifically at fuel system parts that may be 

considered potentially susceptible to B20 use, 

maintenance items found in the data included the 

following: 

• Fuel pump 

• Fuel pump gasket 

• Fuel injector 

• Fuel injector O-ring 

• Fuel lines 

• Fuel filter. 

These parts are all categorized as fuel system parts, with 

the exception of the fuel filter. The fuel filter is grouped 

with a suite of preventative maintenance repair checks 

and part replacements. Preventative maintenance events 

are scheduled by RTD to occur every 6,000 miles of use. 

The fuel system replacement parts installed during the 

study are shown in Table 7 for both diesel and B20 

buses. During the evaluation period, fuel pumps were 

replaced for two diesel buses (2203 and 2206), and a 

fuel pump was replaced for a B20 bus (2209). A single 

fuel injector was replaced for one diesel bus (2205) and 

one B20 bus (2208). However, B20 Bus 2211 had all six 

injectors replaced due to a “no-start” condition in May 

2006. In June 2006, Bus 2211 had a cylinder head 

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 

second replacement of 6 valves was actually necessary. 

The labor and parts costs associated with the May – 

June 2006 fuel injector and cylinder head replacements 

of Bus 2211 make up the difference in engine and fuel 

maintenance costs between the B20 and diesel groups 

in this study. Further analysis of the replaced parts is 

ongoing to determine if B20 use is related to their failure. 

These results highlight certain challenges inherent in 

controlled fleet evaluations, and in particular the high 

variability in maintenance costs from vehicle to vehicle, 

independent of the fuel used. For this group of vehicles 

transmission repairs that were unrelated to fuel use 

caused the total maintenance costs for the diesel group 

to be higher. At the same time, a single maintenance 

event for Bus 2211 caused the running average engine 

and fuel system maintenance costs for the B20 group to 

spike during the last few months of the study. To mitigate 

the impact of the high variability in maintenance costs 

between vehicles, future studies will need to assess a 

larger fleet or a similar sized fleet for a significantly 

longer period of time. 

Table 7. Replacement Fuel System Parts 

Bus 

Number

Part 

Description

Date 

Replaced 

Quantity Total 

Cost 

Diesel Group 

2203 Gasket Fuel 

Pump 07/13/05 1 $4.81 

2203 Gasket Fuel 

Pump 07/14/05 1 $4.91 

2203 Fuel Pump 07/14/05 1 $622.67 

2203 Tube Fuel 

Supply 01/24/05 1 $15.31 

2204 

Gasket Fuel 

Pump 07/15/05 1 $4.91 

2205 Fuel Injector 01/27/06 1 $548.49 

2206 

Gasket Fuel 

Pump 10/21/04 1 $1.76 

2206 Fuel Pump 10/21/04 1 $555.37 

2206 

Gasket Fuel 

Pump 02/20/06 1 $4.94 

Total $1,763 

B20 Group 

2208 Fuel Injector 07/08/05 1 $709.2 

2208 

Fuel 

Solenoid 

S/Off 07/08/05 1 $36.47 

2208 Oring Injector 07/11/05 1 $1.59 

2208 Oring Injector 07/11/05 1 $1.61 

2208 Oring Injector 07/11/05 1 $1.78 

2209 

Gasket Fuel 

Pump 10/26/04 1 $1.76 

2209 Fuel Pump 10/26/04 1 $555.37 

2209 

Tube Fuel 

Supply 10/13/05 1 $15.82 

2211 Oring Injector 05/24/06 6 $10.32 

2211 Fuel Injector 05/24/06 6 $2,479.14

2211 Fuel Injector 06/14/06 6 $2,479.14

Total $6,293 

ROAD CALLS 

A road call is defined as a call-in to dispatch reporting a 

mechanical problem. Depending on the nature of the 

problem, dispatch may instruct operators to continue 

driving their routes. However, a road call may stem from 

an issue that requires the bus to stop driving, allowing for 

roadside mechanical repair or towing back to the 

maintenance facility. Road calls and average miles 

(driven) between road calls (MBRC) are an important 

reliability indicator for the transit industry. For the 

purposes of this analysis, data received from RTD 

indicating the occurrence of a road call was recorded as 

such, regardless of its relative severity. 
Figure 8 shows the cumulative MBRC for all road calls 

for the B20 and diesel baseline groups. Average MBRC 

values over 24 months are 3,197 and 3,632 for diesel 

and B20 groups, respectively. The B20 buses are 

apparently 14% higher, but the difference is not 

significant (p-value = 0.59) and there is no evidence in 

the data to suggest this improvement is attributable to 

fuel use. Differences during the first months of the study 

are related to the variability of MBRC month-to-month, 

with a few months required for the running average of 

each group to settle. After 24 months of evaluation, there 

is no negative impact on MBRC from the use of B20. 

However, in April 2005 two buses reported road calls for 

engine misfiring and stalling caused by plugged fuel 

filters. The first incident happened with Bus 2210 on April 

8, 2005. The plugged fuel filter was removed from the 

vehicle and cut open for examination. A brown “greaselike” material was found in the filter pleats and was the 

suspected cause of the filter plugging (Figure 9). 

Running Miles Between Road Calls (MBRC)

0

2,000

4,000

6,000

8,000

10,000

Aug04Oct04Dec04Feb05Apr05Jun05Aug05Oct05Dec05Feb06Apr06Jun06Aug06

Miles

Diesel Group B20 Group

Figure 8. Running Average MBRC Comparison by Fuel Group 

Figure 9. Bus 2210 Plugged Filter Examination 

The second incident with Bus 2208 occurred three days 

later on April 11, 2005. The plugged filter from this bus 

also contained the brown material. The filters on the 

other three B20 buses were changed as a precautionary 

measure, and inspection of these used filters also 

revealed signs of the brown material, but not of the 

quantity and consistency of the plugged filters. 

Fuel was removed from the vehicle fuel tanks and tested 

for several properties as shown in Table 8. None of the 

fuel samples exhibited excessively high levels of 

biodiesel or cold filter plugging point (CFPP, determined 

by ASTM D6371). Water determined by Karl Fischer 

method (ASTM D6304) indicated higher levels than 

typical of a No. 2 diesel fuel but not excessively high. 

The Bug AlertTM ATP test for microbial growth does not 

indicate that microbial contamination is an issue. 

Table 8. Vehicle Fuel Testing Results (April 2005) 

Sample 

Percent 

Biodiesel

CFPP 

oC 

Water, 

ppm 

Bug AlertTM

ATP 

2207 18.4 -24 72 139 (med) 

2208 16.9 -25 77 27 (low) 

2209 19.2 -25 88 57 (low) 

2210 20.3 -25 97 1 (very low) 

2211 15.0 -30 78 93 (low-med)

The dark brown gelatinous residue coating RTD fuel filter 

No. 2210 was analyzed by gas chromatography mass 

spectrometry (GC-MS) (Agilent 6890 GC equipped with a 

5890 MSD mass selective detector). The sample was 

prepared by scraping 23 mg of residue from a filter pleat 

and dissolving this in 1.0 mL of toluene. 1.0 μL of the 

solution was injected into the GC-MS using a split 

injection (100:1) onto a 30m x 0.25mm column, (0.50 μm 

DB-5 film). 

The resulting chromatographic data are shown as the 

total ion current (TIC) signal from the MSD, as a function 

of component elution time in Figure 10. The multiple 

peaks in the 8-17 minute region are identified as diesel 

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. 

Retention Time, min

0 10 20 30 40 50

Total Ion Current

Diesel Hydrocarbons

FAME

Phytosterols

39 40 41 42 43 44 45 Campesterol StigmasterolSitosterol Stigmast-4-en-3-one

Figure 10. GC-MS results for brown material found on plugged fuel 

filter from Bus 2210 

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. 
Campesterol, stigmasterol, sitosterol and stigmast-4-en3-one were the major species identified at retention 

times 39.59, 40.06, 41.30 and 44.34 minutes, 

respectively. The relative amounts are in rough 

agreement with those reported in soybean oil [10]. While 

this analysis is semi-quantitative, the total level of sterols 

is significantly higher than expected for B20. Thus, this 

analysis suggests that high levels of plant sterols might 

be responsible for the filter plugging. These sterols are 

much higher molecular weight (≥400 amu) and would 

have a higher freezing point than typical of FAME or 

diesel fuel components. However, based on this analysis 

alone we cannot rule out other potential causes. 

Two other filter plugging events occurred during the 

remaining study period. One happened about two 

months later on June 14, 2005. Bus 2209 filled from the 

B20 dispenser with a report of slow fueling by the 

dispenser operator. The fuel filter on Bus 2209 plugged 

shortly thereafter. It was later discovered that the B20 

tank was nearly empty. Bus 2209 required several fuel 

filter changes and vehicle tank drainage to correct the 

plugging problem. The remaining B20 was also drained 

from the storage tank, the tank cleaned, and then refilled 

for continued B20 service. 

A final plugging event happened during the last month of 

the study period. Two buses, 2207 and 2211, were road 

called for plugged fuel filters on July 7, 2006. The fuel 

storage tank was again near empty in anticipation of 

completion of the study and removal of the tank. 

Because the tank is drawn from the bottom, this implies 

that a material less dense than biodiesel was floating on 

top and was pumped into the vehicles as the tank 

became nearly empty. One well known quality issue with 

biodiesel is the presence of soap, which in large enough 

concentration will float on top of a fuel tank. 

As mentioned previously, fuel filters are not included in 

the analysis of engine and fuel system labor and parts 

costs because they are considered preventative 

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 

number of extra fuel filters replaced, and indicates the 

associated labor and parts cost. In addition, labor for 

diagnosis and related work (draining and refilling fuel 

tanks) amounted to $712.50 for a total cost of $1,054.81. 

These additional fuel filter replacements were not 

significant additions to the maintenance analysis, adding 

an additional maintenance cost per mile to the B20 group 

of only $0.002. However, disruptions in transit service 

and related costs (bus substitution, affected ridership) 

are not captured in the maintenance costs, and were 

considered significant to RTD. 

Table 9. Extra Fuel Filter Maintenance Costs 

Bus 

Number 

Extra 

Fuel 

Filters 

Labor 

Hours 

Parts 

Cost 

Total 

Cost 

B20 Group 

2207 3 1 $19.45 $69.45 

2208 3 0.8 $18.84 $58.84 

2209 4 1 $25.12 $75.12 

2210 2 0.5 $12.56 $37.56 

2211 4 1.5 $26.34 $101.34 

Totals 16 4.8 $102.31 $342.31 

FUEL TESTING 

Fuel delivery load samples were collected for analysis of 

biodiesel blend content. Fuel was delivered about twice a 

month with samples starting in September 2004. 

Biodiesel content was determined by infrared 

spectroscopy (FTIR). The fuel blender employed splash 

blending to mix biodiesel with petroleum diesel to make 

the required B20 blend. Biodiesel from a heated storage 

tank was loaded into the delivery truck and then driven to 

the fuel terminal where No. 2 diesel fuel (or both No. 2 

and No. 1 diesel in the winter months) was typically 

bottom loaded to make the blend. From the terminal, the 

delivery truck would drive to the RTD Boulder facility and 

offload all of the B20 into the outdoor storage tank 

(typically 2,500 gallons per delivery). Load samples were 

taken from the top of the delivery truck tank at the 

terminal, prior to delivery. 

In examining the delivery load samples for biodiesel 

content, blend levels were found to range from less than 

1% to over 80% (Figure 11). After the discovery of erratic 

blend levels from the first group of samples tested in May 

2005, the blender reported changing its blending 

procedure to include recirculation of fuel within the 

delivery truck tank prior to delivery. Later samples 

seemed consistently B20 for a short period then again 

became erratic. 

Delivery Sample Biodiesel Content (Volume %)

0

10

20

30

40

50

60

70

80

90

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 
the fuel from each delivery truck was offloaded into the 

storage tank, and it appeared to have been completely 

blended in this process as evidenced by vehicle tank 

samples taken in April 2005 (Table 8) and March 2006 at 

or near B20 (Table 10). 

In addition to blend level, several other properties were 

tested to examine fuel quality in March 2006. Acid value, 

peroxides, and aldehydes (or alkanals) were determined 

using the SaftestTM method. Acid and peroxides are 

consistently low in comparison to levels observed in the 

B20 quality survey [9]. Alkanals indicate some oxidative 

degradation of the biodiesel but are not high. 

Table 10. B20 Vehicle Sample Testing March 2006 

Vehicle 

Number 

B100 

Content 

Volume % 

Acid 

Value 

mgKOH/g 

Peroxide 

SaftestTM

ppm 

Aldehyde 

SaftestTM

nmol/mL 

2207 20.3 <0.1 -- 58.212 

2208 18.4 <0.1 13.22 57.902 

2209 17.4 <0.1 11.59 55.696 

2210 18.7 <0.1 16.75 73.35 

2211 19.7 <0.1 11.42 61.546 

Samples collected in March 2006 from both the diesel 

and biodiesel vehicle fuel tanks were combined to 

produce composite diesel and B20 samples. These were 

subjected to more detailed analysis as shown in Table 

11. These results show the reduction of fuel sulfur 

content caused by blending in of biodiesel as well as the 

2.4% reduction in energy content. The B20 blends exhibit 

significantly higher cetane number. 

Table 11. Results of Testing of B20 and Diesel Composite samples 

Obtained from Vehicles March 2006 

B20 

Composite 

Diesel 

Composite

Water & 

Sediment vol % 

D2709 0.01 0.01 

Cloud Point oC D2500 -13 -14 

Sulfur ppm D5453 324.1 

Sulfur ppm D2622 272 

Aromatics vol % D1319 25.6 

Olefins vol % 1.3 

Saturates vol % 73.1 

C mass% D5291 84.71 86.6 

H mass% 12.88 13.21 

Derived Cetane 

Number 

D6890 51.0 47.9 

LHV BTU/lb D240 17859.7 18306.6 

OIL TESTING 

Oil was sampled from several of the test vehicles during 

oil drain intervals in March through June of 2006 and at 

the mileage indicated in Table 12. These data provide a 

snapshot of oil performance for the two fuels in terms of 

ZDDP (zinc dialkyldithiophosphate) decay, TBN (total 

base number) decay, oxidation, fuel dilution, viscosity, 

soot loading, and wear metals). 

ZDDP decay and lubricant oxidation were assessed 

using FTIR. For ZDDP the strengths of the C-O-P 

stretching band at 957 cm-1 and of the P=S stretching 

band at 670 cm-1 were measured. The decay of these 

bands with mileage for all samples listed in Table 12 is 

shown in Figure 12. This indicates exponential decay 

with mileage: 

C = C0exp(-αm) (1) 

ln (1 + (C-C0)/C0) = - αm (2) 

Where m = mileage 

 C0 = initial intensity 

 C = intensity at mileage m 

α= exponential decay constant 

Table 12. Oil Samples Taken for Analysis, Miles Indicate Mileage 

Since Last Oil Change 

Vehicle No. Sample Date Miles 

Diesel Group

2203 3/14/06 1295 

2203 4/26/06 7770 

2203 6/8/06 5375 

2204 3/14/06 1392 

2204 4/22/06 6850 

2204 6/6/06 6035 

2205 5/17/06 7463 

2206 6/1/06 5840 

B20 Group 

2207 3/14/06 2900 

2207 6/6/06 7256 

2208 3/14/06 1834 

2208 4/25/06 7314 

2209 4/25/06 802 

2210 4/29/06 3634 

2211 5/24/06 8800 

-10

-8

-6

-4

-2

0

0 2000 4000 6000 8000 10000

Mileage

ZDDP decay

957 cm-1

670 cm-1

Figure 12. ZDDP Decay in Lubricants from All Vehicles. C-O-P 

Monitored at 957 cm-1 and P=S Monitored at 670 cm-1
Fitting of the data to equation 2 (Figure 13) yields 

exponential decay constants of 0.00007/mile for the C-OP stretch and 0.0001/mile for the P=S stretch, which are 

consistent with previous testing at Cummins and with 

expectations for oil decay for this engine. Differences 

between the fuels for ZDDP decay were small and 

probably not significant. The buildup of oxidation 

products was monitored via the acid carbonyl IR band at 

1700 cm-1. Oxidation products grow exponentially with 

mileage as shown in Figure 14. Lubricants contain overbased detergent, which neutralize acids by formation of 

carboxylate. The formation of carboxylate was also 

monitored via the IR band at 1640 cm-1 which also shows 

exponential growth. As seen in Figure 14, both acid and 

carboxylate growth has essentially the same slope or 

exponential growth constant. This indicates that the 

acids being formed are being adequately neutralized by 

the detergent. No difference was observed between 

diesel and B20. 

TBN also decays exponentially with mileage, as shown in 

Figure 15. The decay constant is consistent with 

Cummins experience for conventional diesel fuel. TBN is 

a sensitive function of fuel sulfur content. As noted, 

biodiesel dilutes the sulfur content of the diesel fuel. 

Table 13 lists TBN values and other oil properties as a 

function of mileage. Examination of these values 

suggests that TBN decay is occurring more slowly for the 

B20 blends; however, this cannot be proven based on 

this limited dataset. 

Fuel dilution of the lubricant was examined by a gas 

chromatography method for some samples. This method 

yields the amount of petroleum diesel fuel in the lubricant 

and results are shown in Table 13. Fuel dilution by 

lubricant is low in all cases, and even lower for the B20 

blends. Additionally, lubricant dilution by methyl ester 

from the biodiesel fuel blend was examined by FTIR. No 

ester was detected in the lubricant from the B20 buses 

except for the 7314 mile sample from bus 2208, which 

contained 0.6% methyl ester. 

y = -7E-05x + 0.0124

R2

 = 0.7142

y = -0.0001x - 0.0352

R2

 = 0.7035

-2

-1.5

-1

-0.5

0

0.5

0 2000 4000 6000 8000 10000

Mileage

Ln(ZDDP decay)

Ln(957)

Ln(670)

y = -2E-05x + 0.6024

R2 = 0.5653

0.2

0.4

0.6

0.8

0 2000 4000 6000 8000 10000

Mileage

Ln(TBN decay)

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 

y = 0.0002x + 0.2317

R2 = 0.8788

y = 0.0003x - 0.5963

R2 = 0.9268

-2

-1

0

1

2

3

0 2000 4000 6000 8000 10000

Mileage

Ln(oxidation)

ln(fC=O)

Ln(carboxy)

Metals analysis was performed by ICP-AA (inductively 

coupled plasma/atomic absorption). Calcium, zinc, and 

phosphorus increase in concentration as the lubricant is 

consumed by evaporation, thus can be used to track oil 

consumption. However for these oil samples the 

concentrations of these metals do not exhibit any trend 

with mileage and show no discernable difference for 

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. 

Finally, sodium levels are low in all cases indicating no 

coolant leak or contamination with high soap content 

biodiesel. 

Soot in oil was determined by a thermogravimetric 

analysis method, and these values are also shown in 

Table 13. Soot levels are low in all cases but about 50% 

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 

Figure 14. Results of Fitting Acid Carbonyl and Carboxylate Growth 

Data to Linearized Exponential Law, Equation 2 
Table 13. Lubricant Properties at Various Mileage 

Vehicle No. Miles TBN Fuel % 

by GC 

Ca, 

ppm 

Zn, 

ppm 

P, 

ppm 

Fe, 

ppm 

Cu, 

ppm 

Cr, 

ppm 

Na, 

ppm 

Soot, 

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 

also tested for viscosity and viscosity index. These 

values did not decay significantly during the oil drain 

interval for lubricant from either group of vehicles. 

Overall the lubricant analysis indicates no negative 

impact from the use of B20 and suggests some potential 

benefits. To quantify these potential benefits will require 

additional study. 

CHASSIS EMISSION TESTS 

Emission testing was conducted on two vehicles: bus 

numbers 2208 and 2211. The buses were tested using 

both the in-use No. 2 diesel and the in-use B20 fuels. 

Properties of these fuels are listed in Table 14. At the 

time of this testing, the B20 was being blended with the 

same diesel fuel used by the diesel buses. The B20 

shows significantly higher cetane number, in part 

because of the relatively high cetane number of biodiesel 

which is typically 54 [9], but this can only account for a 

cetane number increase of about 3. The additional 

increase is caused by the multifunctional diesel additive 

used by the biodiesel supplier, which includes a cetane 

improving additive. The aromatic content of the B20 

appears to be slightly higher than the aromatic content of 

the diesel fuel; however biodiesel is known to interfere in 

method D1319 [11] and may provide falsely high values 

for B20 blends. Two drivers were used to perform the 

test cycles. Testing was performed so that each driver 

performed the same number of test runs on each bus, 

per fuel. 

Six to eight total repeated test cycle runs were driven for 

each bus on each fuel, results for each test run are 

reported in Appendix A; and summary results analyzed 

statistically for each bus and for both buses combined 

are reported in Table 15. The data show that for these 

vehicles on this test cycle, operation on B20 reduced all 

regulated pollutants, including NOx. Fuel economy on a 

mpg basis was decreased by roughly 2%, in agreement 

with the lower energy content of B20. In most cases 

p<0.05, indicating changes are significant with 95% or 

better confidence. 

Table 14. Fuel Properties of Fuel Used for Emissions Testing 

B20 Diesel 

B100 Content (Vol %) 21.1 NA 

Distillation T90, oF (D86) 644.4 617.4 

Flash Point, oF (D93) 159 151 

Copper Corrosion (D130) 1a 1a 

Kinematic Viscosity, cSt@40oC 

(D445) 

2.726 2.438 

Ash, %Mass (D482) 0 0 

Carbon Residue, %mass (D524) <0.010 0.04 

Cetane Number (D613) 47 40 

Cloud Point, oF (D5773) 6 0 

Total Sulfur, ppm (D5453) 280 364 

Water & Sediment, %Vol 

(D2709) 

0 0 

Aromatics, %Vol (D1319) 28.5 27.1 

Heat of Combustion, BTU/gal 

(D240) 

134,650 137,720

Acid Number, mg KOH/gram 

(D664) 

0.16 0.01 

Peroxide Number, ppm (D3703) 8.1 0 

Clearly the oxygen content provided by the biodiesel is 

primarily responsible for the reductions in THC, CO, and 

PM; in agreement with previous studies [1]. The situation 

for NOx emissions is less clear. B20 exhibited a cetane 
number of 47 versus 40 for the in-use diesel fuel. For 

fuels that are otherwise identical, increasing cetane 

number from 40 to 47 could produce a NOx reduction of 

3% [12]. However, in testing of a third mechanically 

identical transit bus where neither fuel contained 

additives we have observed NOx reductions ranging from

2 to 5% [13]. Thus it seems unlikely that additive effects 

are the main cause of the NOx reduction observed here. 

By testing two buses, with two drivers each, on both 

fuels, some information regarding variability between 

vehicles and between drivers can also be acquired. This 

analysis can be useful to evaluate whether the results 

can be interpreted to likely apply generally to the Skip 

bus fleet or if vehicle-to-vehicle variability prevents such 

a generalized conclusion. 

The data show good agreement between the two test 

vehicles, with no statistically significant difference (at 

alpha of 0.05), between the two buses for any of the 

regulated emissions. There was a statistically significant 

difference between the two buses for fuel economy with 

bus 2211 achieving approximately 3% better fuel 

economy than bus 2208, on average. 

There was no statistically significant difference between 

drivers (alpha of 0.05) for NOx emissions, THC 

emissions, or fuel economy. There was a statistically 

significant difference between drivers for CO and PM 

emissions, with these emissions from driver A averaging 

about 22%-23% higher than driver B. 

Table 15. Summary Results for Bus Emission Testing 

Bus NOx

g/mile 

THC 

g/mile 

CO 

g/mile 

PM 

g/mile 

FE 

mpg 

2211 

Base 19.81 0.871 3.60 0.274 4.70 

B20 18.65 0.625 2.63 0.226 4.59 

%Δ -5.8 -28.2 -26.8 -17.3 -2.4 

p-value <0.001 0.001 <0.001 0.042 0.032 

2208 

Base 19.44 0.807 3.44 0.2648 4.60 

B20 18.67 0.571 2.73 0.2150 4.45 

%Δ -3.9 -28.0 -20.3 -19.9 -2.1 

p-value 0.039 0.002 0.071 0.153 0.018 

CONCLUSIONS 

In the 100,000-mile evaluation of transit buses operated 

on B20, the following operational differences were found 

related to vehicle fuel: 

• The fuel economy for both petroleum diesel and B20 

groups was 4.41 mpg based on in-use fleet data. An 

approximately 2% reduction in fuel economy for B20 

was measured in laboratory emission testing. 

• Total maintenance costs per mile were $0.54 for the 

diesel group and $0.51 for the B20 group, and 

maintenance costs specific to the engine and fuel 

systems were $0.05 and $0.07 per mile, 

respectively. Because of high variability in 

maintenance costs between vehicles, the engine and 

fuel system maintenance costs for the two groups 

are not significantly different with a high degree of 

confidence (p~0.5). The increased engine and fuel 

system costs for the B20 group were due to fuel 

injector and cylinder head replacements on Bus 

2211. Further study will be required to determine if 

B20 fuel use caused these failures. 

• Miles between road calls averaged 3,197 for the 

diesel group and 3,632 for the B20 group. There is 

no evidence in the data to suggest this difference is 

related to fuel use. 

• Fuel filter plugging on the B20 buses caused road 

calls, and required extra filter replacements in the 

B20 group. Although the additional maintenance cost 

was small, adding only $1,054.81 to the B20 group 

or $0.002 per mile, the events were significant to the 

transit district because of resulting disruptions to 

normal bus service. Fuel filter plugging may have 

been caused by the presence of high levels of plant 

sterols in the biodiesel or other fuel quality issues. 

• Measurement of biodiesel blend level showed erratic 

biodiesel content for delivery load samples. Vehicle 

samples, however, were consistently at or near B20 

indicating complete blending had occurred during 

delivery and offloading of the fuel. 

• Oil analysis results indicate no additional wear 

metals from the use of B20, with similar rates of TBN 

and ZDDP decay, oxidation, fuel dilution, and 

viscosity. Soot levels in the lubricant were 

significantly lower for the B20 vehicles. Quantifying 

potential benefits will require additional study. 

• Laboratory chassis testing on the CSHVC cycle 

using the in-use fuels found that B20 reduced 

emissions of all regulated pollutants and caused a 

small fuel economy decrease. 

ACKNOWLEDGMENTS 

The FreedomCAR and Vehicle Technologies Program, 

Fuels Technologies Subprogram of the U.S. Department 

of Energy sponsored NREL’s participation in this project. 

The authors thank the Technical Services group and the 

Boulder maintenance staff of RTD. Special thanks are 

extended to Dean Shaklee from Operations; Dave 

Richardson and Dave Ober from Maintenance Reporting 

Systems; and Hugh Willis and Ray Fernandez from 

Boulder Maintenance. Additionally the authors wish to 

thank Dave Forrester of Power Service Products for B20 

testing during the fuel filter plugging incident; Dale Rains 

of Gray Oil; and Sean Lafferty and Ryan Lafferty of Blue 

Sun Biodiesel. 
REFERENCES 

1. United States Environmental Protection Agency. “A 

Comprehensive Analysis of Biodiesel Impacts on 

Exhaust Emissions.” Draft Technical Report, 

EPA420-P-02-001, 2002. 

2. Sheehan J., Camobreco V., Duffield J., Graboski M., 

Shapouri H. “An Overview of Biodiesel and 

Petroleum Diesel Life Cycles.” National Renewable 

Energy Laboratory, NREL/TP-580-24772, May 1998. 

3. Graboski M., McCormick R. “Combustion of Fat and 

Vegetable Oil Derived Fuels in Diesel Engines.” 

Prog. Energy Combust. Sci.; Vol. 24, 1998; p. 125-

164. 

4. Bickel K., Strebig K. “Soy-Based Diesel Fuel Study.” 

Final report to Legislative Commission on Minnesota 

Resources and Minnesota Soygrowers Association, 

2000. 

5. Chase C.L., Peterson C.L., Lowe G.A., Mann P., 

Smith J.A., Kado N.Y. “A 322,000 Kilometer 

(200,000 Mile) Over the Road Test with HySEE 

Biodiesel in a Heavy Duty Truck.” SAE Technical 

Paper No. 2000-01-2647, 2000. 

6. Biodiesel Demonstration and Assessment with the 

Société de Transport de Montréal (STM), Final 

Report http://www.stm.info/English/info/a-biobusfinal.pdf, May 2003. 

7. Fraer R., Dinh H., Proc K., McCormick R.L., 

Chandler K., Buchholz B. “Operating Experience and 

Teardown Analysis for Engines Operated on 

Biodiesel Blends (B20).” SAE Technical Paper No. 

2005-01-3641, 2005. 

8. Dtn Energy’s Alternative Fuels Index; Vol. 4, Issue 

32, 2006; p. 2. 

9. McCormick R.L., Alleman T.L., Ratcliff M., Moens L., 

Lawrence R. “Survey of the Quality and Stability of 

Biodiesel and Biodiesel Blends in the United States 

in 2004.” National Renewable Energy Laboratory, 

NREL/TP-540-38836, October 2005. 

10. Tarandjiiska, R.B., Marekov, I.N., NikolovaDamyanova, B.M., Amidzhin, B.S. “Determination of 

Triacylglycerol Classes and Molecular Species in 

Seed Oils with High Content of Linoleic and Linolenic 

Fatty Acids” J. of the Sci. of Food and Agriculture; 

Vol. 24, 1996 p. 403-410.

11. Active Standard: D1319-03e1 “Standard Test 

Method for Hydrocarbon Types in Liquid Petroleum 

Products by Fluorescent Indicator Adsorption”, 

ASTM International, West Conshohocken, PA; 

www.astm.org. 

12. United States Environmental Protection Agency. 

“The Effect of Cetane Number Increase Due to 

Additives on NOx Emissions from Heavy-Duty 

Highway Engines”, Final Technical Report, EPA420-

R-03-002, February 2003. 

13. McCormick, R.L., Hayes, R.R., Williams, A., Ireland, 

J. “Effect of Biodiesel Blends on Vehicle Emissions”, 

Milestone Report. National Renewable Energy 

Laboratory, NREL/MP-540-40554, October 2006. 

CONTACT 

Ken Proc can be contacted at kenneth_proc@nrel.gov. 

DEFINITIONS, ACRONYMS, ABBREVIATIONS

AA: Atomic absorption 

ASTM: ASTM International 

B20: A blend of 20% biodiesel with diesel fuel 

CFPP: Cold filter plugging point 

CFR: Code of Federal Regulations 

CSHVC: City-suburban heavy-vehicle cycle 

CO: Carbon monoxide 

CO2: Carbon dioxide 

DC: Direct current 

FAME: Fatty acid methyl esters 

FE: Fuel economy 

FTIR: Infrared spectroscopy 

GCMS: Gas chromatography mass spectrometry 

ICP: Inductively coupled plasma 

MBRC: Miles between road calls 

NOx: Oxides of nitrogen 

NREL: National Renewable Energy Laboratory 

PM: Particulate matter 

RC: Road call 

RTD: Regional Transportation District 

TBN: Total base number 

THC: Total hydrocarbon 

VIN: Vehicle identification number 

ZDDP: Zinc dialkyldithiophosphate 
APPENDIX A 

Table A1. Chassis Emissions Test Data 

Bus 2211

Fuel 

 

Run 

 

NOx 

g/mile 

THC 

g/mile 

CO 

g/mile 

PM 

g/mile 

Fuel 

Economy 

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 

 

Run 

 

NOx 

g/mile 

THC 

g/mile 

CO 

g/mile 

PM 

g/mile 

Fuel 

Economy 

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