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2012-01-1984 

Published 09/24/2012

doi:10.4271/2012-01-1984

saefuel.saejournals.org 

Effect of B20 and Low Aromatic Diesel on Transit Bus NOx
	

Emissions Over Driving Cycles with a Range of Kinetic 
 

Intensity
	

Michael P. Lammert, Robert L. McCormick, Petr Sindler and Aaron Williams
	

National Renewable Energy Laboratory
	

ABSTRACT 

The objective of this research project was to compare the emissions of oxides of nitrogen (NOx) from transit buses on 

as many as five different fuels and three standard transit duty cycles to establish if there is a real-world biodiesel NOx 

increase for transit bus duty cycles and engine calibrations. Prior studies have shown that B20 can cause a small but 

significant increase in NOx emissions for some engines and duty cycles. Six buses spanning engine build years 1998 to 

2011 were tested on the National Renewable Energy Laboratory's Renewable Fuels and Lubricants research laboratory's 

heavy-duty chassis dynamometer with certification diesel, certification B20 blend, low aromatic [California Air Resources 

Board (CARB)] diesel, low aromatic B20 blend, and B100 fuels over the Manhattan, Orange County and UDDS test 

cycles. The buses selected represented the majority of the current national transit fleet as well as including hybrid and 

selective catalyst reduction (SCR) systems that are increasing penetration in the fleet. 

The engine emissions certification level had the dominant effect on NOx, with the kinetic intensity of the tested duty 

cycle being the secondary driving factor. The biodiesel effect on NOx emissions was not statistically significant for most 

buses and duty cycles for blends with certification diesel, with the exception being a 2008 model year bus. CARB fuel had 

many more instances of a statistically significant effect of biodiesel increasing NOx. SCR systems proved effective at 

reducing NOx to near the detection limit on all duty cycles and fuels, including B100. A hybrid system proved to 

significantly increase NOx emissions over a same model year bus with a conventional drivetrain and the same engine. As 

all but one test bus were equipped with diesel particulate filter aftertreatment PM emissions were negligible and trends 

could not be drawn. On the oldest sample bus without aftertreatment PM emissions were reduced with B20 blends. Fuel 

economy was not significantly changed by engine certification level except that the 2008 conventional bus had the best 

performance on all cycles while all other buses had very similar results on each cycle. All buses had lower fuel economy 

with increased kinetic intensity of the cycle. 

CITATION: Lammert, M., McCormick, R., Sindler, P. and Williams, A., "Effect of B20 and Low Aromatic Diesel on
	

Transit Bus NOx Emissions Over Driving Cycles with a Range of Kinetic Intensity," SAE Int. J. Fuels Lubr. 5(3):2012, doi:
	

10.4271/2012-01-1984.
	

Early studies revealed that blending biodiesel into diesel INTRODUCTION 

fuel caused emissions of oxides of nitrogen (NOx) to increase Biodiesel is an important petroleum displacement fuel that in many engines [2,3]. It was also shown that biodiesel from is produced from various fats, oils, and greases. It consists of unsaturated feedstocks (having more double bonds) caused a fatty acid methyl esters produced from the various feedstocks larger increase in NOx [4]. Several different hypotheses have by transesterification with methanol. Biodiesel used in the been presented for the cause of this NOx increase [5]. United States must meet the quality requirements of ASTM Mueller and coworkers have shown that the primary cause is Specification D6751. Over 1 billion gallons of biodiesel were shifting of the combustion stoichiometry to be less rich at produced in the United States in 2011 [1]. Typically, ignition and in the standing premixed autoignition zone near biodiesel is used as a blend with petroleum diesel at levels the flame liftoff-length [6]. However, Eckerle and coworkers ranging from 2% to 20% by volume (B20). Energy content demonstrated that engine control systems could respond to per gallon is slightly lower for B20, resulting in a small the lower energy content of biodiesel in ways that reduction in peak torque and fuel economy, but no change in compensated for the NOx increase under some engine thermal efficiency [2]. 

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Lammert et al / SAE Int. J. Fuels Lubr. / Volume 5, Issue 3(November 2012) 

operating conditions [7]. Yanowitz and McCormick reviewed 

all available data and tried to relate differences in the effect 

of biodiesel on NOx to specific engines and fuel injection 

technologies [8]. They concluded that biodiesel could 

increase or reduce NOx emissions depending upon engine 

technology and driving cycle, and that there were inadequate 

data to determine an average effect. Transit buses represent a 

large niche market with a specific engine calibration. 

Therefore, it is important to determine if using biodiesel 

blends could increase emissions in this application. The 

objective of this research project was to evaluate the NOx 

emissions of a range of transit buses on as many as five 

different fuels and three standard transit duty cycles to 

establish if there is a discernible real-world biodiesel NOx 

increase for transit bus duty cycles and engine calibrations. 

BACKGROUND AND METHODS 

Bus Selection 

An analysis of the 2009 American Public Transportation 

Association transit authority survey [9] was used to select 

buses to test based on the prevalence of different models in 

the nation's transit fleet. Figure 1 shows the population by 

model year of different engine makes. Beyond representing 

the current population, an effort was made to represent up 

and coming technologies such as hybrid powertrains and 

newer emissions control systems. While Detroit Diesel 

Engines used to dominate the market, in the late 1990s and 

early 2000s Cummins Engines began to dominate the transit 

bus market. Also of note is the distinct rise in hybrid 

powertrains starting in 2003. Table 1 breaks down the percent 

of the transit population represented by each engine for each 

engine certification period. Table 2 lists the actual buses 

tested to represent the biggest categories and those increasing 

in percent of bus population. Two Colorado transit agencies, 

Denver Regional Transportation District (RTD) and the 

Roaring Fork Transportation Authority (RFTA), loaned buses 

for these tests. 

Figure 1. U.S. transit bus population.
	

Table 1. Population Breakdown of Current U.S. Transit
	

Bus Fleet.
	

Table 2. Selected Test Buses.
	

Fuel Selection 

The purpose of this study was to identify the effect on 

NOx emissions from different buses on different duty cycles 

and using different fuels. U.S. Environmental Protection 

Agency certification diesel, a nominally 10% aromatic Tier 2 

certification diesel meeting requirements for CARB diesel, a 

B100 derived from soy oil, and B20 blends of each diesel 

with the biodiesel were used. The CARB diesel has a lower 

aromatic content (8.5% vs. 29.9%) and a higher cetane 

number (48.7 vs. 42.5) than the certification diesel and was 

included in addition to the certification diesel both because 

CARB has great interest in NOx emissions and because the 

low-aromatic fuel has been shown to reduce NOx emissions 

[10]. Additionally, early results suggested that the effect of 

biodiesel on NOx may be greater for lower aromatic, higher 

cetane diesel fuels [3]. The biodiesel was typical of soy 

biodiesel. Detailed fuel properties are provided in the 

Appendix. 

Test Cycle Selection 

Dynamometer test cycles representative of common U.S. 

transit bus usage were desired to best represent real in-use 

emissions. Also taken into consideration was that the cycles 

chosen should be common enough that data from other 

researchers could be compared. The Manhattan Bus (MAN), 

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Orange County Transit Authority (OCTA), and Urban 

Dynamometer Driving Schedule (UDDS) cycles were 

chosen. These cycles are also being used as part of the 

Altoona new bus testing program at Pennsylvania State 

University [11]. Table 3 shows some of the representative 

statistics for the cycles, and Figures 2,3,4 show the speedtime traces of the cycles. “Average speed” is the average 

speed over the full time a bus engine is on and is commonly 

used to describe transit routes while “average driven speed” 

does not consider zero speed time and is more indicative of 

the intensity of a duty cycle. Stops per mile is another metric 

that is commonly used to describe transit routes. The kinetic 

intensity metric [12] reflects the nature of energy 

consumption along a given drive cycle. A large kinetic 

intensity value corresponds to drive cycles where the energy 

consumed is more strongly influenced by vehicle acceleration 

rather than vehicle driving speed. Similarly a small kinetic 

intensity value corresponds to drive cycles where vehicle 

speed dominates acceleration with regards to energy 

consumption. Lower average speeds and higher kinetic 

intensity and stops per mile tend to indicate a duty cycle that 

has more aggressive transient conditions typical of a heavy 

traffic urban environment and typically gets lower fuel 

economy than duty cycles with higher average speeds and 

lower kinetic intensity and stops per mile. The Manhattan 

Bus cycle represents transit bus operating conditions in a very 

dense urban environment. Of the three drive cycles, it has the 

most stops, lowest speed, and highest kinetic intensity. The 

OCTA cycle represents urban transit bus operation in the Los 

Angeles area and is characterized by higher driving speeds 

and less frequent stops than the MAN cycle. The UDDS 

cycle has the highest operating speeds and lowest kinetic 

intensity. This cycle represents generic urban and suburban 

operating conditions for heavy-duty vehicles and is the basis 

for the heavy-duty engine dynamometer federal test 

procedure cycle. 

Table 3. Selected Representative Statistics of Chosen
	

Dynamometer Test Cycles.
	

Figure 2. MAN chassis dynamometer test cycle.
	

Figure 3. OCTA chassis dynamometer test cycle.
	

Figure 4. UDDS chassis dynamometer test cycle.
	

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Laboratory Chassis Dynamometer Testing 

Procedures 

When tested, the vehicle is secured to the dynamometer 

with the drive axle(s) over the rollers. The vehicle is 

exercised by a driver following a prescribed speed trace on 

the test aid monitor. Test weight was determined by assuming 

half the rated passenger load and a driver with an average 

weight of 150 lbs per person in addition to the curb weight. 

Vehicle weights are given in Table 4. A large fan is used to 

evacuate the space behind the vehicle where the hot air tends 

to accumulate. The engine exhaust stream is collected by the 

emissions measurement system for analysis, and various 

vehicle parameters are monitored and logged by the data 

acquisition system. 

Table 4. Test Bus Weight. 

To assure the accuracy and consistency of road load 

simulation, the dynamometer is subjected to various 

procedures and checks. From a practical perspective, the 

daily testing routine consists of following steps. In the 

morning the vehicle is lifted off the rollers and the 

dynamometer is subjected to its warm-up procedure until the 

parasitic loses stabilize. Then the unloaded coastdown 

procedure is used to verify that the parasitic loses did not 

change from the previous test due to component failure and 

that the load cell calibration has not drifted. Following this 

verification, the vehicle is dropped on the rollers and driven 

for roughly 20 minutes to warm up. After the warm-up, a 

conditioning test run is performed to stabilize the vehicle 

temperature for a given test cycle. At this point, the system is 

ready to either set or verify correct road load simulation 

through a loaded coastdown procedure. The following test 

runs are considered usable in terms of data validity provided 

the road load simulation proves consistent. This is verified 

after each test to ensure that changing conditions (e.g., test 

facility temperature) are not affecting vehicle loading. To 

maximize the chances of consistency, the soak period 

between one test end and following test start is kept at 20 

minutes. 

Emissions measurement 

The emissions measurement system is based on 

recommendations in Code of Federal Regulations Section 40, 

Part 86, Subpart N. It consists of a full flow dilution tunnel 

with a constant volume sampling system for mass flow 

measurement. The tunnel flow rate is measured and 

controlled using critical flow venturis. The dilution and 

engine combustion air is supplied by an air handling unit that 

maintains the desired air temperature of 20°C and dew point 

of 12°C. 

The diluted engine exhaust was sampled for gravimetric 

PM analysis and by a Horiba MEXA 7100 series system for 

gaseous analysis including total hydrocarbons, NOx, carbon 

monoxide, and carbon dioxide. The gas analytical system was 

verified prior to the beginning of the testing period including 

linearization checks and NOx efficiency test. The analyzers 

were zero and span calibrated on a daily basis, and each test 

was bracketed by zero and span response reading used for 

corrections. The emissions measurement data were then 

reduced to distance-specific mass results using the Code of 

Federal Regulations-recommended calculations, including 

humidity, dry to wet, zero, span and background corrections. 

Fuel consumption measurement 

The fuel consumption measurement in this project relied 

on a gravimetric approach. The engine fuel supply and return 

lines were connected to a fuel container placed on a scale. 

The scale mass measurements were recorded in real time 

along with all the test data. The difference between the 

beginning and the end test mass measurement indicated the 

mass of fuel consumed during the test. Prior to testing the 

scale calibration was verified with a known calibration 

weight. The instrument used for this test was a Sartorius 

Midrics MAPP1U-60ED-L. The fuel consumption 

measurement was also backed up using the carbon balance 

method back-calculating the mass of fuel consumed from 

measurement of exhaust emissions constituents. 

RESULTS 

Bus NOX Emissions 

1998 Neoplan / DDC series 50 NOx emissions 

A 1998 model year Neoplan 40-ft bus with a Detroit 

Diesel Series 50 275 HP engine from RFTA was selected to 

represent the pre Heavy Duty Diesel Engine Consent Decree 

engine certification period that included no EGR or exhaust 

aftertreatment. Figure 5 shows the NOx g/mile results of each 

test cycle and fuel. Note that individual run data for NOx 

emissions, PM emissions, and fuel economy are provided in 

the Appendix. The B20 blend with certification diesel 

showed no statistically significant effect on NOx emissions 

on any cycle. The B20 blend with CARB diesel showed a 3% 

to 5% increase in NOx on the OCTA and UDDS cycles, but 

no increase on the MAN cycle (the most kinetically intense 

cycle). The test cycle had a larger effect with the UDDS cycle 

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producing 50% less NOx than the MAN cycle on certification 

diesel. The B20 blends demonstrated a 24% reduction in 

particulate emissions (PM) on the UDDS cycle with 

certification diesel and a 19-38% reduction with CARB 

diesel. Figure 6 shows the PM g/mile results of each test 

cycle and fuel. 

Figure 5. NOx emissions from a 1998 transit bus.
	

Figure 6. PM emissions from a 1998 transit bus.
	

certification diesel. The B20 blend with CARB diesel showed 

no increase in NOx on the MAN cycle, but a 3% to 5% 

increase on the OCTA and UDDS cycles as compared to 

CARB diesel. CARB diesel showed a 9% to 13% reduction 

in NOx as compared to certification diesel. The test cycle had 

a larger effect with the UDDS cycle producing 55% less NOx 

than the MAN cycle on certification diesel. 

Figure 7. NOx emissions from a 2005 transit bus. 

2008 Gillig / Cummins ISL NOx emissions 

2005 Gillig / Cummins ISM NOx emissions 

A 2005 model year Gillig 40-ft bus with a Cummins ISM 

280 HP engine from Denver RTD was selected to represent 

the post Heavy Duty Diesel Engine Consent Decree engine 

certification period that included EGR but not DPF units. 

However, Denver RTD, like most transits, had installed 

aftermarket DPF units on its buses that did not have active 

regeneration, and particulate emissions were below detection 

limits. Figure 7 shows the NOx g/mile results of each test 

cycle and fuel. Note that NOx emissions for this bus were 

approximately 50% lower than observed for the 1998 model 

year bus. The B20 blend with certification diesel showed no 

effect on NOx for the MAN and UDDS cycles and a 5% 

increase in NOx on the OCTA cycle as compared to 

Figure 8. NOx Emissions from a 2008 transit bus. 

A 2008 model year Gillig 40-ft bus with a Cummins ISL 

280 HP engine from Denver RTD was selected to represent 

the 2007 engine certification period that included higher EGR 

rates and active DPF units. Figure 8 below shows the NOx g/ 

mile results of each test cycle and fuel. Again, note that this 

next vintage of emission control technology represented 

another approximately 50% reduction in NOx emissions 

compared to the 2005 model year bus. The B20 blend with 

certification diesel showed statistically significant NOx 

increases of 9% to 12% on all three test cycles as compared 

to certification diesel. The B20 blend with CARB diesel 

showed 9% and 12% NOx increases on the UDDS and MAN 

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Lammert et al / SAE Int. J. Fuels Lubr. / Volume 5, Issue 3(November 2012) 

cycles, respectively, as compared to CARB diesel. Still, the 

test cycle had a larger affect with UDDS producing 37% less 

NOx than MAN on certification diesel. 

2008 Gillig / Allison hybrid / Cummins ISL NOx 

emissions 

A 2008 model year Gillig 40-ft bus with a Cummins ISL 

260 HP engine from Denver RTD was selected to represent 

the rapidly expanding hybrid bus segment of the population. 

This bus also has 2007 engine certification, including higher 

EGR rates and an active DPF unit. Because the relationship 

between certification diesel and CARB diesels had been 

sufficiently demonstrated, this bus was tested only on 

certification diesel and certification diesel B20 fuels. Figure 9 

shows the NOx g/mile results of each test cycle and fuel. The 

B20 blend with certification diesel showed no statistically 

significant effect on any of the test cycles. However, it must 

be noted that this bus demonstrated an increase of 28% to 

141% in NOx as compared to the 2008 bus with the same 

engine and a conventional drivetrain. The hybrid also 

demonstrated a stronger duty cycle effect on NOx than the 

conventional bus with the UDDS cycle producing 66% less 

NOx than the MAN cycle on certification diesel. The hybrid 

also had 11% to 18% worse fuel economy (11% on MAN, 

18% on OCTA,13% on UDDS). This result was counter to 

expectations and to other published studies of showing that 

bus hybridization can significantly improve fuel economy 

[13, 14]. It is unknown if there were calibration or parts 

failures, and the bus operator was unaware of any problems. 

No other issues were apparent during testing. 

Figure 9. NOx emissions from a 2008 hybrid transit bus. 

2010 Gillig / Cummins ISL (with SCR) NOx 

emissions 

A 2010 model year Gillig 40-ft bus with a Cummins ISL 

280 HP engine from RFTA was selected to represent the 

2010 engine certification period that includes SCR NOx 

aftertreatment in addition to DPF units. Figure 10 shows the 

NOx g/mile results of each test cycle and fuel. SCR 

aftertreatment provided the engine makers with flexibility on 

how to operate the engine and the ability to increase engineout NOx to improve fuel economy. B100 fuel was used on 

the MAN cycle to check the effectiveness of eliminating even 

the NOx increase from straight biodiesel. The SCR 

aftertreatment system was so effective that no statistically 

significant differences were identified among any of the fuels 

on the MAN and OCTA cycles. Interestingly CARB diesel 

demonstrated a statistically significant increase in NOx on the 

UDDS cycle compared to certification diesel, and a B20 

blend showed a statistically significant decrease in NOx 

compared to CARB diesel. While statistically significant, the 

value of these differences were still very small. NOx 

emissions were on the order of 90% lower than observed for 

the 2008 model year (non-hybrid) bus. For much of the cycle 

NOx would be at or near the detection limit of the laboratory 

equipment, which resulted in a 95% confidence interval error 

that was high relative to the value of the cycle emissions. 

Figure 10. NOx emissions from a 2010 transit bus. 

2011 Gillig / Cummins ISL (with SCR) NOx 

emissions 

A late 2011 model year Gillig 30-ft bus with a Cummins 

ISL 280 HP engine from RFTA was selected to verify the 

results of the 2010 bus and to investigate the warmup or 

adaptation noticed during those tests. Figure 11 shows the 

NOx g/mile results of each test cycle and fuel. Only the MAN 

and UDDS cycles were tested to bracket the duty cycle 

spectrum, and B100 fuel was used on both cycles to 

investigate any prolonged adaptation by the system to 

eliminate even the NOx increase from straight biodiesel. The 

SCR aftertreatment system was so effective that the only 

statistically significant difference identified among any of the 

fuels and cycles was a reduction in NOx of 47% for B20 on 

the MAN cycle. For much of the cycle NOx would be at or 

near the detection limit of the laboratory equipment, which 

resulted in 95% confidence interval error that was high 

relative to the value of the cycle emissions. 

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Lammert et al / SAE Int. J. Fuels Lubr. / Volume 5, Issue 3(November 2012) 

Figure 11. NOx emissions from a 2011 transit bus. 

SCR Warmup / adaptation behavior 

On both the 2010 and 2011 (SCR equipped) buses, 

achieving a repeatable g/mile result was complicated by 

apparent aftertreatment warmup and/or ammonia storage 

behavior. It is known that copper/zeolite SCR catalysts have 

the ability to store significant amounts of ammonia at low 

temperatures. This improves the NOx conversion efficiency 

of the catalyst at low temperatures and simplifies urea 

injection strategies over highly transient conditions [15]. 

However, this ammonia storage can complicate catalyst 

testing because the NOx conversion can change as it stores or 

releases ammonia. Testing of the 2011 bus was adjusted 

specifically to investigate this behavior. A DPF regeneration 

was completed prior to each test campaign, causing exhaust 

temperature to reach over 500°C. This high temperature event 

purges any stored ammonia off of the SCR catalyst [15]. 

Following the DPF regeneration event, the test cycle was 

repeated at least nine times to allow for complete stabilization 

of ammonia storage. The final three runs were used to 

calculate average hot start tailpipe g/mile NOx emissions. 

This procedure was repeated for each test fuel and drive 

cycle. The bus was also instrumented with extra sensors: SCR 

temperature, pre and post SCR raw exhaust NOx emissions in 

addition to the dilute tailpipe NOx. Figures 12 and 13 show 

the NOx g/mile and SCR NOx conversion based on the raw 

exhaust results of each test cycle and fuel. 

The MAN cycle showed a continued reduction of g/mi 

NOx at least until the fourth hot start run on B100 and well 

beyond that for certification diesel and certification diesel 

B20. As seen in Figure 12, this steady decline in tailpipe NOx 

emissions is accompanied by an increase in NOx conversion. 

This result is likely caused by the catalyst storing increasing 

amounts of ammonia under the low exhaust temperature 

conditions created by the MAN cycle. Average catalyst 

temperatures for this cycle were 249°C for certification 

diesel, and peak temperatures did not exceed 284°C. For the 

catalyst tested by Cavataio and coworkers [15], NOx 

conversion at 250°C more than doubled the amount of stored 

ammonia from 0.1 g/L to 0.5 g/L. The UDDS cycle (Figure 

13), however, showed relatively stable results on certification 

diesel and certification diesel B20 from the first hot run on 

until the end of the test campaign. This could be because the 

SCR catalyst temperatures are much higher with peak 

temperatures reaching 340°C to 370°C during the high-speed 

portion of the cycle. At these high temperatures, stored 

ammonia is much less of a factor in achieving high NOx 

conversion [15], so steady-state conversion is achieved much 

more rapidly. In addition, at 350°C ammonia storage should 

be about 50% lower than at 250°C [15]. On both cycles, the 

results for B100 proved less consistent than the other fuels, 

but the SCR system still kept emissions under 2 g/mile after 

the warm-up run. 

Figure 12. NOx emissions from a 2011 transit bus.
	

Figure 13. NOx emissions from a 2011 transit bus. 

Discussion 

These results show three consistent trends. First, the 

emissions certification level of a transit bus engine is the 

largest determining factor for NOx emissions. Each 

certification level reduced NOx emissions roughly in half 

from the previous certification level, and 2010 SCR NOx 

aftertreatment effectively reduced NOx to near detection 

levels at many engine operating modes. The exception is the 

2008 hybrid bus, which had emissions similar to a pre-2007 

bus, a possible effect of hybridization that has been observed 

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before [8]. Figure 14 shows the MAN cycle NOx emissions 

from each bus. 

Figure 14. NOx emissions from transit buses on the 

Manhattan cycle. 

Second, the kinetic intensity of a drive cycle determines 

the NOx emissions of any individual bus. All buses without 

NOx aftertreatment demonstrated higher NOx emissions on 

more intense stop and go, slow average speed, high stops per 

mile, and high kinetic intensity test cycles. NOx emissions on 

the Manhattan cycle were roughly twice that on the UDDS 

cycle for most buses without SCR - roughly the same change 

in output as an emission certification level. Figure 15 shows 

the NOx emissions of each test cycle of each bus by the 

kinetic intensity of the cycles. The engine certification level 

and duty cycle effects on NOx emissions are clearly evident. 

In every case, without NOx aftertreatment the duty cycle 

effect is clearly larger than the fuels effect, and most of the 

fuels effect is from certification diesel vs. CARB, not 

biodiesel content. SCR NOx aftertreatment appears to negate 

the effect of duty cycle and fuel on NOx emissions in transit 

buses. 

Finally, the fuel used on each bus had a minor or not 

statistically significant change in NOx emissions. CARB 

diesel generally lowered NOx emissions while a B20 blend of 

either certification diesel or CARB diesel did not consistently 

show a statistically significant increase. The exception was 

the 2008 Cummins ISL, which had a statistically significant 

increase with B20 certification diesel on each cycle in the 9% 

to 12% range. The other exception was the 2011 Cummins 

ISL, which had a statistically significant 47% reduction in 

NOx emissions with B20 (a high percentage with little realworld effect given the low baseline emissions compared to 

other buses in the study). SCR NOx aftertreatment appears to 

nearly negate the effect of fuels on NOx emissions in transit 

buses. 

Figure 15. NOx emissions from transit buses vs. kinetic 

intensity. 

SUMMARY/CONCLUSIONS 

The choice by a transit agency to fuel its buses with a 

biodiesel blend has an insignificant effect on the transit's 

overall NOx emissions in comparison to the age of its bus 

fleet and the duty cycle mix of the routes it serves. Even the 

selection of which bus in the fleet serves the high intensity 

route or the lower intensity route has a far greater effect on 

NOx than the fuel used to power the buses. Transit bus fleets 

could reduce their overall NOx inventory impact by carefully 

matching buses to drive cycles with newer buses placed on 

the highest kinetic intensity routes. And, while hybrids are 

usually shown to save fuel, hybrids without NOx 

aftertreatment may have an unintended adverse effect on 

NOx production. 

• Engine emissions certification level has the largest impact 

on transit bus NOx emissions. 

• Route duty cycle has a very strong influence on transit NOx 

emissions, but is largely outside the control of a transit 

agency with the exception of assigning newer buses to the 

most kinetically intense routes. 

• For buses without NOx emissions control: 

◦ There is little or no effect of B20 on NOx for ultralow 

sulfur diesel. 

◦ There is a statistically significant, but relatively small 

effect of B20 on NOx for CARB. 

◦ CARB fuel consistently lowered NOx emissions on the 

OCTA and UDDS cycles. 

• NOx emissions control appears to eliminate any fuel or duty 

cycle effect on NOx, even for B100. 

◦ Result validated by testing a second SCR-equipped bus 

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• Parallel hybrid drivetrain configuration can increase NOx 

emissions as compared to a conventional drivetrain bus of the 

same engine model and certification year. 

REFERENCES 

1.		 http://www.biodiesel.org/production/production-statistics. Accessed 

April 7, 2012.

2.		 Graboski, M., and McCormick, R., “Combustion of Fat and Vegetable

Oil Derived Fuels in Diesel Engines.” Prog. Energy Combust. Sci.; Vol. 

24, 1998; pp. 125-164.

3.		 U.S. Environmental Protection Agency, A Comprehensive Analysis of 

Biodiesel Impacts on Exhaust Emissions, Draft Technical Report,

EPA420-P-02-001 (2002).

4.		 McCormick, R., Alleman, T., Graboski, M., Herring, A., and Tyson, K.,

“Impact Of Biodiesel Source Material and Chemical Structure on 

Emissions of Criteria Pollutants from a Heavy-Duty Engine,”

Environmental Science & Technology 35 1742-1747 (2001).

5.		 McCormick, R., Ross, J., and Graboski, M., “Effect of Several 

Oxygenates on Regulated Emissions from Heavy-Duty Diesel Engines,”

Environmental Science & Technology 31, 1144-1150 (1997).

6.		 Mueller, C., Boehman, A., and Martin, G., “An Experimental

Investigation of the Origin of Increased NOx Emissions When Fueling a

Heavy-Duty Compression-Ignition Engine with Soy Biodiesel,” SAE 

Int. J. Fuels Lubr. 2(1):789-816, 2009, doi:10.4271/2009-01-1792. 

7.		 Eckerle, W., Lyford-Pike, E., Stanton, D., LaPointe, L. et al., “Effects of

Methyl Ester Biodiesel Blends on NOx Emissions,” SAE Int. J. Fuels 

Lubr. 1(1):102-118, 2009, doi:10.4271/2008-01-0078. 

8.		 Sandoval, J.A., Wayne, W.S., Posada, F., Schiavone, J., Pigman, E.,

Bluestone, M., Rudd, R., Stanton, H.J., Pereira, R., and D'Amore, J., 

“Emissions Reduction in Transit Buses: Westchester County's Proactive

Approach,” Journal of the Transportation Research Forum, Vol. 47, 

No. 3 (Public Transit Special Issue 2008), pp. 127-148 http://

www.trforum.org/journal

9.		 American Public Transportation Association, 2009 Public 

Transportation Vehicle Database, American Publication Transportation

Association: Washington, DC, June (2009).

10.		McCormick, R., Alvarez, J., Graboski, M., Tyson, K. et al., “Fuel

Additive and Blending Approaches to Reducing NOx Emissions from

Biodiesel,” SAE Technical Paper 2002-01-1658, 2002, doi: 

10.4271/2002-01-1658. 

11.		 “Bus Testing; Phase-In of Brake Performance and Emissions Testing,

and Program Updates.” Federal Register 74:191 (October 5, 2009) p. 

51083. 

12.		O'Keefe, M., Simpson, A., Kelly, K., and Pedersen, D., “Duty Cycle

Characterization and Evaluation Towards Heavy Hybrid Vehicle 

Applications,” SAE Technical Paper 2007-01-0302, 2007, doi: 

10.4271/2007-01-0302. 

13.		King County Metro Hybrid Electric Buses; http://www.nrel.gov/

vehiclesandfuels/fleettest/research_hybrid_king.html

14.		 “In-Use Performance Comparison of Hybrid Electric, CNG, and Diesel

Buses at New York City Transit” http://www.nrel.gov/vehiclesandfuels/

fleettest/pdfs/42534.pdf

15.		Cavataio, G., Girard, J., Patterson, J., Montreuil, C. et al., “Laboratory

Testing of Urea-SCR Formulations to Meet Tier 2 Bin 5 Emissions,”

SAE Technical Paper 2007-01-1575, 2007, doi:10.4271/2007-01-1575. 

CONTACT INFORMATION 

michael.lammert@nrel.gov 

1617 Cole Blvd. 

Golden, CO 80401 

(303) 275-4067 

robert.mccormick@nrel.gov 

1617 Cole Blvd. 

Golden, CO 80401 

(303) 275-4432 

petr.sindler@nrel.gov 

1617 Cole Blvd. 

Golden, CO 80401 

(303) 275-3142 

aaron.williams@nrel.gov 

1617 Cole Blvd. 

Golden, CO 80401 

(303) 275-3153 

ACKNOWLEDGMENTS 

This work was supported by the U.S. Department of 

Energy (DOE) Vehicle Technologies Program/Clean Cities 

Program and would not have been possible without the 

generous donation of time and buses from the Denver 

Regional Transportation District and the Roaring Fork 

Transportation Authority. The authors wish to thank Dennis 

Smith at DOE, Lou Ha at Denver RTD, and Dan Blankenship 

and Kenny Osier at RFTA for their support. 

DEFINITIONS/ABBREVIATIONS 

B100 - 100% biofuel 

B20 - blend of 20% biodiesel and 80% petroleum-based 

diesel 

CARB - California Air Resources Board 

DPF - diesel particulate filter 

EGR - exhaust gas recirculation 

MAN - Manhattan Bus cycle 

MFR - manufacturer 

NOx - oxides of nitrogen 

OCTA - Orange County Transit Authority cycle 

PM - particulate matter 

RFTA - Roaring Fork Transportation Authority 

RTD - Regional Transportation District 

SCR - selective catalyst reduction 

UDDS - Urban Dynamometer Driving Schedule 

9
Property Method Units CERT CARB 

Distillation 90% ASTMD86 op 580 573 

Gravity ASTMD4052 0 API 35.4 37.7 

Specific Gravity ASTMD4052 0.848 0.8363 

Flash Point ASTMD93 op 163 162 

Cloud Point ASTMD2500 op -13 -18 

Pour point ASTMD97 op -22 NA 

Viscosity, 40°C ASTMD445 eSt 2.4 2.52 

Sulfur ASTMD5453 ppm 12 0 

Carbon ASTMD5291 wt% 86.79 NA 

Hydrogen ASTMD5291 wt% 13.21 NA 

Nitrogen ASTMD4629 ppm NA 0.6 

Composition, ASTMD5186 wt% 29.9 8.5 aromatics 

Polycyclic ASTMD5186 wt% NA <0.5 Aromatics 

Composition, ASTMD1319 vol% 29 NA aromatics 

Composition, ASTMD1319 vol% 4 NA ole fins 

Composition, ASTM D1319 vol% 67 NA 

saturates 

Cetane Number ASTM D613 42.5 48.7 

Net heat content ASTMD240 btu/lb 18435 NA 

Lammert et al / SAE Int. J. Fuels Lubr. / Volume 5, Issue 3(November 2012) 

APPENDIX 

Table A1. Diesel Fuel Properties. 

10
Property Units Soy 8100 D675llimits 

Acid value mg KOH/g 0.24-0.35 0.5 max 

Rancimat hours 7.33- 11.2 3 min 

KF water ppm 264-380 500 max 

Free Glycerin %wt 0.007 - 0.010 0.020 max 

Total Glycerin %wt 0.105-0.126 0.240 max 

CSFT seconds 117-572 360 max 

Ca ppm <0.1-0.3 5 ppm max combined Ca and Mg 

Mg ppm <0.1 5 ppm max combined Ca and Mg 

Na ppm <1- 2.3 5 ppm max combined Na and K 

K ppm <1 5 ppm max combined Na and K 

Flash Point degC 137- 182 130 min 

Flash Point degF 281-361 

Cloud Point degC -0.4--0.2 Report 

Lammert et al / SAE Int. J. Fuels Lubr. / Volume 5, Issue 3(November 2012) 

Table A2. B100 Properties. 

11
Bus Nox (g/mile) 

Fuel Cycle 1998 2005 2008 2008H 2010 2011 

Cert MAN 43.6 22.6 11.1 27.0 0.4 1.4 

Cert MAN 44.8 22.4 11.3 27.5 0.3 1.2 

Cert MAN 46.9 21.9 11.4 27.0 0.2 1.1 

Cert B20 MAN 47.0 22.4 12.5 26.5 0.5 0.8 

Cert B20 MAN 44.9 22.5 12.9 27.5 0.4 0.7 

Cert B20 MAN 46.5 22.1 12.4 27.7 0.3 0.6 

CARB MAN 44.0 19.9 11.0 0.6 

CARB MAN 40.5 18.9 10.6 0.5 

CARB MAN 41.8 19.4 11.0 0.3 

CARB B20 MAN 42.0 20.0 12.2 1.0 

CARB B20 MAN 44.0 20.0 12.1 0.7 

CARB B20 MAN 42.0 20.1 12.3 0.3 

B100 MAN 0.5 1.5 

B100 MAN 0.4 1.5 

B100 MAN 0.3 1.6 

Cert OCTA 32.0 13.3 9.0 16.3 0.0 

Cert OCTA 31.6 13.1 8.9 16.4 0.1 

Cert OCTA 31.8 13.3 8.7 15.3 0.2 

Cert B20 OCTA 32.8 14.1 9.8 16.3 0.2 

Cert B20 OCTA 32.8 13.9 9.6 16.3 0.1 

Cert B20 OCTA 31.6 13.8 9.9 16.1 0.2 

CARB OCTA 28.6 12.2 8.9 0.1 

CARB OCTA 28.0 12.1 8.9 0.1 

CARB OCTA 28.3 11.9 8.7 0.1 

CARB B20 OCTA 29.4 12.8 9.0 0.1 

CARB B20 OCTA 29.1 12.9 9.3 0.1 

CARB B20 OCTA 29.3 12.9 9.6 0.1 

Cert UDDS 22.7 9.9 7.1 9.2 0.4 0.7 

Cert UDDS 22.7 10.3 7.2 9.1 0.4 0.8 

Cert UDDS 22.1 10.2 7.1 9.2 0.3 1.0 

Cert B20 UDDS 22.4 10.8 7.8 9.1 0.5 0.5 

Cert B20 UDDS 22.5 10.4 7.7 9.9 0.4 0.8 

Cert B20 UDDS 22.7 10.1 7.8 9.2 0.4 0.7 

CARB UDDS 20.2 9.0 6.9 0.7 

CARB UDDS 20.4 9.0 6.8 0.6 

CARB UDDS 20.4 9.1 6.7 0.5 

CARB B20 UDDS 21.6 9.6 7.4 0.2 

CARB B20 UDDS 21.4 9.9 7.2 0.2 

CARB B20 UDDS 20.9 9.8 7.5 0.2 

B100 UDDS 1.3 

B100 UDDS 1.1 

B100 UDDS 0.9 

Lammert et al / SAE Int. J. Fuels Lubr. / Volume 5, Issue 3(November 2012) 

Table A3. NOx Emission Results for All Tests. 

12
P value of NOx results compared to baseline 

Fuels Compared Cycle 1998 2005 2008 2008H 2010 2011 

Cert B20vs Cert baseline MAN 0.471 0.728 0.018 0.838 0.109 0.019 

CARB B20vs CARB baseline MAN 0.772 0.161 0.004 0.173 

CARB vs Cert baseline MAN 0.221 0.009 0.153 0.103 

B100vs Cert baseline MAN 0.114 0.126 

Cert B20vs Cert baseline OCTA 0.282 0.017 0.019 0.433 0.409 

CARB B20vs CARB baseline OCTA 0.006 0.016 0.216 0.572 

CARB vs Cert baseline OCTA 0.000 0.008 0.872 0.828 

Cert B20vs Cert baseline UDDS 0.968 0.466 0.011 0.502 0.282 0.362 

CARB B20vs CARB baseline UDDS 0.059 0.020 0.035 0.021 

CARB vs Cert baseline UDDS 0.011 0.013 0.047 0.006 

B100vs Cert baseline UDDS 0.379 

Lammert et al / SAE Int. J. Fuels Lubr. / Volume 5, Issue 3(November 2012) 

Table A4. P value table for all NOx results comparisons. 

13
Bus PM (g/mile) 

Fuel Cycle 1998 2005 2008 2008H 2010 2011 

Cert MAN 6.663 0.003 0.004 0.003 0.005 0.006 

Cert MAN 6.231 0.003 0.007 0.001 0.002 0.008 

Cert MAN 5.807 0.005 0.004 0.003 0.004 0.004 

Cert B20 MAN 5.055 0.003 0.004 na 0.010 0.001 

Cert B20 MAN 4.697 0.002 0.004 0.000 0.026 0.001 

Cert B20 MAN 5.506 0.004 0.003 0.003 0.012 0.001 

CARB MAN 5.093 0.001 0.001 0.008 

CARB MAN 5.916 0.005 0.005 -0.014 

CARB MAN 5.246 0.002 0.005 0.008 

CARB B20 MAN 4.466 0.002 0.006 0.014 

CARB B20 MAN 4.138 0.005 0.003 0.001 

CARB B20 MAN 4.273 0.001 0.003 0.001 

B100 MAN na 

B100 MAN na 

B100 MAN na 

Cert OCTA 4.518 0.002 0.002 0.001 0.004 

Cert OCTA 4.250 0.001 0.002 0.000 0.003 

Cert OCTA 3.709 0.002 0.003 0.001 0.005 

Cert B20 OCTA na 0.001 0.001 0.001 0.004 

Cert B20 OCTA 2.937 0.001 0.001 0.001 0.006 

Cert B20 OCTA 3.507 0.001 0.001 0.001 0.003 

CARB OCTA 3.570 0.002 0.002 0.003 

CARB OCTA 3.915 0.000 0.001 0.004 

CARB OCTA 3.174 0.001 0.001 0.002 

CARB B20 OCTA 2.935 0.005 0.002 0.007 

CARB B20 OCTA 2.852 0.001 0.001 0.013 

CARB B20 OCTA 2.819 0.001 0.001 0.004 

Cert UDDS 5.721 0.004 0.002 0.002 0.011 0.001 

Cert UDDS 5.586 0.004 0.001 0.000 0.004 0.001 

Cert UDDS 5.767 0.006 0.002 0.001 0.002 0.001 

Cert B20 UDDS 4.413 0.005 0.001 0.001 0.002 0.001 

Cert B20 UDDS 4.516 0.002 0.001 0.002 0.007 0.000 

Cert B20 UDDS 4.113 0.002 0.000 0.001 0.005 0.001 

CARB UDDS 6.085 0.004 0.002 0.001 

CARB UDDS 6.337 0.003 0.002 0.029 

CARB UDDS 5.701 0.004 0.002 0.014 

CARB B20 UDDS 3.628 0.001 0.001 0.004 

CARB B20 UDDS na 0.001 0.001 0.006 

CARB B20 UDDS 3.834 0.001 0.001 0.004 

B100 UDDS 0.001 

B100 UDDS 0.001 

B100 UDDS 0.002 

Lammert et al / SAE Int. J. Fuels Lubr. / Volume 5, Issue 3(November 2012) 

Table A5. PM Emission Results for All Tests. 

14
Bus Fuel Economy (mpg) 

Fuel Cycle 1998 2005 2008 2008H 2010 2011 

Cert MAN 2.67 2.66 3.13 2.77 2.65 2.69 

Cert MAN 2.62 2.65 3.15 2.78 2.61 2.72 

Cert MAN 2.60 2.82 3.13 2.84 2.61 2.69 

Cert B20 MAN 2.43 2.70 2.88 2.78 2.64 2.63 

Cert B20 MAN 2.44 2.63 2.96 2.72 2.57 2.63 

Cert B20 MAN 2.46 2.65 2.88 2.68 2.63 2.70 

CARB MAN 2.43 2.67 3.20 2.66 

CARB MAN 2.42 2.62 3.18 2.69 

CARB MAN 2.47 2.67 3.26 2.68 

CARB B20 MAN 2.48 2.70 3.09 2.64 

CARB B20 MAN 2.48 2.75 3.09 2.57 

CARB B20 MAN 2.49 2.74 3.10 2.59 

B100 MAN 2.29 

B100 MAN 2.29 

B100 MAN 2.29 

Cert OCTA 3.76 3.70 4.62 3.79 3.71 

Cert OCTA 3.80 4.07 4.64 3.75 3.70 

Cert OCTA 3.83 4.05 4.55 3.79 3.70 

Cert B20 OCTA 3.77 4.13 4.31 3.70 3.70 

Cert B20 OCTA 3.82 4.08 4.19 3.67 3.71 

Cert B20 OCTA 3.78 4.09 4.38 3.68 3.71 

CARB OCTA 3.82 3.95 4.63 3.74 

CARB OCTA 3.80 3.95 4.64 3.74 

CARB OCTA 3.81 3.96 4.66 3.72 

CARB B20 OCTA 3.84 4.00 4.54 3.27 

CARB B20 OCTA 3.83 3.97 4.22 3.30 

CARB B20 OCTA 3.86 3.97 4.53 3.29 

Cert UDDS na 5.24 5.78 4.99 4.67 5.03 

Cert UDDS 5.89 5.25 5.67 4.98 4.73 5.01 

Cert UDDS 5.20 5.25 5.64 4.95 4.72 4.71 

Cert B20 UDDS 5.47 4.98 5.76 4.73 4.78 4.88 

Cert B20 UDDS 5.47 5.23 5.74 4.97 4.80 4.87 

Cert B20 UDDS 5.47 5.15 5.75 4.87 4.84 4.89 

CARB UDDS 5.41 5.29 5.97 6.53 

CARB UDDS 5.41 5.30 5.95 5.03 

CARB UDDS 5.43 5.30 5.81 4.97 

CARB B20 UDDS 5.54 5.22 5.65 4.78 

CARB B20 UDDS 5.50 5.06 5.89 4.80 

CARB B20 UDDS 5.47 5.00 5.80 4.79 

B100 UDDS 4.54 

B100 UDDS 4.58 

B100 UDDS 4.55 

Lammert et al / SAE Int. J. Fuels Lubr. / Volume 5, Issue 3(November 2012) 

Table A6. Fuel Economy Results for All Tests. 

15