meta-analysis of transit bus exhaust emissionsdocs.trb.org/prp/13-4618.pdfmeta-analysis of transit...

Meta-analysis of Transit Bus Exhaust Emissions

Corresponding author:

Erin Cooper, Research Analyst

EMBARQ/WRI

10 G. St. NE

Washington, DC 20002

202.729.7730

[email protected]

Magdala Arioli, Transport Engineer

EMBARQ Brasil/ Universidade Federal do Rio Grande do Sul

R. Luciana de Abreu, 471/801

90570-060 Porto Alegre, RS, Brazil

+55 51 3312-6324

[email protected]

Aileen Carrigan, Senior Transport Planner

EMBARQ/WRI

10 G. St. NE

Washington, DC 20002

202.729. 7896

[email protected]

Umang Jain, Transport Specialist

EMBARQ India

Godrej and Boyce Premises

Gaswork Lane

Lalbaug Parel Mumbai 400012 India

+91 22 24713565

[email protected]

Total Word Count: 7433

Table Count: 6

Words in Text: 5933

Submission Date: 11/14/2012

TRB 2013 Annual Meeting Paper revised from original submittal.

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Abstract 2

Though many specific studies of bus emissions exist, this paper addresses the need for a comparative 3

analysis of emissions associated with a variety of fuel types, specifically for developing countries. This 4

paper compiles a large data set of in-use transit bus tests for commonly regulated transportation emissions 5

including carbon monoxide, hydro-carbons, nitrogen oxides, and particulate matter. Carbon dioxide is 6

also included to understand greenhouse gas emissions. A meta-analysis technique was used with 25 7

studies to find a range of emissions values for different fuel and exhaust after treatment combinations to 8

determine which combinations provide the greatest emissions reduction. The fuels considered for this 9

report are diesel with various concentrations of sulfur, biodiesel (100 percent and 20 percent blend with 10

diesel), compressed natural gas, liquefied natural gas, and ethanol. The technologies considered are 11

standard internal combustion engines, hybrid ICE-electric and a variety of exhaust after-treatment 12

technologies. The analysis shows that no single fuel is best at reducing all emissions if the appropriate 13

exhaust after-treatment technologies are used. The technologies which show the lowest emissions in 14

important categories, NOx, PM, and CO2 equivalents, are compressed natural gas with a three-way 15

catalyst, 100 percent biodiesel, and ultra-low sulfur diesel with selective catalyst reduction. Other factors 16

explored, such as altitude, drive cycle, and mileage, also have an impact on emissions values. Overall, 17

there is a wide range of emissions values even for the same fuel and technology. These variations and 18

factors should be understood in order to accurately evaluate results from further emissions testing. 19

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1 Cooper, Arioli, Carrigan, Jain

INTRODUCTION 1

Even with the abundance of information available in recent decades regarding alternative fuels 2

and vehicles, it is often unclear which fuel and vehicle types a transit agency should choose for their bus 3

fleet. The existing research on fuels and vehicles provides in depth information on exhaust emissions for 4

specific fuels and technologies in specific locations. Many major transit agencies world-wide, especially 5

in the US and Europe, have also done extensive fuel and vehicle testing and cost-benefit analyses for local 6

and national programs. However, because results of individual bus emissions tests can vary greatly, a 7

small sample size of bus tests, which are typically performed by agencies, may not be representative of 8

the fuel type generally. In addition, each approach to analysis, as well as the fuels and technologies 9

considered in the tests, can vary significantly. Thus, the results may not be easily comparable to other 10

agencies’ studies or applicable in other locations. 11

This research aims to create a better understanding of the exhaust emissions impacts of fuel and 12

exhaust after treatment technology combinations, or to make the results of transit bus testing more easily 13

comparable and applicable in different locations. The report also focused on data that is relevant to Brazil, 14

India, and Mexico, as part of a larger transit fleet research program, by compiling data and research from 15

a variety of transit and government agencies from different countries and considering fuels and 16

technologies that are relevant to these countries. Data was collected from testing performed on in-use or 17

previously used transit buses to create a large database of emissions testing results from which agencies 18

can make choices. Because this relies on available data from in-use buses, it does not represent all of the 19

newest bus technologies available in the US or Europe and alone should not be used as a final guide to 20

making fleet decisions. However, comparing a variety of fuels, as well as having a broad range of sources 21

and locations represented, makes the results of the analysis more relevant for agencies which have not 22

performed their own emissions testing and more representative of average emissions values for fuel types. 23

This paper is part of a broader Sustainable Fuels and Vehicles program which will subsequently 24

address the important issues of lifecycle costs and lifecycle emissions not included in this paper. However 25

for context, exhaust emissions comprise varying percentages of the lifecycle GHG emissions depending 26

on the fuel type; these emissions can also increase or decrease with respect to diesel emissions based on 27

fuel type and fuel pathway. The lifecycle emissions and issues related to fuel production will significantly 28

impact final fuel choice recommendations. 29

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BACKGROUND 32

There are many possible fuels and emissions-reducing technology combinations which have been 33

tested and are available for use. However, not all of these combinations are relevant for the next decade in 34

all countries and at all transit agencies. The fuels addressed in this report are all available, or soon to be 35

available, in Brazil, India, or Mexico. Brazil has a wide variety of fuels available, including diesel, 36

compressed natural gas (CNG), ethanol and biodiesel. Cities in Mexico use diesel, hybrids and CNG. In 37

India, as a result of a Supreme Court order, thirteen major cities must use CNG vehicles, while diesel fuel 38

is still available for buses in other cities (1). In selecting bus fleets, agencies must balance many factors 39

including fuel and vehicle availability, local conditions, service needs, as well as meeting emissions 40

standards. 41

Alternative fuels provide more options for meeting requirements, especially emissions standards. 42

Emissions standards can have a direct impact on public health through reducing air pollution, which is a 43

major environmental and health problem affecting people worldwide. Exposure to air pollutants is often 44



beyond the control of individuals and requires action by public authorities at the national, regional and 1

international levels. According to the World Health Organization (WHO), more than two million 2

premature deaths each year can be attributed to the effects of urban outdoor air pollution, at least partly 3

caused by fuel combustion (2). WHO shows that there are significant health impacts related to nitrogen 4

oxides and sulfur dioxides, while there are specific quantifiable mortality impacts related to particulate 5

matter and ozone. There is roughly a six percent increase in mortality for each 10 µg/m3 increase in PM 6

and a three to five percent increase in mortality for each 60 µg/m3 increase in ozone (2). The health and 7

environmental impacts of some regulated and unregulated emissions are detailed further in publications 8

from the WHO and EPA (2; 3). 9

To reduce harmful emissions, many national governments use emissions standards and testing to 10

control the amount and types of emissions that are released into the environment as a direct result of fuel 11

combustion. The exhaust emissions considered in this report are based both on the European Emissions 12

Standards’ (EURO) and EPA standards’ regulated emissions: nitrogen oxides (NOx), total hydrocarbons 13

(THC) or non-methane hydrocarbons (NMHC), particulate matter (PM), and carbon monoxide (CO). 14

Required emissions tests are performed using specific drive cycles on a heavy-duty chassis dynamometer. 15

Emissions tests performed for research purposes, some of which are used as data in this report, may use 16

different drive cycles or take place in the field. 17

This paper also looks at GHG emissions, or CO2 equivalent emissions (CO2e) for which 18

regulation is more recent in the US and Europe. In the United States, the EPA and the National Highway 19

Traffic Safety Administration (NHTSA) are developing the first GHG regulations for heavy-duty engines 20

and vehicles. The proposed standards are expected to save more than six billion barrels of oil through 21

2025 and reduce more than 3,100 million metric tons of CO2 emissions (3). Since there is currently no 22

after-treatment technology that can reduce CO2 emissions from road vehicles, CO2 reductions are 23

achieved through fuel efficiency improvements (4). 24

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Emissions related to fuels and technologies 26

In order to meet emissions standards, exhaust after treatment technologies for different fuels have 27

been developed. The existing literature on fuel types and emissions characteristics of fuels shows the 28

generally expected emissions reductions from each fuel and technology. Diesel emissions are affected by 29

the amount of sulfur in the diesel as well as the emissions reduction technologies. CO and CO2 emissions 30

are generally low for diesel engines. THC emissions from diesel are generally non-methane, and less of a 31

concern for global warming. The major concerns for diesel fuel are NOx and PM emissions (5). 32

Developing countries commonly have sulfur content levels above 500 parts per million; sulfur levels 33

below this value allow for the use oxidation catalysts (6). At present, the US and Europe have ultra-low 34

sulfur diesel available (10-15 ppm), while low sulfur diesel (50 ppm) is available in some locations in 35

Mexico, India, and Brazil. Sulfur in fuel contributes to formation of particulates and reduces the 36

effectiveness of emissions reduction technologies. 37

Diesel emission technologies, such as diesel particulate filter (DPF), tend to have a greater effect 38

on reducing large particle emissions (5). They can only be used with less than 50 ppm of sulfur (6). A 39

diesel oxidation catalyst (DOC) reduces PM, HC, and CO emissions and can only be used below 500 ppm 40

sulfur content in diesel (7; 6). Exhaust gas recirculation (EGR) recirculates exhaust (mainly containing 41

inert nitrogen, CO2, and water vapor) into the engine cylinders, which cools the engine, thereby reducing 42

NOx emissions and possibly PM (8). Selective catalyst reduction (SCR) combines urea and water to 43



produce ammonia and CO2 which then combines with NOx to produce nitrogen and water (8). SCR can 1

reduce NOx emissions by 75 to 90 percent (9). 2

A hybrid-electric vehicle can draws energy from two sources of stored energy: a consumable fuel 3

and a rechargeable energy storage system (11). Combustion emissions associated with a hybrid are the 4

same as the emissions associated with internal combustion engines. There is a reduction in emissions due 5

to hybrid systems achieving lower fuel consumption. These benefits are possible through regenerative 6

braking and reductions in engine transient operation through power management systems (12). 7

The composition of biodiesel is very similar to the composition of diesel and therefore can be 8

used in diesel engines with minor modifications. The difference between emissions for diesel and 9

biodiesel depends on the percent of the blend, or the portion that diesel versus biodiesel. Biodiesel is 10

naturally lower in sulfur than diesel, which can also reduce PM emissions (7). For 20 percent blends 11

(B20), NREL (10) shows biodiesel can reduce NOx emissions between 3 to 6 percent and reduce PM 12

emissions between 15 and 20 percent, though data from various studies show more uncertainty in the 13

magnitude and direction of change for these emissions. It can also have reductions in HC, non-methane 14

HC, and CO. However, EPA 2010 emission standards will make these differences between diesel and 15

biodiesel almost insignificant, because diesel and B20 are very similar, many newer bus models can run 16

on both diesel and B20 (9). As B20 use the same bus model, they have many of the same emissions 17

reduction technologies as diesel buses. 18

In general, ethanol buses have PM emissions similar to diesel with DPF. Ethanol has lower NOx 19

emissions compared to diesel, but higher amounts of HC and CO than diesel. Newer models developed to 20

meet stronger emissions standards can also have low HC and CO values (13). 21

CNG emissions are mainly in the form of methane (CH4) and NOx. Compared to diesel, the PM 22

and NOx emissions are lower, though the amount of reduction varies by bus (14). Though particulate 23

emissions are low, the particles are still harmful to health, and with higher loads on buses, the amount of 24

PM can increase to levels comparable to diesel (5; 15). CNG also emits higher quantities of 25

formaldehydes, even with oxidation catalysts. Liquefied Natural Gas is cooled natural gas that has a 26

higher energy content than compressed natural gas. CNG and LNG vehicles use the same engines and 27

therefore meet the same emissions standards and use the same emissions reduction technologies (9). 28

The technologies associated with the CNG are oxidation catalysts (OC) and three-way catalysts 29

(3WC). OCs perform oxidation of both CO and HC which results in the production of CO2. Oxidation 30

catalysts can reduce HC, CO and CH4 emissions (5; 7; 16). Three-way catalysts perform oxidation of both 31

CO and HC and reduction of NOx. This results in the production of CO2, nitrogen, and water (16). 32

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Additional factors that impact emissions 34

Research also revealed that there are many factors which can contribute to increases in emissions. 35

These concepts are tested in analysis of the dataset. In general, it is known that emissions vary based on 36

drive cycles. More aggressive, or urban, drive cycles result in higher emissions. Emissions can also vary 37

based on mileage of the bus, as wear is expected as engines or other components age. 38

Emissions impacts of altitude are important as Mexico City and other Mexican cities are at high 39

altitude (more than 2000 meters above the sea level). This is less of a concern in Brazil and India, where 40

most major cities are at low altitude (below 500 meters above the sea level). Himalayan India is also at 41

high altitude, though with few urban areas. Studies show that there are higher emissions of HC, CO, and 42

PM (17) at higher altitudes. The relationship between altitude and emissions is poorly quantified and 43

though there can be an increase in emissions at high altitude, they may be lower than the often predicted 44



values for emissions of HC, CO, and PM for buses at high altitude (18). Also, altitude does not appear to 1

have an effect on NOx emissions. These altitude studies do not take into account newer bus emissions 2

standards and technologies. These factors, as well as the available data on fuels and technologies, were 3

taken into account in order to develop the analysis methodology. 4

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METHODOLOGY 7

A meta-analysis method was chosen to reflect the range of possible emissions for each pollutant, 8

as there is significant variation between emissions for buses of the same fuel type, technology, and model 9

for all fuel categories (19; 15). A large data set with 368 entries was compiled in order to provide the 10

most representative values possible for each fuel and technology combination. This analysis looks at 11

combustion emissions which result from a combination of fuels and technologies. The data set includes 12

many different lab and field tests conducted in different locations. The results of each study are not 13

directly comparable due to various testing conditions which are not controlled for, such as the age of the 14

bus, specific terrain, or drive cycle. However the meta-analysis method allows for generalized results 15

from a variety of buses tested in a variety of conditions and the normalization of data through increasing 16

the sample size for each fuel and technology in the study (20). 17

18

Data Collection 19

Combustion emissions data was collected from a total of 25 sources including reports by cities 20

which conducted emissions testing, government laboratories, institutes with bus testing facilities, or 21

similar reports in peer-reviewed journals. Reports include field or lab tests for generally 12 m transit 22

buses. Stand-alone engine tests were not included. An initial effort was made to find data on as many 23

types of fuels as may be applicable to Mexico, India, or Brazil. However, only fuels which were currently 24

relevant to these locations were maintained in the final dataset. The studies also reflect data from tests 25

performed within the last decade, except for fuels where recent testing data was unavailable. 26

The dataset includes countries where the test was completed, the year the study was completed 27

(or published if the study year was not available), fuels, and emissions standards. A large portion of the 28

studies were done between 2002 and 2006. This is not representative of newer technologies, but can 29

account for the lag time in uptake of new technologies in developing countries. There was also limited 30

data on EPA certified buses because certification years were not available in the reports. The majority of 31

the data represents the US, Europe, and Canada, due to limited availability of testing facilities and 32

therefore testing data, though data from China, India, and Mexico are included. 33

As was available from the reports, data was collected on drive cycle, mileage, location of test (for 34

altitude), field test or lab test, Euro or EPA standard of vehicle, model, and motor type. Bus technologies 35

were also identified to the extent possible including presence of particulate filters, catalysts, or exhaust 36

gas recirculation. Emissions data was collected on CO2, CO, NOx, total HC, CH4, non-methane HC, PM, 37

acetaldehydes, and formaldehydes. All units of emissions were converted to grams per kilometer. 38

Data was also collected on fuel consumption and fuel quality. Due to a variety of definitions of 39

ultra-low sulfur diesel, low-sulfur diesel, and conventional diesel, the sulfur content of the fuel (parts per 40

million ) was recorded from the reports. Where this data was not available, estimations were made based 41

on the study year and fuel standards by country or by agency at the time. The fuels were then re-42

categorized based on which common sulfur ppm content values (15, 50, 150+) most closely matched the 43



sulfur content of the fuel. There are also many qualities of CNG, but specific standards have not been 1

developed. In general, biodiesel has many different pathways. Because of the available reports, the 2

biodiesel represented here is hydrotreated renewable NExBTL diesel (from vegetable oil or animal fat) 3

and rapeseed and soybean methyl esters. 4

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Data Analysis Approach 7

Because emissions vary widely, this analysis looks at a range of emissions values as well as 8

average emissions values through two methods: an Interquartile Range (IQR) and a confidence interval of 9

the mean. The IQR provides a likely range of emissions values for given fuel types and represents the 10

middle 50 percent of data points bounded by the upper and lower quartiles (75th and 25

th percentiles) (20). 11

In developing a confidence interval, it is assumed that the data represent a normal distribution (21), then 12

average values are determined and standard deviations to find the range that includes, in many cases, 95 13

percent of the emissions values for each type of fuel. Using these two techniques, the emissions are 14

compared based on the following criteria, some tests have further explanation: 15

Specific fuel type and relevant technologies: compares emissions from different fuel quality, fuel 16

types, and technologies. 17

Euro standard: shows if buses are meeting required standards 18

Mileage: compares emissions to kilometers on bus odometer 19

Altitude: compares emissions to altitude of test 20

Field vs. lab tests: compares tests performed in lab versus field routes. Most of the studies in this 21

data set present emissions results from lab tests. For these tests the vehicle is driven onto the chassis 22

dynamometer. The bus then follows a specific drive cycle while emissions data is collected. Field tests 23

involve collecting emissions data while a bus is being driven on a designated route in a city. It does not 24

follow a standard drive cycle and collects data from buses operating under normal conditions. For both 25

types of tests, different loads are often tested, which does have an effect on emissions, but is not 26

addressed in this report. 27

Drive cycles: compares steady-state, urban, and urban/suburban cycles. Drive cycles may 28

represent urban environments only, meaning there are many stops and starts, and often large variations in 29

speed. Suburban cycles have fewer stops and starts and buses are capable of achieving higher operational 30

speeds. Steady-state cycles ramp up to a speed and stay for a given period of time and may repeat the 31

process at different speeds. Because there is a large variety of drive cycles, the cycles were grouped into 32

the environments they represent: urban cycles, urban to suburban, and steady state. 33

CO2equivalent: includes carbon dioxide equivalent for many pollutants to demonstrate effect on 34

global warming. CO2 equivalent combines the amount of a pollutant with its 100-year global warming 35

potential. CO2 equivalent was calculated based on CO2, CH4, and N2O exhaust. The difference in 36

calculation between natural gas fuels and other fuels stems from the fact that the total hydrocarbon (THC) 37

value for natural gas fuels contains approximately 90 percent methane. 38

Comparing NOx, CO2 equivalent, and PM emissions: this comparison consists of plotting NOx 39

and CO2e versus PM to show the fuel and technologies that perform best among these pollutants 40

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DATA ANALYSIS 1

The analysis looks at fuel and technology combinations as well as the additional factors. The 2

graphs presented here show some of the main findings of the research, while further graphs are available 3

in the detailed research report. First the analysis looked at individual emissions (CO, PM, NOx, THC and 4

CO2e) according to the technology and fuel. Figure 1 shows results from the confidence interval analysis 5

of fuel and technology combinations, specifically for regulated emissions. Euro standards limits are 6

shown as a references, however this data represents many different Euro standards. 7

For carbon monoxide, the lowest CO emissions are from B100 and D15 with EGR, D15 with OC, 8

DPF, or Hybrid, and CNG with OC. This is reasonable considering that oxidation catalysts and diesel 9

particulate filters are meant to reduce CO emissions. Both of the SCR technologies shown have higher 10

CO emissions than similar fuels without SCR. The highest CO emissions are from fuels without 11

emissions reduction technologies: Ethanol, LNG and CNG. 12

Regarding total hydrocarbons, due to the composition of diesel, it has very low THC emissions. 13

This is reflected in Figure 1. THC is important for CNG, LNG, and Ethanol. An oxidation catalyst 14

reduces CNG emissions by close to 50 percent while a 3WC reduces emission by close to 100 percent. 15

With a 3WC, the THC emissions from a CNG vehicle are comparable to THC emissions from diesel and 16

biodiesel. 17

CNG with a 3WC has the lowest NOx emissions, followed by B100 with EGR and SCR, and D15 18

with EGR and SCR. This confirms an expected result as 3WC, EGR and SCR are all meant to reduce 19

NOx. The NOx value for E93 is also comparable to D15 with EGR. Figure 1 also shows that oxidations 20

catalysts are also effective at reducing NOx while DPFs have little effect or increase NOx. The highest 21

NOx emitters are D >150, CNG, and E95 without technologies. 22

CNG and LNG are naturally low in particulate emissions. For diesel fuels, the data shows that 23

there is a significant reduction in PM as a result of all technologies, especially DPFs. However, other 24

fuels will still have lower quantities of PM. B20 has a 50 percent reduction in PM compared to D15, and 25

CNG with 3WC is 25 percent lower than D15 with DPF. 26

Figure 2 shows the results for the confidence interval analysis of fuel and technology 27

combinations for CO2 emissions. The mean shows that there is a wide range of CO2 emissions. It also 28

shows that technologies used to reduce local pollution have varying effects on CO2 emissions. The CO2 29

equivalent also shows that technologies may increase overall GHG emissions, and that emissions 30

standards do not regulate GHG emissions. 31

The summary provided in Figure 3 shows that generally emissions reduction technologies are 32

very effective for reducing CO, THC, and PM. The technologies are less effective at reducing NOx and 33

CO2 emissions. Only DPF and hybrids increase emissions in the categories of NOx and THC respectively. 34

Additional Factors 35

Though graphs are not presented here, the analysis of additional factors gave the following results 36

in each category. 37

Euro Standards 38

Buses tested are shown to meet the emissions standards for THC. However, they are not meeting 39

the Euro standard for CO emissions in all cases. The median emission rate for Euro III and IV-rated 40

vehicles meet the standard, but some values are higher than the standard. In general, buses are not 41

meeting NOx emissions standards, and not all buses are meeting emissions standards for PM. 42



FIGURE 1 Mean for Regulated emissions by technology (in g/km) 1

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FIGURE 2 Mean for CO2 emissions by technology (g/km) 1

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FIGURE 3 Summary of emissions results for USLD plus technologies with respect to ULSD alone. 5

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Field versus Lab tests 9

CNG and diesel are the only fuels with significant numbers of lab and field tests to compare. The 10

field tests tend to show larger ranges of emissions than the lab tests, and the median value for NOx and 11

CO2 lab tests are clearly lower than field tests. The varied results in field and lab tests are important to 12

understand when comparing future tests results. Comparing one field test to one lab test may show 13

skewed results. 14



Drive Cycles 1

For all emissions, the urban cycles show a wider range and higher emissions values. Steady state 2

cycles and urban to suburban cycles generally show lower emissions by at least 30 percent and 20 percent 3

respectively. There is not a clear trend by individual fuels or technology. When comparing future test 4

data, this respective difference in emissions by drive cycle should be taken into account. 5

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Mileage 7

Though all emissions were plotted versus mileage, only mileage versus NOx emissions shows that 8

kilometers traveled is a good predictor of increased NOx emissions. Plotting CO2 equivalent versus bus 9

kilometers traveled also shows some correlation, though the relationship is not as strong as with NOx 10

emissions. 11

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Altitude 13

The analysis shows a correlation between CO, THC, PM and altitude, though the analysis lacks 14

sufficient data at higher altitudes to show a strong correlation. The range of expected values varies for 15

each fuel type. CO shows an increase for diesel and hybrids of approximately 2 g/km per 1500 meter of 16

altitude increase. The range of CO values for diesel and hybrid is roughly 15 g/km, therefore an increase 17

in CO by 2 g/km (Figure 4) would be a 10 percent increase over 1500 m altitude increase. For THC, only 18

CNG showed an increase correlated with altitude. A similar analysis to CO shows that a 1500 m increase 19

in altitude would result in approximately a 10 percent increase in THC. Considering biodiesel and diesel 20

for PM, there is roughly a 10 percent increase also with a 1500 m increase. Figure 4 shows the relation 21

between altitude and emissions for CO, THC and PM . 22

FIGURE 4 CO, THC, and PM emissions versus altitude 23

24 25

Comparing NOx, CO2 equivalent, and PM emissions 26

Both NOx and PM are considered some of the most harmful local pollutants, while CO2 27

equivalent is important for global warming. Plotting NOx and CO2e versus PM shows the fuel and 28

technologies that perform best among these pollutants. Figure 5 shows the means and the 95 percent 29

confidence intervals for each better performing combinations of fuels and technologies. This shows that 30

CNG + 3WC is the best in terms of NOx and in some cases PM. Figure 6 look at CO2 equivalent, and 31

show that B100 + SCR is generally the best fuel comparing CO2e and PM. The figures show however, 32

that the range of possible results does not make one fuel and technology combination always better than 33

others. Some of the overall best benefits come from CNG + 3WC, B100 +SCR, D15 + SCR, and B100 34

+EGR. 35



FIGURE 5 NOx versus PM, confidence interval ranges for selected fuels 1

2 3

FIGURE 6 CO2e vs. PM, confidence interval ranges for selected fuels 4

5 6

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RESULTS OF ANALYSIS 1

Overall, the meta-analysis emissions results validate reports with similar expected values for 2

individual fuels and are similar to emissions standards for each type of fuel and technology. The 3

technologies also produce the expected changes to emissions, both regulated and unregulated. This is seen 4

clearly with D15, LNG, CNG, and Ethanol, where the data analyzed for these fuels does not include 5

technologies and emissions are high for each. Because exhaust after treatment technologies are often built 6

to meet emissions standards, the data shows the emissions standards are generally effective. However, the 7

data also shows that not all buses are meeting their expected emissions standard, specifically for NOx and 8

PM. The emissions standards, which do not yet consider GHGs, do not make an impact on CO2 9

equivalent emissions. 10

There are many factors which can impact emissions. Drive cycle does have an effect on 11

emissions, as also shown in some of the source reports for this study. The urban drive cycle, with many 12

stops and starts, shows higher emissions in all categories, but the effect is roughly consistent across all 13

fuels and emission types. Field and lab tests also show different emissions values, though there is not a 14

clear trend for all emissions, and the differences are within the same order of magnitude. In field and lab 15

tests, CO2 emissions and NOx emissions are 10 percent and 20 percent higher for field tests compared to 16

lab tests. 17

The analysis also shows there is a correlation between altitude and CO, PM, and THC. Each 18

category showed roughly a 10 percent increase in emissions over a 1500 meter increase in altitude for 19

specific fuel types. In all cases, more data is needed to make a more accurate estimation of the effects of 20

different driving cycles, field tests and altitude. Looking at the kilometers traveled by a vehicle versus the 21

emissions shows that a good predictor of NOx emissions is increased mileage on a vehicle. This is likely 22

because older buses will not have the best technologies, and worn out engines can have higher emissions 23

(5). There is also a correlation between kilometers traveled and CO2 equivalent emissions. 24

Overall, four technologies show the lowest emissions in important categories affecting pollution, 25

health, and GHGs (NOx, PM, and CO2 equivalence): Compressed Natural Gas with three way catalyst 26

(CNG + 3WC), 100percent Biodiesel with selective catalyst reduction (B100 + SCR), Diesel with 15 ppm 27

sulfur content with SCR (D15 + SCR) and 100 percent Biodiesel with exhaust gas recirculation (B100 + 28

EGR). No one fuel shows a distinct advantage over the other fuels in all categories, but control 29

technologies are an important factor in reducing emissions. 30

CONCLUSION 31

Much research, included that presented here, shows that there can be a variety of emissions, even 32

for similar fuels and technologies, under different conditions. Though the report makes data more relevant 33

to fuels and technologies available in Brazil, India, and Mexico, detailed analysis by fleet is required to 34

make specific recommendations. This also shows how increased mileage can affect certain emissions 35

types, even if the technology is not meant to deteriorate over time. In general, high quality emissions 36

testing data, on a variety of technologies under a variety of conditions, altitudes, driving cycles, field or 37

lab tests, and in specific countries is not always readily available due to the cost of testing. There is an 38

opportunity to improve results as additional data is gathered. However, the large data set presented here 39

takes advantage of the existing data to give agencies a summary of the most relevant existing data on 40

fuels and control technologies. 41



The analysis shows that no one fuel is significantly better at reducing all emissions than the others 1

if the right control technologies are used. These control technologies are a necessary part of reducing 2

emissions. At the same time, fuels or technologies that may reduce one pollutant may increase other 3

emissions, especially in the case of CO2 and PM. Though all emissions are important, NOx, CO2e, and 4

PM are particularly harmful emissions for global warming and public health. Efforts to improve 5

emissions standards, which often drive new technology, are an important part of achieving key emissions 6

reductions. Much of the focus of emissions standards has been on reducing local pollution, so including 7

CO2 into these standards, as the EU and US have begun to do, may result in CO2e reducing technologies 8

or increased fuel efficiency. 9

In addition, understanding how fuels and technologies contribute to combustion emissions is only 10

a first step in understanding the true cost, and impact of urban bus fleets. The results of this analysis, 11

which does not highlight one single fuel as the best, shows the need to understand the other two 12

components mentioned previously, lifecycle costs and lifecycle emissions. Costs and emissions raise 13

many possible factors, either global or locally-specific, which can have an impact on final fuel and 14

vehicle recommendations. 15

It is also important to make existing data relevant by understanding the local context. Specifically 16

for developing countries, the factors analyzed in this report should be considered prior to making fuel 17

recommendations based on data from the US or Europe. Additionally, it may be more important to focus 18

resources on improving fuel quality and improving uptake of new technologies, as well as vehicle 19

maintenance education, rather than changing fuel types. In all cases, the decisions should be based on 20

local conditions as well as transit agency needs. 21

22

The authors are grateful for the generous support of FedEx, which made the research possible, along with 23

initial guidance from FedEx Fuels and Vehicles experts Keshav Sondhi and Jimmy Mathis. We would 24

also like to acknowledge the contributions from the Fuels and Vehicles team: Jorge Macias, Hilda 25

Martinez, Cynthia Menendez, and Georg Schmid, CTS-EMBARQ Mexico, as well as valuable comments 26

from Marco Balam Almanza, Amit Bhatt, Dario Hidalgo, Luis Antonio Lindau, Karl Peet, and Rodolfo 27

Lacy Tamayo. 28

References: 29 1. Roychowdhry, A. “CNG Programme in India: Future Challenges.” Center for Science and Environment. 2010. 30 2. (WHO) World Health Organization. WHO Air quality guidelines for particulate matter, ozone, nitrogen dioxide 31 and sulfur dioxide. Global update 2005. Summary of risk assessment. 2006. 32 3. (EPA) Environmental Protection Agency. Air Pollutants. http://www.epa.gov/air/airpollutants.html. Accessed Jul. 33 31, 2012. 34 4. Lindqvist, K. “Emission standards for light and heavy road vehicles” Air Pollution & Climate Secretariat. 35 Gotenburg, Sweden, 2012. 36 5. Nylund, N. K., Erkkila, M. Lappi, and M. Ikonen. “Transit Bus Emission Study: Comparison of Emissions from 37 Diesel and Natural Gas Buses”. VTT, Finland. 2004. 38 6. (UNEP) United Nations Environment Programme. Opening the Door to Cleaner Vehicles in Developing and 39 Transition Countries: The Role of Lower Sulphur Fuels. Nairobi, Kenya. 2007. 40 7. TransLink. Bus Technology & Alternative Fuels Demonstration Project, Phase 1 –Test Program Report. 41 TransLink. Vancouver, Canada. 2006 42



8. Murtonen, T. and P. Aakko-Saksa. “Alternative fuels with heavy-duty engines and vehicles”. VTT. Finland. 2009. 1 9. Transit Cooperative Research Program. TCRP Report 146: Guidebook for Evaluating Fuel Choices for Post-2010 2 Transit Bus Procurements. Washington, DC: Transportation Research Board. 2011. 3 10. Mccormick R.L., A. Williams, J. Ireland, M. Brimhall, and R.R. Hayes. “Effects of Biodiesel Blends on Vehicle 4 Emissions.” National Renewable Energy Laboratory. Golden, CO. 2006 5 11. Wayne, W. S., Clark, N. N., et al. “A Comparison of Emissions and Fuel Economy from Hybrid-Electric and 6 Conventional-Drive Transit Buses” Energy & Fuels 2004;18:257-270 7 12. (WBCSD) World Business Council for Sustainable Development. Mobility2030. London, 2004. 8 13. Motta, R. P., Norton, and K. Kelly. Alternative Fuel Transit Buses. National Renewable Energy Laboratory. 9 Golden, CO. 1996. 10 14. Melendez, M., Taylor, J. et al. “Emission Testing of Washington Metropolitan Area Transit Authority 11 (WMATA) Natural Gas and Diesel Transit Buses”. National Renewable Energy Laboratory Innovation for Our 12 Energy Future, 2005. 13 15. Jayaratne, E., Ristovski, Z., et al. “Particle and gaseous emissions from compressed natural gas and ultralow 14 sulphur diesel-fuelled buses at four steady engine loads” Science of the Total Environment 2009; 407:2845-2852. 15 16. Johnson Matthey. http://ect.jmcatalysts.com/emission-control-technologies-oxidation-catalysts Accessed Jul 30, 16 2012 17 17. Yanowitz, J., R. McCormick, and M. Graboski. “In-Use Emissions from Heavy-Duty Diesel Vehicles” 18 Environmental Science & Technology 2000; 34: 729-740 19 18. McCormick, R.L., Graboski, M.S., et al. “Idle Emissions from Heavy-Duty Diesel and Natural Gas Vehicles at 20 High Altitude”. Journal of the Air & Waste Management Association 2011. 21 19. (SFMTA) San Francisco Municipal Transportation Agency. Alternative Fuel Pilot Program: Initial 6 month 22 Evaluation Results. 2002. http://www.sfmta.com/cms/rclean/altpilot.htm. Accessed Jul 30, 2012 23 20. Healey, J. (2005) Statistics: A tool for social research. Thomson Wadsworth. Belmont, CA. 24 21. Borenstein, Hedges, Higgins, and Rothstein. Introduction to Meta-analysis. John Wiley & Sons, Ltd. Chichester, 25 UK. 2009 26


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