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technology. The CNG-powered bus was included in this studysince CNG is considered an alternative to CD and is used toachieve emission reductions for school bus fleets.2 EPA regu-lated emissions, related unregulated emissions, and toxic aircontaminant (TAC) emissions were sampled, including the 41TACs listed by the California Air Resources Board (CARB) asassociated with diesel exhaust.3

METHODSAll three bus configurations were tested on a chassis dynamom-eter operated over a representative driving cycle and using thesame test weight and road load settings and drivetrain configu-ration. The technical details of this study are reported elsewhere.4

Since many of the emissions measured were expected to be atvery low concentrations, sampling times were extended andflow rates optimized to achieve detectable levels.

To meet new federal requirements for particulate emissions that gointo effect in 2007, manufacturers of heavy-duty diesel enginesanalyzed the effectiveness of catalyzed diesel particulate filter (CDPF)technology to reduce diesel fuel emissions. The objective of the studydescribed in this article was to perform a detailed characterization ofexhaust emissions from school buses powered by conventionaldiesel, low-emitting diesel with CDPF, and compressed natural gastechnology. The results indicate that CDPF technology can reduceemissions of regulated, related unregulated, and air toxic pollutants.

INTRODUCTIONIn its 2002 Health Assessment Document for Diesel Engine Ex-haust,1 the U.S. Environmental Protection Agency (EPA) con-cluded that “long-term (i.e., chronic) exposure to DE [dieselexhaust] is likely to pose a lung cancer hazard, as well as dam-age the lungs in other ways, depending on exposure.” EPAalso observed that the “composition of exhaust particulatesand the gases also will change” and that “as cleaner dieselengines replace a substantial number of existing engines, thegeneral applicability of the conclusions in this health assess-ment document will need to be reevaluated.” To comply withnew EPA emission standards for particulate matter (PM), whichgo into effect in 2007, all manufacturers of heavy-duty dieselengines have decided to employ catalyzed diesel particulatefilter (CDPF) technology, which requires the use of ultra-low-sulfur (i.e., less than 15 parts per million, or ppm) diesel fuel,to reduce emissions. This article summarizes the results of achemical characterization study, which was conducted in 2003by Southwest Research Institute for International Truck andEngine Corporation to better understand the effect of CDPFtechnology on diesel emissions. The objective of this studywas to perform a detailed characterization of exhaust emis-sions from school buses powered by conventional diesel (CD),low-emitting diesel (LED), and compressed natural gas (CNG)

Table 1. Diesel fuel properties.

ASTM Ultra-Low

Test Sulfur Conventional

Fuel Property Method Diesel Fuel Diesel Fuel

Sulfur (ppm) D 5453 14 371

Aromatics (vol%) D 1319 31.8 27.5

Aromatics (wt%) D 5186 30.9 33.1

Polynuclear aromatic D 5186 7.6 13.3

hydrocarbons (wt%)

Cetane numbera D 613 47.7 47.5

Cetane indexb D 976 46.1 47.5

aThe cetane number is a measure of how easily diesel fuel ignites in an engine; bThe cetane index is anestimation of the cetane number calculated from distillation data and density.

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Bus ConfigurationsThe CD-powered bus configuration was a 1998 IC Corpora-tion (wholly owned subsidiary of International Truck andEngine Corporation) rear-engine school bus, equipped witha 2001 model-year International DT530 diesel engine, ModelC275, using the standard on-highway electronic control mod-ule calibration, PRD5AS00. This engine configuration met1998 emission standards and was run on CD (D-2) fuel (seeTable 1).

The LED-powered bus configuration used the same bus asfor the CD bus configuration, but this time it was poweredusing Green Diesel Technology, which uses an Engelhard DPXfilter (i.e., CDPF) and a low-NOx (nitrogen oxides) engine con-trol module. Ultra-low-sulfur (<15 ppm) diesel fuel was usedto enable proper operation of the CDPF (see Table 1). Thisengine and filter system is EPA- and CARB-certified for the2007 emissions standards for particulate and hydrocarbonsand is very similar to the heavy-duty diesel engines that willbe available in 2007. The Green Diesel Technology has beenincorporated in buses commercially sold and delivered toCalifornia school districts.

The CNG-powered bus configuration was a 2000 model-year Blue Bird All-American fitted with CNG storage tanks tofuel a rear-mounted Deere 8.1-l natural gas engine. The bus,which was borrowed from an Arizona school district for thisstudy, was of a similar style to the diesel bus used in the twoother configurations and had approximately the same enginepower rating. The engine met 1998 emission standards andwas the most common CNG engine used in school buses. Itwas not equipped with an exhaust catalyst, since they are notcommonly available for natural gas school buses. The CNGbus used a gas mixture blended composition that met CARBspecifications for CNG certification fuel.

RESULTSThe study’s results show that emissions of PM,NOx, nitric oxide (NO), carbon monoxide(CO), total hydrocarbons (THCs), and non-methane hydrocarbons (NMHCs) were all sig-nificantly lower for the LED-powered bus thanfor either the CD or CNG configurations (seeFigure 1). The LED configuration produced thehighest levels of nitrogen dioxide (NO2)because the CDPF technology relies on excessNO2 to oxidize PM. The LED configurationreduced PM emissions to an average of 0.010g/mi, approximately 19% of the level obtainedfor the CNG bus and 5% obtained for the CDbus. Due to the low sulfur concentrations inthe fuel, sulfate portions of the total PM forboth the CNG and LED bus configurationswere below levels of detection.

More than half (21) of the 41 CARB-listedTACs were not detected for any of the configurations (see Table2). Concentrations for the other 20 TACs for all three busconfigurations are shown in Figure 2. There was no significantdifference between the three bus configurations for bis(2-ethylhexyl)phthalate, cyanide compounds, total dioxins and

Figure 1. Measured emissions (g/mi) for regulated and related unregulated emissions.

Table 2. TACs not found in this study.

Detection Limit (µg/mi) TACa

2 Aniline380 Antimony compounds190 Arsenic94 Berylium compounds94 Cadmium

3200 Chlorine (as chloride)b

56 Chlorobenzene and derivatives94 Chromium compoundsc

94 Cobalt compounds1000 Ethylbenzene (20)d

94 Lead (inorganic)94 Manganese7.5 Mercury94 Nickel4 4-Nitrobiphenyl94 Selenium

1000 Styrened

1000 Xylene isomers and mixturesd

1000 o-Xylened

1000 m-Xylened

1000 p-Xylened

aNot detected or a value of zero; bChloride was detected in one of the three samples for the CD busconfiguration, and the result was considered an outlier; cIncluding chromium VI, having a minimumdetection level of 80 µg/mi; dNot detected in sample or background.

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furans, hexane, and phosphorus. The LED bus produced loweremissions of acetaldehyde, acrolein, benzene, formaldehyde,methyl ethyl ketone, and propionaldehyde than either the CDor CNG configurations. Emissions of biphenyl, naphthalene,phenol, total polycyclic organic matter, and toluene were sig-nificantly higher for the CD bus than for the LED, but for theseemissions, there was no significant difference between the LEDand CNG configurations. The 1,3-butadiene level for the CDbus was significantly lower than for the CNGconfiguration.

The CD bus showed higher polycyclic aro-matic hydrocarbon (PAH) emissions than eitherthe LED or CNG configurations (see Figures 3and 4). The LED bus showed lower levels of allPAHs than the CNG bus, except for naphthalene,2-methyl naphthalene, benzo(b)fluoranthrene,indeno(1,2,3-cd) pyrene, dibenz(a,h)anthracene,and 1- nitropyrene; however, the differencesbetween the two configurations were not statisti-cally significant. Several of the dioxins and furanswere not detected. For total tetra-dioxins, levelsfor both the LED and CD configurations were sig-nificantly lower than for the CNG bus. With theexception of total tetra-dioxins, there were no sig-nificant differences among the three configura-tions for the other dioxins and furans detected.

This study is the first to report detailed emis-sions comparing LED- and CNG-poweredschool buses. Despite differences in test cycles

and vehicle and enginesizes, the comparisons re-ported in this study forschool buses are in agree-ment with recent studiesof transit buses.5-7 In thesestudies, none of the CNGbuses included after-treat-ment devices, such ascatalysts; however, recentwork conducted by CARBsuggests that oxidation cata-lysts can significantly reduceemissions of CO, formalde-hyde, and NMHCs.8 In otherstudies, grocery trucks7

and passenger cars9 pow-ered by diesel enginesequipped with CDPF tech-nology have also pro-duced similar emissionreductions.

CONCLUSIONSA detailed emission characterization of three school bus con-figurations was conducted. Each configuration was poweredusing a specific fuel—CD, LED, and CNG—having propertiesconsidered representative of practical fuels. Emission measure-ments, sampling, and analysis followed accepted practices, em-phasizing improved limits of detection to quantify emissionsof concern. The following results were found:

Figure 2. Measured emissions (mg/mi) for CARB-listed TACs found in this study.

Figure 3. Measured emissions (µg/mi) for more prevalent PAHs.

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About the AuthorsWilliam B. Bunn, III, MD, JD, MPH, is vice president of health,safety, security, and productivity, Tom W. Hesterberg, Ph.D., DABT,MBA, is director of product stewardship, and Warren Slodowskeis manager of environmental staff, all with International Truck andEngine Corporation, Chicago, IL. Charles Lapin, Ph.D., DABT, is atoxicology consultant. Address correspondence to: Charles Lapin,Lapin and Associates, 1870 Calafia St., Glendale, CA 91208; phone:1-818-802-2980; fax: 1-818-548-1606; e-mail: calapin@aol.com.

• The LED bus in this study generated the lowest emis-sions of PM, NOx, NO, THC, NMHC, methane, andCO, but the highest emissions of CO2 and NO2.

• The CNG bus produced the lowest emissions of CO2

and lower emissions of NO2 than the LED bus, but italso generated the highest emissions of NOx, NO,THC, NMHC, methane, and CO.

• The total PM emissions measured for the LED buswere approximately one-fifth the level obtained forthe CNG bus and one-eighteenth the level obtainedfor the CD bus.

• Both the LED and CNG bus configurations showedessentially no sulfate emissions.

• THC and NMHC levels were essentially zero for theLED configuration.

• 21 of the 41 TACs listed by CARB as being present indiesel exhaust were not found in the exhaust of anyof the vehicles studied, even though special samplingprovisions were used to detect low levels of thesecontaminants.

• Compared to LED, the CNG bus configurationshowed higher levels for six of the TACs (acetalde-hyde, acrolein, benzene, formaldehyde, methyl ethylketone, and propionaldehyde) and did not producelower emissions for any TAC.

• Compared to CD, the CNG bus configuration showedhigher emissions of five of the TACs (acetaldehyde, ac-rolein, 1,3-butadiene, formaldehyde, propionaldehyde),although it also produced lower emissions for severalother TACs.

• For all PAH compounds, the highest emission levelswere generated by the CD configuration. The LED

Figure 4. Measured emissions (µg/mi) for less prevalent PAHs.

and CNG configurations produced similarlow emissions of PAHs.

Overall, in this study, the LED configu-ration, using CDPF technology and pow-ered by ultra-low-sulfur diesel fuel,produced the lowest emissions of mostcriteria pollutants and TACs as specified byCARB. The LED configuration generatedmuch lower emissions than the CD con-figuration, and also resulted in lower emis-sions than the CNG configuration. Theseresults, in addition to similar studies con-ducted for other vehicles,5-9 support theview that the health hazard conclusionsmade by EPA in its 2002 Health AssessmentDocument do not apply to LED and CDPFtechnology and that further testing isneeded to better assess the potential healtheffects of this emerging technology.

REFERENCES1. Health Assessment Document for Diesel Engine Exhaust; EPA/600/8-90/057F;

U.S. Environmental Protection Agency, 2002; pp xiv-xv.2. Rule 1195—Clean On-Road School Buses; South Coast Air Quality Manage-

ment District, adopted April 20, 2001; available at http://www.aqmd.gov/rules/html/r1195.html.

3. Report to the Air Resources Board on the Proposed Identification of Diesel Ex-haust as a Toxic Air Contaminant. Part A. Exposure Assessment; CaliforniaAir Resources Board. As Approved by the Scientific Review Panel on April22, 1998; Table III-1, p A-7.

4. Ullman, T.L.; Smith, L.R.; Anthony, J.W.; Slodowske, W.J.; Trestrail, B;Cook, A.L.; Bunn, W.B.; Lapin, C.A.; Wright, K.J.; Clark, C.R. Compari-son of Exhaust Emissions, Including Toxic Air Contaminants, FromSchool Buses in Compressed Natural Gas, Low-Emitting Diesel, and Con-ventional Diesel Engine Configurations; Society of Automotive EngineersTechnical Paper Series 2003; Paper No. 2003-01-1381.

5. Ayala, A.; Kado, N.Y; Okamoto, R.A.; Holmén, B.A.; Kuzmicky, P.A.;Kobayashi, R.; Stiglitz, K.E. Diesel and CNG Heavy-Duty Transit Bus Emis-sions over Multiple Driving Schedules; Regulated Pollutants and ProjectOverview; Society of Automotive Engineers Technical Paper Series 2002;Paper No. 2002-01-1722.

6. Lanni, T.; Frank, B.P.; Tang, S.; Rosenblatt, D.; Lowell, D. Performanceand Emissions Evaluation of Compressed Natural Gas and Clean DieselBuses at New York City’s Metropolitan Transit Authority; Society of Auto-motive Engineers Technical Paper Series 2003; Paper No. 2003-01-0300.

7. Lev-On, M.; LeTavec, C.; Uihlein, J. Speciation of Organic Compoundsfrom the Exhaust of Trucks and Buses: Effect of Fuel and After-Treatmenton Vehicle Emission Profiles; Society of Automotive Engineers Technical PaperSeries 2002; Paper No. 2002-01-2873.

8. Ayala, A.; Gebel, M.E.; Okamoto, R.A.; Rieger, P.L.; Kado, N.Y.; Cotter, C.;Verma, N. Oxidation Catalyst Effect on CNG Transit Bus Emissions; Societyof Automotive Engineers Technical Paper Series 2003; Paper No. 2003-01-1900.

9. Ahlvik, P. Environmental and Health Impact from Modern Cars; EcotrafficR&D3 AB, Vägverket, May 2002; Publikation 2002:62.