Emissions Comparisons fromAlternative Fuel Buses and DieselBuses with a Chassis DynamometerTesting FacilityW E N G . W A N G , * N . N . C L A R K ,D . W . L Y O N S , R . M . Y A N G , M . G A U T A M ,R . M . B A T A , A N D J . L . L O T H

Department of Mechanical and Aerospace Engineering, WestVirginia University, Morgantown, West Virginia 26506-6106

The use of alternative fuels is considered to be an effectivemeasure to meet strict emissions regulations ofparticulate matter (PM) and oxides of nitrogen (NOx). Inresponse to these requirements, emissions data from in-use alternative fuel and diesel-powered heavy-duty vehicleshave been measured and collected from 32 transit agenciesin 17 states using the two West Virginia University (WVU)transportable heavy-duty vehicle emissions testing labo-ratories (THDVETLs). More than 600 tests have been performedon over 300 buses and heavy trucks operating on alternativefuels such as natural gas, methanol, and ethanol and alsooperating on conventional fuel diesel. Regulated emissionsof PM, NOx, carbon monoxide (CO), and total hydrocarbon(HC) have been measured and analyzed. In this study,emissions data from alternative fuel buses and diesel controlbuses are carefully compared. The results show thatnatural gas, methanol, and ethanol have a strong potentialto reduce PM and NOx emissions levels.

IntroductionOur increasing concern about the cleanliness of the environ-ment leads us to demand a significant reduction in theemissions from vehicles, especially from heavy-duty vehicles.In addition to engine improvements and installation ofaftertreatment devices, the use of alternative fuels to replaceconventional diesel fuel is considered to be an effectivemeasure to meet strict emissions regulations of particulatematter (PM) and oxides of nitrogen (NOx). In response tothese requirements, emissions data from in-use alternativefuel and diesel-powered heavy-duty vehicles have beenmeasured and collected from 32 transit agencies in 17 statesusing the two West Virginia University (WVU) transportableheavy-duty vehicle emissions testing laboratories (THD-VETLs). So far, more than 600 tests have been performed onover 300 buses and heavy trucks operating on alternativefuels such as natural gas, methanol, ethanol, and biodieseland also operating on conventional fuel diesel. Regulatedemissions of PM, NOx, and carbon monoxide (CO) and totalhydrocarbon (HC) have been measured and analyzed.

In this study, emissions data from alternative fuel busesand diesel control buses are carefully compared. The resultsshow that natural gas, methanol, and ethanol have a strongpotential to reduce PM and NOx emissions levels. The majorreasons are explained by the PM and NOx formation mech-anisms in which the fuel composition and molecular structure

play important roles in PM formation, while combustiontemperature is a significant factor in NOx formation. However,mixture control and ignition timing for natural gas andinjection timing for diesel will influence the comparison ofthe fuels.

Emissions from heavy-duty vehicles, especially from citybuses and trucks, are recognized as some of the major sourcesthat contribute to air pollution and the formation of low-level ozone. The U.S. Environmental Protection Agency (EPA)Emissions Certification Standards for 1998 Urban Bus Enginesand 1998 Heavy Truck Engines place specific emphasis onthe reductions of NOx emissions. To meet the goals set bythese increasingly stringent emissions regulations, inten-sive research and developmental efforts have been madeby engine manufacturers and research institutions. Themajority of effort is focused on engine improvements, after-treatment devices, and most importantly, the use of alternativefuels.

The U.S. Congress enacted the Alternative Motor FuelsAct (AMFA) in 1988, which requires the U.S. Department ofEnergy (DOE) to collect emissions data, operating data, andcapital cost data on alternative fuel vehicles. In this program,WVU undertakes the emissions test of heavy-duty vehiclesusing their two THDVETLs.

A THDVETL has many advantages over a stationary chassisdynamometer or an engine dynamometer testing facilitybecause it minimizes the cost of determining emissions of avehicle in the field. The tests conducted at the actualoperating site of the vehicle with in-use fuel by a THDVETLcan obtain more realistic emissions data than the regulatedtests by an engine dynamometer.

The two WVU THDVETLs have been described elsewhere(1, 2). For conducting emissions testing, a THDVETL is drivento the site of the fleet owner, and the selected heavy dutybuses or trucks are tested. During a test, a driving patternis chosen to represent a speed-time trace that includes bothtransient and steady-state operations. The road load of thevehicle is simulated by a function of speed, and generally, thegrade of the road is excluded.

The WVU THDVETLs have been operated for 4 years. Themap in Figure 1 shows the areas and agencies visited by theWVU THDVETLs. Most of the tested vehicles were poweredwith alternative fuels such as natural gas (NG), representingboth compressed natural gas (CNG) and liquefied naturalgas (LNG); alcohol fuels including M100 (100% methanol),E93 (93% ethanol, 5% methanol, 2% K-1 kerosene by volume),E95 (95% ethanol, 5% gasoline), BD20 (20% soy biodiesel,80% no. 2 diesel by volume), and BD35 (35% soy biodiesel,65% no. 2 diesel by volume); and conventional no. 1 and no.2 diesels (D1 and D2). Table 1 summarizes the numbers oftests conducted on different fuels and engines.

Some earlier field emissions data obtained from the WVUTHDEVTLs have been presented in previous papers (3, 4).However, with the data collected from over 600 tests, morecomprehensive comparisons and discussions of emissionsbetween natural gas, alcohol fuels, and conventional dieselbecome possible in the present paper. The reader mustrecognize that fuel comparisons occur within constraints ofthe engine technologies available and that alternative fuelengine technology is still in its infancy relative to diesel enginedesign.

Descriptions of Test Facility and Test ProceduresThe THDVETL is composed of two trailers. One trailer holdsthe dynamometer, and the other carries the instrumentation.The dynamometer can simulate vehicle test weights rangingfrom 4000 to 34 000 kg with different flywheel settings, and

* Corresponding author phone: 304-293-3111, ext. 357; fax: 304-293-6689; e-mail: wang@cemr.wvu.edu.

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vehicle road load is simulated through the use of eddy currentair-cooled power absorbers. Although the vehicle wheels runon rollers during the testing, power is extracted from thevehicle hubs using adapter plates and shafts that drive thedynamometer directly.

The instrumentation trailer contains the exhaust analyzers,data acquisition system, and control system. Instantaneousconcentrations of CO, HC, NOx, and carbon dioxide (CO2) aremeasured at a frequency of 10 Hz, while samples of for-maldehyde (HCHO), methanol (CH3OH), ethanol (C2H5OH),methane (CH4), and PM emissions are gathered over theduration of the test. A photograph of the THDVETL operatingat a test site in Minnesota is presented in Figure 2.

To perform a test, the THDVETL is driven to the test sitewhere a transit agency is located. The dynamometer may beset up indoors or outdoors depending on the space available.To make sure that the analyzers and the associated systemsare functioning properly, leak checks and calibrations areconducted whenever the facility is moved to a new location.Prior to the actual testing, all gearboxes in the powertrain ofthe dynamometer are warmed up to minimize variability dueto the viscosity of the oil in the drivetrain.

During a test, the driver is provided with a visual trace ofthe scheduled speed versus time on a monitor. The driveris expected to follow the speed trace closely to minimize theerrors introduced by the operating conditions. Each test

includes several repeat test runs in order to guarantee thatthe exhaust emissions measured are a true representation ofthe test vehicle performance. The emissions data reportedfor each vehicle test are the average values of at least fourrepeat test runs.

Analysis of Selected Test ResultsMeasured emissions are influenced by the engine technologyused, the test cycle employed, the presence or absence ofafter-treatment devices, atmospheric conditions, and fueltypes. The test results also have shown that some of thefactors, such as after-treatment devices and test cycles, mayhave much larger effects than the type of fuel (8). In addition,poor state-of-tune of a vehicle may easily mask the benefitsof an alternative fuel, and technology changes may alteremissions performance substantially.

In this study, a special driving cyclesthe central businessdistrict (CBD) cycleswas used for all buses tested. The drivingpattern for the CBD cycle was developed as a generalrepresentation of transit vehicle operation in a downtownbusiness district, and it was included in SAE RecommendedPractice, SAE J1376. The cycle consists of 14 identicalsegments as shown in Figure 3. Each segment includes 10s of acceleration, 18.5 s of 20 mph cruise, 4.5 s of deceleration,and 7 s of idle. The total driving distance is 2 mi.

FIGURE 1. Test sites and agencies covered by the THDVETLs.

TABLE 1. Number of Emissions Tests Summarized by Engine Type and Fuel Type (1992-1995)

diesel CNG LNG M100 E93/E95 BD20/BD35 dual other total

DDC 179 16 57 86 15 15 3 371Cummins 71 88 12 4 4 1 180Caterpillar 33 5 2 2 42M.A.N. 12 12other 7 10 2 4 3 26total 302 119 12 57 86 21 25 9 631

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In order to lessen the obfuscating effect of other uncon-trollable factors, emissions tests with diesel control vehiclesfor each fuel type on each test site were also conducted. Thediesel control vehicles selected were as similar as possible,in equipment, to their alternative fuel counterpart vehicles(that was, the vehicles were of the same manufacturer, thesame models, and close in model year).

The emissions performance difference between each pairof fuels can be obtained by hypothesis testing on a certainconfidence level, which is usually taken as 95%. The meanand 95% confidence intervals of PM, NOx, CO, and HCemissions are presented in Figures 4-7. Statistical com-parison results are listed in Tables 2-5. In the tables, “yes”indicates that the difference of the corresponding pair of fuelsis statistically significant, while “no” indicates that there isno significant difference between the corresponding pair of

fuels on a 95% confidence level. Tables 6-9 give the averagevalues and sample size of test results. A line is drawn underthese fuels if there is no significant difference between theiremissions values. The fuels are arranged according to theiraverage test result values, ranging from the smallest on theleft to the largest on the right. The mean result and the samplesize used for comparison of emissions are listed for eachparticular fuel. Emissions data for biodiesel blends are notincluded in those tables due to a small sample size, and moredata on biodiesel buses is desired by the authors at time ofwriting.

DiscussionFrom the emissions results obtained by the WVU THDVETLs,the following observations and discussions can be made.

PM Emissions. NG has the lowest PM emissions levelwhen compared with all the other fuels. The average PM

FIGURE 2. Transportable heavy-duty vehicle emissions testing laboratory at a test site in Minnesota.

FIGURE 3. Sketch of CBD cycle.

FIGURE 4. Mean and 95% confidence interval of PM emissions.

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emissions level of NG is only 0.03 g/mi on the CBD drivingcycle. M100 and E93/E95 have the second and third lowestlevels. Although their averages (0.26 and 0.47 g/mi, respec-tively) are much higher than the average for NG, their levelsare still significantly lower than those for diesel. Based onthese results, NG and alcohol fuels are considered to be verypromising alternative fuels because they satisfy the envi-ronmental need for PM reduction.

PM consists mainly of combustion-generated carbon-aceous material (soot) on which some organic or hydrocarbon

compounds and sulfates have become adsorbed (5). Sinceall these organic and inorganic compounds in PM emissionsoriginate from fuel and from engine lubricants, fuel com-position plays an important role in determining PM emissions(6). In addition, fuel molecular weight and molecularstructure influence engine-out hydrocarbon compositionsand thus they affect PM emissions (7).

The major component of NG is methane that has the lowestmolecular weight (only 16) and simplest structure (one carbonatom and four hydrogen atoms in a molecule) among allfuels in this study. Hence, the simplest components andsmallest molecular sizes of the unburned and partially oxi-dized hydrocarbons are generated in emissions from NGbuses. This explains why NG has the lowest PM emissionslevel.

FIGURE 5. Mean and 95% confidence interval of NOx emissions.

FIGURE 6. Mean and 95% confidence interval of CO emissions.

FIGURE 7. Mean and 95% confidence interval of HC emissions.

TABLE 2. Statistical Comparison on PM Emission

NG D1 D2 E93/E95

D1 yesD2 yes yesE93/E95 yes yes yesM100 yes yes yes yes

TABLE 3. Statistical Results on NOx Emission

NG D1 D2 E93/E95

D1 noD2 no noE93/E95 yes yes yesM100 yes yes yes yes

TABLE 4. Statistical Results on CO Emission

NG D1 D2 E93/E95

D1 yesD2 no yesE93/E95 yes yes yesM100 no yes no yes

TABLE 5. Statistical Results on HC Emission

NG D1 D2 E93/E95

D1 yesD2 yes yesE93/E95 yes yes yesM100 no yes yes no

TABLE 6. PM Emissions Comparisons (g/mi)

NG M100 E93/E95 D1 D2

mean 0.03 0.26 0.49 0.96 1.48sample size 60 46 28 61 70

TABLE 7. NOx Emissions Comparisons (g/mi)

M100 E93/E95 NG D2 D1

mean 14.7 18.2 30.0 31.8 32.0sample size 46 28 60 70 61

TABLE 8. CO Emissions Comparisons (g/mi)

D1 NG D2 M100 E93/E95

mean 9.7 15.3 16.5 19.9 31.9sample size 61 60 70 46 28

TABLE 9. HC Emissions Comparisons (g/mi)

D2 D1 E93/E95 M100 NG

mean 2.1 2.6 10.5 14.7 14.8sample size 70 61 28 46 60

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For alcohol, the molecular weight of methanol and ethanolis 32 and 46, respectively. The molecular structure of alcoholis not as simple as NG but not as complicated as diesel. Inconsequence, the PM emissions level of alcohol shown inFigure 4 and Table 6 lies between NG and diesel.

In contrast, diesel fuel consists of a blend of complex,heavy molecules that include aromatics and a wide range ofunsaturated compounds. The carbon to hydrogen ratio islow, and there is a tendency under pyrolysis and combustionto form smoke precursors. Unburned carbon atoms are morelikely to occur, and the composition of the unburned andpartially oxidized hydrocarbons in the diesel exhaust are muchmore complex and extend over a larger range of molecularsize than those for NG and alcohol and, consequently, increasethe PM emissions (6). Also, NG and alcohol do not containany inorganic materials such as sulfur, so that their inorganicPM emissions will be lower than those of diesel engines.

NOx Emissions. M100 has the lowest level of NOx emis-sions, and E93/E95 has the second lowest level, while NGand diesel have relatively the same levels of NOx emissions.These results support the position that alcohol fuels offer anadvantage as alternative fuels for heavy-duty vehicles.

The reason for lower NOx emissions from alcohol fuelscan be explained as follows: most NOx emissions fromcombustion engines are formed by the oxidation of atmo-spheric nitrogen at high temperatures; therefore, the flametemperature is significant in determining NOx emissions (5).NOx production rates are highly sensitive to in-cylindertemperature. The latent heats of vaporization of methanol,ethanol, and diesel are 510, 362, and 250 BTU/lb, respectively,and the heating values of methanol, ethanol, and diesel are8600, 11600, and 18500 BTU/lb, respectively (8). Therefore,more heat is needed for methanol and ethanol than for dieselin order to evaporate a certain mass of fuel, while the heatprovided by methanol and ethanol is lower than that providedby diesel. As a result, the flame temperature is lower formethanol and ethanol than for diesel. For this reason, muchless NOx will be formed during the premixed combustionperiod in methanol and ethanol engines. Test results alsoindicate that NOx emissions could be reduced to some ex-tent even at increased compression ratios used for compres-sion ignition of alcohol, which generally tend to increase NOx

levels (9).Since most NG buses operate under lean premixed con-

ditions, where flame temperatures are lower than in dieselengines, the NOx emissions from NG buses tend to be lowerthan those for diesel. NOx control in NG engines relies onboth the spark timing and maintenance of the fuel-air mixturewithin a narrow band. As with gasoline, retarding the timingwill reduce NOx formation but will also have an adverse effecton engine economy. The air-fuel ratio window is criticaland calls for approximately 40% excess air in the cylinder(often termed a λ ratio of 1.4). If the mixture is allowed tobecome significantly leaner, misfire with associated highhydrocarbon emissions will arise. On the other hand, betweenthe desired air-fuel ratio and a stoichiometric air-fuel ratiolies a significant “NOx peak”, which can yield NOx emissionsand order of magnitude higher than those anticipated at thedesign air-fuel ratio. Experience has shown that NG leanburn NOx emissions may be held to a low value with carefultiming and mixture control, but that cylinder-to-cylindermixture variations and straying of the air-fuel ratio from thedesign point often bring the NOx emissions to the level ofdiesel engines. It is noteworthy, however, that the NOx

emissions from diesel engines are weighed against engineefficiency in determining the diesel injection timing. In thisway it is seen that the engine control technology often playsa higher role than the fuel in determining emissions.

CO Emissions. CO emissions are the result of impropermixing and incomplete combustion and are controlledprimarily by the global or local air/fuel equivalence ratio.

Alcohol fuels have a lower flame temperature and a lowerburn velocity than those of diesel, due to their lower net massheating value and high vaporization cooling effect. With lowerflame temperature and burn velocity, a fraction of methanoland ethanol may be found in a rich air-fuel ratio range oreven liquid state as the flame front spreads too slowly toreach them or wall quenching may take effect. Therefore,they result in incomplete combustion and cause relativelyhigh CO emissions.

CO emissions of NG buses exhibit a high variance over thefleet, but on the average, the numbers are still lower thanthose of diesel buses. After investigation, it was found thatmost of the engines exhibiting high CO levels were earlyuncertified versions of NG engines. All of the later modelsof NG engines have significantly lower CO levels of less than1 g/mi (10). Although most NG-fueled buses run under leanburn condition to take advantage of NG’s lean flammabilitylimits, the effect of air-fuel ratio on CO emissions is still verysignificant. In many cases, analysis of second-by-second datashowed that high gross CO arose due to an insufficiently leanidle condition rather than due to emissions under load.However, with proper air-fuel mixing and proper enginemodification, these high CO emissions may be avoided (11).

HC Emissions. Methanol, ethanol, and NG tend to havehigher HC emissions levels than diesel. However, it must benoted that HC data in this study were reported as totalhydrocarbon measured by a heated flame ionization detector(FID). However, FID response varies for oxygenated com-pounds. For alcohol-fueled vehicles, organic material hy-drocarbon mass equivalent (OMHCE) is the designation oftenused to denote the total hydrocarbon mass emitted from anengine as unburned and partially burned fuel. OMHCE maybe calculated by adding the mass contribution of unburnedalcohol, formaldehyde, aldehydes, and residual hydrocarbon(RHC), which represents the remaining fraction. For NG-fueled vehicles, HC consists mainly of the unburned methane.Methane is considered to be non-reactive in the formationof ozone in the atmosphere.

Under normal combustion conditions, HC emissions arecaused primarily by unburned mixtures, which indicateimproper mixing and incomplete combustion. Flame quench-ing at the wall, particularly for homogeneous charge enginessuch as CNG lean burn engines, and adsorption and deposi-tion from the oil film on the cylinder wall may also have aneffect. Due to the slow flame front of alcohol, a fraction ofalcohol will not completely burn in the cylinder when theexhaust valve opens. For a lean burn NG bus, as the load onthe engine is reduced, the air-fuel mixtures may be too leanto burn efficiently, and partial misfire may occur. All theseconditions result in high HC emissions. Through carefuldesign of charge motion, HC emissions from NG and alcoholengines can be lowered to an acceptable level through properair-fuel mixing.

AcknowledgmentsThe authors wish to thank the U.S. Department of Energy, inparticular the Office of Transportation Technologies, for thegrant that made this study possible and the NationalRenewable Energy Laboratory (NREL) for collaboration ofthis project. Thanks also go to Byron Rapp and the staff atthe THDVETL for conducting the field tests.

Literature Cited(1) Bata, R.; Clark, N.; Gautam, M.; Howell, A.; Long, T.; Loth, J.;

Lyons, D.; Palmer, G.; Smith, J.; Wang, W. A Transportable HeavyDuty Engine Testing Laboratory, SAE Paper 912668. SAE Trans.1991, 100, 433-440.

(2) Clark, N.; Gautam, M.; Bata, R.; Wang, W.; Loth, J.; Palmer, G.;Lyons, D. Design and Operation of a New TransportableLaboratory for Emission Testing of Heavy Duty Trucks and Buses.Int. J. Vehicle Des. 1995, 2 (3/4), 308-322.

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(3) Wang, W.; Gautam, M.; Sun, X.; Bata, R.; Clark, N.; Palmer, G.;Lyons, D. Emission Comparisons of Twenty-six Heavy-dutyVehicles Operated on Conventional and Alternative Fuels, SAEPaper 932952. SAE Trans. 1993, 102, 566-575.

(4) Clark, N.; Gadapati, C.; Lyons, D.; Wang, W.; Bata, R.; Gautam,M.; Kelly, K.; White, C. Comparative Emissions from NaturalGas and Diesel Buses. SAE Paper 952746; SAE InternationalAlternative Fuels Congress and Exposition, San Diego, CA,December 6-8, 1995.

(5) Heywood, J. B. Internal Combustion Engine Fundamentals;McGraw-Hill Book Company: New York, 1988.

(6) Johnson, J. H.; Bagley, S. T.; Gratz, L. D.; Leddy, D. G. A Reviewof Diesel Particulate Control Technology and Emissions Ef-fectss1992 Horning Memorial Award Lecture. SAE Paper 940233;SAE International Congress and Exposition, Detroit, MI, February28-March 3, 1994.

(7) Shore, P.; Humphries, D.; Hadded, O. Speciated HydrocarbonEmissions from Aromatic, Olefinic and Paraffinic Model Fuel.SAE Paper 930373; SAE International Congress and Exposition,Detroit, MI, March 1-5, 1993

(8) Yang, R. Uncertainty and Comparison Analyses of Heavy DutyVehicle Emissions Test Results. M.S. Thesis, West VirginiaUniversity, Morgantown, WV, 1995.

(9) Rideout, G.; Kirshenblatt, M.; Prakash C. Emissions fromMethanol, Ethanol, and Diesel Powered Urban Transit Buses.SAE Paper 942261; SAE International Truck & Bus Meeting &Exposition, Seattle, WA, November 7-9, 1994.

(10) Chandler, K.; Malcosky, N.; Motta, R.; Norton, P.; Kelly, K.;Schumacher, L.; Lyons, D. Alternative Fuel Transit Bus Evalua-tion Program Results. SAE Paper 961082; SAE InternationalSpring Fuels & Lubricants Meeting, Detroit, MI, May 6-8,1996.

(11) Clark, N.; Wang, W.; Lyons, D.; Gautam, M.; Bata, R. Trouble-shooting High Emissions from In-Service Alternative FueledBuses. Presented at Windsor Workshop on Alternative Fuels,Toronto, Canada, June, 1996.

Received for review February 6, 1997. Revised manuscriptreceived July 10, 1997. Accepted July 21, 1997.X

ES9701063

X Abstract published in Advance ACS Abstracts, September 1, 1997.

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