Environ. Sci. Technol. 1997, 31, 3132-3137
Emissions Comparisons from Alternative Fuel Buses and Diesel Buses with a Chassis Dynamometer Testing Facility WEN G. WANG,* N. N. CLARK, D. W. LYONS, R. M. YANG, M. GAUTAM, R. M. BATA, AND J. L. LOTH Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506-6106
The use of alternative fuels is considered to be an effective measure to meet strict emissions regulations of particulate matter (PM) and oxides of nitrogen (NOx). In response to these requirements, emissions data from inuse alternative fuel and diesel-powered heavy-duty vehicles have been measured and collected from 32 transit agencies in 17 states using the two West Virginia University (WVU) transportable heavy-duty vehicle emissions testing laboratories (THDVETLs). More than 600 tests have been performed on over 300 buses and heavy trucks operating on alternative fuels such as natural gas, methanol, and ethanol and also operating on conventional fuel diesel. Regulated emissions of 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 control buses are carefully compared. The results show that natural gas, methanol, and ethanol have a strong potential to reduce PM and NOx emissions levels.
Introduction Our increasing concern about the cleanliness of the environment leads us to demand a significant reduction in the emissions from vehicles, especially from heavy-duty vehicles. In addition to engine improvements and installation of aftertreatment devices, the use of alternative fuels to replace conventional diesel fuel is considered to be an effective measure to meet strict emissions regulations of particulate matter (PM) and oxides of nitrogen (NOx). In response to these requirements, emissions data from in-use alternative fuel and diesel-powered heavy-duty vehicles have been measured and collected from 32 transit agencies in 17 states using the two West Virginia University (WVU) transportable heavy-duty vehicle emissions testing laboratories (THDVETLs). So far, more than 600 tests have been performed on over 300 buses and heavy trucks operating on alternative fuels such as natural gas, methanol, ethanol, and biodiesel and also operating on conventional fuel diesel. Regulated emissions of PM, NOx, and carbon monoxide (CO) and total hydrocarbon (HC) have been measured and analyzed. In this study, emissions data from alternative fuel buses and diesel control buses are carefully compared. The results show that natural gas, methanol, and ethanol have a strong potential to reduce PM and NOx emissions levels. The major reasons are explained by the PM and NOx formation mechanisms in which the fuel composition and molecular structure * Corresponding author phone: 304-293-3111, ext. 357; fax: 304293-6689; e-mail:
[email protected].
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play important roles in PM formation, while combustion temperature is a significant factor in NOx formation. However, mixture control and ignition timing for natural gas and injection timing for diesel will influence the comparison of the fuels. Emissions from heavy-duty vehicles, especially from city buses and trucks, are recognized as some of the major sources that contribute to air pollution and the formation of lowlevel ozone. The U.S. Environmental Protection Agency (EPA) Emissions Certification Standards for 1998 Urban Bus Engines and 1998 Heavy Truck Engines place specific emphasis on the reductions of NOx emissions. To meet the goals set by these increasingly stringent emissions regulations, intensive research and developmental efforts have been made by engine manufacturers and research institutions. The majority of effort is focused on engine improvements, aftertreatment devices, and most importantly, the use of alternative fuels. The U.S. Congress enacted the Alternative Motor Fuels Act (AMFA) in 1988, which requires the U.S. Department of Energy (DOE) to collect emissions data, operating data, and capital cost data on alternative fuel vehicles. In this program, WVU undertakes the emissions test of heavy-duty vehicles using their two THDVETLs. A THDVETL has many advantages over a stationary chassis dynamometer or an engine dynamometer testing facility because it minimizes the cost of determining emissions of a vehicle in the field. The tests conducted at the actual operating site of the vehicle with in-use fuel by a THDVETL can obtain more realistic emissions data than the regulated tests by an engine dynamometer. The two WVU THDVETLs have been described elsewhere (1, 2). For conducting emissions testing, a THDVETL is driven to the site of the fleet owner, and the selected heavy duty buses or trucks are tested. During a test, a driving pattern is chosen to represent a speed-time trace that includes both transient and steady-state operations. The road load of the vehicle is simulated by a function of speed, and generally, the grade of the road is excluded. The WVU THDVETLs have been operated for 4 years. The map in Figure 1 shows the areas and agencies visited by the WVU THDVETLs. Most of the tested vehicles were powered with alternative fuels such as natural gas (NG), representing both compressed natural gas (CNG) and liquefied natural gas (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 of tests conducted on different fuels and engines. Some earlier field emissions data obtained from the WVU THDEVTLs have been presented in previous papers (3, 4). However, with the data collected from over 600 tests, more comprehensive comparisons and discussions of emissions between natural gas, alcohol fuels, and conventional diesel become possible in the present paper. The reader must recognize that fuel comparisons occur within constraints of the engine technologies available and that alternative fuel engine technology is still in its infancy relative to diesel engine design.
Descriptions of Test Facility and Test Procedures The THDVETL is composed of two trailers. One trailer holds the dynamometer, and the other carries the instrumentation. The dynamometer can simulate vehicle test weights ranging from 4000 to 34 000 kg with different flywheel settings, and
S0013-936X(97)00106-5 CCC: $14.00
1997 American Chemical Society
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
DDC Cummins Caterpillar M.A.N. other
179 71 33 12 7
16 88 5
12
total
302
119
M100
E93/E95
BD20/BD35
dual
other
total
57
86
15 4
15 4 2
3 1 2
371 180 42 12
2
4
3
57
86
21
25
9
26 631
10 12
vehicle road load is simulated through the use of eddy current air-cooled power absorbers. Although the vehicle wheels run on rollers during the testing, power is extracted from the vehicle hubs using adapter plates and shafts that drive the dynamometer directly. The instrumentation trailer contains the exhaust analyzers, data acquisition system, and control system. Instantaneous concentrations of CO, HC, NOx, and carbon dioxide (CO2) are measured at a frequency of 10 Hz, while samples of formaldehyde (HCHO), methanol (CH3OH), ethanol (C2H5OH), methane (CH4), and PM emissions are gathered over the duration of the test. A photograph of the THDVETL operating at a test site in Minnesota is presented in Figure 2. To perform a test, the THDVETL is driven to the test site where a transit agency is located. The dynamometer may be set up indoors or outdoors depending on the space available. To make sure that the analyzers and the associated systems are functioning properly, leak checks and calibrations are conducted whenever the facility is moved to a new location. Prior to the actual testing, all gearboxes in the powertrain of the dynamometer are warmed up to minimize variability due to the viscosity of the oil in the drivetrain. During a test, the driver is provided with a visual trace of the scheduled speed versus time on a monitor. The driver is expected to follow the speed trace closely to minimize the errors introduced by the operating conditions. Each test
includes several repeat test runs in order to guarantee that the exhaust emissions measured are a true representation of the test vehicle performance. The emissions data reported for each vehicle test are the average values of at least four repeat test runs.
Analysis of Selected Test Results Measured emissions are influenced by the engine technology used, the test cycle employed, the presence or absence of after-treatment devices, atmospheric conditions, and fuel types. The test results also have shown that some of the factors, such as after-treatment devices and test cycles, may have much larger effects than the type of fuel (8). In addition, poor state-of-tune of a vehicle may easily mask the benefits of an alternative fuel, and technology changes may alter emissions performance substantially. In this study, a special driving cyclesthe central business district (CBD) cycleswas used for all buses tested. The driving pattern for the CBD cycle was developed as a general representation of transit vehicle operation in a downtown business district, and it was included in SAE Recommended Practice, SAE J1376. The cycle consists of 14 identical segments as shown in Figure 3. Each segment includes 10 s 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.
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FIGURE 2. Transportable heavy-duty vehicle emissions testing laboratory at a test site in Minnesota.
FIGURE 4. Mean and 95% confidence interval of PM emissions. FIGURE 3. Sketch of CBD cycle. In order to lessen the obfuscating effect of other uncontrollable factors, emissions tests with diesel control vehicles for each fuel type on each test site were also conducted. The diesel 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, the same models, and close in model year). The emissions performance difference between each pair of fuels can be obtained by hypothesis testing on a certain confidence level, which is usually taken as 95%. The mean and 95% confidence intervals of PM, NOx, CO, and HC emissions are presented in Figures 4-7. Statistical comparison results are listed in Tables 2-5. In the tables, “yes” indicates that the difference of the corresponding pair of fuels is statistically significant, while “no” indicates that there is no significant difference between the corresponding pair of
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fuels on a 95% confidence level. Tables 6-9 give the average values and sample size of test results. A line is drawn under these fuels if there is no significant difference between their emissions values. The fuels are arranged according to their average test result values, ranging from the smallest on the left to the largest on the right. The mean result and the sample size used for comparison of emissions are listed for each particular fuel. Emissions data for biodiesel blends are not included in those tables due to a small sample size, and more data on biodiesel buses is desired by the authors at time of writing.
Discussion From the emissions results obtained by the WVU THDVETLs, the following observations and discussions can be made. PM Emissions. NG has the lowest PM emissions level when compared with all the other fuels. The average PM
TABLE 3. Statistical Results on NOx Emission D1 D2 E93/E95 M100
NG
D1
D2
E93/E95
no no yes yes
no yes yes
yes yes
yes
TABLE 4. Statistical Results on CO Emission
FIGURE 5. Mean and 95% confidence interval of NOx emissions.
D1 D2 E93/E95 M100
NG
D1
D2
E93/E95
yes no yes no
yes yes yes
yes no
yes
TABLE 5. Statistical Results on HC Emission D1 D2 E93/E95 M100
NG
D1
D2
E93/E95
yes yes yes no
yes yes yes
yes yes
no
TABLE 6. PM Emissions Comparisons (g/mi) mean sample size
FIGURE 6. Mean and 95% confidence interval of CO emissions.
NG
M100
E93/E95
D1
D2
0.03 60
0.26 46
0.49 28
0.96 61
1.48 70
TABLE 7. NOx Emissions Comparisons (g/mi) mean sample size
M100
E93/E95
NG
D2
D1
14.7 46
18.2 28
30.0 60
31.8 70
32.0 61
TABLE 8. CO Emissions Comparisons (g/mi) mean sample size
D1
NG
D2
M100
E93/E95
9.7 61
15.3 60
16.5 70
19.9 46
31.9 28
TABLE 9. HC Emissions Comparisons (g/mi) FIGURE 7. Mean and 95% confidence interval of HC emissions. mean sample size
TABLE 2. Statistical Comparison on PM Emission D1 D2 E93/E95 M100
NG
D1
yes yes yes yes
yes yes yes
D2
yes yes
D2
D1
E93/E95
M100
NG
2.1 70
2.6 61
10.5 28
14.7 46
14.8 60
E93/E95
yes
emissions level of NG is only 0.03 g/mi on the CBD driving cycle. M100 and E93/E95 have the second and third lowest levels. Although their averages (0.26 and 0.47 g/mi, respectively) are much higher than the average for NG, their levels are still significantly lower than those for diesel. Based on these results, NG and alcohol fuels are considered to be very promising alternative fuels because they satisfy the environmental need for PM reduction. PM consists mainly of combustion-generated carbonaceous material (soot) on which some organic or hydrocarbon
compounds and sulfates have become adsorbed (5). Since all these organic and inorganic compounds in PM emissions originate from fuel and from engine lubricants, fuel composition plays an important role in determining PM emissions (6). In addition, fuel molecular weight and molecular structure influence engine-out hydrocarbon compositions and thus they affect PM emissions (7). The major component of NG is methane that has the lowest molecular weight (only 16) and simplest structure (one carbon atom and four hydrogen atoms in a molecule) among all fuels in this study. Hence, the simplest components and smallest molecular sizes of the unburned and partially oxidized hydrocarbons are generated in emissions from NG buses. This explains why NG has the lowest PM emissions level.
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For alcohol, the molecular weight of methanol and ethanol is 32 and 46, respectively. The molecular structure of alcohol is not as simple as NG but not as complicated as diesel. In consequence, the PM emissions level of alcohol shown in Figure 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 of unsaturated compounds. The carbon to hydrogen ratio is low, and there is a tendency under pyrolysis and combustion to form smoke precursors. Unburned carbon atoms are more likely to occur, and the composition of the unburned and partially oxidized hydrocarbons in the diesel exhaust are much more complex and extend over a larger range of molecular size than those for NG and alcohol and, consequently, increase the PM emissions (6). Also, NG and alcohol do not contain any inorganic materials such as sulfur, so that their inorganic PM emissions will be lower than those of diesel engines. NOx Emissions. M100 has the lowest level of NOx emissions, and E93/E95 has the second lowest level, while NG and diesel have relatively the same levels of NOx emissions. These results support the position that alcohol fuels offer an advantage as alternative fuels for heavy-duty vehicles. The reason for lower NOx emissions from alcohol fuels can be explained as follows: most NOx emissions from combustion engines are formed by the oxidation of atmospheric nitrogen at high temperatures; therefore, the flame temperature is significant in determining NOx emissions (5). NOx production rates are highly sensitive to in-cylinder temperature. 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 are 8600, 11600, and 18500 BTU/lb, respectively (8). Therefore, more heat is needed for methanol and ethanol than for diesel in order to evaporate a certain mass of fuel, while the heat provided by methanol and ethanol is lower than that provided by diesel. As a result, the flame temperature is lower for methanol and ethanol than for diesel. For this reason, much less NOx will be formed during the premixed combustion period in methanol and ethanol engines. Test results also indicate that NOx emissions could be reduced to some extent even at increased compression ratios used for compression ignition of alcohol, which generally tend to increase NOx levels (9). Since most NG buses operate under lean premixed conditions, where flame temperatures are lower than in diesel engines, the NOx emissions from NG buses tend to be lower than those for diesel. NOx control in NG engines relies on both the spark timing and maintenance of the fuel-air mixture within a narrow band. As with gasoline, retarding the timing will reduce NOx formation but will also have an adverse effect on engine economy. The air-fuel ratio window is critical and calls for approximately 40% excess air in the cylinder (often termed a λ ratio of 1.4). If the mixture is allowed to become significantly leaner, misfire with associated high hydrocarbon emissions will arise. On the other hand, between the desired air-fuel ratio and a stoichiometric air-fuel ratio lies a significant “NOx peak”, which can yield NOx emissions and order of magnitude higher than those anticipated at the design air-fuel ratio. Experience has shown that NG lean burn NOx emissions may be held to a low value with careful timing and mixture control, but that cylinder-to-cylinder mixture variations and straying of the air-fuel ratio from the design point often bring the NOx emissions to the level of diesel engines. It is noteworthy, however, that the NOx emissions from diesel engines are weighed against engine efficiency in determining the diesel injection timing. In this way it is seen that the engine control technology often plays a higher role than the fuel in determining emissions. CO Emissions. CO emissions are the result of improper mixing and incomplete combustion and are controlled primarily by the global or local air/fuel equivalence ratio.
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Alcohol fuels have a lower flame temperature and a lower burn velocity than those of diesel, due to their lower net mass heating value and high vaporization cooling effect. With lower flame temperature and burn velocity, a fraction of methanol and ethanol may be found in a rich air-fuel ratio range or even liquid state as the flame front spreads too slowly to reach them or wall quenching may take effect. Therefore, they result in incomplete combustion and cause relatively high CO emissions. CO emissions of NG buses exhibit a high variance over the fleet, but on the average, the numbers are still lower than those of diesel buses. After investigation, it was found that most of the engines exhibiting high CO levels were early uncertified versions of NG engines. All of the later models of NG engines have significantly lower CO levels of less than 1 g/mi (10). Although most NG-fueled buses run under lean burn condition to take advantage of NG’s lean flammability limits, the effect of air-fuel ratio on CO emissions is still very significant. In many cases, analysis of second-by-second data showed that high gross CO arose due to an insufficiently lean idle condition rather than due to emissions under load. However, with proper air-fuel mixing and proper engine modification, these high CO emissions may be avoided (11). HC Emissions. Methanol, ethanol, and NG tend to have higher HC emissions levels than diesel. However, it must be noted that HC data in this study were reported as total hydrocarbon measured by a heated flame ionization detector (FID). However, FID response varies for oxygenated compounds. For alcohol-fueled vehicles, organic material hydrocarbon mass equivalent (OMHCE) is the designation often used to denote the total hydrocarbon mass emitted from an engine as unburned and partially burned fuel. OMHCE may be calculated by adding the mass contribution of unburned alcohol, formaldehyde, aldehydes, and residual hydrocarbon (RHC), which represents the remaining fraction. For NGfueled vehicles, HC consists mainly of the unburned methane. Methane is considered to be non-reactive in the formation of ozone in the atmosphere. Under normal combustion conditions, HC emissions are caused primarily by unburned mixtures, which indicate improper mixing and incomplete combustion. Flame quenching at the wall, particularly for homogeneous charge engines such as CNG lean burn engines, and adsorption and deposition from the oil film on the cylinder wall may also have an effect. Due to the slow flame front of alcohol, a fraction of alcohol will not completely burn in the cylinder when the exhaust valve opens. For a lean burn NG bus, as the load on the engine is reduced, the air-fuel mixtures may be too lean to burn efficiently, and partial misfire may occur. All these conditions result in high HC emissions. Through careful design of charge motion, HC emissions from NG and alcohol engines can be lowered to an acceptable level through proper air-fuel mixing.
Acknowledgments The authors wish to thank the U.S. Department of Energy, in particular the Office of Transportation Technologies, for the grant that made this study possible and the National Renewable Energy Laboratory (NREL) for collaboration of this project. Thanks also go to Byron Rapp and the staff at the 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 Heavy Duty 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 Transportable Laboratory for Emission Testing of Heavy Duty Trucks and Buses. Int. J. Vehicle Des. 1995, 2 (3/4), 308-322.
(3) Wang, W.; Gautam, M.; Sun, X.; Bata, R.; Clark, N.; Palmer, G.; Lyons, D. Emission Comparisons of Twenty-six Heavy-duty Vehicles Operated on Conventional and Alternative Fuels, SAE Paper 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 Natural Gas and Diesel Buses. SAE Paper 952746; SAE International Alternative 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 Review of Diesel Particulate Control Technology and Emissions Effectss1992 Horning Memorial Award Lecture. SAE Paper 940233; SAE International Congress and Exposition, Detroit, MI, February 28-March 3, 1994. (7) Shore, P.; Humphries, D.; Hadded, O. Speciated Hydrocarbon Emissions 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 Duty Vehicle Emissions Test Results. M.S. Thesis, West Virginia University, Morgantown, WV, 1995.
(9) Rideout, G.; Kirshenblatt, M.; Prakash C. Emissions from Methanol, 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 Evaluation Program Results. SAE Paper 961082; SAE International Spring Fuels & Lubricants Meeting, Detroit, MI, May 6-8, 1996. (11) Clark, N.; Wang, W.; Lyons, D.; Gautam, M.; Bata, R. Troubleshooting High Emissions from In-Service Alternative Fueled Buses. Presented at Windsor Workshop on Alternative Fuels, Toronto, Canada, June, 1996.
Received for review February 6, 1997. Revised manuscript received July 10, 1997. Accepted July 21, 1997.X ES9701063 X
Abstract published in Advance ACS Abstracts, September 1, 1997.
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