Critical Review A Comparison of Emissions from Vehicles Fueled with Diesel or Compressed Natural Gas T H O M A S W . H E S T E R B E R G , † C H A R L E S A . L A P I N , * ,‡ AND WILLIAM B. BUNN† Navistar, Inc. 4201 Winfield Road, P.O. Box 1488, Warrenville, Illinois 60555 and Lapin and Associates, 1870 Calafia Street, Glendale, California 91208
Received July 12, 2007. Revised manuscript received April 7, 2008. Accepted April 15, 2008.
A comprehensive comparison of emissions from vehicles fueled with diesel or compressed natural gas (CNG) was developed from 25 reports on transit buses, school buses, refuse trucks, and passenger cars. Emissions for most compounds were highest for untreated exhaust emissions and lowest for treatedexhaust.CNGbuseswithoutafter-treatmenthadthehighest emissions of carbon monoxide, hydrocarbons, nonmethane hydrocarbons (NMHC), volatile organic compounds (VOCs; e.g., benzene, butadiene, ethylene, etc.), and carbonyl compounds (e.g., formaldehyde, acetaldehyde, acrolein). Diesel buses without after-treatment had the highest emissions of particulate matter and polycyclic aromatic hydrocarbons (PAHs). Exhaust after-treatments reduced most emissions to similar levels in diesel and CNG buses. Nitrogen oxides (NOx) and carbon dioxide (CO2) emissions were similar for most vehicle types, fuels, and exhaust after-treatments with some exceptions. Diesel school buses had higher CO2 emissions than the CNG bus. CNG transit buses and passenger cars equipped with three-way catalysts had lower NOx emissions. Diesel buses equipped with traps had higher nitrogen dioxide emissions. Fuel economy was best in the diesel buses not equipped with exhaust aftertreatment.
advances have occurred to control emissions from diesel and natural gas engines. The biggest emissions reduction advance for diesel engines came with the use of catalyzed diesel particulate filters in conjunction with ultralow sulfur diesel fuel, sometimes referred to as “New Diesel Technology” (NTD) (3). This paper will show that NTD exhaust is substantially lower in regulated and non regulated emissions compared to both preregulation Traditional Diesel Exhaust (i.e., pre-1988) and transitional-phase diesel exhaust (i.e., 1988-2006). A study completed by the California Air Resources Board (4–7) found that natural gas fueled transit buses emitted high levels of formaldehyde, a known human carcinogen. To reduce formaldehyde emissions, many natural gas engines are now equipped with oxidation catalysts. There have been 25 recent studies that compared emissions from diesel and compressed natural gas (CNG) fueled vehicles (4–31). However, a comprehensive assessment of these studies and the full impact of the new emissions control technologies on the chemical characteristics of engine emissions has not been published. The following review provides the first comprehensive assessment of emission benefits using the latest emissions controls for diesel and CNG fueled vehicles.
Methods Introduction In an effort to improve air quality, the U.S. Environmental Protection Agency has enacted increasingly more stringent emissions standards for vehicles with diesel engines (1). Diesel emissions of particulate matter (PM) and nitrogen oxides (NOx) have been reduced by more than 90% over the past decade. In 2007, emissions of PM were reduced by another 90%, and by 2010, NOx will be reduced by another 90% (1). In several metropolitan areas it has been particularly difficult to reach attainment of air quality standards despite the latest engine emission standards. Starting in 2000, to further work toward attainment, the South Coast Air Quality Management District, which covers the California counties of Los Angeles, Orange, Riverside, and San Bernardino, enacted rules encouraging the purchase of natural gas vehicles for government fleets (2). Based on engine certification data, it was felt natural gas engines would emit less PM and NOx than corresponding diesel engines. Over the last 7 years, significant * Corresponding author e-mail:
[email protected]; phone: 818802-2980; fax: 818-548-1606. † Navistar, Inc. ‡ Lapin and Associates. 10.1021/es071718i CCC: $40.75
Published on Web 07/25/2008
2008 American Chemical Society
Twenty-five reports were identified that had contemporaneously collected data suitable for comparing emissions from engines fueled with either diesel or CNG. Four groups of emissions endpoints were evaluated including (1) regulated and related compounds and fuel economy, (2) volatile organic compounds (VOCs), (3) carbonyls (formaldehyde, acetaldehyde, acrolein), and (4) total 2-ring, 3-ring, 4-ring and higher ring polycyclic aromatic hydrocarbons (PAHs). Regulated and related compounds included carbon monoxide (CO), total hydrocarbons (HC), particulate matter (PM), nitrogen oxides (NOx), nitrogen dioxide (NO2), carbon dioxide (CO2), nonmethane hydrocarbons (NMHC), and methane. VOCs included benzene, butadiene, ethylene, propylene, ethylbenzene, toluene, and xylenes. The majority of the studies (17) involved transit buses (4–8, 11, 14–21, 23–26, 28, 30, 31) compared to three studies each for refuse trucks (12, 13, 31) and passenger cars (9, 10, 27) and two for school buses (22, 29). For refuse trucks and passenger cars, there was only comparative data on regulated emissions and fuel economy. Most studies ran the vehicles on chassis dynamometers to generate emissions, while two measured emissions while the transit buses (28)or refuse trucks (31)were driven in traffic. Traffic measurements are less reproducible than measureVOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Regulated and Related Emissions and Fuel Economy in Transit Buses g/mi compound
diesel
diesel + oc
diesel + trap
cng
cng + oc
cng + twc
CO
7.71 ( 1.91 (21)b [3,4]c
3.46 ( 1.83 (23) [4]
0.62 ( 1.66 (28) [1,4,5]
14.16 ( 1.63 (29) [1-3,5,6]
5.43 ( 1.57 (31) [3,4]
4.93 ( 2.34 (14) [4]
HC
1.11 ( 1.93 (21) [4,5]
0.22 ( 1.84 (23) [4,5]
0.10 ( 1.84 (23) [4,5]
18.95 ( 1.67 (28) [1-3,5,6]
7.56 ( 1.77 (25) [1-4]
3.45 ( 2.36 (14) [4]
PM
0.63 ( 0.04 (18) [2-6]
0.43 ( 0.04 (23) [1,3-6]
0.03 ( 0.04 (28) [1,2]
0.05 ( 0.04 (25) [1,2]
0.03 ( 0.03 (31) [1,2]
0.04 ( 0.06 (11) [1,2]
NOx
27.7 ( 3.1 (24) [5,6]
23.5 ( 3.1 (23) [6]
26.2 ( 2.9 (27) [5,6]
26.6 ( 2.8 (29) [5,6]
18.1 ( 2.7 (31) [1,3,4,6]
7.7 ( 4.0 (14) [1-5]
NO2
1.68 ( 2.13 (6) [3]
1.17 ( 3.01 (3) [3]
11.61 ( 1.35 (15) [1,2,4-6]
4.12 ( 1.74 (9) [3]
1.67 ( 1.60 (12) [3]
0.10 ( 3.69 (2) [3]
NO2/NOx
0.05 ( 0.03 (6) [3,4]
0.05 ( 0.05 (3) [3]
0.38 ( 0.02 (15) [1,2,4-6]
0.15 ( 0.03 (9) [1,3]
0.12 ( 0.02 (12) [3]
0.03 ( 0.06 (2) [3]
CO2
2384 ( 286 (17)
2448 ( 245 (23)
2836 ( 231 (26)
2703 ( 231 (26)
2558 ( 215 (30)
2291 ( 372 (10)
NMHC
0.85 ( 0.65 (2)
0.09 ( 0.65 (2) [4]
0.03 ( 0.32 (8) [4]
1.64 ( 0.25 (13) [2,3,5,6]
0.80 ( 0.20 (17) [4]
0.29 ( 0.46 (4) [4]
methane
0.03 ( 12.63 (2)
0.02 ( 8.0 (5) [5]
0.00 ( 6.31 (8) [5]
9.97 ( 10.30 (3)
20.98 ( 3.90 (21) [2,3]
2.75 ( 8.92 (4)
MPG
4.03 ( 0.29 (11) [3-5]
4.51 ( 0.33 (8) [3-5]
3.22 ( 0.24 (16) [1,2]
2.67 ( 0.29 (11) [1,2]
2.54 ( 0.26 (13) [1,2]
3.43 ( 0.55 (8)
a
a Mean ( standard error. b Number of data points. diesel + trap, 4 - cng, 5 - cng + oc, or 6 - cng + twc.
c
Significantly different at P < 0.05 from 1 - diesel, 2 - diesel + oc, 3 -
ments on vehicles tested on chassis dynamometers. In the case of the transit buses where the data set was sufficiently large, the in-traffic measurements were excluded from the present analysis. In the case of the refuse trucks the in-traffic measurements were included as they represented nearly half the data points available for analysis. In the present analysis, comparisons were made within vehicle groups to minimize differences due to engine size and vehicle weight (load). Detailed vehicle information can be found in Tables SI-1 through SI-4 in the Supporting Information. Vehicles were grouped by the fuel type and type of exhaust after-treatment: none, oxidation catalysts (oc), catalyzed particulate filters (traps), or three-way catalysts (twc). Although not all engine and exhaust after-treatment configurations were tested by all test cycles, in the current review all test cycles were pooled to enlarge the sample size and to identify overall emission patterns. Then recognizing that emissions can differ among different test cycles, results were grouped according to test cycles where there were sufficient studies to provide useful comparisons (see Supporting Information for test cycle patterns and comparisons). Additionally, transit bus CO2 emission data were used to normalize regulated emission rates to minimize the impact of different test cycles. The vehicles used in this analysis were manufactured during a time of changing and increasingly stringent emission standards. While there was not sufficient engine emission certification information to group vehicles, there was suf6438
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ficient transit bus data to separate earlier (1994-1999) and later (2000-2004) model years for comparisons (see Supporting Information). An analysis of variance was first performed, and when the F statistic was significant, pairwise Tukey-Kramer multiple-comparison tests were performed. Statistical software was obtained from NCSS, Kaysville, UT, 84037.
Results Carbon Monoxide. In transit buses, diesel fueled vehicles had lower carbon monoxide (CO) emissions than their CNG fueled counterparts (Table 1). Exhaust after-treatment lowered emissions for both diesel and CNG fueled buses. Diesel transit buses with traps and CNG+twc buses had statistically similar CO emissions. The school bus results were consistent with transit bus results (Table 2). In contrast, the refuse trucks and passenger cars showed no differences in CO emissions regardless of fuel type or exhaust after-treatment (Tables 3 and 4). Hydrocarbons. As with CO emissions, diesel fueled transit buses had lower hydrocarbons (HC) emissions than their CNG fueled counterparts (Table 1). Exhaust after-treatment lowered HC emissions for both diesel and CNG fueled buses to levels that were not statistically different. In comparison to the transit bus data, the data for the other vehicle groups is less complete, in some cases lacking similar groups of uncontrolled and controlled vehicles. Despite these limita-
TABLE 2. Regulated and Related Emissions and Fuel Economy in School Buses g/mi compound
diesel + oc
diesel
diesel + trap
cng
CO
1.76 ( 0.28 (3)b [2,3,4]c
5.22 ( 0.13 (15) [1,3]
0.12 ( 0.12 (16) [1,2,4]
4.78 ( 0.28 (3) [1,3]
HC
0.39 ( 0.04 (3) [3,4]
0.44 ( 0.02 (15) [3,4]
0.02 ( 0.02 (16) [1,2,4]
9.33 ( 0.04 (3) [1,2,3]
PM
0.184 ( 0.015 (3) [3,4]
0.19 ( 0.007 (14) [3,4]
0.003 ( 0.006 (16) [1,2,4]
0.052 ( 0.015 (3) [1,2,3]
NOx
14.1 ( 0.4 (3) [2,3,4]
9.9 ( 0.2 (15) [1,3,4]
8.2 ( 0.2 (14) [1,2,4]
16.2 ( 0.4 (3) [1,2,3]
NO2
1.4 ( 0.2 (3) [3]
5.3 ( 0.2 (3) [1,4]
1.8 ( 0.2 (3) [3]
NO2/NOx
0.10 ( 0.01 (3) [3]
0.53 (3) [1, 4]
0.11 ( 0.01 (3) [3]
CO2
1526 ( 15 (3) [2,3,4]
1680 ( 6 (16) [1,2,4]
1199 ( 15 (3) [1,2,3]
NMHC
0.39 ( 0.05 (3) [3,4]
0.00 ( 0.05 (3) [1,4]
0.65 ( 0.05 (3) [1,3]
methane
0.00 ( 0.16 (3) [43]
0.01 ( 0.16 (3) [4]
8.94 ( 0.16 (3) [1,3]
MPG
6.6 ( 0.1 (3) [2,3,4]
5.9 ( 0.0 (16) [1,4]
4.3 ( 0.1 (3) [1,2,3]
a
a Mean ( standard error. diesel + trap, 4 - cng.
b
Number of replicates.
1626 ( 7 (15) [1,3,4]
5.9 ( 0.0 (15) [1,4] c
Significantly different at P < 0.05 from 1 - diesel, 2 - diesel + oc, 3 -
tions, the results from school buses, refuse trucks, and passenger cars were consistent with the transit bus results (Tables 2–4). Particulate Matter. In contrast to CO and HC emissions, diesel fueled transit buses had the highest particulate matter (PM) emissions (Table 1). CNG buses with and without exhaust after treatment and diesel buses with traps had statistically similar emissions. Within the limitations noted above, results from school buses, refuse trucks, and passenger cars were consistent with the transit bus results (Tables 2–4). Nitrogen Oxides. Transit bus diesels with and without after-treatment and CNG vehicles without after-treatment had similar nitrogen oxides (NOx) emissions. CNG + oc and CNG + twc buses had lower NOx emissions (Table 1). However, in school buses, NOx emissions were highest in the CNG bus (Table 2) followed by the diesel bus, while the trap-equipped diesel had the lowest NOx emissions. The lower NOx emissions in the trap equipped diesel were due to a low-NOx calibration of the engine control module and not the trap (28). While no statistically significant differences in NOx emissions were noted in refuse trucks, passenger car diesel + oc had higher NOx emissions than the CNG + twc vehicles (Tables 3 and 4). Nitrogen Dioxide. Diesel transit and school buses equipped with traps had the highest nitrogen dioxide (NO2) emissions and NO2/NOx ratios (Tables 1 and 2). This was
expected as catalyzed diesel particulate filters are designed to generate excess NO2 to oxidize the PM trapped by the filter. In transit buses without after treatment, CNG buses had higher NO2/NOx ratios than diesel buses. This difference was not observed in school buses without after treatment. Passenger cars with diesel + oc had higher NO2 emissions and NO2/NOx ratios than CNG + twc vehicles (Table 4). Carbon Dioxide. In transit buses, carbon dioxide (CO2) emissions were similar for all fuels and after treatments (Table 1). In contrast in school buses, CO2 emissions were highest in the diesel + trap bus (Table 2) followed by the diesel bus with the CNG having the lowest CO2 emissions. The refuse truck results were consistent with the transit bus results (Table 3). Nonmethane Hydrocarbons. The nonmethane hydrocarbon emissions (NMHC) were similar to the HC emissions. CNG fueled transit buses had the highest NMHC emissions (Table 1). Exhaust after-treatment lowered NMHC emissions by CNG engines to levels that were not statistically different from diesel engines. School bus results were consistent with the transit bus results (Table 2). Methane. Methane emissions data mirrored the HC and NMHC data. Again CNG fueled transit buses had higher methane emissions than diesels (Table 1). However, only the high methane emissions from the CNG + oc vehicles were statistically different from the diesels. As with most of VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Regulated Emissions and Fuel Economy in Refuse Trucks g/mi compound
diesel
diesel + oc
diesel + trap
cng + oc
cng + twc
5.10 ( 4.13 (3)
1.39 ( 3.58 (4)
17.75 ( 5.06 (3)
17.89 ( 7.01 (2)
CO
7.11 ( 2.92 (6)b
HC
5.58 ( 0.98 (4) [2,3,4]c
1.50 ( 1.13 (3) [1,4]
1.02 ( 1.12 (3) [1,4]
12.45 ( 1.38 (2) [1,2,3,5]
5.3 ( 1.96 (1) [4]
PM
1.51 ( 0.36 (6) [2,3,5]d
0.038 ( 0.51 (3) [1]d
0.24 ( 0.44 (4) [1]d
0.01 ( 0.63 (2)
0.02 ( 0.51 (3) [1]d
NOx
63.8 ( 15.0 (6)
34.1 ( 21.1 (3)
82.4 ( 18.3 (4)
58.9 ( 25.9 (2)
8.7 ( 21.2 (3)
CO2
4369 ( 993 (6)
MPG
1.9 ( 0.3 (4)
a
5814 ( 1216 (4) 3.0 ( 0.3 (3)
1.9 ( 0.3 (4)
1999 ( 1719 (2) 2.7 ( 0.4 (2)
2.5 ( 0.6 (1)
a Mean ( standard error. b Number of data points. c Significantly different at P < 0.05 from 1 - diesel, 2 - diesel + oc, 3 diesel + trap, 4 - cng + oc, or 5 - cng + twc. d Significant at P < 0.1.
TABLE 4. Regulated Emissions in Passenger Cars g/mi compound
diesel + oc
CO
1.12 ( 0.30 (7)b
HC
diesel + trap
cng + twc>
0.11 ( 0.57 (2)
0.68 ( 0.36 (5)
0.20 ( 0.18 (7) [3]c
0.02 ( 0.34 (2) [3]
0.95 ( 0.21 (5) [1,2]
PM
0.136 ( 0.018 (8) [2,3]
0.000 ( 0.037 (2) [1]
0.018 ( 0.021 (6) [1]
NOx
1.30 ( 0.16 (8) [3]
0.82 ( 0.31 (2)
0.29 ( 0.18 (6) [1]
NO2
0.43 ( 0.07 (5) [3]
0.02 ( 0.07 (5) [1]
NO2/NOx
0.27 ( 0.01 (5) [3]
0.06 ( 0.01 (5) [1]
a
a Mean ( standard error. b Number of data points. Significantly different at P < 0.05 from 1 - diesel + oc, 2 diesel + trap, or 3 - cng + twc.
c
the other emissions, school bus results were consistent with the transit bus results (Table 2). Volatile Organic Compounds. Several volatile organic compound emissions were highest in CNG transit and school buses. Exhaust after treatment reduced CNG bus emissions to levels that were not statistically different from diesels equipped with traps (Tables 5 and 6). Benzene emissions were higher in CNG transit buses and diesel school buses than in diesels equipped with traps. After-treatments lowered benzene emissions in CNG transit buses to levels not different from those found in diesels equipped with traps. Butadiene emissions were elevated in the CNG school bus, but no significant elevations were observed with transit buses. Ethylene emissions were highest in CNG transit and school 6440
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buses without after-treatment. Exhaust after-treatments reduced ethylene emissions to similar levels between diesel and CNG fueled transit buses. Propylene emissions were higher, but not statistically significant, in the CNG transit buses. Similarly, the propylene emissions in the CNG school bus were significantly higher compared to the diesel buses with and without a trap. Toluene emissions were also higher in CNG transit buses than in diesels. After-treatments lowered toluene emissions in CNG vehicles to levels found in diesels with and without traps. In contrast, toluene emissions were similar for CNG and diesel school buses. Ethylbenzene, o-xylene, and m- and p-xylene levels were low and similar in all transit buses and were not detected in school buses. Carbonyls. Carbonyl emissions (formaldehyde, acetaldehyde, and acrolein) were highest in CNG transit and school buses (Table 7). Diesel buses with traps and CNG buses with exhaust after-treatment had much lower and similar carbonyl emissions. Polycyclic Aromatic Hydrocarbons. Emissions of total 2-ring, 3-ring, and 4- and higher ring PAH compounds were highest in diesel transit and school buses not equipped with after-treatment (Tables 8 and 9). For most PAH compounds, diesels equipped with after-treatments had low emissions statistically similar to CNG buses with and without aftertreatment. Total Group B 4-ring and higher PAHs were lower in trap-equipped diesel school bus than in the CNG bus. Fuel Economy. Diesel transit and school buses without exhaust after-treatments and diesel + oc transit buses had the best fuel economy (i.e., highest miles/gallon (MPG)) (Tables 1 and 2). Exhaust after-treatment with traps lowered diesel fuel economy to CNG levels for transit buses. The trapequipped diesel school bus had better fuel economy than the CNG school bus. For refuse trucks, no differences in fuel economy were found (Table 3).
Discussion The engine emission control technologies evaluated in this review provided significant emission reductions. These improvements were readily evident when comparing the untreated engine emissions to engines with exhaust aftertreatment regardless of the fuel. This was particularly true comparing diesel engines without exhaust after-treatment, sometimes referred to as “traditional diesels”, to diesel
TABLE 5. Volatile Organic Compound Emissions in Transit Buses mg/mi diesel + oc
diesel
diesel + trap
cng
cng + oc
cng + twc
1.76 ( 1.54a (7)b
2.01 ( 1.66 (6)
0.41 ( 1.18 (12) [4]c
benzene 5.35 ( 0.94 (19) [3,5]
0.47 ( 1.54 (7) [4]
0.00 ( 2.88 (2)
4.26 ( 2.50 (7)
1.45 ( 2.70 (6)
2.32 ( 1.91 (12)
butadiene 3.57 ( 1.51 (19)
0.00 ( 2.50 (7)
0.00 ( 4.67 (2)
37.42 ( 120.6 (7) [4]
6.71 ( 225.63 (2) [4]
ethylene 3.35 ( 112.81 653.58 ( 96.21 (8) (11) [4] [1-3,5,6]
3.52 ( 130.27 (6) [4]
0.00 ( 225.63 (2) [4]
2.66 ( 70.4 (5)
0.00 ( 111.39 (2)
0.23 ( 64.31 (6)
0.32 ( 64.31 (6)
0.00 ( 111.39 (2)
2.05 ( 0.74 (4)
0.10 ( 0.66 (5)
ethylbenzene 0.09 ( 0.52 0.74 ( 0.37 (8) (16)
0.12 ( 0.56 (7)
1.36 ( 1.05 (2)
0.33 ( 1.71 (4) [4]
0.31 ( 1.53 (5) [4]
0.10 ( 1.21 (8) [4]
toluene 4.79 ( 0.85 (16) [1-3,5]
0.84 ( 1.29 (7) [4]
2.33 ( 2.41 (2)
0.40 ( 0.26 (4)
0.12 ( 0.23 (5)
0.36 ( 0.18 (8)
o-xylene 0.29 ( 0.13 (16)
0.13 ( 0.19 (7)
0.39 ( 0.36 (2)
0.88 ( 0.63 (4)
1.79 ( 0.56 (5)
m- and p-xylene 0.18 ( 0.45 0.75 ( 0.32 (8) (16)
0.32 ( 0.48 (7)
0.87 ( 0.89 (2)
a Mean ( standard error. b Number of data points. diesel + trap, 4 - cng, 5 - cng + oc, or 6 - cng + twc.
c
propylene 184.08 ( 52.51 (9)
Significantly different at P < 0.05 from 1 - diesel, 2 - diesel + oc, 3 -
TABLE 6. Volatile Organic Compound Emissions in School Buses mg/mi diesel
diesel + trap
benzene
4.67 ( 0.16a (3)b [2,3]c
0.00 ( 0.16 (3) [1,3]
4.33 ( 0.16 (3) [1,2]
butadiene
0.00 ( 0.98 (3) [3]
1.33 ( 0.98 (3) [3]
4.50 ( 0.98 (3) [1,2]
ethylene
27.07 ( 3.75 (3) [2,3]
2.57 ( 3.75 (3) [1,3]
361.67 ( 3.75 (3) [1,2]
propylene
7.83 ( 0.34 (3) [2,3]
0.00 ( 0.34 (3) [1,3]
66.00 ( 0.34 (3) [1,2]
toluene
4.33 ( 0.77 (3)
2.00 ( 0.77 (3)
3.23 ( 0.77 (3)
compound
cng
a Mean ( standard error. b Number of replicates. Significantly different at P < 0.05 from 1 - diesel, 2 diesel + trap, 3 - cng.
c
engines equipped with catalyzed particulate filters or “new technology diesels” (3). Since exhaust after-treatment systems are installed in newly manufactured vehicles and retrofitted
in older vehicles, it is of interest to compare the emissions from CNG and diesel fueled engines equipped with exhaust after-treatment. A side-by-side comparison of emissions from CNG and diesel vehicles equipped with the best performing emissions controls reveals some distinctions (Table 10). For regulated emissions, diesels tended to have lower HC and methane emissions, but diesel NOx and NO2 emissions were higher. PM and CO levels were quite low in both CNG and diesels. The school bus data for PM and CO are not quite in line with the other vehicles as the CNG school bus was not equipped with an exhaust after-treatment system. For unregulated emissions, the best emissions controls lead to a convergence of similar low emission levels for CNG and diesel transit buses (Table 10). Again the CNG school bus data for VOCs and aldehydes are high in part because the vehicle tested was not equipped with an exhaust aftertreatment system. For the most part, emissions of CO2, a greenhouse gas, were similar for CNG and diesel fueled vehicles. While the diesel school bus equipped with a trap had higher CO2 emissions than a CNG bus (29), this difference was not observed in a larger sample of 18 diesel buses and 68 CNG buses (32). The tailpipe emission results are consistent with several life-cycle analyses that compared greenhouse gas emissions between CNG and diesel transit buses (33) and passenger cars (34–39). One concern in this review was the incompleteness of the data available for statistical analyses. Not all after-treatment systems were tested by all test cycles. As expected our analyses found a strong test cycle effect (see Supporting Information). VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 7. Carbonyl Emissions in Transit and School Buses mg/mi vehicle transit bus school bus
transit bus school bus
transit bus school bus
diesel
diesel + oc
59 ( 209a (7)b [4]c 27 ( 8.6 (3) [4]
18 ( 276 (4) [4]
27.91 ( 7.92 (7) [3] 9.4 ( 0.9 (3) [3,4]
18.61 ( 10.48 (4)
0.52 ( 1.25 (5) [4] 3.27 ( 0.32 3 [3,4]
0.70 ( 1.98 (2)
diesel + trap
cng
cng + oc
cng + twc
formaldehyde 3.42 ( 175 (10) [4] 5.2 ( 8.6 (3) [4]
1113 ( 148 (14) [1-3,5,6] 495 ( 8.6 (3) [1,3]
25 ( 225 (6) [4]
0 ( 391 (2) [4]
acetaldehyde 1.93 ( 6.63 (10) [1,4] 2.7 ( 0.9 (3) [1,4]
37.28 ( 5.60 (14) [3,5,6] 24.3 ( 0.9 (3) [1,3]
11.05 ( 8.56 (6) [4]
0.4 ( 14.82 (2) [4]
acrolein 0.11 ( 1.06 (7) [4] 0.45 ( 0.32 3 [1,4]
5.03 ( 0.77 (13) [1,3,5] 4.90 ( 0.32 3 [1,3]
0.00 ( 1.40 (4) [4]
a Mean ( standard error. b Number of data points (transit buses) or replicates (school buses). c Significantly different at P < 0.05 from 1-diesel, 2-diesel+oc, 3-diesel+trap, 4-cng, 5-cng+oc, or 6-cng+twc.
TABLE 8. Total 2-Ring and 3-Ring Polycyclic Aromatic Hydrocarbon Emissions in Transit and School Buses (µg/mi) diesel
7200 ( 950a (3)b [2-6]c
diesel+oc
339 ( 738 (5) [1]
2320 ( 74 (3) [3,4]
650 ( 135 (6) [2-6] 818 ( 15 (3) [3,4]
91 ( 150 (5) [1]
diesel + trap
cng
total 2 ring PAHs transit bus 216 ( 623 385 ( 497 (7) (11) [1] [1] school bus 19 ( 74 105 ( 74 (3) (3) [1] [1] total 3 ring PAHs transit bus 98 ( 105 102 ( 80 (10) (17) [1] [1] school bus 1.36 ( 15 53.1 ( 15 (3) (3) [1] [1]
cng + oc
cng + twc
49 ( 673 (6) [1]
10 ( 1170 (2) [1]
5.6 ( 135 (6) [1]
1.9 ( 234 (2) [1]
a Mean ( standard error. b Number of data points (transit buses) or replicates (school buses). c Significantly different at P < 0.05 from 1 - diesel, 2 - diesel + oc, 3 - diesel + trap, 4 - cng, 5 - cng + oc, or 6 - cng + twc.
A recent study of CNG transit buses found prominent test cycle effects on regulated emissions and fuel economy (40). We normalized transit bus regulated emissions against CO2 emissions to offset differences in amount of work required by the three most common test cycles. Normalized data demonstrated trends consisted with the pooled data. The main reason we chose to pool all test cycles was to enlarge the sample size and to identify overall emission patterns. While the pooling of test cycles undoubtedly increased group variability diminishing the ability to make distinctions between test groups, clear trends were observed for improved emissions performance with exhaust after-treatment systems. The overall trends observed in the pooled transit bus data were for the most part also 6442
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observed when the data were analyzed by model year or test cycle. However diesel + trap NOx emissions varied between being similar to or higher than the CNG + twc buses. And under one test cycle, the Central Business District cycle, CNG + twc buses had higher CO emissions. Additionally, a recent study using a composite test cycle found similar PM emissions for heavy duty diesel trucks compared to the transit buses reported here (41). While test cycle is probably the largest source of variability in the studies reviewed here, other sources of variability include engine age, manufacturer, and size (42). For transit buses, we were able to separate older and newer buses which would help take into account engine wear and the lower emission standards for the newer buses. We
TABLE 9. Total 4-Ring and Higher Polycyclic Aromatic Hydrocarbon Emissions in Transit and School Buses (µg/mi) diesel + oc
diesel
119 ( 26 (6)b [3-6]c
a
89 ( 1.2 (3) [3,4]
243 ( 32 (3) [2-6]
diesel + trap
total 4- and higher ring PAHs: Group A transit bus 46 ( 29 9.9 ( 20 (5) (10) [1] school bus 2.5 ( 1.2 (3) [1]
9.9 ( 20 (5) [1]
8.4 ( 0.2 (3) [3,4]
cng + oc
cng
cng + twc
(fluoranthenes, pyrenes) 21 ( 16 (17) [1]
7.4 ( 26 (6) [1]
0.10 ( 45 (2) [1]
0.6 ( 18 (6) [1]
0.5 ( 32 (2) [1]
5.4 ( 1.2 (3) [1]
total 4 and higher ring PAHs Group B (others) transit bus 6.3 ( 17 2.9 ( 14 (7) (11) [1] [1] school bus 0.2 ( 0.2 3.2 ( 0.2 (3) (3) [1,4] [1,3]
a Mean ( standard error. b Number of data points (transit buses) or replicates (school buses). c Significantly different at P < 0.05 from 1 - diesel, 2 - diesel + oc, 3 - diesel + trap, 4 - cng, 5 - cng + oc, or 6 - cng + twc.
TABLE 10. Comparisons of Diesel and CNG Fueled Vehicles Equipped with the Best Emission Control Technologies Tested transit bus diesel + trap vs CNG + twc CNG had higher emissions
methanea
refuse truck
passenger car
school bus
diesel + oc vs diesel + trap vs CNG + oc CNG + twc COa, HCa
HC
diesel + trap vs CNGb CO, HC, NMHC, PM, NOx, methane, VOCs, aldehydes, 4+ring Group B total PAHs
CO, PM, NOx CO, PM, NOx diesel and CNG had CO, HC, NMHC, PM, CO2, VOCs, aldehydes, total PAHs similar emissions
2, 3, and 4+ring Group A total PAHs
diesel had higher emissions
NO2, CO2
NOx, NO2
a Absolute value substantially higher for CNG, but not statistically significant due to high variance. not have exhaust after-treatment.
found emission trends within older or newer bus groups that were consistent with the pooled data (see Supporting Information). We did not compare older bus groups to the newer bus groups as there were a disproportionate number of older buses tested by the New York Bus test cycle which produces the highest emissions due to greater demands on the engine (see Supporting Information). Even when all sources of vehicle variability are controlled, variability between testing facilities can still result in variability (43). Testing facility variability arises from the fact that actual drivers are used to accelerate and brake the vehicles over the tests’ cycles while on the vehicle dynamometers. Despite limitations in the data, a clear benefit of reduced emissions was found in engines equipped with emission controls. For most emission components compared, vehicles equipped with emission controls had the lowest emissions regardless of fuel used or vehicle size. Importantly, the application of emission controls to meet mandated emission requirements has had the additional benefit of reducing unregulated emissions of toxicological interest such as benzene, butadiene, formaldehyde, and polycyclic aromatic hydrocarbons. These data also support efforts to reduce emissions from older vehicles by retrofitting them with exhaust after-treatment systems.
b
CNG school bus did
Acknowledgments T.W.H. and W.B.B. are employed by Navistar, Inc., a major manufacturer of diesel engines and vehicles. C.A.L. is a consultant to Navistar, Inc.
Supporting Information Available Additional details of the study are shown in 24 tables and 15 figures. This information is available free of charge via the Internet at http://pubs.acs.org.
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