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Compressed Natural Gas and Clean Diesel Buses

Performance and Emissions Evaluation of Compressed Natural Gas and Clean Diesel Buses at New York City's Metropolitan Transit Authority

  • Thomas Lanni, Brian P. Frank, Shida Tang, Division of Air Resources, New York State Department of Environmental Conservation
  • Deborah Rosenblatt, Environment Canada
  • Dana Lowell, New York City Metropolitan Transit Authority


The New York City Metropolitan Transit Authority (MTA) has initiated a program to utilize various diesel emission control, alternative fuel, and hybrid electric drive technologies as part of its ongoing effort to provide environmentally friendly bus service. The New York State Department of Environmental Conservation (DEC) has joined with the MTA and Environment Canada in evaluating this program, and has established a protocol for measuring both regulated and unregulated emissions, as well as other operational parameters. This paper compares and contrasts the emissions of buses powered by Detroit Diesel Series 50 diesel engines with and without continuously regenerating diesel particulate filters (CRDPFs), and by Series 50G Compressed Natural Gas (CNG) engines. All buses were tested for regulated emissions at the Emissions Research and Measurement Division of Environment Canada, in Ottawa, Ontario, and unregulated emissions measurements, including particle size distributions and chemical analysis, were supported by DEC staff.

CNG buses were generally found to produce more carbonyls, up to an order of magnitude higher than diesel buses. Greater than 96% of the total carbonyl emissions from CNG buses were formaldehyde, in contrast to 60-65% formaldehyde for the diesel buses. Reduction of carbonyls by the CRDPF was so efficient that most were below the method detection limit. Compared to CRDPF buses, higher emission of carbonyl, benzene, ethylene, propylene, toluene and PAHs from CNG buses without oxidation catalysts may contribute to the reported unfavorable toxic potentials from CNG buses. The reduction in PM of the CNG buses was approximately equal to that of the Series 50 CRDPF buses, i.e., a 90% reduction from Series 50 buses without CRDPF using 30 or 300 ppm sulfur fuel.

Regulated Emissions from CNG and Series 50 Diesel Buses
Bus ID Test Cycle Config Fuel FE
NYCTA #824 CBD OEM CNG 3.5 2126 46.7 17.5 22.8 0.017
NYCTA #854 CBD OEM CNG 3.4 2230 16.6 17.9 11.4 0.013
NYCTA #975 CBD OEM CNG 3.0 2506 19.1 17.9 11.5 0.019
NYCTA #6019 CBD CRDPF ULSD 3.2 3079 25.9 0.035 0.159 0.035
NYCTA #6065 CBD CRDPF ULSD 3.7 2672 21.8 0.0803 0.107 0.047
NYCTA #824 NYB OEM CNG 1.4 5222 73.3 78.1 68.9 0.065
NYCTA #854 NYB OEM CNG 1.3 5870 24.2 63.3 40.2 0.040
NYCTA #975 NYB OEM CNG 1.3 5672 30.9 33.7 33.4 0.055
NYCTA #6019 NYB CRDPF ULSD 1.4 6946 68.6 0.04 0.16 0.04
NYCTA #6065 NYB CRDPF ULSD 1.6 6326 60.0 0.08 0.11 0.05
Bar chart comparison of carbonyl emissions from various transit buses with CBD driving cycles
Transit Bus Carbonyl Emissions with CBD Driving Cycles