Physiochemical and Toxicological Properties of Emissions from CNG and Diesel Buses

This page last reviewed April 8, 2011

Background

The Air Resources Board (ARB) has led a multi-agency research effort to collect emissions data from late-model heavy-duty transit buses in five different configurations. The objectives of the study were 1) to assess driving cycle effects, 2) to evaluate toxicity between new and "clean" heavy duty engine technologies in use in California, and 3) to investigate total PM and ultrafine particle emissions.

Chassis dynamometer testing was conducted at ARB's Heavy-duty Emissions Testing Laboratory (HDETL) in Los Angeles. The impetus behind this work was to compare the emissions from transit buses powered by similar engines and fueled by ARCO (a BP company) Low Sulfur Emission Control Diesel (ECD-1) and compressed natural gas (CNG). Follow-on work focused on the assessment of aftertreatment control for CNG applications. Five vehicle configurations were investigated: 1) a CNG bus equipped with a 2000 DDC Series 50G engine certified for operation without an oxidation catalyst, 2) the same CNG bus retrofitted with an OEM oxidation catalyst, 3) a diesel bus equipped with a 1998 DDC Series 50 engine and a catalyzed muffler, 4) the same diesel vehicle retrofitted with a Johnson Matthey Continuously Regenerating Technology (CRT) diesel particulate filter (DPF) in place of the muffler, and 5) a CNG bus equipped with a 2001 Cummins Westport C Gas Plus engine and OEM-equipped oxidation catalyst.

The duty cycles were: 1) idle operation, 2) a 55 mph steady-state (SS) cruise condition, 3) the Central Business District (CBD) cycle, 4) the Urban Dynamometer Driving Schedule (UDDS), and 5) the New York City Bus Cycle (NYBC). Collection of PM over multiple cycles was performed to ensure sufficient sample mass for subsequent chemical analyses. Information on regulated (NOx, HC's, PM, and CO) and non-regulated (CO2, NO2, gas-phase toxic HC's, carbonyl compounds, polycyclic aromatic hydrocarbons, elements, and elemental and organic carbon) emissions was collected. Size-resolved PM mass and number emission measurements were conducted and extracts from diesel and CNG total PM samples were tested in the Ames mutagenicity bioassay analysis to determine mutagen emission factors.

An overview of results are reported below.


References

Results

  • ARB Board Presentation, (April 22, 2004). Study of CNG and Diesel Transit Bus Emissions. Presentation (PDF - 595KB)
  • CNG Oxidation Catalyst Evaluation
  • Robert A. Okamoto et al., Chemical and Bioassay Analyses of Emissions from Two CNG Buses with Oxidation Catalyst, Presentation (PDF - 278KB)
  • Alberto Ayala et al., (August 22, 2002). Partial Results: Emissions from Two CNG Buses with Oxidation Catalyst, Presentation (PDF - 1,543KB); Briefing Paper (PDF - 19KB)
  • CNG and Diesel Comparison
  • Alberto Ayala et al., ARB's Study of Emissions from Late Model Diesel and CNG Transit Buses - Toxic Compounds and PM Emissions, Presentation (PDF - 624KB)
  • Alberto Ayala et al., (November 16, 2001). ARB's Study of Emissions from Late Model Diesel and CNG Transit Buses, Presentation (PDF - 623KB)
  • ARB, (April 18, 2002). Air Board Tests Compare Diesel and CNG Bus Emissions. News Release; Briefing Paper (PDF - 34KB)

Publications for Peer-reviewed Journals

  • Norman Y. Kado et al., (2005). Emissions of Toxic Pollutants from Compressed Natural Gas and Low Sulfur Diesel-Fueled Heavy-Duty Transit Buses Tested over Multiple Driving Cycles. Environmental Science and Technology. Link
  • Robert A. Okamoto et al., (2005). Unregulated Emissions from Compressed Natural Gas (CNG) Transit Buses Configured with and without Oxidation Catalyst. Environmental Science and Technology. Link
  • Britt A. Holmén et al., (2004). Chemical Uncertainty in Particle Number Modal Analysis during Transient Operation of Compressed Natural Gas, Diesel, and Trap-Equipped Diesel Transit Buses. Environmental Science and Technology. Link
  • Alberto Ayala et al., (2003). Oxidation Catalyst Effect on CNG Transit Bus Emissions. SAE Technical Paper. Link
  • Alberto Ayala et al., (2002). Diesel and CNG Heavy-Duty Transit Bus Emissions Over Multiple Driving Schedules: Regulated Pollutants and Project Overview. SAE Technical Paper. Link
  • Britt A. Holmén et al., (2002). Ultrafine PM Emissions from Natural Gas, Oxidation-Catalyst Diesel, and Particle-Trap Diesel Heavy-Duty Transit Buses. Environmental Science and Technology. Link

Conference Presentations

  • M. Katherine Nanzetta-Converse et al., (2005). Size-Selective Measurements of Ultrafine Particle Emissions from a CNG and a Trap-Equipped Diesel Bus during Transient Operation. 15th CRC On-Road Vehicle Emissions Workshop, April 4-6, 2005, San Diego, CA. Presentation (PDF - 433KB)
  • Michael E. Gebel et al., (2004). Matrix Effects on the Stability of Toxic Hydrocarbons Stored in Tedlar Bags for Vehicle Exhaust Emissions Testing. 14th CRC On-Road Vehicle Emissions Workshop, March 29-31, 2004, San Diego, CA. Poster (PDF - 69KB)
  • Norman Y. Kado et al., (2003). Emissions of Toxic Pollutants from Compressed Natural Gas and Ultra-Low Sulfur Diesel - Fueled Transit Buses. ARB Chairman's Seminar Series, October 24, 2003, El Monte, CA. Presentation (PDF - 499KB)
  • Norman Y. Kado et al., (2003). Size-Segregated PM Emissions from Diesel with After-Treatment and from CNG-Fueled Transit Buses. 2003 American Association for Aerosol Research (AAAR) Annual Conference, October 20-24, 2003, Anaheim, CA. Presentation (PDF - 400KB)
  • Alberto Ayala et al., (2003). CNG and Diesel Transit Bus Emissions in Review. 9th Diesel Engine Emissions Reduction Conference, August 18-20, 2003, Newport, RI. Presentation (PDF - 223KB); Paper (PDF - 150KB)
  • Alberto Ayala et al., (2003). CNG and Diesel Transit Bus Emissions in Review. 7th ETH International Conference on Combustion Generated Nanoparticles, August 18-20, 2003, Zürich. Poster (PDF - 391KB)
  • Alberto Ayala et al., (2003). Comparison of Particle Size Distributions for Diesel and CNG Transit Buses with After-Treatment. 13th CRC On-Road Vehicle Emissions Workshop, April 7-9, 2003, San Diego, CA. Presentation (PDF - 1,874KB)
  • Alberto Ayala et al., (2003). Oxidation Catalyst Effect on CNG Transit Bus Emissions. 13th CRC On-Road Vehicle Emissions Workshop, April 7-9, 2003, San Diego, CA. Presentation (PDF - 422KB)
  • Alberto Ayala et al., (2003). Comparison of Emissions from Diesel and CNG Buses with After-Treatment. 4th World Truck Conference, March 2-4, 2003, Monterey, CA. Presentation (PDF - 168KB)
  • Alberto Ayala et al., (November 18-20, 2002). Highlights from CARB's Study of Emissions from Transit Buses. SAE International Truck & Bus Meeting and Exhibition, November 18-20, 2002, Detroit, MI. Presentation (PDF - 250KB)
  • Britt A. Holmén et al., (2002). Natural Gas Transit Bus Ultrafine Particle Emissions: Laboratory Dilution & Number Distributions. 2002 American Association for Aerosol Research (AAAR) Annual Conference, October 7-11, 2002, Charlotte, NC. Presentation (PDF - 2,036KB)
  • ARB, (2002). Diesel Emission Reductions: A Study of Transit Bus Alternatives for California. 2002 American Public Transportation Association Annual Meeting, September 22, 2002, Las Vegas, NV. Presentation (PDF - 411KB)
  • Alberto Ayala et al., (2002). ARB's Study of Emissions from Diesel and CNG Heavy-Duty Transit Buses. 8th Diesel Engine Emissions Reduction Conference, August 25-29, 2002, San Diego, CA. Presentation (PDF - 457KB)
  • Britt A. Holmén et al., (2002). Oxidation Catalyst Effects on Natural Gas Transit Bus Ultrafine Particle Emissions. 6th ETH International Conference on Combustion Generated Nanoparticles, August 19-21, 2002, Zürich. Presentation (PDF - 1,008KB)
  • Alberto Ayala et al., (2002). ARB's Study of Emissions from Two "Late-Model" Diesel and CNG Heavy-Duty Transit Buses. 12th CRC On-Road Vehicle Emissions Workshop, April 15-17, 2002, San Diego, CA. Presentation (PDF - 108KB)
  • Britt A. Holmén et al., (August 6-8, 2001). ARB's Study of Emissions from "Late-Model" Diesel and CNG Heavy-Duty Transit Buses: Preliminary Nanoparticle Measurement Results. 5th ETH International Conference on Combustion Generated Nanoparticles, August 6-8, 2001, Zürich. Presentation (PDF - 1,736KB); Abstract (PDF - 23KB)

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