California Air Resources Board

CARB 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 to

• Assess driving cycle effects
• Evaluate toxicity between new and "clean" heavy duty engine technologies in use in California
• Investigate total PM and ultrafine particle emissions

Phase I

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
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 (NO_x, 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.

Phase II

Phase II provided a 'snapshot' of what emissions would look like in 2010 and beyond by testing vehicles with advanced after-treatment, such as particulate filters and NO_x scrubbers, and using various types of fuel, such as diesel, CNG and ethanol blends. The major objective of this 4-year study focused on the physicochemical and toxicological properties of the semi-volatile and non-volatile fractions of PM from vehicular emissions. Knowledge of how the toxicity of vehicular PM varies with particle component volatility evaluated the design of emissions control technologies and provided a better understanding of exposure. This in turn provided the information needed to better protect public health.

Heavy-Duty Diesel Vehicles (HDDV)

Four heavy-duty diesel vehicles in seven configurations were tested at ARB's heavy-duty diesel emission testing laboratory (HDETL) in downtown Los Angeles. The facility included a heavy-duty chassis dynamometer, a constant volume sampling (CVS) system for measurement of THC, NO_x, CO, CO₂, and PM as described in 40 CFR 86, and included specialized aerosol sampling equipment. The after-treatment devices were selected to represent technologies available to the California on-road fleet to meet the US07 PM and US2010 NO_x emission standards. The configurations included:

1. A baseline vehicle without any emission control technology
2. The same vehicle retrofitted with a combination oxidation catalyst (OC) and diesel particulate filter (DPF)
3. The same vehicle retrofitted as in configuration 2 with further addition of zeolite and vanadium based selective catalytic reduction systems (SCR) respectively
4. A Caltrans truck retrofitted with diesel particulate trap with a catalytic wash-coat
5. A diesel hybrid electric bus equipped with an OC followed by a catalyzed trap
6. A school bus equipped with an electrically regenerated uncatalyzed DPF

The heavy-duty diesel vehicles were tested over three driving cycles: 1) a 50 mph steady state cruise, 2) a transient cycle EPA urban dynamometer driving schedule (UDDS) and 3) idle to simulate various real-world driving conditions. Regulated emissions including PM were measured using standard CFR methods for each test. Each cycle was repeated until sufficient PM mass was loaded onto a separate set of collection substrates for chemical and toxicological analysis. Chemical speciation included PAH's, inorganic ions, organic and elemental carbon, and trace metals. Particle sizing and counting was conducted with real-time spectrometers and particle counters. Size-resolved samples were collected by a MOUDI- NanoMOUDI loaded with aluminum foil substrates. Two parallel thermal denuders were used to remove the volatile fraction of PM at temperature of 150 and 230^oC, respectively. Particulate-bound PAHs and active surface area of emissions were measured in real time with commercial instruments.

CNG Transit Buses

Testing on two CNG transit buses with 2010 compliant engines for 0.2g/bhp-hr NOx and 0.01g/bhp-hr PM standards were conducted using West Virginia University's transportable chassis dynamometer laboratory located at University of California, Riverside. Both CNG buses were equipped with exhaust after-treatment devices, three-way catalytic converters. The same testing protocols used for the heavy-duty diesel vehicles were applied for CNG transit buses over two driving cycles: 1) a 45 mph steady state cruise and 2) a transient cycle EPA urban dynamometer driving schedule (UDDS).

Light-Duty Gasoline Vehicles (LDGV)

Five light-duty gasoline vehicles in a total of eight separate configurations were tested at ARB's Hagen-Smit Laboratory in El Monte, California. These included both a newer and an older, "high-emitting" gasoline vehicle, a light-duty diesel truck with a diesel particulate filter using both diesel and biodiesel fuel, vehicles running on both compressed natural gas and 85% ethanol, and a gasoline direct injection vehicle both with and without a gasoline particulate filter.

Results

• CARB Contract 05-308 Main page and Final Report
• CARB Contract 07-340 Main page and Final Report
• CARB Board Presentation, (April 22, 2004). Study of CNG and Diesel Transit Bus Emissions
• CNG Oxidation Catalyst Evaluation
  • Robert A. Okamoto et al., Chemical and Bioassay Analyses of Emissions from Two CNG Buses with Oxidation Catalyst
• CNG and Diesel Comparison
  • Alberto Ayala et al., CARB's Study of Emissions from Late Model Diesel and CNG Transit Buses - Toxic Compounds and PM Emissions
  • Alberto Ayala et al., CARB's Study of Emissions from Late Model Diesel and CNG Transit Buses
  • CARB, (April 18, 2002). Air Board Tests Compare Diesel and CNG Bus Emissions. News Release; Briefing Paper

Peer-reviewed Journal Publications

• Arvind Thiruvengadam et al., (2013). Characterization of Particulate Matter Emissions from a Current Technology Natural Gas Engine
• Seungju Yoon et al., (2013). Chemical and Toxicological Properties of Emissions from CNG Transit Buses Equipped with Three-Way Catalysts Compared to Lean-Burn Engines and Oxidation Catalyst Technologies
• Seungju Yoon et al., (2013). Criteria Pollutant and Greenhouse Gas Emissions from CNG Transit Buses Equipped with Three-Way Catalysts Compared to Lean-Burn Engines and Oxidation Catalyst Technologies
• Shaohua Hu et al., (2013). Emissions of Polycyclic Aromatic Hydrocarbons (PAHs) and Nitro-PAHs from Heavy-Duty Diesel Vehicles with DPF and SCR
• Martin Shafer et al., (2011). Chemical Speciation of Vanadium and Application to Particulate Matter Emitted from Diesel Vehicles and Urban Atmospheric Aerosols
• Jorn Herner et al., (2011). Effect of Advanced Aftertreatment for PM and NO_x Reduction on Heavy-Duty Diesel Engine Ultrafine Particle Emissions. Environmental Science and Technology.
• Vishal Verma, et al., (2010). Contribution of Transition Metals in the Reactive Oxygen Species Activity of PM Emissions from Retrofitted Heavy-Duty vehicles. Atmospheric Environment.
• Jorn Herner et al., (2009). The Effect of Advanced Aftertreatment for PM and NO_x Control on Heavy Duty Diesel Truck Emissions. Environmental Science and Technology.
• Payam Pakbin et al., (2009). Characterization of Particle Bound Organic Carbon from Diesel Vehicles Equipped with Advanced Emission Control Technologies. Environmental Science and Technology.
• Subhasis Biswas et al., (2009). Oxidative Potential of Semi-Volatile and Non Volatile Particulate Matter (PM) from Heavy-Duty Vehicles Retrofitted with Emission Control Technologies.
• Rongsong Li et al., (2009). Ultrafine Particles from Diesel Engines Induce Vascular Oxidative Stress via JNK Activation. Free Radical Biology and Medicine.
• Shaohua Hu et al., (2009). Metals Emitted from Heavy-duty Diesel Vehicles Equipped with Advanced PM and NO_x Emission Controls. Atmospheric Environment.
• Subhasis Biswas et al., (2008). Chemical Speciation of PM Emissions from Heavy-duty Diesel Vehicles Equipped with Diesel Particulate Filter (DPF) and Selective Catalytic Reduction (SCR) Retrofits. Atmospheric Environment.
• Subhasis Biswas et al., (2008). Physical Properties of Particulate Matter (PM) from Newer Heavy Duty Diesel Vehicles Operating with Advanced PM and NOx Emission Control Technologies. Atmospheric Environment.
• Andrea Polidori et al., (2008). Real-time Characterization of Particle-bound Polycyclic Aromatic Hydrocarbons in Ambient Aerosols and from Motor-vehicle Exhaust. Atmospheric Chemistry and Physics.
• 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.
• 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.
• Britt A. Holmen 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.
• Alberto Ayala et al., (2003). Oxidation Catalyst Effect on CNG Transit Bus Emissions. SAE Technical Paper.
• 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.
• Britt A. Holmen et al., (2002). Ultrafine PM Emissions from Natural Gas, Oxidation-Catalyst Diesel, and Particle-Trap Diesel Heavy-Duty Transit Buses. Environmental Science and Technology.