Tuesday, August 8, 2006

CARB CNG and Diesel Transit Bus Emissions Research


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.