Diesel and CNG Heavy-duty Transit Bus Emissions over Multiple Driving Schedules: Regulated Pollutants and Project Overview 2002-01-1722

The Air Resources Board (ARB) led a multi-division, multi-agency research effort to collect emissions data from two late-model heavy-duty transit buses in three 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 Emission Control Diesel (ECD-1) and compressed natural gas (CNG). Three vehicle configurations were investigated: 1) a CNG bus equipped with a 2000 DDC Series 50G engine, 2) a diesel bus equipped with a 1998 DDC Series 50 engine and a catalyzed muffler, and 3) the same diesel vehicle retrofitted with a Johnson Matthey Continuously Regenerating Technology (CRT™) diesel particulate filter (DPF) in place of the muffler. The CNG engine was certified for operation without an oxidation catalyst. The diesel vehicle was fueled by ARCO (a BP company) ECD-1. 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 (PAH), 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. Some preliminary results were reported in [1].

Emissions of measured pollutants showed cycle dependence. The shortest cycle, the NYBC, resulted consistently in the highest g/mi emissions of all regulated, CO₂, and NO₂ (for the CRT) emissions for all three vehicle configurations. Diesel (OxiCat) total PM emissions were the highest, as expected, compared to the CRT and CNG configurations. But the CRT was able to achieve an average reduction of 85% across all cycles based on data uncorrected for tunnel blanks. The CNG without oxidation catalyst resulted in the highest emissions of THC and CO relative to the diesel configurations. Total Diesel (OxiCat) NO_x levels were essentially unchanged by the CRT and were higher compared to the CNG. However, during the CNG re-test, total NO_x showed a considerable increase relative to earlier results. The CRT catalyst generated NO₂/NO_x fractions that ranged from 40 % to 50 % across all cycles. This is in contrast to the NO₂ emission fractions from the Diesel (OxiCat), which were in the single-digit percentage range.

This paper presents results for the regulated, NO₂ and CO₂ emissions for all cycles, except idle, and an overview of the entire project.