Project at a Glance

Title: Effect of Maintenance Practices and Fuel Quality on Transit Bus Smoke and Particulate Emissions

Principal Investigator / Author(s): Crawford, R.

Contractor: Sierra Research, Inc.

Contract Number: A2-065-32

Research Program Area: Economic Analysis

Topic Areas: Impacts, Mobile Sources & Fuels


An analysis of smoke emissions from Diesel transit buses indicates that while most buses have peak smoke levels which are invisible or just barely visible, some buses emit peak smoke levels of 40-45% opacity. Buses with the lowest smoke levels tend to be equipped with older, naturally aspirated engines. Buses with the highest smoke levels are usually equipped with later model, turbocharged engines. However, some of these late model engines exhibit very low smoke levels, while others have relatively high smoke levels.

Transit district officials believe that late model, turbocharged engines are more difficult to keep in proper tune. In addition, the variability in smoke emissions from late model turbocharged engines seems to indicate that late model engines are quite sensitive to the manner in which they are maintained. The incorporation of routine smoke measurements may be effective in detecting those engines which are in need of further adjustment.

Analysis of bus engine maintenance histories indicates that the frequency of smoke related maintenance is a very poor predictor of exhaust opacity in customer service. This fact supports the position that bus smoke would be controlled more effectively through an "inspection and maintenance" (I/M) approach, rather than a "mandatory maintenance" approach. Identification and correction of the highest smoke buses of each engine type could reduce smoke and particulate emissions from the entire fleet by approximately 35%.

The use of turbocharging and engine calibrations used to meet current NOx emission standards appears to be related to increased smoke levels; however, the newest turbocharged engines have significantly lower smoke levels than 1980-1982 models. Improved turbocharger matching and other improvements incorporated into new engines may offer some potential for improving the performance of earlier model engines. Because the components used on late model engines are interchangeable with those used on earlier models, the "upgrading" or retrofit of engines during rebuilding may be an effective means of reducing smoke and particulate emissions.

Differences in the engine models used by transit districts made it impossible to determine whether differences in fuel specifications are having a significant effect on smoke emissions from buses. However, a literature review indicates that fuels with lower aromatics content and lower 90% boiling point reduce smoke and particulate emissions. Although the cost of requiring all Diesel fuel to meet more stringent fuel quality specifications may be relatively high, costs would be significantly lower if only the limited amount of Diesel fuel used by transit districts were subject to more stringent requirements.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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