Masters Theses

Date of Award

12-1997

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

J. W. Hodgson

Committee Members

Ke Nguyen, J. R. Parsons

Abstract

With a growing awareness of the global environmental impact of conventional petroleum fuels, restrictions on vehicle exhaust emissions are becoming more stringent at both the state and federal levels. Due primarily to areas of dense population and the associated commuter traffic in California, the California Air Resources Board (CARB) has led the development of stringent emissions standards in an effort to stimulate the production of cleaner, more environmentally friendly vehicles. One method for reducing exhaust emissions from motor vehicles is the use of alternative fuels.

The University of Tennessee Department of Mechanical and Aerospace Engineering and Engineering Science owns a 1991 Saturn SL1 sedan which has been converted to dedicated compressed natural gas (CNG) operation. The vehicle had previously met the emissions requirements of the CARB Ultra-Low Emissions Vehicle (ULEV) standard while calibrated for very low levels of exhaust gas recirculation (EGR). It is important to note, however, that the ULEV standard requires that the specified emission levels also be met after the vehicle has reached 100,000 miles of service, which the UTK Saturn has not accumulated yet. Based on the emissions testing results, it was thought that a greater level of EGR might reduce the level of oxides of nitrogen (NO) in the exhaust products. The vehicle was evaluated to determine the effects of stoichiometry and increased EGR on NO, emissions, spark timing, and driveability. The vehicle was then emissions tested over the Federal Test Procedure (FTP) Urban Driving Cycle (UDC) and the results were compared to the goal, the California ULEV standard. Once these emissions levels were achieved, the acceleration performance of the vehicle was evaluated against that of a comparable gasoline-fueled model.

This study shows that for a conventional spark-ignition (SI) engine using CNG as its dedicated fuel, a substantial amount of exhaust gas recirculation may be used to decrease NOx levels in the exhaust while maintaining levels of other exhaust constituents below those of the California ULEV. NOx levels achieved were less than 25% of ULEV. A vehicle performance evaluation indicates that the increased level of EGR does not substantially affect the driveability of the vehicle but that its acceleration potential is reduced relative to that of the production gasoline-fueled vehicle. An investigation of initial catalyst aging after 4,000 vehicle miles also reveals the importance of high catalyst activity to meeting the stringent ULEV standard.

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