Masters Theses

Date of Award

5-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

J. W. Hodgson

Committee Members

Ke Nguyen, J. R. Parsons

Abstract

Due primarily to environmental concerns, the California Air Resources Board has developed an aggressive series of transitional emission standards for automobiles operating in that state. With many other states following California’s lead, a substantial amount of research has been initiated, both by federal and industry leaders, to lower the regulated emissions of production vehicles. One facet of this research focuses on obtaining these low vehicle emissions through the use of an alternative fuel. This study demonstrates that compressed natural gas through some specific engine modifications can be used to meet the 1997 Ultra Low Emission Vehicle (ULEV) standard. An evaluation was performed to determine what effect these modifications had on the performance characteristics of the vehicle. The conversion vehicle utilized was a 1991 Saturn SL1 sedan dedicated to natural gas operation. This vehicle was modified to use a very small amount of exhaust gas recirculation and a Palladium/Rhodium catalytic converter developed for the natural gas fuel. Utilizing multi-point fuel injection and an exhaust gas oxygen sensor for closed-loop control over fuel metering, an air/fuel ratio providing the highest overall catalytic conversion efficiency was implemented. With this configuration and a detailed analysis of hydrocarbon emissions, the conversion vehicle was found to meet the 1997 ULEV standard. With these engine modifications, the vehicle was subjected to two types of vehicle energy efficiency evaluations. These tests, in addition to supplying a miles per equivalent gallon (mpge) value, were used to determine a vehicle range estimate. The vehicle was also compared to the gasoline-fueled stock model during two types of vehicle acceleration tests. These different vehicle performance tests demonstrated that the vehicle’s energy efficiency increased by 12 to 25 mpge. Yet with this improvement in energy efficiency, the vehicle’s range decreased by 200 to 150 miles because of the restrictions on vehicle fuel storage. The vehicle’s acceleration was several seconds slower than the gasoline-fueled vehicle but still quite acceptable for a vehicle of this type. It was shown by this research, through a more precise fuel metering system and emission control devices designed specifically for the natural gas fuel, that the conversion vehicle could meet the California 1997 Ultra Low Emission Vehicle standard. It was also shown that with the engine modifications required for this level of vehicle emissions, the vehicle’s performance characteristics were above acceptable levels and proved to be comparable to the gasoline-fueled model.

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