1993-03-01

Fuel Composition Effects on Automotive Fuel Economy - Auto/Oil Air Quality Improvement Research Program 930138

Fuel economy measurements from portions of Phase I of the Auto/Oil Air Quality Improvement Research Program were analyzed. The following fuel variables were examined: aromatics, olefins, T90, RVP, and various oxygenates (MTBE, ETBE and ethanol). Two vehicle fleets were tested: twenty 1989 vehicles and fourteen 1983-1985 vehicles. Three measures of fuel economy were analyzed. EPA Fuel Economy used the calculation defined in the Federal Register and is an attempt to correct for changes in fuel properties. Volumetric Fuel Economy is based on a carbon balance calculation and is a measure of the actual volume of gasoline burned. Energy Specific Fuel Economy is a measure of fuel economy based on energy content.
The following fuel changes resulted in reductions of Volumetric Fuel Economy in both fleets: reduced aromatics, reduced olefins, reduced T90, and addition of oxygenates. Changes in RVP did not have a significant effect on fuel economy. Changes in fuel economy were highly correlated with either fuel heat of combustion or with a combination of fuel density and oxygen content.
Analysis of R, a measure of the sensitivity of the vehicle/fuel system to changes in fuel energy content showed that the values should be in the range of 0.93-0.94 rather than the 0.6 currently used to calculate EPA Fuel Economy.
CO2 emissions were directly related to the carbon-hydrogen ratio of the fuel.

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