Determination of the R Factor for Fuel Economy Calculations Using Ethanol-Blended Fuels over Two Test Cycles

During the 1980s, the U.S. Environmental Protection Agency (EPA) incorporated the R factor into fuel economy calculations in order to address concerns about the impacts of test fuel property variations on corporate average fuel economy (CAFE) compliance, which is determined using the Federal Test Procedure (FTP) and Highway Fuel Economy Test (HFET) cycles. The R factor is defined as the ratio of the percent change in fuel economy to the percent change in volumetric heating value for tests conducted using two differing fuels. At the time the R-factor was devised, tests using representative vehicles initially indicated that an appropriate value for the R factor was 0.6. Reassessing the R factor has recently come under renewed interest after EPA's March 2013 proposal to adjust the properties of certification gasoline to contain significant amounts of ethanol. This proposed change will likely result in a significant deviation from the CAFE baseline test fuel heating value, and thus increased importance of the R factor. This paper reports on the analysis of fuel economy data from three relatively large vehicle studies recently conducted using ethanol-blended fuels; the analysis determines the value of R for Tier-1 and Tier-2 compliant vehicles. These data sets include the DOE Intermediate Ethanol Blends Immediate Effects Study, the EPAct/V2/E-89 project results, and the DOE Intermediate Ethanol Blends Catalyst Durability Program. The Immediate Effects Study and EPAct test program yielded measurements for R of 0.891ŷ0.075 and 0.921ŷ0.010, respectively for the LA92 drive cycle. Both studies using the LA92 cycle observed a tendency for the R value to rise for E15 and E20 fuels. The results of the DOE Intermediate Ethanol Catalyst Durability Program produced an average R factor of 0.949ŷ0.041 for the FTP cycle. Increasing fuel ethanol content did not have a significant effect on FTP R factor values for the FTP cycle.