Table 8.1 compares the physical properties of E85 and gasoline. The property differences that exert the most influence on vehicle performance are: octane, energy density, Reid vapor pressure, stoichiometric A/F mixture, heat of vapor­ization, and flammability limits. The effects of these different properties are described below.

Fuel Economy

The energy density on a mass basis for E85 is only about 68% of the level for gasoline. Fortunately, the specific gravity of E85 is slightly greater than that for gasoline leading to an E85 energy density of 71% of that for gasoline on a volume basis. Therefore, in order to achieve the same level of power, and assuming no change in engine efficiency, a vehicle operating on E85 would have to consume about 1.4 times as much fuel on a volume basis. This would lead directly to a 29% loss in fuel economy. However, in practice, this reduction is limited as the E85 fuel burns more cleanly, and the engine calibration is adjusted to advance the spark timing, further improving engine efficiency.

Actual test values for FFVs are published by the U. S. federal government; a portion of this is shown in Table 8.3. This data reveals an average loss of about 25% in fuel economy on both the federal highway and city tests when going from gasoline to E85 in FFVs.16 It is interesting to note that the test data reveal losses as high as 29% and as low as 20%, demonstrating the effect that proper engine

calibrations can have when using E85 with the same FFV. Other sources have suggested lower losses in fuel economy (miles per gallon) of only a 5% to 12% during real-world driving conditions.13

A dedicated E85 vehicle could perform better by taking advantage of the higher octane of E85 compared to gasoline. As shown in Table 8.1, E85 enjoys about a 10% advantage in octane rating. Studies have shown that engines could then be designed with higher compression ratios, increasing their operating effi­ciency by up to 10%.14 This efficiency, coupled with the increased power extrac­tion during the expansion stroke of the engine due to the increased volume of the combustion products results, in a total efficiency increase of up to 15% compared to gasoline engines. If vehicles were designed to take full advantage of E85, they would probably experience a fuel economy penalty of about 14% on a volume basis when compared with a gasoline powered vehicle. Although it is important to note that engine calibrations, as shown earlier, can have a dramatic impact upon this value.

In conclusion, it is important to note that the actual energy efficiency for vehicles using E85 is higher than those using gasoline; however, the fuel economy, expressed on a miles-per-gallon basis, is lower due to the lower energy density of E85 on a volumetric basis. Thus, it is environmentally beneficial to use E85 even though its use will probably result in higher fuel usage on a volumetric basis.