The potential work that a fuel can do is determined by its energy content. One measure of energy content is British thermal units (Btu) per gallon. Oxygenated gasolines contain less energy per gallon than conventional gasoline1:
| Fuel |
Energy Content2, Btu per Gallon |
Difference from Conventional Gasoline, % |
| Ethanol |
76,000 |
— |
| MTBE |
93,500 |
— |
| Conventional Gasoline |
115,0003 |
0 |
| Conventional Gasoline + 10 Vol % Ethanol |
111,100 |
-3.4 |
| Conventional Gasoline + 15 Vol % MTBE |
111,800 |
-2.8 |
1 Lower heating value (liquid fuel at 60°F to water vapor).
2 Typical value.
The values in the table for the reduced energy content of oxygenated gasolines represent only the volumetric effect of replacing higher-energy-content conventional gasoline with lower-energy-content oxygenate. Often, particularly for gasolines oxygenated with MTBE, there is an additional energy content reduction related to changes in the gasoline portion of the blend. Since adding oxygenate increases octane, refiners adjust the octane of the gasoline portion so that the octane of the finished oxygenated gasoline will be the same as the octane of the conventional gasoline it replaces. The adjustment involves substituting lower-octane gasoline components for higher-octane components. Since some of the higher octane components, particularly aromatics, have higher energy contents, the adjustment lowers the energy content of the gasoline. The additional decrease in energy content is about 0.5 percent.
So, since oxygenated gasolines contain 3.5 percent less energy, they would be expected to yield a proportional reduction in fuel economy.
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