“EIA has adjusted its estimates of the energy content of retail motor gasoline in the Monthly Energy Review (MER) to reflect its changing composition. Ethanol and other oxygenates, which have lower energy content than petroleum-based gasoline components, have seen their share of total gasoline volumes increase from 2% in 1993 to nearly 10% in 2013. As a result, EIA’s estimate of motor gasoline’s average energy content per gallon has declined by about 3% over this 20-year period,” writes the Energy Information Administration (EIA) in its latest Monthly Energy Review.
To better understand the changes, a recent “Today in Energy” looked at how higher U.S. ethanol use has cut the average energy content of a gallon of gasoline.
The EIA explains that the adjustment of the average energy content per gallon of motor gasoline reflects changes in response to 1990 Clean Air Act (CAA) regulations that split the U.S. gasoline market into three segments: conventional, oxygenated, and reformulated. Oxygenated and reformulated gasoline was required to be blended with compounds that contained oxygen, such as MTBE (methyl tert-butyl ether) or ethanol. This Act was designed to reduce toxic air emissions in cities and it was successful. However, EIA states that while these additives reduced air pollution, they also resulted in lower heating value compared with conventional gasoline, translating to fewer miles per gallon, because they have lower energy density.
In response to these regulations, EIA began collecting separate data on the production of conventional, oxygenated, and reformulated gasoline in 1994. The gasoline heating value was estimated based on the relative volumes of conventional, oxygenated, and reformulated gasoline in the total motor gasoline product supplied to the United States. Continue reading