Gasoline-equivalent gallon

A gasoline-equivalent gallon is a concept used to compare the energy content of gasoline and alternative fuels, which often have different caloric values than gasoline. Therefore, in order to properly compare prices between fuels, they may be sold in quantities energetically equivalent to a gallon of gasoline.

The energy content is determined by a fuel's enthalpy of combustion. This, however, can be problematic. Different fuels tend to be more efficient than others. Ethanol, for example burns more slowly and at a lower temperature than gasoline. This makes it easier to extract the chemical energy stored inside it. While a gallon of gasoline has about 50% more energy than ethanol, cars that run on ethanol can get very similar mileage to gasoline powered vehicles when they are optimized to take advantage of ethanol's higher octane rating. This optimization involves increasing the gasoline engines compression ratio from a normal of 9 or 10 to one, to as high as 16 to one. This involves significant and expensive refitting with different pistons (to reduce the combustion chamber size) and mechanical alterations to assure valve to piston clearance. When this optimization is completed, the engine is no longer suitable to operate on 100% gasoline, as the high compression ratio will cause severe knocking (which is very harmful to the engine). Ordinary consumers driving a "flex-fuel" vehicle can expect a substantial drop in fuel mileage when using 85% ethanol products (the compression ratio is fixed mechanically, and electronic sensors can only modify the timing of the spark and allow the electronic fuel injectors to provide more of the reduced BTU value fuel).