Thursday, May 26, 2011
As it is used in the fuel industry, "energy density" means the amount of energy stored in a given liquid, per gallon.
Ethanol has a higher energy density, for example, than methanol. You can drive more miles on a gallon of ethanol than a gallon of methanol.
From ASE Fuel Technology:
Ethanol, and especially methanol, fall short on BTU energy content compared to gasoline. This translates to an estimated 27% to 30% loss of vehicle miles-per-gallon traveled compared to gasoline; it’s even worse for methanol (51%). On the positive side, with ethanol and especially methanol’s higher octane ratings, vehicle operators are impressed with improved torque and horsepower over much of the engine speed range.
From Energy Quest:
The amount of energy in alcohol fuels is different than gasoline. A gallon of gasoline in California contains approximately 111,500 Btu (which stands for British thermal units). By comparison, M85 contains approximately 65,000 Btu/gallon and E85 contains approximately 81,000 Btu/gallon. The lower energy content of these fuels will result in fewer miles per gallon or a shorter driving range. Still, with larger fuel tanks, FFVs often have driving ranges equivalent to conventional gasoline cars. Currently, the Ford Ranger ethanol FFV, beginning with the 1999 model, is the only vehicle available in the U.S. that can run on an alcohol fuel.
One advantage shared by all four alcohols is their high octane rating. This tends to increase fuel efficiency and largely offsets the lower energy density of alcohol fuels (as compared to petrol/gasoline and diesel fuels), thus resulting in comparable "fuel economy" in terms of distance per volume metrics, such as kilometers per liter, or miles per gallon.
From Popular Mechanics:
Methanol has only 51 percent of the BTU content of gasoline by volume.