Energy Balance

• The energy balance for a fuel is the ratio of energy it took you to produce the fuel vs. the energy you obtain by burning it. The energy balance for corn ethanol has been disputed in the past by advocates and opponents, and the figures remain contentious. According to a 2007 study by researchers at MIT, the energy balance for corn ethanol is close to 1, meaning that you need to put in nearly as much energy to produce the ethanol as you obtain by burning it. According to a 2007 National Geographic article, the energy balance is about 1:1.3. Since the energy needed to distill the ethanol is supplied by fossil fuel, this also means the process emits a substantial amount of carbon dioxide (CO2). Sugarcane ethanol has a better energy balance and reduces CO2 emissions, largely because the waste biomass, such as fiber, from the sugarcane is burned to supply energy for distillation.
  
  

Energy Density

• The energy density of a fuel is the amount of energy it delivers per unit volume or mass. Although ethanol is denser than gasoline (0.79 grams per milliliter for ethanol as opposed to 0.7 for gasoline), the amount of energy per gram for ethanol is 40 percent less than the energy per gram for gasoline. Consequently, it takes more ethanol to deliver the same amount of energy, decreasing the miles per gallon on your car and making ethanol a less-efficient fuel.
  
  

Transportation

• Unlike gasoline, ethanol can absorb water from the air, and it's also fairly good at picking up impurities. Consequently, ethanol cannot be pumped through pipelines together with gasoline; it has to be transported by truck, train or barge instead. These modes of transport are more expensive and can increase the CO2 emissions associated with production and use. The water absorbed by ethanol can lead to rusting and corrosion on metal surfaces in pipelines and storage tanks during transport.
  
  

Engines

• The ethanol inside your gas tank can absorb water. At low concentrations, this won't cause a problem; at higher concentrations, however, some of the water may separate from the fuel and cause rusting, which could potentially damage your engine. Most engines can handle blends of 90 percent gasoline/10 percent ethanol, but richer blends with more ethanol can lead to lean running conditions and cause corrosion, possibly damaging the engine. Engines need to be converted before they can run on blends like E85, which contains 85 percent ethanol.