How Green Is Ethanol?

Does Corn-Based Ethanol Help the Environment?

How much energy do you save if you run your car, truck or SUV on an ethanol blend instead of ordinary gasoline?

None. It takes the same amount of energy to move you and your vehicle no matter what form of fuel you use. The only way to save energy is to get a more efficient vehicle (that gets higher mileage) or to drive less.

How much fossil petroleum (oil) do you save if you run your car, truck or SUV on an ethanol blend instead of ordinary gasoline?

Not much. One gallon of corn ethanol displaces 0.67 gallons of gasoline. (Ethanol has 0.7 times the energy content of gasoline, and it takes about 0.03 gallons of petroleum fuels to make a gallon of ethanol from corn.) So if you use E10 (a blend of 10% ethanol with 90% gasoline) you would save 0.0028 gallons of fossil petroleum for every mile you drive (if you get 25 mpg). If you drive the U.S. average of 12,500 miles per year, you would reduce fossil petroleum consumption by close to 35 gallons. Per year. Total.

If you used E85 (85% ethanol and 15% gasoline, for which you would need a "flex-fuel" vehicle, which make up only 2% of the U.S. passenger and light truck vehicle fleet) you would save 0.28 0.03 gallons of fossil petroleum fuel for each mile you drive (if you got 18 mpg with E85--remember ethanol has less energy per gallon than gasoline. You'd have to buy about 15% more gallons to get through your yearly driving). For 12,500 miles a year, that is 3,700 395 gallons. If all the 4 million flex-fuel vehicles in the U.S. actually used E85 (less than 1% do), together they would reduce demand for gasoline from fossil petroleum by 15 million 1.6 billion gallons annually. That is about 0.014% 1.4% of total gasoline consumption in the U.S. (one one-hundredth of one percent). [Updated 2012-07-28]

How much greenhouse gas emissions do you save by using ethanol?

We don't know. If you use E10 you either increase greenhouse gas emissions by about 2%, or reduce them by about 2%, or somewhere in between, depending on how the greenhouse gas contributions of the byproducts of ethanol production from corn are accounted for. Basically, using E10 instead of gasoline cuts your greenhouse gas output by very little, and might even increase it a bit.

If you use E85, you either increase greenhouse gas emissions by about 20% or reduce them by about 20%, again depending on how you define the system boundaries. How much is a matter of opinion. Of course, practically nobody uses E85 – There are no E85 stations within 100 miles of Los Angeles, for example. (see E85 Refueling)

Moral

Do the math. If you want to reduce petroleum use and greenhouse gas emissions you have to change what you drive, how you drive, and how much you drive. Switching fuels won’t make much difference.

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A principal source for the analysis above was Review of Corn Based Ethanol Energy Use and Greenhouse Gas Emissions. T. Groode. LFEE Working Paper 07-1, June 2006, from The Laboratory For Energy and the Environment at the Massachusetts Institute of Technology.

Good summary of Groode’s analysis in energy & environment, October 2006, the newsletter of the MIT Laboratory for Energy and the Environment.