Ethanol Vehicle Emissions

When blended with gasoline for use as a vehicle fuel, ethanol can offer some emissions benefits depending on vehicle type, engine calibration, and blend level. As with conventional fuels, the use and storage of ethanol blends can result in emissions of regulated pollutants, toxic chemicals, and greenhouse gases (GHGs).

Today's emissions standards require ethanol/gasoline-capable flexible fuel vehicles (FFVs) to meet the same emissions standards as conventional vehicles, regardless of the fuel used.

Life Cycle Emissions

Life cycle analysis is used to assess the environmental impacts of all stages of a product's life, including raw material extraction, processing, manufacturing, distribution, use, and disposal or recycling. When comparing fuels, a life cycle analysis may focus on particular portions of a fuel's life cycle, such as from extraction-to-use, also referred to as well-to-wheels, to determine the merits or problems associated with each fuel.

Using ethanol as a vehicle fuel has measurable GHG emissions benefits when considering the life cycle steps required for gasoline. Carbon dioxide (CO₂) released when ethanol is used in vehicles is offset by the CO₂ captured when crops used to make the ethanol are grown. As a result, FFVs running on high-level blends of ethanol produce less net CO₂ than conventional vehicles per mile traveled.

An analysis by Argonne National Laboratory found that using corn-based ethanol in place of gasoline reduces life cycle GHG emissions on average by 40%. Using cellulosic ethanol provides an even greater benefit. A 2012 study by Argonne National Laboratory found that when fuel life cycles are considered, average emissions reductions of cellulosic ethanol compared to conventional gasoline range from 88% to 108% depending on feedstocks used.

In addition to life cycle GHG emissions and petroleum use, numerous studies have examined the life cycle energy balance of ethanol. These studies are addressed in the Ethanol Energy Balance section.

Evaporative and Tailpipe Emissions

Evaporative emissions include the emissions that evaporate from fuel in open-air conditions. They are highly dependent on temperature, vehicle activity, and vehicle system materials. Most of these emissions occur when the car is parked or refueling. Because low levels of ethanol can cause gasoline to evaporate more easily, low-level ethanol blends can increase evaporative emissions in vehicles. However, vapor pressure for low-level ethanol blends can be adjusted to adhere to the same volatility standards as gasoline.

E85 (or flex fuel)—a high-level, gasoline-ethanol blend—is less volatile than gasoline and low-level ethanol blends and results in lower evaporative emissions.

Tailpipe emissions result from fuel combustion in a vehicle's engine. Emissions of primary concern include hydrocarbons, oxides of nitrogen (NO_x), carbon monoxide (CO), air toxics, and CO₂. Numerous studies have compared the emissions of E85 and gasoline. E85 decreases the emissions of CO₂, as well as the emissions of many harmful toxics, such as benzene—a known carcinogen. However, it increases acetaldehyde emissions, which the National Institutes of Health describe as "reasonably anticipated to be a human carcinogen" and is moderately reactive for ground level ozone formation.