Ethanol Vehicle Emissions

When blended with gasoline for use as a vehicle fuel, ethanol can offer some emissions benefits over gasoline, 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). However, when compared to gasoline, the use of high-level ethanol blends, such as E85, generally result in lower emissions levels.

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 a technique 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 or well-to-wheels, to determine the merits or problems associated with each fuel.

Using ethanol as a vehicle fuel has measurable GHG emissions benefits compared with using 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 ethanol produce less net CO2 than conventional vehicles per mile traveled.

A 2007 study by Argonne National Laboratory found that when these entire fuel life cycles are considered, using corn-based ethanol instead of gasoline reduces life cycle GHG emissions by 19% to 52%, depending on the source of energy used during ethanol production (see graph). Using cellulosic ethanol provides an even greater benefit—reducing GHG emissions by up to 86%. Recent studies have shown the importance of incorporating assumptions about future crop production rates and land use into life cycle analyses. These factors can substantially affect net GHG emission calculations.

Graph indicating high greenhouse gas emissions for gasoline, then reductions with corn ethanol and further reductions with sugarcane biomass at a 78% reduction and cellulosic biomass at a 86% reduction.
Source: Life cycle Energy and Greenhouse Gas Emission Impacts
of Different Corn Ethanol Plant Types (2007)

and DOE Bioenergy Technologies Office

The Argonne study also found the well-to-wheel petroleum-use reduction to be more than 70%, regardless of the ethanol production pathway 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. The U.S. Environmental Protection Agency is currently assessing an additional variable—indirect land use—and the effect it will have on the overall GHG emissions from corn-based ethanol.

Evaporative and Tailpipe Emissions

Evaporative emissions are highly dependent on temperature, vehicle activity, and vehicle system materials. The majority of these emissions occur when the car is sitting 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—a high-level, gasoline-ethanol blend—is less volatile than gasoline and low-level ethanol blends and results in lower evaporative emissions. (See the Coordinating Research Council's Fuel Permeation from Automotive Systems report.)

Tailpipe emissions result from fuel combustion in a vehicle's engine and are emitted from its exhaust system. Emissions of primary concern include hydrocarbons, oxides of nitrogen (NOx), carbon monoxide (CO), air toxics, and CO2. Numerous studies have compared the emissions of E85 and gasoline. Although E85 increases the emissions of acetaldehyde in vehicles, it has been found to reduce emissions of CO2, as well as the emissions of many harmful toxics, such as benzene.