Comparison Methodology
Most emissions studies compare alternative fuels to reformulated gasoline (RFG). RFG is a gasoline designed to reduce ozone-forming and toxic pollutants from vehicle emissions. Therefore, the actual benefits of alternative fuels may be greater than what these studies indicate because more than half of all drivers use conventional gasoline, which emits more emissions than RFG.
Due to the variability in these studies and the multiple metrics that they use, this Web section does not attempt to compare all alternative fuels directly. However, the following studies do attempt this through the use of models, adjustments, and assumptions.
Emissions of Criteria Pollutants, Toxic Air Pollutants, and Greenhouse Gases from the Use of Alternative Transportation Modes and Fuels, a report available on the University of California-Davis Web site, directly compares emission from gasoline, diesel, E85, compressed natural gas, liquefied petroleum gas, and M85.
Argonne National Laboratory's GREET Model measures pollutants emitted per mile by various fuels in different vehicles. It compares specific fuel and vehicle combinations, including all the combinations covered in this Web section. However, the methodology and assumptions documented in the GREET 1.5 Report (PDF 5.8 MB) are needed to determine if the database is applicable to your fleet or location. Download Adobe Reader.
Full Fuel Cycle Assessment: Well-to-Wheels Energy Inputs, Emissions, and Water Impacts (PDF 1.5 MB), a report available on the California Energy Commission Web site, compares the GHGs, criteria pollutants, air toxics, and energy balances of many fuel and vehicle combinations. Download Adobe Reader. In addition to alternative fuels, it includes premarket fuels such as cellulosic ethanol, renewable diesel, biomass-to liquids, coal-to-liquids, and tar-sand gasoline. Appendix A shows emissions-per-mile data for each fuel and vehicle combination in a directly comparable format. However, the report uses California-based factors and parameters, so its numbers are not entirely applicable to other locations.
Lifecycle GHG Emissions and Petroleum Use
This Web section judges fuels according to their lifecycle (well-to-wheels) petroleum use. This metric is a primary indicator of a fuel's energy security since much of U.S. petroleum is imported from unstable regions. A previous metric, fossil energy balance, is not emphasized in this Web section because it can be misleading and cause people to assume that fuels with the same fossil energy balance have the same GHG emissions. This assumption is false because each fossil fuel has different GHG emission rates per unit energy. The fossil-fuel-use metric is only useful beyond the GHG and petroleum metrics if someone were trying to conserve all nonpetroleum fossil fuels that are domestically produced and secure, merely for the sake of conservation—not for the sake of emissions reductions.