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Green Vehicle Guide

Fuels: Learn More about Fuel Assumptions in the Choose a Path Tool

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Fuel pump thumbnailVehicles that operate primarily on gasoline or diesel have historically accounted for over 99% of cars and passenger truck sales. However, sales of cars that operate on alternative fuels like ethanol, natural gas, and electricity are growing. Many alternative fuels "burn" cleaner than gasoline or diesel, so there are fewer tailpipe emissions. Total emissions from fuel also accounts for how they are produced. For example, electricity produced by coal has much higher emissions than electricity produced from wind or solar power, which we count as a zero emissions fuel in this tool.

  • Fuel volumes are shown as billion gallon gasoline equivalents (BGGe), an energy-equivalent metric. For example, 33.705 kilowatt-hours of electricity contains the same energy as 1 gallon of gasoline; therefore 1 BGGe of electricity is equal to 33.705 billion kilowatt-hours of electricity.
  • Certain scenarios will increase, rather than decrease, GHG emissions. For instance, if you select a fuels mix that has lifecycle emissions that are worse than today, it will increase overall emissions, unless you have offset those emissions through activity reductions or MPG improvements.

Assumptions & Calculations

Baseline assumption: We calculate GHG reductions relative to today’s fuel mix, based on DOE’s Annual Energy Outlook (AEO) 2015 reference case for 2015, adjusted to ensure that the 2015 volumes required by the Renewable Fuel Standard program are met. As a simplifying assumption, the categories for advanced, cellulosic, and biomass-based diesel are all considered as “advanced biofuels” for the purpose of the tool. We made a further simplifying assumption that three-quarters of the 2015 biofuel volumes (on an energy basis) are consumed by passenger cars and trucks while the other quarter is consumed by freight trucks and other vehicles not covered by the tool. This results in a fuel mix of roughly 92.5% gasoline, 5.5% conventional biofuels, 1.1% advanced biofuels, and <1% each of diesel, natural gas, propane, and electricity.

Lifecycle GHG assumptions: The fuel lifecycle GHG estimates in the table below represent a combination of tailpipe emissions and upstream emissions from individual motor fuels. Upstream emissions include emissions associated with fuel production such as feedstock extraction, feedstock transport to a processing plant, and conversion of feedstock to motor fuel, as well as distribution of the motor fuel.

Fuels % Reduction Compared to 2015 Baseline gCO2e/KJ Sources
Baseline Fuel Mix 0% 0.091 See lifecycle GHG estimates for individual fuels in the baseline mix below. For the 0.1% of propane in the baseline fuel mix, we assume an emissions factor of 0.082 gCO2e/KJ.
Gasoline -2% 0.093 Gasoline and diesel lifecycle GHG estimates are based on a 2005 baseline from Renewable Fuel Standard (RFS2) Program: Final Rule (March 2010).
Diesel -1% 0.092
Compressed Natural Gas 17% 0.076 Approximate based on estimates from Argonne National Laboratory’s GREET Model, GREET1_2015, available at: https://greet.es.anl.gov/.
Conventional biofuel 19% 0.074 Conventional biofuels may include corn ethanol, ethanol and biodiesel from other feedstocks, or other fuels generated from renewable biomass. The emissions factor represents a 20% reduction below 2005 gasoline lifecycle GHG emissions, which is the threshold for fuels to qualify for the “Renewable fuel” category in the RFS2 program. Some conventional biofuels may have significantly lower emissions. However, biofuels with emissions at least 50% lower than gasoline or diesel are counted as advanced biofuels (described below).
Electricity 21% 0.072 There are no tailpipe emissions. Upstream emissions rate is based on e-GRID (eGRID2012 Version 1.0, available at: eGRID) and represents a long-term 2% reduction per year in the upstream grid GHG emissions rates.

Electricity can be generated from many feedstocks including natural gas, coal, wind, solar, and hydropower as well as biomass. Electricity generated from biomass may qualify as renewable fuel under the RFS2 program and count toward minimum advanced biofuel requirements.
Hydrogen 7% 0.085 There are no tailpipe emissions. Upstream emissions rate is based on the GREET1_2015 estimate of hydrogen produced from steam-methane reforming of natural gas and represents our assumption of a long-term 1% reduction per year in GHG emissions rates due to improvements in hydrogen production process.

Hydrogen can be produced from sources other than natural gas, including water electrolysis and biomass. The latter may qualify as renewable fuel under the RFS2 program and count toward minimum advanced biofuel requirements.
Advanced biofuel 49% 0.047 Represents a 50% reduction below 2005 gasoline lifecycle GHG emissions, which is the threshold GHG reduction for fuels to qualify for the “Advanced biofuel” category in the RFS2 program. For the purposes of this tool we also count cellulosic biofuels, which are required to meet a 60% reduction under RFS2, in this category. Some advanced and cellulosic biofuels may have significantly lower GHG emissions rates than assumed here.

Note that this category can include both ethanol and biodiesel as well as hydrogen, electricity, natural gas, or other fuels produced from biomass that meet the GHG reduction targets.
Zero-emissions fuel 100% 0.000 Renewably produced hydrogen and electricity, as well as certain fuels produced from biomass could have zero or near-zero emissions. Fuels produced from biomass could also help meet RFS2 requirements.

Renewable Fuel Standard (RFS2) requirements: EPA is responsible for developing and implementing regulations to ensure that transportation fuel sold in the United States contains a minimum volume of renewable fuel. The Energy Independence and Security Act of 2007 (EISA) established four categories of renewable fuel, and set separate volume requirements for each one. EISA requires EPA to apply lifecycle greenhouse gas performance threshold standards to ensure that each category of renewable fuel emits fewer greenhouse gases than the petroleum fuel it replaces. These categories are: renewable fuel (20% GHG reduction), advanced biofuel (50% GHG reduction), biodiesel (50% GHG reduction), and cellulosic biofuel (60% reduction). As a simplifying assumption, the categories for advanced, cellulosic, and biomass-based diesel are all considered as “advanced biofuels” for the purpose of the tool and are assumed to meet a 50% GHG reduction threshold. Learn more about RFS2.

We made a further simplifying assumption that 3/4 of the biofuels (on an energy basis) required by the 2015 Renewable Fuel Standard are consumed by passenger vehicles while the other quarter is consumed by freight trucks and other vehicles not covered by the tool. Therefore the default fuel mix at the start of the tool includes approximately 7 BGGE of conventional biofuels and 1 BGGE of advanced biofuels. We intend to update this tool as new biofuel volume thresholds are set in the future.

We additionally assumed biofuels will generally displace gasoline (rather than diesel) in the light-duty sector and therefore estimate lifecycle GHG emissions rates for biofuels based on a reduction relative to the 2005 gasoline baseline.