Gas vs. Diesel: why the European move to Diesel wasn’t eco-friendly
The big difference is when you account for Oxidation, so scroll down to the IPCC figures.
One of the primary determinants of carbon dioxide (CO2) emission from mobile sources is the amount of carbon in the fuel. Carbon content varies, but typically we use average carbon content values to estimate CO2 emissions.
The Code of Federal Regulations (40 CFR 600.113) provides values for carbon content per gallon of gasoline and diesel fuel which EPA uses in calculating the fuel economy of vehicles:
Gasoline carbon content per gallon: 2,421 grams
Diesel carbon content per gallon: 2,778 grams
Note that for the “Inventory of U.S. Greenhouse Gas Emissions and Sinks,” EPA estimates CO2 emissions from fuel from the heat content of the fuel and carbon content coefficients in terms of carbon content per quadrillion BTU (QBTU), using data from the Energy Information Administration (EIA). EIA’s numbers are derived from carbon content by mass, and equate to roughly the same carbon content per gallon of fuel as the values provided in 40 CFR 600.113. EPA uses heat content data from Energy Information Administration’s (EIA) “Annual Energy Outlook 2003” Exit EPA disclaimer and carbon content from EIA’s “Emissions of Greenhouse Gases in the United States, 2000.” Exit EPA disclaimer
Note also that these estimates are based only on an average carbon content of conventional gasoline and diesel fuel, and do not specifically address the impact of fuel additives such as ethanol or methyl tertiary butyl ether (MTBE) that may depend on the feedstock.
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Calculating CO2 emissions
The Intergovernmental Panel on Climate Change (IPCC) Exit EPA disclaimer guidelines for calculating emissions inventories require that an oxidation factor be applied to the carbon content to account for a small portion of the fuel that is not oxidized into CO2. For all oil and oil products, the oxidation factor used is 0.99 (99 percent of the carbon in the fuel is eventually oxidized, while 1 percent remains un-oxidized.)[1.]
Finally, to calculate the CO2 emissions from a gallon of fuel, the carbon emissions are multiplied by the ratio of the molecular weight of CO2 (m.w. 44) to the molecular weight of carbon (m.w.12): 44/12.
CO2 emissions from a gallon of gasoline = 2,421 grams x 0.99 x (44/12) = 8,788 grams = 8.8 kg/gallon = 19.4 pounds/gallon
CO2 emissions from a gallon of diesel = 2,778 grams x 0.99 x (44/12) = 10,084 grams = 10.1 kg/gallon = 22.2 pounds/gallon
Note: These calculations and the supporting data have associated variation and uncertainty. EPA may use other values in certain circumstances, and in some cases it may be appropriate to use a range of values.