The three most common types of fossil fuel used for transportation are as follows:
- Gasoline (cars, trucks)
- Diesel (cars, trucks, buses, rail locomotives, ships)
- Jet fuel (aviation)
Although these fuels differ greatly in how they are used and handled, for the purposes of energy and GHG calculations, they are all more-or-less the same.
Gasoline is the most common liquid fuel for internal combustion engines. Its chemical composition is complicated and variable, but a simple approximation removes all these uncertainties and allows to make sufficiently accurate energy and CO$_2$ production calculations.
The most important characteristics of gasoline for the purposes of our calculations are:
1. Enthalpy of combustion (“energy content”) in terms of MJ/kg and MJ/L. These two quantities are linked via the density in kg/L.
Engineering Toolbox[note]Engineering Toolbox https://www.engineeringtoolbox.com/fuels-higher-calorific-values-d_169.html [2019-09-13][/note] gives an enthalpy value of 46.4 MJ/kg noting that this value may vary either way by 5-10%. The density of 737 kg/m$^3$ (0.737 kg/L) thus gives an enthalpy of 34.2 MJ/L.
2. Hydrogen to carbon ratio (H/C): the three main components of gasoline are isooctane, butane and 3-ethyltoluene [note]Gasoline: chemical analysis and production, https://en.wikipedia.org/wiki/Gasoline#Chemical_analysis_and_production [2019-09-13][/note], whose H/C ratios are 18/8, 12/9 and 12/5 respectively. In other words, H/C $\approx$ 2.
Using H/C = 2, we can calculate the CO$_2$ production for a given quantity of fuel burned.
CH$_2$ + 3/2 O$_2 \rightarrow$ CO$_2$ + H$_2$O $\tag{1}$
Taking the molar masses of CH$_2$ to be 14 and CO$_2$ to be 44, this means that 14 kg of gasoline (14/0.737 = 19.0 L) burns to 44 kg of CO$_2$. Thus 1 L of gasoline burns to 44/14 = 2.3 kg of CO$_2$. This is the generally accepted value[note] “How much carbon dioxide is produced by burning gasoline and diesel fuel?”. U.S. Energy Information Administration (EIA). [2013-10-27][/note], i.e. H/C = 2 is a perfectly usable approximation.
Diesel
Diesel fuel is a heavier fraction of crude oil and consists of a mixture of different hydrocarbons with an average chemical composition of C$_{12}$H$_{24}$[note]Diesel, chemical analysis, https://en.wikipedia.org/wiki/Diesel_fuel#Chemical_analysis [2019-09-16].[/note]. Thus H/C = 2 is as good an assumption as for gasoline, and CO$_2$ emissions in terms of kg/kg will be the same.
The enthalpy of combustion is 45.6 MJ/kg, 38.6 MJ/L with a density of 846 kg/m$^3$[note]Engineering Toolbox https://www.engineeringtoolbox.com/fuels-higher-calorific-values-d_169.html [2019-09-13][/note].
Jet Fuel
Jet fuel, a.k.a. aviation kerosene or Jet-A, has a mean chemical composition of C$_{12}$H$_{26}$[note]Kerosene, https://en.wikipedia.org/wiki/Kerosene#Engines [2019-09-16].[/note], i.e. H/C = 2.17 or a little larger than 2. The CO$_2$ emissions in terms of kg/kg will be a few % less than for gasoline or diesel.
The enthalpy of combustion is 43 MJ/kg, 35 MJ/L with a density of around 800 kg/m$^3$[note]Jet Fuel, https://en.wikipedia.org/wiki/Jet_fuel [2019-09-13][/note].
| Fuel | C:H ratio | Enthalpy MJ/kg | Enthalpy MJ/L | Density kg/L | kg(CO$_2$)/L |
| Gasoline | CH$_2$ | 46.4 | 34.2 | 0.74 | 2.3 |
| Diesel | CH$_2$ | 45.6 | 38.6 | 0.85 | 2.7 |
| Jet A | CH$_{2.17}$ | 43 | 35 | 0.80 | 2.5 |
Table 1. Summary of liquid fossil fuels used in transportation.
The values shown in the summary Table 1 are typical, not definitive. The bottom line is that they are all about the same. If you assume that any given liquid fuel has an enthalpy of combustion of 35 MJ/L, a density of 0.8 kg/L and produces when burnt 2.5 kg of CO$_2$/L, you will not be far wrong.
Updated (CEW) 2019-10-03
