Incomplete Combustion – Why Petroleum is More Polluting
The second chart at left shows the molecular structure typical of gasoline – you can see that it’s much more complex than the simple structure of methane (natural gas) shown above at left . You’ll notice that there are eight (8) carbon atoms and eighteen (18) hydrogen atoms in each molecule. Because of the complexity of the molecule, gasoline tends not to completely combust in an engine.
When a hydrocarbon fuel (that is one that’s made of a combination of hydrogen and carbon) burns completely, the oxygen in our air combines with the hydrogen to form water (H2O) and with the carbon to form carbon dioxide (CO2). If the combustion is not complete, then some of the carbon atoms only combine with one oxygen instead of two, and form carbon monoxide (CO) as a result – a poisonous gas that is lethal to humans if concentrated.
The third chart below shows the very complex molecule structure typical of diesel. There are sixteen (16) carbon molecules and thirty-four (34) hydrogens. The ratio of carbon to hydrogen is nearly double to the ratio of carbon to hydrogen found in methane (CNG). Because of its complex structure, diesel fuel does not completely combust in an engine.
Along with carbon monoxide pollution resulting from incomplete combustion, other carbon atoms may remain stuck together with each other and with some of the hydrogen atoms also; these unburned hydrocarbon molecules come out of the tailpipe. The unburned hydrocarbons then react with nitrogen oxides (another pollutant from combustion) in the presence of sunlight to form ozone, which is a lung irritant.
FACT: high level ozone in the stratosphere is a shield against the sun’s ultraviolet light, but ground level ozone is the main component of “smog”. Carbon atoms can also remain stuck to one another with few or no hydrogen atoms attached, especially during incomplete combustion of diesel fuel, producing soot. Soot has been linked to a significant increase in respiratory illness in school age children. Find out more about health risks of diesel from the EPA:
http://www.epa.gov/mobile-source-pollution/ (link opens in new window)
Because alternative fuels such as CNG have simpler molecules, they burn more completely in an engine, so that less carbon monoxide, soot, and unburned hydrocarbons come out the tailpipe. Methane, or natural gas, in particular is almost incapable of forming smog. These are reasons that CNG is much less polluting than gasoline and diesel.