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It is an indisputable fact that bioethanol reduces automotive carbon-based tailpipe emissions relative to gasoline, despite some petroleum zealots who may try to argue otherwise. I’d tell those individuals to go back to school and take a class in organic chemistry (or, in the spirit of practicality, to read this article) before stating their case!
There are two mechanisms through which bioethanol reduces emissions relative to gasoline. The first is the ratio of hydrogen to carbon in the fuel. The following equations show the complete combustion of one average gasoline molecule (weighted carbon and hydrogen average content of gasoline) and one bioethanol molecule:
Gasoline: C7H13 + 10.25O2 → 7CO2 + 6.5H2O
Bioethanol: C2H5OH + 3.5O2 → 2CO2 + 3H2O
Fuels produce power through the exothermic oxidation of carbon (C) and hydrogen (H) to carbon dioxide (CO2) and water (H2O). Note that gasoline produces more carbon dioxide than water, whereas bioethanol produces 50 percent more water than carbon dioxide. Water during combustion improved atomization and mixing, which leads to an increase in the combustion efficiency and thus, a higher engine output.
Bioethanol has more water because it contains three hydrogen atoms for every carbon atom, whereas gasoline contains 1.85 hydrogen atoms for every carbon atom. Oxidation of hydrogen produces water, hence the dreams of environmentalists who don’t drive electric cars to develop hydrogen-powered propulsion systems. Bioethanol does not produce zero-carbon emissions as pure hydrogen would, but it does derive significantly more of its power from non-carbon water production than gasoline does because of its increased hydrogen content.
The second mechanism of carbon-based emissions reduction is proximity to oxygen. Gasoline combustion requires every carbon atom in every gasoline molecule to find two oxygen atoms from the air to form carbon dioxide (CO2). This creates a giant game of “Where’s Waldo,” which is only successful if the gasoline and air are mixed into a fully homogeneous mixture at the molecular level.
Carbon atoms that don’t find two oxygen atoms are emitted as carbon monoxide (CO) or elemental carbon particulate emissions (smoke). This is a challenge in modern gasoline direct-injection engines, where fuel is injected directly into the combustion chamber just before ignition with little time for mixing with air. Bioethanol, on the other hand, has an oxygen atom embedded in the fuel molecule, reducing the likelihood of carbon atoms unable to find oxygen atoms. A stark example is the bright yellow color and smoke emissions when burning heavy-carbon, liquid hydrocarbon fuels like gasoline compared to the light blue, virtually smoke-free flame when burning bioethanol.
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Put simply, gasoline generates more carbon dioxide, while bioethanol produces more water, enhancing combustion efficiency and engine output. Bioethanol’s higher hydrogen content results in more water production and fewer carbon emissions. Additionally, bioethanol contains an oxygen atom, facilitating more complete combustion and reducing emissions like carbon monoxide and soot.
In conclusion, increased hydrogen content and oxygen embedded in the fuel cause bioethanol to produce fewer carbon-based tailpipe emissions than gasoline. Blends of gasoline and bioethanol burn cleaner than pure gasoline, but not as clean as pure bioethanol. Indeed, carbon emissions decrease as the percentage of bioethanol increases. This pathway to reduced emissions is available to us now, today. Federal approval of higher bioethanol blends, like E15, is the next step toward healthier air and a cleaner environment!
