Oxygen Content

Some alternative fuels are not hydrocarbons; alcohols and biodiesel contain oxygen atoms as well as carbon and hydrogen. Here are the chemical structures of the common alcohol fuels:

Methanol, CH3OHmethanol (CH3OH) and
Ethanol, C2H5OHethanol (C2H5OH).

(Biodiesel molecules are "monoalkyl esters", but I haven't been able to trace down anything more specific. The "ester" part of that name, however, indicates that the molecules include oxygen atoms.)

In many parts of the USA, gasoline is "oxygenated" during at least part of the year; this means that oxygen-bearing compounds are added to the fuel mixture. The reason for doing this is that having some oxygen as part of the fuel molecules to start with promotes more complete combustion, so that less carbon monoxide, soot, and unburned hydrocarbons come out the tailpipe, as described above. Alcohol fuels and biodiesel carry this one step further, in that the oxygen-bearing compound is not an additive at the 5 to 10 percent level, but a major constituent of the fuel, which increases the benefits of oxygenation.