The illustration above shows a methane molecule that has
oxidized completely.
Sometimes the combustion of a fuel is not complete. For
example, if the fuel mixture is too rich—that is, the
fuel-to-oxygen ratio is too high—there will not be
enough oxygen to allow the fuel to fully oxidize. This can
result both in the formation of carbon monoxide (CO) instead
of CO2 and in a sooty flame.
