4.5 What are oxygenates?

Oxygenates are just preused hydrocarbons :-). They contain oxygen, which can
not provide energy, but their structure provides a reasonable antiknock
value, thus they are good substitutes for aromatics, and they may also reduce
the smog-forming tendencies of the exhaust gases [15]. Most oxygenates used
in gasolines are either alcohols ( Cx-O-H ) or ethers (Cx-O-Cy), and contain
1 to 6 carbons. Alcohols have been used in gasolines since the 1930s, and
MTBE was first used in commercial gasolines in Italy in 1973, and was first
used in the US by ARCO in 1979. The relative advantages of aromatics and
oxygenates as environmentally-friendly and low toxicity octane-enhancers are
still being researched.

Oxygenates that are added to gasoline function in two ways. Firstly they
have high blending octane, and so can replace high octane aromatics
in the fuel. These aromatics are responsible for disproportionate amounts
of CO and HC exhaust emissions. This is called the "aromatic substitution
effect". Oxygenates also cause engines without sophisticated engine
management systems to move to the lean side of stoichiometry, thus reducing
emissions of CO ( 2% oxygen can reduce CO by 16% ) and HC ( 2% oxygen can
reduce HC by 10%) [17], and other researchers have observed similar
reductions also occur when oxygenates are added to reformulated gasolines
on older and newer vehicles, but have also shown that NOx levels may
increase, as also may some regulated toxins [18,19,20].

However, on vehicles with engine management systems, the fuel volume will be
increased to bring the stoichiometry back to the preferred optimum setting.
Oxygen in the fuel can not contribute energy, consequently the fuel has less
energy content. For the same efficiency and power output, more fuel has to
be burnt, and the slight improvements in combustion efficiency that
oxygenates provide on some engines usually do not completely compensate for
the oxygen.