Ask A Scientist© Chemistry Archive

name         Heather S.
status       other
age          20s

Question -   How do oxygenates in gasoline help reduce NOx
emissions?  I understand that the presence of more oxygen makes
combustion more complete, reducing CO and hydrocarbons.  But how does
increased oxygen lower the temperature of combustion for reduced NOx
emissions?
I would like to know how the increased percentage of oxygen in gasoline
reduces NOx.  As I understand it, NOx are reduced when the air/fuel mixture
in a combustion engine burns at a lower temperature.  Adding oxygen to
combustion makes the process more complete but does not necessarily lower
the temperature.  Doesn't it acutally increase the temperature?!
-----------------------------
Heather,

High compression engines require a superior grade fuel or antiknock 
compounds in
the their formulation. In any case (as you correctly stated) the reactions
occurring when such an engine operates with suitable fuel are producing high
temperatures that encourage formation of NOx. Lowering the compression ratio
also lowers combustion temperatures and thereby reduces NOx formation.

Oxygenates in the fuel simply allow poorer quality fuel to serve at lower
combustion temperatures without producing destructive engine knock.

Because there is a limit to the amount of oxygen available in the air, and so
little time for the fuel to burn as the engine runs, oxygenated fuel 
assists in
more complete combustion by adding oxygen as part of the fuel itself. Thus,
lower CO and hydrocarbon emissions.

Lower combustion temperature reduces NOx, oxygenates reduce CO and hydrocarbon
emissions, and everyone seems to be happy. However, engine efficiency and fuel
economy suffers. This makes no one happy.

Regards,
ProfHoff
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