Fundamentals
5-13
Oxidation of any compound requires an abundance of O
2
. Some
engines use Air Injection Reaction (AIR) or pulse air type systems
to provide the supplemental O
2
needed. These systems supply
additional air to the exhaust manifold or catalytic converter.
As converter technology has progressed, catalysts treating NO
x
compounds were developed. With a reduction reaction, these
catalyst reduced NO
x
to N
2
and O
2
. Three-way converters oxidize
HC and CO and reduce NO
x
. These converters are even more
sensitive to air-fuel ratio because the reduction of NO
x
is efficient
only at stoichiometric air-fuel ratio.
Two-way and three-way catalytic converters only lower HC, CO
and NO
x
by a certain amount. When an engine operates properly,
the converter decreases emissions to levels lower than the amount
specified by state and federal regulations.
Figure 5-11
: HC and CO Oxidation in a Catalytic Converter
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