Fundamentals
5-3
Oxides of Nitrogen (NO
X
)
Air in the atmosphere, and air admitted into the combustion
chamber of an engine, consists of about 78% nitrogen (N
2
) and
about 21% oxygen (O
2
) by volume. Nitrogen does not contribute to,
or detract from, combustion. When exposed to heat above 2000°F
(1093°C), oxygen and nitrogen combine. Since combustion
chamber temperatures easily exceed 2500°F (1371°C) under
some engine conditions, such as under load, O
2
combines with N
to form harmful oxides, including nitrogen monoxide (NO) and
nitrogen dioxide (NO
2
). These are part of a group of oxides
commonly known as oxides of nitrogen (NO
x
).
NO
x
is an engineering term for an unknown mixture of nitrogen
oxides. NO
x
includes all the N compounds formed in the
combustion chamber of an engine, especially NO and NO
2
. The X
subscript in place of numbers indicates that all NO compounds are
included.
Of the nitrogen oxide compounds, NO is the only compound with
appreciable importance to engine combustion. NO formed in the
combustion chamber persists during the exhaust stroke where it
reacts with additional O
2
, forming NO
2
.
NO is one of the gases that may be checked to determine if the
vehicle complies with specifications.
Figure 5-3
shows the
concentration of NO
x
in relation to air-fuel ratio before treatment in
a three-way catalytic converter.
When based only on air-fuel ratio, NO
x
levels peak at about 15.8:1.
Many other factors affect actual NO
x
formation besides air-fuel
ratio.
The formation of NO
x
does not affect engine performance.
However, some devices used to prevent the formation of NO
x
can
affect engine performance and contribute to higher levels of HC
and CO when not functioning properly.
Figure 5-3
: NO
X
Versus Air-Fuel Ratio
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